**Meet the editor**

Claudia Belviso is a researcher at the Institute of Methodologies of Environmental Analysis (IMAA) of CNR. Since 2002 she has been carrying out her research activity in the field of mineralogy and geochemistry aimed at environmental protection. She is responsible for research activity on zeolite synthesis from waste materials as well as natural sources and their application to solve

environmental problems. This activity has allowed her to be the inventor of an international patent about zeolite synthesis from fly ash, to publish numerous scientific articles in peer-reviewed journals; to participate in many national and international conferences; to take part in the organisation of international congresses and national and international schools and to carry out scientific researches in national and international projects supported by public and private institutions.

### Contents

#### **Preface XI**



### Preface

Chapter 8 **XAFS and IR Studies on Luminescent Silver Zeolites 153** Takafumi Miyanaga, Yushi Suzuki and Sho Narita

**the Fischer‐Tropsch Synthesis 167**

**Biodegradable Wastes 185**

Aleksandra Badora

**Connection and Food Safety 203**

Arroyo

**VI** Contents

Chapter 9 **Zeolites as Potential Structures in Obtaining Jet Fuel Through**

Chapter 10 **Effects of Natural Zeolites on Bioavailability and Leachability of Heavy Metals in the Composting Process of**

Jiwan Singh, Ajay S. Kalamdhad and Byeong-Kyu Lee

Chapter 11 **The Influence of Zeolites on Quality Indicators of Soil-Plant**

Lidiane Sabino da Silva, Victor Luis dos Santos Teixeira da Silva, Maria Angélica Simões Dornellas de Barros and Pedro Augusto

> The book collects recent results about research activities on zeolites, from synthesis to appli‐ cation. It is composed of two sections. The first is devoted to articles and brief review articles on the synthesis of zeolite from fly ash and final application of these newly formed minerals to solve environmental problems. The second part of the book provides useful information on different applications both of natural and synthetic zeolites ranging from environmental pollution to industrial and commercial applications. The performance of zeolite molecular sieves, hollow titanium zeolites and luminescent zeolites is interesting considering the new frontiers reached by the research on zeolites.

> This book is a useful instrument for readers interested in investigating innovative aspects of the studies on zeolite, and I want to thank all the authors for their significant contribution.

> Zeolites are hydrated aluminosilicate minerals that can be synthesised using several source materials.

> A number of water purification processes have been developed in recent years based on the utilisation of low-cost materials with high pollutant removal efficiency.

> Zeolites are microporous crystalline hydrated aluminosilicates characterised by a three-di‐ mensional network of tetrahedral (Si, Al)O4 units that form a system of interconnected pores. The aluminium ion produces a net negative charge, which is balanced by the pres‐ ence of an extra cation in the framework. One important property of zeolite is its ability to exchange cations. Due to their similar structure and physicochemical properties, synthetic zeolites have replaced natural zeolites in a variety of applications.

> Zeolites are hydrated aluminosilicate minerals consisting of a three-dimensional open struc‐ ture with excellent ion-exchange and sorption properties, which makes them notably useful for resolving the mobility of toxic elements in a number of environmental applications. Their effectiveness is strictly connected to their typical structural characteristics, which facil‐ itate pollutant adsorption and encapsulation.

> Fly ash is a product of coal combustion waste that consists of an amorphous aluminosilicate glass, quartz and mullite. The high glass content makes fly ash a suitable material for the synthesis of zeolites, which are hydrated aluminosilicate minerals with a three-dimensional open structure. Zeolites are useful in a number of environmental applications.

> Coal fly ash is the most abundant coal combustion by-product, partly used in concrete and cement manufacturing yet more than half of it disposed of in landfills since it finds no other application.

The huge production of fly ash is extremely of concern because of this kind of disposal, and several investigations have been carried out to try to exploit this waste material. Over the last few years, fly ash has been gaining ground in the synthesis of zeolites. This is due to the fact that the main fly ash component is amorphous aluminosilicate glass, which is the preva‐ lent reactive phase.

> **Dr. Claudia Belviso** Institute of Methodologies for Environmental Analysis CNR-IMAA, Italy

**Section 1**

## **Fly Ash and Zeolite**

The huge production of fly ash is extremely of concern because of this kind of disposal, and several investigations have been carried out to try to exploit this waste material. Over the last few years, fly ash has been gaining ground in the synthesis of zeolites. This is due to the fact that the main fly ash component is amorphous aluminosilicate glass, which is the preva‐

**Dr. Claudia Belviso**

CNR-IMAA, Italy

Institute of Methodologies for Environmental Analysis

lent reactive phase.

VIII Preface

## **Upscaling of Zeolite Synthesis from Coal Fly Ash Waste: Current Status and Future Outlook**

James Philip Brassell, Tunde Victor Ojumu and Leslie Felicia Petrik

Additional information is available at the end of the chapter

http://dx.doi.org/ 10.5772/63792

#### **Abstract**

Coal fly ash has been recognised as suitable feedstock for zeolite synthesis, and with this discovery, there has been much literature produced over the years on various synthesis processes with a view to address the disposal problems associated with fly ash wastes. The different synthesis processes can be divided into a pre‐synthesis activation with an alkali solution and a hydrothermal synthesis of heating the resulting aluminosilicate gel. However, the challenge lies in up scaling the synthesis to allow production on a commercial scale. The most common pre‐synthesis involving high‐ temperature fusion of fly ash with sodium hydroxide may not be feasible in an economic context, given that a large‐scale furnace that would be needed. In this review, the successes recorded in the application of acoustic cavitation by using ultrasonication technique were discussed as a possible alternative to the fusion process on one hand. On the other hand, jet‐loop reactor system provides hydrodynamic cavitation, which may be relatively scalable compared to ultrasonication and may offer an economic advantage, if systematically explored, over fusion process for large‐scale synthesis of zeolites. It is also recommended that agitation is critical during the crystallisation process and the suitability of an impeller type that offers a minimum shearing to be explored during the hydrothermal process with respect to quality and yield of the zeolite produced. The understanding gleaned from these recommendations may be useful in designing an appropriate scale‐up operation for zeolite synthesis from coal fly ash.

**Keywords:** fly ash, zeolites, up scale, jet loop, hydrothermal agitation

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **1. Introduction**

It has been 30 years since the discovery by Holler and Wirsching that zeolites can be synthes‐ ised from coal fly ash [1]. It was found that the coal fly ash contained the necessary ele‐ ments, namely silicon and aluminium, which are building blocks of the zeolitic structure. This has spawned substantial research into zeolite synthesis from coal fly ash with a view to create outlets for fly ash utilisation, thereby solving the worldwide problem of coal fly ash dispos‐ al. Coal fly ash is produced during the coal combustion process, and it is the most predomi‐ nant coal combustion by‐product [2]. The electricity generated from coal combustion accounted for 29.9% of the world's electricity supply in 2011, and this is expected to increase 46% by 2030 [3]. Therefore, there are concerns with the high volumes of coal fly ash waste being produced.

In South Africa alone, approximately 36 million tonnes are produced per annum with 95% of that being dumped in ash dumps [4]. When it is seen that South Africa only accounts for 2.4% of the world coal consumption, it becomes apparent of the scale of coal fly ash produced worldwide. Especially, concerning the large‐scale world coal consumers, such as China (50.2%), United States (11.7%), and India (8%) [3], it has been reported that China produced 540 million tonnes of coal fly ash in 2011 with only 68% being reused, while about 61% of the 163.56 million tonnes of fly ash produced in India between 2012 and 2013 was reused [3]. This poses risks to the environment and surrounding populations of these ash dumps. This is due to coal fly ash having the potential to leach toxic heavy elements into the groundwater system [5] and poses a threat to person's health if inhaled [6]. Due to the problem of coal fly ash disposal, many studies investigated its use with regard to different applications, such as in agriculture, in construction, in wastewater treatment, and for the synthesis of geopolymers and zeolites as can be seen in a recent review [3]. The views of these studies were to find outlets for the utilisation of this waste, thereby addressing the disposal problems. However, these applications have not solved the problem of coal fly ash disposal, as they are not at the scale required to deal with the high volumes of coal fly ash being produced.

There is a need to investigate further into the feasibility of scaling up some of these applications. Although recent studies have been focusing on some fundamental investigations regarding scale‐up development for the synthesis of zeolite from coal fly ash [7, 8], there remains a significant amount of studies to be conducted in order to realise the end goal of synthesis on a commercial scale. This book chapter aims at giving a review of the work that has been produced regarding the synthesis of zeolites from coal fly ash with a view to recommending outstanding investigations to be carried out regarding the up scale of zeolite synthesis from coal fly ash. It will justify the necessary procedures that need to be studied in the future to realise the end goal of large‐scale zeolite synthesis from coal fly ash.

#### **2. Coal fly ash**

Coal fly ash is mainly composed of silica (SiO2), alumina (Al2O3), ferrous oxide (Fe2O3) and calcium oxide (CaO) with also some unburned carbon detected through loss on ignition tests (LOI) [9, 10]. The chemical properties depend on the type of coal being burned, along with handling and storage procedures [3, 9, 10]. This gives rise to the high variability in coal fly ash compositions obtained from different batches of coal processed in the same power station [8]. The major and minor elements in coal fly ash are expressed as oxides, with the main ones being already mentioned previously. However, it also consists of trace elements, with some being As, B, Ba, Be, Cd, Co, Cr, Cu, Ge, Hg, Li, Mo, Ni, Pb, Rb, Sb, Se, Sn, Sr, Th, U, V and Zn [10]. The major phases of coal fly ash are amorphous glass, mullite and quartz with various others in smaller amounts [10, 11]. A recent comprehensive study by Musyoka [12] on the mineral phases of South African coal fly ash reported that the ash yielded percentage amounts ranging from the more amorphous content of 57.54% to less amorphous content of 41.55%. The mullite ranged from 34.55 to 22.27% and the quartz from 21.38 to 11.92%. Another comprehensive study was performed by Vassilev and Vassileva [11] on coal fly ash from Spain, Bulgaria, the Netherlands, Italy, Turkey and Greece. The amorphous content ranged from 85 to 34%, the mullite from 40 to 0.3% and the quartz from 14 to 2%.

**1. Introduction**

4 Zeolites - Useful Minerals

being produced.

**2. Coal fly ash**

It has been 30 years since the discovery by Holler and Wirsching that zeolites can be synthes‐ ised from coal fly ash [1]. It was found that the coal fly ash contained the necessary ele‐ ments, namely silicon and aluminium, which are building blocks of the zeolitic structure. This has spawned substantial research into zeolite synthesis from coal fly ash with a view to create outlets for fly ash utilisation, thereby solving the worldwide problem of coal fly ash dispos‐ al. Coal fly ash is produced during the coal combustion process, and it is the most predomi‐ nant coal combustion by‐product [2]. The electricity generated from coal combustion accounted for 29.9% of the world's electricity supply in 2011, and this is expected to increase 46% by 2030 [3]. Therefore, there are concerns with the high volumes of coal fly ash waste

In South Africa alone, approximately 36 million tonnes are produced per annum with 95% of that being dumped in ash dumps [4]. When it is seen that South Africa only accounts for 2.4% of the world coal consumption, it becomes apparent of the scale of coal fly ash produced worldwide. Especially, concerning the large‐scale world coal consumers, such as China (50.2%), United States (11.7%), and India (8%) [3], it has been reported that China produced 540 million tonnes of coal fly ash in 2011 with only 68% being reused, while about 61% of the 163.56 million tonnes of fly ash produced in India between 2012 and 2013 was reused [3]. This poses risks to the environment and surrounding populations of these ash dumps. This is due to coal fly ash having the potential to leach toxic heavy elements into the groundwater system [5] and poses a threat to person's health if inhaled [6]. Due to the problem of coal fly ash disposal, many studies investigated its use with regard to different applications, such as in agriculture, in construction, in wastewater treatment, and for the synthesis of geopolymers and zeolites as can be seen in a recent review [3]. The views of these studies were to find outlets for the utilisation of this waste, thereby addressing the disposal problems. However, these applications have not solved the problem of coal fly ash disposal, as they are not at the scale

There is a need to investigate further into the feasibility of scaling up some of these applications. Although recent studies have been focusing on some fundamental investigations regarding scale‐up development for the synthesis of zeolite from coal fly ash [7, 8], there remains a significant amount of studies to be conducted in order to realise the end goal of synthesis on a commercial scale. This book chapter aims at giving a review of the work that has been produced regarding the synthesis of zeolites from coal fly ash with a view to recommending outstanding investigations to be carried out regarding the up scale of zeolite synthesis from coal fly ash. It will justify the necessary procedures that need to be studied in the future to

Coal fly ash is mainly composed of silica (SiO2), alumina (Al2O3), ferrous oxide (Fe2O3) and calcium oxide (CaO) with also some unburned carbon detected through loss on ignition tests

required to deal with the high volumes of coal fly ash being produced.

realise the end goal of large‐scale zeolite synthesis from coal fly ash.

Furthermore, the combustion rate and temperature of the coal combustion process affects the morphology of the coal fly ash particles [10, 13]. Coal fly ash mainly consists of spherical particles, such as solid spheres and hollow spheres (cenospheres), with some irregular unburned carbon, as determined by scanning electron microscopy (SEM) analysis [3, 13]. The colour of coal fly ash ranges from water white to yellow, orange to deep red or brown to opaque and is due to the iron and unburned carbon content [3, 14]. Coal fly ash containing more than 70% SiO2+Al2O3+Fe2O3 and having a low lime content of 5% is classified as class F fly ash, while if the SiO2+Al2O3+Fe2O3 content is between 50 and 70% and a high lime content of between 10 and 35%, it is classified as class C fly ash [9, 10]. It follows from the aforementioned that class C fly ash can be produced from lignite and sub‐bituminous coal while class F from bituminous and anthracite coal [10, 11].

There have been numerous studies investigating the leachability of coal fly ash into the groundwater system that surrounds the ash dumpsites [5, 15–26]. In a more recent article by Nyale et al. [5], it was reported that the disposed ash at the dump site was able to release trace elements into the environment over time, and the study revealed that all the trace elements studied (As, Zn, Pb, Ni, Mo, Cr and Cu) were significantly leached from the labile phases (4.42– 27.43%). The inhomogeneity of the ash dump was also attributed to the observed differences in the elements leached from varying core samples. Besides this type of environmental problem, the potential threat of coal fly ash to the health of the surrounding population and the mine workers exposed to the ash has been reported [6]. The leachability of genotoxic compounds from fly ash into human lungs if the ash dust is inhaled has been reported [6], and conditions such as lung cancer, asthma and many others are typical diseases attributed to fly ash inhalation [6, 27].

However, there have been several studies that investigated potential application of fly ash with a view to mitigate the disposal of this fly ash; some of the applications investigated include uses in agriculture, construction, extraction of rare earth elements, wastewater treatment and synthesis of geopolymers and zeolites [3]. Furthermore, only 5% of coal fly ash is being re‐used as reported by Eskom [4]. A comprehensive and recent review of some of the applications of coal fly ash is reported by Yao et al. [3]; however, these authors recommended the need to explore commercialisation of the applications. The following section will focus on the various synthesis methods employed in the application of coal fly ash for zeolites synthesis and assess studies aimed towards the up scale of the synthesis process.

#### **3. Zeolites**

Zeolites are crystalline aluminosilicates of which possess cavities and pores on a molecular scale [28]. They do occur as natural minerals but are more useful being synthetically produced in laboratories for uses such as sorbents, catalysts and exchange materials [28]. The zeolite consists of SiO4 and AlO4 tetrahedra of which gives it an anionic framework with the negative charge of Al being compensated by extra framework cations, some being Na+ , K+ , Ca2+ and Mg2+ [29]. This can be illustrated in **Figure 1**.

**Figure 1.** (a) Basic tectosilicate structure of zeolite where dark (i.e. vertex in) and light (i.e. vertex out) shades to add three‐dimensional (3D) effect and upside down orientation of the tetrahedra for vertex sharing between two rings of the zeolite structure in its 2D view on a picture plane; (b) single‐ring tetrahedron structure and framework of a zeolitic mineral; (c) <*i* >*SiO*<sup>4</sup> <sup>−</sup> <sup>&</sup>lt; / *<sup>i</sup>* > and <*<sup>i</sup>* <sup>&</sup>gt; *AlO*<sup>4</sup> <sup>−</sup> < / *i* > in a ring of sodium zeolite; and (d) pictorial representation of a 3D view of a tetrahedra with centrally located Si or Al atoms, exhibited by dotted lines drawn to represent the portion within the body of the mineral [30].

The primary building units of the zeolite structure are shown in **Figure 1(d)**, and they are TO4, tetrahedra of silicon and aluminium mentioned previously [29]. There is a concept of infinite component units with the zeolite structure regarded as being made of finite component units with infinite component unit‐like chains or layers, these are known as the secondary building units, some illustrated in **Figure 2**. The secondary building units can consist of single or double rings with each of those consisting of four‐, five‐, six‐ or eight‐linked tetrahedral, including the silicon and aluminium atoms [30].

Upscaling of Zeolite Synthesis from Coal Fly Ash Waste: Current Status and Future Outlook http://dx.doi.org/ 10.5772/63792 7

**Figure 2.** Secondary building units (SBU's) with SBU codes below figures [29].

coal fly ash is reported by Yao et al. [3]; however, these authors recommended the need to explore commercialisation of the applications. The following section will focus on the various synthesis methods employed in the application of coal fly ash for zeolites synthesis and assess

Zeolites are crystalline aluminosilicates of which possess cavities and pores on a molecular scale [28]. They do occur as natural minerals but are more useful being synthetically produced in laboratories for uses such as sorbents, catalysts and exchange materials [28]. The zeolite consists of SiO4 and AlO4 tetrahedra of which gives it an anionic framework with the negative

**Figure 1.** (a) Basic tectosilicate structure of zeolite where dark (i.e. vertex in) and light (i.e. vertex out) shades to add three‐dimensional (3D) effect and upside down orientation of the tetrahedra for vertex sharing between two rings of the zeolite structure in its 2D view on a picture plane; (b) single‐ring tetrahedron structure and framework of a zeolitic

3D view of a tetrahedra with centrally located Si or Al atoms, exhibited by dotted lines drawn to represent the portion

The primary building units of the zeolite structure are shown in **Figure 1(d)**, and they are TO4, tetrahedra of silicon and aluminium mentioned previously [29]. There is a concept of infinite component units with the zeolite structure regarded as being made of finite component units with infinite component unit‐like chains or layers, these are known as the secondary building units, some illustrated in **Figure 2**. The secondary building units can consist of single or double rings with each of those consisting of four‐, five‐, six‐ or eight‐linked tetrahedral,

<sup>−</sup> < / *i* > in a ring of sodium zeolite; and (d) pictorial representation of a

, K+

, Ca2+ and

charge of Al being compensated by extra framework cations, some being Na+

studies aimed towards the up scale of the synthesis process.

Mg2+ [29]. This can be illustrated in **Figure 1**.

<sup>−</sup> <sup>&</sup>lt; / *<sup>i</sup>* > and <*<sup>i</sup>* <sup>&</sup>gt; *AlO*<sup>4</sup>

including the silicon and aluminium atoms [30].

**3. Zeolites**

6 Zeolites - Useful Minerals

mineral; (c) <*i* >*SiO*<sup>4</sup>

within the body of the mineral [30].

The varying framework structures seen above give rise to the unique three‐letter code system to name each unique zeolite, some of these codes along with the pore sizes of some zeolites are shown in **Figure 3**.

**Figure 3.** A comparison of the different framework pore sizes [31].

#### **4. Synthesis of zeolites from coal fly ash**

As mentioned previously, the synthesis of zeolites from coal fly ash began with the pioneering work of Holler and Wirsching in 1985 [1]. Researchers have found a number of synthesis methods to produce various types of zeolites from the coal fly ash [32–42]. All known synthetic methods can be found on the website of International Zeolite Association (IZA) at URL: http:// www.iza‐online.org/synthesis/default.htm [43]. However some of the prominent methods are reviewed in this section.

All the methods developed mainly involve the dissolution Al‐Si bearing fly ash phases with alkaline solutions such as NaOH and KOH with the proceeding precipitation of zeolitic material [2]. The two most well‐known routes for fly ash conversion to zeolite are the fusion with sodium hydroxide process developed by Shigemoto et al. [32] and a two‐step process developed by Hollman et al. [35]. The two‐step process developed by Hollman et al. [35], which followed from the traditional one‐step process produced more "pure" zeolites; however, the process was relatively more costly. A comparison of the one‐step and two‐step process can be seen in **Figures 4** and **5**, respectively. Shigemoto et al. [32] developed the fusion process where coal fly ash is fused with sodium hydroxide using a high temperature, prior to hydrothermal treatment. A ratio of CFA:NaOH of 1:1.2 was optimum with a fusion temperature of 550°C, and this converted the fly ash particles into sodium salts. These salts (silicate and aluminate) after the hydrothermal treatment favoured the formation of zeolite Na‐X. The process is illustrated in **Figure 6**.

**Figure 4.** Flow diagram of the traditional one‐step process [30].

Upscaling of Zeolite Synthesis from Coal Fly Ash Waste: Current Status and Future Outlook http://dx.doi.org/ 10.5772/63792 9

**Figure 5.** Flow diagram of two‐step process [30].

**4. Synthesis of zeolites from coal fly ash**

reviewed in this section.

8 Zeolites - Useful Minerals

illustrated in **Figure 6**.

**Figure 4.** Flow diagram of the traditional one‐step process [30].

As mentioned previously, the synthesis of zeolites from coal fly ash began with the pioneering work of Holler and Wirsching in 1985 [1]. Researchers have found a number of synthesis methods to produce various types of zeolites from the coal fly ash [32–42]. All known synthetic methods can be found on the website of International Zeolite Association (IZA) at URL: http:// www.iza‐online.org/synthesis/default.htm [43]. However some of the prominent methods are

All the methods developed mainly involve the dissolution Al‐Si bearing fly ash phases with alkaline solutions such as NaOH and KOH with the proceeding precipitation of zeolitic material [2]. The two most well‐known routes for fly ash conversion to zeolite are the fusion with sodium hydroxide process developed by Shigemoto et al. [32] and a two‐step process developed by Hollman et al. [35]. The two‐step process developed by Hollman et al. [35], which followed from the traditional one‐step process produced more "pure" zeolites; however, the process was relatively more costly. A comparison of the one‐step and two‐step process can be seen in **Figures 4** and **5**, respectively. Shigemoto et al. [32] developed the fusion process where coal fly ash is fused with sodium hydroxide using a high temperature, prior to hydrothermal treatment. A ratio of CFA:NaOH of 1:1.2 was optimum with a fusion temperature of 550°C, and this converted the fly ash particles into sodium salts. These salts (silicate and aluminate) after the hydrothermal treatment favoured the formation of zeolite Na‐X. The process is

**Figure 6.** Flow diagram of the fusion‐assisted process [30].

Murayama et al. [36] investigated the synthesis of zeolites from coal fly ash using different alkali sources, namely NaOH, Na2CO3 and KOH. It was found that regarding the reaction mechanism that three steps exist in an alkali hydrothermal reaction, namely dissolution, condensation and crystallisation. The dissolution step begins at a temperature range of 298– 393 K with the amount of OH in the alkali solution making a great contribution to the reaction. The concentration of Na+ in the alkali solution mainly determined the total reaction rate of the zeolite synthesis.

Not only the degree of zeolitisation is affected by the NaOH concentration but also the type of zeolite formed. This is according to Molina and Poole [44] who noted the difference found in the zeolite product was due to the increase in supersaturation achieved from the higher proportion of soluble species from the rise of the NaOH concentration. This phenomenon can be described by the Ostwald's rule of successive transformation with the higher the supersa‐ turation, better the conditions are to nucleate metastable phases [44]. An example being zeolite X which later recrystallises and is replaced by the more stable zeolite hydroxysodalite. Some other factors affecting zeolite formation from coal fly ash are the SiO2/Al2O3 ratio in the feedstock, the NaOH concentration as mentioned previously, the extra framework cations, the water content, the synthesis time and temperature and the agitation used in the zeolite synthesis.

Regarding the SiO2/Al2O3 ratio Inada et al. [38] demonstrated the dependence of this ratio in zeolite synthesis. Two differing zeolites were formed from a silica rich fly ash (zeolite Na‐P1) and an alumina‐rich fly ash or silica lean fly ash (hydroxysodalite) with the ratio being controlled with SiO2 and Al2O3 aerosil powders. According to Querol et al. [45], this ratio depends on the compositions of these species within the aluminosilicate glass phase of the coal fly ash and not on their bulk composition in the fly since this phase is the first to be activated.

Murayama et al. [36] also showed how the choice of cations could significantly alter the reaction to form one zeolite crystal over another. The Na+ ions promoted the formation of zeolite P, while the K+ ions promoted the formation of chabazite. Indeed, zeolite synthesis was shown to be favoured by the use of Na+ ions over K+ ions [46, 47]. This is due to Na+ ions stabilising the secondary building units of the zeolite frameworks, while K+ ions act as a suppressor due to their promotion of slow crystallisation rates [47].

There is a problem when it comes to zeolite synthesis regarding the water content used. This is due to the increase in water content resulting in an increase in yield when using fly ash as the starting material. This is because the higher water content results in the increase in the crystalline and amorphous phases dissolution rates in the coal fly ash [46, 48]. This led to investigation of using alternate water sources, such as investigating using tap water, distilled water and acid mine drainage (AMD) in the replacement of ultra‐pure water. It was reported by Mainganye [8] that tap water and distilled water could produce similar products as the ultra‐pure water, while AMD water, however, was not suitable. Musyoka et al. [49] established that AMD could yield hydroxysodalite—a zeolite with less application. However, it was found that circumneutral mine water produced a comparable quality of Na‐P1 and X zeolites to ultra‐ pure synthesis. Zeolite X and ZK‐5 were successfully synthesised using seawater at lower temperatures with hydroxysodalite also being formed; however, zeolite A could not be achieved [42]. Artificial seawater was used at room temperature (25°) to crystallise zeolites A and X in a later study [50].

The thermodynamic stability of zeolite has been reported by Boycheva et al. [51] to follow the sequence in the order of Linda --> Faujasite--> Chabazite--> Na‐P1 --> Hydroxysodalite. It has been observed previously in the transformation of faujasite into more stable phases such as chabazite and Na‐P1 at longer synthesis times in the reactant liquor [37, 51]. Due to this metastable behaviour of zeolite X, its formation is always competitive with hydroxysodalite, zeolite X favouring the lower temperatures [42, 51]. It is also known that a higher temperature produces zeolites of a greater particle size [44]. It was found that aluminium dissolution tends to be faster than silicon, of which the two need to be continuously dissolved to promote and feed crystal growth, the silicon dissolution increasing with temperature [44]. Therefore, the Si/ Al ratio will favour the formation of zeolite X over A at higher temperatures. When the glassy content is high, the synthesis time drops, while a high content of quartz and mullite requires longer reaction times [2].

Inhomogeneous mixtures can result due to inadequate mixing which in turn has a dramatic effect on zeolite synthesis, this is pertaining to the viscous gel produced before the crystal growth step [52]. These inhomogeneous mixtures produce "pockets" of gel each with differing compositions and each acting like a "mini‐reactor", producing phases based on that "mini‐ reactor's" composition. The effect of agitation is not well understood and this is due to mixing having to perform the following tasks during crystallisation [52]:

**•** Reagent dissolution

Murayama et al. [36] investigated the synthesis of zeolites from coal fly ash using different alkali sources, namely NaOH, Na2CO3 and KOH. It was found that regarding the reaction mechanism that three steps exist in an alkali hydrothermal reaction, namely dissolution, condensation and crystallisation. The dissolution step begins at a temperature range of 298–

The concentration of Na+ in the alkali solution mainly determined the total reaction rate of the

Not only the degree of zeolitisation is affected by the NaOH concentration but also the type of zeolite formed. This is according to Molina and Poole [44] who noted the difference found in the zeolite product was due to the increase in supersaturation achieved from the higher proportion of soluble species from the rise of the NaOH concentration. This phenomenon can be described by the Ostwald's rule of successive transformation with the higher the supersa‐ turation, better the conditions are to nucleate metastable phases [44]. An example being zeolite X which later recrystallises and is replaced by the more stable zeolite hydroxysodalite. Some other factors affecting zeolite formation from coal fly ash are the SiO2/Al2O3 ratio in the feedstock, the NaOH concentration as mentioned previously, the extra framework cations, the water content, the synthesis time and temperature and the agitation used in the zeolite

Regarding the SiO2/Al2O3 ratio Inada et al. [38] demonstrated the dependence of this ratio in zeolite synthesis. Two differing zeolites were formed from a silica rich fly ash (zeolite Na‐P1) and an alumina‐rich fly ash or silica lean fly ash (hydroxysodalite) with the ratio being controlled with SiO2 and Al2O3 aerosil powders. According to Querol et al. [45], this ratio depends on the compositions of these species within the aluminosilicate glass phase of the coal fly ash and not on their bulk composition in the fly since this phase is the first to be activated. Murayama et al. [36] also showed how the choice of cations could significantly alter the reaction

while the K+ ions promoted the formation of chabazite. Indeed, zeolite synthesis was shown

the secondary building units of the zeolite frameworks, while K+ ions act as a suppressor due

There is a problem when it comes to zeolite synthesis regarding the water content used. This is due to the increase in water content resulting in an increase in yield when using fly ash as the starting material. This is because the higher water content results in the increase in the crystalline and amorphous phases dissolution rates in the coal fly ash [46, 48]. This led to investigation of using alternate water sources, such as investigating using tap water, distilled water and acid mine drainage (AMD) in the replacement of ultra‐pure water. It was reported by Mainganye [8] that tap water and distilled water could produce similar products as the ultra‐pure water, while AMD water, however, was not suitable. Musyoka et al. [49] established that AMD could yield hydroxysodalite—a zeolite with less application. However, it was found that circumneutral mine water produced a comparable quality of Na‐P1 and X zeolites to ultra‐ pure synthesis. Zeolite X and ZK‐5 were successfully synthesised using seawater at lower temperatures with hydroxysodalite also being formed; however, zeolite A could not be

in the alkali solution making a great contribution to the reaction.

ions promoted the formation of zeolite P,

ions stabilising

ions [46, 47]. This is due to Na+

393 K with the amount of OH-

to form one zeolite crystal over another. The Na+

to their promotion of slow crystallisation rates [47].

to be favoured by the use of Na+ ions over K+

zeolite synthesis.

10 Zeolites - Useful Minerals

synthesis.


The substantial change in viscosity during synthesis is also an important factor to consider. The solution can go from a viscous gel to something resembling water during the course of synthesis [52]. Marrot et al. [53] explained another important factor in agitation, that of shearing, of which can have harmful effects on zeolite synthesis. This can affect the stability of the zeolite and the purity of which can be synthesised. The highest crystallinity was produced by the Archimedes screw impeller due to the low rate of shearing, while higher shearing resulted in lower crystallinities [53]. Agitation during the ageing step of synthesis was investigated by Mainganye et al. [54]. It was shown that agitation during the hydrothermal step of synthesis can have a harmful effect on the stability and purity of the zeolite regarding the shearing produced [53, 54]. It was thought that, during the aging step, shearing may favour the zeolite formation due to it facilitating the dissolution of coal fly ash into the alkaline solution. The four‐blade impeller was found to optimum for zeolite synthesis, applied during the ageing step [54]. It is noted however that regarding the up scale of zeolite synthesis, the hydrothermal reactor used would also need to implement a form of agitation given the need to maintain a homogeneous mixture [55]. From the above analysis, Archimedes screw may be a favourable impeller during the hydrothermal synthesis due to its low rate of shearing; however, further research is required to ascertain this assumption. The synthesis gel that is used in the stage of hydrothermal synthesis varies considerably from the solution during aging. There is a need to understand the effect of agitation during hydrothermal synthesis. Homogenous mixing is critical if large‐scale production is envisioned, a large‐scale reactor would need to implement some form of agitation.

There have been significant studies on the use of class F fly ash for zeolite synthesis, which is pioneered Musyoka et al. [12, 56, 57]. The more recent studies have focused on using alternate techniques such as ultrasound to either used after fusing to decrease the hydrothermal synthesis time [12, 58], and the temperature needed in hydrothermal synthesis [58, 59] or complete replacement of the fusion process [60] A reduction in synthesis time was also noted by Musyoka et al. [61] when using ultrasonic synthesis using AMD and circumneutral mine waters in place of ultra‐pure water. In addition, studies have also shown that ultrasound techniques can be used to investigate the formation mechanism of zeolites from coal fly ash using in‐situ ultrasound techniques, namely of the zeolites A and X (zeolite X with novel hierarchal morphology) [57]. According to Murayama et al. [62], the three main steps involved in the mechanism of zeolite formation from coal fly ash are as follows:


The above process can be represented pictorially in the following **Figure 7**.

Another technique used is microwave‐assisted synthesis as reported by Querol et al. [64]. The activation time need during hydrothermal conversion was radically decreased from 24–48 hours to 30 minutes. It was further explained by Inada et al. [65] that continuous microwave irradiation retards the formation of zeolite in crystalline form, inhibiting the zeolite in the intermediate gel. It was found that early microwave irradiation enhances the zeolite formation. Heating during the middle stage, however, significantly inhibits zeolite formation. Therefore, it was found to be advisable to heat with microwave irradiation early on and then follow with conventional heating.

The feasibility though of scaling‐up these alternate processes, such as microwave and ultra‐ sonic‐assisted synthesis, would prove challenging regarding the scale needed for large‐scale zeolite synthesis. Ultrasound seems promising given its use of cavitation to produce high temperatures and pressures or "hot spots" under nearly ambient conditions [66]. The main form of cavitation being studied is acoustic cavitation generated by sound waves in which ultrasound generates, grows and collapses the cavities in microseconds. These hot spots can

Upscaling of Zeolite Synthesis from Coal Fly Ash Waste: Current Status and Future Outlook http://dx.doi.org/ 10.5772/63792 13

**Figure 7.** Illustration of the zeolite process of a coal fly ash particle [63].

the ageing step [54]. It is noted however that regarding the up scale of zeolite synthesis, the hydrothermal reactor used would also need to implement a form of agitation given the need to maintain a homogeneous mixture [55]. From the above analysis, Archimedes screw may be a favourable impeller during the hydrothermal synthesis due to its low rate of shearing; however, further research is required to ascertain this assumption. The synthesis gel that is used in the stage of hydrothermal synthesis varies considerably from the solution during aging. There is a need to understand the effect of agitation during hydrothermal synthesis. Homogenous mixing is critical if large‐scale production is envisioned, a large‐scale reactor

There have been significant studies on the use of class F fly ash for zeolite synthesis, which is pioneered Musyoka et al. [12, 56, 57]. The more recent studies have focused on using alternate techniques such as ultrasound to either used after fusing to decrease the hydrothermal synthesis time [12, 58], and the temperature needed in hydrothermal synthesis [58, 59] or complete replacement of the fusion process [60] A reduction in synthesis time was also noted by Musyoka et al. [61] when using ultrasonic synthesis using AMD and circumneutral mine waters in place of ultra‐pure water. In addition, studies have also shown that ultrasound techniques can be used to investigate the formation mechanism of zeolites from coal fly ash using in‐situ ultrasound techniques, namely of the zeolites A and X (zeolite X with novel hierarchal morphology) [57]. According to Murayama et al. [62], the three main steps involved

**•** Condensation of silicate and aluminate ions in the alkali solution making the aluminosilicate

Another technique used is microwave‐assisted synthesis as reported by Querol et al. [64]. The activation time need during hydrothermal conversion was radically decreased from 24–48 hours to 30 minutes. It was further explained by Inada et al. [65] that continuous microwave irradiation retards the formation of zeolite in crystalline form, inhibiting the zeolite in the intermediate gel. It was found that early microwave irradiation enhances the zeolite formation. Heating during the middle stage, however, significantly inhibits zeolite formation. Therefore, it was found to be advisable to heat with microwave irradiation early on and then follow with

The feasibility though of scaling‐up these alternate processes, such as microwave and ultra‐ sonic‐assisted synthesis, would prove challenging regarding the scale needed for large‐scale zeolite synthesis. Ultrasound seems promising given its use of cavitation to produce high temperatures and pressures or "hot spots" under nearly ambient conditions [66]. The main form of cavitation being studied is acoustic cavitation generated by sound waves in which ultrasound generates, grows and collapses the cavities in microseconds. These hot spots can

would need to implement some form of agitation.

**•** Dissolution of Si4+ and Al3+ in coal fly ash

gel

12 Zeolites - Useful Minerals

conventional heating.

in the mechanism of zeolite formation from coal fly ash are as follows:

The above process can be represented pictorially in the following **Figure 7**.

**•** Crystallisation of the aluminosilicate gel‐forming zeolites

produce temperatures greater than 5000 K and a cooling rate greater than 107 °C/s [60]. In a recent study by Ojumu et al. [60], the 90‐min high‐temperature fusion was completely replaced by a 10 minutes of high intensity ultrasound irradiation. It was shown that 24% of silicon was extracted from the fly ash, comparable to the 32% from fusion. It also reduced the crystallisation time of zeolite A and would be promising for up‐scaling. On industrial scale, there is hardly any process carried out, which allows for up scale of the process. This is due to the lack of expertise in differing fields such as material science, acoustics, chemical engineering, etc. [66]. Hydrodynamic cavitation offers a cheaper alternative. It concerns generating cavitation from the flow of a liquid under controlled conditions through obstructions such as venture tubes and orifice plates. It is generated when the pressure at the throat falls below the vapour pressure of the liquid, the liquid then flashes and generates a number of cavities. When the pressure is recovered downstream of the mechanical constriction, these cavities then collapse [66–68]. The hydrodynamic cavitation is applied on a pilot plant scale within a jet loop reactor. In a study involving the scaling‐up of a process to reduce sulphate concentrations within neutral mine drainage, a jet‐loop reactor was used [69]. The study showed that the impinge‐ ment and cavitation mixing techniques applied within the jet‐loop reactor played an important role in enhancing the sulphate removal. This was due to the superior mixing in a jet‐loop reactor compared to the laboratory‐scale experiments that were also performed. The feasibility of scaling up a jet‐loop pilot plant to full‐scale seems more easily achieved than what would be needed to scale‐up the high‐intensity ultrasound process. Therefore, it may be necessary to investigate the application of a jet‐loop reacting system incorporating fly ash and sodium hydroxide to ascertain the dissolution rates of the silicon and aluminium that can be achieved. A recent study by Nyale et al. [27] showed that the intense mixing within the jet‐loop reactor

increased the dissolution of the amorphous content of the fly ash used in the production of geopolymer from a slurry obtained from a jet‐loop system. This may prove favourable for zeolite synthesis; however, systematic studies that investigate the potential to use jet‐loop reactor as a process to completely replace the high‐temperature fusion need to be conducted. This would involve comparison of jet‐loop reacting system to the recent studies of involving ultrasound utilisation to determine which process might be more lucrative to up scale.

#### **5. Up‐scaling zeolite synthesis from coal fly ash**

Querol et al. [48] ran experiments at pilot scale using a 10 m3 R‐410‐A reactor vessel; the optimum conditions were generated from laboratory‐scale experiments using Parr digestion bombs: they were found to be 2 M NaOH solution, solution/fly ash ratio of 21 kg-1, *T* = 150°C, and time of 24 hours. It was concluded that the optimisation of synthesis yields would have to be specific for each type of fly ash from the differing power plants due to their difference in mineralogical and chemical compositions. There were reproducible results, however, in synthesising zeolite NaP1 from a specific power station fly ash. Also, high cation‐exchange capacities (CEC) were reported for ammonium and heavy metals for the varying zeolites achieved.

Moriyama et al. [70] compared the CEC values of a conventional method of synthesis to a test unit and a pilot plant. The conventional method conditions were a liquid–solid ratio of 8 dm3 /kg, a temperature of 373 K and a holding time of 24 hours. The test unit consisted of a high‐pressure kneader with a volume of 5 dm3 fitted with a hot oil jacket around the pres‐ sure vessel. It was equipped with two sigma‐shaped blades for kneading and a steam ex‐ haust line to remove the water. The pilot plant was also a high‐pressure kneader but at the volume of 0.6 m3 . This process had benefits of water removal during operation eliminating wastewater treatment. It was concluded that the influence of the pressure on the CEC val‐ ues obtained depends on the original coal fly ash characteristics and the type of zeolite formed was gismodine (GIS) type.

Mainganye et al. [54] investigated the effect of the impeller design concerning agitation during the ageing step of zeolite NaP1 synthesis. Three impellers were tested (four flat blade, anchor and Archimedes screw) at varying agitation speeds (150, 200 and 300 rpm). The conditions were 48 hours of aging at 47°C and static hydrothermal treatment of 48 hours at 140°C. It was shown that the phase purity of the zeolite was strongly affected by the agitation rate and type of impeller used during the ageing step of synthesis. The optimal impeller was the four‐blade impeller at an agitation rate of 200 rpm under the previous conditions mentioned, with a product yield of 0.98 ± 0.05 g zeolite/g fly ash and a space time yield of 15 ± 0.4 kg d‐1m-3.

Du Plessis et al. [71] developed waste minimisation protocols for zeolite synthesis from coal fly ash. The study effectively constructed two protocols for the minimisation of the waste generated in zeolite synthesis from South African coal fly ash. It was shown that to successfully synthesise zeolites with the waste solution, there needed to be a pH adjustment giving the opportunity to recycle 40% if the supernatant waste back in to the system. With this pH adjustment, zeolites NaP1 and analcime were successfully synthesised. Due to a high Si/Al ratio in the waste after reusing the supernatant, the more predominant phase was zeolite analcime. It was also shown that it is possible to recycle 100% of the supernatant waste without adjusting the alkalinity but by adjusting the basic synthesis process slightly.

increased the dissolution of the amorphous content of the fly ash used in the production of geopolymer from a slurry obtained from a jet‐loop system. This may prove favourable for zeolite synthesis; however, systematic studies that investigate the potential to use jet‐loop reactor as a process to completely replace the high‐temperature fusion need to be conducted. This would involve comparison of jet‐loop reacting system to the recent studies of involving ultrasound utilisation to determine which process might be more lucrative to up scale.

Querol et al. [48] ran experiments at pilot scale using a 10 m3 R‐410‐A reactor vessel; the optimum conditions were generated from laboratory‐scale experiments using Parr digestion bombs: they were found to be 2 M NaOH solution, solution/fly ash ratio of 21 kg-1, *T* = 150°C, and time of 24 hours. It was concluded that the optimisation of synthesis yields would have to be specific for each type of fly ash from the differing power plants due to their difference in mineralogical and chemical compositions. There were reproducible results, however, in synthesising zeolite NaP1 from a specific power station fly ash. Also, high cation‐exchange capacities (CEC) were reported for ammonium and heavy metals for the varying zeolites

Moriyama et al. [70] compared the CEC values of a conventional method of synthesis to a test unit and a pilot plant. The conventional method conditions were a liquid–solid ratio of 8

wastewater treatment. It was concluded that the influence of the pressure on the CEC val‐ ues obtained depends on the original coal fly ash characteristics and the type of zeolite

Mainganye et al. [54] investigated the effect of the impeller design concerning agitation during the ageing step of zeolite NaP1 synthesis. Three impellers were tested (four flat blade, anchor and Archimedes screw) at varying agitation speeds (150, 200 and 300 rpm). The conditions were 48 hours of aging at 47°C and static hydrothermal treatment of 48 hours at 140°C. It was shown that the phase purity of the zeolite was strongly affected by the agitation rate and type of impeller used during the ageing step of synthesis. The optimal impeller was the four‐blade impeller at an agitation rate of 200 rpm under the previous conditions mentioned, with a product yield of 0.98 ± 0.05 g zeolite/g fly ash and a space time yield of 15 ± 0.4 kg d‐1m-3.

Du Plessis et al. [71] developed waste minimisation protocols for zeolite synthesis from coal fly ash. The study effectively constructed two protocols for the minimisation of the waste generated in zeolite synthesis from South African coal fly ash. It was shown that to successfully synthesise zeolites with the waste solution, there needed to be a pH adjustment giving the opportunity to recycle 40% if the supernatant waste back in to the system. With this pH

/kg, a temperature of 373 K and a holding time of 24 hours. The test unit consisted of a high‐pressure kneader with a volume of 5 dm3 fitted with a hot oil jacket around the pres‐ sure vessel. It was equipped with two sigma‐shaped blades for kneading and a steam ex‐ haust line to remove the water. The pilot plant was also a high‐pressure kneader but at the

. This process had benefits of water removal during operation eliminating

**5. Up‐scaling zeolite synthesis from coal fly ash**

achieved.

14 Zeolites - Useful Minerals

volume of 0.6 m3

formed was gismodine (GIS) type.

dm3

Du Plessis et al. [72] determine the distributional fate of elements during two of the most prominent zeolite synthesis methods from South African coal fly ash, namely the fusion process and the two‐step process. An overall elemental material balance was performed around the two process routes. The results indicated that in the two‐step method, almost all the elements were concentrated in the solid zeolite product, while in the fusion‐assisted process, the elements mainly report to the solid waste. It was shown that toxic elements such as Pb, Hg, Al, As and Nb were found in both the supernatant waste and washing water within both processes.

Wdowin et al. [55] developed a technological line for converting coal fly ash to zeolites on a pilot plant scale. It could be divided into four stages: a reactor loading stage; a reaction stage; separation of the reaction products stage; and a final stage for processing the material obtained. The total volume of the reaction tank was 130 dm3 (with a working volume of 100 dm3 ). The tank was equipped with three 2‐kW heaters, a temperature and tank filling probe and a stirrer. The stirrer was responsible for the homogenisation of the material and prevented aggregation of the material during the reaction process. The zeolite NaP1 was synthesised with a zeolite content of 81% under the conditions of: 20 kg fly ash; 12 kg NaOH; 90 dm3 of H2O; temperature of 80°C; and duration of 36 hours. The line was fully automated and was able to synthesise zeolites Na‐X and Linde‐A after changing of the synthesis conditions.

#### **6. Future outlook towards up‐scaling zeolite synthesis from coal fly ash**

As can be seen from majority of the literature regarding zeolite up‐scale synthesis from coal fly ash, the main zeolite being investigated is Na‐P1. It is proposed that future investigation should focus on whether the faujasite zeolite (zeolite X) can be synthesised and targeted for large‐scale production. Zeolite X is important, being the most widely employed zeolites on industrial scale and used extensively as a fluid catalytic cracking catalyst for refining oil and as materials for adsorbing and removing gaseous emissions [73]. In view of the recent study by Musyoka [12] on the synthesis of zeolite X with novel hierarchical morphology from the clear extract of fused fly ash with NaOH, a significant breakthrough would be to target the zeolite with such novel morphology for up scale. The hierarchical pore structure enables the zeolites to have maximum structural functions in a limited space and volume, thus conferring a high degree of diffusion efficiency [73]. This synthesis route would be much more advanta‐ geous over the complicated procedures reported in literature to obtain the hierarchical structured zeolites.

The hydrodynamic cavitation provided in jet‐loop reacting system should be explored as suitable alternative to replace the high‐temperature fusion process for a large‐scale production process. Following from Nyale [27], who showed that the jet loop increased the amorphous content within the fly ash and reduced the amount of quartz and mullite, which would favour the synthesis of zeolites. It is reasoned that the same method could therefore be used for zeolite synthesis in the replacement of the fusion pre‐synthesis. Although in Nyale's study, the goal was to synthesise geopolymers. However, zeolites and geopolymers are very similar regarding their atomic structures of silicon and aluminium atoms. Also, the jet‐loop geopolymer produced leached significant amount of silicon which could be used for zeolite synthesis after a curing/leaching process of the slurry produced by the jet loop. This in turn could increase the yield of zeolite by creating a filtrate concentrated in the necessary silicon and aluminium.

Agitation is critical in the up scale of zeolite synthesis from coal fly ash; however, the challenge is to determine the effect of the impeller type and agitation during the hydrothermal treatment stage of the process. Although it has been shown that impeller type and agitation rate have a remarkable effect during the ageing step of zeolite synthesis [54], there is need to investigate these effects during the crystallisation process. It is postulated that impeller type that provide minimum shearing effect would be suitable during the hydrothermal process. At an industrial scale, stirring would need to be required not only to ensure homogeneity but also to ensure uniform heat distribution necessary for the crystallisation step. There have not been in depth studies to show the effect of agitation during the hydrothermal treatment process. Although the recent agitation used by Wdowin et al. [55] to synthesise zeolites from coal fly ash recorded 81% pure phase product, the purity could be due to the shear stress effect of the mechanical stirrer during the hydrothermal treatment process [54]. It is also not clear whether a pure zeolite would have been obtained if the clear filtrate was used instead of the total slurry. The afore‐ mentioned highlights the need for an in‐depth studies into agitation during the hydrothermal process. The results of such studies might hold the promise for a large‐scale synthesis that provides the highest and most pure yield of zeolite.

#### **7. Conclusion**

This review highlighted the present status and future outlook with respect to up‐scaling of zeolite synthesis from coal fly ash. Although current studies have demonstrated that the seemingly energy‐intensive fusion pre‐synthesis step can be avoided by using the acoustic cavitation technique provided by ultrasound devices, the scalability of this device for large‐ scale production remains a subject of debate. The hydrodynamic cavitation provided by a jet‐ loop reacting system may be a suitable alternative. However, there is a need for systematic studies that would investigate the potential of jet‐loop system, in comparison with the fusion and ultrasound pre‐synthesis steps, for the dissolution of the silicon and aluminium from the coal fly ash. The critical need for agitation and suitable impeller type, during the hydrothermal synthesis on a large scale, was outlined with an emphasis on the requirement for minimum shearing during crystallisation in large hydrothermal stirred reactors. It is expected that these recommendations would provide understanding towards the design of an appropriate scale up operation for zeolite synthesis from coal fly ash.

#### **Author details**

content within the fly ash and reduced the amount of quartz and mullite, which would favour the synthesis of zeolites. It is reasoned that the same method could therefore be used for zeolite synthesis in the replacement of the fusion pre‐synthesis. Although in Nyale's study, the goal was to synthesise geopolymers. However, zeolites and geopolymers are very similar regarding their atomic structures of silicon and aluminium atoms. Also, the jet‐loop geopolymer produced leached significant amount of silicon which could be used for zeolite synthesis after a curing/leaching process of the slurry produced by the jet loop. This in turn could increase the yield of zeolite by creating a filtrate concentrated in the necessary silicon and aluminium.

Agitation is critical in the up scale of zeolite synthesis from coal fly ash; however, the challenge is to determine the effect of the impeller type and agitation during the hydrothermal treatment stage of the process. Although it has been shown that impeller type and agitation rate have a remarkable effect during the ageing step of zeolite synthesis [54], there is need to investigate these effects during the crystallisation process. It is postulated that impeller type that provide minimum shearing effect would be suitable during the hydrothermal process. At an industrial scale, stirring would need to be required not only to ensure homogeneity but also to ensure uniform heat distribution necessary for the crystallisation step. There have not been in depth studies to show the effect of agitation during the hydrothermal treatment process. Although the recent agitation used by Wdowin et al. [55] to synthesise zeolites from coal fly ash recorded 81% pure phase product, the purity could be due to the shear stress effect of the mechanical stirrer during the hydrothermal treatment process [54]. It is also not clear whether a pure zeolite would have been obtained if the clear filtrate was used instead of the total slurry. The afore‐ mentioned highlights the need for an in‐depth studies into agitation during the hydrothermal process. The results of such studies might hold the promise for a large‐scale synthesis that

This review highlighted the present status and future outlook with respect to up‐scaling of zeolite synthesis from coal fly ash. Although current studies have demonstrated that the seemingly energy‐intensive fusion pre‐synthesis step can be avoided by using the acoustic cavitation technique provided by ultrasound devices, the scalability of this device for large‐ scale production remains a subject of debate. The hydrodynamic cavitation provided by a jet‐ loop reacting system may be a suitable alternative. However, there is a need for systematic studies that would investigate the potential of jet‐loop system, in comparison with the fusion and ultrasound pre‐synthesis steps, for the dissolution of the silicon and aluminium from the coal fly ash. The critical need for agitation and suitable impeller type, during the hydrothermal synthesis on a large scale, was outlined with an emphasis on the requirement for minimum shearing during crystallisation in large hydrothermal stirred reactors. It is expected that these recommendations would provide understanding towards the design of an appropriate scale

provides the highest and most pure yield of zeolite.

up operation for zeolite synthesis from coal fly ash.

**7. Conclusion**

16 Zeolites - Useful Minerals

James Philip Brassell1 , Tunde Victor Ojumu1\* and Leslie Felicia Petrik2

\*Address all correspondence to: ojumut@cput.ac.za

1 Department of Chemical Engineering, Cape Peninsula University of Technology, Cape Town, South Africa

2 Environmental Nano Sciences, Department of Chemistry, University of the Western Cape, Bellville, South Africa

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22 Zeolites - Useful Minerals

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### **Hydrothermal Conversion of South African Coal Fly Ash into Pure Phase Zeolite Na‐P1**

Mugera W. Gitari, Leslie F. Petrik and Nicholas M. Musyoka

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63399

#### **Abstract**

South African coal combustion power utilities generate huge amounts of coal fly ash that can be beneficiated into zeolitic products. This chapter reports on the optimiza‐ tion of the presynthesis and synthesis conditions for a pure‐phase zeolite Na‐P1 from selected South African coal fly ashes. The hydrothermal treatment time, temperature, and molar quantities of water during the hydrothermal treatment step were success‐ fully optimized. The optimum hydrothermal treatment time and temperature were 48 h and 140°C, respectively. Pure‐phase zeolite Na‐P1 was obtained with a molar regime of 1 SiO2:0.36 Al2O3:0.59 NaOH:0.49 H2O at an aging temperature of 47°C for 48 h. The optimized conditions were applied to two fly ashes from two coal‐fired power utilities, and high‐purity zeolite Na‐P1 was obtained. The third coal fly ash with a different chemical composition gave a low‐quality Na‐P1 under the optimized conditions. The cation exchange capacity for the high‐purity zeolite phase was 4.11 mEq/g, indicating that the adjustment of reactant composition and presynthesis or synthesis parameters leads to yields of high‐quality zeolite Na‐P1. The results also show that conversion of the coal fly ash into high‐purity zeolite also depends on the chemical and mineralogi‐ cal composition of the coal fly ash.

**Keywords:** zeolite Na‐P1, coal fly ash, hydrothermal synthesis, optimization, aging step, CEC

#### **1. Introduction**

South Africa relies mainly on coal combustion for power generation [1]. Low‐grade bitumi‐ nous coal is combusted for power generation; this in turn generates huge volumes of waste

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

materials such as bottom ash, fly ash, boiler slag, flue gas desulfurization sludge, and noncaptured particles [1, 2]. Of most concern is fly ash, which is mainly collected from flue gases by means of mechanical devices [3, 4]. After collection, fly ash is hydraulically trans‐ ported to holding ponds, lagoons, landfills, and slag heaps, where it can be reacquired for treatment purposes or discarded or conveyed to ash heaps in dry disposal systems. The disposal and management of the huge quantities of coal fly ash has been a concern to industrial environmental managers and the scientific community and are in constant search for bulky beneficial utilization of coal fly ash to offset management costs and protect the environment. Approximately 27 Mt of fly ash is produced annually in South African power utilities [1]. Of this, only 5% is utilized beneficially. The remainder is discarded in surface impoundments, such as ash retention dams, where it continues to evolve chemically and mineralogically with possible environmental impacts [5]. Several researchers have proposed strategies for bulky utilization of fly ash, which include (i) applications as an additive in the manufacturing of cement, concrete, construction materials, and road pavements [6, 7], (ii) utilization of fly ash in zeolite synthesis for wastewater treatment [8–14], and (iii) use of fly ash in the neutraliza‐ tion of acid mine drainage and mine backfill [1, 15]. Most of these approaches are an at‐ tempt to beneficially use coal fly ash to supplement the cost of disposal and management and to reduce the negative environmental impact.

The application of coal fly ash in zeolite synthesis is of major interest due to the many industrial applications of zeolites, such as catalysis or catalyst carriers, adsorbents for the removal of inorganic and organic contaminants from wastewaters, management of radioactive wastes, gas separation, slow‐release fertilizer, and in the manufacture of detergents [14]. The conver‐ sion of these low‐cost waste products into products of higher value such as zeolites would allow the beneficiation of fly ash in an environment‐friendly condition in addition to economic benefits. South African class F coal fly ash has been confirmed to be a good feedstock of Al and Si for zeolite synthesis because of its compositional dominance of aluminosilicate and silicate phases [14, 16]. Zeolite synthesis from fly ash is one of the potential environmentally useful applications of fly ash to produce high‐value industrial products [10]. The major potential applications of zeolites synthesized from fly ash are based on their use as high capacity ion exchangers in industrial water waste treatment due to their large pore volumes [8, 10, 17]. Zeolites have also proven to be good candidates for use in soil decontamination [18] and have also shown great potential for use in the removal of postcombustion gases such as SOx and NOx [19].

Through various synthesis methods, researchers have synthesized various types of zeolites from fly ash, such as zeolite Na‐P1 [20], zeolite A [21, 22], and zeolite ZSM‐5 [23]. However, few studies have been successful in the conversion of fly ash into pure‐phase zeolites [24]. Querol et al. [13] evaluated the synthesis of zeolites from fly ash at pilot scale using various Spanish fly ashes. The authors observed that, to obtain pure‐phase zeolites, the optimum synthesis conditions have to be established for each coal fly ash. They attributed this to differences in mineralogical and chemical composition. South African coal fly ashes differ significantly in the chemical and mineralogical composition and this formed the basis of this work, as they have not been evaluated before as feedstocks to synthesize pure‐phase zeolites. The zeolite Na‐P1 was chosen as the model zeolite to test our hypothesis. Zeolite Na‐P1 belongs to an important group of zeolites that can be synthesized under mild hydrothermal synthesis conditions without using templates, making them potentially economically viable and a green product. The narrow particle size distribution of the zeolite Na‐P1 coupled with the micron‐ sized crystallites and the unusual framework flexibility gives this zeolite unique, favorable ion exchange and water sorption properties [25]. Very few studies have reported on the synthesis of pure‐phase zeolites, although numerous studies have been carried out in the synthesis of zeolites from coal fly ash using modified synthesis procedures [13, 21, 26]. Most of the studies have yielded low‐quality zeolites due to the incomplete conversion of the coal fly ash matrix into zeolitic phase. Successful transformation of South African fly ash into impure zeolite P using the mild temperature method (100°C) was achieved by Petrik et al. [27].

Szostak [28] classified the factors affecting the zeolite crystallization as composition of the reaction matrix, time of reaction, temperature, and history‐dependent factors such as stirring, aging, nature of mixture, and order of mixing. Casci [29] observed that the variation of individual reaction components and reaction variables, such as reaction temperature, alkaline concentration, and time of synthesis, influence the type of zeolite synthesized, the quality of the zeolite, and also the efficiency of the synthesis process. To achieve the complete dissolution of coal fly ash matrix and conversion into the zeolitic phase, the modification of presynthesis steps and synthesis and postsynthesis conditions is recommended. A two‐step synthesis procedure developed by Hollman et al. was adapted for our optimization procedures using South African coal fly ashes [30]. The optimized parameters were hydrothermal treatment time, temperature, and water content during hydrothermal treatment. This chapter reports on the successful conversion of South African coal fly ash into pure‐phase Na‐P1 zeolite.

#### **2. Sampling and experimental procedures**

#### **2.1. Sample handling**

materials such as bottom ash, fly ash, boiler slag, flue gas desulfurization sludge, and noncaptured particles [1, 2]. Of most concern is fly ash, which is mainly collected from flue gases by means of mechanical devices [3, 4]. After collection, fly ash is hydraulically trans‐ ported to holding ponds, lagoons, landfills, and slag heaps, where it can be reacquired for treatment purposes or discarded or conveyed to ash heaps in dry disposal systems. The disposal and management of the huge quantities of coal fly ash has been a concern to industrial environmental managers and the scientific community and are in constant search for bulky beneficial utilization of coal fly ash to offset management costs and protect the environment. Approximately 27 Mt of fly ash is produced annually in South African power utilities [1]. Of this, only 5% is utilized beneficially. The remainder is discarded in surface impoundments, such as ash retention dams, where it continues to evolve chemically and mineralogically with possible environmental impacts [5]. Several researchers have proposed strategies for bulky utilization of fly ash, which include (i) applications as an additive in the manufacturing of cement, concrete, construction materials, and road pavements [6, 7], (ii) utilization of fly ash in zeolite synthesis for wastewater treatment [8–14], and (iii) use of fly ash in the neutraliza‐ tion of acid mine drainage and mine backfill [1, 15]. Most of these approaches are an at‐ tempt to beneficially use coal fly ash to supplement the cost of disposal and management and

The application of coal fly ash in zeolite synthesis is of major interest due to the many industrial applications of zeolites, such as catalysis or catalyst carriers, adsorbents for the removal of inorganic and organic contaminants from wastewaters, management of radioactive wastes, gas separation, slow‐release fertilizer, and in the manufacture of detergents [14]. The conver‐ sion of these low‐cost waste products into products of higher value such as zeolites would allow the beneficiation of fly ash in an environment‐friendly condition in addition to economic benefits. South African class F coal fly ash has been confirmed to be a good feedstock of Al and Si for zeolite synthesis because of its compositional dominance of aluminosilicate and silicate phases [14, 16]. Zeolite synthesis from fly ash is one of the potential environmentally useful applications of fly ash to produce high‐value industrial products [10]. The major potential applications of zeolites synthesized from fly ash are based on their use as high capacity ion exchangers in industrial water waste treatment due to their large pore volumes [8, 10, 17]. Zeolites have also proven to be good candidates for use in soil decontamination [18] and have also shown great potential for use in the removal of postcombustion gases such as SOx and

Through various synthesis methods, researchers have synthesized various types of zeolites from fly ash, such as zeolite Na‐P1 [20], zeolite A [21, 22], and zeolite ZSM‐5 [23]. However, few studies have been successful in the conversion of fly ash into pure‐phase zeolites [24]. Querol et al. [13] evaluated the synthesis of zeolites from fly ash at pilot scale using various Spanish fly ashes. The authors observed that, to obtain pure‐phase zeolites, the optimum synthesis conditions have to be established for each coal fly ash. They attributed this to differences in mineralogical and chemical composition. South African coal fly ashes differ significantly in the chemical and mineralogical composition and this formed the basis of this work, as they have not been evaluated before as feedstocks to synthesize pure‐phase zeolites.

to reduce the negative environmental impact.

NOx [19].

26 Zeolites - Useful Minerals

Pulverized coal fly ashes were collected from the ash collection systems at three different coal power utilities in Mpumalanga, South Africa. These coal fly ashes were used as feedstock for the zeolite synthesis. The sampled coal fly ash was stored in tightly locked plastic containers to prevent the ingress of moisture and CO2 and out of direct sunlight. Coal fly ash consists of metastable phases and reactive components, which were formed during high‐temperature combustion, and they are likely to react with CO2 and moisture in the atmosphere forming new products. This might alter the chemical and mineralogical composition of the coal fly ash [30].

#### **2.2. Synthesis procedures and equipment**

A two‐step process for the synthesis of zeolites from fly ash adopted from Hollman et al. [11] was followed, whereby a mixture of coal fly ash and alkaline solution was subjected to (1) aging and (2) hydrothermal treatment.

The setup for the aging process is shown in **Figure 1**. In this aging step, coal fly ash was mixed with sodium hydroxide (NaOH) pellets in a ratio of 1:1. The NaOH pellets were first dissolved in 100 mL ultra‐pure water in a separate beaker and then added to the coal fly ash in a 250 mL HDPC temperature‐resistant sealable bottle, and a magnetic bar was added and the mixture in the sealed bottle was then heated on a magnetic stirrer hotplate. The hotplate was adjusted to a predetermined temperature of 47°C, which was controlled by a temperature probe, while the speed of rotation was set at 800 rpm. The aging temperature and time were kept constant at 47°C and 48 h, respectively, as reported from a previous study [27], to be optimum for the dissolution of aluminosilicate matrix of the coal fly ash in alkaline media.

**Figure 1.** Experimental set‐up for the aging process.

For hydrothermal treatment, varying amounts of ultra‐pure water were added to the slurry after aging, the mixture was stirred, and the resulting homogenous solution was transferred in aliquots of 10 mL into a 23 mL Parr bomb. The crystallization of the feedstock was achieved by placing the mixture in sealed Parr bombs in a thermostated Memmert hot air oven for a predetermined time and temperature (100–160°C at intervals of 20°C with parallel time variation of between 12 and 48 h at intervals of 12 h).

After hydrothermal treatment at the predetermined time and temperature, the sample bearing Parr bombs were removed from the oven and allowed to cool down to room temperature. The cooled mixture was filtered to obtain the filtrates and the solid residue products. The pH of the filtrate was determined, whereas the solid product was washed thoroughly with ultra‐pure water until the washing attained a pH of 9 to 10. The supernatant solution was acidified and kept refrigerated until analysis for chemical species. When the pH of the rinse solution had reached 9, the phases were separated and the solid product was recovered and dried overnight at 90°C and then transferred into airtight plastic containers before characterization. The pH and EC of solutions and supernatants were measured using HANNA HI 991301 portable pH/ EC/TDS/temperature meter, which was calibrated before taking the measurements.

#### **2.3. Characterization**

The setup for the aging process is shown in **Figure 1**. In this aging step, coal fly ash was mixed with sodium hydroxide (NaOH) pellets in a ratio of 1:1. The NaOH pellets were first dissolved in 100 mL ultra‐pure water in a separate beaker and then added to the coal fly ash in a 250 mL HDPC temperature‐resistant sealable bottle, and a magnetic bar was added and the mixture in the sealed bottle was then heated on a magnetic stirrer hotplate. The hotplate was adjusted to a predetermined temperature of 47°C, which was controlled by a temperature probe, while the speed of rotation was set at 800 rpm. The aging temperature and time were kept constant at 47°C and 48 h, respectively, as reported from a previous study [27], to be optimum for the

For hydrothermal treatment, varying amounts of ultra‐pure water were added to the slurry after aging, the mixture was stirred, and the resulting homogenous solution was transferred in aliquots of 10 mL into a 23 mL Parr bomb. The crystallization of the feedstock was achieved by placing the mixture in sealed Parr bombs in a thermostated Memmert hot air oven for a predetermined time and temperature (100–160°C at intervals of 20°C with parallel time

After hydrothermal treatment at the predetermined time and temperature, the sample bearing Parr bombs were removed from the oven and allowed to cool down to room temperature. The cooled mixture was filtered to obtain the filtrates and the solid residue products. The pH of the filtrate was determined, whereas the solid product was washed thoroughly with ultra‐pure water until the washing attained a pH of 9 to 10. The supernatant solution was acidified and

dissolution of aluminosilicate matrix of the coal fly ash in alkaline media.

**Figure 1.** Experimental set‐up for the aging process.

28 Zeolites - Useful Minerals

variation of between 12 and 48 h at intervals of 12 h).

The chemical composition of the coal fly ash and the synthesized zeolitic materials was carried out using X‐ray fluorescence spectroscopy (XRF; Philips PW 1480 X‐ray spectrometer). The samples were prepared by mixing 9 g of coal fly ash or zeolitic material with 2 g of a binder (10% C‐wax binder and 90% EMU powder). The mixture was then thoroughly shaken, poured into the mould, and pelletized at a pressure of 15 tons for ∼1 min using a Dickie and Stockler manual pelletizer. Loss on ignition (LOI) was measured by placing the samples in the furnace at 1000°C for at least 45 min. The instrument operating conditions for major element analysis were on a fused glass bead at 40 kV and 50 mA tube, whereas those for trace species were on a powder briquette at 50 kV and 40 mA tube operating conditions.

The samples for X‐ray diffraction (XRD) were ground to a fine powder. The qualitative and quantitative XRD analyses of the coal fly ash and synthesized zeolite samples were done by placing the powder samples in a sample holder. The sample pattern was generated by a Philips X‐ray diffractometer with Cu Kα radiation. The crystalline phases were identified by matching the obtained XRD profile with the powder diffraction file data (Joint Committee of Powder Diffraction Standards) files for inorganic compounds. The quantification of crystalline and amorphous phases was done as follows: After the addition of 20% Si (Aldrich; 99% purity) for the determination of amorphous content, the samples were milled in a McCrone micronizing mill. XRD profiles were acquired with a PANalytical X'Pert Pro powder diffractometer with X'Celerator detector and variable divergence and receiving slits with Fe filtered Co Kα radiation generated at 20 mA and 40 kV. The phases were identified using X'Pert Highscore Plus software. The relative phase amounts (wt.%) were estimated using the Rietveld method (Autoquan Program).

The morphology of the coal fly ash and the zeolitic products were examined using a scanning electron microscope (Hitachi X‐650, Tokyo, Japan) with a compact detecting unit lead detector at 25 kV. The samples were mounted on aluminum pegs and sput coated with a thin film of gold for conductivity. For analysis by transmission electron microscopy (TEM), the samples were prepared by diluting a suspension of the synthesis products in ethanol, ultrasonicating, and depositing a drop onto S147‐4 Holey carbon film 400 mesh Cu grids. A 200 kV field emission gunlens 1 was used with spotsize 3 at 200 kV using high‐resolution TEM (HRTEM) Tecnai G2 F20 XTwin MAT.

The cation exchange capacity (CEC) was determined following the methods of Amrhein et al. [31]. The untreated coal fly ash was first saturated through three repeated rinsing steps with 1.0 M sodium acetate (pH 8.2) followed by four washings with ultra‐pure water. The extraction of the exchangeable cations was done with three aliquots of 1.0 M ammonium acetate (pH 8.2). A 0.5 g of the zeolitic products was extracted in succession with four 25 mL aliquots of 1.0 M ammonium acetate (pH 8.2). On the addition of the 25 mL extracting solution, the mixture was then continuously agitated for 15 min, centrifuged for 15 min, and decanted. The procedure was repeated four times and the cumulative extract was collected for each sample. The concentrations of exchangeable cations (Na+ , Mg2+, Ca2+, and K+ ) in the final solution were determined by inductively coupled plasma‐atomic emission spectrometry. The CEC was then reported as mEq/g sample.

The surface area and pore size determination of the zeolitic products were carried out using the Brunauer‐Emmett‐Teller (N2‐BET) surface analysis technique. The samples (0.35–0.5 g) were first outgassed at 110°C using helium gas. Micromeritics Tristar instrument (Tristar3000, Micromeritics, Norcross, GA, USA) was used with nitrogen as the analysis gas based on a 5 point with 30 adsorption and 30 desorption points.

### **3. Results and discussion**

#### **3.1. Characterization of coal fly ash and zeolitic products**

The chemical composition of the coal fly ash feedstock and the zeolitic products reported as oxides is presented in **Table 1**.


**Table 1.** Chemical composition of coal fly ash and synthesized zeolites.

For all coal fly ash feedstock, the SiO2+Al2O3+Fe2O3 is ≥70%, meaning the fly ashes can be classified as class F fly ash (ATSM method C 618). This is consistent with coal fly ash from the combustion of bituminous coal from South Africa [32]. The LOI ranged from 3.85% to 6.59% (w/w). LOI represents the unburned carbon in coal fly ash. The mean SiO2/Al2O3 ratio ranged from 1.65 for coal fly ash A, 1.57 for B, and 2.01 for C. The SiO2/Al2O3 ratio is important in that it directly governs both the Si/Al ratio of the zeolite product and the incorporation of Al in the framework structure. The presence of CaO and MgO plays a significant role in zeolite synthesis. The Ca2+ and Mg2- ions act as competing cations during synthesis [33]. The lower content of CaO in coal fly ash C could result in the lower alkalinity of the solution during aging and hydrothermal synthesis, which could affect the rate of depolymerization and monomer‐ ization process [33]. The SiO2/Al2O3 for zeolite A (from coal fly ash A) was 1.45 less than in the feedstock fly ash, indicating the inefficient conversion of the feedstock. Na2O was observed to be higher in all the zeolites compared to the feedstock; this is because of the incorporation of Na as a charge‐balancing cation, as NaOH was used as the alkaline coal fly ash matrix dissolution agent.

ammonium acetate (pH 8.2). On the addition of the 25 mL extracting solution, the mixture was then continuously agitated for 15 min, centrifuged for 15 min, and decanted. The procedure was repeated four times and the cumulative extract was collected for each sample. The

determined by inductively coupled plasma‐atomic emission spectrometry. The CEC was then

The surface area and pore size determination of the zeolitic products were carried out using the Brunauer‐Emmett‐Teller (N2‐BET) surface analysis technique. The samples (0.35–0.5 g) were first outgassed at 110°C using helium gas. Micromeritics Tristar instrument (Tristar3000, Micromeritics, Norcross, GA, USA) was used with nitrogen as the analysis gas based on a 5

The chemical composition of the coal fly ash feedstock and the zeolitic products reported as

SiO2 50.91 49.79 54.92 36.69 35.16 33.95 Al2O<sup>3</sup> 30.91 31.75 27.27 25.17 22.57 31.81 Fe2O<sup>3</sup> 3.46 3.17 4.78 2.28 1.78 2.28 MnO 0.02 0.00 0.05 0.06 0.04 0.04 MgO 1.48 0.98 1.07 2.21 1.48 1.12 CaO 6.2 4.62 3.69 6.07 4.36 3.65 Na2O 0.10 0.09 0.07 6.53 9.17 5.49 K2O 0.60 0.63 0.66 0.13 0.13 0.14 P2O5 0.56 0.67 0.61 0.04 0.06 0.04 TiO2 1.65 1.46 0.3 1.58 1.24 1.95 SO<sup>3</sup> 0.24 0.23 1.71 0.06 0.06 0.03 LOI 3.85 6.59 4.44 11.92 13.91 10.99 Total 99.99 99.98 99.58 99.81 98.2 99.21 SiO2/Al203 1.65 1.57 2.01 1.45 1.56 1.07

**Coal fly ash C (%, w/w)**

**Zeolite A (%, w/w)**

**Zeolite B (%, w/w)**

**Zeolite C (%, w/w)**

, Mg2+, Ca2+, and K+

) in the final solution were

concentrations of exchangeable cations (Na+

point with 30 adsorption and 30 desorption points.

**3.1. Characterization of coal fly ash and zeolitic products**

**Table 1.** Chemical composition of coal fly ash and synthesized zeolites.

**Coal fly ash B (%, w/w)**

reported as mEq/g sample.

30 Zeolites - Useful Minerals

**3. Results and discussion**

oxides is presented in **Table 1**.

**Coal fly ash A (%, w/w)**

**Chemical component** The qualitative XRD results of coal fly ash A are presented in **Figure 2** (note that the XRD profile is similar for coal fly ashes B and C and hence not shown), whereas **Figure 3** shows a summary of the quantitative analysis of the three coal fly ashes.

**Figure 2.** XRD profile of coal fly ash A (Q = quartz, M = mullite, H = hematite, Mag = magnetite).

**Figure 3.** Relative quantitative XRD analysis of the three coal fly ashes.

The qualitative XRD analysis indicated the presence of the following mineral phases in all coal fly ash, quartz, mullite, magnetite, and hematite (**Figure 2**). The presence of the amorphous phase can be identified as the broad diffraction "hump" in the region between 18° and 32° 2*θ* [21, 26]. The quantitative analysis results show that mullite, quartz, and amorphous materials comprised 96% of the total mineral composition (**Figure 3**). It is noticeable in **Figure 3** that coal fly ash A has a higher content of the amorphous phase and the lowest quantity of quartz, mullite, hematite, and magnetite phases. Rayalu et al. [22] pointed out that the low levels of mullite promote zeolite synthesis and this is attributed to the fact that mullite is resistant to dissolution during hydrothermal treatment. The presence of the aluminosilicate phases in all the evaluated coal fly ashes qualifies their potential for the conversion to zeolite Na‐P1.

#### **3.2. Optimization of zeolite synthesis conditions**

The optimization of the zeolite synthesis conditions was carried out using coal fly ash A. These optimized conditions were then applied for zeolite synthesis using coal fly ashes B and C and the properties of the zeolitic material compared to that resulting from coal fly ash A. The optimized conditions were the hydrothermal treatment temperature, time, and amount of water during the hydrothermal treatment process. The results are presented in terms of the chemical and mineralogical analysis of the zeolitic products, morphology, CEC, surface area, and pore volume.

#### *3.2.1. Optimization of the hydrothermal treatment temperature and time*

XRD was employed as a tool to monitor the evolution of zeolitic phases during the hydro‐ thermal treatment process. Several authors pointed out that the increase of temperature leads to an increase in mullite dissolution, nucleation, and growth in zeolite crystals and an increased in Si dissolution from coal fly ash leading to an increase in Si/Al ratio, which influences the crystallinity of the final zeolitic product [10, 33, 34]. **Figure 4** shows the XRD profile of zeolitic products for a concurrent variation of hydrothermal treatment time and temperature for coal fly ash A feedstock. **Figure 4a** shows the XRD profile of the transformation of different phases of the coal fly ash as a function of time at hydrothermal treatment temperature of 100°C. **Figure 4a** shows the disappearance of the broad hump between 18° and 32° 2*θ* (**Figure 2**), signifying the dissolution of the amorphous glassy phase during the hydrothermal treatment. The zeolitic phase produced was identified as zeolite Na‐P1 (Na6Al6Si10O32ċ12H2O). Some quartz, mullite, and hematite phases remained undissolved from the matrix even after 48 h hydrothermal treatment time at 100°C. **Figure 4b** shows the XRD profile of the transformation of different phases of the coal fly ash as a function of time at the hydrothermal treatment temperature of 120°C. The hydrothermal treatment at 120°C for various times did not result in pure‐phase zeolite. An incomplete dissolution of the coal fly ash matrix was also observed with mullite and quartz being identified. However, a new zeolitic phase was identified as hydroxy‐sodalite formed in all treatment times. At an increased hydrothermal treatment temperature of 140°C (**Figure 4c**), an increased dissolution of the coal fly ash aluminosilicate matrix was observed with increased hydrothermal treatment time, leading to almost complete dissolution at 48 h. The hematite mineral phase was completely dissolved. At 160°C hydro‐ thermal treatment temperature, the mullite and quartz phases were gradually dissolved as the treatment time increased with complete dissolution at 48 h. However, another zeolitic phase was observed (hydroxyl‐sodalite, whose peaks increased in intensity with increasing time; **Figure 4d**). The optimization of the hydrothermal treatment time and temperature identified

**Figure 3.** Relative quantitative XRD analysis of the three coal fly ashes.

**3.2. Optimization of zeolite synthesis conditions**

*3.2.1. Optimization of the hydrothermal treatment temperature and time*

Na‐P1.

32 Zeolites - Useful Minerals

and pore volume.

The qualitative XRD analysis indicated the presence of the following mineral phases in all coal fly ash, quartz, mullite, magnetite, and hematite (**Figure 2**). The presence of the amorphous phase can be identified as the broad diffraction "hump" in the region between 18° and 32° 2*θ* [21, 26]. The quantitative analysis results show that mullite, quartz, and amorphous materials comprised 96% of the total mineral composition (**Figure 3**). It is noticeable in **Figure 3** that coal fly ash A has a higher content of the amorphous phase and the lowest quantity of quartz, mullite, hematite, and magnetite phases. Rayalu et al. [22] pointed out that the low levels of mullite promote zeolite synthesis and this is attributed to the fact that mullite is resistant to dissolution during hydrothermal treatment. The presence of the aluminosilicate phases in all the evaluated coal fly ashes qualifies their potential for the conversion to zeolite

The optimization of the zeolite synthesis conditions was carried out using coal fly ash A. These optimized conditions were then applied for zeolite synthesis using coal fly ashes B and C and the properties of the zeolitic material compared to that resulting from coal fly ash A. The optimized conditions were the hydrothermal treatment temperature, time, and amount of water during the hydrothermal treatment process. The results are presented in terms of the chemical and mineralogical analysis of the zeolitic products, morphology, CEC, surface area,

XRD was employed as a tool to monitor the evolution of zeolitic phases during the hydro‐ thermal treatment process. Several authors pointed out that the increase of temperature leads to an increase in mullite dissolution, nucleation, and growth in zeolite crystals and an increased

**Figure 4.** XRD profile of zeolitic products on the concurrent variation of hydrothermal treatment time (12–48 h) and temperature (100–160°C) for coal fly ash A (P = zeolite Na‐P1, Q = quartz, M = mullite, H = hematite, HS = hydroxy‐ sodalite). Reproduced with permission from Taylor & Francis Group, LLC, the copyright owners.

140°C temperature and 48 h as the optimum conditions for the synthesis of pure‐phase zeolite Na‐P1. However, traces of mullite and quartz were still observed in the almost pure phase obtained.

#### *3.2.2. Effect of variation of water during hydrothermal treatment process*

During the aging process, the dissolution of the aluminosilicate matrix of the coal fly ash occurs, releasing Si and Al, which are the main building blocks of the zeolite. Depending on the conditions employed, the saturation of the ionic precursors can occur, hindering more dissolution. The addition of water will tend to dilute the concentration of the ionic precursors leading to the further dissolution of the coal fly ash matrix. **Figure 5** shows the XRD profile of zeolitic products obtained on varying the water added during the hydrothermal treatment at 140°C and 48 h treatment time.

**Figure 5.** XRD profile of zeolitic products synthesized at 140°C for 48 h with variation of water during the hydrother‐ mal treatment (P = zeolite Na‐P1, HS = hydroxy‐sodalite, Q = quartz). Reproduced with permission from Taylor & Francis Group, LLC, the copyright owners.

The additional water was added after the aging process, as equal amounts of water (100 mL) had been used during the aging process. It is observed that the addition of more water after aging enabled the formation of relatively pure phase of zeolite Na‐P1. A balance has to be struck on the amount of water to be added, as too much water can also change the degree of supersaturation, which can slow the crystallization kinetics in addition to increasing the cost of zeolite synthesis due to the need to manage the disposal of large amounts of effluent generated.

#### *3.2.3. Relative percent crystallinity of the zeolitic products*

140°C temperature and 48 h as the optimum conditions for the synthesis of pure‐phase zeolite Na‐P1. However, traces of mullite and quartz were still observed in the almost pure phase

During the aging process, the dissolution of the aluminosilicate matrix of the coal fly ash occurs, releasing Si and Al, which are the main building blocks of the zeolite. Depending on the conditions employed, the saturation of the ionic precursors can occur, hindering more dissolution. The addition of water will tend to dilute the concentration of the ionic precursors leading to the further dissolution of the coal fly ash matrix. **Figure 5** shows the XRD profile of zeolitic products obtained on varying the water added during the hydrothermal treatment at

**Figure 5.** XRD profile of zeolitic products synthesized at 140°C for 48 h with variation of water during the hydrother‐ mal treatment (P = zeolite Na‐P1, HS = hydroxy‐sodalite, Q = quartz). Reproduced with permission from Taylor &

The additional water was added after the aging process, as equal amounts of water (100 mL) had been used during the aging process. It is observed that the addition of more water after aging enabled the formation of relatively pure phase of zeolite Na‐P1. A balance has to be struck on the amount of water to be added, as too much water can also change the degree of supersaturation, which can slow the crystallization kinetics in addition to increasing the cost

*3.2.2. Effect of variation of water during hydrothermal treatment process*

obtained.

34 Zeolites - Useful Minerals

140°C and 48 h treatment time.

Francis Group, LLC, the copyright owners.

The relative percent crystallinity of the zeolitic products was done by summing and normal‐ izing the peak heights of five major peaks (2*θ*=12°, 23°, 28°, 33°, and 46°; **Figure 4c**). The normalized peaks of zeolite Na‐P1 synthesized at 140°C for 48 h was assumed to be 100% crystalline for the purpose of comparing to the spectra of the zeolites synthesized at 12, 24, and 36 h. A comparison of the percent crystallinity (peak heights) of the zeolite Na‐P1 with quartz peaks was observed to be inversely proportional (**Figure 6**). This suggested the dissolution of quartz phase and the release of Si, which was then made available for the zeolitization.

**Figure 6.** Comparison of the relative percent crystallinity of quartz and zeolite Na‐P1 synthesized at 140°C (48 h) by varying the H2O/SiO2 molar ratio during the hydrothermal treatment.

#### **3.3. Synthesis of zeolite Na‐P1 using other coal fly ashes at optimized conditions**

To test the success of the optimization process, the synthesis of zeolite Na‐P1 from different coal fly ash feedstock was attempted at the optimized conditions. The XRD and XRF charac‐ terization of these coal fly ashes (coal fly ashes B and C) were presented previously (**Fig‐ ures 2** and **3**; **Table 1**). **Figure 7** presents a comparison of the XRD profile of zeolite Na‐P1 synthesized from coal fly ashes A, B, and C at the previously optimized conditions (140°C hydrothermal temperature and 48 h treatment time). **Figure 7** indicates that the zeolite Na‐P1 produced with coal fly ash B was similar to that of coal fly ash A. However, coal fly ash C showed an incomplete dissolution of mullite and quartz. This could be attributed to the difference in chemical and mineralogical composition of the coal fly ash C compared to coal fly ashes A and C (**Figures 2** and **3**; **Table 1**). This indicates that the conditions employed were not optimum for the dissolution of quartz and mullite in coal fly ash C. Another observable fact about the incomplete zeolitization of coal fly ash C could be due to the low CaO, which is known to contribute to the alkalinity of the solution. Coal fly ash C had the lowest CaO content (**Table 1**).

**Figure 7.** XRD profile of zeolites synthesized from coal fly ashes A, B, and C at 140°C and 48 h treatment time. Repro‐ duced with permission from Taylor & Francis Group, LLC, the copyright owners.

#### **3.4. Morphological evolution of the coal fly ashes during the synthesis process**

The morphological evolution of the coal fly ash feedstock after aging and hydrothermal treatment is depicted in **Figure 8**. The coal fly ash has a smooth spherical appearance and typically consists of microspheres. Gitari et al. [15] have previously reported the smooth spherical appearance of South African coal fly ash. The coal fly ash particles are observed to be smooth spheres, as the glassy phase covers the particles. The coal fly ash after the aging process appears to show a partial disintegration of the spheres, indicating the conditions employed during aging are not sufficient to completely break the matrix. This could also point to the resistance of mullite and quartz to dissolution. However, after hydrothermal treatment, a transformation of the disintegrating microspheres was observed, with granular crystalline particles being observed on the surface of the spheres. These were identified by XRD to be the zeolitic phases. Walek et al. [24] reported that the hydrothermal crystallization process begins on the surface of the undissolved or partially dissolved coal fly ash particle. These granular particles represented the initial zeolite formation process. A further illustration of the evolution of the crystalline zeolitic phases is depicted during the variation of water during the hydro‐ thermal treatment at 140°C temperature for 48 h (**Figure 9**). Crystalline granules are clearly evident on the surface of the microspheres, which increases as the amount of water is increased. The crystallinity is also evident from the increasing agglomeration of the crystalline granules. The formation of the well‐ordered crystalline zeolitic phase formed during aging conditions of 47°C for 48 h and hydrothermal treatment for 48 h at 140°C with H2O/SiO2 ratio of 0.49 was confirmed by use of HRTEM (**Figure 10**).

difference in chemical and mineralogical composition of the coal fly ash C compared to coal fly ashes A and C (**Figures 2** and **3**; **Table 1**). This indicates that the conditions employed were not optimum for the dissolution of quartz and mullite in coal fly ash C. Another observable fact about the incomplete zeolitization of coal fly ash C could be due to the low CaO, which is known to contribute to the alkalinity of the solution. Coal fly ash C had the lowest CaO content

**Figure 7.** XRD profile of zeolites synthesized from coal fly ashes A, B, and C at 140°C and 48 h treatment time. Repro‐

The morphological evolution of the coal fly ash feedstock after aging and hydrothermal treatment is depicted in **Figure 8**. The coal fly ash has a smooth spherical appearance and typically consists of microspheres. Gitari et al. [15] have previously reported the smooth spherical appearance of South African coal fly ash. The coal fly ash particles are observed to be smooth spheres, as the glassy phase covers the particles. The coal fly ash after the aging process appears to show a partial disintegration of the spheres, indicating the conditions employed during aging are not sufficient to completely break the matrix. This could also point to the resistance of mullite and quartz to dissolution. However, after hydrothermal treatment, a transformation of the disintegrating microspheres was observed, with granular crystalline particles being observed on the surface of the spheres. These were identified by XRD to be the zeolitic phases. Walek et al. [24] reported that the hydrothermal crystallization process begins on the surface of the undissolved or partially dissolved coal fly ash particle. These granular particles represented the initial zeolite formation process. A further illustration of the evolution

**3.4. Morphological evolution of the coal fly ashes during the synthesis process**

duced with permission from Taylor & Francis Group, LLC, the copyright owners.

(**Table 1**).

36 Zeolites - Useful Minerals

**Figure 8.** Morphological illustration of the reaction mechanism for the formation of zeolite phases.

**Figure 9.** Morphological evolution of the zeolitic phases with variation of water content during the hydrothermal treat‐ ment.

**Figure 10.** HRTEM micrograph showing the well‐ordered crystalline zeolitic phase obtained from coal fly ash A feed‐ stock.

In conclusion, the observations on the morphological transformation/evolution of the coal fly ash indicate that the glassy phase was dissolved in alkaline media and subsequently trans‐ formed into zeolite crystals, which were deposited on the surface of the disintegrating microsphere. The zeolite formation mechanism can therefore be described as (1) the dissolution of the SiO2 and Al2O3 from the glassy phase/mullite/quartz in alkaline media, (2) the formation of aluminosilicate gel as zeolite precursor from SiO4 and AlO4 species, and (3) the crystalliza‐ tion of the zeolite phase at supersaturation of the alkaline media. The addition of water during hydrothermal treatment lowers the supersaturation and allows more dissolution of the glassy phase/mullite/quartz and the release of more SiO4 and AlO4 species leading to increased formation of the zeolite phase. The disintegration of the fly ash microsphere is predicted to continue with increased dissolution of the resistant mullite and quartz phases, as more hydroxyl ions migrate through the microsphere with continued hydrothermal treatment and subsequent formation of crystalline zeolitic phases (**Figure 9**).

#### **3.5. CEC and surface area**

The CEC of the zeolitic products was determined to provide further evidence to the purity of the phases. An increase in CEC was observed in the zeolitic products compared to the feedstock coal fly ash. Coal fly ash had a CEC of 0.48 mEq/g, zeolite product at 100°C/48 h of 2.98 mEq/g, and zeolite product at 140°C/48 h of 3.91 to 4.11 mEq/g. These results corre‐ late well with the increasing crystallinity observed in the optimization stages (**Figures 4** and **9**). The results also compare closely to the CEC value of commercial zeolite Na‐P1 (5 mEq/g). The addition of water during the hydrothermal treatment enhanced the CEC of the zeolitic products (zeolitic product obtained at 140°C/48 h with a H2O/SiO<sup>2</sup> molar ratio of 0.49 was 4.10 mEq/g). This again confirms the high crystallinity and purity of the zeolitic phase obtained at this synthesis conditions. The BET analysis of surface area showed that the surface area of the material obtained after aging was low (30.5693 m2 /g), but this was observed to increase significantly on hydrothermal treatment at 24 h/140°C (58.6358 m2 /g) and 48 h/140°C (67.6329 m2 /g), which again confirms the high‐purity and crystallinity of the zeolitic phase.

#### **4. Conclusions**

**Figure 10.** HRTEM micrograph showing the well‐ordered crystalline zeolitic phase obtained from coal fly ash A feed‐

In conclusion, the observations on the morphological transformation/evolution of the coal fly ash indicate that the glassy phase was dissolved in alkaline media and subsequently trans‐ formed into zeolite crystals, which were deposited on the surface of the disintegrating microsphere. The zeolite formation mechanism can therefore be described as (1) the dissolution of the SiO2 and Al2O3 from the glassy phase/mullite/quartz in alkaline media, (2) the formation of aluminosilicate gel as zeolite precursor from SiO4 and AlO4 species, and (3) the crystalliza‐ tion of the zeolite phase at supersaturation of the alkaline media. The addition of water during hydrothermal treatment lowers the supersaturation and allows more dissolution of the glassy phase/mullite/quartz and the release of more SiO4 and AlO4 species leading to increased formation of the zeolite phase. The disintegration of the fly ash microsphere is predicted to continue with increased dissolution of the resistant mullite and quartz phases, as more hydroxyl ions migrate through the microsphere with continued hydrothermal treatment and

The CEC of the zeolitic products was determined to provide further evidence to the purity of the phases. An increase in CEC was observed in the zeolitic products compared to the feedstock coal fly ash. Coal fly ash had a CEC of 0.48 mEq/g, zeolite product at 100°C/48 h of 2.98 mEq/g, and zeolite product at 140°C/48 h of 3.91 to 4.11 mEq/g. These results corre‐ late well with the increasing crystallinity observed in the optimization stages (**Figures 4** and **9**). The results also compare closely to the CEC value of commercial zeolite Na‐P1 (5 mEq/g). The addition of water during the hydrothermal treatment enhanced the CEC of the zeolitic products (zeolitic product obtained at 140°C/48 h with a H2O/SiO<sup>2</sup> molar ratio of 0.49 was 4.10 mEq/g). This again confirms the high crystallinity and purity of the zeolitic phase obtained at this synthesis conditions. The BET analysis of surface area showed that

/g), but this was

/g)

the surface area of the material obtained after aging was low (30.5693 m2

observed to increase significantly on hydrothermal treatment at 24 h/140°C (58.6358 m2

subsequent formation of crystalline zeolitic phases (**Figure 9**).

**3.5. CEC and surface area**

stock.

38 Zeolites - Useful Minerals

South African coal fly ashes have been proven to be suitable feedstock for hydrothermal conversion into high‐purity zeolite Na‐P1. The compositional and mineralogical analysis of the South African coal fly ashes identified mineral phases quartz, mullite, and amorphous glassy phase, which acts as crucial ingredients for the zeolite synthesis process. The optimi‐ zation of the hydrothermal treatment process identified the conditions for the synthesis of high‐purity zeolite Na‐P1 as molar regime of 1 SiO2:0.36 Al2O3:0.59 NaOH:0.49 H2O and syn‐ thesis conditions of aging slurry at 47°C for 48 h. This was followed by hydrothermal treat‐ ment at 48 h and 140°C. The addition of water after the aging step expressed as H2O/SiO<sup>2</sup> molar ratio was observed to play an important role in the hydrothermal conversion of coal fly ash into high‐purity zeolites. The results show that a simple adjustment of reactant com‐ position and presynthesis or synthesis parameters leads to almost complete dissolution of coal fly ash matrix and the conversion of the Si/Al ionic precursors into high‐purity zeolite Na‐P1. The high purity and CEC (4.11 mEq/g) imply that the zeolites from the class F South African coal fly ash can meet high‐end applications such as wastewater treatment or appli‐ cation in detergents.

#### **Acknowledgements**

The authors would like to acknowledge the Water Research Commission (WRC), National Research Foundation (NRF), and Coaltech 2020 for funding this research work.

#### **Author details**

Mugera W. Gitari1\*, Leslie F. Petrik<sup>2</sup> and Nicholas M. Musyoka<sup>3</sup>

\*Address all correspondence to: mugera.gitari@univen.ac.za

1 Environmental Remediation and Water Pollution Chemistry Group, School of Environmental Sciences, University of Venda, Thohoyandou, South Africa

2 Environmental and Nano Science Research Group, Department of Chemistry, University of the Western Cape, Cape Town, South Africa

3 Materials Science and Manufacturing, Council for Scientific and Industrial Research, Pretoria, South Africa

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## **Synthesis of Zeolite from Fly Ash and their Use as Soil Amendment**

Maria Harja, Sorin Mihai Cimpeanu, Marcel Dirja and Daniel Bucur

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http://dx.doi.org/10.5772/64126

#### **Abstract**

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The amendments may be natural or synthetic and by their origin organic or inorganic. Among amendments, the inorganic ones are recommended, such as gypsum, lime, zeolites and altered rocks, including sand, ceramic, perlite, ash, slag, zeolite, pyrites, dolomite, calcined clay, etc. Fly ash-based zeolites can be used as amendments to improve the use of the soil for agricultural purposes. Generally, there are different methods for the synthesis of new materials using solid waste – fly ashes. Methods are known to be composed of a single stage or two stages. The first method – the conven‐ tional – is direct hydrothermal conversion of the mixture of ash and alkaline solution (NaOH or KOH), but only 50% of ashes can be converted into zeolite. The second method consists in mixing the ash with KOH and fusion at elevated temperature. The method leads to an advanced conversion – the type of zeolite depending on treatment conditions. Other studies recommend the conversion of ash using microwave ovens or ultrasound bath; in this case, the conversion time is reduced to 1–2 hours. From these methods, the direct method was proposed to be used in synthesizing zeolites for agriculture, with a part in controlling release of fertilizers and as a soil amendment. The synthesized materials were characterized with respect to microstructure (electronic microscopy SEM) and chemical and mineralogical composition (EDAX, X-ray diffraction). By analyzing the obtained data, one may observe the destruction of the ash network and crystallization of the zeolitic phase, especially in the case of treatment by diffusion. The XRD analysis confirms the presence of zeolite in materials obtained. Good results were also obtained in the case of using ultrasound treatment for zeolite synthesis; also, the time of treatment significantly decreased in this case.

**Keywords:** amendment, fly ash, heavy metals, characterization, synthesis, soil, zeolite

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **1. Introduction**

Soil is the most important component of the surface crust along with water and air of which it is composed of the environment. The mineral composition of soils is very diverse, usually includingmineralsfromthesilicategroup,oxides,andhydroxides,andmineralsorderedmainly on the grain size criteria [1]. The usual minerals are quartz, feldspar, mica and calcite, mullite, kaolinite, montmorillonite, illite, vermiculite, and chlorite. Organic and mineral constituents of soils are represented by organic complexes of metal ions (Cu2+, Fe2+, Mn2+) and associations between clay and organic humic and nonhumic substances.

In the last decade, there has been a decrease in the areas with agricultural uses due to decreased fertility, compaction, acidification, heavy metal pollution, etc. To achieve higher agricultural production, measures must be taken to improve the physical, chemical, and biological properties of soils. Due to the acidity in the soil, nitrification and nitrogen-fixing soil bacteria do not develop; therefore, the content of nitrogen is low [2].

For improving soil characteristics, the research on soil quality being oriented toward finding measures to improve the structure, air, and water regime, restoring the content of organic matter, improving activity of soil microorganisms, knowing the transformation to which applied chemicals are exposed, critical limits of the pollution, reducing the content of heavy metal ions, and evaluating the risks to human health and domestic animals and wildlife.

A soil amendment describes any substances added to a soil to improve or modify its qualities and the conditions including physical, chemical, and biological conditions. An amended soil is a conditioned and a fertilized one. Choosing the amendment depends on what we seek to improve, the degree of soil degradation, its composition, the climate, etc. A suitable amend‐ ment to a situation may not be useful in other conditions [3].

The main objective of the amendments is to improve the use of the soil for agricultural purposes. The amendments may be natural or synthetic and by their origin organic or inorganic. Among amendments the inorganic ones are recommended, such as gypsum, lime, zeolites, altered rocks, ceramic, perlite, ash, slag, zeolite, pyrites, dolomite, calcined clay, etc. They contain a variable amount of nutrients, which can affect the pH.

Among inorganic substances used, the study focuses on thermal power station ash and its use in the synthesis of zeolites.

Among industrial wastes, ash plays a leading role because of the large volume and the consequences, due to a growing accumulation. Annually about 600 million tons of ash are produced, 75–80% of which is the one resulted from thermal plants, being considered the fifth as a source of prime mover [4].

The ash is used in agriculture since it contains nearly all of micro- and macro-elements, except for the organic carbon and nitrogen [5]. Also, the ash can contain moderate amounts of heavy metals, affecting groundwater and may be retained by plants. This material is used as the filling material, the recovery of acid soils, depending on the pH.

Zeolites are crystalline form of alkaline aluminosilicates, with a stable structure. This structure containing cavities has the ability to retain water and negative charge capably to change heavy metals [6]. This negative charge results in the ability of the zeolites to change plant nutrients and eliminate them from the structure. This ability to exchange cations allows nutrients such as nitrogen, calcium, iron, magnesium, potassium, and water to be available in the plant and to be removed at a low speed in time. In this way, the plant can absorb nutrients from the soil whenever it is required. Zeolites improve performance of fertilizers by increasing resistance to wash, immobilization, and gas losses. They increase soil water reserves needed during dry periods and are able to reversibly lose and retain water and to change network's cations without the change in the structure. They are chemically inert, nontoxic to plants, animals, humans, and the environment and regulate the balance of soil through the immobilization of toxic contaminants.

#### **2. Methods and types of amendments used for reduction of heavy metals**

The contamination of soil with heavy metals has been occurring from the last century, but its extent has increased in the last years, due to industrial progresses and amplified user of supplies containing these metals. In the category, heavy metals enter a series of chemical elements, with high toxicity to living organisms. The toxic effect occurs above a certain threshold below which some of them (Co, Cd, Cu, Fe, Ni, Zn, Hg, and Pb) may even be essential components of proteins involved in various metabolic pathways. Thus, if the food would be completely devoid of metals, it would appear nutritional deficiencies.

The heavy metals are found in various concentrations in the soil, water, air, foods of vegetable, or animal origin, depending on the various factors that determine their pollution. Air can be a source of contamination representing a way of handling metals and their deposition on soil and plants (e.g., lead emissions from cars).

Accumulation of heavy metals in soils has implications over human health [7]. The heavy metals from different ecosystems would migrate into groundwater or bioaccumulate in herbs and get into the food chain [8]. In general, heavy metals may affect the soil ecosystem, not only agricultural produce and groundwater quality but also the human health [9].

Heavy metal contamination of the air is the result of numerous anthropogenic activities: combustion of coal, petroleum, nonferrous metal production, production of iron, steel and cement production, installations for waste gas treatment and incineration of waste accumula‐ tion, etc. The sources of metals in the soil can be use of fertilizers, pesticides containing metals (fungicides containing mercury, copper, arsenic, zinc, etc.). Of course, depending on the type of soil and geographic location, it can contain high amounts of heavy metals or they can be deficient.

#### **2.1. Methods used for reduction of heavy metals**

**1. Introduction**

44 Zeolites - Useful Minerals

Soil is the most important component of the surface crust along with water and air of which it is composed of the environment. The mineral composition of soils is very diverse, usually includingmineralsfromthesilicategroup,oxides,andhydroxides,andmineralsorderedmainly on the grain size criteria [1]. The usual minerals are quartz, feldspar, mica and calcite, mullite, kaolinite, montmorillonite, illite, vermiculite, and chlorite. Organic and mineral constituents of soils are represented by organic complexes of metal ions (Cu2+, Fe2+, Mn2+) and associations

In the last decade, there has been a decrease in the areas with agricultural uses due to decreased fertility, compaction, acidification, heavy metal pollution, etc. To achieve higher agricultural production, measures must be taken to improve the physical, chemical, and biological properties of soils. Due to the acidity in the soil, nitrification and nitrogen-fixing soil bacteria

For improving soil characteristics, the research on soil quality being oriented toward finding measures to improve the structure, air, and water regime, restoring the content of organic matter, improving activity of soil microorganisms, knowing the transformation to which applied chemicals are exposed, critical limits of the pollution, reducing the content of heavy metal ions, and evaluating the risks to human health and domestic animals and wildlife.

A soil amendment describes any substances added to a soil to improve or modify its qualities and the conditions including physical, chemical, and biological conditions. An amended soil is a conditioned and a fertilized one. Choosing the amendment depends on what we seek to improve, the degree of soil degradation, its composition, the climate, etc. A suitable amend‐

The main objective of the amendments is to improve the use of the soil for agricultural purposes. The amendments may be natural or synthetic and by their origin organic or inorganic. Among amendments the inorganic ones are recommended, such as gypsum, lime, zeolites, altered rocks, ceramic, perlite, ash, slag, zeolite, pyrites, dolomite, calcined clay, etc.

Among inorganic substances used, the study focuses on thermal power station ash and its use

Among industrial wastes, ash plays a leading role because of the large volume and the consequences, due to a growing accumulation. Annually about 600 million tons of ash are produced, 75–80% of which is the one resulted from thermal plants, being considered the fifth

The ash is used in agriculture since it contains nearly all of micro- and macro-elements, except for the organic carbon and nitrogen [5]. Also, the ash can contain moderate amounts of heavy metals, affecting groundwater and may be retained by plants. This material is used as the filling

between clay and organic humic and nonhumic substances.

do not develop; therefore, the content of nitrogen is low [2].

ment to a situation may not be useful in other conditions [3].

material, the recovery of acid soils, depending on the pH.

in the synthesis of zeolites.

as a source of prime mover [4].

They contain a variable amount of nutrients, which can affect the pH.

Many methods such as solidification, cementation, phytoremediation, earth-swap, and soil flushing can be used to reduce the heavy metal from contaminated soils. Among them, fixation of heavy metal using amendments is a promising technology for removing heavy metals from contaminated soils. The application of soil amendments modified the properties of the contaminating heavy metals that allow their immobilization [10–12].

The fixation of heavy metals in soils had been greatly carried out with different amendments, such as zeolite, phosphates, cements, limes, and carbonates, or in the last years, with synthetic amendments: zeolites [13, 14], geopolymers, and phosphate [15–17]. In recent years, there were a lot of studies regarding amendments including by-products, waste, and low-cost synthetic materials for fixation of heavy metals. Low-cost amendments are required for the enhancement of fixing efficiency.

The fixation treatment is based on the reduction of metal mobility and availability as a result of applying amendments. In this way, the unfavorable effects of heavy metals to environmental receptors are reduced [18]. The mechanism of heavy metals removal is based on adsorption, precipitation, cation exchange, and complexation.

#### **2.2. Types of amendments**

The fixation efficiency depends on the quantity of the amendments and on the properties of these and soils. The chemical and physical properties are also the influencing factors in the fixation process. The cation mobility is the greatest property that can determine their envi‐ ronmental impact. The heavy metal ions have the individual mobilities and are difficult to find the amendments for reducing the bioavailability.

Among the numerous treatment compounds, some are recommendable for several heavy metals: limes, phosphates, organic matter, industrial by-products, and low-cost zeolites.


**Table 1.** Types of inorganic amendments for metal immobilization [18].

Firstly, additives used for heavy metal immobilization can be classified into inorganic, organic, and both inorganic-organic materials. According to the nature of different materials, there are three resources including natural materials, artificial synthetics, and by/co-products used for fixing process. Natural materials that are available in a large quantity or are waste products from industrial or agricultural sources may have potential as low-cost sorbents. Due to their low cost, after utilization they can be disposed of without expensive regeneration. In addition, inorganic or organic amendments are used to improve soils' substrate characteristics. The types of inorganic amendments are presented in **Table 1**.

of heavy metal using amendments is a promising technology for removing heavy metals from contaminated soils. The application of soil amendments modified the properties of the

The fixation of heavy metals in soils had been greatly carried out with different amendments, such as zeolite, phosphates, cements, limes, and carbonates, or in the last years, with synthetic amendments: zeolites [13, 14], geopolymers, and phosphate [15–17]. In recent years, there were a lot of studies regarding amendments including by-products, waste, and low-cost synthetic materials for fixation of heavy metals. Low-cost amendments are required for the enhancement

The fixation treatment is based on the reduction of metal mobility and availability as a result of applying amendments. In this way, the unfavorable effects of heavy metals to environmental receptors are reduced [18]. The mechanism of heavy metals removal is based on adsorption,

The fixation efficiency depends on the quantity of the amendments and on the properties of these and soils. The chemical and physical properties are also the influencing factors in the fixation process. The cation mobility is the greatest property that can determine their envi‐ ronmental impact. The heavy metal ions have the individual mobilities and are difficult to find

Among the numerous treatment compounds, some are recommendable for several heavy metals: limes, phosphates, organic matter, industrial by-products, and low-cost zeolites.

Firstly, additives used for heavy metal immobilization can be classified into inorganic, organic, and both inorganic-organic materials. According to the nature of different materials, there are three resources including natural materials, artificial synthetics, and by/co-products used for fixing process. Natural materials that are available in a large quantity or are waste products from industrial or agricultural sources may have potential as low-cost sorbents. Due to their low cost, after utilization they can be disposed of without expensive regeneration. In addition,

**Materials Heavy metals References** Fly ash Cd, Pb, Cu, Zn, Cr [22, 23] Lime Cd, Cu, Ni, Pb, Zn, Cr [18, 19] Phosphate salt Pb, Zn, Cd, Cu [3, 16] Hydroxyapatite Zn, Pb, Cu, Cd [15] Slag Cd, Pb, Zn, Cr [24] Ettringite Cd, Cu, Pb, Zn, Cr [25]

contaminating heavy metals that allow their immobilization [10–12].

precipitation, cation exchange, and complexation.

the amendments for reducing the bioavailability.

**Table 1.** Types of inorganic amendments for metal immobilization [18].

of fixing efficiency.

46 Zeolites - Useful Minerals

**2.2. Types of amendments**

The materials including CaO, Ca(OH)2, CaCO3, CaMgCO3, CaHPO3, Ca(H2PO3)2, K2HPO4, H3PO4, and (NH4)HPO4 are the typical amendments used for the immobilization of heavy metals. On the other hand, zeolites decrease the metal concentration in soil solution, the metal mobility, and leachability and transform heavy metal cations from soluble into insoluble cations [16, 19, 20]. Wang et al. [21] reported that Ca(H2PO4)2 could replace CaHPO4 due to lower cost and higher solubility in the field study. The combination of Ca(H2PO4)2 and CaCO3 significantly reduces the extractable metal concentration and successfully immobilize heavy metals on the site. Quicklime (CaO) was a more effective additive because it is readily soluble and available for reactions, and its addition to a soil material results in larger pH increases.


**Table 2.** Chemical composition of zeolite obtained from fly ash [31, 32].

Zeolites are crystalline aluminosilicate form alkali [26, 27]. They have a stable structure with three-dimensional five silica tetrahedra in a unique symmetrical arrangement [28]. This structure containing cavities has the ability to retain water and negative charge inside the opening cation binds to change [29]. This negative charge results in the ability of zeolites to change plant nutrients and eliminate them from the structure [29].

Zeolites are recognized as "intelligent fertilizers" because of the ion exchange capacity (CEC) and high porosity [30]. The ability to exchange cations allows nutrients such as nitrogen, calcium, iron, magnesium, potassium, and water to be available in the plant and be eliminated slowly [26, 29, 30]. In this way, plants can absorb nutrients from the soil when required. Zeolites improve performance by increasing resistance to wash fertilizers, immobilization, and gas leaks. They increase water reserves in soil needed during dry periods and are able to reversibly lose and retain water and change cations' network without changing the structure. They are chemically inert, nontoxic to plants, animals, humans, and the environment and regulate the balance of soil by immobilization of toxic contaminants to the plant [26].

As a result, the application of the zeolite increases ion-exchange capacity, which allows the retention followed by a controlled release of nutrients, reduces the consumption of fertilizer and water necessary to complete the potassium, increases the availability of the use of phosphorous adsorbed toxic contaminants, acts as a buffering agent pH and alkalinity, reduces the effects, improves the aeration, allows the activity of microorganisms, establishes the alkali metal balances, and is cheap. It can have nutritional value that helps germination, root development, and plant growth and grain [28–30].

Zeolites with chemical composition presented in **Table 2** are used, due to the special properties such as adsorption, ion exchange, molecular sieve, hydration, dehydration, bulk density, porosity, and structure [27, 28], to interact with other minerals from the soil, resulting in improved soil structure [26].

#### **3. The methods to obtain zeolites from fly ash**

One possibility to recover the ash is the conversion in zeolites. The zeolites prepared by alkali activation of fly ash are aluminum-silicates and include three classes of inorganic polymers, depending on the ratio of silica/alumina: (─Si─O─Al─O─), poly-sialic – SiO2/Al2O3 = 2; (─Si─O─Al─O─Si─O─), poly-sialic siloxo – SiO2/Al2O3 = 4; and (─Si─O─Al─O─Si─O─Si─O─), poly-sialic disiloxo – SiO2/Al2O3 = 6. The composition of the mixture and the reaction conditions such as SiO2/Al2O3 ratio, hydroxide concentration, activation temperature, curing time, ratio of the solid/liquid, and pH significantly affect the formation and properties of zeolites. Limits of variation in these parameters are broad enough: ratio SiO2/Al2O3 2–6, activation temperature 80–150°C, concentration of (Na or K) hydroxide 1–5 M, curing time of 4–48 hours, and the ratio of the solid/liquid 1/1–1/4 [33, 34].

Generally, there are two main methods for the synthesis of new materials using solid waste – fly ashes. Methods are known to be composed of a single stage or two stages.


Zeolites are crystalline aluminosilicate form alkali [26, 27]. They have a stable structure with three-dimensional five silica tetrahedra in a unique symmetrical arrangement [28]. This structure containing cavities has the ability to retain water and negative charge inside the opening cation binds to change [29]. This negative charge results in the ability of zeolites to

Zeolites are recognized as "intelligent fertilizers" because of the ion exchange capacity (CEC) and high porosity [30]. The ability to exchange cations allows nutrients such as nitrogen, calcium, iron, magnesium, potassium, and water to be available in the plant and be eliminated slowly [26, 29, 30]. In this way, plants can absorb nutrients from the soil when required. Zeolites improve performance by increasing resistance to wash fertilizers, immobilization, and gas leaks. They increase water reserves in soil needed during dry periods and are able to reversibly lose and retain water and change cations' network without changing the structure. They are chemically inert, nontoxic to plants, animals, humans, and the environment and regulate the

As a result, the application of the zeolite increases ion-exchange capacity, which allows the retention followed by a controlled release of nutrients, reduces the consumption of fertilizer and water necessary to complete the potassium, increases the availability of the use of phosphorous adsorbed toxic contaminants, acts as a buffering agent pH and alkalinity, reduces the effects, improves the aeration, allows the activity of microorganisms, establishes the alkali metal balances, and is cheap. It can have nutritional value that helps germination, root

Zeolites with chemical composition presented in **Table 2** are used, due to the special properties such as adsorption, ion exchange, molecular sieve, hydration, dehydration, bulk density, porosity, and structure [27, 28], to interact with other minerals from the soil, resulting in

One possibility to recover the ash is the conversion in zeolites. The zeolites prepared by alkali activation of fly ash are aluminum-silicates and include three classes of inorganic polymers, depending on the ratio of silica/alumina: (─Si─O─Al─O─), poly-sialic – SiO2/Al2O3 = 2; (─Si─O─Al─O─Si─O─), poly-sialic siloxo – SiO2/Al2O3 = 4; and (─Si─O─Al─O─Si─O─Si─O─), poly-sialic disiloxo – SiO2/Al2O3 = 6. The composition of the mixture and the reaction conditions such as SiO2/Al2O3 ratio, hydroxide concentration, activation temperature, curing time, ratio of the solid/liquid, and pH significantly affect the formation and properties of zeolites. Limits of variation in these parameters are broad enough: ratio SiO2/Al2O3 2–6, activation temperature 80–150°C, concentration of (Na or K) hydroxide

Generally, there are two main methods for the synthesis of new materials using solid waste –

1–5 M, curing time of 4–48 hours, and the ratio of the solid/liquid 1/1–1/4 [33, 34].

fly ashes. Methods are known to be composed of a single stage or two stages.

change plant nutrients and eliminate them from the structure [29].

balance of soil by immobilization of toxic contaminants to the plant [26].

development, and plant growth and grain [28–30].

**3. The methods to obtain zeolites from fly ash**

improved soil structure [26].

48 Zeolites - Useful Minerals

**Table 3.** Zeolites from fly ash as a function of hydroxide (NaOH/KOH) concentration, temperature and L/s ratio.

In 1985, Holler and Wirsching synthesized, for the first time, zeolites from fly ash by direct activation method [35]. As a result of studies carried out by Holler and Wirsching, several researchers have proposed various methods to obtain new materials based on fly ash in articles and patents:


**Table 3** presents the main types of new materials, which can be obtained as a function of zeolitization conditions.

**Figure 1.** The mechanism of zeolitization of fly ash.

The first method – the conventional one, it is direct hydrothermal conversion of the mixture of ash and alkaline solution. Only 50% of ashes can be converted into P-zeolite or hydroxy sodalite. The second method consists in mixing the ash with alkaline reagents and fusion at elevated temperature (typically 550°C) to convert the ash into a soluble potassium silicate and potassium aluminate, followed by dissolution in an alkaline solution. Fusion method is performed with KOH and NaOH. The method leads to an advanced conversion, the type of zeolite depending on treatment conditions. The third method consists in the extraction of silicon from ashes in the first stage, after that solution was added in a well-established proportion, which leads to a very pure zeolite. Also, experimental studies recommend the conversion of ash using microwave ovens; in this case, the conversion time is reduced from 24–48 hours to 30 minutes. To use this method, extensive research is required because not every type of ash can be changed. Even if the direct method was considered, it is disappointing regarding the zeolite purity and simplicity, and synthesis in a single step was proposed to be used in synthesizing zeolites for agriculture, with a part in controlling release of fertilizers and as a soil amendment.

The main limitation of the synthesis process of zeolites is the temperature (125–200°C) to dissolve Si and Al from the ash particles. If the temperature is reduced, then the yield of the synthesis is considerably reduced, and it is necessary to have a greater reaction time. However, KM, herschelite, K-chabazite, and Linde F can be obtained in the range of 125–200°C.

Fly ash mainly consists of amorphous and subordinate crystalline phases such as quartz, mullite, and hematite. The amorphous material represents the most reactive phase during the zeolitization process. If there is a big content of vitreous phase, the activation time is reduced, and the alkaline hydroxide solution is less concentrated.

After treatment, the samples were allowed to cool for 20 hours at room temperature *T* = 20°C, filtered, washed, and dried.

Murayama et al. [52] and Bukhari et al. [53] showed that the conversion mechanism implicates the steps: dissolution, crystallization, and/or condensation (**Figure 1**).

Either method is used; the most important parameters in the modifying process are activation time; concentration of the solutions used for modification; temperature; and ratio of ashsolution.

#### **3.1. Direct activation method (hydrothermal method)**

**•** direct activation method (hydrothermal method): [36–44]; **•** fusion method followed by direct activation method: [45–49];

**Table 3** presents the main types of new materials, which can be obtained as a function of

The first method – the conventional one, it is direct hydrothermal conversion of the mixture of ash and alkaline solution. Only 50% of ashes can be converted into P-zeolite or hydroxy

**•** microwave followed by direct activation method.

**•** ultrasound method: [13, 50, 51];

**Figure 1.** The mechanism of zeolitization of fly ash.

zeolitization conditions.

50 Zeolites - Useful Minerals

The experimental results demonstrated that the fly ash consists predominantly of quartz (SiO2), mullite (2SiO2·3Al2O3), hematite (Fe2O3), and magnetite (Fe3O4) can be modified by direct activation method. The method consists in conversion of fly ash into zeolite by treatment of these with alkaline solution, such as NaOH or KOH for extraction of the components (aluminum and silica), and heat treatment for growth of zeolite crystals.

For obtaining one pre-established type of zeolite, it can be possible to insert with aluminum or silica compounds. By the direct activation method, materials were obtained as zeolite X [45, 54, 55], zeolite A, analcime, chabazite, or hydroxy-sodalite as a function of temperature, pressure, Si/Al ratio, alkaline solution, reaction time, pH, etc. [37, 56, 57].

Scheme for the preparation of new materials by direct activation method is shown in **Figure 2**.

**Figure 2.** Modification scheme of ash through direct activation.


**Table 4.** Zeolites obtained by direct activation method.

Direct activation method comprises one step in seeking to use the content of silica in ash to produce a new material. Treatment takes place in an autoclave at temperatures between 80 and 200°C with agents brought to KOH and NaOH for 3–96 hours contact solution concen‐ tration: 0.5–5 M and the ratio L/s 1–3 mL/g and 10–18 mL/g (**Table 4**).

**Figure 3.** SEM micrographs of zeolites synthesized by direct activation method.

**Figure 2.** Modification scheme of ash through direct activation.

52 Zeolites - Useful Minerals

**Zeolite References**

NaA, S [45, 60] Na-Cha [33, 61] Na-X [41] NaP1, T, HC, Her, HS [58] NaP1, A, P, Nep [36] NaP1, HS [37] NaP1, HS, T, A [38] NaP1, HS, A [38, 62] NaP1, F [63] Na-A [40] Gis, S [47] Y [33] A [64] ZSM-5 [65] HS, NaA, Linde F [43] HS, NaP1, X [66] Na-HEU [42]

**Table 4.** Zeolites obtained by direct activation method.

NaP1 [33, 36–38, 45, 58, 59]

The main disadvantage of the direct conversion method is relatively low-conversion degree. Several studies have focused on the relationship between composition of the raw materials and the synthesis conditions. However, conditions are not clearly established, and the combination of crystalline phases obtained is often unpredictable.

In **Figure 3**, the particles' morphology of zeolites synthesized by direct activation method is shown.

From **Figure 3**, it can be seen that ash morphology changes significantly, increasing mainly newly formed crystals on the surface of the particles. Following zeolitization, surface area increases significantly.

#### **3.2. Fusion method followed by direct activation method**

To obtain ash zeolites, the fusion step was proposed before the hydrothermal treatment. The ash mixed with solid NaOH is heated at high temperature, after which it is subjected to direct conversion. The alkaline fusion step before direct activation significantly improves zeolitiza‐ tion [45, 69].

Ash changing using the fusion method followed by direct activation has been reported by several authors [49, 69].

This method is known as the indirect method, as it consists of two phases:

**•** the fusion phase (450–600°C temperature, contact time 1–3 hours)

**•** the direct activation phase (temperature 60–100°C for 4–96 hours contact)

After the conclusion of the fusion phase, the materials were allowed time to age (room temperature, contact time of approximately 20 hours).


**Table 5.** New materials obtained by the fusion method followed by direct activation.

Previous studies have also shown that the ash-synthesized materials by this method have a higher crystallinity and cation exchange capacity compared to the materials synthesized by the conventional hydrothermal method. The fusion method followed by direct activation method allows for a higher amount of alkaline extract ash silicates and alumina. The formation of new materials depends heavily on the SiO2/Al2O3 ratio of the base material.

**Table 5** presents the main types of new materials, which are obtained.

**Figure 4** provides a few examples of new materials obtained by the fusion method followed by the direct activation.

**Figure 4.** SEM images of materials obtained by fusion.

Obtaining new Na-A and Na-X type materials is also possible by direct activation, but when obtained by the fusion followed by direct activation method, these have higher cation exchange capacity and larger pores. The disadvantage of this method consists in the fact that the method involves the separation of the solid residues after a high content of silicon and aluminum has been dissolved in the alkaline solution. Disposal of the residues increases the possibility of obtaining a desired type of zeolite with high purity and regular particles, while leaving a new solid waste, with a very low production yield [45].

#### **3.3. Ultrasound method**

**•** the direct activation phase (temperature 60–100°C for 4–96 hours contact)

temperature, contact time of approximately 20 hours).

54 Zeolites - Useful Minerals

**Material References** Na-X, Na-A [70] Na-X [49] Na-A [21, 49] Na-Y [47] Na-X, Na-A, Na-P [46, 67] Fau [47] Zeolite 13X [69] ZFA [48] X, A, P, HS [41]

**Table 5.** New materials obtained by the fusion method followed by direct activation.

of new materials depends heavily on the SiO2/Al2O3 ratio of the base material.

**Table 5** presents the main types of new materials, which are obtained.

by the direct activation.

**Figure 4.** SEM images of materials obtained by fusion.

After the conclusion of the fusion phase, the materials were allowed time to age (room

Previous studies have also shown that the ash-synthesized materials by this method have a higher crystallinity and cation exchange capacity compared to the materials synthesized by the conventional hydrothermal method. The fusion method followed by direct activation method allows for a higher amount of alkaline extract ash silicates and alumina. The formation

**Figure 4** provides a few examples of new materials obtained by the fusion method followed

Ultrasounds (USs) are a form of mechanical energy that propagates in all directions in the form of beams. The ultrasound method has been reported primarily by Schmachtl et al. [72]. The ultrasound method is an indirect method, which is based on the investigation of the degree of interaction of the wave transmission ultrasonic properties with the precursor species of new materials, as they pass through the synthesis mixture. The method has been used in the synthesis of a wide range of nano- and microparticles.

**Figure 5.** SEM images obtained using ultrasound equipment. 3 - NaP zeolite synthesized by: (a) the sonochemical method at room temperature, 3 hours, (b) the sonochemical method followed by hydrothermal treatment for 12 hours at 100°C (c) sonochemical method followed by hydrothermal treatment for 24 hours at 100°C [78].

Of the various applications of the ultrasound treatment obtaining new ash-based materials requires special attention because it offers several advantages, the most significant being a reduced reaction time and crystallization temperature [21, 50, 51, 67, 73–75].

Feng et al. [76] showed that this method increases the compressive strength and thermal stability of the material. By accelerating the dissolution of the Al-Si, it improves polyconden‐ sation and the formation of semicrystalline phases. Ultrasound accelerates the chemical reactions of the components, the method being used to obtain new materials from sources, which contain Al and Si [72].

Ultrasound influences the physicochemical properties of the crystals, which result in following the synthesis. The effects of ultrasound are due to cavitation, a phenomenon may be defined as the occurrence of bubbles, which rise to the surface and then break. This can be explained by the fact that rapid successive expansions and compressions in liquid give rise to excessive stress in some areas.

The molar ratio of reactants, contact time, and temperature is important parameters that determine the type and properties of the new material. The disadvantage of this method is that the application of ultrasound changes the morphology of the crystals (crystal size decreases with power increase – **Figure 5**).

By applying different temperatures, concentrations of alkaline solution, the solid/liquid ratio, and the contact time, one can obtain materials with specific applications (**Table 6**).


**Table 6.** New materials obtained by the ultrasound method.

#### **3.4. Microwave method followed by hydrothermal activation**

Querol et al. [36] obtained a zeolite using ash with a Si/Al ratio of 1.85 with a 62% amorphous phase, while the crystalline phases included: mullite (13%), magnetite (13%), quartz (7%), anhydrite (0.7%), and anorthite (0.3%). Two alkaline solutions of NaOH and KOH were used to obtain the zeolite crystals. The concentration of the solutions ranged from 1.0 to 5.0 M, and the temperature ranged between 150°C and 225°C, using both the conventional method and the microwave method (microwave oven with the available power of 1000 W). The crystalli‐ zation time was reduced from conventional conversion of 8–98 hours to 10–30 minutes of microwave-assisted conversion. By changing the above-mentioned parameters, different stages of zeolite were obtained: Na-P1, hydroxy-sodalite, analcime, Linde F, tobermorite and kalsilit. Generally speaking, microwaves play a positive role in obtaining Na-P1. Experimen‐ tally, it was found that microwaves applied in the first 15 minutes, followed by 2 hours of synthesis led to the best results in as far as the ion-exchange capacity was concerned [53].


**Table 7.** New materials obtained by the ultrasound method.

Of the various applications of the ultrasound treatment obtaining new ash-based materials requires special attention because it offers several advantages, the most significant being a

Feng et al. [76] showed that this method increases the compressive strength and thermal stability of the material. By accelerating the dissolution of the Al-Si, it improves polyconden‐ sation and the formation of semicrystalline phases. Ultrasound accelerates the chemical reactions of the components, the method being used to obtain new materials from sources,

Ultrasound influences the physicochemical properties of the crystals, which result in following the synthesis. The effects of ultrasound are due to cavitation, a phenomenon may be defined as the occurrence of bubbles, which rise to the surface and then break. This can be explained by the fact that rapid successive expansions and compressions in liquid give rise to excessive

The molar ratio of reactants, contact time, and temperature is important parameters that determine the type and properties of the new material. The disadvantage of this method is that the application of ultrasound changes the morphology of the crystals (crystal size decreases

By applying different temperatures, concentrations of alkaline solution, the solid/liquid ratio,

Querol et al. [36] obtained a zeolite using ash with a Si/Al ratio of 1.85 with a 62% amorphous phase, while the crystalline phases included: mullite (13%), magnetite (13%), quartz (7%), anhydrite (0.7%), and anorthite (0.3%). Two alkaline solutions of NaOH and KOH were used to obtain the zeolite crystals. The concentration of the solutions ranged from 1.0 to 5.0 M, and the temperature ranged between 150°C and 225°C, using both the conventional method and the microwave method (microwave oven with the available power of 1000 W). The crystalli‐ zation time was reduced from conventional conversion of 8–98 hours to 10–30 minutes of microwave-assisted conversion. By changing the above-mentioned parameters, different stages of zeolite were obtained: Na-P1, hydroxy-sodalite, analcime, Linde F, tobermorite and kalsilit. Generally speaking, microwaves play a positive role in obtaining Na-P1. Experimen‐

and the contact time, one can obtain materials with specific applications (**Table 6**).

reduced reaction time and crystallization temperature [21, 50, 51, 67, 73–75].

which contain Al and Si [72].

with power increase – **Figure 5**).

**Materials References** 4A [51, 77, 79] NaP [78, 80] MCM-22 [21] X [56] Na-X [81]

**Table 6.** New materials obtained by the ultrasound method.

**3.4. Microwave method followed by hydrothermal activation**

stress in some areas.

56 Zeolites - Useful Minerals

**Figure 6.** SEM images of the materials obtained by the microwave method at different molar ratios of SiO2/Al2O3: (a) 0.4, (b) 0.8, (c) 4.0, and (d) 10.0 [83].

Fukui et al. [82] used ash with a Si/Al ratio of 2.39 and NaOH at 100°C using the conventional method and the microwave method and obtained preponderantly filipsite. The particles of filipsite were smaller in the case of microwave treatment. Experimentally it has been demon‐ strated that microwaves increase the rate of nucleation, but result in a slower growth of the germ. The microwaves used for the first 2 hours, followed by conventional heating, positively influence the mechanism of germ formation and growth.

Tanaka et al. [83] used a 600 W microwave oven and the two-step synthesis method in order to obtain Na-A zeolite. The ashes used in the process had a Si/Al ratio of 2.76, a vitreous phase content of 49.4% and the NaOH solution had a concentration of 2.2 M. The mixture was irradiated in the oven for 60 minutes. The material obtained had an ion-exchange capacity of 4.70 meq/g, lower than the commercial material.

**Figure 7.** SEM images of zeolites synthesized from ashes [62, 67, 71, 74, 90, 91].

Kim and Lee [79] obtained Na-4A zeolite with an ion-exchange capacity of 5.5 meq/g. The literature indicates that microwave irradiation for 2 hours dissolves the compounds containing Si and Al and P zeolite and hydroxyl sodalite were obtained (**Table 7**). SEM images of the zeolites synthesized by the microwave method are shown in **Figure 6**.

**Figure 7** presents the studies in the literature in the field of the materials obtained by modifying the ashes, depending on the morphology of the preponderant zeolite [62, 67, 71, 89, 90, 92]. **Figure 7** shows also that the morphology differs from one material to another from welldefined tetrahedral crystals to clusters of crystals.

Analysis of the literature data shows that alkaline attack of power plant ash allows one to obtain new materials, a mixture of zeolites, but the type and degree of zeolitization are influenced by the working conditions.

### **4. Conclusions**

Tanaka et al. [83] used a 600 W microwave oven and the two-step synthesis method in order to obtain Na-A zeolite. The ashes used in the process had a Si/Al ratio of 2.76, a vitreous phase content of 49.4% and the NaOH solution had a concentration of 2.2 M. The mixture was irradiated in the oven for 60 minutes. The material obtained had an ion-exchange capacity of

4.70 meq/g, lower than the commercial material.

58 Zeolites - Useful Minerals

**Figure 7.** SEM images of zeolites synthesized from ashes [62, 67, 71, 74, 90, 91].

defined tetrahedral crystals to clusters of crystals.

zeolites synthesized by the microwave method are shown in **Figure 6**.

Kim and Lee [79] obtained Na-4A zeolite with an ion-exchange capacity of 5.5 meq/g. The literature indicates that microwave irradiation for 2 hours dissolves the compounds containing Si and Al and P zeolite and hydroxyl sodalite were obtained (**Table 7**). SEM images of the

**Figure 7** presents the studies in the literature in the field of the materials obtained by modifying the ashes, depending on the morphology of the preponderant zeolite [62, 67, 71, 89, 90, 92]. **Figure 7** shows also that the morphology differs from one material to another from wellZeolites from ashes are recognized as "intelligent fertilizer" because they have the ability to exchange cations allowing nutrients such as nitrogen, calcium, iron, magnesium, potassium, and water to be available in the plant and be eliminated slowly. The plants can absorb nutrients from the soil when required. Zeolites increase water reserves in soil needed during dry periods and are able to reversibly lose and retain water and change cations' network without changing the structure. They are chemically inert, nontoxic to plants, animals, humans, and the envi‐ ronment and regulate the balance of soil by immobilization of toxic contaminants to the plant.

The waste – fly ash can be capitalized by conversion in zeolites. The zeolites prepared by alkali activation of fly ash are aluminum-silicates and include three classes of inorganic polymers, depending on the ratio of silica/alumina. The composition of the mixture, and the reaction conditions such as SiO2/Al2O3 ratio, hydroxide concentration, activation temperature, curing time, ratio of the solid/liquid, and pH significantly affect the formation and properties of zeolites. Limits of variation in these parameters are broad enough: ratio SiO2/Al2O3 2–6, activation temperature 80–150°C, concentration NaOH or KOH solution 1–5 M, curing time of 4–48 hours and the ratio of the solid/liquid 1/1–1/4.

The methods used for the synthesis of new materials using solid waste – fly ashes are direct activation method (hydrothermal method), fusion method followed by direct activation method, ultrasound method, and microwave followed by direct activation method.

The main limitation of the synthesis process of zeolites is the relatively high temperature to dissolve Si and Al from the ash particles. If the temperature is reduced, then the yield of the synthesis is considerably reduced, and it is necessary to have a greater reaction time. However, KM, herschelite, K-chabazite, and Linde F can be obtained in the range of 125–200°C.

#### **Author details**

Maria Harja1 , Sorin Mihai Cimpeanu2 , Marcel Dirja3 and Daniel Bucur4\*

\*Address all correspondence to: dbucur@uaiasi.ro

1 "Gheorghe Asachi" Technical University of Iasi, Faculty of Chemical Engineering and Environmental Protection, Iasi, Romania

2 University of Agronomical Sciences and Veterinary Medicine, Bucharest, Romania

3 University of Agricultural Sciences and Veterinary Medicine, Cluj Napoca, Romania

4 University of Agricultural Sciences and Veterinary Medicine, Iasi, Romania

#### **References**


[12] Diels L., van der Lelie N., Bastiaens L. New developments in treatment of heavy metal contaminated soils. Reviews in Environmental Science Bio/Technology, 2002;1:75–82.

3 University of Agricultural Sciences and Veterinary Medicine, Cluj Napoca, Romania

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### **Fly Ash Derived Zeolites in the Removal of Toxic Compounds**

Magdalena Wdowin and Wojciech Franus

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/64362

#### **Abstract**

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The present paper describes the previous obtained results concerning purification of gases, water and sewages from toxic components such as mercury, radionuclides and petroleum substances. The tested materials constitute synthetic zeolites obtained in hydrothermal reaction, i.e., Na-X, Na-P1, and natural zeolite clinoptilolite. The zeolites (Na-X, Na-P1 and clinoptilolite) needed for mercury capture were activated by silver ions; however, for the removal of petroleum substances (Na-X and Na-P1) as well as radionuclides (Na-P1) raw zeolite materials were used. The review of the results have shown that zeolites used both for mercury capture and the removal of radionuclides, as well as petroleum substances, can be considered promising sorbent materials.

**Keywords:** fly ash, na-X, na-P1, clinoptilolite, mercury, radionuclides, petroleum substances

#### **1. Introduction**

The chemical impact on the environment as well as human health constitutes an increasing concern [1]. Therefore, the awareness of relationships between ecosystem and human health and their distribution in the environment [2] is growing worldwide. Considering chemicals that are toxic and harmful for the organisms, Plant et al. [1] distinguished four main groups of pollutants, i.e., (1) inorganic elements such as heavy metals, i.e., As, Cd, Hg, or Pb but also Se, I, NOx, Ga and PGEs; (2) radioactive substances such as 238U, 226Ra, 222Rn as well as process materials, i.e., depleted uranium or isotopes 137Cs, 90Sr, 243Am and 240Pu; (3) persistent organic pollutants such as dichlorodiphenyltrichloroethane (DDT), polychlorinated biphenyls (PCBs),

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

polybrominated diphenyl ethers (PBDEs), perfluorooctane sulphonate (PFOS); and (4) human and veterinary pharmaceuticals.

The European Environment Agency and the United Nations Environment Programme articulated the problem of chronic exposure to potentially toxic chemicals [3]. The increasing concern about harmful compounds in the environment is also reflected by the growing number of European Union (EU) Directives and national and international conventions related to chemicals [1].

For the removal of toxic pollutants in the environment, many sorbents and materials are investigated. One of the promising materials considered over the years is zeolites (alumino‐ silicates of alkali and earth metals). Due to their crystalline, infinite and three-dimensional crystal structure, they possess many important properties that are helpful in environmental and chemical sciences among others, such as, adsorption, dehydratation-rehydratation, ionexchange molecular sieve and catalytic properties [4].

The various potential applications of both synthetic and natural zeolites depend on their fundamental physical and chemical properties that are in turn directly related to the chemical composition and crystal structure of individual species [5].

The present work describes the evaluation of zeolite materials for three toxic compounds, namely, mercury from flue gases, and radionuclides from mine waters and petroleum substances.

#### **2. Zeolites as mercury sorbents**

#### **2.1. Characteristic of mercury pollutions in flue gas**

Mercury in various forms (for example, Hg0 , HgCl and HgO) is one of the most important and dangerous environmental contaminants and has received special attention because of its high volatility, high toxicity and bioaccumulation ability [6].

It is believed that coal combustion is one the main sources of mercury emissions in the atmosphere, accounting for 60%, or even more, of the total mercury emissions [7]. The rest of the emissions originate from cement clinker production, and primary production of ferrous and non-ferrous metals is a predominant source of global anthropogenic mercury emission. Annual emissions of mercury to air form current anthropogenic sources are responsible for about 30% [8].

Considering combustion of fuels, the mercury in coal is volatilized and converted into elemental mercury (Hg0 ) vapour in the high-temperature regions of coal-fired boilers. During cooling of flue gas, a series of complex reactions take place converting Hg0 into ionic mercury (Hg2+) compounds and/or Hg compounds (Hgp) that are in a solid phase at flue gas cleaning temperatures or Hg that is adsorbed onto the surface of other particles. The presence of chlorine in flue gas causes the formation of mercuric chloride (HgCl2) at flue gas cleaning temperatures. Nevertheless, Hg0 is speciation; as a result, Hg enters the flue gas cleaning device(s) as a mixture of Hg0 , Hg2+ and Hgp. It is thought that majority of gaseous mercury in bituminous coal-fired boilers constitutes Hg2+. But, the majority of gaseous mercury in subbituminous- and lignite-fired boilers is Hg0 [9].

In conclusion, mercury in flue gas is found in three major chemical forms: gaseous elemental mercury (Hg0 ), gaseous oxidized mercury (Hg2+) and particulate-bound mercury (Hgp). The mercury amount adsorbed on the particles of ash ranges from 5 to 10% of the total quantity of Hg released during the combustion processes. The remaining 90–95% of the mercury is released in the gaseous form. Mercury concentrations of flue gas from coal combustion generally range from 5 to 10 μg/m3 , with a large range in the relative proportions of Hg0 , Hg2+ and Hgp. The most dangerous and the most stable mercury form that accounts for over 90% of the total mercury in the atmosphere is the elemental form. It is hazardous due to long residence time in the atmosphere (from 6 months to 2 years) but it could also be as short as hours to weeks under specific environmental conditions [10, 11] and due to migrating hundreds of kilometres from the source of emission. The gaseous form of Hg2+ lasts for only several days to several weeks in the atmosphere [12–14].

Another problem is that mercury is insoluble in the water, thus making inefficient the existing methods of their removal from flue gases [15]. To remove mercury-activated carbons (AC) commercially, their high cost and instability of the structure (degradation) in the exhaust gas temperatures are the major problems. In addition, these materials are not very efficient for removing mercury in the elemental form. Consequently, a large number of studies have focused on the possibility of using new techniques based on zeolites for removing gaseous elemental mercury species [16, 17]. The other forms of mercury (oxidized and gaseous divalent mercury Hg2+) can react with ash particles to form the solids. These two forms, as compared to Hg0 , are efficiently removed from the gas stream by the conventional purification methods (wet desulphurization or bag filter) [14, 18–21].

#### **2.2. Sorption mechanism of Hg on zeolites**

polybrominated diphenyl ethers (PBDEs), perfluorooctane sulphonate (PFOS); and (4) human

The European Environment Agency and the United Nations Environment Programme articulated the problem of chronic exposure to potentially toxic chemicals [3]. The increasing concern about harmful compounds in the environment is also reflected by the growing number of European Union (EU) Directives and national and international conventions related to

For the removal of toxic pollutants in the environment, many sorbents and materials are investigated. One of the promising materials considered over the years is zeolites (alumino‐ silicates of alkali and earth metals). Due to their crystalline, infinite and three-dimensional crystal structure, they possess many important properties that are helpful in environmental and chemical sciences among others, such as, adsorption, dehydratation-rehydratation, ion-

The various potential applications of both synthetic and natural zeolites depend on their fundamental physical and chemical properties that are in turn directly related to the chemical

The present work describes the evaluation of zeolite materials for three toxic compounds, namely, mercury from flue gases, and radionuclides from mine waters and petroleum

dangerous environmental contaminants and has received special attention because of its high

It is believed that coal combustion is one the main sources of mercury emissions in the atmosphere, accounting for 60%, or even more, of the total mercury emissions [7]. The rest of the emissions originate from cement clinker production, and primary production of ferrous and non-ferrous metals is a predominant source of global anthropogenic mercury emission. Annual emissions of mercury to air form current anthropogenic sources are responsible for

Considering combustion of fuels, the mercury in coal is volatilized and converted into

cooling of flue gas, a series of complex reactions take place converting Hg0 into ionic mercury (Hg2+) compounds and/or Hg compounds (Hgp) that are in a solid phase at flue gas cleaning temperatures or Hg that is adsorbed onto the surface of other particles. The presence of chlorine in flue gas causes the formation of mercuric chloride (HgCl2) at flue gas cleaning temperatures.

) vapour in the high-temperature regions of coal-fired boilers. During

is speciation; as a result, Hg enters the flue gas cleaning device(s) as a

, HgCl and HgO) is one of the most important and

and veterinary pharmaceuticals.

exchange molecular sieve and catalytic properties [4].

**2.1. Characteristic of mercury pollutions in flue gas**

volatility, high toxicity and bioaccumulation ability [6].

**2. Zeolites as mercury sorbents**

Mercury in various forms (for example, Hg0

composition and crystal structure of individual species [5].

chemicals [1].

68 Zeolites - Useful Minerals

substances.

about 30% [8].

elemental mercury (Hg0

Nevertheless, Hg0

Many materials are under examination because of their potential oxidation, capture and catalysis of mercury. The most frequently studied are, among others, palladium, gold, iridium, platinum, iron, fly ash and activated carbons [22].

Previous studies on removing gaseous forms of mercury (mainly Hg0 ) from zeolites showed negligible adsorption of mercury vapour compared with the commercially used activated carbons. This may be related to the lack of chemical affinity of zeolites to mercury or a stronger affinity to the other, main components of exhaust gas (including water vapour present in the exhaust gas, which is rapidly adsorbed in the zeolite structure). Therefore, it can be concluded that the adsorption of gaseous forms of mercury is not a pure physical adsorption. In addition to zeolites, sulphur-activated carbons did not effectively adsorbed mercury, whereas sulphurimpregnated activated carbons are good sorbents of Hg and currently they are commercially used [23].

Silver activation of zeolite materials significantly improves their adsorption capacity with respect to mercury [17, 24–26], which is related to chemical affinity of silver to mercury. The magnetic properties of Fe-activated zeolites also significantly influence the adsorption of mercury [27]. Manganese-oxidized mercury, Hg0 to HgO, compounds are more easily adsorbed [28]. On the other hand, silver has a high affinity for mercury [17]. Mercury in contact with the silver-activated zeolite is bonded to the zeolite structure as a silver amalgam. Then, mercury is released from the structure of zeolite during the heating of zeolite to a temperature of about 400°C, indicating the possibility of regeneration of the material. An example for silverimpregnated zeolites is that the magnetic composite zeolite-containing silver nanoparticles were able to capture mercury from the flue gases in large scale from coal-fired power plants [26]. In order to improve the sorption of mercury on different sorbents, the bromine chloride (BrCl) to oxidize Hg0 to HgCl2 was used [29]. Nowadays, the introduction of selective catalytic reduction (SCR) increases the efficiency of the oxidation of Hg0 to Hg2+ [30].

#### **2.3. Results of mercury capture**

Considering the results obtained by other authors, so far many materials have been observed and studied for their potential to remove mercury from exhaust gases. Others activated carbons (impregnated with iodine, bromine and sulphur) and carbon-based materials are fly ash, metals (such as silver, gold, copper and lead), and their oxides and sulphides, calcium species (lime) and zeolites [10, 15, 21, 31–35]. Unfortunately, most of these sorbents are less effective at higher temperature, in addition they have low capacities, cannot be regenerated and are easily deactivated by flue gas components such as sulphur oxides (SO*x*) [36, 37], which means that the search for the ideal mercury sorbent is far from over [38].

Other raw materials for the capture of mercury zeolites were tested both synthetically and naturally. However, compared with the activated carbon, they showed much less adsorption capacity relative to the gaseous form of mercury. Therefore, significant attempts have been taken to activate them in order to improve their adsorption capacity.

The sorption tests of mercury compounds were performed by Wdowin et al. [17] at laboratory scale in pure mercury stream wherein nitrogen was used as carrier gas. Similar experiments were carried out by Wdowin et al. [6] in flue gas stream wherein gas mixtures were composed of SO*x*, NO*x*, CO2, HCl, HF and CO.

The tested materials in the experiments carried out by Wdowin et al. [17] constitute Na-X, Na-P1, clinoptilolite. In order to enhance their sorption capacity to mercury, the investigated materials were activated with silver by using the ion-exchange method.

The first experiment was carried out in a pure stream of Hg0 wherein nitrogen was used as carrier gas. The tested raw zeolite materials were not able to remove more than 10% (by weight) of mercury; therefore, their sorption capacity at the breakthrough of the bed could not be calculated.

**Table 1** clearly shows that the addition of Ag+ to the tested samples significantly improves the performance of the zeolite with respect to mercury capture. The best results were observed for the sample of Ag-X, which is associated with the highest BET surface area and the largest volume of micro- and mesopores. This result suggests that both micro- and mesopores are essential for the mercury capture. Moreover, in this case multilayer adsorption is an important parameter, but only to some extent, because Ag-P1 possesses a larger volume of meso- and macropores and Ag+ ions, but this zeolite captures lower amount of mercury. The results were compared with commercially available activated carbons.


Breakthrough when 5% of Hg passed through the sorbent bed without being captured.

**Table 1.** The results of Hg0 capture on the raw and silver-activated zeolites [17].

magnetic properties of Fe-activated zeolites also significantly influence the adsorption of

adsorbed [28]. On the other hand, silver has a high affinity for mercury [17]. Mercury in contact with the silver-activated zeolite is bonded to the zeolite structure as a silver amalgam. Then, mercury is released from the structure of zeolite during the heating of zeolite to a temperature of about 400°C, indicating the possibility of regeneration of the material. An example for silverimpregnated zeolites is that the magnetic composite zeolite-containing silver nanoparticles were able to capture mercury from the flue gases in large scale from coal-fired power plants [26]. In order to improve the sorption of mercury on different sorbents, the bromine chloride

Considering the results obtained by other authors, so far many materials have been observed and studied for their potential to remove mercury from exhaust gases. Others activated carbons (impregnated with iodine, bromine and sulphur) and carbon-based materials are fly ash, metals (such as silver, gold, copper and lead), and their oxides and sulphides, calcium species (lime) and zeolites [10, 15, 21, 31–35]. Unfortunately, most of these sorbents are less effective at higher temperature, in addition they have low capacities, cannot be regenerated and are easily deactivated by flue gas components such as sulphur oxides (SO*x*) [36, 37], which

Other raw materials for the capture of mercury zeolites were tested both synthetically and naturally. However, compared with the activated carbon, they showed much less adsorption capacity relative to the gaseous form of mercury. Therefore, significant attempts have been

The sorption tests of mercury compounds were performed by Wdowin et al. [17] at laboratory scale in pure mercury stream wherein nitrogen was used as carrier gas. Similar experiments were carried out by Wdowin et al. [6] in flue gas stream wherein gas mixtures were composed

The tested materials in the experiments carried out by Wdowin et al. [17] constitute Na-X, Na-P1, clinoptilolite. In order to enhance their sorption capacity to mercury, the investigated

carrier gas. The tested raw zeolite materials were not able to remove more than 10% (by weight) of mercury; therefore, their sorption capacity at the breakthrough of the bed could not be

performance of the zeolite with respect to mercury capture. The best results were observed for the sample of Ag-X, which is associated with the highest BET surface area and the largest volume of micro- and mesopores. This result suggests that both micro- and mesopores are essential for the mercury capture. Moreover, in this case multilayer adsorption is an important

to HgCl2 was used [29]. Nowadays, the introduction of selective catalytic

to HgO, compounds are more easily

to Hg2+ [30].

wherein nitrogen was used as

to the tested samples significantly improves the

mercury [27]. Manganese-oxidized mercury, Hg0

reduction (SCR) increases the efficiency of the oxidation of Hg0

means that the search for the ideal mercury sorbent is far from over [38].

taken to activate them in order to improve their adsorption capacity.

materials were activated with silver by using the ion-exchange method.

The first experiment was carried out in a pure stream of Hg0

(BrCl) to oxidize Hg0

70 Zeolites - Useful Minerals

**2.3. Results of mercury capture**

of SO*x*, NO*x*, CO2, HCl, HF and CO.

**Table 1** clearly shows that the addition of Ag+

calculated.

From **Table 1** it can be concluded that Ag-X zeolite found very promising results because it has achieved five times better result of Hg capture as compared with AC/Br zeolite material.

Moutsatsou et al. [39] obtained similar results of Hg capture on clinoptilolite; it was very low especially compared with the synthetic zeolites. It also was not improved by silver-activated clinoptilolite.

The removal of the gaseous forms of mercury from exhaust gases was carried out by Wdowin et al. [6] on the zeolite Na-X-activated silver (Ag-X) in an especially designed oven. The tested zeolite beds were in the granular and powder form. The Hg flow rate was established at 85 dm3 /h. The estimated exhaust gas composition at an oxygen concentration of the order of 4– 8% was as follows: CO2 = 11–17%, CO = 20–50 ppm, NO = 70–100 ppm, SO2 = 650–1000 ppm. The fixed gas stream was passed through a sorbent bed.

Studies that remove gaseous forms of mercury from the exhaust gas at the Ag-X zeolite in the form of powder and granulate showed that granulation enhances the efficiency of the tested material. The average penetration of mercury was 15% in the test lasting about two hours, whereas the presence of other components (mainly sulphur compounds in the flue gases) reduces the efficiency of the tested sorbent in comparison to tests carried out in a clean gas stream of Hg (carrier gas N2). The better results were obtained for granulated zeolite materials.

Hsi et al. [40] studied fly ash and zeolites of types A and Y in terms of the removal of elemental Hg and HgCl2 in the flue gases. They studied both starting materials and sulphur-activated materials. Studies have shown negligible adsorption of Hg compounds by initial fly ash and sulphur-activated materials. These results explained that the adsorbents' porous structure, mainly as micropores, is one of the important factors for Hg0 and HgCl2 adsorption. In addition, the authors point out that the reason for these results is the water content in the structure of the zeolite. Therefore, prior to impregnation it is necessary to get rid of H2O from the zeolite structure. Studies show that too small size of the zeolite channels is also a cause of low adsorption of Hg compounds.

Morency et al. [41] investigated the use of a low-cost additive to improve the capability of a high surface area sorbent such as zeolite to remove all forms of mercury vapour from power plant flue gases. They examined treated zeolite wherein preliminary research led to the conclusion that a zeolite material could achieve substantial mercury removal at a significant cost reduction over current mercury control technologies. They carried out the adsorption test of Hg compounds at 150°C on activated natural zeolite (clinoptilolite) and synthetic (A, X, Y, ZSM-5) zeolites. The results proved to be similar to the results obtained on the active carbons. In addition, the zeolites used in the experiments captured all forms of mercury present in the flue gas. This material is also much cheaper than the commercially used activated carbons for this purpose. These authors also found that synthetic zeolites are better for the removal of mercury compounds due to the constant size of the channels and chambers, whereas sizes of natural zeolites are variable and are often associated with a number of network defects; however, natural zeolites are much cheaper material. In addition, after the Hg adsorption experiment on zeolites, these authors conducted leachability test on this material wherein they obtained positive results, i.e., the tested sorbent can be safely disposed in a landfill if sale of the ash is not an option.

In addition, Morency et al. [42] conducted a successful Hg adsorption test on two types of treated zeolites. The experiment was conducted at a temperature of 130–200°C of flue gas and a concentration of 30–60 μm/m3 of Hg. Studies have shown almost 100% adsorption of Hg at an Hg ratio of 25,000.

Morency et al. [16] carried out experiment on adsorption of Hg compounds of the flue gases on activated carbons and raw and treated zeolites using appropriate chemical reagents. The results showed that in the case of treated zeolites, they show similar results to those of active carbons whereas their efficiency is greater than 90% at a ratio of injected mercury 25,000 g/g.

Pavlish et al. [43] have studied the adsorption of mercury compounds wherein A class of magnetic zeolite composites with supported silver nanoparticles has been tested for elemental mercury removal from power plant flue gas. As a result, this type of activation mercury capture levels of 45–92% was achieved at high ratios of sorbent to mercury ranging from 20,000 to 100,000, with no decrease in capture observed and an increase in gas temperature between 130 and 200°C. In Addition, high dose rates of zeolites, which are composed of aluminosilicate, will not degrade the commercial value of fly ashes used as a substitute for cement in concrete or as filler in plastics.

In many reports on the removal of mercury from flue gas, it is underlined that zeolite-based sorbents are not as efficient as activated carbons but can be much more economical to use than the commonly used carbons.

Mercury sorption experiments on the non-carbon materials at 110°C were conducted by Jurng et al. [23]. For this purpose, they used zeolite and bentonite materials with a relatively low surface area (less than 30 m2 /g), which were sulphur activated in order to improve the sorption. As an activating agent, CS2 solution was used. The results showed 50% of the sorption of Hg compounds for both the starting and activated zeolites and bentonites.

#### **3. Zeolites as radionuclide sorbents**

the authors point out that the reason for these results is the water content in the structure of the zeolite. Therefore, prior to impregnation it is necessary to get rid of H2O from the zeolite structure. Studies show that too small size of the zeolite channels is also a cause of low

Morency et al. [41] investigated the use of a low-cost additive to improve the capability of a high surface area sorbent such as zeolite to remove all forms of mercury vapour from power plant flue gases. They examined treated zeolite wherein preliminary research led to the conclusion that a zeolite material could achieve substantial mercury removal at a significant cost reduction over current mercury control technologies. They carried out the adsorption test of Hg compounds at 150°C on activated natural zeolite (clinoptilolite) and synthetic (A, X, Y, ZSM-5) zeolites. The results proved to be similar to the results obtained on the active carbons. In addition, the zeolites used in the experiments captured all forms of mercury present in the flue gas. This material is also much cheaper than the commercially used activated carbons for this purpose. These authors also found that synthetic zeolites are better for the removal of mercury compounds due to the constant size of the channels and chambers, whereas sizes of natural zeolites are variable and are often associated with a number of network defects; however, natural zeolites are much cheaper material. In addition, after the Hg adsorption experiment on zeolites, these authors conducted leachability test on this material wherein they obtained positive results, i.e., the tested sorbent can be safely disposed in a landfill if sale of

In addition, Morency et al. [42] conducted a successful Hg adsorption test on two types of treated zeolites. The experiment was conducted at a temperature of 130–200°C of flue gas and a concentration of 30–60 μm/m3 of Hg. Studies have shown almost 100% adsorption of Hg at

Morency et al. [16] carried out experiment on adsorption of Hg compounds of the flue gases on activated carbons and raw and treated zeolites using appropriate chemical reagents. The results showed that in the case of treated zeolites, they show similar results to those of active carbons whereas their efficiency is greater than 90% at a ratio of injected mercury 25,000 g/g. Pavlish et al. [43] have studied the adsorption of mercury compounds wherein A class of magnetic zeolite composites with supported silver nanoparticles has been tested for elemental mercury removal from power plant flue gas. As a result, this type of activation mercury capture levels of 45–92% was achieved at high ratios of sorbent to mercury ranging from 20,000 to 100,000, with no decrease in capture observed and an increase in gas temperature between 130 and 200°C. In Addition, high dose rates of zeolites, which are composed of aluminosilicate, will not degrade the commercial value of fly ashes used as a substitute for cement in concrete

In many reports on the removal of mercury from flue gas, it is underlined that zeolite-based sorbents are not as efficient as activated carbons but can be much more economical to use than

Mercury sorption experiments on the non-carbon materials at 110°C were conducted by Jurng et al. [23]. For this purpose, they used zeolite and bentonite materials with a relatively low

adsorption of Hg compounds.

72 Zeolites - Useful Minerals

the ash is not an option.

an Hg ratio of 25,000.

or as filler in plastics.

the commonly used carbons.

#### **3.1. Characteristics of radionuclides in mine water**

The development in the field of nuclear science and technology, generation of electricity in nuclear power plants, nuclear weapons testing, and the use of radionuclides in a research and medicine have led to the increase in the production of various wastes (both liquid and solid) containing radionuclides and release of artificial radionuclides into the environment. It is thought that radiocaesium 134Cs and 137Cs are one of the most abundant radionuclides in nuclear fission products and is considered as hazardous element [44, 45].

One of the most important sources of radionuclide pollution is the mine waters. Some mines have a serious problem with utilization of mine water rich in the radionuclide elements among others Silesia mines in Poland. There are also other areas all over the world where significant radioanomalies are present, including a coal mining district in Germany where radiumbearing waters occurred with 226Ra concentration up to 63 kBq·m-3. One of the major sources of radiation in such areas is related with hard coal mining activity. Sometimes in radiumbearing mine water barium ions are also present, in some mines in concentrations of up to 2 g·dm−3. Contrary, in other waters no barium can be found but radium and sulphate ions are present. The presence of barium in mine waters is the most important factor for the further behaviour of radium isotopes in mine galleries or on the surface. From waters with radium and barium these elements always co-precipitate as sulphates, when such waters are mixed with any water, containing sulphate ions [46]. Coal, deep circulation groundwater, and deposits contain significant quantities of 226Ra and 228Ra as a member of the uranium chain.

In addition, natural waters with similarly high radium concentrations are observed, especially in oil fields. For example, hot springs in Iran contain up to 330 kBq·m-3 of 226Ra [47].

#### **3.2. Sorption mechanisms of radionuclides on zeolites**

In the case of radionuclide sorption onto zeolite such mechanisms are quite simple in com‐ parison to mercury capture. Zeolites due to their ion-exchange properties have received great attention, especially for application in radioactive liquid waste treatment [48, 49]. The radioi‐ sotopes in the form of cations, present in the liquid effluents of low and intermediate levels of liquid waste, are removed by the ion exchange with the Na+ ions present in the zeolite structure in ion-exchange positions. Zeolite materials are characterized by high exchange capacity, selectivity and specificity, and good resistant to radiation, which is very important with respect to the immobilization of radionuclides and final disposal when compared with organic ion exchangers [45, 49–51].

The results of Remenárová et al. [52] on the removal of radionuclide from solution have shown that maximum sorption capacity is achieved though high values indicating that the physical adsorption is not the main binding mechanism. So far many studies [45, 53–56] based on the chemical nature of zeolites stated that ion exchange is the dominating mechanism of metal cation sorption in which cations in the zeolite framework are replaced by metal cations present in water.

Some author modified the surface properties of zeolite in order to enhance the sorption capacity for the removal of radionuclides. For example, Singh and Dutta [57] modified the surface of the zeolite Y type using *n*-octadecyltrichlorosilane for the extraction of 137Cs and 90Sr from aqueous to organic phases [35, 57].

The removal of radionuclides such as 137Cs and 134Ba onto zeolites was investigated by Chmielewská-Horváthová [58]. The examinations were carried out on synthetic aqueous solutions using clinoptilolite and mordenite. In this process, the predominant reaction was also related to the ion exchange of cations that occur in mineral structure, i.e., Ca2+, Mg2+, K+ and Na+ . Results have also shown that pH of tested solutions is also important in the sorption capacity of tested zeolites, where the best pH values of sorption are almost neutral.

In addition, Elizondo et al. [59] studied the effects of pH solution and particle size on the removal of 137Cs and 90Sr by natural zeolite (clinoptilolite) from liquid radioactive wastes and observed that natural zeolite is an effective filter for the radionuclides from a liquid radioactive waste solution with a pH value of 8.

Consequently, Faghihian [60] used modified zeolites for the removal of radioactive iodine (121I and 131I) with different quaternary alkylammonium ions to replace the exchangeable cations from the zeolite surface.

#### **3.3. Results of radionuclide sorption**

The third experiment on radionuclides for the removal of (226Ra and 228Ra) from mine water taken from a hard coal mine KWK Piast was carried out by Chałupnik et al. [61]. The effec‐ tiveness of tested zeolite (Na-P1) with regards to the removal of radionuclides was measured in an experiment performed under dynamic conditions.

The effectiveness of zeolite sorbent with regards to the removal radionuclides was measured under dynamic conditions wherein the used mine water (KWK Piast, Poland horizon 650 m) contained the following concentrations of isotopes: 226Ra 4.216 ± 0.534 [kBq·m-3], 228Ra 7.81 ± 1.46 [kBq·m-3] (thermal conductivity was 119,000 μS·cm-1 and salinity was 110 g·dm-3). In this experiment, the column of about 30 cm in diameter was filled with 26 kg of sand and 2 kg of tested zeolite.

The results obtained by Chałupnik et al. [61] on the removal radionuclides (226Ra and 228Ra) have shown high efficiency of up to 98% of the tested zeolites. After passing 120 dm3 of mine water through the zeolite bed in the column, the concentration of 226Ra decreased to a value of 0.017 kBq·m-3, and in the case of isotope 228Ra decreased to a value of 0.05 kBq·m-3. It has been concluded that despite the adsorption of about 500 Bq of 226Ra and about 1000 Bq of 228Ra still no breakthrough of zeolite bed had occurred. They also carried out preliminary laboratory tests on "synthetic waters" and found that for waters with a similar concentration of radium ions as in the experiment a column breakthrough could only occur after passing approximately 1200 dm3 of water through it with similar characteristics as in the present experiment [61].

The results of Remenárová et al. [52] on the removal of radionuclide from solution have shown that maximum sorption capacity is achieved though high values indicating that the physical adsorption is not the main binding mechanism. So far many studies [45, 53–56] based on the chemical nature of zeolites stated that ion exchange is the dominating mechanism of metal cation sorption in which cations in the zeolite framework are replaced by metal cations present

Some author modified the surface properties of zeolite in order to enhance the sorption capacity for the removal of radionuclides. For example, Singh and Dutta [57] modified the surface of the zeolite Y type using *n*-octadecyltrichlorosilane for the extraction of 137Cs and

The removal of radionuclides such as 137Cs and 134Ba onto zeolites was investigated by Chmielewská-Horváthová [58]. The examinations were carried out on synthetic aqueous solutions using clinoptilolite and mordenite. In this process, the predominant reaction was also related to the ion exchange of cations that occur in mineral structure, i.e., Ca2+, Mg2+, K+

In addition, Elizondo et al. [59] studied the effects of pH solution and particle size on the removal of 137Cs and 90Sr by natural zeolite (clinoptilolite) from liquid radioactive wastes and observed that natural zeolite is an effective filter for the radionuclides from a liquid radioactive

Consequently, Faghihian [60] used modified zeolites for the removal of radioactive iodine (121I and 131I) with different quaternary alkylammonium ions to replace the exchangeable cations

The third experiment on radionuclides for the removal of (226Ra and 228Ra) from mine water taken from a hard coal mine KWK Piast was carried out by Chałupnik et al. [61]. The effec‐ tiveness of tested zeolite (Na-P1) with regards to the removal of radionuclides was measured

The effectiveness of zeolite sorbent with regards to the removal radionuclides was measured under dynamic conditions wherein the used mine water (KWK Piast, Poland horizon 650 m) contained the following concentrations of isotopes: 226Ra 4.216 ± 0.534 [kBq·m-3], 228Ra 7.81 ± 1.46 [kBq·m-3] (thermal conductivity was 119,000 μS·cm-1 and salinity was 110 g·dm-3). In this experiment, the column of about 30 cm in diameter was filled with 26 kg of sand and 2 kg of

The results obtained by Chałupnik et al. [61] on the removal radionuclides (226Ra and 228Ra)

water through the zeolite bed in the column, the concentration of 226Ra decreased to a value of 0.017 kBq·m-3, and in the case of isotope 228Ra decreased to a value of 0.05 kBq·m-3. It has been concluded that despite the adsorption of about 500 Bq of 226Ra and about 1000 Bq of 228Ra still

of mine

have shown high efficiency of up to 98% of the tested zeolites. After passing 120 dm3

capacity of tested zeolites, where the best pH values of sorption are almost neutral.

. Results have also shown that pH of tested solutions is also important in the sorption

in water.

74 Zeolites - Useful Minerals

and Na+

90Sr from aqueous to organic phases [35, 57].

waste solution with a pH value of 8.

**3.3. Results of radionuclide sorption**

in an experiment performed under dynamic conditions.

from the zeolite surface.

tested zeolite.

The analysis of the water samples obtained after the process of removing radium ions showed that using Na-P1 zeolite for the removal of radionuclides has no effect on the level of calcium or magnesium concentration in the purified water where their concentrations before and after the experiment are similar. The same tested zeolite also did not cause significant changes in the concentration of monovalent metals (sodium and potassium) as well as iron, strontium and manganium in the solution. Moreover, no change was observed in the concentrations of anions (chloride, sulphate, or bicarbonate ions), which are not adsorbed by Na-P1 that is related to the negative charge on their surface [61].

Other authors carried out the removal of radiounuclides using different zeolites among others natural zeolites. Gallant and Prakash [62] carried out dynamic experiment onto the removal of radionuclides such as 137Cs, 60Co and 90Sr from water effluents also rich in Cu, Cd and Zn from nuclear operations using clinoptilolite and chabazite. Experiments have shown that in multication solution caesium is easily removed by both zeolites, so it would be economical to remove caesium first from such solutions using low-cost zeolites [62].

In 2014, Remenárová et al. [45] used synthetic zeolites from fly ash for the removal of caesium from the solution of CsCl under static conditions. They have examined the influence of pH on the sorption of radionuclides and the results have shown that the better sorption was found at pH values ranging from 4 to 5. Nevertheless, the carried out experiments have shown that synthesized zeolite materials could be suitable sorbents for the removal of radiocaesium from liquid radioactive wastes. Moreover, Moattar and Hayeripour [46] have examined natural zeolite (clinoptilolite) for the removal of 137Cs, 60Co and 54Mn from low-level radioactive liquid waste under static and dynamic conditions. In such experiments, zeolites showed the same promising results in the removal of caesium.

El-Naggar et al. [49] used A type synthesized zeolite and evaluated it as an inorganic ionexchange material for the removal of 137Cs and 90Sr ions from aqueous solutions in both batch and fixed bed column operations. The results have shown that the total ion uptake of these metals and the overall bed capacity decreased by increasing the flow rate and increased by increasing the initial ion concentrations and bed depth. The removal of caesium was also investigated by the International Atomic Energy Agency [63] where tests of the removal of 137Cs ions for aqueous solution were carried out using synthetic NaA-X zeolite. This synthetic zeolite showed a promising sorption behaviour.

Soelberg et al. [64] impregnated zeolite of silver in order to enhance the uptake of volatile radioactive iodine and krypton. Silver impregnated zeolites showed high iodine decontami‐ nation factors as high as 105, iodine loading capacities and other adsorption parameters including adsorption rates.

#### **4. Zeolites as petroleum substance sorbents**

#### **4.1. Characteristics of petroleum substances**

Crude oil belongs to the group of natural hydrocarbon fuels, such as natural gas or coals, which was created as a result of biochemical, chemical and geochemical transformations of organic matter derived from dead plant and animal organisms. It is the liquid mixture of hydrocarbons, oxygen and sulphur compounds, containing trace amounts of various inorganic compounds and elements. The conventional crude oil is mainly composed of hydrogen and carbon in an atomic relation H/C ~ 1.85. Other elements such as sulphur, nitrogen and oxygen are usually not less than 3% (by weight) of its content. The remaining components of the crude oil are usually trace amounts of phosphorus, arsenic, selenium and heavy metals such as vanadium, nickel, iron, cobalt, copper and mercury.

The composition of the hydrocarbons in crude oil consist of paraffinic hydrocarbons (aliphat‐ ic), cycloparaffinic (naphthenic) and aromatic among which the cycloparaffins dominate (approximately 50% content). Aliphatic hydrocarbons are the second group of the hydrocar‐ bons present in crude oil in substantial quantities. It is assumed that crude oil comprises *n*aliphatic (straight) hydrocarbons with the number of carbon atoms from 1 to 60. Except *n*aliphatic hydrocarbons, branched chain hydrocarbons are present in significant amounts. Their boiling point is lower than the boiling point of unbranched equivalents. Aliphatic hydrocarbons (straight and branched) have lower density and are more volatile than their naphthenic analogues.

Aromatic hydrocarbons contain at least one benzene ring in the molecule. Crude oil usually contains up to 15% of arenes. This type of compounds are typically concentrated in fractions boiling at a higher temperature, such as lubricating oils and residues, where the concentration may exceed 50%. Among the aromatic hydrocarbons, toluene and *m*-xylene are present in the crude oil to the largest quantities. Monocyclic aromatic hydrocarbons constitute a valuable component of the fuel. It should be emphasized that the benzene and its alkyl derivatives (ethyl benzene, toluene and xylene) are compounds with carcinogenic properties and very high toxicity. These features and the relatively good water solubility make them a group of substances that is very hazardous to the environment. On the other hand, polycyclic aromatic hydrocarbons (PAHs) are concentrated in petroleum products such as diesel oils boiling at high temperatures, lubricating oils and residues distillation. They have the ability to accumu‐ late in tissues of living organisms and exhibit the carcinogenic properties [65].

Petroleum distillates are components of commonly used refined products such as gasoline, aviation fuel, diesel fuel and motor oils, hydraulic oils, fuel oils, base oils for lubricants, petroleum waxes, asphalt, petroleum coke and petrochemical raw materials.

Currently, there is a special emphasis on the preparation of such petroleum products that have a minimum negative impact on the environment. Interaction depends on many factors that include the physical, chemical and toxic properties of the product, the manner of its use, scale of consumption and method of disposal. Fuels accounts for about 80% V/V of the processed crude oil and therefore they are the biggest source of oil pollution of the environment [52]. Other products representing approximately 1.5% of processed crude oil pose a smaller risk, albeit not without significance for the environment. In fact, there is still a problem with waste oil and other petroleum waste, which are the substances of potentially dangerous status from the point of view of ecology.

Undesirable effects of petroleum substances are due to their physicochemical properties, including viscosity, density, lipophilicity and hydrocarbon composition that determines these properties.

Among the petroleum compounds particularly dangerous to life and health of living organ‐ isms, volatile aromatic hydrocarbons, such as benzene, toluene, ethylbenzene and xylene (BTEX), can be distinguished. These compounds are components of many solvents or fuels, such as petrol or diesel oil [66–68]. They tend to accumulate in groundwater and are also soil or air pollutants [69]. They are carcinogenic, irritate the mucous membranes, cause changes in haematology, central nervous system disorders, respiratory problems, abnormal liver function and kidney function. They act toxic even in small concentrations, and their inhalation or ingestion may result in death [69, 70].

#### **4.2. Sorption mechanisms of petroleum substances on zeolites**

**4. Zeolites as petroleum substance sorbents**

Crude oil belongs to the group of natural hydrocarbon fuels, such as natural gas or coals, which was created as a result of biochemical, chemical and geochemical transformations of organic matter derived from dead plant and animal organisms. It is the liquid mixture of hydrocarbons, oxygen and sulphur compounds, containing trace amounts of various inorganic compounds and elements. The conventional crude oil is mainly composed of hydrogen and carbon in an atomic relation H/C ~ 1.85. Other elements such as sulphur, nitrogen and oxygen are usually not less than 3% (by weight) of its content. The remaining components of the crude oil are usually trace amounts of phosphorus, arsenic, selenium and heavy metals such as vanadium,

The composition of the hydrocarbons in crude oil consist of paraffinic hydrocarbons (aliphat‐ ic), cycloparaffinic (naphthenic) and aromatic among which the cycloparaffins dominate (approximately 50% content). Aliphatic hydrocarbons are the second group of the hydrocar‐ bons present in crude oil in substantial quantities. It is assumed that crude oil comprises *n*aliphatic (straight) hydrocarbons with the number of carbon atoms from 1 to 60. Except *n*aliphatic hydrocarbons, branched chain hydrocarbons are present in significant amounts. Their boiling point is lower than the boiling point of unbranched equivalents. Aliphatic hydrocarbons (straight and branched) have lower density and are more volatile than their

Aromatic hydrocarbons contain at least one benzene ring in the molecule. Crude oil usually contains up to 15% of arenes. This type of compounds are typically concentrated in fractions boiling at a higher temperature, such as lubricating oils and residues, where the concentration may exceed 50%. Among the aromatic hydrocarbons, toluene and *m*-xylene are present in the crude oil to the largest quantities. Monocyclic aromatic hydrocarbons constitute a valuable component of the fuel. It should be emphasized that the benzene and its alkyl derivatives (ethyl benzene, toluene and xylene) are compounds with carcinogenic properties and very high toxicity. These features and the relatively good water solubility make them a group of substances that is very hazardous to the environment. On the other hand, polycyclic aromatic hydrocarbons (PAHs) are concentrated in petroleum products such as diesel oils boiling at high temperatures, lubricating oils and residues distillation. They have the ability to accumu‐

Petroleum distillates are components of commonly used refined products such as gasoline, aviation fuel, diesel fuel and motor oils, hydraulic oils, fuel oils, base oils for lubricants,

Currently, there is a special emphasis on the preparation of such petroleum products that have a minimum negative impact on the environment. Interaction depends on many factors that include the physical, chemical and toxic properties of the product, the manner of its use, scale of consumption and method of disposal. Fuels accounts for about 80% V/V of the processed crude oil and therefore they are the biggest source of oil pollution of the environment [52].

late in tissues of living organisms and exhibit the carcinogenic properties [65].

petroleum waxes, asphalt, petroleum coke and petrochemical raw materials.

**4.1. Characteristics of petroleum substances**

76 Zeolites - Useful Minerals

nickel, iron, cobalt, copper and mercury.

naphthenic analogues.

Several mechanical, biological and chemical methods are used to liquidate oil spills from water and paved roads [11, 33, 71–76]. Among these methods, one of the most popular is the sorption method using sorbents of different origins. Sorbents are solids with developed surface and able to bond molecules on their surface (from liquid or gaseous phase). These sorption features are characteristics of zeolite minerals.

The most commonly used adsorbents for oil spills on paved surfaces (roads, asphalt and land) are those of mineral origin. For this type of materials, the sorption process takes place according to the physical sorption including two main mechanisms. The first one is a stage of sorption associated with filling of the available pores in the material and the capillary pores between the grains of the sorbent according to mechanism involving capillary (mass transport). Capillarity depends on, among others, the effective diameter of the capillary, or surface energy of the interior wall of the capillary and viscosity of the oil. After this step, sorption occurs on the outer surface and an oily film layer is created around the grain of sorbent (or optionally around the agglomerate grains). On the surface of the sorbent grains, oil can create a uniform layer (film) or irregular aggregates, depending on the morphology of the material surface, its irregularities and the properties of the oil [77].

The adsorption of petroleum compounds from aqueous solutions (e.g., solubilized form of organic compounds such as BTEX) on porous sorbents involves diffusion process, the course of which may include transport of the molecules in the vicinity of the adsorbent, the diffusion in a boundarylayer,thediffusioninsorbent'scapillary,surfacediffusionandintra-particlediffusion.

#### **4.3. Results of petroleum substances sorption**

Some relevant papers concerning removal of petroleum substances indicate the possibility of using zeolites for this purpose, including those derived from fly ash.

A comparison of sorption properties in relation to diesel fuel (Biodiesel and Verva ON) of zeolites from fly ash on the zeolite types of structures of Na-P1 and Na-X and natural clinop‐ tilolite as well as on commercial silica-based sorbent (Absodan) was presented by Bandura et al. [78, 79]. They received the sorption capacity of the order of 1.24–1.40 g/g for Na-P1 zeolite, 0.91–1.13 g/g for Na-X zeolite and 0.31 g/g for clinoptilolite. For commercial sorbent, the sorption capacity was 0.83–0.95 g/g (**Figure 1**).

**Figure 1.** Sorption capacity of zeolite materials.

Na-P1 and Na-X zeolites also showed good sorption properties with respect to oils similar to sepiolite and vermiculite and some commercial sorbents. Importantly, Na-P1 and Na-X zeolites have higher sorption capacity with respect to oil in comparison of zeolites tested in the works of Carmody et al. [77] and Ankowski [80]. The zeolites from the fly ash, i.e., Na-X and Na-P1, can also be used as absorbents of volatile organic compounds [81].

In this direction, Ankowski [80] examined zeolites from fly ash granulated with clay, whose capacity to diesel fuel was 0.51–0.57 g/g. In addition, the capacity of sodium aluminium silicate and commercial sorbents Eco-dry and dry-Lite was determined, which amounted to 0.27–0.43, 1.11, 0.77 g/g, respectively.

Sakthivel et al. [82] synthesized zeolites from fly ash by hydrothermal conversion and fusion methods, and then zeolite products were modified by organic compounds. The sorption capacity of zeolites without modification was 0.37 g/g for zeolite obtained by the hydrothermal method and 1.22 for zeolite obtained by fusion. It is worth noting that the sorption capacity of zeolites modified was in the range of 0.21–1.15 g/g, lower than those that were not modified. These results indicate that organic modification of zeolites is not in all cases profitable with respect to the removal of petroleum substances. It turned out that this type of modification of the sorbent surface blocks the pores of the zeolite, thus reducing its specific surface area and limiting the access of oil into the pores.

The results of this research indicate the possibility of the use of zeolites from fly ash as a mineral sorbent to remove oil spills from paved surfaces.

### **5. Conclusions**

A comparison of sorption properties in relation to diesel fuel (Biodiesel and Verva ON) of zeolites from fly ash on the zeolite types of structures of Na-P1 and Na-X and natural clinop‐ tilolite as well as on commercial silica-based sorbent (Absodan) was presented by Bandura et al. [78, 79]. They received the sorption capacity of the order of 1.24–1.40 g/g for Na-P1 zeolite, 0.91–1.13 g/g for Na-X zeolite and 0.31 g/g for clinoptilolite. For commercial sorbent, the

Na-P1 and Na-X zeolites also showed good sorption properties with respect to oils similar to sepiolite and vermiculite and some commercial sorbents. Importantly, Na-P1 and Na-X zeolites have higher sorption capacity with respect to oil in comparison of zeolites tested in the works of Carmody et al. [77] and Ankowski [80]. The zeolites from the fly ash, i.e., Na-X

In this direction, Ankowski [80] examined zeolites from fly ash granulated with clay, whose capacity to diesel fuel was 0.51–0.57 g/g. In addition, the capacity of sodium aluminium silicate and commercial sorbents Eco-dry and dry-Lite was determined, which amounted to 0.27–0.43,

Sakthivel et al. [82] synthesized zeolites from fly ash by hydrothermal conversion and fusion methods, and then zeolite products were modified by organic compounds. The sorption capacity of zeolites without modification was 0.37 g/g for zeolite obtained by the hydrothermal method and 1.22 for zeolite obtained by fusion. It is worth noting that the sorption capacity of zeolites modified was in the range of 0.21–1.15 g/g, lower than those that were not modified. These results indicate that organic modification of zeolites is not in all cases profitable with respect to the removal of petroleum substances. It turned out that this type of modification of the sorbent surface blocks the pores of the zeolite, thus reducing its specific surface area and

The results of this research indicate the possibility of the use of zeolites from fly ash as a mineral

and Na-P1, can also be used as absorbents of volatile organic compounds [81].

sorption capacity was 0.83–0.95 g/g (**Figure 1**).

78 Zeolites - Useful Minerals

**Figure 1.** Sorption capacity of zeolite materials.

1.11, 0.77 g/g, respectively.

limiting the access of oil into the pores.

sorbent to remove oil spills from paved surfaces.

The present results have shown that zeolite can be considered promising materials for the removal of toxic compounds, such as mercury, radionuclides, and petroleum substances.

The results of mercury removal in pure stream of gas have shown that silver impregnation greatly improves zeolites efficiencies. The highest uptake achieved Na-X Ag-impregnated that adsorbed significantly more mercury than the commercial available activated carbon with bromine. For the removal of mercury in flue gas only Na-X Ag-impregnated was tested (powder and granulate). The tested zeolite reduced the level of mercury in the flue gas where the long-time mercury breakthrough ranges from 15 to 40% depending on the experimental conditions. The better results were obtained for granulated material.

Results of removing radium isotopes indicate high efficiency of the process at a level of 98% which indicates a high application potential of synthetic zeolites for the purification of mine water from radionuclides. Other radionuclides, mainly caesium and strontium, tested by other authors are also removed perfectly from various solutions using zeolites (natural and syn‐ thetic).

Results have shown that the sorption of diesel fuels and used oil on clinoptilolite, Na-P1 and Na-X, and commercial sorbent mainly constitutes physical characters. Synthetic zeolites have around twice higher sorption capacity for oils than commercial sorbents. Higher sorption capacities were achieved for oils of higher densities.

#### **Acknowledgements**

This work was financed by the National Centre for Research and Development (NCBR) under contract number LIDER/384/L-6/14/NCBR/2015 and statutory fund no. S12/II/B/2016.

#### **Author details**

Magdalena Wdowin1\* and Wojciech Franus2

\*Address all correspondence to: wdowin@meeri.pl

1 Mineral and Energy Economy Research Institute, Polish Academy of Sciences, Kraków, Poland

2 Civil Engineering and Architecture Faculty, Lublin University of Technology, Lublin, Poland

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84 Zeolites - Useful Minerals


**Natural and Synthetic Zeolites: Applications**

### **Industrial Zeolite Molecular Sieves**

Kristin Gleichmann, Baldur Unger and Alfons Brandt

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63442

#### **Abstract**

Zeolite molecular sieves are used in industrial applications since more than 60 years, mainly as highly efficient adsorbents for separation processes in gas or liquid phase. Zeolite molecular sieves may be applied in powder form, preferably in static applica‐ tions, but to a much larger extent as shaped material in both static and dynamic (flowing media) applications. Many shaping technologies for molecular sieves have been developed over the last decades, reflecting the different requirements for molecular sieves in different applications. This review deals with the influence of the applied zeolite synthesis and shaping methods for hydrophilic zeolite molecular sieves (procedures, materials, recipes) on the potential industrial applications; thereby considering powders, binder-containing shapes as well as binderless shapes (includ‐ ing compact structures such as honeycombs, multi-channel, and foam-like structures). Due to new challenges from the market, more specialized, tailor-made types of zeolite molecular sieves are required. Such a higher specialization can be achieved by applying new types of zeolites or zeolite-like materials, modified synthesis and/or post synthe‐ sis treatments, and by modified, to the needs of the application adjusted shaping processes.

**Keywords:** zeolite, molecular sieve, adsorption, fixed bed, compact zeolite bodies

#### **1. Introduction**

Zeolite molecular sieves are used in industrial applications since more than 60 years, mainly as highly efficient adsorbents for separation processes in gas or liquid phase. The current market volume for molecular sieves (especially for zeolites for adsorption applications) is 150.000– 200.000 t with a sales volume of 450–550 Mio. USD with an annual growth forecast up to 2020 of 3–5% [1–3]. Growth drivers are environmental protection and conservation of resources (deeper processing of crude oil, using of coal and natural gas (especially from off-shore and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

shale gas deposits coming [4–6]) as raw material for petrochemical products in the so-called coal-to-liquid (CTL) or gas-to-liquid (GTL) processes) [7–9].

Zeolite molecular sieves can be applied in powder form, preferably in static applications. During the polyurethane production, zeolite 3A powder is mixed with the polyol component to enable exclusion of moisture and prevent the chemical reaction of water with the isocyanate component [10]. To support the material handling, and to avoid, for example, dust formation molecular sieve beads are used in non-regenerative applications, too. Dry sealing plug or small dry bags filled with zeolite A (or silica) [11, 12] to guaranty a dry transport or storage of humidity sensitive goods such as electronical products, leather ware, or pharmaceuticals are common [11, 12]. The application of zeolite 3A in insulating glass panes is well-known, too. By putting the adsorbent material as desiccant in the perforated spacer between the two or three window panes moisture condensation between the panes (fogging) is prevented [13]. Refrigerant drying [14, 15] is also a static adsorption process, where in the coolant circuit formed, water and acidic components are removed [14, 15].

Besides the mentioned application examples, shaped zeolite molecular sieve material in flowing media is used to a much larger extent. For instance, the purification of technical hydrogen (removal of nitrogen and carbon monoxide) on zeolite 5A [16], the pre-purification of raw air on zeolite NaX [17] for the cryogenic air separation or the adsorptive oxygen enrichment, for example, on zeolite type 5A [18] or LiX [19, 20] should be mentioned in this place. Applied technologies for purification and drying of natural gas are operating with zeolite molecular sieves [21] of zeolite 5A and/or NaX type [22]. In petro chemistry applications especially in steam cracking plants, light hydrocarbons are separated by low temperature rectification. To avoid ice crystal and gas hydrate formation, cracked gases have to be dried to a residual water content <1 ppm [23]. Zeolite molecular sieves of type 3A meet the requirements for that complex process [22]. For the sake of completeness, the principle of the simulating moving bed (SMB) should also be mentioned. This practically continuous liquid phase adsorption process is based on the countercurrent extraction principle [24]. The so-called Molex process, where paraffin mixtures (zeolite molecular sieve of type 5A [25, 26]) are separated, and the so-called Parex processes, where *p*-xylen is separated from C8 aromatic mixture (zeolite molecular sieve of type KY or BaX [27]) are known SMB processes. Further‐ more, the dehydration of preferably bio-ethanol above the azeotropic point to more than 99.5% on zeolite 3A is common practice [28, 29]. Besides the mentioned examples, the use of zeolite molecular sieves in automotive applications such as drying of brake air in rail vehicles, trucks, or busses is also a well-known application. To avoid corrosion or freezing caused by humidity, such pneumatically working breaking systems are equipped with zeolite 4A molecular sieve or silica gel-containing desiccant cartridges [22].

Another field for the application of zeolite molecular sieves is the utilization of the adsorption heat during the adsorption process. Different applications such as building heating [30–32] or enhanced household devices such as dish washers [33, 34] based on the thermo-chemical energy properties of zeolites are introduced into the market.

The above-mentioned examples clearly demonstrate the demand for zeolite molecular sieves with special properties depending on the different requirements in the application.

shale gas deposits coming [4–6]) as raw material for petrochemical products in the so-called

Zeolite molecular sieves can be applied in powder form, preferably in static applications. During the polyurethane production, zeolite 3A powder is mixed with the polyol component to enable exclusion of moisture and prevent the chemical reaction of water with the isocyanate component [10]. To support the material handling, and to avoid, for example, dust formation molecular sieve beads are used in non-regenerative applications, too. Dry sealing plug or small dry bags filled with zeolite A (or silica) [11, 12] to guaranty a dry transport or storage of humidity sensitive goods such as electronical products, leather ware, or pharmaceuticals are common [11, 12]. The application of zeolite 3A in insulating glass panes is well-known, too. By putting the adsorbent material as desiccant in the perforated spacer between the two or three window panes moisture condensation between the panes (fogging) is prevented [13]. Refrigerant drying [14, 15] is also a static adsorption process, where in the coolant circuit

Besides the mentioned application examples, shaped zeolite molecular sieve material in flowing media is used to a much larger extent. For instance, the purification of technical hydrogen (removal of nitrogen and carbon monoxide) on zeolite 5A [16], the pre-purification of raw air on zeolite NaX [17] for the cryogenic air separation or the adsorptive oxygen enrichment, for example, on zeolite type 5A [18] or LiX [19, 20] should be mentioned in this place. Applied technologies for purification and drying of natural gas are operating with zeolite molecular sieves [21] of zeolite 5A and/or NaX type [22]. In petro chemistry applications especially in steam cracking plants, light hydrocarbons are separated by low temperature rectification. To avoid ice crystal and gas hydrate formation, cracked gases have to be dried to a residual water content <1 ppm [23]. Zeolite molecular sieves of type 3A meet the requirements for that complex process [22]. For the sake of completeness, the principle of the simulating moving bed (SMB) should also be mentioned. This practically continuous liquid phase adsorption process is based on the countercurrent extraction principle [24]. The so-called Molex process, where paraffin mixtures (zeolite molecular sieve of type 5A [25, 26]) are separated, and the so-called Parex processes, where *p*-xylen is separated from C8 aromatic mixture (zeolite molecular sieve of type KY or BaX [27]) are known SMB processes. Further‐ more, the dehydration of preferably bio-ethanol above the azeotropic point to more than 99.5% on zeolite 3A is common practice [28, 29]. Besides the mentioned examples, the use of zeolite molecular sieves in automotive applications such as drying of brake air in rail vehicles, trucks, or busses is also a well-known application. To avoid corrosion or freezing caused by humidity, such pneumatically working breaking systems are equipped with zeolite 4A molecular sieve

Another field for the application of zeolite molecular sieves is the utilization of the adsorption heat during the adsorption process. Different applications such as building heating [30–32] or enhanced household devices such as dish washers [33, 34] based on the thermo-chemical

The above-mentioned examples clearly demonstrate the demand for zeolite molecular sieves

with special properties depending on the different requirements in the application.

coal-to-liquid (CTL) or gas-to-liquid (GTL) processes) [7–9].

90 Zeolites - Useful Minerals

formed, water and acidic components are removed [14, 15].

or silica gel-containing desiccant cartridges [22].

energy properties of zeolites are introduced into the market.

**Figure 1.** Overview of the manufacturing of different zeolite molecular sieves (figure compact zeolite bodies [35]).

Many synthesis and shaping technologies for molecular sieves were developed over the last decades. Within the following descriptions, the influence of the applied synthesis and shaping methods for hydrophilic zeolite molecular sieves of zeolite A, X, and NaY on their properties, and thus, on their potential applicability in industrial adsorption processes is discussed. The procedures, materials, and recipes for the manufacturing of zeolite powders, binder-contain‐ ing shapes as well as binderless shapes are described. Also compact structures such as honeycombs and multi-channel tubes are considered.

**Figure 1** summarizes different manufacturing procedures and materials dealt with in this review.

#### **2. Zeolite powder with different particle sizes**

Within the manufacturing of synthetic zeolite molecular sieves, the synthesis of the related zeolite powder is first step. There are two different principles for the synthesis of aluminumrich zeolites in industrial processes:


Usually, in the industrial production of zeolite A, X, and NaY, the sodium form is manufac‐ tured (except the zeolite LSX synthesis [36]). Therefore, different raw materials as silicon source such as silicic acid, colloidal silica, sodium silicate, or silica gel [37] are used. Possible alumina sources are alumina trihydrate, sodium aluminate, activated alumina, gamma alumina. Sodium may be obtained from sodium hydroxide which controls simultaneously the pH value [37]. A convenient way to prepare such a reactant mixture is to pour under more or less intensive stirring an aqueous solution containing the aluminum source and sodium hydroxide into an aqueous solution containing the silica source. After gel formation and generating a homogeneous mixture, the mixture is heated up to maximal 120°C at atmospheric pressure or moderate overpressure [37]. After crystallization, the zeolite suspension is filtered at temper‐ atures between room temperature and reaction temperature [37], for example, in a filter press [38] or a centrifuge [39, 40] or a vacuum belt filter [40]. The zeolite crystals are washed directly on the filter device till a pH value between 9 and 12 [37] is achieved. If desired, the zeolite powder in its (virgin) sodium form can be ion exchanged or otherwise modified. For example, the filter cake can be suspended in a suitable salt solution and stirred at a suitable temperature. Alternatively, the exchange step is done in a column [41] or directly on the filter press [42] with a suitable salt solution at a suitable temperature. After the ion-exchange procedure is com‐ pleted, the zeolite is washed as described above. Afterwards, the filter cake is dried in a direct or indirect heated thermal treatment process, as, for example, spray drying, spin flash drying, or long gap mill drying [40, 43]. The dried zeolite powder is processed in the following steps. The mother liquor from the filtration step can usually (at least to a certain extent) be reused in the following zeolite synthesis batch [37, 44].

The manufacturing of zeolites by conversion of minerals is not really a commonly used process. In several papers which deal with the synthesis of zeolite A, X, and NaY from minerals, kaolin or metakaolin is used as aluminum source [45–47]. A disadvantage of such procedures is the presence of impurities in the applied minerals [45], but some advantages (simple treatment of the mother liquor and a nearly waste-free production) support the application of that synthesis method especially in the case of zeolite NaY [46].

The properties of zeolite molecular sieves can be influenced by the properties of the starting zeolite powder. For instance, the hydrothermal stability or desorption characteristics of FAU vary with the chemical composition, especially the aluminum content [48]. The particle size, particle size distribution, and the surface characteristics of single zeolite crystals affect the adsorption properties as well as the mechanical stability of zeolite molecular sieves. By varying the synthesis gel composition, time, or temperature or by adding additives to the synthesis gel, the mentioned properties of the zeolite powder can be changed [49, 50]. Depending on the intended applications, the particle size distribution is adjusted. Besides the described classical gel synthesis [51] resulting in particles of about 5 μm, microparticles (particle size of approx. 1 μm) and nanoparticles (particle sizes between 50 and 100 nm) can be achieved via new synthesis routes [52].

#### **3. Binder-containing zeolite molecular sieves**

Many synthesis and shaping technologies for molecular sieves were developed over the last decades. Within the following descriptions, the influence of the applied synthesis and shaping methods for hydrophilic zeolite molecular sieves of zeolite A, X, and NaY on their properties, and thus, on their potential applicability in industrial adsorption processes is discussed. The procedures, materials, and recipes for the manufacturing of zeolite powders, binder-contain‐ ing shapes as well as binderless shapes are described. Also compact structures such as

**Figure 1** summarizes different manufacturing procedures and materials dealt with in this

Within the manufacturing of synthetic zeolite molecular sieves, the synthesis of the related zeolite powder is first step. There are two different principles for the synthesis of aluminum-

Usually, in the industrial production of zeolite A, X, and NaY, the sodium form is manufac‐ tured (except the zeolite LSX synthesis [36]). Therefore, different raw materials as silicon source such as silicic acid, colloidal silica, sodium silicate, or silica gel [37] are used. Possible alumina sources are alumina trihydrate, sodium aluminate, activated alumina, gamma alumina. Sodium may be obtained from sodium hydroxide which controls simultaneously the pH value [37]. A convenient way to prepare such a reactant mixture is to pour under more or less intensive stirring an aqueous solution containing the aluminum source and sodium hydroxide into an aqueous solution containing the silica source. After gel formation and generating a homogeneous mixture, the mixture is heated up to maximal 120°C at atmospheric pressure or moderate overpressure [37]. After crystallization, the zeolite suspension is filtered at temper‐ atures between room temperature and reaction temperature [37], for example, in a filter press [38] or a centrifuge [39, 40] or a vacuum belt filter [40]. The zeolite crystals are washed directly on the filter device till a pH value between 9 and 12 [37] is achieved. If desired, the zeolite powder in its (virgin) sodium form can be ion exchanged or otherwise modified. For example, the filter cake can be suspended in a suitable salt solution and stirred at a suitable temperature. Alternatively, the exchange step is done in a column [41] or directly on the filter press [42] with a suitable salt solution at a suitable temperature. After the ion-exchange procedure is com‐ pleted, the zeolite is washed as described above. Afterwards, the filter cake is dried in a direct or indirect heated thermal treatment process, as, for example, spray drying, spin flash drying, or long gap mill drying [40, 43]. The dried zeolite powder is processed in the following steps. The mother liquor from the filtration step can usually (at least to a certain extent) be reused in

**•** Manufacturing of highly pure crystalline zeolite powders from a synthesis gel and

honeycombs and multi-channel tubes are considered.

**2. Zeolite powder with different particle sizes**

**•** Conversion of natural minerals into zeolite matter.

the following zeolite synthesis batch [37, 44].

rich zeolites in industrial processes:

review.

92 Zeolites - Useful Minerals

In dynamic adsorption processes, where the adsorbent has to be regenerated frequently, the fluid dynamics (e.g., the pressure drop over the fixed bed of molecular sieves) has to be taken into account. Therefore, the zeolite has to be applied in a suitable particle size (usually in the form of shaped mechanically stable macroscopic particles such as beads/spheres or extrudates/ cylinders).

Because pure zeolite powder does not exhibit binding properties, an appropriate binder needs to be applied to form the above-mentioned macroscopic particles. **Figure 2** shows the main principles for the manufacturing of conventional binder-containing zeolite molecular sieves (bulk material).

The zeolite component is the main component of the pre-mixture and can be applied as dried zeolite powder (occasionally as filtercake). Usually, the zeolite component is modified (as described above, e.g., ion exchanged or dealuminated) prior to the shaping step. Besides the zeolite component a binder material is put into the pre-mixture. Mineral binders such as bentonite, attapulgite, or kaolin are common [53, 54] as well as synthetic binders such as alumina, silica [54, 55], or a mixture thereof [56]. The type of binder material to be applied depends on the shaping procedure and on the final application of the zeolite molecular sieve. Bentonite (montmorillonite) for example is commonly used for extrusion [57, 58], attapulgite for beading [59]. It should be noted that (natural) mineral binders may contain impurities (e.g., catalytically active heavy metals), which could cause undesired side reactions in the final application. If necessary, additives such as shaping aids (e.g., mineral oil, PVA, starch derivate solutions [60]) are put into the pre-mixture to make the shaping process most effective. Such mainly organic compounds are burnt out in subsequent thermal treatment steps. In order to create a defined secondary pore system (see below) thermally or chemically, removable spacers (e.g., cellulose fibers or soluble salts) are added to the pre-mixture. In most of the bulk zeolite molecular sieves, a binder content of around 20% (anhydrous basis) is used. But also lower (down to around 5%) and higher (above 30%) binder amounts are described [59].

**Figure 2.** Scheme of the principle manufacturing procedures for binder-containing zeolite molecular sieve shapes (bulk material).

After the (mainly dry) mixing of all aforementioned materials in the pre-mixture usually a certain amount of water is added in order to adjust the viscosity and plasticity of the mixture for the related shaping process that can be agglomeration granulation [59], spray granulation [61], or extrusion [62]. After the shaping process, the molecular sieve has to be dried and activated in a thermal step, for example, in a rotary kiln [63] or belt calciner [64] to remove the water and other adsorbed compounds. The applied temperature is often higher than required for the zeolite activation, since the binder system needs such higher temperatures for setting. The zeolite component in zeolite NaX beads with attapulgite as binder materials, for example, needs not more than 350–400°C for a complete activation (residual water content <1 wt% measured by Karl-Fischer titration) [65]. But the attapulgite binder needs approximately 500 °C for getting converted into a mechanically stable structure [53, 59]—a temperature which can already be critical for the structural stability of the zeolite [66, 95]. Due to the limited hydrothermal stability [67–69, 95] of hydrophilic zeolites, the activation step has to be carried out carefully. To avoid (partial) zeolite destruction during the activation step, the released water needs to be removed from the shape as fast as possible in order to avoid the appearance of hydrothermal conditions. The higher the temperature and higher the moisture concentration directly at the zeolite centers the higher the probability for (partial) zeolite destruction [70]. By applying a controlled temperature increase, a low height of the granules bed and/or a dry purge gas can be met these conditions. Depending on the particular conditions, a residual water content of less than one percent can be reached. During the cooling step as well as during the storage of the activated material, the presence of humidity or other adsorptive components should be avoided to prevent undesired adsorption on the material. Therefore, activated zeolite molecular sieves are usually packed in hermetically sealed packaging units, as, for example, sealed steel drums or big bags equipped with sealed inliners.

mainly organic compounds are burnt out in subsequent thermal treatment steps. In order to create a defined secondary pore system (see below) thermally or chemically, removable spacers (e.g., cellulose fibers or soluble salts) are added to the pre-mixture. In most of the bulk zeolite molecular sieves, a binder content of around 20% (anhydrous basis) is used. But also lower

**Figure 2.** Scheme of the principle manufacturing procedures for binder-containing zeolite molecular sieve shapes (bulk

After the (mainly dry) mixing of all aforementioned materials in the pre-mixture usually a certain amount of water is added in order to adjust the viscosity and plasticity of the mixture for the related shaping process that can be agglomeration granulation [59], spray granulation [61], or extrusion [62]. After the shaping process, the molecular sieve has to be dried and activated in a thermal step, for example, in a rotary kiln [63] or belt calciner [64] to remove the water and other adsorbed compounds. The applied temperature is often higher than required for the zeolite activation, since the binder system needs such higher temperatures for setting. The zeolite component in zeolite NaX beads with attapulgite as binder materials, for example, needs not more than 350–400°C for a complete activation (residual water content <1 wt% measured by Karl-Fischer titration) [65]. But the attapulgite binder needs approximately 500

material).

94 Zeolites - Useful Minerals

(down to around 5%) and higher (above 30%) binder amounts are described [59].

The mentioned use of a binder system opens a large variety of zeolite molecular sieves with different properties. Depending on the final application, the properties of the single shapes or the bulk material have to be optimized. For instance, with regard to the particle size, two limits should be considered: The bigger the single shape, the higher the probability for a possible limitation of the mass transfer within the shape. That means the diffusion within the single shape might be so slow that the active adsorptive matter is not completely used if the full adsorption/desorption cycle is very fast (e.g., in (vacuum) pressure swing processes [71]) [72]. But the smaller the single shapes, the larger the pressure drop over the fixed bed and the higher the risk of fluidization. Therefore, the adjustment of the size of the shapes is a critical parameter in the design of an adsorption process. Besides the size also other characteristics of the bulk material are to be adjusted such as mechanical properties such as dust formation caused by attrition/abrasion between shapes or the shapes and the wall of the adsorber vessel. Because dust may cause problems in a running adsorption process (e.g., blocking of valves), the dust formation has to be avoided by applying an optimal type and amount of binder as well as by generating single shapes with as smooth as possible surfaces. To receive smooth surfaces on single shapes of bulk material, the shaping process has to be run accurately. So, for example, in the case of extruded material an optimal plasticity of the extrusion mixture (pre-mixture), and in the case of agglomeration granulation, a post-granulation surface polishing step may help avoid/lower attrition/dust formation. Besides dusting, the mechanical stability (crush resistance) of the shapes has to be considered. The higher the bulk bed in the adsorber the bigger the weight and force that affects the lowest single shapes in the bed, which could finally result in a destruction of the shapes, and hence, generation of fines and an increased pressure drop. The binding mechanism of activated binder-containing zeolite molecular sieves is based on the generation of a network of binder material, wherein the zeolite crystals are embedded. **Figure 2** shows SEM pictures of the raw materials. The zeolite crystal agglomerates (top left) and the binder (top right) form a physically strong bound shape, which is demonstrated in **Figure 2** downright. As mentioned earlier, the use of (adsorption inert) binder material reduces the adsorption capacity zeolite molecular sieves by approximately the percentage of binder in the shape (see **Table 1**).


**Table 1.** Adsorption capacities for zeolite NaX (pure zeolite NaX powder and binder-containing bulk material with 82.5 wt% zeolite NaX and 17.5 wt% attapulgite).

In addition to the mechanical properties, the type and amount of binder (and the shaping process applied) have an essential influence on the structure of the secondary pore system of the shape—the part of the shape through which the transport of the molecules to and from the zeolites crystals within the shape is realized [66, 73]. Said secondary (or transport) pore system strongly influences the kinetics of the adsorbent, and hence, the adsorption process. So the zeolite NaX beads mentioned in **Table 1** show an attrition of <0.1 wt% (determined using Chemiewerk Bad Köstritz GmbH method) but relatively tight transport pores, which can lead to a slower diffusion within the zeolite beads. Using a lower amount of binder material, the adsorption capacity increases and the transport pores become slightly wider; see **Figure 3**. A faster diffusion is possible. But using a lower amount of binder material leads to an increased attrition value (0.2 wt%, determined using Chemiewerk Bad Köstritz GmbH method); for some applications that might be a high risk for an operating plant.

**Figure 3.** Hg intrusion curves of binder-containing zeolite NaX bulk material with 17.5 and 10.0 wt% of attapulgite binder.

#### **4. Binderless zeolite molecular sieves**

**Zeolite NaX powder**

31.5 26.5

Water adsorption capacity at 55% relative

82.5 wt% zeolite NaX and 17.5 wt% attapulgite).

N2 adsorption capacity at 1000 mbar, 25°C/cc(STP)/g 10.7 8.0 CO2 adsorption capacity at 333 mbar, 25°C/cc(STP)/g 120.7 95.4

applications that might be a high risk for an operating plant.

**Table 1.** Adsorption capacities for zeolite NaX (pure zeolite NaX powder and binder-containing bulk material with

In addition to the mechanical properties, the type and amount of binder (and the shaping process applied) have an essential influence on the structure of the secondary pore system of the shape—the part of the shape through which the transport of the molecules to and from the zeolites crystals within the shape is realized [66, 73]. Said secondary (or transport) pore system strongly influences the kinetics of the adsorbent, and hence, the adsorption process. So the zeolite NaX beads mentioned in **Table 1** show an attrition of <0.1 wt% (determined using Chemiewerk Bad Köstritz GmbH method) but relatively tight transport pores, which can lead to a slower diffusion within the zeolite beads. Using a lower amount of binder material, the adsorption capacity increases and the transport pores become slightly wider; see **Figure 3**. A faster diffusion is possible. But using a lower amount of binder material leads to an increased attrition value (0.2 wt%, determined using Chemiewerk Bad Köstritz GmbH method); for some

**Figure 3.** Hg intrusion curves of binder-containing zeolite NaX bulk material with 17.5 and 10.0 wt% of attapulgite

humidity, 25°C/wt%

96 Zeolites - Useful Minerals

binder.

**Binder-containing zeolite NaX beads**

> To avoid the above-mentioned disadvantages of binder-containing zeolite molecular sieve bulk materials such as adsorption capacity reduction by adsorption inert binders or influence of the secondary pore structure by the binder material, the so-called binderless zeolite molecular sieves were developed. There are different manufacturing procedures for binderless molecular sieves described in the open or patent literature. In most of those processes, the same shaping principles as mentioned earlier for the manufacturing of binder-containing zeolite molecular sieve are applied. They differ in the raw materials, respectively, the composition especially the type and amount of the so-called temporary binder—of the starting pre-mixture, or the conditions for the conversion of the temporary binder into zeolite matter. Important for the generation of binderless zeolite molecular sieve is that the temporary binder contains only those elements, which are present in the target zeolite matter; that means in most of the cases silicon, aluminum, and sodium, for example, for binderless molecular sieves of A, NaX, and NaY type (or mixtures thereof). Only for the preparation of binderless zeolite LSX-type molecular sieves, potassium-containing systems are used [74].

> Thus, it is possible to generate binderless zeolite shapes using temporary binder material such as kaolin as starting material [75]. A mixture of temporary binder material such as kaolin, metakaolin or silica, and zeolite powder or zeolite filter cake is also mentioned [76–83]. The conversion step can be a wet chemical [75, 79] or an at least partially autogenic thermal reaction [84].

> **Figure 4** shows the principle manufacturing procedure starting with shaping a pre-mixture using the above-mentioned technologies agglomeration granulation, spray granulation, or extrusion. Taggart [75] reported about shaping a mixture of kaolin and sodium hydroxide followed by drying of the shapes. It should be considered that kaolin has good binding properties but nearly no chemical reactivity. Due to that fact, the kaolin needs to be converted thermally (about 600 °C) into the chemically more reactive metakaolin. Depending on the target zeolite type, a suitable reaction solution comprising of sodium, aluminum, and/or silicon components is prepared. Usually, the shapes are aged and further processed in that reaction solution to convert the temporary binder into the desired zeolite matter. Finally, the shapes are washed, dried, and activated. Taggart mentioned that, due to the limited accessibility of the interior of the shapes by the mentioned reaction solution (blocked or too tight secondary pore system), the degree of the conversion of the temporary binder into zeolite matter, and hence, the adsorption capacity of the resulting material is limited, and can be enhanced if zeolite powder is used in the starting mixture. However, the mechanical stability of the resulting shapes is lower. Therefore, Howell et al. [85] propose the utilization of pore-forming agents (spacers), which are burned out in the following thermal treatment, thereby leaving space for the better accessibility of the interior of the shapes. Goytisolo et al. [76], Barrer et al. [77], Nozemack et al. [78], Brandt et al. [79], and Schumann et al. [80] described the manufac‐ turing of binderless zeolite molecular sieves using a mixture of metakaolin, zeolite powder of such zeolite type, in which the metakaolin should be converted into, and additional sodium and silicon compounds such as sodium hydroxide, silicic acid or sodium silicate as feed for

the shaping process. The zeolite powder in the mixture has obviously crystallization triggering properties and supports the generation of an open, for the reaction solution accessible pore system. It has to be considered that metakaolin has practically no binding properties. Thus, one has to make sure, that the process is carried out in a way, that the shapes, which are coming out of the shaping process, are mechanically stable enough until the conversion of the non-

**Figure 4.** Scheme of the principle manufacturing procedure of binderless zeolite molecular sieve shapes (bulk materi‐ al).

zeolitic compounds into zeolite matter in the reaction solution is completed. Said conversion is the basis for the mechanical stability of binderless molecular sieve shapes (see below). If it is possible to put all necessary compounds into the starting mixture for the shaping the reaction solution can be water [77, 78], otherwise all missing components for the desired zeolite formation have to be present in the reaction solution [75, 79, 80]. Another way to manufacture binderless zeolite molecular sieves is the use of silica as synthetic temporary binder. The shapes obtained are aged to achieve a certain water stability [81] followed by the conversion of the temporary binder in a solution consisting of aluminum and sodium components [82, 83].

the shaping process. The zeolite powder in the mixture has obviously crystallization triggering properties and supports the generation of an open, for the reaction solution accessible pore system. It has to be considered that metakaolin has practically no binding properties. Thus, one has to make sure, that the process is carried out in a way, that the shapes, which are coming out of the shaping process, are mechanically stable enough until the conversion of the non-

**Figure 4.** Scheme of the principle manufacturing procedure of binderless zeolite molecular sieve shapes (bulk materi‐

al).

98 Zeolites - Useful Minerals

The wet chemical reaction for the conversion of the temporary binder is preferably carried out by recirculating the reaction solution over the bulk material at suitable temperatures without moving the single shapes in order to avoid attrition between the still relatively soft shapes.

In the course of the chemical conversion of the non-zeolitic components into zeolite matter, the binding mechanism changes. In the case of kaolin/metakaolin-based systems, the mechan‐ ical stability of the shapes is now based on intergrowths between zeolite crystals in the single shape (see **Figure 4** bottom). Said intergrowths are formed during the conversion of the metakaolin into a polycrystalline zeolite matter [86]. Such unusual crystallization behavior can be explained by considering the available space for crystallization in a shape. On the outer surface of a single shape, there is enough space for a conventional and epitaxial crystal growth (see **Figure 4** down left for zeolite NaX), whereas in the interior, the space is obviously limited in a way that typical zeolite crystals such as octahedrons or cubes with rounded corners cannot be formed, but polycrystalline structures consisting of zeolite in untypical shape only (see **Figure 4** bottom right for zeolite NaX) [86]. After the chemical conversion, the bulk material is separated from the mother liquor (which can reused for further reactions) and washed until a desired pH or conductivity of the effluent is achieved. Optionally, an ion exchange (applying a suitable ion exchange solution) can be affiliated. Finally, the material is dried and thermally activated in a suitable device (e.g., rotary kiln or belt calciner). As the mechanical stability of binderless molecular sieve shapes is already achieved during the temporary binder conversion step said thermal treatment is just needed to activate the zeolite, and not anymore to set a binder component. Thus, usually lower temperatures can be applied, which saves costs and is more gentle to the thermally sensitive zeolite structure. Despite the different manufacturing procedures of binder-containing and binderless zeolite bulk material, the mechanical stability of the resulting shapes is similar. In the case products produced according to [79] and [80], a higher mechanical strength for bigger binderless zeolite spheres in comparison with bindercontaining zeolite spheres of the same size and zeolite type is observed. Using comparable analytical methods for both kinds of zeolite bulk material, the attrition/dust formation is also similar.

As can be shown exemplary in **Table 2**, the static adsorption capacity of binderless zeolite NaX bulk material is similar to that of pure zeolite NaX powder. That indicates that the binderless zeolite bulk material consists of nearly 100% of zeolite matter.


**Table 2.** Adsorption capacities for zeolite NaX (pure zeolite NaX powder, binder-containing bulk material with 82.5 wt% zeolite NaX and 17.5 wt% attapulgite and binderless zeolite NaX bulk material).

A further interesting observance is, that in the above-described special manufacturing process for binderless molecular sieves, a very open secondary pore system is generated, which allows faster kinetics as compared to conventional binder-containing zeolite molecular sieves with the (usual) binder content of around 20% [86], **Figure 5** proves the presence of a larger amount of macro pores. While the ratio of meso-pore to the sum of meso- and macro-pores is 30% for conventional binder-containing zeolite molecular sieves (pre-mixture consisting only of zeolite and attapulgite) the same ratio is only 5% for binder-less zeolite molecular sieves (produced according to [80]) [66].

**Figure 5.** Hg intrusion curves of binder-containing and binderless zeolite NaY bulk material.

Due to the mentioned open secondary pore system, the above-mentioned problems concerning the inner shape transport limitations in binder-containing shapes depending on the size are eased in the case of binderless shapes.

It should, however, besides all obvious advantages of binderless over binder-containing molecular sieves mentioned earlier, noted, that, at least in the case of the kaolin/metakaolinbased systems, the structure of the binderless shapes is more rigid (less flexible) than the one of related binder-containing shapes. This can easily be explained by the different binding mechanisms: intergrowths in the case of the binderless structures, embedment in the case of binder-containing structures. This is a fact which must not be underestimated and needs to be taken into consideration especially in applications, which are connected with structural compressions/expansions (e.g., as a result of fast heat release and/or lattice changes by adsorption/desorption).

#### **5. Compact zeolite bodies**

**Zeolite NaX powder**

N2 adsorption capacity at 1000 mbar, 25°C/cc(STP)/g 10.7 8.0 10.5 CO2 adsorption capacity at 333 mbar, 25°C/cc(STP)/g 120.7 95.4 119.4

82.5 wt% zeolite NaX and 17.5 wt% attapulgite and binderless zeolite NaX bulk material).

**Figure 5.** Hg intrusion curves of binder-containing and binderless zeolite NaY bulk material.

eased in the case of binderless shapes.

Due to the mentioned open secondary pore system, the above-mentioned problems concerning the inner shape transport limitations in binder-containing shapes depending on the size are

**Table 2.** Adsorption capacities for zeolite NaX (pure zeolite NaX powder, binder-containing bulk material with

A further interesting observance is, that in the above-described special manufacturing process for binderless molecular sieves, a very open secondary pore system is generated, which allows faster kinetics as compared to conventional binder-containing zeolite molecular sieves with the (usual) binder content of around 20% [86], **Figure 5** proves the presence of a larger amount of macro pores. While the ratio of meso-pore to the sum of meso- and macro-pores is 30% for conventional binder-containing zeolite molecular sieves (pre-mixture consisting only of zeolite and attapulgite) the same ratio is only 5% for binder-less zeolite molecular sieves (produced

Water adsorption capacity at 55% relative humidity,

25°C/wt%

100 Zeolites - Useful Minerals

according to [80]) [66].

**Binder-containing zeolite NaX beads**

31.5 26.5 31.2

**Binderless zeolite NaX beads**

> In order to improve the overall effectivity of a given adsorption process, the increase of the volume-related adsorption capacity of the adsorbent used is often desirable. Said volumerelated adsorption capacity is depending on the (bulk) density of the bulk material and on the amount of adsorption active matter within the shape (binder-containing or binderless zeolite bulk material). The bulk density itself depends on the density of the single shape(s) and on the void space between the shapes. It is known, that, for example, the packing density of a closepacking of spheres of equal diameter is ca. 74%; thus, the void space is about 26%. Theoretically, an increase of the packing density (and thus, of the bulk density) can be achieved by mixing spheres of optimal adjusted sizes, but it is certainly very difficult and expensive to produce and arrange such bed consisting of spheres of optimal sizes, the more, as the influence of the packing density on the pressure drop has always to be considered. An alternative to dense packed bulk beds can be compact zeolite bodies such as multi-channel tubes or honeycombs. By choosing appropriate geometries of such compact zeolite bodies with optimal channel dimensions a space filling (packing density) of much more, the mentioned 74% (theoretically up to 100% in the case of a void free compact body) is possible. In addition, such compact bodies can be modified in many ways to meet the requirements of the related adsorption process. Thus, a gas flow can be guided straight through the parallel arranged channels of a honeycomb-like structure, but through alternately blocking of the channels, a compulsorily perfusion is also possible [35].

> Most of the manufacturing procedures of such compact zeolite bodies described in the literature are based on the use of (adsorption inert) binder material. As in the above-mentioned cases of binder-containing zeolite bulk material, synthetic binder [87] and mineral binder materials [88] are applied in the case of compact shapes as well. Consequently, the same disadvantageously impacts on the properties of the related adsorbents need to be considered: The introduction of the mentioned binder material causes a dilution of the adsorptive active matter and consequentially a reduction of adsorption capacities, and the binder material could affect the desired adsorption and desorption processes by forming a disadvantageous secondary pore system [66], an effect which at least partly can be cured by applying thermally removable pore-forming substances [89], and the fact, that impurities present in mineral binders may support undesired chemical side reactions.

At that point, it seems to be worth to remember, what was discussed above in this regard—to overcome the disadvantages of binder-containing molecular sieves bulk material. The solution (at least to a certain extent) was the application of binderless systems. And recently it could clearly be demonstrated that the same principle concept works in the case of zeolite-containing compact bodies as well: A high volume-related adsorption capacity can be reached [35]. The related honeycombs, multi-channel tubes, cylinder, etc. exhibit a remarkable adjustable high space filling (up to 100%), but at the same time a similar open secondary pore system such as mentioned earlier for binderless zeolite bulk material [90, 35].

For both types of compact zeolite bodies (binder-containing and binderless structures), the production process starts with mixing the ingredients zeolite powder, (temporary) binder, water and if needed shaping aids (such as e.g., glycerol or cellulose derivates) or in case of binder-containing compact bodies pore-forming substances if necessary (see **Figure 1**). Shaping technologies can be extrusion, pressing, or molding—depending on the desired geometry of the compact zeolite body. It should be noted that in the case of the binderless systems, the application of kaolin instead of metakaolin as main non-zeolitic component has been shown as being advantageous. In case of the binder-containing compact zeolite bodies, the following and final manufacturing step is the thermal treatment for drying and activation, where the organic compounds are burned out as well as the all water and other adsorbed compounds present in the system is removed and the binder is set. In case of binderless compact zeolite bodies, a first thermal treatment is needed for removing the organic shaping aids and converting kaolin into metakaolin [35, 91]. The generated metakaolin is converted into zeolite during a following wet chemical reaction. The related reaction solution consists of all ingredients necessary for the conversion of the metakaolin into the desired zeolite type (A, X or NaY) similar to the manufacturing of binderless zeolite bulk material. The conversion can be followed by further modification processes, for example, ion exchange. The completeness of the conversion of the non-zeolitic into zeolite matter can be demonstrated, for example, by adsorption measurements (in combination with XRD [35]). The static water adsorption capacity for zeolite 4A honeycombs in comparison with the fully crystalline zeolite 4A powder is shown in **Table 3** proving that the honeycomb consists of nearly 100% zeolite 4A matter.


**Table 3.** Adsorption capacities for zeolite 4A (pure zeolite 4A powder and binderless zeolite 4A honeycomb).

Just for completion it should be noted that compact zeolite-containing bodies can also be manufactured by washcoating [92, 93], where mainly for catalytic applications, the catalyti‐ cally active zeolite component is deposited onto monolithic support structures, as, for exam‐ ple, ceramic (in most cases cordierite) honeycombs by washcoating. The result is a compact body with a thin zeolite layer. Furthermore, the crystallization of zeolite matter directly on

aluminum foams is published [94]. Such structures exhibit interesting flow characteristics, but due to the rather large void fraction and a relatively thin zeolite layer onto the alumi‐ num support, a low volume-related zeolite amount, and hence, a low volume-related ad‐ sorption capacity is observed.

#### **6. Conclusion**

At that point, it seems to be worth to remember, what was discussed above in this regard—to overcome the disadvantages of binder-containing molecular sieves bulk material. The solution (at least to a certain extent) was the application of binderless systems. And recently it could clearly be demonstrated that the same principle concept works in the case of zeolite-containing compact bodies as well: A high volume-related adsorption capacity can be reached [35]. The related honeycombs, multi-channel tubes, cylinder, etc. exhibit a remarkable adjustable high space filling (up to 100%), but at the same time a similar open secondary pore system such as

For both types of compact zeolite bodies (binder-containing and binderless structures), the production process starts with mixing the ingredients zeolite powder, (temporary) binder, water and if needed shaping aids (such as e.g., glycerol or cellulose derivates) or in case of binder-containing compact bodies pore-forming substances if necessary (see **Figure 1**). Shaping technologies can be extrusion, pressing, or molding—depending on the desired geometry of the compact zeolite body. It should be noted that in the case of the binderless systems, the application of kaolin instead of metakaolin as main non-zeolitic component has been shown as being advantageous. In case of the binder-containing compact zeolite bodies, the following and final manufacturing step is the thermal treatment for drying and activation, where the organic compounds are burned out as well as the all water and other adsorbed compounds present in the system is removed and the binder is set. In case of binderless compact zeolite bodies, a first thermal treatment is needed for removing the organic shaping aids and converting kaolin into metakaolin [35, 91]. The generated metakaolin is converted into zeolite during a following wet chemical reaction. The related reaction solution consists of all ingredients necessary for the conversion of the metakaolin into the desired zeolite type (A, X or NaY) similar to the manufacturing of binderless zeolite bulk material. The conversion can be followed by further modification processes, for example, ion exchange. The completeness of the conversion of the non-zeolitic into zeolite matter can be demonstrated, for example, by adsorption measurements (in combination with XRD [35]). The static water adsorption capacity for zeolite 4A honeycombs in comparison with the fully crystalline zeolite 4A powder is shown in **Table 3** proving that the honeycomb consists of nearly 100% zeolite 4A matter.

> **Zeolite 4A powder**

**Table 3.** Adsorption capacities for zeolite 4A (pure zeolite 4A powder and binderless zeolite 4A honeycomb).

Just for completion it should be noted that compact zeolite-containing bodies can also be manufactured by washcoating [92, 93], where mainly for catalytic applications, the catalyti‐ cally active zeolite component is deposited onto monolithic support structures, as, for exam‐ ple, ceramic (in most cases cordierite) honeycombs by washcoating. The result is a compact body with a thin zeolite layer. Furthermore, the crystallization of zeolite matter directly on

24.8 24.7

**Binderless**

**zeolite 4A honeycomb**

mentioned earlier for binderless zeolite bulk material [90, 35].

Water adsorption capacity at 55% relative

humidity, 25°C/wt%

102 Zeolites - Useful Minerals

Zeolite molecular sieves play an important role as highly effective adsorbents in major industrial adsorption processes. The properties of such molecular sieves can be tailored according to the requirements of the related applications. Besides the common bindercontaining molecular sieves, binderless grades are applied. Due to the particular binding mechanism and the related special secondary pore system, binderless molecular sieves are mainly applied where fast kinetics is required, thus, especially in (vacuum) pressure swing adsorption processes. Furthermore, the principle concept for the manufacturing of binderless zeolite bulk material can be applied to the manufacturing of compact zeolite bodies, too. This opens ways for a further improvement of the related adsorption processes.

#### **Acknowledgements**

The authors gratefully acknowledge Gundula Fischer, Hannes Richter, and Andreas Häusler from Fraunhofer IKTS Hermsdorf, Germany for the contribution to this work.

#### **Author details**

Kristin Gleichmann\* , Baldur Unger and Alfons Brandt

\*Address all correspondence to: k.gleichmann@cwk-bk.de

Chemiewerk Bad Köstritz GmbH, Bad Köstritz, Germany

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### **Hollow Titanium Silicalite Zeolite: From Fundamental Research to Commercial Application in Environmental-Friendly Catalytic Oxidation Processes**

Changjiu Xia, Min Lin, Bin Zhu, Yibin Luo and Xingtian Shu

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63699

#### **Abstract**

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No. 12, 2237–2243. doi:10.1002/cite.201100151

2012

108 Zeolites - Useful Minerals

The systematical investigation on the synthesis, characterization, formation mecha‐ nism, and catalytic application of hollow titanium silicalite (HTS) zeolite has been reviewed. HTS is prepared through a "dissolution–recrystallization" post-treatment in the presence of template under hydrothermal conditions. Compared with TS-1, HTS is of unique hollow voids and with high framework Ti content, which significantly increase the mass diffusion and the amount of active sites, respectively. Thus, HTS zeolite displays high catalytic activity and stability in many oxidation processes with H2O2 oxidant, that is, cyclohexanone ammoximation, phenol hydroxylation, propy‐ lene epoxidation, Baeyer-Villiger oxidation of cyclohexanone, and selective oxidation of aromatics and cycloalkanes. The former three ones have been commercialized and run smoothly, which have promising economic and environmental significance.

**Keywords:** Hollow Titanium Silicate, Environmental friendly, Post synthesis, Selective oxidation, Commercialization

#### **1. Introduction**

Zeolites are regular microporous materials composed by TO4 tetrahedra (T stands for Al, Si, Sn, Ti, and B) through sharing the interconnected oxygen atoms in the framework positions. They possess abundant of ordered channels and cages within molecular dimensional range, which can reversibly uptake the molecules (smaller than 1 nm) depending on the structure topology

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

[1–3]. Aluminosilicate zeolites display Brönsted acidity, due to the isomorphous substitution of tetrahedral Si atom by trivalent Al atoms in the framework positions, needing one proton ion to keep electron charge balance. As a result, Si–Al zeolites are widely used as acid catalysts, adsorbents, and ion-exchanges in petroleum refining and chemical manufacture [4–6]. Furthermore, to extend the application scope of zeolite, transition metal ions have been incorporated into the framework sites, with the endowment of redox property [7–9]. Up to now, the MFI-typed titanium silicalite-1 (TS-1) zeolite is the most successful and profoundly investigated one in the area of isomorphously substituted heteroatom zeolites. Since it is originally claimed by Enichem in 1983, TS-1 zeolite has attracted intensive attentions from both academic and industrial practices, owing to its high catalytic performance in hydrocarbon oxidation process. That is because the framework Ti species can enhance the nucleophilicity capability of H2O2 molecules to attack electrophilic groups during heterogeneous catalysis, through the acceptor–donator interaction between the empty 3*d* orbits of tetrahedral Ti species and electron pairs in H2O2 molecules [10–13]. Compared with traditional oxidation processes, TS-1 catalyzed routes are much more environmental friendly, with H2O as the only side product. For example, the one-step cyclohexanone ammoximation approach by direct reacting with H2O2 and NH3 has already replaced the conventionally four-step hydroxylamine routes in commer‐ cial ε-caprolactam production units [14–17].

However, at the early stage, TS-1 zeolite usually exhibited poor catalytic activity and repro‐ ducibility, due to the mismatch of hydrolysis and crystallization rates between Si and Ti spe‐ cies under hydrothermal conditions. To solve this problem, hollow titanium silicate (HTS) zeolite was developed by Lin and his coworkers at SINOPEC in 1990s, through a post-syn‐ thesis treatment in the presence of organic structure directing agents (OSDAs) [18–21]. It is found that HTS zeolite is of unique hollow cavities inside crystal, which are preferentially to enhance the mass diffusion property of reagents and products during catalytic reactions. Be‐ sides, the post-treatment promotes the hydrated condensation between Ti-OH and Si-OH groups, forming much more framework Ti species. Thus, HTS zeolite shows much higher catalytic performance than TS-1 zeolite in many oxidation reactions, such as phenol hydrox‐ ylation, cyclohexanone ammoximation, and propylene epoxidation and aromatic oxidation. On the other hand, HTS zeolite has its own intellectual property, which is very important to overcome patent restriction for its commercial production. Since 2000, SINOPEC has suc‐ cessfully completed the commercialization of HTS zeolite at a scale of 100 t/a. After that, HTS zeolite was initially used as catalyst for the environmental friendly cyclohexanone am‐ moximation process in the 140 kt/a commercial manufacture of ε-caprolactam at Baling Branch, SINOPEC, to replace the four-step HPO process, with low formation of toxic waste and side-products [22–25]. Thus, this route has been considered as one of the most impor‐ tant cases in green chemistry, due to its high atom utilization efficiency and low source con‐ sumption. Moreover, the HTS zeolite has also been employed in other industrial oxidation reactions, such as phenol hydroxylation and propylene epoxidation, with low concentration H2O2 solution as oxidant. Very recently, the 100 kt/a industrial demonstration plant of pro‐ pylene epoxidation has been run smoothly, with high propylene oxide yield and H2O2 con‐ version.

Above all, HTS zeolite shows interesting physicochemical properties and great performance in the catalytic oxidation process, and we have done abundant of fundamental investigations to understand the basic principles in this topic, such as the hollow cavity characterization, chemical status of Ti species and the formation mechanism of constricted hierarchical struc‐ ture. However, until now, the synthesis, characterization and catalytic application of HTS zeolite is still not reviewed systematically, especially for its catalytic performance in industry. Herein, we try to describe the main achievements on all aspects of HTS zeolite in brief, and we do hope it can provide a novel view for modifying zeolite materials to meet specific needs.

### **2. Synthesis and characterization of HTS zeolite**

[1–3]. Aluminosilicate zeolites display Brönsted acidity, due to the isomorphous substitution of tetrahedral Si atom by trivalent Al atoms in the framework positions, needing one proton ion to keep electron charge balance. As a result, Si–Al zeolites are widely used as acid catalysts, adsorbents, and ion-exchanges in petroleum refining and chemical manufacture [4–6]. Furthermore, to extend the application scope of zeolite, transition metal ions have been incorporated into the framework sites, with the endowment of redox property [7–9]. Up to now, the MFI-typed titanium silicalite-1 (TS-1) zeolite is the most successful and profoundly investigated one in the area of isomorphously substituted heteroatom zeolites. Since it is originally claimed by Enichem in 1983, TS-1 zeolite has attracted intensive attentions from both academic and industrial practices, owing to its high catalytic performance in hydrocarbon oxidation process. That is because the framework Ti species can enhance the nucleophilicity capability of H2O2 molecules to attack electrophilic groups during heterogeneous catalysis, through the acceptor–donator interaction between the empty 3*d* orbits of tetrahedral Ti species and electron pairs in H2O2 molecules [10–13]. Compared with traditional oxidation processes, TS-1 catalyzed routes are much more environmental friendly, with H2O as the only side product. For example, the one-step cyclohexanone ammoximation approach by direct reacting with H2O2 and NH3 has already replaced the conventionally four-step hydroxylamine routes in commer‐

However, at the early stage, TS-1 zeolite usually exhibited poor catalytic activity and repro‐ ducibility, due to the mismatch of hydrolysis and crystallization rates between Si and Ti spe‐ cies under hydrothermal conditions. To solve this problem, hollow titanium silicate (HTS) zeolite was developed by Lin and his coworkers at SINOPEC in 1990s, through a post-syn‐ thesis treatment in the presence of organic structure directing agents (OSDAs) [18–21]. It is found that HTS zeolite is of unique hollow cavities inside crystal, which are preferentially to enhance the mass diffusion property of reagents and products during catalytic reactions. Be‐ sides, the post-treatment promotes the hydrated condensation between Ti-OH and Si-OH groups, forming much more framework Ti species. Thus, HTS zeolite shows much higher catalytic performance than TS-1 zeolite in many oxidation reactions, such as phenol hydrox‐ ylation, cyclohexanone ammoximation, and propylene epoxidation and aromatic oxidation. On the other hand, HTS zeolite has its own intellectual property, which is very important to overcome patent restriction for its commercial production. Since 2000, SINOPEC has suc‐ cessfully completed the commercialization of HTS zeolite at a scale of 100 t/a. After that, HTS zeolite was initially used as catalyst for the environmental friendly cyclohexanone am‐ moximation process in the 140 kt/a commercial manufacture of ε-caprolactam at Baling Branch, SINOPEC, to replace the four-step HPO process, with low formation of toxic waste and side-products [22–25]. Thus, this route has been considered as one of the most impor‐ tant cases in green chemistry, due to its high atom utilization efficiency and low source con‐ sumption. Moreover, the HTS zeolite has also been employed in other industrial oxidation reactions, such as phenol hydroxylation and propylene epoxidation, with low concentration H2O2 solution as oxidant. Very recently, the 100 kt/a industrial demonstration plant of pro‐ pylene epoxidation has been run smoothly, with high propylene oxide yield and H2O2 con‐

cial ε-caprolactam production units [14–17].

110 Zeolites - Useful Minerals

version.

#### **2.1. Synthesis of HTS zeolite through the "dissolution-recrystallization" treatment**

TS-1 zeolite is the first crystalline transition metal inserted microporous materials (also referred to as heteroatom zeolites). As reported by Taramasso et al., TS-1 zeolite was synthesized using tetraethyl orthosilicate (TEOS), tetrapropylammonium hydroxide (TPAOH), and tetrabutyl orthotitanate (TBOT) as raw materials, under hydrothermal conditions and autogeneous pressure [18, 20]. The high pH value of TPAOH favors the existence of isolated Ti species coordinated by low aggregated Si species. The most important recipe for preparing particularly active TS-1 zeolite lies in the accurate control of the hydrolysis of Ti and Si sources in aqueous alkaline solution, to promote the formation of Si–O–Ti bonds, rather than Ti–O–Ti bonds. Although many alternative synthesis routes had been proposed by researchers over the past 30 years, except for Enichem Company, it is still difficult to prepare the active and stable TS-1 zeolite through direct method [26–29]. Consequently, to improve the catalytic performance of TS-1 zeolite, a novel post-synthesis treatment method has been designed by the researchers at SINOPEC in 1990s. In a typical procedure, the parent TS-1 zeolite is synthesized following the classic Enichem route as previously mentioned; and then the as-made TS-1 zeolite is calcined at high temperature in air to remove the organic templates inside micropores. In the next step, the calcined TS-1 zeolite is washed by specific additive agents, for trying to remove the extraframework Ti species (majorly in the form of anatase TiO2); after that, this TS-1 zeolite is mixed with aqueous TPAOH solution, and sealed in a Teflon autoclave. Then, the autoclave is continuously heated at high temperature (usually over 120°C) under statistic state for several days, with the formation of white HTS zeolite powder. It is worthy noted that the as-made TS-1 zeolite, which containing TPAOH template, cannot be used as precursor to prepare HTS zeolite treated under the same procedure as calcined TS-1 zeolite. Thus, it is inferred that part of TS-1 zeolite is dissolved in the alkaline solution, and the soluble Si and Ti species are recrystallized under the impact of TPAOH molecules during the post-synthesis processing. Most importantly, this post-synthesis treatment is very convenient and available in both laboratory and industry, on which allows the commercial manufacture of HTS zeolite at large scale.

#### **2.2. Confirmation of hollow cavities and reincorporation of Ti species**

It is well known that the intracrystalline porosity is of great importance to enhance the catalytic performance of zeolites during heterogeneous reactions [30–32]. Different from TS-1 zeolite, HTS zeolite shows a typical H2 hysteresis loop at the region of P/P0 between 0.45 and 1 in the N2 physisorption curve (**Figure 1**), which suggests the existence of ink-bottle alike hierarchical pores. It is attributed to the tensile strength effect, owing to the difference between capillary condensation for adsorption and capillary evaporation for desorption. The hysteresis loop starting from P/P0 = 0.45 usually reveals the constricted mesopores and/or macropores inside zeolite crystal [33–35]. Thus, to further confirm the feature of hierarchical pores, transmission electron microscopy (TEM) method has been used, as illustrated in **Figure 2**. Obviously, abundant of hollow cavities (around several to tens nm in size) have been observed in the HTS zeolite, while the conventional TS-1 zeolite is completely solid. Moreover, to learn the three dimensional distribution of hollow cavities, HTS zeolite was characterized by using electron tomography (ET) technology [36–38]. As displayed in **Figure 2b** (insert), intracrystalline cavities are highly dispersed inside HTS zeolite with different size and shape. Consequently, we can infer that intracrystalline pores have been created via the post-synthesis treatment in the presence of template under hydrothermal conditions. Herein, it is noted that HTS zeolite is still of high crystallinity and uniform morphology, which are favor of the high stability during catalytic process. Therefore, this post-synthesis route takes much more advantages than base/acid leaching method and provides a novel strategy to design and modify zeolite materials for meeting specific demands in heterogeneous catalysis.

**Figure 1.** Low-temperature N2 adsorption-desorption curves of HTS and TS-1 zeolites.

Hollow Titanium Silicalite Zeolite: From Fundamental Research to Commercial Application in Environmental-Friendly Catalytic Oxidation Processes http://dx.doi.org/10.5772/63699 113

**2.2. Confirmation of hollow cavities and reincorporation of Ti species**

112 Zeolites - Useful Minerals

materials for meeting specific demands in heterogeneous catalysis.

**Figure 1.** Low-temperature N2 adsorption-desorption curves of HTS and TS-1 zeolites.

It is well known that the intracrystalline porosity is of great importance to enhance the catalytic performance of zeolites during heterogeneous reactions [30–32]. Different from TS-1 zeolite, HTS zeolite shows a typical H2 hysteresis loop at the region of P/P0 between 0.45 and 1 in the N2 physisorption curve (**Figure 1**), which suggests the existence of ink-bottle alike hierarchical pores. It is attributed to the tensile strength effect, owing to the difference between capillary condensation for adsorption and capillary evaporation for desorption. The hysteresis loop starting from P/P0 = 0.45 usually reveals the constricted mesopores and/or macropores inside zeolite crystal [33–35]. Thus, to further confirm the feature of hierarchical pores, transmission electron microscopy (TEM) method has been used, as illustrated in **Figure 2**. Obviously, abundant of hollow cavities (around several to tens nm in size) have been observed in the HTS zeolite, while the conventional TS-1 zeolite is completely solid. Moreover, to learn the three dimensional distribution of hollow cavities, HTS zeolite was characterized by using electron tomography (ET) technology [36–38]. As displayed in **Figure 2b** (insert), intracrystalline cavities are highly dispersed inside HTS zeolite with different size and shape. Consequently, we can infer that intracrystalline pores have been created via the post-synthesis treatment in the presence of template under hydrothermal conditions. Herein, it is noted that HTS zeolite is still of high crystallinity and uniform morphology, which are favor of the high stability during catalytic process. Therefore, this post-synthesis route takes much more advantages than base/acid leaching method and provides a novel strategy to design and modify zeolite

**Figure 2.** (a) TEM image of conventional TS-1 zeolite, (b) TEM image of HTS zeolite, and the restructured image of hollow cavities (insert).

Besides the formation of hollow cavities, the reincorporation of Ti species into framework positions has also been found in our previous work. Firstly, the chemical status of Ti species in the HTS zeolite has been distinguished by X-ray photoelectron spectroscopy (XPS) through peak fitting treatment [39, 40]. Apparently, two kinds of Ti species, referred to as framework Ti and extraframework Ti species, have been detected in the external surface of HTS zeolite (as presented in **Figure 3**). That is because the chemical environmental of Ti atoms in the framework positions are different from that in the extraframework Ti species, especially for octahedral anatase TiO2 crystal.

Additionally, it is widely accepted that the band at around 960 cm−1 in the FT-IR spectrum is attributed to the stretching vibration of Si–O–Ti bonds, indicating the incorporation of tetrahedral framework Ti species. And the band intensity ratio between 960 and 800 cm−1 (I960/ I800) is relevant with the framework Ti content [41, 42]. Thus, it is concluded more tetrahedral Ti species have been inserted into framework sites after the post-synthesis treatment, due to the higher I960/I800 value of HTS than that of TS-1 zeolite (as shown in **Figure 4**). Furthermore, to verify the reincorporation of Ti species, 29Si MAS NMR spectroscopy has been employed to detect the chemical environmental of Si species, which can indirectly give the information on the chemical status of Ti species. Generally, the bands at around −113 and −116 ppm are ascribed to the framework Si(OSi)4 and Si(OTi)(OSi)3 Species, respectively, owing to induction effect of framework Ti atoms substituted Si atoms [43–45]. As shown in **Figure 5**, HTS zeolite has a larger peak at −116 ppm than TS-1 zeolite, and thus it is well demonstrated that "disso‐ lution-recrystallization" treatment favors the reincorporation of tetrahedral Ti species into framework sites.

**Figure 3.** XPS spectrum of HTS zeolite with peak fitting to confirm framework and extraframework Ti species.

**Figure 4.** FT-IR spectra of both TS-1 and HTS zeolites.

Hollow Titanium Silicalite Zeolite: From Fundamental Research to Commercial Application in Environmental-Friendly Catalytic Oxidation Processes http://dx.doi.org/10.5772/63699 115

**Figure 5.** 29Si MAS NMR spectra of both TS-1 and HTS zeolites.

**Figure 3.** XPS spectrum of HTS zeolite with peak fitting to confirm framework and extraframework Ti species.

**Figure 4.** FT-IR spectra of both TS-1 and HTS zeolites.

114 Zeolites - Useful Minerals

#### **2.3. The formation mechanism of hollow voids inside HTS zeolite**

The detailed formation mechanism of hollow cavities inside zeolite is still not fully understood, but huge investigations on this post-synthesis treatment have been carried out. To certify the "dissolution-recrystallization" process, the as-made (organic template containing) TS-1 zeolite was treated by using three different methods, as following: (a) the as-made TS-1 zeolite was post-synthesized in the TPAOH solution at high temperature; (b) the as-made TS-1 zeolite was calcined to remove organic template, and then treated in the TPAOH solution at 80°C in air;

**Figure 6.** Low-temperature N2 adsorption–desorption curves of different titanium containing zeolites.

(c) the as-made TS-1 zeolite was calcined to remove organic template, and then postsynthesized in the TPAOH solution at 170°C under autogeneous pressure. **Figures 6** and **7** show the N2 physisorption curve and TEM characterization results of these mentioned samples. We can observe that only the calcined TS-1 zeolite can be used as starting material to prepare HTS zeolite under hydrothermal conditions.

**Figure 7.** TEM images of different titanium containing zeolites.

On the formation mechanism of hollow cavities, Mielby et al. has ever used cetyltrimethy‐ lammonium bromide (CTAB) alkaline solution as organic structure directing agent (OSDA) to produce the Silicalite-1 zeolite with hollow voids [46–48]. They consider that the CTAB molecules can enter into the bulk body of zeolite to form micelles, which severs as the meso‐ porous template for hollow structure via directing the soluble Si and Ti species dissolved from zeolite crystal. However, this viewpoint is not fully reasonable enough to demonstrate the generation of hollow cavities inside HTS zeolite, owing to the larger molecular diameter of TPAOH than that of CTAB molecules. On the other hand, CTAB is of poor capability to cause the formation of MFI topology. Based on these mechanistic models, one plausible mechanism has been proposed to explain the formation of hollow cavities inside zeolite dur‐ ing "dissolution–recrystallization" process, but it still need further detailed analysis results to support it. We consider that, in the initial stage, the calcined TS-1 zeolite can be dissolved in the TPAOH solution to, owing the hydrolysis of T–O–T bonds (T stands for the frame‐ work atoms) catalyzed by OH<sup>−</sup> anions in both external surface and bulk body of zeolite. It is worthy to note that only OH<sup>−</sup> anions, rather than TPA+ ions, go into zeolite body along mi‐ croporous channels. Due to the absence of TPA+ ions, the recrystallization of soluble Ti and Si sources cannot be occurred, without the formation of new TS-1 inside zeolite crystal. At the same time, the "dissolution–recrystallization" process happens in the external surface, with the changing of zeolite morphology. That is why the hollow voids cannot be produced for the post-synthesis of template-containing TS-1 zeolite, which is ascribed to the restriction

of mass transfer of OH<sup>−</sup> group within zeolite. Above all, this proposed mechanism provides a novel view for describing the formation hollow structure, which is in favor of the inner diffusion of raw materials and products during catalytic oxidation processing [49–51].

#### **3. Applications of HTS zeolite in catalytic oxidation processes**

#### **3.1. Cyclohexanone ammoximation process**

(c) the as-made TS-1 zeolite was calcined to remove organic template, and then postsynthesized in the TPAOH solution at 170°C under autogeneous pressure. **Figures 6** and **7** show the N2 physisorption curve and TEM characterization results of these mentioned samples. We can observe that only the calcined TS-1 zeolite can be used as starting material to

On the formation mechanism of hollow cavities, Mielby et al. has ever used cetyltrimethy‐ lammonium bromide (CTAB) alkaline solution as organic structure directing agent (OSDA) to produce the Silicalite-1 zeolite with hollow voids [46–48]. They consider that the CTAB molecules can enter into the bulk body of zeolite to form micelles, which severs as the meso‐ porous template for hollow structure via directing the soluble Si and Ti species dissolved from zeolite crystal. However, this viewpoint is not fully reasonable enough to demonstrate the generation of hollow cavities inside HTS zeolite, owing to the larger molecular diameter of TPAOH than that of CTAB molecules. On the other hand, CTAB is of poor capability to cause the formation of MFI topology. Based on these mechanistic models, one plausible mechanism has been proposed to explain the formation of hollow cavities inside zeolite dur‐ ing "dissolution–recrystallization" process, but it still need further detailed analysis results to support it. We consider that, in the initial stage, the calcined TS-1 zeolite can be dissolved in the TPAOH solution to, owing the hydrolysis of T–O–T bonds (T stands for the frame‐

worthy to note that only OH<sup>−</sup> anions, rather than TPA+ ions, go into zeolite body along mi‐ croporous channels. Due to the absence of TPA+ ions, the recrystallization of soluble Ti and Si sources cannot be occurred, without the formation of new TS-1 inside zeolite crystal. At the same time, the "dissolution–recrystallization" process happens in the external surface, with the changing of zeolite morphology. That is why the hollow voids cannot be produced for the post-synthesis of template-containing TS-1 zeolite, which is ascribed to the restriction

anions in both external surface and bulk body of zeolite. It is

prepare HTS zeolite under hydrothermal conditions.

116 Zeolites - Useful Minerals

**Figure 7.** TEM images of different titanium containing zeolites.

work atoms) catalyzed by OH<sup>−</sup>

Cyclohexanone oxime, produced by cyclohexanone ammoximation, is one of the most important chemical intermediates to produce ε-caprolactam via a Beckmann intramolecular rearrangement [52–54]. ε-caprolactam is widely used to manufacture nylon-6 worldwide, which acts as the precursor for producing fiber, plastic, resins, so on. As the fast development of human society, especially for recent 10 years, the demands of ε-caprolactam and cyclohex‐ anone oxime are becoming rapidly increasing. The traditional cyclohexanone oxime produc‐ tion methods majorly focus on the hydroxylamine phosphate (HPO) or hydroxylamine sulfate (HSO) routes, as shown in **Figure 8a** [55–57]. Obviously, it is very complex, highly polluted and corrosive to prepare hydroxylamine, which is used as oxidant for cyclohexanone ammox‐ imation reaction, due to the usage of strong acid solutions and ammonia. Large amount of low value-added side-products (particularly ammonium sulfate) are formed in this approach, and thus the traditional methods obey the fundamental principles of green chemistry and need to be remarkably improved in both academia and industry.

$$\langle \mathbf{O\_5^\* \star \cdot \mathbf{H^{\bullet}}} \rangle \xrightarrow[\mathsf{R^0 \to \mathsf{R^1}}]{} \bigoplus\_{\mathbf{O^0 \to \mathbf{O^1}}} \mathsf{C^0 \to \mathsf{R^1} \star \mathsf{R^1} \star \mathsf{O^0}} \supset \mathsf{O^\bullet}$$

**Figure 8.** Reaction pathways of traditional HPO route (a) and one-step route catalyzed by TS-1 (b).

To overcome these drawbacks in HPO and HSO routes, a new liquid phase one-step process was developed by Enichem in 1987 [58–60]. It is based on the "one-step" cyclohexanone ammoximation of catalyzed by TS-1 zeolite, with ammonia and hydrogen peroxide solutions as reactants, as demonstrated in **Figure 8b**. After that, extensive researches have been carried out with make this novel route become economic and technological available in industry. Compared with traditional routes, there is no hazard side-product generated in the TS-1 catalyzed route, with water as the only by product and the atom efficiency over 85%. Here, the cyclohexanone oxime can be achieved by one-step method, which is operated under mild condition, with low reaction temperature and low pressure. Therefore, cyclohexanone ammoximation reaction catalyzed by TS-1 zeolite is considered as a completely environmentalfriendly process.

**Figure 9.** Catalytic performance of TS-1 and HTS zeolites in the cyclohexanone ammoximation with low concentration H2O2 solution as oxidant.

However, the TS-1 zeolite synthesized by traditional method is usually of poor catalytic performance in cyclohexanone ammoximation process, due to the presence of alkaline ammonia, causing the hydrolysis of Ti–O–Si bonds. Thus, in the early years, the stability of TS-1 zeolite was considered as the main obstacle to prevent this process be commercialized at large scale, which was attributed to the low lifetime of TS-1 zeolite in ammoximation system. That is why only first one12 kt/a cyclohexanone ammoximation demonstration installation was established by Enichem in 1994 at Porto Margherita, Italy [61]. With respect to the deactivation of TS-1 zeolite, three main aspects were proposed by Petrini et al., such as (a) pore filling by heavy organic side-products, (b) the transformation of framework Ti atoms into extraframework Ti species, and (c) dissolution of framework Si atoms in the alkaline solution [62–65]. And very recently, we have confirmed this phenomenon using multiple characteri‐ zation methods and catalytic evaluation. Therefore, to enhance the stability of TS-1 zeolite in ammoximation, the HTS zeolite was developed by Lin et al., at SINOPEC through a novel postsynthesis of conventional TS-1 zeolite in the presence of TPAOH under hydrothermal condi‐ tion. The catalytic performance of HTS and TS-1 zeolites in cyclohexanone ammoximation process is outlined in **Figure 9**, which suggests that HTS zeolite displays much higher catalytic activity and longer lifetime than conventional TS-1 zeolite under the same reaction conditions. In our opinion, the fast cyclohexanone conversion rate is attributed to the generation of hollow voids inside crystal, which favors the accessibility of raw material molecules to tetrahedral framework Ti sites, and improves inner mass diffusion, thus reducing the formation of heavy organic deposit. And the high stability of HTS zeolite is associated with the reincorporation of Ti species into the framework positions via the dehydrated condensation between Ti-OH and Si-OH groups under hydrothermal conditions. As shown in **Figure 9**, the lifetime of HTS zeolite in ammoximation reaction is much longer than that of TS-1 zeolite, with high cyclo‐ hexanone conversion (>95%) for over 110 h. As a result, the commercial cyclohexanone ammoximation units catalyzed by HTS zeolite had been builded in Baling Petrochemical Company, SINOPEC at 140 kt/a scale since 2000, to replace the conventional four-step HPO process. In industrial process, the cyclohexanone conversion and oxime selectivity are all higher than 99.5%, whereas the utilization efficiency of H2O2 is about 90%. It is worthy to note that the size of HTS zeolite is about 200–350 nm, which means these particles are hard to be separated from reaction system. Thus, the separation process of HTS zeolite by inorganic member was been successfully developed and widely applied in industrial ammoximation units. In a word, this novel route takes promising advantages for the manufacture of εcaprolactam, with significant economic and environmental meaning. Additionally, in very recent years, several cyclohexanone ammoximation units catalyzed by HTS zeolite have been established or are still under construction, with even larger scale (>200 kt/a) in China.

#### **3.2. Phenol hydroxylation process**

as reactants, as demonstrated in **Figure 8b**. After that, extensive researches have been carried out with make this novel route become economic and technological available in industry. Compared with traditional routes, there is no hazard side-product generated in the TS-1 catalyzed route, with water as the only by product and the atom efficiency over 85%. Here, the cyclohexanone oxime can be achieved by one-step method, which is operated under mild condition, with low reaction temperature and low pressure. Therefore, cyclohexanone ammoximation reaction catalyzed by TS-1 zeolite is considered as a completely environmental-

**Figure 9.** Catalytic performance of TS-1 and HTS zeolites in the cyclohexanone ammoximation with low concentration

However, the TS-1 zeolite synthesized by traditional method is usually of poor catalytic performance in cyclohexanone ammoximation process, due to the presence of alkaline ammonia, causing the hydrolysis of Ti–O–Si bonds. Thus, in the early years, the stability of TS-1 zeolite was considered as the main obstacle to prevent this process be commercialized at large scale, which was attributed to the low lifetime of TS-1 zeolite in ammoximation system. That is why only first one12 kt/a cyclohexanone ammoximation demonstration installation was established by Enichem in 1994 at Porto Margherita, Italy [61]. With respect to the deactivation of TS-1 zeolite, three main aspects were proposed by Petrini et al., such as (a) pore filling by heavy organic side-products, (b) the transformation of framework Ti atoms into extraframework Ti species, and (c) dissolution of framework Si atoms in the alkaline solution [62–65]. And very recently, we have confirmed this phenomenon using multiple characteri‐ zation methods and catalytic evaluation. Therefore, to enhance the stability of TS-1 zeolite in ammoximation, the HTS zeolite was developed by Lin et al., at SINOPEC through a novel postsynthesis of conventional TS-1 zeolite in the presence of TPAOH under hydrothermal condi‐ tion. The catalytic performance of HTS and TS-1 zeolites in cyclohexanone ammoximation process is outlined in **Figure 9**, which suggests that HTS zeolite displays much higher catalytic activity and longer lifetime than conventional TS-1 zeolite under the same reaction conditions. In our opinion, the fast cyclohexanone conversion rate is attributed to the generation of hollow voids inside crystal, which favors the accessibility of raw material molecules to tetrahedral

friendly process.

118 Zeolites - Useful Minerals

H2O2 solution as oxidant.

Phenol hydroxylation to prepare catechol (*o*-dihydroxybenzene) and hydroquinone (*p*dihydroxybenzene), which are widely applied to manufacture agrichemical, photography chemicals, antioxidants, polymerization inhibitors, is one of most important reactions in fine chemical industry [66, 67]. Traditionally, catechol, usually together with hydroquinone, is produced by oxidation of phenol with various oxidants, such as 60–70% aqueous H2O2 solutions, performic acid, and ketone peroxides (such as α-hydro hydroperoxides). Heavy transition metal compounds and strong phosphoric acid are selected as catalysts for these hydroxylation reactions, but they are very difficult to be separated from products [68, 69]. Obviously, these processes are very complicated and far beyond environmental friendly to be carried out at commercial scale, with very low atom utilization efficiency. There are huge amount of liquid and/or solid phase wastes need to be disposed, which are highly toxic and corrosive for the installations. Fortunately, one completely green and clean phenol hydroxy‐ lation process via using TS-1 zeolite as catalyst has been developed by EniChem, which uses low concentration (usually 30 wt%) H2O2 solution, and is carried out under benign conditions (lower than 100°C and atmosphere pressure). **Figure 10** shows the comparison of catalytic performance of HTS and TS-1 zeolite in phenol hydroxylation under the same conditions. We observe that HTS zeolite can faster catalyze the transformation of phenol than TS-1 zeolite, which is correlated with its inner hierarchical structure. That is to say, hollow cavities promote the mass diffusion inside zeolite crystal, while the higher phenol conversion is dependent on the more framework Ti content, as previously characterized. Based on the excellent catalytic performance, several phenol hydroxylation installations have been builded and run smoothly in China.

**Figure 10.** Catalytic performance of HTS and TS-1 zeolite in phenol hydroxylation reaction.

Furthermore, the phenol hydroxylation is chosen as probe reaction to reveal the kinetic activity of framework Ti sites, owing to its mechanism on the activation of H2O2, rather than functional groups in phenol. Although high-concentration (>60 wt%) H2O2 solutions are of strong nucleophilic attack capability during catalytic oxidation reaction, it is considerably difficult and dangerous to store, transfer and use them at large scale. Interestingly, the activated H2O2 molecules (in the form of Ti-OOH species) by HTS zeolite are of as strong nucleophilicity as 60 wt% H2O2 solution or peroxide acids. And the solid HTS zeolite crystal is easy to be separated from liquid phase, and thus the heterogeneous hydroxylation process has attracted kind attentions from the researchers in both academic and industrial sections.

#### **3.3. Propylene epoxidation (HPPO) process**

Propylene oxide (PO) is also one of the most important chemical intermediates to produce polyesters and polyurethane in industry, with an increasingly huge market worldwide (over 7.5 million tons in 2010) [70–73]. Generally, two basic strategies are majorly employed for commercial PO production, including hydrochlorination route and co-oxidation route. The former one involves the transformation of propylene to chloropropanols and dehydrochlori‐ nation, which need to use the Cl2 and Ca(OH)2 as basic materials, respectively, with the formation of corrosive and low value-added wastes. The other one lies in the co-oxidation of propylene and other organic compounds (such as ethylbenzene or isobutane) with air or O2 as oxidant, in the presence of catalyst. The coproducts in this route are t-butyl alcohol, MTBE or styrene, but the main drawback is the high capital and material cost and long reaction pathway. To avoid these disadvantages, an ultra environmental-friendly propylene epoxida‐ tion process with hydrogen peroxide as oxidant (referred to as HPPO route) was exploited by using TS-1 zeolite as catalyst, and in 2009 a new HPPO plant was started up by BASF and Dow chemical at 300 kt/a scale in Antwerp [74–77]. Later on, several HPPO plants were established in many countries, such as Korea, Thailand, and China. At the same time, the HPPO process catalyzed by HTS zeolite has been developed by SINOPEC, and one 100 kt/a propylene epoxidation industrial initialization has been established and run smoothly in China.

Hollow Titanium Silicalite Zeolite: From Fundamental Research to Commercial Application in Environmental-Friendly Catalytic Oxidation Processes http://dx.doi.org/10.5772/63699 121

**Figure 11.** Reaction network of propylene epoxidation via HPPO process.

**Figure 10.** Catalytic performance of HTS and TS-1 zeolite in phenol hydroxylation reaction.

kind attentions from the researchers in both academic and industrial sections.

**3.3. Propylene epoxidation (HPPO) process**

120 Zeolites - Useful Minerals

Furthermore, the phenol hydroxylation is chosen as probe reaction to reveal the kinetic activity of framework Ti sites, owing to its mechanism on the activation of H2O2, rather than functional groups in phenol. Although high-concentration (>60 wt%) H2O2 solutions are of strong nucleophilic attack capability during catalytic oxidation reaction, it is considerably difficult and dangerous to store, transfer and use them at large scale. Interestingly, the activated H2O2 molecules (in the form of Ti-OOH species) by HTS zeolite are of as strong nucleophilicity as 60 wt% H2O2 solution or peroxide acids. And the solid HTS zeolite crystal is easy to be separated from liquid phase, and thus the heterogeneous hydroxylation process has attracted

Propylene oxide (PO) is also one of the most important chemical intermediates to produce polyesters and polyurethane in industry, with an increasingly huge market worldwide (over 7.5 million tons in 2010) [70–73]. Generally, two basic strategies are majorly employed for commercial PO production, including hydrochlorination route and co-oxidation route. The former one involves the transformation of propylene to chloropropanols and dehydrochlori‐ nation, which need to use the Cl2 and Ca(OH)2 as basic materials, respectively, with the formation of corrosive and low value-added wastes. The other one lies in the co-oxidation of propylene and other organic compounds (such as ethylbenzene or isobutane) with air or O2 as oxidant, in the presence of catalyst. The coproducts in this route are t-butyl alcohol, MTBE or styrene, but the main drawback is the high capital and material cost and long reaction pathway. To avoid these disadvantages, an ultra environmental-friendly propylene epoxida‐ tion process with hydrogen peroxide as oxidant (referred to as HPPO route) was exploited by using TS-1 zeolite as catalyst, and in 2009 a new HPPO plant was started up by BASF and Dow chemical at 300 kt/a scale in Antwerp [74–77]. Later on, several HPPO plants were established in many countries, such as Korea, Thailand, and China. At the same time, the HPPO process catalyzed by HTS zeolite has been developed by SINOPEC, and one 100 kt/a propylene

epoxidation industrial initialization has been established and run smoothly in China.

With respect to the reaction network of HPPO process, except for the major propylene epoxidation, other side reactions (as shown in **Figure 11**) are also happened, causing the decrease of PO yield. Thus, it is necessary to use large amount of methanol as solvent and operate this route at low temperature. A 1.0 kt/a HPPO process, schematic flow diagram is demonstrated in **Figure 12**. The epoxidation reaction is carried out in a fixed-bed reactor, with the mixture of propylene and H2O2 injecting into the reactor. When the epoxidation reaction at 1.0 kt/a scale is carried out in a wide range, T = 30–70°C, P = 0.5–2.0 MPa, WHSV of H2O2 molecules is 0.12–1.2 h−1, *n*(CH3OH)/*n*(H2O2) is 5–25, *n*(Propylene)/*n*(H2O2) is 1.2–2.5, the TOF of H2O2 is around 31–32 mmol g−1 h−1(*X*H2O2 is 96–99%) and selectivity of PO is 96–99%, which is much better than that reported by BASF and DOW (the selectivity of PO is 93.2%). Further‐ more, the raw propylene oxide produced in the fixed-bed reactor is purified in the PO purification column, obtaining the final PO product at 99.97% purity. And then, the purified PO has been used as chemical intermediates to manufacture 1,2-ethyl glycerol, oilfield demulsifier, propylene carbonate, triisopropanolamine, hydroxypropy, and dimethyl carbo‐ nate [78].

**Figure 12.** Schematic process flow diagram of propylene epoxidation at 1.0 kt/a pilot scale [78].

#### **3.4. Baeyer–Villiger oxidation of cyclohexanone**

Baeyer–Villiger (B–V) oxidation is an important reaction to produce esters or lactones from ketones via inserting one O atom in C–C bond. Especially, ε-carprolactone, prepared via cyclohexanone BV oxidation, is widely used to synthesize polycaprolactone and poliglecap‐ rone [79–83]. Traditionally, many kinds of peracids, such as *m*-chloroperbenzoic acid, trifluor‐ operacetic acid and perbenzoic acid, are chosen as oxidizing agents and catalyst to motivate BV reaction. The main advantages are the high ketone conversion and fast transformation rate, but there are still many significant drawbacks, such as low selectivity of target products, low atomic utilization, being harmful to the environment and dangerous to handle. Thus, up to now, only three companies can commercially produce ε-carprolactone based on peracids oxidant in the world, owing its long and complex reaction pathway. To beat these drawbacks, low concentration (usually 30–50 wt%) H2O2 solution (which cheap and easy to handle) is considered as the best candidate to hazardous replace peracids. But it is of poor nucleophilic capability to attack the carbonyl group in ketones during BV oxidation reaction. As a result, two basic strategies are proposed to accelerate the heterogeneous B–V oxidation of cyclohex‐ anone using aqueous H2O2 solution: (a) activation the carbonyl groups to make the C atoms in carbonyl group easy to be attacked by H2O2 molecules; (b) increasing the nucleophilic capability of H2O2 molecules by forming M-OOH species (M represents transition metal element, which could accept the lone electron pairs by employing their empty *d* orbits) [84, 85].

**Figure 13.** BV oxidation of ketones with different zeolite catalysts under mild reaction conditions [86, 87].

For activation of carbonyl group, Sn-beta zeolite reported by Corma and his coworkers [88– 90] shows the most promising catalytic performance in BV oxidation of cyclohexanone with low concentration H2O2 solution, with almost 100% ε-carprolactone selectivity. It is attribut‐ ed to the tetrahedral coordinated Sn atoms that were incorporated into the framework of beta zeolite can accept the electron pairs in carbonyl group of cyclohexanone. On the other hand, TS-1 zeolite is extensively confirmed as the Lewis catalyst to activate H2O2 molecules.

Consequently, HTS zeolite is taken as catalyst to promote the BV oxidation of cyclohexa‐ none under mild conditions, as show in **Figure 13**. It suggests that HTS zeolite has the dif‐ ferent product distribution from the Sn-beta zeolite, with high selectivity of deep-oxidative products, which are 6-hydroxyhexanoic acid and adipic acid. 6-hydroxyhexanoic acid is produced through the ring-opening reaction of lactone, due to the poor stability of 7-mem‐ ber ring of ε-carprolactone and high water content in reactor. Then, the C-OH groups in 6 hydroxyhexanoic acid can be further oxidized by Ti-OOH species, to obtain the C-OOH group in adipic acid [86]. Compared with Sn-beta catalyzed route, HTS-catalyzed reaction system does not use any specific organic solvents, which lower the concentration of oxidant and are high-energy consumed to be separated. Thus, it is in favor of the formation of a series of important chemical intermediates in this "one-pot" reaction. However, there are still many kinds of undesirable by-products formed at the same time, reducing the yields of target product. As a consequence, the BV oxidation of cyclohexanone catalyzed by HTS zeo‐ lite is still under investigation in the laboratory to minimize other side reactions.

#### **3.5. Catalytically selective oxidation of cyclohexane**

**3.4. Baeyer–Villiger oxidation of cyclohexanone**

122 Zeolites - Useful Minerals

Baeyer–Villiger (B–V) oxidation is an important reaction to produce esters or lactones from ketones via inserting one O atom in C–C bond. Especially, ε-carprolactone, prepared via cyclohexanone BV oxidation, is widely used to synthesize polycaprolactone and poliglecap‐ rone [79–83]. Traditionally, many kinds of peracids, such as *m*-chloroperbenzoic acid, trifluor‐ operacetic acid and perbenzoic acid, are chosen as oxidizing agents and catalyst to motivate BV reaction. The main advantages are the high ketone conversion and fast transformation rate, but there are still many significant drawbacks, such as low selectivity of target products, low atomic utilization, being harmful to the environment and dangerous to handle. Thus, up to now, only three companies can commercially produce ε-carprolactone based on peracids oxidant in the world, owing its long and complex reaction pathway. To beat these drawbacks, low concentration (usually 30–50 wt%) H2O2 solution (which cheap and easy to handle) is considered as the best candidate to hazardous replace peracids. But it is of poor nucleophilic capability to attack the carbonyl group in ketones during BV oxidation reaction. As a result, two basic strategies are proposed to accelerate the heterogeneous B–V oxidation of cyclohex‐ anone using aqueous H2O2 solution: (a) activation the carbonyl groups to make the C atoms in carbonyl group easy to be attacked by H2O2 molecules; (b) increasing the nucleophilic capability of H2O2 molecules by forming M-OOH species (M represents transition metal element, which could accept the lone electron pairs by employing their empty *d* orbits) [84, 85].

**Figure 13.** BV oxidation of ketones with different zeolite catalysts under mild reaction conditions [86, 87].

For activation of carbonyl group, Sn-beta zeolite reported by Corma and his coworkers [88– 90] shows the most promising catalytic performance in BV oxidation of cyclohexanone with low concentration H2O2 solution, with almost 100% ε-carprolactone selectivity. It is attribut‐ ed to the tetrahedral coordinated Sn atoms that were incorporated into the framework of beta zeolite can accept the electron pairs in carbonyl group of cyclohexanone. On the other hand, TS-1 zeolite is extensively confirmed as the Lewis catalyst to activate H2O2 molecules.

Cyclohexane oxidation is an ultra important reaction to product KA oil (including cyclohex‐ anone and cyclohexanol) in industry, for manufacturing various of chemical precursors and final products, as shown in **Figure 14**. Generally, the KA oil is usually obtained via these methods: hydrogenation of phenol, hydration of cyclohexene, hydrogenation of benzene, and air oxidation of cyclohexane (>90% for KA oil production) [91–94]. The cyclohexane oxidation with air or O2 can be carried out in the presence of various catalysts or in the absence of catalyst, with high temperature (>100°C) and autogeneous pressure. Although air is very cheap and easy to be obtained, this route also suffers serious restrictions in industrial practice, such as low yield of target product, high pollution and waste generation, and particularly high-energy consumption needed for separation of products.

**Figure 14.** Reaction pathway of oxidation of cyclohexane catalyzed by HTS zeolite with H2O2 solution.

For solving these limitations, cyclohexane oxidation with H2O2 catalyzed by TS-1 zeolite has also been investigated. It is found that the KA oil can be achieved through this approach, with very high cyclohexane conversion and selectivity of KA oil under mild conditions. Herein, HTS zeolite is also introduced to catalyze the cyclohexane oxidation, as illustrated in **Figure 15**. We observe that the total yield of KA oil is increasing as a function of reaction time, which is up to 70% after reaction time more than 10 h. The optimized reaction parameters are as follow: T = 60–80°C, catalyst content = 5–10%, cyclohexane to H2O2 molar ratio = 0.5, acetone to cyclohexane molar ratio = 10–15, reaction time = 6–10 h. Under this conditions, the yield of KA oil is over 40%, with a cylohexanone/cyclohexanol ratio larger than 2. Above all, the cyclohexane oxidation catalyzed by HTS zeolite shows great industrial potential, and the further study on scale up of this route is still under carrying out [95, 96].

**Figure 15.** Catalytic performance of HTS zeolite in cyclohexane oxidation with H2O2 solution [95].

#### **3.6. Catalytic oxidation of aromatic compounds**

Aromatic oxidation is of great important, but still a big challenge in the chemical industry, because the kinds and quantity of synthetic aromatic compounds demanded are so huge in the world. Several synthesis methods have been developed for the oxyfunctionalization of aromatics, such as the partial oxidation of isopropylbenzene, neutralization of benzenesulfo‐ nate, hydrolysis of chlorobenzene, direct oxidation with N2O, and direct oxidation with H2O2 solution [97–100]. among them, the last two routes are considered as typical green chemistry process, but the origination of N2O is so limited.

Hollow Titanium Silicalite Zeolite: From Fundamental Research to Commercial Application in Environmental-Friendly Catalytic Oxidation Processes http://dx.doi.org/10.5772/63699 125

**Figure 16.** Reaction pathway of aromatic oxidation with H2O2 solution catalyzed by HTS zeolite.

In our laboratory, both benzene and toluene oxidation reactions catalyzed by HTS zeolite have been examined (**Figure 16**), which show some aromatic conversion and high selectivity of target product [101, 102]. And it is found that polar solvents are useful for high conversion rate, but the high intersolubility between aromatics and H2O2 is still very difficult. Thus, it is necessary to intensify the mixing inside various reactor.

#### **4. Conclusion and perspective**

For solving these limitations, cyclohexane oxidation with H2O2 catalyzed by TS-1 zeolite has also been investigated. It is found that the KA oil can be achieved through this approach, with very high cyclohexane conversion and selectivity of KA oil under mild conditions. Herein, HTS zeolite is also introduced to catalyze the cyclohexane oxidation, as illustrated in **Figure 15**. We observe that the total yield of KA oil is increasing as a function of reaction time, which is up to 70% after reaction time more than 10 h. The optimized reaction parameters are as follow: T = 60–80°C, catalyst content = 5–10%, cyclohexane to H2O2 molar ratio = 0.5, acetone to cyclohexane molar ratio = 10–15, reaction time = 6–10 h. Under this conditions, the yield of KA oil is over 40%, with a cylohexanone/cyclohexanol ratio larger than 2. Above all, the cyclohexane oxidation catalyzed by HTS zeolite shows great industrial potential, and the

further study on scale up of this route is still under carrying out [95, 96].

124 Zeolites - Useful Minerals

**Figure 15.** Catalytic performance of HTS zeolite in cyclohexane oxidation with H2O2 solution [95].

Aromatic oxidation is of great important, but still a big challenge in the chemical industry, because the kinds and quantity of synthetic aromatic compounds demanded are so huge in the world. Several synthesis methods have been developed for the oxyfunctionalization of aromatics, such as the partial oxidation of isopropylbenzene, neutralization of benzenesulfo‐ nate, hydrolysis of chlorobenzene, direct oxidation with N2O, and direct oxidation with H2O2 solution [97–100]. among them, the last two routes are considered as typical green chemistry

**3.6. Catalytic oxidation of aromatic compounds**

process, but the origination of N2O is so limited.

In summary, HTS zeolite, which synthesized via "dissolution–recrystallization" treatment, is an innovative catalytic material for the conversion of oxygenated hydrocarbons under mild conditions. It has been well demonstrated that highly dispersed intracrystalline mesopores and/or macropores are formed inside zeolite, which favors the inner mass diffusion and high conversion rate confirmed by physicochemical adsorption of N2 and catalytic evaluation. The mechanism of the formation of these hollow cavities has been proposed to further understand the "dissolution–recrystallization" process, which involves the diffusion of OH<sup>−</sup> group inside zeolite crystal to form hollow voids, and the recrystallization of soluble Ti and Si sources cause the change of zeolite morphology. At the same time, the reincorporation of Ti species into framework positions has been verified using multiple characterization methods, with the significantly decreasing of framework defects (Q3 Si species) and increasing of framework Ti content. That is why HTS zeolite shows even longer lifetime than conventional TS-1 zeolite in alkaline cyclohexanone ammoximation reaction, owing to relative prefect framework atoms tolerated to basic corrosion, except for the fast mass transfer via hierarchical structure. Since 2000, HTS zeolite has been successfully commercialized by SINOPEC in China.

Based on the unique hollow feature and high framework content, HTS zeolite displays very excellent catalytic performance and stability in many selective oxidation reactions of organic compounds, that is, cyclohexanone ammoximation, phenol hydroxylation, propylene epoxidation, Baeyer–Villiger oxidation of cyclohexanone, selective oxidation of cyclohexane and benzene, so on. The first three items have been already put into commercialization to produce bulk chemicals by SINOPEC, of great social and environmental significance to reduce pollution in industry. The others are still under investigation in the laboratory, but some ones have present good potential to replace these hazard and low atom efficiency process. For further development of HTS zeolite material and its catalytic application, we consider that the synthesis and modification of HTS zeolite should be paid more attention, because catalytic materials are the fundamentally original power for the development of heterogeneous catalysis. Moreover, the fast development of chemistry science and human society provides so many opportunities for catalytic materials with specific characteristics. Thus, from the viewpoint of application, HTS zeolite still has considerable applications in the conversion of oxygenated hydrocarbons, such as in fine chemical, environmental catalysis, and biomass utilization. Additionally, combining the redox property of HTS with other functions (such as hydrogenation of noble metal, Pt or Pd) is also a good way for updating its application. In a word, this chapter is of both theoretical and practical meanings to design and produce catalytic materials for facing specific demands. We have described a systematical investigation on zeolite synthesis and environmental-friendly heterogeneous catalysis processes from laboratory to large commercial scale.

#### **Author details**

Changjiu Xia, Min Lin\* , Bin Zhu, Yibin Luo and Xingtian Shu

\*Address all correspondence to: minlin@tom.com

State Key Laboratory of Catalytic Materials and Reaction Engineering, Research Institute of Petroleum Processing, SINOPEC, Beijing, People's Republic of China

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**Chapter 7**

### **Luminescent Zeolites**

Hui Lin and Minoru Fujii

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63805

#### **Abstract**

Compared with the intensive fundamental research and practical application development of zeolites in the fields of photocatalysis, separation, ion exchange, and antimicrobial, research on the luminescence from optically functionalized zeolites is still at a juvenile stage. So far, plenty of important potential applications based on luminescent zeolites, such as light harvesting, phosphors, optical encoding/record‐ ing, micro-lasers, chemical sensors, and bio-imaging, have been demonstrated. What's more, it has been shown that the porous crystalline nature of zeolites makes them an excellent matrix for investigating the fundamental physics of the sub-nm-scale luminescent metal clusters, which is still not sufficiently clear till now. Due to its significance, luminescent zeolites functionalized by luminescence-active guest species in various types including organic dye molecules, rare earth (RE) ions, bismuth ions, and d10 metal ions are reviewed in this chapter, dedicating to presenting some representative work on luminescent zeolites during the past few decades, from materials design, preparation, to application demonstrations, in hope to boost more research interest and strength in this community.

To date, optically activated luminescent zeolites are mainly focused on several types of aluminosilicate zeolites with the dimension of pores or channels in the scale of about 0.3–1.5 nm, such as the FAU-type, the LTL-type, the LTA-type, the ZSM-5 type, and the SOD-type zeolites. Here, in this chapter, the zeolites mainly refer to these nanoporous zeolites functionalized by luminescent centers.

**Keywords:** luminescence, zeolites, rare earth ions, bismuth ions, noble and coinage metal clusters

#### **1. Introduction**

In Section 1, luminescence of zeolites functionalized by organic dye molecules and quantum dots incorporated within the cavities is reviewed. The strong capacity of incorporating and confining organic dye molecules and quantum dots into zeolites' cavities is enabled by the

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

large inner surface area, the exchangeable extra-framework cations, the built-in static electrical field between the extra-framework cations and the framework oxygen, and the well-confined periodically distributed cavities. Previous work on organic dye molecules or quantum dots containing zeolites with designed alignment of the guest dye molecules and sometimes the host zeolite micro-/nanocrystals is briefly introduced. Since most of these important work has been well summarized in some previous book chapters, reviews, or feature articles (Gion Calzaferri, *Top Catal*, 2010, 53:130–140; Gion Calzaferri, *Chapter 7* in *Photofunctional zeolites: synthesis, characterization, photocatalysis reactions, light harvesting*; & Calzaferri, *Langmuir*, 2012, 28:6216–6231; Gion Calzaferri, *Advances in Photochemistry*, 2002, 27:1–50; Thomas Bein, *Chapter 18* in *Introduction to Zeolite Molecular Sieves*, 3rd Edition, 2007), here we generally review some typical methods for processing the dye molecules containing zeolites and introduce some new achievements in this area within the recent years.

The extra-framework cations of zeolites such as Na+ , K+ , and Ca2+ are not as stable as the framework Si4+ and Al3+ and are exchangeable to other cations, for instance, the usage of zeolite 4A as water softener via the Na+ → Ca2+ or Mg2+ exchange. The extra-framework cations can also be replaced by optically active centers such as rare earth (RE) ions, the transition metal (TM) ions, and some luminescence-active main group elements ions, for instance, bismuth (Bi) ions, etc. The porous crystalline nature of zeolites benefits the homogeneous dispersion of luminescence-active centers in the zeolite matrices. To present efficient luminescence from the RE ion- and Bi ion-exchanged zeolites, relieving or eliminating the influence of the hydrated water molecules is a prerequisite. In Section 2, recent achievements on the visible and NIR emission from the rare earth (RE) ion- and bismuth (Bi) ion-exchanged zeolites and their potential optical and photonics applications are reviewed.

d10 ion-exchanged zeolites, especially Ag+ ion-exchanged zeolites, show excellent photocatal‐ ysis capacity. Comprehensive work on summarizing the photocatalysis properties of transition metal ion-exchanged zeolites has been conducted in a previous book "Photofunctional Zeolites: Synthesis, Characterization, Photocatalytic Reactions, and Light Harvesting." In Section 3, we mainly focus on the luminescence properties of these materials, aiming for potential optical and photonics applications. Due to the closed-shell electronic structure, usually there exist weak metallophilic interactions between the d10 ions, which exhibit many intriguing luminescence phenomena. Amazingly, the capability of adsorption/desorption of small polar molecules of the aluminosilicate nanoporous zeolites can have strong influence on the coordination of the extra-framework d10 ions and the metallophilic interaction between the adjacent d10 ions; in result reversible emission variation from the d10 ion-exchanged zeolites has been observed.

#### **1.1. Incorporating organic dye molecules into zeolites' cavities**

Dye molecules incorporated within the cavities of zeolites are representative guest-host materials which have significance to both fundamental research and technical applications. The distinct feature of zeolite matrices is their nature-born, spatially perfectly confined cages and channels enabled by its porous crystalline structure, which makes them superior to porous glasses or dense crystalline matrices. Dye molecules are often encapsulated within zeolites' cavities via physisorption, self-assembly, ion exchange, crystallization inclusion, or direct synthesis in the cages or channels of zeolites (also called the ship-in-a-bottle method). The most frequently adopted zeolite matrices for organic dye molecules are those with strictly parallel one-dimensional (1D) channels, such as the LTL-type (short for L-type) and AFI-type zeolites, which can help simplify the investigation on the energy transfer process due to the strong 1D confinement of the dye molecules. By choosing proper organic dye molecules and zeolite matrix guest-host pairs, emission quenching due to the aggregation of dye molecules can be prevented by spatially confining the dye molecules at the monomer scale. Meantime, this features the high quantum yield emission by high concentration loading of organic dye molecules without aggregation-induced quenching. Anisotropic energy transfer among organic dye molecule guests and polarized emission parallel/perpendicular to the *c*-axis is possible to achieve within the dye molecules/1D-channel zeolites guest-host systems by proper spatial confinement of the dye molecules [1]. So far, various kinds of neutral, cationic organic dye molecules and their combination have been incorporated into the cavities of zeolites, including some dyes for laser generation [2]. To achieve patterned incorporation of the dye molecules into the orientation of well-aligned 1D-channel zeolite monolayers or arrays, sometimes micro-contact printing (mCP), which is usually used for fabricating or transferring the *c*-axis-oriented L-type or AFI-type zeolite monolayers or arrays, has been conducted [3].

large inner surface area, the exchangeable extra-framework cations, the built-in static electrical field between the extra-framework cations and the framework oxygen, and the well-confined periodically distributed cavities. Previous work on organic dye molecules or quantum dots containing zeolites with designed alignment of the guest dye molecules and sometimes the host zeolite micro-/nanocrystals is briefly introduced. Since most of these important work has been well summarized in some previous book chapters, reviews, or feature articles (Gion Calzaferri, *Top Catal*, 2010, 53:130–140; Gion Calzaferri, *Chapter 7* in *Photofunctional zeolites: synthesis, characterization, photocatalysis reactions, light harvesting*; & Calzaferri, *Langmuir*, 2012, 28:6216–6231; Gion Calzaferri, *Advances in Photochemistry*, 2002, 27:1–50; Thomas Bein, *Chapter 18* in *Introduction to Zeolite Molecular Sieves*, 3rd Edition, 2007), here we generally review some typical methods for processing the dye molecules containing zeolites and introduce some new

framework Si4+ and Al3+ and are exchangeable to other cations, for instance, the usage of zeolite

also be replaced by optically active centers such as rare earth (RE) ions, the transition metal (TM) ions, and some luminescence-active main group elements ions, for instance, bismuth (Bi) ions, etc. The porous crystalline nature of zeolites benefits the homogeneous dispersion of luminescence-active centers in the zeolite matrices. To present efficient luminescence from the RE ion- and Bi ion-exchanged zeolites, relieving or eliminating the influence of the hydrated water molecules is a prerequisite. In Section 2, recent achievements on the visible and NIR emission from the rare earth (RE) ion- and bismuth (Bi) ion-exchanged zeolites and their

ysis capacity. Comprehensive work on summarizing the photocatalysis properties of transition metal ion-exchanged zeolites has been conducted in a previous book "Photofunctional Zeolites: Synthesis, Characterization, Photocatalytic Reactions, and Light Harvesting." In Section 3, we mainly focus on the luminescence properties of these materials, aiming for potential optical and photonics applications. Due to the closed-shell electronic structure, usually there exist weak metallophilic interactions between the d10 ions, which exhibit many intriguing luminescence phenomena. Amazingly, the capability of adsorption/desorption of small polar molecules of the aluminosilicate nanoporous zeolites can have strong influence on the coordination of the extra-framework d10 ions and the metallophilic interaction between the adjacent d10 ions; in result reversible emission variation from the d10 ion-exchanged zeolites

Dye molecules incorporated within the cavities of zeolites are representative guest-host materials which have significance to both fundamental research and technical applications. The distinct feature of zeolite matrices is their nature-born, spatially perfectly confined cages and channels enabled by its porous crystalline structure, which makes them superior to porous glasses or dense crystalline matrices. Dye molecules are often encapsulated within zeolites'

, K+

→ Ca2+ or Mg2+ exchange. The extra-framework cations can

ion-exchanged zeolites, show excellent photocatal‐

, and Ca2+ are not as stable as the

achievements in this area within the recent years.

4A as water softener via the Na+

136 Zeolites - Useful Minerals

The extra-framework cations of zeolites such as Na+

potential optical and photonics applications are reviewed.

**1.1. Incorporating organic dye molecules into zeolites' cavities**

d10 ion-exchanged zeolites, especially Ag+

has been observed.

Synthesis of *c*-axis-oriented 1D-channel zeolite monolayers or arrays with the channels open at the off-substrate ends and ready to load dye molecules has been of particular interest and intensively investigated [4]. To seal the entrance(s) to the 1D channels in case of dye molecule leakage after loading, stuffing at the end(s) of the 1D channels with molecules (phthalocyanine, for instance) serving as stopcock or surface coating is often performed [5]. In some cases, the stopcock molecules can also act as linkages so that they assemble the dye molecules containing 1D-channel zeolite micro-/nano-crystals end to end into chains.

#### **1.2. Aligning the orientation of the 1D-channel zeolite micro-/nanocrystals in favor of photonics applications**

Besides the controllable assembly of the organic dye molecules in the parallel 1D channels, orienting the micro-/nano-zeolite matrix grains to obtain well-aligned arrays or thin films is also highly expected to facilitate certain optical or photonic applications. Chemical modifica‐ tion [6], seeding-epitaxial growth [4], external magnetic field application [7], and micro-contact imprinting [8] have been utilized to align the zeolite micro-/nanocrystals along the 1D channel direction.

Obtaining zeolite micro-/nano-crystals monolayer via chemical interactions such as covalent linkages, ionic bonding, and hydrogen bonding has been summarized by Yoon [6]. Here we mainly introduce the orientation alignment via seeding-epitaxial growth and via external magnetic field assistance.

A typical process to obtain 1D-channel zeolite thin films with well-aligned grains by first loading a seeding layer on the substrate followed by the further epitaxial growth has been described as follows by Xue et al. [4]: first by surface modification of the quartz slice substrate using cation polymer, for example, poly(diallyldimethylammonium chloride) (PDDA), to get a positively charged substrate surface as template for the zeolite seed crystals; then seeds loading via immersing the substrate horizontally in the seed crystals suspension, followed by rinsing and drying to get the seeds slice; growth of the *c*-axis-oriented AFI film by a further hydrothermal crystallization in the autoclave; organic template (PDDA) removal by calcinations; and finally dye molecules loading by adding the calcined *c*-axis-oriented AFI thin film in an azo dye molecule (DPPA) solution. This azo dye molecule containing *c*-axis-oriented AFI zeolite film shows ability for detecting heavy metal ions by monitoring the optical absorption variation, particularly efficient for the detection of Zn2+ ions.

Cucinotta et al. depicted a simple experimental layout for aligning the orientation of the zeolite L micro-cylinders via applying external magnetic field [9]. After dye molecule loading, monodisperse Fe3O4 nanoparticles prepared by chemical co-precipitation were coated on the surface of zeolite L microcrystals via natural assembly without any surface modification. The attached Fe3O4 nanoparticles can assist the alignment and the orientation of the zeolite L microcrystals.

Amazingly, alignment and the orientation of organic dye molecules incorporated into zeolite L microcrystals decorated by the Fe3O4 nanoparticles via external magnetic field can be achieved in aqueous solution, on solid substrates, and even in polymer matrices. The most interesting point may be that anisotropy in the optical absorption and emission spectra was observed and assigned to the fine spatial alignment of both the hexagonal zeolite L microcrystal matrices and the organic dye molecules within the matrices' 1D channels.

#### **1.3. Featured luminescence from the dye molecules incorporated into zeolites**

The most intensively conducted work on the luminescence properties of organic dye molecules incorporated into 1D channel zeolites is regarding the design and fulfillment of the efficient energy transfer from the donors to acceptor dyes via the Förster resonant energy transfer (FRET), typically aiming for artificial photosynthesis. An extreme fast non-radiative energy transfer from the donor to the acceptor (emitter) is required in case of energy loss via other pathways [10].

It is interesting that some organic dyes, hostasol red (HR), for instance, show efficient lumi‐ nescence in aprotic solvents but becomes inefficient when it's incorporated into zeolite L's channels. By partial or complete exchanging the extra-framework K+ ions with imidazolium cation, the emission quantum yield of the hostasol red (HR) dyes containing zeolite L can be enhanced to a large extent by reducing the proton activity of the extra-framework cations in water molecules environment [11]. Meantime, the high-emission quantum yield can be together ensured by locating the HR dyes in the middle of the 1D channels and in the envi‐ ronment of perylene dye serving as the antenna donor, in which condition a highly efficient donor-acceptor system can be formed.

#### **1.4. Quantum dots (clusters) embedded in zeolite matrices**

Due to the tiny sizes of the nanoporous zeolites' cavities (0.3–1.5 nm), the sizes of the semi‐ conductor nanospecies confined within the zeolites' cavities are smaller than typical semicon‐ ductor quantum dots and they can be at the scale which consists of several "molecules" [12]. At this scale, the tiny semiconductor crystals are sometimes called quantum clusters. Zeolite matrices have the advantages of excellent size confinement and periodical spacing between the quantum dots. Owing to the size well-defined cages and channels, zeolites have also been demonstrated for screening out the semiconductor dots or clusters with narrow size distribu‐ tion.

#### **1.5. Possible future work**

a positively charged substrate surface as template for the zeolite seed crystals; then seeds loading via immersing the substrate horizontally in the seed crystals suspension, followed by rinsing and drying to get the seeds slice; growth of the *c*-axis-oriented AFI film by a further hydrothermal crystallization in the autoclave; organic template (PDDA) removal by calcinations; and finally dye molecules loading by adding the calcined *c*-axis-oriented AFI thin film in an azo dye molecule (DPPA) solution. This azo dye molecule containing *c*-axis-oriented AFI zeolite film shows ability for detecting heavy metal ions by monitoring the optical

Cucinotta et al. depicted a simple experimental layout for aligning the orientation of the zeolite L micro-cylinders via applying external magnetic field [9]. After dye molecule loading, monodisperse Fe3O4 nanoparticles prepared by chemical co-precipitation were coated on the surface of zeolite L microcrystals via natural assembly without any surface modification. The attached Fe3O4 nanoparticles can assist the alignment and the orientation of the zeolite L

Amazingly, alignment and the orientation of organic dye molecules incorporated into zeolite L microcrystals decorated by the Fe3O4 nanoparticles via external magnetic field can be achieved in aqueous solution, on solid substrates, and even in polymer matrices. The most interesting point may be that anisotropy in the optical absorption and emission spectra was observed and assigned to the fine spatial alignment of both the hexagonal zeolite L microcrystal

The most intensively conducted work on the luminescence properties of organic dye molecules incorporated into 1D channel zeolites is regarding the design and fulfillment of the efficient energy transfer from the donors to acceptor dyes via the Förster resonant energy transfer (FRET), typically aiming for artificial photosynthesis. An extreme fast non-radiative energy transfer from the donor to the acceptor (emitter) is required in case of energy loss via other

It is interesting that some organic dyes, hostasol red (HR), for instance, show efficient lumi‐ nescence in aprotic solvents but becomes inefficient when it's incorporated into zeolite L's channels. By partial or complete exchanging the extra-framework K+ ions with imidazolium cation, the emission quantum yield of the hostasol red (HR) dyes containing zeolite L can be enhanced to a large extent by reducing the proton activity of the extra-framework cations in water molecules environment [11]. Meantime, the high-emission quantum yield can be together ensured by locating the HR dyes in the middle of the 1D channels and in the envi‐ ronment of perylene dye serving as the antenna donor, in which condition a highly efficient

absorption variation, particularly efficient for the detection of Zn2+ ions.

matrices and the organic dye molecules within the matrices' 1D channels.

**1.3. Featured luminescence from the dye molecules incorporated into zeolites**

microcrystals.

138 Zeolites - Useful Minerals

pathways [10].

donor-acceptor system can be formed.

At present, the applied magnetic field for aligning the orientation of zeolite microcrystals in the recent reports is very high (1 T). Actually, even for the paramagnetic materials, Al2O3, or Ca5(PO4)3F, for instance, under such strong magnetic field, aligning grains along the *c*-axis can be also be achieved [13, 14]. Efforts may be deserved on providing other novel structures or architectures or decorating methods which can reduce the applied magnetic field to facilitate the orientation alignment with moderate magnetic field.

### **2. Rare earth (RE) ion- and bismuth (Bi) ion-exchanged nanoporous zeolites and their derivatives**

#### **2.1. Factors influencing the luminescence quantum efficiency and possible solutions**

#### *2.1.1. Phonon energy of zeolites' frameworks*

The framework of aluminosilicate zeolites consists of [SiO4] 4− and [AlO4] 5− tetrahedrons sharing the O2− corners; the lattice vibration energy of the (dehydrated) zeolite framework itself is suitable to serve as a host material for luminescence.

#### *2.1.2. Hydrate water molecules*

As mentioned above, for these zeolites with the aluminosilicate composition, to keep charge neutral, cations such as Na+ , K+ , and Ca2+ are located in the channels and cages and are coordinated by the framework oxygen. The electric field existing between the extra-framework cations and the framework oxygen has strong attraction for small polar molecules. As a result, water molecules are easily adsorbed in this type of zeolites such as the LTA and FAU types, usually with the H atoms bonded to the framework oxygen or with the O atoms coordinated to the extra-framework cations.

Hydrated water molecules are the most influencing factor on the emission efficiency of the optically functionalized luminescent zeolites. On the one hand, due to the high vibrational energy (maximal ~3600–3700 cm−1), hydrated water molecules are generally detrimental to the emission efficiency of the RE and Bi ion-exchanged zeolites. To achieve efficient luminescence from these types of optically activated zeolites and its derivatives, dehydration and then preventing rehydration via stuffing the entrance for water molecules [15], or filling the cavities with molecules or radicals of low vibrational energy [16], are two often adopted methods.

On the other hand, interestingly, it has been demonstrated in our recent work that the hydrated water molecules are essential to the strong yellow-green emission from the Ag+ ion-exchanged LTA zeolites, which can be attributed to the coordination to the Ag+ ions that ensures an efficient ligand to metal charge transfer (LMCT) process. This part will be discussed with more details in Section 3.

#### **2.2. Down-conversion near-infrared (NIR) emission from Nd3+ ion-exchanged zeolite Y**

Nd3+ is a representative RE ion as the NIR luminescence-active center in various matrices (one peak at ~1.06 μm and the other at ~1.35 μm), for instance, the Nd3+-doped phosphate glass or the Nd3+-doped yttrium aluminum garnet (usually denoted as Nd3+:YAG) single crystal for solid-state laser generation.

**Figure 1.** TEM morphology of pure zeolite Y (a), Nd3+ singly exchanged zeolite Y (b), and Nd3+, Ag+ ion-exchanged zeolite Y (c) individual particles treated at 900°C [17] (reproduced from American Institute of Physics).

As mentioned in Section 2.1.1, toward efficient emission from the RE or Bi ion-activated nanoporous zeolites, reducing or removing the hydrated water molecules is the key point. We found that co-loading Ag+ ions into the cavities of zeolite Y can help collapse zeolite Y's structure; see **Figure 1**. Partial vitrification of the Nd3+ ion-exchanged zeolite Y was achieved at 900°C without extra crystalline phase introduction at the meantime. Partial elimination of water molecules can be identified from the FTIR spectra, where the absorption intensity due to the vibration of water molecules and hydroxyls both decreased for the partial vitrified sample, as shown in **Figure 2** (left). Due to the nonreversible dehydration in the vitrified parts of the partially collapsed Nd3+ ion-exchanged zeolite Y microcrystals, a six-fold enhancement of the 1053 nm NIR emission was achieved, as shown in **Figure 2** (right).

**Figure 2.** Left, FTIR spectra for the pure zeolite Y, Nd3+ ion-exchanged zeolite Y, and Nd3+, Ag+ ion-exchanged zeolite Y under different thermal treatment conditions; right, NIR emission spectra of the Nd3+ (and Ag+ ) ion-exchanged zeolite Y samples under different thermal treatment conditions [17] (reproduced from American Institute of Physics).

#### **2.3. Up-conversion emission**

emission efficiency of the RE and Bi ion-exchanged zeolites. To achieve efficient luminescence from these types of optically activated zeolites and its derivatives, dehydration and then preventing rehydration via stuffing the entrance for water molecules [15], or filling the cavities with molecules or radicals of low vibrational energy [16], are two often adopted methods.

On the other hand, interestingly, it has been demonstrated in our recent work that the hydrated water molecules are essential to the strong yellow-green emission from the Ag+ ion-exchanged LTA zeolites, which can be attributed to the coordination to the Ag+ ions that ensures an efficient ligand to metal charge transfer (LMCT) process. This part will be discussed with more

**2.2. Down-conversion near-infrared (NIR) emission from Nd3+ ion-exchanged zeolite Y**

Nd3+ is a representative RE ion as the NIR luminescence-active center in various matrices (one peak at ~1.06 μm and the other at ~1.35 μm), for instance, the Nd3+-doped phosphate glass or the Nd3+-doped yttrium aluminum garnet (usually denoted as Nd3+:YAG) single crystal for

**Figure 1.** TEM morphology of pure zeolite Y (a), Nd3+ singly exchanged zeolite Y (b), and Nd3+, Ag+ ion-exchanged

As mentioned in Section 2.1.1, toward efficient emission from the RE or Bi ion-activated nanoporous zeolites, reducing or removing the hydrated water molecules is the key point. We

structure; see **Figure 1**. Partial vitrification of the Nd3+ ion-exchanged zeolite Y was achieved at 900°C without extra crystalline phase introduction at the meantime. Partial elimination of water molecules can be identified from the FTIR spectra, where the absorption intensity due to the vibration of water molecules and hydroxyls both decreased for the partial vitrified sample, as shown in **Figure 2** (left). Due to the nonreversible dehydration in the vitrified parts of the partially collapsed Nd3+ ion-exchanged zeolite Y microcrystals, a six-fold enhancement

ions into the cavities of zeolite Y can help collapse zeolite Y's

zeolite Y (c) individual particles treated at 900°C [17] (reproduced from American Institute of Physics).

of the 1053 nm NIR emission was achieved, as shown in **Figure 2** (right).

details in Section 3.

140 Zeolites - Useful Minerals

solid-state laser generation.

found that co-loading Ag+

Generally, there are few reports on the up-conversion (UC) emission from the RE ionexchanged zeolites. This is probably due to the detrimental influence from hydrated water in the cavities, which can further decrease the low quantum efficiency of the up-conversion emission (usually less than 2%). Still, efforts have been devoted to this field, via extra coupling of molecules or radicals of low vibrational quanta, such as F<sup>−</sup> and WO4 2− [18], or partial dehydration by thermal treatment, or complete dehydration via collapsing the crystalline zeolite structure into glass-phase derivatives [19].

By employing the same method and taking advantage of blockade against water molecules by bismuth aggregates, that is, co-loading of bismuth ions into the zeolite Y's cavities combined with post-thermal treatment at 900°C, up-conversion green and red emissions have been achieved for the Er3+, Yb3+, and Bi3+ ions co-loaded zeolite Y powders [20].

#### **2.4. Bismuth ion-exchanged zeolites for broad NIR emissions**

#### *2.4.1. Significance of the broad NIR emission from bismuth-activated materials*

Since the first demonstration of broad NIR emission from bismuth-doped silica glass by Y. Fujimoto in 2001, research on the bismuth-activated broad NIR emitters has been conducted in various host lattices, involving nanocrystals, glasses, single crystals, and fibers (including both glass and crystalline fibers). The broad NIR (and mid-IR) emission from bismuthactivated luminescent materials falls well in the fiber-optic communication bands and hence is very promising as an alternative to the present commercial erbium (Er3+)- or thulium (Tm3+) doped fibers and the erbium-doped fiber amplifiers (EDFA) for fiber-optic communication.

#### *2.4.2. Broad NIR luminescence from bismuth ion-exchanged FAU zeolites and exploration on the NIR luminescent bismuth species*

Exploration on the specific luminescent bismuth species is essential to promote the practical application of bismuth-doped materials, though it is complicated and challenging due to the rich oxidation states of bismuth ions. On the bismuth ion-exchanged zeolites, Sun and coworkers have contributed a series of work to the efficient broad NIR emission and the exploration on the bismuth species for the broad NIR emissions [15, 21, 22]. FAU zeolites, especially the Y-type possessing high-temperature structural duration due to the high Si/Al ratio, which is in favor of the dehydration process via relatively high-temperature thermal treatment, are chosen as the typical matrix for bismuth ions. Meantime, some by-product generated during the thermal treatment process may act as blockades for the Re-entry of water molecules. For instance, bismuth aggregates were proposed to be formed during the thermal annealing at 950°C and can prevent the water molecules from Re-entering the sodalite cages of zeolite Y; hence, strong NIR emission was achieved for the bismuth ion-exchanged zeolite Y powders, as depicted in **Figure 3**.

**Figure 3.** (a–c) Schematic illustration of the structure transformation of the zeolites annealed at different temperatures. (d) 2D representation of the structure of an aluminosilicate network. Note that "loose" and "dense" just represent small and large possibilities of sealing the pores of the zeolites with Bismuth agglomerates [15] (reproduced from John Wiley and Sons).

By high-resolution synchrotron X-ray diffraction (HR-XRD) (see **Figure 4**), photoluminescence (PL) spectra, electron spin resonance (ESR), nuclear magnetic resonance (NMR), and quantum chemistry calculation based on the spin-restricted relativistic time-dependent density func‐ tional theory (TDDFT), the broad emissions are assigned to several types of Bi+ substructures in the sodalite cages of zeolite Y. Bi+ , Bi2 2+, Bi3 3+, and Bi4 4+ are proposed to be the active lumi‐ nescence centers in the sodalite cages of zeolite Y. Specifically, NIR emissions at 1050 nm, 1135 nm, 1145 nm, and 1240/1285 nm are assigned to Bi+ , Bi2 2+, Bi3 3+, and Bi4 4+, respectively (**Figure 5**).

*2.4.2. Broad NIR luminescence from bismuth ion-exchanged FAU zeolites and exploration on the NIR*

Exploration on the specific luminescent bismuth species is essential to promote the practical application of bismuth-doped materials, though it is complicated and challenging due to the rich oxidation states of bismuth ions. On the bismuth ion-exchanged zeolites, Sun and coworkers have contributed a series of work to the efficient broad NIR emission and the exploration on the bismuth species for the broad NIR emissions [15, 21, 22]. FAU zeolites, especially the Y-type possessing high-temperature structural duration due to the high Si/Al ratio, which is in favor of the dehydration process via relatively high-temperature thermal treatment, are chosen as the typical matrix for bismuth ions. Meantime, some by-product generated during the thermal treatment process may act as blockades for the Re-entry of water molecules. For instance, bismuth aggregates were proposed to be formed during the thermal annealing at 950°C and can prevent the water molecules from Re-entering the sodalite cages of zeolite Y; hence, strong NIR emission was achieved for the bismuth ion-exchanged zeolite

**Figure 3.** (a–c) Schematic illustration of the structure transformation of the zeolites annealed at different temperatures. (d) 2D representation of the structure of an aluminosilicate network. Note that "loose" and "dense" just represent small and large possibilities of sealing the pores of the zeolites with Bismuth agglomerates [15] (reproduced from John

By high-resolution synchrotron X-ray diffraction (HR-XRD) (see **Figure 4**), photoluminescence (PL) spectra, electron spin resonance (ESR), nuclear magnetic resonance (NMR), and quantum chemistry calculation based on the spin-restricted relativistic time-dependent density func‐

nescence centers in the sodalite cages of zeolite Y. Specifically, NIR emissions at 1050 nm,

3+, and Bi4

substructures

4+ are proposed to be the active lumi‐

tional theory (TDDFT), the broad emissions are assigned to several types of Bi+

, Bi2 2+, Bi3

*luminescent bismuth species*

142 Zeolites - Useful Minerals

Y powders, as depicted in **Figure 3**.

Wiley and Sons).

in the sodalite cages of zeolite Y. Bi+

**Figure 4.** Rietveld fit to the high-resolution synchrotron XRD pattern of dehydrated Bi-embedded zeolite Y. Plots show the observed and calculated powder patterns. A difference curve is shown at the bottom of the diagram. Inset: a loga‐ rithmic-scale version of the graph [21] (reproduced from American Chemical Society).

**Figure 5.** Theoretical absorption spectra for Bi4 4+ (a), Bi3 3+ (b), and Bi2 2+ (c). The energy-level diagram of Bi+ (d) was drawn according to the experimental absorption spectrum of AlCl3-NaCl molten salt containing Bi+ [21] (reproduced from American Chemical Society).

#### **2.5. Potential applications of the down- and up-conversion emission from RE and Bi ionexchanged zeolites**

Sun et al. demonstrated the bio-imaging in the second biological window with the amorphous aluminosilicate nanoparticles derived from FAU and LTA zeolites nanocrystals. The FAU and LTA zeolite nanocrystals are synthesized by the hydrothermal growth [23]. Bismuth ions were incorporated into the zeolite FAU and LTA nanocrystals' cavities via ion exchange and then subjected to thermal treatment at 870°C for 15 min, after which amorphous Bi ion-doped aluminosilicate nanoparticles were obtained. Finally, a 22 nm-thick silica layer was coated on the surface of Bi ion-doped aluminosilicate amorphous nanoparticles to improve the biocom‐ patibility. In vivo bio-imaging in the second biological window (1–1.35 μm) for a mouse was successfully demonstrated by employing the silica-coated Bi ion-doped aluminosilicate nanoparticles derived from FAU and LTA zeolite nanocrystals.

Two main features of this material for in vivo bio-imaging applications are the relatively long emission decay time (hundreds of microseconds), which is in favor of the fluorescence lifetime imaging microscopy, and the emission wavelength in the second biological window, which provides higher spatial resolution than that of bio-imaging based on fluorescence in the first biological window (0.65–0.95 μm) [24].

Besides bio-imaging, down-conversion NIR emission from RE and Bi ion-exchanged zeolites which can split one UV-vis photon into two or more NIR photons may also have application as the converter layer to improve the conversion efficiency of crystalline silicon solar cells.

#### **2.6. Possible future work**

So far, average sizes of most commercial available zeolite powder matrices are in the range of micrometer or sub-micrometer scales; for instance, the typical edge length of the LTA zeolite cube (micro-single crystal) is about 1–2 μm. This size is too large for bio-imaging applications. Nanoporous zeolite nanocrystals with the average size from tens of, to several hundred nanometers, may be possible to get dispersed in water to facilitate the bio-applications.

Further detailed structure characterization of the RE and Bi ion-exchanged and thermal-treated zeolite micro-/nanocrystals is worth conducting, since the luminescent active centers incor‐ porated in the glass-phase parts may also contribute to the emission.

It is worth trying whether it is possible to observe up-conversion emission color tuning via adsorption/de-adsorption of molecules, for example, small polar molecules such as H2O, H2S, and NH3 or small organic molecules. Tuning of the UC emission color is expected to be enabled by the overall phonon energy change of the whole host-guest system upon adsorption/deadsorption of different guest molecules of different phonon energies.

### **3. d10 ion-exchanged zeolites**

**2.5. Potential applications of the down- and up-conversion emission from RE and Bi ion-**

Sun et al. demonstrated the bio-imaging in the second biological window with the amorphous aluminosilicate nanoparticles derived from FAU and LTA zeolites nanocrystals. The FAU and LTA zeolite nanocrystals are synthesized by the hydrothermal growth [23]. Bismuth ions were incorporated into the zeolite FAU and LTA nanocrystals' cavities via ion exchange and then subjected to thermal treatment at 870°C for 15 min, after which amorphous Bi ion-doped aluminosilicate nanoparticles were obtained. Finally, a 22 nm-thick silica layer was coated on the surface of Bi ion-doped aluminosilicate amorphous nanoparticles to improve the biocom‐ patibility. In vivo bio-imaging in the second biological window (1–1.35 μm) for a mouse was successfully demonstrated by employing the silica-coated Bi ion-doped aluminosilicate

Two main features of this material for in vivo bio-imaging applications are the relatively long emission decay time (hundreds of microseconds), which is in favor of the fluorescence lifetime imaging microscopy, and the emission wavelength in the second biological window, which provides higher spatial resolution than that of bio-imaging based on fluorescence in the first

Besides bio-imaging, down-conversion NIR emission from RE and Bi ion-exchanged zeolites which can split one UV-vis photon into two or more NIR photons may also have application as the converter layer to improve the conversion efficiency of crystalline silicon solar cells.

So far, average sizes of most commercial available zeolite powder matrices are in the range of micrometer or sub-micrometer scales; for instance, the typical edge length of the LTA zeolite cube (micro-single crystal) is about 1–2 μm. This size is too large for bio-imaging applications. Nanoporous zeolite nanocrystals with the average size from tens of, to several hundred nanometers, may be possible to get dispersed in water to facilitate the bio-applications.

Further detailed structure characterization of the RE and Bi ion-exchanged and thermal-treated zeolite micro-/nanocrystals is worth conducting, since the luminescent active centers incor‐

It is worth trying whether it is possible to observe up-conversion emission color tuning via adsorption/de-adsorption of molecules, for example, small polar molecules such as H2O, H2S, and NH3 or small organic molecules. Tuning of the UC emission color is expected to be enabled by the overall phonon energy change of the whole host-guest system upon adsorption/de-

porated in the glass-phase parts may also contribute to the emission.

adsorption of different guest molecules of different phonon energies.

nanoparticles derived from FAU and LTA zeolite nanocrystals.

biological window (0.65–0.95 μm) [24].

**2.6. Possible future work**

**exchanged zeolites**

144 Zeolites - Useful Minerals

#### **3.1. One question about the reversible color change of Ag+ ion-exchanged LTA zeolites**

The reversible color change of Ag+ ion-exchanged LTA was reported by Rálek et al. [25]. This color change was generally attributed to formation of Agm n+ clusters [26]. Till 1998, this phenomenon was explained in the way of charge transfer transition from the framework oxygen to the empty 5s orbit of the extra-framework Ag+ ions [27]. This reversible color change can be achieved under very low Ag+ ion loading concentration (Ag+ -Na+ exchanging ratio equals 1/96) and dehydration at room temperature in vacuum without thermal treatment, in which condition, in principle Ag clusters were almost impossible to be formed due to the considerably light loading of Ag+ ions. Interestingly, except for the assignment of the reversible color change, most of the later work on luminescence from the Ag+ ion-exchanged zeolites still assigned the few-atom silver clusters [28–30], such as Ag3 <sup>+</sup> and Ag6 <sup>+</sup> to the luminescence origin other than the charge transfer transition [31, 32].

#### **3.2. Methods to activate the Ag+ ion-exchanged nanoporous luminescent zeolites**

So far, various methods have been tried to activate the luminescence from the Ag+ ionexchanged nanoporous zeolites, such as UV laser illumination, X-ray irradiation, and the most often adopted thermal treatment. The role of UV laser in activating the visible emission can be observed by sharp contrast for one piece of Ag+ ion-exchanged micro-crystal through com‐ paring the regions with/without light irradiation. The mechanism of activation by UV illumination [33] and X-ray irradiation [34] was explained in the way of photo-reduction of Ag+ into Ag0 atoms and then the formation of Agn m+ clusters. It should be pointed out that besides the explanation from the view of formation of Agn m+ clusters, dehydration by the heat effect under the irradiation of high energy light beam may also contribute to the luminescence activation.

#### **3.3. Reversible emission variation of Ag+ ion-exchanged LTA and SOD zeolites and possible mechanisms for this process**

It should be noted that the terminology "cluster" is widely adopted among different com‐ munities for different types of metal nanospecies. For instance, for metal-organic complexes, aggregates of metal ions in close contact via the weak metallophilic interaction are also called clusters (short for cluster A hereinafter). During the past two decades, few-atom, sub-nm scale (below 1 or 2 nm) noble and coinage (Au, Ag, Pt, Pd, and Cu) metal nanospecies bonded via the metal-metal bonds as the same as in bulk metals or molecular-like bonds are also called clusters (short for cluster B hereinafter).

So far, the origin of emission from cluster B is still not sufficiently clear. At present, explanations from the following two ways have been mainly proposed: one is based on the free electron theory combined with quantum confinement effect; the other is based on the super atom theory. To explore the emission mechanisms of the metal clusters, first the interaction type between the neighboring metal ions or atoms needs to be clarified. It is a pity that this issue did not seem to be clearly addressed in lots of the previous work.

**Figure 6.** PL evolution of Ag4.4, Na7.6-A from air to vacuum then to water vapor (a) or oxygen (b) [35] (reproduced from American Institute of Physics).

**Figure 7.** Temperature-dependent PL spectra for Ag4.4, Na7.6-A under the 355 nm excitation [35] (reproduced from American Institute of Physics).

Recently, H2O molecule-sensitive, reversible green/red-dominant emission evolution was observed for the Ag+ ion-exchanged LTA zeolite (see **Figure 6**), and different from most of the previous reports, the yellow-green emission is not assigned to Agn m+ cluster [35]. Confirmation of the role of H2O molecules in the reversible emission evolution was achieved with the assistance of a designed vacuum chamber, into which different gas species such as N2, O2, and H2O vapor can be introduced. The reversible emission evolution process observed from the Ag+ ion-exchanged LTA zeolites upon desorption/adsorption of H2O molecules led us to reflect that the interaction between the neighboring Ag+ -Ag+ ions does not seem to be the metal bonds while it may be the weak argentophilic (Ag+ ---Ag+ ) interaction.

between the neighboring metal ions or atoms needs to be clarified. It is a pity that this issue

**Figure 6.** PL evolution of Ag4.4, Na7.6-A from air to vacuum then to water vapor (a) or oxygen (b) [35] (reproduced from

**Figure 7.** Temperature-dependent PL spectra for Ag4.4, Na7.6-A under the 355 nm excitation [35] (reproduced from

Recently, H2O molecule-sensitive, reversible green/red-dominant emission evolution was observed for the Ag+ ion-exchanged LTA zeolite (see **Figure 6**), and different from most of the

of the role of H2O molecules in the reversible emission evolution was achieved with the assistance of a designed vacuum chamber, into which different gas species such as N2, O2, and

m+ cluster [35]. Confirmation

previous reports, the yellow-green emission is not assigned to Agn

did not seem to be clearly addressed in lots of the previous work.

American Institute of Physics).

146 Zeolites - Useful Minerals

American Institute of Physics).

**Figure 8.** Single-crystal emission characterization for the Ag3 + clusters inside zeolite A upon UV excitation. (a) The indi‐ vidual crystals have sizes ranging from 2 to 5 μm. (b) Emission spectra of Ag6,Na-A zeolites upon 400-nm illumination. The black curve corresponds to the bulk spectrum (measured in a fluorimeter), while the colored curves represent spectra from individual crystals measured on a confocal fluorescence microscope [28] (reproduced from the American Chemical Society).

There is no obvious narrowing of the emission band in the PL spectra of the Ag+ ion-exchanged LTA zeolites taken at cryogenic temperature as low as 10 K (**Figure 7**). Pioneering work on comparing the PL spectra of the Ag+ ion-exchanged LTA zeolite micro-crystals' ensemble and individual micro-crystals proves that there is no obvious inhomogeneous broadening contri‐ buting to the broad emission, as shown in **Figure 8**.

The H2O molecule-sensitive, reversible emission evolution and the broad emission band at 10 K, together with the negligible contribution from inhomogeneous broadening, suggest that the emission from the Ag+ ions containing LTA zeolites is not attributed to the quantum confinement effect. Corresponding to the origin of the reversible color change of Ag+ ionexchanged LTA zeolites upon dehydration/hydration, the green emission can be assigned to the framework O2− → Ag<sup>+</sup> (coordinated by water) ligand to metal charge transfer (LMCT) transition. Since there are two sets of cages (α-cages and β-cages) in LTA zeolites, it is difficult to propose further detailed physical scenario for the H2O molecule-sensitive reversible green/ red dominant emission evolution. So SOD zeolite, which consists of the sodalite cages and has very similar chemical composition to LTA zeolite, was employed as the matrix for Ag+ ions, in hope to provide more specific information about the H2O molecule-sensitive emission phenomenon.

**Figure 9.** Excitation-emission 3D contours of non-exchanged SOD-B250°C (a), Ag1Na7-SOD-B250°C (b), and Ag8Na0-SOD-B250°C (c) [36] (reproduced from Royal Society of Chemistry).

The PL characteristics of the Ag+ ion-exchanged SOD zeolites with different Ag+ loading concentrations are similar to the Ag+ ion-exchanged LTA counterparts. That is, under low Ag <sup>+</sup> loading concentration, only green emission can be observed, while under high Ag+ loading concentration, red emission can be observed, as shown in **Figure 9**. Though not being as obvious as the situation for the Ag+ ion-exchanged LTA zeolites, similar H2O moleculesensitive, reversible emission variation of the Ag+ ion-exchanged SOD zeolites in vacuum/air or water vapor environment was observed [36], as shown in **Figure 10**.

**Figure 10.** PL spectra recorded in different environmental conditions for Ag1Na7-SOD-B250°C (a) and Ag8Na0-SOD-B250°C (b) [38] (reproduced from Royal Society of Chemistry).

The low-lying excited states of Ag+ ions (aggregates) enabling the visible or even NIR emission are commonly seen in the Ag(I)-organic complexes. In contrast, this phenomenon is rare for inorganic materials. So far, several types of inorganic materials have been reported with this kind of capability, such as the Ag+ -doped (or containing) oxyfluoride glasses, K[Ag(I)CN2] [37], or simply the Ag+ ion-exchanged LTA and SOD zeolites. Note that for the Ag+ ion-exchanged zeolites, it is shown that the amount of negative charges in zeolites' framework seems to have a key role in tuning down the emission energy, since sharp contrast has been observed between the emission energy of Ag+ ion-exchanged LTA (or SOD) and Ag+ ion-exchanged ZSM-5 zeolites. Specifically, green or red visible emissions can be observed from the Ag+ ionexchanged LTA (or SOD) zeolites (Al/Si ratio = 1), while only emissions in the UV region were observed from the Ag+ ion-exchanged ZSM-5 zeolites (Al/Si ratio = 11.65). This is in good accordance with conclusions that the high-lying excited state of Ag+ can be modified via ligand to metal or metal to metal charge transfer transition, especially when a reducing ligand is present [38].

With the complementary information obtained from the Ag+ ion-exchanged SOD zeolites, a specific physical scenario for the H2O molecule-sensitive reversible emission variation is proposed: the yellow-green emission is due to the ligand to metal (framework O2− → Ag<sup>+</sup> ) charge transfer transition within the partial hydration state, and the red emission is attributed to the ligand to metal-metal (framework O2− → Ag<sup>+</sup> -Ag+ ) or metal-metal (Ag+ -Ag+ ) charge transfer transition upon dehydration when the water molecules which block the interaction between the neighboring Ag+ ions are removed (see **Figure 11**).

**Figure 9.** Excitation-emission 3D contours of non-exchanged SOD-B250°C (a), Ag1Na7-SOD-B250°C (b), and Ag8Na0-SOD-

concentrations are similar to the Ag+ ion-exchanged LTA counterparts. That is, under low Ag

concentration, red emission can be observed, as shown in **Figure 9**. Though not being as

**Figure 10.** PL spectra recorded in different environmental conditions for Ag1Na7-SOD-B250°C (a) and Ag8Na0-SOD-B250°C

are commonly seen in the Ag(I)-organic complexes. In contrast, this phenomenon is rare for inorganic materials. So far, several types of inorganic materials have been reported with this

zeolites, it is shown that the amount of negative charges in zeolites' framework seems to have a key role in tuning down the emission energy, since sharp contrast has been observed between

ion-exchanged LTA (or SOD) and Ag+

or simply the Ag+ ion-exchanged LTA and SOD zeolites. Note that for the Ag+

<sup>+</sup> loading concentration, only green emission can be observed, while under high Ag+

or water vapor environment was observed [36], as shown in **Figure 10**.

ion-exchanged SOD zeolites with different Ag+ loading

ion-exchanged LTA zeolites, similar H2O molecule-

ions (aggregates) enabling the visible or even NIR emission


ion-exchanged SOD zeolites in vacuum/air

loading

ion-exchanged

ion-exchanged ZSM-5

B250°C (c) [36] (reproduced from Royal Society of Chemistry).

sensitive, reversible emission variation of the Ag+

The PL characteristics of the Ag+

148 Zeolites - Useful Minerals

obvious as the situation for the Ag+

(b) [38] (reproduced from Royal Society of Chemistry).

The low-lying excited states of Ag+

kind of capability, such as the Ag+

the emission energy of Ag+

**Figure 11.** Proposed emission mechanisms in Ag+ ion-exchanged LTA (a) and SOD (b) zeolites [36] (reproduced from Royal Society of Chemistry).

Though the emission variation tendency for the Ag+ ion-exchanged SOD zeolites is similar to that of the Ag+ ion-exchanged LTA zeolites, the recovery time for the emission variation from vacuum to H2O vapor is much slower (~30 min) than the counterpart of Ag+ ion-exchanged LTA zeolites (≤30 s). This can be attributed to the slow diffusion of H2O molecules in SOD zeolites consisting of only the β-cages without straight, large channels (critical size 2.53 Å). In LTA, H2O molecules can fast permeate along the 3D channels with the critical size of 4.21 Å confined by the 8-rings of the α-cage.

Location preference of the Ag+ ions and Agn m+ clusters in LTA zeolite (either in the α-cage or the β-cage) is still controversial [39, 40]. From the contrasting response time of emission variation between the Ag+ ion-exchanged LTA and SOD zeolites, it is suggested that for LTA zeolite Ag+ ions are mainly located in the α-cage.

#### **3.4. Potential optical and photonic applications of the Ag+ ion-exchanged nanoporous zeolites**

So far, by employing Ag+ ion-exchanged nanoporous zeolites, promising optical and photonic applications have been demonstrated, such as optical recording and LED phosphors. Besides, the emission color change accompanied with reversible adsorption/de-adsorption of H2O may have application as photoluminescence responsive sensors for small polar molecules or molecular tweezers.

#### **Author details**

Hui Lin and Minoru Fujii\*

\*Address all correspondence to: fujii@eedept.kobe-u.ac.jp

Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Kobe, Japan

#### **References**


**3.4. Potential optical and photonic applications of the Ag+**

\*Address all correspondence to: fujii@eedept.kobe-u.ac.jp

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**zeolites**

150 Zeolites - Useful Minerals

So far, by employing Ag+

molecular tweezers.

**Author details**

Hui Lin and Minoru Fujii\*

University, Kobe, Japan

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applications have been demonstrated, such as optical recording and LED phosphors. Besides, the emission color change accompanied with reversible adsorption/de-adsorption of H2O may have application as photoluminescence responsive sensors for small polar molecules or

Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe

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32, 29.

152 Zeolites - Useful Minerals

In this chapter, studies of structures and optical properties of Ag-zeolite A by means of X-ray absorption fine structure (XAFS) and infrared (IR) spectra are presented. XAFS is a powerful tool to study the local structure of Ag nanoclusters in the zeolite cavity and IR spectra are quite sensitive to the change in zeolite lattice affected by the production of Ag clusters. First, we focus on the creation of Ag clusters in zeolite A by heat treatment under atmosphere and vacuum. Second, we discuss the mechanism of photolumines‐ cence for Ag-zeolite composite. It is widely believed that the emitting point is Ag clusters in the Ag zeolite; on the other hand, our recent result is contradicted that Ag clusters are broken down in the strongly emitting species.

**Keywords:** silver zeolite A, Ag cluster, photoluminescence, XAFS, IR spectroscopy

#### **1. Introduction**

Ag zeolites are versatile family of functional materials: application to catalysis, anti-bacterial materials, information storage, pressure, and chemical sensors. In the field of nanoscaled science, the metal nanoparticles and nanoclusters are quite quite attractive. Using the zeolite cavity, nanoparticles and clusters are produced simply and the particle size is easily controlled [1–4]. However, the structural effect of zeolite lattice is not so well known that the structure and properties of nanoparticles and clusters are quite complicated.

Optical properties of small metal clusters are intriguing research topics today. Luminescence from the metal clusters is an attractive subject and especially silver and gold clusters show brilliant emission. This luminescent behavior can be expected by industry market without rareearth metals. Recently, the characterization of photo luminescence from heat-treated Ag zeolites is extensively investigated [5–7]. So far, the luminescent species are widely believed

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

to be Ag clusters or oligomers produced in the cavity of zeolite. However, there are still ambiguous points and hot discussions in the field of luminescent Ag zeolite.

X-ray absorption fine structure (XAFS) has characteristic features that elemental selectively analyzed with local structure for such complex systems. In the case of Ag zeolites, the local and electronic structure around Ag atoms has been extensively investigated [8–11]. On the other hand, infrared (IR) spectra are sensitive to molecular atomic vibration of zeolite lattice. In this chapter, we present the introduction of XAFS and IR spectroscopy first and then discuss (1) the creation of Ag clusters in the cavity of zeolite A and (2) the mechanism of photolumi‐ nescence (PL) of Ag-zeolite composites by the use of XAFS and IR spectra [12].

#### **2. X-ray absorption fine structure and infrared spectra**

In this section, we show the outline of the tools for the investigation of the structure and electronic states of Ag zeolites: XAFS and IR.

#### **2.1. XAFS spectroscopy**

We present the basic description of XAFS technique theoretically and experimentally. XAFS is the fine structure observed on X-ray absorption coefficient spectra near the absorption edge of a particular element. When monochromatic X-rays are irradiated on a sample, the intensity of the transmitted X-ray, *I*<sup>t</sup> , depends on the photon energy *E* and sample thickness *d* (**Figure 1**). According to the Beer-Lambert law, *I*t is expressed as

**Figure 1.** Schematic representation of transmission and fluorescence mode for X-ray absorption.

$$I\_t = I\_0 e^{-\mu(E)d} \tag{1}$$

Here, *I*0 is the intensity of the incident X-ray and *μ* (*E*) is the energy-dependent X-ray absorption coefficient. Since *μ*(*E*) is approximately proportional to *a Z*<sup>4</sup> /*m E*<sup>3</sup> where *a*, *Z*, and *m* are, respectively, the density of the sample, atomic number, and electron mass, the *μ*(*E*) decreases monotonically as a function of X-ray energy. When the X-ray energy has an exciting energy of a core electron, the core electron ejects to continuum state. In that time, the X-ray photon is annihilated in order to create a photoelectron and a core hole. As the result, *μ*(*E*) shows stepwise increase at the energy of core electron excitation (absorption edge). The core hole recombines with an electron from the higher energy level; X-ray fluorescence or the creation of an Auger electron is emitted.

to be Ag clusters or oligomers produced in the cavity of zeolite. However, there are still

X-ray absorption fine structure (XAFS) has characteristic features that elemental selectively analyzed with local structure for such complex systems. In the case of Ag zeolites, the local and electronic structure around Ag atoms has been extensively investigated [8–11]. On the other hand, infrared (IR) spectra are sensitive to molecular atomic vibration of zeolite lattice. In this chapter, we present the introduction of XAFS and IR spectroscopy first and then discuss (1) the creation of Ag clusters in the cavity of zeolite A and (2) the mechanism of photolumi‐

In this section, we show the outline of the tools for the investigation of the structure and

We present the basic description of XAFS technique theoretically and experimentally. XAFS is the fine structure observed on X-ray absorption coefficient spectra near the absorption edge of a particular element. When monochromatic X-rays are irradiated on a sample, the intensity

, depends on the photon energy *E* and sample thickness *d*

ambiguous points and hot discussions in the field of luminescent Ag zeolite.

nescence (PL) of Ag-zeolite composites by the use of XAFS and IR spectra [12].

**2. X-ray absorption fine structure and infrared spectra**

(**Figure 1**). According to the Beer-Lambert law, *I*t is expressed as

**Figure 1.** Schematic representation of transmission and fluorescence mode for X-ray absorption.

electronic states of Ag zeolites: XAFS and IR.

**2.1. XAFS spectroscopy**

154 Zeolites - Useful Minerals

of the transmitted X-ray, *I*<sup>t</sup>

In the X-ray energy region generally 60–1000 eV from the absorption edge, the equation of the absorption coefficient can be simplified under the following approximations as the Muffin-tin approximation for the atomic potential, the one-electron process in the absorption, the dipole approximation for the interaction between X-ray and matter, the plane-wave approximation for the propagating photoelectron, and the single scattering process considered in the scatter‐ ing process. The X-ray absorption profile in this high-energy region is called Extended XAFS (EXAFS), where function *χ*(*k*) is expressed as

$$\chi(k) = \sum\_{j} \frac{N\_j S\_0^2}{k r\_j^2} \left| f\_j(k, \pi) \right| e^{\left( \frac{-2(\sigma\_j^2 k^2 + r\_j/\lambda(k))}{\lambda} \right)} \sin \left( 2kr\_j + 2\delta\_i(k) + \phi\_j(k) \right), \tag{2}$$

where *rj* , *Nj* , and *σ<sup>j</sup>* are the interatomic distance between X-ray absorbing atom *i* and electron scattering atom *j*, the coordination number of scattering atom *j*, and the root-mean-square relative displacement (or Debye-Waller factor) of the atomic pair ij, respectively. These unknown structural parameters can be determined by the use of the nonlinear least-squarefitting (the so-called curve-fitting) method; for example, XANADU [13] code and Athena & Artemis code [14]. The phase shift of the photoelectron *ϕ<sup>j</sup> k*), the electron backscattering amplitude *fj* (*k*), and the electron mean free path "lambda(symbol)" (k) in the medium are generally calculated from theoretically, for example, FEFF code [15].

The X-ray absorption coefficient can be determined from the ratio of intensities for the incident and transmitted X-ray as transmission mode, X-ray fluorescence as fluorescence mode, or Auger electrons as electron yield mode.

The intensity of incident and transmitted X-ray is measured using ionization chambers filled by inert gas. For transmission measurements, appropriate concentration is important for sample preparation. And then, the samples are required to be homogeneous and to have a constant thickness without pinholes. the samples are prepared by grinding and mixed with a suitable binder such as BN as transparent to X-rays in general.

We apply the transmission mode for the XAFS measurement of Ag zeolites. The Ag *K*-edge (25 keV) XAFS were measured at NW10A on PF-AR at KEK using Si(311) monochromator. We use in situ sample chamber shown in **Figure 2**, in which temperature can be controlled from LN2 temperature to 773 K and the introduction of the various gasses is available.

**Figure 2.** In situ cell chamber for the measurement of Ag *K*-edge XAFS and IR spectra for Ag zeolites.

#### **2.2. IR spectroscopy: basics and measurements**

In this subsection, the method of infrared spectra is described. Vibrational spectroscopy has gained wide recognition for its ability to determine the chemical nature of materials. Three techniques have emerged to be most useful, namely Infrared Spectroscopy (IR), Electron Energy Loss Spectroscopy (EELS), and Raman Spectroscopy. Infrared spectroscopy has achieved high sensitivity at high resolution. Furthermore, this technique has proven to be most valuable due to its ability to work at ambient pressure from ultra-high vacuum (UHV) to atmosphere. EELS is restricted to UHV pressures and homogeneous (i.e., orders) surface. Since in the measurement of zeolite the coexistence gas is important, EELS is not suitable. Raman spectroscopy commonly used visible light as the excitation light, the energy is close to the PL excitation energy of Ag zeolite, and it would give a perturbation to the system. In addition, Raman spectroscopy has the disadvantage of sensitivity and is not suitable for the zeolite.

In XAFS, high-energy X-rays are used as probes, whereas low-energy infrared light is used in the IR as a probe. In the measurement of the Ag zeolite, XAFS detects the local structure around Ag atom but IR does that of the framework of zeolite. Thus, on measuring Ag zeolite, XAFS and IR can be said to be best partner for obtaining the complementary structural information.

The IR light absorption spectra were obtained at normal incidence of radiation. The infrared measurements were performed on an ABB BOMEM MB-100 spectrometer equipped with a DTGS detector at 4 cm−1 resolution with 128 interferometric scans. Zeolite powder was deposited on the Si wafer, and then an excessive specimen was blown off with a hand blower. A small quantity of zeolite was stuck electrostatically. The Si was installed with a sample into the in situ chamber cell as shown in **Figure 2**. The principle of signal collection is similar to the XAFS, as described in Eq. (1).

### **3. Sample preparation**

We apply the transmission mode for the XAFS measurement of Ag zeolites. The Ag *K*-edge (25 keV) XAFS were measured at NW10A on PF-AR at KEK using Si(311) monochromator. We use in situ sample chamber shown in **Figure 2**, in which temperature can be controlled from

LN2 temperature to 773 K and the introduction of the various gasses is available.

156 Zeolites - Useful Minerals

**Figure 2.** In situ cell chamber for the measurement of Ag *K*-edge XAFS and IR spectra for Ag zeolites.

In this subsection, the method of infrared spectra is described. Vibrational spectroscopy has gained wide recognition for its ability to determine the chemical nature of materials. Three techniques have emerged to be most useful, namely Infrared Spectroscopy (IR), Electron Energy Loss Spectroscopy (EELS), and Raman Spectroscopy. Infrared spectroscopy has achieved high sensitivity at high resolution. Furthermore, this technique has proven to be most valuable due to its ability to work at ambient pressure from ultra-high vacuum (UHV) to atmosphere. EELS is restricted to UHV pressures and homogeneous (i.e., orders) surface. Since in the measurement of zeolite the coexistence gas is important, EELS is not suitable. Raman spectroscopy commonly used visible light as the excitation light, the energy is close to the PL excitation energy of Ag zeolite, and it would give a perturbation to the system. In addition, Raman spectroscopy has the disadvantage of sensitivity and is not suitable for the zeolite.

**2.2. IR spectroscopy: basics and measurements**

The fully Ag+ -exchanged zeolite A was prepared by immersing the hydrated 12Na-A in 0.1-M AgNO3 solution for 24 h at 25 °C. The solution was stirred every 1 h. After careful filtration, the Ag-zeolite-A powder was dried under air at room temperature (RT) in the dark space.

We apply two kinds of methods for producing Ag clusters in the cavity as follows:


For both processes, the samples were kept for long time (about 24 h) after cooling to room temperature.

### **4. Creation of Ag clusters in zeolite A cavity**

In this section, we discuss the creation of Ag clusters in the zeolite cavity of zeolite A with heat treatment under atmosphere or vacuum studied by using XAFS.

**Figure 3** shows the typical example of Fourier transform (FT) of the Ag *K*-edge EXAFS spectra for the hydrated Ag-zeolite A measured at RT atmosphere and after heating at 300 °C under vacuum. We can find two main peaks in FT: the first peak at around 2.2 Å and the second one at around 3.0 Å. The first peak is higher than the second one for the hydrated Ag-zeolite A, while the second peak is higher than the first one for the dehydrated Ag-zeolite A under vacuum. This phenomenon indicates that the Ag clusters are produced by heating at least 300°C under vacuum.

**Figure 3.** Fourier transforms (FT) of the Ag *K*-edge EXAFS spectra for the hydrated Ag-zeolite A measured at RT at‐ mosphere and measured at RT after heating at 300 °C under vacuum [11].

**Figure 4** shows *k*<sup>2</sup> *χ*(*k*) spectra measured by quick EXAFS mode in the process of heating to 500°C atmosphere. Each spectrum was measured about 50 s and the corresponding sample temperatures are presented. The large structural change appears at around 150–300°C.

**Figure 4.** *k*<sup>2</sup> *χ*(*k*) spectra for quick EXAFS measurement in the process of heating to 500°C atmosphere.

**Figure 3.** Fourier transforms (FT) of the Ag *K*-edge EXAFS spectra for the hydrated Ag-zeolite A measured at RT at‐

500°C atmosphere. Each spectrum was measured about 50 s and the corresponding sample temperatures are presented. The large structural change appears at around 150–300°C.

*χ*(*k*) spectra for quick EXAFS measurement in the process of heating to 500°C atmosphere.

*χ*(*k*) spectra measured by quick EXAFS mode in the process of heating to

mosphere and measured at RT after heating at 300 °C under vacuum [11].

**Figure 4** shows *k*<sup>2</sup>

158 Zeolites - Useful Minerals

**Figure 4.** *k*<sup>2</sup>

**Figure 5.** (a) Plots of the interatomic distances, *r*, and (b) that of the coordination number, *N*, as a function of measure‐ ment temperature.

The structural parameters obtained from the curve-fitting method by the use of Eq. (2) are plotted in **Figure 5(a)** for the interatomic distance, *r,* and **(b)** for the coordination number, *N*.

The Ag clustering in zeolite A occurs in the temperature range from 150 to 300°C. In this temperature range, the distance of Ag-O (short) decreases because of dehydration, and that of Ag-Ag also decreases because of less repulsion force between Ag+ and Ag0 when Ag+ reduced by the dehydration. At the same temperature range, the increase of *N* for Ag-Ag indicates the Ag clustering process.

#### **5. Mechanism of photoluminescence from Ag-zeolite A composites**

In this section, we present the observation of PL and discuss the origin of the luminescent point in the Ag zeolite by the use of XAFS and IR. Many researchers believe that the luminescent points are Ag clusters themselves in the zeolite, but our result suggests that the origin of luminescence is the zeolite lattice after affected by the formation of Ag clusters in it.

First, we discuss the result of the PL measurement. **Figure 6** shows the PL curves for various introduction gases after heating of Ag-zeolite A in vacuum. The wavelength of the excitation light is 405 nm. The strongest intensity of PL band around 2.1 eV was observed when the air or the mixture of the H2O + N2 is introduced. It is suggested that the combination of H2O + N2 is important for the strong PL band. On the other hand, PL is not observed when the O2 was introduced. In the present stage, the role of the gases of O2, N2, and H2O for these phe‐ nomena has been unclear.

**Figure 6.** PL curves for various introduction gases after heating of Ag-zeolite A. The wave length of the excitation light is 405 nm [12].

**Figure 7** shows the EXAFS *k*<sup>2</sup> *χ*(*k*) spectra for Ag-zeolite A after following gases were intro‐ duced: Air, O2, N2, H2O + O2, H2O + N2 with unheated state. The structure around 3.5 Å−1 is returned to unheated state when air or H2O+N2 was introduced. This reversible change in *k*2 *χ*(*k*) spectra indicates the breakdown of the Ag clusters. On the other hand, the *k*<sup>2</sup> *χ*(*k*) spectra were not changed when O2 gas was introduced.

**5. Mechanism of photoluminescence from Ag-zeolite A composites**

luminescence is the zeolite lattice after affected by the formation of Ag clusters in it.

nomena has been unclear.

160 Zeolites - Useful Minerals

is 405 nm [12].

*k*2

**Figure 7** shows the EXAFS *k*<sup>2</sup>

were not changed when O2 gas was introduced.

In this section, we present the observation of PL and discuss the origin of the luminescent point in the Ag zeolite by the use of XAFS and IR. Many researchers believe that the luminescent points are Ag clusters themselves in the zeolite, but our result suggests that the origin of

First, we discuss the result of the PL measurement. **Figure 6** shows the PL curves for various introduction gases after heating of Ag-zeolite A in vacuum. The wavelength of the excitation light is 405 nm. The strongest intensity of PL band around 2.1 eV was observed when the air or the mixture of the H2O + N2 is introduced. It is suggested that the combination of H2O + N2 is important for the strong PL band. On the other hand, PL is not observed when the O2 was introduced. In the present stage, the role of the gases of O2, N2, and H2O for these phe‐

**Figure 6.** PL curves for various introduction gases after heating of Ag-zeolite A. The wave length of the excitation light

duced: Air, O2, N2, H2O + O2, H2O + N2 with unheated state. The structure around 3.5 Å−1 is returned to unheated state when air or H2O+N2 was introduced. This reversible change in

*χ*(*k*) spectra indicates the breakdown of the Ag clusters. On the other hand, the *k*<sup>2</sup>

*χ*(*k*) spectra for Ag-zeolite A after following gases were intro‐

*χ*(*k*) spectra

**Figure 7.** *k*<sup>2</sup> *χ*(*k*) spectra for Ag-type zeolite A after the following gases were introduced: Air, O2, N2, H2O + O2, H2O + N2 with unheated state.

**Figure 8** shows the Ag *K*-edge Fourier transform spectrum for Ag-zeolite A measured at room temperature in atmosphere and that after following gases were introduced: Air, O2, N2, H2O + O2, H2O + N2 with unheated state. The peak at ca 1.7 Å is assigned to the first nearest neighbor (1NN) O1 atom, and the second peak at ca 2.7 Å is to the second nearest neighbor (2NN) O2 and Ag atom. The three-shell curve-fitting technique was applied and the unknown structural parameters, the atomic distance *r* and the coordination number *N* for Ag-O1, Ag-O2, and Ag-Ag, were determined.

**Figure 8.** Ag *K*-edge Fourier transform spectrum for Ag-zeolite A measured at room temperature in atmosphere and that after following gases were introduced: Air, O2, N2, H2O + O2, H2O + N2 with unheated state [12].

**Table 1** shows the structural parameters for Ag-zeolite A after the various gases are intro‐ duced. We can see in **Figure 6** that after the Air is introduced the strong PL band is observed. The Ag clusters were broken down according to the introduction of the Air. The parameters of *r*Ag and *N*Ag are very close to those in atmospheric pressure for the room temperature, which mean no Ag clusters. It takes enough time as 24 hours to breakdown Ag cluster by the introduction of Air. The present PL and XAFS results lead to the fact that the existence of the Ag clusters inhibits the strong PL band. When the O2 gas is introduced, no PL band is observed. The *r*O1 and *r*Ag are constant after the O2 gas is introduced, and the *N*Ag is still large (*N*Ag ∽2.35), which indicates that the Ag clusters still exist. We consider the cluster change after the mixed gases of H2O + O2, and H2O + N2 are introduced. When H2O + O2 mixture gases were introduced, weak PL band is detected. On the other hand, strong PL band is observed in the case of H2O + N2. The *N*Ag-Ag evaluated from the curve-fitting method shows the breakdown of the Ag clusters similar to the case of the air or both cases H2O + O2 and H2O + N2 introduction.


**Table 1.** The structural parameters, *r* and *N* after the various gases are introduced [12].

**Figure 9** shows the IR result of Ag-zeolite A. The black line shows the IR transmission spectrum of the unheated Ag zeolite. Since the zeolite has a complicated structure and bands are superimposed on the various Si-O and Al-O, it is extremely difficult to distinguish between them. Therefore, assignment of the absorption band around 1000 cm−1 region is only repre‐ sented as ν(T-O) (T = Si or Al) [16]. Bands appearing at relatively high wavenumber are assigned to the νασ mode, and the low wavenumber region is assigned to νσ mode. Bending mode appears in the lower-energy region, not shown here.

The green line shows that the spectrum of the Ag zeolite was cooled to RT after heating for 24 h at 500 °C in vacuum (measured in a vacuum). As compared to the case of unheated, absorption band has been changed drastically. While individual band assignment is not known as mentioned above, considering the results of XAFS, it is reasonable to assume the change due to the formation of Ag clusters. For cluster formation, it is necessary that Ag+ ions (at least partially) are reduced to Ag<sup>0</sup> ; also Ag which is to be aggregated away from the site of Ag+ ion was located. Reduction of ions and changing the position also affect vibration of the framework in each case. (The distance changes between the framework and the Ag atoms and changes in the electrostatic attraction of Ag+ can perturb the vibration of the T-O. In other words, it can be said that the IR is a sensitive technique to such a change.)

The red line is the spectrum in the case of introducing Air into the zeolite, cooling to RT after the vacuum heating. The spectrum greatly changed again, and is much closer to the spectrum of the unheated spectrum indicated in the black line. While XAFS for the state that Air intake after heating substantially coincide with that unheated, IR spectra for both state do not mach completely each other. Distinct band appeared at 1140 cm−1 and the strength of the main band of 1000–900 cm−1 decreased slightly. The cause of this change is not made apparent. However, the state where PL is expressed, that is, in the state where the excitation light is illuminated, the strength of the main band increases and 1140 cm−1 band decreases (**Figure 10**). For these reasons, this band change is evident and directly related to PL. As a cause of this change, considering that a change in XAFS is not almost observed, there may be two hypotheses as follows:

**Table 1** shows the structural parameters for Ag-zeolite A after the various gases are intro‐ duced. We can see in **Figure 6** that after the Air is introduced the strong PL band is observed. The Ag clusters were broken down according to the introduction of the Air. The parameters of *r*Ag and *N*Ag are very close to those in atmospheric pressure for the room temperature, which mean no Ag clusters. It takes enough time as 24 hours to breakdown Ag cluster by the introduction of Air. The present PL and XAFS results lead to the fact that the existence of the Ag clusters inhibits the strong PL band. When the O2 gas is introduced, no PL band is observed. The *r*O1 and *r*Ag are constant after the O2 gas is introduced, and the *N*Ag is still large (*N*Ag ∽2.35), which indicates that the Ag clusters still exist. We consider the cluster change after the mixed gases of H2O + O2, and H2O + N2 are introduced. When H2O + O2 mixture gases were introduced, weak PL band is detected. On the other hand, strong PL band is observed in the case of H2O + N2. The *N*Ag-Ag evaluated from the curve-fitting method shows the breakdown of the Ag clusters similar to the case of the air or both cases H2O + O2 and H2O + N2 introduction.

*r***O1(Å)** *N***O1** *r***O2(Å)** *N***O2** *r***Ag(Å)** *NAg*

Air 2.38 4.57 2.86 2.96 2.86 1.40 O2 2.30 4.14 2.85 4.85 2.84 2.35 H2O + O2 2.39 6.73 2.86 5.19 2.88 0.94 H2O + N2 2.39 5.75 2.86 4.23 2.87 1.23

**Figure 9** shows the IR result of Ag-zeolite A. The black line shows the IR transmission spectrum of the unheated Ag zeolite. Since the zeolite has a complicated structure and bands are superimposed on the various Si-O and Al-O, it is extremely difficult to distinguish between them. Therefore, assignment of the absorption band around 1000 cm−1 region is only repre‐ sented as ν(T-O) (T = Si or Al) [16]. Bands appearing at relatively high wavenumber are assigned to the νασ mode, and the low wavenumber region is assigned to νσ mode. Bending

The green line shows that the spectrum of the Ag zeolite was cooled to RT after heating for 24 h at 500 °C in vacuum (measured in a vacuum). As compared to the case of unheated, absorption band has been changed drastically. While individual band assignment is not known as mentioned above, considering the results of XAFS, it is reasonable to assume the change due to the formation of Ag clusters. For cluster formation, it is necessary that Ag+ ions (at least

was located. Reduction of ions and changing the position also affect vibration of the framework in each case. (The distance changes between the framework and the Ag atoms and changes in the electrostatic attraction of Ag+ can perturb the vibration of the T-O. In other words, it can

The red line is the spectrum in the case of introducing Air into the zeolite, cooling to RT after the vacuum heating. The spectrum greatly changed again, and is much closer to the spectrum

; also Ag which is to be aggregated away from the site of Ag+

ion

**Table 1.** The structural parameters, *r* and *N* after the various gases are introduced [12].

mode appears in the lower-energy region, not shown here.

be said that the IR is a sensitive technique to such a change.)

partially) are reduced to Ag<sup>0</sup>

162 Zeolites - Useful Minerals


Since Ag loading into zeolite performed in solution for a sufficient time, Ag+ ions can be positioned at a stable site. By contrast, disintegration after cluster formation is in atmosphere at room temperature and may not exceed a potential barrier existing between the stable site and the metastable sites.

**Figure 9.** IR spectra for Ag-zeolite; unheated, after heating to 500 °C in vacuum and air introduced after heating.

**Figure 10.** IR spectra for Ag-zeolite; Air introduced after heating (before irradiation), irradiated by excited light (405 nm, 1 min) and irradiated by excited light (405 nm, 2 min).

#### **6. Summary**

In this chapter, we discussed the formation of Ag clusters in the Ag-zeolite A cavity and the origin of strong PL after the formation of Ag clusters in zeolite A.

Ag clusters are formed in the cavity of Ag-substituted zeolite A after heat treatment under atmosphere or in vacuum. In the cavity, Ag+ is reduced by the removal of the water molecule and Ag-O distance decreases. The Ag-Ag distance is also decreased because the repulsion between Ag and Ag is weakened. The coordination number of Ag-Ag is increased to around 2.35 after producing Ag clusters.

On the other hand, strong PL appears at 2.1 eV excited by 405-nm laser with Air or mixed gas of H2O and N2. The present Ag *K*-edge XAFS study indicated the local structure of Ag atoms in chemical species, which shows that the strong PL is almost the same as unheated one and Ag clusters are broken down. The IR spectra in the region of zeolite framework vibration of the species after Ag cluster breakdown are different from the unheated one. This indicates that the framework structure of zeolite changed by the production and breakdown of Ag clusters in the cavity. The effect to zeolite framework has a key for the strong PL.

#### **Acknowledgements**

The authors are grateful to Prof. H. Hoshino for the fruitful discussion of XAFS and PL in Agzeolite A. The synchrotron radiation experiments were performed at the Photon Factory in KEK.

#### **Author details**

Takafumi Miyanaga\* , Yushi Suzuki and Sho Narita

\*Address all correspondence to: takaf@hirosaki-u.ac.jp

Department of Mathematics and Physics, Hirosaki University, Aomori, Japan

#### **References**

**Figure 10.** IR spectra for Ag-zeolite; Air introduced after heating (before irradiation), irradiated by excited light (405

In this chapter, we discussed the formation of Ag clusters in the Ag-zeolite A cavity and the

Ag clusters are formed in the cavity of Ag-substituted zeolite A after heat treatment under atmosphere or in vacuum. In the cavity, Ag+ is reduced by the removal of the water molecule and Ag-O distance decreases. The Ag-Ag distance is also decreased because the repulsion between Ag and Ag is weakened. The coordination number of Ag-Ag is increased to around

On the other hand, strong PL appears at 2.1 eV excited by 405-nm laser with Air or mixed gas of H2O and N2. The present Ag *K*-edge XAFS study indicated the local structure of Ag atoms in chemical species, which shows that the strong PL is almost the same as unheated one and Ag clusters are broken down. The IR spectra in the region of zeolite framework vibration of the species after Ag cluster breakdown are different from the unheated one. This indicates that the framework structure of zeolite changed by the production and breakdown of Ag clusters

nm, 1 min) and irradiated by excited light (405 nm, 2 min).

origin of strong PL after the formation of Ag clusters in zeolite A.

in the cavity. The effect to zeolite framework has a key for the strong PL.

**6. Summary**

164 Zeolites - Useful Minerals

2.35 after producing Ag clusters.


### **Zeolites as Potential Structures in Obtaining Jet Fuel Through the Fischer‐Tropsch Synthesis**

Lidiane Sabino da Silva, Victor Luis dos Santos Teixeira da Silva, Maria Angélica Simões Dornellas de Barros and Pedro Augusto Arroyo

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63662

#### **Abstract**

[9] T. Miyanaga, H. Hoshino, H. Endo. Local structure of silver clusters in the channels of

[10] Y. Suzuki, T. Miyanaga, H. Hoshino, N. Matsumoto, T. Ainai. In-situ XAFS study of

[11] H. Hoshino, Y. Sannohe, Y. Suzuki, T. Azuhata, T. Miyanaga, K. Yaginuma, M. Itoh, T. Shigeno, Y. Osawa, Y. Kimura. Photoluminescence of the dehydrated Ag-type zeolite

[12] A. Nakamura, M. Narita, S. Narita, Y. Suzuki, T. Miyanaga. In-situ XAFS study of Ag

[13] H. Sakane, T. Miyanaga, I. Watanabe, N. Matsubayashi, S. Ikeda, Y. Yokoyama. Reproducibility tests of extended X-ray absorption fine structure for aqua and amine complexes of first transition metals in solid and aqueous solution. Jpn. J. Appl. Phys.

[15] A.L. Ankudinov, B. Ravel, J.J. Rehr, S.D. Conradson. Real space multiple scattering

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Spectrosc. 1995;9:225.

Ag clusters in zeolite 4A. Phys Scr. 2005;T115:765.

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clusters in Ag-type zeolite-A. J. Phys. Conf. Ser. 2014;502:012033.

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calculation of XANES. Phys. Rev. B. 1998;58:7565.

Due to the constant expansion of the aviation sector, the global air industry has performed the search for alternative fuels to petroleum‐derived aviation kerosene, which present low emission of greenhouse gases and other pollutants, in addition to supplying the engines and aircraft already in operation using the existing distribution infrastructure. Thus, one alternative that has been studied is the synthetic kerosene derived from gasification processes followed by Fischer‐Tropsch synthesis. Several features seem to be crucial in controlling product selectivity in the aviation fuel range, among them are the zeolitic support, acidity, and promoters. Therefore, the effect of these parameters in the Fischer‐Tropsch synthesis is discussed in this chapter and, finally, the zeolitic catalysts that, according to the literature, have significant potential in obtaining synthetic aviation fuel are evidenced.

**Keywords:** zeolites, aviation kerosene, Fischer‐Tropsch synthesis, acidity, promoters

#### **1. Introduction**

The air transport sector, due to constant expansion, is being pressured to reduce their CO2 emissions, since aviation is responsible for 10% of the emissions in transport sector. In addition, because of the prospects for growth in this sector, there is evidence of a further increase in CO2 emissions in the coming years [1]. For this reason, the airlines associated with

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the International Air Transport Association (IATA) pledged to improve fuel efficiency by 1.5% annually to 2020, cap net emissions from 2020 with carbon‐neutral growth and cut net emissions in half by 2050 compared to 2005 [2]. Such obligations, in the future, may limit the civil aviation market and stimulate the search for alternatives. However, the demand for fuels tends to expand, since the passenger demand will be more than doubling from 3.3 to 7.3 billion by 2034 [3], even with the possible gains in energy efficiency (estimated between 30 and 50%), from improved aerodynamics, reducing the weight of the aircraft, the advances in technology of thrusters, and the adoption of best practices in air traffic management [4].

In fact, historically, since the first aircraft was manufactured, the airline industry has been trying to decrease the fuel consumption through the improvement of technologies used. Although there is a decrease in emissions over the years, due to new technologies employed, the speed of which can perform this benefit is compromised by the fact that airlines maintain their assets in use for about 40 years, due to the high investment cost. Thus, the new technol‐ ogies have to wait until the fleet is replaced [5].

In this context, the global air industry has performed the search for alternative fuels to petroleum‐derived aviation kerosene, which present low emission of greenhouse gases and other pollutants, in addition to supplying the engines and aircraft already in operation using the existing distribution infrastructure [6]. Therefore, one alternative that has been studied is the synthetic kerosene derived from gasification processes followed by Fischer‐Tropsch synthesis (FTS). In this synthesis route, the raw material (coal, natural gas, or biomass) is transformed into syngas, a mixture of carbon monoxide and hydrogen, which is converted by means of a metal catalyst on FTS, forming hydrocarbons of different sizes. The final step is the processing and fractionation of products to produce a fuel with appropriate properties to be used in the airline industry [7].

The literature shows that the structure of the support used in the catalyst can be one of the key features in obtaining products in the aviation fuel range. When the catalytic reaction occurs within the pores of a zeolite, the size and shape of the canals and cavities can be used to select the desired reaction path, making use of "restricted transition state selectivity" or, also, through the "product selectivity." In the first case, the reaction intermediates and/or transition states are sterically limited due to the shape and size of the microporous lattice [8]. In the second case, the product selectivity refers to discrete diffusivities of the reaction products formed with respect to the microporous architecture and crystal size of the catalyst particles. If the molecules formed are very large compared to the pore size, the product diffusion may not occur, be extremely slow, or be completed after the occurrence of secondary reactions, such as cracking and isomerization, among others [8, 9]. In this way, specific structures with unique character‐ istics can direct the formation of products with particular chain length.

Another feature that seems to be crucial in controlling product selectivity is the acidity. The acid sites of the zeolites can promote cracking, isomerization and aromatization reactions. This property can help in overcoming the limitation of Anderson‐Schulz‐Flory (ASF) distribution and adjustment of the Fischer‐Tropsch products in a specific distribution [10]. This can also reduce the need for subsequent treatments to Fischer‐Tropsch synthesis, which could lead to a significant reduction in process costs [11].

Finally, one must take into consideration the use of specific promoters, which can provide desirable characteristics to the catalysts, such as increasing the reaction rate and greater stability and selectivity of the hydrocarbons C5+ [12, 13]. The promoters may direct the formation of products at a particular chain length, especially when combined with a structure and a specific acidity [11, 14].

However, despite the wide literature directing the production of synthetic fuels, there is still much to be studied on the aviation kerosene, since the vast majority of articles have focused on the production of gasoline and/or diesel [15–18], or simply the formation of products in the range of liquid hydrocarbons C5+ [13, 19–23]. For this reason, the main objective of this chapter is to analyze zeolitic structures studied in the literature, besides the influence of acidity and promoters, which have significant potential in obtaining fuel in the range of aviation kerosene (C8–C16) through the Fischer‐Tropsch synthesis.

### **2. Conventional aviation fuel**

the International Air Transport Association (IATA) pledged to improve fuel efficiency by 1.5% annually to 2020, cap net emissions from 2020 with carbon‐neutral growth and cut net emissions in half by 2050 compared to 2005 [2]. Such obligations, in the future, may limit the civil aviation market and stimulate the search for alternatives. However, the demand for fuels tends to expand, since the passenger demand will be more than doubling from 3.3 to 7.3 billion by 2034 [3], even with the possible gains in energy efficiency (estimated between 30 and 50%), from improved aerodynamics, reducing the weight of the aircraft, the advances in technology of thrusters, and the adoption of best practices in air traffic management [4].

In fact, historically, since the first aircraft was manufactured, the airline industry has been trying to decrease the fuel consumption through the improvement of technologies used. Although there is a decrease in emissions over the years, due to new technologies employed, the speed of which can perform this benefit is compromised by the fact that airlines maintain their assets in use for about 40 years, due to the high investment cost. Thus, the new technol‐

In this context, the global air industry has performed the search for alternative fuels to petroleum‐derived aviation kerosene, which present low emission of greenhouse gases and other pollutants, in addition to supplying the engines and aircraft already in operation using the existing distribution infrastructure [6]. Therefore, one alternative that has been studied is the synthetic kerosene derived from gasification processes followed by Fischer‐Tropsch synthesis (FTS). In this synthesis route, the raw material (coal, natural gas, or biomass) is transformed into syngas, a mixture of carbon monoxide and hydrogen, which is converted by means of a metal catalyst on FTS, forming hydrocarbons of different sizes. The final step is the processing and fractionation of products to produce a fuel with appropriate properties to be

The literature shows that the structure of the support used in the catalyst can be one of the key features in obtaining products in the aviation fuel range. When the catalytic reaction occurs within the pores of a zeolite, the size and shape of the canals and cavities can be used to select the desired reaction path, making use of "restricted transition state selectivity" or, also, through the "product selectivity." In the first case, the reaction intermediates and/or transition states are sterically limited due to the shape and size of the microporous lattice [8]. In the second case, the product selectivity refers to discrete diffusivities of the reaction products formed with respect to the microporous architecture and crystal size of the catalyst particles. If the molecules formed are very large compared to the pore size, the product diffusion may not occur, be extremely slow, or be completed after the occurrence of secondary reactions, such as cracking and isomerization, among others [8, 9]. In this way, specific structures with unique character‐

Another feature that seems to be crucial in controlling product selectivity is the acidity. The acid sites of the zeolites can promote cracking, isomerization and aromatization reactions. This property can help in overcoming the limitation of Anderson‐Schulz‐Flory (ASF) distribution and adjustment of the Fischer‐Tropsch products in a specific distribution [10]. This can also reduce the need for subsequent treatments to Fischer‐Tropsch synthesis, which could lead to

istics can direct the formation of products with particular chain length.

a significant reduction in process costs [11].

ogies have to wait until the fleet is replaced [5].

used in the airline industry [7].

168 Zeolites - Useful Minerals

The petroleum refining occurs through a process of fractional distillation, in which crude oil is heated and its different components come boiling at various temperatures, and later being condensed. Therefore, specific components are retrieved in different temperature ranges.

The lowest boiling fraction, taken from the top of the distillation column, is called naphtha, which is mainly processed further to make motor gasoline. The second fraction of about 33% of the crude oil input contains the raw material for jet fuel production. This fraction is further processed in the distillate hydrotreater to obtain the desired products. Basically, kerosene is originated as a straight‐run (distilled) petroleum fraction with boiling temperature ranging from 205 to 260°C [24]. Subsequent fractions correspond to components in the range of diesel fuel and residues, and the latter can be distilled under vacuum, yielding high‐value distillate components [25].

The petroleum processing through fractional distillation is still the dominant technology in the production of aviation kerosene; besides, this fuel accounts for 6.3% of the total amount of crude oil consumed [24]. However, the large fluctuations in the price of petroleum, the concern about the pollutants emitted, and the increase in passenger demand show a strong incentive for the airline industry to invest in alternative sources of fuel.

#### **3. Synthetic aviation fuel**

The airline industry bet on fuels that are chemically indistinguishable from conventional fuel and can be mixed in order to provide similar performance and security, without change in the aircraft systems and engines or on distribution and storage infrastructure. One of the most promising alternative processes for obtaining aviation kerosene is the Fischer‐Tropsch process.

Currently, Semi‐Synthetic Jet Fuel (SSJF) obtained by Fischer‐Tropsch process and hydropro‐ cessing, mixed with JET‐A1, is already approved by ASTM D7655, limited to 50% proportion. One of the reasons that the SSJF fuel cannot be used without restriction in civil aviation, especially those derived from natural gas or biomass, is the fact that these products do not present aromatic hydrocarbons, not answering, so, to the specifications imposed by the existing standard (minimum of 8% aromatics) [26].

The lack of aromatic components in the fuel results in higher freezing points and lower densities. The freeze point of a fuel composition is an important factor in determining whether it is suitable for aviation use, for which low‐temperature conditions are experienced at high altitudes. Therefore, it is vital that the fuel composition does not freeze or cause flow to be restricted during operation; otherwise, the consequences could be disastrous [27]. In addition, the deficiency of aromatics in aviation kerosene can also cause shrinking of elastomers present in the storage and distribution system, which can lead to fuel leaks [28].

One of the advantages of using semi‐synthetic kerosene is cleaner burning, since there is a lower emission of sulfur compounds [28], leading to an increase in the useful life of the combustor and turbine. Moreover, the emission of particulate compounds is lower compared to conventional fuel [24]. Therefore, the synthetic fuel blend with the conventional allows aggregating the advantages of synthetic fuel, while it is appropriate in relation to pre‐ established standards.

Currently, there is also the Fully Synthetic Jet Fuel (FSJF). The SASOL was the first and only one to produce this type of fuel for unrestricted use in civil aviation, from 2008, proving that the fuel produced behaved as the conventional in all aspects of storage, handling, and flight performance [7].

Coal‐derived fuels are rich in naphthene compounds [29]. Due to the historic necessity by SASOL to produce diesel and gasoline fuel from their coal reserves located in South Africa, the catalysts and operating conditions have a different focus than other plants currently online and/or being developed [7]. This fact probably contributed to the success in obtaining the Fully Synthetic Jet Fuel from the Fischer‐Tropsch synthesis.

The process consists of a mixture of five main blends, of which fractions of kerosene can be obtained. These blends come from the Fischer‐Tropsch process or products generated in coal gasification. Therefore, the blend in greater amount is obtained from the Fischer‐Tropsch process. The other four blends are obtained from heavy fractions of naphtha, commonly used in the production of gasoline, or light fractions of distillate, usually used in the production of diesel [7].

#### **4. The Fischer‐Tropsch synthesis**

#### **4.1. The Fischer‐Tropsch reaction**

The Fischer‐Tropsch process has four main steps. The first stage corresponds to the production of synthesis gas, which is a mixture of hydrogen and carbon monoxide. In the second step, the removal of unwanted compounds and carbon dioxide from syngas occurs. The third step corresponds to the Fischer‐Tropsch reaction, in which the formation of hydrocarbons occurs. After this step, the products undergo treatments of hydrocracking and isomerization in order to acquire the necessary features to be used as fuel.

Among the products obtained in Fischer‐Tropsch synthesis are the alkenes (olefins), alkanes (paraffins), and oxygenates (e.g., alcohols, aldehydes, carboxylic acids, and ketones), in addition to water [30]. The nature of oxygenated compounds is determined by the catalyst type and reaction conditions. The paraffins are generally straight chains, forming products in the range of gasoline, diesel oil, aviation kerosene, and waxes [31].

The temperature and reaction pressure are some of the most important parameters in the SFT to obtain products at a specific desired range. Currently, there are some well‐defined condi‐ tions: the Low‐Temperature Fischer‐Tropsch (LTFT), the High‐Temperature Fischer‐Tropsch (HTFT), and pressures in the range of 10–40 bar. In LTFT process, iron or cobalt catalysts are typically used at approximately 230°C, yielding mainly products in the kerosene range, diesel, and waxes. In the HTFT process, iron catalysts are typically used at approximately 340°C, yielding mainly products in the range of gasoline and olefins [32]. These different behaviors occur because at elevated temperatures the reaction rate and quantity of secondary products increases, which leads to a reduction in the average chain length of the molecules, with a corresponding increase in methane formation [33].

On the operating pressure, the literature shows that the increase in the value of this parameter provides an increase in carbon monoxide conversion and C5+ selectivity [11, 34–36]. The increased conversion of carbon monoxide is explained by the greater amount of C1 intermedi‐ ates, which increase the propagation rate [35]. In the same context, the increase in C5+ selectivity occurs due to the higher concentration of syngas, leading to a higher propagation rate of the carbon monoxide polymerization reaction and, therefore, higher concentrations of long‐chain hydrocarbons [37].

The overall reaction of Fischer‐Tropsch synthesis is highly exothermic and is subject to limitation diffusional, which can affect the distribution of final products and lead to the deactivation of catalysts [38]. The overall reaction is represented below:

$$2nCO + 2nH\_2O \rightarrow \left(CH\_2\right)n + nH\_2O \qquad\qquad\qquad \Delta = -167.4kJ/mol\tag{1}$$

Secondary reactions, according to the literature [39, 40], are represented as

Paraffins

One of the reasons that the SSJF fuel cannot be used without restriction in civil aviation, especially those derived from natural gas or biomass, is the fact that these products do not present aromatic hydrocarbons, not answering, so, to the specifications imposed by the

The lack of aromatic components in the fuel results in higher freezing points and lower densities. The freeze point of a fuel composition is an important factor in determining whether it is suitable for aviation use, for which low‐temperature conditions are experienced at high altitudes. Therefore, it is vital that the fuel composition does not freeze or cause flow to be restricted during operation; otherwise, the consequences could be disastrous [27]. In addition, the deficiency of aromatics in aviation kerosene can also cause shrinking of elastomers present

One of the advantages of using semi‐synthetic kerosene is cleaner burning, since there is a lower emission of sulfur compounds [28], leading to an increase in the useful life of the combustor and turbine. Moreover, the emission of particulate compounds is lower compared to conventional fuel [24]. Therefore, the synthetic fuel blend with the conventional allows aggregating the advantages of synthetic fuel, while it is appropriate in relation to pre‐

Currently, there is also the Fully Synthetic Jet Fuel (FSJF). The SASOL was the first and only one to produce this type of fuel for unrestricted use in civil aviation, from 2008, proving that the fuel produced behaved as the conventional in all aspects of storage, handling, and flight

Coal‐derived fuels are rich in naphthene compounds [29]. Due to the historic necessity by SASOL to produce diesel and gasoline fuel from their coal reserves located in South Africa, the catalysts and operating conditions have a different focus than other plants currently online and/or being developed [7]. This fact probably contributed to the success in obtaining the Fully

The process consists of a mixture of five main blends, of which fractions of kerosene can be obtained. These blends come from the Fischer‐Tropsch process or products generated in coal gasification. Therefore, the blend in greater amount is obtained from the Fischer‐Tropsch process. The other four blends are obtained from heavy fractions of naphtha, commonly used in the production of gasoline, or light fractions of distillate, usually used in the production of

The Fischer‐Tropsch process has four main steps. The first stage corresponds to the production of synthesis gas, which is a mixture of hydrogen and carbon monoxide. In the second step, the removal of unwanted compounds and carbon dioxide from syngas occurs. The third step

in the storage and distribution system, which can lead to fuel leaks [28].

existing standard (minimum of 8% aromatics) [26].

Synthetic Jet Fuel from the Fischer‐Tropsch synthesis.

**4. The Fischer‐Tropsch synthesis**

**4.1. The Fischer‐Tropsch reaction**

established standards.

performance [7].

170 Zeolites - Useful Minerals

diesel [7].

$$nCO + \left(2n + 1\right)H\_2 \to C\_nH\_{2n+2} + nH\_2O\tag{2}$$

Olefins

$$nCO + 2nH\_2 \rightarrow C\_nH\_{2n} + nH\_2O\tag{3}$$

Alcohols

$$nCO + 2nH\_2 \rightarrow C\_nH\_{2n+1}OH + (n-1)H\_2O \tag{4}$$

Methane

$$nCO + 3nH\_2 \rightarrow CnH\_{4n} + nH\_2O\tag{5}$$

Water‐gas shift reaction

$$nCO + nH\_2O \rightarrow nCO\_2 + nH\_2\tag{6}$$

Coke deposition

$$nCO + nH\_2 \rightarrow nC + nH\_2O\tag{7}$$

Boudouard reaction

$$nCO + nCO \rightarrow nC + nCO\_2 \tag{8}$$

The distribution of products in the Fischer‐Tropsch synthesis follows the Anderson‐Schulz‐ Flory distribution [41], which is expressed as follows:

$$W\_{n/n} = \left(1 - \alpha\right)^2 \alpha^{\binom{n-1}{2}} \tag{9}$$

where *Wn*/*<sup>n</sup>* is the weight fraction of hydrocarbon molecules containing *n* carbon atoms, and α the chain growth probability or the probability of molecule to continue reacting to form a longer chain.

The product selectivity is then determined by an ideal *α* value. Smaller *α* values are obtained when short‐chain hydrocarbons are produced (C1–C4), while higher values of α are obtained when heavier hydrocarbons are formed (C21+). Therefore, the Anderson‐Schulz‐Flory model is not selective for the hydrocarbon's intermediate range, which is in the vast majority of the cases the most desired products [42]. For example, according to the Anderson‐Schulz‐Flory model, the maximum product selectivity in gasoline range (C5–C11) and diesel (C10–C20) is approxi‐ mately 45 and 35%, respectively.

According to Zhang et al. [42], the catalytic factors that exert greater influence in the activity and selectivity of catalytic in the FTS are the type of support, the promoters, and the active phase and its size, chemical state, and microenvironment in which it inserts. Therefore, the development of selective catalysts, which can direct the formation of hydrocarbons in the desired range, is one of the biggest challenges in the synthesis of catalysts for Fischer‐Tropsch synthesis.

#### **4.2. Influence of zeolitic support**

Alcohols

172 Zeolites - Useful Minerals

Methane

Water‐gas shift reaction

Coke deposition

Boudouard reaction

longer chain.

mately 45 and 35%, respectively.

Flory distribution [41], which is expressed as follows:

2 21 ( ) <sup>2</sup> 2 1 *n n nCO nH C H OH n H O* + ® +- <sup>+</sup> (4)

2 42 3 *<sup>n</sup> nCO nH CnH nH O* +® + (5)

2 22 *nCO nH O nCO nH* +®+ (6)

2 2 *nCO nH nC nH O* + ®+ (7)

<sup>2</sup> *nCO nCO nC nCO* + ®+ (8)


The distribution of products in the Fischer‐Tropsch synthesis follows the Anderson‐Schulz‐

( )<sup>2</sup> ( ) <sup>1</sup> / 1 . *<sup>n</sup> Wn n* a a

where *Wn*/*<sup>n</sup>* is the weight fraction of hydrocarbon molecules containing *n* carbon atoms, and α the chain growth probability or the probability of molecule to continue reacting to form a

The product selectivity is then determined by an ideal *α* value. Smaller *α* values are obtained when short‐chain hydrocarbons are produced (C1–C4), while higher values of α are obtained when heavier hydrocarbons are formed (C21+). Therefore, the Anderson‐Schulz‐Flory model is not selective for the hydrocarbon's intermediate range, which is in the vast majority of the cases the most desired products [42]. For example, according to the Anderson‐Schulz‐Flory model, the maximum product selectivity in gasoline range (C5–C11) and diesel (C10–C20) is approxi‐

According to Zhang et al. [42], the catalytic factors that exert greater influence in the activity and selectivity of catalytic in the FTS are the type of support, the promoters, and the active The support structure used in the catalyst can be one of the key features in obtaining products in a specific range. When the catalytic reaction occurs within the pores of zeolite, the size and shape of the channels can be used to select the desired reaction path making use of the "shape‐ selectivity." This occurs when the spatial configuration around reactants, products, transition state, or a reaction intermediate located in the intra‐crystalline volume is such that only certain configurations are possible [8]. Therefore, specific supports may direct the formation of products with a particular chain length.

Molecular shape selective is observed frequently when using zeolite catalysts because of their two‐ or three‐dimensional nature. Zeolites have unique advantages which render them particularly attractive for a variety of conversions and processes of industrial interest [9]:


An advantage of the catalysts with shape selectivity is the formation of products not predicted in the equilibrium composition. The thermodynamic equilibrium laws are valid only if the catalytic and non‐catalytic reactions are truly the same, and yield identical products. However, these considerations may not be obeyed in chemical processes. The catalytic reactions inside the pore channel of a shape‐selective catalyst can be substantially different from those that occur in homogeneous catalytic or non‐catalytic reactions involving the same reactants, due to the selectivity mechanisms characteristic of each structure [43].

The selectivity mechanisms of catalysts with shape selectivity are described briefly below:


this allows these molecules to undergo secondary reactions of isomerization and cracking, among others;

**•** Restricted transition state‐type selectivity: it occurs when the spatial configuration around a transition state or a reaction intermediate located in the intra‐crystalline volume is such that only certain configurations are possible; this means that the formation of reaction intermediates and/or transition states is sterically limited due to the shape and size of the microporous lattice allowing the access of the species formed to interact with the active sites [8].

Despite the vast majority of articles presenting the formation of gasoline or diesel fuel through the Fischer‐Tropsch synthesis as an objective, some zeolites can be identified with considerable potential in the production of hydrocarbons in the aviation kerosene range. This stems from the fact that these structures present structural and acidic properties that give significant shape selectivity to products or transition states to the catalysts.

Bessel [44] investigated bifunctional catalysts of cobalt supported in zeolites ZSM‐5, ZSM‐11, ZSM‐12, and ZSM‐34 to the Fischer‐Tropsch synthesis. The use of bifunctional catalysts composed of active metal and zeolite in acidic form aimed to overcome the typical nonselective Anderson‐Schulz‐Flory product distribution arising from conventional catalysts. In this type of catalyst, the active metal is responsible for the growth of the primary straight‐chained hydrocarbon product, which undergoes further restructuring at the zeolite acid sites to produce a more branched hydrocarbon product of limited chain length. Despite having the lowest conversion of carbon monoxide (45%), among the catalysts studied, Co/ZSM‐34 showed the highest selectivity to hydrocarbons in the jet fuel range (over 30%). This result was not expected because the ZSM‐34 was the zeolite that showed the highest acidity; however, this also presents the more complex structure, with the largest (6.7 Å) and smallest (3.6 Å) diameters of channels among the zeolites used. In this case, the accessibility to the active sites and pore structure appears to be more important than the acidity in obtaining products in the aviation kerosene range.

Zola et al. [45] synthesized cobalt catalysts with 10% metallic phase supported through wet impregnation in zeolites Beta, USY, mordenita, and ZSM‐5. The catalyst supported on zeolite USY presented the highest carbon monoxide conversion values (19%) and activity in Fischer‐ Tropsch synthesis. Hydrocarbon production in the jet fuel range also appears to be greater for this catalyst. The USY zeolite has a three‐dimensional system of channels and large micropores, which favors the accessibility to catalytic sites. Therefore, these characteristics seem to favor the formation of products with a specific chain length in Fischer‐Tropsch synthesis.

Ngamcharussrivichai et al. [17] investigated the performance of cobalt‐based catalysts supported on silica, alumina, montmorillonite, and three zeolites (ZSM‐5, USY, and MCM‐22) in Fischer‐Tropsch synthesis, 10 bar, and 230°C. The catalysts supported on USY and mont‐ morillonite showed the carbon number distribution to be narrower and lower occurrence of waxes compared to catalysts supported on silica and alumina, because the zeolitic structure limits the growth of hydrocarbon chain. The material acidity also contributed to narrow the carbon number distribution; however, the zeolitic structure was still the most relevant feature. Among the Co/SiO2, Co/USY, Co/ZSM‐5, and Co/MCM‐22 catalysts, the second and the third ones showed the distribution of carbon number more similar to the aviation kerosene (C8–C16). In addition, when a support with acidic characteristics was used, the formation of C12+ significantly decreased, probably due to the higher occurrence of cracking reactions.

Martínez et al. [18] studied the catalytic properties of ZSM‐5, MCM‐22, IM‐5, and ITQ‐2 zeolites, all with 10‐membered ring pore openings (10‐MR) and Si/Al ratio similar. The study aimed to form products in the gasoline range through the Fischer‐Tropsch synthesis. Hybrid catalysts (zeolite + silica) were tested for at least 7–8 h of reaction showing conversions around 50%. The presence of zeolites in hybrid catalyst increases the selectivity to products C2–C4 and gasoline (C5–C12), as well as reduces the selectivity to diesel (C13–C22) and waxes (C23+). The hybrid catalysts with zeolites MCM‐22 and IM‐5 showed very similar products and the lower yields to iso‐C5–8. The decreased formation of C23+ and the greater and lesser C13–22 formation and C5–12, respectively, among the hybrid catalysts, suggest that the MCM‐22 and IM‐5 zeolites may have a narrower distribution of products, which can provide a higher yield of hydrocar‐ bons in the aviation kerosene range (C8–C16). These properties are probably related to the unique features of the zeolitic structures used, as, for example, the presence of supercages in the case of MCM‐22.

Kibby et al. [46] performed the synthesis of a hybrid catalyst, which was composed of Ru and Co, as active metals, ZSM‐12 and ZSM‐5 zeolites, as supports, and alumina as base. The intention was to synthesize a catalyst which maximizes the formation of C5+ products, were free of the solid phase (waxes), and minimize the formation of gaseous products (C1–C4) in the Fischer‐Tropsch synthesis, so the product would not need to undergo further hydrocracking process. The hybrid catalyst using ZSM‐5 produced a greater quantity of gaseous products and the average molar mass of hydrocarbons was lower when compared to hybrid catalyst with ZSM‐12. In addition, the catalyst using ZSM‐12 yielded a larger amount of products in the aviation kerosene range (C8–C16) than the hybrid catalyst with ZSM‐5. Apparently, ZSM‐ 5 is more effective in the isomerization of C4–C9 olefins, while ZSM‐12 is more active in isomerization of C10 olefins, the latter leading to the formation of products with a longer length of chain. The reasons for this behavior are still not very well known, although the straight channels of ZSM‐12 can carry heavier olefins faster. The authors concluded that the under‐ standing of metal deposition rules on the support, the morphology and structure of zeolites, optimization of acid functions, activation, and process conditions are essential for obtaining the desired hydrocarbon range during the Fischer‐Tropsch synthesis.

#### **4.3. Influence of acidity**

this allows these molecules to undergo secondary reactions of isomerization and cracking,

**•** Restricted transition state‐type selectivity: it occurs when the spatial configuration around a transition state or a reaction intermediate located in the intra‐crystalline volume is such that only certain configurations are possible; this means that the formation of reaction intermediates and/or transition states is sterically limited due to the shape and size of the microporous lattice allowing the access of the species formed to interact with the active sites

Despite the vast majority of articles presenting the formation of gasoline or diesel fuel through the Fischer‐Tropsch synthesis as an objective, some zeolites can be identified with considerable potential in the production of hydrocarbons in the aviation kerosene range. This stems from the fact that these structures present structural and acidic properties that give significant shape

Bessel [44] investigated bifunctional catalysts of cobalt supported in zeolites ZSM‐5, ZSM‐11, ZSM‐12, and ZSM‐34 to the Fischer‐Tropsch synthesis. The use of bifunctional catalysts composed of active metal and zeolite in acidic form aimed to overcome the typical nonselective Anderson‐Schulz‐Flory product distribution arising from conventional catalysts. In this type of catalyst, the active metal is responsible for the growth of the primary straight‐chained hydrocarbon product, which undergoes further restructuring at the zeolite acid sites to produce a more branched hydrocarbon product of limited chain length. Despite having the lowest conversion of carbon monoxide (45%), among the catalysts studied, Co/ZSM‐34 showed the highest selectivity to hydrocarbons in the jet fuel range (over 30%). This result was not expected because the ZSM‐34 was the zeolite that showed the highest acidity; however, this also presents the more complex structure, with the largest (6.7 Å) and smallest (3.6 Å) diameters of channels among the zeolites used. In this case, the accessibility to the active sites and pore structure appears to be more important than the acidity in obtaining products in the aviation

Zola et al. [45] synthesized cobalt catalysts with 10% metallic phase supported through wet impregnation in zeolites Beta, USY, mordenita, and ZSM‐5. The catalyst supported on zeolite USY presented the highest carbon monoxide conversion values (19%) and activity in Fischer‐ Tropsch synthesis. Hydrocarbon production in the jet fuel range also appears to be greater for this catalyst. The USY zeolite has a three‐dimensional system of channels and large micropores, which favors the accessibility to catalytic sites. Therefore, these characteristics seem to favor

Ngamcharussrivichai et al. [17] investigated the performance of cobalt‐based catalysts supported on silica, alumina, montmorillonite, and three zeolites (ZSM‐5, USY, and MCM‐22) in Fischer‐Tropsch synthesis, 10 bar, and 230°C. The catalysts supported on USY and mont‐ morillonite showed the carbon number distribution to be narrower and lower occurrence of waxes compared to catalysts supported on silica and alumina, because the zeolitic structure limits the growth of hydrocarbon chain. The material acidity also contributed to narrow the carbon number distribution; however, the zeolitic structure was still the most relevant feature.

the formation of products with a specific chain length in Fischer‐Tropsch synthesis.

selectivity to products or transition states to the catalysts.

among others;

174 Zeolites - Useful Minerals

[8].

kerosene range.

The microporous structure of zeolites provides a large internal surface area that allows a large concentration of active sites accessible to reactants, together with notable selectivity effects related both to diffusion of reactants and products inside the pore system and to steric constraints on intermediates and transition states. All these features influence the overall catalytic performance of acid zeolites, and make difficult to establish an acidity scale and a clear correlation between acidity and catalytic activity [47].

Two types of acid sites, Brönsted and Lewis, can be formed on the zeolite surface. Thus, Brönsted acid sites are generated when Si4+ is isomorphically replaced by a trivalent metal cation such as Al3+. This substitution creates a negative charge in the lattice that can be compensated by a proton [48]. These protons can be present in the medium depending on the reactants that were used in the material synthesis, exerting the function of compensation cations. To replace these cations and consequently form a Brönsted acid site, ion exchange procedure is conducted with an aqueous solution of ammonium chloride, followed by washing to remove residual chloride, and calcination [49]. At high temperatures, the remaining protons can acquire mobility and be lost as water molecules, followed by the formation of Lewis acid sites, due to the isomorphic substitution of Al by Si [8].

It is evident that the amount of acid sites in the zeolite is related to the Si/Al ratio. The smaller the Si/Al ratio, more aluminum is present in the structure and increases the amount of compensation cations and the number of acid sites when the zeolite is in protonic form. However, the generation of acidic sites is associated with the imbalance of charges generated by isomorphic substitution of different load cations. Therefore, the greater the number of aluminum atoms, more unbalanced will be the zeolitic framework [49]. Due to this fact, the reduction of Si/Al ratio makes the zeolite structure to become less stable and may even damage the crystalline arrangement, besides having influence on the strength of acid sites.

In fact, not all acid sites have the same acid strength, because this feature depends on the number of aluminum atoms near the aluminum atom, which supports the acid site [50]. The more isolated is the trivalent atom, the greater the acid strength.

From the change of the Si/Al ratio of the zeolitic structure, through either synthesis or post‐ synthesis treatments, it is possible to change not only the total number but also the electronic density on the bridging hydroxyl group, and consequently to change the acid strength of the Brönsted acid site [51]. The control of acid strength as well as the density of acid sites of zeolite catalysts has also led to successful catalysts and processes in the field of oil refining and petrochemistry [48].

For the incorporation and/or suitability of the acidic species of Lewis or Brönsted, there are many synthetic strategies that can be developed, such as impregnation, deposition, precipita‐ tion, calcination, and reduction, among others. Structural variations are diverse and numerous chemical species can be incorporated into the support, in composition, oxidation states, and chemical environments in different ways [49].

In order to evaluate the influence of acidity on catalysts, Bessel et al. [52] showed that acid zeolitic structures have a tendency to catalyze secondary reactions in the Fischer‐Tropsch synthesis, from olefinic products. Secondary reactions include oligomerization, isomerization, cracking, and hydrogen transfer, thus obtaining more branched products with a shorter chain length compared to the catalysts using supports with less or no acidity.

Ravishankar et al. [53] synthesized cobalt catalysts using MCM‐22 as support, through wet impregnation. The Fischer‐Tropsch synthesis was performed at 280°C, 12.5 bar, and H2/CO = 2. Samples with lower Si/Al ratio showed the most selectivity to methane. These results also confirm the higher cracking of the products and, consequently, a lower selectivity to C5+ when using catalysts that are more acidic. It is well known that Fischer‐Tropsch synthesis selectivity depends on the conversion level and one would expect an increase of C5+ selectivity with increasing the carbon monoxide conversion. However, the C5+ selectivity values obtained for the cobalt catalyst supported on MCM‐22 with high Si/Al ratio (200) and different cobalt loadings seem to be independent of the conversion level. The authors justify the increased C5+ selectivity to the particular properties of the support structure related to shape selectivity, active metal, and Si/Al ratio.

Therefore, it is possible to provide, in most cases, the occurrence of secondary reactions through the presence of acid sites, which may be desirable to obtain hydrocarbons in the aviation kerosene range. The amount and type of sites will depend on intrinsic characteristics of the support, the reaction conditions, and the metallic phase.

#### **4.4. Influence of promoters**

Two types of acid sites, Brönsted and Lewis, can be formed on the zeolite surface. Thus, Brönsted acid sites are generated when Si4+ is isomorphically replaced by a trivalent metal cation such as Al3+. This substitution creates a negative charge in the lattice that can be compensated by a proton [48]. These protons can be present in the medium depending on the reactants that were used in the material synthesis, exerting the function of compensation cations. To replace these cations and consequently form a Brönsted acid site, ion exchange procedure is conducted with an aqueous solution of ammonium chloride, followed by washing to remove residual chloride, and calcination [49]. At high temperatures, the remaining protons can acquire mobility and be lost as water molecules, followed by the formation of Lewis acid

It is evident that the amount of acid sites in the zeolite is related to the Si/Al ratio. The smaller the Si/Al ratio, more aluminum is present in the structure and increases the amount of compensation cations and the number of acid sites when the zeolite is in protonic form. However, the generation of acidic sites is associated with the imbalance of charges generated by isomorphic substitution of different load cations. Therefore, the greater the number of aluminum atoms, more unbalanced will be the zeolitic framework [49]. Due to this fact, the reduction of Si/Al ratio makes the zeolite structure to become less stable and may even damage

In fact, not all acid sites have the same acid strength, because this feature depends on the number of aluminum atoms near the aluminum atom, which supports the acid site [50]. The

From the change of the Si/Al ratio of the zeolitic structure, through either synthesis or post‐ synthesis treatments, it is possible to change not only the total number but also the electronic density on the bridging hydroxyl group, and consequently to change the acid strength of the Brönsted acid site [51]. The control of acid strength as well as the density of acid sites of zeolite catalysts has also led to successful catalysts and processes in the field of oil refining and

For the incorporation and/or suitability of the acidic species of Lewis or Brönsted, there are many synthetic strategies that can be developed, such as impregnation, deposition, precipita‐ tion, calcination, and reduction, among others. Structural variations are diverse and numerous chemical species can be incorporated into the support, in composition, oxidation states, and

In order to evaluate the influence of acidity on catalysts, Bessel et al. [52] showed that acid zeolitic structures have a tendency to catalyze secondary reactions in the Fischer‐Tropsch synthesis, from olefinic products. Secondary reactions include oligomerization, isomerization, cracking, and hydrogen transfer, thus obtaining more branched products with a shorter chain

Ravishankar et al. [53] synthesized cobalt catalysts using MCM‐22 as support, through wet impregnation. The Fischer‐Tropsch synthesis was performed at 280°C, 12.5 bar, and H2/CO = 2. Samples with lower Si/Al ratio showed the most selectivity to methane. These results also confirm the higher cracking of the products and, consequently, a lower selectivity to C5+ when

length compared to the catalysts using supports with less or no acidity.

the crystalline arrangement, besides having influence on the strength of acid sites.

more isolated is the trivalent atom, the greater the acid strength.

petrochemistry [48].

176 Zeolites - Useful Minerals

chemical environments in different ways [49].

sites, due to the isomorphic substitution of Al by Si [8].

In addition to a support with adequate characteristics and good Si/Al ratio, the use of specific promoters may offer desirable features to catalysts, such as, for example, increased reaction rate, greater stability, and C5+ selectivity. According to Iglesia [54], promoters can be classified as chemical and structural, increasing the amount of active sites and the activity per site, respectively. The general effect has been postulated to be due to many factors, including (1) intimate electronic contact changing the local band structure of the metal, (2) ensemble‐type geometric effects, (3) reducing deactivation by carbonaceous deposits, and (4) enabling more surface sites to be reduced by hydrogen spillover during the initial activation [55].

According to the literature, one of the promoters who presented good results when incorpo‐ rated into cobalt catalysts was manganese. Dinse et al. [12] investigated the use of this promoter for cobalt catalysts in the Fischer‐Tropsch synthesis. The catalytic performance of the samples was carried out at 220°C, H2/CO = 2, and operating pressure between 1 and 10 bar. At low pressures, the addition of promoter in the catalyst led to a decrease in methane selectivity and increased C5+ selectivity, while the formation of C2–C<sup>4</sup> hydrocarbons suffered no influence. At high pressures, in addition to a decrease in methane selectivity and increased C5+ selectivity, there was also an increase in C2–C<sup>4</sup> selectivity, as well as a large increase in olefins/paraffins in C2–C4 range.

Dinse et al. [12] attributed the selectivities obtained for Co/SiO2 and Mn‐Co/SiO2 catalysts to the equilibrium constant for CO adsorption, which is greater than the equilibrium constant for H2 adsorption in the presence of manganese, resulting in a reduction in H2/CO ratio and a consequent increase in chain length of products. Selectivity differences in different pressures are also assigned mostly to the H2/CO ratio, which decreases with the increase of the total pressure of reaction and, also, to the increase of α‐olefins reincorporation in chain growth process. Therefore, the manganese exerts significant influence in obtaining long‐chain hydrocarbons, and again the importance of reaction conditions in obtaining hydrocarbons in a specific range is evident.

Khobragade et al. [13] synthesized CO/SiO2 catalysts using K and CeO2 as promoters, from the sol‐gel method, in order to evaluate the performance in Fischer‐Tropsch synthesis. Among the samples tested, the catalyst promoted with cerium presented the highest C5+ selectivity, while the catalyst promoted with potassium presented higher methane selectivity. The behavior of the catalyst promoted with potassium was expected by its use to be more suitable when using iron as active metal, due to the higher reaction temperatures required for this catalyst [56]. In addition, potassium can cause poisoning of catalysts using cobalt as active phase [20]. According to Khobragade et al. [13], the behavior of the catalyst promoted with CeO2 occurs due to a decrease in the hydrogenation rate during the Fischer‐Tropsch synthesis, in which the adsorption of H2 is suppressed in favor of chain growth. This can occur due to partial reduction of CeO2, which would change the electronic properties of cobalt atoms through oxidation. It was also verified that most of the products were in the diesel range, probably due to the increase of active carbon species (‐CH2‐) in the catalyst. Cerium probably decreases the surface of metallic cobalt, resulting in a different chain growth and more favorable to the formation of C5+ hydrocarbons. This effect of the promoter is obtained because of the close interaction with the species of cobalt.

A way of aggregating the advantages of various promoters and brackets in the same catalyst is the use of hybrids and multifunctional catalysts. Teiseh et al. [14] showed that the use of a multifunctional hybrid catalyst (Co‐SiO2/Mo‐Pd‐Pt‐HZSM‐5) produces more hydrocarbons in kerosene range than that of a conventional catalyst (Co/SiO2). In this case, the presence of molybdenum in the hybrid catalyst leads to a combination of carbonium ions (oligomerization of olefins), which contributes to the formation of paraffin in the kerosene range.

In this same context, Yan et al. [11] studied the performance of multifunctional catalysts in the Fischer‐Tropsch synthesis, using alumina as support, cobalt and iron as active metals, and potassium and molybdenum as promoters (K‐Fe‐Co‐Mo/γ‐Al2O3). In this case, the potassium can increase the selectivity and stability of the catalysts in the presence of iron. The multifunc‐ tional catalyst was used with the intention to reduce the number of steps necessary to obtain a jet fuel by Fischer‐Tropsch synthesis. The syngas used was obtained from biomass, which typically has a low H2/CO ratio [57]. In this case, the simultaneous use of Fe and Co is advan‐ tageous because, due to the iron having activity in the water‐gas shift reaction, there is a setting in H2/CO ratio for higher values, in which the cobalt is more active.

After obtaining the products, distillation was conducted for the removal of short‐chain hydrocarbons and waxes. The samples were collected on boiling‐point range between 110 and 310°C. The products obtained after distillation step presented features quite similar to the conventional fuel, in relation to both the distribution of organic functions and the number of carbons. Furthermore, the properties of the distilled fraction have become quite similar to jet fuel.

#### **5. Potential zeolitic catalysts in obtaining synthetic aviation kerosene**

**Table 1** presents the catalysts with significant potential in the production of jet fuel through the Fischer‐Tropsch synthesis. The Si/Al ratio of zeolites, the temperatures and pressure


reaction, and the H2/CO ratio of the syngas and the carbon monoxide conversion are presented in the table.

**Table 1.** Catalysts with significant potential in the production of jet fuel through the Fischer-Tropsch synthesis.

According to a review carried out, the zeolitic supports used in catalysts for Fischer‐Tropsch synthesis wield great influence on the product's chain length. This occurs due to the fact that the support structure can inhibit or head the formation of certain products. In this case, the zeolite ZSM‐34, USY, ZSM‐5, ZSM‐12, MCM‐22, and IM‐5 have significant potential in obtaining products in the aviation kerosene range. In addition, the Si/Al ratio in the catalyst, especially in the range of 7–220, involves the presence of acid sites, which may promote secondary reactions that reduce the formation of waxes and narrow the product's selectivity. Another factor that should be considered is the use of promoters, which in mass concentrations of about 1% may confer upon a major activity to the catalyst and assist in the formation of long‐ chain hydrocarbons. In addition, the Fischer‐Tropsch process at low temperatures and pressures between 10 and 20 bar shows more appropriate in obtaining hydrocarbons C8–C16. Thus, the use of supports with appropriate structural characteristics, specific Si/Al ratio, specific promoters, and suitable conditions of operation shows promise to obtain products with features similar to the jet fuel through the Fischer‐Tropsch synthesis.

#### **Acknowledgements**

samples tested, the catalyst promoted with cerium presented the highest C5+ selectivity, while the catalyst promoted with potassium presented higher methane selectivity. The behavior of the catalyst promoted with potassium was expected by its use to be more suitable when using iron as active metal, due to the higher reaction temperatures required for this catalyst [56]. In addition, potassium can cause poisoning of catalysts using cobalt as active phase [20]. According to Khobragade et al. [13], the behavior of the catalyst promoted with CeO2 occurs due to a decrease in the hydrogenation rate during the Fischer‐Tropsch synthesis, in which the adsorption of H2 is suppressed in favor of chain growth. This can occur due to partial reduction of CeO2, which would change the electronic properties of cobalt atoms through oxidation. It was also verified that most of the products were in the diesel range, probably due to the increase of active carbon species (‐CH2‐) in the catalyst. Cerium probably decreases the surface of metallic cobalt, resulting in a different chain growth and more favorable to the formation of C5+ hydrocarbons. This effect of the promoter is obtained because of the close interaction

A way of aggregating the advantages of various promoters and brackets in the same catalyst is the use of hybrids and multifunctional catalysts. Teiseh et al. [14] showed that the use of a multifunctional hybrid catalyst (Co‐SiO2/Mo‐Pd‐Pt‐HZSM‐5) produces more hydrocarbons in kerosene range than that of a conventional catalyst (Co/SiO2). In this case, the presence of molybdenum in the hybrid catalyst leads to a combination of carbonium ions (oligomerization

In this same context, Yan et al. [11] studied the performance of multifunctional catalysts in the Fischer‐Tropsch synthesis, using alumina as support, cobalt and iron as active metals, and potassium and molybdenum as promoters (K‐Fe‐Co‐Mo/γ‐Al2O3). In this case, the potassium can increase the selectivity and stability of the catalysts in the presence of iron. The multifunc‐ tional catalyst was used with the intention to reduce the number of steps necessary to obtain a jet fuel by Fischer‐Tropsch synthesis. The syngas used was obtained from biomass, which typically has a low H2/CO ratio [57]. In this case, the simultaneous use of Fe and Co is advan‐ tageous because, due to the iron having activity in the water‐gas shift reaction, there is a setting

After obtaining the products, distillation was conducted for the removal of short‐chain hydrocarbons and waxes. The samples were collected on boiling‐point range between 110 and 310°C. The products obtained after distillation step presented features quite similar to the conventional fuel, in relation to both the distribution of organic functions and the number of carbons. Furthermore, the properties of the distilled fraction have become quite similar to jet

**5. Potential zeolitic catalysts in obtaining synthetic aviation kerosene**

**Table 1** presents the catalysts with significant potential in the production of jet fuel through the Fischer‐Tropsch synthesis. The Si/Al ratio of zeolites, the temperatures and pressure

of olefins), which contributes to the formation of paraffin in the kerosene range.

in H2/CO ratio for higher values, in which the cobalt is more active.

with the species of cobalt.

178 Zeolites - Useful Minerals

fuel.

The authors are grateful to the Coordination for the Improvement of Higher Education Personnel (CAPES/Brazil) for financial support.

#### **Author details**

Lidiane Sabino da Silva1 , Victor Luis dos Santos Teixeira da Silva2 , Maria Angélica Simões Dornellas de Barros1 and Pedro Augusto Arroyo1\*

\*Address all correspondence to: arroyo@deq.uem.br

1 Departament of Chemical Engineering, State University of Maringá, Maringá, Brazil

2 Technology Center, Chemical Engineering Program, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil

#### **References**


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**Author details**

180 Zeolites - Useful Minerals

Lidiane Sabino da Silva1

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10.1243/09544062JMES1515

17. DOI: 10.1115/1.3028234

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Maria Angélica Simões Dornellas de Barros1

\*Address all correspondence to: arroyo@deq.uem.br

, Victor Luis dos Santos Teixeira da Silva2

1 Departament of Chemical Engineering, State University of Maringá, Maringá, Brazil

2 Technology Center, Chemical Engineering Program, Federal University of Rio de Janeiro,

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