α-TiP-Supported Vanadium Oxide Catalysts: Influence of Calcination Pretreatments on Structure and Performance for o-Xylene Oxidation

The changes induced by increasing the calcination temperature in vanadium oxide supported on α-Ti(HPO4)2H2O (α-TiP) are presented. The characterization on samples calcined at low temperature by TGA, DTA, XRD, XPS, and Raman spectroscopy showed that a mixture of poorly dispersed VOPO4 is present at the surface of α-TiP after calcination. At higher calcination temperatures vanadium does not spread over the surface. On the contrary, a cubic TiVxP2−xO7 solid solution is formed that induces the migration of V cations into the bulk Ti phosphate and the consequent phosphorous segregation to the surface. The conversion of o-xylene to phthalic anhydride is worse on the samples calcined at higher temperatures, and was ascribed to the drastic decrease in the specific surface. The drop in phthalic anhydride selectivity is related to the increase in P concentration at the surface due to the migration of P cations induced by the formation of the solid solution.