Preparation method of monovalent / divalent selective cation exchange membrane based on crown ether

Abstract

The invention relates to the field of ion separation, and particularly discloses a crown ether-based monovalent / divalent selective cation exchange membrane preparation method, which comprises: carrying out super acid catalysis copolymerization on dibenzo crown ether and a pre-sulfonated monomer to obtain a precursor, and carrying out graft modification on a main chain by using a side chain on the basis to obtain the crown ether-based monovalent / divalent selective cation exchange membrane. The separation performance of monovalent / divalent cations is optimized, and the side chain type cation exchange membrane is prepared. The ion exchange membrane provided by the invention can be used for screening various monovalent / divalent ion systems, the number of # imgabs 0 can reach 370, the number of # imgabs 1 can reach 207, and the number of # imgabs 2 can reach 11125.

Description

Technical Field

[0001] The invention belongs to the field of membrane technology, and in particular relates to a method for preparing a monovalent / divalent selective cationic membrane based on crown ether. Background Art

[0002] Selective ion separation is a common requirement in important chemical processes, including energy conversion and storage, environmental pollution control and monitoring, clean industrial production, and resource recycling. It is a crucial step in processes such as lithium extraction from salt lakes, the chlor-alkali industry, high-salinity wastewater treatment in coal chemical industry, and hydrometallurgy, and represents a pressing technical challenge for the chemical industry. Membrane separation, as a highly efficient and environmentally friendly method, has garnered widespread attention. Membrane separation technologies include concentration-driven diffusion dialysis, pressure-driven microfiltration, ultrafiltration, nanofiltration, and reverse osmosis, and electrically driven electrodialysis. Electrodialysis, a more efficient ion separation technology, offers simple equipment and assembly processes, lower overall operating energy consumption, longer service life, and higher long-term economic and environmental benefits. It plays a vital role in a variety of applications, including seawater desalination, concentration, and saline wastewater treatment. The core component of electrodialysis is the ion exchange membrane. Traditional ion exchange membranes are categorized as cation exchange membranes and anion exchange membranes. These membranes utilize the electrostatic interaction between fixed ionic groups on the membrane and ions, allowing counterions to pass while blocking the passage of ions with the same ion. However, the selective separation ability of traditional ion exchange membranes for ions of the same name is limited. + / Mg 2+ , K + / Mg 2+ Separation of Na + / Ca 2+ 、Na + / Mg 2+ Separation of Cl in chemical high-salt wastewater - / SO4 2-When carrying out processes such as separation, it is necessary to develop more effective ion exchange membranes, namely monovalent / divalent selective ion exchange membranes. Currently, commercial monovalent / divalent selective ion exchange membranes are mainly based on the principles of charge repulsion and pore size screening, and a selective layer is constructed on the membrane surface to achieve the purpose of ion screening. For example, Japanese commercial cationic membranes and commercial anionic membranes add a charge layer to the surface of the base membrane, utilizing the charge repulsion effect to achieve the retention of divalent ions and the selective passage of monovalent ions, or by adjusting the density of the ion membrane and controlling the free volume of the membrane, utilizing the pore size screening effect to separate monovalent / divalent ions. In addition, by introducing a hydrophobic modification layer on the membrane surface, it helps to repel divalent ions with higher Gibbs hydration energy and allow monovalent cations with lower hydration energy to pass. To achieve better separation effects, researchers have also drawn inspiration from biological membrane processes and introduced groups that bind to ions into or on the membrane surface. Through the interaction between ions and channels within the membrane, the energy barrier for ion distribution into the membrane is reduced, thereby promoting the transport of specific ions. For example, the literature (ACS Applied Materials & Interfaces, 2019, 11 (19), 17730-17741.) reported a series of cation exchange membranes with co-deposition of dopamine and 4'-aminobenzo-15-crown-5 on sulfonated polysulfone-based membranes. Due to the pore size screening effect of the dense membrane and the interaction with K + The host-guest molecular recognition effect of the complexed crown ether, the membrane showed good ion separation performance. The literature (Nature Communication, 2023, 14 (1), 4907.) reported a biomimetic channel using Ti3C2T x The sheets are stacked to simulate the pore size of biological potassium ion channels, and EDTA is introduced to simulate the binding site of potassium ion channels. The charge density is adjustable by changing the pH. By simulating biological channels in terms of pore size, binding site, and charge density, a good separation effect is achieved. + / Mg 2+ The selectivity can reach 121.2.

[0003] Comprehensive data show that through the rational design of membrane structure and the utilization of the basic principles of ion separation, effective separation of monovalent and divalent ions can be achieved. However, currently used surface modification methods lead to increased membrane resistance and complex preparation steps. At the same time, the bonding strength between the modified layer and the base membrane affects the durability of the ion exchange membrane. Summary of the Invention

[0004] To address the shortcomings of the aforementioned prior art, the present invention provides a method for preparing a monovalent / divalent selective cation exchange membrane based on crown ethers. Utilizing the fundamental principles of ion separation, crown ether sites that bind to ions are introduced, along with three types of side chains: an alkyl chain to regulate hydrophilicity and hydrophobicity, a fluorinated side chain that interacts with lithium ions to achieve selective separation of lithium ions, and an ether chain that weakly binds to alkali metal ions to promote the separation of alkali metal ions from high-valent metal ions. This method enables the preparation of a high-performance, homogeneous monovalent / divalent selective cation exchange membrane. The present invention discloses for the first time a method for preparing a monovalent / divalent selective cation exchange membrane with a simple preparation method, significant selective separation performance, and high ion flux.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for preparing a monovalent / divalent selective cation membrane based on crown ethers comprises the following steps:

[0007] Step 1: dissolving a sulfonic acid group-substituted aromatic monomer, a dibenzocrown ether, and a carbonyl compound containing a nitrogen heterocycle in a solvent, adding a first catalyst after complete dissolution, and performing ternary copolymerization to obtain a base polymer;

[0008] Step 2: dissolving the base polymer in dimethyl sulfoxide, adding the side chain monomer and the second catalyst under heating conditions to modify the base polymer;

[0009] Step 3: Pour the product obtained in step 2 into water for purification, and obtain a modified polymer material after washing and drying;

[0010] Step 4: After mixing the modified polymer material and the polar organic solvent, the mixture is coated and dried to obtain a monovalent / divalent selective cation membrane.

[0011] Furthermore, in step 1: the solvent is dichloromethane, the first catalyst includes trifluoroacetic acid and trifluoromethanesulfonic acid, the reaction temperature of the ternary copolymerization is 0°C to 19°C, and the polymerization time is 1h to 6h. The molar ratio of the sulfonic acid-substituted aromatic monomer to the dibenzocrown ether is 1:1-3, and the molar amount of the nitrogen heterocycle-containing carbonyl compound is 1 to 1.5 times the sum of the molar amounts of the sulfonic acid-substituted aromatic monomer and the dibenzocrown ether. The amount of the solvent added is based on ensuring the dissolution of the three raw materials, and the volume ratio of the first catalyst to the solvent (trifluoroacetic acid:trifluoromethanesulfonic acid:dichloromethane) is 3.75:1:1.5.

[0012] Furthermore, in step 2, the heating condition is 60-80°C, the second catalyst is sodium tert-butoxide or sodium hydride, the reaction time is 12-24 hours, and the reactive grafting site is the -NH site. The molar ratio of the base polymer, the side chain monomer, and the second catalyst is 1:0.2-1:0.2-1. The amount of dimethyl sulfoxide added is based on the solubility of the base polymer.

[0013] Furthermore, in step 4, the polar organic solvent is dimethyl sulfoxide, the drying temperature is 60° C. to 80° C., and the drying time is 8 to 12 hours.

[0014] In step 1, the structural formula of the base polymer can be expressed as:

[0015]

[0016] In step 3, the structural formula of the modified polymer can be expressed as:

[0017]

[0018] Wherein: Ai comes from a sulfonic acid-substituted aromatic monomer, Aj comes from a dibenzocrown ether, Ac comes from a carbonyl compound containing a nitrogen heterocycle, m and n represent the molar ratios of Ai and Aj in the base polymer, respectively, m+n=1, Ae represents the grafted side chain, v+u=m, g+j=n, v+u+j+g=1.

[0019] In the preparation method of the crown ether-based mono / divalent selective cationic membrane of the present invention, the raw materials used are as follows:

[0020] 1. In step 1, the structural formula of the sulfonic acid group-substituted aromatic monomer is shown in formula (1):

[0021]

[0022] In the formula, Aq is a disubstituted phenyl group, a naphthyl group or an anthracenyl group, x and y are any integers selected from 1 to 5, and x=y.

[0023] The group Ai formed by the monomer shown in formula (1) in the polymer is shown in formula (1a):

[0024]

[0025] Furthermore, the structural formula of the sulfonic acid group-substituted aromatic monomer is as shown in any one of formula (1-1) or formula (1-2):

[0026]

[0027] The groups Ai formed by the above monomers in the polymer are shown in formulas (1-1-a) to (1-2-a):

[0028]

[0029] The compounds of formula (1-1) and formula (1-2) are obtained by sulfonating bisphenol monomers. The synthesis method can be found in the literature "J. Mater. Chem., 2012, 22, 13996-14000".

[0030] 2. In step 1, the structural formula of the dibenzocrown ether is shown in formula (2):

[0031]

[0032] In formula (2), q and z are each independently selected from any integer from 1 to 4, and q=z or q=z+1.

[0033] The group Aj formed by the dibenzocrown ether shown in formula (2) in the polymer is shown in formula (2a):

[0034]

[0035] Furthermore, the structural formula of the dibenzocrown ether is shown in any one of Formula (2-1) to Formula (2-6):

[0036]

[0037] The groups Aj formed by the dibenzocrown ethers represented by formula (2-1) to formula (2-6) in the polymer are represented by formula (2-1-a) to formula (2-6-a):

[0038]

[0039]

[0040] The compounds represented by formula (2-1) to formula (2-6) are dibenzo-18-crown-6, dibenzo-12-crown-4, dibenzo-15-crown-5, dibenzo-21-crown-7, dibenzo-24-crown-8, and dibenzo-30-crown-10, respectively. Among them, dibenzo-18-crown-6, dibenzo-15-crown-5, dibenzo-21-crown-7, dibenzo-24-crown-8, and dibenzo-30-crown-10 can all be purchased from commercial sources, and dibenzo-12-crown-4 can be prepared by referring to the literature "Chinese Chemical Letters 2009, 20(8), 924-926".

[0041] 3. The structural formula of the carbonyl compound containing a nitrogen heterocycle is shown in any one of Formula (3-1) to Formula (3-4):

[0042]

[0043] The group Ac formed by the nitrogen-containing heterocyclic carbonyl compound represented by formula (3-1) to formula (3-4) in the polymer is represented by formula (3-1-a) to formula (3-4-a):

[0044]

[0045] The compounds represented by the above formulas (3-1) to (3-4) are respectively isatin, 4-piperidone, 2,3-dihydro-1H-quinolin-4-one, and 3-pyrrolidone, which are commercially available.

[0046] 4. The structural formula of the side chain monomer is shown in any one of formulas (4) to (6):

[0047]

[0048] In the formula, t is any integer from 1 to 5, s is any integer from 1 to 6, and k is any integer from 1 to 10.

[0049] The group Ae formed by the side chain monomers represented by formula (4) to formula (6) in the polymer is as shown in formula (4a) to formula (6a):

[0050]

[0051] Furthermore, the structural formula of the side chain monomer is shown in any one of the following formulas (4-1) to (6-4):

[0052]

[0053]

[0054] The groups Ae formed by the above side chain monomers in the polymer are shown below:

[0055]

[0056]

[0057] Any compound of formula (4-1) to formula (6-4) can be purchased commercially.

[0058] The present invention provides a method for preparing a homogeneous monovalent / divalent selective cation exchange membrane based on crown ethers. Unlike commonly used surface-modified ion-selective membranes, the functional groups of the homogeneous membrane described in the present invention are chemically bonded to the main chain, making it less susceptible to detachment. This method also improves the service life of surface-modified membrane materials. Furthermore, the preparation process is simple and the operating conditions are mild. Through rational structural design, efficient monovalent and divalent cation separation is achieved, and its effectiveness is specifically reflected in the following aspects:

[0059] 1. The present invention provides superacid catalytic reaction sites and cation exchange sites through aromatic monomer compounds containing side chain sulfonic acid groups. In aqueous solution, the hydrophilic, highly polar sulfonic acid side chains and hydrophobic segments form a hydrophilic-hydrophobic microphase separation structure, forming a continuous ion transport channel and ensuring the free transport of ions. The sulfonic acid groups, as fixed negatively charged groups, repel anions while providing sites for the free movement of cations in the membrane, ensuring full utilization of current efficiency.

[0060] 2. The present invention provides a superacid catalytic reaction site and an ion binding site by introducing a dibenzocrown ether ring. By changing the ratio of Aj to Ai, the hydrophilicity and hydrophobicity of the membrane can be adjusted. Since the oxygen on the crown ether ring has a lone pair of electrons, different crown ethers have matching pore sizes with different alkali metal ions, enabling them to produce a host-guest binding effect with alkali metal ions, reducing the energy barrier for alkali metal ions to enter the membrane, and improving the selectivity for alkali metal ions. After the crown ether binds to the alkali metal ions, the positively charged complex and the negatively charged sulfonic acid group can form a cooperative ion transport network by generating electrostatic interactions, promoting the transmission of alkali metal ions. At the same time, the positively charged complex produces a stronger electrostatic repulsion of high-valent metal ions, hindering the transmission of high-valent ions and further improving the monovalent / divalent ion selectivity.

[0061] 3. The nitrogen-containing heterocyclic carbonyl compound of the present invention introduces abundant -NH- sites while ensuring the superacid catalytic reaction, which facilitates the side chain modification: (1) The introduced hydrophobic side chain structure makes the membrane material itself more hydrophobic. Divalent cations with higher hydration energy bind more tightly to water molecules. When passing through the membrane, the hydrophobic alkyl chain causes the ions to generate additional transmembrane energy barriers, increasing the resistance of high-valent ions to pass through the membrane layer; while monovalent ions are less affected by the hydrophobic alkyl chain due to their lower hydration energy. Therefore, the introduction of the alkyl chain enhances the membrane's monovalent and divalent ion separation performance. (2) The introduced fluorine-containing side chain has both hydrophobic modification and specific recognition effects. The fluorine atoms of the fluorine-containing side chain can recognize lithium ions, thereby reducing the energy barrier for lithium ions to enter the membrane, promoting the transmission of lithium ions, and enhancing the membrane material's selectivity for lithium ions. (3) The introduced ether chain is connected to the main chain by a chemical bond. Similar to the crown ether ring, the ether chain can also utilize electron-rich oxygen to produce a weak binding effect with alkali metal ions, and at the same time, it can play the advantage of its flexible chain and act as a "transporter" for ion transport, transporting ions between each crown ether ring, shortening the ion jump distance, and further reducing the overall energy barrier that monovalent ions need to overcome to pass through the membrane, thus completing the efficient transmission of monovalent ions. Divalent ions, due to their stronger "Lewis acidity", are more tightly bound to ether chains. The introduction of a certain amount of ether chains actually enhances their diffusion energy barrier during the diffusion process within the membrane. Therefore, an appropriate amount of ether chains increases the energy barrier difference in the transmission process of monovalent and divalent ions, giving the membrane excellent monovalent / divalent ion selectivity. Adjusting the ratio of ether chain branches can adjust the hydrophilicity, pore structure and ion separation selectivity of the membrane. In one embodiment of the present invention, the prepared ether chain-modified polymer membrane P (K+ / Mg 2+ ) Up to 370, P (Na+ / Mg 2+ ) Up to 207. Since the main chain structure and side chain structure of the present invention are rich in oxygen, when the structure is filled with water, the oxygen atoms can form hydrogen bonds with water molecules, thereby forming a hydrogen bond network in the membrane, forming a hydrogen ion transmission channel, and promoting the rapid transmission of hydrogen ions. In one embodiment of the present invention, the prepared ether chain modified polymer membrane P (H+ / Fe 2+ ) It can reach 11125. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 2 is a diagram of the ion selectivity testing device used in the present invention.

[0063] Figure 2 This is the membrane material prepared in Example 1 of the present invention.

[0064] Figure 3The embodiment 1 to embodiment 3 of the present invention 1 HNMR characterization results.

[0065] Figure 4 is the embodiment 3-embodiment 8 of the present invention 1 HNMR characterization results.

[0066] Figure 5 These are XPS spectra of the base polymer obtained in Example 3 and the modified polymer obtained in Example 8 of the present invention, wherein (a) and (b) are the full spectrum and fine spectrum of the base polymer, respectively, and (c) and (d) are the full spectrum and fine spectrum of the modified polymer, respectively.

[0067] Figure 6 It is a schematic diagram of the separation principle of the monovalent / divalent cation selective membrane constructed in the present invention. DETAILED DESCRIPTION

[0068] The present invention discloses a method for preparing and modifying a monovalent / divalent selective cation membrane. Those skilled in the art can refer to the disclosure herein and appropriately modify the process parameters to achieve the desired results. It should be noted that these technical solutions are intended only to facilitate understanding by those skilled in the art and are not intended to limit the scope of the present invention.

[0069] The present invention is further described below with reference to the following examples.

[0070] The following examples adopt Figure 1 The device shown in the figure was used to perform electrodialysis tests on the obtained polymer membrane: the test device includes a cathode plate and an anode plate, and an anion exchange membrane (AMX) is arranged in sequence between the two electrode plates. The polymer membrane to be tested and the anion exchange membrane (AMX) form a cathode chamber, a concentration chamber, a desalination chamber, and an anode chamber. The cathode chamber is connected to the anode chamber, and the membrane effective area is 7.07 cm 2 The specific test conditions are:

[0071] KCl-MgCl2 system: 100 mL of 0.1 mol L -1 KCl and 0.1 mol L -1 MgCl2 mixed solution, the concentration chamber contains 100mL of 0.01mol L -1 KCl solution, 0.3 mol L in the electrode compartment -1 Na2SO4 solution, current density 2 mAcm -2 , test time 1h.

[0072] HCl-FeCl2 system: 100mL 1mol L in the desalination chamber -1 HCl and 0.267 mol L -1 or 0.2 mol L -1FeCl2 mixed solution (0.2 mol L FeCl2 was used in the tests of Examples 7, 8 and Comparative Example 1) -1 The concentration of the other examples is 0.267 molL -1 ), the concentration chamber is 100mL 0.01mol L -1 HCl solution, 0.3 mol L in the electrode compartment -1 Na2SO4 solution, current density 10 mA cm -2 , test time 1h.

[0073] NaCl-MgCl2 system: 100 mL 0.1 mol L in the desalination chamber -1 NaCl and 0.1 mol L -1 MgCl2 mixed solution, the concentration chamber contains 100mL of 0.01mol L -1 NaCl solution, 0.3 mol L -1 Na2SO4 solution, current density 2 mAcm -2 , test time 1h.

[0074] LiCl-MgCl2 system: 100 mL 0.1 mol L in the desalination chamber -1 LiCl and 0.1 mol L -1 MgCl2 mixed solution, the concentration chamber contains 100mL of 0.01mol L -1 LiCl solution, polarization chamber is 0.3 mol L -1 Na2SO4 solution, current density 2 mAcm -2 , test time 1h.

[0075] The calculation method of ion flux is shown in the following formula:

[0076]

[0077] J represents the flux, C0 represents the initial cation concentration in the concentration chamber, and C t represents the cation concentration in the concentrating compartment when electrodialysis is performed for t hours, V represents the solution volume in the concentrating compartment, A represents the effective area of the membrane, and t represents the test duration.

[0078] The calculation method of permeation selectivity is shown in the following formula:

[0079]

[0080] P represents permeation selectivity, J M + represents the flux of monovalent ions, C M0 + represents the initial concentration of monovalent ions in the concentration chamber, JD 2+ represents the flux of divalent ions, C D0 2+ Represents the initial concentration of divalent ions in the concentrating compartment.

[0081] Specific test results are shown in Tables 1 to 4.

[0082] 1. Preparation of base polymer membrane

[0083] Example 1

[0084] Preparation of the base polymer: Weigh 12.25 g of dibenzo-18-crown-6 ether (structural formula shown in Formula (2-1)) and 16.13 g of 2,2'-bis(3-sulfonated propoxy)biphenyl disodium (structural formula shown in Formula (1-1)) into an open-jacketed reactor. Dissolve them in 75 mL of trifluoroacetic acid and 30 mL of dichloromethane. Once completely dissolved, add 11.02 g of isatin (structural formula shown in Formula (3-1)). Once completely dissolved, cool the reaction system to 0°C. Add 20 mL of trifluoromethanesulfonic acid and allow the mixture to react with stirring for 5 hours. The product is precipitated in water, washed thoroughly with deionized water, and dried in an electric forced-air drying oven at 60-80°C for 24 hours to obtain the base polymer.

[0085] Take 0.5g of the above-mentioned basic polymer material, add 10mL of dimethyl sulfoxide, and fully dissolve it to obtain a uniform and transparent membrane liquid. Apply the membrane liquid on a clean glass plate and dry it at 80°C for 12h. Place the glass plate in deionized water and fully immerse it. The membrane swells and falls off on the glass plate to obtain a basic polymer membrane.

[0086] Example 2

[0087] Preparation of the base polymer: Weigh 16.22 g of dibenzo-18-crown-6 ether and 10.91 g of 2,2'-bis(3-sulfonated propoxy)biphenyl disodium into an open-jacketed reactor. Dissolve the mixture in 75 mL of trifluoroacetic acid and 30 mL of dichloromethane. Once dissolved, add 11.02 g of isatin. Once dissolved, cool the reaction system to 0°C. Add 20 mL of trifluoromethanesulfonic acid and allow the mixture to react with stirring for 5 hours. The product is precipitated in water, washed thoroughly with deionized water, and dried in an electric forced-air drying oven at 60-80°C for 24 hours to obtain the base polymer.

[0088] Take 0.5g of the above-mentioned basic polymer material, add 10mL of dimethyl sulfoxide, and fully dissolve it to obtain a uniform and transparent membrane liquid. Apply the membrane liquid on a clean glass plate and dry it at 80°C for 12h. Place the glass plate in deionized water and fully immerse it. The membrane swells and falls off on the glass plate to obtain a basic polymer membrane.

[0089] Example 3

[0090] Preparation of the base polymer: 18.39 g of dibenzo-18-crown 6 ether and 8.07 g of 2,2'-bis(3-sulfonated propoxy)biphenyl disodium were weighed into an open-jacketed reactor. 75 mL of trifluoroacetic acid and 30 mL of dichloromethane were added to dissolve the mixture. Once dissolved, 11.02 g of isatin was added. Once dissolved, the reaction temperature was lowered to 0°C. 20 mL of trifluoromethanesulfonic acid was added and the mixture was stirred for 5 hours. The product was precipitated in water, washed thoroughly with deionized water, and dried in an electric forced-air drying oven at 60-80°C for 24 hours to obtain the base polymer.

[0091] Take 0.5g of the above-mentioned basic polymer material, add 10mL of dimethyl sulfoxide, and fully dissolve it to obtain a uniform and transparent membrane liquid. Apply the membrane liquid on a clean glass plate and dry it at 80°C for 12h. Place the glass plate in deionized water and fully immerse it. The membrane swells and falls off on the glass plate to obtain a basic polymer membrane.

[0092] The base polymers obtained in Examples 1 to 3 were subjected to nuclear magnetic resonance testing, and the results were as follows: Figure 3 As shown. H NMR spectrum ( 1 H NMR spectra were recorded on an AVANCE AV400, Bruker, using deuterated dimethyl sulfoxide (DMSO-d6, with tetramethylsilane as the internal standard) as the solvent. The NMR results indicate that base polymers containing varying ratios of dibenzocrown ether and sulfonic acid-substituted aromatic monomers were successfully synthesized.

[0093] The selectivity of the basic polymer ion selective separation membranes obtained in Examples 1 to 3 was tested in the KCl-MgCl2 system. The results are shown in Table 1. It can be seen that with the increase of crown ether content, the monovalent / divalent ion selectivity increased sharply. In Example 3, the potassium ion flux increased and the magnesium ion flux decreased significantly, indicating that the introduction of dibenzo-18-crown-6 ether can promote the transmission of potassium ions, proving the effectiveness of the "host-guest interaction".

[0094] 2. Preparation of modified polymer membrane

[0095] Example 4

[0096] Preparation of modified polymer: Take 3 g of the base polymer of Example 3 above and place it in a round-bottom flask, add 30 mL of dimethyl sulfoxide, and fully dissolve it at 50°C. Then add 0.10 g of sodium tert-butoxide and fully dissolve it at 80°C. After complete dissolution, weigh 0.20 g of 1-bromo-2-(2-methoxyethoxy)ethane (structural formula as shown in formula (4-4)) and add it dropwise to the flask. After 24 hours, precipitate the product in deionized water and wash it thoroughly, then dry it in an electric blast drying oven at 60-80°C for 24 hours to obtain a modified polymer. The obtained modified polymer was subjected to nuclear magnetic resonance testing, and the results are as follows. Figure 4 shown.

[0097] Preparation of modified polymer membrane: Take 0.5g of the above-mentioned modified polymer material, add 10mL of dimethyl sulfoxide, and fully dissolve it to obtain a uniform and transparent membrane liquid. Apply the membrane liquid on a clean glass plate, dry it at 80°C for 12h, and fully immerse the glass plate in deionized water. The membrane swells and falls off on the glass plate to obtain a modified polymer membrane.

[0098] Example 5

[0099] Preparation of modified polymer: Take 3g of the base polymer of Example 3 above and place it in a round-bottom flask, add 30mL of dimethyl sulfoxide, and fully dissolve it at 50°C. Then add 0.31g of sodium tert-butoxide and fully dissolve it at 80°C. After complete dissolution, weigh 0.59g of 1-bromo-2-(2-methoxyethoxy)ethane and add it dropwise to the flask. After 24 hours, precipitate the product in deionized water and wash it thoroughly, then dry it in an electric blast drying oven at 60-80°C for 24 hours to obtain a modified polymer. The obtained modified polymer was subjected to nuclear magnetic resonance testing, and the results are as follows. Figure 4 shown.

[0100] Preparation of modified polymer membrane: same as Example 4.

[0101] Example 6

[0102] Preparation of modified polymer: Take 3g of the base polymer of Example 3 above and place it in a round-bottom flask, add 30mL of dimethyl sulfoxide, and fully dissolve it at 50°C. Then add 0.52g of sodium tert-butoxide and fully dissolve it at 80°C. After complete dissolution, weigh 0.98g of 1-bromo-2-(2-methoxyethoxy)ethane and add it dropwise to the flask. After 24 hours, precipitate the product in deionized water and wash it thoroughly, then dry it in an electric blast drying oven at 60-80°C for 24 hours to obtain a modified polymer. The obtained modified polymer was subjected to nuclear magnetic resonance testing, and the results are as follows. Figure 4 shown.

[0103] Preparation of modified polymer membrane: same as Example 4.

[0104] Example 7

[0105] Preparation of modified polymer: Take 2g of the base polymer of Example 3 above and place it in a round-bottom flask, add 20mL of dimethyl sulfoxide, and fully dissolve it at 50°C. Then add 0.04g of sodium hydride and fully dissolve it at 80°C. After complete dissolution, weigh 0.54g of perfluorooctyl bromide and add it dropwise to the flask. After 24 hours, precipitate the product in deionized water and wash it thoroughly, then dry it in an electric blast drying oven at 60-80°C for 24 hours to obtain a modified polymer. The obtained modified polymer was subjected to nuclear magnetic resonance testing, and the results are as follows Figure 4 shown.

[0106] Preparation of modified polymer membrane: same as Example 4.

[0107] Example 8

[0108] Preparation of modified polymer: Take 2g of the base polymer of Example 3 above and place it in a round-bottom flask, add 20mL of dimethyl sulfoxide, and fully dissolve it at 50°C. Then add 0.04g of sodium hydride and fully dissolve it at 80°C. After complete dissolution, weigh 0.21g of 1-bromooctane and add it dropwise to the flask. After 24 hours, precipitate the product in deionized water and wash it thoroughly, then dry it in an electric blast drying oven at 60-80°C for 24 hours to obtain a modified polymer. The obtained modified polymer was subjected to nuclear magnetic resonance testing, and the results are as follows: Figure 4 shown. Figure 5 Comparison of the XPS spectra of the modified polymer obtained in this example with the base polymer obtained in Example 3 shows successful grafting of the side chains.

[0109] Preparation of modified polymer membrane: same as Example 4.

[0110] Comparative Example 1

[0111] A commercial cation selective separation membrane CIMS was selected as a comparison.

[0112] The selectivity of the separation membranes obtained in Examples 3 to 8 and Comparative Example 1 in the KCl-MgCl2 system is shown in Table 1, the selectivity in the NaCl-MgCl2 system is shown in Table 2, the selectivity in the HCl-FeCl2 system is shown in Table 3, and the selectivity in the LiCl-MgCl2 system is shown in Table 4.

[0113] Table 1

[0114]

[0115] Table 2

[0116]

[0117]

[0118] Table 3

[0119]

[0120] Table 4

[0121]

[0122] * In Tables 1 to 4, the unit of J is mol m -2 h -1 , P has no unit and is rounded to one decimal place.

[0123] As shown in Tables 1-3, from Examples 3-6, the preferred ratio of added ether chains is approximately 20%. Under conditions of 20% ether chain modification, the separation selectivity for potassium and magnesium increased by 13 times compared to the pre-modification state, nearly double that of commercial membranes; the separation selectivity for sodium and magnesium increased by 11 times compared to the pre-modification state, more than three times that of commercial membranes; and the separation selectivity for hydrogen and iron increased by 7 times compared to the pre-modification state, more than 21 times that of commercial membranes. Meanwhile, the flux of monovalent ions remained stable. These results demonstrate that the introduction of a certain amount of ether chains can hinder the transmission of divalent ions while ensuring the transmission of monovalent ions, thereby improving the separation selectivity of the membrane.

[0124] In summary, if Figure 6 As shown, the present invention introduces ion exchange sites by introducing sulfonic acid groups. The introduction of crown ethers leverages host-guest interactions, lowering the energy barrier for alkali metal ion distribution into the membrane. The positively charged complexes formed by the crown ethers and ions generate electrostatic repulsion against high-valent ions, hindering their transport and thus enhancing separation selectivity. The weak binding of the introduced ether chains to monovalent ions shortens the ion hopping sites and promotes monovalent ion transport, while the strong binding to high-valent ions increases the barrier to high-valent ion transport, enhancing the membrane's monovalent / divalent ion selectivity. During hydrogen ion transport, the oxygen-rich polymer can form hydrogen bonds with water, creating a hydrogen bond network that further facilitates hydrogen ion transport. At the same time, it can be seen from Examples 3, 7, and 8 in Tables 1 to 3 that the introduction of alkyl chains and fluorinated side chains significantly improves the potassium-magnesium, sodium-magnesium, and hydrogen-iron selectivity of the polymer membrane, illustrating the effectiveness of hydrophobic modification of the polymer. From the comparison of the lithium-magnesium separation performance of the base polymer of Example 3 and the fluorinated side chain-modified polymer of Example 7 in Table 4, it can be seen that the introduction of fluorinated side chains enhances the lithium-magnesium separation performance of the polymer membrane, illustrating that the introduction of fluorine that has a recognition effect on lithium ions can effectively reduce the transmembrane energy barrier of lithium ions, thereby improving the selectivity.

[0125] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for preparing a monovalent / divalent selective cation exchange membrane based on crown ethers, characterized in that: The steps include: Step 1: dissolving a sulfonic acid group-substituted aromatic monomer, a dibenzocrown ether, and a carbonyl compound containing a nitrogen heterocycle in a solvent, adding a first catalyst after complete dissolution, and performing ternary copolymerization to obtain a base polymer; Step 2: dissolving the base polymer in dimethyl sulfoxide, adding the side chain monomer and the second catalyst under heating conditions to modify the base polymer; Step 3: Pour the product obtained in step 2 into water for purification, and obtain a modified polymer material after washing and drying; Step 4: After mixing the modified polymer material and the polar organic solvent, the mixture is coated and dried to obtain a monovalent / divalent selective cation membrane.

2. The preparation method according to claim 1, wherein: In step 1, the structural formula of the sulfonic acid group-substituted aromatic monomer is shown in formula (1): In the formula, Aq is a disubstituted phenyl group, a naphthyl group or an anthracenyl group, x and y are any integers selected from 1 to 5, and x=y.

3. The preparation method according to claim 2, wherein: The structural formula of the sulfonic acid group-substituted aromatic monomer is shown in Formula (1-1) or Formula (1-2):

4. The preparation method according to claim 1, wherein: The structural formula of the dibenzocrown ether is shown in formula (2): In formula (2), q and z are each independently selected from any integer from 1 to 4, and q=z or q=z+1.

5. The preparation method according to claim 4, characterized in that: The structural formula of the dibenzocrown ether is shown in any one of Formula (2-1) to Formula (2-6):

6. The preparation method according to claim 1, wherein: The structural formula of the carbonyl compound containing a nitrogen heterocycle is shown in any one of Formula (3-1) to Formula (3-4):

7. The preparation method according to claim 1, wherein: The structural formula of the side chain monomer is shown in any one of formulas (4) to (6): In the formula, t is any integer from 1 to 5, s is any integer from 1 to 6, and k is any integer from 1 to 10.

8. The preparation method according to claim 7, characterized in that: The structural formula of the side chain monomer is shown in any one of the following formulas (4-1) to (6-4):

9. The preparation method according to claim 1, wherein: In step 1, the solvent is dichloromethane, the first catalyst includes trifluoroacetic acid and trifluoromethanesulfonic acid, the reaction temperature of the ternary copolymerization is 0° C. to 19° C., and the polymerization time is 1 h to 6 h.

10. The preparation method according to claim 1, characterized in that: In step 2, the heating condition is 60-80° C., the second catalyst is sodium tert-butoxide or sodium hydride, the reaction time is 12 h to 24 h, and the reaction grafting site is the -NH site.

11. The preparation method according to claim 1, characterized in that: In step 4, the polar organic solvent is dimethyl sulfoxide, the drying temperature is 60° C. to 80° C., and the drying time is 8 to 12 hours.

12. A monovalent / divalent selective cation membrane based on crown ethers prepared by the preparation method according to any one of claims 1 to 11.

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