Ionic Liquids Based Two-Dimensional Hierarchically Porous Metal-Organic Framework Composites And Targeted Regulation Mechanism

Metal-organic frameworks(MOFs)have shown wide-spread potential applications in gas adsorption and separation,heterogeneous catalysis,etc.,benefiting from their large specific surface area,high porosity,and flexible and adjustable structure.Zeolitic imidazolate frameworks(ZIFs)belong to a common subclass of MOFs,which have excellent thermal and chemical stabilities combining the advantages of MOFs and zeolites.Compared with traditional microporous ZIFs,hierarchically porous ZIFs have micropores,mesopores,or macropores at the same time,which organically combines the advantages of traditional microporous ZIFs and mesoporous materials,showing significant advantages in practical applications.However,there are some challenging issues in the construction of 2D ZIFs and hierarchically porous ZIFs:1)Limited methods and Harsh synthesis conditions,2)Multistep synthesis and Instability,3)Controlling the pore structure for hierarchically porous ZIFs remains challenging.Therefore,it is highly necessary to establish a simple and green procedure for the synthesis of hierarchically porous ZIFs and 2D ZIFs with controllable/tailorable structures and porosities.In this paper,the formation of a typical 2D ZIFs(ZIF-L)and its influencing factors are firstly explored.It is found that changing the molar ratio of zinc nitrate hexahydrate/2-methylimidazole(Zn/2-Me Im)in ZIFs could adjust the deprotonation ability of 2-Me Im and affect the spatial structure of ZIFs.In addition,the formation mechanism of 2D nanosheets is further explored,and it is speculated that the formation of 2D nanosheets is mainly due to the that hydrogen bonds are formed between layers by unprotonated 2-Me Im in the aqueous solution,which provides theoretical guidance for the controllable construction of porous ZIFs.Based on the above theories,from the perspective of effective regulation of the pore structure of ZIFs,sulfo-functionalized zwitterionic ionic liquids(SFILs)have been designed as both functional and structure-oriented agents to direct the formation of microporous ZIF-SFILs nanosheet in aqueous solution.The grain size and pore structure of the 2D microporous ZIF-SFILs could be adjusted by changing the content and reaction time of SFILs.The formation mechanism of 2D spatial structure and microporous structure are speculated by FT-IR,XPS characterization analysis,and structural optimization simulation:On the one hand,SFILs,acting as monodentate ligands,are demonstrated to coordinate with Zn(II)to induce micropores,coordination to form a 2D structure;on the other hand,SFILs act as deprotonation agents,enhancing the deprotonation of the 2-Me Im,breaking the hydrogen bonds between the layers formed by ZIF-L in water,and promoting the conversion of non-porous ZIF-L to microporous ZIF-SFILs.Finally,SFILs containing characteristic anions(BF4~-,PF6~-,NTf2~-)have been used as modulators for the rapid synthesis of 2D hierarchically porous ZIF-SFILAnions in water(25°C)based on competitive coordination of anions with different polarities.The results show that different types of anions can modulate the thickness and porous properties of the as-synthesized 2D hierarchically porous ZIF-SFILAnions.When SFILBF4 is introduced,the thickness of the fabricated nanosheets decreases to 58 nm with a hierarchical pore structure.the mesoporous volume of ZIF-SFILBF4 becomes larger as the SFILBF4 content increases.Combining the comparative experiments with DFT calculation analysis,the formation process of hierarchical pore ZIF-SFILBF4 materials is further explored,and the mechanism of hierarchical pore structure formation is hypothesized:based on microporous ZIF-SFIL formation,BF4~-tends to partially replace the pre-coordinated large-size molecules SFILs and coordinate with Zn(II)to produce defective mesopores.Benefiting from the unique physical and chemical properties of ionic liquids and the flexible designability of their structures,this paper proposes a novel and versatile method for the fabrication of 2D hierarchically porous ZIFs induced by functional ionic liquids at room temperature.The dual functional-structural regulation mechanism of anions and cations of ionic liquids on 2D ZIFs is revealed,providing a theoretical reference for the further development of simple,green,and controllable construction methods for functional 2D hierarchical porous MOFs.