The roles of acidity and structure of zeolite for catalyzing toluene alkylation with methanol to xylene

A variety of zeolites, including MOR, MCM-22, SAPO-34, SAPO-11, SAPO-5 and ZSM-5 with different SiO2/Al2O3 ratio, were used for catalyzing toluene alkylation with methanol to produce xylene. The catalytic activity of zeolite for the toluene alkylation is approximately proportional to the number of its mid-strength acidic sites, except for SAPO-34 with small channels and MOR with strong acidity. Moreover, the acidic sites for catalyzing toluene alkylation are weaker in strength than those for catalyzing toluene disproportionation, but slightly stronger than those for catalyzing the reaction of methanol to hydrocarbons. On the other hand, zeolites with 12-membered ring channels may lead to the further alkylation of xylene and the rapid deactivation by coking. Zeolites with 8-membered ring channels restrict toluene alkylation, only favorable for forming non-aromatic hydrocarbons. As a result, zeolites with 10-membered ring channels and mid-strength acidity show high catalytic selectivity and activity for methylation of toluene.