Crosslinked graphene oxide / covalent organic framework nanofiltration membrane and preparation method therefor

Abstract

Disclosed in the present invention is a graphene oxide / covalent organic framework nanofiltration membrane. The nanofiltration membrane is prepared by mixing a graphene oxide nanosheet dispersion liquid and an imine covalent organic framework nanosheet dispersion liquid, and performing ultrasonic treatment, suction filtration and heat treatment. Further disclosed are a preparation method for the nanofiltration membrane and a use thereof. According to the present invention, a polymer film is used as a support layer, a covalent organic framework and graphene oxide are loaded on the support layer by means of vacuum filtration, and a cross-linked graphene / covalent organic framework nanofiltration membrane is prepared by a thermal crosslinking reaction after drying. The method involves simple and convenient operations and ingenious design, and the obtained membrane has excellent separation performance. By means of covalent bond crosslinking between the graphene oxide and the covalent organic framework, the structure of the membrane is more compact, thereby increasing the retention rate of the membrane for sodium sulfate and sodium chloride. In addition, covalent organic framework nanosheets provide straight channels for the nanofiltration membrane, thereby significantly improving the water permeability of the nanofiltration membrane.

Description

A cross-linked graphene oxide / covalent organic framework nanofiltration membrane and its preparation method Technical Field

[0001] The invention relates to a cross-linked graphene oxide / covalent organic framework nanofiltration membrane and a preparation method thereof, belonging to the technical field of porous two-dimensional nanofiltration membrane materials and preparation methods thereof. Background Art

[0002] With increasing population, environmental pollution, and extreme weather, water shortages are becoming increasingly serious. Among the numerous desalination water treatment technologies, membrane separation has attracted considerable attention due to its high efficiency, low energy consumption, and wide range of applications. Graphene and its derivatives are considered ideal candidates for separation membranes due to their atomic thickness, versatility, flexibility, and stability. Graphene oxide (GO), a chemically modified form of graphene, contains numerous functional groups, such as carboxyl, hydroxyl, carbonyl, and epoxy groups, which can be used to construct tunable interlayer subnanochannels for water molecule transport. However, strong hydrogen bonding exists between oxygen-containing functional groups and water molecules. The intercalation of water molecules into the nanosheets increases the interlayer spacing, reducing the GO membrane's selectivity for salt ions and weakening the interlayer interactions, which can easily lead to destabilization and even structural collapse of the GO membrane. Reduced graphene oxide (rGO), due to its reduced oxygen-containing functional groups, exhibits more stable interlayer nanochannels. However, the reduction of oxygen-containing functional groups will narrow the interlayer nanochannels and reduce the hydrophilicity, resulting in poor water permeability of the membrane.

[0003] In order to improve the permeability of graphene-based membranes, other materials can be inserted between layers to adjust the interlayer spacing (P.Su, F.Wang, Z.Li, CYTang, W.Li, Graphene oxide membranes: controlling their transport pathways, Journal of Materials Chemistry A, 8 (2020) 15319-15340.). However, the resulting membrane material is still subject to the "trade-off" effect, that is, as the interlayer spacing expands, different substance molecules indiscriminately permeate through the membrane, and the membrane selectivity decreases (the retention rate of Na2SO4 is <80%, and the retention rate of NaCl is <50%). Therefore, it is of great significance to develop a graphene-based membrane material with both high permeability and high selectivity.

[0004] Summary of the Invention

[0005] The present invention aims to provide a cross-linked graphene oxide / covalent organic framework nanofiltration membrane to solve the problems of narrow interlayer nanochannels of existing graphene-based membrane materials and low water permeability and selectivity of the membrane.

[0006] To address these issues, the present invention provides a cross-linked graphene oxide / covalent organic framework nanofiltration membrane. This membrane is produced by mixing a dispersion of graphene oxide nanosheets and a dispersion of imine-based covalent organic framework nanosheets, subjecting them to ultrasonic treatment, filtration, and subsequent heat treatment. During the heat treatment, the graphene oxide undergoes thermal reduction, which then cross-links with the imine-based COF to form a cross-linked graphene oxide / covalent organic framework nanofiltration membrane.

[0007] In one embodiment of the present invention, the mass ratio of the graphene oxide to the imine covalent organic framework material is 7:3 to 5:5.

[0008] In one embodiment of the present invention, the imine covalent organic framework material includes any one of TpPa, TpBD, and TpBD(OH)2.

[0009] The present invention also provides a method for preparing the above-mentioned filter membrane, comprising the following steps:

[0010] (1) Preparation of graphene oxide nanosheet dispersion: dispersing graphene oxide nanosheets in deionized water to obtain a graphene oxide nanosheet dispersion;

[0011] (2) Preparation of a covalent organic framework nanosheet dispersion: dissolving an imine covalent organic framework monomer in glycerol, stirring and mixing the mixture, and then adding a catalyst to perform a solvothermal reaction to obtain an imine covalent organic framework nanosheet dispersion;

[0012] (3) Preparation of cross-linked graphene oxide / covalent organic framework nanofiltration membrane: The graphene oxide nanosheet dispersion obtained in step (1) and the imine covalent organic framework nanosheet dispersion obtained in step (2) are mixed, ultrasonically treated, and loaded on the surface of a polymer support membrane by vacuum filtration. After drying, heat treatment is performed to obtain a cross-linked graphene oxide / covalent organic framework nanofiltration membrane.

[0013] In one embodiment of the present invention, the method for preparing graphene oxide nanosheets comprises the following steps: mixing 1-4 g of flake graphite, 120-480 ml of concentrated sulfuric acid, and 13.33-53.33 ml of concentrated phosphoric acid, slowly adding 6-24 g of potassium permanganate, reacting in a 60°C oil bath for 24 hours, adding deionized water, then adding hydrogen peroxide dropwise until the solution turns golden yellow, letting it stand overnight, and then centrifuging. The centrifuged product is washed with dilute hydrochloric acid and deionized water to a pH of 5-6, and freeze-dried to obtain the graphene oxide nanosheets.

[0014] In one embodiment of the present invention, the size of the flake graphite used in the process of preparing graphene oxide nanosheets is 100-400 mesh, the concentrations of concentrated sulfuric acid and concentrated phosphoric acid are 98% and 85%, respectively, and the volume ratio of the two is 9:1.

[0015] In one embodiment of the present invention, in step (1), graphene oxide nanosheets are dispersed in deionized water to form a graphene oxide nanosheet dispersion, and the concentration of graphene oxide nanosheets in the graphene oxide nanosheet dispersion is 0.01 to 0.1 mg / mL.

[0016] In one embodiment of the present invention, in step (2), the imine covalent organic framework monomer includes monomer A and monomer B, the monomer A includes 1,3,5-triformylphloroglucinol (Tp) and / or trimesicol (Tb), and the monomer B includes at least one of p-phenylenediamine (Pa), benzyldiamine (BD) and 3,3-dihydroxybenzidine (BD(OH)2).

[0017] In one embodiment of the present invention, in step (2), the molar ratio of monomer A to monomer B is 1:1 to 1:2, the volume of glycerol is 15 to 30 mL, the catalyst is acetic acid, and the amount of the catalyst is 0.5 to 1 mL.

[0018] In one embodiment of the present invention, the temperature of the solvent thermal reaction in step (2) is 100-140° C., and the reaction time is 24-72 hours.

[0019] In one embodiment of the present invention, the concentration of the imine covalent organic framework nanosheets in the imine covalent organic framework nanosheet dispersion in step (2) is 0.01-0.1 mg / mL.

[0020] In one embodiment of the present invention, the mixing ratio of the graphene oxide nanosheet dispersion and the imine covalent organic framework nanosheet dispersion in step (3) is 7:3 to 3:7, preferably 7:3 to 5:5, the ultrasonic power is 60 to 180 W, the ultrasonic time is 5 to 30 min, and the volume of the mixed dispersion used for filtration is 5 to 50 mL.

[0021] In one embodiment of the present invention, the polymer support membrane in step (3) is any one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyethersulfone (PES), and mixed cellulose (MCEM) microporous filter membrane.

[0022] In one embodiment of the present invention, the polymer support membrane has a pore size of 0.22 to 0.44 μm and a diameter of 25 to 50 cm.

[0023] In one embodiment of the present invention, the drying time in step (3) is 12 to 24 hours, the drying temperature is 40 to 60°C, and during heat treatment, the heat treatment temperature is 80°C and the heat treatment time is 48 to 72 hours; alternatively, the heat treatment temperature is 150°C and the heat treatment time is 1 to 2 hours.

[0024] The present invention also provides a separation device comprising the cross-linked graphene oxide / covalent organic framework nanofiltration membrane.

[0025] The present invention also provides application of the cross-linked graphene oxide / covalent organic framework nanofiltration membrane in the field of water treatment.

[0026] Beneficial effects of the present invention:

[0027] (1) The present invention inserts imine covalent organic framework nanosheets between graphene oxide nanosheet layers, and constructs more vertical water channels in the membrane through the pore structure in the covalent organic framework, thereby improving the permeability of the membrane to water molecules.

[0028] (2) The present invention uses the unreacted amino groups on the imine covalent organic framework nanosheets to react with the carboxyl groups on the graphene oxide nanosheets to prepare the nanofiltration membrane. The amino groups on the imine covalent organic framework nanosheets can chemically react with the carboxyl groups on the graphene oxide to form a cross-linked network, which reduces the interlayer spacing and inhibits the excessive swelling of the membrane in water, making the prepared nanofiltration membrane structure more compact, thereby improving the selectivity of the membrane.

[0029] (3) The nanofiltration membrane prepared by the method of the present invention has excellent membrane permeability and its pure water flux can be as high as 4.38 L·m -2 ·h -1 bar -1 At the same time, the prepared nanofiltration membrane has a high retention rate for salt ions, with a maximum retention rate of 95.3% for sodium sulfate and 71.3% for sodium chloride. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG1 is a Fourier transform infrared spectrum of the nanofiltration membrane before and after heat treatment in Example 1.

[0031] FIG2 is a scanning electron microscope image of the surface of the cross-linked graphene oxide / covalent organic framework nanofiltration membrane in Example 1.

[0032] FIG3 is a scanning electron microscope image of a cross-section of the cross-linked graphene oxide / covalent organic framework nanofiltration membrane in Example 1. DETAILED DESCRIPTION

[0033] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0034] Example 1

[0035] A method for preparing a graphene oxide / covalent organic framework nanofiltration membrane comprises the following steps:

[0036] (1) Preparation of graphene oxide nanosheet dispersion: 4 g of 325 mesh flake graphite, 480 mL of 98% concentrated sulfuric acid, and 53.33 mL of 85% concentrated phosphoric acid were mixed and stirred, 24 g of potassium permanganate was slowly added, and the mixture was reacted in an oil bath at 60°C for 24 h. 800 mL of deionized water was then added, followed by dropwise addition of hydrogen peroxide until the solution turned golden yellow. The mixture was allowed to stand overnight and then centrifuged. The centrifuged product was washed with dilute hydrochloric acid and deionized water to a pH of 5-6, and freeze-dried to obtain graphene oxide nanosheets. The graphene oxide nanosheets were redispersed in deionized water to obtain a graphene oxide nanosheet dispersion with a concentration of 0.01 mg / mL.

[0037] (2) Preparation of a covalent organic framework nanosheet dispersion: 0.03 mmol Tp (6.3 mg) and 0.045 mmol Pa (4.8 mg) were dissolved in 15 mL of glycerol, respectively. The Tp solution was then added dropwise to the Pa solution, and 0.5 mL of glacial acetic acid was added. After purging, the mixture was solvothermally reacted at 120°C for 72 h. Subsequently, 970 mL of deionized water was added for dilution to obtain a covalent organic framework nanosheet dispersion with a concentration of 0.01 mg / mL.

[0038] (3) Preparation of cross-linked graphene oxide / covalent organic framework nanofiltration membrane: The graphene oxide nanosheet dispersion obtained in step (1) and the covalent organic framework nanosheet dispersion obtained in step (2) were mixed in a ratio of 1:1, and the dispersions were uniformly mixed by ultrasonication at 180W for 5 minutes. 50 mL of the uniformly mixed dispersion was filtered onto a PES microporous filter membrane with a pore size of 0.22 μm and a diameter of 50 mm by vacuum assisted filtration, dried at 40°C for 24 hours, and heat treated at 80°C for 72 hours to obtain a cross-linked graphene oxide / covalent organic framework nanofiltration membrane. The separation performance of the membrane was tested using a dead-end filtration device, and its pure water flux was 4.38 L·m -2 ·h -1 bar -1 The retention rate of sodium sulfate is 95.3%, and the retention rate of sodium chloride is 71.3%.

[0039] Structural and morphological characterization of cross-linked graphene oxide / covalent organic framework nanofiltration membranes:

[0040] The cross-linked graphene oxide / covalent organic framework nanofiltration membrane prepared in Example 1 was characterized. The results are shown in FIG1 . In the Fourier transform infrared spectrum, the membrane after heat treatment has a peak at 1255 cm -1 The corresponding CN bond peak is significantly enhanced, indicating that the oxygen-containing functional groups on graphene oxide react with the amino groups in the covalent organic framework to produce more CN bonds.

[0041] Figure 2 is a scanning electron micrograph of the surface of the cross-linked graphene oxide / covalent organic framework nanofiltration membrane prepared in Example 1. The image shows that the membrane surface is dense and intact, without defects. This helps the membrane retain salt ions.

[0042] Figure 3 is a scanning electron microscope image of a cross-section of the cross-linked graphene oxide / covalent organic framework nanofiltration membrane prepared in Example 1. As can be seen from the figure, the membrane thickness is approximately 226 nm, and the membrane is continuous, complete and defect-free.

[0043] Example 2

[0044] A method for preparing a cross-linked graphene oxide / covalent organic framework nanofiltration membrane comprises the following steps:

[0045] (1) Preparation of graphene oxide nanosheet dispersion: The preparation method of the graphene oxide nanosheet dispersion is the same as that in Example 1.

[0046] (2) Preparation of a COF nanosheet dispersion: 0.03 mmol Tp (6.3 mg) and 0.045 mmol BD (8.2 mg) were dissolved in 15 mL of glycerol. The Tp solution was then added dropwise to the BD solution, followed by the addition of 0.5 mL of glacial acetic acid. After purging, the mixture was solvothermally reacted at 120°C for 72 h. The mixture was then diluted with 1275 mL of deionized water to obtain a COF nanosheet dispersion with a concentration of 0.01 mg / mL.

[0047] (3) Preparation of cross-linked graphene oxide / covalent organic framework nanofiltration membrane: The graphene oxide dispersion obtained in step (1) of Example 1 and the covalent organic framework dispersion obtained in step (2) were prepared into 50 ml of mixed dispersion at a ratio of 5:5, and ultrasonicated at 180W for 5 minutes to mix evenly. The 50 ml mixed dispersion was filtered onto a PES microporous filter membrane with a pore size of 0.22 μm and a diameter of 50 mm by vacuum assisted filtration, dried at 40°C for 24 hours, and heat treated at 80°C for 72 hours to obtain a cross-linked graphene oxide / covalent organic framework nanofiltration membrane. Its pure water flux was 5.34 L·m -2 ·h -1 bar -1The retention rate of sodium sulfate is 92.5%, and the retention rate of sodium chloride is 68.3%.

[0048] Example 3

[0049] A method for preparing a cross-linked graphene oxide / covalent organic framework nanofiltration membrane comprises the following steps:

[0050] (1) Preparing a graphene oxide dispersion: Graphene oxide nanosheets were dispersed in deionized water to obtain a graphene oxide nanosheet dispersion with a concentration of 0.1 mg / mL, wherein the preparation method of the graphene oxide nanosheets was the same as that in Example 1.

[0051] (2) Preparation of a covalent organic framework nanosheet dispersion: 0.03 mmol Tp (6.3 mg) and 0.045 mmol Pa (4.8 mg) were dissolved in 15 mL of glycerol, and the Tp solution was added dropwise to the Pa solution. 0.5 mL of glacial acetic acid was then added. After purging, the mixture was solvothermally reacted at 120°C for 72 h. The mixture was then diluted with 70 mL of deionized water to obtain a covalent organic framework nanosheet dispersion with a concentration of 0.1 mg / mL.

[0052] (3) Preparation of cross-linked graphene oxide / covalent organic framework nanofiltration membrane: The graphene oxide dispersion obtained in step (1) and the covalent organic framework dispersion obtained in step (2) were prepared into a 5 ml mixed dispersion at a ratio of 5:5, and ultrasonically mixed for 5 min. 50 mL of the mixed dispersion was filtered onto a PES microporous filter membrane with a pore size of 0.22 μm and a diameter of 50 mm by vacuum assisted filtration, dried at 40°C for 24 h, and heat treated at 80°C for 72 h to obtain a cross-linked graphene oxide / covalent organic framework nanofiltration membrane. Its pure water flux is 5.35 L·m -2 ·h -1 bar -1 The retention rate of sodium sulfate is 90.2%, and the retention rate of sodium chloride is 64.3%.

[0053] Example 4

[0054] A method for preparing a cross-linked graphene oxide / covalent organic framework nanofiltration membrane comprises the following steps:

[0055] (1) Preparation of graphene oxide nanosheet dispersion: The preparation method of the graphene oxide nanosheet dispersion is the same as that in Example 1.

[0056] (2) Preparation of covalent organic framework nanosheet dispersion: The preparation method of the covalent organic framework nanosheet dispersion is the same as that in Example 1.

[0057] (3) Preparation of cross-linked graphene oxide / covalent organic framework nanofiltration membrane: The graphene oxide dispersion obtained in step (1) and the covalent organic framework dispersion obtained in step (2) were prepared into 50 ml of mixed dispersion in a ratio of 7:3, and ultrasonically mixed for 5 minutes. The 50 ml mixed dispersion was filtered onto a PES microporous filter membrane with a pore size of 0.22 μm and a diameter of 50 mm by vacuum assisted filtration, dried at 40°C for 24 hours, and heat treated at 80°C for 72 hours to obtain a cross-linked graphene oxide / covalent organic framework nanofiltration membrane. Its pure water flux is 2.83 L·m -2 ·h -1 bar -1 The retention rate of sodium sulfate is 86.9%, and the retention rate of sodium chloride is 50.3%.

[0058] Example 5

[0059] A method for preparing a cross-linked graphene oxide / covalent organic framework nanofiltration membrane comprises the following steps:

[0060] (1) Preparation of graphene oxide nanosheet dispersion: The preparation method of the graphene oxide nanosheet dispersion is the same as that in Example 1.

[0061] (2) Preparation of covalent organic framework nanosheet dispersion: The preparation method of the covalent organic framework nanosheet dispersion is the same as that in Example 1.

[0062] (3) Preparation of cross-linked graphene oxide / covalent organic framework nanofiltration membrane: The graphene oxide dispersion obtained in step (1) and the covalent organic framework dispersion obtained in step (2) were mixed into 50 ml of a mixed dispersion at a ratio of 5:5, and ultrasonically mixed for 5 minutes. The 50 ml mixed dispersion was filtered onto a PES microporous filter membrane with a pore size of 0.22 μm and a diameter of 50 mm by vacuum assisted filtration, dried at 40°C for 24 hours, and heat treated at 150°C for 2 hours to obtain a cross-linked graphene oxide / covalent organic framework nanofiltration membrane. Its pure water flux is 4.95 L·m -2 ·h -1 bar -1 The retention rate of sodium sulfate is 92.9%, and the retention rate of sodium chloride is 67.7%.

[0063] Comparative Example 1

[0064] A method for preparing a cross-linked graphene oxide / covalent organic framework nanofiltration membrane comprises the following steps:

[0065] (1) Preparation of graphene oxide nanosheet dispersion: The preparation method of the graphene oxide nanosheet dispersion is the same as that in Example 1.

[0066] (2) Preparation of covalent organic framework nanosheet dispersion: The preparation method of the covalent organic framework nanosheet dispersion is the same as that in Example 1.

[0067] (3) Preparation of cross-linked graphene / covalent organic framework nanofiltration membrane: The graphene oxide dispersion obtained in step (1) and the covalent organic framework dispersion obtained in step (2) were mixed into 50 ml of a mixed dispersion at a ratio of 5:5, and ultrasonically mixed for 5 minutes. The 50 ml mixed dispersion was filtered onto a PES microporous filter membrane with a pore size of 0.22 μm and a diameter of 50 mm by vacuum assisted filtration, dried at 40°C for 24 hours, and heat treated at 80°C for 24 hours to obtain a cross-linked graphene / covalent organic framework nanofiltration membrane. Its pure water flux is 6.1 L·m -2 ·h -1 bar -1 The retention rate of sodium sulfate is 81.4%, and the retention rate of sodium chloride is 38.7%.

[0068] Comparative Example 2:

[0069] Graphene oxide nanosheets were dispersed in deionized water to obtain a graphene oxide nanosheet dispersion with a concentration of 0.01 mg / mL. The preparation method of graphene oxide nanosheets was the same as that in Example 1. 50 mL of the graphene oxide nanosheet dispersion was taken and filtered onto a PES microporous filter membrane with a pore size of 0.22 μm and a diameter of 50 mm using vacuum assisted filtration. The membrane was dried at 40°C for 24 h to obtain a graphene oxide nanofiltration membrane. Its pure water flux was 4.5 L·m -2 ·h -1 bar -1 The rejection rate of sodium sulfate is 60.7%, and the rejection rate of sodium chloride is 21.2%. The pure GO membrane has low water permeability and low rejection rate for sodium chloride.

[0070] Comparative Example 3:

[0071] Graphene oxide nanosheets were dispersed in deionized water to obtain a 0.01 mg / mL graphene oxide nanosheet dispersion. The preparation method of graphene oxide nanosheets was the same as that in Example 1. 50 mL of the graphene oxide nanosheet dispersion was taken and filtered onto a PES microporous filter membrane with a pore size of 0.22 μm and a diameter of 50 mm using vacuum assisted filtration. The membrane was dried at 40°C for 24 h and heat treated at 80°C for 72 h to obtain a reduced graphene oxide nanofiltration membrane. Its pure water flux was 1.2 L·m -2 ·h -1 bar -1 , the rejection rate for sodium sulfate is 83.7%, and the rejection rate for sodium chloride is 45.2%. The pure rGO membrane has low water permeability and low rejection rate for sodium chloride.

[0072] Comparative Example 4:

[0073] Take 50 mL of the covalent organic framework nanosheet dispersion and filter the 50 mL of the uniformly mixed dispersion onto a PES microporous filter membrane with a pore size of 0.22 μm and a diameter of 50 mm using vacuum assisted filtration, and dry at 40°C for 24 h. The preparation method of the covalent organic framework nanosheet dispersion is the same as step (2) in Example 1. Its pure water flux is 121.2 L·m -2 ·h -1 bar -1 , the rejection rate for sodium sulfate is 4.1%, and the rejection rate for sodium chloride is 5.3%. Pure COF has high water permeability, but has a low rejection rate for salt ions.

[0074] Comparative Example 5

[0075] The graphene oxide nanosheet dispersion and the covalent organic framework nanosheet dispersion were prepared into a 50 ml mixed dispersion at a ratio of 5:5, and mixed evenly under 180W ultrasonication for 5 minutes. The 50 ml mixed dispersion was filtered onto a PES microporous filter membrane with a pore size of 0.22 μm and a diameter of 50 mm by vacuum assisted filtration, and dried at 40°C for 24 hours to obtain a graphene oxide / covalent organic framework nanofiltration membrane. The preparation method of the graphene oxide nanosheet dispersion and the covalent organic framework nanosheet dispersion was the same as in Example 1. Its pure water flux was 13.1 L·m -2 ·h -1 bar -1 The retention rate of sodium sulfate is 43.8%, and the retention rate of sodium chloride is 10.3%.

[0076] Comparative Example 6

[0077] A method for preparing a cross-linked graphene oxide / covalent organic framework nanofiltration membrane comprises the following steps:

[0078] (1) Preparation of graphene oxide nanosheet dispersion: The preparation method of the graphene oxide nanosheet dispersion is the same as that in Example 1.

[0079] (2) Preparation of boron covalent organic framework nanosheet dispersion: 9.9 mg (0.06 mmol) of 1,4-phenylenediboronic acid was dissolved in 15 ml of glycerol, and after removing bubbles, the solution was subjected to solvent thermal reaction at 120°C for 72 h. Subsequently, 985 mL of deionized water was added for dilution to obtain a boron covalent organic framework nanosheet dispersion with a concentration of 0.01 mg / mL.

[0080] (3) Preparation of cross-linked graphene oxide / covalent organic framework nanofiltration membrane: 50 ml of mixed dispersion was prepared by mixing graphene oxide nanosheet dispersion and boron covalent organic framework nanosheet dispersion in a ratio of 5:5. The mixture was uniformly mixed by ultrasonic treatment at 180W for 5 minutes. The 50 ml of uniformly mixed dispersion was filtered onto a PES microporous filter membrane with a pore size of 0.22 μm and a diameter of 50 mm by vacuum assisted filtration. The membrane was dried at 40°C for 24 hours and heat treated at 80°C for 72 hours to obtain a cross-linked graphene oxide / covalent organic framework nanofiltration membrane. The separation performance of the membrane was tested using a dead-end filtration device, and its pure water flux was 17.42 L·m -2 ·h -1 bar -1 The retention rate of sodium sulfate is 39.3%, and the retention rate of sodium chloride is 9.7%.

Claims

1. A cross-linked graphene oxide / covalent organic framework nanofiltration membrane, characterized in that: The nanofiltration membrane is prepared by mixing a graphene oxide nanosheet dispersion and an imine covalent organic framework nanosheet dispersion, and then performing ultrasonic and suction filtration followed by heat treatment.

2. The nanofiltration membrane according to claim 1, characterized in that The mass ratio of graphene oxide to imine covalent organic framework is 7:3 to 5:

5.

3. The nanofiltration membrane according to claim 1 or 2, characterized in that: The imine-based covalent organic framework includes any one of TpPa, TpBD and TpBD(OH)2.

4. A method for preparing a nanofiltration membrane according to any one of claims 1 to 3, characterized in that: The following steps are involved: (1) Preparation of graphene oxide nanosheet dispersion: dispersing graphene oxide nanosheets in deionized water to obtain a graphene oxide nanosheet dispersion; (2) Preparation of a covalent organic framework nanosheet dispersion: dissolving an imine covalent organic framework monomer in glycerol, stirring and mixing the mixture evenly, and then adding a catalyst to perform a solvothermal reaction to obtain an imine covalent organic framework nanosheet dispersion; (3) Preparation of cross-linked graphene oxide / covalent organic framework nanofiltration membrane: The graphene oxide nanosheet dispersion obtained in step (1) and the imine covalent organic framework nanosheet dispersion obtained in step (2) are mixed, ultrasonically treated, loaded on the surface of the polymer support membrane by vacuum filtration, dried and then heat-treated to obtain a cross-linked graphene oxide / covalent organic framework nanofiltration membrane.

5. The preparation method according to claim 4, characterized in that: In step (2), the imine covalent organic framework monomer comprises monomer A and monomer B, wherein monomer A comprises 1,3,5-triformylphloroglucinol and / or trimesic acid, and monomer B comprises at least one of p-phenylenediamine, benzyl diamine and 3,3-dihydroxybenzidine.

6. The preparation method according to claim 5, characterized in that: In step (2), the molar ratio of monomer A to monomer B is 1:1-1:2, the volume of glycerol is 15-30 mL, the catalyst is acetic acid, the amount of the catalyst is 0.5-1 mL, the solvent thermal reaction temperature is 100-140° C., and the reaction time is 24-72 h.

7. The preparation method according to claim 4, characterized in that: The concentration of the graphene oxide nanosheets in the graphene oxide nanosheet dispersion is 0.01-0.1 mg / mL, and the concentration of the imine covalent organic framework nanosheets in the imine covalent organic framework nanosheet dispersion is 0.01-0.1 mg / mL.

8. The preparation method according to claim 4, characterized in that: The polymer support membrane includes any one of polyvinylidene fluoride, polytetrafluoroethylene, polyethersulfone, and mixed cellulose microporous filter membrane. The heat treatment time is 2 to 72 hours, and the heat treatment temperature is 80 to 150°C.

9. A separation device comprising the cross-linked graphene oxide / covalent organic framework nanofiltration membrane according to any one of claims 1 to 3.

10. Use of the cross-linked graphene oxide / covalent organic framework nanofiltration membrane according to any one of claims 1 to 3 in the field of water treatment.

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