A self-microporous polymer ion exchange membrane, a preparation method and application thereof

Abstract

This invention discloses a self-microporous polymer, a self-microporous ion exchange membrane, its preparation method, and its applications, belonging to the field of polymer materials technology. The polymer uses 1,3,5-tris(4-aminophenyl)benzene (TAPB), o-toluidine (DMBP), and dimethylformaldehyde as monomers. Utilizing the rigid, distorted, and non-densely stacked molecular structures of TAPB and DMBP, a polymer material with an inherent sub-nanometer microporous structure and its corresponding homogeneous ion exchange membrane are prepared. The formed self-microporous structure constructs continuous ion transport channels within the membrane, where the sub-nanometer-scale micropores impart a significant size sieving effect, thereby achieving selective separation of monovalent / polyvalent anions. Furthermore, by quaternizing the membrane material, fixed positively charged groups are introduced onto the polymer backbone. These positively charged groups generate significant electrostatic repulsion, enabling selective separation of monovalent / polyvalent cations, particularly suitable for the efficient separation of lithium and magnesium ions.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a self-microporous polymer, a self-microporous ion exchange membrane prepared based thereon, a preparation method thereof, and its application in magnesium-lithium separation. Background Technology

[0002] With the growing global demand for lithium in lithium-ion batteries for electric vehicles and renewable energy storage, there is an urgent need to advance lithium resource recycling and extraction technologies from unconventional sources. Traditional lithium extraction methods (such as hard rock mining) face environmental challenges and are limited in scale. Direct extraction of lithium from various water resources (such as salt lake brines or geothermal brine solutions) offers a promising alternative that improves efficiency, reduces environmental impact, and addresses economic considerations.

[0003] Among existing separation methods, membrane separation technology is widely used due to its advantages such as simplified process, low operating energy consumption, and low dependence on chemical reagents. As a key functional component in membrane separation technology, determining its applicability and separation efficiency, ion exchange membranes play a central role. Ion exchange membranes are polymeric membrane materials containing fixed ion exchange groups. The polymeric framework provides the membrane with necessary mechanical strength and structural stability, while the introduced ion exchange groups enable selective ion transport. The ion transport capacity and separation selectivity of ion exchange membranes directly affect their application scope and level in relevant separation fields.

[0004] The ion separation process of ion exchange membranes mainly relies on the following three basic mechanisms: (1) sieving effect based on pore size difference; (2) electrostatic repulsion effect based on fixed charge groups; and (3) based on the difference in ion transmembrane hydration energy. In the prior art, perfluorosulfonic acid type ion exchange membranes (such as Nafion membranes) are widely used in energy storage devices, water treatment and other fields due to their high proton conductivity and excellent chemical stability. These membrane materials usually use hydrophobic perfluorinated backbones to maintain the dimensional stability of the membrane structure. The hydrophilic sulfonic acid groups introduced on their side chains self-assemble in the membrane to form microphase-separated ion transport channels. This channel structure is generally considered to be the key structural basis for achieving high ion conductivity. However, while suppressing membrane swelling, these membrane materials may be accompanied by a decline in mechanical properties. In addition, Nafion membranes also have technical limitations such as limited proton selectivity, high methanol permeability, and decreased conductivity under low humidity or high temperature environments. Furthermore, because its synthesis process involves fluorine chemistry systems, the material preparation cost is relatively high, which to some extent restricts the large-scale commercial application of Nafion series membranes.

[0005] Traditional polymeric ion exchange membrane materials typically rely on microscale phase separation mechanisms in their structural design. This involves constructing phase-separated ion transport channels through the self-assembly of block or comb-type polymers, or utilizing the difference in hydrophilicity and hydrophobicity between the polymer backbone and side chains to spontaneously form microphase-separated morphologies, thereby achieving rapid ion transport. However, cation exchange membranes based on aromatic backbone structures generally exhibit low ionic conductivity, and their synthesis and processing costs are high, making it difficult to meet the comprehensive performance and economic requirements for industrial applications. Furthermore, due to the limited hydrophilic / hydrophobic phase separation driving force of the aromatic polymer backbone itself, it is difficult to construct continuous, interconnected ion transport channel structures through self-assembly. Therefore, even under high ion exchange capacity conditions, the ionic conductivity of this type of membrane material is still significantly lower than that of perfluorosulfonic acid ion exchange membranes; and further increasing the ion exchange capacity easily leads to excessive swelling of the membrane, thus significantly weakening the mechanical stability and service life of the ion exchange membrane.

[0006] To overcome the shortcomings of traditional polymeric ion exchange membrane materials in applications, there is an urgent need to develop novel, low-cost membrane materials to improve ion transport and selectivity. Porous polymers (PIMs) are a special class of polymers with high specific surface areas. Due to the rigidity and twisted structure within their molecules, these polymers cannot effectively entangle and stack during close packing, resulting in a large number of micropores. Intrinsic microporous polymers (PIMs) are an emerging class of amorphous, ultraglassy, ​​solution-processable microporous polymer materials used to describe a continuous network of intermolecular voids caused by the rigid structure of the macromolecules. They have attracted widespread attention due to their unique intrinsic microporosity. PIMs are typically composed of bulky, ladder-like building blocks connected by spirocyclic centers. These rigid and inflexible main-chain structures, due to their low chain segment stacking efficiency and numerous twisted sites, provide a large free volume without pores. "Solution processability" refers to the ability of a material to be easily dissolved or dispersed in a solvent to obtain the desired form or concentration.

[0007] In existing technologies, a series of anion exchange membranes and cation exchange membranes have been developed around PIM materials. For example, Professor Sun Shipeng's team at Nanjing University of Technology (Microporous membrane with ionized sub-nanochannel senabling highly selective monovalent and divalent anion separation[J]. Nature Communications, 2024, 15(1): 7271.) introduced a randomly twisted V-shaped structure of Trog base units and quaternary ammonium groups to construct charged sub-nanochannels, providing an efficient template for constructing functionalized sub-nanochannels in PIM membranes; Professor Xu Tongwen's team at the University of Science and Technology of China (Near-frictionless ion transport within triazine framework membranes[J]. Nature, 2023, 617(7960): 299-305.) constructed rigidly constrained ion channels through covalently bonded polymer frameworks, enabling large-area self-supporting synthetic membranes to approach the ion diffusion limit in water and achieve near-frictionless ion flow. However, there are few novel membranes in existing technologies that can simultaneously remove multivalent anions and cations from complex water systems. Summary of the Invention

[0008] To address the aforementioned technical deficiencies and application limitations, the present invention aims to provide a self-microporous polymer, a self-microporous ion exchange membrane, and a method for preparing the same. The polymer uses 1,3,5-tris(4-aminophenyl)benzene and o-toluidine, which have strong structural rigidity and are difficult to densely stack, as monomers. Through polymerization, a polymer material with an inherent microporous structure is formed, and a self-microporous ion exchange membrane is prepared based on this polymer. Furthermore, after quaternization modification of the polymer membrane, fixed quaternary ammonium groups are introduced into the polymer backbone, significantly improving the hydrophilicity of the membrane material and promoting the construction of continuous internal hydration channels within the membrane, thereby effectively accelerating the selective transport of monovalent ions. Multivalent ions are almost completely blocked due to size effects and electrostatic repulsion. Simultaneously, the electrostatic repulsion effect generated by the fixed charge further enhances the transport efficiency of anions. Based on the aforementioned ion transport mechanism synergistically dominated by structural and charge regulation, when this ion exchange membrane is integrated into a multi-stage electrodialysis process, it can achieve the direct separation and enrichment of lithium ions from high-salinity salt lake brines, possessing significant industrial application potential.

[0009] A self-porous ion exchange membrane, wherein the ion exchange membrane uses a self-porous polymer or a quaternary ammonium compound of the self-porous polymer as the main component; the self-porous polymer is prepared by copolymerization and crosslinking using 1,3,5-tris(4-aminophenyl)benzene, o-toluidine, and dimethylformaldehyde as monomers; the structural formula of the self-porous polymer is shown in Formula I.

[0010] The value of n ranges from 15000 to 30000.

[0011] The structural formula of the quaternary ammonium compound of the self-porous polymer is shown in Formula II:

[0012] The value of n ranges from 15000 to 30000.

[0013] The thickness of the ion exchange membrane is 70-90 μm; and / or, the ion exchange membrane has a sub-nanometer microporous structure; and / or, the ion exchange membrane is a homogeneous ion exchange membrane.

[0014] A method for preparing the self-microporous ion exchange membrane includes the following steps: S1, performing a polymerization and crosslinking reaction of o-toluidine and dimethylformaldehyde in a reaction solvent containing a catalyst, and then adding 1,3,5-tris(4-aminophenyl)benzene to continue the crosslinking reaction to obtain a casting solution containing a self-microporous polymer; S2, casting the casting solution and heating and drying it, and then performing a deprotonation treatment after demolding to obtain the self-microporous ion exchange membrane.

[0015] In step S1, the reaction solvent is a halocarbon solvent and / or a fluorinated organic acid, preferably one or both of dichloromethane and trifluoroacetic acid; the catalyst is a fluorinated organic acid, preferably trifluoroacetic acid.

[0016] In step S1, the molar ratio of 1,3,5-tris(4-aminophenyl)benzene to dimethylformaldehyde is 1:6-10; the molar ratio of o-toluidine to dimethylformaldehyde is 1:5-8; the mass-to-volume ratio of 1,3,5-tris(4-aminophenyl)benzene to the catalyst is 0.1-0.3 g:0.5-5 mL; and the mass-to-volume ratio of o-toluidine to the catalyst is 0.2-0.5 g:0.5-5 mL.

[0017] In step S1, the monomer dissolution and mixing process is carried out under ice-water bath conditions, and the stirring time is 30-60 min; the polymerization and crosslinking reaction time is 1-36 h, and the crosslinking reaction continues for 1-18 h; in step S2, the heating and drying temperature is 30-60℃, and the time is 3-12 h; the deprotonation treatment is to immerse the demolded membrane in an alkali metal hydroxide solution for neutralization, preferably in a 0.1 mol / L KOH solution.

[0018] The method also includes step S3, which involves preparing a quaternized microporous ion exchange membrane: S3, the microporous ion exchange membrane obtained in step S2 is immersed in a solution containing a quaternization functional reagent to undergo a quaternization reaction, followed by anion exchange treatment and washing; the quaternization functional reagent is a haloalkane reagent, preferably iodomethane; the solvent of the solution containing the quaternization functional reagent is a C1-C4 lower fatty alcohol, preferably methanol; the concentration of iodomethane in the solution containing the quaternization functional reagent is 20-50 wt%, preferably 40 wt%; the temperature of the quaternization reaction is 50-70℃, preferably 60℃; the time of the quaternization reaction is 4-12 h, preferably 4-8 h; the anion exchange treatment involves immersing the membrane in an alkali metal halide solution, preferably in a 1 mol / L KCl aqueous solution, for a treatment time of 24-48 h.

[0019] The application of the self-porous ion exchange membrane in electrodialysis or diffusion dialysis separation processes.

[0020] The application is in the separation of magnesium and lithium by electrodialysis; and / or, the application is in the selective separation of monovalent and polyvalent anions, as well as monovalent and polyvalent cations, in complex aqueous systems.

[0021] The beneficial effects of this invention are as follows: 1. This application uses 1,3,5-tris(4-aminophenyl)benzene (TAPB), o-toluidine (DMBP), and dimethylformaldehyde (DMM) as polymerization monomers. The TAPB-DMBP monomers have rigid and twisted structural characteristics, making it difficult for them to stack together, thus giving the polymer a microporous feature. 2. This application utilizes self-porous polymers to prepare homogeneous ion exchange membranes with inherent sub-nanometer channels. By introducing a large number of rigid twisted structures, the polymer chains cannot be tightly packed, thereby forming microporous channels within the membrane. The sub-nanometer microporous channels of different sizes have a size sieving effect. This restriction leads to significant size repulsion-induced selectivity, which is beneficial for achieving selective separation of monovalent / polyvalent anions. In addition, even under high ion exchange capacity, the swelling of the membrane can be greatly suppressed due to the existence of the rigid microporous framework structure. 3. Through feasible functional group modification, such as quaternization (e.g., iodomethane) to convert into positively charged quaternary ammonium groups with anion conduction properties, the membrane can conduct anions more efficiently and repel cations. Based on the strong electrostatic repulsion, selective separation of monovalent / polyvalent cations can be achieved, and the selective separation property of the membrane can be further controlled by adjusting the membrane thickness. 4. After quaternization modification of the polymer membrane with its own microporous framework, the quaternary ammonium groups on the TAPB-DMBP framework improve the hydrophilicity of the membrane and can effectively improve the transport efficiency of anions. In addition, the rigid polymer framework also provides confined ion transport channels. The interaction of anions within the confined channels is further enhanced, promoting their transport and making it possible to apply it to the separation of monovalent / polyvalent anions and cations. 5. The self-polymerized microporous polymer disclosed in this application can be used as a self-polymerized microporous polymer ion exchange membrane material in the fields of electrodialysis or diffusion dialysis. It can be applied to the separation process of monovalent / polyvalent anions and cations. The corresponding membrane material has high selectivity and permeability, and low membrane resistance. 6. This application uses cost-effective chemical raw materials and simple membrane preparation methods to prepare polymers and corresponding ion exchange membranes. The process route is mature and simple, the conditions are mild, and it is easy to scale up production, resulting in significant economic benefits.

[0022] 7. Based on the synergistic effect of the above-mentioned structural sieving effect and electrostatic repulsion effect, the self-porous polymer ion exchange membrane disclosed in this invention can be applied to the separation process of monovalent / polyvalent anions and cations at the same time. In practical applications, it exhibits comprehensive performance with high selectivity, large permeation flux and excellent long-term operational stability, and has good application prospects. Attached Figure Description

[0023] Figure 1 These are scanning electron microscope (SEM) images of the P_DMBP, DMBP-TAPB, Q-DMBP-TAPB, and P_TAPB films prepared in Examples 1-3. Figure 2 These are the XPS spectra of the DMBP-TAPB and Q-DMBP-TAPB membranes prepared in Example 1; Figure 3 These are the Zeta potential test curves of the P_DMBP, DMBP-TAPB, Q-DMBP-TAPB, and P_TAPB films prepared in Example 1; Figure 4 These are the water contact angle test results of the P_DMBP, DMBP-TAPB, Q-DMBP-TAPB, and P_TAPB membranes prepared in Examples 1-3; Figure 5This is a schematic diagram of ion transport within the membrane channel during selective sieving using the Q-DMBP-TAPB membrane prepared in Example 1. Figure 6 The P_DMBP, DMBP-TAPB, Q-DMBP-TAPB, and P_TAPB films prepared in Examples 1-3 are at 5 mA cm⁻¹ -2 Statistical analysis of cation separation performance data under the specified conditions; Figure 7 The P_DMBP, DMBP-TAPB, Q-DMBP-TAPB, and P_TAPB films prepared in Examples 1-3 are at 5 mA cm⁻¹ -2 Statistical analysis of anion separation performance under certain conditions. Detailed Implementation

[0024] To enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0025] In this invention, a microporous polymer framework is constructed based on the specific molecular structures of two monomers, DMBP and TAPB. TAPB provides a rigid three-dimensional framework. Based on its three-dimensional rigid triphenylbenzene core, its highly twisted configuration acts as a strong, rigid branching crosslinking point in the polymer network, supporting a permanent sub-nanometer microporous structure. This effectively inhibits excessive swelling of the membrane in aqueous solution and ensures the stability of the sieving channels. DMBP has a biphenyl structure, and its molecular chain segments are rigid and rotationally restricted. In the polymer, it mainly increases the twist of the chain segments, preventing tight entanglement and dense stacking of polymer chains. This helps regulate the pore size distribution, achieves connectivity between micropores, and provides channels for rapid ion transport.

[0026] Some embodiments of this patent include the following technical solutions: An ion exchange membrane based on a self-porous polymer is obtained by casting a casting solution with the self-porous polymer or its quaternary ammonium compounds as the main components. The microporous polymer described above was prepared using 1,3,5-tris(4-aminophenyl)benzene, o-toluidine, and dimethylformaldehyde as polymeric monomers, and the polymer structure is shown in Figure I. (I) The molecular structure of its quaternary ammonium compounds is shown in Figure II: (II) Where n takes values ​​ranging from 15000 to 30000.

[0027] The preparation steps of the self-contained microporous polymer include: dissolving monomers 1,3,5-tris(4-aminophenyl)benzene and o-toluidine in a reaction solvent, adding a catalyst, stirring and mixing under an ice-water bath until the monomers are dissolved, adding dimethylformaldehyde at low temperature to carry out a polymerization reaction, and crosslinking occurs.

[0028] The molar ratio of 1,3,5-tris(4-aminophenyl)benzene to dimethylformaldehyde is 1:6 to 10, the molar ratio of o-toluidine to dimethylformaldehyde is 1:5 to 8, the reaction solvent is one or both of dichloromethane and trifluoroacetic acid, the catalyst is trifluoroacetic acid, the mass-to-volume ratio of 1,3,5-tris(4-aminophenyl)benzene to trifluoroacetic acid is 0.1 to 0.3 g: 0.5 to 5 mL, and the mass-to-volume ratio of o-toluidine to trifluoroacetic acid is 0.2 to 0.5 g: 0.5 to 5 mL.

[0029] The preparation steps of the quaternary ammonium compound include: dissolving the self-porous polymer shown in Formula I in a solvent to obtain solution A, adding a quaternization functional reagent to solution A, carrying out the quaternization reaction in the dark to obtain solution B, washing with deionized water or methanol, filtering out the solid, and drying to obtain the quaternary ammonium compound.

[0030] The quaternization functional reagent is iodomethane, the solvent is methanol, the concentration of iodomethane is 20-50 wt%, the reaction temperature is 50-70℃, and the reaction time is 4-12 h.

[0031] Quaternization modification of the obtained microporous ion exchange membrane introduces positively charged groups, which can enhance the membrane's hydrophilicity and electrostatic repulsion, improve the membrane's transport efficiency for anions, and enhance its ability to block high-valence cations.

[0032] The quaternization modification is achieved by first immersing a microporous ion exchange membrane in deionized water and then immersing it in a methanol solution of iodomethane for quaternization reaction. The reaction temperature is 50-70℃ and the reaction time is 4-12h.

[0033] The application of the ion exchange membrane based on a self-porous polymer in the separation of lithium and magnesium in electrodialysis.

[0034] The self-porous ion exchange membrane can be used for selective electrodialysis to separate monovalent / polyvalent anions and monovalent / polyvalent cations. Example 1

[0035] Weigh 0.2 g of o-toluidine (DMBP) monomer and dissolve it in a 10 mL round-bottom flask. Add 1.5 mL of trifluoroacetic acid (TFA) and 1.5 mL of dichloromethane (DCM). Add a magnetic stir bar and immerse the flask in an ice-water bath. Stir for 30 min to 1 h until the monomer is completely dissolved. While the flask temperature is low, add 0.34 g of dimethylformaldehyde (DMM) to the round-bottom flask. React for 1 to 2 h until the reactants in the flask become viscous. Add an additional 2 mL of TFA, then quickly drop the diluted solution onto a clean glass dish. Heat at 60°C for 3 to 6 h until the membrane is completely dry, resulting in a deep red membrane. After the glass dish returns to room temperature, immerse it in clean deionized water for a short time. The membrane will detach naturally from the glass dish and then be immersed in 0.1 M KOH solution to neutralize any residual TFA, thus achieving membrane deprotonation. After the membrane turns completely yellow, remove it from the alkaline solution, rinse the membrane surface with deionized water to remove any remaining alkaline solution, and then soak the membrane in deionized water for later use. The membrane obtained in this step is denoted as P_DMBP.

[0036] The prepared P_DMBP membrane was immersed in a 40 wt% iodomethane methanol solution, and then the mixture was placed on a shaker for quaternization under light-protected conditions. The reaction temperature was 60℃ and the time was 4 h. The membrane was then immersed in a 1 M KCl aqueous solution for anion exchange for 24–48 h (I - Exchange for Cl - Finally, the membrane is washed with excess water to remove adsorbed salts from the membrane surface and inside the membrane. The resulting ion exchange membrane has a thickness of approximately 70 μm and is denoted as Q-P_DMBP. Example 2

[0037] o-Toluidine (DMBP, 0.168 g) was reacted with dimethoxymethane (DMM, 0.305 g) in a round-bottom flask at 0 °C with trifluoroacetic acid (TFA, 3.0 mL) as a catalyst for 24 h. Subsequently, 1,3,5-tris(4-aminophenyl)benzene (TAPB, 0.028 g) was added as a branched linker, and the reaction was continued at room temperature for 12 h to obtain a branched copolymer. A Trog base framework (TBF) membrane was prepared by the sol-gel method. The resulting viscous solution was cast onto a flat glass vessel and heat-treated at 30 °C for 12 h until completely dry. The dried membrane was immersed in 0.1 M KOH aqueous solution to remove excess TFA. After the membrane turned completely yellow, it was removed from the alkaline solution, rinsed with deionized water to remove residual alkaline solution, and then immersed in deionized water for later use. The membrane obtained in this step was designated DMBP-TAPB.

[0038] The prepared DMBP-TAPB membrane was immersed in a 40 wt% iodomethane-methanol solution, and then the mixture was placed on a shaker for quaternization under light-protected conditions. The reaction temperature was 60 °C and the time was 4 h. The membrane was then immersed in a 1 M KCl aqueous solution for anion exchange for 24–48 h (exchanging I- for Cl-). Finally, it was washed with excess water to remove adsorbed salts from the membrane surface and inside the membrane. The resulting ion exchange membrane had a thickness of approximately 90 μm and was denoted as Q-DMBP-TAPB. Example 3

[0039] Weigh 0.2 g of 1,3,5-tris(4-aminophenyl)benzene (TAPB) monomer and dissolve it in a 10 mL round-bottom flask. Add 1.5 mL of trifluoroacetic acid (TFA) and 1.5 mL of dichloromethane (DCM). Add a magnetic stir bar and immerse the flask in an ice-water bath. Stir for 30 min to 1 h until the monomer is completely dissolved. While the flask temperature is low, add 0.28 g of dimethylformaldehyde (DMM) to the round-bottom flask. React for 1–2 h until the reactants in the flask become viscous. Add an additional 2 mL of TFA, then quickly drop the diluted solution onto a clean glass dish. Heat at 60°C for 3–6 h until the membrane is completely dry, yielding a deep red membrane. After the glass dish returns to room temperature, immerse it in clean deionized water for a short time. The membrane will detach naturally from the glass dish and then be immersed in 0.1 M KOH solution to neutralize any residual TFA, thus achieving membrane deprotonation. After the membrane turns completely yellow, remove it from the alkaline solution, rinse the membrane surface with deionized water to remove any remaining alkaline solution, and then soak the membrane in deionized water for later use. The membrane obtained in this step is denoted as P_TAPB.

[0040] The prepared P_TAPB membrane was immersed in a 40 wt% iodomethane methanol solution, and then the mixture was placed on a shaker for quaternization under light-protected conditions. The reaction temperature was 60℃ and the time was 4 h. The membrane was then immersed in a 1 M KCl aqueous solution for anion exchange for 24–48 h (I - Exchange for Cl - Finally, the membrane is washed with excess water to remove adsorbed salts from the membrane surface and inside the membrane. The resulting ion exchange membrane has a thickness of approximately 85 μm and is denoted as Q-P_TAPB.

[0041] Related performance tests 1. The surface of the P_DMBP, DMBP-TAPB, Q-DMBP-TAPB, and P_TAPB films prepared in Examples 1-3 was characterized using scanning electron microscopy. The results are referenced. Figure 1 As can be seen from the figure, the membrane maintains a flat and dense surface at the nanoscale, and there is no significant difference in the surface structure of ion exchange membranes with different degrees of cross-linking.

[0042] 2. Figure 2 The XPS characterization results of the DMBP-TAPB and Q-DMBP-TAPB membranes prepared in Example 1 are shown in the figures. As can be seen, after introducing quaternary ammonium groups into the DMBP-TAPB membrane and performing ion exchange, the C1s peak shows very little change, indicating a stable framework. In contrast, the appearance of the N1s peak indicates a significant change in the membrane's ionic environment, suggesting enhanced functionality after the introduction of quaternary ammonium groups.

[0043] 3. Figure 3 The zeta potential of the DMBP-TAPB membrane before and after the introduction of quaternary ammonium groups is shown. At pH 3, the zeta potential of the DMBP-TAPB membrane is -2.56 mV, and that of the Q-DMBP-TAPB membrane is 5.21 mV. This significant positive charge is attributed to the introduction of the quaternary ammonium groups, which significantly increases the density of cation sites. Therefore, the Q-DMBP-TAPB membrane holds promise for enhancing electrostatic interactions and improving ion conductivity.

[0044] 4. The water contact angle of the P_DMBP, DMBP-TAPB, Q-DMBP-TAPB, and P_TAPB membranes prepared in Examples 1-3 was tested, and the results are shown in [reference]. Figure 4 As shown in the figure, the water contact angle of the DMBP-TAPB membrane is 88°. After introducing quaternary ammonium groups, the water contact angle of the Q-DMBP-TAPB membrane decreased to 76°. The water contact angles of the quaternized P_DMBP and P_TAPB membranes prepared in Examples 2-3 are 83° and 80°, respectively, which are significantly lower than those of the uncrosslinked membranes. This demonstrates that quaternization and crosslinking treatments effectively improve the hydrophilicity of the membrane. The ion transport process within the membrane channels during selective sieving using the Q-DMBP-TAPB membrane can be found in [reference needed]. Figure 5 .

[0045] 5. Characterization of the selectivity of monovalent / divalent cations in the electrodialysis cation separation process By measuring Li during selective electrodialysis + For Mg 2+ The selective migration of the membrane was studied to determine its ability to separate magnesium and lithium. The electrodialysis cell consisted of four compartments: a concentration compartment, a desalination compartment, and two electrode compartments. Two ruthenium-coated titanium electrodes were used as the anode and cathode, respectively. The desalination compartment contained 200 mL of a pentagonal mixed solution, in which the concentrations of LiCl and MgCl₂ were both 0.1 mol / L. -1 The concentration chamber contains 200 mL of 0.3 mol L⁻¹ -1 NH4Cl; other electrode areas are injected with 200 mL of Na2SO4 (0.3 mol L). -1The effective membrane area is 7.0 cm². 2 The membrane is sandwiched between the desalination and concentration chambers, and the corresponding solutions are dispensed at 30 mL / min. -1 The flow rate was circulated in each chamber. Before measurement, the membrane was equilibrated in the electrodialysis cell for 10 minutes. Subsequently, a flow rate of 5 mA cm⁻¹ was applied. -2 A constant current is applied and maintained for 1 hour.

[0046] Table 1. Cation diffusion rate and selectivity in ion-exchange membrane electrodialysis process

[0047] As can be seen from the data in Table 1, Q-DMBP-TAPB exhibits the strongest electrostatic repulsion against high-valence metal ions and the highest selectivity for monovalent / divalent cations.

[0048] See electrodialysis test results Figure 6 ,from Figure 6 The current density is 5 mA cm⁻¹ -2 At this time, due to the size sieving effect and the Donnan effect, monovalent cations with smaller hydration radii and lower charge densities experience less resistance and have a higher flux. Meanwhile, ultramicropores facilitate the flow of Mg... 2+ Al 3+ The resistance increases significantly, and the corresponding membrane has a high selectivity for monovalent / divalent cations.

[0049] 6. Characterization of the selectivity of membranes for mono / divalent anions during electrodialysis anion separation. By measuring Cl during selective electrodialysis - For SO4 2- The selective migration of the membrane was studied to determine its ability to separate anions. The electrodialysis cell consisted of four compartments: a concentration compartment, a desalination compartment, and two electrode compartments. Two ruthenium-coated titanium electrodes were used for both the anode and cathode; the desalination compartment contained 200 mL of a pentagonal mixed solution, in which the concentrations of NaCl and Na₂SO₄ were both 0.1 mol / L. -1 The concentration chamber contains 200 mL of 0.1 mol L⁻¹ -1 KNO3; other electrode areas are injected with 200 mL of Na2SO4 (0.3 mol L). -1 The effective membrane area is 7.0 cm². 2 The membrane is sandwiched between the desalination and concentration chambers, and the corresponding solutions are dispensed at 30 mL / min. -1 The flow rate was circulated in each chamber. Before measurement, the membrane was equilibrated in the electrodialysis cell for 10 minutes. Subsequently, a flow rate of 5 mA cm⁻¹ was applied. -2 A constant current is applied and maintained for 1 hour.

[0050] Table 2. Anion diffusion rate and selectivity in ion-exchange membrane electrodialysis process

[0051] As can be seen from the data in Table 2, Q-DMBP-TAPB has a stronger blocking effect on anions with large hydration radius and the highest selectivity for monovalent and divalent anions.

[0052] See electrodialysis test results Figure 7 ,from Figure 7 The current density is 5 mA cm⁻¹ -2 At this time, due to the size sieving effect and the smaller hydration radius of monovalent anions, the resistance is less, resulting in a higher flux. Meanwhile, ultramicropores facilitate the flow of SO42-. 2- WO4 2- The resistance increases significantly, and the corresponding membrane has a high selectivity for monovalent / divalent anions.

[0053] Based on the above comparison, it can be seen that if only TAPB monomer is used, the excessive rigidity of the skeleton and the high crosslinking density will lead to overly strict pore size restriction. Although there is a certain degree of selectivity, the ion transport resistance is high, resulting in low flux. If only DMBP is used, although the twisted chain segments form micropores with good pore connectivity and high flux, the lack of strong support from three-dimensional rigid nodes results in insufficient uniformity of micropore size, leading to insufficient selectivity for size sieving. By using the rigid nodes of TAPB to lock the micropore size and combining them with the twisted chain segments of DMBP to form transport channels, a stable and interconnected sub-nanometer ion channel network can be constructed inside the polymer.

[0054] The above tests show that the ion exchange membrane prepared in the examples has high conductivity, high swelling resistance and mechanical properties, and high ion selectivity. It performs excellently in the separation of monovalent and polyvalent anions and cations and has broad application prospects.

Claims

1. A microporous ion exchange membrane, characterized by, The ion exchange membrane uses a microporous polymer or a quaternary ammonium compound of the microporous polymer as its main component; the microporous polymer is prepared by copolymerization and crosslinking of 1,3,5-tris(4-aminophenyl)benzene, o-toluidine, and dimethylformaldehyde as monomers; the structural formula of the microporous polymer is shown in Formula I. ； The value of n ranges from 15000 to 30000.

2. The self-porous ion exchange membrane according to claim 1, characterized in that, The structural formula of the quaternary ammonium compound of the self-porous polymer is shown in Formula II: ； The value of n ranges from 15000 to 30000.

3. The self-porous ion exchange membrane according to claim 1 or 2, characterized in that, The thickness of the ion exchange membrane is 70-90 μm; the ion exchange membrane has a sub-nanometer microporous structure inside; the ion exchange membrane is a homogeneous ion exchange membrane.

4. A method for preparing a self-porous ion exchange membrane as described in any one of claims 1-3, characterized in that, Includes the following steps: S1. Toluidine and dimethylformaldehyde are polymerized and crosslinked in a reaction solvent containing a catalyst, and then 1,3,5-tris(4-aminophenyl)benzene is added to continue the crosslinking reaction to obtain a casting solution containing a self-microporous polymer; S2. The casting solution is cast and heated to dry, and after demolding, it is deprotonated to obtain the self-microporous ion exchange membrane.

5. The method according to claim 4, characterized in that, In step S1, the reaction solvent is a halocarbon solvent and / or a fluorinated organic acid; the catalyst is a fluorinated organic acid.

6. The method according to claim 4, characterized in that, In step S1, the molar ratio of 1,3,5-tris(4-aminophenyl)benzene to dimethylformaldehyde is 1:6-10; the molar ratio of o-toluidine to dimethylformaldehyde is 1:5-8; the mass-to-volume ratio of 1,3,5-tris(4-aminophenyl)benzene to the catalyst is 0.1-0.3 g:0.5-5 mL; and the mass-to-volume ratio of o-toluidine to the catalyst is 0.2-0.5 g:0.5-5 mL.

7. The method according to claim 4, characterized in that, In step S1, the monomer dissolution and mixing process is carried out under ice-water bath conditions, and the stirring time is 30-60 min; the polymerization and crosslinking reaction time is 1-36 h, and the crosslinking reaction continues for 1-18 h; in step S2, the heating and drying temperature is 30-60℃, and the time is 3-12 h; the deprotonation treatment is to immerse the demolded membrane in an alkali metal hydroxide solution for neutralization.

8. The method according to claim 4, characterized in that, The method also includes step S3, which involves preparing a quaternized microporous ion exchange membrane: S3, the microporous ion exchange membrane obtained in step S2 is immersed in a solution containing a quaternization functional reagent to undergo a quaternization reaction, followed by anion exchange treatment and washing; the quaternization functional reagent is a haloalkane reagent; the solvent of the solution containing the quaternization functional reagent is a C1-C4 lower fatty alcohol; the concentration of iodomethane in the solution containing the quaternization functional reagent is 20-50 wt%; the temperature of the quaternization reaction is 50-70℃; the time of the quaternization reaction is 4-12 h; the anion exchange treatment involves immersing the membrane in an alkali metal halide solution for 24-48 h.

9. The application of a self-porous ion exchange membrane as described in any one of claims 1-3 in electrodialysis or diffusion dialysis separation processes.

10. The application according to claim 9, characterized in that, The application is in the separation of magnesium and lithium by electrodialysis; or the application is in the selective separation of monovalent and polyvalent anions, and monovalent and polyvalent cations.

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