Structure and properties of vanadium oxide zirconia catalysts for propane oxidative dehydrogenation

The structure of vanadia species supported on zirconia depends on VOx surface density and on the temperature of catalyst oxidation pretreatments. X-ray diffraction and Raman and UV-visible spectroscopies show that supported VOx species form polyvanadate domains of increasing size and ultimately monolayers and clusters as vanadiun surface density increases. Initial propene selectivities in oxidative dehydrogenation of propane at 606 K increase with increasing VOx surface density and reach constant values of 80% at surface densities of about 2-3 VOx/nm(2). High selectivities to COx products at low surface densities and on bulk ZrV2O7 appear to be associated with exposed unselective V-O-Zr and Zr-O-Zr sites. Propane oxidative dehydrogenation rates increase initially as the size of polyvanadate domains increases with increasing VOx surface density. Oxidative dehydrogenation rates decreases eventually as the formation of V2O5 clusters at high surface densities leads to a decrease in the fraction of VOx exposed at cluster surfaces. The ratio of rate constants for propane oxidative dehydrogenation and propene combustion to COx remains constant throughout the entire range of VOx surface density (0.4-100 VOx/nm(2)), suggesting that primary oxidative dehydrogenation steps and secondary oxidation reactions of desired propene products require identical polyvanadate structures. (C) 1998 Academic Press.