The selective oxidation of light alkanes is an attractive alternative to produce value-added chemicals such as maleic acid and acrylic acid since they are cheaper and more abundant feedstocks than the corresponding alkenes. The selective oxidation of alkanes, however, presents a very difficult challenge since the conditions that are required to activate the relatively inert alkane also promote its total oxidation to carbon-dioxide. Molybdenum-based heteropolyacids are known to actively catalyze different selective oxidation reactions where the addition of V and Nb greatly improve the activity and selectivity of these reactions. [1] The temperatures and conditions of operation however tend to lead to a breakdown in the primary Keggin structure into active amorphous nanoscale structures. Polyoxometalates possess unique molecular architecture that can be atomically tuned by changing the central atoms, the addendum atoms or the counter ions which can control redox as well as acid/base functionality and in addition may enhance stability. Despite the tremendous number of experimental investigations on mixed metal oxide systems, there have been relatively few theoretical analyses.

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