Enantioselective hydrogenation of pyruvate esters in the mesoporous environment of Pt-MCM-41

Pt-MCM-41 catalysts having loadings of up to 2% Pt have been synthesised using three strategies: (i) direct synthesis from a Pt2+-containing gel, (ii) exchange of Na+ in Al-MCM-41 (containing 8% aluminium) for Pt2+, (iii) exchange of H+ in H-MCM-41 (containing 1% aluminium) for Pt2+. HRTEM confirmed the retention of the mesoporous structure in the active catalysts and gave information on Pt particle size and location. Al-27 NMR provided information on the movement of aluminium within the structure during catalyst preparation. Enantioselective hydrogenation of methyl and ethyl pyruvate was catalysed by cinchonidine-modified Pt-MCM-41 at 293 K and elevated hydrogen pressures; performance was compared to that provided by the standard reference catalyst EUROPT-1. Catalysts prepared by strategies (i) and (ii) performed best, giving values of the enantiomeric excess comparable to those afforded by EUROPT-1 at rates moderated by an order of magnitude by mass transfer effects. Performance was impaired when Pt particle size in the mesopores was so small that conditions favoured racemic reaction.