Heterogeneous asymmetric hydrogenation of ethyl pyruvate on chirally modified Pt/Al₂O₃, catalyst with fixed-bed reactor

The enantioselective hydrogenation of ethyl pyruvate on the cinchonidine modified Pt/Al₂O₃ catalyst was investigated using a high-pressure reaction system with a fixed-bed reactor, on purpose to produce the chiral product without separating the catalyst from the reaction system. The reaction was also investigated in a batch reactor for comparison. About 60% e.e. and 90% e.e. were obtained with the fixed-bed reactor and the batch reactor respectively, demonstrating the possibility for the heterogeneous asymmetric hydrogenation in the fixed-bed reactor. Some adsorbed chiral modifier, cinchonidine, were slowly removed from the surface of Pt/Al₂O₃ under the conditions of continuous flow reaction, as a result, the e.e. value drops with the reaction time for the fixed-bed reactor. The reaction was also tested using supercritical fluids (CO₂ and C₂H₆). It was found that the reaction was significantly accelerated in the presence of supercritical C₂H₆ but some adsorbed cinchonidine was extracted from the catalyst surface by the supercritical fluids. CO was used as a molecular probe to characterize the adsorbed cinchonidine on the catalyst surface by IR spectroscopy. A red shift observed for the IR band of coadsorbed CO with cinchonidine suggests that the cinchonidine adsorption is mainly through the π-interaction with platinum surface and donates electron to the platinum surface.