A thermo-responsive MOF-based H₂O₂-in-oil “pickering emulsion in situ catalytic system” for highly efficient olefin epoxidation

A newly functionalized MIL-101(Fe) type MOF, namely NaPT@MIL-101-NH-PNIPAM, decorated with a thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) brush outside and loaded with sodium phosphotungstate particles (NaPT) inside has been successfully synthesized. The new thermally responsive amphiphilic NaPT@MIL-101-NH-PNIPAM powder can well serve as a highly efficient Pickering emulsifier to stabilize the H₂O₂-in-oil emulsion. The resultant H₂O₂-in-oil Pickering emulsion displayed good catalytic performance in the epoxidation of olefins at room temperature, promoting a greener reaction with good yield and selectivity. Notably, the constitutes (H₂O₂, olefin) of the Pickering emulsion is also the substrate of the epoxidation of the olefin reaction, which will facilitate further expansion of the reaction scale. Moreover, the catalyst of the Pickering emulsion featured intriguing thermo-responsive properties of swelling/shrinking behavior that can trigger emulsification at room temperature (25 °C) and accomplish demulsification at an elevated temperature (50 °C), mainly benefitting from the thermally responsive amphiphilic nature of the attached PNIPAM polymer. This thermo-responsive feature of the Pickering emulsion catalyst renders the catalyst-product separation and catalyst recycling feasible during the epoxidation of alkenes. This work reveals new insights for the design of a highly efficient heterogeneous catalytic system in H₂O₂-based epoxidation and may have potential for application in industry in the future.