Activity measurements and spectroscopic studies on the catalytic oxidation of toluene over vanadium oxides supported on silica–alumina COK84

Abstract

We describe a study in which a mixture of silica and alumina (Degussa Aerosil COK84) was impregnated with varying amounts of vanadium. These catalysts were investigated for their activities and selectivities in the oxidation of toluene. The catalytic properties were correlated with the features shown by X.r.d., ESCA and u.v.–visible and i.r. spectroscopies of adsorbed CO. The pure support produces benzene in initial selectivity exceeding 50% one acidic Al—O—Si sites formed during the preparation procedure. The benzene selectivity decreases with time owing to coking effects. Introduction of vanadium decreases the selectivity for benzene and simultaneously selectivity for benzaldehyde appears. The surface area decreases considerably with a loading of 10% V, which is due to a decrease in volume of pores <80 Å. I.r. spectra revealed that the initially introduced vanadium adsorbs on alumina sites. U.v.–visible data, especially at low vanadium loadings, were similar to those for V/alumina. Comparison between experimental and calculated activities indicated that the first 0.2% V is adsorbed exclusively on alumina. At 0.5% V loading, a vanadium species on silica is also detected by i.r. spectroscopy. This loading possesses the most active vanadium, i.e. large agglomerates on alumina. The activity per g V at higher loadings levels off owing to the greater amount of low-activity vanadium on silica. An increased benzaldehyde selectivity was found for the 2 and 10% V catalysts. On these catalysts V₂O₅ crystallites are present. This is evident from the X.r.d. results and the product pattern of the 10% V sample.