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

Abstract

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 (C 3 im) to prepare a series of functionalized Zr-MOFs (MOF-C 3 im). It is found that MOF-C 3 im is an excellent emulsifier to construct stable and CO 2 -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 CO 2 and N 2 alternatively at room temperature because CO 2 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.

Multiresponsive Behavior of the Pickering Emulsifier and Its Application for Collecting Small Oil Droplets in Water

“…In recent years, stimuli-responsive nanoparticle surfactants have attracted much attention due to their controllable surface activity . The emulsifier reversibly switches between active and inactive forms to achieve emulsion breaking and emulsification by applying external stimuli to the emulsion, such as CO 2 , pH, light, and redox reactions. , The pH response can be quickly and easily controlled by the addition of acids and bases to control the breaking emulsification behavior of Pickering emulsions. − However, CO 2 triggering have superior appeal because CO 2 is cheap, green, nontoxic, and does not pollute the system . Therefore, CO 2 and pH are more suitable for practical industrial applications than other stimuli.…”