Molecular insight into the dynamics of chiral modification of Pt/alumina

A simple transient method—replacement of the chiral modifier in the reactor feed with a second modifier that gives the opposite enantiomer of the product in excess—was used to investigate the competition of cinchona alkaloids on a commercial Pt/Al2O3 catalyst. Ethyl pyruvate was hydrogenated in a continuous-flow fixed-bed reactor with high enantiomeric excess (up to 89%) and reaction rate (TOF 12,700 h−1) at a modifier/substrate molar ratio of only 307 ppm. The rate of modifier replacement indicates the following order of adsorption strength on Pt: cinchonidine > cinchonine > quinidine, which is in line with that observed with other techniques. The dynamics of the competition was investigated under different reaction conditions. The study supports the contention that the origin of “ligand acceleration” is not the suppression of catalyst deactivation by addition of the chiral modifier, because under appropriate conditions, catalyst deactivation is negligible in pyruvate hydrogenation. The catalytic experiments were completed with an ATR-IR study in a high-pressure, continuous-flow reactor cell in which the formation of (R)- and (S)-lactate, and the competition of cinchonidine and quinidine at the Pt surface, could be followed in situ.