The influence of external acid strength of hierarchical ZSM-5 zeolites on n-heptane catalytic cracking

The hierarchical ZSM-5 zeolite is expected to enhance mass transfer performance and accessibility of acid sites. The acid sites distributed in different locations (mesopore/outer surface and micropore) play different roles in catalytic reactions. In this work, a series of hierarchical ZSM-5 zeolites with similar porous structures but different external acid strength was successfully prepared through chemical vapor deposition (CVD) method by adjusting the reaction temperature and treatment time. Then the n-heptane (the model compound of naphtha) catalytic cracking was employed as a model to test the catalytic performance. The results show that the external environment is an important place for primary cracking reaction and bimolecular reaction. The weakening of the acid strength is conducive to suppress the bimolecular reactions of light olefins. At the same time, the change of selectivity of key products indicates that the microporous pores are the main place to generate ethylene compared with external environment which lacks the confinement effect.