Theoretical Investigation of the Hydrodeoxygenation of Levulinic Acid to γ-Valerolactone over Ru(0001)

The reaction mechanism of the hydrodeoxygenation (HDO) of levulinic acid (LA) to γ-valerolactone (GVL) has been investigated over a Ru(0001) model surface by a combination of plane-wave density functional theory (DFT) calculations and mean-field microkinetic modeling. Catalytic pathways involving the direct hydrogenation of LA to GVL with and without formation of the experimentally proposed 4-hydroxypentanoic acid (HPA) intermediate have been considered. In the low reaction temperature range of 323-373 K, the activity of the model Ru(0001) surface is low, owing to a very small number of free sites available for catalysis. As a result, it is unlikely that Ru(0001) is the active site for the experimentally observed catalysis at low temperatures. In contrast, in the medium- to high-temperature range (423-523 K), the HDO of LA is facile over Ru(0001) and we predict at 423 K a turnover frequency, apparent activation barrier, and forward reaction orders that are fairly close to prior experimental observations, leading us to suggest that Ru(0001) sites might constitute the active site for high-temperature catalysis. Finally, our microkinetic analysis suggests that the HDO of LA occurs by LA adsorption, hydrogenation of LA to an alkoxy intermediate, surface ring closure, and -OH group removal: i.e., it does not occur via HPA production as previously suggested. The first hydrogenation step of LA toward the formation of an alkoxy intermediate is the most rate controlling step over Ru(0001).