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

Area of Science:

• Materials Science
• Green Chemistry
• Catalysis

Background:

• Stimuli-responsive Pickering emulsions are emerging as advanced catalytic systems for sustainable chemical transformations.
• Hierarchically porous metal-organic frameworks (H-MOFs) offer tunable porosity, high surface area, and structural diversity, making them suitable for Pickering emulsion fabrication.
• The development of CO2-switchable Pickering emulsions using hierarchically porous zirconium-based MOFs remains an unexplored area.

Purpose of the Study:

• To develop novel amine-functionalized H-MOFs for creating CO2-switchable Pickering emulsions.
• To investigate the mechanism behind the CO2-induced switching behavior of these emulsions.
• To demonstrate the application of these emulsions as recyclable catalysts in a Knoevenagel condensation reaction for green chemistry.

Main Methods:

• Post-synthetic modification of H-UiO-66-(OH)2 with various amine-functionalized silanes (APTMS, AEAPTMS, AEAEAPTMS).
• Fabrication of Pickering emulsions using the functionalized H-MOFs as stabilizers for toluene/water mixtures.
• Investigation of CO2-induced reversible switching between emulsion and demulsification.
• Characterization of the switching mechanism using spectral analysis.
• Application in Knoevenagel condensation reactions and assessment of catalyst recyclability.

Main Results:

• Successfully synthesized amine-functionalized H-UiO-66-(OH)2 capable of stabilizing Pickering emulsions at low concentrations (0.25 wt %).
• Demonstrated reversible switching of Pickering emulsions between emulsification and demulsification states upon alternate addition/removal of CO2.
• Elucidated the switching mechanism: CO2 reacts with amine groups, forming hydrophilic salts that reduce MOF wettability.
• Achieved a highly efficient and controlled Knoevenagel condensation reaction using the emulsion as a mini-reactor and the MOF as a catalyst.
• Enabled seamless coupling of catalysis, product isolation, and MOF recyclability.

Conclusions:

• Developed a novel strategy for creating CO2-switchable Pickering emulsions using amine-functionalized H-MOFs.
• The CO2-responsive nature of these emulsions facilitates controlled reaction conditions and efficient separation.
• The developed system offers a promising platform for sustainable chemical processes, combining catalysis, separation, and recyclability.