Isotopic Tracer and Kinetic Studies of Oxidative Dehydrogenation Pathways on Vanadium Oxide Catalysts

Abstract

Kinetic analysis and isotopic tracer studies were used to identify elementary steps and their reversibility in the oxidative dehydrogenation of propane on VOx/ZrO2 catalysts with VOx surface densities between 1.6 and 6 VOx/nm2. Competitive reactions of C3H6 and CH313CH2CH3 showed that CO forms via secondary combustion of propene intermediates. CO2 formed via this reaction and also via the direct combustion of propane. Reactions of 18O2/C3H8 mixtures on supported V216O5 led to the preferential initial appearance of lattice 16O atoms in all oxygen-containing products, as expected if lattice oxygens were required for the activation of C–H bonds. Isotopically mixed O2 species were not detected during reactions of C3H8–18O2–16O2 reactant mixtures. Therefore, dissociative O2 chemisorption steps are irreversible. Similarly, C3H8–C3D8–O2 reactants undergo oxidative dehydrogenation without forming C3H8−xDx mixed isotopomers, suggesting that C–H bond activation steps are also irreversible. Normal kinetic isotopic effects (kC–H/kC–D=2.5) were measured for primary oxidative dehydrogenation reactions. Kinetic isotope effects were slightly lower for propane and propene combustion steps (1.7 and 2.2, respectively). These data are consistent with kinetically relevant steps involving the dissociation of C–H bonds in propane and propene. C3H6–D2O and C3D6–H2O cross exchange reactions occur readily during reaction; therefore, OH recombination steps are reversible and nearly equilibrated. These isotopic tracer results are consistent with a Mars–van Krevelen redox mechanism involving two lattice oxygens in irreversible C–H bond activation steps. The resulting alkyl species desorb as propene and the remaining O–H group recombines with neighboring OH groups to form water and reduced V centers. These reduced V centers reoxidize by irreversible dissociative chemisorption of O2. The application of pseudo-steady-state and reversibility assumptions leads to a complex kinetic rate expression that describes accurately the observed water inhibition effects and the kinetic orders in propane and oxygen when surface oxygen and OH groups are assumed to be the most abundant surface intermediates.