ENANTIOSELECTIVE HYDROGENATION OF ETHYL PYRUVATE - KINETIC MODELING OF THE MODIFICATION OF PT CATALYSTS BY CINCHONA ALKALOIDS

The effect of modifier concentration on rate and optical yield is described for the enantioselective hydrogenation of ethyl pyruvate using 5% Pt/Al2O3 catalysts in the presence of cinchona alkaloids. Results are reported for three systems using two commercial catalysts, 10,11-dihydrocinchonidine (HCd) and O-methyl-HCd as modifiers, and ethanol. toluene, and acetic acid as solvents. The modifier concentration was varied between 0 and 6 mmol/liter and the reactions were carried out at room temperature at 20 and 100 bar hydrogen pressure. The fully modified catalysts displayed enantioselectivities up to 95%. A simple kinetic model was developed which is able to describe the observed dependence of both rate and enantioselectivity on the modifier concentration. The main feature of the model is the reversible formation of modified active sites by adsorption of single cinchona molecules on the Pt surface whereby the modified sites display high enantioselectivity and an enhanced catalytic activity. The scope and limitations of the simple two-site model together with the underlying assumptions are discussed and a three-site model is proposed in order to rationalize the effects observed at high modifier concentrations. © 1993 by Academic Press, Inc.