Influence of Al Coordinates on Hierarchical Structure and T Atoms Redistribution during Base Leaching of ZSM-5

Al coordinates in ZSM-5 showed significant effects on hierarchical structure and T atoms redistribution during NaOH/TPAOH treatment and different catalytic performances in the methanol-to-propylene reaction. NaOH treatment of parent ZSM-5 with a few extra-framework Al sites mainly involved the desilication process, while an additional alumination process was observed when some Al³⁺ was added into the NaOH solution, which resulted in mesoporous crystals with intact outer shells. Dealumination-realumination and desilication processes occurred with ZSM-5 with distorted four-coordinate, five-coordinate, or six-coordinate Al sites. For TPAOH treatment, the desilication-recrystallization process occurred with parent ZSM-5 and resulted in hollow crystals with ultrasilica shells via Si redistribution, consequently 2.5 times the lifetime of parent ZSM-5 by restricting external coke deposition. By comparison, desilication, dealumination, and (Si, Al)-recystallization were observed for ZSM-5 with distorted four-coordinate Al sites, involving both Si and Al atoms redistribution processes resulting in abundant mesopores and a high-silica outer surface, which showed almost 5 times the lifetime of parent ZSM-5.