SELECTIVE OXIDATION OF ETHANOL TO ACETALDEHYDE ON V2O5 TIO2/SIO2 CATALYSTS - EFFECT OF TIO2-COATING OF THE SILICA SUPPORT/

The effect of coating the SiO2 support of a V2O5 oxidation catalyst wi th different amounts of TiO2 has been studied. X-ray photoelectron spe ctroscopy of the resulting V2O5/TiO2/SiO2 showed that the highest disp ersion was reached for the low TiO2 coverages. The catalysts were prep ared by depositing vanadia in a quantity equivalent to one monolayer o nto SiO2, TiO2, or TiO2-coated SiO2 samples, by impregnation with a va nadyl acetylacetonate solution in ethanol, drying at 348 K, and calcin ing at 773 K in air. This procedure decreased the surface area by up t o 30%. TPR profiles of binary and ternary samples were similar, showin g only a single peak, suggesting the presence of just one vanadium spe cies. Electron spin resonance spectra of partially reduced TiO2-coated silica carriers showed that, at very low titania content, titanium io ns were present in the silica surface in small clusters. In the high c ontent sample, TiO2 covered the silica surface, as in a crystalline Ti O2 phase. The characteristics of supported vanadium oxide were affecte d by the TiO2 distribution. These V/xTiSi catalysts were found to be a ctive for the selective oxidation of ethanol. The titania coating of t he support increased the activity of the supported vanadia as compared to that of the oxide supported on silica, but maintained a similar pr oduct distribution. This increase of activity depended on the number o f titania monolayers (x), being maximum for x greater than or equal to 1. The selectivity to acetaldehyde also increased with x, being maxim um for the same composition. The combination of the two factors allowe d the attainment of acetaldehyde yields of 74% per pass at temperature s as low as 473 K. The modifications in the catalytic behavior are dis cussed in terms of the changes of the support surface and its interact ion with the supported vanadia caused by the presence of the titania c oating. (C) 1997 Academic Press.