Synthesis of hierarchical GaZSM-5 zeolites by a post-treatment method and their catalytic conversion of methanol to olefins

A number of hierarchical GaZSM-5 zeolite molecular sieves with diverse amounts of mesoporosity and acidity were prepared by post-synthetic desilication with different concentrations of alkali. The zeolite structure and acidity have an important effect on the catalyst lifetime and the product distribution of the methanol to olefins (MTO) reaction, which have been investigated. The changes in pores and acidity by post-treatment were characterized by XRD, SEM, TEM, NH3-TPD, FTIR, Py-IR, etc. An alkali treatment of an appropriate concentration can increase the mesoporosity and reduce the strong acid density, which improves catalytic stability by suppressing coke formation. However, excessive alkali concentrations cause low crystallinity and excessive acidity despite a high mesopomsity, which is conducive to the formation of aromatic products and coking. The desilication significantly changes acidity of GaZSM-5 zeolites and affects the product selectivity; the selectivities of C-2 and C-3 olefin are consistent with the change of Lewis acid amount.