Oxidation of Toluene and o-Xylene on Ti Phosphate-Supported Vanadium Oxide Catalysts

Abstract

The oxidation of toluene and o-xylene on α-Ti(HPO4)2·H2O-supported vanadium oxide catalysts is reported. Vanadium is incorporated at different loadings by VOCl3 grafting and by vanadyl oxalate wet impregnation. Their catalytic properties and the results of the physico-chemical characterisation (Laser Raman, XRD, SEM, N2 adsorption, XPS, and thermal analysis) were compared with those of a conventional V2O5/TiO2 catalyst (with an 8 V2O5 wt% loading). It is shown that toluene oxidation is as selective in Ti-phosphate-based catalysts as in the conventional system, whereas o-xylene oxidation is less selective: a much lower phthalic anhydride yield is obtained. Vanadium uptake on Ti phosphate reaches a limit value when the grafting method is used: V2O5 loadings higher than 5 wt% cannot be surpassed. A vanadium oxide–Ti phosphate support interaction exists at V loadings lower than 5 V2O5 wt% as suggested by the fact that the same VOPO4-like environment is detected irrespective of the method used to incorporate V. Higher vanadium loadings by using wet impregnation result in the formation of larger V2O5 crystallites than in the case of the conventional V2O5/TiO2 catalyst. The lack of interaction at high V loadings leaves the support uncovered. The poorly covered Ti phosphate surface exposing acid surface sites must account for the lower o-xylene oxidation to phthalic anhydride.