Solvent effects in heterogeneous selective hydrogenation of acetophenone: differences between Rh/C and Rh/Al2O3 catalysts and the superiority of water as a functional solvent

Abstract

Selective hydrogenation of acetophenone (AP) to 1-phenylethanol (PhE) was investigated over Rh/Al₂O₃ and Rh/C catalysts in 13 solvents including water and conventional organic solvents. Strong solvent effects on the overall rate of AP conversion were observed in different manners depending on the catalysts used. The conversion obtained is correlated with hydrogen-bond-donation (HBD) capability for Rh/C but with hydrogen-bond-acceptance (HBA) capacity for Rh/Al₂O₃. The solvent effects should result from interactions between the carbonyl group of AP and the solvent molecules through hydrogen bonding for Rh/C and from those between the solvent molecules and the catalyst surface for Rh/Al₂O₃ having HBD hydroxyl groups on its surface. Water is the most effective functional solvent in the selective hydrogenation of AP for C and Al₂O₃-supported Rh catalysts due to its high HBD capability (α) and low HBA capability (β), respectively. For the hydrogenation with Rh/Al₂O₃ in water, its large polarity/polarizability index (π*) may contribute to the high selectivity to PhE.