Green and low-cost synthesis of zeolites from kaolin: a promising technology or a delusion?

Aluminosilicate zeolites are indispensable in many branches of the chemical industry. Conventionally, they are synthesised by the hydrothermal method from pure sources of SiO2 and Al2O3, whose production is relatively expensive and associated with waste generation. Recently, there has been a growing interest in kaolin as a low-cost alternative natural raw material for zeolite synthesis. In this case, the synthesis route includes kaolin pretreatment steps. For the development of feasible technology, it is necessary to have insight into new and basic synthesis steps and prove that the kaolin-based process is really green and low-cost. These aspects are often silent in research papers, which makes one doubt the idea. We have conducted an extensive literature review to fill these gaps. Collected data has been systematised and put in an easy-to-compare form. Synthesis routes from the literature have been divided into separate steps (grinding and screening, purification, metakaolinisation, alkali fusion, dealumination, hydrothermal process) to elucidate their conditions, theoretical basis and mechanisms, research progress, effect on the overall process, and compliance with economic and environmental requirements. It has turned out that alkali fusion is impracticable, and dealumination is not industrially applicable economically and environmentally (additional SiO2 is a feasible alternative). The other steps are essential. As a result, we have found that the synthesis of only low-silica zeolites can be considered green and low-cost. So, involved researchers should focus on this direction. Also, the review provides in-depth details about hydrothermal synthesis and may become information support for any researchers studying zeolite synthesis. This review demonstrates that kaolin-based synthesis of zeolites is not always as green and low-cost as it may first appear. Here, we discuss both kaolin pretreatment methods and zeolite synthesis per se.