Preparation of cinchonidine-modified palladium catalysts for the enantioselective hydrogenation of (E)-alpha-phenylcinnamic acid

The enantioselective hydrogenation of the C=C double bond in (E)-alpha -phe nylcinnamic acid has been studied with different supported Pd catalysts pre pared by a precipitation-deposition method and modified with cinchonidine. The influences of the support materials and the texture, Pd loadings, preci pitation procedures, and reduction conditions on the hydrogenation activity and enantioselectivity are described. The texture of the supports has a de cisive influence on the behavior of the resulting catalysts, especially whe n it is pre-reduced at elevated temperatures. Nonporous titania with a rela tively small specific surface area was found to be the most suitable. The u se of a large excess amount of precipitant resulted in an increase in the a mount of residual Na and in a drastic decrease in the amount of residual Cl , leading to a remarkable increase in the activity. The elimination of both the residual Na and Cl from the catalyst precursor was found to be prefera ble for the enantioselectivity. A considerable increase in the enantioselec tivity was observed when a 5wt%Pd/TiO2 catalyst, prepared with nonporous ti tania, was reduced in a hydrogen Row at elevated temperatures up to 473 K. The influences of the surface area of nonporous supports, Pd loadings, and reduction conditions strongly suggest that the enantioselectivity of the mo dified Pd/TiO2 catalysts depends on the Pd dispersion; a relatively low dis persion around 0.2-0.3 was found to be optimal.