Preparation of Vanadium–Phosphorus Mixed Oxide (P/V=1) Catalysts and Their Application to Oxidation of Butane to Maleic Anhydride

Several vanadium–phosphorus mixed oxides with a P/V ratio=1 have been prepared and the relationships between the structure of catalyst and the catalytic function for the selective oxidation of butane to maleic anhydride were examined. As revealed from XRD, IR, XPS, and redox titration studies, five among them were presumably composed of crystalline single phases, that is, α-VOPO4, β-VOPO4, and (VO)2P2O7 as well as unknown phases X1 and X2. The valences of the vanadium of these catalysts were close to five, except for (VO)2P2O7 (the valence was four). These phases were prepared, respectively, by controlling the kind of precursors and the calcination conditions. The selectivity to maleic anhydride differed markedly among these catalysts, although the rate did not change much when it was normalized to surface area. The selectivity at 440 °C was less than 20% for α- and β-VOPO4, 40% for X1 and X2, and 70% for (VO)2P2O7. Amorphous phases having V4+ were much less selective than (VO)2P2O7. No structural changes of the catalysts after use were observed in the cases of α-, β-VOPO4, and (VO)2P2O7. Therefore, the effective phase for selective oxidation of butane to maleic anhydride was speculated to be a phase like (VO)2P2O7 in which the valence of vanadium is four and the structure is stable under the reaction conditions.