LIQUID-PHASE ASYMMETRIC HYDROGENATION ON PT-CONTAINING ZEOLITE-Y CATALYSTS

5 wt.-% Pt/MNaY catalysts (M=rare earths, Ca, Mg and H with ion exchange degrees of alpha approximate to 0.65) combined with (-)cinchonidine as the chiral auxiliary were used successfully for the enantioselective hydrogenation of ethyl pyruvate to R(+)ethyl lactate which resulted in enantiomeric excesses of about 73% (with cyclohexane as the solvent) and 85% (with acetic acid as the solvent). The acidity of the zeolite carrier and the dispersion of the active Pt component do not influence the enantioselectivity of the reaction within the range investigated, but both distinctly control the catalytic activity, which increases with decreasing dispersion and rising acidity. The activities of the catalysts were obtained from kinetic data of the hydrogenation related to the specific surfaces of the active Pt components. The specific Pt surfaces and the dispersion data were derived from volumetric CO chemisorption measurements. 30 to 50 vol.-% of ethyl pyruvate are regarded as the optimal range for the initial concentration concerning both enantioselectivity and reaction rate. Furthermore, initial hydrogen pressure of at least 3 MPa is necessary in order to achieve good enantioselectivities and catalytic activities. The temperature range of 293 to 303 K was found to be optimal. With respect to the dependence of the initial reaction rate on temperature, a change in reaction kinetics was observed between 323 and 333 K.