The use of biomass as a feedstock for the production of fuels and chemicals requires the selective removal of oxygen from biomass-derived molecules. One popular approach for deoxygenation is C-O hydrogenolysis, catalyzed by transition metals, in a process commonly referred to as hydrodeoxygenation (HDO). While HDO is often catalyzed by precious metals (e.g., Pt or Ru) or sulfides (e.g., CoMoS or NiMoS), the reaction can also proceed via what is hypothesized to be a reverse Mars-van-Krevelen mechanism over transition metal oxides (e.g., MoO3 or WO3). Here, we use the conversion of acrolein via HDO (to yield propene) or hydrogenation (to yield allyl alcohol and 1-propanol) as a model system to study the factors governing selective C-O bond cleavage in the presence of C=C bonds using WO3 as a catalyst. We use a combination of density functional theory calculations, reaction kinetics measurements, and microkinetic analysis to elucidate the elementary steps that lead to allyl alcohol, 1-propanol, and propene products. Under the reaction conditions applied here (623 K with 1 atm H2 pressure), the WO3 catalyst exists as a hydrogen bronze with an apparent stoichiometry of H1.29WO2.77. Oxygen vacancies on the surface of the bronze are hypothesized to be the active sites for both C-O hydrogenolysis and C=C hydrogenation. The microkinetic model constructed here includes reaction steps that lead to the formation of the active oxygen vacancy sites, and it accurately describes the results of gravimetric experiments that show the formation of the hydrogen bronze. Moreover, the model describes accurately the production of both propene (40% selectivity, balance hydrogenation products, at 623 K) and allyl alcohol and 1-propanol (100% selectivity at 325K). We will also present an analysis of the model indicating which elementary steps govern the rates of HDO and hydrogenation and, thereby, the observed selectivities.

Reaction kinetics analysis of acrolein hydrodeoxygenation over a WO3 catalyst

2019 Spring National Meeting

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