Isotopic tracer and kinetic studies of oxidative dehydrogenation pathways on vanadium oxide catalysts

Kinetic analysis and isotopic tracer studies mere used to identify elementa ry steps and their reversibility in the oxidative dehydrogenation of propan e on VOx/ZrO2 catalysts with VOx surface densities between 1.6 and 6 VOx/nm (2). Competitive reactions of C3H6 and (CH3CH2CH3)-C-13 showed that CO form s via secondary combustion of propene intermediates. CO2 formed via this re action and also via the direct combustion of propane. Reactions of O-18(2)/ C3H8 mixtures on supported (V2O5)-O-16 led to the preferential initial appe arance of lattice O-16 atoms in all oxygen-containing products, as expected if lattice oxygens were required for the activation of C-H bonds. Isotopic ally mixed Oz species were not detected during reactions of C3H8-O-18(2)-O- 16(2) reactant mixtures. Therefore, dissociative Oz chemisorption steps are irreversible. Similarly, C3H8-C3D8-O-2 reactants undergo oxidative dehydro genation without forming C3H8-xDx mixed isotopomers, suggesting that C-H bo nd activation steps are also irreversible. Normal kinetic isotopic effects (k(C-H)/k(C-D) = 2.5) were measured for primary oxidative dehydrogenation r eactions. Kinetic isotope effects were slightly lower for propane and prope ne 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 r eadily during reaction; therefore, OH recombination steps are reversible an d nearly equilibrated. These isotopic tracer results are consistent with a Mars-van Krevelen redox mechanism involving two lattice oxygens in irrevers ible C-H bond activation steps. The resulting alkyl species desorb as prope ne and the remaining O-H group recombines with neighboring OH groups to for m water and reduced V centers. These reduced V centers reoxidize by irrever sible dissociative chemisorption of O-2. The application of pseudo-steady-s tate and reversibility assumptions leads to a complex kinetic rate expressi on 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. (C) 1999 Academic Pr ess.