Bulk and surface properties of highly dispersed VOx/ZrO2, VOx/SiO2 and VOx/TiO2/SiO2 systems and their relevance for propane oxidation

Abstract

Highly dispersed vanadium-doped metal oxides such as VO_x/ZrO₂, VO_x/SiO₂ and VO_x/TiO₂/SiO₂ with vanadium contents between 0 and 25 mole% were prepared by special bulk preparation methods (coprecipitation and sol–gel, followed by freeze-drying). Bulk and surface properties of the obtained mixed oxide solid solutions were thoroughly investigated by different analytical methods (Raman and FTIR spectroscopy, TPD, H₂-TPR, oxygen isotope measurements etc.). Moreover, the catalytic behaviour of the oxides was studied for the example of the oxidative dehydrogenation (ODH) of propane to propylene. Independent of the preparation method, the catalytic behaviour of vanadium-doped ZrO₂ and TiO₂ phases is very similar. Both metal oxide solid solutions are very active in propane ODH whereas the catalytic activity of VO_x/SiO₂ is relatively low. On the other hand, the reduction of the catalytic activity is accompanied by an improved selectivity for the formation of propylene. The correlation between the catalytic activity and the acidity of the oxide systems is discussed. Oxidation experiments with ¹⁸O₂ clearly show that the ODH reaction occurs according to the Mars–van Krevelen mechanism.