A light-responsive covalent organic framework nanosheet and a preparation method thereof, a light-responsive covalent organic framework composite film and a preparation method and application thereof

Abstract

The application provides a kind of photoresponsive covalent organic framework nanosheet and its preparation method, photoresponsive covalent organic framework composite film and its preparation method and application, it is related to nanofiltration technical field.The application first uses solvothermal synthesis to have the covalent organic framework material with high crystallinity, then by ultraviolet light irradiation, small molecule containing photoresponsive group and charged group is introduced into covalent organic framework material by post-modification method, under light condition, photoresponsive group occurs cis-trans isomerization, resulting in the change of molecular size and configuration, the steric hindrance effect and charged repulsion effect generated by isomerization are utilized in the application, synergistically weaken the interlayer pi-pi interaction of covalent organic framework, realize the nondestructive self-peeling of photoresponsive covalent organic framework nanosheet.The photoresponsive covalent organic framework nanosheet obtained by the preparation method provided by the application has high crystallinity and large horizontal-vertical ratio.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration technology, specifically to a photoresponsive covalent organic framework nanosheet and its preparation method, a photoresponsive covalent organic framework membrane and its application, and a photoresponsive covalent organic framework composite membrane and its preparation method and application. Background Technology

[0002] Nanofiltration, as a highly efficient, green, and environmentally friendly membrane separation technology, possesses unique separation advantages and application prospects. Membrane materials are the core of membrane separation technology, and high-performance membrane materials are key to achieving efficient separation. Currently, traditional polymer membrane materials, represented by polyamides, have become the mainstream materials for nanofiltration membranes due to their good film-forming properties and strong processability. However, limited by the mutual constraints between the rigidity of chain segments and the spacing between chain segments in the polymer structure, polymer membranes struggle to simultaneously achieve high permeability and high selectivity, becoming a bottleneck problem restricting the development of nanofiltration membranes.

[0003] Covalent organic frameworks (COFrames) are a class of crystalline porous polymers formed by the covalent bonding of organic molecules through reactions. Due to their diverse structural units, large specific surface area, regular framework structure, and ease of modification and control, coupled with the fact that the pore sizes of COFrames are mostly between 1 and 4 nm, they hold great promise for nanofiltration membrane separation. Methods for preparing COFrame membranes, including in-situ growth, interfacial polymerization, and nanosheet self-assembly, have been developed. However, in-situ growth and interfacial polymerization methods involve monomer polymerization and polymer nucleation and crystallization processes, which are strongly coupled, making process control challenging. In contrast, the method of preparing COFrame nanosheets and then self-assembling them into membranes allows for flexible control of the membrane formation process, while ensuring good orientation, effectively reducing mass transfer resistance and increasing permeation flux. Therefore, it is considered an effective method for constructing COFrame membranes.

[0004] However, traditional methods such as mechanical and chemical exfoliation result in low yields / monolayer ratios and small sheet sizes for obtaining covalent organic framework nanosheets. Furthermore, the exfoliation process may disrupt covalent bonds, reducing the crystallinity of the covalent organic framework and leading to defects during subsequent film assembly. Therefore, obtaining non-destructive covalent organic framework nanosheets with high crystallinity and high aspect ratio is crucial for the preparation of nanosheet self-assembly films. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a photoresponsive covalent organic framework nanosheet and its preparation method, a photoresponsive covalent organic framework film and its application, and a photoresponsive covalent organic framework composite film and its preparation method and application. The photoresponsive covalent organic framework nanosheets prepared by the method provided by this invention have high crystallinity and large aspect ratio, and the photoresponsive covalent organic framework nanosheets are undamaged.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for preparing photoresponsive covalent organic framework nanosheets, comprising the following steps:

[0008] An amino monomer, an aldehyde monomer, a catalyst, and a first organic solvent are mixed and subjected to a solvothermal reaction to obtain a covalent organic framework material.

[0009] The covalent organic framework material, a second organic solvent, triethylamine, and a compound containing photoresponsive groups are mixed and subjected to a photochemical reaction under ultraviolet light irradiation to obtain photoresponsive covalent organic framework nanosheets.

[0010] Preferably, the molar ratio of the amino group in the amino monomer to the aldehyde group in the aldehyde monomer is 1:1;

[0011] Both the amino monomer and the aldehyde monomer contain active functional groups, which independently include one or more of phenolic hydroxyl, imine and methyl groups;

[0012] The catalyst includes one or more of acetic acid, octanoic acid, scandium trifluoromethanesulfonate, and p-toluenesulfonic acid;

[0013] The first organic solvent includes one or more of dioxane, mesitylene, N,N-dimethylformamide, dichloromethane, acetonitrile, o-dichlorobenzene, n-butanol, and ethyl acetate;

[0014] The solvothermal reaction was carried out at a temperature of 120°C for 72 hours.

[0015] The covalent organic framework material has a pore size of 2–5 nm.

[0016] Preferably, the compound containing the photoresponsive group includes p-phenylazobenzoyl chloride and / or azobenzene-4,4'-dicarbonyl chloride;

[0017] The mass ratio of the covalent organic framework material to the photoresponsive compound is 1:1.5 to 3;

[0018] The mass ratio of the covalent organic framework material to triethylamine is 1:30-50;

[0019] The second organic solvent includes one or more of tetrahydrofuran, N,N-dimethylformamide, and dimethyl sulfoxide;

[0020] The photochemical reaction is carried out at a temperature of 0–80°C for a time of 12–48 h.

[0021] This invention provides photoresponsive covalent organic framework nanosheets prepared by the preparation method described above.

[0022] This invention provides a photoresponsive covalent organic framework membrane, which is obtained by forming a film from the photoresponsive covalent organic framework nanosheets described in the above technical solution.

[0023] The present invention provides a photoresponsive covalent organic framework composite film, comprising a support layer and the photoresponsive covalent organic framework film described in the above-mentioned technical solution located on the surface of the support layer.

[0024] Preferably, the support layer comprises a polymer ultrafiltration membrane or an inorganic porous carrier.

[0025] The present invention provides a method for preparing the photoresponsive covalent organic framework composite membrane described above, comprising the following steps: a dispersion of covalent organic framework nanosheets is subjected to water dialysis and then placed on the surface of a support material to form a film, thereby obtaining a photoresponsive covalent organic framework composite membrane; wherein the dialysis bag used for water dialysis has a molecular weight cutoff ≥10000 Da.

[0026] Preferably, the film formation includes vacuum-assisted self-assembly.

[0027] This invention provides the application of the photoresponsive covalent organic framework membrane, the photoresponsive covalent organic framework composite membrane, or the photoresponsive covalent organic framework composite membrane prepared by the preparation method described above in nanofiltration separation.

[0028] This invention first synthesizes highly crystalline covalent organic framework materials using solvothermal methods. Then, through ultraviolet light irradiation, small molecules containing photoresponsive and charged groups are introduced into the covalent organic framework material via a post-modification method. Under light irradiation, the photoresponsive groups undergo cis-trans isomerization, leading to changes in molecular size and configuration. This invention utilizes the steric hindrance effect and charged repulsion generated by isomerization to synergistically weaken the interlayer π-π interactions of the covalent organic framework, achieving non-destructive self-exfoliation of photoresponsive covalent organic framework nanosheets. The photoresponsive covalent organic framework nanosheets obtained by the preparation method provided by this invention have high crystallinity and a large aspect ratio.

[0029] The photoresponsive covalent organic framework nanosheets provided by this invention have high crystallinity and high aspect ratio. Through light stimulation (UV / Vis light switching), the pore size, charge distribution, and hydrophilicity of photoresponsive covalent organic framework membranes and composite membranes based on these nanosheets can be varied and precisely controlled within a certain range. For example, after UV irradiation, the pore size increases, the charge density decreases, and the hydrophobicity of the pores increases; after visible light irradiation, the pore size decreases, the charge density increases, and the hydrophilicity of the pores increases. This achieves a synergistic enhancement of selectivity and permeability in nanofiltration separation, improving the separation performance of the photoresponsive covalent organic framework membrane and demonstrating excellent application prospects in nanofiltration separation.

[0030] As shown in the test results of the examples, the photoresponsive covalent organic framework composite membrane provided by the present invention exhibits a methylene blue removal rate of 88.3%–99.4% and an ethanol flux of 7.3–27.1 L·m⁻¹ under ultraviolet light irradiation. -1 ·h -1 ·bar -1 Under visible light irradiation, the removal rate of methylene blue was 91.5%–99.5%, and the ethanol flux was 4.5–17.4 L·m⁻¹. -1 ·h -1 ·bar -1 . Attached Figure Description

[0031] Figure 1 Transmission electron microscope images of the covalent organic framework material (a) and photoresponsive covalent organic framework nanosheets (b) prepared in Example 1;

[0032] Figure 2 Atomic force microscopy image and nanosheet thickness of the photoresponsive covalent organic framework nanosheets prepared in Example 1;

[0033] Figure 3 XRD pattern of the photoresponsive covalent organic framework nanosheets prepared in Example 1;

[0034] Figure 4 Scanning electron microscope images of the surface (a) and cross-section (b) of the photoresponsive covalent organic framework membrane prepared for Example 1;

[0035] Figure 5 The cis-trans structures of the photoresponsive covalent organic framework nanosheets and photoresponsive covalent organic framework films prepared in Example 1 after ultraviolet irradiation and visible light irradiation;

[0036] Figure 6 The cis-trans structures of the photoresponsive covalent organic framework nanosheets and photoresponsive covalent organic framework films prepared in Example 5 after ultraviolet irradiation and visible light irradiation;

[0037] Figure 7 The cis-trans structures of the photoresponsive covalent organic framework nanosheets and photoresponsive covalent organic framework films prepared in Example 6 after ultraviolet irradiation and visible light irradiation. Detailed Implementation

[0038] This invention provides a method for preparing photoresponsive covalent organic framework nanosheets, comprising the following steps:

[0039] An amino monomer, an aldehyde monomer, a catalyst, and a first organic solvent are mixed and subjected to a solvothermal reaction to obtain a covalent organic framework material.

[0040] The covalent organic framework material, a second organic solvent, triethylamine, and a compound containing photoresponsive groups are mixed and subjected to a photochemical reaction under ultraviolet light irradiation to obtain photoresponsive covalent organic framework nanosheets.

[0041] Unless otherwise specified, the materials and equipment used in this invention are all commercially available products in the field.

[0042] This invention involves mixing an amino monomer, an aldehyde monomer, a catalyst, and a first organic solvent, and then carrying out a solvothermal reaction to obtain a covalent organic framework material.

[0043] In this invention, both the amino monomer and the aldehyde monomer preferably contain active functional groups, and the active functional groups preferably include one or more of phenolic hydroxyl, imine and methyl groups.

[0044] In this invention, the amino monomer preferably includes one or more of 1,3,5-tris(4-aminophenyl)benzene (TAPB), 4,4'-diaminobenzoylaniline (DABA), and 2,5-dihydroxyterephthalamide (DBDH).

[0045] In this invention, the aldehyde monomer preferably includes one or more of trimethylbenzaldehyde (BTCA), trialdehyde phloroglucinol (TFP), and 2,5-dihydroxyterephthalaldehyde (DHTP).

[0046] In this invention, the molar ratio of the amino group in the amino monomer to the aldehyde group in the aldehyde monomer is preferably 1:1.

[0047] In this invention, the catalyst preferably includes one or more of acetic acid, octanoic acid, scandium trifluoromethanesulfonate, and p-toluenesulfonic acid, and the catalyst acts to catalyze the Schiff base reaction between aldehyde monomers and amino monomers.

[0048] In this invention, the first organic solvent serves to dissolve the aldehyde monomer and the amino monomer. The organic solvent is selected based on the types of aldehyde and amino monomers. Preferably, the first organic solvent includes one or more of dioxane, mesitylene, N,N-dimethylformamide, dichloromethane, acetonitrile, o-dichlorobenzene, n-butanol, and ethyl acetate. More preferably, it is a n-butanol-o-dichlorobenzene mixed solvent or a mesitylene-1,4-dioxane mixed solvent. The volume ratio of n-butanol to o-dichlorobenzene in the n-butanol-o-dichlorobenzene mixed solvent is preferably 1:0.5–5, more preferably 1:1–2. The volume ratio of mesitylene to 1,4-dioxane in the mesitylene-1,4-dioxane mixed solvent is preferably 1:0.5–10, more preferably 1:1–3.

[0049] In this invention, the mixing preferably includes: dissolving the amino monomer and the aldehyde monomer in a first organic solvent, adding a catalyst, and then remixing. In this invention, the dissolution is preferably performed under ultrasonic conditions, and the dissolution time is preferably 5–30 min, more preferably 10–20 min. In this invention, the remixing is preferably performed under ultrasonic conditions, and the remixing time is preferably 5–30 min, more preferably 10–20 min.

[0050] In this invention, oxygen is preferably removed from the obtained mixture before a solvothermal reaction is carried out. The oxygen removal method preferably includes a freeze-thaw cycle. Specifically, the freeze-thaw cycle includes: freezing the mixture in liquid nitrogen for 5-10 minutes, then evacuating the air from the reaction flask for 5-10 minutes, and then thawing the reaction flask at room temperature under sealed conditions; this process is repeated three times.

[0051] In this invention, the temperature of the solvothermal reaction is preferably 120°C, and the time is preferably 72 h.

[0052] After completing the solvothermal reaction, the present invention preferably further includes: cooling the system obtained from the solvothermal reaction to room temperature, performing solid-liquid separation, washing and drying the resulting precipitate to obtain a covalent organic framework material. In the present invention, the solid-liquid separation preferably includes centrifugal separation. In the present invention, the washing preferably includes sequential tetrahydrofuran washing and acetone washing; the number of tetrahydrofuran washing and acetone washing is preferably 2 to 5 times, more preferably 3 to 4 times. In the present invention, the drying temperature is preferably 50 to 120°C, more preferably 60 to 80°C; the drying time is preferably 20 to 40 hours, more preferably 24 to 30 hours.

[0053] In this invention, the pore size of the covalent organic framework material is preferably 2-5 nm, more preferably 3-4 nm.

[0054] After obtaining the covalent organic framework material, the present invention mixes the covalent organic framework material, a second organic solvent, triethylamine and a compound containing a photoresponsive group, and carries out a photochemical reaction under ultraviolet light irradiation to obtain photoresponsive covalent organic framework nanosheets.

[0055] In this invention, the compound containing the photoresponsive group preferably includes p-phenylazobenzoyl chloride and / or azobenzene-4,4'-dicarbonyl chloride. In this invention, the mass ratio of the covalent organic framework material to the photoresponsive compound is preferably 1:1.5–3, more preferably 1:1.5–2.5, and even more preferably 1:1.5–2.

[0056] In this invention, the mass ratio of the covalent organic framework material to triethylamine is preferably 1:30-50, more preferably 1:35-45, and even more preferably 1:35-40.

[0057] In this invention, the second organic solvent preferably includes one or more of tetrahydrofuran, N,N-dimethylformamide, and dimethyl sulfoxide. In this invention, the mass ratio of the covalent organic framework material to the second organic solvent is preferably 1:1500–2500, more preferably 1:1800–2200, and even more preferably 1:2000.

[0058] In this invention, the mixing preferably includes: dispersing a covalent organic framework material in a portion of a second organic solvent, adding triethylamine and mixing to obtain solution A; dissolving a compound containing a photoresponsive group in the remaining second organic solvent to obtain solution B; and adding solution B dropwise to solution A under stirring conditions. In this invention, the volume ratio of the portion of the second organic solvent to the remaining second organic solvent is preferably 5–50:1, more preferably 10–20:1. In this invention, the temperature of solution A during dropwise addition is preferably 0–80°C, more preferably 0–60°C, and even more preferably 30–60°C; the temperature of solution A during dropwise addition is preferably the same as the temperature of the photochemical reaction.

[0059] In this invention, the temperature of the photochemical reaction is preferably 0–80°C, more preferably 0–60°C, and even more preferably 30–60°C; the time of the photochemical reaction is preferably 12–48 h, more preferably 24–48 h, and the time of the photochemical reaction begins when the B solution is completely added; the wavelength of the ultraviolet light is preferably 365 nm. In this invention, during the photochemical reaction, under ultraviolet light irradiation, the active functional groups in the covalent organic framework material react chemically with compounds containing photoresponsive groups. Specifically, small molecules of compounds containing photoresponsive groups and charged groups are introduced into the covalent organic framework material through a post-modification method. Under ultraviolet light irradiation, the photoresponsive groups undergo cis-trans isomerization, resulting in changes in molecular size and configuration. This invention utilizes the steric hindrance effect and charged repulsion generated by isomerization to synergistically weaken the π-π interaction forces between the layers of the covalent organic framework, thereby achieving non-destructive self-exfoliation of photoresponsive covalent organic framework nanosheets.

[0060] After the photochemical reaction is completed, the present invention preferably further includes: centrifuging the reaction system obtained by the photochemical reaction and taking the supernatant to obtain photoresponsive covalent organic framework nanosheets (a dispersion of photoresponsive covalent organic framework nanosheets).

[0061] This invention provides photoresponsive covalent organic framework nanosheets prepared by the method described above. In this invention, the photoresponsive covalent organic framework nanosheets contain azobenzene photoresponsive groups.

[0062] This invention provides a photoresponsive covalent organic framework membrane, which is obtained by forming a film from the photoresponsive covalent organic framework nanosheets described in the above technical solution.

[0063] In this invention, the preferred method for preparing the photoresponsive covalent organic framework membrane includes the following steps: water dialysis of a dispersion of covalent organic framework nanosheets to form a membrane, thereby obtaining a photoresponsive covalent organic framework composite membrane; wherein the dialysis bag used for water dialysis has a molecular weight cutoff of ≥10000 Da.

[0064] In this invention, the dispersion of the covalent organic framework nanosheets is preferably the supernatant after centrifugation of the reaction system obtained from the photochemical reaction.

[0065] In this invention, the dialysis bag used for water dialysis has a molecular weight cutoff of ≥10000 Da, preferably 10000–30000 Da, and more preferably 10000–15000 Da; the water used for water dialysis is preferably deionized water; and the dialysis time is preferably 1–5 days, more preferably 2–3 days. This invention uses a dialysis bag with a molecular weight cutoff of ≥10000 Da for filtration, aiming to obtain high aspect ratio covalent organic framework nanosheets, thereby preparing a dense covalent organic framework membrane.

[0066] After water dialysis, the present invention preferably further includes dilution to obtain a diluted nanosheet dispersion, which is then used to form a film. In the present invention, the dilution factor is preferably 50 to 300 times, more preferably 200 to 250 times.

[0067] In this invention, the film formation preferably includes vacuum-assisted self-assembly, and the vacuum degree of the vacuum-assisted self-assembly is preferably -100 to -50 kPa. This invention does not have a special limitation on the time of the vacuum-assisted self-assembly; the solvent, including the pores, can be dried.

[0068] This invention provides a method for preparing highly crystalline photoresponsive covalent organic framework nanosheets and self-assembled membranes (photoresponsive covalent organic framework membranes). Through photo-assisted exfoliation, under ultraviolet light irradiation, a post-modification strategy is employed to introduce photoresponsive groups and charged groups into the side chains of solvothermal synthesized covalent organic frameworks. This fully utilizes steric hindrance and charge repulsion to synergistically weaken the interlayer π-π interactions of the covalent organic framework, achieving non-destructive self-exfoliation of the nanosheets. This invention uses a vacuum-assisted self-assembly method to obtain photoresponsive covalent organic framework membranes. In nanofiltration separation applications, the physicochemical microenvironment, including pore size, charge distribution, and hydrophilicity, can be controlled by freely switching between ultraviolet and visible light irradiation, thereby improving the separation performance of the covalent organic framework membrane.

[0069] The present invention provides a photoresponsive covalent organic framework composite film, comprising a support layer and the photoresponsive covalent organic framework film described in the above-mentioned technical solution located on the surface of the support layer.

[0070] In this invention, the support layer comprises a polymeric ultrafiltration membrane or an inorganic porous carrier; the polymeric ultrafiltration membrane is preferably made of one or more of polyacrylonitrile, polyethersulfone, and polyvinylidene fluoride; the inorganic porous carrier is preferably made of one or more of alumina, silicon oxide, and zirconium oxide.

[0071] The present invention provides a method for preparing the photoresponsive covalent organic framework composite membrane described above, comprising the following steps: a dispersion of covalent organic framework nanosheets is subjected to water dialysis and then placed on the surface of a support material to form a film, thereby obtaining a photoresponsive covalent organic framework composite membrane; wherein the dialysis bag used for water dialysis has a molecular weight cutoff ≥10000 Da.

[0072] In this invention, the dispersion, water dialysis, and film formation of the covalent organic framework nanosheets are the same as those for the aforementioned photoresponsive covalent organic framework membranes, and will not be repeated here.

[0073] This invention provides the application of the photoresponsive covalent organic framework membrane, the photoresponsive covalent organic framework composite membrane, or the photoresponsive covalent organic framework composite membrane prepared by the preparation method described above in nanofiltration separation.

[0074] The photoresponsive covalent organic framework membrane and photoresponsive covalent organic framework composite membrane provided by this invention can, in nanofiltration separation applications, freely switch between ultraviolet and visible light to regulate the physicochemical microenvironment of the covalent organic framework membrane, such as pore size, charge distribution, and hydrophilicity. This enables the synergistic enhancement of the selectivity and permeability of the covalent organic framework membrane in nanofiltration separation, thereby improving the separation performance of the photoresponsive covalent organic framework membrane and showing great application prospects in nanofiltration separation.

[0075] To further illustrate the present invention, the following detailed descriptions, in conjunction with embodiments, describe photoresponsive covalent organic framework nanosheets and their preparation methods, photoresponsive covalent organic framework films and their applications, and photoresponsive covalent organic framework composite films and their preparation methods and applications. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0076] Example 1

[0077] Step 1: Mix 56 mg of 2,5-dihydroxyterephthalaldehyde (DHTP) and 40 mg of 1,3,5-tris(4-aminophenyl)benzene (TAPB), dissolve in a mixed solvent of 1 mL n-butanol and 1 mL o-dichlorobenzene, sonicate for 10 min, add 0.2 mL of 6 mol / L acetic acid aqueous solution, continue sonicating for 10 min to mix, remove oxygen by freezing and thawing, then react at 120 °C for 72 h. After the reaction is completed, cool to room temperature, collect the precipitate by centrifugation, wash 3 times with tetrahydrofuran and 3 times with acetone, and vacuum dry at 60 °C for 24 h to obtain TAPB-DHTP type covalent organic framework material;

[0078] Step 2: 20 mg of TAPB-DHTP type covalent organic framework powder was ultrasonically dispersed in 50 mL of tetrahydrofuran, and 1 mL of triethylamine was added and mixed thoroughly to obtain solution A; 30 mg of azobenzene-4,4'-dicarbonyl chloride was dissolved in 5 mL of tetrahydrofuran to obtain solution B. Then, solution A was heated to 60 °C, and while maintaining this reaction temperature, solution B was added dropwise to solution A under stirring. After the addition was completed, the reaction was continued under 365 nm ultraviolet light irradiation for 24 h with stirring to obtain a TAPB-DHTP type covalent organic framework nanosheet dispersion.

[0079] Step 3: The TAPB-DHTP type covalent organic framework nanosheet dispersion was placed in a 10000Da dialysis bag, immersed in deionized water, and dialyzed for three days to obtain an aqueous dispersion of TAPB-DHTP type covalent organic framework nanosheets; 200μL of the above dispersion was taken and diluted with water to 50mL to obtain a diluted nanosheet dispersion. A polyacrylonitrile ultrafiltration membrane was selected as the filter membrane substrate, and the diluted nanosheet dispersion was added. The membrane was then filtered under vacuum to obtain a TAPB-DHTP type covalent organic framework membrane.

[0080] Figure 1 The images show transmission electron microscopy (TEM) images of the covalent organic framework material (a) and the photoresponsive covalent organic framework nanosheets (b) prepared in Example 1. It can be seen that the prepared covalent organic framework material is in the form of powder particles, while the prepared covalent organic framework nanosheets are in the form of sheets. The comparison shows that the present invention successfully exfoliates nanosheets from the covalent organic framework material.

[0081] Figure 2The image shows an atomic force microscope image and the thickness of the photoresponsive covalent organic framework nanosheets prepared in Example 1. It can be seen that the prepared material is a covalent organic framework nanosheet. Three positions were randomly selected, and the thickness of each nanosheet is about 2 nm.

[0082] Figure 3 The XRD pattern of the photoresponsive covalent organic framework nanosheets prepared in Example 1 is shown in Figure 3. The XRD pattern shows a strong (100) diffraction peak, indicating that the photoresponsive covalent organic framework nanosheets have high crystallinity.

[0083] Moreover, from Figures 1-3 It is known that the photoresponsive covalent organic framework sheet prepared by this invention has a high aspect ratio.

[0084] Figure 4 Scanning electron microscope images of the surface (a) and cross-section (b) of the photoresponsive covalent organic framework membrane prepared in Example 1 show that the membrane surface is smooth and free of particles, and the membrane thickness is approximately 220 nm.

[0085] Example 2

[0086] The only difference from Example 1 is that the photochemical reaction temperature in step two is 0°C.

[0087] Example 3

[0088] The only difference from Example 1 is that the photochemical reaction time in step two is 48 hours.

[0089] Example 4

[0090] The only difference from Example 1 is that the solvent used to disperse the TAPB-DHTP type covalent organic framework powder in step two is replaced with dimethyl sulfoxide instead of tetrahydrofuran.

[0091] Example 5

[0092] Step 1: 102 mg of 4,4'-diaminobenzoylaniline (DABA) and 63 mg of trialdehyde phloroglucinol (TFP) were dissolved in a mixed solvent of 2 mL of mesitylene and 2 mL of 1,4-dioxane. The mixture was sonicated for 10 min, and 0.2 mL of 6 mol / L acetic acid aqueous solution was added. The mixture was sonicated for another 10 min to mix. Oxygen was removed by a freeze-thaw cycle. The mixture was then reacted at 120 °C for 72 h. After the reaction was completed, the mixture was cooled to room temperature, and the precipitate was collected by centrifugation. The precipitate was washed 6 times with tetrahydrofuran and acetone and then dried under vacuum at 60 °C for 24 h to obtain the TFP-DABA type covalent organic framework material.

[0093] Step 2: 20 mg of TFP-DABA covalent organic framework powder was ultrasonically dispersed in 50 mL of tetrahydrofuran, and 1 mL of triethylamine was added and mixed thoroughly to obtain solution A. 45 mg of azobenzene-4,4'-dicarbonyl chloride was dissolved in 5 mL of tetrahydrofuran to obtain solution B. Then, solution A was heated to 60 °C, and while maintaining this reaction temperature, solution B was added dropwise to solution A under stirring. After the addition was completed, the reaction was continued under 365 nm ultraviolet light irradiation for 24 h with stirring to obtain a TFP-DABA covalent organic framework nanosheet dispersion.

[0094] Step 3: The above TFP-DABA type covalent organic framework nanosheet dispersion was placed into a 10000Da dialysis bag, immersed in deionized water and dialyzed for three days to obtain an aqueous dispersion of TFP-DABA type covalent organic framework nanosheets; 300μL of the above dispersion was taken and diluted with water to 50mL to obtain a diluted nanosheet dispersion; a polyacrylonitrile ultrafiltration membrane was selected as the filter membrane substrate, the diluted nanosheet dispersion was added, and the membrane was filtered under vacuum to obtain a TFP-DABA type covalent organic framework membrane.

[0095] Example 6

[0096] The only difference from Example 1 is that in step two, azobenzene-4,4'-dicarbonyl chloride is replaced with p-phenylazobenzoyl chloride.

[0097] Comparative Example 1

[0098] Step one is the same as step one in Example 1;

[0099] Step 2: 20 mg of TAPB-DHTP type covalent organic framework powder was ultrasonically dispersed in 50 mL of tetrahydrofuran, 1 mL of triethylamine was added, and the mixture was thoroughly mixed. The solution was heated to 60 °C and the reaction temperature was maintained. The mixture was stirred for 24 h under 365 nm ultraviolet light irradiation to obtain TAPB-DHTP type covalent organic framework dispersion.

[0100] Step 3: The above TAPB-DHTP type covalent organic framework dispersion is placed into a 10000Da dialysis bag, immersed in deionized water and dialyzed for three days to obtain an aqueous dispersion of TAPB-DHTP type covalent organic framework; 200μL of the above dispersion is taken and diluted with water to 50mL to obtain a diluted dispersion; a polyacrylonitrile ultrafiltration membrane is selected as the filter membrane substrate, the diluted dispersion is added, and the membrane is filtered under vacuum to obtain a TAPB-DHTP type covalent organic framework membrane.

[0101] Comparative Example 2

[0102] Step one is the same as step one in Example 1;

[0103] Step two differs from Step two in Example 1 only in that 365nm ultraviolet light irradiation is not used in this step;

[0104] Step three is the same as step three in Comparative Example 1.

[0105] The covalent organic framework composite membranes prepared in Examples 1-6 and Comparative Examples 1-2 were irradiated with 365 nm ultraviolet light for 10 min or with 455 nm visible light for 10 min, and then applied to organic solvent nanofiltration performance tests. A 100 ppm methylene blue ethanol solution was selected as the separation target. At 4 bar, the dye removal rate and ethanol permeation flux of the covalent organic framework membranes were measured. The results are as follows: Figures 5-7 As shown in Table 1.

[0106] Figure 5 , Figure 6 and Figure 7 The cis-trans structures of photoresponsive covalent organic framework nanosheets and photoresponsive covalent organic framework films prepared in Examples 1, 5 and 6, respectively, after being irradiated with ultraviolet light or visible light.

[0107] Table 1. Test data of covalent organic framework membranes prepared in the examples and comparative examples.

[0108]

[0109] According to Table 1 and Figures 5-7 It can be seen that the pore size increased after ultraviolet light irradiation. The difference is that the side chains of the photoresponsive covalent organic framework nanosheets in Examples 1 and 5 contain acyl chloride groups, which dissociate into negatively charged COO groups after hydrolysis. - Therefore, compared to Example 6 without charged groups, the photoresponsive covalent organic framework nanosheets and photoresponsive covalent organic framework membranes with charged groups are of superior quality. This invention employs a post-modification method, introducing cis-trans isomerized photoresponsive groups into the side chains of the covalent organic framework material. Photoresponsive covalent organic framework nanosheets can be obtained under 365nm ultraviolet light irradiation. The resulting self-assembled photoresponsive covalent organic framework membrane can achieve precise control of membrane separation performance through simple ultraviolet / visible light switching. In contrast, without the addition of post-modifiers or without 365nm ultraviolet light irradiation, covalent organic framework nanosheets cannot be obtained, and the resulting covalent organic framework membrane has numerous defects and almost no separation performance.

[0110] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, not all embodiments. People can obtain other embodiments based on the present invention without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing photoresponsive covalent organic framework nanosheets, comprising the following steps: An amino monomer, an aldehyde monomer, a catalyst, and a first organic solvent are mixed and subjected to a solvothermal reaction to obtain a covalent organic framework material. Both the amino monomer and the aldehyde monomer contain active functional groups, which independently include one or both of phenolic hydroxyl and imine groups. The molar ratio of the amino group in the amino monomer to the aldehyde group in the aldehyde monomer is 1:

1. The covalent organic framework material, a second organic solvent, triethylamine, and a compound containing photoresponsive groups are mixed and subjected to a photochemical reaction under ultraviolet light irradiation to obtain photoresponsive covalent organic framework nanosheets; the compound containing photoresponsive groups includes p-benzoazobenzoyl chloride and / or azobenzene-4,4'-dicarbonyl chloride; the mass ratio of the covalent organic framework material to the photoresponsive compound is 1:1.5~3.

2. The preparation method according to claim 1, characterized in that, The catalyst includes one or more of acetic acid, octanoic acid, scandium trifluoromethanesulfonate, and p-toluenesulfonic acid; The first organic solvent includes one or more of dioxane, mesitylene, N,N-dimethylformamide, dichloromethane, acetonitrile, o-dichlorobenzene, n-butanol, and ethyl acetate; The solvothermal reaction was carried out at a temperature of 120°C for 72 hours. The covalent organic framework material has a pore size of 2~5 nm.

3. The preparation method according to claim 1, characterized in that, The mass ratio of the covalent organic framework material to triethylamine is 1:30~50; The second organic solvent includes one or more of tetrahydrofuran, N,N-dimethylformamide, and dimethyl sulfoxide; The photochemical reaction is carried out at a temperature of 0~80℃ for a time of 12~48 h.

4. Photoresponsive covalent organic framework nanosheets prepared by the preparation method according to any one of claims 1 to 3.

5. A photoresponsive covalent organic framework film, obtained by forming a film from the photoresponsive covalent organic framework nanosheets as described in claim 4.

6. A photoresponsive covalent organic framework composite film, comprising a support layer and the photoresponsive covalent organic framework film of claim 5 located on the surface of the support layer.

7. The photoresponsive covalent organic framework composite film according to claim 6, characterized in that, The support layer includes a polymer ultrafiltration membrane or an inorganic porous carrier.

8. A method for preparing the photoresponsive covalent organic framework composite film according to claim 6 or 7, comprising the following steps: A photoresponsive covalent organic framework composite membrane is obtained by water dialysis of a dispersion of covalent organic framework nanosheets and then placing it on the surface of a support material to form a film; the dialysis bag used for water dialysis has a molecular weight cutoff of ≥10000 Da.

9. The preparation method according to claim 8, characterized in that, The film formation includes vacuum-assisted self-assembly.

10. The application of the photoresponsive covalent organic framework membrane of claim 5, the photoresponsive covalent organic framework composite membrane of any one of claims 6-7, or the photoresponsive covalent organic framework composite membrane prepared by the preparation method of any one of claims 8-9 in nanofiltration separation.

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