Abstract:

MgO-modified ZSM-5 was prepared by impregnation of ZSM-5 with an aqueous solution of magnesium nitrate. Selective synthesis of p-xylene by alkylation of toluene with dimethyl carbonate（DMC）was carried out in a continuous flow fixed-bed rector over MgO-modified ZSM-5 zeolite. Physico-chemical properties of MgO/ZSM-5 catalysts were characterized by means of XRD，NH₃-TPD，2,4-dimethylquinoline-IR and pyridine-IR.The results of experiments indicated that alkylation of toluene with DMC occurred mainly on Brønsted acid sites. With increasing MgO content，there was a decrease in toluene conversion and increase in selectivity for p-xylene. With further increase in MgO content from 9% to 12%，no significant change in toluene conversion and selectivity for p-xylene was observed. The results of FT-IR characterization with 2,4- dimethylquinoline adsorption indicated that the MgO was only dispersed over the external surface of ZSM-5 zeolite. With increasing MgO content，the amount of Brønsted acid sites decreased. When the amount of MgO was >9%，Brønsted acid sites on the external surface almost disappeared.