Preparation method of side chain type cation selective separation membrane

Abstract

The invention discloses a preparation method of a side chain type cation selective separation membrane, which is based on the ultra-micropore characteristic of an intrinsic microporous polymer, and respectively realizes the ion exchange function and selective construction of the polymer through a side chain grafted sulfonated monomer and a long alkyl side chain small molecular monomer. Based on the hydrophobic property of a long alkyl chain, the transmembrane transmission difference of different ions is realized, the dehydration process of ion transmembrane transmission is regulated and controlled, and monovalent / divalent cation selectivity is constructed. Accurate construction of a selective ion channel is guaranteed through the confinement effect of grafted side chain micromolecules in an ultramicropore channel, meanwhile, the preparation route of the grafting reaction can be matched with an industrial reel-to-reel production route, and industrial application is expected to be achieved.

Description

Technical Field

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

[0002] Chemical separation technology plays an important role in the fields of seawater resource utilization, lithium extraction from salt lakes, and high-salt wastewater treatment. Among them, the application demand for accurate 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 system 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 a wide range of application needs 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, the current commercial cation-selective separation 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 the intrinsic properties of ions with the same valence but different charges are not significantly different, and the size difference of different ions is only at the sub-nanometer level, their precise 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) Using pore size screening to achieve selectivity for ions of different sizes by increasing density. Current research based on this strategy mainly constructs ion selectivity by increasing the density of the membrane itself or constructing a dense thin layer on the surface of the base membrane, such as CN112546872B, which 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 polyimide on the membrane surface, a monovalent selective cation selective separation membrane is prepared. However, the surface modification method often increases the membrane resistance, resulting in increased energy consumption during actual application. At the same time, the problem of shedding of the surface layer 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, crown ether nanomaterials are incorporated into the membrane liquid by blending to form a mixed matrix membrane. By utilizing the specific effect of crown ethers and monovalent ions, the membrane is prepared by casting 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 the current membrane material production line, making it difficult to achieve large-scale preparation. In addition, current cation selective separation membranes still have problems such as poor selectivity, low limiting current density, and poor stability. Further research and development of cation selective separation membranes are still needed to meet the growing demand for industrial applications. Summary of the Invention

[0004] In order to overcome the shortcomings of the above-mentioned prior art, the present invention is based on the intrinsic micropores of the microporous polymer, utilizes flexible long alkyl side chain grafting, further regulates the size of the micropores through the flexible chain, and utilizes the hydrophobic properties of the long alkyl chain to regulate the hydrophilicity and hydrophobicity of the microporous channel, thereby promoting the formation of ion channels. The regulation of the hydrophobic chain also achieves high selectivity of the membrane material, ultimately realizing the preparation of a side chain-type cation selective separation membrane with high flux, high selectivity and simple preparation process.

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

[0006] A method for preparing a side chain type cation selective separation membrane 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: The PN polymer with micropores is reacted and grafted with a sulfonated monomer to prepare a polymer SPN with a cation exchange group;

[0010] Step 3: The polymer SPN with a cation exchange group is reacted and grafted with a small molecule monomer with a long alkyl side chain to prepare an SPNC polymer;

[0011] Step 4: dissolving the SPNC polymer in an organic solvent to prepare a membrane solution, coating the membrane solution on a substrate and drying it to prepare a side chain type cation selective separation membrane.

[0012] Furthermore, in step 1, the molecular weight range m of the PN polymer is 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 sites on the PN polymer are -OH sites or -NH- sites, 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, and the reaction time is 12 hours-48 hours. A hydrogenation reagent (such as NaH, potassium carbonate, sodium tert-butoxide, etc.) is added during the reaction, and the molar ratio of the hydrogenation reagent added to the total number of reactive sites in the PN polymer is 0.1-1:1.

[0018] Further, in step 3, the long alkyl side chain small molecule monomer is Wherein, A is —CH3 or —C═C, and y is any integer from 0 to 10.

[0019] Furthermore, in step 3, the sites of reactive grafting on the SPN polymer are -OH sites or -NH- sites, and the molar ratio of the total number of reactive sites in the SPN polymer (i.e., the total molar amount of -OH and -NH-) to the long alkyl side chain small molecule monomer is 1:0.1-1. The reactive grafting temperature range is 50°C-120°C, and the reaction time is 12 hours-48 hours. A hydrogenation reagent (such as NaH, potassium carbonate, sodium tert-butoxide, etc.) is added during the reaction, and the molar ratio of the 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., the substrate can be a glass plate, a PET substrate, a polytetrafluoroethylene substrate, etc., and the drying temperature is between 30°C and 120°C.

[0021] The present invention provides a method for preparing a side-chain cation-selective separation membrane. This method introduces functional side chains through side-chain grafting, avoiding the problems of increased membrane resistance, reduced stability, and complex processes typically associated with surface modification. The beneficial effects are specifically reflected in the following aspects:

[0022] (1) Based on the inherent microporous properties of PN polymer, sulfonated monomers and long alkyl side chain small molecule monomers are confined to grow in the intrinsic micropores of the polymer, and selective ion transport channels are constructed using the inherent confined pores of the microporous polymer, avoiding the uncontrollable ion transport channel problem caused by the random distribution of ion exchange sites in traditional random polymers. Specifically, first, the grafted sulfonated monomer forms fixed ion exchange sites in the micropores, realizing the ion exchange function and ensuring the ability of the membrane material to transport ions. Secondly, the grafting of long alkyl side chain small molecule monomers is used to regulate the micropore diameter of the microporous polymer and the hydrophilicity and hydrophobicity of the membrane phase, forming transmission differences between different ions and realizing the construction of ion separation performance. In addition, the rigidity of PN as the base polymer of the membrane material also ensures excellent swelling resistance after membrane formation. At the same time, the intrinsic π-π interaction and abundant hydrogen bonding sites of the PN polymer promote hydrogen bond self-assembly, promote the formation of orderly stacking of molecular chains in the membrane, and promote the construction of ion transport channels in the membrane.

[0023] (2) Based on the flexibility of the long alkyl side chain small molecule monomer itself, it can effectively adjust the micropore diameter in the micropores of the rigid self-microporous polymer PN, further reduce the pore size, and enhance the size screening effect of the membrane material on ions. At the same time, based on the hydrophobic properties of the long alkyl chain, the overall hydrophobicity of the membrane material can be further regulated, and for ions with high hydration energy (such as Mg 2+ , Ca 2+Divalent ions, such as ions with a higher hydration energy, will encounter a greater energy barrier when entering the membrane from the solution, making it more difficult for these divalent ions to transport across the membrane. Based on the regulation of hydrophilicity and hydrophobicity, ions with a higher hydration energy will encounter a greater energy barrier when entering the membrane from the solution, making their transport across the membrane more difficult than ions with a lower hydration energy. Based on the regulation of the pore size by long alkyl side chains, the degree of freedom of divalent ions with a larger ionic radius is more significantly reduced, the entropy barrier is larger, and the probability of them entering the membrane pores is lower. According to this design, it is ultimately possible to achieve an obstruction to the transmembrane transport of multivalent ions and construct a cation-selective separation membrane with high selectivity.

[0024] (3) In the grafted side chains of the present invention, the connection between polymer chains can be further increased by cross-linking the terminal double bonds through heat treatment. On the one hand, this increases the strength of the membrane material, ensuring that it can meet the mechanical performance requirements of the electrodialysis operation process. On the other hand, cross-linking also increases the density within the membrane, which can further increase the size screening effect on the basis of micropore regulation and improve the selectivity of the membrane material.

[0025] (4) The present invention utilizes the characteristics of long alkyl side chain small molecule monomers, and through size screening and membrane phase hydrophilicity and hydrophobicity regulation, ensures high flux, high selectivity and low membrane resistance during the use of membrane materials, and finally realizes the preparation of side chain type cation selective separation membranes. The membrane synthesis routes designed by the present invention can be achieved through liquid phase grafting reactions. The functional small molecule monomers are introduced into the polymer structure through the grafting method, making it a part of the polymer, and finally a homogeneous membrane material is prepared, which simplifies the membrane production process and matches the current industrial membrane material production route. The membrane preparation route avoids the introduction of surface modification methods, reduces membrane resistance, reduces the energy consumption of membrane materials in the application process of membrane group devices, and also increases the stability of the membrane material. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0027] Figure 2 These are the nuclear magnetic resonance hydrogen spectrum characterization results of the SPNC polymers prepared in Examples 1-3 of the present invention.

[0028] Figure 3 This is the H NMR structural analysis of the SPNC polymer prepared in Example 4 of the present invention.

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

[0030] Figure 5These are the ion selectivity test results of the cation selective separation membranes SPN and SPNC-1 prepared in Example 1 of the present invention and the comparative commercial membranes CXP-S and CIMS.

[0031] Figure 6 These are the ion selectivity test results of the side chain type cation selective separation membranes prepared in Examples 2-6 of the present invention.

[0032] Figure 7 This is a schematic diagram of constructing an ion-selective membrane based on the microporous polymer to control the micropore size and pore environment after membrane formation. 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 side chain type 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 and stir until the solid drug is completely dissolved. Add 18.6g of indigo carmine, then add 15mL of trifluoroacetic acid dropwise, and continue stirring until the indigo carmine is completely dissolved to form a light red uniform solution. Set the condensation cycle 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 the 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 polymer PN: 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 SPNC polymer:

[0042] Weigh 5g of SPN polymer into a single-necked round-bottom flask and place the round-bottom flask in an oil bath. Add a magnet to the round-bottom flask and stir to dissolve the organic solvent, DMSO. Once dissolved, add 2g of octanoyl chloride and 0.67g of NaH as a hydrogen extraction agent. Raise the oil bath temperature to 60°C and allow to react for 12 hours. Then, precipitate the reactants and dry them to obtain a crude SPNC product. Dissolve the crude SPNC-1 product in the organic solvent DMSO and precipitate again. Repeat this process three times to obtain a high-purity SPNC polymer.

[0043] Where p=0.5, 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 SPNC polymer prepared in Step 3 into a 20 mL round-bottom flask. Add 9 g of the organic solvent, DMF, and a magnetic stirrer. Stir the mixture 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 membrane formed a side-chain cation-selective separation membrane, designated SPNC-1.

[0047] The photo of the membrane material prepared in this example is as follows Figure 1 The chemical structure of the SPNC polymer prepared in this example was characterized by hydrogen nuclear magnetic resonance spectroscopy, as shown in FIG. Figure 2 shown.

[0048] Example 2

[0049] The steps for preparing the side chain type 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 SPNC polymer: The same preparation method as in Example 1 was used, except that octanoyl chloride was replaced with hexanoyl chloride and the added amount was adjusted to 1.28 g.

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

[0054] The chemical structure of the SPNC polymer prepared in this example was characterized by hydrogen nuclear magnetic resonance spectroscopy. Figure 2 shown.

[0055] Example 3

[0056] The steps for preparing the side chain type cation selective separation membrane of this embodiment are as follows:

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

[0058] Preparation of SPN polymer: The same preparation method as in Example 1 was used.

[0059] Preparation of SPNC polymer: The same preparation method as in Example 1 was used, except that octanoyl chloride was replaced with butyryl chloride, and the added amount was adjusted to 1.03 g.

[0060] Preparation of membrane: The same preparation method as in Example 1 was used, and the prepared membrane material was named SPNC-3.

[0061] The chemical structure of the SPNC polymer prepared in this example was characterized by hydrogen nuclear magnetic resonance spectroscopy. Figure 2 shown.

[0062] Example 4

[0063] The steps for preparing the side chain type cation selective separation membrane of this embodiment are as follows:

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

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

[0066] Preparation of SPNC polymer: The same preparation method as in Example 1 was used, except that octanoyl chloride was replaced with bromoheptene in an amount of 1.37 g.

[0067] Preparation of membrane: The same preparation method as in Example 1 was used, and the prepared membrane material was named SPNC-4.

[0068] The chemical structure of the SPNC polymer prepared in this example was characterized by hydrogen nuclear magnetic resonance spectroscopy. Figure 3 shown.

[0069] Example 5

[0070] The steps for preparing the side chain type cation selective separation membrane of this embodiment are as follows:

[0071] Preparation of PN polymer: The same preparation method as in Example 1 was used.

[0072] Preparation of SPN polymer: The same preparation method as in Example 1 was used.

[0073] Preparation of SPNC polymer: The same preparation method as in Example 1 was used, except that octanoyl chloride was replaced by bromhexene in an amount of 1.17 g.

[0074] Preparation of membrane: The same preparation method as in Example 1 was used, and the prepared membrane material was named SPNC-5.

[0075] Example 6

[0076] The steps for preparing the side chain type cation selective separation membrane of this embodiment are as follows:

[0077] Preparation of PN polymer: The same preparation method as in Example 1 was used.

[0078] Preparation of SPN polymer: The same preparation method as in Example 1 was used.

[0079] Preparation of SPNC polymer: The same preparation method as in Example 1 was used, except that octanoyl chloride was replaced by bromobutene in an amount of 0.89 g.

[0080] Preparation of membrane: The same preparation method as in Example 1 was used, and the prepared membrane material was named SPNC-6.

[0081] 2. Membrane flux and ion selectivity test

[0082] Use Figure 4 The device shown is at 10mAcm -2 The Li ion selective separation membrane obtained in each embodiment was tested under the current density + / Mg 2+ Ion selectivity was determined, and commercial monovalent / divalent cation selective separation membranes CIMS, CXP-S, and the SPN membrane synthesized in Example 1 were used as controls. The specific method is as follows:

[0083] 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 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:

[0084]

[0085] 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:

[0086]

[0087] in, is the initial lithium ion concentration in the desalination chamber, is the initial magnesium ion concentration in the desalination chamber.

[0088] The side chain type cation selective separation membrane and SPN based membrane obtained in Examples 1-3 have Li + / Mg 2+ Ion selectivity test results such as Figure 5 and Figure 6 As shown, the results show that: Mg 2+ Flux greater than Li + ions, the selectivity is less than 1, but after the grafting of the alkyl chain, it has a significant effect on the construction of lithium magnesium selectivity. As the length of the alkyl chain gradually shortens, the selectivity of the membrane material tends to decrease. The shortening of the length of the flexible alkyl chain may make the pore density control in the membrane weaker, and the hydrophobicity is relatively low, which ultimately leads to a weakening of the difference in the process of dehydration into the membrane, causing the selectivity to show a certain downward trend. However, Examples 1-3 as a whole are still superior to commercial membranes CIMS and CXP-S in terms of ion flux and selectivity, which reflects the effectiveness of the design ideas of the present invention.

[0089] The side chain type cation selective separation membrane obtained in Examples 4-6 has a Li + / Mg 2+Ion selectivity test results such as Figure 6 As shown, the results show that after adding double bonds on the basis of the alkyl chain, the ion selectivity is significantly improved compared with Examples 1-3. This is because while maintaining the characteristics of the alkyl chain, the cross-linking of the double bonds during the membrane formation process further strengthens the connection between the chain segments in the membrane, and the shorter the alkyl chain length, the higher the selectivity after double bond cross-linking. This also shows that the membrane has a higher density after the formation of short chains, but at the same time, the increase in density causes a certain degree of loss of ion flux. The final regulation of selectivity and flux can be selected according to the actual application.

[0090] The structures related to the synthetic routes proposed in the embodiments of the present invention are verified by NMR results, such as Figure 2 and Figure 3 As shown in the figure, the NMR results show that the target structure can be correctly prepared according to this method. Figure 1 It can be seen that the prepared membrane material is completely homogeneous and transparent, and is prepared by a simple solution casting method. This further verifies the operability of the membrane material preparation route designed by the present invention, and is expected to achieve industrial production. As can be seen from the results of the examples, the introduction of long alkyl side chain small molecule monomers has an effective effect on the selective construction of mono / divalent selective cation selective separation membranes. Figure 7 As shown, by introducing flexible long alkyl side chains into the micropores of the rigid self-microporous polymer, the control of the micropore size can be effectively achieved. At the same time, based on its high hydrophobic properties, the hydrophobicity of the membrane can be further regulated, so that ions with a large number of hydrated ions and ion hydration energy need to remove more water before they can enter the membrane, significantly increasing the difficulty of such ions entering the membrane material, and ultimately making it difficult for ions with large hydration energy to complete the transmembrane transport process of ions, thereby forming transmembrane transport differences between different ions. From the perspective of ion separation performance, the performance of the SPNC membrane material under each embodiment is better than that of the imported Japanese membrane CIMS and CXP-S, and has a higher target ion flux, which further illustrates the important role of long alkyl side chains in ion selectivity construction. In summary, the effectiveness of the cation selective separation membrane preparation route provided by the present invention is explained from multiple angles such as preparation process, synthesis process, and ion selectivity. The cation selective separation membrane preparation method provided by the present invention is simple in process and has higher performance than the currently commercialized cation selective separation membrane, and is expected to be widely used.

Claims

1. A method for preparing a side chain type 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 a cation exchange group; Step 3: The polymer SPN with a cation exchange group is reacted and grafted with a small molecule monomer with a long alkyl side chain to prepare an SPNC polymer; Step 4: dissolving the SPNC polymer in an organic solvent to prepare a membrane solution, coating the membrane solution on a substrate and drying it to prepare a side chain type 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 step 1, the structural formula of R is shown in any one of formula (2), where * is 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 long alkyl side chain small molecule monomer is Wherein, A is —CH3 or —C═C, and y is any integer from 0 to 10.

7. The preparation method according to claim 1, wherein: In step 3, the reaction grafting site on the SPN is the -OH site or the -NH- site, the molar ratio of the total number of reaction sites in the SPN polymer to the long alkyl side chain 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 side chain type cation selective separation membrane prepared by the preparation method according to any one of claims 1 to 8.

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