CO₂-Responsive Multistage Pore MOFs Stabilized Pickering Emulsion

The Pickering emulsion is a kind of emulsion system using solid particles as emulsifier,which has wide application prospects in food,medicine,chemical material synthesis,cosmetics,petroleum transportation and catalysis.In some cases,people want the emulsion to be stable,and the longer it lasts,the better.However,in cases like asphalt emulsification,oil transportation,fuel production,liquid phase multiphase catalysis and nanomaterials preparation,the emulsion is usually only needed to achieve temporary stability,and then demulsified for purposes like product separation and catalyst recovery.Therefore,how to solve the long-term stability of Pickering emulsion and its convenient reversible switch from emulsification to demulsification is an important scientific problem in the field of emulsion research.The emergence of environmentally responsive Pickering emulsion provides a new way to solve this problem.CO₂ has special advantages and importance,such as non-toxic and harmless,environmentally friendly,simple and easy to obtain,low price,and easy to remove from the emulsion system without interfering with the emulsion system.Metal-organic frameworks（MOFs）are a kind of relatively new porous crystal materials composed of organic ligands and inorganic metal ions or metal clusters.They have the advantages of high specific surface area,high porosity and structural diversity,which makes them have great potential for emulsifier applications.Nearly all the reported pore sizes of MOFs to date are in the micropore range（defined by iupac as<2nm）,which is a disadvantage for their development applications because the relatively small coordination channels（micropores）inhibit the molecular diffusion rate and limit the accessibility of macromolecules.Multistage porous metal-organic framework materials（H-MOFs）have 3D porous structure,pores containing both micropores and mesoporous pores,and ultra-high specific surface area.Compared with single-stage porous metal-organic framework in scientific research,they have natural advantages,so they are widely welcomed and sought after by the scientific community.Based on the analysis above,this paper intends to design and synthesize a novel CO₂-responsive functional porous MOFs material,then use the material as an emulsifier to prepare CO₂-responsive Pickering emulsion.Also,the reversible regulation mechanism and rule of CO₂/N₂ on emulsification and demulsification of MOFs stabilizing Pickering emulsion at room temperature and pressure and some possible applications were studied.The main contents include the following:（1）In order to develop an excellent CO₂-responsive Pickering emulsion system,a CO₂-responsive quaternary ammonium salt ionic liquid[C₁₀DMEA][Im]was designed and synthesized.The response of Pickering emulsion to CO₂,which was formed by co-emulsification of the ionic liquid and H-ZIF-8 with ethyl acetate and water mixture,was also studied in this paper.Meanwhile.the effect of the concentration of ionic liquid on the formation and microstructure of Pickering emulsion was studied.The possible mechanism of the reversible transformation between emulsification and demulsification of Pickering emulsion driven by CO₂ was studied by ¹³C NMR.Based on this,the Knoevenagel condensation reaction was co-catalyzed by[C₁₀DMEA][Im]@H-ZIF-8 and the high efficiency reaction separation coupling was realized.（2）In this paper,a series of imidazolyl ionic liquids[C_nMIM][2-CN-Pyc]（n=4,6,8）in response to CO₂ were designed and synthesized.Therefore,the ionic liquid[C_nMIM][2-CN-Pyc]（n=4,6,8）was embedded into H-Ui O-66 material by impregnation method to form the[C_nMIM][2-CN-Pyc]@H-Ui O-66 complex.Also,the response of Pickering emulsion formed by emulsifying n-hexane-water system with[C_nMIM][2-CN-Pyc]@H-Ui O-66 composite to CO₂ stimulation was studied.The effects of alkyl chain length of ionic liquid and concentration of composite material on the phase behavior and microstructure of emulsion were studied.The ¹³C NMR was used to study the mechanism of CO₂ regulating the reversible transition of Pickering emulsion from demulsification to reemulsification.The application of the CO₂in response to Pickering emulsion as a microreactor in Knoevenagel reaction was explored.（3）In this work,a novel kind of the amine-functionalized hierarchically porous Ui O-66-（OH）₂（H-Ui O-66-（OH）₂）has been developed through post-synthetic modification of H-Ui O-66-（OH）₂ by（3-aminopropyl）trimethoxysilane（APTMS）,3-（2-aminoethylamino）propyltrimethoxysilane（AEAPTMS）and3-[2-（2-aminoethylamino）ethylamino]propyl-trimethoxysilane（AEAEAPTMS）,and employed as emulsifiers for the construction of Pickering emulsions.It was found that the functionalized H-Ui O-66-（OH）₂ could stabilize the 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 by 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 reduction of MOF wettability.Based 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.