Water-assisted hydrogenation of aromatics under ambient conditions over Ru catalyst: A combined experimental and computational investigation

Abstract

Conventional hydrogenation of lignin-derived compounds requires high H2 pressures and temperatures, and yet, achieving the desired conversion and selectivity remains a challenge. Herein, a novel reaction system with a ruthenium catalyst and water as a solvent is developed for the selective hydrogenation of lignin-derived aromatics to corresponding ring-saturated products under ambient conditions (room temperature, and 1 bar H2 pressure). Using a synergistic combination of catalytic experiments, advanced characterization techniques and quantum mechanical simulations, we elucidate that Ru catalyst switches its selectivity from deoxygenation in gas phase to ring hydrogenation in the condensed phase. Water partially dissociates and adsorbs on the catalyst surface as a combination of hydroxyl fragments, H atoms, and physisorbed molecules, and this is critical for Ru to flip its selectivity in the aqueous phase. Experimental results demonstrate a high conversion (>99 %) and >75 % selectivity towards the total hydrogenated products in the presence of water, corroborating the computational results in which kinetic free energy barriers for direct hydrogenation steps reduced to 70 kJ/mol and barrier for direct dehydroxylation increased from 63 kJ/mol to 202 kJ/mol in the case of phenol. Furthermore, H from dissociated water molecules is utilized in the hydrogenation and water also gets regenerated utilizing external hydrogen supply, thus acting as a shuttler for the external hydrogen.