Investigations of catalysis by palladium nanoparticles embedded in multilayer polyelectrolyte films

Palladium nanoparticles immobilized in multilayer polyelectrolyte films were prepared by alternating adsorption of poly(acrylic acid) (PAA) and a poly(ethyleneimine)-Pd(II) (PEI-Pd(II)) complex on alumina followed by in situ reduction of Pd(II) by NaBH4. These polymer-encapsulated nanoparticle catalysts showed order of magnitude selectivities in hydrogenation of substituted unsaturated alcohols of different sizes. Hydrogenation experiments with [PAA/PEI]3PAA-coated commercial 5% Pd on alumina suggest that selectivity likely occurs due to differential rates of diffusion to embedded nanoparticles.; In Heck reactions, a [PAA/PEI-Pd(0)]3PAA film on alumina allowed selective coupling of methyl acrylate to iodobenzene in the presence of di(ethylene glycol) ethyl ether acrylate (DEGA), but there was little selectivity between methyl acrylate and tert-butyl acrylate. The high temperature (95°C) may limit selectivity. Pd nanoparticles were also prepared by alternating adsorption of PEI and PdCl42- followed by reduction of Pd(II). Such films were nonselective in Heck coupling, probably because the active catalyst in these reactions was Pd that was leached from the film. Atomic emission spectroscopy confirmed the leaching of Pd.