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

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. The catalytic behaviour of ternary systems in the selective oxidation of o-xylene depends strongly on the characteristics of the support. An increase in the content of TiO2 leads to a decrease in the temperature required to achieve total conversion. The use of SiO2 as support causes formation of large amounts of undesirable C-8 products, whereas the presence of TiO2 deposited on SiO2 leads generally to the formation of lower amounts of such products. The formation of both CO2 and tar is strongly favoured on the support composed of two monolayers of TiO2 on SiO2. (C) 2002 Elsevier Science B,V. All rights reserved.