Metastable fcc α-MoC1-x supported on HZSM5:: preparation and catalytic performance for the non-oxidative conversion of methane to aromatic compounds

There exist two forms of molybdenum carbide: the stable hexagonally close packed (hcp) structure beta -Mo2C and the metastable face centered cubic (fcc) structure alpha -MoC1-x. Bulk fee alpha -MoC1-x can be prepared by controlled preactivation in hydrogen or hydrogen/hydrocarbon mixtures, at moderate temperature (623 K), of MoO3. Fee structure molybdenum oxyhydride or molybdenum oxycarbide are initially formed, respectively, by a topotactic transformation and they can be further carburized to yield fee alpha -MoC1-x. However, zeolite-supported alpha -MoC1-x cannot be prepared by this route when hydrogen is used. probably as a result of MoO3-zeolite interactions or a too small size of the MoO3 crystals which facilitate the reduction of MoO3 to MoO2. Carbon stabilization, leading to the fee structure oxycarbide is necessary, which can be achieved by activating MoO3 with a hydrogen/n-butane mixture at 623 K. The latter is then easily carburized to yield fee alpha -MoC1-x. Molybdenum carbide species were identified to be active components in Mo-modified catalysts used for the direct dehydroaromatization of methane. The catalytic performance of HZSM5 modified by either hcp beta -Mo2C, the classical catalyst well described in the literature. and fee alpha -MoC1-x, the new catalytic system we prepared, have been compared. The latter shows superior performance: higher activity, higher selectivity to benzene, and higher stability as a function of time-on-stream. (C) 2000 Elsevier Science B.V. All rights reserved.