Rates of oxidative conversion of organic molecules on metal oxide catalysts are often limited by C-H bond activation steps. Here, descriptors of C-H activation energies are derived from mechanistic interpretations of measured rates and benchmarking of experiments and computation on Polyoxometalate clusters that represent solids retaining their known structures during catalysis. The effects of these descriptors on selectivity to products containing weaker C-H bonds than reactants are derived for different systems including the example of strong non-allylic and weak allylic C-H bonds in propane and propene, respectively, which is relevant to energy efficient propene production via oxidative dehydrogenation.<br/><br/>C-H activation at lattice O-atoms are mediated by “late” transition states that exhibit near complete cleavage and formation of C-H and O-H bonds, respectively. As a result, density functional theory derived C-H bond dissociation energy (BDE) of molecules and H-addition energies (HAE) at lattice O-atoms represent relevant descriptors of rates, when properly averaged over all types of C-H bonds and lattice O-atoms. Linear scaling relations of transition states with HAE and BDE, however, do not predict accurate activation energies for diverse C-H bonds in C1-C4 hydrocarbons and oxygenates using lattice O-atoms with a broad range of HAE, because they omit interaction between organic radicals and surface O-H species that significantly affect stability of transition states and the dependence of the sensitivity of transition states energies to BDE and HAE on the lateness of transition states. Such effects are included through estimates of radical-surface interaction energies at the product state, which differ markedly for allylic and non-allylic C-H bonds along reaction coordinates, and use of crossing potential models that rigorously account for the lateness of transition states. The strength of radical-surface interactions depends strongly on the localization of unpaired electrons in the organic radicals and the surfaces with added H-atoms. Localized surface densities favor stronger interactions with localized alkyl radicals than delocalized allyl radicals and stronger H-abstractors weaken BDE dependence of activation energies, suggesting that strong and highly localized H-abstracting surfaces best compensate for weak product bonds to improve selectivity to products with weak allylic C-H bonds.

Effects of abstractor strengths, C-H bond strengths and radical-surface interactions on reactivity and selectivity in reactions limited by C-H activation on metal oxides

2019 Fall National Meeting

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