The use of TiO2 contaminated with phosphorus and potassium as a support for V2O5 catalysts in the selective oxidation of toluene

Abstract

The structure of V₂O₅–TiO₂ catalysts, both of the supported type and the fused mixed oxide type, has been investigated by a variety of physicochemical techniques and correlated with their activity and selectivity in the oxidation of toluene. Special emphasis is put on the influence of phosphorus and potassium contaminants of commercial TiO₂ preparations. These contaminants are mainly present in the TiO₂ surface and prevent the formation of ideal monolayer catalysts. There are no results indicating V–P–O phase formation, but unselective potassium vanadate crystallites could be observed at a combination of high potassium and Iow vanadium concentrations. For fused catalysts of high vanadium contents, no such compound formation appears to occur. All preparations of low vanadium concentrations have a much lower activity in toluene oxidation than those of higher concentration containing bulk vanadium oxide crystallites. The selectivity for benzaldehyde could, however, be as high as for the most active catalyst prepared by the decomposition of NH₄VO₃. Selectivity for oxidative coupling products appears with the presence of vanadium oxide crystallites in catalysts with higher vanadium concentrations. Activity and selectivity data for the bulk V₂O₅–TiO₂ catalysts emphasizes the importance of defects and reduced phases.