Synthesis of hierarchical GaZSM-5 zeolites by the HCl treatment method and their catalytic performance in methanol aromatization

Abstract

A series of ZSM-5 samples exhibiting isomorphic substitution with Ga and containing both micropores and mesopores were synthesized by a posttreatment method involving different concentrations of hydrochloric acid. Various techniques were used to analyse the posttreatment changes in pores and acidity of GaZSM-5. The effects of pores and acidity on the catalytic lifetime of GaZSM-5 zeolites and on the product distribution for MTA were investigated. Significant amounts of mesopores were introduced by HCl treatment, and the micropore mass transfer rate of the products of the MTA reaction increased as the selectivity for aromatics obviously increased. In addition, skeleton Ga and nonskeletal species were removed simultaneously by HCl treatment, and the strong acid density and weak acid density of GaZSM-5 were decreased significantly. The catalytic lifetimes of the modified samples were obviously extended, and the conversion rate remained at 100% after 30 h at a reaction temperature of 400 °C and a space velocity of 10 h⁻¹. The improvement of catalyst lifetime in the MTA reaction is due to the combination of decreased acidity and increased mesoporosity.