Promoting the alkylation of toluene to para-xylene over MCM-22 by the steric effects of trimethylsilyl groups

Owing to the low energy consumption and few by-products, toluene alkylation is regarded as a promising way to produce p-xylene. The zeolites with the characteristics of 10-MR were usually employed to catalyze the toluene alkylation after being modified. However, the catalyst activity is liable to sharp decrease by common modifying strategies. In this study, the alkylation of toluene with dimethyl carbonate was catalyzed by the MCM-22 zeolites with selective dealumination and silanization treatment. The Al sites on the MCM-22 external surface were first eliminated by oxalic acid, and then trimethylsilyl groups were grafted at the external surface of MCM-22 by bonding with Si–OH formed from dealumination. This modification strategy can not only reduce the Brønsted acid sites on the external surface and narrow the pore entrance size but also retain more internal active sites, leading to improving PX selectivity without a significant decrease in activity. Compared to parent MCM-22, the PX selectivity was doubled, and the toluene conversion only decreased from 39 to 34% over 200-DeMCM-22 catalyst at 400 °C.