Effect of promoters on o-xylene oxidation pathways reveals nature of selective sites on TiO2 supported vanadia

Abstract

The o-xylene oxidation pathways on TiO2 supported vanadium oxide catalysts are affected by Cs+ and Sb4+ promoters. Reaction kinetics change remarkably at 360 °C: while at lower temperatures the rate determining step is the C-H bond cleavage in xylene methyl groups, the catalyst reoxidation becomes kinetically limiting at higher temperatures. Promotion by Sb4+ is related to an increase in the availability of oxygen at high temperatures for the reoxidation of V sites. The presence of Cs+ enhances the reoxidation of V sites and increases the ratio of nucleophilic to electrophilic oxygen on the surface. Catalytic tests of benzene oxidation confirm the lack of electrophilic sites on Cs-promoted VOx/TiO2 catalyst. Combined evidence points to a key role of the bridging lattice oxygen in V-O-V species as active site for the activation of o-xylene and its selective transformation to phthalic anhydride.