Recyclable amphiphilic metal nanoparticle colloid enabled atmospheric oxidation of alcohols.

Developing amphiphilic colloid catalysts are essentially important for realizing environmentally benign biphasic catalysis under atmospheric conditions. Herein, a linear structured plant polyphenol was employed amphiphilic stabilizer for preparing a series of amphiphilic Pd nanoparticles (PdNPs) colloids. For the as-prepared PdNPs colloids, the phenolic hydroxyls of plant polyphenols were responsible for the stabilization of PdNPs, while the rigid aromatic scaffold of plant polyphenols effectively suppressed the PdNPs from aggregation by providing high steric effect. Thanks to the coexistence of hydrophilic phenolic hydroxyls and hydrophobic aromatic rings, the plant polyphenols provided the PdNPs tunable amphiphilic properties, which allowed an easier wetting of PdNPs to the substrate molecules. By tuning the content of plant polyphenols in the colloid, the particle size (3.17-4.73 nm) and dispersity of PdNPs were facilely controlled. When applied for atmospheric oxidation of insoluble alcohols in water by air, the amphiphilic PdNPs preferentially absorbed the alcohol substrates to create a relatively high substrate concentration microenvironment, which improved the mass transfer in the biphasic catalysis, allowing the proceeding of low-temperature (50 oC) atmospheric oxidation of diverse alcohols with high catalytic conversion, including aliphatic alcohols, cyclic aliphatic alcohols and aromatic alcohols. Furthermore, the amphiphilic PdNPs colloid also exhibited excellent reusability with a conversion yield high up to 97.96% in the 5th cycle. In contrast, the control catalysts of PVP-and PEG-stabilized PdNPs were completely inactived in the 5th cycle. As a consequence, our findings provided a new route for developing environmentally benign aqueous colloid catalyst that are both highly active and recyclable for mild biphasic oxidation reaction systems.