A monovalent and divalent anion separation membrane based on microporous polymer, and a preparation method and use thereof

Abstract

The present application relates to a kind of based on microporous polymer's one or two valence anion separation membrane and its preparation method and purposes, belong to ion exchange membrane technical field.The present application uses quaternary ammonium TB polymer as film-forming matrix, so that free I- in membrane will be more uniform, adsorption is stronger, so that the anion exchange membrane of the resulting charge is stronger, and therefore reduce the ion exchange capacity, and effectively reduce the membrane swelling ratio, significantly improve the selectivity of one or two valence anion separation, increase the flux of monovalent ion, especially when separating Cl ‑ With CO3 2‑ When, ‑ Flux reaches 16.13x10 ‑4 mol / m 2 / s, permeation selectivity performance reaches 106.

Description

Technical Field

[0001] This invention relates to a membrane material, specifically to an anion exchange membrane based on a microporous polymer. Background Technology

[0002] Due to the significant pressure that rapid industrial development places on energy security, excessive carbon emissions can easily lead to the greenhouse effect and global warming. Many researchers are focusing on reducing carbon emissions and increasing their recycling through membrane separation processes. Converting carbon-containing materials into carbonates for reuse can effectively recover carbon resources. Separating and extracting carbonate ions from other valuable ions can also reduce chemical consumption and alleviate energy pressure. Typically, membranes with highly uniform nanoscale or even sub-nanometer pore sizes can achieve the separation of similar ions. Their nanochannels can act as channels for the selective transport of small molecules / ions, making them ideal for energy storage and conversion. However, when the ions have small radius differences and similar physical properties, nanochannels struggle to achieve specific separation of such ions. As the pore size of the channel decreases to the sub-nanometer scale, enhanced interactions between ions and channels, such as friction and slip flow, can effectively improve the selectivity of monovalent and divalent ions, achieving more precise separation. An electric field, as an external force, can drive ions, promoting their directional migration and increasing their permeation rate, thereby accelerating the transport of specific ions in sub-nanometer channels. Anion exchange membranes (AEMs) are a key element in electro-driven processes, and their permeability determines the separation efficiency. Many polymer materials have been developed to improve the membrane performance of AEMs. However, ion selectivity remains limited by the membrane material structure, and traditional polymer materials generally suffer from wide pore sizes and pore collapse, leading to poor separation selectivity and chemical stability.

[0003] Microporous polymers are a relatively new type of amorphous microporous material with a rigid and randomly twisted structure. The inefficient stacking of solid molecular chains can generate nanochannels. Due to the twisting of its polymer chains, the main chain cannot rotate freely, resulting in a high free volume fraction in the membrane. At the same time, the membrane exhibits a microporous structure (most pore sizes are less than 2 nm) to induce the transport and separation of small molecules or ions. The central component of the group is a methane[1,5]diazopyrimidine bridge connecting the two sides of the aromatic hydrocarbon, giving TB a rigid V-shaped structure. The introduction of TB units can twist the polymer backbone, thereby increasing the distance between polymer chains, optimizing the inherent microporosity, facilitating the formation of sub-nanometer cavities to allow for rapid ion transport, and also improving the membrane's alkali stability and inhibiting swelling. However, since there are no ion-interacting groups within the sub-nanometer channels, the ion entry barrier is relatively large, resulting in very low membrane conductivity. This indicates a trade-off between ion selectivity and conductivity. Summary of the Invention

[0004] To address the aforementioned issues, this invention introduces hydrophilic quaternary ammonium groups into the sub-nanometer channels of the membrane (TB), significantly increasing the flux of monovalent ions while simultaneously improving the selectivity for separating monovalent and divalent anions. Although the free I- ions near the quaternary ammonium groups have strong adhesion, making ion exchange on the membrane difficult, the low ion exchange capacity effectively reduces membrane swelling. Due to the mutual repulsion between I- ions and anions, anions cannot adsorb onto the membrane surface but migrate from within the sub-nanometer channels.

[0005] The present invention provides an anion exchange membrane based on a microporous polymer, wherein the membrane matrix of the anion exchange membrane is a quaternized TB polymer, and the TB polymer is formed by the condensation reaction of o-toluidine (DMB) and dimethoxymethane (DMM).

[0006] Preferably, the condensation reaction uses trifluoroacetic acid (TFA) as a solvent and acid catalyst.

[0007] Preferably, dioctyl phthalate (DOP) is added to the anion exchange membrane to improve the mechanical properties of the membrane.

[0008] The present invention also provides a method for preparing the above-mentioned microporous polymer-based anion exchange membrane, the method comprising the following steps:

[0009] S1: TB polymer is generated by condensation reaction of o-toluidine (DMB) and dimethoxymethane (DMM);

[0010] S2: Using iodomethane as a quaternizing agent, QA-TB polymer is generated by reacting with TB polymer;

[0011] S3: The QA-TB polymer from step S2 is mixed with a film-forming solvent to form a casting solution, and an anion exchange membrane is formed.

[0012] Preferably, step S1 specifically includes: adding appropriate amounts of o-toluidine and dimethoxymethane to a container and stirring until homogeneous; adding trifluoroacetic acid dropwise in an ice bath; stirring at room temperature for 48-120 hours to allow for complete reaction; mixing the reactants with deionized water to precipitate the polymer; further washing with deionized water to remove residual acid until the pH is neutral; then filtering; drying the obtained solid; pulverizing it in a ball mill; and collecting the TB polymer; the mass ratio of o-toluidine, dimethoxymethane, and trifluoroacetic acid is 1:1.8:14.3.

[0013] Preferably, step S2 specifically includes: adding an appropriate amount of N-methylpyrrolidone (NMP) solution, TB polymer, and anhydrous K2CO3 to a container; adding iodomethane solution dropwise under completely dark conditions; and stirring at room temperature for 5-48 hours to complete the quaternization of the TB polymer; pouring the quaternized mixture into deionized water to precipitate it; and washing the anhydrous K2CO3 with deionized water until no white particles are observed in the mixture; then filtering, drying, and grinding to obtain the quaternized QA-TB polymer; the mass ratio of TB polymer, N-methylpyrrolidone (NMP) solution, anhydrous K2CO3, and iodomethane is 1:20.56:1.6:3.648.

[0014] Preferably, step S3 specifically includes: mixing QA-TB polymer with N-methylpyrrolidone (NMP) solution to prepare a casting solution, filtering, pouring onto a clean and dry glass plate, forming a film at 40-80°C, and after the solvent has completely evaporated, placing it in a container containing deionized water to allow it to fall off naturally, thereby forming an anion exchange membrane; the mass concentration of QA-TB polymer in the casting solution in step S3 is 2-10 wt%, and a 0.2-0.6 μm injection filter is used for filtration.

[0015] The present invention also provides an application of anion exchange membrane based on microporous polymers for the separation of chloride ions and divalent anions.

[0016] Preferably, the divalent anion is a carbonate ion or a sulfate ion.

[0017] Compared to existing technologies, this invention uses a quaternized TB polymer as the film-forming matrix, enabling free I... - The membrane becomes more uniform and has stronger adsorption, resulting in a more charged anion exchange membrane. This reduces the ion exchange capacity and effectively decreases the membrane swelling ratio, significantly improving the selectivity for separating monovalent and divalent anions and increasing the flux of monovalent ions, especially in the separation of Cl. - With CO3 2- At that time, Cl - The flux reached 16.13 x 10⁻⁶. -4 mol / m 2 / s, with a permeation selectivity of 106. Furthermore, this invention effectively addresses the problem of high membrane brittleness by using additives, thereby enhancing mechanical strength. Attached Figure Description

[0018] Figure 1 Diagram of the test setup;

[0019] Figure 2(a) Synthesis and quaternization route of TB polymer; (b) Thermomechanical analysis of TB and QA-TB; (c) Water absorption and swelling ratio of TB and QA-TB membranes; (d) Zeta potential of TB and QA-TB; (e) Tensile strength test of QA-TB membrane and QA-TB membrane with added DOP.

[0020] Figure 3 (a) Working principle of microporous membranes, Cl - Migration hinders CO3 2- (b) Test Cl concentration chamber within 1 hour - and CO3 2- (c) Test Cl within 1 hour - Changes in flux and membrane permeation selectivity; (d) Testing of Cl in the 12-day concentration chamber - and CO3 2- Changes in concentration; (e) Testing Cl over 12 days - Changes in flux and membrane permeation selectivity;

[0021] Figure 4 (a) Working principle of microporous membranes, Cl - Migration hinders SO4 2- (b) Test Cl concentration chamber within 1 hour - and SO4 2- (c) Test Cl within 1 hour - Changes in flux and membrane permeation selectivity; (d) Testing of Cl in the concentration chamber for 48 hours - and SO4 2- Changes in concentration; (e) Test 48h Cl - Changes in flux and membrane permeation selectivity;

[0022] Figure 5 AFM characterization tests were performed on the membranes: (a) TB membrane, (b) QA-TB membrane; (c) Cl - Migration occurs from the hydrophobic regions in the membrane; dynamic water vapor adsorption (DVS) tests are performed on the membranes: (d) TB membrane, (e) QA-TB membrane; (f) Comparison of the performance of this work with that reported in the literature (all data in the literature are Cl... - With SO4 2- (separation) Figures 3-5 The image shows an anion exchange membrane with added DOP in Example 2. Detailed Implementation

[0023] Example 1

[0024] The anion exchange membrane prepared in this embodiment was prepared using the following steps:

[0025] S1:TB polymer is prepared by the condensation reaction of o-toluidine (DMB) and dimethoxymethane (DMM), with trifluoroacetic acid (TFA) added as a solvent and acid catalyst. 7.83 g of o-toluidine and 14.09 g of dimethoxymethane were added to a round-bottom flask and stirred until homogeneous. 75 mL of trifluoroacetic acid was added dropwise in an ice bath, and the mixture was stirred at room temperature for 96 h to allow for complete reaction. The polymer precipitated in deionized water, and the solution was washed with deionized water to remove residual acid until the pH was neutral. The solution was then filtered, dried, and pulverized in a ball mill to obtain TB polymer.

[0026] S2: The TB polymer was further subjected to a quaternization reaction. 100 mL of N-methylpyrrolidone (NMP) solution was added to a round-bottom flask, followed by 5 g of TB polymer and 8 g of anhydrous K₂CO₃. 8 mL of iodomethane solution was added dropwise under completely dark conditions, and the mixture was stirred at room temperature for 24 h to complete the quaternization of the TB polymer. The stirred mixture was then poured into deionized water to precipitate the TB polymer. The anhydrous K₂CO₃ was washed away with deionized water until no white particles were observed in the mixture. The mixture was then filtered, dried, and ground to obtain the quaternized QA-TB polymer.

[0027] S3: A casting solution was prepared by mixing a self-made QA-TB microporous polymer at a mass fraction of 4% with an N-methylpyrrolidone (NMP) solution. The stirred casting solution was then filtered through a 0.45 μm injection filter to obtain a cleaner casting solution. The filtered casting solution was uniformly poured onto a clean, dry glass plate, and the membrane was formed in a 60°C oven using the solvent evaporation method. After the solvent had completely evaporated, the membrane was removed from the oven and placed in a foam basin containing deionized water to allow it to detach naturally. The resulting anion exchange membrane (AEM) was removed from the water, dried, and stored in anhydrous methanol to protect the micropores of the membrane. Before use, it was thoroughly soaked and washed with deionized water to remove the methanol solution from the membrane surface.

[0028] Example 2:

[0029] The difference between this embodiment and Example 1 is that 0.4g of dioctyl phthalate (DOP) was added as an additive to the S3 casting solution.

[0030] Comparative Example 1

[0031] The difference between this comparative example and Example 1 is that step S2 is omitted, and the TB polymer prepared in step S1 is directly mixed with N-methylpyrrolidone (NMP) solution at a mass fraction of 4% to prepare the casting solution.

[0032] Based on the anion exchange membranes prepared in the above embodiments and comparative examples, the present invention also designs a cation exchange membrane, the preparation method of which is as follows:

[0033] S1: Dissolve 9g of polyether ether ketone (PEEK) in 150mL of concentrated sulfuric acid (H2SO4), stir thoroughly at room temperature for 48h, and then precipitate the polymer in an ice-water bath. Wash the solution with deionized water to neutral pH, filter, and then dry at 50℃. Pulverize the dried polymer to obtain sulfonated polyether ether ketone powder (SPEEK).

[0034] S2: The above SPEEK polymer is mixed with NMP solution at a mass fraction of 5% to prepare a casting solution. A cation exchange membrane (CEM) is obtained by the same casting method. The cation exchange membrane is stored directly in deionized water.

[0035] Characterization results

[0036] o-Toluidine (DMB) and dimethoxymethane (DMM) undergo a condensation reaction to obtain TB polymer, which is then quaternized. Figure 2 a). Dynamic thermomechanical analysis (DMA) test results of TB and QA-TB membranes ( Figure 2 (b) indicates that both exhibit excellent thermomechanical properties, consistent with the properties of microporous polymers. Data shows that their storage moduli reach 1267 MPa and 1629 MPa, respectively, at 145 °C, indicating that the thermomechanical properties of QA-TB are slightly higher than those of TB.

[0037] Polymer membranes used for ion separation need to consider the swelling ratio and ion exchange capacity of ion exchange membranes, because a high swelling ratio typically reduces the dimensional stability of the membrane. Test data from this work show ( Figure 2 c) The water absorption rates of the TB membrane and the QA-TB membrane were 5.3% and 3.61%, respectively, and the swelling rates were 0.87% and 0.86%, respectively, both relatively low. The ion exchange capacity was also found to be very low, at 0.0167 meq / g, consistent with the water absorption and swelling rate test results. High ion exchange capacity (IEC) typically leads to high water content, which can result in a high membrane swelling ratio and reduced membrane selectivity. In this study, the membrane swelling ratio was low, leading to a low IEC. In addition, the Zeta potential test ( Figure 2 d) Test results show that the membrane surface is significantly negatively charged under alkaline conditions, and the negative charge is even stronger after quaternization, indicating that I- ions successfully attach around the quaternary ammonium groups after quaternization. This is because I- ions are adsorbed in a free state on the N-... + The surrounding area is covered with strong adhesion, making it difficult to detach and hindering ion exchange, resulting in a lower IEC (internal ion exchange rate). Furthermore, comparing the ion separation results of TB membranes and QA-TB membranes, it was found that QA-TB membranes can indeed better migrate monovalent ions and achieve better monovalent ion flux in a shorter time.

[0038] Considering the common problem of brittleness in microporous polymer membranes, studies have found that adding dioctyl phthalate (DOP) can improve membrane flexibility without affecting its separation performance. The mechanical strength before and after the addition of DOP was compared. Figure 2 e) By comparing the elongation at break and tensile strength, it can be found that the membrane with added DOP exhibits better tensile strength, and the Young's modulus increases from 56.25 to 60 MPa, indicating that the mechanical stability of the membrane is significantly enhanced.

[0039] The anion exchange membranes and cation exchange membranes of the above embodiments and comparative examples were prepared according to... Figure 1 The test device is assembled in a manner that allows it to be used in various ways. Figure 1 In the testing apparatus, the dilution chamber contains a 0.1 mol / L mixed salt solution of NaCl and Na₂CO₃, the concentration chamber contains a 0.1 mol / L Na₂SO₄ salt solution, and the cathode and anode chambers share a 0.2 mol / L Na₂SO₄ solution circulating together, under an adjustable voltage of 4V and an A / m... 2 Ion separation tests were conducted at a specific current density. The current efficiency and energy consumption ratio during the test were calculated, and the separation of Cl- / CO3- was also performed. 2- At that time, the current efficiency dropped to 69% after 12 hours, and the separation of Cl- / SO4 was achieved. 2- At that time, the current efficiency dropped to 62% after 13 hours, which is a relatively good efficiency.

[0040] Cl- / CO3 2- The separation test results are as follows Figure 3 As shown, Cl was found within 1 hour - The migration speed is faster than that of CO3 2- Quick, Cl - The flux can reach 16.13 x 10⁻⁶ in a short period of time. -4 mol / m 2 / s, and no CO3 was detected in the concentration chamber within the first 25 minutes. 2- Osmotic selectivity could not be calculated. A small amount of CO3 could be detected starting at 30 minutes. 2- The mixture migrates to the concentration chamber, but when counted over a short period of time, the concentration chamber contains Cl- and CO3. 2- The amounts were all very small, so the osmotic selectivity decreased gradually and irregularly. Subsequent sampling every 12 hours revealed a slowdown in ion migration rate and Cl... - Flux decreases with increasing time, but the system's permeability selectivity increases, reaching 10⁶ after 12 days of operation. This is because during long-term operation, Cl... - The CO3 continuously migrates into the concentration chamber, accumulating over time, and... 2- The incremental migration was minimal, even plateauing. Furthermore, in the titration test of CO3... 2-It was observed that the pH of the solution in the concentration chamber and the dilution chamber remained almost unchanged during the test, indicating that the CO3 in the dilution chamber was relatively stable. 2- It did not generate excessive CO2 or HCO3 due to hydrolysis. - Cause CO3 2- A decrease in concentration can affect the migration rate or the quality of migration.

[0041] Membranes for Cl under the same equipment conditions - SO4 2- The separation performance was tested. Figure 4 The test system was changed only: the dilution chamber was replaced with a mixed salt solution of 0.1 mol / L NaCl and Na₂SO₄, the concentration chamber was replaced with a 0.1 mol / L Na₂CO₃ salt solution, and the cathode and anode chambers continued to circulate a 0.2 mol / L Na₂SO₄ solution, with all other conditions remaining unchanged. The test results showed that Cl… - The flux still reached its maximum value within a short period of time, reaching 20.37 x 10⁻⁶. -4 mol / m 2 / s. However, comparing the two separation systems, it is clear that separating Cl... - With SO4 2- The migration speed is faster, which means SO4 2- Also followed Cl - Migration occurred. Therefore, Cl was separated. - SO4 2- The permeability selectivity reached its optimal point of 82.11 within a short time. The concentration changes within the concentration chamber show that almost all Cl- migration was completed within 48 hours, while SO4... 2- Nearly half of the CO3 migrated, compared to only a small fraction of the CO3 that ultimately migrated. 2- In comparison, Cl - With SO4 2- The separation ratio of Cl- to CO3 2- The separation is not ideal, but it is still far superior to existing technologies.

[0042] By using dynamic vapor adsorption curves ( Figure 5 (d, 5e) Observing the change of water adsorption capacity of the membrane with relative humidity, it was found that the water adsorption capacity of both TB membrane and QA-TB membrane was low, not exceeding 20%. The water adsorption capacity of QA-TB membrane was slightly higher than that of TB membrane. This is because the introduction of quaternary ammonium groups enhances hydrophilicity, leading to increased water absorption. The internal structure of the membrane was observed using AFM (Anaerobic Membrane Phosphate). Figure 5(a) and (b) , the TB membrane has only a hydrophobic backbone, exhibiting a uniform structure. In contrast, the QA-TB membrane, due to the introduction of ammonium groups, alters the charge, causing specific aggregation of hydrophilic ion clusters that induces microphase separation, resulting in distinct bright and dark regions. The bright areas are hydrophobic regions caused by the polymer backbone, while the dark areas are hydrophilic regions caused by quaternary ammonium groups. Furthermore, the amorphous stacking of polymer chains and the presence of more hydrophobic backbones in the structure result in more hydrophobic regions in the membrane, allowing for better migration of easily dehydrated ions.

[0043] Furthermore, by comparing the hydration radius, hydration free energy, and hydration entropy of the ions, it was found that the order of magnitude was Cl- <SO4 2- <CO3 2- This indicates that Cl- has the lowest hydration free energy, making it most likely to detach from surrounding water molecules and thus more easily attracted to the membrane surface by quaternary ammonium groups. SO4 2- and CO3 2- CO3 has a relatively high hydration free energy and a stronger ability to bind with water. It easily attracts water molecules around its ions, forcing the ions to break through the surrounding water molecules to reach the membrane surface. Furthermore, CO32-... 2- It is easily hydrolyzed and highly polarizable; it can be polarized and therefore easily combines with polar water molecules, so CO3... 2- It needs to overcome the resistance of more water molecules to reach the membrane surface, resulting in a slower speed. Therefore, Cl... - It can reach the membrane surface faster and easier. However, Cl- in the system is attracted to the membrane surface by quaternary ammonium groups. - Unable to compete with the surrounding free-floating I - Ions that cannot react with the N-type quaternary ammonium group + The CO3 molecules bind and adsorb onto the membrane surface, then rapidly desorb, migrating from the hydrophobic regions of the membrane under the influence of an electric field. Simultaneously, CO3... 2- Due to the influence of water molecules, the rate at which water reaches the membrane surface is slower, and the ionic radius is also slightly larger than that of Cl. - Therefore, the amount of ions migrating from the membrane under the influence of an electric field is small and slow. This invention maintained a 12-day testing period, during which Cl... - The migration rate and ion quantity are both higher than those of CO3. 2- The faster and more abundant the solution, the greater the osmotic selectivity of the system will be. Sulfuric acid, as a strong acid, completely ionizes in water, producing SO42-. 2- It will not undergo hydrolysis and has a higher water binding capacity than CO3. 2- Slightly weaker, but better than Cl - To be strong, therefore Cl - SO4 2- The selectivity of separation compared to Cl - / CO3 2-While the selectivity during separation is low, the overall separation performance is still good.

Claims

1. A method for preparing anion exchange membrane based on microporous polymers, characterized in that... The anion exchange membrane is formed by a quaternized TB polymer, which is obtained by a condensation reaction of o-toluidine (DMB) and dimethoxymethane (DMM); the anion exchange membrane is used for the separation of chloride ions and divalent anions. The method includes the following steps: S1: TB polymer is generated by condensation reaction of o-toluidine (DMB) and dimethoxymethane (DMM); S2: Using iodomethane as a quaternizing agent, it reacts with TB polymer to generate QA-TB polymer; S3: The QA-TB polymer from step S2 is mixed with a film-forming solvent to form a casting solution, and an anion exchange membrane is formed. Step S2 specifically includes: adding an appropriate amount of N-methylpyrrolidone (NMP) solution, TB polymer and anhydrous K2CO3 to a container, adding iodomethane solution dropwise under completely dark conditions, and stirring at room temperature for 5-48 h to complete the quaternization of TB polymer, pouring the quaternized mixture into deionized water to precipitate, and washing the anhydrous K2CO3 with deionized water until no white particles are observed in the mixture, then filtering, drying and grinding to obtain the quaternized QA-TB polymer; Step S3 specifically includes: mixing QA-TB polymer with N-methylpyrrolidone (NMP) solution to prepare casting solution, filtering and pouring it onto a clean and dry glass plate, forming a film at 40-80℃, and after the solvent has completely evaporated, placing it in a container containing deionized water to allow it to fall off naturally to form an anion exchange membrane.

2. The method according to claim 1, characterized in that... The condensation reaction uses trifluoroacetic acid (TFA) as a solvent and acid catalyst.

3. The method according to claim 1, characterized in that... Dioctyl phthalate (DOP) is added to the anion exchange membrane to improve its mechanical properties. The addition range of DOP is 0.1-6 wt.

4. The method according to claim 1, characterized in that... Step S1 specifically includes: adding appropriate amounts of o-toluidine and dimethoxymethane to a container and stirring evenly; adding trifluoroacetic acid dropwise in an ice bath; stirring at room temperature for 48-120 hours to allow for complete reaction; mixing the reactants with deionized water to precipitate the polymer; further washing with deionized water to remove residual acid until the pH is neutral; then filtering; drying the obtained solid; pulverizing it in a ball mill; and collecting the TB polymer; the mass ratio of o-toluidine, dimethoxymethane, and trifluoroacetic acid is 1:1.8:14.

3.

5. The method according to claim 1, characterized in that, The mass ratio of TB polymer, N-methylpyrrolidone (NMP) solution, anhydrous K2CO3 and iodomethane was 1:20.56:1.6:3.

648.

6. The method according to claim 1, characterized in that, In step S3, the mass concentration of QA-TB polymer in the casting solution is 2-10 wt%, and a 0.2-0.6 μm injection filter is used for filtration.

7. The method according to claim 1, characterized in that... The divalent anion mentioned is either carbonate or sulfate.

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