Transient kinetics of methane dehydrogenation and aromatisation: experiments and modelling

Isotopic transient experiments combined with in situ DRIFT spectroscopy and TAP experiments were used to study the methane dehydrogenation and aromatisation over a Mo/HZSM-5 catalyst. The stoichiometric reduction of both external and internal molybdenum oxide into carbide-like species was clearly observed in the early stage of the reaction. The active sites for the catalytic dimerization of methane into ethylene are in very low concentration and could be internal carbide-like clusters in tight interaction with the zeolite. The methane molecules are also strongly interacting with most of the OH groups in the zeolite via a reversible activation process leading to the OH/OD exchange. However this activation process is not directly related to the aromatisation process. A large reservoir of benzene precursors is also present in the zeolite, also strongly interacting with the OH groups. Benzene appears as a secondary product formed from ethylene. The aromatisation steps could occur directly on the acid sites of the zeolite, available after the intial reduction and clustering of the Mo ions.