Butylene catalytic cracking to propylene over a hierarchical HZSM-5 zeolite: Location of acid sites controlling the reaction pathway

Abstract

A hierarchical ZSM-5 zeolite was prepared by steam-assisted crystallization (SAC) method and then the acidity of zeolites was modified by using organic silane compounds (catalytic cracking of silane, or the CCS method). The obtained zeolite was finally applied in the process of butylene catalytic cracking to produce propylene. High yield of propylene, 40 wt%, was achieved and it was much higher than that achieved on other prepared zeolites. This rise in propylene yield is mainly ascribed to the specific location of framework aluminum of zeolite in crystal sphere. Compared with the ZSM-5 zeolite synthesized by hydrothermal method, the ZSM-5 zeolite prepared from SAC method shows more acid sites in straight and sinusoidal channels rather than the intersection of channels, which is beneficial to reinforcing the reaction pathway of producing propylene and simultaneously suppressing the main side reactions in butylene catalytic cracking. And since the external acid sites of zeolite were covered by using the CCS method, the main side reactions were further inhibited. These results also help to elucidate the catalytic behavior of acid sites in different locations of HZSM-5 in butylene catalytic cracking and better understand the processing of SAC method.