Conventional hydrogenation of lignin-derived compounds requires high H 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 H 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.

中文翻译：

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环境条件下 Ru 催化剂上芳烃的水辅助加氢：实验与计算相结合的研究


木质素衍生化合物的传统氢化需要高氢气压力和温度，然而，实现所需的转化率和选择性仍然是一个挑战。在此，开发了一种以钌催化剂和水为溶剂的新型反应系统，用于在环境条件（室温和 1 bar H 压力）下将木质素衍生的芳族化合物选择性加氢为相应的环饱和产物。通过催化实验、先进表征技术和量子力学模拟的协同组合，我们阐明了 Ru 催化剂将其选择性从气相脱氧转变为凝聚相环氢化。水部分解离并以羟基碎片、H 原子和物理吸附分子的组合形式吸附在催化剂表面，这对于 Ru 改变其在水相中的选择性至关重要。实验结果表明，在水存在下，总氢化产物的转化率很高（>99%），选择性>75%，这证实了计算结果，其中直接氢化步骤的动能自由能势垒降低至 70 kJ/mol，对于苯酚，直接脱羟基从 63 kJ/mol 增加到 202 kJ/mol。此外，在氢化过程中利用了离解水分子中的H，并且水还利用外部氢气供应进行再生，从而充当外部氢气的穿梭者。