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.