CO2-Switchable Hierarchically Porous Zirconium-Based MOF-Stabilized Pickering Emulsions for Recyclable Efficient Interfacial Catalysis

Abstract

Stimuli-responsive Pickering emulsions are recently being progressively utilized as advanced catalyzed systems for green and sustainable chemical conversion. Hierarchically porous metal–organic frameworks (H-MOFs) are regarded as promising candidates for the fabrication of Pickering emulsions because of the features of tunable porosity, high specific surface area and structure diversity. However, CO2-switchable Pickering emulsions formed by hierarchically porous zirconium-based MOFs have never been seen. In this work, a novel kind of the amine-functionalized hierarchically porous UiO-66-(OH)2 (H-UiO-66-(OH)2) has been developed using a post-synthetic modification of H-UiO-66-(OH)2 by (3-aminopropyl)trimethoxysilane (APTMS), 3-(2-aminoethylamino)propyltrimethoxysilane (AEAPTMS) and 3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane (AEAEAPTMS), and employed as emulsifiers for the construction of Pickering emulsions. It was found that the functionalized H-UiO-66-(OH)2 could stabilize a mixture of toluene and water to give an emulsion even at 0.25 wt % content. Interestingly, the formed Pickering emulsions could be reversibly transformed between demulsification and re-emulsification with alternate addition or removal of CO2. Spectral investigation indicated that the mechanism of the switching is attributed to the reaction of CO2 with amino silane on the MOF and the generation of hydrophilic salts, leading to a reduction in MOF wettability. Based on this strategy, a highly efficient and controlled Knoevenagel condensation reaction has been gained by using the emulsion as a mini-reactor and the emulsifier as a catalyst, and the coupling of catalysis reaction, product isolation and MOF recyclability has become accessible for a sustainable chemical process.

CO2-Switchable colloids

“…For example, MOF (metal-organic framework) particles with surface amine groups are able to stabilize toluene-in-water emulsions in the absence of CO 2 (neutral, moderately hydrophobic surface state) but not in its presence (charged, highly hydrophilic state), likely because the charged state is far too hydrophilic to be wetted by toluene. 20 Finally, CO 2 -switching can change the chemical reactivity of particles. For example, a heterogeneous hydrogenation catalyst can be prepared by putting a polymeric amine coating on silica particles, and then embedding Ru nanoparticles into that coating.…”

“…These can either be switchable small molecules or switchable polymers grafted to or from the surface. For example (3-aminopropyl)trimethoxysilane (H 2 NCH 2 CH 2 CH 2 Si(OMe) 3 ) can be grafted to a MOF particle surface 20 or PDMAPMAm (poly(dimethylaminopropyl methacrylamide)) can be grafted from a silica particle (Scheme 4). ''Grafting from,'' meaning growing a polymer chain from a surface-bound initiator, tends to generate a polymer coating with a greater density of chains per unit surface area but unknown molecular weights, while ''grafting to,'' meaning growing a polymer chain in solution and then attaching one end of it to the surface, gives lower grafting density but with known molecular weights.…”

“…For example, MOF (metal–organic framework) particles with surface amine groups are able to stabilize toluene-in-water emulsions in the absence of CO 2 (neutral, moderately hydrophobic surface state) but not in its presence (charged, highly hydrophilic state), likely because the charged state is far too hydrophilic to be wetted by toluene. 20…”

“…The potential utility of the CO 2 -switchable MOFs was demonstrated using the Knoevenagel condensation reaction as a model system, showing that inherent disadvantages of a homogeneous reaction could be overcome using a Pickering emulsion to achieve effective interfacial catalysis followed by facile separation of the two phases to enable both product recovery as well as re-use of the modified ZIF-90. Citing the limited acid and alkaline resistance of the TETA-modified ZIF-90, Pei et al 20 proposed a new CO 2 -switchable MOF also designed for Pickering stabilization but synthesized using the zirconium-based UiO-66-OH MOF that was functionalized with (3-aminopropyl)triethoxysilane (APTES). This UiO-66-O-APTES MOF was shown to reversibly stabilize toluene-in-water emulsions (as well as benzene and n -hexane as the oil phase), with stable emulsions generated under N 2 and demulsification occurring under CO 2 , opposite behaviour to the ZIF-90/TETA MOFs.…”

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

“…However, among the reported MOF emulsifiers, − most of them are restricted to a micropore range that would slow down diffusion and impede large substrate molecules from approaching the active sites of a host MOF. Hierarchically porous MOFs have gained extensive attention because of their multiscale structures, enhanced molecular accessibility, especially as heterogeneous platforms to encapsulate or immobilize functional species, and novel properties for multiple important application potentials in industrial processes, , but they are under-developed owing to their relatively poor stability and low structure tunability. It is of great significance to develop functionalized MOFs with hierarchical pores for the construction of switchable Pickering emulsions for broader applications.…”