CO₂-Switchable Emulsification and Demulsification of Pickering Emulsion Stabilized by Zirconium-Based Metal–Organic Frameworks

Abstract

Stimuli-responsive Pickering emulsions have aroused considerable amounts of attention recently because of their potential for on-demand demulsification. Metal–organic frameworks (MOFs) are promising emulsifiers for the construction of Pickering emulsions due to their ultrahigh specific surface area, tunable porosity, and designable structures. However, CO2-responsive zirconium-based MOF-stabilized Pickering emulsions have been rarely reported up to now. Herein, a new series of amine-functionalized UiO-66-O-APTES were developed via the covalent post-modification of UiO-66-OH with different amounts of (3-aminopropyl)­triethoxysilane (APTES) and were then applied to construct CO2-switchable Pickering emulsions. It was shown that UiO-66-O-APTES could emulsify toluene and water well to form an emulsion even at the content of 0.1 wt %. Significantly, the Pickering emulsion could be reversibly demulsified and re-emulsified by alternate CO2 and N2 bubbling under constant pressure. Mechanism results revealed that the CO2-switchable phase transition based on the formation of hydrophilic ammonium salts resulting from the reaction of APTES with CO2, which lowered the wettability of particles, reduced emulsion stability, and resulted in emulsion breaking. After CO2 was removed, the Pickering emulsion could be rebuilt by the reverse reaction. By using this strategy, a controllable and highly effective CuI-catalyzed cycloaddition reaction has been achieved, and the coupling of efficient condensation, product separation, and recovery of MOF catalysts has been demonstrated to give rise to a sustainable reaction process.

CO2-Induced Phase Transfer of Amino-Modified ZIF-90 between Immiscible Liquid Phases for Recyclable Efficient Synergistic Catalysis

“…As shown in Figure c, a new peak for C 1 s of DEPDA-ZIF-90 appeared at 292.0 eV after adding CO 2 and was ascribed to the bicarbonate . Meanwhile, a new signal at 401.2 eV was observed for the reacted DEPDA-ZIF-90 and belonging to N 1 s transformation in ammonium salts (Figure d), demonstrating the production of ammonium salts . In addition, XPS was also determined for the recycled amino-modified ZIF-90 resulting from N 2 bubbling, and the results were shown in Figure S23.…”

“…42 Meanwhile, a new signal at 401.2 eV was observed for the reacted DEPDA-ZIF-90 and belonging to N 1s transformation in ammonium salts (Figure 4d), demonstrating the production of ammonium salts. 43 In addition, XPS was also determined for the recycled amino-modified ZIF-90 resulting from N 2 bubbling, and the results were shown in Figure S23. It is clear that the signals at 292.0 and 401.2 eV disappeared from the high-resolution C 1s and N 1s XPS spectra of the recycled DEPDA-ZIF-90, which was consistent with its original sample, suggesting that the reaction between the amine of the functionalized ZIF-90 and CO 2 in aqueous solution was completely reversible.…”

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

“…As given in Figure S1, all Pd­(OAc) 2 -[C n MIM]­[2-CN-Pyr]@H-UiO-66 were composed of Zr, C, N, O, and Pd elements. Based on their high-resolution XPS spectra, the peaks at 178.9 and 181.3 eV were attributed to Zr 3d 5/2 and Zr 3d 3/2 (Figure S2), respectively. The binding energies at 284.9, 286.6, and 288.4 eV were attributed to C–C/CC, C–N, and CO (Figure S3), respectively.…”

“…Of particular note is that CO 2 is cheap, environmentally benign, has good biocompatibility and easy removal, and thus is an alluring trigger for many switching processes. − Metal–organic frameworks (MOFs), as a burgeoning type of porous crystalline material, feature large surface areas, tailorable porosity, and highly diversified structures, , which make them particularly important in the manufacture of advanced Pickering emulsifiers. However, a majority of the MOF stabilizers reported are limited to the microporous range. − These micropores inherently restrict the diffusion and mass transfer of chemical species and thus are hardly accessible to anchor molecular catalysts, impregnate catalyst precursors, or transport large numbers of drug molecules, which greatly restrains the diverse applications of emulsions formed by them. Therefore, hierarchical porous MOFs (H-MOFs) have been taken into account due to their hierarchical structures of the micropore, mesopore, and macropore, molecular accessibility, and a multitude of functions in a variety of applications. , However, few reports about CO 2 -triggered H-MOF-stabilized Pickering emulsions are currently available due to the difficulty in synthesizing H-MOFs with highly structural tunability and good stability.…”

“…Zirconium­(IV)-containing MOFs are of great interest and significance due to their superior performance such as high thermostability, resistance to common solvents, and high porosity. , Herein, by immobilizing Pd­(OAc) 2 -ILs onto the hierarchical-pore zirconium­(IV)-containing MOF UiO-66, namely, H-UiO-66, through PSM, a novel class of Pd­(OAc) 2 -IL@H-UiO-66 composites have been developed and utilized as emulsifiers to construct CO 2 -switchable Pickering emulsions. It was found that Pd­(OAc) 2 -IL@H-UiO-66 could emulsify n -hexane–water mixtures to build stable Pickering emulsions even at an amount of 0.5 wt %, and reversible demulsification and re-emulsification of the formed emulsions could be easily achieved via alternate adding of CO 2 and N 2 (Scheme ).…”