Depletion of fossil fuel combined with environmental and political concerns with fossil fuels have accelerated research on technologies to convert biomass into liquid fuels, i.e. biofuels. Aqueous-phase hydrogenation (APH) reactions are crucial for biofuel production, including ethanol production by hydrogenation of fermentation products (organic acids) [1], gasoline production by hydrogenation of pyrolysis oils [2], and alkane production by aqueous-phase dehydration/hydrogenation of carbohydrates [3]. APH reactions involve hydrogenating a range of functionalities, including acids, sugars, alkenes, and aldehydes. The objective of this project is to develop a fundamental understanding for aqueous-phase hydrogenations of organic acids by a combined theoretical and experimental approach.

We have measured the catalytic activity of aqueous-phase hydrogenation of acetic acid on supported monometallic Ni, Cu, Ru, Rh, Pd, Ir, and Pt catalysts in a fixed bed reactor. Ru/carbon is by far the most active catalyst, with an apparent activation energy of 44 kJ/mol. It is also the most selective toward ethanol at moderate temperatures, with the selectivity topping at 74% at 175 °C. Rh and Pt are the next most active, but their activity is an order of magnitude lower than that of Ru, with the rest of the metals being still less active.

For additional mechanistic insight, we have investigated the hydrogenation of acetic acid on the monometallic surfaces using density functional theory (DFT) calculations. By determining the energetics and kinetics of probable elementary steps, and taking experimental evidence into account, we tentatively identify the formation of acetyl (CH₃CO) as the main rate-determining step in the conversion of acetic acid, and the binding energy of acetyl as an important parameter of the overall activity. The effect of the aqueous phase on the reaction mechanism will be discussed. This study will provide valuable insight for designing a more versatile, alloy-based APH catalyst in the future.

[1] T. Eggeman, D. Verser, Recovery of organic acids from fermentation broths, Appl. Biochem. Biotechnol. 2005, 121-124, 605.

[2] G. W. Huber, S. Iborra, A. Corma, Synthesis of transportation fuels from biomass: chemistry, catalysts and engineering, Chem. Rev. 2006, 106, 4044.

[3] G. W. Huber, J. A. Dumesic, An overview of aqueous-phase catalytic processes for production of hydrogen and alkanes in a biorefinery, Catal. Today 2006, 111, 119.