Effects of hydrogen and water on the activity and selectivity of acetic acid hydrogenation on ruthenium

Kinetic flow reactor experiments have been carried out to study acetic acid hydrogenation on a Ru/C catalyst in both three-phase (catalyst, aqueous, and gaseous) and two-phase (catalyst and gaseous) regimes. In addition, density functional theory calculations have been performed and combined with microkinetic modeling to better understand the activity and selectivity observed in the experiments. Our experiments show that ethanol selectivity varies strongly from <10% to a maximum of ∼70% with increasing hydrogen partial pressure (pH2) at 185 °C in the three-phase reactor. Co-fed water also enhances ethanol selectivity, from ∼60% to ∼70% in the two-phase reactor and ∼40% to ∼65% in the three-phase reactor, at 185 °C, but only up to a certain concentration. The aqueous phase is not necessary for high ethanol selectivity. The first-principles microkinetic analysis is able to reasonably capture the apparent activation energy, ethanol selectivity, and reaction orders of acetic acid and ethanol with respect to pH2, providing a theoretical explanation for the crucial role that hydrogen plays in the selectivity of this reaction. Our findings provide insights into why high activity and selectivity for acetic acid hydrogenation to ethanol can be achieved on Ru, which may have general relevance to the catalytic hydrogenation of organic oxygenates on Ru and other metals.