Why phenol is selectively hydrogenated to cyclohexanol on Ru (0001): An experimental and theoretical study

Ruthenium shows unique catalytic properties in selective hydrodeoxygenation of lignin-derived phenols to alkyl cyclohexanols, but seldom exploration has been devoted to the mechanism of phenol hydrogenation on Ru. Herein, the mechanism of phenol hydrogenation to cyclohexanol on Ru is explored combining experiments and density functional theory (DFT) calculations. The experimental results show that the selectivity to cyclohexanol is higher than cyclohexanone over 3% RuSiO2 when the conversion is higher than 25%. Mechanistic study by DFT shows that after forming cyclohexenol as an intermediate, the hydrogenation can proceed along both the classical tautomerization pathway and direct hydrogenation pathway, with tautomerization pathway being the dominant one. Further, the hydrogenation pathways producing cyclohexanol without the generation of cyclohexenol could also run parallel to the two pathways through it. These pathways together lead to the high selectivity to cyclohexanol on Ru catalyst. The refreshed hydrogenation pathway can contribute to a new insight into the mechanism of phenol hydrogenation on Ru based catalyst.