Covalent organic framework composite membrane prepared by gel steam assisted method as well as preparation method and application of covalent organic framework composite membrane

Abstract

The invention relates to the technical field of polymer separation membranes, and discloses a method for preparing a covalent framework composite membrane under the assistance of gel steam, which comprises the following steps: mixing an aminoguanidine derivative or a water-soluble amine monomer, 1, 3, 4-trimethyl-1, 3, 4-trimethyl-1, 3, 4-trimethyl-1, 3, 4-trimethyl-1, 3, the preparation method comprises the following steps: respectively dissolving 1, 3, 5-triformyl phloroglucinol derivative monomers in pure water, uniformly mixing, dropwise adding onto a basilar membrane subjected to alkalization pretreatment to form hydrogel, and finally carrying out steam-assisted reaction. According to the preparation method disclosed by the invention, the uniform and compact membrane structure is formed on the substrate membrane by the polymer through gel steam assisted membrane formation, the continuous and uniform high-efficiency nanofiltration COF composite membrane is rapidly and simply prepared, and high-efficiency nanofiltration separation can be realized.

Description

Technical Field

[0001] The present invention relates to a preparation method and application of a covalent organic framework composite membrane by a gel vapor-assisted method, belonging to the technical field of polymer separation membranes. Background Art

[0002] Membrane separation technology is efficient, energy-saving, and widely used in various fields. With its high selectivity, low energy consumption, and environmental friendliness, it has become the key to industrial purification and separation. As an important branch, nanofiltration membrane technology has a pore size of about 1-10 nanometers and can achieve efficient and precise separation. It has significant advantages in water treatment, seawater desalination, wastewater recycling, and biopharmaceuticals. It can remove dissolved organic matter, heavy metal ions, and some salts, maintain low operating pressure and energy consumption, and support water resource management and environmental protection.

[0003] As an emerging membrane material, covalent organic framework (COF) exhibits a precisely defined topological structure and ordered nanopores due to its layered structure connected by strong planar covalent bonds and out-of-plane π-π interactions, which can effectively alleviate the "selectivity-permeability" trade-off problem commonly found in traditional polymer membranes. The highly crystalline nature and tunable pore size of COF membranes endow them with great application potential in fields such as gas separation, liquid separation, and catalysis. Despite the significant performance advantages of COF membranes, their preparation still faces many challenges. Traditional COF synthesis methods (such as solvothermal method and microwave heating method) 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 by chemical vapor deposition. By evaporating monomers at high temperature (usually several hundred degrees Celsius), in a vacuum or inert gas environment, and allowing them to react to form a film on a high-temperature-resistant substrate (such as metal, ceramic), but this method has problems such as easy local overreaction due to gas-phase transport, uneven film thickness, the need for precise parameter control, and the possible formation of irreversible bonds during the gas-phase transport and reaction of monomers, resulting in structural defects. Chinese Patent CN114478967B reports a method for preparing a covalent organic framework thin film with a single-sided microsphere structure by liquid-liquid interfacial polymerization to prepare COF membranes. However, due to the long preparation time and relatively cumbersome reaction steps of this method, its application in nanofiltration separation membranes is limited. Chinese Patent CN115010884B reports a covalent organic framework membrane assembled by aqueous two-phase interface, achieving a breakthrough in the interface assembly membrane of a non-organic solvent system. However, due to the low flux (1.7~15.4L h -1 m -2 bar -1 ), its practical application in nanofiltration separation membranes is limited. Solving the poor membrane uniformity and improving the flux of covalent organic framework membranes have become the key technical problems in solving their application in the field of nanofiltration separation. Summary of the Invention

[0004] Aiming at the defects existing in the prior art, the purpose of the present invention is to provide a preparation method of a covalent organic framework composite membrane by a gel vapor-assisted method. This method first dissolves two monomers in water to form a gel layer, and uses the gel state as an intermediate step to allow the monomers to slowly and orderly arrange. Then, it helps them to crystallize and reconstruct through steam evaporation, thereby preparing a covalent organic framework membrane. Compared with the traditional chemical vapor deposition method, this method uses water as a solvent to avoid organic solvents, is more environmentally friendly, has a mild preparation process, does not require high temperature and high vacuum, thus reducing equipment requirements and costs, decouples the polymerization and crystallization processes, and solves the problem of defect accumulation in the rapid gas-phase reaction of the traditional CVD method by pre-organizing the spatial distribution of monomers through the gel layer and regulating the reaction kinetics in the vapor phase. It can also precisely control the thickness and porosity of the membrane to optimize the performance by adjusting the gel layer thickness and steam evaporation conditions, and is applicable to large-area and flexible membranes with low requirements for the substrate.

[0005] To achieve the above technical purpose, the present invention provides a preparation method of a covalent organic framework composite membrane by a gel vapor-assisted method, including the following steps: an amino-functional monomer and a 1,3,5-triformylphloroglucinol derivative monomer are respectively dissolved in pure water, mixed evenly, and dropped onto a basified and pretreated substrate membrane to form a hydrogel, and finally a vapor-assisted reaction is carried out.

[0006] The technical solution of the present invention uses a gel vapor-assisted method to prepare a covalent organic framework composite membrane. The core principle is to achieve the controllable preparation of highly crystalline and low-defect membranes under mild conditions through the combination of water solubility and dynamic regulation. Specifically, in this method, an amino-functional monomer and a 1,3,5-triformylphloroglucinol derivative monomer are first dissolved in water to form a homogeneous mixed solution, which is then dropped onto the surface of a basified and 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. At the same time, dynamic covalent bonds (such as the pre-equilibrium state of imine bonds) are introduced to reversibly break and recombine to dynamically regulate the three-dimensional network structure, endowing the gel layer with stability and adaptability. This gel layer serves as a "pre-organization platform", using the "spatial confinement" effect to promote the ordered arrangement of monomers, and regulating the subsequent crystallization and reconstruction process through the mild driving force of steam evaporation. At this stage, steam evaporation not only provides the dynamic environment required for the reaction, but also promotes the gradual formation of a highly ordered COF lattice of monomers on the substrate surface by slowly removing water, realizing the decoupling of the polymerization and crystallization processes, thereby overcoming the problem of defect accumulation caused by rapid gas-phase reactions in the traditional chemical vapor deposition method. Compared with the CVD method, this solution has multiple advantages: First, the reaction conditions are mild. Under normal pressure, water is used as a solvent instead of organic solvents, without the need for high temperature, vacuum or inert gas environment, significantly reducing equipment costs and energy consumption, and at the same time avoiding the use of toxic gases, which conforms to the concept of green chemistry; Second, more active functional groups are introduced through the basification modification of the substrate membrane surface, enhancing the adsorption ability and spreading uniformity of COF monomers, ensuring the tight binding between the substrate and the COF separation layer, and improving the membrane structure stability; Third, the selection of water-soluble monomers and the dynamic regulation of the intermediate gel layer solve the contradictions between crystallinity and defect rate, flexibility of the substrate adaptability and high-temperature process in the traditional method, making the membrane thickness and porosity can be precisely adjusted through the gel layer thickness and steam conditions, suitable for large-area preparation of flexible (polymers, fibers) and rigid (silicon wafers, glass) substrates; Fourth, the uniform monomer distribution in the gel layer and the steam-assisted slow nucleation mechanism effectively avoid the problem of local over-reaction caused by uneven gas-phase transport in the CVD method. The obtained COF membrane has few defects and uniform thickness, and has more advantages in terms of separation efficiency, mechanical strength and other properties. In summary, through the innovative combination of solvent selection, dynamic gel regulation and steam-driven crystallization, this technology provides an efficient, environmentally friendly and highly versatile solution for the large-scale preparation of high-performance COF membranes.

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

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

[0009] As a preferred embodiment, the molar ratio of the 1,3,5-triformylphloroglucinol 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; wherein 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-triformylphloroglucinol derivative monomer is one of the following structural formulas:

[0015]

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

[0017] As a preferred embodiment, the alkalization pretreatment is to soak the base membrane with sodium hydroxide for 1 to 2 h; the concentration of the sodium hydroxide is 1 to 3 mol / L.

[0018] The present invention also provides a method for preparing a covalent organic framework composite membrane by a gel vapor-assisted method, and the composite membrane has a uniform and dense structure.

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

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

[0021] Compared with the prior art, the innovation and beneficial technical effects brought by the technical solution of the present invention:

[0022] (1) The present invention uses an impregnation method to simply alkalize and modify the substrate, improves the hydrophilicity of PVDF, and improves the uniform spreading of the aqueous monomer on the base membrane.

[0023] (2) Since the present invention pre-forms a macroporous gel layer and then removes the liquid medium by gas-phase evaporation, this process is accompanied by a large volume shrinkage, resulting in the densification of the gel layer and completing the process from an amorphous 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 the present application can efficiently separate antibiotics.

[0025] (4) The present invention first uses the gel vapor-assisted method to prepare a two-dimensional covalent organic framework composite membrane. Description of the Drawings

[0026] Figure 1 It is a comparative schematic diagram of the COF composite membrane prepared based on the gel vapor-assisted method in Example 1 and the COF composite membrane prepared without the 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 It is an optical photograph of the COF composite membranes prepared in the examples and comparative examples of the present invention. Among them, (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; (d) is the composite membrane prepared in Comparative Example 3.

[0028] Figure 3 It is a scanning electron micrograph of the COF composite membranes prepared in the examples and comparative examples of the present invention. Among them, (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; (d) is the composite membrane prepared in Comparative Example 3.

[0029] Figure 4 It is a cross-sectional scanning electron micrograph of the COF composite membranes prepared in the examples and comparative examples of the present invention. Among them, (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; (d) is the composite membrane prepared in Comparative Example 3.

[0030] Figure 5 It is a side-by-side comparison view of the surface water contact angles of the substrate and the COF composite membrane of the present invention. Among them, in (a), PVDF is the unmodified substrate and CPVDF is the modified substrate, and in (b), M1TG Cl -CPVDF is the composite membrane prepared in Example 1, and M1TG Cl -PVDF is the composite membrane prepared in Comparative Example 1.

[0031] Figure 6 It is the interception effect diagram of the composite membrane prepared in Example 1 for antibiotics with different molecular weights.

[0032] Figure 7 It is the interception effect diagram of the composite membrane prepared in Example 1 for the desalination of tetracycline. Detailed Embodiments

[0033] The technical solution of the present invention will be further described below in conjunction with specific embodiments of the present invention. It should be noted that the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments; these embodiments are only for better understanding of the present invention, rather than limiting the scope protected by the present invention.

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

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

[0036] The preparation method of the trione enamine type trimethanoylphloroglucinol monomer (M1) of the present invention is as follows:

[0037] Under the protection of nitrogen, 90 mL of trifluoroacetic acid was added to hexamine (15.1 g, 108 mmol). After cooling to room temperature, phloroglucinol (9 g, 48 mmol) was added to the mixed solution. After the mixed solution was heated at 100 °C for 2.5 h, 150 mL of 3 M hydrochloric acid solution was added and the reaction continued at 100 °C for 1 h. The reaction solution was filtered through diatomaceous earth while it was hot. After cooling to room temperature, it was extracted with 3 × 200 mL of DCM. The extract was dried over anhydrous sodium sulfate, filtered, and the solvent was removed under vacuum to obtain an orange solid. The pink crude product was obtained by rinsing with a large amount of ethanol. The crude product was dissolved in 160 mL of DCM, and 0.7 g of activated carbon was mixed and stirred at 40 °C for 0.5 h, filtered, and the solvent in the filtrate 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 conditions, 1.91 g of guanidine hydrochloride was added to 10 mL of 1,4-dioxane. 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 the excess hydrazine hydrate, and finally dried to obtain TG Cl 。

[0042] The chemical reaction equation is as follows:

[0043]

[0044] The base membranes used in the examples and comparative examples of the present invention are polyvinylidene fluoride membranes, purchased from Haining Deli New Materials Technology Co., Ltd., and the membrane pore size is 100 nm.

[0045] The test method of the present invention using a scanning electron microscope (SEM) is as follows: The COF composite film sample is sputtered with gold in an argon atmosphere and then tested under the condition of an acceleration voltage of 10 kV. Manually tear off the substrate of the COF composite film to obtain the M1TG Cl -COF layer sample. After soaking it in water, quench it in liquid nitrogen in a wet state to obtain a complete cross-section sample. After drying it in a drying oven at 35 °C for 24 h, it is used to observe its cross-sectional morphology and measure the thickness of the separation layer.

[0046] The method for testing the hydrophilicity and hydrophobicity of the film surface of the present invention using a water contact angle measuring instrument (SDC-100 China, Dongguan) is as follows: The size of the test water droplet is 3 μL. The water contact angle is measured by the goniometry method, and 5 different parts of the film are measured. After removing the maximum and minimum values, the average value is taken. When testing the film performance, the interception rate is measured using an ultraviolet-visible spectrophotometer (UV 8100A).

[0047] The test method for the liquid phase separation performance of the film of the present invention is as follows: A self-built cross-flow filtration device is used to test the interception rate and water flux of the film. The concentrations of pollutant molecules on the feed side and the permeate side are measured using an ultraviolet-visible spectrophotometer (UV 8100A). The pressure on the feed side of the film is maintained at 1.0 MPa, and the measurement is carried out at room temperature. The effective area of the film is about 19.625 cm 2 . Record the water flux under steady-state conditions, collect the liquid on the permeate side, and use the average value of at least 3 observations to ensure the accuracy of the value.

[0048] Example 1

[0049] This example is the preparation of a covalent organic framework composite film (M1TG Cl -CPVDF) by a gel vapor-assisted method, which is specifically as follows:

[0050] At room temperature, in a glass petri dish, first alkalize and modify polyvinylidene fluoride. Immerse polyvinylidene fluoride in a 2 M NaOH solution for 1.5 h for modification, and then wash the modified film with deionized water until it is neutral and set aside. Dissolve 0.04 mmol of 2,4,6-tris(pyrrolidin-1-ylmethylene)cyclohexane-1,3,5-trione monomer (M1) and 0.04 mmol of guanidine triamine chloride in 0.5 mL of ultrapure water respectively. Mix the two monomer solutions and ultrasonicate until uniform. Drop the mixed monomer solution evenly into the glass petri dish containing the modified polyvinylidene fluoride base film, react for 10 minutes, and a hydrogel layer is formed on the surface. Transfer the gel film to a self-built vapor-assisted device, add 20 ml of 6 mol / L HOAc at the bottom of the device, and react at 90 °C for 12 h. Take it out and wash and soak it with ethanol.

[0051] The specific synthesis route is as follows:

[0052]

[0053] Example 2

[0054] This example is the preparation of a covalent organic framework composite membrane (M1DG Cl -CPVDF) by a gel vapor-assisted method, which is as follows:

[0055] At room temperature, in a glass petri dish, first, polyvinylidene fluoride is alkalized and modified by impregnating it in a 2 M NaOH solution for 1.5 h. Then, the modified membrane is washed with deionized water until neutral and reserved. 0.04 mmol of 2,4,6-tris(pyrrolidin-1-ylmethylene)cyclohexane-1,3,5-trione monomer (M1) and 0.04 mmol of diaminoguanidine chloride are respectively dissolved in 0.5 mL of ultrapure water. The two monomer solutions are mixed and sonicated until homogeneous. The mixed monomer solution is evenly dropped into the glass petri dish containing the modified polyvinylidene fluoride base membrane and reacted for 10 minutes to form a hydrogel layer on the surface. The gel membrane is transferred to a self-built vapor-assisted device, and 20 ml of 6 mol / L HOAc is added to the bottom of the device, and the reaction is carried out at 90 °C for 12 h. Take it out and wash and soak it with ethanol.

[0056] The specific synthesis route is as follows:

[0057]

[0058] Example 3

[0059] This example is the preparation of a covalent organic framework composite membrane (M1Pa-CPVDF) by a gel vapor-assisted method, which is as follows:

[0060] At room temperature, in a glass petri dish, first, polyvinylidene fluoride is alkalized and modified by impregnating it in a 2 M NaOH solution for 1.5 h. Then, the modified membrane is washed with deionized water until neutral and reserved. 0.04 mmol of 2,4,6-tris(pyrrolidin-1-ylmethylene)cyclohexane-1,3,5-trione monomer (M1) and 0.04 mmol of p-phenylenediamine are respectively dissolved in 0.5 mL of ultrapure water. The two monomer solutions are mixed and sonicated until homogeneous. The mixed monomer solution is evenly dropped into the glass petri dish containing the modified polyvinylidene fluoride base membrane and reacted for 10 minutes to form a hydrogel layer on the surface. The gel membrane is transferred to a self-built vapor-assisted device, and 20 ml of 6 mol / L HOAc is added to the bottom of the device, and the reaction is carried out at 90 °C for 12 h. Take it out and wash and soak it with ethanol.

[0061] The specific synthesis route is as follows:

[0062]

[0064] Comparative Example 1

[0065] This comparative example is a preparation method of an M1TG Cl -PVDF composite membrane:

[0066] The difference between this comparative example and Example 1 is that the base polyvinylidene fluoride membrane is not alkalized and modified. At room temperature, in a glass petri dish, commercial polyvinylidene fluoride is placed. 0.04 mmol of 2,4,6-tris(pyrrolidin-1-ylmethylene)cyclohexane-1,3,5-trione monomer (M1) and 0.04 mmol of guanylthiourea chloride are respectively dissolved in 0.5 mL of ultrapure water. The two monomer solutions are mixed and ultrasonicated until uniform. The mixed monomer solution is uniformly dropped into the glass petri dish containing the unmodified polyvinylidene fluoride base membrane. At room temperature, the reaction is carried out for 15 minutes, and a gel layer is formed on the surface. 20 ml of HOAc with a concentration of 6 mol / L is added to the bottom of the device, and the reaction is carried out at 90 °C for 12 h. It is taken out and washed and soaked with ethanol.

[0067] The specific synthesis route is the same as that of Example 1.

[0068] Comparative Example 2

[0069] This comparative example is a preparation method of a TPTG Cl -CPVDF composite membrane:

[0070] The difference between this comparative example and Example 1 is that the water-soluble triketone enamine-form trimesoyl monomer (M1) is changed to trimesoyl monomer (TP), and the reaction system does not form a gel. 0.04 mmol of TP monomer and 0.04 mmol of guanylthiourea chloride are respectively dissolved in 1,4-dioxane and ultrapure water. The two monomer solutions are mixed and ultrasonicated until uniform. The mixed monomer solution is dropped into the glass petri dish containing the modified polyvinylidene fluoride base membrane, and it is directly transferred to a self-built steam-assisted device. 20 ml of HOAc with a concentration of 6 mol / L is added to the bottom of the device, and the reaction is carried out at 90 °C for 12 h. It is taken out and washed and soaked with ethanol.

[0071] The specific synthesis route is as follows:

[0072]

[0073] Comparative Example 3

[0074] This comparative example is a preparation method of an M1TG Cl -CPVDF-L composite membrane:

[0075] The difference between this comparative example and Example 1 is that after the sol-gel initial film is formed, it is directly heated to volatilize the solvent into a dry gel film without steam assistance. 0.04 mmol of 2,4,6-tris(pyrrolidin-1-ylmethylene)cyclohexane-1,3,5-trione monomer (M1) and 0.04 mmol of guanidine triamine chloride are respectively dissolved in 0.5 mL of ultrapure water. The two monomer solutions are mixed and ultrasonicated until uniform. The mixed monomer solution is evenly dropped into a glass petri dish containing a modified polyvinylidene fluoride membrane and reacted for 10 minutes to form a hydrogel layer on the surface. The initial gel film is transferred to an 80 °C oven until the water in the gel film evaporates completely.

[0076] The specific synthesis route is the same as that in Example 1.

[0077] The following is the application of the COF membranes prepared in Examples 1-3 and Comparative Examples 1-3 for antibiotic separation:

[0078] 300 mL of aqueous solutions of antibiotic molecules with 4 different sizes are respectively passed through a cross-flow filtration device with a pressure of 0.5 Mpa. The effective separation area of each membrane is 19.625 cm 2 . Among them, the 4 different aqueous solutions of antibiotic feeds are 50 mg of vancomycin (VA) with a molecular size of 2.3 * 1.9 nm, 50 mg of rifampicin (RFP) with a molecular size of 1.7 * 1.3 nm, 50 mg of erythromycin (ERY) with a molecular size of 1.48 * 1.26 nm, and 50 mg of tetracycline (TC) with a molecular size of 1.3 * 0.7 nm. They are respectively dissolved in 1 L of water to obtain aqueous solutions of antibiotic feeds with a concentration of 50 mg / L. 5 mL of the filtrate is collected for testing.

[0079] The antibiotic concentrations of the feed and permeate are measured by ultraviolet-visible spectroscopy. The rejection percentage (R%) is calculated according to the formula:

[0080]

[0081] Among them, C F represents the antibiotic concentration in the feed solution; C P represents the antibiotic concentration in the permeate.

[0082] The test results are shown in Table 1 and Figures 6-7 as follows.

[0083] Table 1 Comparative data on the performance of the COF membranes prepared in Examples 1-3 and Comparative Examples 1-3.

[0084]

[0085] From Table 1 above and Figures 6-7It can be seen that the gel vapor-assisted method of the present invention realizes the high densification and interfacial stability of the COF separation layer through the synergistic effect of three steps: 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 to 3, this method precisely regulates the pore structure at the sub-nanometer scale, enabling the rejection rate of 1.3 - 2.3 nm antibiotic molecules to exceed 99%, while maintaining the practical flux, providing a scalable preparation scheme for high-precision molecular sieve membranes.

[0086] Optical comparison was carried out between the COF composite membrane prepared by the gel vapor-assisted method in Example 1 and the COF composite membrane prepared by the non-gel vapor-assisted method in Comparative Example 2. As Figure 1 shown, it can be seen from the figure that the M1TG Cl system formed a uniformly mixed and smooth gel layer (right), while in the TPTG Cl system (left), solid particles precipitated rapidly, resulting in surface particle protrusions and uneven distribution. This shows that the gel layer formed by the M1TG Cl system provided a template for subsequent film formation, being more conducive to the preparation of a uniform and continuous film.

[0087] Optical photos of the COF composite membranes prepared in Example 1 and Comparative Examples 1 to 3 were compared. As Figure 2 shown, it can be seen from Figure 2 a and 2b that it is a comparison between Example 1 and Comparative Example 1. When preparing the COF composite membrane on the unmodified PVDF substrate membrane, the bonding force between the substrate membrane and the separation layer was weak, resulting in the separation layer peeling off easily with a slight touch. While in Example 1, when preparing the COF composite membrane on the modified PVDF substrate membrane, the bonding force between the COF separation layer and the modified substrate membrane was stronger, which is beneficial for subsequent use in liquid separation.

[0088] The COF composite membranes prepared in Example 1 and Comparative Examples 1 to 3 were analyzed by scanning electron microscopy. The results are as Figure 3 shown. Under high-magnification scanning electron microscopy, the morphology of the COF growth on the surface of the PVDF substrate membrane can be seen. Through SEM comparison, the M1TG Cl -CPVDF membrane prepared by the gel vapor-assisted method has the best uniformity.

[0089] The cross-sectional scanning electron micrographs of the COF composite membranes prepared in Example 1 and Comparative Examples 1 to 3 were analyzed. The results are as Figure 4 shown. It can be seen that the thickness of the separation layer in the M1TG Cl -CPVDF composite membrane provided by the present invention is 230 nm, while the thicknesses of the composite membranes prepared in Comparative Examples 1 to 3 are all greater than that of the composite membrane prepared in Example 1, indicating that the thinner and more uniform membrane brings better permeability and rejection separation performance.

[0090] The water contact angles of the substrates after treatment and before treatment in the examples and comparative examples, as well as the corresponding composite films prepared, were tested, and the results are shown in Figure 5 As can be seen, the hydrophilicity of the base film after alkaline modification is greatly improved, which improves the hydrophilicity of the base film and makes the combination between the COF layer and the CPVDF base film closer.

[0091] The above embodiments are only for better explaining the principle and practical application of the present invention, so that those skilled in the relevant technical fields can better understand and utilize the present invention. It does not limit the patent scope of the present invention. Any equivalent transformation made by using the content of the present invention is within the patent protection scope of the present invention.

Claims

1. A preparation method of a covalent organic framework composite membrane by a gel vapor-assisted method, characterized in that, It includes the following steps: Dissolve the amino-functional monomer and the 1,3,5-triformylphloroglucinol derivative monomer in pure water respectively, then mix them evenly and drop them onto the basified and pretreated substrate membrane to form a hydrogel, and finally carry out a steam-assisted reaction.

2. The preparation method of a covalent organic framework composite membrane prepared by a gel vapor-assisted method according to claim 1, characterized in that: The steam source for the steam-assisted reaction is an aqueous acetic acid solution with a concentration of 3 - 6 mol / L.

3. The preparation method of a covalent organic framework composite membrane prepared by a gel vapor assisted method according to claim 2, characterized in that, The conditions for the steam-assisted reaction are: the temperature is 80 - 100 °C, and the reaction time is 9 - 24 h.

4. The preparation method of a covalent organic framework composite membrane prepared by a gel vapor-assisted method according to claim 1, characterized in that, The molar ratio of the 1,3,5-triformylphloroglucinol derivative monomer to the amino-functional monomer is 1 - 1.

2.

5. The preparation method of a covalent organic framework composite membrane prepared by a gel vapor-assisted method according to claim 4, characterized in that, The amino-functional monomer is one of Formula I, Formula II and Formula III; Formula III; where R1 is H, SO3H, OH or COOH, x is Cl or Br; R is H or NH2.

6. The preparation method of a covalent organic framework composite membrane prepared by a gel vapor-assisted method according to claim 4, wherein The 1,3,5-triformylphloroglucinol derivative monomer is one of the following structural formulas:

7. The preparation method of a covalent organic framework composite membrane prepared by a gel vapor-assisted method according to claim 1, characterized in that, The concentration of the 1,3,5-triformylphloroglucinol derivative monomer is 0.02 - 0.06 mmol / L.

8. The preparation method of a covalent organic framework composite membrane prepared by a gel vapor-assisted method according to claim 6, wherein, The basified pretreatment is to soak the substrate membrane with sodium hydroxide for 1 - 2 h; the concentration of the sodium hydroxide is 1 - 3 mol / L.

9. A method for preparing a covalent organic framework composite membrane by gel vapor assisted method, characterized in that, Prepared by the preparation method according to any one of claims 1 - 8.

10. Use of a method for preparing a covalent organic framework composite membrane by gel vapor assisted method according to claim 9, characterized in that, It is used in the liquid nanofiltration separation technology.

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