(602g) Investigation of Pore Accessibility and Connections to Selectivity in Alkane Oxidation on M1 Phase Oxide

M1 phase mixed oxides contain pores of similar accessible diameters as cross-sectional molecular diameters of linear alkanes (~0.4 nm, Fig.1a), and are well-known for high selectivity in oxidative conversion of ethane to ethylene and propane to acrylic acid. Our past experiments involving ethane/cyclohexane rate-ratios on M1 phase MoVTeNb oxide and DFT-based computations demonstrated that ethane predominantly activates inside the pores and established connections of high selectivity to van der Waals stabilization of C-H activation and hindrance to undesired C-O bond formations in molecules confined in pores. This work focuses on propane and propene with significantly different practical limitations and mechanistic details than ethane (due to the involvement of weak allylic C-H bonds in propene), and systematically investigates pore confinement effects in longer linear and branched alkanes.

Propane/cyclohexane rate-ratio on MoVTeNb oxide is nearly an order of magnitude higher than on oxides without micropores, suggesting significant pore confinement of propane (albeit smaller than ethane, which showed two orders of magnitude higher ratios). Fig.1 shows measured and DFT-derived rate-constant-ratios representing selectivity in parallel and sequential steps for propane-O₂ and propene-O₂ reactions as a function of reciprocal temperature. The rate-constant-ratio comparing C-O bond formation versus C-H activation in propane (k₂/k₁) is nearly an order or magnitude lower on MoVTeNbO than on VO_x/SiO₂—used as a non-microporous reference. The overall sequential oxidation of propene relative to propane C-H activation (k₃/k₁) is similar on both oxides, but C-O bond formation to allylic C-H activation ratio (k_3,2/k_3,1) is much less than unity on MoVTeNb (consistent with high acrolein and acrylic acid selectivity) and greater than unity on VO_x/SiO₂. These findings support the suppression of C-O bonds in the pores observed previously. Systematic decrease in alkane/cyclohexane rate-ratio with alkane length and branching as well as impacts on selectivity provide comprehensive insights on confinement effects in selective oxidation.