Transfer Hydrogenation of Cinnamaldehyde Catalyzed by Al₂O₃Using Ethanol as a Solvent and Hydrogen Donor

Allylic alcohols produced from α,β-unsaturated aldehydes by selective hydrogenation are useful intermediates and additives in the fine chemicals industry. An efficient process for the catalytic transfer hydrogenation (CTH) of cinnamaldehyde (CMA) to cinnamyl alcohol (CMO) was developed by using ethanol as a solvent and hydrogen donor over Al2O3. Under optimized reaction conditions (120 °C, 12 h), 97.8% conversion of CMA and 96.8% selectivity to CMO were obtained. The solvent effect on catalytic performance was investigated with different primary alcohols, and the results showed that the activity decreased with decreasing polarity of the C2-C5 alcohols, which was ascribed to decreased alcohol-CMA and alcohol-catalyst interaction. In addition, on the basis of the results of poisoning experiments and catalyst characterizations, a reaction mechanism involving a six-membered ring intermediate on acid-base pair active sites of Al2O3 was proposed for the CTH of CMA in ethanol, which was further confirmed by density functional theory calculations. Furthermore, this catalytic system demonstrated good stability and wide compatibility for a range of α,β-unsaturated aldehydes.