Factors controlling the selectivity of V2O5 supported catalysts in the oxidative dehydrogenation of propane

The surface properties of a series of V2O5 catalysts supported on different oxides (Al2O3, H-Na/Y zeolite, MgO, SiO2, TiO2 and ZrO2) were investigated by transmission electron microscopy and FTIR spectroscopy augmented by CO and NH3 adsorption. In the case of the V2O5/SiO2 system TEM images evidenced the presence of V2O5 crystallites, whereas such segregated phase was not observed for the other samples. VOx species resulted widely spread on the surface of Al2O3 H-Na/Y zeolite, MgO and SiO2, whereas on Tier and ZrO2 they are assembled in a layer covering almost completely the support, Furthermore, evidences for the presence in this layer of V-OH Bronsted acid sites close to the active centres were found. It is proposed that propene molecules primarily produced by oxydehydrogenation of propane can be adsorbed on this acid centres and then undergo an overoxidation by reaction with redox centres in the neighbourhood. This features could account for the low selectivity of V2O5/TiO2 and V2O5/ZrO2 catalysts.