Mechanisms of initial propane activation on molybdenum oxides: a density functional theory study

We report the first detailed density functional theory study on the mechanisms of initial propane activation on molybdenum oxides. We consider 6 possible mechanisms of the C-H bond activation on metal oxides, leading to 17 transition states. We predict that hydrogen abstraction by terminal Mo=O is the most feasible reaction pathway. The calculated activation enthalpy and entropy are 32.3 kcal/mol and -28.6 cal/(mol/K), respectively, in reasonably good agreement with the corresponding experimental values (28.0 kcal/mol and -29.1 cal/(mol/K)). We find that activating the methylene C-H bond is 4.7 kcal/mol more favorable than activating the methyl C-H bond. This regioselectivity is correlated with the difference in strength between a methylene C-H bond and a methyl C-H bond. Our calculations suggest that a combined effect from both the methylene and the methyl C-H bond cleavages leads to the experimentally observed overall kinetic isotopic effects from propane to propylene on the MoO(x)/ZrO(2) catalysts.