Activity and selectivity in the oxidation of benzene on supported vanadium oxide catalysts

Abstract

The dependences of activity and selectivity in the oxidation of benzene on unsupported and supported V2O5 catalysts on the catalyst structure have been investigated. It was found that the reaction rate at various concentrations of O2 was proportional to the amount of V5+O species in the catalyst, indicating that surface VO is an active oxygen species for the reaction. The specific activity of surface VO species on unsupported V2O5 was the same as that of the fused catalyst, while the surface structures were markedly different from each other. This means that the oxidation of benzene on V2O5 is a structure-insensitive reaction. The activity of the V2O5/TiO2 and V2O5/Al2O3 catalysts was mainly determined by the number of surface VO species. The specific activity of surface VO species of V2O5/TiO2 was greater than that of unsupported V2O5 or V2O5/Al2O3, indicating the promoting effect of the TiO2 support on the activity of surface VO species. The selectivity to maleic anhydride was found to be determined only by the number of V2O5 layers on the support for both V2O5/TiO2 and V2O5/Al2O3 catalysts. When the number of V2O5 layers was 1 or 2, the selectivity to maleic anhydride was very low, while it increased markedly on increasing the number of V2O5 layers to 5 and attained a constant value above 5 layers. The change in the oxidation state of the catalyst did not affect the selectivity.