Mechanistic aspects of propane oxidation over Ni-Co-molybdate catalysts

Mechanistic aspects of propane oxidation over Ni0.5CO0.5MoO4 supported on SiO2 reveal, that the homolytic C-H bond breaking of the methylene hydrogen abstraction is the rate limiting step of the reaction, involving most probably an electron rich oxygen of a molybdenum located adjacent to the catalyst's cation (A---O-Mo), with the second hydrogen abstraction involving an oxygen of an adjacent molybdenum to the moiety of the first abstraction (A---O-Mo-O-Mo-O), where Ni comprises a more efficient A atom than Co, and where the transition state is relatively symmetric in nature. The reaction is first order in propane and zero order in oxygen, consistent with a Mars-van-Krevelen mechanism involving lattice oxygen of the catalyst. Because this catalyst produces propylene as the exclusive first formed product from propane at low conversions, and since propane and propylene compete for the metal oxide catalytic sites with similar effectiveness, the named paraffin activating catalyst or a modification therefrom show promise, when combined with known multicomponent Bi-molybdates efficient in olefin oxidation, for the direct conversion of propane to acrylic acid or in presence of ammonia to acrylonitrile.