Mof-on-mof structure composite ion separation membrane with cross-linking-metal ion system, preparation method and application thereof

Abstract

This invention provides a MOF-On-MOF composite ion separation membrane with a cross-linked metal ion system, its preparation method, and its application. First, an anolyl porous alumina membrane plate is immersed in a solution containing metal and a cross-linking agent, shaken, and then removed. Next, a first MOF membrane layer is grown hydrothermally. Then, metal and a cross-linking agent are deposited on the surface of the first MOF membrane layer. Finally, a second MOF membrane layer is deposited in situ on the surface. This invention, by constructing a cross-linked metal ion system, provides more stable and reliable binding sites for MOF membrane growth, enhances the binding stability between MOF membranes of different sizes, and enables the separation membrane to maintain high flux and ion selectivity even during long-term operation. Furthermore, by optimizing the addition of functional groups, the energy barrier differences between different metal ions across the membrane are strengthened, enhancing the selectivity for monovalent and polyvalent ions. The preparation method is simple and easily industrialized.

Description

Technical Field

[0001] This invention belongs to the field of separation membranes, specifically relating to a MOF-On-MOF structure composite ion separation membrane with a cross-linked metal ion system, its preparation method, and its application. It is a novel bilayer composite metal-organic framework membrane containing a cross-linked metal ion system and having monovalent and multivalent ion separation performance. Background Technology

[0002] In recent years, with the increasing prominence of global environmental problems, addressing global warming and meeting the growing energy demand have become paramount issues. Building a low-carbon economic model has become an important development direction.

[0003] Lithium metal resources are widely used in lithium battery manufacturing and energy storage industries due to their excellent conductivity and energy storage properties. Therefore, in the new energy era, lithium resources are increasingly becoming a focus of attention for countries worldwide. However, the total amount of lithium ore resources is limited, and the production of lithium batteries still generates significant carbon emissions. Therefore, developing efficient recycling technologies for spent lithium batteries has become a key focus of future research and development.

[0004] During the acid release process of cathode materials in retired batteries, a large amount of ionic solution is often obtained, which makes lithium recovery difficult. Taking lithium iron phosphate as an example, after acid release, a solution containing Fe is formed. 3+ Li + Plasma mixtures. Monovalent and multivalent ions can be separated using conventional methods such as chemical precipitation, extraction, adsorption extraction, and membrane separation. However, these methods generally suffer from complex separation processes, low recovery rates, and high costs.

[0005] Metal-organic frameworks (MOFs), as novel hybrid materials, are widely used in various research fields due to their stability and unique structure, such as catalysis, gas adsorption, drug delivery, and ion separation. Currently, membranes such as ZIF-8, HKUST-1, and UIO-66 have been successfully prepared using MOFs, all capable of separating ions of different valence states.

[0006] A patent published on April 16, 2021, with publication number CN 112657350 A, discloses a method for preparing MOF@MOF hybrid films using heteroepitaxial growth induced by two-dimensional nanosheet films. First, a ZnO nanoparticle active layer is introduced onto the surface of a porous support via a sol-gel method. The support is then placed in a metal-free organic ligand synthesis solution, where it self-transforms into a two-dimensional Zn2(bIm)4 nanosheet film. Next, the support with the two-dimensional nanosheet film is placed in another MOF synthesis solution, where the two-dimensional nanosheet base film induces the formation of another type of MOF crystal, filling the gaps between the nanosheets and generating a continuous MOF@MOF hybrid film. However, its application to the separation of lithium iron phosphate is not disclosed. Summary of the Invention

[0007] The purpose of this invention is to provide a MOF-On-MOF composite ion separation membrane with a cross-linked metal ion system and its preparation method. By constructing a cross-linked metal ion system, more stable and reliable binding sites are provided for the growth of the MOF membrane, enhancing the binding stability between MOF membranes of different sizes, enabling the separation membrane to maintain high flux and ion selectivity even during long-term operation. Furthermore, by optimizing the addition of functional groups, the energy barrier differences between different metal ions across the membrane are strengthened, enhancing the selectivity for monovalent and multivalent ions. Moreover, the preparation method of this invention is simple and easily industrialized.

[0008] Another objective of this invention is to provide the application of a MOF-On-MOF structured composite ion separation membrane with a cross-linked-metal ion system for the recycling of waste lithium-ion batteries.

[0009] The specific technical solution of this invention is as follows:

[0010] This invention provides a method for preparing a MOF-On-MOF structured composite ion separation membrane with a cross-linked metal ion system, comprising the following steps:

[0011] 1) Immerse the porous anodized aluminum membrane plate in a solution containing metal and crosslinking agent, shake, and then remove it;

[0012] 2) The first MOF film is hydrothermally grown on the surface of the anodic aluminum porous membrane plate after step 1);

[0013] 3) Deposit metal and crosslinking agent on the surface of the first MOF film after treatment in step 2);

[0014] 4) Deposit a second MOF film in situ on the surface of the film after step 3).

[0015] In step 1), the anodized aluminum porous membrane plate is cleaned with deionized water before use;

[0016] In step 1), the metal is selected from a zirconium source, preferably zirconium chloride;

[0017] In step 1), the crosslinking agent is dopamine hydrochloride (PDA) and polyethyleneimine (PEI); the mass ratio of the two is 1:1.

[0018] In step 1), the concentration of dopamine hydrochloride in the solution containing the metal and the cross-linking agent is 1.8-2.2 mg / ml;

[0019] In step 1), the concentration of the metal in the solution containing the metal and the crosslinking agent is 12-18 mmol / L, preferably 15 mmol / L;

[0020] In step 1), the solvent of the solution containing the metal and the crosslinking agent is a 0.05M Tris-HCl buffer solution with a pH of 8.5;

[0021] The oscillation described in step 1) was performed at 130 rpm and room temperature for 6 hours.

[0022] In step 1), oscillation is performed to allow the anodic aluminum oxide porous membrane (AAO membrane) to fully combine with the crosslinking agent, so that metal ions can be stably attached to the membrane surface, providing more binding sites for the next step of MOF membrane growth.

[0023] After the AAO membrane is removed in step 1), it is soaked in deionized water for 12 hours to remove residual PDA-PEI on the membrane surface.

[0024] Step 1) Modify the surface of the commercially available anodized aluminum porous film to obtain more sites for connection with MOF particles and improve the stability of the surface MOF film.

[0025] In step 2), the first MOF membrane is either a UIO-66-NH2 MOF membrane or a UIO-66-NH2 PSS MOF membrane;

[0026] In step 2), the method for hydrothermal growth of the first MOF membrane is as follows:

[0027] The anodic aluminum oxide porous membrane plate after step 1) is placed in a metal ion solution and an organic ligand solution, and then subjected to hydrothermal reaction in a high-pressure autoclave with a polytetrafluoroethylene lining to obtain the first MOF membrane.

[0028] The metal ion solution is prepared by dissolving the metal source in N,N-dimethylformamide (DMF) and hydrochloric acid solution, and mixing the solutions thoroughly.

[0029] The ratio of the metal source to N,N-dimethylformamide is 0.01-0.02 mol / L;

[0030] The volume ratio of N,N-dimethylformamide (DMF) to hydrochloric acid solution is 5-6:1; the mass concentration of hydrochloric acid solution is 37%.

[0031] The metal source is selected from zirconium sources, preferably ZrCl4;

[0032] The concentration of the organic ligand in the organic ligand solution is 0.016-0.020 mol / L;

[0033] In the organic ligand solution, the organic ligand is selected from 2-aminoterephthalic acid, and the solvent is N,N-dimethylformamide (DMF).

[0034] The molar ratio of metal in metal ion solution to organic ligand in organic ligand solution is 1:1.2-1.5;

[0035] Preferably, sodium polystyrene sulfonate is added to the metal ion solution, and the ratio of sodium polystyrene sulfonate to N,N-dimethylformamide is 0.5-0.7 mg / mL. The addition of sodium polystyrene sulfonate provides sulfonic acid groups, which have a greater affinity for lithium ions. This is mainly because sulfonic acid groups have a high affinity for lithium ions, allowing monovalent lithium to pass more easily and restricting the passage of polyvalent iron ions during the separation of lithium and iron, thereby enhancing the selectivity of the separation between monovalent and polyvalent ions.

[0036] In step 2), the hydrothermal reaction is carried out by heating at 80°C for 2 hours;

[0037] In step 2), the prepared first MOF membrane is washed several times with deionized water and then removed before the subsequent deposition of the second MOF membrane.

[0038] Step 3) Specifically, the surface of the first MOF film after the treatment in step 2) is placed in a solution of metal and crosslinking agent for deposition;

[0039] In step 3), the concentration of the metal in the metal and crosslinking agent solution is 0.01-0.025 mol / L; the concentration of the crosslinking agent is 1.5-2.5 mg / ml; the metal is selected from anhydrous zinc acetate, the crosslinking agent is selected from dopamine hydrochloride, and the solvent is 0.05 M Tris-HCl buffer solution with pH 8.5;

[0040] In step 3), the deposition is carried out at room temperature for 1 hour.

[0041] In step 3), a surface of the first MOF film after the treatment in step 2) is deposited.

[0042] In step 3), the membrane prepared in step 2) is first deposited in a mixed solution of zinc acetate and dopamine hydrochloride for 1 hour to provide binding sites for the growth of the ZIF-8 membrane. When dopamine hydrochloride interacts with the surface of the MOF material, its active groups are mainly the functional groups on the dopamine molecule. The structure of the dopamine molecule contains multiple functional groups, including phenolic hydroxyl groups (-OH), amino groups (-NH2), and aromatic rings. These functional groups and the functional groups on the surface of the MOF material can undergo various interactions, such as hydrogen bonds, ionic bonds, and van der Waals forces, thereby inducing and promoting MOF growth. In addition, the loading of PDA mixed with Zn ions on the membrane surface increases the surface roughness, and the organic ligands readily bind with the metal ions anchored on the membrane surface, forming crystal nuclei and promoting membrane growth.

[0043] Step 4) The second MOF film deposited is a ZIF-8 film;

[0044] Step 4) specifically involves mixing anhydrous zinc acetate aqueous solution with a ligand solution containing PVP, and depositing the mixture onto the membrane prepared in step 3), thus obtaining the final product.

[0045] In step 4), the concentration of the anhydrous zinc acetate aqueous solution is 0.09-1.1 mol / L;

[0046] In the ligand solution containing PVP, the concentration of the ligand is 0.6-0.8 mol / L; the ligand is selected from dimethylimidazole (HMIM); and the concentration of PVP is 0.7-0.8 mg / mL.

[0047] The molar ratio of anhydrous zinc acetate in the anhydrous zinc acetate aqueous solution to the ligand in the PVP-containing ligand solution is 1:60-80.

[0048] In step 4), the deposition is carried out at room temperature for 1 hour.

[0049] Preferably, in step 4), the deposition is performed twice using the same method, and the two depositions result in better compactness.

[0050] In step 4), the second MOF film is deposited on one surface of the film treated in step 3), i.e. the side treated in step 3).

[0051] The present invention provides a MOF-On-MOF structured composite ion separation membrane with a cross-linked metal ion system, which is prepared by the above method. The MOF-On-MOF structured composite ion separation membrane with a cross-linked metal ion system includes an AAO substrate support membrane, a first MOF membrane grown on the AAO substrate support membrane, and a second MOF membrane grown on the first MOF membrane.

[0052] The thickness of the AAO substrate support film is 200-250 nm; the first MOF film is a UIO-66-NH2 MOF film or a UIO-66-NH2 PSS MOF film with a thickness of 190-210 nm; the second MOF film is a ZIF-8 MOF film with a thickness of 140-160 nm.

[0053] The porous alumina anode plate (AAO)-MOF-MOF composite membrane structure provided by this invention incorporates a metal ion crosslinking agent, forming a crosslinked-metal ion system. The AAO, modified with the crosslinking agent and metal ions, provides more sites for MOF attachment and growth. The first MOF layer is a hydrothermally grown MOF membrane containing functional groups; specific groups are introduced into UIO-66-NH2 to enhance the energy barrier differences across the membrane for different metal ions, thereby improving selectivity for monovalent and polyvalent ions. A second MOF layer is deposited in situ on the first MOF layer. Before depositing this second layer, a crosslinking agent and metal ions are pre-deposited to form a crosslinked-metal ion system. This provides more sites for the deposition of the second MOF layer while strengthening the stability of the bond between the two MOF layers, resulting in a tightly bonded MOF membrane with no structural differences.

[0054] The porous alumina anode plate (AAO)-MOF-MOF composite membrane structure provided by this invention incorporates a metal ion crosslinking agent, forming a crosslinked-metal ion system. When PDA interacts with the MOF material surface, its active groups are primarily the functional groups on the PDA. The PDA molecule contains multiple functional groups, including phenolic hydroxyl groups (-OH), amino groups (-NH2), and aromatic rings. These functional groups and the functional groups on the MOF material surface can interact in various ways, such as hydrogen bonds, ionic bonds, and van der Waals forces, thereby inducing and promoting MOF growth. Furthermore, the PDA mixed with metal ions loaded on the membrane surface increases the surface roughness, making it easier for organic ligands to bind with the metal ions anchored on the membrane surface, forming crystal nuclei and promoting MOF membrane growth. The porous alumina anode plate (AAO) modified with the crosslinking agent-metal ion provides more sites for MOF attachment and growth. The first MOF membrane is a hydrothermally grown MOF membrane containing sulfonic acid groups. The addition of sulfonic acid groups enhances the energy barrier differences across membranes for different metal ions, thus improving selectivity for monovalent and polyvalent ions. This is mainly due to the combined effects of the electron affinity, charge density, and coordination ability of the sulfonic acid group. The sulfonic acid group (-SO3-) contains sulfur, and sulfur atoms have high electron affinity, attracting surrounding electrons to form a relatively stable negative charge. This gives the sulfonic acid group strong electrophilic properties, especially for lithium ions with their small ionic radius and high charge, where it can form relatively stable coordinate bonds. Secondly, the charge density of the sulfonic acid group is relatively uniform, resulting in a high charge density. Lithium ions, with their smaller charge and higher charge density, exhibit a stronger electrostatic attraction between the sulfonic acid group and lithium ions. In contrast, iron ions have a larger charge and lower charge density, leading to a relatively weaker electrostatic attraction with the sulfonic acid group. Furthermore, the coordination ability of the sulfonic acid group with lithium ions is significant: the sulfur atom in the sulfonic acid group has a high coordination ability, providing multiple lone pairs of electrons to form coordinate bonds with metal ions. Lithium ions, with their smaller radius, form relatively stable complexes by forming tighter coordination bonds with the sulfur atoms of sulfonic acid groups. In contrast, iron ions, with their larger radius, form looser complexes with sulfonic acid groups, resulting in relatively lower stability. Therefore, sulfonic acid groups have a higher affinity for lithium ions than iron ions. A second MOF layer is deposited in situ on the first MOF layer. Before depositing this second layer, a crosslinking agent and metal ions are pre-deposited to form a crosslinked-metal ion system, providing more sites for the second MOF deposition. During the second MOF deposition, a structure-directing agent, PVP, is added to enhance the stability of the bond between the two MOF layers, resulting in a tightly bonded MOF film with no structural differences.

[0055] Different types of MOF-On-MOF membranes can be prepared by selecting MOFs with different size properties to meet different separation requirements.

[0056] This invention provides an application of a MOF-On-MOF structured composite ion separation membrane with a cross-linked-metal ion system for lithium-ion battery recycling.

[0057] This invention prepares a stable and uniform MOF-on-MOF separation membrane on an alumina template, optimizing the MOF membrane preparation process and addressing the issue of uneven MOF distribution and cracks on the membrane surface. Simultaneously, while constructing the bilayer MOF-on-MOF structure, a cross-linked metal ion system is built, providing more stable and robust binding sites for MOF membrane growth, enabling two MOFs of different sizes to be stably and densely bonded together. Furthermore, by adding functional groups, the MOF-on-MOF membrane maintains high ion selectivity while retaining high flux, improving the trade-off effect of single-layer MOF separation membranes due to their single pore size, and enhancing both ion selectivity and flux. This invention prepares a MOF-on-MOF membrane with a composite structure, improving the recovery rate of lithium-ion batteries at a low economic cost.

[0058] Compared with existing technologies, this invention overcomes the technical defects in the preparation of MOF-on-MOF membranes through a designed preparation method, changing the situation where the MOF distribution on the membrane surface is uneven, with cracks and easy detachment. An improved method for preparing stable and uniform MOF-on-MOF separation membranes on a metal alumina template is proposed. By constructing a cross-linked metal ion system, more stable and reliable binding sites are provided for the growth of the MOF membrane, enhancing the binding stability between MOF membranes of different sizes, enabling the separation membrane to maintain high flux and ion selectivity even under long-term operation. Furthermore, by optimizing the addition of functional groups, the energy barrier differences between different metal ions across the membrane are strengthened, enhancing the selectivity for monovalent and multivalent ions. This improves the lithium recovery rate in lithium-ion batteries at a low economic cost. Attached Figure Description

[0059] Figure 1 This is a schematic diagram of the composite ion separation membrane with MOF-On-MOF structure of the present invention. Detailed Implementation

[0060] This invention provides a method for preparing a MOF-On-MOF structured composite ion separation membrane with a cross-linked metal ion system, comprising the following steps:

[0061] 1) The surface of a commercially available anode porous aluminum oxide (AAO) membrane plate is modified to obtain more binding sites for MOF particles, thereby improving the stability of the surface MOF membrane. This invention uses dopamine hydrochloride (PDA) and polyethyleneimine (PEI) as crosslinking agents. Tris-HCl buffer solutions of dopamine hydrochloride, polyethyleneimine, and zirconium chloride are prepared and mixed to form a homogeneous crosslinked-metal ion system. Subsequently, the AAO membrane, after being washed with deionized water, is placed in this solution and shaken at room temperature on a shaker to allow the AAO membrane to fully bind with the crosslinking agent, enabling the metal ions to stably attach to the membrane surface and providing more binding sites for the subsequent MOF membrane growth. After shaking, the AAO membrane is removed and immersed in deionized water for 12 hours to remove residual PDA-PEI from the membrane surface.

[0062] 2) Preparation of UIO-66-NH2(PSS) MOF membrane and obtaining a uniform membrane on an AAO substrate via hydrothermal growth: ZrCl4 was weighed as the metal source and dissolved in a solution of DMF and HCl to form a metal ion solution; preferably, PSS was weighed and added to the metal ion solution to provide specific functional groups and enhance the energy barrier difference for different ions; 2-aminoterephthalic acid was weighed and dissolved in a DMF solution as the ligand solution. After the metal ion solution and organic ligand solution were mixed evenly, the AAO membrane was transferred together to a polytetrafluoroethylene-lined autoclave and heated at 80°C for 2 hours to obtain the UIO-66-NH2(PSS) membrane.

[0063] 3) ZIF-8 membranes were grown on UIO-66-NH2(PSS) MOF membranes, using polyvinylpyrrolidone (PVP) as a crosslinking agent and structure-directing agent between the two MOF membranes to achieve structural matching: the membrane prepared in step 2) was first deposited in zinc acetate solution and then naturally deposited with dopamine hydrochloride for 1 hour. At this time, a uniform crosslinked-metal ion system was formed on the membrane surface, providing binding sites for the growth of ZIF-8 membranes. The structure of PDA molecules contains multiple functional groups, including phenolic hydroxyl groups (-OH), amino groups (-NH2), and aromatic rings. These functional groups and functional groups on the surface of MOF materials can interact in various ways, such as hydrogen bonds, ionic bonds, and van der Waals forces, thereby inducing and promoting MOF growth. In addition, the loading of PDA mixed with metal ions on the membrane surface increases the surface roughness, making it easier for organic ligands to bind with metal ions anchored on the membrane surface, forming crystal nuclei on the membrane surface and promoting the growth of MOF membranes.

[0064] 4) Mix anhydrous zinc acetate aqueous solution with a ligand solution containing PVP, deposit the mixture onto the membrane prepared in step 3) for 1 hour, and repeat the deposition process twice to obtain the loaded UIO-66-NH2(PSS)-ZIF-8@AAO membrane;

[0065] Before successfully preparing a bilayer MOF separation membrane on an AAO substrate membrane through in-situ growth or natural deposition, this invention constructs a cross-linked metal ion system, providing more robust and stable binding sites for MOF membrane growth. Furthermore, by introducing functional groups such as PSS into the MOF, the energy barriers required for different ions to cross the membrane are strengthened, while the hydrophilicity of the MOF membrane is enhanced, more water channels are constructed, making the membrane more hydrophilic and maintaining high flux while retaining high ion sieving performance.

[0066] The specific embodiments of the present invention will be further described below with reference to specific examples.

[0067] Example 1

[0068] A method for preparing a MOF-On-MOF composite ion separation membrane with a cross-linked metal ion system includes the following steps:

[0069] 1) Using Tris-HCl buffer (0.05M, pH 8.5) as solvent, dopamine hydrochloride (PDA) and polyethyleneimine (PEI) were used as crosslinking agents, and zirconium chloride was added at concentrations of 2 mg / ml dopamine hydrochloride, 2 mg / ml polyethyleneimine, and 15 mmol / L zirconium chloride to form a homogeneous crosslinking-metal ion system. Subsequently, the AAO membrane (25 mm in diameter, 0.2 μm pore size) after being washed with deionized water was placed in this solution and shaken on a shaker at room temperature (130 rpm) for 6 h to allow the AAO membrane to fully bind with the crosslinking agent-metal ions. After shaking, the AAO membrane was removed and soaked in deionized water for 12 h to remove residual PDA-PEI from the membrane surface.

[0070] 2) Preparation of UIO-66-NH2(PSS) MOF membrane and obtaining a uniform membrane on an AAO substrate via hydrothermal growth: 0.0591 g of ZrCl4 was weighed as the metal source and dissolved in 16.7 ml of DMF and 3.3 ml of 37% HCl solution until fully dissolved; then 10.04 mg of PSS was added to provide specific functional groups, forming a metal ion solution; 0.065 g of 2-aminoterephthalic acid was weighed and dissolved in 20 ml of DMF solution as the ligand solution. After the metal ion solution and organic ligand solution were mixed evenly, the AAO membrane was transferred together to a polytetrafluoroethylene-lined autoclave and heated at 80°C for 2 hours to obtain the UIO-66-NH2(PSS) membrane; the prepared UIO-66-NH2(PSS) membrane was washed several times with deionized water and then removed.

[0071] 3) Add 0.13761g of zinc acetate and 100mg of PDA to 50ml of 0.05M, pH 8.5 Tris buffer to form a metal crosslinking system. Deposit one side of the membrane taken out in step 2) using the metal crosslinking system for 1h to provide binding sites for the growth of ZIF-8 membrane.

[0072] 4) Growth of ZIF-8 membrane on UIO-66-NH2(PSS) MOF membrane: 0.0345 g of anhydrous zinc acetate was dissolved in 2 ml of deionized water to obtain a metal ion solution; 1.132 g of dimethylimidazole (HMIM) and 0.0136 g of PVP were dissolved in 18 ml of deionized water to obtain an organic ligand solution. The metal ion solution and the organic ligand solution were mixed. The resulting solution was then rapidly deposited onto the synthesized UIO-66-NH2 membrane for 1 hour, and this deposition process was repeated twice to obtain the loaded UIO-66-NH2(PSS)-ZIF-8@AAO membrane.

[0073] The AAO substrate membrane of this invention has a pore size of approximately 220 nm; the UIO-66-NH2(PSS) MOF membrane has a thickness of approximately 200 nm; and the ZIF-8 MOF membrane has a thickness of approximately 150 nm.

[0074] The application of the UIO-66-NH2(PSS)-ZIF-8@AAO membrane prepared in Example 1 above is for lithium-ion battery recycling, specifically as follows:

[0075] An H-type electrolyzer was used, with a room-temperature dried UIO-66-NH2(PSS)-ZIF-8@AAO membrane as the separation membrane. A 10mM LiCl-FeCl3 mixed solution was used on the feed side, and deionized water was used on the permeate side. A voltage of 1V was applied to enhance ion drive. After 24 hours of operation, the lithium iron phosphate separation coefficient reached 23.

[0076] Comparative Example 1

[0077] A method for preparing a non-crosslinked metal ion system MOF-On-MOF structured composite ion separation membrane includes the following steps:

[0078] 1) The modification of the substrate AAO film is the same as step 1) in Example 1;

[0079] 2) The ZIF-8 / ZIF-L bilayer MOF-on-MOF membrane was prepared as follows: 1.132 g of HMIM was dissolved in 18 mL of deionized water, and 0.054 g of Zn(NO3)2·6H2O was dissolved in 2 mL of deionized water. After mixing the two parts, the mixture was rapidly deposited onto the PDA-PEI modified AAO membrane for 1 hour. The ZIF-8 coating process was repeated twice.

[0080] 3) Add 0.5968g HMIM to 18mL of deionized water and 0.1352g Zn(NO3)2·6H2O to 2mL of deionized water. Mix the two solutions and deposit the mixed solution on the ZIF-8 layer for 1h to synthesize a ZIF-L membrane. Then rinse with deionized water and store the prepared membrane under dry conditions.

[0081] Experimental results show that the MOF-on-MOF membrane is relatively unstable due to the lack of a crosslinking system. Under conditions of using a 5 mM ferric chloride-lithium chloride mixed solution as feed and applying a 1 V voltage, severe membrane detachment occurred. After 24 hours of operation, the lithium-iron separation effect was poor, and a large amount of Fe... 3+ Once the membrane reaches the permeate side, it has no practical application value and cannot achieve the separation of lithium iron phosphate.

[0082] The above description of the embodiments is intended to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for preparing a MOF-On-MOF structured composite ion separation membrane having a cross-linking-metal ion system, characterized by, The preparation method includes the following steps: 1) Immerse the porous anodized aluminum membrane in a solution containing metal and crosslinking agent, shake, and then remove it; 2) The first MOF film is hydrothermally grown on the surface of the anodic aluminum porous membrane plate after step 1); 3) Deposit metal and crosslinking agent on the surface of the first MOF film after treatment in step 2); 4) Deposit a second MOF film in situ on the surface of the film treated in step 3); In step 1), the crosslinking agent is dopamine hydrochloride (PDA) and polyethyleneimine (PEI); the mass ratio of the two is 1:1; the metal is selected from zirconium source; In step 2), the method for hydrothermal growth of the first MOF membrane is as follows: The anodic aluminum oxide porous membrane plate treated in step 1) is placed in a metal ion solution and an organic ligand solution, and hydrothermally reacted in a high-pressure autoclave with a polytetrafluoroethylene lining to obtain the first MOF membrane. In step 2), sodium polystyrene sulfonate is added to the metal ion solution.

2. The preparation method according to claim 1, characterized in that, In step 1), the concentration of dopamine hydrochloride in the solution containing the metal and the cross-linking agent is 1.8-2.2 mg / ml; the concentration of the metal in the solution containing the metal and the cross-linking agent is 12-18 mmol / L.

3. The preparation method according to claim 1, characterized in that, In step 2), the hydrothermal reaction is carried out by heating at 80°C for 2 hours.

4. The preparation method according to claim 1, characterized in that, Step 3) specifically involves placing the surface of the first MOF membrane after the treatment in step 2) in a solution of metal and crosslinking agent for deposition; the metal is selected from anhydrous zinc acetate, and the crosslinking agent is selected from dopamine hydrochloride.

5. The preparation method according to claim 1 or 4, characterized in that, Step 4) specifically involves: Anhydrous zinc acetate aqueous solution is mixed with a ligand solution containing PVP and deposited onto the membrane prepared in step 3) to obtain the final product.

6. A MOF-On-MOF structured composite ion separation membrane with a cross-linked-metal ion system prepared by the preparation method according to any one of claims 1-5.

7. The application of the MOF-On-MOF structure composite ion separation membrane with a cross-linked-metal ion system as described in claim 6, characterized in that, Used for lithium-ion battery recycling.

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