Catalytic performance of surface silicon-rich ZSM-5 zeolites in the co-production of lower olefins and p-xylene from methanol

Surface silicon-rich ZSM-5 zeolites were prepared by surface chemical modification; their pore structure and acid properties were characterized by XRD, nitrogen sorption, TEM, NH₃-TPD and Py-FTIR spectroscopy. The catalytic performance of modified ZSM-5 zeolites in the conversion of methanol to p-xylene and lower olefins was investigated. The results show that the introduction of Zn in ZSM-5 can change part of the strong acid sites into the medium ones and increase the Zn-Lewis acid sites with dehydrogenation capacity, which can enhance the selectivity to ethene and propene. The modification with Mg can not only adjust the pore shape selectivity, but also increase the amount of Lewis acid sites, which is beneficial to the formation of p-xylene. Through multiple silicon depositions from different silicon sources, SiO₂ is uniformly deposited on the outer surface of modified ZSM-5 catalysts, which can modulate the acid properties and pore structure and thereby further improve the selectivity to p-xylene and ethene and propene. By using these modification approaches, the selectivity to p-xylene and ethene and propene reaches 61%, with 87.1% of p-xylene in the xylenes product, 97.8% of ethene in C₂ hydrocarbons, and 90.6% of propene in C₃ hydrocarbons.