Clay minerals can be used as raw materials for the production of various industrial products. However, most bentonite and kaolinite deposits contain a significant quantity of non-clay mineral impurities. These impurities often affect the quality of clay minerals for adsorption and catalytic application. Therefore, in order to be used as adsorbents and catalysts, those clay minerals need some beneficiation, activation processing to improve their properties or conversion to a porous materials. In this chapter, an overview of the current state, the properties, the beneficiation, activation as well as the conversion of bentonite, kaolinite… to porous materials such as pillared clays, zeolites and their intended applications were presented. In addition, in this review, the challenges and difficulty in the conversion of bentonite and kaolinite to porous materials were also discussed.
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Clays have attracted great attention for sustainable materials or environmental materials. Clay minerals such as bentonite and kaolinite have characteristics of high surface area, layered structure, chemical and mechanical stability, cation exchange ability, abundant in nature and low cost and can be used for many industrial applications. The structure and composition of these clays usually influence their physical and chemical properties and determine the applications of the clay minerals (Muray, 1991). Industrial uses of bentonite depend on quality and quantity of their smectites (especially montmorillonite content), as well as the type, valence and the amount of exchangeable cations (Önal & Sarikaya, 2007). Most bentonite contains a significant quantity of non-clay mineral impurities or other clay minerals, therefore, bentonite needs some beneficiation to the activation processing to improve their properties or conversion to a porous materials.
In order to increase the surface area and acidity of the clay minerals, as well as eliminate several mineral impurities, acid treatment process is needed. Treatments of clay minerals with inorganic acids of rather high concentration and usually at high temperature are known as acid activation. The acid treatment changes surface area and porous structure of the clays. Key parameters which influence the acid treatment process are: the present of other clay minerals and non-clay minerals, the chemical composition, the type of cations between the layers, the type of the acid, the activating temperature and time, acid to bentonite ratio and so on (Nguetnkam, Kamga, Villiéras, Ekodeck, Razafitianamaharavo, & Yvon, 2005;Bhattacharyya & Gupta, 2008;Amari, Chlendi, Gannouni, & Bellagi, 2010). However at high acid concentration, aluminum component in the framework can also be dissolved and leads to the collapse of the clay structure, resulting in the decrease of the surface area. So the activation reaction condition should vary depending on the treatment condition. The cations between the layers of montmorillonite could be exchanged with large polyoxocations which is then converted to oxide clusters upon calcination to form pillared clays. Zeolites are crystalline aluminosilicates that have uniform microporous system. Therefore this kind of clays can be used as adsorbents or catalyst supports. Most research papers described zeolites synthesized from pure and expensive chemicals containing organic aluminum and silicium (such as aluminum isopropoxide, tetraethyl orthosilicate TEOS) and organic templates (tetramethylammonium hydroxide TMAOH, tetramethylammonium bromide TMABr and so on) in alkaline media on certain conditions (Don, Huyen, Hong, 2013). Recently, kaolinite-based zeolites are reported using cheap natural materials compared to conventional system which are therefore more cost efficient (Johnson & Arshad, 2014).