Investigation of solvent effects in the hydrodeoxygenation of levulinic acid to γ-valerolactone over Ru catalysts

Liquid phase, reductive deoxygenation of biomass derived platform chemicals over transition metal surfaces constitutes an efficient scheme for upgrading lignocellulosic biomass. The solvation effects on the reaction kinetics of the hydrodeoxygenation (HDO) of levulinic acid (LA) towards the formation of γ-valerolactone (GVL) over Ru(0 0 0 1) has been studied in three condensed phase media, i.e., liquid water, methanol, and 1,4-dioxane. Detailed microkinetic models have been developed incorporating various catalytic pathways including formation of 4-hydroxypentanoic acid (HPA) and α-angelicalactone (AGL) to simulate the catalytic activity of Ru(0 0 0 1) under various reaction conditions of solvent, temperature, and partial pressures. Our microkinetic models suggest that direct catalytic conversion with alkoxy formation is the preferred reaction mechanism in all reaction environments. Furthermore, we find that water facilitates the reaction kinetics significantly and that the solvent effect is strongest at lower temperatures (T < 373 K). Here, rate increases due to liquid water solvation effects of 2–4 orders of magnitude are observed. All solvents increase the rate of reaction relative to the gas phase; however, solvation effects decrease with decrease in polarity. 1,4-dioxane increases the rate only minimally due to competitive adsorption of the solvent molecules despite facilitating the partially rate controlling step of the LA hydrogenation to an alkoxy intermediate.