Thermoresponsive Polymeric Nanoparticles as Efficient Pickering Interfacial Catalysts for Selective Oxidation of Sulfides

Pickering emulsions provide a versatile platform for liquid–liquid two-phase reactions; however, the separation and recovery of solid catalysts have always been plagued by strong adsorption at the interfaces. In this study, we developed thermoresponsive polymeric nanoparticles by simply copolymerizing the monomers of N-isopropylacrylamide (NIPAM) and 2,2,6,6-tetramethyl-4-piperidinyl methacrylate (TMPM) and oxidizing the precursor to 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). Structural design and molecular modulation give the polymeric catalysts excellent amphiphilicity, which allows them to be tightly adsorbed at the interfaces, thus stabilizing oil-in-water (O/W) Pickering emulsions in biphasic reaction systems. Such emulsion systems with abundant interfacial areas can enhance reaction mass transfer and promote catalytic transformation. More importantly, modulating the wettability of the polymers leads to an oriented distribution of the catalysts at the interfaces, which in turn affects the contact between the substrate molecules and catalyst centers, avoiding overoxidation of the sulfides and improving the selectivity of sulfoxides. As expected, the selectivity of methyl phenyl sulfoxide in Pickering emulsion microreactors was 97.8% with a turnover frequency (TOF) of 10.0 mol mol–1 min–1, which is higher than previously reported results. When the systems warm to 37 °C at the end of the reactions, the amine groups of NIPAM immobilized on the polymer catalysts shrink, causing the particles to become hydrophobic and thus spontaneously desorbed from the interfaces, resulting in demulsification. After recycling ten runs, the polymeric nanocatalysts showed no loss on either conversion or selectivity and maintained stable catalytic activity. The conceptually temperature-responsive Pickering emulsions provide a clean and effective channel to efficiently recycle the polymeric catalysts and promote the catalytic activity of biphasic reactions.