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 with different amounts of TiO2 has been studied. X-ray photoelectron spectroscopy of the resulting V2O5/TiO2/SiO2 showed that the highest dispersion was reached for the low TiO2 coverages. The catalysts were prepared by depositing vanadia in a quantity equivalent to one monolayer onto SiO2, TiO2, or TiO2-coated SiO2 samples, by impregnation with a vanadyl acetylacetonate solution in ethanol, drying at 348 K, and calcining at 773 K in air. This procedure decreased the surface area by up to 30%. TPR profiles of binary and ternary samples were similar, showing only a single peak, suggesting the presence of just one vanadium species. Electron spin resonance spectra of partially reduced TiO2-coated silica carriers showed that, at very low titania content, titanium ions were present in the silica surface in small clusters. In the high content sample, TiO2 covered the silica surface, as in a crystalline TiO2 phase. The characteristics of supported vanadium oxide were affected by the TiO2 distribution. These V/xTiSi catalysts were found to be active for the selective oxidation of ethanol. The titania coating of the support increased the activity of the supported vanadia as compared to that of the oxide supported on silica, but maintained a similar product distribution. This increase of activity depended on the number of titania monolayers (x), being maximum for x greater than or equal to 1. The selectivity to acetaldehyde also increased with x, being maximum for the same composition. The combination of the two factors allowed the attainment of acetaldehyde yields of 74% per pass at temperatures as low as 473 K. The modifications in the catalytic behavior are discussed in terms of the changes of the support surface and its interaction with the supported vanadia caused by the presence of the titania coating. (C) 1997 Academic Press.