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

Abstract

The structure of vanadia species supported on zirconia depends on VO{sub x} surface density and on the temperature of catalyst oxidation pretreatments. X-ray diffraction and Raman and UV-visible spectroscopies show that supported VO{sub x} species form polyvanadate domains of increasing size and ultimately monolayers and clusters as vanadium surface density increases. Initial propene selectivities in oxidative dehydrogenation of propane at 606 K increase with increasing VO{sub x} surface density and reach constant values of 80% at surface densities of about 2--3 VO{sub x}/nm{sup 2}. High selectivities to CO{sub x} products at low surface densities and on bulk ZrV{sub 2}O{sub 7} 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 VO{sub x} surface density. Oxidative dehydrogenation rates decreases eventually as the formation of V{sub 2}O{sub 5} clusters at high surface densities leads to a decrease in the fraction of VO{sub x} exposed at cluster surfaces. The ratio of rate constants for propane oxidative dehydrogenation and propene combustion to CO{sub x} remains constant throughout the entire range of VO{sub x} surface density (0.4--100 VO{sub x}/nm{sup 2}), suggesting that primary oxidative dehydrogenation steps and secondary oxidationmore » reactions of desired propene products require identical polyvanadate structures.« less