Selective hydrogenation of ethyl-benzoylacetate to 3-hydroxy-3-phenyl-propionate catalyzed by Pd/C in EtOH as a solvent in the presence of KOH: The role of the enolate ion on the reaction mechanism

The selective hydrogenation of ethyl-benzoylacetate to 3-hydroxy-3-phenyl-propionate catalyzed by Pd/C in EtOH in a solution of KOH has been investigated. Mass transfers as well as adsorption and desorption stages do not influence reaction kinetics. A kinetic model is proposed based on the best fitting of the experimental data with Langmuir–Hinshelwood type kinetics equation. The mechanism implies that the enolate of the ethyl-benzoylacetate adsorbs strongly on two sites, thus occupying a large part of the surface Pd atoms without any reaction. The ethyl-benzoylacetate adsorbs also on two sites but with adsorption equilibrium constant almost three order of magnitude lower than that of the enolate anions. Also the hydrogen is poorly adsorbed, however, it forms Pd–H and reacts with the adsorbed keto-ester by a step hydrogenation mechanism in which the first hydride insertion is the rate-determining step. Furthermore, due to the low surface Pd–H availability and the fast desorption of the 3-hydroxy-3-phenyl-propionate the consecutive hydrogenolysis of the C–OH bond of the product is practically suppressed, thus achieving selectivity close to 100%.