Effect of promoters for n-butane oxidation to maleic anhydride over vanadium-phosphorus-oxide catalysts: comparison with supported vanadia catalysts

The oxidation of n-butane to maleic anhydride was investigated over model N b-, Si-, Ti-, V-, and Zr-promoted bulk VPO and supported vanadia catalysts. The promoters were concentrated in the surface region of the bulk VPO cata lysts. For the supported vanadia catalysts, the vanadia phase was present a s a two-dimensional metal oxide overlayer on the different oxide supports ( TiO2, ZrO2, Nb2O5, Al2O3, and SiO2). No correlation was found between the e lectronegativity of the promoter or oxide support cation and the catalytic properties of these two catalytic systems. The maleic anhydride selectivity correlated with the Lewis acidity of the promoter cations and oxide suppor ts. Both promoted bulk VPO and supported vanadia catalysts containing surfa ce niobia species were the most active and selective to maleic anhydride. T hese findings suggest that the activation of n-butane on both the bulk and supported vanadia catalysts probably requires both surface redox and acid s ites, and that the acidity also plays an important role in controlling furt her kinetic steps of n-butane oxidation.