Deoxygenation of organic molecules is arguably the most important catalytic reaction necessary to enable large-scale, efficient biomass usage within our energy and chemical infrastructure. In this contribution we examine the fundamental mechanisms of hydrodeoxygenation (HDO) over reducible zinc oxide catalysts, and present experimental and theoretical evidence of a cooperative mechanism between a vacancy site, responsible for breaking the C-O bond, and a surface site, responsible for hydrogenating the product. We performed density functional theory calculations that suggest that a vacancy-free catalyst is relatively unreactive for C-O bond splitting, while the presence of an oxygen vacancy greatly improves the reaction energetics for this portion of the reaction. This suggests that the role of exogenous H<sub>2</sub> in this reaction is primarily to regenerate surface vacancies, not for the direct dehydrogenation itself. To experimentally investigate this hypothesis, we conducted catalysis in an H<sub>2</sub>-free environment, instead beginning with a reduced Zn catalyst; that is, in a looped-oxide catalysis (LOC) process. Experiments comparing HDO and LOC showed a striking agreement in deoxygenated products (for carboxylic acids feeds); since vacancies must be the reactive sites in LOC, this directly suggests that the same sites are active for C-O bond breaking in HDO. In that context, metallic zinc with numerous vacant sites on the surface showed a high activity in promoting the deoxygenation reaction, while the oxidized zinc without any oxygen vacancy was fully deactivated. We argue that by making use of two very different reactive sites, this reaction can proceed with much higher efficiency than a uniform site-type catalyst would allow.

Cooperative defect/surface mechanism in deoxygenation reactions over reducible metal oxides

254th National Meeting (2017)

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