Fluorinated covalent organic framework anti-pollution nanofiltration membrane and preparation method thereof

Abstract

The invention discloses a preparation method of a fluorinated covalent organic framework anti-pollution nanofiltration membrane, which comprises the following steps: firstly, generating a free amino group in situ in a covalent organic framework membrane through two aldehyde group monomers in a certain molar ratio, taking the free amino group as a grafting site for subsequent surface fluorination, and enabling the two aldehyde group monomers to comprise 2, 5-dihydroxy benzaldehyde and 2, 5-dihydroxy benzaldehyde; the catalyst is 2, 5-dihydroxy terephthalaldehyde; then perfluoroalkyl chains are grafted on the surface of the membrane by utilizing the reaction of perfluorooctanoyl chloride and free amino groups, and fluorine-containing chain segments with low surface energy can reduce the interaction between pollutants and the surface of the membrane, so that the pollutants are more easily removed from the surface of the membrane under the action of water flow shearing force, and the membrane is endowed with excellent anti-pollution performance. The modification operation of the covalent organic framework membrane is simple, and compared with a traditional polymer nanofiltration membrane, the fluorine content on the surface of the membrane prepared by the method is fine and adjustable, and the membrane has high water flux, separation performance and anti-pollution performance.

Description

Technical Field

[0001] The invention belongs to the field of anti-pollution nanofiltration membrane preparation, and in particular relates to a method for preparing a fluorinated covalent organic framework anti-pollution nanofiltration membrane. Background Art

[0002] Nanofiltration technology, as an energy-saving and efficient membrane separation technology, has been widely used in the fields of wastewater deep treatment and resource utilization. Covalent Organic Frameworks (COF) membranes are expected to achieve high permeability and high selectivity at the same time due to their designable structure and long-range ordered pores, and have received extensive attention in the membrane field.

[0003] Membrane fouling is an important problem in the actual water treatment process. Membrane fouling will not only lead to a serious decline in the separation performance of the membrane, but frequent cleaning will also shorten the membrane life and increase the cost of the membrane separation process. Taking seawater desalination plants as an example, about 10% of the membranes are usually replaced every year due to fouling. In addition, at high flux, the concentration of dirt on the membrane surface will increase due to concentration polarization, resulting in more serious membrane fouling. Therefore, the development of covalent organic framework nanofiltration membranes with high-flux anti-fouling properties is an important demand in the field of water treatment.

[0004] By introducing fluorine-containing materials (such as perfluoroalkyl chains) to modify the membrane to low surface energy, the interaction between pollutants and the membrane surface can be reduced, which is an effective strategy for preparing anti-pollution separation membranes. Fine adjustment of the surface chemical structure is a key factor affecting the anti-pollution performance of the membrane. For the anti-pollution modification strategy of low surface energy of the membrane, the anti-pollution performance of the membrane mainly depends on the density and molecular structure of the fluorine chain, which puts high demands on the fine regulation of the surface fluorination of the membrane. However, unlike traditional polymer membrane materials such as polyamide membranes, the active groups in covalent organic framework materials are mainly used to form a framework structure, and there is a lack of active groups that are easy to undergo grafting reactions. How to controllably introduce perfluoroalkyl chains into the surface of covalent organic framework membranes remains a challenge. Summary of the invention

[0005] In view of the development demand of anti-pollution covalent organic framework nanofiltration membrane in the above-mentioned background technology, the present invention proposes a method for preparing a fluorinated covalent organic framework anti-pollution nanofiltration membrane. First, an amino-functionalized covalent organic framework membrane is prepared by using a "defect engineering" strategy, and then a fluorinated chain segment is grafted onto the membrane surface to reduce the interaction between pollutants and the membrane surface, thereby preparing an anti-pollution covalent organic framework nanofiltration membrane.

[0006] In order to solve the above technical problems, the present invention proposes a method for preparing a fluorinated covalent organic framework anti-pollution nanofiltration membrane. First, two aldehyde monomers with a certain molar ratio are used to in-situ generate free amino groups in the covalent organic framework membrane. The free amino groups serve as grafting sites for subsequent surface fluorination. The two aldehyde monomers include 2,5-dihydroxybenzaldehyde and 2,5-dihydroxyterephthalaldehyde; then, perfluorooctanoyl chloride is reacted with the free amino groups to graft perfluoroalkyl chains on the membrane surface to prepare a fluorinated covalent organic framework anti-pollution nanofiltration membrane. The specific steps are as follows:

[0007] Step 1, synthesis of amino-functionalized covalent organic framework nanosheets: weigh 2,5-dihydroxybenzaldehyde and 2,5-dihydroxyterephthalaldehyde according to a molar ratio of X:Y, wherein X=1-4, Y=6-9, and X+Y=10; dissolve the weighed aldehyde monomer in dichloromethane to obtain an aldehyde monomer solution, wherein the molar volume ratio of the aldehyde monomer to dichloromethane is 3mmol / 1600mL; then, add 3M acetic acid aqueous solution on top of the aldehyde monomer solution according to a volume ratio of 3:8; then, dissolve an appropriate amount of triaminoguanidine hydrochloride in N,N- In dimethylformamide, a triaminoguanidine hydrochloride-N,N-dimethylformamide solution with a molar volume ratio of 1 mmol / 500 mL is prepared, which is recorded as solution A, and the solution A is slowly added dropwise onto the acetic acid aqueous solution; the system is allowed to react at room temperature for 7 days, and after the reaction is completed, the upper solution A is removed to collect the aqueous nanosheet dispersion; using a dialysis membrane with a molecular weight cutoff of 30000Da, the obtained nanosheets are dialyzed in deionized water for 3 days to remove residual unreacted monomers and small molecule oligomers, and an amino-functionalized covalent organic framework nanosheet solution is obtained;

[0008] Step 2, preparation of amino-functionalized covalent organic framework membrane: the amino-functionalized covalent organic framework nanosheet solution prepared in the above step 1 is dispersed in anhydrous ethanol at a volume ratio of 1 to 5:1000, and a polyacrylonitrile membrane is used as a substrate to assemble into a membrane by vacuum-assisted self-assembly method, and then dried at 25°C for 12 hours to obtain an amino-functionalized covalent organic framework membrane;

[0009] Step 3, preparation of fluorinated covalent organic framework membrane: immerse the amino-functionalized covalent organic framework membrane prepared in the above step 2 in a 10% by volume perfluorooctanoyl chloride solution, carry out grafting reaction at 25°C for 0.5 to 5 minutes, and then wash with an organic solvent to obtain a fluorinated covalent organic framework anti-pollution nanofiltration membrane.

[0010] Furthermore, the preparation method of the present invention, wherein:

[0011] In step 1, the molar ratio of 2,5-dihydroxybenzaldehyde to 2,5-dihydroxyterephthalaldehyde is preferably 4:6.

[0012] In step 2, the volume ratio of the amino-functionalized covalent organic framework nanosheet solution to anhydrous ethanol is preferably 2.5:1000.

[0013] In step 3, the organic solvent of the perfluorooctanoyl chloride solution is any one of anhydrous ethanol, n-heptane, and n-hexane, preferably anhydrous ethanol.

[0014] In step 3, the grafting reaction time is preferably 1 min.

[0015] In step 3, the organic solvent selected for cleaning is the same as the organic solvent of the perfluorooctanoyl chloride solution.

[0016] Compared with the anti-pollution covalent organic framework membrane prepared by the prior art, the fluorinated covalent organic framework anti-pollution nanofiltration membrane prepared by the present invention can achieve fine regulation and optimization of the density of fluorinated chain segments on the membrane surface by optimizing the ratio of the two aldehyde monomers, thereby improving the anti-pollution performance of the membrane and ensuring the water permeability of the membrane. The water flux of the fluorinated covalent organic framework anti-pollution nanofiltration membrane prepared by the present invention is 75.7-115.9 L m -2 h -1 bar -1 The flux attenuation rate was as low as 16.8% to 10.4%, and the flux recovery rate was 88.4% to 98.3%. It also showed good anti-pollution performance for a variety of pollutants (oils, humic acid), and also showed good interception performance for a variety of dyes, such as Figure 5 shown. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is the reaction formula of the grafting reaction in the process of preparing the nanofiltration membrane of the present invention;

[0018] Figure 2-1 This is a surface electron microscope image of the nanofiltration membrane prepared in Example 4;

[0019] Figure 2-2 This is a cross-sectional electron microscope image of the nanofiltration membrane prepared in Example 4;

[0020] Figure 3-1 This is a surface electron microscope image of the nanofiltration membrane prepared in the comparative example;

[0021] Figure 3-2 This is a cross-sectional electron microscope image of the nanofiltration membrane prepared in the comparative example;

[0022] Figure 4 The anti-pollution performance diagram of the nanofiltration membranes prepared in the comparative example and Examples 1-4, the simulated pollutant is pump oil emulsion.

[0023] Figure 5The graphs show the water flux of the nanofiltration membranes prepared in the comparative example and Examples 1-4 and the retention rate of aqueous solutions of various dyes (Congo red, chrome black T, methyl blue, Coomassie brilliant blue, methyl orange, all with a mass fraction of 0.01%). DETAILED DESCRIPTION

[0024] The invention provides a method for preparing a fluorinated covalent organic framework anti-pollution nanofiltration membrane, and the design concept is as follows: first, by introducing two aldehyde monomers in a certain ratio, amino-functionalized covalent organic framework nanosheets are prepared, and the two introduced aldehyde monomers generate free amino groups in situ in the covalent organic framework membrane, which serve as subsequent surface fluorination grafting sites; then, on a polyacrylonitrile-based membrane, the nanosheets are filtered to form a membrane by a vacuum-assisted self-assembly method; finally, by grafting free amino groups on the surface of the amino-functionalized covalent organic framework membrane with fluorinated acyl chloride, fluorinated segments are grafted on the membrane surface. Figure 1 The reaction of amino groups with perfluorooctanoyl chloride is shown. The low surface energy fluorinated segments can reduce the interaction between pollutants and the membrane surface, making it easier for pollutants to be removed from the membrane surface under the shear force of the water flow, giving the membrane excellent anti-pollution performance. The modification of the covalent organic framework membrane by the present invention is simple to operate, and compared with the traditional polymer nanofiltration membrane, the fluorine content of the membrane surface prepared by the method is finely adjustable, and has high water flux, separation performance and anti-pollution performance, such as Figure 4 , Figure 5 And as shown in Table 1.

[0025] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but the following embodiments are by no means intended to limit the present invention in any way.

[0026] Example 1

[0027] The steps for preparing the fluorinated covalent organic framework anti-fouling nanofiltration membrane are as follows:

[0028] Step 1: Synthesis of amino-functionalized covalent organic framework nanosheets:

[0029] Weigh 2.07 mg of 2,5-dihydroxybenzaldehyde and 22.43 mg of 2,5-dihydroxyterephthalaldehyde, and dissolve them in 80 mL of dichloromethane to obtain an aldehyde monomer solution. Then, add 30 mL of 3M acetic acid aqueous solution on top of the aldehyde monomer solution; then, dissolve 14.0 mg (0.1 mmol) of triaminoguanidine hydrochloride in 50 mL of N,N-dimethylformamide, and slowly add it dropwise to the top of the acetic acid aqueous solution. The system was allowed to stand at room temperature to react for 7 days. After the reaction was completed, the upper N,N-dimethylformamide solution was removed and the aqueous nanosheet dispersion was collected. Using a dialysis membrane with a molecular weight cutoff of 30,000 Da, the obtained nanosheets were dialyzed in deionized water for 3 days to remove residual unreacted monomers and small molecule oligomers to obtain an amino-functionalized covalent organic framework nanosheet solution;

[0030] Step 2. Preparation of amino-functionalized covalent organic framework membrane: Take 25 μL of the amino-functionalized covalent organic framework nanosheet solution prepared in the above step 1, disperse it in 10 mL of anhydrous ethanol, assemble it into a membrane by vacuum-assisted self-assembly using polyacrylonitrile membrane as the substrate, and dry it at 25°C for 12 hours to obtain an amino-functionalized covalent organic framework membrane.

[0031] Step 3. Preparation of fluorinated covalent organic framework membrane: Immerse the amino-functionalized covalent organic framework membrane prepared in the above step 2 in an ethanol solution of perfluorooctanoyl chloride with a volume fraction of 10%, react at 25°C for 1 minute, and then wash with anhydrous ethanol to obtain a fluorinated covalent organic framework anti-pollution nanofiltration membrane, which is recorded as membrane 1.

[0032] The water flux of membrane 1 at an operating pressure of 1.5 bar is 75.7 L m -2 h -1 bar -1 The retention rate of chrome black T (mass fraction 0.01%) aqueous solution is 99.5%. When filtering 1000ppm pump oil emulsion at an operating pressure of 1.5 bar, the flux attenuation rate is 16.8% and the flux recovery rate is 88.4%, as shown in Table 1.

[0033] Example 2

[0034] A fluorinated covalent organic framework anti-pollution nanofiltration membrane was prepared. The preparation process was basically the same as that in Example 1, except that in step 1, the molar ratio of 2,5-dihydroxybenzaldehyde to 2,5-dihydroxyterephthalaldehyde was 2:8. Finally, a fluorinated covalent organic framework anti-pollution nanofiltration membrane was obtained, which was recorded as Membrane 2.

[0035] The water flux of membrane 2 at an operating pressure of 1.5 bar was 77.9 L m -2 h -1 bar -1The retention rate of chrome black T (mass fraction 0.01%) aqueous solution is 99.5%. When filtering 1000ppm pump oil emulsion at an operating pressure of 1.5 bar, the flux attenuation rate is 15.2% and the flux recovery rate is 91.0%, as shown in Table 1.

[0036] Example 3

[0037] A fluorinated covalent organic framework anti-pollution nanofiltration membrane was prepared. The preparation process was basically the same as that in Example 1, except that in step 1, the molar ratio of 2,5-dihydroxybenzaldehyde to 2,5-dihydroxyterephthalaldehyde was 3:7. Finally, a fluorinated covalent organic framework anti-pollution nanofiltration membrane was obtained, which was recorded as Membrane 3.

[0038] The water flux of membrane 3 at an operating pressure of 1.5 bar is 101.7 L m -2 h -1 bar -1 The retention rate of chrome black T (mass fraction 0.01%) aqueous solution is 99.5%. When filtering 1000ppm pump oil emulsion at an operating pressure of 1.5 bar, the flux attenuation rate is 13.1% and the flux recovery rate is 95.9%, as shown in Table 1.

[0039] Example 4

[0040] A fluorinated covalent organic framework anti-pollution nanofiltration membrane was prepared. The preparation process was basically the same as that in Example 1, except that in step 1, the molar ratio of 2,5-dihydroxybenzaldehyde to 2,5-dihydroxyterephthalaldehyde was 4:6. Finally, a fluorinated covalent organic framework anti-pollution nanofiltration membrane was obtained, which was recorded as membrane 4. Figure 2-1 and Figure 2-2 They are the surface electron microscope image and cross-sectional electron microscope image of membrane 4, respectively. The membrane surface and cross-sectional structure are intact and free of defects.

[0041] The water flux of membrane 4 at an operating pressure of 1.5 bar is 115.9 L m -2 h -1 bar -1 The retention rate of chrome black T (mass fraction 0.01%) aqueous solution is 99.4%. When filtering 1000ppm pump oil emulsion at an operating pressure of 1.5 bar, the flux attenuation rate is 10.4% and the flux recovery rate is 98.3%, as shown in Table 1.

[0042] Figure 4 The anti-pollution performance diagram of the comparative example and embodiments 1-4, the simulated pollutant is pump oil emulsion. It can be seen that with the increase of the perfluoroalkyl chain grafting amount, the flux attenuation rate of the membrane decreases significantly, and the flux recovery rate increases significantly. Membrane 4 shows excellent anti-pollution performance.

[0043] Comparative Example

[0044] The preparation process of the comparative example is basically the same as that of Example 1, except that in step 1, only 0.15 mmol of 2,5-dihydroxyterephthalaldehyde is used as the aldehyde monomer, and a comparative film is finally obtained. Figure 3-1 and Figure 3-2 The surface electron microscope image and cross-sectional electron microscope image of the comparative film are respectively: the surface and cross-sectional structures of the film are intact and have no defects, which is consistent with Figure 2-1 and Figure 2-2 There is no significant difference between the two groups, indicating that the grafting reaction has no effect on the macrostructure of the membrane.

[0045] The water flux of the comparative membrane at an operating pressure of 1.5 bar is 72.3 L m -2 h -1 bar -1 The retention rate of chrome black T (mass fraction 0.01%) aqueous solution is 99.6%. When filtering 1000ppm pump oil emulsion at an operating pressure of 1.5 bar, the flux attenuation rate is 20.7% and the flux recovery rate is 86.7%, as shown in Table 1.

[0046] Table 1 Preparation process conditions and properties of the resulting membranes

[0047]

[0048] By comparing Examples 1 to 4 and the comparative example, it can be concluded that by changing the ratio of the monoaldehyde monomer, an adjustable amount of free amino groups is generated in situ in the covalent organic framework membrane, thereby optimizing the content of perfluoroalkyl chains. Compared with the comparative example, as the content of perfluoroalkyl chains on the membrane surface increases, the membrane exhibits excellent anti-pollution performance, the flux recovery rate is significantly improved, and the flux attenuation rate is significantly reduced. In addition, due to the introduction of the monoaldehyde monomer, more mass transfer channels are generated in the membrane, and the water flux of the membrane 4 prepared in Example 4 reaches 115.9 L m -2 h -1 bar -1 , and also exhibits good retention performance for a variety of dyes, such as Figure 5 As shown, the interception rate of Congo red, chrome black T and Coomassie brilliant blue is more than 99%. The present invention provides a simple and effective method for designing and manufacturing high-performance anti-pollution covalent organic framework membranes and other types of organic molecular sieve membranes.

[0049] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the above-mentioned specific embodiments, which are merely illustrative rather than restrictive. Under the guidance of the present invention, ordinary technicians in this field can make many improvements and changes without departing from the purpose of the present invention, which are all within the protection of the present invention.

Claims

1. A method for preparing a fluorinated covalent organic framework anti-pollution nanofiltration membrane, characterized in that: Firstly, free amino groups are in situ generated in the covalent organic framework membrane by using two aldehyde monomers in a certain molar ratio, and the free amino groups serve as grafting sites for subsequent surface fluorination. The two aldehyde monomers include 2,5-dihydroxybenzaldehyde and 2,5-dihydroxyterephthalaldehyde. Then, the perfluorooctanoyl chloride reacts with free amino groups to graft perfluoroalkyl chains on the membrane surface to prepare a fluorinated covalent organic framework anti-pollution nanofiltration membrane.

2. The preparation method according to claim 1, characterized in that: The following steps are involved: Step 1: Synthesis of amino-functionalized covalent organic framework nanosheets: 2,5-dihydroxybenzaldehyde and 2,5-dihydroxyterephthalaldehyde are weighed according to a molar ratio of X:Y, wherein X=1-4, Y=6-9, and X+Y=10; dissolved in dichloromethane to obtain an aldehyde monomer solution, wherein the molar volume ratio of the aldehyde monomer to dichloromethane is 3mmol / 1600mL; then, a 3M acetic acid aqueous solution is added above the aldehyde monomer solution according to a volume ratio of 3:8; then, an appropriate amount of triaminoguanidine hydrochloride is dissolved in N,N-dimethylformamide to prepare a molar volume ratio A 1mmol / 500mL triaminoguanidine hydrochloride-N,N-dimethylformamide solution, denoted as solution A, is slowly dripped onto the acetic acid aqueous solution; the system is allowed to react at room temperature for 7 days, and after the reaction is completed, the upper solution A is removed to collect the aqueous nanosheet dispersion; a dialysis membrane with a molecular weight cutoff of 30000Da is used to dialyze the obtained nanosheets in deionized water for 3 days to remove the residual unreacted monomers and small molecule oligomers, and an amino-functionalized covalent organic framework nanosheet solution is obtained; Step 2: Preparation of amino-functionalized covalent organic framework membrane: The amino-functionalized covalent organic framework nanosheet solution prepared in the above step 1 is dispersed in anhydrous ethanol at a volume ratio of 1 to 5:1000, and a polyacrylonitrile film is used as a substrate to form a film by vacuum-assisted self-assembly, and the film is dried at 25° C. for 12 hours to obtain an amino-functionalized covalent organic framework film; Step 3: Preparation of fluorinated covalent organic framework membrane: The amino-functionalized covalent organic framework membrane prepared in the above step 2 is immersed in a 10% by volume perfluorooctanoyl chloride solution, and subjected to a grafting reaction at 25° C. for 0.5 to 5 minutes, followed by washing with an organic solvent to obtain a fluorinated covalent organic framework anti-pollution nanofiltration membrane.

3. The preparation method according to claim 1, characterized in that: In step 1, the molar ratio of 2,5-dihydroxybenzaldehyde to 2,5-dihydroxyterephthalaldehyde is 4:

6.

4. The preparation method according to claim 1, characterized in that: In step 2, the volume ratio of the amino-functionalized covalent organic framework nanosheet solution to anhydrous ethanol is 2.5:1000.

5. The preparation method according to claim 1, characterized in that: In step 3, the organic solvent of the perfluorooctanoyl chloride solution is any one of anhydrous ethanol, n-heptane and n-hexane.

6. The preparation method according to claim 5, characterized in that: In step 3, the organic solvent of the perfluorooctanoyl chloride solution is anhydrous ethanol.

7. The preparation method according to claim 1, characterized in that: In step 3, the grafting reaction time is 1 min.

8. The preparation method according to claim 1, characterized in that: In step 3, the organic solvent selected for cleaning is the same as the organic solvent of the perfluorooctanoyl chloride solution.

9. A fluorinated covalent organic framework anti-pollution nanofiltration membrane prepared according to the preparation method according to any one of claims 1 to 8, characterized in that: The water flux of the membrane is 75.7-115.9 L m -2 h -1 bar -1 , the flux attenuation rate dropped from 16.8% to 10.4%, and the flux recovery rate ranged from 88.4% to 98.3%.

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