(437a) Determining the Effect of Water during the Catalytic Reduction of Propanal to Alcohols on Ru-Based Catalysts

Carbonyl compounds such as aldehydes and ketones, that occur during pyrolysis of biomass-derived sources, can be catalytically hydrogenated to value-added chemicals and sustainable fuels. Reduction of such oxygenates requires the sequential hydrogenation of the carbonyl functionality leading to either the Alkoxy or Hydroxy pathway. The alkoxyl intermediate is formed via the formation of a C-H bond while the hydroxyl intermediate occurs via the formation of an O-H bond (see Figure 1). The presence of water may solvate the reactive hydrogen adatoms and therefore alter the underlying free energy landscape, the overall mechanism and in turn, the catalytic turnover. Here, we use density functional theory (DFT) combined with kinetic and isotopic experiments to elucidate the role of water toward the promotion of the hydrogenation rate by facilitating the formation of an O-H bond through hydrogen bonding.

Our results suggest that, in the vapor phase, hydride adspecies are present on the Ru surface and react with adsorbed propanal by preferentially attacking the positively charged carbon to rapidly and selectively form a C-H bond, which results in an Alkoxy pathway (Figure1). Co-feeding water during the reduction of propanal in the vapor phase produces an environment with a low water concentration, which bridges the vapor phase and the condensed water phase. Increasing the water pressures and the number of water molecules around the active sites in our model, induces a switch in the hydrogen addition order by stabilizing the transition state via hydrogen bonding and results in a reduction of the O-H formation barrier and a different hydrogen addition order (Figure 1). Overall, this work provides theoretical and experimental insights demonstrating the systematic tunability of the solvent with regard to the underlying energetics of the elementary steps during the catalytic sequence and the reaction pathway for propanal reduction to alcohols.