Acid-driven architecture of hierarchical porous ZSM–5 with high acidic quantity and its catalytic cracking performance

Abstract

Controllable synthesis of ZSM–5 zeolites with hierarchical pores and high acidities is an efficient way for cracking of naphtha to produce light olefins. Here, nanocrystalline ZSM-5 aggregates (NCs) with n(Si)/n(Al) of ∼18 were hydrothermally synthesized via acid hydrolysis of a silicon source for the cracking of n–pentane. Compared with conventional microporous ZSM–5 zeolite, NCs catalysts with preserved micropores and newly generated intercrystalline mesopores promote cracking conversion as well as selectivity of light olefins. Moreover, enhanced acidities with high Al concentration in zeolite benefit the conversion of n–pentane. Interestingly, it is found that with moderate amount of framework Fe, the strength of the Brønsted acid sites of the catalysts increases and Lewis acid sites are formed in zeolite, while maintaining the total acidities. Owing to the restricted bimolecular reaction of the olefins on the adjacent acid sites, the catalysts with an Al–Fe co–framework (NC–5Al–1Fe) affords increased yield of light olefins (ethylene and propylene) up to ∼40 wt% with enhanced catalytic stability after 16 h reaction at 600 °C.