A review on synthesis of zeolites from natural clay resources and waste ash: Recent approaches and progress

Zeolites, known as molecular sieves, are crystalline hydrated aluminosilicates of alkali earth cations. They have a three-dimensional structure that contains frameworks of [SiO4]4- and [AlO4]5- tetrahedral, which are connected to each other by oxygen atoms. Zeolites are utilized in several fields such as catalysis, environmental remediation, gas separation, biotechnological and medical applications. Despite the availability of naturally occurring zeolites, there is an increased interest in zeolite synthesis from primary and secondary sources since it allows the production of zeolitic structures with high purity, better ion exchange capacity, higher thermal resistance, and more uniform size. Furthermore, the utilization of low-cost precursors makes a significant contribution to reducing the process cost and mitigating important environmental problems. Among different zeolite synthesis methods, conventional hydrothermal and alkali-assisted hydrothermal methods have been frequently applied in the last decades due to their high efficiency and simplicity. This study systematically reviews the data for the synthesis of Zeolite A, X, Y, and ZSM-5 from different primary and secondary sources, including kaolin, diatomite, bentonite, coal fly ash and biomass ash. Thus, the specific objective of this review is to provide critical information on zeolite synthesis from these materials together with their applications in environmental remediation and gas separation.