Elucidation of the CH4 coupling mechanism to C2H4 over GaN catalysts under non-oxidative conditions

Abstract

The second C−H bond cleavage in methane to form methylene (CH2*) from methyl (CH3*) is the rate-limiting step for the direct non-oxidative methane coupling to ethylene over gallium-nitride catalysts. Herein, we use isotope labelled experiments, in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), 13C solid state-NMR, as well as kinetic modelling to elucidate the reaction mechanism. Our experiments showed that supported GaN/SBA-15 had more aliphatic surface intermediates (CH3* and CH2*) than unsupported GaN catalysts, which had higher amounts of aromatics intermediates and coke deposition. Isotope labelling experiments and the kinetic study confirmed that the most likely pathway is via the fast abstraction of the first H from CH4 to form methyl (CH3*) surface intermediates, followed by the slow second C−H bond cleavage to form methylene (CH2*) and subsequent coupling to ethylene. The intrinsic activation energy predicted by kinetic modelling was rather low at around 20 kJ mol−1.