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 for the 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, can be slowly removed from the surface of Pt/Al₂O₃ under the continuous flow reaction, as a result, the e. e. values drops with the reaction time in the fixed-bed reactor. The enantio-selectivity is higher in the fixed-bed reactor, but lower in the batch reactor when ethanol was used as solvent than that when acetic acid as solvent. CO was used as molecular probe to characterize the adsorption of cinchonidine on the catalyst surface by IR spectroscopy. A red shift observed in IR spectra of coadsorbed CO with cinchonidine suggests that the cinchonidine adsorption is mainly through the π-interaction with platinum surface and donating electron to the platinum surface.