Influence of potassium on the catalytic properties of V2O5/TiO2 catalysts for toluene oxidation

Abstract

Monolayer-type vanadium catalysts, 2 wt % vanadium on TiO₂(Degussa P25), were prepared with potassium additions ranging from a potassium : vanadium atom ratio of 0.125 up to 10.1. The catalytic performance in the oxidation of toluene and of mixed oxidation products was investigated and correlated with features shown by X.r.d., ESCA and i.r. studies. Spectroscopic and activity results suggests predominant formation of monolayer vanadium with adsorbed potassium species at low K/V ratios. ESCA data indicate a very high dispersion of the major part of the vanadium in all samples. At K/V 1 small quantities of KVO₃ were detected, and at K/V = 10.1 the additional presence of external crystalline potassium compounds were found. Potassium in excess of the amount bonded to vanadium results in a reduced surface area and an increased anatase to rutile transformation. The activity for toluene oxidation decreases rapidly and linearly with potassium addition. It is < 2 % of the original rate at a K/V atom ratio of 0.4. At higher K/V ratios the further decrease is low. The activity changes may be explained by an increasing coverage of adsorbed potassium species, exerting both a geometric and an electrostatic effect. Selectivities also show great changes, but in a more complex manner. With increasing K/V ratio oxidative coupling products, and all acidic products, disappear whereas carbon oxide formation increases. The selectivity for benzaldehyde is much less affected. The selectivity changes may be explained by altered acid–base properties affecting the oxidation of products. For the formation of phthalic anhydride and anthraquinone different mechanisms involving base or acid catalysed oxidation of benzoylbenzoic acid, a proposed intermediate, are suggested.