Programmable multilayer covalent organic framework film and preparation method and application thereof

Abstract

The invention relates to a programmable multilayer covalent organic framework film and a preparation method and application thereof, and belongs to the technical field of film material preparation. A programmable multilayer covalent organic framework membrane comprises a supporting base membrane and at least two organic framework layers sequentially deposited on the supporting base membrane, and each organic framework layer is prepared by reacting an amine monomer and an aldehyde monomer under the action of a catalyst, any adjacent organic framework layers are connected through covalent bonds formed by condensation reaction of amino groups and aldehyde groups which are not completely reacted at interfaces. By regulating and controlling the monomer type, concentration and reaction time of each layer of film, programmable construction of multiple layers of films in the aspects of layer sequence, thickness and chemical structure is achieved, and the method is suitable for precise design and preparation of high-performance separation films, sensors and catalytic materials.

Description

Technical Field

[0001] This invention relates to a programmable multilayer covalent organic framework membrane, its preparation method, and its application, belonging to the field of membrane material preparation technology. Background Technology

[0002] Covalent organic frameworks (COFs) are a class of porous crystalline materials constructed through covalent bonds between organic monomers. They possess characteristics such as strong structural designability, high specific surface area, and tunable pore size, showing broad application prospects in gas separation, liquid separation, catalysis, and energy storage. Preparing COFs into membrane structures can fully utilize their molecular sieving properties to achieve highly efficient separation processes.

[0003] Currently, the main methods for preparing COF membranes include solvothermal methods, interfacial polymerization, layer-by-layer self-assembly, and in-situ growth. Among these, the solvothermal method typically requires high temperature and high pressure conditions, resulting in harsh reaction conditions and making it difficult to precisely control the membrane thickness and microstructure. While interfacial polymerization can be carried out under relatively mild conditions, it usually only produces single-layer or structurally simple COF membranes, making it difficult to achieve precise construction of multilayer composite structures. Layer-by-layer self-assembly offers a possibility for preparing multilayer COF membranes. For example, patent CN110698710B discloses a method for quantitatively preparing covalent organic framework material thin films through layer-by-layer self-assembly. This method introduces anchoring groups on the substrate surface through substrate pretreatment, then quantitatively sprays a monomer solution onto the substrate for reaction, repeating the spraying reaction process to obtain a covalent organic framework material thin film. However, this method requires complex substrate pretreatment and makes it difficult to precisely control the thickness and chemical structure of each layer. In-situ polymerization is another commonly used method for preparing COF membranes. Patent CN114259879B discloses a covalent organic framework composite membrane and its in-situ polymerization preparation method. This method involves contacting an acidified support membrane with a two-component organic solution containing amine and aldehyde monomers, causing the monomers to undergo in-situ polymerization on the surface of the acidified support membrane. While this method is relatively simple to operate, it is difficult to achieve controllable preparation of multilayer COF membranes, especially in independently controlling the chemical composition and thickness of each layer.

[0004] In recent years, novel preparation methods such as electrochemical and microwave-assisted methods have also been applied to the preparation of COF membranes. Patent CN115105971B discloses a method for electrochemically preparing covalent organic framework composite membranes and its application, which uses electric current to promote the growth of amino and aldehyde monomers on a base membrane to form a COF membrane layer. Patent CN116785952B discloses a covalent organic framework nanofiltration membrane and its preparation method, which uses microwave assistance to achieve a more uniform temperature distribution in a shorter time, thus increasing the synthesis rate. However, these methods are mainly aimed at the preparation of single-layer COF membranes and are difficult to use for the precise construction of multilayer composite structures.

[0005] For separation applications requiring specific functions, such as ion separation, gas separation, or molecular sieving, the surface chemistry and pore structure of membrane materials have a decisive influence on separation performance. Especially in cutting-edge fields such as water treatment, salinity gradient conversion, and carbon dioxide capture, traditional single-layer COF membranes face inherent limitations such as large pore sizes (typically >1 nm) and weak interlayer interactions, making it difficult to effectively sieve high-precision separation targets such as water molecules (0.3 nm) and hydrated salt ions (>0.7 nm). Therefore, introducing multilayer structures has become an important approach to improve the separation performance of covalent organic framework membranes.

[0006] Wang et al. used alternating layers of positively charged DhaTGCl and negatively charged TpPa-COOH COF nanosheets to form a structure with compressed channels (0.7 × 0.4 nm). 2 The composite membrane structure, with its multilayer structure, has an effective pore size reduced to a scale suitable for ion sieving, resulting in significantly improved performance compared to a single-layer structure. Wang, Jingfeng, et al. "Staggered- stacking two-dimensional covalent organic framework membranes for molecular and ionic sieving." ACS nano, 18 (51), 34698-34707. However, membranes prepared by alternating nanosheet assembly have poor mechanical properties.

[0007] In summary, the existing COF multilayer membrane preparation methods have the following main problems: (1) The preparation process is complex and cumbersome, making it difficult to achieve industrial production; (2) It is difficult to accurately control the thickness and chemical structure of each layer in the multilayer COF membrane, which limits its performance optimization in specific application scenarios; (3) The interaction force between the multilayers is weak, making it difficult to meet the requirements for high stability of membrane materials in complex environments.

[0008] Therefore, developing multilayer COF membranes with programmable layer order, controllable thickness and strong interlayer interaction is not only a key way to break through the performance bottleneck of existing COF membranes in precision separation, but also provides new ideas for the design of high-performance membrane materials for energy and environmental applications. Summary of the Invention

[0009] To address the problems of complex preparation processes and insufficient interlayer stability in existing multilayer COF membranes, this invention provides a programmable multilayer covalent organic framework membrane, its preparation method, and its applications. This invention employs a programmable layer-by-layer deposition strategy. First, amine and aldehyde monomers are dissolved in an organic solvent, and an acid catalyst is added to react with the base membrane to form the first covalent organic framework membrane. Subsequently, using this layer as a substrate, amine and / or aldehyde monomers are independently selected for the deposition of subsequent layers. By controlling the type, concentration, and reaction time of each monomer in each layer, the programmable construction of the multilayer membrane in terms of layer sequence, thickness, and chemical structure can be achieved. This is suitable for the precise design and preparation of high-performance separation membranes, sensors, and catalytic materials.

[0010] A programmable multilayer covalent organic framework membrane, the organic framework membrane comprising a supporting substrate membrane and at least two organic framework layers sequentially deposited on the supporting substrate membrane, wherein the organic framework layers are prepared by reacting amine monomers and aldehyde monomers under the action of a catalyst, and any adjacent organic framework layers are connected by covalent bonds formed by the condensation reaction of unreacted amino and aldehyde groups at the interface.

[0011] The programmability described in this invention means that the preparation of each COF membrane layer is regarded as an independent "programming unit". By sequentially executing different "unit operations" (i.e., adjusting the type and concentration of reactants, catalysts and controlling the reaction time), the structure and thickness of each COF membrane layer can be freely designed to achieve functional customization.

[0012] This invention achieves independent control of the thickness of each layer by setting different concentrations of reactant monomers, catalyst types and concentrations, and reaction times for different deposition layers. The thickness control range of a single layer is 1 nm to 10 μm.

[0013] This invention achieves gradient variations or regional specificity of the chemical functions of multilayer membranes by selecting the same or different amine monomers and aldehyde monomers for different deposition layers.

[0014] This invention controls the molar ratio of amino and aldehyde groups in amine monomers and aldehyde monomers to retain unreacted amino or aldehyde functional groups on the surface of the layer. These unreacted functional groups can then react with the amino or aldehyde functional groups in the next covalent organic framework layer to form covalent bonds, thereby enhancing interlayer interactions.

[0015] Furthermore, during at least one deposition process, the reaction solution is mixed according to the molar ratio of amino to aldehyde groups in the amine monomers and aldehyde monomers not being equal to 1:1.

[0016] Preferably, during at least one deposition process, the molar ratio of amino to aldehyde groups is between 1:0.5 and 1.5, excluding 1:1.

[0017] In the above technical solution, the amine monomers are 2,7-diaminofluorene, 1,3,5-tris(4-aminophenyl)benzene, 3,3′-dimethyl-[1,1′-biphenyl]-4,4′-diamine, 2,5-diamino-1,4-benzenediol, tris(4-aminophenyl)triazine, 2,4,6-trihydroxy-1,3,5-benzenetrialdehyde, 5,10,15,20-tetra(4-aminophenyl)porphyrin, [1,1′-biphenyl]-3,3′,5,5′-tetraamine, 2,5-dichloro-4-phenylenediamine, 2,5-dibromo-p-phenylenediamine, 3,3-diaminofluorene, 1,3,5-tris(4-aminophenyl)benzene ... One or more of the following: pyridine-6,6-diamino, 5,5′,5″-(1,3,5-triazine-2,4,6-triyl)tris(pyridine-2-amine), 1,3,6,8-tetra(5-amino-2-pyridyl)pyrene, 2,2′-difluoro-4,4′-diaminobiphenyl, 4,4′-diaminooctafluorobiphenyl, 2,3,6,7,10,11-hexa(4′-aminophenyl)triphenylene, 3,6-diaminocarbazole, 2,6-naphthyldiamine, or 4,4′′-diamino-[1,1′:4′,1′′-triphenyl]-2′,5′-diol.

[0018] Alternatively, the chemical structural formula of the amine monomer is shown below:

[0019] In the above technical solution, the aldehyde monomers are 1,3,5-benzenetriformaldehyde, terephthalaldehyde, 2,5-dimethoxy-1,4-benzenetriformaldehyde, 2,4,6-trihydroxy-1,3,5-benzenetriformaldehyde, 1,2,4,5-benzenetetraaldehyde, 1,10-phenanthroline-2,9-diformaldehyde, 2,5-dibromo-terephthalaldehyde, 2,5-diiodo-terephthalaldehyde, 2,5-difluoro-terephthalaldehyde, 2,3-difluoro-terephthalaldehyde, octafluorobiphenyldiformaldehyde, 4,4′-(1-methyl-1H-benzo[d]imidazol-4,7-diyl)dibenzaldehyde, 2,5-dibutyl-1,4-terephthalaldehyde, and 2,5-bis(2-ethoxyethoxy)-terephthalaldehyde. One or more of the following: dimethylformaldehyde, 2,5-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)terephthalaldehyde, 1,3,5-tris(p-formylphenyl)benzene, 3,3′,3′′-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde, 5,5′,5′′-(benzene-1,3,5-triyl)tri(thiophene-2-carboxaldehyde), 1,3,6,8-tetra(4-carboxylphenyl)pyrene, benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene-2,5,8-trialdehyde, or 3,3′-((2,5-diformyl-1,4-phenylene)bis(oxy))bis(propane-1-sulfonic acid).

[0020] Alternatively, the chemical structural formula of the aldehyde monomer is shown below:

[0021] In the above technical solution, the catalyst is one of acetic acid, p-toluenesulfonic acid, trifluoroacetic acid, or trifluoromethanesulfonic acid.

[0022] In the above technical solution, the material of the supporting base film is selected from metal, ceramic, metal oxide, sulfide, glass or polymer.

[0023] Preferably, the supporting base membrane is a polyacrylonitrile ultrafiltration membrane (PAN) or a polyethersulfone microporous membrane (PES).

[0024] In the above technical solution, the organic framework layer is prepared by the following reaction: amine monomers and aldehyde monomers are dissolved in organic solvents respectively, mixed evenly, a catalyst is added, the mixture is transferred to a container in which the base film has been placed, reacted, washed, and dried to obtain the final product.

[0025] Furthermore, the reaction conditions are: reaction at 10~80℃ for 0.5~6 h.

[0026] Further, the organic solvent is one or more selected from isopropanol, acetonitrile, o-dichlorobenzene, ethylene glycol, toluene, ethyl acetate, dichloromethane, or mesitylene.

[0027] Furthermore, the reaction is filtered every 5 to 30 minutes. This operation helps to remove the nascent crystal nuclei in the solution in a timely manner, promoting the uniform nucleation and growth of the COF layer on the surface of the base film.

[0028] Preferably, the reaction is filtered every 15 minutes.

[0029] Preferably, the volume ratio of the catalyst to the mixed solution is 0.001 to 0.08:1.

[0030] Another object of the present invention is to provide a method for preparing the above-mentioned programmable multilayer covalent organic framework membrane, comprising the following steps: (1) First layer deposition: Amine monomer A and aldehyde monomer A are dissolved in organic solvents respectively, mixed evenly, catalyst is added, and then they are quickly brought into contact with the supporting substrate membrane, reacted, washed, and the first layer of covalent organic framework composite membrane is obtained. (2) Second layer deposition: Amine monomer B and aldehyde monomer B are dissolved in organic solvents respectively, mixed evenly, catalyst is added, and then they are quickly contacted with the first layer covalent organic framework composite membrane obtained in step (1), reacted, washed, and a double layer covalent organic framework composite membrane is obtained; wherein, the amine monomer B and aldehyde monomer B are not completely the same as the amine monomer A and aldehyde monomer A in step (1); (3) Deposition of the nth layer: Optionally, repeat step (2) to construct a multilayer covalent organic framework membrane with a predetermined sequence, independent thickness of each layer and chemical structure by independently selecting the type, concentration and reaction time of amine monomers and aldehyde monomers in each layer.

[0031] Furthermore, the reaction conditions are: reaction at 10~80℃ for 0.5~6 h.

[0032] Furthermore, the solvent used for washing is the same organic solvent used to dissolve amine and aldehyde monomers.

[0033] Furthermore, the composite membrane obtained after the reaction is immersed and washed 3 to 5 times, each time for 30 to 240 minutes.

[0034] Furthermore, the above preparation method also includes a drying step, namely, drying the multilayer covalent organic framework membrane obtained from the last deposition.

[0035] Preferably, the drying conditions are: first drying at 25~100℃ for 6~48 h, and then vacuum drying at 25~100℃ for 6~48 h.

[0036] This invention requires washing each COF membrane layer, but drying is only necessary after the final COF membrane layer has been deposited. The purpose of the washing step described in this invention is to remove excess amine and aldehyde monomers; however, unreacted aldehyde and amino functional groups will remain on the surface of the COF membrane after washing.

[0037] In this invention, when dissolving monomers, each monomer must first be dissolved separately in the same solvent before mixing and shaking. When adding the catalyst, the action should be rapid, and the mixture should be immediately poured into the reaction vessel for film formation using a deposition method. By optimizing experimental conditions to match the reaction rate with the sedimentation rate, COF nanocrystals are uniformly deposited on the surface of the base film, thereby forming a continuous COF layer through subsequent reactions.

[0038] This invention, through adjusting the monomer ratio design, retains unreacted amino or aldehyde functional groups on the surface of each COF membrane layer. These functional groups undergo amine-aldehyde condensation reactions with the corresponding aldehyde or amino monomers during the deposition of the next layer, forming high-energy covalent bonds, thereby achieving strong chemical anchoring between layers. This design effectively avoids interlayer delamination and significantly improves the structural stability and service life of multilayer membranes under harsh conditions such as high pressure, high temperature, or chemical corrosion.

[0039] Another object of the present invention is to provide the application of the above-described programmable multilayer covalent organic framework membrane in gas separation.

[0040] The beneficial effects of this invention are: (1) High programmability: This invention allows for the independent selection of monomers with different functional groups (such as sulfonic acid groups, fluorine groups, carboxyl groups, etc.), control of reaction time, and free design of the type and thickness of each COF layer, thereby constructing functional regions with chemical gradients. Compared with existing technologies, this method can precisely control the chemical composition and thickness of each layer in a multilayer COF film, providing a new approach for designing complex functional materials.

[0041] (2) High mechanical stability: During the layer-by-layer deposition process, this invention can retain unreacted amino or aldehyde functional groups on the surface of each COF film by adjusting the molar ratio of amine monomers to aldehyde monomers away from the equimolar ratio. These functional groups undergo amine-aldehyde condensation reaction with the corresponding aldehyde or amine monomers during the deposition of the next layer to form covalent bonds, thereby achieving strong anchoring between layers, enhancing interlayer bonding, and significantly improving the structural integrity and mechanical stability of the multilayer film.

[0042] (3) Excellent flexibility: The supporting base membrane used in this invention has flexible properties, and the covalent organic framework membrane itself has a certain degree of extensibility, which makes the entire composite membrane exhibit good flexibility, high mechanical strength and long-term stability, and can withstand the high pressure environment (pressure difference: 0~5 bar) during the gas separation test.

[0043] (4) Performance synergistic improvement: This invention achieves effective compression of sub-nanometer pores by alternating stacking of COF layers with different chemical structures to form a "pore misalignment" effect. The pore structure is unobstructed and the size is uniform. While maintaining high gas flux, it significantly improves the selectivity of gas separation (e.g., the selectivity of CO2 / CH4 can be increased from 15.4 to 38.1).

[0044] (5) Simple process and environmentally friendly: The preparation method of the multilayer covalent organic framework membrane provided by the present invention is simple to operate, has simple steps, and is environmentally friendly, and can realize industrial mass production. Attached Figure Description

[0045] Figure 1 A schematic diagram illustrating the fabrication of a programmable multilayer covalent organic framework membrane via in-situ deposition.

[0046] Figure 2 The image shows a cross-sectional view of the COF membrane obtained in steps (1)(a) and (2)(b) of Example 2.

[0047] Figure 3 The images show actual COF membranes obtained in steps (1)(a) and (2)(b) of Example 2.

[0048] Figure 4 The graph shows the CO2 / CH4 binary gas separation performance of the multilayer COF membranes obtained in Examples 1 and 2 under a pressure difference of 0.5 bar.

[0049] Figure 5 The graph shows the CO2 / CH4 binary gas separation performance of the multilayer COF membranes obtained in Example 2 and Comparative Example 1 under a pressure difference of 0.5 bar.

[0050] Figure 6 These are photographs of the multilayer COF membranes obtained in Example 2 and Comparative Example 1 before and after rinsing. Detailed Implementation

[0051] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the invention, but do not limit the invention in any way.

[0052] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0053] Example 1 A method for preparing a COF-1 / PAN multilayer covalent organic framework membrane includes the following steps: (1) First layer deposition: Accurately weigh 0.026 mmol of 1,3,5-tris(4-aminophenyl)benzene and 0.035 mmol of 2,5-difluoroterephthalaldehyde, and dissolve them separately in 5 mL of ethyl acetate / trimethylbenzene (volume ratio 1:1) mixed solvent. Sonicate for 10 min to ensure complete dissolution. Mix the two solutions thoroughly and place them in a 50℃ constant temperature oven for later use. Add 80 μL of acetic acid as a catalyst to the mixed solution, shake quickly, and transfer to a pre-placed area of ​​4 cm². 2 The rectangular PAN substrate membrane was placed in a 15 mL beaker and allowed to react at 50 °C for 1 h. During the reaction, the solution in the beaker was filtered every 15 min to effectively remove primary crystal nuclei and promote uniform growth of the COF layer on the substrate membrane surface. After the reaction, the membrane material was immersed and washed three times with a 1:1 v / v mixed solvent of ethyl acetate / trimethylbenzene, each time for 30 min, finally obtaining a first covalent organic framework membrane COF-1 with a thickness of approximately 0.6 μm.

[0054] (2) Second layer deposition: Accurately weigh 0.026 mmol of 1,3,5-tris(4-aminophenyl)benzene and 0.035 mmol of 2,5-difluoroterephthalaldehyde, and dissolve them separately in 5 mL of ethyl acetate / trimethylbenzene (volume ratio 1:1) mixed solvent. Sonicate for 10 min to ensure complete dissolution. Mix the two solutions thoroughly and place them in a 50℃ constant temperature oven for later use. Add 80 μL of acetic acid as a catalyst to the mixed solution, shake quickly, and transfer it to a beaker containing the COF-1 membrane prepared in step (1). Let it stand at 50℃ for 1 h. Filter the solution in the beaker every 15 min during the reaction. After the reaction was completed, the membrane material was immersed and cleaned three times with a mixed solvent of ethyl acetate / trimethylbenzene (1:1, v / v), each time for 30 min. Then it was dried at 60 °C for 12 h and then vacuum dried at 60 °C for 12 h to obtain a COF-1 / PAN multilayer covalent organic framework membrane with a thickness of about 1.2 μm.

[0055] Example 2 A method for preparing a COF-1 / COF-2 / PAN multilayer covalent organic framework membrane includes the following steps: (1) First layer deposition (COF-1): Accurately weigh 0.026 mmol of 1,3,5-tris(4-aminophenyl)benzene and 0.035 mmol of 2,5-difluoroterephthalaldehyde, and dissolve them separately in 5 mL of ethyl acetate / trimethylbenzene (volume ratio 1:1) mixed solvent. Sonicate for 10 min to ensure complete dissolution. Mix the two solutions thoroughly and place them in a 50℃ constant temperature oven for later use. Add 80 μL of acetic acid as a catalyst to the mixed solution, shake quickly, and transfer to a pre-placed area of ​​4 cm². 2 The rectangular PAN substrate membrane was placed in a 15 mL beaker and allowed to react at 50 °C for 1 h. During the reaction, the solution in the beaker was filtered every 15 min to effectively remove primary crystal nuclei and promote uniform growth of the COF layer on the substrate membrane surface. After the reaction, the membrane material was immersed and washed three times with a 1:1 v / v mixed solvent of ethyl acetate / trimethylbenzene, each time for 30 min, finally obtaining a first covalent organic framework membrane COF-1 with a thickness of approximately 0.6 μm.

[0056] (2) Second layer deposition (COF-2): Weigh 0.026 mmol of 1,3,5-tris(4-aminophenyl)benzene and 0.035 mmol of 2,5-dibromo-terephthalaldehyde, and dissolve them separately in 5 mL of a mixed solvent of ethyl acetate / trimethylbenzene (volume ratio 1:3). Sonicate for 10 min to ensure complete dissolution, and then mix the two solutions thoroughly. Add 40 μL of acetic acid as a catalyst to the mixture, shake quickly, and pour into a 15 mL beaker containing the above COF-1 membrane. Let it stand at 50 °C for 1 h. Filter the system solution every 15 min during the reaction. Since the surface of the first COF membrane retains unreacted amino functional groups, they can undergo amine-aldehyde condensation reaction with aldehyde monomers during the second layer deposition process to form covalent bonds with a bond energy of about 200 kJ / mol, thereby achieving strong interlayer chemical anchoring. After the reaction, the composite membrane was immersed and washed three times with a mixed solvent of ethyl acetate / trimethylbenzene (1:3, v / v), each time for 30 min. It was then dried at 60℃ for 12 h, followed by vacuum drying at 60℃ for 12 h, to obtain a COF-1 / COF-2 / PAN multilayer covalent organic framework membrane with a thickness of approximately 1.3 μm. Cross-sectional images of the COF / PAN obtained in steps (1) and (2) are shown below. Figure 2 See actual product photos Figure 3 .

[0057] Gas separation performance testing of the multilayer covalent organic framework membranes obtained in Examples 1 and 2: The separation performance of a CO2 / CH4 (volume ratio 1:1) binary gas mixture was tested at an operating pressure difference of 0.5 bar. The results are shown in [Figure 1]. Figure 4 As can be seen, the COF-1 / COF-2 / PAN multilayer covalent organic framework membrane with interlayer covalent anchoring achieves a CO2 / CH4 selectivity of 38.1, which is a significant improvement of approximately 147.4% compared to the COF-1 / PAN multilayer covalent organic framework membrane in Example 1 (selectivity of 15.4). This result verifies that the gas sieving performance of COF membranes can be effectively enhanced through programmable multilayer construction and interlayer covalent anchoring strategies.

[0058] Example 3 The preparation method of a high-thickness COF-1 / COF-2 / PAN multilayer covalent organic framework membrane is the same as that in Example 2, except that the standing reaction time in steps (1) and (2) is increased to 2 h, and a COF-1 / COF-2 / PAN multilayer covalent organic framework membrane with a thickness of more than 2.4 μm is obtained.

[0059] Example 4 The preparation method of a COF-1 / COF-2 / PES multilayer covalent organic framework membrane is the same as that in Example 2, except that PAN in step (1) is replaced with a PES membrane.

[0060] Example 5 A method for preparing a COF-1 / COF-2 / COF-3 / PAN multilayer covalent organic framework membrane, wherein steps (1) and (2) are the same as steps (1) and (2) in Example 2, the difference being that step (3) is added based on step (2), and the specific process is as follows: The third layer deposition (COF-3): Weigh 0.051 mmol of 1,3,5-tris(4-aminophenyl)benzene and 0.069 mmol of 3,3'-((2,5-dicarboxy-1,4-phenylene)bis(oxy))bis(propane-1-sulfonic acid), and dissolve them separately in 5 mL of ethyl acetate / trimethylbenzene (v / v) mixed solvent. Sonicate for 10 min to ensure complete dissolution, and then mix the two solutions evenly. Add 20 μL of acetic acid as a catalyst to the mixed solution, shake quickly, and pour into a 15 mL beaker containing the COF-1 / COF-2 / PAN composite membrane obtained in step (2). Let it stand at 50 °C for 1 h. Filter the solution in the beaker every 15 min during the reaction. After the reaction, soak and clean the membrane material three times with ethyl acetate / trimethylbenzene (3:v / v) mixed solvent for 30 min each time, and then dry it at 60 °C for 12 h. The membrane was then vacuum dried at 60°C for 12 h to obtain a COF-1 / COF-2 / COF-3 / PAN multilayer covalent organic framework membrane with a thickness of approximately 1.8 μm.

[0061] Comparative Example 1 The preparation method of a COF-1 / COF-2 / PAN multilayer covalent organic framework membrane without interlayer covalent anchoring is the same as in Example 2, except that the amount of 2,5-dibromo-terephthalaldehyde and 2,5-difluoro-terephthalaldehyde in steps (1) and (2) is adjusted to 0.039 mmol, so that the molar ratio of amine monomers to aldehyde monomers is strictly maintained at 1:1.

[0062] A comparative analysis of the performance of the multilayer covalent organic framework membranes obtained in Example 2 and Comparative Example 1 is presented. The results of the CO2 / CH4 binary gas separation test at a pressure difference of 0.5 bar are shown below. Figure 5 The CO2 / CH4 selectivity of the membrane obtained in this comparative example is 26.3, which is about 30.2% lower than the COF membrane with interlayer covalent anchoring prepared in Example 2 (selectivity is 38.1).

[0063] A comparative analysis of the interlayer bonding strength of the multilayer covalent organic framework membranes obtained in Example 2 and Comparative Example 1 is shown in the figure. Figure 6 The COF membranes obtained in Example 2 and Comparative Example 1 were rinsed. Figure 6ab are actual images of the multilayer COF membrane obtained in Comparative Example 1 before and after rinsing. Figure 6 cd correspond to the actual images of the multilayer COF membrane obtained in Example 2 before and after rinsing. It can be seen that the COF membrane obtained in Comparative Example 1 has a low bonding strength at the interlayer interface, which makes the second layer of COF easy to fall off, while the COF membrane obtained in Example 2 does not have this phenomenon.

[0064] This result fully demonstrates that by controlling the non-equimolar ratio of monomer feed and introducing covalent bonds between layers, the interfacial bonding strength of multilayer COF membranes can be significantly enhanced, thereby effectively improving their selectivity and structural stability in the gas separation process.

Claims

1. A programmable multilayer covalent organic framework membrane, characterized in that: The organic framework membrane includes a supporting base membrane and at least two organic framework layers sequentially deposited on the supporting base membrane. The organic framework layers are prepared by reacting amine monomers and aldehyde monomers under the action of a catalyst. Any adjacent organic framework layers are connected by covalent bonds formed by the condensation reaction of unreacted amino and aldehyde groups at the interface.

2. The programmable multilayer covalent organic framework membrane according to claim 1, characterized in that: The amine monomers are 2,7-diaminofluorene, 1,3,5-tris(4-aminophenyl)benzene, 3,3′-dimethyl-[1,1′-biphenyl]-4,4′-diamine, 2,5-diamino-1,4-benzenediol, tris(4-aminophenyl)triazine, 2,4,6-trihydroxy-1,3,5-benzenetrialdehyde, 5,10,15,20-tetra(4-aminophenyl)porphyrin, [1,1′-biphenyl]-3,3′,5,5′-tetraamine, 2,5-dichloro-4-phenylenediamine, 2,5-dibromo-p-phenylenediamine, and 3,3-bipyridine-6- One or more of the following: 6-diamino, 5,5′,5″-(1,3,5-triazine-2,4,6-triyl)tri(pyridin-2-amine), 1,3,6,8-tetra(5-amino-2-pyridyl)pyrene, 2,2′-difluoro-4,4′-diaminobiphenyl, 4,4′-diaminooctafluorobiphenyl, 2,3,6,7,10,11-hexa(4′-aminophenyl)triphenylene, 3,6-diaminocarbazole, 2,6-naphthyldiamine, or 4,4′′-diamino-[1,1′:4′,1′′-triphenyl]-2′,5′-diol.

3. The programmable multilayer covalent organic framework membrane according to claim 1, characterized in that: The aldehyde monomers are 1,3,5-benzyltricarboxaldehyde, terephthalaldehyde, 2,5-dimethoxy-1,4-benzyltricarboxaldehyde, 2,4,6-trihydroxy-1,3,5-benzyltricarboxaldehyde, 1,2,4,5-benzyltetraaldehyde, 1,10-phenanthroline-2,9-dicarboxaldehyde, 2,5-dibromo-terephthalaldehyde, 2,5-diiodo-terephthalaldehyde, 2,5-difluoro-terephthalaldehyde, 2,3-difluoro-terephthalaldehyde, octafluorobiphenylcarboxaldehyde, 4,4′-(1-methyl-1H-benzo[d]imidazol-4,7-diyl)dibenzaldehyde, 2,5-dibutyl-1,4-terephthalaldehyde, 2,5-bis(2-ethoxyethoxy)terephthalaldehyde, One or more of the following: 2,5-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)terephthalaldehyde, 1,3,5-tris(p-formylphenyl)benzene, 3,3′,3′′-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde, 5,5′,5′′-(benzene-1,3,5-triyl)tri(thiophene-2-carboxaldehyde), 1,3,6,8-tetra(4-carboxylphenyl)pyrene, benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene-2,5,8-trialdehyde, or 3,3′-((2,5-diformyl-1,4-phenylene)bis(oxy))bis(propane-1-sulfonic acid).

4. The programmable multilayer covalent organic framework membrane according to claim 1, characterized in that: The catalyst is one of acetic acid, p-toluenesulfonic acid, trifluoroacetic acid, or trifluoromethanesulfonic acid.

5. The programmable multilayer covalent organic framework membrane according to claim 1, characterized in that: The material of the supporting base membrane is selected from metals, ceramics, metal oxides, sulfides, glass, or polymers.

6. The programmable multilayer covalent organic framework membrane according to claim 1, characterized in that: The organic framework layer is prepared by the following reaction: amine monomers and aldehyde monomers are dissolved in organic solvents, mixed evenly, a catalyst is added, the mixture is transferred to a container with a base film already placed, reacted, and washed to obtain the final product.

7. The programmable multilayer covalent organic framework membrane according to claim 6, characterized in that: The reaction conditions are 10~80℃ for 0.5~6 h.

8. The programmable multilayer covalent organic framework membrane according to claim 6, characterized in that: The organic solvent is one or more of isopropanol, acetonitrile, o-dichlorobenzene, ethylene glycol, toluene, ethyl acetate, dichloromethane, or mesitylene.

9. A method for preparing a programmable multilayer covalent organic framework membrane according to any one of claims 1 to 8, characterized in that: Includes the following steps: (1) First layer deposition: Amine monomer A and aldehyde monomer A are dissolved in organic solvents respectively, mixed evenly, catalyst is added, and then they are quickly brought into contact with the supporting substrate membrane, reacted, washed, and the first layer of covalent organic framework composite membrane is obtained. (2) Second layer deposition: Amine monomer B and aldehyde monomer B are dissolved in organic solvents respectively, mixed evenly, catalyst is added, and then they are quickly contacted with the first layer covalent organic framework composite membrane obtained in step (1), reacted, washed, and a double layer covalent organic framework composite membrane is obtained; wherein, the amine monomer B and aldehyde monomer B are not completely the same as the amine monomer A and aldehyde monomer A in step (1); (3) Deposition of the nth layer: Optionally, repeat step (2) to construct a multilayer covalent organic framework membrane with a predetermined sequence, independent thickness of each layer and chemical structure by independently selecting the type, concentration and reaction time of amine monomers and aldehyde monomers in each layer.

10. The application of the programmable multilayer covalent organic framework membrane according to claim 1 in gas separation.

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