Preparation Of Supported Metal Nanoclusters And Their Catalytic Performance In The Hydrogenation Of Pyruvates And Acetophenone

The homogeneous asymmetric hydrogenation catalyzed by organometallic complexes exhibits high catalytic activity and high selectivity under mild conditions, but the separation of metal complexes from the products is difficult This inherent defect makes that the applications of homogeneous asymmetric catalysts are limited. However, the heterogeneous asymmetric catalytic hydrogenation connected with the practical advantages of easy separation and handling, has become a rapidly growing and interesting field. The nanoclusters, which are new catalytic materials, are expected to be able to combine the merits of both homogeneous and heterogeneous hydrogenation. Therefore, they attract more and more attentions.The metal nanoclusters and their supported catalysts have been prepared with an improved method. The stabilizer, which always is the macromolecule, was added in the preparation of metal nanoclusters. It could protect the small metal particles from aggregation and precipitation, and the metal colloid can steady dispersed in the solution. The support not only provides the large surface for fixing metal nanoclusters, but also improves the catalytic properties through a synergistic effect.XRD and XPS show that the electric structure of the metal and the texture of the support are changed during the pretreatment and modification, as well as the metal nanoclusters are well reduced. These would cause the changes of the reaction micro-circumstance.Cinchona (cinchonidine or quinine) is used as chiral modifier in the enantioselective hydrogenation of methyl and ethyl pyruvate. The effects of the pretreatment, modification and reaction parameters have been investigated in detail. We found that the optimum pretreatment conditions are PH2=5.0 Mpa, 1400C, t=1.0h and the optimum modification conditions arePH2=5.0 Mpa, 250C, t=30min and [cinchonidine]=4.0 10-3rnol/L. When the asymmetric hydrogenation is carried out under PH2=5.0 Mpa, 250C for 1 hour, the substrate is almost completely conversed and the e.e can reached to 38% for methyl pyruvate and 64% for ethyl pyruvate. The optimal molar ratio of substrate, rhodium and cinchonidine is 1000:1:8.The results show that cinchona not only has good enantiodifferentiating ability but also a considerable accelerating effect for the reaction. The hydrogenation of quinoline ring of cinchonidine or quinine would reduce its chiral induction effect and its adsorption strength on the metal surface, which would cause a drop of the enantioselectivity of the catalyst. It is expected that the enantioselectivity could keep good if the chiral modifier is suitably compensated during the catalytic hydrogenation recycle.The PVP stabilized platinum nanoclusters are prepared under gentle conditions. They are supported on y-aluminua surface and reduced in solution. The catalyst is used in acetophenone hydrogenation. The effects of reaction parameters have been investigated. The platinum nanocluster catalyst exhibits very high activity and the selectivity of 100% for the formation of -phenylethanol. Under the optimum reaction conditions: 60 , PH2 =5MPa, the concentration of KOH 0.02mol/L, acetophenone : Pt = 60000:1 (molar ratio), the conversion of acetophenone and TOP can reach 97.0% and 101.6 molAnol-min, respectively. The study of reaction mechanism suggests that the hydrogen atom in acetophenone hydrogenation comes from activated hydrogen molecule rather than hydrogen transfer reagent (isopropanol) and the activation and dissociation of hydrogen molecule are not the rate-determining step in the hydrogenation.