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

Article abstract of DOI:10.1246/bcsj.73.2635

The enantioselective hydrogenation of the C=C double bond in (E)-α-phenylcinnamic acid has been studied with different supported Pd catalysts prepared by a precipitation-deposition method and modified with cinchonidine. The influences of the support materials and the texture, Pd loadings, precipitation procedures, and reduction conditions on the hydrogenation activity and enantioselectivity are described. The texture of the supports has a decisive influence on the behavior of the resulting catalysts, especially when it is pre-reduced at elevated temperatures. Nonporous titania with a relatively small specific surface area was found to be the most suitable. The use of a large excess amount of precipitant resulted in an increase in the amount 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 preferable for the enantioselectivity. A considerable increase in the enantioselectivity was observed when a 5wt%Pd/TiO₂ catalyst, prepared with nonporous titania, was reduced in a hydrogen flow 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 modified Pd/TiO₂ catalysts depends on the Pd dispersion; a relatively low dispersion around 0.2-0.3 was found to be optimal.