Polyelectrolyte / Covalent Organic Framework Nanosheet Hybrid Membrane and Its Preparation Method and Application

Abstract

The present invention discloses a polyelectrolyte / covalent organic framework nanosheet hybrid membrane, which is composed of covalent organic framework nanosheets and polyelectrolytes. The preparation steps mainly include: preparing COF nanosheets through interfacial polymerization; respectively preparing solutions of appropriate concentrations of COF nanosheets and polyelectrolytes, adjusting the pH to a specific value, mixing the two solutions and stirring well to obtain an assembly dispersion; depositing an appropriate amount of the dispersion on a polyacrylonitrile substrate through vacuum-assisted self-assembly, and drying to obtain the hybrid membrane. Based on the characteristics that the polyelectrolyte has different conformations and charged states with the change of pH, the structure of the assembly formed by the polyelectrolyte and COF can be regulated. The present invention has the advantages of mild preparation conditions, strong controllability in the preparation process, and universality of the method. When used in the CO2 / N2 system, the membrane has an ultra-high CO2 permeation rate, good CO2 / N2 selectivity and excellent operational stability. This ultrathin hybrid membrane has good application prospects in carbon capture.

Description

Technical Field

[0001] The present invention relates to the preparation and application of an ultrathin polyelectrolyte / covalent organic framework nanosheet hybrid membrane, belonging to the technical field of covalent organic framework membranes. Background Art

[0002] The excessive emission of greenhouse gases has brought great pressure to the ecological environment of the earth. In the past decade, the average annual emission of carbon dioxide into the atmosphere has been close to 35 Gt, and the concentration of CO 2 in the atmosphere has increased sharply to 420 ppm and shows a continuing upward trend. Currently, the commonly used methods mainly include cryogenic distillation, absorption, adsorption, and membrane separation. Among them, membrane-based carbon dioxide separation technology has the advantages of high energy efficiency, compact modules, simple operation, easy expansion and coupling, etc., and is considered to be one of the most promising technologies for efficient carbon capture. High-performance membrane materials are the core of membrane technology. At present, the performance of traditional polymer membranes that dominate is limited by the inherent trade-off effect between permeability and selectivity. At the same time, effects such as plasticization and aging also limit the application of polymer membranes. Therefore, it is crucial to develop advanced membrane materials with both high permeability, high selectivity, and excellent stability.

[0003] Covalent organic framework (COF) is a crystalline framework material formed by covalent bonding of organic monomers based on the design principle of "network chemistry". The periodic framework structure and long-range ordered pores endow the COF membrane with high permeability. Its diverse physical / chemical structures and rich functionalization sites enable it to be designed according to the characteristics of gas molecules to enhance the separation process. The rigid framework structure connected by covalent bonds endows the COF membrane with excellent stability. These excellent properties make COF an ideal gas separation membrane material. However, currently, the pore sizes of COF materials are generally larger than 0.5 nm, and the sieving ability for the CO 2 / N 2 system is poor. Secondly, the processability of COF materials is poor, making it difficult to prepare defect-free COF thin films. Therefore, the preparation of defect-free ultrathin COF membranes with high selectivity remains a key challenge. Summary of the Invention

[0004] The present invention provides a method for preparing and applying a polyelectrolyte / covalent organic framework nanosheet hybrid membrane. The preparation method is simple and controllable, and the prepared polyelectrolyte / covalent organic framework nanosheet hybrid membrane is used for the separation of the CO 2 / N 2 system, and has excellent CO 2 / N 2 separation performance and stability.

[0005] The present invention is achieved through the following technical solutions. A polyelectrolyte / covalent organic framework nanosheet hybrid membrane is composed of covalent organic framework (COF) nanosheets and a polyelectrolyte. The polyelectrolyte modifies and induces the COF nanosheets to form an assembly with excellent mechanical properties and film-forming properties. An ultra-thin hybrid membrane with a thickness of 8 - 200 nm is prepared from the assembly on a porous polyacrylonitrile substrate by a vacuum-assisted self-assembly method.

[0006] Furthermore, in the polyelectrolyte / COF nanosheet hybrid membrane of the present invention, the COF nanosheets are any one of TpPa-SO 3 H, TpBd-(SO 3 H) 2 、TpPa-COOH、TpEB、TpTGCl、TpPa-PO 3 H 2 ; the COF nanosheets are prepared by interfacial polymerization. The polyelectrolyte is any one of polyethyleneimine, polyacrylic acid, polyvinylsulfonic acid, polyvinylphosphonic acid, and polyvinylamine, and its molecular weight is any one of 10k, 50k, and 70k. The polyelectrolyte bridges the COF nanosheets through electrostatic interaction to form an assembly.

[0007] Meanwhile, the present invention also provides a method for preparing the above-mentioned polyelectrolyte / COF nanosheet hybrid membrane. The charged state and conformation of the polyelectrolyte determine the assembly mode of the polyelectrolyte and the COF nanosheets. The charged state and conformation of the polyelectrolyte are regulated by changing the pH value of the polyelectrolyte aqueous solution. The polyelectrolyte aqueous solution and the COF nanosheet dispersion are stirred and blended to form an assembly dispersion. The assembly is deposited on a porous polyacrylonitrile substrate by a vacuum-assisted self-assembly method using the assembly dispersion, and an ultra-thin hybrid membrane is obtained after drying. The specific steps are as follows:

[0008] Step 1: Preparation of the COF nanosheet dispersion: Dissolve the amino monomer in deionized water to prepare an amino monomer aqueous solution with a concentration of 0.005 M, and take an appropriate amount of the amino monomer aqueous solution and place it in a container. Dissolve the aldehyde monomer in an organic solvent to prepare an aldehyde monomer aqueous solution with a concentration of 0.005 M, and drop the aldehyde monomer aqueous solution above the amino monomer aqueous solution. The volume ratio of the aldehyde monomer aqueous solution to the amino monomer aqueous solution is 2:3, and the reaction is carried out at 20 °C for 7 days. After the reaction, take the aqueous solution and perform dialysis treatment for 5 days to obtain the COF nanosheet dispersion;

[0009] Disperse the covalent organic framework nanosheet dispersion prepared in Step 1 to prepare a covalent organic framework nanosheet dispersion with a concentration of 0.02 mg / ml, denoted as Solution A; prepare an aqueous polyelectrolyte solution with a concentration of 0.01 - 0.06 mg / mL, denoted as Solution B; adjust the pH of the above-mentioned Solution A and Solution B to 3 - 11 respectively by sodium hydroxide solution or hydrochloric acid solution, and the pH values of Solution A and Solution B are equal; mix equal amounts of Solution A and Solution B with equal pH values and stir for 1 hour to obtain an assembly dispersion.

[0010] Step 3: Preparation of the ultrathin hybrid membrane: According to the relationship between the dosage of the assembly dispersion and the membrane area, which is 0.2 - 4 mL / cm 2 , dilute the assembly dispersion prepared in Step 2 to a dilution with a volume concentration of 1 - 10%, deposit the assembly on the surface of a porous polyacrylonitrile substrate by vacuum-assisted self-assembly method to form a covalent organic framework nanosheet membrane, and then dry it at room temperature to obtain an ultrathin hybrid membrane with a thickness of 8 - 200 nm.

[0011] Use the ultrathin hybrid membrane prepared by the present invention above for CO 2 / N 2 separation. At 25°C, a raw material gas pressure of 2 bar, and 100% relative humidity, the CO 2 permeation rate is 97 - 1371 GPU, and the CO 2 / N 2 selectivity is 10 - 125.

[0012] Compared with the prior art, the advantages of the present invention are as follows:

[0013] Based on the interaction between the polyelectrolyte and the COF nanosheets, the present invention utilizes the characteristics that the charged state and conformation of the polyelectrolyte change with pH, and regulates the assembly of the COF nanosheets through the polyelectrolyte to form assemblies with different characteristics. The assemblies have stronger mechanical properties and film-forming ability, overcome the defect of poor film-forming property of the original nanosheets, and prepare a hybrid membrane with a minimum thickness of 8 nm. At the same time, the polyelectrolyte covering the surface of the COF nanosheets reduces the pore size of the COF material and introduces CO 2 -philic groups, effectively improving the CO 2 separation performance.

[0014] In the present invention, the preparation process of the membrane is simple and controllable, the structure of the membrane is diverse, and the structure is stable. It can be extended to other two-dimensional materials and polymer materials, and has strong universality. Applying the membrane prepared by the present invention to CO 2 / N 2The material system has high permeability, high selectivity and high stability, and its separation performance exceeds that of all current COF membranes. So far, there is no literature report on the strategy of polyelectrolyte-mediated assembly membranes for the preparation of COF membranes. Description of the Drawings

[0015] Figure 1 It is the scanning electron micrograph of the assemblies prepared in Examples 1 and 4;

[0016] Figure 2 It is the cross-sectional electron micrograph of Membrane 1 prepared in Example 1;

[0017] Figure 3 It is the surface electron micrograph of Membrane 1 prepared in Example 1;

[0018] Figure 4 It is the cross-sectional electron micrograph of Membrane 2 prepared in Example 2;

[0019] Figure 5 It is the surface electron micrograph of Membrane 2 prepared in Example 2;

[0020] Figure 6 It is the scanning electron micrograph of the assemblies prepared in Example 3;

[0021] Figure 7 It is the cross-sectional electron micrograph of Membrane 3 prepared in Example 3;

[0022] Figure 8 It is the surface electron micrograph of Membrane 3 prepared in Example 3;

[0023] Figure 9 It is the thickness scanning electron micrograph of Membrane 4 prepared in Example 4;

[0024] Figure 10 It is the surface electron micrograph of Membrane 4 prepared in Example 4;

[0025] Figure 11 It is the scanning electron micrograph of the assemblies prepared in Example 5;

[0026] Figure 12 It is the scanning electron micrograph of the nanosheets in Comparative Example 1;

[0027] Figure 13 It is the cross-sectional electron micrograph of the comparative membrane prepared in Comparative Example 1;

[0028] Figure 14 It is the surface electron micrograph of the comparative membrane prepared in Comparative Example 1;

[0029] Figure 15 It is the CO 2 permeation rate and CO 2 / N 2 selectivity performance comparison chart. Detailed implementation mode

[0030] The design concept of the polyelectrolyte / covalent organic framework nanosheet hybrid membrane of the present invention is as follows: The membrane is composed of COF nanosheets and polyelectrolytes. The polyelectrolytes modify and induce the COF nanosheets to form an assembly with excellent mechanical properties and film-forming properties. Further, a porous polyacrylonitrile is used as a substrate, and an ultrathin hybrid membrane with a thickness of 8 - 200 nm is prepared from the assembly by a vacuum-assisted self-assembly method. Among them, the COF nanosheets are prepared by interfacial polymerization of monomers; the polyelectrolytes bridge the COF nanosheets through electrostatic interactions, and an assembly is obtained through the assembly of the polyelectrolytes and the COF nanosheets. In the formed membrane, the amount of the polyelectrolyte is 10 - 50% of the mass of the COF nanosheets. Based on the characteristics that the polyelectrolytes have different conformations and charged states with the change of pH, the structure of the assembly formed by the polyelectrolytes and the COF can be regulated. By changing the pH value of the polyelectrolyte aqueous solution, the charged state and conformation of the polyelectrolyte are regulated. The charged state and conformation of the polyelectrolyte determine the assembly mode of the polyelectrolyte and the COF nanosheets. Different assembly modes are induced by electrostatic interaction to form different assemblies. The assemblies with various morphologies form membranes with different stacking structures and thicknesses on the polyacrylonitrile substrate through vacuum-assisted self-assembly, and the thickness of the membrane can be adjusted between 8 - 200 nm. The preparation steps are mainly as follows: Prepare a COF nanosheet dispersion by interfacial polymerization between monomers, respectively prepare COF nanosheet dispersions and polyelectrolyte aqueous solutions with appropriate concentrations, and after adjusting to a specific pH, stir and blend them to form an assembly dispersion; use this assembly dispersion, deposit the assembly on a porous polyacrylonitrile substrate by a vacuum-assisted self-assembly method, and obtain an ultrathin hybrid membrane after drying. The present invention has the advantages of mild preparation conditions, strong controllability in the preparation process, diverse and adjustable structures, and general methods. This membrane is used for the CO 2 / N 2 system, with extremely high CO 2 permeation rate, good CO 2 / N 2 selectivity and excellent operational stability. This ultrathin hybrid membrane has good application prospects in post-combustion carbon capture.

[0031] In the present invention, the COF nanosheets are any one of TpPa-SO 3 H, TpBd-(SO 3 H) 2 , TpPa-COOH, TpEB, TpTGCl, TpPa-PO 3 H 2 , and the polyelectrolytes are any one of polyethyleneimine, polyacrylic acid, polyvinylsulfonic acid, polyvinylphosphonic acid, and polyvinylamine, and their molecular weights are any one of 10k, 50k, and 70k.

[0032] The technical solution of the present invention is further described in detail below in conjunction with specific embodiments and appended tables. The described specific embodiments are only used to explain the present invention and are not intended to limit the present invention.

[0033] Example 1: Preparation of polyelectrolyte / covalent organic framework nanosheet hybrid membrane, the steps are as follows:

[0034] Step 1, prepare COF nanosheet dispersion: dissolve 2,5-diaminobenzenesulfonic acid and sodium carbonate in deionized water, the concentrations of the two are 0.005M and 0.0075M respectively, stir at room temperature for 20 minutes to fully dissolve them, the resulting solution is an aqueous solution of amine monomer with a concentration of 0.005M, take 30mL of the solution and place it in a beaker; dissolve trialdehyde phloroglucinol in n-octanoic acid, prepare a 0.005M aldehyde monomer solution, take 20ml of the solution and drop it on top of the aqueous solution of amine monomer in the above beaker; react at 20°C for 7 days; after the reaction, take the lower aqueous solution and dialyze it for five days to obtain TpPa-SO 3 H nanosheet dispersion.

[0035] Step 2: Preparation of assembly dispersion: Take the TpPa-SO prepared in step 1 3 0.1 mL of H nanosheet dispersion was diluted to 10 mL; 20 mg of polyethyleneimine (PEI, M w =70k) was dissolved in 500mL of deionized water to prepare a PEI aqueous solution with a concentration of 0.04mg / mL; the diluted TpPa-SO 3 The H nanosheet dispersion and PEI aqueous solution were adjusted to pH 7, and 10 mL of the diluted TpPa-SO 3 The H nanosheet dispersion was fully mixed with 10 mL of PEI aqueous solution with a pH value of 7 and stirred at room temperature for 1 hour to obtain an assembly dispersion. The SEM image of the assembly is shown in FIG. Figure 1 shown.

[0036] Step 3: Preparation of ultrathin hybrid membrane: Take 2 mL of the above assembly dispersion and dilute it to 20 mL; deposit the assembly on an area of ​​1 cm 2 The obtained film was air-dried at room temperature for 12 h to obtain a hybrid film with a thickness of 105 nm, which was recorded as Film 1. Figure 2 is a cross-sectional electron microscope image of membrane 1. Figure 3 is the surface electron microscope image of membrane 1.

[0037] The ratio of nitrogen and sulfur elements in the membrane was measured by X-ray photoelectron spectroscopy (XPS), and the mass fraction of PEI relative to COF in Membrane 1 was calculated to be 32.01 wt%.

[0038] Membrane 1 was used for the separation of the CO 2 / N 2 system. At 25 °C, a feed gas pressure of 2 bar, and 100% relative humidity, the CO 2 permeance was 133 GPU, and the CO 2 / N 2 selectivity was 125.

[0039] Example 2: Preparation of a polyelectrolyte / covalent organic framework nanosheet hybrid membrane. The preparation process was basically the same as that of Example 1, except that in Step 2, the pH values of the diluted TpPa-SO 3 H nanosheet dispersion and the PEI aqueous solution were changed from 7 to 3. The thickness of the finally obtained membrane was 103 nm. The finally obtained membrane was designated as Membrane 2. Figure 4 is the cross-sectional electron micrograph of Membrane 2, Figure 5 and is the surface electron micrograph of Membrane 2.

[0040] Membrane 2 was used for the separation of the CO 2 / N 2 system. At 25 °C, a feed gas pressure of 2 bar, and 100% relative humidity, the CO 2 permeance was 358 GPU, and the CO 2 / N 2 selectivity was 10.

[0041] Example 3: Preparation of a polyelectrolyte / covalent organic framework nanosheet hybrid membrane. The preparation process was basically the same as that of Example 1, except that in Step 2, the pH values of the diluted TpPa-SO 3 H nanosheet dispersion and the PEI aqueous solution were changed from 7 to 11. The scanning electron micrograph of the nanosheets in the assembly dispersion is shown in Figure 6 Figure [X]. The thickness of the finally obtained membrane was 105 nm. The finally obtained membrane was designated as Membrane 3. Figure 7 is the cross-sectional electron micrograph of Membrane 3, Figure 8 and is the surface electron micrograph of Membrane 3.

[0042] Membrane 3 was used for the separation of the CO 2 / N 2 system. At 25 °C, a feed gas pressure of 2 bar, and 100% relative humidity, the CO 2 permeance was 203 GPU, and the CO 2 / N 2 selectivity was 18.

[0043] Example 4: Preparation of a polyelectrolyte / covalent organic framework nanosheet hybrid membrane. The preparation process is basically the same as that in Example 1, except that: in step three, instead of diluting 2 mL of the assembly dispersion to 20 mL, 0.2 mL of the assembly dispersion is diluted to 20 mL. The scanning electron microscope image of the assembly is as shown in Figure 1 . The thickness of the finally obtained membrane is 8 nm, and the finally obtained membrane is denoted as membrane 4. Figure 9 Figure Figure 9 is the scanning electron microscope image of the thickness of membrane 4. Figure 10 Figure Figure 10 is the surface electron microscope image of membrane 4.

[0044] Membrane 4 is used for the separation of the CO 2 / N 2 system. Under the conditions of 25 °C, a raw gas pressure of 2 bar, and 100% relative humidity, the CO 2 permeation rate is 1371 GPU, and the CO 2 / N 2 selectivity is 33.

[0045] Example 5: Preparation of a polyelectrolyte / covalent organic framework nanosheet hybrid membrane. The preparation process is basically the same as that in Example 1, except that: in step two, instead of dissolving 20 mg of polyethyleneimine (PEI, M w = 70k) in 500 mL of deionized water to prepare a PEI aqueous solution with a concentration of 0.04 mg / mL, 5 mg of polyethyleneimine (PEI, M w = 70k) is dissolved in 500 mL of deionized water to prepare a PEI aqueous solution with a concentration of 0.01 mg / mL, that is, the concentration of the PEI solution is changed from 0.04 mg / mL to 0.01 mg / mL. The scanning electron microscope image of the assembly is as shown in Figure 11 . The thickness of the finally obtained membrane is 104 nm, and the finally obtained membrane is denoted as membrane 5.

[0046] By testing the ratio of nitrogen and sulfur elements in the membrane through X-ray photoelectron spectroscopy technology, the mass fraction of PEI relative to COF in membrane 5 is calculated to be 11 wt%.

[0047] Membrane 5 is used for the separation of the CO 2 / N 2 system. Under the conditions of 25 °C, a raw gas pressure of 2 bar, and 100% relative humidity, the CO 2 permeation rate is 336 GPU, and the CO 2 / N 2 selectivity is 50.

[0048] Example 6: Preparation of a polyelectrolyte / covalent organic framework nanosheet hybrid membrane. The preparation process is basically the same as that in Example 1, except that: in step two, instead of taking 20 mg of polyethyleneimine (PEI, M wDissolve 30 mg of polyethyleneimine (PEI, M w = 70k) in 500 mL of deionized water to prepare a PEI aqueous solution with a concentration of 0.04 mg / mL. Instead, take 30 mg of polyethyleneimine (PEI, M w = 70k) and dissolve it in 500 mL of deionized water to prepare a PEI aqueous solution with a concentration of 0.06 mg / mL, that is, the concentration of the PEI solution is changed from 0.04 mg / mL to 0.06 mg / mL; the final thickness of the obtained membrane is 109 nm, and the finally obtained membrane is denoted as membrane 6.

[0049] The ratio of nitrogen and sulfur elements in the membrane was measured by X-ray photoelectron spectroscopy, and the mass fraction of PEI relative to COF in membrane 6 was calculated to be 42.5 wt%.

[0050] Membrane 6 was used for CO 2 / N 2 system separation. At 25 °C, a raw gas pressure of 2 bar, and 100% relative humidity, the CO 2 permeation rate was 97 GPU, and the CO 2 / N 2 selectivity was 116.

[0051] Comparative Example 1: Preparation of a covalent organic framework nanosheet membrane. The steps include the preparation of a TpPa-SO 3 H nanosheet dispersion and the preparation of an ultrathin COF membrane, where:

[0052] Step 1: The process of preparing the TpPa-SO 3 H nanosheet dispersion was the same as in Example 1. The scanning electron micrograph of the nanosheets is as shown in Figure 12 Figure.

[0053] Step 2: Preparation of the ultrathin COF membrane: Dilute 10 μL of the above-mentioned TpPa-SO 3 H nanosheet dispersion to 20 mL; deposit the assembly on a 1 cm 2 polyacrylonitrile substrate by vacuum-assisted self-assembly; air-dry the obtained membrane at room temperature for 12 h to obtain a COF membrane with a thickness of 97 nm. This COF membrane is denoted as the comparative membrane. Figure 13 Figure is the cross-sectional electron micrograph of the comparative membrane, Figure 14 Figure is the surface electron micrograph of the comparative membrane.

[0054] The comparative membrane was used for CO 2 / N 2 system separation. At 25 °C, a raw gas pressure of 2 bar, and 100% relative humidity, the CO 2 permeation rate was 1205 GPU, and the CO 2 / N 2 selectivity was 2.6.

[0055] By comparing the examples and comparative examples, it can be seen that the assembly of COF nanosheets mediated by polyelectrolytes in the present invention makes a great contribution to the ultrathin preparation of the membrane and the improvement of the separation performance. For cationic polyelectrolytes, as the pH decreases, the degree of ionization of the polyelectrolyte gradually increases, the conformation gradually unfolds, and the structural rigidity gradually enhances. The variation law of anionic polyelectrolytes is opposite. At an appropriate pH, the polyelectrolyte has an unfolded worm-like conformation and an appropriate amount of positive charges. The unfolded conformation provides binding opportunities, and the appropriate number of charge sites ensures the binding strength. During the assembly process, the polyelectrolyte can connect COF nanosheets together through strong electrostatic interactions to form a super-large assembly with an aspect ratio exceeding 10,000 and a lateral size exceeding 40 microns, as Figure 1 shown. Compared with the original nanosheets, the super-large nanosheets formed by assembly have more excellent mechanical properties and film-forming properties, and are more inclined to stack layer by layer in the vertical direction during the film-making process, which is beneficial to reducing defects in the membrane. Therefore, the thickness of the COF layer can be reduced to 8 nm (as Figure 9 shown), while maintaining high separation performance. Figure 15 Figure 2 shows the comparison of the CO 2 permeation rate and the CO 2 / N 2 selectivity performance of Membranes 1-6 and the comparative membrane. In addition, the shielding effect of the polyelectrolyte on the COF pores reduces the effective pore size of the membrane, and the CO 2 affinity groups are more conducive to the diffusion of CO 2 / N 2 separation performance of the membrane is comprehensively enhanced. In summary, the present invention has the advantages of simple and controllable preparation process, diverse and adjustable stacking structures (see Figure 3 , the surface of Membrane 1 is smooth, dense and defect-free, see Figure 13 , the surface of the comparative membrane has obvious cracks), and strong generality of the preparation method. The prepared membrane is used for the CO 2 / N 2 system, obtaining high selectivity and high flux, achieving a breakthrough in the separation performance of COF membranes, and also having strong competitiveness compared with the most advanced gas separation membranes, showing great application potential in CO 2 / N 2 separation.

[0056] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the above specific embodiments. The above specific embodiments are merely illustrative and not restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many modifications without departing from the purpose of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A preparation method of a polyelectrolyte / covalent organic framework nanosheet hybrid membrane, the polyelectrolyte / covalent organic framework nanosheet hybrid membrane being composed of covalent organic framework nanosheets and a polyelectrolyte, the polyelectrolyte modifying and inducing the covalent organic framework nanosheets to form an assembly with excellent mechanical properties and film-forming properties, and preparing an ultrathin hybrid membrane with a thickness of 8 - 200 nm from the assembly on a porous polyacrylonitrile substrate by a vacuum-assisted self-assembly method; The covalent organic framework nanosheets described above are TpPa-SO 3 H, TpBd-(SO 3 H) 2 , TpPa-COOH, TpEB, TpTGCl, TpPa-PO 3 H 2 Any one of them, and the covalent organic framework nanosheets are prepared by interfacial polymerization; The polyelectrolyte is any one of polyethyleneimine, polyacrylic acid, polyvinylsulfonic acid, polyvinylphosphonic acid, and polyvinylamine, and the polyelectrolyte bridges the covalent organic framework nanosheets through electrostatic interaction to form an assembly; It is characterized in that: The charged state and conformation of the polyelectrolyte determine the assembly mode of the polyelectrolyte and the covalent organic framework nanosheets. The charged state and conformation of the polyelectrolyte are regulated by changing the pH value of the polyelectrolyte aqueous solution; the polyelectrolyte aqueous solution and the covalent organic framework nanosheet dispersion are stirred and blended to form an assembly dispersion; the assembly is deposited on the porous polyacrylonitrile substrate by a vacuum-assisted self-assembly method using the assembly dispersion, and an ultrathin hybrid membrane is obtained after drying.

2. According to the preparation method described in claim 1, It is characterized in that, The specific steps are as follows: Step 1: Preparation of the covalent organic framework nanosheet dispersion: Dissolve the amino monomer in deionized water to prepare an amino monomer aqueous solution with a concentration of 0.005 M, and take an appropriate amount of the amino monomer aqueous solution and place it in a container; dissolve the aldehyde monomer in an organic solvent to prepare an aldehyde monomer solution with a concentration of 0.005 M, and drop the aldehyde monomer solution above the amino monomer aqueous solution. Among them, the volume ratio of the aldehyde monomer solution to the amino monomer aqueous solution is 2:3, react at 20 °C for 7 days, take the aqueous phase solution after the reaction, and perform dialysis treatment for 5 days to obtain the covalent organic framework nanosheet dispersion; Step 2: Preparation of the assembly dispersion: Prepare a covalent organic framework nanosheet dispersion with a concentration of 0.02 mg / ml from the covalent organic framework nanosheet dispersion prepared in Step 1, denoted as Solution A; prepare a polyelectrolyte aqueous solution with a concentration of 0.01 - 0.06 mg / mL, denoted as Solution B; adjust the pH of Solution A and Solution B to 3 - 11 respectively with sodium hydroxide solution or hydrochloric acid solution, and the pH values of Solution A and Solution B are equal; mix equal amounts of Solution A and Solution B with equal pH values and stir for 1 hour to obtain the assembly dispersion; Step 3: Preparation of the ultrathin hybrid membrane: The relationship between the dosage of the assembly dispersion and the membrane area is 0.2 - 4 mL / cm 2 , dilute the assembly dispersion prepared in Step 2 to a dilution with a volume concentration of 1 - 10%, deposit the assembly on the surface of the porous polyacrylonitrile substrate to form a covalent organic framework nanosheet membrane by vacuum-assisted self-assembly method, and then dry it at room temperature to obtain an ultrathin hybrid membrane with a thickness of 8 - 200 nm.

3. An application of a polyelectrolyte / covalent organic framework nanosheet hybrid membrane prepared by the preparation method described in claim 1 or 2, It is characterized in that, For CO 2 / N 2 Separation, at 25 °C, feed gas pressure of 2 bar and 100% relative humidity, the CO 2 permeation rate is 97 - 1371 GPU, and the CO 2 / N 2 selectivity is 10 - 125.

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