A series of Pd-Pt/alumina catalysts were prepared by consecutive deposition of Pd onto supported Pt particles. Electrochemical model studies indicated that under very mild conditions in aqueous acetic acid the reduction of Pd2+ ions occurred partly via the ionization of hydrogen adsorbed on Pt and partly by the slow oxidation of acetic acid. There was only a moderate change in the surface Pd/ Pt atomic ratio during heat treatment in flowing hydrogen at 400°C. as determined by XPS analysis. The catalytic performance of the bimetallic catalysts was tested in the enantioselective hydrogenation of ethyl pyruvate and ketopantolactone, in the presence of cinchonidine. Pd was virtually inactive and acted as a site blocker, which decreased the size of Pt ensembles and hindered the formation of the bulky transition complex between the reactant and the chiral modifier. The intrinsically low activity and selectivity of Pd are discussed in the light of H/D exchange studies in deuterated ethanol.
- Enantioselective hydrogenation
- Ethyl pyruvate
- Preparation of pd-pt/alumina
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