(560gl) Selective Oxidations of Alkanes Via Gas-Phase Catalytic Cycles of NOx in Alkane-O2-NO Mixtures

Leelavathi Annamalai, Yilang Liu, Prashant Deshlahra, Tufts University

Selective formation of alkenes and oxygenates via alkane activation in exothermic oxidative routes remains challenging because products undergo more facile secondary C-H bond activation or C-O bond formation than reactants. This work shows that small concentrations of NOx molecules efficiently catalyze gas-phase oxidative dehydrogenation of propane [1], and employs kinetic measurements and density functional theory (DFT) calculations to distinguish mechanistic details in NO_x mediated oxidative conversions from those in the conversions on V₂O₅ solid catalyst. The redox conversions of NOx in C₃H₈-O₂-NO mixtures generate unstable OH radicals that readily abstract C-H bonds in hydrocarbons, but do so with suppressed preference toward activating weaker C-H bonds. This suppression improves selectivity to the desired product propene because it contains much weaker C-H bonds than propane.

C₃H₈ conversions are enhanced by up to 35 times when merely 0.005 kPa NO is assed to C₃H₈-O₂ mixtures (3-20 kPa each), suggesting that NO acts as a catalyst and mediates the generation of reactive species for facile C-H activation. The C₃H₈ conversion rates increase strongly with increasing NO concentrations. These conversions exhibit C₃H₆ selectivity values among the highest reported at similar oxidative conditions because NO_x mediated reactive species lower the preference to activate weak allylic C-H bonds in C₃H₆. Such reactions also exhibit significant selectivity to C₂H₄, in addition to C₃H₆, and these selectivities decrease weakly with conversions, analogous to recent reports of propane activations on boron nitride catalysts.

Previous kinetic simulations for homogeneous CH₄-O₂-NO_x reactions suggest that NO₂ formed via NO oxidation facilitates pathways that generate OH radicals and re-form NO to complete NO_x catalytic cycles [2]. Here, the rates of C₃H₆ activation 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 C-H bonds in C₃H₈ and allylic C-H bonds in C₃H₆ show activation energy differences among strong and weak C-H bonds is much smaller in NO_x mediated routes than on V₂O₅. These effects are consistent with density functional theory estimates of these differences for NO_x­-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. DFT calculations are used to probe propane and propene activations on all plausible reactive species generated in NOx cycles and show that only abstractors as strong as OH radicals can lead to very early transition states and activation energy differences as strong as those derived from measured effects of temperature on selectivity.

NO_x 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. These findings defy the intuition that soft oxidants are needed for selective oxidative conversion of hydrocarbons and instead show the strong H-abstractor oxidants such as OH radicals are important in such conversion. The propene selectivity is further enhanced by higher O₂ concentrations in propane-O₂-NO mixtures, due to the enhancement of C-H activation over C-C cracking in propyl radicals, which contrasts past attempts to improve selectivity by lowering O₂ concentrations via staged feeding. Optimized reaction conditions lead to high reaction rates and point to opportunities for developing industrially relevant high-productivity selective hydrocarbon oxidation processes that may utilize NO_x from industrial flue gases instead of expensive solid catalysts.

References

1. Annamalai, Y. Liu and P. Deshlahra, "Selective C-H Bond Activation via NOx Mediated Generation of Strong H-Abstractors," Revised version submitted to ACS Catal..
2. Zalc, J. M.; Green, W. H.; Iglesia, E. NO_x-Mediated Homogeneous Pathways for the Synthesis of Formaldehyde from CH₄-O₂Ind. Eng. Chem. Res. 2006, 45, 2677-2688.