Selective oxidation of o-xylene by monolayer V2O5–TiO2catalysts

Abstract

The reaction of VOCl3 in benzene with the hydroxyl groups at the surface of low-area (ca. 10 m2 g–1) anatase leads, after hydrolysis and calcination, to a monolayer of vanadium species having approximately the same density as a two-dimensional layer of V2O5. Drying the anatase at 100–150 °C apparently removes only physisorbed water, while drying at 200 °C causes partial dehydroxylation. The concentration of VOCl3 in benzene has been varied, the optimum vanadium content being given by 0.20 cm3 in 50 cm3 benzene for the treatment of 5 g anatase. The best temperature for the reaction is 75 °C and the best time is 5 h.Catalysts prepared in this way are active and selective for the oxidation of o-xylene to phthalic anhydride. Good catalysts contain 0.9–1.4 wt % vanadium as V2O5; they give >95 % conversion at 315 °C and selectivities exceeding 80 %. Activation energies are 100–140 kJ mol–1. Activity and selectivity improve with increasing vanadium content up to the limit of 1.4 wt % V2O5, and the best catalysts give yields of phthalic anhydride of 85–87 %, which can be further improved by the use of Rb and P promoters. The possible structure of the vanadium monolayer is discussed, but neither it nor subsequently deposited layers show the Raman line at 996 cm–1 which is characteristic of the VO bond.