Recyclable Amphiphilic Metal Nanoparticle Colloid Enabled Atmospheric Oxidation of Alcohols

Developing amphiphilic colloid catalysts is essentially important for realizing environmentally benign biphasic catalysis under atmospheric conditions. Herein, a linear structured plant polyphenol was employed as an 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, whereas the rigid aromatic scaffold of plant polyphenols effectively suppressed the PdNPs from aggregation by providing a high steric effect. Thanks to the coexistence of hydrophilic phenolic hydroxyls and hydrophobic aromatic rings, the plant polyphenols induced tunable amphiphilic properties into the PdNPs, allowing an easier wetting of PdNPs with the substrate molecules. By tuning the content of plant polyphenols in the colloid, the particle size (3.17-4.73 nm) and the dispersity of the 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 °C) 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 fifth cycle. In contrast, the control catalysts of poly(vinylpyrrolidone)- and poly(ethylene glycol)-stabilized PdNPs were completely inactivated in the fifth cycle. As a consequence, our findings provided a new route for developing an environmentally benign aqueous colloid catalyst that is both highly active and recyclable for mild biphasic oxidation reaction systems.