Despite the availability of many water-soluble organometallic and nanoparticle catalysts, the direct application of water-soluble catalysts for the reaction of immiscible and hydrophobic substrates has been hindered by the low solubility of nonpolar reactants in water. Herein, we introduce two different strategies employing ligand-capped Pd nanoparticles as a platform for facile biphasic hydrogenation of olefins. The first approach is based on water-soluble alkanethiolate-capped Pd nanoparticles that have micelle-like structures. The Pd nanoparticle core is surrounded by thiolated alkyl chains, maintaining hydrophobic characteristics near catalytically active sites, and terminated with COO<sup>-</sup> groups, providing the solubility in aqueous phase. The catalytic reactions of various hydrophobic substrates with C=C bonds (e.g. 1-octen-3-ol and 1-octene) are performed. The micellar characteristics of Pd nanoparticles allow the hydrophobic substrates to momentarily enter the hydrophobic region of the catalysts with adequate stirring force. The kinetic studies indicate that the rate of reaction increases for water-soluble Pd nanoparticle catalysts compared to organic-soluble Pd nanoparticle counterparts, indicating the hydrophobic effects of micelle-like structure. The second approach employs the protocol of encapsulating hydrophobic alkanethiolate-capped Pd nanoparticles in the spherical vesicle of phosphatidylcholine (PC) lipids. The produced liposome-encapsulated Pd nanoparticles also maintain active surface sites in hydrophobic environments showing a good catalytic activity toward olefin hydrogenation under atmospheric pressure and at room temperature. After the reactions, the resulting hydrophobic products from both biphasic systems are subsequently separated and the remaining aqueous phase containing Pd nanoparticles become immediately ready for the recycled uses. The recycling tests of catalysts indicated that the aqueous phase containing both Pd nanoparticles could be reused multiple times with only minimum decreases in the overall reaction rate. No other surfactants and co-solvents are necessary for the complete reaction of various olefin substrates, making the recycling of catalysts easier and the purification of products simpler.

Colloidal Pd nanoparticles with hydrophobic catalytic sites: Biphasic hydrogenation of olefins in water

256th National Meeting (2018)

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