Oxygenated compound such as aldehydes or ketones generated during fast pyrolysis of biomass, can be selectively reduced to produce value-added products like alcohols. Catalytic hydrogenation of these carbonyl compounds on transition metals involves two sequential hydrogen additions onto the C=O bond via two common pathways: (1) Hydroxy, where the O-H bond forms first or (2) Alkoxy, where the C-H bond forms first. Here, we employ density functional theory (DFT) to probe the mechanistic effects of a polar protic solvent (i.e. water) on the energetically dominant reaction pathway for propanal hydrogenation to propanol on Ru (0001). Critical factors influencing reaction pathways such as high concentration of reacting species, hydrogen pressure, presence of solvent and undesired side products (CO molecules) is also examined in our study. Our results indicate that polar, protic solvent can promote the hydrogenation of carbonyl containing compounds by stabilizing the transition state and intermediates via hydrogen-bonding. Furthermore, these model results are correlated with experimental, hydrogenation kinetic-isotopic analyses, developing accurate atomistic insights to the reaction mechanism and improving aldehyde hydrogenation catalysts. Alkali atoms have been shown to extend the reactivity of transition metal complexes in the liquid phase. As such, we also investigate Cs as a precious metal alternative to precious metals, which have been shown to create their own local electric field and thus enhance the hydrodeoxygenation of phenolics. To elucidate the effect of Cs on the underlying reaction pathway in the presence of water, water is co-adsorbed with Cs on Fe in the underlying model. Finally, we investigate the HDO of phenol and guaiacol under the aqueous environment present in bio-oil. We demonstrate that protic solvents such as water can ionize hydrogen adatoms (H*), generating protons (H+) at the solid-liquid interface, which alters the kinetically relevant paths. Our study shows that, by choosing an appropriate pair of solvent and metal, we can manipulate the charge of the reactive hydrogen species and in turn the HDO reactivity and selectivity towards the desired C-O bond cleavage.

Unravelling the role of reaction environment at the metal-reactant interface

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.