The importance of modifier pH in the generation of enantioselective nickel catalysts

Abstract

Enantioselective Ni/SiO₂ catalysts have been prepared by modification with aqueous solutions of (R)-(+)-tartaric acid (TA) and used in the asymmetric hydrogenation of a prochiral β-keto ester (methylacetoacetate) to a β-hydroxy ester ((R)-(-)-methyl-3-hydroxybutyrate). The simultaneous adsorption of TA and corrosive leaching of surface nickel metal are graphically illustrated and the progress of TA buildup on the catalyst surface with the duration of modification is presented. Variations of modifier pH were found to strongly affect the modification process and influence the ultimate hydrogenation rate and enantioselectivity. The surface coverage by TA is correlated to the asymmetric activity and an optimum fractional coverage of 0.2 is identified; at higher coverages modification with basic TA solutions yielded superior enantioselectivities. While TA treatment in basic media was less corrosive, a proportion of the surface enantioselective nickel sites was leached into solution during the hydrogenation step. The difference in the response of nickel precursors, prepared by impregnation and homogeneous precipitation/deposition, to TA treatment is compared and discussed in terms of metal/support interaction.