Preparation And Characterization Of MOFs-Based Ions Perm-selective Membranes

Metal organic frameworks(MOFs)are a type of three-dimensional pore structure framework materials with broad application prospects,which are widely used in the fields of energy storage,catalysis and separation.However,it has always been difficult to apply the metal organic framework to the aqueous phase for material separation.With the in-depth study of the metal organic framework,many scholars have begun to pay attention to the application research of the MOFs in the separation of aqueous phase substances,and the MOFs-based ion separation membrane is one of them.MOFs is used in the preparation of separation membranes due to the sieve action of different size ions in the internal channels of its crystal structure is the fundamental impact for MOFs-based separation membrane for ion separation.Through a series of methods,we have constructed different MOFs-based ion selective separation membranes,mainly used in the process of separation of monovalent and divalent cations.The main contents are as follows:(1)Using the characteristics of the stable cavity inside the structure of MOFs,heteropoly acid molecules are introduced into the cavity of MOFs by a one-pot method to form a composite material,Heteropoly acid@MOFs.Then the mixed matrix membranes which are used for ion separation are prepared by blending the composite material with polymer solution.This topic combines phosphotungstic acid(HPW)with MIL-101.Adds HPW to the synthetic liquid of MIL-101,and prepares HPW-filled MIL-101(HPW@MIL-101)in a one-pot method.After blending with sulfonated polyphenylene ether/polyvinyl alcohol(SPVA)as the filler of the mixed matrix film,HPW@MIL-101/SPVA mixed matrix films with different HPW contents were prepared.The membrane exhibits excellent performance in H+flux and H+/Fe2+.Specifically,with the increase of HPW content,the ion permeation fluxes of H+and H+/Fe2+permselectivity obviously increased.The resulting representative HWS-3 membrane offers excellent H+ permeation(1.27×10-6 mol·cm-2·s-1)and impressive H+/Fe2+selectivity(>210).(2)By modifying and polymerizing organic ligands,polymer ligands are prepared to coordinate with metal ions to form poly-metal organic framework(polyMOFs)composites with far greater ductility and processability than MOFs.A MOFs-based ion separation membrane was constructed on a porous alumina-based membrane by a simple coating method.Among them,polyMOF-5(10)/AAO ion selective separation membrane has impressive selectivity in the separation of K+/Mg2+ and Na+/Mg2+.In the K+/Mg2+ separation,the K+flux reaches 6.34×10-9mol·cm-2·s-1,and the selectivity is as high as 9.46;while in the Na+/Mg2+ separation,the Na+flux is 7.31 ×10-9 mol·cm-2·s-1 and the selectivity reached 12.41.We also analyzed the possible ion transport and separation mechanism of the ion-selective separation membrane constructed in this way,which was further analyzed and verified by photoelectron spectroscopy.At the same time,we have also conducted research and verification on the organic solvent solubility of this kind of polyMOF-5,which is expected to be found and extended to more types of MOFs(3)Further extend and expand based on the ideas in the previous part,using Grubbs reaction to prepare polymer ligands with higher stability,and at the same time react with various metal ions to prepare different types of composite materials.We mainly prepared polyMOF-5,polyUIO-66,polyMIL-101(Cr)and polyMIL-101(Fe),and directly combined the in-situ growth method and the secondary growth method to construct metal on the porous polybenzimidazole-based membrane.MOFs-based separation layer is expected to construct a more stable MOFs-based ion selective separation membrane by this method and has been applied to the separation of monovalent and divalent cations.Among them,the ion flux of polyMOF-5/PBI ion separation membrane is the highest of all ion separation membranes to be tested.In addition,its ion selectivity also stays at a high level.During the separation of Na+/Mg2+,the Na+flux was 8.47 × 10-8mol·cm-2·s·-1,and the selectivity of Na+/Mg2+for polyMOF-5/PBI was as high as 21.3.In the Li+/Mg2+ separation process,the Li+flux reached 5.12 × 10-8 mol·cm-2·s-1,and the Li+/Mg2+selectivity reached 7.34.However,the construction of the polyMIL-101(Cr)/PBI and polyMIL-101(Fe)/PBI two ion separation membranes is not satisfactory.There are still many details in the synthesis of poly-metal organic frameworks that need to be explored and solved in order to prepare a more stable and better metal organic framework-based composite membrane.