Selective oxidation of o-xylene over ternary V-Ti-Si catalysts Original Research Article

The selective oxidation of o-xylene to phthalic anhydride (PA) has been studied on a series of ternary V-Ti-Si oxides. Fresh and used catalysts, containing one monolayer of V2O5 on TiO2-SiO2 supports with 0.5, 1, 2 and 3 monolayers of TiO2 on SiO2, were characterised by Raman spectroscopy, X-ray diffraction (XRD) and temperature-programmed reduction (TPR). The characteristics and catalytic behaviour of these catalysts were compared with those of catalysts containing one monolayer of V2O5 supported on SiO2 and on TiO2. In the ternary V-Ti-Si catalysts, the presence of V2O5 and TiO2 (anatase) was detected, TiO2 (rutile) being also found in those with two and three monolayers of TiO2. The formation of TiO2 (rutile) due to exposure to reaction conditions was not observed. A moderate rearrangement of surface vanadium oxide species into crystalline V2O5 is observed. In addition, the formation of V2O4 was detected by XRD in the used catalyst containing one monolayer of V2O5 on the support with one monolayer of TiO2 on silica. The TPR of used and fresh samples showed that the reduction of vanadium occurs during reaction, such reduction being partially reversible after reoxidation in air flow at high temperature. This partial reduction may favour the aggregation of dispersed vanadia species into crystalline V2O5.