Studies On The Preparation And Asymmetric Hydrogenation Of Platinum-based Bimetallic Catalysts

Cinchonidine modified Pt/γ-Al₂O₃catalysts exhibit excellent properties in theenantioselective hydrogenations of prochiral ketones. Asymmetric hydrogenation ofethyl2-oxo-4-phenylbutyrate to ethyl （R）-2-hydroxy-4-phenylbutyrate（EHPB）, Theproduct is a key intermediate for the synthesis of angiotensin converting enzymeinhibitors（ACEI）. In Master’s thesis, a series of γ-Al₂O₃supported Pt and Pt-M（Ni,Co, Cr, Ru, Ir） catalysts are prepared and investigated their properies for the theenantioselective hydrogenation of EOPB. Some main conclusions drawn from ourwork are follows:（1） A series of catalysts PVP-Pt/γ-Al₂O₃were prepared by different reductionmethods （ethanol, HCHO, HCOONa, and NaBH₄） in the presence ofpolyvinylpyrrolidone （PVP） as stabilizer, the catalytic performance for asymmetrichydrogenation of ethyl2-oxo-4-phenylbutyrate was tested. The catalyst prepared byusing NaBH₄as reducing agent showed the best overall performance. The effect ofvarious catalyst preparation conditions, such as reduction temperature, NaBH₄concentration, the medium in which reduction was carried out, PVP/Pt mole ratio andreaction solvent and time was also investigated. Optically pure （R）-EHPB could beobtained with100%yield and76.8%e.e. when catalyst was prepared at0oC using0.5mol/L NaBH₄in a medium of ethanol.（2） Pt/γ-Al₂O₃catalyst and Pt-M（Ni, Co, Cr）/γ-Al₂O₃bimetal catalysts wereprepared under optimum reduction conditions of PVP-Pt/γ-Al₂O₃catalyst. Heattreatment, Pt/M mole ratios, solvent and amount of cinchonidine was tested in theasymmetric hydrogenation of EOPB. It is found that the heat treatment increase Ptparticles size and enhance catalytic performance. The catalytic acitivity of Pt-Mcatalysts would decreases with M loading. XRD、TEM、XPS and H2-O2titrationanalysis showed strong interactions exist between Pt and M metal species on thecatalyst surface. M were virtually inactive and acted as a site blocker, whichdecreased the surface Pt sites and hindered the formation of the bulky transitioncomplex between the reactant and the chiral modifier and the loss in activity and selectivity.（3） Pt/γ-Al₂O₃catalyst and Pt-M（Ru, Ir）/γ-Al₂O₃bimetal catalysts were preparedby impregnation、calcination、hydrogen reduction in high temperature, Pt/M massratios, solvent and reaction temperature was tested in the asymmetric hydrogenationof EOPB. Both Ru/γ-Al₂O₃and Ir/γ-Al₂O₃catalysts show inferior catalyticperformance to Pt/γ-Al₂O₃catalysts in this reaction. TPR、XPS、XRD、UV-Vis、benzene/ethanol-TPDanalysis results showed cinchonidine is adsorbed on Pt, Ru andIr via the quinoline moiety approximately parallel to the metal surface by π-bondingand an additional interaction of the alkaloid molecule with thesurface occurs athydroxy oxygen atom C9. The former strength of force: F（Pt）>F（Ir）>F（Ru）; latterstrength of force: F（Ru）>F（Ir）>F（Pt）, which would lead to different adsorptiongeometry of quinuclidine on metallic surface. The difference adsorption geometrybetween the three metals induces the different catalytic results in the asymmetrichydrogenation of EOPB.