Roles of Hydroxyl Radicals in Zeolite Synthesis

Zeolites have been widely utilized in chemical industry as the most important solid catalysts because of their tunable acidities and shape selective catalysis. Synthesis of zeolites is usually performed via hydrothermal crystallization in a strong basic medium. Hydroxide ions(OH^-) are considered to catalyze the depolymerization and polymerization of the silicate and aluminate species. Recently, it was found that the hydroxyl radicals(•OH) existed in the hydrothermal synthesis system. By using physical or chemical methods such as ultraviolet irradiation and Fenton's reagent, •OH radicals can be introduced into the zeolite synthesis system, which remarkably accelerated the zeolite crystallization and promoted the formation of zeolites with higher SiO₂/Al₂O₃ ratio. Density functional theory calculations demonstrated that •OH radicals significantly facilitated the depolymerization of the aluminosilicate gel by breaking the Si-O-Si bonds and the polymerization of the aluminosilicate anions around the hydrated cation species by remaking the Si-O-Si bonds, thus speeding up the nucleation and favoring Si incorporation into the zeolite frameworks. This review summarizes the recent progress in the application of •OH radicals in the synthesis of zeolites, with an emphasis on the role and advantage of •OH radicals in zeolite crystallization. The future perspective of the •OH radical route for the synthesis of zeolites is pointed out.