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

Solvent-stabilised Pt and Pd nanoparticles, of size range 2.3-2.8 nm and 3. 7-3.8 nm, respectively, have been prepared by metal vapour synthesis routes , characterised by transmission electron microscopy (TEM), and their behavi our as catalysts for the enantioselective hydrogenation of ethyl pyruvate ( EP) investigated; comparisons have been effected with the performance of st andard supported Pt and Pd catalysts. Cinchona alkaloid-modified Pt nanopar ticles display parallel behaviour to that exhibited by their conventional s upported counterparts both in terms of the sense of the enantioselectivity in the ethyl lactate product and in the acceleration in reaction rate relat ive 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 ret ardation is noted on cinchona alkaloid modification. The Pt particle size d istribution shows a higher degree of monodispersity after use in catalysis, although the average particle site remains essentially unchanged, whereas the behaviour of the Pd nanoparticles shows evidence of concentration depen dence, lower concentrations showing Pt-like behaviour bur more highly conce ntrated preparations showing evidence of significant aggregation during cat alysis. With Pt catalysts, the presence of water as a component of the keto nic solvent system is shown to result in a significant acceleration in over all reaction rate with both conventional supported catalysts and their solv ent-stabilised counterparts. In sharp contrast, totally aqueous-based collo idal platinum preparations, obtained by conventional salt reduction, displa y very low reaction rates and enantioselectivities. (C) 1999 Elsevier Scien ce B.V. All rights reserved.