MECHANISM OF DEHYDROGENATION OF CYCLOHEXANE ON MOO3-AL2O3 CATALYSTS

The conversion of cyclohexane in the dehydrogenation reaction over MoO3 γ-Al2O3 catalysts, measured in a pulse reactor, increases with the metal oxide content. Up to 15 wt% MoO3 only dehydrogenation products are observed (cyclohexene and/or benzene) with cyclohexene being present in limited amounts only and completely absent on catalysts with more than 10% MoO3. Above 15 wt% MoO3 combustion products are formed at the expense of benzene. On oxidized catalysts the reaction proceeds through oxodehydrogenation: carbon oxides mainly stem from octahedral oxomolybdenum configurations, while formation of benzene is favored by a tetrahedral molybdenum stereogeometry. For reduced catalysts an alternative simple dehydrogenation scheme is required in which the catalytic activity is directly related to the d-electron configuration of the metal in its various valence states. The proposed mechanisms are sustained by observations on the influence of reductants [H2, CO), oxidants (O2, CO2) and water and by a study of the catalyst deactivation. © 1979.