Herein we report on the use of ab initio density functional theoretical calculations to examine the activation of propane over the Keggin structure of phosphomolybdic acid. We explicitly examine the effects of vanadium and niobium substitution into the phosphomolybdic Keggin structure on the selective oxidation of propane. Our results indicate that propane activation proceeds with the homolytic cleavage of the C-H bond forming radicals. The propane molecule dissociatively chemisorbs at a single lattice oxygen site forming a propanol intermediate. This is followed by a proton transfer to a second lattice oxygen to form a propoxy species. This is consistent with experimental findings that secondary propane activation proceeds with a substantially lower barrier than primary propane activation. The results are also consistent with previous theoretical results carried out on simple vanadium cluster models.
The substitution of vanadium for molybdenum in the Keggin structure acts to reduce the barrier of alkane activation and thus leads to a more exothermic reaction energy. Incorporating vanadium into the Keggin unit increases the oxidizing power of the HPA molecule as it stabilizes the Lowest Unoccupied Molecular Orbital (LUMO). Alkane activation results in a two electron reduction of the Keggin unit; therefore the stabilization of the LUMO leads to more favorable reaction.
[1] Dillon CJ, Holles JH, Davis ME, Labinger JA, Catal. Today, 81 (2): 189-195, 2003