(399e) Mechanistic Investigation of the Catalytic Hydrogenation of Levulinic Acid over Ru Model Catalyst Surfaces

Understanding the reaction kinetics governing the aqueous phase hydrodeoxygenation (HDO) of levulinic acid (LA) to γ-Valerolactone (GVL) over Ru surfaces is crucial for designing better catalysts for the conversion of LA to GVL considering that Ru/C catalysts are the most often used catalysts for the HDO of LA. In this presentation, we report a computational investigation of the reaction mechanism of LA to GVL using DFT calculations and mean-field microkinetic modeling. Our gas phase and liquid phase results indicate that water (polar solvents) have a beneficial effect on the reaction kinetics of the hydrodeoxygenation of LA. In contrast, non-polar solvents (THF, methanol, & n-heptane etc.) have a detrimental effect on the reaction kinetics. Next, our results show that Ru (0001) is highly active for the hydrogenation of LA to its corresponding alcohol product 4-Hydroxypentanoic acid (HPA) at a high reaction temperature above 423 K. However, at a low temperature (T < 373 K), the hydrogenation rate is sluggish compared to experimental observations. Considering furthermore that the hydrogenation of various short chain ketones (acetone, butanone-2, and pentanone-2) over Ru (0001) also leads to reaction rates much smaller than predicted by experiment, we conclude that Ru (0001) is not the active site at low temperatures. To identify the active Ru site for the experimentally observed low temperature activity, we performed a constrained thermodynamics study and identified surface oxygen species as a possible active site for the hydrogenation of LA. The resulting two-site model can explain the hydrogenation mechanism of ketones and aldehyde to its corresponding alcohol over Ru catalysts.