Selective C–H Bond Activation via NO_x-Mediated Generation of Strong H-Abstractors

Abstract

Mechanistic details and product distributions for C3H8–O2 reactions catalyzed by gas-phase NO x species are compared to reactions on solid V2O5 catalysts. C3H8 conversions are greatly enhanced by addition of small concentrations of NO to C3H8–O2 mixtures without solid catalysts because homogeneous catalytic redox cycles involving oxidation of NO to NO2, reduction by H-addition to form HONO and release of OH radicals facilitate abstraction of H-atoms from strong C–H bonds in C3H8. NO x -mediated conversions exhibit C3H6 selectivity values among the highest reported at similar oxidative conditions because OH radicals are strong abstractors that activate C–H bonds via early transition states that do not exhibit significant bond elongation, which dampens bond-strength sensitivity and the preference to activate weak allylic C–H bonds in C3H6 products over strong bonds in C3H8. C3H8 conversion 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. Temperature dependencies of rate-constant-ratios representing activation energy differences between primary and secondary C3H8 and allylic C3H6 C–H bonds in NO x -catalyzed routes are much weaker than V2O5, consistent with dampened bond-strength sensitivity that is also observed in density functional theory estimates of these differences for OH radicals. NO x mediation enables efficient alkane activation providing high productivity and yields at moderate temperatures, which is important for chemical transformations requiring rate-limiting C–H activation.

Why Boron Nitride is such a Selective Catalyst for the Oxidative Dehydrogenation of Propane

“…We conclude that optimal ODH catalysts should 1) generate reactive H-abstracting species that favor the abstraction of secondary CÀHb onds in propane,a nd 2) not interact strongly with intermediates and products to minimize fast consecutive surface oxidation steps.T hese conclusions are in line with studies by Iglesia et al exploring COH-mediated oxidation of methane, [34] and Deshlahra et al investigating NO x -mediated ODH reactions; [35] moreover,t hey reveal ageneral set of guidelines with which to pursue development of more selective catalysts.…”

Roles of Enhancement of C−H Activation and Diminution of C−O Formation Within M1‐Phase Pores in Propane Selective Oxidation

“…Mole balances for reactants and products show that selectivity at zero‐conversion ( ) and decrease in selectivity with increasing conversion ( exhibit the following dependence on ratios of rate constants ( , )(Eqs. [3] and [4]): [11b,36] …”

Balancing the Activity and Selectivity of Propane Oxidative Dehydrogenation on NiOOH (001) and (010)

“…6) show that the TS for breaking the allylic C-H bond (C-H bond in CH 3 group) of propylene occurs earlier along the reaction coordinate compared with those for the formation of propylene by dehydrogenation (Figs. 4, 5) and therefore has weaker radical-surface interactions at TS [64,65]. Based on these analyses, we predict that the NiOOH-based propane ODH catalysts will result in high selectivity and high propylene yield.…”