Enantioselective hydrogenation of ethyl pyruvate catalysed by Pt/graphite: Superior performance of sintered metal particles

A 5% Pt/graphite catalyst has been sintered in 5% H2/Ar at 400, 500, 600, 700, 800, and 1000 K and cinchonidine-modified samples used to catalyse enantioselective ethyl pyruvate hydrogenation at 293 K and 30 bar pressure. Changes in catalyst morphology on sintering have been investigated by high resolution electron microscopy and cyclic voltammetry. The as-received catalyst contained small Pt particles aggregated into clusters. As the temperature was raised these clusters disaggregated; the resulting small particles grew, first showing enhanced crystallinity and better defined step and terrace topography, and later progressive faceting. Ultimately the particles were large and hexagonal. These processes were accompanied by a loss of surface area and a reduction in catalytic activity. Enantiomeric excess rose from 43% for as-received material to 63% for catalyst sintered at 700 K, and declined for higher sintering temperatures. The particle size distribution for the optimal catalyst peaked at 8 nm and most particles were substantially faceted. Chiral performance is interpreted in terms of enhanced enantioselectivity provided by edge sites in the Pt surface. Modification of chiral kink sites by alkaloid, to give diastereomeric enantioselective sites may have further enhanced the enantioselectivity. The investigation provides clear objectives for future catalyst design.