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Abstract: Selective synthesis of p-xylene by vapor phase alkylation of toluene with dimethyl carbonate, over MgO-modified MCM-22, was carried out to demonstrate the relationship between acidity and catalytic performance. Characterization results show that the number of Brønsted acid sites on MCM-22 decreased significantly after MgO modification, which resulted in a decrease in activity. Selectivity for p-xylene was improved greatly over MgO-modified MCM-22, which can be attributed to effective suppression of xylene isomerization by the reduction in external surface Brønsted acid sites. The additional Lewis acid sites generated after MgO modification had no apparent effect on the catalytic performance. [Copyright &y& Elsevier]

Abstract: A novel, shape-selective H-MCM-22/MCM-41 composite catalyst is synthesized for the first time by overgrowing MCM-41 over the external surface of H-MCM-22. Results of FT-IR characterization with 2,4-dimethylquinoline adsorption indicate that the Brønsted acidic sites of H-MCM-22 located on the external surface can virtually be completely covered by the generated layer of MCM-41. When applied to the synthesis of p-xylene by the alkylation of toluene with dimethyl carbonate, the H-MCM-22/MCM-41 composite exhibits significant enhancement in p-xylene selectivity, which can be ascribed to the decrease in the number of external Brønsted acid sites of the composite. [ABSTRACT FROM AUTHOR]