Preparation and characterisation of solvent-stabilised nanoparticulate platinum and palladium and their catalytic behaviour towards the enantioselective hydrogenation of ethyl pyruvate

Abstract

Solvent-stabilised Pt and Pd nanoparticles, of size range 2.3–2.8 nm and 2.7–3.8 nm, respectively, have been prepared by metal vapour synthesis routes, characterised by transmission electron microscopy (TEM), and their behaviour as catalysts for the enantioselective hydrogenation of ethyl pyruvate (EP) investigated; comparisons have been effected with the performance of standard supported Pt and Pd catalysts. Cinchona alkaloid-modified Pt nanoparticles display parallel behaviour to that exhibited by their conventional supported counterparts both in terms of the sense of the enantioselectivity in the ethyl lactate product and in the acceleration in reaction rate relative to the unmodified system. With Pd, however, significant differences are noted. Here, the sense of the enantioselectivity relative to that reported previously over conventional supported catalysts is reversed, i.e., an ( R )- vs. ( S )-enantiomer switch occurs, and a rate acceleration rather than retardation is noted on cinchona alkaloid modification. The Pt particle size distribution shows a higher degree of monodispersity after use in catalysis, although the average particle size remains essentially unchanged, whereas the behaviour of the Pd nanoparticles shows evidence of concentration dependence, lower concentrations showing Pt-like behaviour but more highly concentrated preparations showing evidence of significant aggregation during catalysis. With Pt catalysts, the presence of water as a component of the ketonic solvent system is shown to result in a significant acceleration in overall reaction rate with both conventional supported catalysts and their solvent-stabilised counterparts. In sharp contrast, totally aqueous-based colloidal platinum preparations, obtained by conventional salt reduction, display very low reaction rates and enantioselectivities.