The rate-limiting step in alkane activation is the breaking of the carbon-hydrogen bond. While oxides can catalyze C-H bond breaking, the yield of valuable products is too low for commercial application. We used density functional theory to study whether substitutional doping of the cations in an oxide surface, or in the subsurface, can increase the efficiency of breaking of the C-H bond of an alkane. Dopants whose valence is lower than that of the cations in the host oxide tend to activate the oxygen atoms in the oxide surface, which helps break the C-H bond. Dopants having higher valence adsorb oxygen and activate it, making it capable of reacting with an alkane. The calculations show that the effect of the dopant is not entirely controlled by the difference in the valence. Higher-valence dopants from the left-hand-side of the periodic table (Nb, Ta, Ti, Zr) are much more efficient in binding and activating oxygen than dopants of the same valence from the right-hand side of the periodic table (Ge, Sn, As, Sb). The calculations also show that the effect of a dopant can be described as a Lewis acid-base interaction and since this involves charge transfer, the effect of the dopant is long-ranged: dopants in the third layer from the surface can be as active as dopants in the top layer. Finally we present some experiments based on the idea that oxidative dehydrogenation of alkanes can be improved by using iodine (and perhaps bromine) as a catalyst. The iodine can be delivered in the gas phase or produced in situ by the reaction of oxygen with molten LiI or other iodides. It is still an open question whether the molten iodide is a catalyst or merely a source of iodine. References 1. Chemistry of Lewis acid-base pairs on oxide surfaces, Horia Metiu, Steeve Chrétien, Bo Li, Zhenpeng Hu, and Xiao Ying Sun, J. Phys. Chem. C 116, 10439-10450 (2012) 2. Catalysis by doped oxides, Eric W. McFarland and Horia Metiu, Chemical Reviews 113, 4391-4427 (2013)

Activation of the carbon-hydrogen bond by oxides and halides

251st National Meeting (2016)

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