Enantioselective hydrogenation I. Surface conditions during methyl pyruvate hydrogenation catalyzed by cinchonidine-modified platinum/silica (EUROPT-1)

Abstract

An investigation has been made of surface conditions during the enantioselective hydrogenation of methyl pyruvate, McCOCOOMe, to R-(+)-methyl lactate, McCH(OH)COOMe, at 10 bar pressure catalyzed by 6.3% Pt/silica EUROPT-1 modified by cinchonidine. Isotherms for the adsorption of cinchonidine and of methyl pyruvate at 298 K are reported. During modification, cinchonidine adsorbs both on the platinum active phase and on the silica support; co-adsorption of ethanol with cinchonidine on the platinum enhances the effectiveness of the modifier. Methyl pyruvate is strongly chemisorbed and forms physically adsorbed multilayers on EUROPT-1. Cinchonidine promotes the rate of methyl pyruvate hydrogenation at 293 K by a factor of 25. Some characteristics of the reaction of cinchonidine with adsorbed hydrogen are reported. The effectiveness of cinchonidine both as a modifier and as a promoter collapses above 322 K. The kinetics of methyl pyruvate hydrogenation are reported as are the optical yields obtained over the catalyst preconditioned in four ways before modification. The best activity and enantioselectivity were observed over EUROPT-1 reduced at 373 K from which hydrogen had been desorbed before modification. A mechanism is proposed which interprets both the observed enantioselectivity and the promoting effect of cinchonidine on the rate of reaction.