Importance of Angelica Lactone Formation in the Hydrodeoxygenation of Levulinic Acid to γ-Valerolactone over a Ru(0001) Model Surface

Using mean-field microkinetic modeling with parameters derived from density functional theory calculations and harmonic transition state theory, we investigated the steady-state catalytic hydrodeoxygenation of levulinic acid (LA) to gamma-valerolactone (GVL) on a Ru(0001) model surface. Focusing on the importance of intramolecular esterification of LA to its stable derivative a-angelica lactone (AGL) during the HDO to GVL, we studied various reaction pathways for GVL production that involve AGL and 4-hydroxypentanoic acid (HPA). We find that in a nonpolar reaction environment these pathways are not kinetically relevant but that GVL can be produced from LA by a single hydrogenation step, followed by ring closure and C-OH bond cleavage. However, AGL reaction pathways lead to surface poisoning at temperatures above 423 K when these pathways become kinetically accessible. As a result of surface poisoning possibly at low temperatures by hydrogen and at high temperatures by AGL derivatives we observe two different activity regimes characterized by significantly different activation barriers. Overall, simulation results agree well with experimental observations except at low temperatures of 323 K where our model significantly underestimates the turnover frequency, questioning whether Ru(0001) sites are active at these low temperatures.