A method for preparing a covalent organic framework membrane for molecular separation and applications thereof

Abstract

The application belongs to the field of membrane separation technology and new materials, and discloses a preparation method of a covalent organic framework membrane for molecular separation and application thereof. A continuous, defect-free, high-crystallinity ultrathin COF active separation layer is formed on a porous support base membrane under mild conditions through a synergistic reaction of ligand exchange and interfacial polymerization under acid catalysis. COF powder is dispersed in an acid-containing aqueous solution or organic solution, and monomers are dissolved in another mutually insoluble solvent; the two-phase solutions are contacted at the interface of the two sides of the porous base membrane, and the reaction is carried out under heating conditions; after washing and drying, a COF membrane with separation function is obtained. The COF membrane obtained by the application has a dye molecule rejection rate of greater than 99.5%, a drug molecule rejection rate of greater than 98%, a water flux of higher than 150 L·m ‑2 ·h ‑1 ·bar ‑1 , and excellent performance in dye / salt selective separation and long-term operation, effectively breaking the trade-off effect between permeability and selectivity of traditional polymer membranes.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology and new materials, and discloses a method for preparing COF membranes based on a dynamic conversion strategy from covalent organic framework (COF) powder to membrane, and its application in water pollutant treatment, molecular separation and other fields. Background Technology

[0002] Dyes are widely used in the textile, printing and dyeing, and leather industries. The large amounts of wastewater generated during their production and use are complex in composition, highly chromatic, and have strong biological toxicity. Improper treatment can cause serious water pollution. In the pharmaceutical industry, the efficient separation and purification of target drug molecules (such as antibiotics and hormones) from fermentation broths or synthetic mixtures directly affects the purity, potency, and economic benefits of the product. Traditional separation methods, such as adsorption, flocculation, chemical oxidation, distillation, and chromatographic separation, generally suffer from problems such as complex processes, high energy consumption, limited selectivity, easy generation of secondary pollution, or difficulty in treating low-concentration wastewater.

[0003] Membrane separation technology, especially nanofiltration, has shown great potential in fields such as dye desalination, dye fractionation, and drug concentration and purification due to its non-phase-change process, simple operation, relatively low energy consumption, and ease of scale-up. This technology achieves separation through the pore size sieving of the membrane material and the charge interaction between the membrane material and the solute (the Doonnan effect). However, most mainstream commercial separation membranes are made from polymers such as polyamide, polysulfone, and polyethersulfone via phase inversion or interfacial polymerization. These polymer membranes generally suffer from an intrinsic performance limitation, namely the well-known "permeability-selectivity trade-off." This stems from the fact that the separation layer of the polymer membrane is composed of randomly cross-linked polymer chains, forming a pore structure with a wide pore size distribution, tortuous mass transfer pathways, and non-uniformity. This disordered structure leads to two contradictory results: firstly, to achieve high selective retention of small molecules, a denser membrane structure is needed, but this significantly increases mass transfer resistance, causing a sharp decrease in membrane water flux; conversely, increasing membrane pore size or decreasing density to improve flux sacrifices separation accuracy. Precise separation of pollutants in water (such as dye desalination, dye fractionation, and drug concentration) places higher demands on the pore size uniformity, surface charge characteristics, and chemical stability of membranes, while the disordered channels of traditional polymer membranes are difficult to meet such "precision sieving" requirements.

[0004] Covalent organic frameworks (COFs) exhibit great potential in molecular separation membranes due to their highly ordered pores, ultra-high porosity, functionalizable structure, and good stability, and are expected to overcome the limitations of the traditional polymer trade-off effect. Therefore, COFs are considered ideal candidates for constructing next-generation high-performance molecular separation membranes to overcome the traditional "trade-off effect." However, the key to their practical application lies in how to prepare continuous, defect-free, highly crystalline, and thin-layered COF separation membranes. Existing preparation methods (such as solvothermal methods, interfacial polymerization methods, and vacuum filtration methods) often face challenges such as demanding conditions, complex processes, difficulty in forming a complete active layer on a large-area porous support, or disordered stacking of membrane layers that prevent the utilization of intrinsic pore advantages. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention proposes a method for preparing COF membranes based on a dynamic transformation strategy from covalent organic framework (COF) powder to membrane, and its applications. This method utilizes crystalline COF powder to form a continuous, defect-free, and highly crystalline ultrathin COF active separation layer on a porous support substrate under mild conditions through a synergistic reaction of ligand exchange and interfacial polymerization under acid catalysis. This method is simple, operates under mild conditions, and offers controllable structure, enabling the preparation of various 2D / 3D COF membranes with different pore sizes. The prepared COF membranes have pore sizes of 0.5 nm–2.0 nm and thicknesses of 200–400 nm, exhibiting a good balance between permeability and selectivity, making them suitable for various water pollutant treatment and molecular separation applications.

[0006] The steps for preparing the COF membrane using this method are as follows: Step 1: Preparation of COF powder dispersion: Add COF powder to deionized water or a solvent insoluble in water, then add acetic acid as a catalyst, and sonicate to form a uniform dispersion. Step 2, Preparation of monomer solution: Dissolve the aldehyde or amino monomer to be reacted in an organic or aqueous solvent that is immiscible with the dispersion from Step 1, and sonicate to form a clear monomer solution. Step 3, Interface Conversion and Film Formation: The polyacrylonitrile ultrafiltration membrane treated with alkali hydrophilization is fixed in the middle of a two-chamber diffusion cell; the dispersion prepared in Step 1 and the monomer solution prepared in Step 2 are injected into both sides of the membrane in the diffusion cell to ensure that the interface between the two phases is in contact with the surface of the membrane; the entire reaction apparatus is placed in an oven at 40-80 °C and reacted for 12-36 hours to convert the COF powder interface into a continuous COF membrane layer; after the reaction is completed, the membrane with the COF membrane layer is taken out and washed repeatedly with methanol 3-5 times to remove residual solvent and unreacted monomer, and then dried at room temperature for 12-24 hours to obtain the COF membrane.

[0007] Furthermore, in the preparation method described above: In step 1, the COF powder is one of LZU1, TbBD, and TpPa, and its synthesis method includes: using the corresponding aldehyde monomer and amino monomer as reactants, dissolving them in a 1,3,5-dimethylformamide solution with a volume ratio of 3:1. In trimethylbenzene and 1,4-dioxane solvent, under the catalysis of 3 M-6 M acetic acid at 120 °C o After a 3-day solvothermal reaction, the corresponding COF crystal powder was obtained by washing with methanol. LZU1 powder was synthesized using trimesin (Tb) and p-phenylenediamine; TbBD powder was synthesized using trimesin and benzidine; and TpPa powder was synthesized using tricarboxymethyl phloroglucinol and p-phenylenediamine.

[0008] In step 1, the COF crystal powder is prepared into a dispersion with a mass-volume concentration of 0.1-0.3 mg / ml, the dispersion volume is 10-30 mL, the acid accounts for 5%-15% of the dispersed phase volume, and the ultrasonic time is 5-15 minutes.

[0009] In step 1, the COF dispersion preferably has a mass-volume concentration of 0.2 mg / ml and a dispersion volume preferably of 20 ml. The acid is selected from acetic acid, p-toluenesulfonic acid, p-hydroxybenzenesulfonic acid, and n-octanoic acid.

[0010] In some preferred embodiments, the acid is selected from acetic acid, which preferably accounts for 10% of the dispersed phase volume, and the ultrasonic time is preferably 10 minutes.

[0011] The organic solvents used in steps 1 and 2 are selected from 1, 3, and 5. Trimethylbenzene, o-dichlorobenzene, 1,4- One or more of dioxanes, and the solvent in step 1 and the solvent in step 2 are immiscible.

[0012] In some embodiments, such as step 1, the organic solvent is selected from 1, 3, 5. Trimethylbenzene, o-dichlorobenzene, 1,4- If the dioxane is one of the dioxanes, then the aqueous solvent in step 2 is selected from deionized water.

[0013] In step 2, the monomer solution is prepared at a mass-volume concentration of 0.1-0.3 mg / mL, and the sonication time is 5-15 minutes.

[0014] In step 2, the preferred concentration of the monomer solution is 0.2 mg / ml, the preferred volume is 20 ml, the preferred acetic acid content in the dispersed phase is 10%, and the preferred sonication time is 10 minutes.

[0015] In step (2), the aldehyde monomer is selected from any one of the polyaldehyde small molecules such as triformyl phloroglucinol (Tp), pyromellitic pyroxenaldehyde (Tb), tetra(4-aminophenyl)methane (TAM), or tetra(4-formylphenyl)methane (TFM); the amino monomer is selected from any one of the polyamino small molecules such as benzidine (BD), p-phenylenediamine (Pa), 2,5-diaminophenyl-1,4-diol dihydrochloride (DABD), or p-phenylenediamine sulfonic acid (Pa-SO3H). The organic solvent is selected from 1, 3, 5 Trimethylbenzene, o-dichlorobenzene, 1,4- One or more of dioxanes, wherein the aqueous solvent is selected from deionized water.

[0016] By selecting different COF powders in step 1 and different monomers in step 2, COF films with different valence bonds, topologies, and pore sizes can be prepared, including but not limited to: When the COF powder is LZU1 and the aldehyde monomer is triformyl phloroglucinol (Tp), a TpPa film is obtained; When the COF powder is LZU1 and the amino monomer is benzidine (BD), a TbBD film is obtained; When the COF powder is LZU1, the aldehyde monomer is tetra(4) When (formylphenyl)methane (TFM) is used, a COF-303 membrane is obtained; When the COF powder is TbBD and the amino monomer is p-phenylenediamine (Pa), an LZU1 film is obtained; When the COF powder is TpPa and the amino monomer is 2,5-diaminophenyl-1,4-diol dihydrochloride (DABD), a TbBBO membrane is obtained.

[0017] The porous support membrane mentioned in step 3 is a polyacrylonitrile ultrafiltration membrane, which needs to be treated with 1.0-2.0 M sodium hydroxide solution at 50-70℃ for 30-60 minutes before use.

[0018] In step 3, the reaction temperature is preferably 80 °C, and the reaction time is preferably 24 hours. The two-chamber diffusion cell can be replaced with any device capable of achieving stable liquid-liquid interface contact and fixing the base film.

[0019] The method can be used to prepare large-area membranes, and uniform COF membranes with a size of up to 25 cm × 25 cm can be prepared by using a customized molding die.

[0020] The COF membrane prepared according to this invention was used for nanofiltration molecular separation at a flow rate of 100 L / m³. -2Under a feed pressure of 2.5 bar, the rejection rate for dye molecules with a molecular weight greater than 300 Da (including Congo Red, Methyl Blue, Basic Fuchsin, Rhodamine B, Alcian Blue, Acid Orange, etc.) is higher than 99.5%, and the rejection rate for specific drug molecules with a molecular weight greater than 300 Da (such as Indomethacin, Ceftriaxone Sodium, Ofloxacin, Cefoperazone Sodium, Cefepime Sulfate, etc.) can reach more than 98%, with a water flux greater than 150 L·m -2 ·h -1 ·bar -1 It can achieve highly selective separation of dye and inorganic salt mixtures.

[0021] Compared with the prior art, the beneficial effects of the present invention are: The COF powder-to-COF membrane preparation method of this invention is the first to use pre-synthesized COF powder as a reaction template and growth starting point. Utilizing its dynamic covalent exchange and polymerization at the two-phase interface, a continuous, highly crystalline COF separation layer is controllably grown on a porous substrate membrane under mild conditions. This method is simple and operates under mild conditions (atmospheric pressure and medium temperature), avoiding the high temperature and high pressure of traditional solvothermal methods or the stringent kinetic requirements of interfacial polymerization. The raw materials are readily available and highly versatile, allowing for flexible preparation of COF membranes with various pore sizes and structures by changing the monomers. The prepared COF membrane layer has adjustable thickness and possesses characteristics of dense structure, regular pore size, and no macroscopic defects, while also exhibiting excellent chemical and mechanical stability. When used for nanofiltration molecular separation, the membrane water flux exceeds 150 L·m⁻¹. -2 ·h -1 ·bar -1 It exhibits a retention rate of over 99.5% for dye molecules (such as Congo red) and over 98% for drug molecules, and demonstrates high selectivity in dye / salt mixtures. Its separation performance remains stable over long-term operation, providing an innovative material and efficient preparation method to address the "trade-off" effect between permeability and selectivity in traditional polymer membranes. It shows broad application prospects in water treatment and precision molecular separation. Attached Figure Description

[0022] Figure 1 This is a scanning electron microscope image of the membrane obtained in Example 1 of the specific implementation method.

[0023] Figure 2 This is a scanning electron microscope image of the membrane obtained in Example 2 of the specific implementation method.

[0024] Figure 3 This is a scanning electron microscope image of the membrane obtained in Example 3 of the specific implementation method.

[0025] Figure 4 This is a scanning electron microscope image of the membrane obtained in Example 4 of the specific implementation method.

[0026] Figure 5 This is a scanning electron microscope image of the membrane obtained in Example 5 of the specific implementation method.

[0027] Figure 6 The image shown is a scanning electron microscope image of the membrane obtained in Comparative Example 1 of the specific implementation method.

[0028] Figure 7 The X-ray diffraction results are for the films obtained in all embodiments of the specific implementation method.

[0029] Figure 8 These are the Fourier transform micro-infrared spectroscopy results of the films obtained in all embodiments of the specific implementation method. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that the following embodiments are merely preferred implementations of this invention and are not intended to limit the invention. All equivalent changes or modifications made based on the essence of this invention should be included within the scope of protection of this invention.

[0031] This invention proposes a method for preparing covalent organic framework (COF) membranes based on a dynamic transformation strategy from COF powder to COF membranes. Using pre-synthesized crystalline COF powder as a precursor and reaction template, this method achieves in-situ growth of a continuous, defect-free, and highly crystalline ultrathin COF active separation layer on a porous support substrate under mild conditions through acid-catalyzed dynamic covalent exchange and interfacial polymerization. This method is simple to operate, operates under mild conditions, and has strong versatility; the prepared membrane exhibits excellent balance between permeability and selectivity.

[0032] The technical solution of the present invention will be further described in detail below with reference to specific embodiments and comparative examples. The specific implementation examples described are only for explanation and illustration of the present invention and are not intended to limit the present invention.

[0033] Example 1: The steps for preparing TpPa membranes from LZU1 powder by conversion are as follows: Step 1: Synthesis of LZU1 powder and preparation of solution: 48 mg (0.30 mmol) of 1,3,5-tricarboxymethylbenzene (Tb) and 48 mg (0.45 mmol) of p-phenylenediamine (Pa) were weighed and placed in a 20 mL pressure-resistant reaction tube. 0.75 mL of 1,4-dioxane and 2.25 mL of 1,3,5-trimethylbenzene were added as a mixed solvent. Then, 0.6 mL of a 3.0 M aqueous acetic acid solution was added as a catalyst. The reaction tube was subjected to three cycles of freezing-vacuuming-thawing to remove oxygen, and finally, the reaction was carried out in an oil bath at 120 °C for 72 hours. After the reaction was completed, the yellow solid was collected by centrifugation, washed three times with methanol, and dried under vacuum at 80 °C for 12 hours to obtain LZU1 powder.

[0034] Weigh 4 mg (approximately 0.02 mmol) of LZU1 powder synthesized in step 1, add it to 20 mL of deionized water, then add 2 mL of glacial acetic acid, and sonicate for 10 minutes to form a uniform pale yellow suspension, which is used as the aqueous dispersion.

[0035] Step 2, Preparation of monomer solution: Weigh 4.2 mg (0.02 mmol) of tricarboxymethyl phloroglucinol (Tp), dissolve it in a mixed organic solvent consisting of 18 mL of 1,3,5-trimethylbenzene and 2 mL of o-dichlorobenzene, and sonicate for 10 minutes until completely dissolved to obtain a clear organic monomer solution.

[0036] Step 3, Interface Conversion and Film Formation: Take a polyacrylonitrile (PAN) ultrafiltration membrane that has been treated with 1.5 M NaOH solution at 60 °C for 40 minutes and fix it tightly in the middle of a custom two-chamber diffusion cell. Inject the aqueous dispersion prepared in step (1) into one side of the membrane (defined as the back side) and the organic monomer solution prepared in step (2) into the other side of the membrane (defined as the front side), ensuring that the interface between the two phases is in full contact with the surface of the membrane. Place the entire device in an oven at 80 °C and allow it to stand for 24 hours.

[0037] After the reaction was complete, the COF membrane supported by the red COF layer was carefully removed, and then immersed in anhydrous methanol and gently washed three times for 10 minutes each time to remove residual solvent and unreacted monomers. Finally, the membrane was allowed to air dry at room temperature for 12 hours to obtain the TpPa-COF membrane, denoted as membrane 1.

[0038] Membrane 1 was observed using a scanning electron microscope, such as... Figure 1 As shown in the figure, the synthesized COF film is continuous and defect-free, with a thickness of 280 nm.

[0039] Membrane 1 was used for nanofiltration dye retention treatment, with a feed solution of 100 ppm Congo red and a flow rate of 100 L / m³.-2 At a feed pressure of 2.5 bar, the water flux is 178.98 L·m. -2 ·h -1 ·bar -1 The Congo red rejection rate reached 99.47%, and its separation performance remained stable within 72 hours, demonstrating good long-term operational stability.

[0040] Example 2: The preparation steps for TbBD films using LZU1 powder via conversion are basically the same as in Example 1, with the only difference being: In step 1, 4 mg (approximately 0.02 mmol) of synthesized LZU1 powder was weighed and added to 20 mL of 1,3,5-trimethylbenzene, followed by the addition of 1 mL of glacial acetic acid. The mixture was sonicated for 10 minutes to form a uniform, pale yellow suspension, which served as the organic phase dispersion. In step 2, 5.6 mg (0.03 mmol) of benzidine (BD) was weighed and dissolved in 20 mL of deionized water to form the aqueous phase solution. The resulting TbBD membrane was designated as membrane 2.

[0041] Membrane 2 was observed using a scanning electron microscope, such as... Figure 2 As shown in the figure, the synthesized COF film is continuous and defect-free, with a thickness of 280 nm.

[0042] Membrane 2 was used for nanofiltration dye retention treatment, with a feed solution of 100 ppm Congo red and a flow rate of 100 L / m³. -2 At a feed pressure of 2.5 bar, the water flux is 135.31 L·m. -2 ·h -1 ·bar -1 The Congo red interception rate reached 99.03%.

[0043] Example 3: The preparation steps for COF-303 membranes by conversion of LZU1 powder are basically the same as in Example 1, with the only difference being: In step 2, weigh 4.2 mg (0.02 mmol) tetrakis(4) (Formylphenyl)methane (TFM) was dissolved in 20 mL of deionized water as the aqueous phase solution. The resulting COF-303 membrane was designated as membrane 3.

[0044] Membrane 3 was observed using a scanning electron microscope, such as... Figure 3 As shown in the figure, the synthesized COF film is continuous and defect-free, with a thickness of 230 nm.

[0045] Membrane 3 was used for nanofiltration dye retention treatment, with a feed solution of 100 ppm Congo red and a flow rate of 100 L / m³.-2 At a feed pressure of 2.5 bar, the water flux is 110.2 L·m. -2 ·h -1 ·bar -1 The Congo red interception rate reached 99.42%.

[0046] Example 4: The preparation steps for LZU1 membranes by conversion of TbBD powder are basically the same as in Example 1, with the only difference being: In step 1, 6 mg (approximately 0.02 mmol) of synthesized TbBD powder was weighed and added to 20 mL of 1,3,5-trimethylbenzene, followed by the addition of 1 mL of glacial acetic acid. The mixture was sonicated for 10 minutes to form a uniform, pale yellow suspension, which served as the organic phase dispersion. In step 2, 2.1 mg (0.02 mmol) of p-phenylenediamine (Pa) was weighed and dissolved in an aqueous solvent consisting of 20 mL of deionized water. The solution was sonicated for 10 minutes until completely dissolved, serving as the aqueous monomer solution. The resulting LZU1 membrane was designated as membrane 4.

[0047] Membrane 4 was observed using a scanning electron microscope, such as... Figure 4 As shown in the figure, the synthesized COF film is continuous and defect-free, with a thickness of 260 nm.

[0048] Membrane 4 was used for nanofiltration dye retention treatment, with a feed solution of 100 ppm Congo red and a flow rate of 100 L / m³. -2 At a feed pressure of 2.5 bar, the water flux is 136.14 L·m. -2 ·h -1 ·bar -1 The Congo red interception rate reached 99.12%.

[0049] Example 5: The preparation steps for TbBBO membranes by conversion of TpPa powder are basically the same as in Example 1, with the only difference being: In step 1, 4 mg (approximately 0.02 mmol) of the synthesized TpPa powder was weighed and added to 20 mL of 1,3,5-trimethylbenzene, followed by the addition of 1 mL of glacial acetic acid. The mixture was sonicated for 10 minutes to form a uniform, pale yellow suspension, which served as the organic phase dispersion. In step 2, 2,5-trimethylbenzene powder was weighed... Diaminobenzene-1,4-diol dihydrochloride (DABD) was dissolved in an aqueous solvent consisting of 20 mL of deionized water and sonicated for 10 minutes until completely dissolved to obtain an aqueous monomer solution. The resulting TbBBO membrane is designated as membrane 5.

[0050] Membrane 5 was observed using a scanning electron microscope, such as... Figure 5As shown in the figure, the synthesized COF film is continuous and defect-free, with a thickness of 290 nm.

[0051] Membrane 5 was used for nanofiltration dye retention treatment. With a feed solution of 100 ppm Congo red, a flow rate of 100 L m⁻², and a pressure of 2.5 bar, the water flux was 156.27 L·m⁻². -2 ·h -1 ·bar -1 The Congo red interception rate reached 99.02%.

[0052] Comparative Example 1: The preparation steps of Comparative Example 1 are basically the same as those of Example 1, with the only difference being: In the monomer solution preparation in step 2, no monomers are added and only the same organic phase solution is retained; the final COF membrane is referred to as the control membrane.

[0053] The contrast membrane was observed using a scanning electron microscope, such as... Figure 5 As shown in the figure, there is no obvious COF film layer on the membrane surface, and there are defects of different sizes.

[0054] Table 1 compares the performance of the membranes prepared in Examples 1-4 and Comparative Example 1.

[0055]

[0056] Table 1 shows the composition and separation performance of different COF membranes prepared from COF powder using an interfacial dynamic conversion strategy. The comparative examples show that a defect-free COF membrane cannot be formed without the addition of exchange monomers. This demonstrates that this method can successfully prepare continuous, defect-free COF membranes, which exhibit excellent dye rejection and water flux in nanofiltration separation.

[0057] The present invention has been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many modifications under the guidance of the present invention without departing from the spirit of the present invention, and these modifications are all within the protection scope of the present invention.

Claims

1. A method for preparing a covalent organic framework membrane for molecular separation, characterized in that, Using pre-synthesized crystalline COF powder as raw material, a COF active separation layer is formed on a porous support membrane through a dynamic transformation reaction at the organic-water two-phase interface; the method includes the following steps: (1) Add COF powder to the solvent, then add acetic acid as a catalyst, and sonicate to form a uniform dispersion; (2) Dissolve the aldehyde monomer or amino monomer in a solvent that is immiscible with the dispersion described in step (1) to form a monomer solution; (3) Fix the porous support base membrane in the reaction device so that the dispersion in step (1) and the monomer solution in step (2) are on both sides of the base membrane and form a liquid-liquid interface. (4) The reaction is carried out under heating conditions, so that the COF powder undergoes dynamic covalent bond exchange and polymerization at the interface, and is transformed into a continuous COF film on the base film; (5) After the reaction is complete, the covalent organic framework membrane is obtained by washing and drying.

2. The preparation method according to claim 1, characterized in that, The COF powder is selected from at least one of LZU1, TbBD, or TpPa; the acid is selected from one of acetic acid, p-toluenesulfonic acid, p-hydroxybenzenesulfonic acid, and n-octanoic acid; the organic solvent is selected from 1, 3, 5 Trimethylbenzene, o-dichlorobenzene, 1,4- One or more of dioxanes, wherein the aqueous solvent is selected from deionized water.

3. The preparation method according to claim 1 or 2, characterized in that, In step (1), the amount of acid added is 5%-15% of the volume of the dispersed phase, and the dispersion concentration of the COF powder is 0.1-0.3 mg / mL.

4. The preparation method according to claim 1, characterized in that, In step (2), the concentration of the monomer solution is 0.1-0.3 mg / mL; The aldehyde monomer is selected from any one of triformylphloroglucinol, pyromellitic pyroxenaldehyde, tetra(4-aminophenyl)methane, or tetra(4-formylphenyl)methane polyaldehyde small molecules; The amino monomer is selected from any one of the polyamino small molecules such as benzidine, p-phenylenediamine, 2,5-diaminophenyl-1,4-diol dihydrochloride or p-phenylenediamine sulfonic acid (Pa-SO3H); The solvent is selected from the organic solvents selected from 1, 3, 5. Trimethylbenzene, o-dichlorobenzene, 1,4- One or more of dioxanes, wherein the aqueous solvent is selected from deionized water.

5. The preparation method according to claim 1, characterized in that, In step (3), the porous support base membrane is subjected to alkaline hydrophilization treatment before use; preferably, the base membrane is a polyacrylonitrile ultrafiltration membrane, the alkaline solution is a 1.0-2.0 M sodium hydroxide solution, the treatment temperature is 50-70℃, and the treatment time is 30-60 minutes.

6. The preparation method according to claim 1, characterized in that, In step (4), the reaction temperature is 40-80℃ and the reaction time is 12-36 hours; preferably, the reaction temperature is 80℃ and the reaction time is 24 hours.

7. The preparation method according to claim 1, characterized in that, By selecting different COF powders and different monomers, COF films with different structures were prepared; including but not limited to: When the COF powder is LZU1 and the monomer is triformylphloroglucinol, a TpPa membrane is obtained. When the COF powder is LZU1 and the monomer is benzidine, a TbBD membrane is obtained; When the COF powder is LZU1 and the monomer is tetra(4-formylphenyl)methane, a COF-303 film is obtained; When the COF powder is TbBD and the monomer is p-phenylenediamine, an LZU1 membrane is obtained; When the COF powder is TpPa and the monomer is 2,5-diaminophenyl-1,4-diol dihydrochloride, a TbBBO membrane is obtained.

8. The preparation method according to claim 1, characterized in that, The method described is applicable to the preparation of large-area COF membranes, and can produce membranes up to 25 cm² in size using customized molds. A uniform film of 25 cm.

9. A covalent organic framework membrane prepared by the method according to any one of claims 1-8, wherein the prepared COF membrane has a pore size of 0.5 nm-2.0 nm and a thickness of 200-400 nm.

10. An application of the covalent organic framework membrane as described in claim 9 in molecular separation, characterized in that, The membrane is used in nanofiltration processes in water treatment and has a rejection rate of over 99.5% for dye molecules with a molecular weight greater than 300 Da. The rejection rate for drug molecules with a molecular weight greater than 300 Da is higher than 98%, and the water flux is higher than 150 L·m⁻¹. -2 ·h -1 ·bar -1 Furthermore, it achieves highly selective separation of dye-inorganic salt mixtures.

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