Zr-Based MOF-Stabilized CO₂-Responsive Pickering Emulsions for Efficient Reduction of Nitroarenes

A Pickering emulsion is a natural microreactor for interfacial catalysis in which an emulsifier is critical. Recently, a metal-organic framework (MOF) has attracted attention to emulsify water-organic mixtures for constructing a Pickering emulsion. However, a few stimuli-responsive Pickering emulsions based on MOFs have been reported, and the MOF emulsifiers cannot be regenerated at room temperature. Herein, the Zr-MOF with a rodlike morphology is synthesized using ionic liquid as a modulator and then modified with n-(trimethoxysilylpropyl)imidazole (C₃im) to prepare a series of functionalized Zr-MOFs (MOF-C₃im). It is found that MOF-C₃im is an excellent emulsifier to construct stable and CO₂-responsive Pickering emulsions even at low content (>0.20 wt %). Notably, the emulsification and demulsification of the emulsions can be easily and reversibly switched by bubbling of CO₂ and N₂ alternatively at room temperature because CO₂ and imidazole molecules anchored on the Zr-MOF underwent a reversible acid-base reaction, resulting in an obvious change in the wettability of the emulsifier. As a proof of concept, the reduction reactions of nitrobenzene have been successfully carried out in these Pickering emulsions, demonstrating the efficient integration as a microreactor for chemical reaction, product separation, and emulsifier recycling under ambient conditions. This strategy provides an innovative option to develop stimulus-responsive Pickering emulsions for sustainable chemical processes.