CO₂-Responsive CuI-Ionic Liquid/Zeolitic Imidazolate Frameworks Stabilized Pickering Emulsions for Azide–Alkyne Click Reaction

Abstract

Metal−organic frameworks (MOFs) are becoming perspective emulsifiers for the building of Pickering emulsions on account of their large specific surface area, adjustable pore size, and tunable structure. Nevertheless, stimulusresponsive MOF-stabilized Pickering emulsions have been rarely presented, despite their great importance in interfacial catalysis, oil transportation, and emulsion polymerization. In this work, a novel kind of CuI-ionic liquid/zeolitic imidazolate frameworks (ZIF-8) composite has been designed and prepared by anchoring CuI on the hierarchical channels of ZIF-8 with the assistance of N-alkyl-N, N-dimethyl ethylamine imidazole salts ([C n DMEA][Im], n = 4, 6, 8, 10, 12, and 14), and been applied for constructing CO 2 -responsive Pickering emulsions. The results showed that even when the content of CuI-IL/ZIF-8 was 0.25 wt %, the functionalized MOFs could still effectively emulsify ethyl acetate and water to constitute a stable Pickering emulsion. Upon adding CO 2 at room temperature and atmospheric pressure, the Pickering emulsion was able to switch from emulsification to demulsification. By uniting various spectral techniques, it can be shown that the driving force for the phase transition came from the chemical reaction of CO 2 with IL on ZIF-8 and the creation of more hydrophilic bicarbonate and carbamate. Once CO 2 was expelled, the emulsion could be restored by a reversible reaction. Through utilizing the switchable Pickering emulsion as a microreactor, an efficient and controllable copper(I)-catalyzed alkyne/azide cycloaddition was realized under the catalysis of CuI-IL/ZIF-8 to provide the effective union of click reaction, product isolation, and recyclability of the MOF-based catalyst for a sustainable reaction procedure.