Enantioselective hydrogenation of α-ketoesters on mesoporous matrix supported platinum catalysts

The asymmetric hydrogenation of ethyl 2-oxo-4-phenyl-butyrate (EOPB) over the Pt catalysts modified by cinchonidine using mesoporous SBA-15, cubic mesoporous silica, and Al₂O₃ modified cubic mesoporous silica as matrix was studied. The results showed that in the channels of mesoporous SBA-15, the enantioselectivity and conversion are enhanced in larger sized channels compared to those of the hydrogenation in smaller sized channels. When the channel size of SBA-15 increases from 6.2 nm to 7.6 nm and to 9.2 nm, the hydrogenation ee is 22.0%, 36.0%, and 50.0%, respectively. The ee (53.8%) on the cubic mesoporous silica support is obviously higher than that (36.0%) on the SBA-15 support with similar channel size, and the ee on the Al₂O₃ modified cubic mesoporous silica support reaches 79.3%. These facts showed that the mass transfer or steric clash has important impacts on the enantioselectivity and conversion of the enantioselective hydrogenation of EOPB. In the enantioselective hydrogenation of ethyl pyruvate, the Al₂O₃-grafted cubic mesoporous silica supported Pt catalyst exhibits high activity and excellent enantioselectivity (ee over 90.2%), which is comparable to that of the best commercial Pt/Al₂O₃ catalysts. The high catalytic performance is attributed not only to the developed cubic channel structure of the Pt catalyst support, but also to the enhancement of the interaction of Pt nanoparticles and the Al₂O₃-grafted support, which would improve the modification of cinchonidine on the surface of Pt particles. These indicate that the Al₂O₃ modified cubic mesoporous silica is a novel, promising support material for the heterogeneous chiral catalysis.