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.