(139a) Methane Dehydro-aromatization (DHA) to Aromatic Liquids and Hydrogen by Supported MOx/ZSM-5 (M=V, Cr, Mo, W and Re) Catalysts: Structure-Activity Relationships

The structure and reactivity of the supported MOx/ZSM-5 catalysts for methane dehydro-aromatization (DHA) were investigated with the aid of in situ UV-vis spectroscopy and temperature programmed operando Raman-MS spectroscopy. The initial surface MO_x sites are dispersed as isolated surface metal oxides on the ZSM-5 support. The surface metal oxide sites undergo reduction and/or carburization during the methane DHA reaction to benzene and hydrogen. The supported CrO₃/ZSM-5 and V₂O₅/ZSM-5 catalysts only reduce to sub-oxides without forming carbides and are not selective to benzene and form only carbon monoxide and hydrogen. Supported Re₂O₇/ZSM-5 catalysts reduce to metallic Re nanoparticles that activate CH₄ for benzene and H₂ production. The supported WO₃/ZSM-5 catalysts only partially reduce and carburize to W-carbides, yielding selective and low activity catalysts for DHA of methane to benzene. The supported MoO₃/ZSM-5 catalysts are the most efficient catalysts for CH₄ DHA because of the complete reduction and carburization of the MoO_x sites to catalytic active MoO_xC_y clusters. These dual site catalysts activate CH₄ to C₂H_x hydrocarbon intermediates on the metal carbide/metal sites and oligomerization of C₂H_x hydrocarbons to aromatics takes place on the Brønsted acid sites of the ZSM-5 support. Consequently, efficient catalysts must possess both sites for activating CH₄ to C₂H_x and sites for oligomerizing C₂H_x to aromatics.