Metastable fcc α-MoC 1− x supported on HZSM5: preparation and catalytic performance for the non-oxidative conversion of methane to aromatic compounds

Abstract

There exist two forms of molybdenum carbide: the stable hexagonally close packed (hcp) structure β-Mo 2C and the metastable face centered cubic (fcc) structure α-MoC 1− x . Bulk fcc α-MoC 1− x can be prepared by controlled preactivation in hydrogen or hydrogen/hydrocarbon mixtures, at moderate temperature (623 K), of MoO 3. Fcc structure molybdenum oxyhydride or molybdenum oxycarbide are initially formed, respectively, by a topotactic transformation and they can be further carburized to yield fcc α-MoC 1− x . However, zeolite-supported α-MoC 1− x cannot be prepared by this route when hydrogen is used, probably as a result of MoO 3-zeolite interactions or a too small size of the MoO 3 crystals which facilitate the reduction of MoO 3 to MoO 2. Carbon stabilization, leading to the fcc structure oxycarbide is necessary, which can be achieved by activating MoO 3 with a hydrogen/ n-butane mixture at 623 K. The latter is then easily carburized to yield fcc α-MoC 1− 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 β-Mo 2C, the classical catalyst well described in the literature, and fcc α-MoC 1− 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.