(645b) Theoretical Investigation of the Hydrogenation of Levulinic Acid over Ru Catalysts

Aqueous-phase hydrogenation of biomass derived levulinic acid (LA) to g-valerolactone (GVL) has been proposed as an important process step for separation and purification of biomass derived platform chemicals such as LA and GVL from aqueous media.  The reaction generally occurs over supported metal catalysts such as Ru/C that are found to be both active and selective for GVL production.  In this presentation, we present recent computational results on the reaction mechanism of this reaction over Ru(0001), Ru(100A), Ru(100B), Ru(101A), and Ru(101B) model surfaces.  After a detailed DFT study of various elementary processes at the gas-solid interface with PBE-D3 functional, we applied our implicit solvation method for solid surfaces (iSMS) to approximately determine the effect of an aqueous reaction environment on elementary reaction and activation free energies.  Finally, we developed mean-field microkinetic models to predict reaction rates, activation barriers, reaction orders, and rate controlling steps.  We find that only at high temperatures above 473 K are active sites on the Ru(0001) surface catalytically active and the first hydrogenation step of LA to an alkoxy intermediate is the rate controlling step.  At lower temperatures only interface sites, i.e., edge sites of two surface facets display significant activity and can possibly explain the experimentally observed catalytic activity.