Methane Dehydrogenation and Coupling to Ethylene over a Mo/HZSM-5 Catalyst: A Density Functional Theory Study

Abstract

In the present paper, we report the density functional theory studies on the structure of the molybdenum active center in Mo/HZSM-5 zeolite catalysts and the reaction mechanisms of methane dehydrogenation and coupling to ethylene (MDHC). Three kinds of active center models, Mo(CH2)2CH3+, Mo2(CH2)42+, and Mo2(CH2)52+, were optimized for the carburized Mo species exchanged on the Brønsted acid sites in HZSM-5 zeolites. The entire catalytic cycle of MDHC was investigated, and the catalytic performances of three different active center models were compared. The catalytic cycle consists of four elementary steps: (1) dissociation of the methane C–H bond; (2) dehydrogenation and C–C coupling; (3) activation of the second methane molecule; (4) elimination of ethylene and molecular hydrogen. It was suggested that methane C–H bond dissociation occurs on the π-orbital of the Mo═CH2 double bond, and C–C coupling proceeds on d-orbitals of the Mo atom. Dehydrogenation is realized by rupture of two C–H bonds from the adjacent methyl groups on the Mo atom, which is the rate-determining step of whole MDHC reactions.