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 γ-valerolactone (GVL) on a Ru(0001) model surface. Focusing on the importance of intramolecular esterification of LA to its stable derivative α-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.