Roles Of Enhancement Of C-H Activation And Diminution Of C-O Formation Within M1-Phase Pores In Propane Selective Oxidation

Propane and propene oxidations on M1 phase MoVTeNb mixed oxide catalysts exhibit relatively high selectivity to acrolein and acrylic acid. We probe the ability of the reactant molecules to access the catalytic sites inside the heptagonal pores of these oxides and analyze elementary steps that limit selectivity. Measured propane/cyclohexane activation rate ratios on MoVTeNbO are nearly an order of magnitude higher than non-microporous VOx/SiO2, which suggests significant contribution of M1 phase pores to propane activation because both molecules react via homologous rate-limiting C-H activation. Density functional theory suggests that desired C3H8 dehydrogenation and C3H6 allylic oxidation to acrolein and acrylic acid are limited by C-H activation steps, while less valuable oxygenates form via steps limited by C-O bond formation. Calculated activation barriers for C-O formation are invariably higher than C-H activation when these activations occur inside the pores, suggesting that reactions restricted within the pores are highly selective to desired products. These results demonstrate the role of pore confinement and a framework to assess selectivity limitation in hydrocarbon oxidations involving a complex network of sequential and parallel steps.