Synthesis and Application of Porous Kaolin-Based ZSM-5 in the Petrochemical Industry

*Ebrahim Mohiuddin, Yusuf Makarfi Isa, Masikana M. Mdleleni and David Key*

## **Abstract**

Zeolites are advanced chemical materials that play a significant role in many petrochemical applications. In recent years, research interest in improving and enhancing the effectiveness of ZSM-5 as a catalyst has grown immensely. In particular, finding cheaper, environmentally friendly alternative starting materials for the synthesis of ZSM-5 has gained much attention. Kaolin has been widely investigated as a zeolite precursor as it comprises the required constituents for an aluminosilicate zeolite material; ubiquitous nature and its benefit in synthesising zeolites are well known as an inexpensive way of obtaining catalysts. This chapter deals with the factors affecting ZSM-5 synthesis when utilising a kaolin precursor. The effects of kaolin crystallinity, kaolinite content and synthesis parameters on ZSM-5 formation and its physicochemical properties are discussed. The potential of kaolin-based ZSM-5 as an oligomerisation catalyst is investigated. Pure, crystalline ZSM-5 could be successfully synthesised from a kaolin precursor. Physicochemical properties such as morphology, porosity and acidity are affected by the kaolin precursor and optimum synthesis conditions are required for synthesis of ZSM-5 from particular kaolin. Kaolin-based ZSM-5 catalyst showed good activity and selectivity to valuable fuel range hydrocarbons.

**Keywords:** kaolin, ZSM-5, synthesis, oligomerisation, cracking

## **1. Introduction**

Access to a variety of energy sources has been fundamental in driving human development. Fossil fuels have been a major source of energy for mankind for more than 5000 years. Today, crude oil continues to be a significant contributor to the energy sector; it accounts for a large percentage of the world's energy consumption. The production of chemicals has also continued to play a pivotal role in our daily activities. Interestingly, the amount of chemicals produced and used for both domestic and industrial purposes is very much related to the growth in global population. However, energy sources such as crude oil are non-renewable sources of fuel and current estimations show that world oil supplies will be depleted in the next century. Apart from the uncertainties in crude oil reserves, a major cause for concern is the impact crude oil extraction and its refining has on the environment; the combustion of fossil fuels leads to a net increase in greenhouse gases (GHG)

leading to global warming. These concerns among others have led researchers in the recent past to explore alternative energy sources to the traditional crude oil for the production of fuels and petrochemicals. Various practises such as the use of efficient catalysts and augmented reactor technology are currently being employed towards ensuring that production technologies are eco-friendly and sustainable.

Nanoporous materials are a large class of materials which consist of either an organic or inorganic framework structure containing ordered porous networks. They are generally classified by having pores sizes less than 100 nanometres and may be subdivided into three categories i.e. Microporous (<2 nm), Mesoporous (2–50 nm) and Macroporous (>50 nm). Their ability to interact or discriminate molecules depending on size has granted them scientific and technological importance. Research interest in nanoporous materials continues to grow as researches attempt to understand structure–property relations and design materials tailored for certain applications. Dependent on the properties of the nanoporous materials, applications range from purification and separation, sorption and drug delivery to energy storage, solar and fuel cells as well as electronic and magnetic devices. Typical examples include activated carbon, metal organic frameworks, ceramics, various polymers, aerogels, silicates and zeolites to name a few. Zeolites are microporous aluminosilicate materials that possess a 3-dimensional pore structure and play a prominent role in the petrochemical industry as ion exchangers, adsorbents, in separation and catalysis [1–4]. Their shape selective properties permit control of product distribution in chemical reactions and as such have become indispensable catalysts in many petrochemical processes [5]. Of particular importance to the petrochemical industry is zeolite ZSM-5. ZSM-5 because of its unique channel structure, acidity, and hydrothermal stability has been used as a shape selective catalyst in isomerization, alkylation, oligomerisation and catalytic cracking reactions [6–9]. It is conventionally synthesised using chemical sources such as sodium silicate solutions or silica gels and aluminium salts as the starting materials. Commercial synthetic zeolites are preferred over their naturally occurring analogues due to higher purity and uniform particle size which makes them more suitable for scientific and industrial applications [10]. However, zeolite synthesis using conventional methods leads to large amounts of waste being produced and chemical sources may be expensive, leading to high costs of zeolite production which limit commercialisation and use in many industrial applications [11]. Recently there have been increased efforts to explore the use of more affordable, natural raw materials possessing the necessary requirements for the synthesis of zeolites. ZSM-5 has been synthesised from natural silica and alumina sources such as rice husk ash [12–14], expanded perlite [15], palygorskite [16], fly ash [17], and kaolinite [18–21]. The main drive to utilise these rich aluminosilicate minerals is their relative abundances, cost effectiveness and overall more environmentally friendly synthetic procedures. Although many natural minerals and manufacturing wastes have been utilised to synthesise a wide variety of zeolite structures, this chapter will focus on kaolin-based ZSM-5 synthesis and possible application in the petrochemical industry.

Kaolin is a white clay composed mainly of kaolinite, a hydrous aluminosilicate mineral containing silica and alumina in a 1:1 ratio as well as impurities such as quartz and mica. Kaolin may require beneficiation to remove impurities depending on its application. Due to its low Si/Al ratio it has been extensively use in the synthesis of low silica zeolites [22, 23]. High silica zeolites such as ZSM-5 have also been synthesised with the addition of supplementary silica sources as well as through dealumination of kaolin via acid treatment [24–26]. Of the extensive range of aluminosilicate minerals used as zeolite precursors, kaolin has been favoured due to its ubiquitous nature. However, the studies of kaolin from different areas are significant since kaolin varies in composition depending on its geological occurrence. The chemical

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*Synthesis and Application of Porous Kaolin-Based ZSM-5 in the Petrochemical Industry*

and composition can thus affect its subsequent chemical reactivity [27, 28].

compositions of materials affect their properties and variations in the kaolin structure

In this chapter the synthesis of kaolin-based ZSM-5 and the factors affecting synthesis are discussed. The work presented will focus on the synthesis of ZSM-5 using kaolin of South African origin. Most studies of kaolin derived ZSM-5 is performed on commercial kaolin. Only few have been done using raw kaolin. Chemical reactivities of kaolins obtained from different geological areas and the need to optimise synthesis conditions tailored to particular kaolin are highlighted. The effects of kaolinite content and synthesis parameters such as crystallisation time and temperature are discussed. The work is extended to include the effects of silica to alumina ratio on the physicochemical properties of ZSM-5 and is the main focus of this chapter. Furthermore, the application of kaolin-based ZSM-5 in important petrochemical processes such as the oligomerisation of olefins to fuel range hydrocarbons is evaluated.

In the search for cheaper and more environmentally friendly alternatives to chemical sources, much research has been conducted on the feasibility of kaolin. By converting kaolin to the more reactive metakaolin via thermal activation and subjecting it to hydrothermal treatment in a NaOH medium, zeolite is produced. The use of kaolin as a source of silica and alumina was reported by Barrer [29] after it was calcined between 700 and 1000°C to form metakaolin. However due to the variations in kaolin composition and structure its subsequent chemical reactivity may be affected. Synthesis of zeolites from kaolin is affected by factors such as degree of crystallinity of the kaolin [23], kaolin composition [30], mineralogical impurities [23, 31], calcination temperature of kaolin [32], specific surface area of kaolin [33], synthesis parameters such as crystallisation temperature, time [30] and

Many studies have been performed employing kaolin as the starting material for zeolite synthesis [34]. Investigations on the effects of kaolin crystallinity are contradictory as some researchers have shown that differences in the reaction kinetics of zeolite formation are observed for kaolin with different crystallinities or structural ordering [23] whereas others have reported that no significant differences were established when synthesising zeolites from kaolin of different crystallinity and reported that reactions of metakaolinites are independent of defects in the original crystal structure [22]. When two kaolins of South African origin from different geological areas i.e. Grahamstown (BK) and Fishoek (SK) were analysed it was shown that they differed in their structural order as well as composition and SK was more crystalline than BK [35]. The ZSM-5 synthesised using the two kaolin precursors resulted in differences in the crystallisation kinetics. The more disordered kaolin (BK) showed faster crystallisation kinetics than the more ordered SK. The physical and chemical properties of the reactive metakaolin of BK and SK were compared. The morphology obtained from SEM analysis shown that BK was composed of highly disordered loose kaolin plates compared to SK which possessed highly ordered stacked layers. The ordering remained even after calcination to form metakaolin. It was suggested that the highly disordered kaolin dissolved at a faster rate into the gel solution compared to SK in which the stacking layers were

*DOI: http://dx.doi.org/10.5772/intechopen.81375*

**2. Kaolin in zeolite synthesis**

silica alumina ratio.

*2.1.1 Kaolin crystallinity*

**2.1 Factors affecting kaolin-based zeolite synthesis**

#### *Synthesis and Application of Porous Kaolin-Based ZSM-5 in the Petrochemical Industry DOI: http://dx.doi.org/10.5772/intechopen.81375*

compositions of materials affect their properties and variations in the kaolin structure and composition can thus affect its subsequent chemical reactivity [27, 28].

In this chapter the synthesis of kaolin-based ZSM-5 and the factors affecting synthesis are discussed. The work presented will focus on the synthesis of ZSM-5 using kaolin of South African origin. Most studies of kaolin derived ZSM-5 is performed on commercial kaolin. Only few have been done using raw kaolin. Chemical reactivities of kaolins obtained from different geological areas and the need to optimise synthesis conditions tailored to particular kaolin are highlighted. The effects of kaolinite content and synthesis parameters such as crystallisation time and temperature are discussed. The work is extended to include the effects of silica to alumina ratio on the physicochemical properties of ZSM-5 and is the main focus of this chapter. Furthermore, the application of kaolin-based ZSM-5 in important petrochemical processes such as the oligomerisation of olefins to fuel range hydrocarbons is evaluated.
