Modified alumina supports for catalytic oxidation of benzene to maleic anhydride over vanadia

Aluminas, intermediate in the transformation of γ to α-phase, have been tested as supports for vanadia in the oxidation of benzene to maleic anhydride. Treatment of γ-alumina in air or steam results in an initial rapid loss of half the original surface area corresponding to closing off of small pores with no development of bulk α-phase. However, ²⁷Al NMR measurements, and determinations of surface Lewis acidity using the dehydration of methanol to dimethyl ether as a test reaction, each show that conversion to the α arrangement is well advanced at the local and surface levels. Further reduction in area parallels formation of crystalline α-phase. Aluminas with surface areas of 20 to 40 m₂ g^-1 contain 30-60% α-phase and have very low surface acidity. Catalyst comprising 9% V₂O₅ on such supports are able to convert benzene to maleic anhydride with selectivities of up to 83%, in contrast to zero selectivity for comparison catalysts using γ-alumina supports. The activity and selectivity patterns with the partially transformed aluminas as supports can be rationalised in terms of an optimal average number of V₂O₅ layers of three to four. With less than this, some support is exposed and selectivity and activity per unit area of catalyst is degraded. Thicker layers lead to lower than optimal activity per unit weight of V₂O₅ and some loss of the favourable effect of an α-alumina surface on the selectivity of overlying V₂O₅ layers.