Gel vapor-assisted method for preparing covalent organic framework composite membrane and preparation method and application thereof

Abstract

The present application relates to the technical field of polymer separation membrane, and discloses a method for preparing covalent framework composite membrane by gel vapor assistance, comprising the following steps: dissolving amino guanidine derivatives or water-soluble amine monomers and 1,3,5-triformylphloroglucinol derivative monomers in pure water respectively, mixing uniformly, and then dropping on the base film after alkalization pretreatment to form a hydrogel, and finally performing vapor assisted reaction. By means of gel vapor assisted film formation, the polymers form a uniform and dense film structure on the base film, and a continuous and uniform high-efficiency nanofiltration COF composite membrane can be quickly and simply prepared, and high-efficiency nanofiltration separation can be realized.

Description

Technical Field

[0001] This invention relates to a method for preparing covalent organic framework composite membranes using a gel vapor-assisted method and its application, belonging to the field of polymer separation membrane technology. Background Technology

[0002] Membrane separation technology is highly efficient and energy-saving, and is widely used in various fields. Its high selectivity, low energy consumption, and environmental friendliness make it a key technology for industrial purification and separation. Nanofiltration membrane technology, as an important branch, has pore sizes of approximately 1-10 nanometers, enabling highly efficient and precise separation. It has significant advantages in water treatment, seawater desalination, wastewater recycling, and biopharmaceuticals, removing dissolved organic matter, heavy metal ions, and some salts while maintaining low operating pressure and energy consumption, thus supporting water resource management and environmental protection.

[0003] Covalent organic frameworks (COFs), as an emerging membrane material, exhibit precisely defined topologies and ordered nanopores due to their layered structure connected by strong planar covalent bonds and out-of-plane π-π interactions. This effectively alleviates the "selectivity-permeability" trade-off commonly found in traditional polymer membranes. The high crystallinity and tunable pore size of COF membranes demonstrate significant application potential in gas separation, liquid separation, and catalysis. Despite these performance advantages, the preparation of COF membranes still faces numerous challenges. Traditional COF synthesis methods (such as solvothermal and microwave heating methods) often result in the formation of insoluble and microcrystalline powders, making it difficult to produce uniform COF membranes on an industrial scale. Chinese patent CN114752028A reports a method for preparing covalent organic framework thin films via chemical vapor deposition. This involves evaporating monomers at high temperatures (typically several hundred degrees Celsius), in a vacuum or inert gas environment, and then reacting them on a high-temperature resistant substrate (such as metal or ceramic) to form a film. However, this method suffers from problems such as localized overreaction due to gas-phase transport, uneven film thickness requiring precise parameter control, and the potential for irreversible bonding during monomer transport and reaction in the gas phase, leading to structural defects. Chinese patent CN114478967B reports a method for preparing covalent organic framework thin films with a single-sided microsphere structure via liquid-liquid interfacial polymerization. However, this method's long preparation time and complex reaction steps limit its application in nanofiltration membranes. Chinese patent CN115010884B reports a covalent organic framework membrane assembled at a two-phase aqueous interface, achieving a breakthrough in interfacial assembly membranes in non-organic solvent systems. However, its flux is relatively low (1.7–15.4 L / h). -1 m -2 bar -1 The poor membrane uniformity and the inability to increase the flux of covalent organic framework membranes have limited their practical application in nanofiltration separation. Solving these problems has become a key technical challenge in their application in nanofiltration separation. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing covalent organic framework composite membranes using a gel vapor-assisted method. This method first dissolves two monomers in water to form a gel layer, utilizing the gel state as an intermediate step to allow the monomers to slowly and orderly arrange themselves. Then, vapor evaporation is used to aid in their crystallization and reconstruction, thereby preparing a covalent organic framework membrane. Compared to traditional chemical vapor deposition (CVD), this method uses water as a solvent, avoiding organic solvents, making it more environmentally friendly. The preparation process is gentle, requiring no high temperature or high vacuum, thus reducing equipment requirements and costs. It decouples the polymerization and crystallization processes, pre-organizes the spatial distribution of monomers in the gel layer, and regulates reaction kinetics through the vapor phase to solve the problem of defect accumulation in the rapid gas-phase reaction of traditional CVD. Furthermore, the thickness and porosity of the membrane can be precisely controlled by adjusting the gel layer thickness and vapor evaporation conditions to optimize performance. It is suitable for large-area, flexible membranes and has low substrate requirements.

[0005] To achieve the above-mentioned technical objectives, the present invention provides a method for preparing covalent organic framework composite membranes by a gel vapor-assisted method, comprising the following steps: dissolving amino functional monomers and 1,3,5-tricarboxymethyl phloroglucinol derivative monomers respectively in pure water, mixing them evenly, and then dropping them onto an alkali-pretreated base membrane to form a hydrogel, and finally performing a vapor-assisted reaction.

[0006] This invention employs a gel vapor-assisted method to prepare covalent organic framework composite membranes. Its core principle is to achieve controllable preparation of high-crystallinity, low-defect membranes under mild conditions through a combination of water solubility and dynamic regulation. Specifically, this method first dissolves an amino functional monomer and a 1,3,5-tricarboxymethyl phloroglucinol derivative monomer in water to form a homogeneous mixture, which is then dropwise added to the surface of an alkali-modified substrate membrane to form a gel layer. The formation of the gel state depends on the self-assembly of non-covalent interactions such as hydrogen bonds and π-π stacking between monomer molecules. Simultaneously, dynamic covalent bonds (such as the pre-equilibrium state of imine bonds) are introduced, and the three-dimensional network structure is dynamically regulated through reversible breakage and recombination, endowing the gel layer with stability and adaptability. This gel layer acts as a "pre-organized platform," utilizing the "spatial confinement" effect to promote the orderly arrangement of monomers, and the subsequent crystallization and reconstruction process is regulated by the mild driving force of vapor evaporation. At this stage, vapor evaporation not only provides the dynamic environment required for the reaction, but also promotes the gradual formation of a highly ordered COF lattice on the substrate surface by slowly removing moisture, thereby decoupling the polymerization and crystallization processes and overcoming the problem of defect accumulation caused by rapid gas phase reaction in traditional chemical vapor deposition. Compared to CVD, this method offers several advantages: First, the reaction conditions are mild, using water as a solvent instead of organic solvents at ambient pressure, eliminating the need for high temperatures, vacuum, or inert gas environments, significantly reducing equipment costs and energy consumption, while avoiding the use of toxic gases, aligning with green chemistry principles. Second, alkalization modification of the substrate surface introduces more active functional groups, enhancing the adsorption capacity and spreading uniformity of COF monomers, ensuring a tight bond between the substrate and the COF separation layer, and improving membrane structural stability. Third, the selection of water-soluble monomers and the dynamic control of the intermediate gel layer resolve the contradictions between crystallinity and defect rate, and between the adaptability of flexible substrates and high-temperature processes in traditional methods. This allows for precise adjustment of membrane thickness and porosity through gel layer thickness and vapor conditions, making it suitable for large-area preparation of both flexible (polymer, fiber) and rigid (silicon wafer, glass) substrates. Fourth, the uniform monomer distribution in the gel layer and the vapor-assisted slow nucleation mechanism effectively avoid the local overreaction problem caused by uneven gas phase transport in CVD, resulting in COF membranes with fewer defects and uniform thickness, exhibiting superior performance in separation efficiency and mechanical strength. In summary, this technology, through the innovative combination of solvent selection, dynamic gel regulation, and vapor-driven crystallization, provides an efficient, environmentally friendly, and universally applicable solution for the large-scale preparation of high-performance COF membranes.

[0007] As a preferred embodiment, the steam source for the steam-assisted reaction is an aqueous solution of acetic acid with a concentration of 3–6 mol / L.

[0008] As a preferred embodiment, the conditions for the steam-assisted reaction are: a temperature of 80–100°C and a reaction time of 9–24 h.

[0009] As a preferred embodiment, the molar ratio of the 1,3,5-tricarboxymethyl phloroglucinol derivative monomer to the amino functional monomer is 1 to 1.2.

[0010] As a preferred embodiment, the amino functional monomer is one of Formula I, Formula II, and Formula III;

[0011]

[0012]

[0013] Formula III; where R1 is H, SO3H, OH or COOH, x is Cl or Br; R is H or NH2.

[0014] As a preferred embodiment, the 1,3,5-tricarboxymethyl phloroglucinol derivative monomer has one of the following structural formulas:

[0015]

[0016] As a preferred embodiment, the concentration of the 1,3,5-tricarboxymethyl phloroglucinol derivative monomer is 0.02–0.06 mmol / L.

[0017] As a preferred embodiment, the alkalization pretreatment involves immersing the base film in sodium hydroxide for 1–2 hours; the concentration of the sodium hydroxide is 1–3 mol / L.

[0018] The present invention also provides a gel vapor-assisted method for preparing covalent organic framework composite membranes, which have a uniform and dense structure.

[0019] As a preferred option, the thickness of the separation layer is controlled to be 200 nm to 1 μm.

[0020] Finally, this invention also provides an application of a gel vapor-assisted method for preparing covalent organic framework composite membranes, which are used in liquid nanofiltration separation technology. The composite membrane prepared by this invention can achieve efficient and precise separation, and can efficiently separate antibiotics.

[0021] Compared with existing technologies, the innovativeness and beneficial technical effects of the present invention are as follows:

[0022] (1) The present invention uses an impregnation method to perform simple alkalization modification on the substrate, which improves the hydrophilicity of PVDF and enhances the uniform spreading of aqueous monomers on the base film.

[0023] (2) In this invention, a macroporous gel layer is pre-formed, and then the liquid medium is removed by gas-phase evaporation. This process is accompanied by a large volume shrinkage, which leads to the densification of the gel layer, completing the process from a disordered gel network structure to a regular and ordered three-dimensional network structure. Therefore, a dense and uniform separation layer can be obtained.

[0024] (3) The two-dimensional covalent organic framework composite membrane prepared by the preparation method of this application can efficiently separate antibiotics.

[0025] (4) This invention is the first to use the gel vapor-assisted method to prepare a two-dimensional covalent organic framework composite membrane. Attached Figure Description

[0026] Figure 1 This is a comparative schematic diagram of the preparation of COF composite membranes based on gel vapor assisted method in Example 1 and the preparation of COF composite membranes based on non-gel vapor assisted method in Comparative Example 2, where (a) is the gel vapor assisted method and the non-gel vapor assisted method; (b) is the non-gel vapor assisted method.

[0027] Figure 2 These are optical photographs of COF composite films prepared in the embodiments and comparative examples of the present invention, wherein (a) is the composite film prepared in Example 1; (b) is the composite film prepared in Comparative Example 1; (c) is the composite film prepared in Comparative Example 2; and (d) is the composite film prepared in Comparative Example 3.

[0028] Figure 3 The images shown are scanning electron microscope (SEM) images of COF composite films prepared in the embodiments and comparative examples of the present invention, wherein (a) is the composite film prepared in Example 1; (b) is the composite film prepared in Comparative Example 1; (c) is the composite film prepared in Comparative Example 2; and (d) is the composite film prepared in Comparative Example 3.

[0029] Figure 4 The images shown are cross-sectional scanning electron microscope (SEM) images of COF composite membranes prepared in the embodiments and comparative examples of the present invention, wherein (a) is the composite membrane prepared in Example 1; (b) is the composite membrane prepared in Comparative Example 1; (c) is the composite membrane prepared in Comparative Example 2; and (d) is the composite membrane prepared in Comparative Example 3.

[0030] Figure 5 These are comparative views of the surface water contact angle of the substrate and the COF composite membrane of the present invention, wherein, in (a) PVDF is the unmodified substrate, CPVDF is the modified substrate, and in (b) M1TG Cl -CPVDF is the composite membrane prepared in Example 1, M1TG Cl -PVDF is the composite membrane prepared in Comparative Example 1.

[0031] Figure 6 The image shows the retention effect of the composite membrane prepared in Example 1 on antibiotics of different molecular weights.

[0032] Figure 7 The image shows the retention effect of the composite membrane prepared in Example 1 on tetracycline desalting. Detailed Implementation

[0033] The technical solution of the present invention will be further described below with reference to specific embodiments. It should be noted that the described embodiments are only some embodiments of the present invention, and not all embodiments; these embodiments are only for better understanding of the present invention, and not for limiting the scope of protection of the present invention.

[0034] Unless otherwise specified, all reagents and raw materials used in this invention are commercially available products or products that can be prepared by known methods.

[0035] The room temperature in this invention is 25°C.

[0036] The preparation method of the triketene-aminotricarboxymethyl phloroglucinol monomer (M1) of the present invention is as follows:

[0037] Under nitrogen protection, 90 mL of trifluoroacetic acid was added to (15.1 g, 108 mmol) hexamethylenetetramine. After cooling to room temperature, phloroglucinol (9 g, 48 mmol) was added to the mixture. The mixture was heated at 100 °C for 2.5 h, and then 150 mL of 3 M hydrochloric acid solution was added, and the reaction was continued at 100 °C for 1 h. The reaction solution was filtered hot through diatomaceous earth, cooled to room temperature, and extracted with 3 x 200 mL DCM. The extract was dried over anhydrous sodium sulfate, filtered, and the solvent was removed under vacuum to obtain an orange-yellow solid. Washing with a large amount of ethanol gave a pink crude product. The crude product was dissolved in 160 mL of DCM, and mixed with 0.7 g of activated carbon. The mixture was stirred at 40 °C for 0.5 h, filtered, and the solvent was removed by rotary evaporation under reduced pressure to obtain a beige solid.

[0038] The chemical reaction equation is as follows:

[0039]

[0040] The preparation method of the aminoguanidine derivative of the present invention is as follows:

[0041] Under stirring, 1.91 g of guanidine hydrochloride was added to 10 mL of 1,4-dioxane. Then, 3.41 g of hydrazine hydrate was added. The mixture was refluxed for 2 h, cooled to room temperature, filtered and washed with 1,4-dioxane to remove excess hydrazine hydrate, and finally dried to obtain TG. Cl .

[0042] The chemical reaction equation is as follows:

[0043]

[0044] The substrate membrane used in the embodiments and comparative examples of this invention is a polyvinylidene fluoride membrane, purchased from Haining Delu New Material Technology Co., Ltd., with a pore size of 100 nm.

[0045] The testing method using scanning electron microscopy (SEM) in this invention is as follows: The COF composite film sample is sputtered with gold under an argon atmosphere and then tested under an accelerating voltage of 10 kV. The substrate of the COF composite film is then manually peeled off to obtain M1TG. Cl -COF layer samples were soaked in water and then quenched in liquid nitrogen in a moist state to obtain complete cross-sectional samples. After drying in a 35℃ drying oven for 24 hours, the cross-sectional morphology was observed and the thickness of the separation layer was measured.

[0046] The method for testing the hydrophilicity and hydrophobicity of the membrane surface using a water contact angle meter (SDC-100China, Dongguan) is as follows: The test water droplet size is 3 μL, and the water contact angle is measured using the angle measurement method. Measurements are taken at five different locations on the membrane, and the maximum and minimum values ​​are removed before taking the average. The membrane performance is tested using a UV-Vis spectrophotometer (UV 8100A).

[0047] The membrane liquid-phase separation performance testing method of this invention is as follows: The membrane interception rate and water flux are tested using a self-contained cross-flow filtration device. The concentrations of pollutant molecules on the feed and permeate sides are measured using a UV-Vis spectrophotometer (UV 8100A). The feed-side pressure is maintained at 1.0 MPa, and measurements are performed at room temperature. The effective membrane area is approximately 19.625 cm². 2 Record the water flux under steady-state conditions, collect the osmotic fluid, and use the average of at least three observations to ensure the accuracy of the values.

[0048] Example 1

[0049] This embodiment describes a gel vapor-assisted method for preparing covalent organic framework composite membranes (M1TG). Cl The preparation of -CPVDF is as follows:

[0050] At room temperature, polyvinylidene fluoride (PVDF) was first modified by alkalization in a glass petri dish. PVDF was immersed in a 2M NaOH solution for 1.5 hours. The modified membrane was then washed with deionized water until neutral and set aside. 0.04 mmol of 2,4,6-tris(pyrrolidine-1-ylmethylene)cyclohexane-1,3,5-trione monomer (M1) and 0.04 mmol of triaminoguanidine chloride were dissolved in 0.5 mL of ultrapure water, respectively. The two monomer solutions were mixed and sonicated until homogeneous. The mixed monomer solution was then uniformly added dropwise to the glass petri dish containing the modified PVDF membrane. The reaction was allowed to proceed for 10 minutes, forming a hydrogel layer on the surface. The hydrogel membrane was transferred to a self-built steam-assisted device. 20 mL of 6 mol / L HOAc was added to the bottom of the device, and the reaction was carried out at 90°C for 12 hours. The membrane was then removed and washed with ethanol.

[0051] The specific synthesis route is as follows:

[0052]

[0053] Example 2

[0054] This embodiment describes a gel vapor-assisted method for preparing covalent organic framework composite membranes (M1DG). Cl The preparation of -CPVDF is as follows:

[0055] At room temperature, polyvinylidene fluoride (PVDF) was first modified by alkalization in a glass petri dish. PVDF was immersed in a 2M NaOH solution for 1.5 hours. The modified membrane was then washed with deionized water until neutral and set aside. 0.04 mmol of 2,4,6-tris(pyrrolidine-1-ylmethylene)cyclohexane-1,3,5-trione monomer (M1) and 0.04 mmol of diaminoguanidine chloride were dissolved in 0.5 mL of ultrapure water, respectively. The two monomer solutions were mixed and sonicated until homogeneous. The mixed monomer solution was then uniformly added dropwise to the glass petri dish containing the modified PVDF membrane. The reaction was allowed to proceed for 10 minutes, forming a hydrogel layer on the surface. The gel membrane was transferred to a self-built steam-assisted device, with 20 mL of 6 mol / L HOAc added to the bottom. The reaction was carried out at 90°C for 12 hours. The membrane was then removed and washed with ethanol.

[0056] The specific synthesis route is as follows:

[0057]

[0058] Example 3

[0059] This embodiment describes the preparation of a covalent organic framework composite membrane (M1Pa-CPVDF) using a gel vapor assisted method, as detailed below:

[0060] At room temperature, polyvinylidene fluoride (PVDF) was first modified by alkalization in a glass petri dish. PVDF was immersed in a 2M NaOH solution for 1.5 hours. The modified membrane was then washed with deionized water until neutral and set aside. 0.04 mmol of 2,4,6-tris(pyrrolidine-1-ylmethylene)cyclohexane-1,3,5-trione monomer (M1) and 0.04 mmol of p-phenylenediamine were dissolved in 0.5 mL of ultrapure water, respectively. The two monomer solutions were mixed and sonicated until homogeneous. The mixed monomer solution was then uniformly added dropwise to the glass petri dish containing the modified PVDF membrane. The reaction was allowed to proceed for 10 minutes, forming a hydrogel layer on the surface. The gel membrane was transferred to a self-built steam-assisted device, with 20 mL of 6 mol / L HOAc added to the bottom. The reaction was carried out at 90°C for 12 hours. The membrane was then removed and washed and soaked in ethanol.

[0061] The specific synthesis route is as follows:

[0062]

[0063] Comparative Example 1

[0064] This comparative example is an M1TG. Cl -Preparation method of PVDF composite membrane:

[0065] The difference between this comparative example and Example 1 is that the polyvinylidene fluoride (PVDF) substrate was not modified with alkali. At room temperature, commercial PVDF was placed in a glass petri dish. 0.04 mmol of 2,4,6-tris(pyrrolidine-1-ylmethylene)cyclohexane-1,3,5-trione monomer (M1) and 0.04 mmol of triaminoguanidine chloride were dissolved in 0.5 mL of ultrapure water, respectively. The two monomer solutions were mixed and sonicated until homogeneous. The mixed monomer solution was then uniformly added dropwise to the glass petri dish containing the unmodified PVDF membrane. The reaction was allowed to proceed at room temperature for 15 minutes, resulting in a gel layer on the surface. 20 mL of 6 mol / L HOAc was added to the bottom of the apparatus, and the reaction was carried out at 90°C for 12 hours. The mixture was then removed and washed and soaked in ethanol.

[0066] The specific synthesis route is the same as in Example 1.

[0067] Comparative Example 2

[0068] This comparative example is a TPTG. Cl Preparation method of CPVDF composite membrane:

[0069] The difference between this comparative example and Example 1 is that the water-soluble triketeneamine tricarboxymethyl phloroglucinol monomer (M1) was replaced with tricarboxymethyl phloroglucinol monomer (TP). The reaction system did not form a gel. 0.04 mmol of TP monomer and 0.04 mmol of triaminoguanidine chloride were dissolved in 1,4-dioxane and ultrapure water, respectively. The two monomer solutions were mixed and sonicated until homogeneous. The mixed monomer solution was then dropwise added to a glass petri dish containing a modified polyvinylidene fluoride membrane. The dish was then directly transferred to a self-assembled steam-assisted device, and 20 ml of 6 mol / L HOAc was added to the bottom. The reaction was carried out at 90°C for 12 hours. The mixture was then removed and washed and soaked in ethanol.

[0070] The specific synthesis route is as follows:

[0071]

[0072] Comparative Example 3

[0073] This comparative example is an M1TG. Cl Preparation method of CPVDF-L composite membrane:

[0074] The difference between this comparative example and Example 1 is that, after the initial sol-gel film was formed, it was not steam-assisted; instead, the solvent was directly heated to evaporate into a dry gel film. 0.04 mmol of 2,4,6-tris(pyrrolidone-1-ylmethylene)cyclohexane-1,3,5-trione monomer (M1) and 0.04 mmol of triaminoguanidine chloride were dissolved in 0.5 mL of ultrapure water, respectively. The two monomer solutions were mixed and sonicated until homogeneous. The mixed monomer solution was then uniformly added dropwise to a glass petri dish containing a modified polyvinylidene fluoride membrane. The reaction was allowed to proceed for 10 minutes, during which a hydrogel layer formed on the surface. The initial gel film was then transferred to an 80°C oven to allow the moisture inside the gel film to evaporate completely.

[0075] The specific synthesis route is the same as in Example 1.

[0076] The COF membranes prepared in Examples 1-3 and Comparative Examples 1-3 were used for antibiotic separation in the following applications:

[0077] Four different sizes of antibiotic molecules were fed into an aqueous solution of 300 mL and filtered through a cross-flow filtration device at 0.5 MPa. The effective separation area of ​​each membrane was 19.625 cm². 2 Four different antibiotics were used as feed solutions: vancomycin (VA) 50 mg (molecular size 2.3*1.9 nm), rifampin (RFP) 50 mg (molecular size 1.7*1.3 nm), erythromycin (ERY) 50 mg (molecular size 1.48*1.26 nm), and tetracycline (TC) 50 mg (molecular size 1.3*0.7 nm). Each was dissolved in 1 L of water to obtain a 50 mg / L antibiotic feed solution. 5 ml of the filtrate was collected for testing.

[0078] The antibiotic concentrations in the feed and permeate were measured using UV-Vis spectroscopy. The percentage rejection (R%) was calculated using the formula:

[0079]

[0080] Among them, C F Indicates the antibiotic concentration in the feed solution; C P This indicates the concentration of antibiotics in the permeate.

[0081] The test results are shown in Table 1 below. Figures 6-7 As shown.

[0082] Table 1 shows the performance comparison data of COF membranes prepared in Examples 1-3 and Comparative Examples 1-3.

[0083]

[0084] From Table 1 above and Figures 6-7As can be seen, the gel vapor-assisted method of this invention achieves high density and interfacial stability of the COF separation layer through the synergistic effect of substrate modification, gel confined growth, and vapor-assisted reaction, and by carefully controlling the monomer concentration (i.e., gel concentration), evaporation time, and evaporation temperature. Compared with comparative examples 1-3, this method precisely controls the pore structure at the sub-nanometer scale, enabling a retention rate of over 99% for antibiotic molecules in the 1.3-2.3 nm range while maintaining practical throughput, providing a scalable preparation scheme for high-precision molecular sieve membranes.

[0085] An optical comparison was made between the COF composite film prepared by the gel vapor-assisted method in Example 1 and the COF composite film prepared by the non-gel vapor-assisted method in Comparative Example 2. Figure 1 As shown in the figure, M1TG Cl The system formed a uniformly mixed, smooth gel layer (right), while TPTG Cl The rapid precipitation of solid particles in the system (left) resulted in surface particle protrusions and uneven distribution. This indicates that M1TG... Cl The gel layer formed by the system provides a template for subsequent film formation, which is more conducive to the preparation of uniform and continuous films.

[0086] Optical photographs were taken comparing the COF composite films prepared in Example 1 and Comparative Examples 1-3. Figure 2 As shown, by Figure 2 As can be seen from a and 2b, this is a comparison between Example 1 and Comparative Example 1. When a COF composite membrane is prepared on an unmodified PVDF base membrane, the bonding force between the base membrane and the separation layer is weak, which causes the separation layer to detach easily. In contrast, when a COF composite membrane is prepared on a modified PVDF base membrane in Example 1, the bonding force between the COF separation layer and the modified base membrane is stronger, which is beneficial for subsequent liquid separation.

[0087] The COF composite films prepared in Example 1 and Comparative Examples 1-3 were analyzed by scanning electron microscopy, and the results are as follows: Figure 3 As shown, the morphology of COF growth on the PVDF substrate surface can be observed under high-magnification scanning electron microscopy. SEM comparison reveals the morphology of M1TG prepared via the gel vapor-assisted method. Cl -CPVDF membranes exhibit the best uniformity.

[0088] Cross-sectional scanning electron microscopy (SEM) images of the COF composite films prepared in Example 1 and Comparative Examples 1-3 were analyzed, and the results are as follows: Figure 4 As shown, the M1TG provided by this invention... Cl The separation layer thickness in the CPVDF composite membrane is 230 nm, while the thickness of the composite membranes prepared in Comparative Examples 1 to 3 is greater than that of the composite membrane prepared in Example 1, indicating that a thinner and more uniform membrane results in better permeability and retention separation performance.

[0089] The water contact angles of the substrates in the examples and comparative examples, after treatment and without treatment, as well as the corresponding composite films, were tested. The results are shown in the figure. Figure 5 As shown, the hydrophilicity of the base film is greatly improved after alkaline modification, which makes the COF layer and CPVDF base film bond more tightly.

[0090] The above embodiments are only for better explaining the principles and practical applications of the present invention, so that those skilled in the art can better understand and utilize the present invention, and are not intended to limit the patent scope of the present invention. All equivalent modifications made using the content of the present invention are within the patent protection scope of the present invention.

Claims

1. A method for preparing covalent organic framework composite membranes using a gel vapor-assisted method, characterized in that, Includes the following steps: The amino functional monomer and the 1,3,5-tricarboxymethyl phloroglucinol derivative monomer were dissolved in pure water, mixed evenly, and then dropped onto the polyvinylidene fluoride membrane after alkali pretreatment to form a hydrogel. Finally, a steam-assisted reaction was carried out. The steam source for the steam-assisted reaction is an aqueous solution of acetic acid; The amino functional monomer is one of Formula I and Formula II; Equation I; Formula II; The structural formula of the 1,3,5-tricarboxymethyl phloroglucinol derivative monomer is Formula IV: Formula IV; Where R1 is H, x is Cl; R is H or NH2.

2. The method for preparing covalent organic framework composite membranes using a gel vapor-assisted method according to claim 1, characterized in that: The concentration of the acetic acid aqueous solution is 3~6 mol / L.

3. The method for preparing a covalent organic framework composite membrane using a gel vapor-assisted method according to claim 2, characterized in that, The conditions for the steam-assisted reaction are: temperature of 80~100℃ and reaction time of 9~24h.

4. The method for preparing a covalent organic framework composite membrane using a gel vapor-assisted method according to claim 1, characterized in that, The molar ratio of the 1,3,5-tricarboxymethyl phloroglucinol derivative monomer to the amino functional monomer is 1 to 1.

2.

5. The method for preparing a covalent organic framework composite membrane using a gel vapor-assisted method according to claim 4, characterized in that, The alkali pretreatment involves immersing the polyvinylidene fluoride membrane in a sodium hydroxide solution for 1-2 hours; the concentration of the sodium hydroxide solution is 1-3 mol / L.

6. A gel vapor-assisted method for preparing covalent organic framework composite membranes, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 5.

7. The application of the gel vapor-assisted method for preparing covalent organic framework composite membranes as described in claim 6, characterized in that, It is used in liquid nanofiltration separation technology.

Images