Preparation method of fluorinated modified cation selective separation membrane

Abstract

The invention discloses a preparation method of a fluorinated modified cation selective separation membrane, which comprises the following steps: taking an intrinsic microporous polymer as a precursor for preparing a membrane material, and realizing an ion exchange function of the membrane material by grafting a sulfonated monomer; on the basis, specific recognition of monovalent cations is further realized through grafting of perfluorinated micromolecular monomers, and ion selectivity is constructed. The industrial production difficulty is remarkably reduced by a membrane synthesis route of a solution grafting reaction, the selectivity of the prepared membrane material is superior to that of a current commercial cation selective separation membrane, and large-scale production and practical application are expected to be realized.

Description

Technical Field

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

[0002] Chemical separation technology plays an important role in the fields of seawater desalination, lithium extraction from salt lakes, and high-salt wastewater treatment. Among them, the application demand for precise separation of ions is urgently needed, especially the separation of ions with the same charge but different valence states, such as Li + / Mg 2+ 、Na + / Ca 2+ 、Na + / Mg 2+ Plasma separation. Current traditional methods mainly focus on technical processes such as nanofiltration, solvent extraction, and chromatography. However, due to the technical limitations of these processes themselves, these processes have problems such as high investment costs, low recovery rates, and high energy consumption. In contrast, electrodialysis technology with ion-selective separation membranes as the core is constantly developing and maturing. Electrodialysis technology has the advantages of high separation efficiency, low equipment cost, and small footprint, and has great application demand in many chemical separation fields. With the continuous development of various industries, higher requirements are also placed on the performance of membrane materials, including cation-selective separation membranes. However, current commercial membranes still have problems such as low selectivity, poor membrane stability, and easy shedding. Therefore, further research and development of cation-selective separation membranes is needed to improve performance to meet application needs, optimize preparation routes to achieve industrial production, and control membrane production costs to achieve cost reduction and efficiency improvement.

[0003] Since ions of the same valence but different charges have similar intrinsic properties, their efficient separation has always been a problem. Based on relevant information, it can be seen that the current research on cation selective separation membranes mainly focuses on: 1) achieving selectivity of ions of different ion sizes by increasing density and using pore size screening. For example, CN112546872B effectively increases the density of the membrane by polymerizing a layer of pyrrole-2-carboxylic acid polymer on the surface of the cation exchange base membrane. At the same time, due to the introduction of ion exchange groups, the ionic conductivity of the membrane itself is guaranteed, thereby achieving cation selectivity. 2) Construct selectivity through electrostatic repulsion. For example, CN114377731B uses Fe 3+Co-deposited with polyethyleneimine on the membrane surface, a cation-selective separation membrane is prepared. However, surface modification methods often increase membrane resistance, leading to increased energy consumption during actual application. At the same time, the problem of surface layer shedding also significantly reduces the stability of the membrane during operation. 3) Some researchers have also constructed selectivity through the specific adsorption of ions and groups within the membrane. For example, CN1179919964A, by blending, crown ether nanomaterials are incorporated into the membrane liquid to form a mixed matrix membrane. By utilizing the specific interaction between crown ethers and monovalent ions, the membrane is prepared by a casting method to achieve monovalent / divalent ion selectivity. However, although this method of preparing a mixed matrix membrane by blending avoids the introduction of a surface modification layer, it increases the complexity of the process and is not compatible with the mature production process of current membrane material production lines, making it difficult to achieve large-scale preparation. Therefore, there is still a need to develop a new cation-selective separation membrane with simple process, high membrane material stability and ion selectivity, and potential for large-scale preparation to meet the needs of industrial development. Summary of the Invention

[0004] In order to overcome the shortcomings of the above-mentioned prior art, the present invention is based on the excellent directional self-assembly ability of the microporous polymer, which is directional assembled to form relatively ordered microporous ion channels during the drying and film-forming process after the polymer is dissolved, and uses the secondary interaction between molecules to regulate the discreteness of the micropore size. At the same time, the fluorinated monomer is grafted into the polymer through the halogen site, thereby introducing fluorine bonds that have specific interactions with monovalent cations, reducing the transmission resistance of the target monovalent ion, and constructing mono / divalent ion selectivity, providing a preparation method for a cation-selective separation membrane that is homogeneous and stable, simple in preparation method, compatible with the current membrane production line production process, high ion flux and excellent ion selectivity.

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

[0006] The method for preparing the fluorinated modified cation selective separation membrane of the present invention comprises the following steps:

[0007] Step 1: using a PN polymer having a structural formula as shown in formula (1) as a precursor for preparing a cation selective separation membrane;

[0008]

[0009] Step 2: reacting and grafting the PN polymer with the sulfonated monomer to prepare a polymer SPN with ion exchange groups;

[0010] Step 3: The polymer SPN with cation exchange groups is reacted and grafted with a fluorinated small molecule monomer to prepare the SPNF polymer.

[0011] Step 4: dissolving the SPNF polymer in an organic solvent to prepare a membrane solution, pouring the membrane solution into a mold and drying it to obtain a fluorinated modified cation selective separation membrane.

[0012] Furthermore, in step 1, the molecular weight range of the PN polymer is m in the range of 10 2 -10 7 between.

[0013] Furthermore, in step 1, R is derived from a bisphenol monomer, and its specific structural formula is shown in any one of formula (2), where * represents the connection position:

[0014]

[0015] Furthermore, the PN polymer used in the present invention is prepared by reacting a bisphenol monomer (such as 1,1-bi(2-naphthol), 2,2'-dihydroxybiphenyl, 3,3,3,3-tetramethyl-1,1-spiro BI(indane)-6,6-diol) with a ketone monomer (such as isatin). The specific reaction method can be referred to the literature "One-step synthesis of hydroxyl-functionalized fully carbon main chain PIMs via a Friedel-Crafts reaction for efficient gasseparation" (Separation and Purification Technology 262(2021)118313).

[0016] Further, in step 2, the sulfonated monomer is Wherein, x is any integer from 1 to 5.

[0017] Furthermore, in step 2, the reactive grafting site on the PN polymer is a -OH site or a -NH- site, the molar ratio of the total number of reactive sites in the PN polymer (i.e., the total molar amount of -OH and -NH-) to the sulfonated monomer is 1:0.1-1, the reaction temperature is 50°C-120°C, the reaction time is 12 hours-48 hours, and a hydrogenation reagent (such as NaH, potassium carbonate, sodium 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 in the PN polymer is 0.1-1:1.

[0018] Furthermore, in step 3, the fluorinated small molecule monomer is Wherein, y is any integer between 0 and 10.

[0019] Furthermore, in step 3, the sites for reactive grafting on the SPN polymer are -OH sites or -NH- sites, the molar ratio of the total number of SPN reactive sites (i.e., the total molar amount of -OH and -NH-) to the fluorinated small molecule monomer is 1:0.1-1, the reaction grafting temperature range is 50°C-120°C, and the reaction time is 12h-48h. A hydrogenation reagent (such as NaH, potassium carbonate, sodium tert-butoxide, etc.) is added during the reaction, and the molar ratio of the amount of hydrogenation reagent added to the total number of reactive sites in the SPN polymer is 0.1-1:1.

[0020] 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 120°C.

[0021] The present invention provides a method for preparing a fluorinated, cation-selective separation membrane that matches existing homogeneous membrane casting processes and avoids the introduction of a surface modification layer, significantly improving the stability of the membrane material. Furthermore, by adjusting the pore size of the inherent micropores, the method further reduces the target ion transport resistance, improves ion flux and limiting current density, and exhibits excellent ion selectivity. Its beneficial effects are specifically reflected in the following aspects:

[0022] (1) Based on the inherent microporous properties of PN polymer, sulfonated monomers and fluorinated small molecule monomers are confined to grow in the intrinsic micropores of the polymer. The confined pores of the inherent microporous polymer are used to construct selective ion transport channels, avoiding the uncontrollable ion transport channel problem caused by the random distribution of ion exchange sites in traditional random polymers. At the same time, the pore environment is regulated based on the grafted fluorinated small molecule monomer to form ion transport differences. Specifically, the grafted sulfonated monomer first forms ion transport sites in the micropores to realize the ion exchange function, ensuring the ion transport capacity of the fluorinated modified cation selective separation membrane and the flux of the target ions. Secondly, the specific interaction of fluorine in the side chain of the fluorinated small molecule with monovalent ions is used to differentiate the transport of monovalent ions and divalent ions, ultimately realizing the preparation of a high-performance cation selective separation membrane.

[0023] (2) Based on the grafting of fluorinated small molecule monomer side chains, the fluorine sites are used to bind Li +The specific interaction of monovalent ions significantly reduces the energy barrier for monovalent ions to enter the membrane from the solution, making it easier for monovalent ions to enter the membrane and be transported across the membrane. At the same time, due to the high electronegativity of fluorine atoms, they usually show a certain degree of negative charge. The negatively charged F site can further form secondary interactions with ions, increase the energy compensation effect of dehydration after the ions enter the membrane, reduce the potential energy change of ions during transport within the membrane, make the transport of monovalent ions within the membrane more stable, and increase their diffusion rate within the membrane. Ultimately, monovalent ions not only have a lower energy barrier for distribution into the membrane than divalent ions, but also have a lower energy barrier for diffusion within the membrane, forming a significant difference in ion transport and achieving high monovalent / divalent ion selectivity.

[0024] (3) The rigid main chain PN is used as the base polymer of the membrane material. The rigidity of its own structure also ensures excellent swelling resistance after the membrane is formed, which significantly increases the stability of the membrane material. At the same time, the π-π interaction and abundant hydrogen bonding sites brought about by the structural characteristics of the PN polymer itself promote multiple weak interactions such as hydrogen bond self-assembly, promote the formation of regular channels in the membrane, avoid the random discrete pore size distribution, and the weak interactions in the membrane can further improve the stability of the molecular structure, avoid the collapse or aging of the microporous channels, and effectively construct the ion transport channel in the membrane. The abundant -OH sites and -NH- sites in the PN structure itself facilitate the modification of the base polymer and lay the foundation for the precise control of the performance of the membrane material. The sulfonic acid monomer is used to modify and graft the PN functional sites, and the ion exchange sites are increased on the basis of ensuring that its main chain remains unchanged.

[0025] (4) By grafting fluorinated small molecule monomers onto the main chain to construct ion selectivity, the problems of reduced stability and increased membrane resistance caused by surface modification and other methods are avoided, the energy consumption of membrane materials in the application process of membrane group devices is reduced, and the service life of membrane materials is increased. Based on the interaction of fluorinated small molecule monomers with cations, the small molecule monomers are introduced by grafting to make them part of the polymer structure, and finally a completely homogeneous membrane material can be prepared. The method of preparing the membrane liquid and then coating the membrane matches the current roll-to-roll production process of industrial membrane materials, is easy to realize industrial production, and is expected to achieve large-scale application. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a photograph of the cation selective separation membrane prepared in Example 1 of the present invention.

[0027] Figure 2 The SPNF polymer prepared in Example 1 of the present invention is 19 F NMR characterization results.

[0028] Figure 3The SPNF polymer prepared in Example 1 of the present invention is 1 H NMR characterization results.

[0029] Figure 4 2 is a diagram of the ion selectivity testing device used in the present invention.

[0030] Figure 5 The results of the selectivity tests of the SPN and SPNF1 ion selective separation membranes prepared in Example 1 of the present invention and the commercial membranes CIMS and CXP-S are compared.

[0031] Figure 6 These are the selectivity test results of the ion selective separation membranes prepared in Examples 2 to 6 of the present invention.

[0032] Figure 7 It is a schematic diagram of the present invention for constructing an ion-selective separation membrane based on the regulation of ion channels by self-microporous polymers. DETAILED DESCRIPTION

[0033] The technical solutions of the present invention are further illustrated by examples below. These technical solutions are only intended to make the present invention easier to understand by researchers in the field, and are not intended to limit the scope of protection of the present invention. All variations and extensions made using the present invention are within the scope of protection of the present invention.

[0034] 1. Preparation of cation selective separation membrane

[0035] Example 1

[0036] The steps for preparing the fluorinated modified cation selective separation membrane of this embodiment are as follows:

[0037] 1. Preparation of PN polymer:

[0038] Measure 28.6g of 1,1-bi(2-naphthol) in a round-bottom three-necked flask, then add 150mL of dichloromethane, place the three-necked flask in an ice bath circulation system, insert a stirring paddle, and start stirring until all the solid drugs are dissolved. Add 18.6g of indigo red, then add 15mL of trifluoroacetic acid dropwise, and continue stirring until all the indigo red is dissolved to form a light red uniform solution. Set the condensation circulation temperature to -10°C. After the temperature stabilizes, slowly add 150mL of trifluoromethanesulfonic acid dropwise to the three-necked flask. After reacting for 48 hours, the viscosity of the system rises sharply. Slowly pour the reactant into pure water to obtain a crude PN product. After drying this product, dissolve it in an organic solvent, DMSO, and pour the dissolved solution into pure water again to precipitate. Repeat this operation three times, and then dry the precipitated polymer to obtain the final PN polymer: where m is 10 2 -10 7 between.

[0039] 2. Preparation of SPN polymer:

[0040] Weigh 2.5g of PN polymer into a single-necked round-bottom flask, place the round-bottom flask in an oil bath, add a magnet, and add the organic solvent DMF to dissolve with stirring. After dissolution is complete, add 0.9g of propane sultone and 0.39g of NaH as a hydrogen extraction agent. Raise the oil bath temperature to 70°C and react for 12 hours. Then, precipitate the reactant and dry it to obtain a crude SPN product. Dissolve the crude SPN product in the organic solvent DMF and precipitate it again. Repeat this process three times to obtain a high-purity SPN polymer: Where y=0.5.

[0041] 3. Preparation of SPNF polymer:

[0042] Weigh 5g of SPN polymer into a single-necked round-bottom flask, place the round-bottom flask in an oil bath, add a magnet, and add the organic solvent DMSO to dissolve with stirring. After dissolution is complete, add 3g of perfluorooctanoyl chloride and 0.91g of NaH as a hydrogen extraction agent. Raise the oil bath temperature to 60°C and react for 12 hours. Then, precipitate the reactant and dry it to obtain a crude SPNF product. Dissolve the crude SPNF product in the organic solvent DMSO and precipitate it again. Repeat this process three times to obtain a high-purity SPNF polymer:

[0043] Where p = 0.5 and q = 0.3.

[0044] 4. Preparation of membrane:

[0045] Weigh 1 g of the SPN polymer prepared in Step 2 into a 20 mL round-bottom flask. Add 9 g of the organic solvent, DMF, to the flask. Stir on a magnetic stirrer until dissolved to obtain a membrane solution. Cast the membrane solution evenly onto a glass plate and dry at 90°C for 24 hours to obtain an SPN membrane, which serves as a control for side chain modification.

[0046] Weigh 1 g of the SPNF polymer prepared in Step 3 into a 20 mL round-bottom flask. Add 9 g of the organic solvent DM to the flask, add a magnetic stirrer, and stir on a magnetic stirrer until dissolved to obtain a membrane solution. This membrane solution was evenly cast onto a glass plate and dried at 90°C for 24 hours to form a membrane. This resulted in a fluorinated, cation-selective membrane, designated SPNF-1.

[0047] The SPNF-1 membrane material prepared in this example is as follows Figure 1 As shown, the SPNF polymer 19 The results of F NMR analysis are as follows Figure 2 As shown,1 The results of H NMR analysis are as follows Figure 3 The F-NMR spectrum results verified the successful grafting of the fluorinated monomer, and the H-NMR spectrum results verified the successful synthesis of the SPNF polymer.

[0048] Example 2

[0049] The steps for preparing the fluorinated modified cation selective separation membrane of this embodiment are as follows:

[0050] Preparation of PN polymer: The same preparation method as in Example 1 was used.

[0051] Preparation of SPN polymer: The same preparation method as in Example 1 was used.

[0052] Preparation of SPNF polymer: The same preparation method as in Example 1 was used, except that perfluorooctanoyl chloride was replaced with perfluorohexanoyl chloride, and the added amount was adjusted to 2.2 g.

[0053] Preparation of membrane: The same preparation method as in Example 1 was adopted, and the prepared membrane material was named SPNF-2.

[0054] Example 3

[0055] The steps for preparing the fluorinated modified cation selective separation membrane of this embodiment are as follows:

[0056] Preparation of PN polymer: The same preparation method as in Example 1 was used.

[0057] Preparation of SPN polymer: The same preparation method as in Example 1 was used.

[0058] Preparation of SPNF polymer: The same preparation method as in Example 1 was used, except that perfluorooctanoyl chloride was replaced with perfluorobutyryl chloride, and the added amount was adjusted to 3.5 g.

[0059] Preparation of membrane: The same preparation method as in Example 1 was adopted, and the prepared membrane material was named SPNF-3.

[0060] Example 4

[0061] The steps for preparing the fluorinated modified cation selective separation membrane of this embodiment are as follows:

[0062] Preparation of PN polymer: The same preparation method as in Example 1 was used.

[0063] Preparation of SPN polymer: The same preparation method as in Example 1 was used.

[0064] Preparation of SPNF polymer: The same preparation method as in Example 1 was used, except that perfluorooctanoyl chloride was replaced with perfluorooctane bromide, and the added amount was adjusted to 2.7 g.

[0065] Preparation of membrane: The same preparation method as in Example 1 was used, and the prepared membrane material was named SPNF-4.

[0066] Example 5

[0067] The steps for preparing the fluorinated modified cation selective separation membrane of this embodiment are as follows:

[0068] Preparation of PN polymer: The same preparation method as in Example 1 was used.

[0069] Preparation of SPN polymer: The same preparation method as in Example 1 was used.

[0070] Preparation of SPNF polymer: The same preparation method as in Example 1 was used, except that perfluorooctanoyl chloride was replaced with perfluorohexyl bromide, and the added amount was adjusted to 2.3 g.

[0071] Preparation of membrane: The same preparation method as in Example 1 was used, and the prepared membrane material was named SPNF-5.

[0072] Example 6

[0073] The steps for preparing the fluorinated modified cation selective separation membrane of this embodiment are as follows:

[0074] Preparation of PN polymer: The same preparation method as in Example 1 was used.

[0075] Preparation of SPN polymer: The same preparation method as in Example 1 was used.

[0076] Preparation of SPNF polymer: The same preparation method as in Example 1 was used, except that perfluorooctanoyl chloride was replaced with perfluoropentane bromide, and the added amount was adjusted to 2.1 g.

[0077] Preparation of membrane: The same preparation method as in Example 1 was used, and the prepared membrane material was named SPNF-6.

[0078] 2. Membrane flux and ion selectivity test

[0079] Use Figure 4 The device shown is at 10mAcm -2 The Li ion selective separation membranes obtained in the above embodiments were tested under the current density + / Mg 2+Ion selectivity was determined, and commercial monovalent / divalent cation selective separation membranes CIMS and CXP-S, as well as a membrane directly made from the SPN polymer synthesized in Example 1, were used as controls. The specific method was as follows:

[0080] The device is divided into four chambers by three membranes: (cathode) pole chamber | concentration chamber | desalination chamber | pole chamber (anode). The membranes between the pole chamber and the concentration chamber, and between the pole chamber and the desalination chamber are commercial common anion exchange membranes AGU, whose main purpose is to separate the pole chamber solution from the internal chamber solution. The membrane to be tested is located between the desalination chamber and the concentration chamber. A 0.3M sodium sulfate solution circulates in the pole chamber, a 0.01M LiCl solution circulates in the concentration chamber, and a mixture of 0.1M LiCl and 0.1M MgCl2 circulates in the desalination chamber. After the device circulates the corresponding solutions, voltage is applied on both sides. Under the action of the electric field, the cations in the desalination chamber migrate from the desalination chamber to the concentration chamber. Finally, the concentrations of different ions in the concentration chamber are measured, and the ion flux is calculated based on the concentration:

[0081]

[0082] 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 selectivity P is calculated by the following formula:

[0083]

[0084] in, is the initial lithium ion concentration in the desalination chamber, is the initial magnesium ion concentration in the desalination chamber.

[0085] Examples 1-3 illustrate the grafting of fluorinated small molecule monomers with acyl chloride groups as terminal groups, and the Li + / Mg 2+ Ion selectivity test results such as Figure 5 and Figure 6 As shown, the results show that the lithium-magnesium selectivity of the SPN-based membrane is 0.90, which cannot achieve effective separation of lithium and magnesium. After grafting fluorinated side chains, the membrane material produces a significant lithium-magnesium separation effect, verifying the effectiveness of the present invention in introducing fluorinated side chains to construct ion-selective separation membranes. At the same time, with the increase in the length of the fluorinated side chains, the ion selectivity and flux both increase to a certain extent, which is consistent with the original intention of the design of the present invention, indicating that the recognition effect of the F site on monovalent ions in the separation system increases with the increase of the F site. The interaction between the fluorinated site and the cation in this small molecule promotes the entry of cations from the solution into the membrane, while divalent ions enter the membrane relatively less due to the weaker interaction force.

[0086] Li of the cation selective separation membrane obtained in Example 4-6 + / Mg 2+ Ion selectivity test results such as Figure 6 As shown, the selectivity and flux of fluorinated small molecule monomers with bromine as the end group also increased with the length of the fluorinated alkyl chain, and the selectivity change trend showed the same change pattern as Examples 1-3, both of which further verified the effectiveness of the technical solution of the present invention. However, the overall selectivity of the membrane prepared with bromine as the end group was relatively improved. This is because the carbonyl group introduced after the acyl chloride grafting limits the ion-dipole interaction with the ions, which limits the ion migration ability.

[0087] The nuclear magnetic results of Example 1 are analyzed as follows: Figure 2 and Figure 3 As shown, the successful synthesis of the proposed target membrane material structure is verified, which illustrates the feasibility of the technical route proposed by the present invention. The prepared homogeneous membrane material avoids the problems of poor stability, many defects, low ion flux, etc. caused by traditional surface modified membranes and mixed matrix membranes, and verifies the advanced nature of the membrane preparation process of this technical route. It can be seen from the examples that the introduction of fluorinated side chains has a significant effect on cation selective separation membranes. The target ion flux and selectivity of the membrane materials prepared in each example are higher than those of the commercial high-performance cation selective separation membranes CIMS and CXP-S. Based on the reactive grafting of sulfonated monomers of the basic self-microporous PN polymer, the prepared SPN polymer itself has sulfonate groups, which makes the polymer have ion exchange function. On this basis, fluorinated side chain monomers are additionally grafted, and the specific interaction of the F site in the CF bond with ions promotes the process of monovalent ions being distributed into the membrane, reducing the difficulty of crossing. And because the PN itself has the confining effect of micropores, the ion exchange groups and fluorinated side chains grow inside the pores (such as Figure 7As shown), the combination of intrinsic micropores and fluorinated monomers further enhances the ion separation effect of each other, and the size of the micropores of the intrinsic micropores is further adjusted under the action of the fluorinated monomers. The monomer is regulated to grow in the pores in a certain direction and position under the confined conditions of the intrinsic micropores, thereby enhancing the interaction effect between it and the ions. This makes it easier for monovalent ions with smaller sizes to enter the membrane, which significantly promotes the transmission of monovalent ions, resulting in a significant increase in the flux of monovalent ions, while divalent ions need to overcome a larger energy barrier to enter, further forming a difference in transmembrane transmission of monovalent and divalent ions. The effectiveness of the selective construction of the fluorinated small molecule monomers introduced by the present invention for the cation selective separation membrane is illustrated. In contrast, the flux of divalent ions is much lower than that of commercial membranes CIMS and CXP-S, which ultimately increases the selectivity of the cation selective separation membrane provided by the present invention. In summary, the monovalent / divalent selective cation selective separation membrane preparation method provided by the present invention is simple in process and superior in performance, and is expected to be industrialized and prepared. It has great application potential in the fields of seawater desalination, salt lake lithium extraction, waste lithium recovery, rare earth separation, etc.

Claims

1. A method for preparing a fluorinated cation selective separation membrane, characterized in that The steps include: Step 1: using a PN polymer having a structural formula as shown in formula (1) as a precursor for preparing a cation selective separation membrane; Step 2: reacting and grafting the PN polymer with the sulfonated monomer to prepare a polymer SPN with ion exchange groups; Step 3: The polymer SPN with cation exchange groups is reacted and grafted with a fluorinated small molecule monomer to prepare an SPNF polymer; Step 4: dissolving the SPNF polymer in an organic solvent to prepare a membrane solution, pouring the membrane solution into a mold and drying it to obtain a fluorinated modified cation selective separation membrane.

2. The preparation method according to claim 1, wherein: In step 1, the molecular weight range of PN polymer is 10 2 -10 7 between.

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

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

5. The preparation method according to claim 1, wherein: In step 2, the reaction grafting site on the PN polymer is a -OH site or a -NH- site, the molar ratio of the total number of reaction sites of the PN polymer to the sulfonated monomer is 1:0.1-1, the reaction temperature is 50°C-120°C, the reaction time is 12h-48h, and a hydrogen extraction reagent is added during the reaction.

6. The preparation method according to claim 1, wherein: In step 3, the fluorinated small molecule monomer is Wherein, y is any integer between 0 and 10.

7. The preparation method according to claim 1, wherein: In step 3, the reaction grafting site on SPN is a -OH site or a -NH- site, the molar ratio of the total number of SPN reaction sites to the fluorinated small molecule monomer is 1:0.1~1, the reaction grafting temperature range is 50℃-120℃, the reaction time is 12h-48h, and a hydrogen extraction reagent is added during the reaction.

8. The preparation method according to claim 1, wherein: In step 4, the drying temperature is between 30°C and 120°C.

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

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