**Introduction**

The book *Zeolites and Their Applications* offers important information sources for researchers

We would like to express our sincere thanks to Dr. A. Geetha, Assistant Professor, Depart‐ ment of Chemistry, Kongu Engineering College, Tamilnadu, India and Prof. Kamal A. Magd, Department of Geology, Faculty of Science, Aswan University, Egypt, for their contribution in the scientific revision of the book chapters. Also, we would like to extend our many thanks to Ms. Lada Bozic, Author Service Manager, IntechOpen, for her kind follow-ups and assistance.

> **M. Nageeb Rashed** Faculty of Science

> > **P. N. Palanisamy**

Aswan University, Egypt

Department of Chemistry Kongu Engineering College

Perundurai, Erode, 638 060, Tamilnadu, India

and professionals working in zeolite preparations and applications.

VIII Preface

**Chapter 1**

**Provisional chapter**

**Introductory Chapter: Adsorption and Ion Exchange**

**Introductory Chapter: Adsorption and Ion Exchange** 

Pachagoundanpalayam Nachimuthugounder Palanisamy

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Mohamed Nageeb Rashed and

Mohamed Nageeb Rashed and Pachagoundanpalayam Nachimuthugounder Palanisamy

http://dx.doi.org/10.5772/intechopen.77190

the domestic, agricultural, and industries.

**1. Introduction**

**1.1. Water pollution**

**1.2. Zeolite minerals**

attractive catalysts [1].

**Properties of Zeolites for Treatment of Polluted Water**

Over the last few decades, the world has become increasingly sensitive toward the protection of the ecosystem and its environment. As a result of the rapid increase in population and economic development; large quantities of waste were generated lead to severe environmental degradation and thereby resulting in pollution. One of the major environmental pollutions is water pollution. Waste water as one of the most reasons for water pollution may come from

Adsorption and ion exchange techniques for wastewater treatment have become more popular in recent years owing to their efficiency in the removal of pollutants. The most common adsorbent materials are alumina, calais, silica, zeolites, metal hydroxides, and activated carbon.

Zeolites form a unique class of oxides, consisting of microporous, crystalline aluminosilicates that can be found in nature, or synthesized artificially. The zeolite framework is very open and contains channels and cages, where cations, water, and adsorbed molecules may reside and react. The specific adsorption and ion exchange properties of zeolites are used in industries, color removal, detergents, toothpaste, and desiccants, whereas their acidity makes them

**Properties of Zeolites for Treatment of Polluted Water**

© 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.

© 2018 The Author(s). Licensee IntechOpen. 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.

DOI: 10.5772/intechopen.77190

#### **Introductory Chapter: Adsorption and Ion Exchange Properties of Zeolites for Treatment of Polluted Water Introductory Chapter: Adsorption and Ion Exchange Properties of Zeolites for Treatment of Polluted Water**

DOI: 10.5772/intechopen.77190

Mohamed Nageeb Rashed and Pachagoundanpalayam Nachimuthugounder Palanisamy Mohamed Nageeb Rashed and Pachagoundanpalayam Nachimuthugounder Palanisamy

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.77190

### **1. Introduction**

#### **1.1. Water pollution**

Over the last few decades, the world has become increasingly sensitive toward the protection of the ecosystem and its environment. As a result of the rapid increase in population and economic development; large quantities of waste were generated lead to severe environmental degradation and thereby resulting in pollution. One of the major environmental pollutions is water pollution. Waste water as one of the most reasons for water pollution may come from the domestic, agricultural, and industries.

Adsorption and ion exchange techniques for wastewater treatment have become more popular in recent years owing to their efficiency in the removal of pollutants. The most common adsorbent materials are alumina, calais, silica, zeolites, metal hydroxides, and activated carbon.

#### **1.2. Zeolite minerals**

Zeolites form a unique class of oxides, consisting of microporous, crystalline aluminosilicates that can be found in nature, or synthesized artificially. The zeolite framework is very open and contains channels and cages, where cations, water, and adsorbed molecules may reside and react. The specific adsorption and ion exchange properties of zeolites are used in industries, color removal, detergents, toothpaste, and desiccants, whereas their acidity makes them attractive catalysts [1].

© 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. © 2018 The Author(s). Licensee IntechOpen. 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.

Zeolites are classified into two classes, one is natural zeolites, and the other is synthetic zeolites. Natural zeolites are hydrated aluminosilicates compounds with a characteristic three-dimensional structure of tetrahedrons as TO<sup>4</sup> (T = Si, Al, B, Ge, Fe, P, and Co) joined by oxygen atoms, with large pore apertures and pore system that allow the relatively easy exchange of cations between aqueous solutions and intracrystalline sites [2, 3]. Zeolites have in its internal structure cavities and channels interconnected of molecular dimensions, where compensation cations allowing the ion exchange [4, 5]. New functional groups may introduce to zeolites through several processes of modification that improve its activity and selectivity on the removal several substances [6–8]. Several authors studied the use of modified natural zeolite on environmental applications, mainly anions uptake from effluents by adsorption processes [9–11]. Due to the excellent ion exchange ability and high surface area, natural zeolites [12, 13], and synthetic zeolites [14–16].

adsorption or chemisorption, which involves chemical bonding. The former is well suited for a

Introductory Chapter: Adsorption and Ion Exchange Properties of Zeolites for Treatment of Polluted Water

http://dx.doi.org/10.5772/intechopen.77190

5

Adsorption techniques have gained favor recently due to their efficiency in the removal of pollutants. Also, adsorption process provides an attractive alternative treatment, especially if the adsorbent is inexpensive and readily available. The advantage of the adsorption process is its sludge free, clean operation, and complete removal of pollutants even from dilute solution. Therefore, adsorption considered as one of a low cost and powerful treatment processes for

Zeolites are the most important inorganic cation exchangers and adsorptive materials. It shows higher cation exchange selectivities, good resistance to temperature and ionizing radiations, and excellent compatibility with the environment. Therefore, zeolites are widely used in modern technologies as selective adsorbents, molecular sieves, and particularly as catalysts. Ion exchange property is employed also as a tool for tailoring the structure in order to obtain specific performances, and so, it competes with cation exchange resins in water pro-

Zeolites have two main properties: adsorption and ion exchange. These two properties are due to reactive surfaces, due to the presence of Al3+ on adsorption sites with a Si4+ ion resides, and the micropores crystalline system. These properties allow the zeolite for several applications. Zeolites are essentially nontoxic and pose no environmental risk. Zeolite is also applied

Because of their favorable ion exchange selectivity for certain cations, zeolite minerals, particular clinoptilolite, are of interest for use in the treatment of nuclear waste waters [21], municipal and industrial waste waters [22], acid mine drainage waters [23] and other construction materials.

Ion exchange is a function of solid and aqueous phase composition and aqueous solution

Ion exchange equilibria occur between two or more phases, one of which is liquid which exchanges two or more ions (cations or anions), more or less strongly bound to each phase. Ions exchangeable quantity by a solid exchanger depending on its structural features and is called the ion exchange capacity, usually expressed in meq/g. Ion transfer from one phase to the other is subject to the observance of electroneutrality and regulated by the ion concentration in both phases. This parameter is a function of both the energy of ion lattice interaction

(z+) + ~−nM (mz

) + + mN"

(s+) (1)

regenerable process, while the latter generally destroys the capacity of the adsorbent [18].

the removal of pollutants from waste water [19].

in toothpaste to bind calcium [20].

concentration.

**2.2. Ion exchange phenomena in zeolites**

and the hydration energy (ion solution interaction).

where, m and n are the valences of exchanging cations.

) + −} − mN"

M and N and subscripts s and z denote solution and zeolite phase, respectively.

A cation exchange reaction may be written as.

nM(ms

cessing and in the purification of wastewater and sewage.

Nearly 600 known zeolites were discovered. International Union of Pure and Applied Chemistry (IUPAC) endorsed a general classification of zeolites structures, that is, FAU for faujasites, mordenite framework inverted (MFI) for ZSM-5, and mordenite zeolite (MOR) for mordenite.

### **2. Adsorption and ion exchange phenomena**

Adsorption and ion exchange, take advantage of many common features in regard to application in batch and fixed-bed processes for a unified treatment. These processes involve the transfer and distribution of solutes between a fluid phase and particles.

Adsorbents are natural or synthetic materials of the amorphous or microcrystalline structure. Those used on a large scale are activated carbon, molecular sieves, silica gel, and activated alumina.

Ion exchange occurs throughout a polymeric solid, which dissolves some fluid-phase solvent. In ion exchange, ions of positive charge in some cases (cations) and negative charge in others (anions) from the fluid, replace dissimilar ions of the same charge initially in the solid. The ion exchanger exhibits permanently bound functional groups of different charge. In ion exchanger, cation exchange resins generally contain bound sulfonic acid groups; less commonly, these groups are carboxylic, phosphonic, phosphinic, and so on. Anionic resins involve quaternary ammonium groups (strongly basic) or other amino groups (weakly basic) [17].

#### **2.1. Adsorption phenomena in zeolites**

Adsorption involves, in general, the accumulation (or depletion) of solute molecules at an interface (including gas–liquid interfaces, as in foam fractionation, and liquid–liquid interfaces, as in detergency).

The most adsorption processes are of gas–solid and liquid–solid interfaces, with solute distributed selectively between the fluid and solid phases. The accumulation per unit surface area is small; thus, highly porous solids with very large internal area per unit volume are preferred. Two classes of adsorption are identified, physical adsorption and chemical adsorption. Physical adsorption or physisorption involves van der Waals forces (as in vapor condensation), and retard chemical adsorption or chemisorption, which involves chemical bonding. The former is well suited for a regenerable process, while the latter generally destroys the capacity of the adsorbent [18].

Adsorption techniques have gained favor recently due to their efficiency in the removal of pollutants. Also, adsorption process provides an attractive alternative treatment, especially if the adsorbent is inexpensive and readily available. The advantage of the adsorption process is its sludge free, clean operation, and complete removal of pollutants even from dilute solution. Therefore, adsorption considered as one of a low cost and powerful treatment processes for the removal of pollutants from waste water [19].

Zeolites are the most important inorganic cation exchangers and adsorptive materials. It shows higher cation exchange selectivities, good resistance to temperature and ionizing radiations, and excellent compatibility with the environment. Therefore, zeolites are widely used in modern technologies as selective adsorbents, molecular sieves, and particularly as catalysts. Ion exchange property is employed also as a tool for tailoring the structure in order to obtain specific performances, and so, it competes with cation exchange resins in water processing and in the purification of wastewater and sewage.

Zeolites have two main properties: adsorption and ion exchange. These two properties are due to reactive surfaces, due to the presence of Al3+ on adsorption sites with a Si4+ ion resides, and the micropores crystalline system. These properties allow the zeolite for several applications. Zeolites are essentially nontoxic and pose no environmental risk. Zeolite is also applied in toothpaste to bind calcium [20].

Because of their favorable ion exchange selectivity for certain cations, zeolite minerals, particular clinoptilolite, are of interest for use in the treatment of nuclear waste waters [21], municipal and industrial waste waters [22], acid mine drainage waters [23] and other construction materials.

### **2.2. Ion exchange phenomena in zeolites**

Zeolites are classified into two classes, one is natural zeolites, and the other is synthetic zeolites. Natural zeolites are hydrated aluminosilicates compounds with a characteristic

by oxygen atoms, with large pore apertures and pore system that allow the relatively easy exchange of cations between aqueous solutions and intracrystalline sites [2, 3]. Zeolites have in its internal structure cavities and channels interconnected of molecular dimensions, where compensation cations allowing the ion exchange [4, 5]. New functional groups may introduce to zeolites through several processes of modification that improve its activity and selectivity on the removal several substances [6–8]. Several authors studied the use of modified natural zeolite on environmental applications, mainly anions uptake from effluents by adsorption processes [9–11]. Due to the excellent ion exchange ability and high surface area,

Nearly 600 known zeolites were discovered. International Union of Pure and Applied Chemistry (IUPAC) endorsed a general classification of zeolites structures, that is, FAU for faujasites, mordenite framework inverted (MFI) for ZSM-5, and mordenite zeolite (MOR) for mordenite.

Adsorption and ion exchange, take advantage of many common features in regard to application in batch and fixed-bed processes for a unified treatment. These processes involve the

Adsorbents are natural or synthetic materials of the amorphous or microcrystalline structure. Those used on a large scale are activated carbon, molecular sieves, silica gel, and activated

Ion exchange occurs throughout a polymeric solid, which dissolves some fluid-phase solvent. In ion exchange, ions of positive charge in some cases (cations) and negative charge in others (anions) from the fluid, replace dissimilar ions of the same charge initially in the solid. The ion exchanger exhibits permanently bound functional groups of different charge. In ion exchanger, cation exchange resins generally contain bound sulfonic acid groups; less commonly, these groups are carboxylic, phosphonic, phosphinic, and so on. Anionic resins involve quaternary ammonium groups (strongly basic) or other amino groups (weakly basic) [17].

Adsorption involves, in general, the accumulation (or depletion) of solute molecules at an interface (including gas–liquid interfaces, as in foam fractionation, and liquid–liquid

The most adsorption processes are of gas–solid and liquid–solid interfaces, with solute distributed selectively between the fluid and solid phases. The accumulation per unit surface area is small; thus, highly porous solids with very large internal area per unit volume are preferred. Two classes of adsorption are identified, physical adsorption and chemical adsorption. Physical adsorption or physisorption involves van der Waals forces (as in vapor condensation), and retard chemical

(T = Si, Al, B, Ge, Fe, P, and Co) joined

three-dimensional structure of tetrahedrons as TO<sup>4</sup>

4 Zeolites and Their Applications

natural zeolites [12, 13], and synthetic zeolites [14–16].

**2. Adsorption and ion exchange phenomena**

**2.1. Adsorption phenomena in zeolites**

interfaces, as in detergency).

alumina.

transfer and distribution of solutes between a fluid phase and particles.

Ion exchange is a function of solid and aqueous phase composition and aqueous solution concentration.

Ion exchange equilibria occur between two or more phases, one of which is liquid which exchanges two or more ions (cations or anions), more or less strongly bound to each phase. Ions exchangeable quantity by a solid exchanger depending on its structural features and is called the ion exchange capacity, usually expressed in meq/g. Ion transfer from one phase to the other is subject to the observance of electroneutrality and regulated by the ion concentration in both phases. This parameter is a function of both the energy of ion lattice interaction and the hydration energy (ion solution interaction).

A cation exchange reaction may be written as.

$$\text{nM(m}\_{\text{s}}\text{)} + \text{ }\frac{\text{}}{\text{}}\text{ }\text{-mN'}\text{(z+)} + \text{ }\neg\text{-nM (m}\_{\text{x}}\text{)} + \text{ }\neg\text{mN'}\text{(s+)}\tag{1}$$

where, m and n are the valences of exchanging cations.

M and N and subscripts s and z denote solution and zeolite phase, respectively.

Ion exchange property in zeolites resulted from the presence of extra cations, located on channels and cages of it [24]. In the zeolite structures, there are various cation sites, which differ from each other in framework position and therefore, in bond energy. When the zeolite contact with an electrolytic solution, its cations escape from their sites and replaced by other cations from the solution [25]. Cation sieving may be due to the inability of the negative charge distribution on the zeolite structure to accommodate a given cation [24] (**Table 1**).

#### **2.3. Cations and acidity in zeolites**

In the tetrahedral crystal, when Al3+ replaces Si4+ ions the units have a net charge of–1, and so the cation with a positive charges, such as Na+ , is neutralizing the negative charge. The number of cations presents within in a zeolite structure equals the number of alumina tetrahedral. A zeolite in its sodium compensated form is presented as Na-ZSM-5, Na-X, and so if Na+ ions are replaced by H+ (yielding H-ZSM-5, H-X,) the zeolite becomes a gigantic poly acid. The structure of an acid site with H<sup>+</sup> on a Si–O–Al bridge is illustrated in **Figure 1**. As zeolite being a proton donor, the site is called a Brønsted acid, and its strength depends on the number of other aluminum ions in the environment and the local environment of the proton [20].

**Author details**

**References**

Mohamed Nageeb Rashed1

**Figure 1.** Example of a solid acid.

\*Address all correspondence to: mnrashed@hotmail.com

Tecnologia Mineral; 1997. p. 68

Biotechnology. 1994;**59**(2):121-126

zeolites. Water Research. 1996;**31**(6):1379-1382

1 Department of Chemistry, Faculty of Science, Aswan University, Egypt

2 Department of Chemistry, Kongu Engineering College, Tamil Nadu, India

of Heterogeneous Catalysis, Vol. 1. Weinheim: VCH; 1997. p. 206

[1] Thomas JM, Bell RG, Catlow CRA. In: Ertl G, Knzinger H, Weitkamp J, editors. Handbook

[2] Da Luz BA. Zeólitas: Propriedades e usos industriais. Rio de Janeiro: Centro de

[3] Inglezakis LV, Grigoropoulou H. Effects of operating conditions on the removal of heavy metals by zeolite in fixed bed reactors. Journal of Hazardous Materials. 2004;**B112**:37-43

[4] Kesraoui-Ouki S, Cheeseman RC, Perry R. Natural zeolite utilization in pollution controls: A review of applications to metals effluents. Journal of Chemical Technology and

[5] Luna JF. Modificação de Zeólitas para Uso em Catálise. Química Nova. 2001;**24**(6):885-892 [6] Curkovic L, Cerjan-Stefanovic S, Filipan T. Metal ion exchange by natural and modified

[7] Dentel KS, Jamrah IA, Sparks LD. Sorption and cosorption of 1,2,3-trichlorobenzene and

[8] Milosevic S, Tomasevic-Canovic, M. Modification of the surface of minerals for development the materials-adsorbents. In: 36th International October Conference on Mining and

tannic acid by organo-clays. Water Research. 1998;**32**(12):3689-3697

Metallurgy; 2004 Sep 2, October, Bor, Serbia and Montenegro; 2004

\* and Pachagoundanpalayam Nachimuthugounder Palanisamy<sup>2</sup>

Introductory Chapter: Adsorption and Ion Exchange Properties of Zeolites for Treatment of Polluted Water

http://dx.doi.org/10.5772/intechopen.77190

7

Calculated from the unit-cell formula.

**Table 1.** Representative formulae, chemical composition, and selected physical properties of important zeolites [25, 26].

Introductory Chapter: Adsorption and Ion Exchange Properties of Zeolites for Treatment of Polluted Water http://dx.doi.org/10.5772/intechopen.77190 7

**Figure 1.** Example of a solid acid.

#### **Author details**

Ion exchange property in zeolites resulted from the presence of extra cations, located on channels and cages of it [24]. In the zeolite structures, there are various cation sites, which differ from each other in framework position and therefore, in bond energy. When the zeolite contact with an electrolytic solution, its cations escape from their sites and replaced by other cations from the solution [25]. Cation sieving may be due to the inability of the negative charge

In the tetrahedral crystal, when Al3+ replaces Si4+ ions the units have a net charge of–1, and

number of cations presents within in a zeolite structure equals the number of alumina tetrahedral. A zeolite in its sodium compensated form is presented as Na-ZSM-5, Na-X, and so

zeolite being a proton donor, the site is called a Brønsted acid, and its strength depends on the number of other aluminum ions in the environment and the local environment of the

**Table 1.** Representative formulae, chemical composition, and selected physical properties of important zeolites [25, 26].

, is neutralizing the negative charge. The

on a Si–O–Al bridge is illustrated in **Figure 1**. As

(yielding H-ZSM-5, H-X,) the zeolite becomes a gigantic poly

distribution on the zeolite structure to accommodate a given cation [24] (**Table 1**).

**2.3. Cations and acidity in zeolites**

6 Zeolites and Their Applications

ions are replaced by H+

acid. The structure of an acid site with H<sup>+</sup>

if Na+

b

Calculated from the unit-cell formula.

proton [20].

so the cation with a positive charges, such as Na+

Mohamed Nageeb Rashed1 \* and Pachagoundanpalayam Nachimuthugounder Palanisamy<sup>2</sup>

\*Address all correspondence to: mnrashed@hotmail.com

1 Department of Chemistry, Faculty of Science, Aswan University, Egypt

2 Department of Chemistry, Kongu Engineering College, Tamil Nadu, India

#### **References**


[9] Stocker K, Ellersdorfer M, Lehner M, Raith JG. Characterization and utilization of natural zeolites in technical applications. BHM. 2017;**162**(4):142-147

[24] Dyer A. An Introduction to Zeolite Molecular Sieves. Chichester: John Wiley; 1988. 49 pp [25] Colella C. Ion exchange equilibria in zeolite minerals. Mineralium Deposita.

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[26] Mumpton FA. Natural zeolites. In: Pond WG, Mumpton FA, editors. Zeo-Agriculture: Use of Natural Zeolites in Agriculture and Aquaculture. Colorado: Westview Press;

1996;**31**:554-562

1983. pp. 33-43


[24] Dyer A. An Introduction to Zeolite Molecular Sieves. Chichester: John Wiley; 1988. 49 pp

[9] Stocker K, Ellersdorfer M, Lehner M, Raith JG. Characterization and utilization of natu-

[10] Kędziora K, Piasek J, Szerement J, Ambrożewicz-Nita A. Use of modified zeolite in envi-

[11] Apreutesei RE, Catrinescu C, Teodosiu C. Surfactant-modified natural zeolites for environmental applications in water purification. Environmental Engineering and

[12] Rožić ŠC-S, Kurajica S, Vančina V, Hodžić E. Ammoniacal nitrogen removal from water

[13] Jung J-Y, Chung Y-C, Shin H-S, Son D-H. Enhanced ammonia nitrogen removal using consistent biological regeneration and ammonium exchange of zeolite in modified SBR

[14] Takami N, Murayama K, Ogawa HY, Shibata J. Water purification property of zeolite

[15] Zhang M, Zhang H, Xu D, et al. Removal of ammonium from aqueous solutions using zeolite synthesized from fly ash by a fusion method. Desalination. 2011;**271**(1-3):111-121

[16] Yusof AM, Keat LK, Ibrahim Z, Majid ZA, Nizam NA. Kinetic and equilibrium studies of the removal of ammonium ions from aqueous solution by rice husk ash-synthesized zeolite Y and powdered and granulated forms of mordenite. Journal of Hazardous

[17] Douglas LeVan M, Carta G, Yon CM. Adsorption and Ion Exchange. New York: McGraw

[18] Ruthven DM. Principal of Adsorption and Desorption Processes. New York: John Wiley

[19] Evans GM, Furlong JC. Environmental Biotechnology: Theory and Application I. K.

[20] Chorkendorff I, Niemantsverdriet JW. Concepts of Modern Catalysis and Kinetics.

[21] Pansini M. Natural zeolites as cation exchangers for environmental protection.

[22] Kallo D. Wastewater purification in Hungary using natural zeolites. In Ming DW, Mumpton FA, editors. Natural Zeolites '93: Occurrence, Properties, Use; International

[23] Zamzow MJ, Schultze LE. Treatment of acid mine drainage using natural zeolites. In: Ming DW, Mumpton FA, editors. Natural Zeolites '93: Occurrence, Properties, Use. International Committee on Natural Zeolites, Brockport, New York; 1995. p. 405413

Committee on Natural Zeolites; Brockport, New York; 1995. pp. 341-350

by treatment with clays and zeolites. Water Research. 2000;**34**(14):3675-3681

synthesized from coal fly ash. Shigen-to-Sozai. 2000;**116**(9):789-794

ral zeolites in technical applications. BHM. 2017;**162**(4):142-147

ronmental engineering. A review. 2014;**781**(C):61-66

Management Journal. 2008;**7**(2):149-161

process. Water Research. 2004;**38**(2):347-354

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Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA; 2003

Hill Comp Inc.; 1999

and Sons; 1984

8 Zeolites and Their Applications


**Section 2**

**Zeolites Morphology and Synthesis**

**Zeolites Morphology and Synthesis**

**Chapter 2**

**Provisional chapter**

**Linde Type L Zeolite: A Privileged Porous Support to**

**Linde Type L Zeolite: A Privileged Porous Support to** 

Among the wide assortment of zeolites based on aluminosilicates, Linde Type L (LTL) zeolite outstands as a support host owing to its porous framework and high adsorption surfaces. Thus, the incorporation of suitable guest molecules (fluorophores or metals) allows the development of photoactive and catalytic nanomaterials. In this chapter, we describe the design of materials based on LTL zeolite to achieve artificial antennae, inspired in the natural photosynthesis, and ecofriendly materials for the catalytic reforming of biogas. First, we describe the microwave-assisted synthesis of LTL zeolite with tunable size and morphology. Afterward, we test the energy transfer probability between the guest fluorophores into the LTL zeolite pores as the key process enabling the antenna behavior of this hybrid material with broadband absorption and tunable emission or predominant red fluorescence. Finally, we also test the behavior of LTL zeolite as a support material for the catalytic reforming of biogas. To this aim, suitable metals were impregnated onto LTL zeolite featuring different shapes and alkaline metal exchange. Activity tests indicated that disk- and cylinder-shaped hosts were the most active ones, especially when bimetallic (Rh-Ni) catalysts were prepared. However, the alkaline metal exchange was ineffective to increase the hydrogen yield.

**Keywords:** LTL zeolite, hydrothermal synthesis, nanomaterials, energy transfer,

Zeolites are crystalline silicates and aluminosilicates linked through oxygen atoms, producing well-defined three-dimensional structures with cavities and channels where water, cations,

> © 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.

© 2018 The Author(s). Licensee IntechOpen. 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.

DOI: 10.5772/intechopen.73135

**Develop Photoactive and Catalytic Nanomaterials**

**Develop Photoactive and Catalytic Nanomaterials**

Leire Gartzia Rivero, Jorge Bañuelos, Kepa Bizkarra,

Leire Gartzia Rivero, Jorge Bañuelos, Kepa Bizkarra,

Jose Francisco Cambra and Iñigo López Arbeloa

Jose Francisco Cambra and Iñigo López Arbeloa

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Urko Izquierdo, Victoria Laura Barrio,

Urko Izquierdo, Victoria Laura Barrio,

http://dx.doi.org/10.5772/intechopen.73135

catalytic support, hydrogen

**1. Introduction**

**Abstract**

#### **Linde Type L Zeolite: A Privileged Porous Support to Develop Photoactive and Catalytic Nanomaterials Linde Type L Zeolite: A Privileged Porous Support to Develop Photoactive and Catalytic Nanomaterials**

DOI: 10.5772/intechopen.73135

Leire Gartzia Rivero, Jorge Bañuelos, Kepa Bizkarra, Urko Izquierdo, Victoria Laura Barrio, Jose Francisco Cambra and Iñigo López Arbeloa Leire Gartzia Rivero, Jorge Bañuelos, Kepa Bizkarra, Urko Izquierdo, Victoria Laura Barrio, Jose Francisco Cambra and Iñigo López Arbeloa

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73135

#### **Abstract**

Among the wide assortment of zeolites based on aluminosilicates, Linde Type L (LTL) zeolite outstands as a support host owing to its porous framework and high adsorption surfaces. Thus, the incorporation of suitable guest molecules (fluorophores or metals) allows the development of photoactive and catalytic nanomaterials. In this chapter, we describe the design of materials based on LTL zeolite to achieve artificial antennae, inspired in the natural photosynthesis, and ecofriendly materials for the catalytic reforming of biogas. First, we describe the microwave-assisted synthesis of LTL zeolite with tunable size and morphology. Afterward, we test the energy transfer probability between the guest fluorophores into the LTL zeolite pores as the key process enabling the antenna behavior of this hybrid material with broadband absorption and tunable emission or predominant red fluorescence. Finally, we also test the behavior of LTL zeolite as a support material for the catalytic reforming of biogas. To this aim, suitable metals were impregnated onto LTL zeolite featuring different shapes and alkaline metal exchange. Activity tests indicated that disk- and cylinder-shaped hosts were the most active ones, especially when bimetallic (Rh-Ni) catalysts were prepared. However, the alkaline metal exchange was ineffective to increase the hydrogen yield.

**Keywords:** LTL zeolite, hydrothermal synthesis, nanomaterials, energy transfer, catalytic support, hydrogen

#### **1. Introduction**

Zeolites are crystalline silicates and aluminosilicates linked through oxygen atoms, producing well-defined three-dimensional structures with cavities and channels where water, cations,

© 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. © 2018 The Author(s). Licensee IntechOpen. 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.

and/or small molecules can allocate [1–5]. The term "zeolite" has its origin in the two Greek words *zeo* and *lithos*, literally meaning "boiling stone." The mineralogist Fredrik Cronstedt who observed that the heating of the material produced large amounts of steam water previously adsorbed by the pores coined this term in 1756 [6]. The zeolites are also known as molecular sieves and, albeit many of them occur naturally as minerals, most of these materials have been synthesized by the scientific community for different purposes in fundamental chemistry and technological fields. Nowadays, there are 191 types of zeolite frameworks registered and over 40 natural zeolite frameworks are known [7].

Likely, zeolites are the most widely used catalysts in the industry for oil refining, petrochemistry, and organic synthesis due to their high surface area and adsorption capacity, as well as their controllable adsorption properties [8]. Moreover, the possibility of designing frameworks *a la carte* with tunable porosity and chemical properties make them strong candidates for hosting different types of guests (such as ions, metals, and organic molecules) and for mass transport and/or occlusion [9].

Among the high diversity of natural and synthetic zeolites, Linde Type L (LTL) zeolite stands out owing to its appealing physicochemical and structural properties and high versatility [10]. LTL zeolite is a crystalline aluminosilicate of well-defined three-dimensional framework and a hexagonal symmetry (**Figure 1**). It is formed by corner sharing TO4 tetrahedra (T being aluminum or silicon) leading to the arrangement of cancrinite cages and the final threedimensional network. The presence of the trivalent aluminum infers an anionic character to the framework and charge-compensating cations are required to balance the charge of the tetrahedra. This is why the stoichiometry of LTL zeolite with monovalent charge compensating cations M+ is M<sup>9</sup> [Al<sup>9</sup> Si27O72]⋅nH2 O, where the number of water molecules n per unit cell equals 21 in fully hydrated materials and is about 16 at 20% relative air humidity. Its framework is characterized by uni-dimensional channels running along the c-axis of the crystal with a pore diameter suitable to host for many molecules of interest (7.1 Å), and hence, ideal to allocate organic fluorophores.

Over recent years, LTL zeolite has been successfully applied in a broad range of fields, including ion exchange and separation [11], catalytic processes [12], artificial antenna materials [13], photosensitizers in solar cells or light emitting diodes [14], luminescent solar concentrators

**Figure 2.** Schematic representation of the photoactive antenna and catalytic nanomaterials. Top: sequential insertion of three different dyes (absorbing and emitting in different ranges of the electromagnetic spectrum and undergoing energy transfer) into the LTL zeolite channels. Bottom: adsorption of metals onto the zeolitic surface to generate hydrogen from

Linde Type L Zeolite: A Privileged Porous Support to Develop Photoactive and Catalytic…

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15

In order to achieve competitive applications in advanced nanodevices is essential to provide high quality LTL zeolite crystals, which means, defined morphology, high crystallinity, and homogeneous size distribution of the particles. LTL zeolite is usually synthesized under hydrothermal conditions using conventional ovens. The crystal size and morphology are straightforwardly controlled by adjusting the starting gel composition (water content, alkalinity, or oxide proportion) as well as the heating conditions (heating rate and/or temperature). LTL zeolites with sizes ranging from nanometers to micrometers and disc-shaped or barrel-

Herein, we present a novel synthetic approach relied on the use of microwave-assisted heating for the production of high quality LTL zeolite crystals with tunable size and morphology. These inorganic crystals will then act as scaffolds for antenna systems (inspired in biosynthetic organisms), as well as environmentally friendly catalytic supports for energetic applications (metalcatalyzed hydrogen production from biogas reforming) (**Figure 2**). Indeed, the pore diameter of the host fits well to allocate organic fluorophores inside, whereas the constrained environment of the channel arranges hierarchically the chromophores, boosting the energy transfer, and hence, the antenna effect. Besides, the high adsorption capability of this support material, owing to its large available surface area, enables the efficient absorption of high amount of met-

[15], color changing media [16], microlasers [17], and in biomedicine [18].

like morphologies have already been reported [19, 20].

biogas by catalyzed reforming processes.

als, thereby, enhancing the catalytic activity.

**Figure 1.** Top view of LTL zeolite framework, illustrating the hexagonal structure and the uni-dimensional pores. Side view of a channel that consist of 7.5 Å long unit cells with a van der Waals opening of 7.1 Å at the smallest and 11.26 Å at the widest place.

Linde Type L Zeolite: A Privileged Porous Support to Develop Photoactive and Catalytic… http://dx.doi.org/10.5772/intechopen.73135 15

and/or small molecules can allocate [1–5]. The term "zeolite" has its origin in the two Greek words *zeo* and *lithos*, literally meaning "boiling stone." The mineralogist Fredrik Cronstedt who observed that the heating of the material produced large amounts of steam water previously adsorbed by the pores coined this term in 1756 [6]. The zeolites are also known as molecular sieves and, albeit many of them occur naturally as minerals, most of these materials have been synthesized by the scientific community for different purposes in fundamental chemistry and technological fields. Nowadays, there are 191 types of zeolite frameworks reg-

Likely, zeolites are the most widely used catalysts in the industry for oil refining, petrochemistry, and organic synthesis due to their high surface area and adsorption capacity, as well as their controllable adsorption properties [8]. Moreover, the possibility of designing frameworks *a la carte* with tunable porosity and chemical properties make them strong candidates for hosting different types of guests (such as ions, metals, and organic molecules) and for

Among the high diversity of natural and synthetic zeolites, Linde Type L (LTL) zeolite stands out owing to its appealing physicochemical and structural properties and high versatility [10]. LTL zeolite is a crystalline aluminosilicate of well-defined three-dimensional frame-

being aluminum or silicon) leading to the arrangement of cancrinite cages and the final threedimensional network. The presence of the trivalent aluminum infers an anionic character to the framework and charge-compensating cations are required to balance the charge of the tetrahedra. This is why the stoichiometry of LTL zeolite with monovalent charge compensating

21 in fully hydrated materials and is about 16 at 20% relative air humidity. Its framework is characterized by uni-dimensional channels running along the c-axis of the crystal with a pore diameter suitable to host for many molecules of interest (7.1 Å), and hence, ideal to allocate

**Figure 1.** Top view of LTL zeolite framework, illustrating the hexagonal structure and the uni-dimensional pores. Side view of a channel that consist of 7.5 Å long unit cells with a van der Waals opening of 7.1 Å at the smallest and 11.26 Å

O, where the number of water molecules n per unit cell equals

tetrahedra (T

work and a hexagonal symmetry (**Figure 1**). It is formed by corner sharing TO4

istered and over 40 natural zeolite frameworks are known [7].

mass transport and/or occlusion [9].

14 Zeolites and Their Applications

cations M+

 is M<sup>9</sup> [Al<sup>9</sup>

organic fluorophores.

at the widest place.

Si27O72]⋅nH2

**Figure 2.** Schematic representation of the photoactive antenna and catalytic nanomaterials. Top: sequential insertion of three different dyes (absorbing and emitting in different ranges of the electromagnetic spectrum and undergoing energy transfer) into the LTL zeolite channels. Bottom: adsorption of metals onto the zeolitic surface to generate hydrogen from biogas by catalyzed reforming processes.

Over recent years, LTL zeolite has been successfully applied in a broad range of fields, including ion exchange and separation [11], catalytic processes [12], artificial antenna materials [13], photosensitizers in solar cells or light emitting diodes [14], luminescent solar concentrators [15], color changing media [16], microlasers [17], and in biomedicine [18].

In order to achieve competitive applications in advanced nanodevices is essential to provide high quality LTL zeolite crystals, which means, defined morphology, high crystallinity, and homogeneous size distribution of the particles. LTL zeolite is usually synthesized under hydrothermal conditions using conventional ovens. The crystal size and morphology are straightforwardly controlled by adjusting the starting gel composition (water content, alkalinity, or oxide proportion) as well as the heating conditions (heating rate and/or temperature). LTL zeolites with sizes ranging from nanometers to micrometers and disc-shaped or barrellike morphologies have already been reported [19, 20].

Herein, we present a novel synthetic approach relied on the use of microwave-assisted heating for the production of high quality LTL zeolite crystals with tunable size and morphology. These inorganic crystals will then act as scaffolds for antenna systems (inspired in biosynthetic organisms), as well as environmentally friendly catalytic supports for energetic applications (metalcatalyzed hydrogen production from biogas reforming) (**Figure 2**). Indeed, the pore diameter of the host fits well to allocate organic fluorophores inside, whereas the constrained environment of the channel arranges hierarchically the chromophores, boosting the energy transfer, and hence, the antenna effect. Besides, the high adsorption capability of this support material, owing to its large available surface area, enables the efficient absorption of high amount of metals, thereby, enhancing the catalytic activity.

### **2. LTL zeolite: microwave-assisted hydrothermal synthesis**

Microwave heating is an emerging technique in modern organic synthesis and in the production of nanoparticles and nanostructures. It usually affords an improvement in the yield and reproducibility of the synthetic processes, reducing the energetic costs, and favoring a friendlier environmental methodology [21, 22]. The main characteristics of microwaveassisted heating rely on the accurate control of the temperature, which ensures homogeneous distribution of the heat, preventing temperature gradients within the oven and samples. Therefore, the heating process is more efficient, side reactions are avoided in great extent, and as a consequence, reaction times are reduced notably.

This section is focused on describing the hydrothermal synthesis of LTL zeolite by extrapolating the optimal conditions referred in previous reports for conventional ovens and applied them to microwave heating [19]. One of the main aims is to improve the quality of the crystals and decrease reaction times, which otherwise takes several days and implies higher energetic costs. In this regard, we have studied the effect of reaction conditions (heating rate, time, temperature, and static/dynamic conditions) on the size, morphology and chemical properties of the resulting crystals. Thus, LTL zeolite crystals with size ranging from nanometers (15 nm) to micrometers (3 μm), and shape varying from disc or coin to barrel have been synthesized by tuning the gel composition and the aforementioned reaction conditions.

The herein used hydrothermal synthetic procedure is described in **Figure 3**. The general protocol consists on mixing two aqueous suspensions, one containing the silica source and the other composed by the aluminum source, in a basic environment, leading to a milky gel phase. Afterward, the gel is heated (at reaction temperatures usually higher than 100°C) in a sealed high-pressure polytetrafluoroethylene (PTFE) vessel for a certain time period (optimal for each kind of zeolite). The size and morphology of the crystals can be modulated by changing the source of reactants, composition of the gel (alkalinity, water content, SiO2 /Al2 O3, among others) reaction time, temperature, and aging time of the gel [23]. To this aim and, starting from a fixed gel composition optimized for each kind of zeolite, we focused on modifying the reaction conditions and gel pre-treatment for a fine-tuning of the size and morphology of the crystals. For all types of zeolites the microwave-assisted heating reduced the synthesis times up to 90% (days to hours) compared to the one afforded by conventional ovens [19, 23]. Moreover, the precise control of the temperature and the homogeneous heating during the nucleation process in the microwave oven are reflected in a narrower size distribution of the particles and higher degree of crystallinity. Therefore, LTL zeolite crystals of different sizes and morphologies, and improved quality, can be attained in few hours, with the consequent time and energetic savings. The properties of these zeolites were further studied and classified to best fit different kind of applications.

The synthesis procedure is described in **Figure 3** (route a) and the oxide molar ratio in the gel

**Figure 3.** Scheme of the hydrothermal synthesis route for (a) nanosized, (b) disk-shaped, and (c) micrometric barrelshaped LTL crystals and its respective morphologies analyzed by (a) transmission electron microscopy (TEM) and (b)

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pressure for 1 h nanocrystals sizes were successfully obtained, where the size can be tuned from 15 to 50 nm by adjusting the synthesis conditions (gel ripening, static/dynamic conditions, or different silica sources). These nanosized LTL zeolites were subsequently used in energetic and photonic applications, such as catalytic supports for biogas reforming and fab-

Disk-shaped crystals, characterized for having a low aspect ratio (length/diameter), were also synthesized in which their thickness and diameters were modified in a controlled way. This morphology is desirable to ease their coupling with external devices (e.g., to produce uniform and well-organized monolayers or membranes [10]), and to afford high number of uni-dimensional pores with short diffusion path lengths (around 100,000 channels in a crystal

The synthetic procedure is presented in the route b of **Figure 3**, starting in all cases from a

ratio and heating at 160°C. The variation of factors controlling the growing of the crystal, such

O:1.00 Al2

O3

:30.00 SiO2

:416.08 H2

O oxides molar

O:5.50 Na2

:412.84 H2

O. After heating the gel at 170°C under

was fixed at 9.34 K<sup>2</sup>

O:1.00 Al2

and (c) scanning electron images (SEM) micrographs.

rication of artificial antenna systems, respectively.

**2.2. Disk-shaped LTL zeolite crystals**

with diameter of 600 nm [24]).

fixed gel composition of 5.40 K<sup>2</sup>

O3

:20.20 SiO2

#### **2.1. LTL zeolite nanocrystals**

The small size of these zeolites (15–50 nm) is suitable to allocate guests deep inside the pores thanks to the favored diffusion within the pores [23]. Furthermore, its large external surface makes them optimal supports for adsorption processes, e.g., adsorption of metal particles for catalytic processes [12].

Linde Type L Zeolite: A Privileged Porous Support to Develop Photoactive and Catalytic… http://dx.doi.org/10.5772/intechopen.73135 17

**Figure 3.** Scheme of the hydrothermal synthesis route for (a) nanosized, (b) disk-shaped, and (c) micrometric barrelshaped LTL crystals and its respective morphologies analyzed by (a) transmission electron microscopy (TEM) and (b) and (c) scanning electron images (SEM) micrographs.

The synthesis procedure is described in **Figure 3** (route a) and the oxide molar ratio in the gel was fixed at 9.34 K<sup>2</sup> O:1.00 Al2 O3 :20.20 SiO2 :412.84 H2 O. After heating the gel at 170°C under pressure for 1 h nanocrystals sizes were successfully obtained, where the size can be tuned from 15 to 50 nm by adjusting the synthesis conditions (gel ripening, static/dynamic conditions, or different silica sources). These nanosized LTL zeolites were subsequently used in energetic and photonic applications, such as catalytic supports for biogas reforming and fabrication of artificial antenna systems, respectively.

#### **2.2. Disk-shaped LTL zeolite crystals**

**2. LTL zeolite: microwave-assisted hydrothermal synthesis**

by tuning the gel composition and the aforementioned reaction conditions.

as a consequence, reaction times are reduced notably.

16 Zeolites and Their Applications

SiO2 /Al2

**2.1. LTL zeolite nanocrystals**

catalytic processes [12].

Microwave heating is an emerging technique in modern organic synthesis and in the production of nanoparticles and nanostructures. It usually affords an improvement in the yield and reproducibility of the synthetic processes, reducing the energetic costs, and favoring a friendlier environmental methodology [21, 22]. The main characteristics of microwaveassisted heating rely on the accurate control of the temperature, which ensures homogeneous distribution of the heat, preventing temperature gradients within the oven and samples. Therefore, the heating process is more efficient, side reactions are avoided in great extent, and

This section is focused on describing the hydrothermal synthesis of LTL zeolite by extrapolating the optimal conditions referred in previous reports for conventional ovens and applied them to microwave heating [19]. One of the main aims is to improve the quality of the crystals and decrease reaction times, which otherwise takes several days and implies higher energetic costs. In this regard, we have studied the effect of reaction conditions (heating rate, time, temperature, and static/dynamic conditions) on the size, morphology and chemical properties of the resulting crystals. Thus, LTL zeolite crystals with size ranging from nanometers (15 nm) to micrometers (3 μm), and shape varying from disc or coin to barrel have been synthesized

The herein used hydrothermal synthetic procedure is described in **Figure 3**. The general protocol consists on mixing two aqueous suspensions, one containing the silica source and the other composed by the aluminum source, in a basic environment, leading to a milky gel phase. Afterward, the gel is heated (at reaction temperatures usually higher than 100°C) in a sealed high-pressure polytetrafluoroethylene (PTFE) vessel for a certain time period (optimal for each kind of zeolite). The size and morphology of the crystals can be modulated by changing the source of reactants, composition of the gel (alkalinity, water content,

O3, among others) reaction time, temperature, and aging time of the gel [23]. To

this aim and, starting from a fixed gel composition optimized for each kind of zeolite, we focused on modifying the reaction conditions and gel pre-treatment for a fine-tuning of the size and morphology of the crystals. For all types of zeolites the microwave-assisted heating reduced the synthesis times up to 90% (days to hours) compared to the one afforded by conventional ovens [19, 23]. Moreover, the precise control of the temperature and the homogeneous heating during the nucleation process in the microwave oven are reflected in a narrower size distribution of the particles and higher degree of crystallinity. Therefore, LTL zeolite crystals of different sizes and morphologies, and improved quality, can be attained in few hours, with the consequent time and energetic savings. The properties of these zeolites were further studied and classified to best fit different kind of applications.

The small size of these zeolites (15–50 nm) is suitable to allocate guests deep inside the pores thanks to the favored diffusion within the pores [23]. Furthermore, its large external surface makes them optimal supports for adsorption processes, e.g., adsorption of metal particles for Disk-shaped crystals, characterized for having a low aspect ratio (length/diameter), were also synthesized in which their thickness and diameters were modified in a controlled way. This morphology is desirable to ease their coupling with external devices (e.g., to produce uniform and well-organized monolayers or membranes [10]), and to afford high number of uni-dimensional pores with short diffusion path lengths (around 100,000 channels in a crystal with diameter of 600 nm [24]).

The synthetic procedure is presented in the route b of **Figure 3**, starting in all cases from a fixed gel composition of 5.40 K<sup>2</sup> O:5.50 Na2 O:1.00 Al2 O3 :30.00 SiO2 :416.08 H2 O oxides molar ratio and heating at 160°C. The variation of factors controlling the growing of the crystal, such as the heating rate and the reaction time, allows the production of crystals with lengths ranging from 130 to 200 nm, and diameters spanning from 600 to 1200 nm.

#### **2.3. Micrometric LTL zeolite barrels**

The synthesis of large LTL zeolite crystals (from 1500 to 3000 nm length) with well-defined morphology and high aspect ratio (barrels-like microcrystals) has proven to be useful in the analysis of the distribution, orientation, and interaction of guest molecules by time- and space-resolved fluorescence measurements, such as fluorescence confocal microscopy at single-particle level [25].

The synthetic procedure is presented in the route c of **Figure 3**, starting in all cases from a fixed gel composition of 2.21 K<sup>2</sup> O: 1.00 Al2 O3 :9.00 SiO<sup>2</sup> :164.60 H2 O oxides molar ratio in the mixture and heating at 175°C.

To avoid undesirable process (mainly with organic dyes) owing to the high acidity of the channels, these synthesized zeolites were cation-exchanged with cesium or sodium.

optical coding, biosensing, catalysis, photosynthesis, logic devices, and theranostics, just to

**Figure 4.** Schematic representation of the designed antenna material by means of the sequential insertion of three different dyes into LTL zeolite channels. The molecular structures of the two tested antenna systems using carbostyril (C165) and an oxazole (Dmpopop) as the blue-emitting energy donors, BODIPYs (PM546 and PM567) for the greenyellow region, and oxazines (Ox4 and Ox1) as red emitting final acceptors are also enclosed. The corresponding fluorescence spectra (upon selective excitation at the blue donor, 350 nm) and image (by fluorescence microscope) for each dye combination are also included. In all cases, the dye amount was the 10% of the available adsorption sites and

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In our particular case, the strategy to develop competitive artificial antenna systems is based on the design of photoactive organic-inorganic hybrid materials. The protein matrix present in natural systems has been replaced by the LTL zeolitic host of nanometric dimensions, which apart from protecting the dyes, provides a constrained environment that induces a high degree of supramolecular organization. Furthermore, regarding the photoactive moiety, responsible for interacting with the light, the chlorophyll molecules have been replaced by small enough fluorescent molecules working in different regions of the electromagnetic spectrum (to ensure broadband absorption across the whole visible spectral region) and susceptible of promoting efficient EET processes. The energy transfer takes place via dipole-dipole coupling, better known as the through-space Förster mechanism (FRET), where one of the main requirements is the proper spectral overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. Therefore, by properly choosing the dyes to be encapsulated and controlling the spectral overlap of the combined dyes, the FRET efficiency can be finely tuned, and hence, the output emission light of the photoactive material will be modulated accordingly (**Figure 4**) [13, 33].

Here we present two antenna systems built by the sequential insertion of three different dyes into the pores of the LTL zeolite (**Figure 4**). As energy donors, oxazole (Dmpopop) and carbostyril (C165) dyes have been considered in the UV-blue region, whereas in the green-yellow part, BODIPYs (PM546 and P567) have been selected as first acceptors and succeeding energy donors. Finally, red-emitting oxazines (Ox1 and Ox4) where chosen as final energy acceptors. The UV-blue donors are placed in the center of the crystal, flanked by the green-yellow dyes, and with the red acceptors localized at the edges of the LTL zeolite crystal. In both antenna systems, the light is efficiently harvested over a broad spectral region and after selective excitation at the UV region, where only the first energy donors absorbs (C165 and Dmpopop,

highlight some of them [30–32].

the donor/acceptor ratio was 1:1:1.

### **3. Photoactive antenna materials**

#### **3.1. Dye-doped LTL zeolite**

This section describes the development of photoactive nanomaterials based on the supramolecular organization of luminescent molecules into the channels of LTL zeolite. Taking inspiration from nature we will focus on one of the most sophisticated and vital processes in the earth, the photosynthesis, to develop a new generation of fluorescent nanomaterials. Using the *modus operandi* of the natural antenna systems present in photosynthetic organisms as a reference point, the main goal focused on the design and construction of artificial antenna systems able to mimic the functions and mechanism of these natural entities.

Many scientists all over the world are active participants in the challenge of developing artificial photoactive nanomaterials able to imitate the perfection and high efficiency of the mechanisms present in nature [26–28]. As a significant example, the photosynthetic organism present in plants appear as the most sophisticated solar energy storage systems in nature due to their unique ability to harvest solar radiation and transform it into chemical energy. The antenna systems, composed by few hundred of chlorophyll molecules are embedded in a protein environment (which keeps the photoactive moieties well arranged), are the responsible for absorbing the light and transfer the excitation energy to a specific reaction center [29]. Therefore, one of the main requirements for the proper operation of an artificial antenna system is the ability to harvest and transport the light to an acceptor moiety with the desired energy (**Figure 4**). The excitation energy transfer (EET) is a key factor ruling the effectiveness of the process. A careful understanding of the parameters controlling this phenomenon is mandatory, such as spectral overlap, ratio of donors and acceptors, interchromophoric distances, and relative orientations. The precise control of these variables enables the development of high-quality artificial antenna systems with potential applications in photonics, Linde Type L Zeolite: A Privileged Porous Support to Develop Photoactive and Catalytic… http://dx.doi.org/10.5772/intechopen.73135 19

as the heating rate and the reaction time, allows the production of crystals with lengths rang-

The synthesis of large LTL zeolite crystals (from 1500 to 3000 nm length) with well-defined morphology and high aspect ratio (barrels-like microcrystals) has proven to be useful in the analysis of the distribution, orientation, and interaction of guest molecules by time- and space-resolved fluorescence measurements, such as fluorescence confocal microscopy at sin-

The synthetic procedure is presented in the route c of **Figure 3**, starting in all cases from a

To avoid undesirable process (mainly with organic dyes) owing to the high acidity of the

This section describes the development of photoactive nanomaterials based on the supramolecular organization of luminescent molecules into the channels of LTL zeolite. Taking inspiration from nature we will focus on one of the most sophisticated and vital processes in the earth, the photosynthesis, to develop a new generation of fluorescent nanomaterials. Using the *modus operandi* of the natural antenna systems present in photosynthetic organisms as a reference point, the main goal focused on the design and construction of artificial antenna

Many scientists all over the world are active participants in the challenge of developing artificial photoactive nanomaterials able to imitate the perfection and high efficiency of the mechanisms present in nature [26–28]. As a significant example, the photosynthetic organism present in plants appear as the most sophisticated solar energy storage systems in nature due to their unique ability to harvest solar radiation and transform it into chemical energy. The antenna systems, composed by few hundred of chlorophyll molecules are embedded in a protein environment (which keeps the photoactive moieties well arranged), are the responsible for absorbing the light and transfer the excitation energy to a specific reaction center [29]. Therefore, one of the main requirements for the proper operation of an artificial antenna system is the ability to harvest and transport the light to an acceptor moiety with the desired energy (**Figure 4**). The excitation energy transfer (EET) is a key factor ruling the effectiveness of the process. A careful understanding of the parameters controlling this phenomenon is mandatory, such as spectral overlap, ratio of donors and acceptors, interchromophoric distances, and relative orientations. The precise control of these variables enables the development of high-quality artificial antenna systems with potential applications in photonics,

:9.00 SiO<sup>2</sup>

:164.60 H2

O oxides molar ratio in the

O3

channels, these synthesized zeolites were cation-exchanged with cesium or sodium.

ing from 130 to 200 nm, and diameters spanning from 600 to 1200 nm.

O: 1.00 Al2

systems able to mimic the functions and mechanism of these natural entities.

**2.3. Micrometric LTL zeolite barrels**

gle-particle level [25].

18 Zeolites and Their Applications

fixed gel composition of 2.21 K<sup>2</sup>

**3. Photoactive antenna materials**

mixture and heating at 175°C.

**3.1. Dye-doped LTL zeolite**

**Figure 4.** Schematic representation of the designed antenna material by means of the sequential insertion of three different dyes into LTL zeolite channels. The molecular structures of the two tested antenna systems using carbostyril (C165) and an oxazole (Dmpopop) as the blue-emitting energy donors, BODIPYs (PM546 and PM567) for the greenyellow region, and oxazines (Ox4 and Ox1) as red emitting final acceptors are also enclosed. The corresponding fluorescence spectra (upon selective excitation at the blue donor, 350 nm) and image (by fluorescence microscope) for each dye combination are also included. In all cases, the dye amount was the 10% of the available adsorption sites and the donor/acceptor ratio was 1:1:1.

optical coding, biosensing, catalysis, photosynthesis, logic devices, and theranostics, just to highlight some of them [30–32].

In our particular case, the strategy to develop competitive artificial antenna systems is based on the design of photoactive organic-inorganic hybrid materials. The protein matrix present in natural systems has been replaced by the LTL zeolitic host of nanometric dimensions, which apart from protecting the dyes, provides a constrained environment that induces a high degree of supramolecular organization. Furthermore, regarding the photoactive moiety, responsible for interacting with the light, the chlorophyll molecules have been replaced by small enough fluorescent molecules working in different regions of the electromagnetic spectrum (to ensure broadband absorption across the whole visible spectral region) and susceptible of promoting efficient EET processes. The energy transfer takes place via dipole-dipole coupling, better known as the through-space Förster mechanism (FRET), where one of the main requirements is the proper spectral overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. Therefore, by properly choosing the dyes to be encapsulated and controlling the spectral overlap of the combined dyes, the FRET efficiency can be finely tuned, and hence, the output emission light of the photoactive material will be modulated accordingly (**Figure 4**) [13, 33].

Here we present two antenna systems built by the sequential insertion of three different dyes into the pores of the LTL zeolite (**Figure 4**). As energy donors, oxazole (Dmpopop) and carbostyril (C165) dyes have been considered in the UV-blue region, whereas in the green-yellow part, BODIPYs (PM546 and P567) have been selected as first acceptors and succeeding energy donors. Finally, red-emitting oxazines (Ox1 and Ox4) where chosen as final energy acceptors. The UV-blue donors are placed in the center of the crystal, flanked by the green-yellow dyes, and with the red acceptors localized at the edges of the LTL zeolite crystal. In both antenna systems, the light is efficiently harvested over a broad spectral region and after selective excitation at the UV region, where only the first energy donors absorbs (C165 and Dmpopop, respectively), its own emission is recorded but followed by that from the BODIPYs (PM546 and PM567) and the last energy acceptors (Ox4 and Ox1) (**Figure 4**).

However, there is a notorious difference in the energy output of each antenna system due to the different FRET process efficiency between the encapsulated dyes. The higher degree of spectral overlap in the antenna, composed by C165-PM546-Ox4 dyes, leaded to an efficient transformation of the harvested light into red emission (**Figure 4**). On the other hand, the less pronounced spectral overlap in the antenna, composed by Dmpopop-PM567-Ox1, decreased the FRET efficiency leading to the simultaneous detection of the emission from the three different chromophores with similar intensity. The sum of all these emissions covering the whole visible spectrum leads to white-light emission in which the energy output can be finely modulated over a broad region just using suitable filters (**Figure 4**). Therefore, by controlling the efficiency of the FRET hops, via the optimization of the magnitude of spectral overlap between the luminescent molecules inside de zeolitic channels, a fine modulation of the outcoming light energy can be reached, and hence either red-light or white-light emitting photoactive materials can be achieved.

#### **3.2. External functionalization of LTL zeolite**

In order to improve the degree of supramolecular organization, we tested a strategy based on the external functionalization of the channel entrances of LTL zeolite with tailor-made molecules (stopcock) via covalent linkage of such fluorophore (**Figure 5**) [13, 14]. These stopcock molecules grafted at the pores plug the channels to avoid the leakage of the guest molecules adsorbed inside and connect the inner space of the zeolite with the outside thanks to FRET processes, making the coupling of the material with other external devices straightforward.

**4. LTL zeolite for hydrogen production through methane reforming** 

Catalyst supports play a key role in the process as it provides properties such as thermal stability, surface area, acidity, or the capacity of maintaining the metal dispersion during the reaction process. Those properties can be key factor for a catalyst to stand out from the existing ones [35]. For methane reforming processes, alumina supports are commonly used as they fit the mentioned properties [35–38]. Nevertheless, those properties are also present on zeolites. Accordingly, some zeolites were tested as support for methane reforming processes, in which, the influence of the nature of the zeolite support on the catalyst performance was analyzed [39, 40]. LTL zeolite has been used as catalyst support for methane reforming for hydrogen production, because hydrogen is considered the fuel of the future. For that purpose, LTL zeolite featuring different shapes (disc (DL), cylinders (CL), and nanocrystals (NL)), sizes (for the CL zeolites, between 1 and 3 μm and 30–60 nm) and alkaline metal (for the DL zeolites, with Cs, Na, or without them) exchange were used as catalyst support. Those supports were impregnated with nickel (13 wt.%) or co-impregnated with nickel and rhodium (13 and 1 wt.%, respectively) in order to prepare the corresponding monometallic or bimetallic catalysts, fol-

**Figure 5.** Schematic view of LTL zeolite channel doped with Dmpopop as energy donor (center), and the BODIPYstopcock attached to the channel entrances as energy acceptor. The molecular structure of the stopcock and a scheme representing its three main components are shown in different colors. The corresponding normalized excitation (emission monitored at 600 nm, dashed line) and emission (excited at 350 nm, solid line) spectra, as well as the fluorescence

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Nickel is a metal widely used for reforming processes because of its availability and relatively low price. On the other hand, although noble metals are more expensive than non-noble metals, it has been reported that the addition of small amounts of a noble metal to a non-noble metal catalyst increases the activity and dispersion of the non-noble metal due to the spill-over effect [41]. Therefore, a low amount of rhodium was incorporated to prepare bimetallic catalysts.

The content of the metal incorporated into the zeolite supports was determined by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) model 2000 DV (Perkin Elmer)

**processes**

microscopy image are also depicted.

lowing the wet impregnation procedure [12, 23].

**4.1. Chemical composition**

The rational design of the stopcock molecule appears as key factor to ensure the suitable performance of it. On one hand, it needs to be small enough to diffuse into the pores and have the required functionalization to be covalently grafted at the channel entrances by reaction with free silanol groups (**Figure 5**). On the other hand, the emission should be placed at the rededge of the visible spectral region to act as proper acceptor of the light harvested into the zeolitic channels. In this regard, we have designed a molecule made up of a label (triethoxysilane), a spacer and a head moiety (the fluorophore itself), taking BODIPY dyes as scaffold, owing to its excellent photophysical signatures and chemical versatility of its dipyrrin core subject to a plethora of organic reactions [34], and modified its backbone accordingly to fulfill the above mentioned perquisites. The next step consisted on the construction of the antenna system by its combination with a suitable energy donor; in this case, the above tested oxazole (Dmpopop), since it shows a broad absorption band in the UV-blue region and its emission overlaps well with the absorption band of the BODIPY-based stopcock. Therefore, the Dmpopop donor has been first inserted in the channels and right after the entrances were plugged with the silylated BODIPY. The selective excitation of the donor leads to a predominant red fluorescence from the stopcock centered at 610 nm (**Figure 5**), proving the ongoing FRET process from the inside to the outside of the crystal along the channel direction. Thus, the stopcock molecules enable the communication between the guest molecules encapsulated inside the pores with external materials or molecules outside. Besides, this closure molecule prevents small molecules like water or oxygen (fluorescence quenchers) from diffusing into the inner space of the crystals.

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**Figure 5.** Schematic view of LTL zeolite channel doped with Dmpopop as energy donor (center), and the BODIPYstopcock attached to the channel entrances as energy acceptor. The molecular structure of the stopcock and a scheme representing its three main components are shown in different colors. The corresponding normalized excitation (emission monitored at 600 nm, dashed line) and emission (excited at 350 nm, solid line) spectra, as well as the fluorescence microscopy image are also depicted.

### **4. LTL zeolite for hydrogen production through methane reforming processes**

Catalyst supports play a key role in the process as it provides properties such as thermal stability, surface area, acidity, or the capacity of maintaining the metal dispersion during the reaction process. Those properties can be key factor for a catalyst to stand out from the existing ones [35]. For methane reforming processes, alumina supports are commonly used as they fit the mentioned properties [35–38]. Nevertheless, those properties are also present on zeolites. Accordingly, some zeolites were tested as support for methane reforming processes, in which, the influence of the nature of the zeolite support on the catalyst performance was analyzed [39, 40].

LTL zeolite has been used as catalyst support for methane reforming for hydrogen production, because hydrogen is considered the fuel of the future. For that purpose, LTL zeolite featuring different shapes (disc (DL), cylinders (CL), and nanocrystals (NL)), sizes (for the CL zeolites, between 1 and 3 μm and 30–60 nm) and alkaline metal (for the DL zeolites, with Cs, Na, or without them) exchange were used as catalyst support. Those supports were impregnated with nickel (13 wt.%) or co-impregnated with nickel and rhodium (13 and 1 wt.%, respectively) in order to prepare the corresponding monometallic or bimetallic catalysts, following the wet impregnation procedure [12, 23].

Nickel is a metal widely used for reforming processes because of its availability and relatively low price. On the other hand, although noble metals are more expensive than non-noble metals, it has been reported that the addition of small amounts of a noble metal to a non-noble metal catalyst increases the activity and dispersion of the non-noble metal due to the spill-over effect [41]. Therefore, a low amount of rhodium was incorporated to prepare bimetallic catalysts.

#### **4.1. Chemical composition**

respectively), its own emission is recorded but followed by that from the BODIPYs (PM546

However, there is a notorious difference in the energy output of each antenna system due to the different FRET process efficiency between the encapsulated dyes. The higher degree of spectral overlap in the antenna, composed by C165-PM546-Ox4 dyes, leaded to an efficient transformation of the harvested light into red emission (**Figure 4**). On the other hand, the less pronounced spectral overlap in the antenna, composed by Dmpopop-PM567-Ox1, decreased the FRET efficiency leading to the simultaneous detection of the emission from the three different chromophores with similar intensity. The sum of all these emissions covering the whole visible spectrum leads to white-light emission in which the energy output can be finely modulated over a broad region just using suitable filters (**Figure 4**). Therefore, by controlling the efficiency of the FRET hops, via the optimization of the magnitude of spectral overlap between the luminescent molecules inside de zeolitic channels, a fine modulation of the outcoming light energy can be reached,

and hence either red-light or white-light emitting photoactive materials can be achieved.

In order to improve the degree of supramolecular organization, we tested a strategy based on the external functionalization of the channel entrances of LTL zeolite with tailor-made molecules (stopcock) via covalent linkage of such fluorophore (**Figure 5**) [13, 14]. These stopcock molecules grafted at the pores plug the channels to avoid the leakage of the guest molecules adsorbed inside and connect the inner space of the zeolite with the outside thanks to FRET processes, making the coupling of the material with other external devices straightforward.

The rational design of the stopcock molecule appears as key factor to ensure the suitable performance of it. On one hand, it needs to be small enough to diffuse into the pores and have the required functionalization to be covalently grafted at the channel entrances by reaction with free silanol groups (**Figure 5**). On the other hand, the emission should be placed at the rededge of the visible spectral region to act as proper acceptor of the light harvested into the zeolitic channels. In this regard, we have designed a molecule made up of a label (triethoxysilane), a spacer and a head moiety (the fluorophore itself), taking BODIPY dyes as scaffold, owing to its excellent photophysical signatures and chemical versatility of its dipyrrin core subject to a plethora of organic reactions [34], and modified its backbone accordingly to fulfill the above mentioned perquisites. The next step consisted on the construction of the antenna system by its combination with a suitable energy donor; in this case, the above tested oxazole (Dmpopop), since it shows a broad absorption band in the UV-blue region and its emission overlaps well with the absorption band of the BODIPY-based stopcock. Therefore, the Dmpopop donor has been first inserted in the channels and right after the entrances were plugged with the silylated BODIPY. The selective excitation of the donor leads to a predominant red fluorescence from the stopcock centered at 610 nm (**Figure 5**), proving the ongoing FRET process from the inside to the outside of the crystal along the channel direction. Thus, the stopcock molecules enable the communication between the guest molecules encapsulated inside the pores with external materials or molecules outside. Besides, this closure molecule prevents small molecules like water or oxygen (fluorescence quenchers) from diffusing into the inner space of the crystals.

and PM567) and the last energy acceptors (Ox4 and Ox1) (**Figure 4**).

**3.2. External functionalization of LTL zeolite**

20 Zeolites and Their Applications

The content of the metal incorporated into the zeolite supports was determined by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) model 2000 DV (Perkin Elmer) instrument. The composition, summarized in **Table 1**, indicates that the measured values were close to the nominal ones.

of the low size pores. Nevertheless, both CL (30–60 nm) supported catalysts presented the

It is worth mentioning that in the cases of the metal impregnation on DLCs and DLNa catalysts, the measured BET area increased (**Table 3**). That happened due to the creation of new

Catalysts reducibility was evaluated by temperature programmed reduction (TPR) using an Autosorb-1C-TCD device. The obtained reduction profiles are summarized in **Figure 6**. The profiles in the left side of **Figure 6** show that for bimetallic catalysts the reduction processes start at lower temperatures than their corresponding monometallic catalysts, probably due to

**Table 3.** Textural properties, specific surface area (SSA), average pore volume (PV) and average pore diameter (PD), of

**/g) PV (cm3**

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**/g) PD (Å) SSA (m2**

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**/g)**

23

area with the metallic particles, as the original LTL zeolite surface area was low.

**/g) PD (Å) SSA (m2**

DL 59 0.07 30 Rh-Ni/DL 38 0.13 72 DLCs 13 0.08 122 Rh-Ni/DLCs 17 0.13 155 DLNa 1 <0.01 264 Rh-Ni/DLNa 3 <0.01 251 NL 210 0.79 79 Rh-Ni/NL 68 0.19 59 NLCs 96 0.65 136 Rh-Ni/NLCs 37 0.15 85 NLNa 144 0.74 106 Rh-Ni/NLNa 43 0.17 77

**4.3. Temperature programmed reduction (TPR)**

**/g) PV (cm3**

LTL zeolite and their corresponding bimetallic catalysts [12].

**Figure 6.** TPR profiles for zeolite L supported catalysts, modified from [12, 23].

highest BET areas.

the spill-over effect [12].

**Support SSA (m2**

#### **4.2. Textural properties**

The textural properties of the LTL zeolite supported catalysts were determined by means of N2 adsorption-desorption isotherms obtained at 77 K using an Autosorb-1C-TCD. The measured textural property values were not only affected by metal incorporation, but also by the calcination carried out on the catalyst preparation procedure.

The calcinations at 1073 K for 4 h before the metal impregnation produced an important decrease in the textural properties of the LTL zeolite, as shown in **Table 2**. As it could be expected, those values were further reduced when the metal impregnation and catalysts calcinations were carried out, as shown in **Tables 2** and **3**.

Due to the low amount of noble metal impregnated on the zeolites (1 wt.%), differences on the textural properties for monometallic and bimetallic catalysts (**Table 2**) were only observed for CL (30–60 nm) supported catalysts. The bimetallic catalyst presented a lower SSA value, in accordance to its higher pore volume and much higher PD, probably due to a higher coverage


**Table 1.** Chemical compositions for catalysts prepared with LTL zeolite expressed as wt.% [12, 23].


**Table 2.** Textural properties, specific surface area (SSA), average pore volume (PV) and average pore diameter (PD), of fresh (F) and calcined (C) LTL zeolite and their corresponding monometallic and bimetallic catalysts [12].

of the low size pores. Nevertheless, both CL (30–60 nm) supported catalysts presented the highest BET areas.

It is worth mentioning that in the cases of the metal impregnation on DLCs and DLNa catalysts, the measured BET area increased (**Table 3**). That happened due to the creation of new area with the metallic particles, as the original LTL zeolite surface area was low.

#### **4.3. Temperature programmed reduction (TPR)**

instrument. The composition, summarized in **Table 1**, indicates that the measured values

The textural properties of the LTL zeolite supported catalysts were determined by means of

The calcinations at 1073 K for 4 h before the metal impregnation produced an important decrease in the textural properties of the LTL zeolite, as shown in **Table 2**. As it could be expected, those values were further reduced when the metal impregnation and catalysts cal-

Due to the low amount of noble metal impregnated on the zeolites (1 wt.%), differences on the textural properties for monometallic and bimetallic catalysts (**Table 2**) were only observed for CL (30–60 nm) supported catalysts. The bimetallic catalyst presented a lower SSA value, in accordance to its higher pore volume and much higher PD, probably due to a higher coverage

**Support Rh Ni Catalyst Rh Ni** Ni/DL — 11.1 Rh-Ni/DL 0.9 12.9 Rh-Ni/DL 0.9 12.4 Rh-Ni/DLCs 0.8 12.4 Ni/CL (1–3 μm) — 12.7 Rh-Ni/DLNa 0.8 11.6 Rh-Ni/CL (1–3 μm) 1.0 14.0 Rh-Ni/NL 0.9 13.8 Ni/CL (30–60 nm) — 11.9 Rh-Ni/NLCs 0.8 13.3 Rh-Ni/CL (30–60 nm) 1.1 13.2 Rh-Ni/NLNa 0.9 11.6

calcination carried out on the catalyst preparation procedure.

cinations were carried out, as shown in **Tables 2** and **3**.

**/g) PV (cm3**

**/g) PD (Å)**

**Table 1.** Chemical compositions for catalysts prepared with LTL zeolite expressed as wt.% [12, 23].

DL (F) 258 0.04 14 Ni/DL 40 0.10 54 DL (C) 134 0.06 17 Rh-Ni/DL 42 0.12 63 CL (1–3 μm) (F) 260 0.05 13 Ni/CL (1–3 μm) 23 0.05 55 CL (1–3 μm) (C) 152 0.04 13 Rh-Ni/CL (1–3 μm) 26 0.07 58 CL (30–60 nm) (F) 419 0.90 54 Ni/CL (30–60 nm) 95 0.45 96 CL (30–60 nm) (C) 335 0.93 67 Rh-Ni/CL (30–60 nm) 64 0.63 198

**Table 2.** Textural properties, specific surface area (SSA), average pore volume (PV) and average pore diameter (PD), of

fresh (F) and calcined (C) LTL zeolite and their corresponding monometallic and bimetallic catalysts [12].

**Catalyst SSA (m2**

**/g) PV (cm3**

**/g) PD (Å)**

 adsorption-desorption isotherms obtained at 77 K using an Autosorb-1C-TCD. The measured textural property values were not only affected by metal incorporation, but also by the

were close to the nominal ones.

**4.2. Textural properties**

22 Zeolites and Their Applications

**Support SSA (m2**

N2

Catalysts reducibility was evaluated by temperature programmed reduction (TPR) using an Autosorb-1C-TCD device. The obtained reduction profiles are summarized in **Figure 6**. The profiles in the left side of **Figure 6** show that for bimetallic catalysts the reduction processes start at lower temperatures than their corresponding monometallic catalysts, probably due to the spill-over effect [12].


**Table 3.** Textural properties, specific surface area (SSA), average pore volume (PV) and average pore diameter (PD), of LTL zeolite and their corresponding bimetallic catalysts [12].

**Figure 6.** TPR profiles for zeolite L supported catalysts, modified from [12, 23].

Both, CL (1–3 μm) supported catalysts and Rh-Ni/DL catalyst, presented the main reduction peak around 700 K due to the reduction of NiO with low interaction with the support [42]. Ni/CL (1–3 μm) catalyst presented other two small peaks at 820 and 1000 K originated by the reduction of NiO particles with high interaction with support and Ni species located on the hexagonal prism, respectively [43]. Bimetallic CL (1–3 μm) and DL supported catalysts did not show those two reduction peaks, probably because they produced a more intense peak attributed to the NiO with low interaction with the surface. However, they presented a low peak and a shoulder at temperatures around 820 K, which could also be produced by the reduction NiO with high interaction with the surface. Similar profiles were produced for Rh-Ni/DL with and without alkali metal exchanged.

Ni/DL, Rh-Ni/CL (30–60 nm), and NL supported catalysts also presented similar profiles. On them, the hydrogen consumption peaks that took place around 700–750 K were produced by the reduction of NiO species [42, 44]. The reductions that took place at higher temperatures (~800 K) could be assigned to NiO particles with stronger interaction with support [43].

Among the studied catalysts, Ni/CL (30–60 nm) catalyst produced its main reduction peaks at the highest temperature (1150 K) because of the reduction of non-stoichiometric and stoichiometric Ni-spinels [45].

The application of Scherrer equation on the most intense nickel diffraction peaks (2θ = 43 and 2θ = 44 for nickel oxide and metallic nickel, respectively) showed that, in general, bimetallic catalysts presented smaller NiO crystallite sizes (~30 nm) than their homologous monometallic catalysts (~60 nm), meaning that the active metal could be better dispersed in the bimetallic samples. The DL supported catalysts were the exception, as both monometallic and bimetallic

XRD analyses did not provide any diffraction peak attributed to rhodium (neither metallic nor oxide). Thus, rhodium particles smaller than the detection limit of the equipment (<4 nm) could be present on the catalysts, as the presence of rhodium on the catalysts was assured by

For the LTL zeolite with and without alkali metal modification (Cs or Na) the estimated crystallite sizes were from 10 to 25 nm. In this case, Rh-Ni/DLNa catalyst was the exception and presented nickel crystals around 100 nm, as it can be observed in the TEM images (**Figure 8**) acquired on a Philips CM 200 transmission electron microscope at an acceleration voltage of

The catalysts prepared with DL and NL with and without alkali modification were also reduced and analyzed by X-ray photoelectron spectroscopy (XPS) in order to compare surface (XPS) and bulk (ICP-AES) nickel and rhodium abundances. Results are summarized in **Table 4**. For that purpose, a VG Escalab 200R spectrometer equipped with a hemispherical electron

Both metals, Ni and Rh, were preferentially located on the external surface of the catalysts as indicated by the Ni/zeolite and Rh/zeolite ratios which were higher for the XPS than for

area and the consequent higher nickel agglomeration on the surface of the catalyst.

analyzer and an Al Kα1 (hν = 1486.6 eV) 120 W X-ray source was used.

filament. The high crystallite size could be related with the low surface

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catalyst containing NiO crystallites around 30 nm.

**Figure 7.** XRD patterns of zeolite L supported catalysts, modified from [12, 23].

**4.5. X-ray photoelectron spectroscopy (XPS)**

ICP-AES.

200 kV with a LaB<sup>6</sup>

Rhodium reduction peaks were only detected for Rh-Ni/CL (1–3 μm) and Rh-Ni/DLNa catalysts at temperatures around 450 K [46]. Therefore, rhodium is probably stronger interacting with support in the other bimetallic catalysts, and thus, its reduction peak could be covered by more intense H2 consumptions produced by Ni species.

### **4.4. X-ray diffraction (XRD)**

Before activity tests, catalysts were also analyzed by X-ray diffraction (XRD) using a Philips X'PERT PRO automatic diffractometer operating at 40 kV and 40 mA, in theta-theta configuration, secondary monochromator with Cu Kα radiation (λ = 1.5418 Å) and a PIXcel solid state detector. Diffraction patterns are shown in **Figure 7**. In the left side of the figure, the XRD patterns of fresh calcined monometallic and bimetallic catalysts are shown, while on the right side the patterns of other reduced zeolite L with and without alkali modification supported catalysts are collected.

All the patterns of the LTL zeolite supported catalysts showed characteristic peaks of the LTL zeolite at 2 theta (θ) degrees from 10 to 42. Nevertheless, the intensity of the peaks is lower for the reduced catalysts. The loss of the intensity could be caused by the loss of the crystallinity happened during the reduction treatment. Accordingly, for some of the reduced catalysts, there is only a broad peak in the range in which LTL zeolite peaks should be placed, i.e., Rh-Ni/DLNa, Rh-Ni/NLCs, or Rh-Ni/NLNa catalysts.

Apart from the diffraction peaks attributed to the LTL zeolite structure, fresh calcined catalysts produced diffraction peaks attributed to NiO (Powder Diffraction File (PDF): 01-044- 1159) at 43 and 63 2 theta degrees.

On the other hand, the XRD analyses for reduced LTL zeolite supported catalysts presented metallic nickel peaks at around 44 and 53 2 theta degrees (PDF: 01-087-0712). For these catalysts, peaks corresponding to nickel oxides were not observed.

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**Figure 7.** XRD patterns of zeolite L supported catalysts, modified from [12, 23].

Both, CL (1–3 μm) supported catalysts and Rh-Ni/DL catalyst, presented the main reduction peak around 700 K due to the reduction of NiO with low interaction with the support [42]. Ni/CL (1–3 μm) catalyst presented other two small peaks at 820 and 1000 K originated by the reduction of NiO particles with high interaction with support and Ni species located on the hexagonal prism, respectively [43]. Bimetallic CL (1–3 μm) and DL supported catalysts did not show those two reduction peaks, probably because they produced a more intense peak attributed to the NiO with low interaction with the surface. However, they presented a low peak and a shoulder at temperatures around 820 K, which could also be produced by the reduction NiO with high interaction with the surface. Similar profiles were produced for

Ni/DL, Rh-Ni/CL (30–60 nm), and NL supported catalysts also presented similar profiles. On them, the hydrogen consumption peaks that took place around 700–750 K were produced by the reduction of NiO species [42, 44]. The reductions that took place at higher temperatures (~800 K) could be assigned to NiO particles with stronger interaction with support [43].

Among the studied catalysts, Ni/CL (30–60 nm) catalyst produced its main reduction peaks at the highest temperature (1150 K) because of the reduction of non-stoichiometric and stoi-

Rhodium reduction peaks were only detected for Rh-Ni/CL (1–3 μm) and Rh-Ni/DLNa catalysts at temperatures around 450 K [46]. Therefore, rhodium is probably stronger interacting with support in the other bimetallic catalysts, and thus, its reduction peak could be covered

Before activity tests, catalysts were also analyzed by X-ray diffraction (XRD) using a Philips X'PERT PRO automatic diffractometer operating at 40 kV and 40 mA, in theta-theta configuration, secondary monochromator with Cu Kα radiation (λ = 1.5418 Å) and a PIXcel solid state detector. Diffraction patterns are shown in **Figure 7**. In the left side of the figure, the XRD patterns of fresh calcined monometallic and bimetallic catalysts are shown, while on the right side the patterns of other reduced zeolite L with and without alkali modification supported catalysts are collected.

All the patterns of the LTL zeolite supported catalysts showed characteristic peaks of the LTL zeolite at 2 theta (θ) degrees from 10 to 42. Nevertheless, the intensity of the peaks is lower for the reduced catalysts. The loss of the intensity could be caused by the loss of the crystallinity happened during the reduction treatment. Accordingly, for some of the reduced catalysts, there is only a broad peak in the range in which LTL zeolite peaks should be placed, i.e.,

Apart from the diffraction peaks attributed to the LTL zeolite structure, fresh calcined catalysts produced diffraction peaks attributed to NiO (Powder Diffraction File (PDF): 01-044-

On the other hand, the XRD analyses for reduced LTL zeolite supported catalysts presented metallic nickel peaks at around 44 and 53 2 theta degrees (PDF: 01-087-0712). For these cata-

consumptions produced by Ni species.

Rh-Ni/DL with and without alkali metal exchanged.

Rh-Ni/DLNa, Rh-Ni/NLCs, or Rh-Ni/NLNa catalysts.

lysts, peaks corresponding to nickel oxides were not observed.

1159) at 43 and 63 2 theta degrees.

chiometric Ni-spinels [45].

24 Zeolites and Their Applications

**4.4. X-ray diffraction (XRD)**

by more intense H2

The application of Scherrer equation on the most intense nickel diffraction peaks (2θ = 43 and 2θ = 44 for nickel oxide and metallic nickel, respectively) showed that, in general, bimetallic catalysts presented smaller NiO crystallite sizes (~30 nm) than their homologous monometallic catalysts (~60 nm), meaning that the active metal could be better dispersed in the bimetallic samples. The DL supported catalysts were the exception, as both monometallic and bimetallic catalyst containing NiO crystallites around 30 nm.

XRD analyses did not provide any diffraction peak attributed to rhodium (neither metallic nor oxide). Thus, rhodium particles smaller than the detection limit of the equipment (<4 nm) could be present on the catalysts, as the presence of rhodium on the catalysts was assured by ICP-AES.

For the LTL zeolite with and without alkali metal modification (Cs or Na) the estimated crystallite sizes were from 10 to 25 nm. In this case, Rh-Ni/DLNa catalyst was the exception and presented nickel crystals around 100 nm, as it can be observed in the TEM images (**Figure 8**) acquired on a Philips CM 200 transmission electron microscope at an acceleration voltage of 200 kV with a LaB<sup>6</sup> filament. The high crystallite size could be related with the low surface area and the consequent higher nickel agglomeration on the surface of the catalyst.

#### **4.5. X-ray photoelectron spectroscopy (XPS)**

The catalysts prepared with DL and NL with and without alkali modification were also reduced and analyzed by X-ray photoelectron spectroscopy (XPS) in order to compare surface (XPS) and bulk (ICP-AES) nickel and rhodium abundances. Results are summarized in **Table 4**. For that purpose, a VG Escalab 200R spectrometer equipped with a hemispherical electron analyzer and an Al Kα1 (hν = 1486.6 eV) 120 W X-ray source was used.

Both metals, Ni and Rh, were preferentially located on the external surface of the catalysts as indicated by the Ni/zeolite and Rh/zeolite ratios which were higher for the XPS than for


**4.6. Catalytic activity**

defined as:

where Vi

respectively.

in and Vi

Dry reforming (DR) Biogas: CH<sup>4</sup>

Steam reforming (SR) Biogas + water at S/C = 1.0

Oxidative reforming (OR) Biogas + air at O/C = 0.25

The catalytic activities of the monometallic and bimetallic catalysts supported on DL, CL (30–60 nm) and CL (1–3 μm) were evaluated for 90 min at 1073 K and atmospheric pressure

A ¼ inch 316 L stainless steel fixed bed reactor was used for the experiments. The catalytic bed was composed of 0.34 g of catalyst (0.42 < particle diameter (dP) < 0.50 mm) mixed with 1.53 g of inert CSi (0.50 < dP < 1.0 mm). The catalytic bed was placed in the middle of the length of the reactor and kept in place by filling the rest of the reactor with inert CSi (1.0 < dp < 3.0 mm). The reactor was placed in a Microactivity Reference bench-scale plant (PID Eng&Tech) to

trollers, while deionized water was injected using an HPLC-Gilson water pump. Catalysts

for 4 h. The composition of the product gases was determined by an online connected micro GC equipped with thermal conductivity detector (TCD). The measured parameters were

(H2 /CO)out molar ratio:(H2 /CO)out = (VH2 /VCO)out (4)

**Process Feed ratios WHSV (h−1)**

Biogas + water at S/C = 2.0

Biogas + air at O/C = 0.50

**Table 5.** Summary of the processes and feed ratios for the tests for zeolite L supported catalysts.

Tri-reforming (TR) Biogas + water + air at S/C = 1.0 and O/C = 0.25 161.5

were reduced at 1073 K before the activity tests using 350 N mL min−1 of a 3:1 N2

), N2

Linde Type L Zeolite: A Privileged Porous Support to Develop Photoactive and Catalytic…

in – VCH4

out/(2 × VCH4

out correspond to the inlet and outlet volumetric flow rate of reactant i (NmL/min),

/CO2 = 1.5 75.0

in – VCO2

out)/VCH4

in + VH2O

out)/VCO2

and O2

were fed by electronic con-

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:H2

in × 100 (1)

in × 100 (2)

in) × 100 (3)

104.8 134.6

131.8 188.6 mixture

27

under different biogas reforming conditions, summarized in **Table 5**.

perform the activity experiments. Biogas (CH<sup>4</sup> + CO2

Methane conversion:XCH4 (%) = (VCH4

Hydrogen yield:H2 yield (%) = VH2

Carbon dioxide conversion:XCO2 (%) = (VCO2

**Figure 8.** TEM images of the calcined catalysts. Scale size for DL and NL of 80 and 30 nm, respectively. Modified from [12].


**Table 4.** Comparison of the surface atomic ratios measured by XPS and the bulk atomic ratios measured by ICP.

the ICP-AES. The difference between the high XPS and lower ICP-AES ratios were more significant for DL with and without Cs and Na modification supported catalysts, because they presented the lowest SSA values. On the other hand, for DL and NL supported catalysts, a similar trend was observed; the lowest metallic content on the external surface was measured to the unmodified support, while highest values were measured for the Cs containing catalysts. Therefore, the presence of Cs and Na hinders the metal incorporation into the pores [23].

#### **4.6. Catalytic activity**

the ICP-AES. The difference between the high XPS and lower ICP-AES ratios were more significant for DL with and without Cs and Na modification supported catalysts, because they presented the lowest SSA values. On the other hand, for DL and NL supported catalysts, a similar trend was observed; the lowest metallic content on the external surface was measured to the unmodified support, while highest values were measured for the Cs containing catalysts. Therefore, the presence of Cs and Na hinders the metal incorporation

**Table 4.** Comparison of the surface atomic ratios measured by XPS and the bulk atomic ratios measured by ICP.

**Ni/zeolite L Rh/zeolite L**

**Figure 8.** TEM images of the calcined catalysts. Scale size for DL and NL of 80 and 30 nm, respectively. Modified from [12].

into the pores [23].

Modified from [23].

26 Zeolites and Their Applications

**Catalyst Atomic ratios XPS/ICP**

Rh-Ni/DL 3.10/0.22 0.89/0.01 Rh-Ni/DLCs 5.61/0.24 1.06/0.01 Rh-Ni/DLNa 5.17/0.25 −/0.01 Rh-Ni/NL 0.47/0.22 0.29/0.01 Rh-Ni/NLCs 2.02/0.26 0.44/0.01 Rh-Ni/NLNa 1.81/0.27 −/0.01

The catalytic activities of the monometallic and bimetallic catalysts supported on DL, CL (30–60 nm) and CL (1–3 μm) were evaluated for 90 min at 1073 K and atmospheric pressure under different biogas reforming conditions, summarized in **Table 5**.

A ¼ inch 316 L stainless steel fixed bed reactor was used for the experiments. The catalytic bed was composed of 0.34 g of catalyst (0.42 < particle diameter (dP) < 0.50 mm) mixed with 1.53 g of inert CSi (0.50 < dP < 1.0 mm). The catalytic bed was placed in the middle of the length of the reactor and kept in place by filling the rest of the reactor with inert CSi (1.0 < dp < 3.0 mm).

The reactor was placed in a Microactivity Reference bench-scale plant (PID Eng&Tech) to perform the activity experiments. Biogas (CH<sup>4</sup> + CO2 ), N2 and O2 were fed by electronic controllers, while deionized water was injected using an HPLC-Gilson water pump. Catalysts were reduced at 1073 K before the activity tests using 350 N mL min−1 of a 3:1 N2 :H2 mixture for 4 h. The composition of the product gases was determined by an online connected micro GC equipped with thermal conductivity detector (TCD). The measured parameters were defined as:

$$\text{Methanol conversion:}\\\text{X}\_{\text{CH4}}\text{(\%)}=\left(\text{V}\_{\text{CH4}}\text{\textdegree V}\_{\text{CH4}}\text{\textdegree}\right)\text{/V}\_{\text{CH4}}\text{\textdegree 100}\tag{1}$$

$$\text{Carbon dioxide conversion:}\\\text{X}\_{\text{CO2}}\text{(\%)}=\left(\text{V}\_{\text{CO2}}-\text{V}\_{\text{CO2}}\right)\text{(V}\_{\text{CO2}}\times 100\tag{2}$$

$$\text{Hydrogen yield:}\\\text{H}\_2\text{ yield (\%)} = \text{V}\_{\text{H2}}^{\text{out}} / \left(2 \times \text{V}\_{\text{CH4}}^{\text{in}} + \text{V}\_{\text{H2O}}^{\text{in}}\right) \times 100\tag{3}$$

$$\text{\{H}\_{2}\text{/CO\text{\textdegree}}\text{\textdegree molar ratio:}\text{\{H}\_{2}\text{/CO\textdegree}\text{\textdegree}\text{\textdegree}=\text{\{V}}\_{\text{H2}}\text{\textdegree V}\_{\text{CO}}\text{\textdegree}\text{\textdegree}\end{\text{\textdegree}}\tag{4}$$

where Vi in and Vi out correspond to the inlet and outlet volumetric flow rate of reactant i (NmL/min), respectively.


**Table 5.** Summary of the processes and feed ratios for the tests for zeolite L supported catalysts.

The experimental results, summarized in **Figures 9** and **10**, showed that even if in some cases there were no important differences in the hydrogen yield produced by monometallic or bimetallic catalysts, the highest hydrogen yields were generally achieved by bimetallic catalysts.

The addition of steam for SR produced an increase in the methane conversion, but a decrease

than in DR conditions for most of the catalysts. However, when the S/C ratio was increased from 1.0 to 2.0 (**Figure 9(c)**) the methane conversion was not increased, but reduced in most

were also lower when an S/C ratio of 2.0 was used. In both SR experiments, the CL (1–3 μm) supported catalysts achieved again the lowest hydrogen yields because of their low methane conversions. At the S/C ratio of 2.0, the Ni/CL (1–3 μm) catalyst produced a negative CO2 conversion, due to a higher selectivity to the water gas shift reaction. The rest of the catalysts achieved hydrogen yields, and methane and carbon dioxide conversion values, close to the

The setting of OR conditions produced some modifications on the activities. When an O/C ratio of 0.25 was used (**Figure 9(d)**), Ni/DL and Ni/CL (1–3 μm) catalysts were the less active

ing hydrogen. Surprisingly, the Rh-Ni/CL (1–3 μm) catalyst showed similar activities than Rh-Ni/DL, Ni/CL (30–60 nm) and Rh-Ni/CL (30–60 nm) catalysts, with a hydrogen yields,

Nevertheless, the feeding of a higher amount of oxygen (air) to the reactor (**Figure 9(e)**), demonstrated again the poor activity of the CL (1–3 μm) supported monometallic and bimetallic catalysts in comparison with the rest of the catalysts. Ni/DL catalyst almost produced as much hydrogen as the three most active catalysts. The O/C increase produced an increase of the methane conversion, but a significant decrease in both equilibrium and experimental CO<sup>2</sup>

could be originated by the lower amount of oxygen available to react with methane (assuming that all the oxygen fed reacts with methane), and therefore, a higher amount of methane is

Finally, when oxygen (air) and steam were fed together with biogas in order to carry out TR experiments (**Figure 9(f)**), results similar to the ones achieved during OR at O/C ratio of 0.25 were obtained. Once again, Ni/DL and Ni/CL (1–3 μm) catalysts were the less active. Surprisingly,

and CO2

Monometallic and bimetallic DL and CL (30–60 nm) supported catalysts achieved the highest methane conversions in most of the studied processes. For those catalysts, the rhodium incorporation improved the methane conversion by reaching values higher than 80%. The methane conversion measured for these two bimetallic catalysts was almost complete when they were tested under OR at O/C = 0.50 and TR conditions. Therefore, the bimetallic catalysts supported on DL and CL (30–60 nm) turned out to be the most appropriates ones for the studied reforming processes. The highest hydrogen yields for the most active catalysts were achieved

Due to the poor methane and carbon dioxide conversions reached by Ni and Rh-Ni/CL (1–3 μm) catalysts, their hydrogen production yield was lower than the rest of the catalysts.

) at S/C = 1.0 (**Figure 9(b)**) Thus, the hydrogen yields were higher

http://dx.doi.org/10.5772/intechopen.73135

29

Linde Type L Zeolite: A Privileged Porous Support to Develop Photoactive and Catalytic…

conversion was even more reduced. Accordingly, hydrogen yields

. Accordingly, they were less active than the rest of the catalysts produc-

conversion values measured in OR at O/C = 0.25 than 0.50

conversions, thus a methanation reaction

conversion.

in the CO2

conversion (X CO<sup>2</sup>

ones predicted by the equilibrium calculations.

methane and carbon dioxide conversion values above 80%.

available to react with carbon dioxide by means of the DR reaction.

was carried out [39]. Ni/CL (1–3 μm) catalyst also produced a negative CO2

of the cases, while the CO2

ones converting CH4

conversion values. The higher CO2

the Ni/DL catalyst produced negative CH4

under OR conditions at the O/C ratio of 0.25.

During DR experiments (**Figure 9(a)**), high conversions and hydrogen yields were achieved. In those conditions, CL (1–3 μm) supported catalysts were the less active ones. On the contrary, Ni/DL catalyst achieved a CH4 conversion (X CH<sup>4</sup> ) and consequently hydrogen yield, much higher than equilibrium values, probably caused by the presence of hot spots inside the reactor [12].

**Figure 9.** Activity results obtained for several biogas reforming processes carried out at 1073 K and atmospheric pressure at different feeding ratios: (a) DR, (b) SR at S/C = 1.0, (c) SR at S/C = 2.0, (d) OR at O/C = 0.25, (e) OR at O/C = 0.50, and (f) TR at S/C = 1.0 and O/C = 0.25. Figure modified from [12].

**Figure 10.** Activity results obtained for two biogas reforming processes at 1073 K and atmospheric pressure: (a) DR and (b) OR at O/C = 0.25. Figure modified from [23].

The addition of steam for SR produced an increase in the methane conversion, but a decrease in the CO2 conversion (X CO<sup>2</sup> ) at S/C = 1.0 (**Figure 9(b)**) Thus, the hydrogen yields were higher than in DR conditions for most of the catalysts. However, when the S/C ratio was increased from 1.0 to 2.0 (**Figure 9(c)**) the methane conversion was not increased, but reduced in most of the cases, while the CO2 conversion was even more reduced. Accordingly, hydrogen yields were also lower when an S/C ratio of 2.0 was used. In both SR experiments, the CL (1–3 μm) supported catalysts achieved again the lowest hydrogen yields because of their low methane conversions. At the S/C ratio of 2.0, the Ni/CL (1–3 μm) catalyst produced a negative CO2 conversion, due to a higher selectivity to the water gas shift reaction. The rest of the catalysts achieved hydrogen yields, and methane and carbon dioxide conversion values, close to the ones predicted by the equilibrium calculations.

The experimental results, summarized in **Figures 9** and **10**, showed that even if in some cases there were no important differences in the hydrogen yield produced by monometallic or bimetallic catalysts, the highest hydrogen yields were generally achieved by bimetallic catalysts.

During DR experiments (**Figure 9(a)**), high conversions and hydrogen yields were achieved. In those conditions, CL (1–3 μm) supported catalysts were the less active ones. On the con-

conversion (X CH<sup>4</sup>

much higher than equilibrium values, probably caused by the presence of hot spots inside the

**Figure 9.** Activity results obtained for several biogas reforming processes carried out at 1073 K and atmospheric pressure at different feeding ratios: (a) DR, (b) SR at S/C = 1.0, (c) SR at S/C = 2.0, (d) OR at O/C = 0.25, (e) OR at O/C = 0.50, and (f)

**Figure 10.** Activity results obtained for two biogas reforming processes at 1073 K and atmospheric pressure: (a) DR and

) and consequently hydrogen yield,

trary, Ni/DL catalyst achieved a CH4

TR at S/C = 1.0 and O/C = 0.25. Figure modified from [12].

(b) OR at O/C = 0.25. Figure modified from [23].

reactor [12].

28 Zeolites and Their Applications

The setting of OR conditions produced some modifications on the activities. When an O/C ratio of 0.25 was used (**Figure 9(d)**), Ni/DL and Ni/CL (1–3 μm) catalysts were the less active ones converting CH4 . Accordingly, they were less active than the rest of the catalysts producing hydrogen. Surprisingly, the Rh-Ni/CL (1–3 μm) catalyst showed similar activities than Rh-Ni/DL, Ni/CL (30–60 nm) and Rh-Ni/CL (30–60 nm) catalysts, with a hydrogen yields, methane and carbon dioxide conversion values above 80%.

Nevertheless, the feeding of a higher amount of oxygen (air) to the reactor (**Figure 9(e)**), demonstrated again the poor activity of the CL (1–3 μm) supported monometallic and bimetallic catalysts in comparison with the rest of the catalysts. Ni/DL catalyst almost produced as much hydrogen as the three most active catalysts. The O/C increase produced an increase of the methane conversion, but a significant decrease in both equilibrium and experimental CO<sup>2</sup> conversion values. The higher CO2 conversion values measured in OR at O/C = 0.25 than 0.50 could be originated by the lower amount of oxygen available to react with methane (assuming that all the oxygen fed reacts with methane), and therefore, a higher amount of methane is available to react with carbon dioxide by means of the DR reaction.

Finally, when oxygen (air) and steam were fed together with biogas in order to carry out TR experiments (**Figure 9(f)**), results similar to the ones achieved during OR at O/C ratio of 0.25 were obtained. Once again, Ni/DL and Ni/CL (1–3 μm) catalysts were the less active. Surprisingly, the Ni/DL catalyst produced negative CH4 and CO2 conversions, thus a methanation reaction was carried out [39]. Ni/CL (1–3 μm) catalyst also produced a negative CO2 conversion.

Monometallic and bimetallic DL and CL (30–60 nm) supported catalysts achieved the highest methane conversions in most of the studied processes. For those catalysts, the rhodium incorporation improved the methane conversion by reaching values higher than 80%. The methane conversion measured for these two bimetallic catalysts was almost complete when they were tested under OR at O/C = 0.50 and TR conditions. Therefore, the bimetallic catalysts supported on DL and CL (30–60 nm) turned out to be the most appropriates ones for the studied reforming processes. The highest hydrogen yields for the most active catalysts were achieved under OR conditions at the O/C ratio of 0.25.

Due to the poor methane and carbon dioxide conversions reached by Ni and Rh-Ni/CL (1–3 μm) catalysts, their hydrogen production yield was lower than the rest of the catalysts.

For all the experiments, the synthesis gas ratio (H2 /CO molar ratio), was also measured. For liquid hydrocarbon production through Fischer-Tropsch for methanol synthesis, a syngas with a ratio close to 2 is desired. On the other hand, a synthesis gas with a ratio of 1 is necessary for oxo or hydroformylation reaction. According to equilibrium calculations, the SR process at S/C = 2.0 is the only process able to produce a syngas ratio higher than 2. The SR at S/C = 1.0 and TR processes can also produce syngas ratios with values close to 2. Therefore, those processes would require an additional step (apart from the purification) for reaching the desired syngas ratio. On the contrary, for obtaining a syngas ratio close to 1, the equilibrium calculations indicate that both OR processes (at O/C = 0.25 and 0.50) and DR process are more suitable. These equilibrium predicted values were, in general, in good agreement with experimental values for most of the catalysts.

the activity of DL catalyst series. However, the modification of the zeolites with Cs or Na was unsuccessful for biogas reforming catalytic applications because the Rh-Ni/DL and Rh-Ni/NL

Linde Type L Zeolite: A Privileged Porous Support to Develop Photoactive and Catalytic…

http://dx.doi.org/10.5772/intechopen.73135

31

Summing up, LTL zeolite arises as a suitable and versatile support to allocate small-enough chromophores into their channels. Such cage effect protects and aligns the fluorophores, whereas the encapsulation enhances the energy transfer efficiency even at low dye loadings, being ideal for antenna or microlaser devices. Besides, its high specific surface allows the incorporation of metals boosting their catalytic efficiency toward the desired reaction, in this case biogas reforming, a long seeking clean energy source as an alternative to polluting fuels.

Financial support from MICINN (MAT2014-51937-C3-3-P), MINECO (OIL2H2. ENE2011- 23950), European Regional Development Fund (ERDF) and Gobierno Vasco (IT912-16 and IT993-16) is acknowledged. L. G.-R. and K. B. thanks Gobierno Vasco for a postdoctoral and predoctoral fellowship, respectively. Dr. Y. Xiao (from Dalian University in China) is grate-

\*, Kepa Bizkarra<sup>2</sup>

1 Department of Physical Chemistry, University of the Basque Country (UPV/EHU), Bilbao,

[1] Ogawa M, Kuroda K. Photofunctions of interaction compounds. Chemical Reviews.

[2] Cheetman AK, Férey G, Loiseau T. Open-framework inorganic materials. Angewandte Chemie, International Edition. 1999;**38**:3268-3292. DOI: 10.1002/(SICI)1521-3773(19991115)

[3] Ramamurthy V. Controlling photochemical reactions via confinement: Zeolites. Journal of Photochemistry and Photobiology C. 2000;**1**:145-166. DOI: 10.1016/S1389-5567(00)

[4] Tao Y, Kanoh H, Abrams L, Kaneko K. Mesopore-modifed zeolites: Preparation, characterization and applications. Chemical Reviews. 2006;**106**:896-910. DOI: 10.1021/cr040204o

2 Faculty of Engineering, University of the Basque Country (UPV/EHU), Bilbao, Spain

, Urko Izquierdo2

and Iñigo López Arbeloa1

,

fully thanked by the synthesis of the silylated-BODIPY stopcock.

, Jose Francisco Cambra2

, Jorge Bañuelos<sup>1</sup>

\*Address all correspondence to: jorge.banuelos@ehu.es

1995;**95**:399-438. DOI: 10.1021/cr00034a005

38:22<3268::AID-ANIE3268>3.0.CO;2-U

catalysts showed higher reforming capacity.

**Acknowledgements**

**Author details**

Leire Gartzia Rivero<sup>1</sup>

Victoria Laura Barrio<sup>2</sup>

Spain

**References**

00010-1

According to the above summarized results, we decided to prepare bimetallic catalysts to study the effect of alkali metals modification. The processes selected to test the bimetallic catalysts were DR and OR (**Figure 10**). On the one hand, the DR process presents the harsher reaction conditions and therefore, it is appropriate to distinguish the activity of the catalysts. On the other hand, the OR process is the most favorable in terms of hydrogen production.

Similar catalytic activity was observed for both studied processes. First, the NL, NLCs, NLNa, and DL supported bimetallic catalysts achieved the highest CH4 and CO2 conversions and H2 yield values, while Ni/DLCs catalyst produced intermediate results. Second, the less active catalyst was Rh-Ni/DLNa, which remained almost inactive in both biogas reforming processes. Therefore, there was a good agreement between the activity, the BET areas, the particle sizes estimated by XRD, and the ones observed by TEM, and the atomic Rh and Ni concentration. Thus, the most active catalysts presented the highest BET areas, the smaller metal particle sizes and the most equilibrated Rh and Ni surface concentrations.

### **5. Conclusions**

The herein reported step-by-step strategy to develop dye- and metal-doped LTL zeolites is suitable to attain materials which can be applied in photonics and eco-friendly catalysis. To this aim, we started from the beginning, by the microwave-assisted synthesis of the LTL zeolite crystals. This kind of heat allows a fine control of the size (from tens of nm to few μm) and shape (from barrels to coins) of the zeolite, which can be tuned just fixing the hydrothermal synthesis conditions. Afterwards, the kind of guest adsorbed into the zeolitic support will rule the application field of the material. On one hand, the allocation of rationally selected fluorophores into the uni-dimensional zeolitic channels, or grafted to the pore entrances, provide a hierarchically ordered material able to harvest light over a broad spectral interval and provide predominant red fluorescence, or alternatively white-light emission, just adjusting the energy transfer efficiency. On the other hand, the deposition of suitable metals onto the zeolite guest boosts the catalysis efficiency in the biogas reforming. Among the different LTL zeolite morphologies used, DL and CL (30–60 nm) supports were the most active ones for the studied reforming processes, especially when Rh and Ni metals were incorporated in order to prepare bimetallic catalysts. High activities are attributed to the high metal dispersion and the strong interactions among NiO and the different supports. The metal exchange of Cs and Na carried out on DL and NL supports mainly affected the morphology, and consequently, the activity of DL catalyst series. However, the modification of the zeolites with Cs or Na was unsuccessful for biogas reforming catalytic applications because the Rh-Ni/DL and Rh-Ni/NL catalysts showed higher reforming capacity.

Summing up, LTL zeolite arises as a suitable and versatile support to allocate small-enough chromophores into their channels. Such cage effect protects and aligns the fluorophores, whereas the encapsulation enhances the energy transfer efficiency even at low dye loadings, being ideal for antenna or microlaser devices. Besides, its high specific surface allows the incorporation of metals boosting their catalytic efficiency toward the desired reaction, in this case biogas reforming, a long seeking clean energy source as an alternative to polluting fuels.

### **Acknowledgements**

For all the experiments, the synthesis gas ratio (H2

30 Zeolites and Their Applications

hydrocarbon production through Fischer-Tropsch for methanol synthesis, a syngas with a ratio close to 2 is desired. On the other hand, a synthesis gas with a ratio of 1 is necessary for oxo or hydroformylation reaction. According to equilibrium calculations, the SR process at S/C = 2.0 is the only process able to produce a syngas ratio higher than 2. The SR at S/C = 1.0 and TR processes can also produce syngas ratios with values close to 2. Therefore, those processes would require an additional step (apart from the purification) for reaching the desired syngas ratio. On the contrary, for obtaining a syngas ratio close to 1, the equilibrium calculations indicate that both OR processes (at O/C = 0.25 and 0.50) and DR process are more suitable. These equilibrium predicted values were, in general, in good agreement with experimental values for most of the catalysts.

According to the above summarized results, we decided to prepare bimetallic catalysts to study the effect of alkali metals modification. The processes selected to test the bimetallic catalysts were DR and OR (**Figure 10**). On the one hand, the DR process presents the harsher reaction conditions and therefore, it is appropriate to distinguish the activity of the catalysts. On the other hand, the OR process is the most favorable in terms of hydrogen production.

Similar catalytic activity was observed for both studied processes. First, the NL, NLCs, NLNa,

yield values, while Ni/DLCs catalyst produced intermediate results. Second, the less active catalyst was Rh-Ni/DLNa, which remained almost inactive in both biogas reforming processes. Therefore, there was a good agreement between the activity, the BET areas, the particle sizes estimated by XRD, and the ones observed by TEM, and the atomic Rh and Ni concentration. Thus, the most active catalysts presented the highest BET areas, the smaller metal

The herein reported step-by-step strategy to develop dye- and metal-doped LTL zeolites is suitable to attain materials which can be applied in photonics and eco-friendly catalysis. To this aim, we started from the beginning, by the microwave-assisted synthesis of the LTL zeolite crystals. This kind of heat allows a fine control of the size (from tens of nm to few μm) and shape (from barrels to coins) of the zeolite, which can be tuned just fixing the hydrothermal synthesis conditions. Afterwards, the kind of guest adsorbed into the zeolitic support will rule the application field of the material. On one hand, the allocation of rationally selected fluorophores into the uni-dimensional zeolitic channels, or grafted to the pore entrances, provide a hierarchically ordered material able to harvest light over a broad spectral interval and provide predominant red fluorescence, or alternatively white-light emission, just adjusting the energy transfer efficiency. On the other hand, the deposition of suitable metals onto the zeolite guest boosts the catalysis efficiency in the biogas reforming. Among the different LTL zeolite morphologies used, DL and CL (30–60 nm) supports were the most active ones for the studied reforming processes, especially when Rh and Ni metals were incorporated in order to prepare bimetallic catalysts. High activities are attributed to the high metal dispersion and the strong interactions among NiO and the different supports. The metal exchange of Cs and Na carried out on DL and NL supports mainly affected the morphology, and consequently,

and DL supported bimetallic catalysts achieved the highest CH4

**5. Conclusions**

particle sizes and the most equilibrated Rh and Ni surface concentrations.

/CO molar ratio), was also measured. For liquid

and CO2

conversions and H2

Financial support from MICINN (MAT2014-51937-C3-3-P), MINECO (OIL2H2. ENE2011- 23950), European Regional Development Fund (ERDF) and Gobierno Vasco (IT912-16 and IT993-16) is acknowledged. L. G.-R. and K. B. thanks Gobierno Vasco for a postdoctoral and predoctoral fellowship, respectively. Dr. Y. Xiao (from Dalian University in China) is gratefully thanked by the synthesis of the silylated-BODIPY stopcock.

### **Author details**

Leire Gartzia Rivero<sup>1</sup> , Jorge Bañuelos<sup>1</sup> \*, Kepa Bizkarra<sup>2</sup> , Urko Izquierdo2 , Victoria Laura Barrio<sup>2</sup> , Jose Francisco Cambra2 and Iñigo López Arbeloa1

\*Address all correspondence to: jorge.banuelos@ehu.es

1 Department of Physical Chemistry, University of the Basque Country (UPV/EHU), Bilbao, Spain

2 Faculty of Engineering, University of the Basque Country (UPV/EHU), Bilbao, Spain

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32 Zeolites and Their Applications

chroma.2012.03.031

6885. DOI: 10.1021/cm503761q


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**Chapter 3**

**Provisional chapter**

**Design and Evaluation of Gas Transport through a**

**Design and Evaluation of Gas Transport through a** 

DOI: 10.5772/intechopen.75545

This chapter details the synthesis and applications of zeolite membranes (gas separation and zeolite membrane reactors). Gas separation is still not carried out at industrial level for zeolite membranes. Related areas, such as the possibility of incorporating a zeolite membrane in a reactor for possible catalytic action of the zeolite particles and scale-up issues are also discussed. The basic concept of mass transport through the zeolite layer has been presented. Zeolites can enhance the selectivity of methane more which can lead

**Keywords:** zeolite membranes, greenhouse gases, catalytic membrane reactor and gas

Volatile organic compounds vapourises easily to the atmosphere due to their high vapour pressure. Light hydrocarbons like methane, ethane and propane are considered as VOCs [1]. As stated previously, methane has a global warming potential (GWP) 21 times greater than

Worldwide emissions of GHG can be presented according to the economic activities that lead

. **Table 1** shows the GWP for methane, carbon dioxide and several hydrocarbon gases,

© 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.

© 2018 The Author(s). Licensee IntechOpen. 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.

**Zeolite Membrane on an Alumina Support**

**Zeolite Membrane on an Alumina Support**

Habiba Shehu, Edidiong Okon,

and Edward Gobina

**Abstract**

separations

**1. Introduction**

CO2

http://dx.doi.org/10.5772/intechopen.75545

Ifeyinwa Orakwe and Edward Gobina

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Habiba Shehu, Edidiong Okon, Ifeyinwa Orakwe

to the reduction of greenhouse gases in the atmosphere.

while **Figure 1(a)** shows the percentage of the GHGs emitted.

to their production, as indicated in **Figure 1(b)** [2]. These include,


#### **Design and Evaluation of Gas Transport through a Zeolite Membrane on an Alumina Support Design and Evaluation of Gas Transport through a Zeolite Membrane on an Alumina Support**

DOI: 10.5772/intechopen.75545

Habiba Shehu, Edidiong Okon, Ifeyinwa Orakwe and Edward Gobina Habiba Shehu, Edidiong Okon, Ifeyinwa Orakwe and Edward Gobina

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.75545

#### **Abstract**

resonance energy transfer of dye loaded on LTL zeolite. Microporous and Mesoporous

[34] Loudet A, Burgess K. BODIPY dyes and their derivatives: Syntheses and spectroscopic

[35] Frontera P, Macario A, Aloise A, Crea F, Antonucci PL, Nagy JB, Frsusteri F, Giordano G. Catalytic dry-reforming on Ni–zeolite supported catalyst. Catalysis Today. 2012;**179**:

[36] Bereketidou OA, Goula MA. Biogas reforming for syngas production over nickel supported on ceria-alumina catalysts. Catalysis Today. 2012;**195**:93-100. DOI: 10.1016/j.

[37] Damyanova S, Pawelec B, Arishtirova K, Fierro JLG. Biogas reforming over bimetallic PdNi catalysts supported on phosphorus-modified alumina. International Journal of

[38] Xu J, Zhou W, Li Z, Wang J, Ma J. Biogas reforming for hydrogen production over a Ni-Co bimetallic catalyst: Effect of operating conditions. International Journal of Hydrogen

[39] Kaengsilalai A, Luengnaruemitchai A, Jitkarnka S, Wongkasemjit S. Potential of Ni supported on KH zeolite catalysts for carbon dioxide reforming of methane. Journal of

[40] Nimwattanakul W, Luengnaruemitchai A, Jitkarnka S. Potential of Ni supported on clinoptilolite catalysts for carbon dioxide reforming of methane. International Journal of

[41] San-José-Alonso D, Juan-Juan J, Illán-Gómez MJ, Román-Martínez MC. Ni, Co and bimetallic Ni–Co catalysts for the dry reforming of methane. Applied Catalysis A: General.

[42] Diskin AM, Cunningham RH, Ormerod RM. The oxidative chemistry of methane over

[43] Luengnaruemitchai A, Kaengsilalai A. Activity of different zeolite-supported Ni catalysts for methane reforming with carbon dioxide. Chemical Engineering Journal. 2008;

[44] Garrido Pedrosa AM, Souza MJB, Silva AOS, Melo DMA, Araujo AS. Synthesis, characterization and catalytic properties of the cobalt and nickel supported on HZSM-12 zeolite. Catalysis Communications. 2006;**7**:791-796. DOI: 10.1016/j.catcom. 2006.02.012 [45] El Doukkali M, Iriondo A, Cambra JF, Jalowiecki-Duhamel L, Mamede AS, Dumeignil F, Arias PL. Pt monometallic and bimetallic catalysts prepared by acid sol–gel method for liquid phase reforming of bioglycerol. Journal of Molecular Catalysis A: Chemical.

reforming. Catalysis Today. 2011;**172**:226-231. DOI: 10.1016/j.cattod.2011.02.057


catalysts for methane dry

Hydrogen Energy. 2011;**36**:10635-10647. DOI: 10.1016/j.ijhydene.2011.05.098

Energy. 2010;**35**:13013-13020. DOI: 10.1016/j.ijhydene.2010.04.075

Power Sources. 2007;**165**:347-352. DOI: 10.1016/j.jpowsour.2006.12.005

Hydrogen Energy. 2006;**31**:93-100. DOI: 10.1016/j.ijhydene.2005.02.005

2009;**371**:54-59. DOI: 10.1016/j.apcata.2009.09.026

**144**:96-102. DOI: 10.1016/j.cej.2008.05.023

supported nickel catalysts. Catalysis Today. 1998;**46**:147-154

2013;**368-369**:125-136. DOI: 10.1016/j.molcata.2012.12.006

[46] Ocsachoque M, Pompeo F, Gonzalez G. Rh-Ni/CeO<sup>2</sup>

properties. Chemical Reviews. 2007;**107**:4891-4932. DOI: 10.1021/cr078381n

Materials. 2017;**241**:372-382. DOI: 10.1016/j.micromeso.2016.12.020

52-60. DOI: 10.1016/j.cattod.2011.07.039

cattod.2012.07.006

34 Zeolites and Their Applications

This chapter details the synthesis and applications of zeolite membranes (gas separation and zeolite membrane reactors). Gas separation is still not carried out at industrial level for zeolite membranes. Related areas, such as the possibility of incorporating a zeolite membrane in a reactor for possible catalytic action of the zeolite particles and scale-up issues are also discussed. The basic concept of mass transport through the zeolite layer has been presented. Zeolites can enhance the selectivity of methane more which can lead to the reduction of greenhouse gases in the atmosphere.

**Keywords:** zeolite membranes, greenhouse gases, catalytic membrane reactor and gas separations

#### **1. Introduction**

Volatile organic compounds vapourises easily to the atmosphere due to their high vapour pressure. Light hydrocarbons like methane, ethane and propane are considered as VOCs [1]. As stated previously, methane has a global warming potential (GWP) 21 times greater than CO2 . **Table 1** shows the GWP for methane, carbon dioxide and several hydrocarbon gases, while **Figure 1(a)** shows the percentage of the GHGs emitted.

Worldwide emissions of GHG can be presented according to the economic activities that lead to their production, as indicated in **Figure 1(b)** [2]. These include,


• Transportation: GHG emissions from transportation include the use of fossil fuels that are burned for rail, road, water and air transportation. Petroleum-based fuels account for about

Design and Evaluation of Gas Transport through a Zeolite Membrane on an Alumina Support

http://dx.doi.org/10.5772/intechopen.75545

37

• Buildings: This area accounts for the smallest GHG emissions (6%) and the emissions mainly arise from onsite energy generation and burning fuels for heat in buildings or cooking

• Other Energy: Other sources of GHG emissions come from the Energy sector which are not directly associated with electricity or heat production. For example, oil and gas extraction,

The work carried out in this research considers the economic sector of GHG emissions namely, transportation and storage of crude oil as well as natural gas processes. A technology to separate the major GHGs methane and carbon dioxide and further utilise them in valuable feed

Zeolites are natural or synthetic compounds that are composed of hydrated alumina-silica structures of alkaline and alkaline-earth metals. They have attracted increased interest because of their similar pore size on the molecular scale, which enables the separation of liquid and gaseous mixtures in a continuous way [3]. Zeolites have good chemical and thermal stability. As such, they can be used for high temperature processes and for processes that employ organic solvents. In addition, zeolite materials exhibit intrinsic catalytic property, which pro-

In the previous two decades, enormous progress has been made on zeolite membrane synthesis. However, only 20 out of approximately 170 zeolite structures are used for the preparation of a membrane [4]. The high cost and poor reproducibility of the synthesis hinders the application of the zeolite membranes on a large industrial scale [5, 6]. Zeolite frameworks are made of silicon oxides and aluminium oxides. Moreover, the silicon and aluminium atom centres have a tetrahedral shape, which are linked to each other by bridging oxygen atoms. The strong acidity and uniformity of the micropores (less than 2 nm in diameter), together with a unique crystal structure ensures that zeolites have a high selectivity for separation based on the shape or chemical configuration of molecules in different chemical reactions. For

example, alkylation, aromatisation, cracking, pyrolysis, and hydrodesulphurisation.

H10) over *i*-butane (*i*-C4

of an MFI-type membrane on an alumina support. This was shown to exhibit a *n*-C4

tion process is one of the vital factors for the application of zeolite membranes.

In comparison to natural zeolites, synthetic zeolites (*i.e.* X, Y and A) are often more applicable in membrane technology due to their uniform particle size and high purity. In addition, they can be designed to separate hydrocarbons. Van Bekkum et al. [7] have previously prepared an MFI-type zeolite membrane on a porous stainless steel disk. These exhibited a high perm-

at 20°C. However, the authors reported no data at elevated temperatures. Yan et al. [9] previously prepared an MF membrane on an alumina porous disk. The authors reported a *n*-C4

permselectivity of 90 at 25°C and 11 at 200°C. Thus, reproducibility in the membrane forma-

H10 permselectivity of 6.2 at 108°C and 9.4 at 185°C. Vroon et al. [10] reported the formation

H10/*i*-C4

H10) at room temperature. Jia et al. [8]

H10 selectivity of approximately 50

H10/*i*-

H10

H10/*i*-C4

motes the use of zeolite membranes as catalytic membrane reactors (CMRs).

96% of global transportation energy; this is mainly from diesel and gasoline.

in homes.

refining, processing, and transportation.

selectivity for *n*-butane (*n*-C4

C4

reported on a zeolite membrane that showed a *n*-C4

stock has been explored using a y-type zeolite membrane.

**Table 1.** Global warming potentials for several VOC components [1].


**Figure 1.** Schematic of the solid-state crystallisation route for y-type zeolite synthesis.


The work carried out in this research considers the economic sector of GHG emissions namely, transportation and storage of crude oil as well as natural gas processes. A technology to separate the major GHGs methane and carbon dioxide and further utilise them in valuable feed stock has been explored using a y-type zeolite membrane.

• Electricity and heat production: This sector accounts for the highest percentage of GHG emissions at 25% (in 2010), as reported by the United States Environmental Protection Agency (EPA). Hence, the burning of fossil fuels (*i.e.* coal, natural gas and oil) for electricity and heat generation are the main activities that contribute to the global increase of GHG. • Industry: GHG emissions from industry primarily involve onsite burning of fossil fuels for energy. This area incorporates emissions from mineral transformation processes which are not as a result of energy consumption, chemical, metallurgical and emissions from activi-

21 5.5 3.3 4

ties of waste management. This sector accounts for 21% of GHG (**Figure 1(b)**).

soils, which reduces about 20% of emissions from this sector.

**Components GWP**

Carbon dioxide 1

**Table 1.** Global warming potentials for several VOC components [1].

Methane Ethane Propane Butane

36 Zeolites and Their Applications

**Figure 1.** Schematic of the solid-state crystallisation route for y-type zeolite synthesis.

• Agriculture, forestry and other land uses: GHG emissions from this sector originate primarily from deforestation and planting of trees. This value does not include carbon dioxide removed from the atmosphere by dead organic matter, carbon sequestering in biomass and Zeolites are natural or synthetic compounds that are composed of hydrated alumina-silica structures of alkaline and alkaline-earth metals. They have attracted increased interest because of their similar pore size on the molecular scale, which enables the separation of liquid and gaseous mixtures in a continuous way [3]. Zeolites have good chemical and thermal stability. As such, they can be used for high temperature processes and for processes that employ organic solvents. In addition, zeolite materials exhibit intrinsic catalytic property, which promotes the use of zeolite membranes as catalytic membrane reactors (CMRs).

In the previous two decades, enormous progress has been made on zeolite membrane synthesis. However, only 20 out of approximately 170 zeolite structures are used for the preparation of a membrane [4]. The high cost and poor reproducibility of the synthesis hinders the application of the zeolite membranes on a large industrial scale [5, 6]. Zeolite frameworks are made of silicon oxides and aluminium oxides. Moreover, the silicon and aluminium atom centres have a tetrahedral shape, which are linked to each other by bridging oxygen atoms. The strong acidity and uniformity of the micropores (less than 2 nm in diameter), together with a unique crystal structure ensures that zeolites have a high selectivity for separation based on the shape or chemical configuration of molecules in different chemical reactions. For example, alkylation, aromatisation, cracking, pyrolysis, and hydrodesulphurisation.

In comparison to natural zeolites, synthetic zeolites (*i.e.* X, Y and A) are often more applicable in membrane technology due to their uniform particle size and high purity. In addition, they can be designed to separate hydrocarbons. Van Bekkum et al. [7] have previously prepared an MFI-type zeolite membrane on a porous stainless steel disk. These exhibited a high permselectivity for *n*-butane (*n*-C4 H10) over *i*-butane (*i*-C4 H10) at room temperature. Jia et al. [8] reported on a zeolite membrane that showed a *n*-C4 H10/*i*-C4 H10 selectivity of approximately 50 at 20°C. However, the authors reported no data at elevated temperatures. Yan et al. [9] previously prepared an MF membrane on an alumina porous disk. The authors reported a *n*-C4 H10/*i*-C4 H10 permselectivity of 6.2 at 108°C and 9.4 at 185°C. Vroon et al. [10] reported the formation of an MFI-type membrane on an alumina support. This was shown to exhibit a *n*-C4 H10/*i*-C4 H10 permselectivity of 90 at 25°C and 11 at 200°C. Thus, reproducibility in the membrane formation process is one of the vital factors for the application of zeolite membranes.

In addition, the effect of the supporting substrate on permeation properties of zeolite membranes is critical. Yan et al. [9] reported that the membrane morphology changed for the same porous substrate, under different synthetic conditions. Kusakabe et al. [11] produced an MFI-type zeolite membrane on the exterior surfaces of a porous alumina support tube using a hydrothermal reaction. The authors found no direct relationship between film morphology and permselectivity. The authors also synthesised a Y-type zeolite membrane on a porous α-alumina support tube and carried out single gas permeation test on CO<sup>2</sup> , N2 , CH4 , C2 H6 and SF6 . The authors found that the selectivity of CO2 /CH4 through the membrane was higher at permeation temperatures that are lower, and tends to decrease with increases in temperature.

where Do

where Pi

which is expressed as:

and ci

assumed [16], giving by:

*<sup>Г</sup>* <sup>=</sup> *dln <sup>p</sup>* \_\_\_\_\_*<sup>i</sup>*

*Do*

*Ng* = −

*Dg* = *dp um e* <sup>−</sup>*Ee*

*um* <sup>=</sup> <sup>√</sup>

where dp is the pore diameter and um is the average velocity.

is the intrinsic or corrected diffusivity and Г is the thermodynamic correction factor,

Design and Evaluation of Gas Transport through a Zeolite Membrane on an Alumina Support

(3)

39

*dz*) (4)

http://dx.doi.org/10.5772/intechopen.75545

*RTdz* (5)

/*RT* (6)

*<sup>M</sup>* (7)

*dln ci*

The transport diffusivity is dependent on the temperature. This is more apparent at higher temperature. The assumption of an Arrhenius type dependence on temperature can be

The dependence on temperature will be affected by the adsorption of the component on the zeolite as well as the operating conditions. Moreover, the adsorption phenomena can be negligible at elevated temperatures. Under these conditions molecules can be considered to be in a quasi-gaseous state in the zeolite framework. This is referred to as activated Knudsen diffu-

*Dg dp* \_\_\_\_\_

where dp/dz is the pressure gradient and also the permeance driving force. The diffusion

For ideal gases, kinetic theory can be used to calculate the molecular velocity given by Eq. (7):

From the equations above, it is clear that gas transport through a zeolite membrane is dependent on the adsorptive interaction between the permeating gas molecule and the zeolite. Moreover, the permeating flux is meant to increase with an increase in temperature. This is true for a defect free zeolite membrane. However, Knudsen and viscous flow can contribute to the overall flux and will strongly influence the expected temperature dependence when defects are present [16].

The ramification of predicting the mass transport and separation through synthesised zeolite membranes, where defects of inter-crystalline nature also need to be considered, is evident even though a simple approach has been used. High selectivity separations can be achieved by using nearly perfect zeolite membranes. In addition to high permselectivity, zeolite membranes should exhibit a high permeation flux in order to be suitable for industrial scale

\_\_\_\_ 8\_\_\_\_ *RT*

*= Dg* \_\_\_ *RT*( *dp*\_\_\_

sion or gas translational diffusion. When this occurs, the flux is expressed as:

coefficient that is dependent on the gas molecular velocity is given by:

are the pressure and concentration of component i.

#### **1.1. Mass transfer through a zeolite membrane**

The process of mass transport through a zeolite layer arises via the five steps listed below [12, 13]:


Adsorption and desorption of species from the outer surface of a zeolite layer depends on the permeation conditions (*i.e.* temperature and pressure), type of crystalline material and the nature of the chemical species. Steps 2, 3 and 4 are usually activated processes [14].

Intra-crystalline permeation through a zeolite membrane can be described using several approaches [15]. The Fickian approach considers the concentration gradient as the driving force in a zeolite membrane. Alternatively, the gradient of the thermodynamic potential is the driving force in the Maxwell-Stefan (MS) approach. The MS approach allows for the approximation of the flux through the membrane for multicomponent gas mixtures by using information about single gas permeations [16]. The Fickian approach can be applied for permeation of single gas components through a zeolite membrane at a wide range of temperatures. Moreover, it can be assumed that the total flux N is the combination of the surface flux N<sup>s</sup> , which takes place at low to medium temperatures, and the activated gaseous flux N<sup>g</sup> , which is prevalent at high temperatures [13–15]. This is given by Eq. (1):

$$N = N\_s + N\_g \tag{1}$$

Fick's diffusivity *Ds* is given by Eq. 32:

$$D\_s = D\_\delta \Gamma \tag{2}$$

where Do is the intrinsic or corrected diffusivity and Г is the thermodynamic correction factor, which is expressed as:

$$
\Gamma = \frac{d\ln p\_i}{d\ln c\_i} \tag{3}
$$

where Pi and ci are the pressure and concentration of component i.

In addition, the effect of the supporting substrate on permeation properties of zeolite membranes is critical. Yan et al. [9] reported that the membrane morphology changed for the same porous substrate, under different synthetic conditions. Kusakabe et al. [11] produced an MFI-type zeolite membrane on the exterior surfaces of a porous alumina support tube using a hydrothermal reaction. The authors found no direct relationship between film morphology and permselectivity. The authors also synthesised a Y-type zeolite membrane on a

higher at permeation temperatures that are lower, and tends to decrease with increases in

The process of mass transport through a zeolite layer arises via the five steps listed below

Adsorption and desorption of species from the outer surface of a zeolite layer depends on the permeation conditions (*i.e.* temperature and pressure), type of crystalline material and the

Intra-crystalline permeation through a zeolite membrane can be described using several approaches [15]. The Fickian approach considers the concentration gradient as the driving force in a zeolite membrane. Alternatively, the gradient of the thermodynamic potential is the driving force in the Maxwell-Stefan (MS) approach. The MS approach allows for the approximation of the flux through the membrane for multicomponent gas mixtures by using information about single gas permeations [16]. The Fickian approach can be applied for permeation of single gas components through a zeolite membrane at a wide range of temperatures. Moreover, it can be assumed that the total flux N is the combination of the surface flux N<sup>s</sup>

nature of the chemical species. Steps 2, 3 and 4 are usually activated processes [14].

which takes place at low to medium temperatures, and the activated gaseous flux N<sup>g</sup>

*N* = *Ns* + *Ng* (1)

/CH4

, N2 , CH4 ,

,

, which

*Г* (2)

through the membrane was

porous α-alumina support tube and carried out single gas permeation test on CO<sup>2</sup>

. The authors found that the selectivity of CO2

**1.** Adsorption of the substance on the outer surface of the membrane.

**2.** Mass transport from the outer surface into the zeolite pore.

**4.** Mass transport out of the zeolite pores to the external surface.

is prevalent at high temperatures [13–15]. This is given by Eq. (1):

is given by Eq. 32:

*Ds* = *Do*

**1.1. Mass transfer through a zeolite membrane**

**3.** Diffusion of intra-crystalline zeolite.

**5.** Desorption from the outer surface to the bulk.

C2 H6

and SF6

38 Zeolites and Their Applications

Fick's diffusivity *Ds*

temperature.

[12, 13]:

The transport diffusivity is dependent on the temperature. This is more apparent at higher temperature. The assumption of an Arrhenius type dependence on temperature can be assumed [16], giving by:

$$D\_o = \frac{D\_x}{RT} \left(\frac{dp}{dz}\right) \tag{4}$$

The dependence on temperature will be affected by the adsorption of the component on the zeolite as well as the operating conditions. Moreover, the adsorption phenomena can be negligible at elevated temperatures. Under these conditions molecules can be considered to be in a quasi-gaseous state in the zeolite framework. This is referred to as activated Knudsen diffusion or gas translational diffusion. When this occurs, the flux is expressed as:

$$N\_g = -\frac{D\_g dp}{RTdz} \tag{5}$$

where dp/dz is the pressure gradient and also the permeance driving force. The diffusion coefficient that is dependent on the gas molecular velocity is given by:

$$\mathcal{D}\_g = d\_p u\_m e^{-\mathbb{E}\beta T} \tag{6}$$

where dp is the pore diameter and um is the average velocity.

For ideal gases, kinetic theory can be used to calculate the molecular velocity given by Eq. (7):

$$
\mu\_m = \sqrt{\frac{8RT}{\pi M}}\tag{7}
$$

From the equations above, it is clear that gas transport through a zeolite membrane is dependent on the adsorptive interaction between the permeating gas molecule and the zeolite. Moreover, the permeating flux is meant to increase with an increase in temperature. This is true for a defect free zeolite membrane. However, Knudsen and viscous flow can contribute to the overall flux and will strongly influence the expected temperature dependence when defects are present [16].

The ramification of predicting the mass transport and separation through synthesised zeolite membranes, where defects of inter-crystalline nature also need to be considered, is evident even though a simple approach has been used. High selectivity separations can be achieved by using nearly perfect zeolite membranes. In addition to high permselectivity, zeolite membranes should exhibit a high permeation flux in order to be suitable for industrial scale applications. This can be achieved with minimal membrane thickness. Regrettably, decreasing the membrane thickness results in negative effect of inter-crystalline defects on permselectivity can be limiting. The thickness of a zeolite layer is dependent on the synthesis routes, conditions and on the number of depositions. For example, White et al. [15] obtained a ZSM-5 membrane by direct *in situ* crystallisation with a two-step deposition and showed a thickness between 30 and 40 μm. At laboratory level, zeolite membranes with a thickness of a few microns can be obtained with sufficient quality. Currently there are ongoing investigations to find a way to avoid, reduce or eliminate the presence of inter-crystalline defects, which, aside from poor synthesis reproducibility, are the main obstacle to the widespread industrial application of zeolite membrane. Moreover, if mixtures of gas and vapour having high molecular masses, or liquid mixtures of two species with different volatility and surface tension, are considered, the separation factors and permeation fluxes can be very interesting. However, these separations cannot be extrapolated from the permeances of the pure gases.

#### **1.2. Membrane transport mechanism**

In order to understand the fundamentals of membrane gas separation, a brief introduction to some laws and processes commonly employed is required.

#### *1.2.1. Graham's law (Thomas Graham in 1848)*

Graham's law states that the rate of diffusion of a gas is inversely proportional to the square root of its molecular weight. This can be written as,

$$\frac{\text{Rate}\_{\text{s}}}{\text{Rate}\_{\text{b}}} = \,^{\prime \text{M}}\!/ \,^{\prime M}\text{J}^{1/2} \tag{8}$$

the gases permeate via convective flow and there is no separation of the gases observed. For mesoporous membranes, separation is based on the collision amongst the molecule and hence molecular diffusion is dominant and the mean free path (which is the average distance travelled by a gas molecule between collisions with another gas molecule) of the gas molecules is greater than the pore size. The diffusion here is governed by Knudsen mechanism and it follows from the kinetic theory of gases that the rate of transport of any gas is inversely proportional to the square root of its molecular weight, which coincides with Graham's law of diffusion [16]. However, for a microporous membrane with pore size less than 2 nm, separation of gases is based mostly on molecular sieving. The transport mechanism in these type of membranes is often rather complex and involves surface diffusion that occurs when the permeating species exhibit a strong affinity for the membrane surface, thus adsorbing on the

Design and Evaluation of Gas Transport through a Zeolite Membrane on an Alumina Support

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41

The permeation of gases through a membrane is dependent on both the diffusion and the concentration gradient of the species along the membrane. The selective transport of a gas molecule through a membrane is often associated with the pressure, temperature, electric potential and concentration gradient. The permeability and selectivity are some of the parameters that are used to determine the performance of a membrane. The permeance, P (mol m−2 s−1 Pa−1), represents the proportionality coefficient with a flux at steady state for a gas passing through

where Q is the gas molar gas flow through the membrane (mol s−1), A is the membrane surface area (m) and Δp is the pressure difference across the membrane (Pa). The permeance is a mea-

The ideal gas selectivity αi,j, is the ratio of the permeability coefficients of two different gases

The selectivity is the measure of the ability of a membrane to separate two gases and is used to determine the purity of the permeate gas, as well as determine the quantity of product that is lost. The permeability coefficient is related to the diffusivity coefficient, D (m<sup>2</sup> s−1), and the

> *Di Dj S* \_\_*i Sj*

*Pj*

are the permeance of the single gases through the membrane respectively.

Pa), for a component, i, [16] and is given by:

sure of the quantity of a component that permeates through the membrane [16].

as they permeate independently through the membrane is given by Eq. (10):

Combining Eqs. (4) and (5), the selectivity of a membrane can be expressed as:

*<sup>A</sup>*.*<sup>p</sup>* (9)

.*Si* (11)

(10)

(12)

walls of the pores [16].

where Pi

and Pj

solubility coefficient, S (mol m<sup>3</sup>

a membrane and is defined by Eq. 9:

*<sup>P</sup>* <sup>=</sup> \_\_\_\_\_ *<sup>Q</sup>*

*<sup>α</sup>ij* <sup>=</sup> *<sup>P</sup>*\_\_*<sup>i</sup>*

*Pi* = *Di*

*<sup>α</sup>ij* <sup>=</sup> \_\_

where *Ratea* is the rate of diffusion of the first gas (volume or number of moles per unit time), *Rateb* is the rate of diffusion for the second gas, and *Ma* and *Mb* are the molar masses of gases a and b in g mol−1.

#### *1.2.2. Fick's first law*

Fick's first law relates the diffusive flux to the concentration under the assumption of steady state. It postulates that the flux goes from regions of high concentration to regions of low concentration. The law fundamentally describes diffusion of species and was enunciated by Adolph E. Fick in 1855, who noted a similarity between diffusion of solutes and Fourier's law describing the flow of heat in solids. Fick's law was theoretically deduced in 1860 by James C. Maxwell from the kinetic theory of gases. The derivation of Fick's law includes the following assumptions: (1) statistical laws apply, (2) the average duration of a collision is short compared to the average time between collisions, a condition pertaining to dilute solutions, (3) particles move independently, (4) classical mechanics can be used to describe molecular collisions, (5) energy, momentum and mass are conserved in every collision, and (6) the diffusing solute particles are much larger than the solvent molecules of the liquid.

The separation of gases in membranes is possible due to the difference in the movement of the different species through the membrane. For membranes having large pore sizes of 0.1–10 μm, the gases permeate via convective flow and there is no separation of the gases observed. For mesoporous membranes, separation is based on the collision amongst the molecule and hence molecular diffusion is dominant and the mean free path (which is the average distance travelled by a gas molecule between collisions with another gas molecule) of the gas molecules is greater than the pore size. The diffusion here is governed by Knudsen mechanism and it follows from the kinetic theory of gases that the rate of transport of any gas is inversely proportional to the square root of its molecular weight, which coincides with Graham's law of diffusion [16]. However, for a microporous membrane with pore size less than 2 nm, separation of gases is based mostly on molecular sieving. The transport mechanism in these type of membranes is often rather complex and involves surface diffusion that occurs when the permeating species exhibit a strong affinity for the membrane surface, thus adsorbing on the walls of the pores [16].

applications. This can be achieved with minimal membrane thickness. Regrettably, decreasing the membrane thickness results in negative effect of inter-crystalline defects on permselectivity can be limiting. The thickness of a zeolite layer is dependent on the synthesis routes, conditions and on the number of depositions. For example, White et al. [15] obtained a ZSM-5 membrane by direct *in situ* crystallisation with a two-step deposition and showed a thickness between 30 and 40 μm. At laboratory level, zeolite membranes with a thickness of a few microns can be obtained with sufficient quality. Currently there are ongoing investigations to find a way to avoid, reduce or eliminate the presence of inter-crystalline defects, which, aside from poor synthesis reproducibility, are the main obstacle to the widespread industrial application of zeolite membrane. Moreover, if mixtures of gas and vapour having high molecular masses, or liquid mixtures of two species with different volatility and surface tension, are considered, the separation factors and permeation fluxes can be very interesting. However, these

In order to understand the fundamentals of membrane gas separation, a brief introduction to

Graham's law states that the rate of diffusion of a gas is inversely proportional to the square

Fick's first law relates the diffusive flux to the concentration under the assumption of steady state. It postulates that the flux goes from regions of high concentration to regions of low concentration. The law fundamentally describes diffusion of species and was enunciated by Adolph E. Fick in 1855, who noted a similarity between diffusion of solutes and Fourier's law describing the flow of heat in solids. Fick's law was theoretically deduced in 1860 by James C. Maxwell from the kinetic theory of gases. The derivation of Fick's law includes the following assumptions: (1) statistical laws apply, (2) the average duration of a collision is short compared to the average time between collisions, a condition pertaining to dilute solutions, (3) particles move independently, (4) classical mechanics can be used to describe molecular collisions, (5) energy, momentum and mass are conserved in every collision, and (6) the dif-

The separation of gases in membranes is possible due to the difference in the movement of the different species through the membrane. For membranes having large pore sizes of 0.1–10 μm,

is the rate of diffusion of the first gas (volume or number of moles per unit time),

and *Mb*

= (*Mb* ⁄ *Ma*)1/2 (8)

are the molar masses of gases

\_\_\_\_\_*<sup>a</sup> Rateb*

fusing solute particles are much larger than the solvent molecules of the liquid.

separations cannot be extrapolated from the permeances of the pure gases.

some laws and processes commonly employed is required.

is the rate of diffusion for the second gas, and *Ma*

**1.2. Membrane transport mechanism**

40 Zeolites and Their Applications

*1.2.1. Graham's law (Thomas Graham in 1848)*

*Rate*

where *Ratea*

a and b in g mol−1.

*1.2.2. Fick's first law*

*Rateb*

root of its molecular weight. This can be written as,

The permeation of gases through a membrane is dependent on both the diffusion and the concentration gradient of the species along the membrane. The selective transport of a gas molecule through a membrane is often associated with the pressure, temperature, electric potential and concentration gradient. The permeability and selectivity are some of the parameters that are used to determine the performance of a membrane. The permeance, P (mol m−2 s−1 Pa−1), represents the proportionality coefficient with a flux at steady state for a gas passing through a membrane and is defined by Eq. 9:

$$P = \frac{Q}{A.\Delta p} \tag{9}$$

where Q is the gas molar gas flow through the membrane (mol s−1), A is the membrane surface area (m) and Δp is the pressure difference across the membrane (Pa). The permeance is a measure of the quantity of a component that permeates through the membrane [16].

The ideal gas selectivity αi,j, is the ratio of the permeability coefficients of two different gases as they permeate independently through the membrane is given by Eq. (10):

$$\alpha\_{\parallel} = \frac{P\_i}{P\_j} \tag{10}$$

where Pi and Pj are the permeance of the single gases through the membrane respectively. The selectivity is the measure of the ability of a membrane to separate two gases and is used to determine the purity of the permeate gas, as well as determine the quantity of product that is lost. The permeability coefficient is related to the diffusivity coefficient, D (m<sup>2</sup> s−1), and the solubility coefficient, S (mol m<sup>3</sup> Pa), for a component, i, [16] and is given by:

$$P\_i = D\_i S\_i \tag{11}$$

Combining Eqs. (4) and (5), the selectivity of a membrane can be expressed as:

$$\alpha\_{\parallel} = \frac{D\_i}{D\_j} \frac{S\_i}{S\_j} \tag{12}$$

For a binary mixture of gases with components i and j, the separation factor SF is given by:

$$\text{SF}\_{\parallel} = \frac{\binom{\alpha \wedge \gamma}{\{1 \vee \overline{\gamma}\}}}{\binom{\alpha \wedge \gamma}{\{1 \vee \overline{\gamma}\}}} \tag{13}$$

that have exhibited these properties are known to be commercially available. However, there have been reports indicating the separation of gases that differ in size by just 0.1× 10<sup>−</sup><sup>10</sup> nm [17].

Design and Evaluation of Gas Transport through a Zeolite Membrane on an Alumina Support

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43

Surface diffusion and adsorption is a further mechanism that governs the permeation of gases through membranes that have small pore sizes. When the pore diameter of a membrane is in the range of 50–100 Å then adsorption on the walls of the membrane is observed. It is often noted that the amount of gas that is absorbed on the membrane pore walls is greater than the amount of gas that is not absorbed. The absorbed gas molecules then move by surface diffu-

Capillary condensation occurs when a porous membrane is in contact with a vapour and the saturation vapour pressure in the pores is different from the saturation vapour pressure of the components [20]. In addition, capillary condensation can occur with increasing gas pressures at temperatures below the critical temperature [21]. Therefore, condensed gas molecules are

For transport measurements, the molecular fluxes of the gases need to be determined from the uneven concentration profile, which can be used to determine the diffusion coefficient [21].

Zeolite membranes are normally synthesised on porous alumina supports or stainless steel, because a self-standing zeolite layer is very fragile. The commonly employed procedures used

The structured pores of zeolites, and the ability of zeolites to withstand high temperatures and pressures have made them a unique material for designing membranes. Significant high-profile research is currently being undertaken to develop the synthesis of zeolite membranes. Several of the developed methods for the synthesis of zeolite membranes are reviewed in this section.

For molecular sieving applications, this method of preparation is most commonly employed. Teflon and cellulose supports are used as temporary supports for the synthesis [22]. This preparation method has been discontinued because of the fragility of the self-supported

sion through the membrane with the flow rate obeying Fick's law [17].

**1.5. Surface diffusion**

**1.6. Capillary condensation**

transported across the membrane pores.

**2. Synthesis of zeolite membrane**

for zeolite membrane synthesis include:

**2.1. Zeolite films that are free-standing**

(a) vapour-phase transport.

(c) secondary growth.

membrane.

(b) direct *in situ* crystallisation.

where Y and X are the percentage concentrations in the permeate and feed end of the membrane. During experiments, the concentration of the gases (Xi and Xj ) are fixed while at the permeate side Yi and Yj are determined using gas chromatography (GC).

#### **1.3. Knudsen diffusion**

Knudsen diffusion arises from differences in the molecular weights of components to be separated. This proceeds at a speed that is inversely proportional to the square root of the molecular weight of the component. Separation by Knudsen diffusion requires that the pore diameter of the membrane to be smaller than the mean free path of the components. Generally, diffusion of gases through porous membranes is dependent on the type of collisions that occur. At low concentrations, where there is predominantly molecule-pore wall collisions then the flow is Knudsen flow. Knudsen flow can be achieved with membranes whose pore size is greater than 4 nm. However, for it to dominate the pore size should be less than 50 nm [17]. In addition, the separation factor for a mixture of binary gases can be estimated from the square root of the ratio of the molecular weights of the gases. This is because gas permeation by Knudsen diffusion varies inversely with the square root of the molecular weights of the gases. Hence an ideal Knudsen separation for a mixture of binary gases is equal to the inverse of the square root of their molecular mass ratio [18]. The transportation equation for Knudsen and viscous flow is given by Eq. (8):

$$J = A\,\tilde{P} + B\tag{14}$$

where *P*̄ is the average pressure across a porous membrane, and A and B are constants relative to the membrane structure, molecular weight and size. According to Eq. (8), A is the constant representing Knudsen flow, while B is the constant representing viscous flow.

#### **1.4. Molecular sieving**

The molecular sieving effect in gas separations occurs when the pores of a membrane decrease to the 5 to 10 × 10<sup>−</sup><sup>10</sup> nm range. If the gases to be separated have different kinetic diameters then the smaller molecules will permeate through the membrane while the larger molecules will be retained. Very high separation can be achieved using this effect [18]. The kinetic diameter of a gas is defined as the intermolecular distance of closest approach for two molecules colliding with zero initial kinetic energy. This is dependent on the molecular shape, size and dipole– dipole interactions [19]. **Table 1** lists the kinetic diameters and molecular weights of several molecules found in natural gas or shuttle tanker exhaust off-gases.

Research in the production of membranes exhibiting these properties has accelerated. Zeolites and ceramic membranes can be modified to achieve these properties. None of the membranes that have exhibited these properties are known to be commercially available. However, there have been reports indicating the separation of gases that differ in size by just 0.1× 10<sup>−</sup><sup>10</sup> nm [17].

#### **1.5. Surface diffusion**

For a binary mixture of gases with components i and j, the separation factor SF is given by:

where Y and X are the percentage concentrations in the permeate and feed end of the mem-

Knudsen diffusion arises from differences in the molecular weights of components to be separated. This proceeds at a speed that is inversely proportional to the square root of the molecular weight of the component. Separation by Knudsen diffusion requires that the pore diameter of the membrane to be smaller than the mean free path of the components. Generally, diffusion of gases through porous membranes is dependent on the type of collisions that occur. At low concentrations, where there is predominantly molecule-pore wall collisions then the flow is Knudsen flow. Knudsen flow can be achieved with membranes whose pore size is greater than 4 nm. However, for it to dominate the pore size should be less than 50 nm [17]. In addition, the separation factor for a mixture of binary gases can be estimated from the square root of the ratio of the molecular weights of the gases. This is because gas permeation by Knudsen diffusion varies inversely with the square root of the molecular weights of the gases. Hence an ideal Knudsen separation for a mixture of binary gases is equal to the inverse of the square root of their molecular mass ratio [18]. The transportation equation for Knudsen and viscous

*J* = *A P*̄+ *B* (14)

where *P*̄ is the average pressure across a porous membrane, and A and B are constants relative to the membrane structure, molecular weight and size. According to Eq. (8), A is the constant

The molecular sieving effect in gas separations occurs when the pores of a membrane decrease to the 5 to 10 × 10<sup>−</sup><sup>10</sup> nm range. If the gases to be separated have different kinetic diameters then the smaller molecules will permeate through the membrane while the larger molecules will be retained. Very high separation can be achieved using this effect [18]. The kinetic diameter of a gas is defined as the intermolecular distance of closest approach for two molecules colliding with zero initial kinetic energy. This is dependent on the molecular shape, size and dipole– dipole interactions [19]. **Table 1** lists the kinetic diameters and molecular weights of several

Research in the production of membranes exhibiting these properties has accelerated. Zeolites and ceramic membranes can be modified to achieve these properties. None of the membranes

representing Knudsen flow, while B is the constant representing viscous flow.

molecules found in natural gas or shuttle tanker exhaust off-gases.

are determined using gas chromatography (GC).

(*Xi* <sup>⁄</sup> *Xj*) (13)

) are fixed while at the

and Xj

*SFij* <sup>=</sup> (*Yi* <sup>⁄</sup> *<sup>Y</sup>* \_\_\_*j*)

and Yj

permeate side Yi

42 Zeolites and Their Applications

**1.3. Knudsen diffusion**

flow is given by Eq. (8):

**1.4. Molecular sieving**

brane. During experiments, the concentration of the gases (Xi

Surface diffusion and adsorption is a further mechanism that governs the permeation of gases through membranes that have small pore sizes. When the pore diameter of a membrane is in the range of 50–100 Å then adsorption on the walls of the membrane is observed. It is often noted that the amount of gas that is absorbed on the membrane pore walls is greater than the amount of gas that is not absorbed. The absorbed gas molecules then move by surface diffusion through the membrane with the flow rate obeying Fick's law [17].

#### **1.6. Capillary condensation**

Capillary condensation occurs when a porous membrane is in contact with a vapour and the saturation vapour pressure in the pores is different from the saturation vapour pressure of the components [20]. In addition, capillary condensation can occur with increasing gas pressures at temperatures below the critical temperature [21]. Therefore, condensed gas molecules are transported across the membrane pores.

For transport measurements, the molecular fluxes of the gases need to be determined from the uneven concentration profile, which can be used to determine the diffusion coefficient [21].

### **2. Synthesis of zeolite membrane**

Zeolite membranes are normally synthesised on porous alumina supports or stainless steel, because a self-standing zeolite layer is very fragile. The commonly employed procedures used for zeolite membrane synthesis include:


The structured pores of zeolites, and the ability of zeolites to withstand high temperatures and pressures have made them a unique material for designing membranes. Significant high-profile research is currently being undertaken to develop the synthesis of zeolite membranes. Several of the developed methods for the synthesis of zeolite membranes are reviewed in this section.

#### **2.1. Zeolite films that are free-standing**

For molecular sieving applications, this method of preparation is most commonly employed. Teflon and cellulose supports are used as temporary supports for the synthesis [22]. This preparation method has been discontinued because of the fragility of the self-supported membrane.

#### **2.2. Supported zeolite membrane**

This is the most commonly synthesised zeolite membrane. An *in-situ* hydrothermal synthesis process is used in the preparation. This method is direct and can produce good membranes. In this process a thin layer of zeolite is crystallised on the pores of the porous support. Various forms of porous inorganic materials can be used as supports. These include titania, alumina, dense glass, carbon and stainless steel. Crystal growth on the support involves the pre-treatment of the support, preparation of zeolite seeds and the seeding. Seeding can be achieved by employing several methods including, rub-coating, dip-coating, vacuum seeding, spin coating and filtration seeding [25].

hydrogen in dehydrogenation reactions. Zeolite membranes having an MFI structure have been used for the conversion of alkanes to olefins. Also, isobutane dehydrogenation has been studied in a membrane reactor combining a platinum/zeolite catalyst and a supported MFI membrane with a tubular configuration [25]. The results provide proof that isobutene yield was found to be about four times greater than the values observed when using a normal reactor. Another study of

Design and Evaluation of Gas Transport through a Zeolite Membrane on an Alumina Support

close to one at a temperature of about 23°C and increased to 70 at 500°C [22]. These results can be related to the fact that, at reduced temperature, permeation is controlled by adsorption and the permeate is enriched in butane. Diffusion becomes the dominant mechanism when the temperature is increased, this is because the butane is adsorbed less. Furthermore, for the various conditions that were considered experimentally, the membrane reactor showed increased isobutane

Qi et al. [17] prepared MFI zeolite membranes that contain partial modification of the zeolite channels are able to obtain a high selectivity and permeance during hydrogen separation following a water gas shift reaction at an elevated temperature. Gu et al. [18] have previously modified a zeolite membrane by *in-situ* catalytic cracking of methyl diethoxysilane.

permeance of 2.94 × 10−7 mol m−2 s−1 Pa−1. The membrane also presented a high stability in the temperature range 400–550°C. Moreover, the membrane reactor achieved a carbon monoxide

Fischer–Tropsch synthesis (FTS) allows for the synthesis of liquid hydrocarbons from various feedstocks, including coal and natural gas. The removal of water from this synthetic process

Different hydrophilic zeolite membranes have been used for the selective removal of water

for the water removal under normal FTS conditions [20]. Mordenite membranes have exhib-

their binary mixtures [21]. The permeance of water vapour in the binary mixture is almost close to the value found in the single gases. However, the permeance of each gas component went down with increasing water content. The results obtained can be used to explain how the adsorbed water molecules in the membrane block the other gas molecules. On raising the temperature, the amount of water adsorbed in the membrane goes slightly lower and the

Zeolite membranes act also as distributors to regulate the number of reactants added to a catalyst and thus limit side reactions. The use of membrane reactors is also highly relevant for carrying out oxidative dehydrogenation of alkanes to control the oxygen feed, in order

and CO. For example, ZSM-5 and mordenite membranes have been used

O fluxes and high permselectivities. An A-type (NaA) zeolite membrane

/CO2

/isobutane mixture separation factor was

http://dx.doi.org/10.5772/intechopen.75545

45

permselectivity of 68.3 with a hydrogen

O vapour, CO, H2

, CH4

and

to long-chain hydrocarbons by shifting the equilibrium

the dehydrogenation of isobutane revealed that the H2

The synthesised zeolite membrane showed a H2

is important for the following reasons:

• To reduce deactivation of the catalysts.

• To increase reactor productivity.

• To increase the conversion of CO2

from mixtures of H2

ited increased H2

of the water gas shift reaction [19].

conversion with respect to the conversion obtained using a normal reactor.

conversion of 81.7% at 550°C. This is higher than that obtained using a PBR.

was used to study the permeation of single components of H2

selectivity for water in the binary mixture reduces.

#### **2.3. Polymeric-zeolite filled membranes**

This method involves embedding zeolite crystals in to a polymer matrix [23]. The space between the zeolite crystals is sealed with a gas-tight polymeric structure. A major concern with this preparation method has been pore sizes that are different across the matrix and poor thermal stability.

### **3. Zeolite membrane characterisation**

The morphological of zeolite membranes can be determined using several techniques. In this work, the thickness and morphology of the zeolite membrane have been determined using the SEM. The outer surface and cross-sectional view shows the thickness of the zeolite layer on the support and a top view shows the size and shape of the crystals. EDAX has been used to determine the Si/Al ratio as well as the elemental compositions of zeolite membranes.

Fluorescence confocal optical microscopy is a good instrument for the non-destructive analysis of zeolite membranes. The defects of the membrane and the grain boundary network of the zeolite can be observed along the thickness of the membranes and defects may be clearly visualised [24]. N2 physisorption experiments are typically used to determine the pore volume and porosity of the zeolite powders and membranes. However, this method is difficult to use for supported zeolite membranes, because the supports generally do not fit inside the sample tubes within commercial equipment.

Therefore, in this work, a witness sample of the supported zeolite was used for all characterisation measurements alongside a mortar and pestle that was used to further grind the samples. An alternative method for the determination of porosity in thin films is the porosimetry, which allows analysis of the contribution of micropores and defects to the overall flux through the membrane.

### **4. Zeolite membrane reactors**

Zeolite membrane reactor concept has been developed for equilibrium-limited reactions, products removal and increased reactant conversion rates. They have been used for the in-situ removal of hydrogen in dehydrogenation reactions. Zeolite membranes having an MFI structure have been used for the conversion of alkanes to olefins. Also, isobutane dehydrogenation has been studied in a membrane reactor combining a platinum/zeolite catalyst and a supported MFI membrane with a tubular configuration [25]. The results provide proof that isobutene yield was found to be about four times greater than the values observed when using a normal reactor. Another study of the dehydrogenation of isobutane revealed that the H2 /isobutane mixture separation factor was close to one at a temperature of about 23°C and increased to 70 at 500°C [22]. These results can be related to the fact that, at reduced temperature, permeation is controlled by adsorption and the permeate is enriched in butane. Diffusion becomes the dominant mechanism when the temperature is increased, this is because the butane is adsorbed less. Furthermore, for the various conditions that were considered experimentally, the membrane reactor showed increased isobutane conversion with respect to the conversion obtained using a normal reactor.

Qi et al. [17] prepared MFI zeolite membranes that contain partial modification of the zeolite channels are able to obtain a high selectivity and permeance during hydrogen separation following a water gas shift reaction at an elevated temperature. Gu et al. [18] have previously modified a zeolite membrane by *in-situ* catalytic cracking of methyl diethoxysilane. The synthesised zeolite membrane showed a H2 /CO2 permselectivity of 68.3 with a hydrogen permeance of 2.94 × 10−7 mol m−2 s−1 Pa−1. The membrane also presented a high stability in the temperature range 400–550°C. Moreover, the membrane reactor achieved a carbon monoxide conversion of 81.7% at 550°C. This is higher than that obtained using a PBR.

Fischer–Tropsch synthesis (FTS) allows for the synthesis of liquid hydrocarbons from various feedstocks, including coal and natural gas. The removal of water from this synthetic process is important for the following reasons:

• To increase reactor productivity.

**2.2. Supported zeolite membrane**

44 Zeolites and Their Applications

ing and filtration seeding [25].

thermal stability.

visualised [24]. N2

through the membrane.

**4. Zeolite membrane reactors**

**2.3. Polymeric-zeolite filled membranes**

**3. Zeolite membrane characterisation**

sample tubes within commercial equipment.

This is the most commonly synthesised zeolite membrane. An *in-situ* hydrothermal synthesis process is used in the preparation. This method is direct and can produce good membranes. In this process a thin layer of zeolite is crystallised on the pores of the porous support. Various forms of porous inorganic materials can be used as supports. These include titania, alumina, dense glass, carbon and stainless steel. Crystal growth on the support involves the pre-treatment of the support, preparation of zeolite seeds and the seeding. Seeding can be achieved by employing several methods including, rub-coating, dip-coating, vacuum seeding, spin coat-

This method involves embedding zeolite crystals in to a polymer matrix [23]. The space between the zeolite crystals is sealed with a gas-tight polymeric structure. A major concern with this preparation method has been pore sizes that are different across the matrix and poor

The morphological of zeolite membranes can be determined using several techniques. In this work, the thickness and morphology of the zeolite membrane have been determined using the SEM. The outer surface and cross-sectional view shows the thickness of the zeolite layer on the support and a top view shows the size and shape of the crystals. EDAX has been used to determine the Si/Al ratio as well as the elemental compositions of zeolite membranes.

Fluorescence confocal optical microscopy is a good instrument for the non-destructive analysis of zeolite membranes. The defects of the membrane and the grain boundary network of the zeolite can be observed along the thickness of the membranes and defects may be clearly

ume and porosity of the zeolite powders and membranes. However, this method is difficult to use for supported zeolite membranes, because the supports generally do not fit inside the

Therefore, in this work, a witness sample of the supported zeolite was used for all characterisation measurements alongside a mortar and pestle that was used to further grind the samples. An alternative method for the determination of porosity in thin films is the porosimetry, which allows analysis of the contribution of micropores and defects to the overall flux

Zeolite membrane reactor concept has been developed for equilibrium-limited reactions, products removal and increased reactant conversion rates. They have been used for the in-situ removal of

physisorption experiments are typically used to determine the pore vol-


Different hydrophilic zeolite membranes have been used for the selective removal of water from mixtures of H2 and CO. For example, ZSM-5 and mordenite membranes have been used for the water removal under normal FTS conditions [20]. Mordenite membranes have exhibited increased H2 O fluxes and high permselectivities. An A-type (NaA) zeolite membrane was used to study the permeation of single components of H2 O vapour, CO, H2 , CH4 and their binary mixtures [21]. The permeance of water vapour in the binary mixture is almost close to the value found in the single gases. However, the permeance of each gas component went down with increasing water content. The results obtained can be used to explain how the adsorbed water molecules in the membrane block the other gas molecules. On raising the temperature, the amount of water adsorbed in the membrane goes slightly lower and the selectivity for water in the binary mixture reduces.

Zeolite membranes act also as distributors to regulate the number of reactants added to a catalyst and thus limit side reactions. The use of membrane reactors is also highly relevant for carrying out oxidative dehydrogenation of alkanes to control the oxygen feed, in order to limit total combustion that is highly exothermic [23]. Zeolite membranes have also been found to be active for the partial oxidation of propane at 550°C. Another possible application of these membranes is to use them as an active contactor, which is catalytically active but not necessarily permselective [24]. Bernado et al. [24] showed how a catalytic zeolite membrane, with catalytically active particles dispersed in to a thin zeolite layer ensures ultimate contact between reactants and the active site of the catalyst. This reduces by-pass problems that occur in PBR and reduces the pressure drop. The same authors have also studied carbon monoxide selective oxidation (Selox) from hydrogen-rich gas streams using catalytic zeolite membranes.

**6. Results and discussion**

nanoparticles of 0.35 to 0.37 nm.

alumina support (d) 24 h crystallisation.

**alumina support**

zeolite to be TO4

**6.1. SEM and EDAX observation of solid-state crystallisation deposition on the** 

SEM and EDAX have been carried out for different synthesis conditions to reveal the solid-state crystallisation of the zeolite on the support. Zeolite nanoparticles have been found to have an average size of 0.18–3.72 nm (**Figure 2a** and **b**). **Figure 2c** shows the SEM of a fresh support. Following crystallising in a mixture of sodium, aluminium and silicone oxides for 24 h the membrane revealed zeolite nanoparticles embedded in the matrix of the support (**Figure 2d**). These nanoparticles began aggregating in several locations that had unclear boundaries. Moreover, the nanoparticles have a spherical shape and a uniform particle size. The high magnification SEM image (**Figure 2b**) revealed that the nanoparticles could be mesoporous. This has been attributed to the assembly of many

**Figure 3** shows an EDAX spectrum for zeolite powder. The EDAX spectrum provides details about the elemental composition of the sample. The results confirm the molecular formula of

percentage weights of O, Al and Si are 138.04, 34.27 and 37.05 respectively. The percentage

**Figure 2.** SEM of the zeolite particle samples at (a) before deposition (b) higher magnification before deposition (c)

sition indicates that the zeolite powder is made up of tetrahedral units of AlO4

weight of Oxygen present is approximately four times that of aluminium and silicon.

, where T is either silicon or/and aluminium. Therefore, the elemental compo-

Design and Evaluation of Gas Transport through a Zeolite Membrane on an Alumina Support

http://dx.doi.org/10.5772/intechopen.75545

and SiO4

. The

47

### **5. Materials and method**

The chemicals, materials and gases used for the experimental work carried out in this chapter are listed as follows:


#### **5.1. Zeolite synthesis**

A y-type zeolite membrane was synthesised by mixing NaOH, Al2 O3 , SiO2 and deionised H2 O with a molar ratio of 1SiO2 :10Al2 O3 :14NaOH:798H2 O. The NaOH and Al2 O3 were first dissolved in H2 O. This was followed by the addition of SiO2 and the mixture was agitated with a magnetic stirrer for 24 h at 293.15 K. 2 g of NaX powder was then added. The γ-alumina support, which consists of 77% alumina and 23% TiO2 and has a permeable length of 348 mm and an internal and external diameter of 7 and 10 mm respectively was subsequently dipped in the resulting sol and kept under magnetic agitation at 343.15 K for 20 h making sure that it was kept central to the measuring cylinder and also vertical. This allowed the solvent to evaporate and resulted in the deposition of y-type zeolite crystals on the support matrix. The resulting membrane was then washed with deionised H2 O until the pH of the wash water was neutral. The membrane was then air dried for 2 h, using a motor powered rotatory drier at room temperature. It was then subjected to thermally treatment at 338.15 K in an oven for 20 h. The α-alumina support was weighed before and after zeolite deposition to determine the amount of zeolite loaded on the support. A schematic of the crystallisation process is shown in **Figure 1**.

### **6. Results and discussion**

to limit total combustion that is highly exothermic [23]. Zeolite membranes have also been found to be active for the partial oxidation of propane at 550°C. Another possible application of these membranes is to use them as an active contactor, which is catalytically active but not necessarily permselective [24]. Bernado et al. [24] showed how a catalytic zeolite membrane, with catalytically active particles dispersed in to a thin zeolite layer ensures ultimate contact between reactants and the active site of the catalyst. This reduces by-pass problems that occur in PBR and reduces the pressure drop. The same authors have also studied carbon monoxide selective oxidation (Selox) from hydrogen-rich gas streams using catalytic zeolite

The chemicals, materials and gases used for the experimental work carried out in this chapter

) supplied by Sigma-Aldrich, UK.

:14NaOH:798H2

a magnetic stirrer for 24 h at 293.15 K. 2 g of NaX powder was then added. The γ-alumina

and an internal and external diameter of 7 and 10 mm respectively was subsequently dipped in the resulting sol and kept under magnetic agitation at 343.15 K for 20 h making sure that it was kept central to the measuring cylinder and also vertical. This allowed the solvent to evaporate and resulted in the deposition of y-type zeolite crystals on the support matrix. The

was neutral. The membrane was then air dried for 2 h, using a motor powered rotatory drier at room temperature. It was then subjected to thermally treatment at 338.15 K in an oven for 20 h. The α-alumina support was weighed before and after zeolite deposition to determine the amount of zeolite loaded on the support. A schematic of the crystallisation process is shown

) supplied by Sigma-Aldrich, UK.

**4.** Gases (Oxygen, Propane, Methane, Nitrogen, Helium, and Carbon dioxide) supplied by

O3 , SiO2

O. The NaOH and Al2

and deionised H2

were first dis-

O3

and the mixture was agitated with

O until the pH of the wash water

and has a permeable length of 348 mm

O

**1.** 0.1 M sodium hydroxide (NaOH) supplied by Sigma-Aldrich, UK.

O3

**6.** Y-type Zeolite powder supplied by Sigma-Aldrich, UK

A y-type zeolite membrane was synthesised by mixing NaOH, Al2

O. This was followed by the addition of SiO2

:10Al2 O3

support, which consists of 77% alumina and 23% TiO2

resulting membrane was then washed with deionised H2

**3.** Deionised Water by Purelab Flex, Elga.

membranes.

46 Zeolites and Their Applications

**5. Materials and method**

are listed as follows:

BOC, UK.

**2.** Aluminium oxide (Al2

**5.** Silicon dioxide (SiO2

**5.1. Zeolite synthesis**

solved in H2

in **Figure 1**.

with a molar ratio of 1SiO2

#### **6.1. SEM and EDAX observation of solid-state crystallisation deposition on the alumina support**

SEM and EDAX have been carried out for different synthesis conditions to reveal the solid-state crystallisation of the zeolite on the support. Zeolite nanoparticles have been found to have an average size of 0.18–3.72 nm (**Figure 2a** and **b**). **Figure 2c** shows the SEM of a fresh support. Following crystallising in a mixture of sodium, aluminium and silicone oxides for 24 h the membrane revealed zeolite nanoparticles embedded in the matrix of the support (**Figure 2d**). These nanoparticles began aggregating in several locations that had unclear boundaries. Moreover, the nanoparticles have a spherical shape and a uniform particle size. The high magnification SEM image (**Figure 2b**) revealed that the nanoparticles could be mesoporous. This has been attributed to the assembly of many nanoparticles of 0.35 to 0.37 nm.

**Figure 3** shows an EDAX spectrum for zeolite powder. The EDAX spectrum provides details about the elemental composition of the sample. The results confirm the molecular formula of zeolite to be TO4 , where T is either silicon or/and aluminium. Therefore, the elemental composition indicates that the zeolite powder is made up of tetrahedral units of AlO4 and SiO4 . The percentage weights of O, Al and Si are 138.04, 34.27 and 37.05 respectively. The percentage weight of Oxygen present is approximately four times that of aluminium and silicon.

**Figure 2.** SEM of the zeolite particle samples at (a) before deposition (b) higher magnification before deposition (c) alumina support (d) 24 h crystallisation.

**Figure 3.** EDAX spectrum of zeolite powder before deposition on alumina support.

In addition, an elemental composition analysis of the y-type zeolite membrane has been determined using EDAX. This is presented in **Figure 4**. The associated data is provided in **Table 2**. presented in **Figures 6** and **7**. The flux is shown to be different for each gas. On increasing the temperature from 273 to 373 K, propane showed an increase of 146% in its flux, whereas there was only a 17% increase for methane. The extent of the effect of temperature is determined by the adsorption of the component on the zeolite. As observed from **Figure 8**, zeolite has a higher affinity towards methane compared to propane. Moreover, the influence of adsorption

Design and Evaluation of Gas Transport through a Zeolite Membrane on an Alumina Support

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49

**Molecular weight**

**(***g mol<sup>−</sup><sup>1</sup>* **)**

44.01 16.04 28.01 28.01 39.95 32.00 64.07 46.01 4.00 2.02

**Figure 4.** EDAX spectrum of y-type zeolite membrane 24 h after deposition.

**(× 10<sup>−</sup><sup>10</sup>** *nm***)**

3.30 3.80 3.64 3.76 3.40 3.46 3.60 3.30 2.60 2.89

**Table 2.** Kinetic diameter and molecular mass of various molecules found in off-gases [19].

**Gas Kinetic diameter**

CO2 CH4 N2 CO Ar O2 SO2 NO2 He H2

#### **6.2. Nitrogen physisorption measurements**

Nitrogen adsorption isotherms of the membrane are shown in **Figure 5**. A summary of the adsorption/desorption data is provided **Table 3**. The pore diameters have been calculated using the Barret-Joyner–Halenda (BJH) model. The BET surface areas for the support and zeolite membranes were found to be 10.69 and 0.106 m2 /g. Zeolites are believed to have large surface areas, however, the synthetic Y-type zeolite has a lower surface area than the support.

#### **6.3. Gas permeation**

#### *6.3.1. Effects of temperature on single gas permeation*

The single gas permeances for CO2 , N2 , O2 , CH4 and C3 H8 have been determined using the gas permeation setup. The permeate stream has been measured at standard temperature and pressure. The flux of the permeate gas has been measured using a volumetric digital flow meter (L min−1). Gas phase conditions have been employed exclusively in the feed and the permeate sides. Subsequently, single gases were fed into the membrane reactor at a pressure range of 0.1 to 1 × 10<sup>5</sup> Pa and at temperatures of 293, 373, 473 and 573 K. Data indicating the change in the flux of the gases through the zeolite membrane, as a function of temperature, are

**Figure 4.** EDAX spectrum of y-type zeolite membrane 24 h after deposition.

**Figure 3.** EDAX spectrum of zeolite powder before deposition on alumina support.

**6.2. Nitrogen physisorption measurements**

**6.3. Gas permeation**

48 Zeolites and Their Applications

range of 0.1 to 1 × 10<sup>5</sup>

zeolite membranes were found to be 10.69 and 0.106 m2

*6.3.1. Effects of temperature on single gas permeation*

The single gas permeances for CO2

In addition, an elemental composition analysis of the y-type zeolite membrane has been determined using EDAX. This is presented in **Figure 4**. The associated data is provided in **Table 2**.

Nitrogen adsorption isotherms of the membrane are shown in **Figure 5**. A summary of the adsorption/desorption data is provided **Table 3**. The pore diameters have been calculated using the Barret-Joyner–Halenda (BJH) model. The BET surface areas for the support and

surface areas, however, the synthetic Y-type zeolite has a lower surface area than the support.

gas permeation setup. The permeate stream has been measured at standard temperature and pressure. The flux of the permeate gas has been measured using a volumetric digital flow meter (L min−1). Gas phase conditions have been employed exclusively in the feed and the permeate sides. Subsequently, single gases were fed into the membrane reactor at a pressure

change in the flux of the gases through the zeolite membrane, as a function of temperature, are

and C3

H8

Pa and at temperatures of 293, 373, 473 and 573 K. Data indicating the

, N2 , O2 , CH4 /g. Zeolites are believed to have large

have been determined using the

presented in **Figures 6** and **7**. The flux is shown to be different for each gas. On increasing the temperature from 273 to 373 K, propane showed an increase of 146% in its flux, whereas there was only a 17% increase for methane. The extent of the effect of temperature is determined by the adsorption of the component on the zeolite. As observed from **Figure 8**, zeolite has a higher affinity towards methane compared to propane. Moreover, the influence of adsorption


**Table 2.** Kinetic diameter and molecular mass of various molecules found in off-gases [19].

**Figure 5.** Pore size distribution of zeolite membrane measured by N2 adsorption/desorption (**Table 4**).


**Table 3.** Elemental composition of the zeolite powder and the synthesised y-type zeolite membrane.

is greater than that of temperature. At elevated temperatures, it is likely that adsorption is negligible and the molecules exist in a quasi-gaseous state in the zeolite framework. Diffusion in this state is referred to as activated Knudsen diffusion or gas translational diffusion.

Selectivity is a measure of the ability of a membrane to separate two gases. It is used to determine the purity of the permeate gas and to determine the quantity of product lost. **Figure 8** shows that C3 H8 /CH4 selectivity increases from 0.3 at 293 K to 0.9 at 373 K. The higher temperature favours the separation of CH4 over C3 H8 . However, changes in temperature did not show much significant difference in the separation factors for the CO<sup>2</sup> /CH4 and N2 /CH4 . Moreover, for O2 /CH4 , separation is found to be more favourable at lower temperature (293 K).

gas and operating conditions are given in Section 5.6.6. The values calculated for the different

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51

It has been previously postulated that the linear proportionality of single gas permeance to the inverse of the square root of the molecular weight of the gases indicates that the mode of transport through the membrane is Knudsen diffusion [3]. **Figure 9** plots the relation between the molar gas flux and the inverse of the square root of the gas molecular weight at 1*x*10<sup>4</sup>

and 293 K. Based on this plot it can be deduced that the gas molar flux is dependent on the

Pa

binary gas pairs are listed in **Table 5**.

molecular weight as previously reported.

*6.3.3. Transport mechanism determination using gas permeation*

**Figure 7.** Flux of gases with increase in temperature at 1 × 105 Pa.

**Figure 6.** Flux of gases with increase in temperature at 1 × 104 pa.

#### *6.3.2. Mixed gas permeation using gas chromatograph mass spectrometer (GCMS)*

The selectivity of mixed gases was determined by the measure of the concentration of feed and permeate gases through the GCMS using Eq. (7). Details of the GCMS column, carrier Design and Evaluation of Gas Transport through a Zeolite Membrane on an Alumina Support http://dx.doi.org/10.5772/intechopen.75545 51

**Figure 6.** Flux of gases with increase in temperature at 1 × 104 pa.

**Figure 7.** Flux of gases with increase in temperature at 1 × 105 Pa.

is greater than that of temperature. At elevated temperatures, it is likely that adsorption is negligible and the molecules exist in a quasi-gaseous state in the zeolite framework. Diffusion in this state is referred to as activated Knudsen diffusion or gas translational diffusion.

Selectivity is a measure of the ability of a membrane to separate two gases. It is used to determine the purity of the permeate gas and to determine the quantity of product lost. **Figure 8**

, separation is found to be more favourable at lower temperature (293 K).

The selectivity of mixed gases was determined by the measure of the concentration of feed and permeate gases through the GCMS using Eq. (7). Details of the GCMS column, carrier

over C3

**Element Zeolite powder weight (%) Synthesised y-type zeolite membrane weight (%)**

**Table 3.** Elemental composition of the zeolite powder and the synthesised y-type zeolite membrane.

*6.3.2. Mixed gas permeation using gas chromatograph mass spectrometer (GCMS)*

much significant difference in the separation factors for the CO<sup>2</sup>

**Figure 5.** Pore size distribution of zeolite membrane measured by N2

C 53.72 3.82 Al 34.27 3.11 O 138.04 53.19 Si 37.05 0.50 Ti — 60.63 Na 30.46 —

50 Zeolites and Their Applications

H8

selectivity increases from 0.3 at 293 K to 0.9 at 373 K. The higher tempera-

. However, changes in temperature did not show

and N2

/CH4

. Moreover,

/CH4

adsorption/desorption (**Table 4**).

shows that C3

/CH4

for O2

H8 /CH4

ture favours the separation of CH4

gas and operating conditions are given in Section 5.6.6. The values calculated for the different binary gas pairs are listed in **Table 5**.

#### *6.3.3. Transport mechanism determination using gas permeation*

It has been previously postulated that the linear proportionality of single gas permeance to the inverse of the square root of the molecular weight of the gases indicates that the mode of transport through the membrane is Knudsen diffusion [3]. **Figure 9** plots the relation between the molar gas flux and the inverse of the square root of the gas molecular weight at 1*x*10<sup>4</sup> Pa and 293 K. Based on this plot it can be deduced that the gas molar flux is dependent on the molecular weight as previously reported.

**Figure 8.** Separation factor of gases with increasing temperature.


**Table 4.** BET surface area, average pore diameter and pore volume of the membrane.


**Table 5.** Selectivity of methane through a zeolite membrane at 293 K.

The order of molecular weight is CH4 > O2 > N2 > CO2 > C3 H8 . However, the R2 value of 0.807 suggests there is a deviation from Knudsen flow mechanism. CO<sup>2</sup> and C3 H8 have a similar molecular weight of 44.01 g/mol but the molar flux of CO<sup>2</sup> is greater than that of C3 H8 , this could be explained by molecular sieving flow mechanism, as the kinetic diameter of CO2 (0.38 nm) is lower than that of C3 H8 (0.43 nm). **Figure 10** shows the relation between the molar flux and the kinetic diameter of the gases at 1*x*10<sup>4</sup> Pa and 293 K. For gases to flow via a molecular sieving mechanism, the smaller molecules must move with a higher molar flux than the larger molecules. There was a deviation to this mechanism, as the order of kinetic diameter is O2 > N2 > CH4 > CO2 > C3 H8 . Moreover, CO2 and C3 H8 are observed to permeate through the membrane layer based on their size as C3 H8 has higher size as compared to CO2 .

**7. Conclusions**

An evaluation of the performance of y-type zeolite/γ-alumina membrane for natural gas processing has been carried out for separation ability. The transport of gases through the mem-

Design and Evaluation of Gas Transport through a Zeolite Membrane on an Alumina Support

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53

and C3

adsorption/desorption showed that

H8 have

brane has been shown to be governed by Knudsen diffusion. However, CO<sup>2</sup>

been shown to exhibit a molecular sieving mechanism. N2

**Figure 10.** Gas molar flux against kinetic diameter of gases.

**Figure 9.** Gas molar flux against the inverse square root of molecular weight.

Design and Evaluation of Gas Transport through a Zeolite Membrane on an Alumina Support http://dx.doi.org/10.5772/intechopen.75545 53

**Figure 9.** Gas molar flux against the inverse square root of molecular weight.

**Figure 10.** Gas molar flux against kinetic diameter of gases.

#### **7. Conclusions**

The order of molecular weight is CH4 > O2 > N2 > CO2 > C3

**Table 5.** Selectivity of methane through a zeolite membrane at 293 K.

**Membrane BET surface area (m2**

**Figure 8.** Separation factor of gases with increasing temperature.

(0.38 nm) is lower than that of C3

the order of kinetic diameter is O2 > N2 > CH4 > CO2 > C3

.

C3 H8

eter of CO2

293 k (mixed gases) 293 k (single gases)

52 Zeolites and Their Applications

size as compared to CO2

0.807 suggests there is a deviation from Knudsen flow mechanism. CO<sup>2</sup>

y-type zeolite membrane 0.106 3.139 0.025

**CH4 /CO2 (1.65)**

1.3 1.1

**Table 4.** BET surface area, average pore diameter and pore volume of the membrane.

between the molar flux and the kinetic diameter of the gases at 1*x*10<sup>4</sup>

observed to permeate through the membrane layer based on their size as C3

, this could be explained by molecular sieving flow mechanism, as the kinetic diam-

H8

gases to flow via a molecular sieving mechanism, the smaller molecules must move with a higher molar flux than the larger molecules. There was a deviation to this mechanism, as

similar molecular weight of 44.01 g/mol but the molar flux of CO<sup>2</sup>

H8

**CH4 /N2 (1.32)**

1.8 1.6

**/g) Pore diameter (nm) Pore volume (cm3**

H8

. However, the R2

**CH4 /C3 H8**

**(1.65)**

2.5 3.1

(0.43 nm). **Figure 10** shows the relation

. Moreover, CO2

and C3

is greater than that of

value of

**/g)**

have a

H8

Pa and 293 K. For

and C3

H8

H8 are

has higher

An evaluation of the performance of y-type zeolite/γ-alumina membrane for natural gas processing has been carried out for separation ability. The transport of gases through the membrane has been shown to be governed by Knudsen diffusion. However, CO<sup>2</sup> and C3 H8 have been shown to exhibit a molecular sieving mechanism. N2 adsorption/desorption showed that at a lower surface area of 0.106 m2 /g, the membrane is more effective at the separation of methane compared to the support. The SEM images revealed asymmetric structure deposition of the zeolite layer. Further studies are planned to demonstrate membrane performance for separating the heavier components of natural gas mixtures that can arise during dew point adjustments, thermal problems during crude oil storage and transportation, and when expanding highly compressed natural gas components.

[11] Kusakabe K, Kuroda T, Murata A, Morooka S. Formation of a Y-type zeolite membrane on a porous α-alumina tube for gas separation. Industrial & Engineering Chemistry

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55

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### **Author details**

Habiba Shehu, Edidiong Okon, Ifeyinwa Orakwe and Edward Gobina\*

\*Address all correspondence to: e.gobina@rgu.ac.uk

Center for Process Integration and Membrane Technology, Robert Gordon University, Aberdeen, United Kingdom

### **References**


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at a lower surface area of 0.106 m2

**Author details**

54 Zeolites and Their Applications

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Aberdeen, United Kingdom

expanding highly compressed natural gas components.

\*Address all correspondence to: e.gobina@rgu.ac.uk

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Habiba Shehu, Edidiong Okon, Ifeyinwa Orakwe and Edward Gobina\*

Center for Process Integration and Membrane Technology, Robert Gordon University,

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. Industrial & Engineering Chemistry Research.

O3

/g, the membrane is more effective at the separation of

methane compared to the support. The SEM images revealed asymmetric structure deposition of the zeolite layer. Further studies are planned to demonstrate membrane performance for separating the heavier components of natural gas mixtures that can arise during dew point adjustments, thermal problems during crude oil storage and transportation, and when


**Chapter 4**

**Provisional chapter**

**Separation of Binary Solutions on the Basis of Zeolites**

In this chapter, the author analyzed binary systems, ethanol + water, methanol + water and benzene + water, and an original mathematical model allowing the determination of the complete adsorption of binary systems on KaA, CaA, CaX, NaA and NaX zeolites using the Gibbs adsorption theory is proposed. The Gibbs equation and the Gibbs-Duhem equation have a number of limitations and do not take into account the properties of the investigated zeolites. Therefore, it is necessary to use the equations obtained by the author as a result of laboratory research, for the theoretical calculation and development

**Keywords:** zeolites, binary solutions, molecular sieve properties, Van der Waals forces,

The study of the processes occurring at the phase interface attracts many researchers from all over the world, and they have enormous practical and theoretical importance. A detailed

Considering these processes can be argued, the issues of adsorption of gas on adsorbents have

But the behavior of the adsorbed liquid on the surface of solid zeolite adsorbent is very difficult to describe and the explanation is very simple - the internal structure of the liquid is much more complex than the internal structure of gases and crystals. Comparing gas and liquid, it can be asserted that the density of the liquid is many times greater than the density of the gas. Considering and comparing the molecular level, the distance between molecules in liquids is

been studied quite deeply and have an extensive theoretical and experimental basis.

description of the processes occurring at the phase interface is given in [1].

**Separation of Binary Solutions on the Basis of Zeolites**

DOI: 10.5772/intechopen.73513

© 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,

© 2018 The Author(s). Licensee IntechOpen. 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.

and reproduction in any medium, provided the original work is properly cited.

Paranuk Arambiy, Saavedra Huayta Jose Angel and

of dehydration and concentration systems for alcohols.

Paranuk Arambiy, Saavedra Huayta Jose Angel and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73513

surface tension of the solution

Khrisonidi Vitaly

**Abstract**

**1. Introduction**

Khrisonidi Vitaly

**Provisional chapter**
