(541f) Water-Assisted Hydrogenation of Aromatics Under Ambient Conditions over Ruthenium Catalyst: A Combined Experimental and Computational Investigation

Catalytic hydrogenation (HYD) of lignin-derived aromatics is of high interest for the sustainable production of commodity chemicals and various polymer building blocks. Several noble and non-noble transition metal catalysts with various solvents have been investigated. However, conventional HYD of lignin-derived monomers requires high H₂ pressures and temperatures, and yet, achieving the desired conversion and selectivity remains a challenge. Herein, a novel reaction system with Ruthenium (Ru) as a catalyst and water as a solvent is developed for the selective HYD of phenol, guaiacol, and other oxygenated aromatic compounds to corresponding ring-saturated products under ambient conditions (room temperature, and 1 bar H₂ pressure). Using a synergistic combination of quantum mechanical DFT simulations, catalysis experiments, and advanced characterization techniques (XRD, SEM, XPS, TEM, and isotope labeling experiments), we elucidate the condensed phase catalytic reaction mechanism. Interestingly, the Ru catalyst favours selective C – O bond dissociation (DDO) in the gas phase for the aforementioned aromatic reactants; however, it switches its selectivity to HYD in the presence of water. Mechanistic DFT simulations reveal that water partially dissociates and absorbs on the catalyst surface as a combination of hydroxyl fragments, H atoms, and intact water molecules, and that this is critical for Ru to flip its selectivity from DDO to HYD in the aqueous phase. Experimental results demonstrate a high selectivity (> 99%) and >90 % conversion towards the fully hydrogenated products in the presence of water, corroborating the computational results. Up to date, no other solvent has been shown to act as a hydrogen donor for the selective HYD of lignin derived aromatics, thus requiring high H₂ pressures and temperatures. Our results conclusively show that water, as a solvent, provides excellent HYD conversion/selectivity at ambient conditions, thereby eliminating the need for energy intensive H₂ pressures and temperatures.