Gas-phase dehydration of glycerol to acrolein over Al2O3-, SiO2- and TiO2-supported Nb- and W-oxide catalysts
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Hypothetical monolayers loadings of niobium and tungsten oxide supported on alumina, silica and titania were investigated as catalysts for the dehydration of glycerol to produce acrolein. Besides activity measurements, to have information about the supported structures the catalysts were characterized by XRD, Raman spectroscopy, XPS, XANES and FTIR combined with adsorption of pyridine and CO2. Compared with silica showing the presence of crystalline Nb2O5 and WO3, alumina and titania gave better dispersion and formation of hydrated niobate and tungstate species. Deactivation of the catalysts occurred with time on stream. However, the stability was improved considerably by adding oxygen to the feed. A dependence of the selectivity to acrolein on the Brønsted acidity of the catalyst was observed. Under anaerobic conditions an alumina-supported mixed Nb-W-oxide catalyst gave a selectivity of more than 70% for glycerol conversions above 40%. Even better selectivity was obtained using a catalyst with tungsten oxide supported on titania, which gave 80% selectivity under aerobic conditions at high conversion.