Metastable fcc α-MoC1−x supported on HZSM5: preparation and catalytic performance for the non-oxidative conversion of methane to aromatic compounds
✍ Scribed by C Bouchy; I Schmidt; J.R Anderson; C.J.H Jacobsen; E.G Derouane; S.B Derouane-Abd Hamid
- Publisher
- Elsevier Science
- Year
- 2000
- Tongue
- English
- Weight
- 792 KB
- Volume
- 163
- Category
- Article
- ISSN
- 1381-1169
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✦ Synopsis
There exist two forms of molybdenum carbide: the stable hexagonally close packed (hcp) structure -Mo 2 C and the metastable face centered cubic (fcc) structure ␣-MoC 1-x . Bulk fcc ␣-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 ␣-MoC 1-x . However, zeolite-supported ␣-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 ␣-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 -Mo 2 C, the classical catalyst well described in the literature, and fcc ␣-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.