Liquid-phase hydrogenation of acetophenone over silica-supported Ni, Co and Cu catalysts: Influence of metal and solvent

• Solvent–metal interactions rule the solvent effect on the acetophenone hydrogenation.
• The magnitude of the solvent effect strongly depends on the metal: Ni > Co > Cu.
• Benzene–metal and toluene–metal interactions hamper acetophenone hydrogenation rate.
• Both 2-propanol–metal and 2-propanol–reactant interactions promote hydrogenation rate.
• Strong solvent–metal interaction diminishes 1-phenylethanol hydrogenolysis.

In this work, we studied the influence of solvent and metal nature on the liquid-phase hydrogenation of acetophenone (AP) over Ni/SiO2, Co/SiO2 and Cu/SiO2. Catalysts were prepared by wetness impregnation method with metal loads of about 7–8 wt%. Catalytic tests were performed in a batch reactor, at 363 K and 10 bar (H2), using 2-propanol (IPA), cyclohexane (CHX), toluene (TOL) and benzene (BEN) as solvents. Considering the three catalysts, the general pattern for the initial hydrogenation rate was: Ni/SiO2 > Co/SiO2 > Cu/SiO2, whereas the trend for selectivity to 1-phenylethanol (PHE) was just the opposite. AP can interact with nickel metal surface through both –C=O group and aromatic ring and thus the aromatic alcohol and saturated compounds were obtained. Instead, cobalt and copper metal surfaces interact preferentially with the –C=O group leading to selective hydrogenation of AP into PHE. In addition, an important interaction between –C–OH group of PHE and Co/SiO2 surface takes place, leading to rapid alcohol hydrogenolysis into ethylbenzene. The general activity pattern with the four solvents was: IPA > CHX ≥ TOL ≥ BEN. The magnitude of solvent influence on the catalytic performance strongly depended on the metal nature. The most significant solvent effect took place with Ni/SiO2, whereas the less noticeable influence was observed in the case of Cu/SiO2. From pseudo-homogenous kinetic modeling and temperature-programmed desorption, the following noteworthy observations arose: (1) IPA has a positive contribution by hydrogen transfer and/or AP activation by polarization; (2) the magnitude of the positive IPA influence on AP hydrogenation rate follows the trend: Ni/SiO2 > Co/SiO2 > Cu/SiO2; (3) CHX has a neutral contribution because of its weak adsorption on the metal phase and low interaction with reactant and products; (4) the effect of TOL and BEN is clearly negative for Ni/SiO2 due to blockage of active sites by strong adsorption of solvent on the metallic surface; (5) the effect due to strong adsorption of TOL and BEN is much less noticeable on Co/SiO2 and Cu/SiO2, as a consequence, the pattern for AP hydrogenation rates in BEN and TOL is Cu/SiO2 > Co/SiO2 > Ni/SiO2. Selectivity to PHE was less influenced by solvent nature. However, in the case of Ni/SiO2 and Co/SiO2, maximum PHE yields and selectivities increased with the solvent–metal interactions, mainly due to inhibition of the PHE hydrogenolysis. Cu/SiO2 was always 100% selective to PHE in all of the solvents. These results are clearly indicating that the magnitude of the solvent effect on catalytic performance strongly depends on the metal nature.

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