C-H bond activation steps at lattice O-atoms typically limit rates of oxidative dehydrogenations of organic molecules on metal oxides. Reactivity descriptors for such activations are derived from interpretations of reactivity in terms of elementary steps and by benchmarking of experiments and computation on solids that retain known structures during catalysis. C-H activation transition states contain a nearly cleaved C-H bond and a nearly-formed surface O-H bond, making density functional theory derived C-H bond dissociation (BDE) of molecules and H-addition energies (HAE) for metal oxides the relevant descriptors of measured rates, when properly averaged over all types of C-H bonds and exposed lattice O-atoms.<br/>C-H activation energies determine maximum attainable selectivities and yields of desired oxidation products, which differ from reactants in their C-H bond strength. The prediction of these activation energies for the diverse C-H bonds in C1-C4 hydrocarbons and oxygenates using lattice O-atoms with a broad range of HAE using assumptions of linear scaling of their values with the C-H BDE the O-atom HAE omits critical interactions between organic radicals and surface OH groups that form at kinetically-relevant transition states and ignores deviations from linear scaling relations caused by the lateness of transition states, which vary with HAE and BDE. These effects are rigorously included here through crossing potential formalisms that account for the position of transition states along the reaction coordinate to give activation energies from HAE and BDE and from the energies of estimates of di-radical interactions at the product state, which differ markedly for allylic and non-allylic C-H bonds. The di-radical interactions have not been previously considered in descriptions of C-H activation reactivity; yet, they are required for accurate descriptions and, in fact, expand the yield limits otherwise predicted by linear scaling relations. These more complete descriptors provide essential guidance in the design of solids with surfaces that preferentially destabilize allylic radicals relative to non-allylic ones.

Reactivity and selectivity descriptors for oxidative dehydrogenation of small hydrocarbons and oxygenates on metal oxides

255th National Meeting (2018)

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