Solvent effect in the liquid-phase hydrogenation of acetophenone over Ni/SiO 2: A comprehensive study of the phenomenon

Abstract

The solvent effect on catalyst activity and selectivity for the liquid-phase hydrogenation of acetophenone (AP) to 1-phenylethanol was thoroughly investigated over Ni/SiO 2. Solvents of different nature were used: protic (C 1–C 3 primary and secondary alcohols), aprotic polar (tetrahydrofuran, γ-butyrolactone, and acetonitrile) and apolar solvents (cyclohexane, toluene, and benzene). The solvent had a strong influence on the AP hydrogenation rate but did not modify significantly the selectivity to 1-phenylethanol that was always higher than 92%. The AP hydrogenation activity followed the order: C 2–C 3 alcohols > cyclohexane > toluene > tetrahydrofuran > γ-butyrolactone > methanol ≫ benzene ≅ acetonitrile. In order to explain this activity pattern, the solvent–AP, solvent–H 2 and solvent–catalyst interactions were analyzed. For the analysis of the solvent–AP interactions in liquid phase, both classical measures of polarity and others based on different solvatochromic scales were considered. The H 2 availability in the liquid phase was estimated from the H 2 solubility at reaction conditions. Solvent–catalyst interactions were characterized by means of the adsorption enthalpies measured calorimetrically. A reasonable correlation between the catalyst activity and some solvatochromic parameters was found only when solvents of similar nature were compared. For protic solvents, the AP hydrogenation rate decreased with the solvent polarity and its ability for H-bond formation with AP. Instead, the solvent–AP interactions were weak when using apolar solvents and thereby the activity pattern was essentially determined by the strength of solvent–catalyst interactions. In the case of aprotic polar solvents, both the solvent–AP interactions in the liquid phase and the solvent adsorption strength on the catalyst surface influenced the hydrogenation activity. The highest catalytic activities were obtained when using C 2–C 3 alcohol solvents. These protic solvents adsorbed dissociatively on metal nickel surface increasing the number of active H available for the hydrogenation reaction; this effect was much more important in the case of 2-propanol.