CO₂-switchable amidine-modified ZIF-90-stabilized Pickering emulsions for controllable Knoevenagel condensation reactions

Stimulus-switchable Pickering emulsions have been widely researched in recent years, and the use of CO2 as an interesting stimulus has been of special interest. However, CO2-switchable MOF-based Pickering emulsions have rarely been reported, although they are important in applications in interfacial catalysis, the synthesis of nanoparticles, and crude oil transport. In this study, a new class of amidine-modified ZIF-90 was developed through a post-synthesis functionalization, and then employed to build CO2-switchable Pickering emulsions. The results showed that the amidine-modified ZIF-90 was capable of emulsifying cyclohexane-water mixtures to fabricate stable emulsions at no less than 0.35 wt% content. Once CO2 was added at a normal pressure and temperature, the Pickering emulsion was found to undergo a switch from the emulsifying to the demulsifying state. Through combining various spectroscopic methods, it was revealed that the mechanism for this phase switching originated from the reaction of amidine and CO2 on the ZIF-90, as well as the production of hydrophilic salts. When CO2 was driven out, the Pickering emulsion was reconstructed via a reverse reaction. By applying the emulsion as a micro-reactor, an efficient and controllable Knoevenagel condensation reaction was obtained, wherein amidine-modified ZIF-90 was employed as the catalyst and controller for the reaction.