Covalent triazine unit-based conjugated microporous polymer membrane with negative charges and preparation method of covalent triazine unit-based conjugated microporous polymer membrane

Abstract

The invention relates to the technical field of conjugated microporous polymer materials, and discloses a covalent triazine unit-based conjugated microporous polymer membrane with negative charges and a preparation method thereof. The method comprises the following steps: firstly, preparing a negatively charged cyanophenyl monomer through nucleophilic substitution reaction of a cyanophenyl-containing compound and 1, 4-butane sultone, then taking the monomer and biphenyl nitrile as comonomers, and carrying out step-by-step polymerization, heating film formation, shock cooling shaping and dual-solvent soaking post-treatment under the catalysis of trifluoromethanesulfonic acid to obtain the cyanophenyl-containing polymer film. And finally obtaining the covalent triazine unit-based conjugated microporous polymer membrane with negative charges. The preparation process is high in controllability, and the obtained film has high stability of a covalent triazine structure and the porous characteristic and negative charge function of conjugated micropores, and has wide application prospects in the fields of gas separation and the like through verification.

Description

Covalent triazine unit-based negatively charged conjugated microporous polymer membrane and its preparation method Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to a covalent triazine unit-based conjugated microporous polymer membrane with negative charge and its preparation method, which can be used in fields such as gas separation. Background Technology

[0002] Covalent triazine polymers (CTPs), a class of conjugated microporous polymers formed from cyano monomers via cyclotrimerization, have shown great potential in gas adsorption and separation, catalysis, and energy storage due to their high specific surface area, excellent thermal and chemical stability, and unique triazine ring electronic structure. However, traditional covalent triazine polymers are mostly electrically neutral frameworks with limited functionality, and their film formation process is prone to structural inhomogeneity and brittleness, which restricts their application in high-precision separation.

[0003] Negative charge functionalization of covalent triazine polymers is an effective way to expand their application scenarios. Introducing negatively charged groups can endow materials with properties such as selective adsorption of cations and selective control of gas separation. Currently, the functionalization of covalent triazine polymers mainly falls into two categories: monomer functionalization and post-modification of the polymer. Post-modification suffers from low reaction efficiency and easy damage to the microporous structure of the material. The key technology for polymerization after monomer functionalization lies in solving the problems of poor compatibility between functional monomers and comonomers and insufficient film stability. Furthermore, existing film-forming processes lack efficient shaping methods and systematic post-processing procedures, easily leading to poor film structure stability and poor separation performance. Therefore, developing a process-controllable method for preparing covalent triazine unit-based films with excellent film quality and both negative charge functionality and high-efficiency separation performance has significant research and application value. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides a covalent triazine unit-based conjugated microporous polymer membrane with negative charge and its preparation method. By designing a specific synthetic route for negatively charged monomers, a stepwise polymerization process, and a post-treatment process combining heating for film formation, rapid cooling in pure water for setting, and immersion in dual solvents, the invention solves the problems of single function, poor film formation stability, and insufficient separation performance of traditional covalent triazine polymer membranes, thereby obtaining a functionalized conjugated microporous polymer membrane with synergistic optimization of structure and performance.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a method for preparing a covalently triazine unit-based conjugated microporous polymer membrane with negative charge, comprising the following steps: S1, preparing a negatively charged cyanophenyl monomer: A cyanophenyl compound, 1,4-butanesulfonyl lactone, and potassium carbonate are dissolved in an organic solvent, mixed evenly, and then heated and reacted at 100-120°C for 12-24 h under nitrogen protection; after the reaction, a white solid is collected by filtration, and the white solid is vacuum dried at 60-80°C and a vacuum degree ≤-0.09 MPa for 24-48 h to obtain a negatively charged cyanophenyl monomer. The molar ratio of the cyanophenyl compound, 1,4-butanesulfonyl lactone, and potassium carbonate is 1:1.05-4.2:3-12.

[0006] S2. Preparation of the polymerization system: First, trifluoromethanesulfonic acid (TFSA) is added to the reaction vessel. The reaction vessel is then placed in liquid nitrogen for freezing, lowering the system temperature to below 0°C to freeze the trifluoromethanesulfonic acid and prevent its volatilization during the vacuuming and nitrogen purging steps. Next, the negatively charged cyanophenyl monomers and biphenyl nitrile monomers obtained in step S1 are added. The reaction vessel is then removed from the liquid nitrogen and stirred for the first time under an N2 atmosphere until the system becomes slightly viscous. Then, trifluoromethanesulfonic acid (TFSA) is added again for dilution, followed by a second stirring to obtain a homogeneous polymerization system. The molar ratio of the negatively charged cyanophenyl monomers, biphenyl nitrile monomers, and total trifluoromethanesulfonic acid is 1:1.1~3.2:84~168, and the amount of trifluoromethanesulfonic acid added in both stages is equal. The first and second stirring times are both 10~30 min, and the process is carried out at room temperature.

[0007] S3. Film Formation: Transfer the polymerization system obtained in step S2 to a polytetrafluoroethylene (PTFE) culture dish in a nitrogen bag and spread it evenly. Place the culture dish on a heating table to heat it for preliminary film formation. The heating table temperature is 70~100℃. Then, place the film together with the PTFE culture dish into pure water at 10~25℃ for rapid cooling and shaping to obtain a complete film.

[0008] S4. Post-treatment: After separating the membrane from the culture dish, it was first soaked in pure water at 10-25℃ for 24-48 hours, and then transferred to N-methylpyrrolidone (NMP) at 10-25℃ for another 24-48 hours. After post-treatment, a covalently triazine unit-based conjugated microporous polymer membrane was finally obtained. During the soaking process, both the pure water and NMP were changed every 12 hours. Pure water soaking was used to remove residual trifluoromethanesulfonic acid from the membrane surface, while NMP soaking was used to further remove residual impurities and optimize the membrane pore structure.

[0009] Further: the cyanophenyl compound is one of 3-hydroxy-[1,1'-biphenyl]-4,4'-dicarboxynitrile as shown in formula (1) or 4,4'-dicyano-4''-hydroxytriphenylamine as shown in formula (2). The biphenyl nitrile monomer is 4,4'-biphenyldinitrile as shown in formula (3). (1); (2); (3).

[0010] The beneficial effects of this invention are as follows: 1. This invention can efficiently prepare negatively charged cyanophenyl monomers through the nucleophilic substitution reaction of cyanophenyl compounds with 1,4-butanesulfonyl lactone. The synthesis process is simple, the product purification is convenient, and the sulfonate group (-SO3) in the monomer is negatively charged. ⁻ The sulfonate group is stable. As a core functional group for achieving efficient CO2 separation in membranes, it can enhance the selective adsorption of CO2 through chemical interactions and regulate the pore structure and surface properties of the membrane, laying a reliable foundation for achieving "high selectivity + high permeability" gas separation performance in membrane materials.

[0011] 2. The present invention adopts a polymerization strategy of adding trifluoromethanesulfonic acid (TFSA) in steps. First, a small amount of catalyst is used to initiate polymerization until the system is moderately viscous. Then, TFSA is added to dilute and adjust the viscosity. This ensures that the cyano ring trimerization reaction is fully carried out and that the polymerization system is uniformly spread, effectively avoiding the problem of uneven film thickness.

[0012] 3. This invention adopts an integrated film formation process of "heating for preliminary film formation - rapid cooling and shaping with pure water - post-treatment by immersion in dual solvents". The rapid cooling treatment can quickly solidify the membrane structure, greatly reducing the generation of internal stress and cracking. The stepwise immersion in pure water and NMP can not only efficiently remove residual catalysts and impurities from the membrane, but also further optimize the membrane pore structure and significantly improve the gas separation performance of the membrane.

[0013] 4. The covalent triazine unit-based conjugated microporous polymer membrane obtained in this invention has a covalent triazine unit as its core framework. It combines the high thermal and chemical stability of the triazine ring structure, the high specific surface area of ​​the conjugated micropores, and the functional characteristics of the sulfonate negative charge group. The membrane exhibits excellent performance in various tests and has important practical application value in fields such as gas separation. Attached Figure Description

[0014] Figure 1 shows the ¹H NMR spectrum of monomer 1 prepared in Example 1.

[0015] Figure 2 shows the FTIR spectra of monomer 1 (SO3KCN) and membrane 1 (CTF-SO3KCN) prepared in Example 1.

[0016] Figure 3 shows the XPS N1s images of monomer 1 (SO3KCN) (Figure 3(a)) and membrane 1 (CTF-SO3KCN) (Figure 3(b)) prepared in Example 1.

[0017] Figure 4 shows the XPS full spectrum of monomer 1 (SO3KCN) and membrane 1 (CTF-SO3KCN) prepared in Example 1.

[0018] Figure 5 is an optical image of film 1 prepared in Example 1.

[0019] Figure 6 is a surface SEM image of membrane 1 prepared in Example 1.

[0020] Figure 7 is a cross-sectional SEM image of membrane 1 prepared in Example 1.

[0021] Figure 8 shows the gas separation test diagrams of membrane 1 prepared in Example 1 and membrane 2 prepared in Comparative Example 2, where (a) is the permeability of each gas through the membrane and (b) is a trade-off diagram of the CO2 permeability of the membrane and the CO2 / N2 selectivity α of the membrane. Detailed Implementation

[0022] The embodiments of the present invention are described in detail below. These embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. Any improvements made without departing from the method of the present invention are considered to be within the scope of protection of the present invention.

[0023] Example 1 This example describes the preparation of a covalently triazine-based conjugated microporous polymer membrane with a negative charge, following these steps: S1. Preparation of negatively charged cyanophenyl monomers: 0.5 g (2.27 mmol) of 3-hydroxy-[1,1'-biphenyl]-4,4'-dicarboxynitrile, 0.35 g (2.57 mmol) of 1,4-butanesulfonyl lactone, 0.955 g (6.92 mmol) of K2CO3, and 10 mL of DMAc (N,N-dimethylacetamide) were added to a high-vacuum reaction tube. After mixing thoroughly, the mixture was suspended and heated to 110°C under nitrogen protection and reacted for 12 h. After the reaction, deionized water was added to the reaction vessel, and the white solid was collected by filtration. The white solid was placed in a vacuum drying oven and dried for 24 hours at 70°C and a vacuum degree ≤-0.09 MPa. h, to obtain a negatively charged cyanophenyl monomer (potassium 3-((4,4'-dicyano-[1,1'-biphenyl]-3-yl)oxy)butane-1-sulfonate, hereinafter referred to as monomer 1), the reaction formula is shown below.

[0024]

[0025] S2. Preparation of the polymerization system: Add 1.5 mL (17.1 mmol) of TFSA to a high-vacuum reaction tube and freeze the tube in liquid nitrogen to lower the system temperature to below 0°C. Then add 0.08 g (0.203 mmol) of monomer 1 and 0.12 g (0.588 mmol) of 4,4'-biphenyl dinitrile obtained in step S1. Remove the reaction vessel from the liquid nitrogen and stir for 20 min under N2 atmosphere to make the system slightly viscous. Then add 1.5 mL (17.1 mmol) of trifluoromethanesulfonic acid (TFSA) for dilution and continue stirring for 30 min to obtain a homogeneous polymerization system. The reaction formula is shown below.

[0026]

[0027] S3. Film Formation: The polymerization system obtained in step S2 is transferred to a polytetrafluoroethylene (PTFE) culture dish with an inner diameter of 6 cm in a nitrogen bag and spread evenly. The culture dish is placed on a heating table and heated at 100°C for 25 min to form a preliminary film. Then, the film, together with the PTFE culture dish, is immersed in pure water at 20°C for rapid cooling and shaping to obtain a complete film.

[0028] S4. Post-treatment: After separating the membrane from the culture dish, it is first soaked in pure water at 20℃ for 24 h (the pure water is changed every 12 h), and then transferred to NMP at 20℃ for 24 h (the NMP is changed every 12 h). After post-treatment, a covalent triazine unit base banded conjugated microporous polymer membrane (hereinafter referred to as membrane 1) is finally obtained.

[0029] Figure 1 shows monomer 1 prepared in Example 1. 1 The ¹H NMR spectrum, tested in deuterated dimethyl sulfoxide (DMSO-d6) solvent, matches the hydrogen environment of the target compound: multiple characteristic peaks in the δ 7.0-8.0 ppm region correspond to aromatic hydrogens on the biphenyl ring, exhibiting different chemical shifts due to the combined effects of electron withdrawal by -CN and electron donation by -O; the characteristic peak in the δ 4.0-4.5 ppm is the α-methylene hydrogen bonded to an oxygen atom; the peaks in the δ 1.5-3.0 ppm correspond to the remaining methylene groups in the side chain, with the methylene group near -SO3K shifting slightly higher due to electron withdrawal; the peaks at δ 3.3 ppm and 3.5 ppm are the DMSO-d6 residue peak and the water (H2O) impurity peak, respectively, both being peaks of the test solvent and not belonging to the target monomer structure. The chemical shifts and peak shapes of each peak are consistent with the chemical environment characteristics of hydrogen in the structure.

[0030] Figure 2 shows the FTIR spectra of monomer 1 and membrane 1 prepared in Example 1. As can be seen from the figure, after monomer polymerization, at 2225 cm⁻¹... -1The characteristic peaks of the cyano group (-CN) almost disappeared, while the peaks at 1300-1500 cm⁻¹ disappeared. -1 The presence of a characteristic absorption peak for the triazine group in the range indicates that the cyano group has successfully undergone a cyclotrimerization reaction to transform into the triazine group, the polymerization reaction is complete, and the product has a high degree of polymerization.

[0031] Figure 3 shows the XPS N1s spectra of monomer 1 and membrane 1 prepared in Example 1. The N1s spectrum of monomer 1 shows a characteristic singlet of -CN at 398.9 eV, while the N1s spectrum of membrane 1 shows characteristic doublets at 398.5 eV (-CN) and 400.0 eV (C=NC), indicating that a triazine ring structure was formed in the material after polymerization, and a small amount of unreacted cyano groups are also present, which is consistent with the FTIR characterization results.

[0032] Figure 4 shows the XPS full spectrum of monomer 1 and membrane 1 prepared in Example 1. The XPS full spectrum of monomer 1 clearly shows characteristic orbital peaks of K2p and S2p, confirming the presence of potassium and sulfur elements on the material surface. Peak fitting of the S2p spectrum reveals characteristic binding energies of 168.5 eV and 169.7 eV, respectively, indicating that sulfur binds in the form of -SO3. ⁻ The presence of this form confirms that the negatively charged sulfonate group has been successfully introduced into the monomer molecule, and that this group is stably retained during the polymerization process.

[0033] Figure 5 shows an optical image of film 1 prepared in Example 1. The film is pale yellow and semi-transparent, with a smooth and flat surface, a relatively hard texture, and a resin-like luster. Slight reflection is visible in some areas. There are no obvious defects such as cracks or pores, and the film formation effect is good.

[0034] Figure 6 shows the surface SEM image of membrane 1 prepared in Example 1. Its surface exhibits an uneven rough morphology with fine bumps and wrinkles. A small number of scattered microparticles and aggregates are attached locally. At the same time, blurred fibrous / flocculent fine structures are intertwined. The overall morphology is random composite and does not form a regular and ordered crystal structure, which is consistent with the morphological characteristics of conjugated microporous polymers.

[0035] Figure 7 shows a cross-sectional SEM image of membrane 1 prepared in Example 1. The cross-section shows a layered / blocky solid structure with a uniform overall thickness of 137.0 μm. The cross-sectional surface has a fine rough texture, with a small amount of particulate aggregates attached in local areas. There are slight undulations at the upper and lower edges, with no obvious porous structure or layer gaps. Overall, it exhibits a relatively dense solid substrate morphology, which can ensure the structural stability and mechanical properties of the membrane.

[0036] Comparative Example 1: This comparative example prepared a covalently triazine-based uncharged conjugated microporous polymer membrane according to the following steps, and compared its film-forming properties with a covalently triazine-based negatively charged conjugated microporous polymer membrane: S1, 4,4'-biphenyladionitrile was selected as the cyanophenyl monomer.

[0037] S2. Preparation of the polymerization system: Add 1.5 mL (17.1 mmol) of TFSA to a high-vacuum reaction tube and freeze the tube in liquid nitrogen to lower the system temperature to below 0°C. Then add 0.24 g (1.176 mmol) of 4,4'-biphenyl dinitrile selected in step S1. Remove the reaction vessel from the liquid nitrogen and stir for 20 min under N2 atmosphere to make the system slightly viscous. Then add 1.5 mL (17.1 mmol) of trifluoromethanesulfonic acid (TFSA) for dilution and continue stirring for 30 min to obtain a homogeneous polymerization system. The reaction formula is shown below.

[0038] .

[0039] S3. Thermal crosslinking: The polymerization system obtained in step S2 is transferred to a polytetrafluoroethylene (PTFE) culture dish with an inner diameter of 6 cm in a nitrogen bag and spread evenly. The culture dish is placed on a heating table and heated at 100°C for 25 min for preliminary crosslinking. Then, the membrane and the PTFE culture dish are immersed in pure water at 20°C to obtain a powdered polymer that cannot form a film.

[0040] Comparative Example 1 illustrates that when monomers without negatively charged groups are used as raw materials and reacted under the same polymerization conditions, the resulting product is only an amorphous powder, unable to form a continuous and complete polymer film. This is likely because the uncharged monomer molecules only exhibit weak van der Waals forces, resulting in weak intermolecular interactions and low cross-linking, making it difficult to form a continuous and dense polymer network structure. Therefore, effective aggregation and solidification cannot be achieved during film formation, ultimately yielding only a loose powdery product, unable to form a self-supporting membrane or a continuous, dense membrane layer. In contrast, the negatively charged polymer membrane prepared using monomers with introduced negatively charged groups not only successfully forms a dense membrane layer with a uniform structure and complete surface, but also exhibits excellent gas permeation and separation performance. The comparative results demonstrate that the introduction of negatively charged groups can significantly improve the polymerization behavior and film-forming properties of monomers, which is key to achieving stable film formation and endowing the material with gas separation performance.

[0041] Comparative Example 2: A covalently triazine-based positively charged conjugated microporous polymer membrane was prepared according to the following steps, and its gas separation performance was compared with that of a covalently triazine-based negatively charged conjugated microporous polymer membrane: S1. Preparation of positively charged cyanophenyl monomers: 10 mL (46.9 mmol) of trimethylamine solution (4.2 M ethanol solution) and 25 g (124 mmol) of 1,3-dibromopropane were weighed and mixed and stirred at room temperature for 12 hours. After the reaction was completed, the reaction mixture was diluted with 500 mL of methyl tert-butyl ether (MTBE), the suspended solid was collected by vacuum filtration, the solid was washed with MTBE, and then dried under vacuum at 70 °C to obtain trimethyl(3-bromopropyl)ammonium bromide. 1.37 g (5.24 mmol) of trimethyl(3-bromopropyl)ammonium bromide, 0.5 g (2.27 mmol) of 3-hydroxy-[1,1'-biphenyl]-4,4'-dicarboxynitrile, 0.95 g (6.87 mmol) of potassium carbonate, and 10 mL of DMAc were weighed and added to a high-vacuum reaction tube. After mixing thoroughly, the mixture was suspended and heated to 80 °C under nitrogen protection and reacted for 12 h. After the reaction was completed, deionized water was added to the reaction vessel, and the white solid was collected by filtration. The white solid was placed in a vacuum drying oven and dried for 24 h at 70 °C and a vacuum degree ≤ -0.09 MPa to obtain the positively charged dicyano monomer 3-((4,4'-dicyano[1,1'-biphenyl]-3-yl)oxy)-N,N,N-trimethylpropyl-1-ammonium bromide (hereinafter referred to as monomer 2). The reaction formula is shown below.

[0042]

[0043] S2. Preparation of the polymerization system: Add 1.5 mL (17.1 mmol) of TFSA to a high-vacuum reaction tube and freeze the tube in liquid nitrogen to lower the system temperature to below 0°C. Then add 0.08 g (0.2 mmol) of monomer 2 and 0.12 g (0.588 mmol) of 4,4'-biphenyl dinitrile obtained in step S1. Remove the reaction vessel from the liquid nitrogen and stir for 20 min under N2 atmosphere to make the system slightly viscous. Then add 1.5 mL (17.1 mmol) of trifluoromethanesulfonic acid (TFSA) for dilution and continue stirring for 30 min to obtain a homogeneous polymerization system. The reaction formula is shown below.

[0044]

[0045] S3. Film Formation: The polymerization system obtained in step S2 is transferred to a polytetrafluoroethylene (PTFE) culture dish with an inner diameter of 6 cm in a nitrogen bag and spread evenly. The culture dish is placed on a heating table and heated at 100°C for 25 min to form a preliminary film. Then, the film, together with the PTFE culture dish, is immersed in pure water at 20°C for rapid cooling and shaping to obtain a complete film.

[0046] S4. Post-treatment: After separating the membrane from the culture dish, it is first soaked in pure water at 20℃ for 24 h (the pure water is changed every 12 h), and then transferred to NMP at 20℃ for 24 h (the NMP is changed every 12 h). After post-treatment, a covalent triazine unit base band positively charged conjugated microporous polymer membrane (hereinafter referred to as membrane 2) is finally obtained.

[0047] To evaluate the permeation performance of pure gas under specific conditions, a series of tests were conducted using a pressure-swing, constant-volume gas permeation cell. Before the experiment, the membrane samples from Example 1 and Comparative Example 2 were mounted on the permeation cell and subjected to overnight vacuum treatment to ensure the purity of the test environment. The temperature was strictly controlled at 25°C throughout the entire testing process.

[0048] The permeability of gas through the membrane is calculated by measuring the steady-state pressure increment (dp / dt) and substituting it into the following formula:

[0049] In this formula, P represents the gas permeability, measured in Barrers (1 Barrer equals 1 × 10⁻⁶). -10 cm³ (STP) cm cm -2 s -1 cmHg -1 V is the downstream reservoir volume, in cm³. 3 A represents the effective membrane area, in cm². 2 ; l is the membrane thickness in cm; T is the test temperature in Kelvin (K); P2 is the upstream pressure of the system; dp / dt is the rate of change of pressure p with time t during the test, i.e., the steady-state pressure increment.

[0050] Furthermore, the membrane's selectivity to two different gases was evaluated using ideal gas selectivity (α), where α is defined as the ratio of the permeability of gas A to that of gas B, calculated using the following formula:

[0051] Among them, P A and P B These represent the permeability of gas A and gas B, respectively.

[0052] These detailed tests and calculations provide a comprehensive understanding of the permeation behavior of different gases in membranes and the selectivity of membranes for specific gases.

[0053] Figure 8 shows the gas separation performance test results of the polymer membranes prepared in Example 1 and Comparative Example 2. The gas flux data show that membrane 1 prepared in Example 1 exhibits high permeability for H2 and CO2, reaching 1322 Barrer and 2278 Barrer respectively, while its permeability for N2, CH4, C2H4, and C2H6 is lower, at 71, 76, 102, and 159 Barrer respectively. Membrane 1 also demonstrates high separation selectivity for CO2, N2, CH4, C2H4, and C2H6, with the ideal gas selectivity α for CO2 / N2 reaching 32. In the 2008 Robeson upper limit diagram, the separation performance of this membrane exceeded the upper limit, indicating that it possesses both high gas permeation flux and high separation selectivity, demonstrating excellent gas separation performance. In contrast, membrane 2 prepared in Example 2 has a CO2 permeability of only 420 and an ideal gas selectivity α for CO2 / N2 of only 15.5. Therefore, compared with positive charge functionalization, negative charge functionalization significantly improves both the permeability of CO2 through the membrane and the selectivity of the membrane for CO2.

[0054] The above description is merely a preferred embodiment of the present invention, and while it is quite specific and detailed, it should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various improvements and equivalent substitutions without departing from the principles of the present invention, and these improvements and substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a covalently triazine unit-based conjugated microporous polymer membrane, characterized in that, Includes the following steps: S1. Preparation of negatively charged cyanophenyl monomers: A cyanophenyl compound, 1,4-butanesulfonyl lactone, and potassium carbonate are dissolved in an organic solvent, mixed thoroughly, and then heated under nitrogen protection. After the reaction is complete, a white solid is collected by filtration and vacuum dried to obtain the negatively charged cyanophenyl monomer. S2. Preparation of the polymerization system: Trifluoromethanesulfonic acid is first added to the reaction vessel, which is then placed in liquid nitrogen for freezing. The negatively charged cyanophenyl monomer and biphenyl nitrile monomer obtained in step S1 are then added. The reaction vessel is removed from the liquid nitrogen and stirred for the first time under N2 atmosphere until... The system is viscous, so trifluoromethanesulfonic acid is added for dilution, followed by a second stirring to obtain a homogeneous polymerization system; S3, film formation: the polymerization system obtained in step S2 is transferred to a polytetrafluoroethylene culture dish in a nitrogen bag and spread evenly. The culture dish is placed on a heating platform for preliminary film formation; then the film, along with the polytetrafluoroethylene culture dish, is placed in pure water for rapid cooling and shaping to obtain a complete film; S4, post-treatment: after separating the film from the culture dish, it is first soaked in pure water, then transferred to N-methylpyrrolidone for further soaking, followed by post-treatment, finally obtaining a covalent triazine unit base with negatively charged conjugated microporous polymer film.

2. The preparation method according to claim 1, characterized in that: The cyanophenyl compounds are 3-hydroxy-[1,1'-biphenyl]-4,4'-dicarboxynitrile as shown in formula (1) or 4,4'-dicyano-4''-hydroxytriphenylamine as shown in formula (2): (1) ； (2)。 3. The preparation method according to claim 2, characterized in that: The biphenyl nitrile monomer is 4,4'-biphenyl dinitrile with the structural formula shown in formula (3): (3)。 4. The preparation method according to claim 1, characterized in that: In step S1, the molar ratio of cyanophenyl compounds, 1,4-butanesulfonyl lactone, and potassium carbonate is 1:1.05~4.2:3~12.

5. The preparation method according to claim 1, characterized in that, In step S1: the heating reaction temperature is 100~120℃ and the reaction time is 12 h~24 h; the vacuum drying temperature is 60~80℃, the vacuum degree is ≤ -0.09MPa, and the drying time is 24 h~48 h.

6. The preparation method according to claim 1, characterized in that: In step S2, the molar ratio of the negatively charged cyanophenyl monomers, biphenyl nitrile monomers, and total trifluoromethanesulfonic acid is 1:1.1~3.2:84~168, and the amount of trifluoromethanesulfonic acid added in the two steps is equal.

7. The preparation method according to claim 1, characterized in that, In step S2: the time for the first and second stirring is 10~30 min.

8. The preparation method according to claim 1, characterized in that: In step S3, the temperature of the heating platform is 70~100 ℃.

9. The preparation method according to claim 1, characterized in that, In step S4: the soaking time in pure water is 24~48h, the pure water temperature is 10~25℃, and the pure water is changed every 12h during the soaking process; the soaking time in N-methylpyrrolidone is 24~48h, the N-methylpyrrolidone temperature is 10~25℃, and the N-methylpyrrolidone is changed every 12h during the soaking process.

10. A covalent triazine unit-based conjugated microporous polymer membrane prepared by the preparation method according to any one of claims 1 to 9.

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