Preparation method of cation selective separation membrane for ion separation

Abstract

The invention discloses a preparation method of a cation selective separation membrane for ion separation, which comprises the following steps: selecting a PNA polymer with pi-pi interaction, hydrogen bond and microporous network structure as a precursor of a membrane material, and introducing a sulfonated side chain into a membrane matrix to construct an efficient ion exchange site; and by introducing an alkoxy chain to promote high-efficiency transmission of the monovalent cations, a dense network structure is formed through ultraviolet crosslinking after the homogeneous membrane liquid is coated and dried, and the transmission channel of the monovalent cations is further optimized. The efficient and convenient one-pot ternary polymerization technology is adopted, the synthesis steps of the membrane material are simplified, the production efficiency is improved, the uniformity of the product is ensured, the prepared separation membrane shows high ion flux and ion selectivity in electrodialysis application, and the separation membrane can be applied to electrodialysis. The method has huge application potential and market prospect in the industrial fields of salt lake lithium extraction, chlor-alkali industry, waste acid treatment and the like.

Description

Technical Field

[0001] The invention belongs to the field of membrane technology, and in particular relates to a method for preparing a cation selective separation membrane for ion separation. Background Art

[0002] Lithium, a key strategic resource, is widely used in a variety of fields, including batteries, ceramics, glass, lubricants, and medicine. It holds an irreplaceable position in new energy batteries, such as lithium-ion batteries. With the rapid development of electric vehicles and energy storage systems, global demand for lithium resources has increased dramatically, making the security of the lithium supply chain and its sustainable use a focus of global attention.

[0003] In nature, lithium usually coexists with magnesium in salt lake brine and ores. However, due to the very similar physical and chemical properties of magnesium ions and lithium ions, especially their hydrated ion radius and charge number are close, traditional separation methods (such as precipitation, solvent extraction, etc.) are difficult to separate lithium and magnesium efficiently. These traditional methods not only have low separation efficiency, but also often require a large amount of chemical reagents, resulting in high costs and environmental pollution. Therefore, finding an efficient, economical and environmentally friendly lithium-magnesium separation technology has become a hot topic in current research. Membrane separation technology, as an emerging separation method, has gradually attracted attention due to its advantages such as high selectivity, low energy consumption and environmental protection. By designing specific membrane materials and structures, highly selective separation of lithium ions can be achieved, thereby effectively solving the problems in lithium-magnesium separation.

[0004] Currently, research on lithium-magnesium separation using ion exchange membranes primarily focuses on methods such as modifying the surface of cation exchange membranes with positive charges. This process introduces positive charges on the membrane surface, increasing the electrostatic repulsion between the membrane surface and divalent magnesium ions, thereby improving the selectivity of monovalent lithium ions over divalent cations. However, this approach also has significant drawbacks: surface modification often increases membrane resistance, leading to increased energy consumption during practical applications. Furthermore, the problem of surface layer shedding significantly reduces membrane stability during operation. The main approaches to achieving selectivity include: 1) Improving density: Utilizing pore size screening to increase the density of the membrane itself or to construct a dense thin layer on the base membrane surface, selective separation of different ion sizes is achieved. 2) Precisely controlling the pore size of the separation membrane: By selecting different organic monomers to control the pore size and functionality, selective adsorption and separation of specific molecules or ions is achieved. Current research based on this strategy primarily focuses on materials such as COFs and MOFs, which can achieve highly selective separation of specific molecules or ions by controlling the pore size and functionality through the selection of different organic monomers. However, the synthesis of materials such as COFs and MOFs typically requires strict control of reaction conditions such as temperature, pressure, and solvent environment, resulting in complex and costly processes. These harsh synthesis conditions and high costs have made large-scale production of these materials challenging, limiting their widespread industrial application.

[0005] In addition, current cation-selective membranes also suffer from problems such as poor selectivity, low monovalent cation flux, and poor stability. Therefore, there is a need to develop a cation-selective membrane with better overall performance to meet practical application needs. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, the present invention uses a PNA polymer containing self-polymerized micropores, utilizes the π-π interaction force between its molecules and the in-situ cross-linking of anthracene monomers, and accurately regulates the self-assembly process of the polymer molecules, thereby constructing an orderly and efficient ion transport channel in the membrane. In addition, these ion channels can promote the transport of monovalent cations in the membrane after film formation. This method designs a homogeneous cation-selective separation membrane, and its preparation method is not only simple and fast, but also can be integrated into the current membrane production line process. The separation membrane has a high lithium ion flux and excellent ion selectivity.

[0007] To achieve the above objectives, the present invention adopts the following technical solutions:

[0008] A method for preparing a cation selective separation membrane for ion separation comprises the following steps:

[0009] Step 1: using a PNA polymer having a structural formula as shown in formula (1) as a precursor;

[0010]

[0011] Step 2: Sulfonation grafting of the PNA polymer to obtain an SPNA polymer with ion exchange groups;

[0012] Step 3: reacting and grafting the SPNA polymer with an alkoxy chain monomer to prepare an SPNAO polymer;

[0013] Step 4: Use an organic solvent to dissolve the SPNA polymer or SPNAO polymer to prepare a membrane liquid, pour the membrane liquid into a membrane mold and dry it, and then cross-link it under ultraviolet light to prepare a cation selective separation membrane.

[0014] Furthermore, in step 1, the molecular weight of the PNA polymer is in the range of 10 4 -10 7 between.

[0015] Furthermore, in formula (1), R is derived from an anthracene monomer, and its specific structural formula is shown in any one of formulas (2), where * represents the connection position:

[0016]

[0017] In formula (1), the structural formula of R' is shown in any one of formulas (3), where * is the connection position:

[0018]

[0019] Furthermore, in step 1, the PNA polymer is prepared by ternary copolymerization of a phenolic monomer (such as 2,2'-dihydroxybiphenyl, 1,1-bi(2-naphthol)), an isatin and an anthracene monomer (such as anthracene, 9,10-diphenylanthracene, 9,10-bis(phenylethynyl)anthracene, 2,6-dihydroxyanthracene) by a super acid catalysis method, wherein the super acid is trifluoroacetic acid and trifluoromethanesulfonic acid (the amount ratio of anthracene monomer to trifluoroacetic acid and trifluoromethanesulfonic acid is 1 mmol: 15-25 mL: 2-5 mL); the reaction temperature of the copolymerization is -15°C to 10°C, and the reaction is carried out until the viscosity of the system increases sharply.

[0020] Furthermore, in step 1, the ratio of m to n is 1:0.1-1, that is, the molar ratio of the phenol monomer to the anthracene monomer is 1:0.1-1, and indigo is in excess.

[0021] Further, in step 2, the sulfonated monomer is Wherein, x is any integer from 1 to 5.

[0022] Furthermore, in step 2, the reactive grafting site on the PNA polymer is an -OH site or an -NH- site, and the molar ratio of the total number of reactive sites of the PNA polymer to the sulfonated monomer is 1:0.1-1; the reaction temperature of the sulfonation grafting is 40°C-110°C, and the reaction time is 12h-48h; a hydrogenation reagent (such as NaH, potassium carbonate, potassium tert-butoxide, etc.) is added during the reaction. The molar ratio of the amount of the hydrogenation reagent added to the total number of reactive sites of the PNA polymer is 0.1-1:1.

[0023] Furthermore, in step 3: the reaction grafting site on the SPNA polymer is a -OH site or a -NH- site, and the molar ratio of the total number of reaction sites of the SPNA polymer to the alkoxy chain monomer (such as 1-bromo-2-(2-methoxyethoxy)ethane) is 1:0.1~1; the reaction temperature of the reaction grafting is 40°C-110°C, and the reaction time is 12h-48h; a hydrogenation reagent is added to the reaction, and the molar ratio of the amount of hydrogenation reagent added to the total number of reaction sites of the SPNA polymer is 0.1~1:1.

[0024] Furthermore, in step 4, the organic solvent includes one of common organic solvents such as tetrahydrofuran, dimethyl sulfoxide, N,N-dimethylformamide, N-methylpyrrolidone, etc., and the drying temperature is between 30°C and 110°C.

[0025] Furthermore, in step 4, cross-linking is performed under ultraviolet light with a wavelength of 365 nm for 15 to 30 minutes.

[0026] The present invention proposes a method for preparing a cation-selective separation membrane for ion separation. This method constructs efficient ion exchange sites by introducing sulfonated side chains into the membrane matrix, and promotes efficient transport of monovalent cations by introducing alkoxy chains. After the homogeneous membrane liquid is coated and dried, it is then cross-linked by ultraviolet light to form a dense network structure, further optimizing the monovalent cation transport channel. This method is fully compatible with the currently established homogeneous membrane casting production process, greatly simplifying the production process and improving reaction efficiency. This preparation technology avoids the use of the common surface modification layer, thereby significantly improving various membrane performance indicators. Its beneficial effects are specifically reflected in the following aspects:

[0027] (1) Utilizing the self-polymerizing microporous properties of PNA polymers, sulfonic acid monomers were introduced, which not only adjusted the molecular skeleton of PNA but also enabled the sulfonic acid monomers to be precisely positioned within the microporous network of the polymer. This strategy effectively circumvented the challenges posed by the disordered distribution of ion exchange sites in traditional random polymers and significantly enhanced the controllability and efficiency of ion transport channels. Using PNA as the core base polymer of the membrane material, its unique ether-free and heteroatom-free structure greatly reduces the risk of degradation under acidic and alkaline environmental conditions. In addition, the inherent rigidity of the PNA polymer itself gives the membrane swelling resistance, making ion transport more stable.

[0028] (2) Based on the π-π interaction of PNA itself, when the interaction force of this non-covalent bond is significantly enhanced, it promotes the formation of ordered ion channels in the membrane. Driven by the π-π interaction, they spontaneously aggregate and arrange to form aggregates with a certain size and shape. These aggregates are further assembled into ordered ion channel structures. Within the ordered polymer structure, the π-π interaction guides the formation of ion channels, making the channels more uniform and precise in size, shape and arrangement. This precisely constructed ion channel can more effectively control the transmission path of ions, reduce unnecessary diffusion and crossover, and thus improve ion selectivity.

[0029] (3) The oxygen atoms in the alkoxy chain can react with Li + , K + Plasmas produce certain interactions, which to a certain extent reduce the energy barrier that monovalent cations need to overcome during transmembrane transport, allowing monovalent cations to be transported in a smoother and more efficient manner. This remarkable feature not only significantly accelerates the transmission rate of monovalent cations, but also helps to reduce the number of collisions between monovalent cations and other particles in the transmission path to a certain extent, further ensuring the continuity and stability of monovalent cation transmission. More importantly, the alkoxy chain's ability to promote the transmission of monovalent cations is not limited to increasing the transmission speed and reducing collisions. More importantly, it can also form a transmission channel that is highly selective for monovalent cations through its unique molecular structure. This selective transmission channel can ensure that monovalent cations are minimally obstructed during transmission, while effectively blocking the passage of other types of ions, thereby significantly enhancing the membrane material's selective separation ability for ions.

[0030] (4) Using anthracene monomers as core photosensitive groups, under appropriate light conditions, the anthracene groups can undergo a [4+4] cycloaddition reaction, forming stable crosslinking points and a complex and orderly crosslinked chemical network within the material. The formation of this network structure not only enhances the mechanical strength and stability of the material, but more importantly, it can influence and optimize the pore structure of the membrane through crosslinking. Specifically, the distribution and density of crosslinking points can control the size and connectivity of the pores, allowing ions of appropriate sizes to pass through efficiently through pore size screening, thereby significantly improving the ion selectivity of the membrane.

[0031] (5) Based on an efficient liquid-phase reaction mechanism, a one-pot process from raw materials to final products is achieved. The prepared homogeneous membrane liquid can be easily converted into membrane materials through a simple coating and drying process. This process avoids the complex surface modification steps, not only greatly improving the overall stability of the membrane material, but also ensuring that the membrane resistance is maintained at a low level, effectively reducing the energy consumption of the membrane material when it is running in the membrane module or device. It is not only limited to simplifying the production process, but also takes into account the optimization of membrane performance. While ensuring the excellent performance of high flux and high selectivity, the slight increase in membrane resistance is almost negligible, shortening the production cycle of the membrane material and reducing the complexity of the production process. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is the HNMR test result of the PNA polymer prepared in Example 1 of the present invention;

[0033] Figure 2 1H NMR test results of the SPNA polymer prepared in Example 1 of the present invention;

[0034] Figure 3 1H NMR test results of the SPNAO polymer prepared in Example 1 of the present invention;

[0035] Figure 4 A diagram of an ion selectivity testing device used in an embodiment of the present invention;

[0036] Figure 5 Li is the separation membrane prepared based on SPNA polymer in Examples 1 to 4 of the present invention + / Mg 2+ Flux and selectivity test results;

[0037] Figure 6 Li is the separation membrane prepared based on SPNAO polymer in Examples 5-6 of the present invention + / Mg 2+ Flux and selectivity test results. DETAILED DESCRIPTION

[0038] The technical scheme of the present invention is further illustrated by the following examples. These examples are only used to help researchers in this field better understand the present invention and do not limit the scope of protection of the present invention. Any changes and improvements made using the present invention are within the scope of protection of the present invention.

[0039] 1. Preparation of cation selective separation membrane

[0040] Example 1

[0041] In this embodiment, a cation selective separation membrane for ion separation is prepared according to the following steps:

[0042] Preparation of PNA polymer: 5.15 g of 1,1-bi(2-naphthol) and 0.356 g of anthracene (equivalent ratio of 9:1) were weighed and placed in a dry 250 mL round-bottom flask. Subsequently, while cooling in an ice bath at -15°C, 35 mL of dichloromethane was slowly added to the flask. A magnetic stirrer was added and stirring was initiated until all solids were completely dissolved. Next, 3.6 g of isatin was added to the solution, and 35 mL of trifluoroacetic acid was added while stirring. Stirring was continued to ensure that all solids were evenly dissolved, resulting in a homogeneous reaction mixture. To control the reaction temperature and promote the reaction, the reaction system was placed in a condensing circulation system at -15°C. Under these conditions, 5 mL of trifluoromethanesulfonic acid was slowly and evenly added. After the addition was complete, the reaction was stirred for 24 hours to ensure full reaction progress. As the reaction proceeded, the viscosity of the system gradually increased. After the reaction was complete, the resulting reaction solution was slowly added dropwise to a large amount of pure water to precipitate the crude PNA product. After the solid is completely precipitated, the crude product is collected by filtration, dried, dissolved in DMSO, and then added dropwise to pure water to precipitate the solid to further purify the product. This step can be repeated two to three times to remove residual solvent and impurities and improve the purity of the PNA polymer. The H NMR test results of the obtained PNA polymer are shown in Figure 2. Figure 1 As shown, the structure is as follows: Where m:n=9:1.

[0043] Preparation of SPNA polymer: Weigh 2g of PNA polymer and place it in a 100mL round-bottom flask. Then, place the flask in an oil bath and add the organic solvent DMF. Add a magnetic stir bar to the flask and start stirring to fully dissolve the PNA polymer in the solvent until a uniform, transparent solution is formed. After confirming complete dissolution of the polymer, adjust the oil bath temperature to 60°C. Add 0.4g of propane sultone as a sulfonation reagent and 0.1g of NaH as a hydrogenation reagent to the flask, and continue stirring to ensure that the propane sultone is evenly dispersed in the reaction system. Maintain the reaction temperature at 60°C and continue stirring for 18 hours. After the reaction is complete, slowly pour the reaction solution into pre-prepared pure water to precipitate the crude SPNA product as a solid. After the solid has completely precipitated, collect the crude product by filtration, dry it, dissolve it in DMF, and then add it dropwise to pure water to precipitate the solid for further purification. This step can be repeated two to three times to remove residual solvent and impurities and improve the purity of the SPNA polymer. The H NMR test results of the obtained SPNA polymer are as follows: Figure 2 As shown, the structure is as follows:

[0044] Where x=0.5, y=0.1.

[0045] Preparation of membrane: Weigh 1g of SPNA polymer and place it in a 25mL round-bottom flask. Subsequently, add a magnetic stirrer and 9g of DMSO to the flask, turn on the stirring device to stir the mixture until the SPNA polymer is completely dissolved in the solvent to form a clear, transparent and uniform membrane liquid. The prepared membrane liquid is evenly and smoothly coated on a pre-cleaned and dried glass plate. Dry continuously at 80°C for 24 hours to ensure that the solvent in the membrane liquid is completely volatilized to promote the interaction between SPNA polymer molecules. The dried membrane is irradiated under 365nm ultraviolet light for 15 minutes to promote in-situ cross-linking of anthracene to form a stable and highly selective cationic separation membrane with a dense cross-linked network structure.

[0046] Example 2

[0047] In this embodiment, a cation selective separation membrane for ion separation is prepared according to the following steps:

[0048] Preparation of PNA polymer: The same preparation method as in Example 1 was used, except that the equivalent ratio of 1,1-bi(2-naphthol) to anthracene was adjusted to 19:1 by increasing the mass of 1,1-bi(2-naphthol).

[0049] Preparation of SPNA polymer: The same preparation method as in Example 1 was used.

[0050] Preparation of the membrane: The same preparation method as in Example 1 was adopted.

[0051] Example 3

[0052] In this embodiment, a cation selective separation membrane for ion separation is prepared according to the following steps:

[0053] Preparation of PNA polymer: The same preparation method as in Example 1 was used, except that anthracene was replaced with an equimolar amount of 9,10-diphenylanthracene. The structure of the resulting PNA polymer is shown below:

[0054] Where m:n=9:1.

[0055] Preparation of SPNA polymer: The same preparation method as in Example 1 was used.

[0056] Preparation of the membrane: The same preparation method as in Example 1 was adopted.

[0057] Example 4

[0058] In this embodiment, a cation selective separation membrane for ion separation is prepared according to the following steps:

[0059] Preparation of PNA polymer: The same preparation method as in Example 1 was used, except that anthracene was replaced by 9,10-diphenylanthracene, and the equivalent ratio of 1,1-bi(2-naphthol) to 9,10-diphenylanthracene was adjusted to 19:1.

[0060] Preparation of SPNA polymer: The same preparation method as in Example 1 was used.

[0061] Preparation of the membrane: The same preparation method as in Example 1 was adopted.

[0062] Example 5

[0063] In this embodiment, a cation selective separation membrane for lithium and magnesium separation is prepared according to the following steps:

[0064] Preparation of PNA polymer: The same preparation method as in Example 1 was used

[0065] Preparation of SPNA polymer: The same preparation method as in Example 1 was used.

[0066] Preparation of SPNAO polymer: Weigh 2g of SPNA polymer and place it in a 100mL round-bottom flask. Subsequently, the flask was placed in an oil bath and the organic solvent DMF was added. A magnetic stirrer was added to the flask, and the stirring device was turned on to fully dissolve the SPNA polymer in the solvent until a uniform and transparent solution was formed. After confirming that the polymer was completely dissolved, the temperature of the oil bath was adjusted to 60°C. At this time, 0.36g of 1-bromo-2-(2-methoxyethoxy)ethane was added to the flask, and 0.1g of NaH was added as a hydrogen extraction agent. The mixture was stirred continuously to uniformly disperse 1-bromo-2-(2-methoxyethoxy)ethane in the reaction system. The reaction temperature was maintained at 60°C and the reaction was stirred continuously for 18 hours. After the reaction was completed, the reaction solution was slowly poured into the pre-prepared pure water to precipitate the SPNAO crude product as a solid. After the solid is completely precipitated, the crude product is collected by filtration, dried, dissolved in DMF, and then added dropwise to pure water to precipitate the solid to further purify the product. This step can be repeated two to three times to remove residual solvent and impurities and improve the purity of the SPNAO polymer. The structure of the resulting SPNAO polymer is shown below:

[0067] Where x=0.4 and y=0.5.

[0068] Preparation of membrane: The same preparation method as in Example 1 was adopted, except that the SPNA polymer was replaced by the SPNAO polymer.

[0069] Example 6

[0070] In this embodiment, a cation selective separation membrane for lithium and magnesium separation is prepared according to the following steps:

[0071] Preparation of PNA polymer: The same preparation method as in Example 5 was used, except that anthracene was replaced with an equimolar amount of 9,10-diphenylanthracene.

[0072] Preparation of SPNA polymer: The same preparation method as in Example 5 was adopted.

[0073] Preparation of SPNAO polymer: The same preparation method as Example 5 was adopted.

[0074] Preparation of the membrane: The same preparation method as in Example 5 was adopted.

[0075] 2. Membrane flux and ion selectivity test

[0076] Use Figure 4 The device shown in the figure was used to test the Li ion exchange membranes obtained in Examples 1 to 6 at a current density of 5 mA cm-2. + / Mg 2+ Ion selectivity was measured using a commercial monovalent / divalent selective cation exchange membrane CIMS tested under the same conditions as a control. The specific method is:

[0077] This device uses three membranes (including two commercial ordinary anion exchange membranes AMX and one SPNA membrane to be tested) to divide the internal space into four independent chambers, from left to right:

[0078] Anode chamber: 0.3 mol L-1 sodium sulfate solution circulates in this chamber.

[0079] Desalination chamber: Adjacent to the anode chamber, separated by an AMX membrane. A mixed solution of 0.1 mol L-1 lithium chloride and 0.1 mol L-1 magnesium chloride circulates in this chamber.

[0080] Concentration chamber: Located between the desalination chamber and the cathode chamber, separated by an SPNA membrane. This chamber circulates a 0.01 mol L-1 lithium chloride solution to receive cations, especially lithium ions, migrating from the desalination chamber.

[0081] Cathode chamber: The chamber on the far right, which also circulates 0.3 mol L-1 sodium sulfate solution.

[0082] When voltage is applied across the device, the electric field causes the cations (lithium and magnesium ions) in the desalination chamber to begin migrating. Due to the ion selectivity of the SPNA membrane, lithium ions can pass through the membrane in large quantities into the concentration chamber, while relatively few magnesium ions pass through. This selective migration process causes the lithium ion concentration in the concentration chamber to gradually increase, while the magnesium ion concentration increases only slightly. Ultimately, the lithium and magnesium ion concentrations in the concentration chamber are measured, and the ion flux is calculated based on the concentrations:

[0083]

[0084] Where C t is the ion concentration after the concentration chamber has been running for t hours, C0 is the initial ion concentration of the concentration chamber, and A m is the effective area of the membrane in the test, t is the operating time, V is the final volume of the concentration chamber, and the final flux ratio of lithium ions to magnesium ions is the selectivity value.

[0085] The test results of the separation membranes prepared based on SPNA polymers in Examples 1 to 4 are as follows: Figure 5 As shown, it can be seen that the cation exchange membranes prepared in Examples 1 and 3 have a + / Mg 2+ The ion selectivity is better than that of the commercial membrane CIM. The improvement of this excellent performance is mainly attributed to the introduction of anthracene components in the membrane material and the increase in its proportion. With the increase in the proportion of anthracene in the polymer matrix, the π-π interaction between molecules is significantly enhanced. This non-covalent interaction force not only promotes the formation of ordered ion channels in the membrane, but also optimizes the channel structure, making the transmission of lithium ions more efficient, while the relatively large magnesium ions are effectively hindered due to the pore size screening effect. Although anthracene monomers are also added to the membrane materials in Examples 2 and 4, the enhancement effect of π-π interaction is not significant enough due to the relatively small amount added, resulting in the ion selectivity of SPNA failing to reach the level of commercial membrane CIM. This result further verifies the key role of π-π interaction in the construction and optimization of ion channels, and also highlights the importance of precisely controlling the composition of membrane materials to achieve high-performance ion selectivity.

[0086] The test results of the separation membranes prepared based on SPNAO polymer in Examples 5 and 6 are as follows: Figure 6 As shown, it can be seen that the cation exchange membranes prepared in Examples 5 and 6 have a + / Mg 2+ The ion selectivity is greatly improved. The alkoxy chain can promote the conduction of lithium ions and promote the membrane material to show a higher molecular structure order during the heating film formation process. This order makes the pore size distribution of the pores in the membrane more uniform and consistent, and effectively excludes larger ions (such as Mg) through the pore size screening mechanism. 2+), which is more conducive to the smaller size of lithium ion (Li + ) rapid transport within the membrane. This demonstrates the effectiveness and innovation of the technical approach of the present invention, which is based on the construction of ion channels in microporous polymer confinement and combines π-π interactions and in-situ cross-linking to promote the formation of ion channels.

[0087] In summary, the present invention proposes the use of a one-pot method to prepare a cation-selective separation membrane. This preparation method is not only simple to operate and highly efficient, but the separation membrane obtained is superior in performance to commercial products on the market. This technology greatly simplifies the production process, making large-scale, industrialized preparation easy, thereby meeting the growing market application needs and potential. More importantly, the separation membrane prepared by the present invention has huge application potential and market prospects in industrial fields such as salt lake lithium extraction, chlor-alkali industry, and waste acid recovery, and has shown great significance in the fields of environmental protection and resource recovery. It can promote the sustainable utilization of resources by efficiently separating key resources such as cations, reduce the negative impact on the natural environment, and make an important contribution to green and sustainable development.

Claims

1. A method for preparing a cation selective separation membrane for ion separation, characterized in that: The steps include: Step 1: using a PNA polymer having a structural formula as shown in formula (1) as a precursor; Step 2: Sulfonation grafting of the PNA polymer to obtain an SPNA polymer with ion exchange groups; Step 3: reacting and grafting the SPNA polymer with an alkoxy chain monomer to prepare an SPNAO polymer; Step 4: Use an organic solvent to dissolve the SPNA polymer or SPNAO polymer to prepare a membrane liquid, pour the membrane liquid into a membrane mold and dry it, and then cross-link it under ultraviolet light to prepare a cation selective separation membrane.

2. The preparation method according to claim 1, wherein: In step 1, the molecular weight of the PNA polymer ranges from 10 4 -10 7 between.

3. The preparation method according to claim 1, characterized in that In formula (1), the structural formula of R is shown in any one of formulas (2), where * represents the connection position: In formula (1), the structural formula of R' is shown in any one of formulas (3), where * is the connection position:

4. The preparation method according to claim 1 or 3, characterized in that The PNA polymer is prepared by ternary copolymerization of phenol, isatin and anthracene monomers through a super acid catalysis method, wherein the super acid is trifluoroacetic acid and trifluoromethanesulfonic acid, and the copolymerization reaction temperature is -15°C to 10°C.

5. The preparation method according to claim 1, wherein: In step 1, the ratio of m to n is 1:0.1~1.

6. The preparation method according to claim 1, wherein: In step 2, the sulfonated monomer is Wherein, x is any integer from 1 to 5.

7. The preparation method according to claim 1, characterized in that In step 2: the reactive grafting site on the PNA polymer is a -OH site or a -NH- site, and the molar ratio of the total number of reactive sites of the PNA polymer to the sulfonated monomer is 1:0.1 to 1; the reaction temperature of the sulfonation grafting is 40°C-110°C, and the reaction time is 12h-48h; a hydrogenation reagent is added during the reaction, and the molar ratio of the amount of the hydrogenation reagent added to the total number of reactive sites of the PNA polymer is 0.1 to 1:

1.

8. The preparation method according to claim 1, characterized in that In step 3: the reaction grafting site on the SPNA polymer is a -OH site or a -NH- site, and the molar ratio of the total number of reaction sites of the SPNA polymer to the alkoxy chain monomer is 1:0.1~1; the reaction temperature of the reaction grafting is 40°C-110°C, and the reaction time is 12h-48h; a hydrogenation reagent is added during the reaction, and the molar ratio of the amount of hydrogenation reagent added to the total number of reaction sites of the SPNA polymer is 0.1~1:

1.

9. A cation selective separation membrane for ion separation prepared by the preparation method according to any one of claims 1 to 8.

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