The Exfoliation Methods And Applications For Covalent Organic Frameworks

In natural biological systems,selective transport of molecules and ions is the most common mass transfer process.The separation of the substances can be realized by the different flow rates of the substances in the different phases.Membranes,especially two-dimensional material membranes,can offer size-limited transportation through nano-or even sub-nano pores in the membrane,thereby improving selectivity.Based on this,researchers have made certain progress in applications of membrane technology for environment,energy and resources.As a porous two-dimensional material,two-dimensional covalent organic frameworks materials(2D COFs)exhibit a unique charm in selective transportation:the porous structure based on topological structure design is highly ordered.According to the size sieving mechanism,COFs can provide transport channels for small molecules and ions while restricting the passage for molecules with larger size.The plane growth of COFs makes it less constrained in the vertical direction,thereby reducing the mass transfer resistance effectively.Beside this,the replaceable structural monomers of COFs can offer different chemical and physical environments,so as to achieve priority or delay transportation for single/mulriple component.Therefore,the applications of COFs materials in ion transport,molecular sieving and other fields have made great progress.However,due to the ?-? stacking of COFs in the conventional preparation process,the nanosheets are seriously agglomerated.The prepared COFs by tradition solvothermal method are usually powder materials,which are difficult to dissolve in organic solvents and have poor workability.The "top-down"and "bottom-up" methods are effective to exfoliate the COFs power into nanosheets(CONs)for the improvement the processing ability of COFs.The composite membranes can be prepared by stacking CONs(layer-by-layer)on the substrate or mixing with polymer to realize the application of COFs in material transport.Due to the unique physical and chemical properties of CONs,composite membranes often exhibit excellent capability for selectivity.My research focused on the exfoliation methods of COFs and investigation of the molecules and ion selective transport capabilities for the CONs membrane,and obtained some innovative discoveries.The results are as follows:1.The method of confined growth is used to prepare CONs films at the two-phase interface.By introducing the sulfonic acid group in structural unit,the CONs channels offer an excellent proton transport environment.After calculation,the theoretical IEC of CONs is as high as 3.42 mmol/g.At the same time,due to its rigid frame and porous characteristics,the CONs film inhibits swelling and improves the performance of water absorption to compare with traditional polymers.Beside this,due to the interfacial polymerization,the nanosheets show good extensibility and continuity in the horizontal direction.The growth on vertical is inhibited,which can improve the transport of protons and reduce the mass transfer hindrance of ions effectively.The conductivity test shows that the data of CONs membrane conductivity is much higher than that of the powder COFs,and is close to that of the Nafion membrane.2.Although CONs film shows high conductivity,but the mechanical property of film is poor.So it is difficult for self-standing film to adapt to the operating environment of the fuel cell.Mixed-matrix membranes is one way to solve the above problems.COFs were exfoliated in mechanical force by"top-down" method.After blending with Nafion polymer,a series of Nafion-SCONs composite membranes with different contents of CONs were prepared.These performances of composite membranes are investigated by water uptake,swelling,conductivity,methanol permeation and other aspects.Finally,the Nafion-SCONs-0.6 membrane with the best comprehensive performance was assemblied into direct methonal fuel cell to increase the maximum power density of the battery by 44%(compared with Nafion).3.Although the mechanical force can effectively prepare the CONs.However,the nanosheets are easily damaged during the exfoliation process,resulting in the reduced size of the CONs.During the process of constructing the CONs membrane,in order to eliminate the local defects caused by the lateral size,it is often necessary to increase the contents of CONs.The large size of CONs is conducive to reducing film defects,thereby improving the transmission capacity.Here,an azobenzene-assisted method was offered to reduce the interlayer force of CONs and destruction of layer.The contents of azobenzene,COFs type,experimental conditions and other factors affect the separation of CONs were investigated and the Tp-AD-50 nanosheet with the largest size is obtained finally.These CONs are filtered on the alumina membrane to realize the sieving of dye molecules while ensuring high water flux.4.Azobenzene-assisted exfoliation can effectively prepare large size CONs,but this method has certain requirements for the types of COFs,which is not completely universal.Instead,micron-sized Azo-CTF nanosheets were prepared through the micro-interface formed by trifluorosulfonic acid and dichloromethane.Abundant azo bonds on nanosheets can be used to combine with hydrophilic/hydrophobic acid and nanosheets with controlled wettability are prepared.The modified Azo-CTF nanosheets were used for oil-water emulsion separation,and a better separation effect was achieved.