Bifunctional group copolymerization separation membrane with main chain containing crown ether as well as preparation method and application of bifunctional group copolymerization separation membrane

Abstract

The invention discloses a bifunctional group copolymerization separation membrane with a main chain containing crown ether as well as a preparation method and application of the bifunctional group copolymerization separation membrane. According to the method, 2, 2 '-bis (3-sulfonated propoxy) biphenyl disodium and isatin are used as monomers, a bifunctional material with a main chain containing crown ether is prepared through a superacid catalyzed polycondensation reaction under the condition of a certain reaction temperature, and a pure product is obtained through a large amount of water washing; secondly, preparing the prepared sample into a film through a non-solvent phase transfer method. The prepared bifunctional group copolymerization separation membrane material with the main chain containing crown ether shows relatively good ion exchange capacity, permeation selectivity and stability for lithium element in waste liquid, is simple and convenient to operate and controllable in cost, and shows a good application prospect.

Description

Technical Field

[0001] This invention relates to the field of membrane preparation and application, specifically to a bifunctional copolymer separation membrane with a main chain containing crown ether, its preparation method, and its application. Background Technology

[0002] Lithium, as an important strategic resource, is widely used in lithium-ion batteries, aerospace, pharmaceuticals, and chemicals. With the rapid development of the new energy industry, global demand for lithium continues to grow, while lithium reserves are limited. Therefore, the separation and extraction of lithium from secondary resources such as salt lake brines and industrial waste has become a current research hotspot. Currently, lithium separation and extraction methods mainly include precipitation, solvent extraction, ion exchange, and membrane separation. Among these, membrane separation has received widespread attention due to its advantages of simple operation, low energy consumption, and environmental friendliness. The performance of the separation membrane is crucial to its separation efficiency. Existing lithium separation membranes are mostly designed based on single functional groups, resulting in problems such as insufficient lithium-ion selectivity, poor mechanical stability, and poor recyclability. Separation membrane technology, due to its high efficiency, energy saving, and environmental friendliness, has been widely used in water treatment, gas separation, and resource recovery. However, traditional separation membranes often suffer from problems such as difficulty in balancing selectivity and flux, weak anti-fouling ability, and poor stability, limiting their application in the separation of complex systems.

[0003] Crown ethers, as a class of macrocyclic compounds with specific cavity structures, possess unique recognition and complexation capabilities for metal ions. Introducing crown ether groups into separation membranes can endow the membrane materials with specific recognition functions for target ions, making them ideal functional units for constructing lithium-selective separation membranes. However, single-crown ether-based separation membranes suffer from defects such as weak mechanical strength, insufficient hydrophilicity, and easy membrane structure collapse, resulting in poor separation flux and long-term stability. Furthermore, poor dispersibility of crown ether groups also affects lithium-ion transport efficiency. Summary of the Invention

[0004] The purpose of this invention is to provide a bifunctional copolymer separation membrane with crown ether in the main chain, its preparation method and application. This separation membrane has both high lithium-ion selectivity and good mechanical stability, and has excellent separation performance in lithium separation and extraction from waste liquid.

[0005] In one aspect of the invention, a method for preparing a bifunctional copolymer separation membrane with a main chain containing crown ether is provided. According to embodiments of the invention, the membrane is prepared by solution polymerization using crown ether (CE), disodium 2,2'-bis(3-sulfonylpropoxy)biphenyl (DSOBP), and indigo as monomers. The dense separation layer of the membrane contains both crown ether ring structures and sulfonic acid groups in its main chain.

[0006] In addition, the method for preparing a bifunctional copolymer separation membrane with a main chain containing crown ether according to the above embodiments of the present invention may also have the following additional technical features:

[0007] In some embodiments of the present invention, the crown ether is at least one selected from dibenzo-18-crown ether-6, dibenzo-15-crown ether-5, and dibenzo-14-crown ether-4. The cavity size of this type of crown ether matches the radius of lithium ions, enabling highly selective recognition of lithium ions.

[0008] In some embodiments of the present invention, the method specifically includes the following steps:

[0009] (1) Mix crown ether, 2,2'-bis(3-sulfonylpropoxy)biphenyl disodium salt and indigo, add organic solvent, stir at room temperature to obtain a mixture, and transfer to an ice water bath for later use;

[0010] Among them, the crown ether is the core functional unit, which endows the membrane with ion recognition performance; 2,2'-bis(3-sulfonylpropoxy)biphenyl disodium is a hydrophilic charged comonomer, which regulates the hydrophilicity, swelling, mechanical strength and ion conductivity of the membrane; indigo is a structural comonomer-crosslinking modification unit, which optimizes the degree of polymerization crosslinking and chemical stability of the membrane, and can also help regulate the morphology and separation selectivity of the membrane. The three work together to achieve the performance adaptation of the copolymer membrane.

[0011] (2) Under a nitrogen atmosphere, the superacid catalyst is added to the mixture. After the addition is complete, the temperature is gradually raised to room temperature and the reaction continues for 1-3 hours. After the copolymerization reaction is completed, the mixture is washed and dried to obtain the copolymer. The inert gas protection can avoid the interference of oxygen on the polymerization reaction. The control of reaction temperature and time can ensure that the monomer reacts fully and forms a copolymer with a suitable molecular weight. If the temperature is too high or the time is too long, the polymer may be over-crosslinked, affecting the membrane permeability. If the temperature is too low or the time is too short, the monomer conversion rate will be insufficient and the membrane performance will be poor.

[0012] (3) Add the copolymer to the solvent, heat and stir, then centrifuge to separate the layers, pour the supernatant into the mold and dry it, then peel off the resulting membrane, and obtain the main chain crown ether-containing bifunctional copolymer separation membrane by non-solvent phase transfer method (NIPs).

[0013] In some embodiments of the present invention, in step (1), the molar ratio of the crown ether to disodium 2,2'-bis(3-sulfonoxy)biphenyl is 1:1-4, and the amount of indigo is the sum of the amounts of the crown ether and disodium 2,2'-bis(3-sulfonoxy)biphenyl. This ratio range ensures sufficient copolymerization of the three monomers, resulting in a uniform distribution of crown ether recognition sites and rigid structural units in the main chain of the separation membrane, thus balancing selectivity and mechanical properties. If the proportion of DSOBP or indigo is too high, it will lead to a decrease in the density of crown ether groups, affecting the lithium-ion separation efficiency; if the proportion is too low, it will not effectively improve the mechanical strength of the separation membrane.

[0014] The copolymerization system described in step (2) uses superacids (trifluoroacetic acid and trifluoromethanesulfonic acid) as protonic acid catalysts, and cationic condensation occurs. The three monomers are covalently linked through three core reaction pathways: aromatic ring electrophilic substitution, carbonyl-imino condensation, and ether bond activation crosslinking, and finally form a three-dimensional crosslinked aromatic polymer copolymer membrane skeleton.

[0015] The organic solvent is at least one of dichloromethane, N,N-dimethylformamide, and dimethyl sulfoxide. These solvents have good solubility for the three monomers, ensuring uniform copolymerization. Excessive monomer concentration leads to uneven mixing, while insufficient concentration reduces reaction efficiency and film-forming properties.

[0016] In some embodiments of the present invention, in step (2), the superacid catalyst is composed of trifluoroacetic acid (TFA) and trifluoromethanesulfonic acid (TFSA), the volume ratio of trifluoroacetic acid to trifluoromethanesulfonic acid is 1:5~10, and the total amount of catalyst is 5%-15% of the total mass of the comonomer.

[0017] In some embodiments of the present invention, in step (2), the washing uses deionized water; the drying temperature is 60-80℃ and the time is 12-24h.

[0018] In some embodiments of the present invention, in step (3), the solvent is at least one of dimethyl sulfoxide (DMSO) or N-methylpyrrolidone (NMP).

[0019] In some embodiments of the present invention, in step (3), the heating and stirring temperature is 60-80℃ and the time is 24-48h. Magnetic stirring is used during the reaction process, and the stirring rate is 100-200r / min. The drying temperature is 50℃-80℃ and the time is 12-24h. The stripping is carried out in deionized water.

[0020] In another aspect of the present invention, the present invention provides a bifunctional copolymer separation membrane with a main chain containing crown ether prepared by the preparation method described above.

[0021] In another aspect of the invention, the invention proposes the application of a bifunctional copolymer separation membrane containing crown ethers in the main chain. According to embodiments of the invention, the separation membrane is used for the selective separation of lithium ions in fly ash acid leaching solutions, or for the extraction of lithium from organic wastewater.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] (1) This invention introduces crown ether lithium ion recognition sites and rigid structural units of DSOBP and indigo into the main chain of the separation membrane by ternary copolymerization of crown ether, DSOBP and indigo, thereby achieving dual-function synergy of "selective recognition-structure enhancement". The resulting separation membrane has a high selectivity coefficient for lithium ions and excellent mechanical strength, and is not easily damaged.

[0024] (2) The use of a composite catalyst of trifluoroacetic acid and trifluoromethanesulfonic acid significantly improved the polymerization rate and monomer conversion rate compared with a single catalyst. The resulting copolymer had a uniform molecular weight distribution, a dense membrane structure, and good permeability, thus ensuring efficient lithium-ion transport. This is mainly because the combination of trifluoroacetic acid and trifluoromethanesulfonic acid achieves efficient activation of all monomers through proton strength complementarity, uniform and controllable chain growth through conjugate base synergy, and precise and dense crosslinking. Ultimately, the copolymer product has both high degree of polymerization and narrow molecular weight distribution. The corresponding copolymer membrane forms an optimal structure with dense framework support, continuous hydrophilic channels, and complete crown ether recognition sites, thereby simultaneously meeting the permeability, selectivity, and stability requirements of lithium-ion transport and achieving efficient transport.

[0025] (3) This invention introduces DSOBP and indigo as core framework monomers. The rigid structure of DSOBP provides stable mechanical support for the membrane, while the strongly hydrophilic sodium sulfonate group enables hydrophilic modification of the entire membrane and precisely controls the membrane's equilibrium swelling behavior. Indigo, as a crosslinking modification unit, constructs a three-dimensional dense crosslinking network through condensation and arylation reactions with the main chain, effectively anchoring the molecular chain and inhibiting membrane structure collapse. This effectively overcomes the defects of single crown ether separation membranes, such as weak mechanical strength, insufficient hydrophilicity, and easy membrane structure collapse, and significantly improves the mechanical properties and stability of the membrane.

[0026] (4) The preparation method of the present invention is simple and mild, and is easy to scale up for industrial production. The separation membrane exhibits high lithium adsorption capacity (≥30mg / g), rapid adsorption kinetics (equilibrium time ≤60min) and good recycling performance (lithium adsorption capacity retention rate ≥90% after 5 cycles) in lithium separation and extraction from waste liquid. The separation efficiency is significantly better than that of existing separation membrane materials. Attached Figure Description

[0027] Figure 1 This is a schematic diagram illustrating the preparation method of a bifunctional copolymer separation membrane with a main chain containing crown ether, as shown in Examples 1-4 of the present invention.

[0028] Figure 2 The above are FTIR analysis diagrams of the bifunctional copolymer separation membranes containing crown ethers in the main chain in Examples 1-4 of this invention.

[0029] Figure 3 The images shown are SEM images of the bifunctional copolymer separation membranes with crown ethers in the main chain in Examples 1-4 of this invention. In the images, a, b, c, and d are cross-sectional SEM images of the SPDI-1, SPDI-2, SPDI-3, and SPDI-4 copolymer membranes, respectively; e, f, g, and h are surface SEM images of the SPDI-1, SPDI-2, SPDI-3, and SPDI-4 copolymer membranes, respectively. SPDI 1-4 refer to membranes with a molar ratio of crown ether to 2,2'-bis(3-sulfonylpropoxy)biphenyl disodium salt of 1:1-4.

[0030] Figure 4 The chemical stability analysis diagrams are shown for the bifunctional group copolymer separation membranes with crown ethers in the main chain in Examples 1(a), 2(b), 3(c), and 4(d) of the present invention. Among them, a, b, c, and d are the chemical stability analysis diagrams for SPDI-1, SPDI-2, SPDI-3, and SPDI-4 copolymer membranes, respectively.

[0031] Figure 5 The contact angle analysis diagrams are shown for the bifunctional copolymer separation membranes with crown ethers in the main chain in Examples 1(a), 2(b), 3(c), and 4(d) of the present invention.

[0032] Figure 6 The graphs show the water absorption rate and swelling degree of the bifunctional copolymer separation membranes with crown ethers in the main chain in Examples 1-4 of this invention.

[0033] Figure 7 This is a diagram of the electrodialysis membrane separation experimental apparatus in an embodiment of the present invention;

[0034] Figure 8 The IV curve is shown for the bifunctional copolymer separation membrane with crown ether in the main chain in an application example of this invention.

[0035] Figure 9 This refers to the ion exchange capacity of the bifunctional copolymer separation membrane with crown ether in the main chain in the application example of this invention;

[0036] Figure 10 A comparison of the permeation rate of the bifunctional copolymer separation membrane with crown ether in the main chain in the application example of this invention with that in the literature;

[0037] Figure 11 This is a permeability selectivity analysis diagram of the bifunctional copolymer separation membrane with crown ether in the main chain in an application example of the present invention. Detailed Implementation

[0038] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0039] Example 1

[0040] like Figure 1 As shown, a method for preparing a bifunctional copolymer separation membrane with a main chain containing crown ethers includes the following steps:

[0041] (1) Dibenzo-18-crown ether-6 (0.18 g, 0.5 mmol), DSOBP (0.237 g, 0.5 mmol) and indigo (0.161 g, 1.1 mmol) were added to a three-necked flask, and 25 mL of dichloromethane was added under ice-water bath conditions. The mixture was stirred for 30 min until the monomers were completely dissolved to obtain a monomer mixed solution.

[0042] (2) Acidic catalyst (1 mL trifluoroacetic acid and 10 mL trifluoromethanesulfonic acid) was slowly added dropwise to the monomer mixture using a constant pressure dropping funnel. Nitrogen gas was introduced for protection. After the addition was completed, the temperature was slowly raised to room temperature and the reaction was kept warm for 2 hours to obtain a copolymer solution. The copolymer solution was introduced into a beaker containing a large amount of deionized water. A large amount of light yellow needle-like solid precipitated out. After filtration, the solid was repeatedly washed with water until neutral and dried under vacuum at 60°C to obtain the bifunctional copolymer polymer SPDI-1 with crown ether in the main chain.

[0043] (3) Accurately weigh 0.1g of copolymer SPDI-1 and add 10mL of solvent DMSO, then heat and stir at 60℃ for 24 h. Then centrifuge the solution to separate the layers, pour the supernatant into a mold, put it in an oven at 50℃ and dry for 12 h, and soak the resulting membrane in deionized water to peel off the main chain containing crown ether copolymer separation membrane SPDI-1.

[0044] Example 2

[0045] A method for preparing a bifunctional copolymer separation membrane with crown ether in the main chain, which differs from Example 1 in that: in step (1), dibenzo-18-crown ether-6 is 0.18 g (0.5 mmol) and DSOBP is 0.47 g (1.0 mmol); in step (2), the bifunctional copolymer polymer SPDI-2 with crown ether in the main chain is obtained; and in step (3), the copolymer separation membrane SPDI-2 with crown ether in the main chain is obtained. The remaining steps and parameters are the same as in Example 1.

[0046] Example 3

[0047] A method for preparing a bifunctional copolymer separation membrane with crown ether in the main chain, which differs from Example 1 in that: in step (1), dibenzo-18-crown ether-6 is 0.18 g (0.5 mmol) and DSOBP is 0.71 g (1.5 mmol); in step (2), the bifunctional copolymer polymer SPDI-3 with crown ether in the main chain is obtained; and in step (3), the copolymer separation membrane SPDI-3 with crown ether in the main chain is obtained. The remaining steps and parameters are the same as in Example 1.

[0048] Example 4

[0049] A method for preparing a bifunctional copolymer separation membrane with crown ether in the main chain, which differs from Example 1 in that: in step (1), dibenzo-18-crown ether-6 is 0.18 g (0.5 mmol) and DSOBP is 0.95 g (2.0 mmol); in step (2), the bifunctional copolymer polymer SPDI-4 with crown ether in the main chain is obtained; and in step (3), the copolymer separation membrane SPDI-4 with crown ether in the main chain is obtained. The remaining steps and parameters are the same as in Example 1.

[0050] The bifunctional copolymer separation membranes containing crown ethers in the main chain prepared in Examples 1-4 were characterized as follows:

[0051] (1) FTIR analysis: such as Figure 2 As shown, four membranes of different proportions were placed at 1700 cm. -1 Characteristic peaks corresponding to carbonyl groups (C=O) appeared at all locations, including at 1127 cm⁻¹. -1 A characteristic peak for ether bonds (COC) appeared at 1033 cm⁻¹. -1 The strong characteristic absorption peak at 2850 cm⁻¹ is attributed to the symmetric and asymmetric stretching vibrations of the sulfonic acid group (-SO₃H) in the membrane material. -1 and 2925cm -1 The peak at 3280 cm⁻¹ is caused by the symmetric and asymmetric stretching vibrations of the CH bond. -1 The peak at that position is attributed to the imino group (NH) on indigo. Since the four prepared films exhibit identical FT-IR results, it indicates that the prepared copolymer films have similar structures.

[0052] (2) SEM analysis: such as Figure 3 As shown, the copolymer films SPDI-1, SPDI-2, SPDI-3, and SPDI-4 are very intact with smooth cross-sections. Observation of the surface reveals that the prepared films are uniform and complete, without any collapse. Furthermore, the morphology of the films remains essentially unchanged as the molar ratio of CE:DSOBP increases.

[0053] (3) Chemical stability analysis: To assess the chemical stability of the membrane, the synthesized SPDI-x membranes were cut to a uniform size and immersed in hydrochloric acid solutions with pH values ​​of 2, 4, and 6, and sodium hydroxide solutions with pH values ​​of 8, 10, and 12, respectively, for 12 h. Subsequently, the samples were removed, dried, and scanned using X-ray diffraction (0-50°) to observe changes in the crystal structure. Figure 4 As shown, the membrane material can maintain the integrity of its crystal structure in both acidic environments (pH 2 to 6) and alkaline environments (pH 8 to 12), indicating that it has excellent chemical and structural stability.

[0054] (4) Contact angle: such as Figure 5 As shown, the contact angle of the copolymer membrane gradually increases with the increase of DSOBP content. This is due to the strong hydrophobicity of the biphenyl skeleton and the hydrophobicity of the benzene ring in dibenzo-18-crown-6. Although -SO3H is a hydrophilic group, the hydrophobicity of the benzene ring plays a major role in the overall structure, making the membrane exhibit weak hydrophobicity.

[0055] (5) Water absorption rate and swelling degree: A certain amount of membrane was dried in an oven at 80℃ for 24 h, and the weight and dimensions were recorded to obtain the mass m1 and length L1 of the membrane sample in the dry state. Then the membrane sample was placed in deionized water for 24 h. After the drying, the moisture on the surface of the membrane was absorbed with filter paper, and the mass m2 and length L2 of the wet membrane were recorded. The water absorption rate (WU%) and swelling degree (LSR%) of the membrane were calculated as shown in formulas (1) and (2):

[0056] (1)

[0057] (2)

[0058] In the formula, m1—dry film mass, g; m2—wet film mass, g; L1—dry film length, cm; L2—wet film length, cm.

[0059] like Figure 6 As shown, with the increase of DSOBP content, water absorption (WU) increased from 17.6% to 100%, indicating that a higher -SO3H / 18C6 group ratio is related to increased water absorption. The linear expansion ratio (LSR) increased from 4.89% to 39.5%, indicating that the DSOBP content is related to membrane expansion; as the CE:DSOBP ratio decreases, the LSR increases.

[0060] Application examples

[0061] A bifunctional copolymer membrane with crown ether-containing main chain is used for the selective separation of lithium ions in fly ash acid leaching solution:

[0062] Based on a comprehensive consideration of the current-voltage curves under different ion concentrations, a 0.05 mol / L solution containing Li was prepared. + Mg 2+ Ca 2+ K + The mixed solution was adjusted to a specific pH value with hydrochloric acid, and then an electrodialysis-membrane separation experiment was conducted. The apparatus was as follows: Figure 7 As shown, this study investigates the effects of different membrane materials on ion transport performance.

[0063] The IV curves of the main-chain crown ether-containing bifunctional copolymer films prepared in Examples 1-4 are as follows: Figure 8 As shown, the conductivity of the membrane increases with the gradual increase of DSOBP content. Particularly in the copolymer membrane SPDI-4, its conductivity is the most outstanding, indicating that this membrane material has the optimal promoting effect on ion transport.

[0064] Ion exchange capacity (IEC) includes all functional groups responsible for ion exchange, and is the content of -SO3H groups in the membrane determined by acid-base titration. The ion exchange capacity of the prepared bifunctional copolymer membrane containing crown ethers in the main chain is shown below. Figure 9 As shown, the IEC gradually increases with the increase of DSOBP content in the membrane. When the CE:DSOBP ratio in the membrane is 1:4, the ion exchange capacity of the membrane is the largest, at 1.373 mmol / g. This is because with the increase of DSOBP content, the content of -SO3H groups in the membrane increases, thus increasing the ion exchange capacity and promoting ion exchange.

[0065] To further investigate the ion transmembrane transport efficiency of membrane materials under electric field driving, a quaternary aqueous solution containing LiCl, Mg(NO3)2·6H2O, CaCl2, and KCl was used as the feed solution, and deionized water was used as the receiving solution. The ion permeation capacity of the prepared bifunctional copolymer membrane with crown ether in the main chain was compared with that in the literature (see references 1, 2, 3): Figure 10 It can be seen that the SPDI-4 copolymer membrane has a very high permeation capacity, and its results are all higher than those in the comparative literature.

[0066] A 0.05 mol / L solution was prepared using LiCl, Mg(NO3)2·6H2O, CaCl2, and KCl to simulate the coexisting ionic environment that may be encountered in actual industrial permeation processes, in order to systematically investigate the effects of different cations on LiCl. + The permeation-selective interference mechanism was investigated. A copolymer membrane (CE:DSOBP=1:4) was selected as the core separation medium, and the permeation-selective interference mechanism of this membrane on Li was determined under strictly controlled experimental conditions. + Mg 2+ Ca 2+ and K +Selective separation performance of four cations. Permeability-selectivity analysis of a main-chain crown ether-containing bifunctional copolymer membrane prepared: (from...) Figure 11 It can be seen that this copolymer film is effective against Li + It exhibits significant selectivity advantages, compared to Mg 2+ Ca 2+ K + The permeability selectivity coefficients reached 1.99, 1.51, and 1.65, respectively.

[0067] The above description is merely an example and illustration of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described, or use similar methods to replace them, as long as they do not deviate from the structure of the present invention or exceed the scope defined in the claims, all of which should fall within the protection scope of the present invention.

Claims

1. A method for preparing a bifunctional copolymer separation membrane with a main chain containing crown ethers, characterized in that: The crown ether, 2,2'-bis(3-sulfopropoxy) biphenyl disodium and indigo are mixed, and an organic solvent is added, and a mixed solution is obtained by stirring at room temperature, and then the mixed solution is transferred to an ice water bath for standby; under a nitrogen atmosphere, a super acid catalyst is added to the mixed solution, and after dropwise addition is completed, the temperature is gradually increased to room temperature, and the reaction is continued for 1-3 hours, and after the copolymerization reaction is completed, washing and drying are performed to obtain a copolymer; the copolymer is added to a solvent, heated and stirred, and then centrifuged to separate layers, and the supernatant is poured into a mold for drying, and then the produced film is peeled off to obtain the bifunctional group copolymer separation film with a crown ether in the main chain.

2. The method for preparing a main chain containing crown ether bifunctional base co-separation membrane according to claim 1, characterized in that: The crown ether is at least one of dibenzo-18-crown-6, dibenzo-15-crown-5 and dibenzo-14-crown-4.

3. The method for preparing a bifunctional copolymer separation membrane with a main chain containing crown ether according to claim 1, characterized in that, The method specifically comprises the following steps: (1) The crown ether, 2,2'-bis(3-sulfopropoxy) biphenyl disodium and indigo are mixed, and an organic solvent is added, and a mixed solution is obtained by stirring at room temperature, and then the mixed solution is transferred to an ice water bath for standby; under a nitrogen atmosphere, a super acid catalyst is added to the mixed solution, and after dropwise addition is completed, the temperature is gradually increased to room temperature, and the reaction is continued for 1-3 hours, and after the copolymerization reaction is completed, washing and drying are performed to obtain a copolymer; the copolymer is added to a solvent, heated and stirred, and then centrifuged to separate layers, and the supernatant is poured into a mold for drying, and then the produced film is peeled off to obtain the bifunctional group copolymer separation film with a crown ether in the main chain. In step (1), the molar ratio of the crown ether and 2,2'-bis(3-sulfopropoxy) biphenyl disodium is 1:1-4, and the amount of indigo is the total amount of substance of the crown ether and 2,2'-bis(3-sulfopropoxy) biphenyl disodium; the organic solvent is at least one of dichloromethane, N,N-dimethylformamide and dimethyl sulfoxide. In step (2), the super acid catalyst is composed of trifluoroacetic acid and triflic acid, and the volume ratio of trifluoroacetic acid to triflic acid is 1:5-10.

4. The method for preparing a bifunctional copolymer separation membrane with a main chain containing crown ether according to claim 3, characterized in that: In step (2), deionized water is used for washing, and the drying temperature is 60-80°C, and the time is 12-24 hours.

5. The method for preparing a bifunctional copolymer separation membrane with a main chain containing crown ether according to claim 3, characterized in that: In step (3), the solvent is at least one of dimethyl sulfoxide or N-methyl pyrrolidone.

6. The method for preparing a bifunctional copolymer separation membrane with a main chain containing crown ether according to claim 3, characterized in that: In step (3), the temperature for heating and stirring is 60-80°C, and the time is 24-48 hours; the drying temperature is 50-80°C, and the time is 12-24 hours; and the peeling is performed in deionized water.

7. The method for preparing a bifunctional copolymer separation membrane with a main chain containing crown ether according to claim 3, characterized in that:

9. A bifunctional group copolymer separation film with a crown ether in the main chain prepared by the preparation method of any one of claims 1-8.

8. The method for preparing a bifunctional copolymer separation membrane with a main chain containing crown ether according to claim 3, characterized in that: The separation film is used for selective separation of lithium ions in fly ash acid leaching solution, or for extraction of lithium in organic wastewater. ​ 10. Use of the main-chain crown ether-containing bifunctional based co-separation membrane according to claim 9, characterized in that: ​

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