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

Mechanistic details and product distributions for C3H8-O-2 reactions catalyzed by gas-phase NOx species are compared to reactions on solid V2O5 catalysts. C3H8 conversions are greatly enhanced by addition of small concentrations of NO to C3H8-O-2 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. NOx-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, O-2, 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 NOx-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. NOx 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.