Isotopic Tracer Studies of Reaction Pathways for Propane Oxidative Dehydrogenation on Molybdenum Oxide Catalysts

Kinetic analysis and isotopic tracer studies were used to identify the elementary steps and their reversibility in the oxidative dehydrogenation of propane over ZrO2-supported MoOx catalysts. Competitive reactions of C3H6 and CH313CH2CH3 showed that propene is the most abundant primary product, and that CO and CO2 are formed via either secondary combustion of propene, or by direct combustion of propane. A mixture of C3H8 and C3D8 undergoes oxidative dehydrogenation without forming C3H8-xDx mixed isotopomers, suggesting that steps involving C−H bond activation are irreversible. Normal kinetic isotopic effects (kC-H/kC-D) were measured for propane dehydrogenation (2.3), propane combustion (1.6) and propene combustion (2.1). These data indicate that the kinetically relevant steps in propane dehydrogenation and propene combustion involve the dissociation of C−H bonds in the respective reactant. H−D exchange occurs readily between C3H6 and D2O or C3D6 and H2O, suggesting that OH recombination steps are reversible and quasi-equilibrated. Reactions of 18O2/C3H8 on supported Mo16Ox species lead to the preferential initial appearance of lattice 16O atoms in H2O, CO, and CO2, indicating that lattice oxygen is required for C−H bond activation and for the ultimate oxidation of the adsorbed products of this reaction. 18O16O was not detected during reactions of C3H8−18O2−16O2 mixtures, consistent with irreversible O2 dissociation steps. These isotopic tracer results are consistent with a Mars−van Krevelen redox mechanism in which two lattice oxygens participate in the irreversible activation of C−H bond in propane. The resulting alkyl species desorb as propene, and the remaining O−H group recombines with neighboring OH groups to form water and reduced Mo centers. The reduced Mo centers finally reoxidize by irreversible dissociative chemisorption of O2. The proposed reaction mechanism leads to a complex kinetic rate expression that accurately describes the observed dependences on the partial pressure of propane, oxygen, and water.