CO₂-Triggered Hierarchical-Pore UiO-66-Based Pickering Emulsions for Efficient and Recyclable Suzuki–Miyaura Cross-Coupling in Biphasic Systems

Hierarchical-pore metal–organic frameworks (H-MOFs) are considered to be emerging stabilizers for Pickering emulsion formation because of their hierarchically arranged pores, tailorable structures, and ultrahigh surface areas. However, stimulus-triggered Pickering emulsions built by H-MOFs have been seldom presented to date despite their great significance in diverse applications. Herein, by grafting Pd(OAc) 2 on the hierarchical-pore zirconium MOF UiO-66, namely, H-UiO-66, with the aid of 1-alkyl-3-methylimidazolium 2-cyanopyrrolide salts ([C n MIM][2-CN-Pyr], n = 4, 6, and 8), a series of Pd(OAc) 2 -[C n MIM][2-CN-Pyr]@H-UiO-66 have been developed and utilized as emulsifiers for constructing CO 2 -switching Pickering emulsions. It was found that Pd(OAc) 2 -[C n MIM][2-CN-Pyr]@H-UiO-66 was able to stabilize the n -hexane–water mixture to form a Pickering emulsion even at an amount of 0.5 wt %. Upon alternate addition of CO 2 and N 2 at normal pressure, Pickering emulsions could be smartly converted between demulsification and re-emulsification. Through combining varieties of spectroscopic techniques, the mechanism of the switchable phase transformation lay in the acid–base reaction of ionic liquids with CO 2 on H-UiO-66 and the creation of more hydrophilic salts, which reduced the wettability of the emulsifier and destabilized the emulsion. As an example of application, the stimulus-triggered Pickering emulsion was employed as a palladium-catalyzed Suzuki–Miyaura cross-coupling microreactor to achieve the combination of chemical reactions, isolation of products, and recovery of catalysts.