Enantioselective Hydrogenation Of α-ketoesters On Chirally-modified Ordered Mesoporous Carbon Supported Pt Catalysts

Cinchona-modified Pt supported catalysts show excellent performance in the enantioselective hydrogenation of a-ketoesters, and it has been regarded as one of the landmarks of the heterogeneous asymmetric catalysis. Due to its periodic mesopores, uniform pore size, high surface areas, adequate pore volume and high thermal, chemical and mechanical stabilities, ordered mesoporous carbons (OMCs) can be used as adsorbents, catalyst support and catalyst, electrode materials and templates for synthesis of nanostructured inorganic materials and mesoporous polymer carbon composites. Herein, CMK-3 mesoporous carbons and CMK-8 mesoporous carbons can be synthesized by a classic two-step nano-casting method, using mesoporous silica SBA-15 (with p6mm symmetry) and KIT-6 (with Ia3d symmetry) as templates, respectively. The resulted two kinds of mesoporous carbons prove to be remarkable supports for Pt nanoparticles in the heterogeneous asymmetric hydrogenation ofα-ketoesters such as ethyl pyruvate and ethyl 2-oxo-4-phenylbutyrate. For comparison, we also applied the commercial Pt/Al2O3 and Pt/C catalysts in the enantioselective hydrogenation of a-ketoesters. The main results are summarized as follows:In the first part, CMK-3-900 carbon is a good replica of SBA-15 and this kind of mesoporous carbon has high surface area and adequate pore volume. Pt/CMK-3-E (E represents that the ethanolic solution of H2PtCl6 was used as Pt precursors) catalyst has the smallest Pt nanoparticles (2.2 nm) and the highest Pt dispersion (52.5%). Pt/CMK-3-W-523 catalyst (W-523 represents that the aqueous solution of H2PtCl6 was used as Pt precursors and the catalyst precursors were calcined at 523 K under vacuum before reduction for 2 h) has the largest Pt nanoparticles (10.7 nm) and the lowest Pt dispersion (10.6%).In the second part, Pt/CMK-3-W catalyst (W represents that the aqueous solution of H2PtCl6 was used as Pt precursors) has high specific surface area, adequate pore volume, ordered mesoporous and small Pt particle size with high dispersion. Based on the characterization results from the diffuse reflectance infrared Fourier-transform spectra using CO as probe molecules and Raman spectrum, the Pt/CMK-3-W catalyst surface with high electron density would increase feedback bonding from d orbital of Pt0 atoms to 2π* orbital of C=O double bond in ethyl pyruvate, so that the C=O bond energy was decreased and the C=O double bond can be easily broken. As a result, the hydrogenation of ETPY achieved 81.0% conversion and 82.2% ee values in 5 min. Obviously, the catalytic performance of Pt/CMK-3 catalyst was much higher than Pt catalysts supported on other carbon materials, such as Pt/C and single-walled carbon nanotubes supported Pt catalyst. The catalytic performance of Pt/CMK-3-W-423 catalyst (W-423 represents that the aqueous solution of H2PtCl6 was used as Pt precursors and the catalyst precursors were calcined at 423 K under vacuum before reduction for 2 h) was comparable with that of the commercial Pt/Al2O3 catalyst, with the TOF reaching 23537 h-1. A little CD or CN would significantly accelerate the reaction and induce higher ee values. However, addition of too much CD will retard the reaction. Furthermore, the stability of Pt/CMK-3-W catalyst is superior to that of both the commercial Pt/Al2O3 and Pt/C catalyst under the same conditions. The Pt leaching amount for Pt/Al2O3 and Pt/C catalyst are 30 and 50 times higher than that of Pt/CMK-3-W and the Pt/CMK-3-W catalyst can be reused for more than 5 times without distinct loss of activity or enantioselectivity in acetic acid for the enantioselective hydrogenation of ethyl pyruvate.In the third part, the Pt/CMK-8 catalyst with much higher pore volume will be helpful to the diffusion of reactants and products. As a result, the hydrogenation of ETPY achieved 97.8% conversion and 74.7% ee values in 5 min and the hydrogenation of EOPB achieved 93.4% conversion and 75.0% ee values in 5 min, with the TOF reaching 19782 h-1 and 23716 h-1, respectively. The Pt/CMK-8-W catalyst showed much higher catalytic performance in the enantioselective hydrogenation of both ETPY and EOPB than commercial Pt/C catalyst. It is suggested that both the physical structure features, including high specific surface area, adequate pore volume, ordered mesopores and small Pt particle size with high dispersion, and the chemical nature of catalyst surface with high electron density would certainly improve the catalytic behaviours of Pt/CMK-8 catalyst.