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.

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.

Phase-transfer processes between immiscible liquids play a prominent part in various applications, particularly for separation and recovery of catalysts. Zeolitic-imidazolate frameworks (ZIFs), as a specific subset of metal–organic frameworks, have been proven to be encouraging candidates for catalysts or cocatalysts because of their appealing characteristics, whereas recovery and reusability of ZIFs still remain a challenge because of their submicrometer-to-micrometer size. In this work, a kind of amino-modified ZIF-90 was designed and prepared, and by adjusting the hydrophobicity and hydrophilicity of surface species, these ZIFs could reversibly transfer between organic solvents and water by alternate bubbling of CO2 and N2. It was found that the mechanism of the phase transfer was aroused from the reversible formation of hydrophilic salts upon CO2 addition. By employing this novel property of the functionalized ZIF-90, an efficient and regulable CuI-catalyzed cycloaddition was fulfilled wherein the amino-modified ZIF-90 was applied to encapsulate, activate and reuse the catalyst, and regulate the reaction.

Amphoteric rare-earth ion dopants can improve ferroelectric ceramic electrical performance because of their amphoteric ion radius and valences. Herein, Ho 3þdoped Ba 0.90 Ca 0.10 (Ti 0.95 Zr 0.05 ) 2 O 5 ceramics are sintered at 1230 °C with assynthesized nanoparticles. The effects of various holmium contents (x) on the phase, structure, dielectric, and piezoelectric characteristics are investigated. For low x (<1.5 mol%), Ho 3þ occupies the Ba 2þ or/and Ca 2þ sites, leading to high dielectric properties due to decreased oxygen vacancies, while the electrical properties weaken with further increase in x. The ferroelectricity and piezoelectricity also deteriorate when x > 1.5 mol% because of the re-emerged oxygen vacancies and interrupted long-range ordering with large compositional fluctuations. The defect chemistry mechanism at different x contents is deduced in accordance with the phase and electrical properties. The activation energy initially elevates to 1.56 eV and then decreases to 1.35 eV with increasing x. Results indicate that the ceramics are promising candidates for lead-free ferroelectric ceramics.

No abstract