Controlling the enantioselective hydrogenation of ethyl pyruvate using zeolites as catalyst support

Abstract

5 wt-% Pt/carrier catalysts (carrier materials: NaY, mordenite, erionite and NaX as well as γ-Al 2 O 3 as a microporous, non-zeolitic support) combined with (-)cinchonidine as the chiral auxiliary were used successfully for the enantioselective hydrogenation of ethyl pyruvate to R(+)ethyl lactate. With acetic acid as the solvent, the catalysts Pt/NaY, Pt/mordenite and Pt/erionite produced high enantiomeric excesses (86 to 90% ee), whereas Pt/NaX (77% ee) and Pt/γ-Al 2 O 3 (67% ee) gave comparatively poor optical yields. With regard to the k 0 values, which were obtained from kinetic data of the hydrogenation related to the mass of the catalysts only, a correlation between enantioselectivity and catalytic activity was not discernible. This changed significantly, if referring to the k'value related to the mass of the catalyst and its specific Pt surface area (derived from volumetric CO chemisorption measurements). Then, the catalysts producing the highest enantiomeric excesses also resulted in the highest catalytic activities. Therefore, catalytic activity and enantioselectivity essentially depend on the specific Pt surface area of the catalyst. This means that Pt particles of appropriate geometry and size are necessary to generate high enantioselectivity and catalytic activity. Zeolites Y, mordenite and erionite turned out to be appropriate templates for the creation of such Pt particles during catalyst preparation. This was confirmed by temperature-programmed CO desorption measurements. Zeolite X was partially destroyed by acid solution during catalyst preparation, which was proved by DTA and XRD measurements. Therefore, zeolite X, as well as γ-Al 2 O 3 , could not act as a template for the creation of appropriate Pt particles.