Kinetics and Mechanism of Oxidative Dehydrogenation of Propane on Vanadium, Molybdenum, and Tungsten Oxides

The effect of cation identity on oxidative dehydrogenation (ODH) pathways was examined using two-dimensional VOx, MoOx, and WOx structures supported on ZrO2. The similar kinetic rate expressions obtained on MoOx and VOx catalysts confirmed that oxidative dehydrogenation of propane occurs via similar pathways, which involve rate-determining C−H bond activation steps using lattice oxygen atoms. The activation energies for propane dehydrogenation and for propene combustion increase in the sequence VOx/ZrO2 < MoOx/ZrO2 < WOx/ZrO2; the corresponding reaction rates decrease in this sequence, suggesting that turnover rates reflect C−H bond cleavage activation energies, which are in turn influenced by the reducibility of these metal oxides. Propane ODH activation energies are higher than for propene combustion. This leads to an increase in maximum alkene yields and in the ratio of rate constants for propane ODH and propene combustion as temperature increases. This difference in activation energy (48−61 kJ/mol) between propane ODH and propene combustion is larger than between bond dissociation enthalpies for the weakest C−H bond in propane and propene (40 kJ/mol) and it increases in the sequence VOx/ZrO2 < MoOx/ZrO2 < WOx/ZrO2. These results suggest that relative propane ODH and propene combustion rates depend not only on C−H bond energy differences but also on the adsorption enthalpies for propene and propane, which reflect the Lewis acidity of cations involved in π bonding of alkenes on oxide surfaces. The observed difference in activation energies between propane ODH and propene combustion increases as the Lewis acidity of the cations increases (V5+ < Mo6+ < W6+).