Preparation Of Novel Organic Covalent Nanosheets And Their Nanofiltration Performance

Covalent organic framework（COF）is a new kind of crystalline material.Compared with traditional polymers,COFs have the advantages of low density,large specific surface area,regular pore channels,simple synthesis and functionalization,etc..Thus,these materials have attracted wide attention in many fields such as gas storage,energy storage,electroactive materials,separation,adsorption and catalytic sensor,etc.However,the insoluble properties of COFs powder have brought many limitations for the current application.Therefore,to achieve COFs evenly dispersed in the solvent and fabricate ordered films is a new direction of COFs research,which also derived the new concept of covalent organic nanosheets（CONs）.The ultrathin CONs with regular channels have further expanded the application range of COFs,especially in membrane separation applications.Compared with other two-dimensional materials,COFs are more stable in water.Besides,COFs have orderly pore arrangement,uniform pore size and high pore density,which can improve the rejection rate of nanofiltration membrane by pore size screening in the separation membrane.COFs are designable,with functional groups attached to the framework to enhance selectivity.These properties make COFs and their CONs promising as materials for the construction of advanced separation membranes.However,the preparation process of COFs nanofiltration membrane is complex,and the obtained nanofiltration membrane cannot reach the balance of the high rejection rate and high flux.Therefore,improving the performance of COFs nanofiltration membrane is still a major problem at present.To solve the above problems,we selected four COFs with different properties（TT-COF,R-COF,S-COF and F-COF）,and explored the influencing factors of COFs stripping by trying solve-assisted ultrasonic exfoliation method in different solutions.It is found that the stripping efficiency of COFs is related to the structure of COFs and the type of solvent.Ultrasonic wave can accelerate the solvent molecules to enter COFs,thus increasing the interlayer space,weakening the interlayer interaction and stripping the multilayer without destroying the original skeleton plane.The crystallinity of S-COF is weak,and the covalent bond in its skeleton is destroyed during ultrasound.F-COF has strong crystallization and strong interlayer interaction,and cannot be exfoliated well.In addition,the larger the surface tension,the more suitable molecular size of the solvent can be inserted into the lamellar to complete the exfoliation.Finally,the best method of exfoliation is summarized based on the microcosmic morphology.Then,TT-COF with excellent characterization was selected to study the performance of nanofiltration,and its dispersions were prepared into TT-COF/GO composite membrane by self-assembly with GO.In order to enhance the interaction between TT-COF and GO and reduce the gap between layers,a simple plastic-coated pressing method was used to the composite membrane.The performance of the membrane was characterized by the dye molecule methyl blue（MB）.The effects of different TT-COF content and different pressing times on the rejection rate and water flux of the composite membrane were investigated.It was found that the rejection rate of the composite membrane increased from 76.5%to 99.5%after the addition of 5 mg TT-COF.This is because the multi-channel structure of TT-COF itself rejects the transmission of dye molecules.After hot pressing,the water flux increases to 309.9 L m^-2 h^-1 bar^-1,indicating that hot pressing reduces the gap of layers and increases the speed of water molecules.After that,the sustainability and repeatability of the composite membrane were further studied.Through continuous filtration,it was found that the rejection of membrane C-5＿2 could still reach 98%,which indicated that the composite membrane had good sustainability and repeatability.In addition to pore size screening,the aldehyde group on the TT-COF also gives negative charges at the nanofiltration membrane,thus selectively intercepting cationic dyes.The composite film has high rejection rate due to the dual action of pore size screening and charge selection.Finally,by comparing four COFs,we found that the TT-COF,with high crystallinity,large specific surface area,appropriate aperture and specific groups,is the only COF that can meet the requirements of high interception rate and high permeability.This result also provides the design strategy of COF nanofiltration material.