Reactions limited by C-H activation occur more rapidly for weaker C-H bonds. Dampening this sensitivity of activation rates to bond-strengths improves selectivity to products with weaker bonds than reactants. Such effects are analyzed here for selective C3H8 oxidative dehydrogenation (ODH), which remains challenging because secondary reactions of products involve activation of C-H bonds much weaker than that in alkanes. Recent studies have shown that boron nitride catalysts catalyze C3H8 ODH with high selectivity. Sustained performance of this catalyst has been been shown to require treatments that restore nitrogen content, suggesting that nitrogen atoms may be important for reactivity. This work shows that selectivity and kinetic trends analogous to boron nitride can be achieved in gas-phase reactions without solid catalysts via addition of small concentration of NOx. C3H8 conversions are enhanced by up to 35 times by addition of merely 0.005 kPa NO to C3H8-O2 mixtures (3-20 kPa each), suggesting that NO mediates generation of reactive species for facile C-H activation. These conversions exhibit C3H6 selectivity values among the highest reported at similar oxidative conditions because NOx mediated strong abstractors lower the preference to activate weak allylic C-H bonds in C3H6. The reactive species driving such conversion involve OH radicals based on reactions networks proposed in combustion chemistry. The rates increase with residence time and NO, O2 and H2O pressures and exhibit supra-linear dependence on C3H8 pressure with trends analogous to homogeneous systems involving H-abstraction by OH radicals, but significantly different from V2O5 catalysts that exhibit linear C3H8 pressure dependence and weak sensitivity to the other parameters. Effects of temperature on reaction branches involving activations of primary and secondary C3H8 and allylic C3H6 C-H bonds show activation energy differences much smaller in NOx mediated routes than on V2O5, consistent with density functional theory estimates of these differences for NOx-generated abstractors and V2O5 O-atoms, and with Hammond’s postulate that exothermic reactions for strong abstractors involve “early” transition states insensitive to the final states of cleaved C-H bond. NOx mediation enables efficient alkane activation providing high productivity and yields at moderate temperature, which can be of significant importance to chemical transformations requiring rate-limiting C-H activation.

NOx mediated enhancement of rates and alkene selectivity in gas-phase alkane-O2 reactions

2019 Fall National Meeting

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