An oxidation-resistant polyvinylidene fluoride microfiltration membrane and its preparation process

Abstract

This invention relates to the field of polymer membrane materials, specifically to an oxidation-resistant polyvinylidene fluoride (PVDF) microfiltration membrane and its preparation process. The preparation process of an oxidation-resistant PVDF microfiltration membrane includes: preparing modified porous nanosheets, preparing a modified Zr-based metal-organic framework, preparing a composite base membrane, and preparing the PVDF microfiltration membrane. This invention involves alkalizing the composite base membrane, grafting glycidyl methacrylate onto its surface, and then mixing it with hyperbranched polyethyleneimine and polyethylene glycol diamine for a crosslinking reaction. This forms a hydrophilic crosslinked network, which creates a stable hydration layer on the membrane surface. This facilitates the formation and maintenance of water molecule channels and reduces the resistance of water molecules passing through the membrane pores, significantly increasing the water molecule permeation flux. Furthermore, it effectively improves the antifouling performance of the PVDF microfiltration membrane.

Description

Technical Field

[0001] This invention relates to the field of polymer membrane materials, specifically to an oxidation-resistant polyvinylidene fluoride microfiltration membrane and its preparation process. Background Technology

[0002] Membrane separation technology utilizes membrane materials with selective permeability to achieve efficient separation of substances. Based on the size differences of the substances being separated, it can be categorized into application areas such as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and gas separation. Among these, microfiltration membranes typically have pore sizes of 0.1-10 μm and possess characteristics such as large pore size, low operating pressure drop, no phase change during the separation process, and low energy consumption. They have been widely applied in water treatment fields such as wastewater treatment, reclaimed water reuse, and drinking water preparation.

[0003] Polyvinylidene fluoride (PVDF) is a crystalline polymer formed by the polymerization of vinylidene fluoride. Due to its advantages such as being non-toxic, odorless, tasteless, chemically stable, and having excellent UV resistance and aging resistance, it is widely used in microfiltration membranes. However, the overall molecular chain of PVDF microfiltration membranes exhibits strong hydrophobicity, resulting in weak affinity with water molecules. Water molecules have poor wettability on the membrane surface, making it difficult for them to pass through the membrane pores quickly, thus forming high mass transfer resistance and reducing water flux. Furthermore, its hydrophobicity leads to stronger interactions with hydrophobic pollutants in the water, making it easier for pollutants to adhere to the membrane surface or inside the pores through hydrophobic adsorption, forming a fouling layer.

[0004] In addition, since polyvinylidene fluoride microfiltration membranes adsorb organic pollutants during use, they need to be cleaned with oxidants regularly to restore flux by oxidizing and decomposing pollutants. If the membrane's oxidation resistance is insufficient, the cleaning process itself will lead to degradation of the membrane material. The molecular chain of polyvinylidene fluoride filter membrane is -(CH2-CF2)-. Although the CF bond has high oxidation resistance, the CC bond connecting the chain segments is easily attacked by strong oxidants, leading to chain breakage, which manifests as membrane embrittlement and decreased porosity.

[0005] Therefore, it is necessary to propose an oxidation-resistant polyvinylidene fluoride microfiltration membrane and its preparation process that can improve water flux and antifouling properties. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide an oxidation-resistant polyvinylidene fluoride microfiltration membrane and its preparation process.

[0007] This invention provides a process for preparing an oxidation-resistant polyvinylidene fluoride microfiltration membrane, comprising the following steps:

[0008] S1: Preparation of modified porous nanosheets

[0009] p-phenylenediamine and p-toluenesulfonic acid were dissolved separately, then mixed and reacted. After carbonization and activation, porous nanosheets were prepared. These nanosheets were then carboxylated and reacted with dimethylaminoethyl N,N-methacrylate to obtain modified porous nanosheets.

[0010] S2: Preparation of modified Zr-based metal-organic frameworks

[0011] Zr-based metal-organic frameworks were first prepared using zirconium tetrachloride and terephthalic acid as raw materials, then dispersed in toluene, and perfluorooctyltriethoxysilane was added to carry out the reaction to obtain modified Zr-based metal-organic frameworks.

[0012] S3: Preparation of composite base film

[0013] Polyvinylidene fluoride, the above-mentioned modified porous nanosheets, the above-mentioned modified Zr-based metal-organic framework and pore-forming agent are added to a solvent and heated and stirred at 90-100℃ to obtain a casting solution. The casting solution is then allowed to stand to degas and uniformly coated onto a non-woven fabric support layer. It is then immersed in deionized water. After a phase inversion process, a solid membrane is formed. The membrane is then immersed in deionized water for 22-24 hours and dried to obtain a composite base membrane.

[0014] S4: Preparation of polyvinylidene fluoride microfiltration membrane

[0015] S4.1: Immerse the above composite base membrane in a 1 mol / L sodium hydroxide solution for 3-4 hours, then wash with deionized water until neutral to obtain an alkalized composite base membrane;

[0016] S4.2: Dissolve glycidyl methacrylate in anhydrous ethanol to prepare a 20wt% glycidyl methacrylate solution. Dissolve benzoyl peroxide in butanone at a ratio of 1g:(20-30)mL, and then add it to the glycidyl methacrylate solution. Stir and mix evenly to obtain the reaction solution.

[0017] S4.3: Add the above-mentioned alkalized composite base film to the above-mentioned reaction solution, deoxygenate with nitrogen for 10-20 min, heat and reflux at 70-80℃ for 5-6 h, cool, and then wash and dry to obtain the primary modified base film.

[0018] S4.4: Add hyperbranched polyethyleneimine and polyethylene glycol diamine to anhydrous ethanol at a ratio of 1g:1g:(90-100)mL. After stirring and dissolving thoroughly, add an equal volume of deionized water to the anhydrous ethanol and adjust the pH to 8-9 with 0.1mol / L hydrochloric acid solution. Then add the above-mentioned primary modified base membrane and crosslink at 50-60℃ for 5-6h. After washing with deionized water and vacuum drying, polyvinylidene fluoride microfiltration membrane is obtained.

[0019] Furthermore, S1 specifically includes the following steps:

[0020] S1.1: Dissolve p-phenylenediamine in methanol at a ratio of 1g:(6-8)mL to obtain a p-phenylenediamine solution, then dissolve p-toluenesulfonic acid in methanol to prepare a p-toluenesulfonic acid solution with a mass fraction of 20-25%.

[0021] S1.2: Add the above p-phenylenediamine solution to the above p-toluenesulfonic acid solution, stir and react for 1-2 hours under nitrogen protection, filter, wash and vacuum dry, carbonize at 650-700℃ for 2-3 hours under nitrogen atmosphere, and then activate with potassium hydroxide at 700-750℃ under nitrogen atmosphere for 1-2 hours. After washing and drying, porous nanosheets are obtained.

[0022] S1.3: Disperse the above porous nanosheets in a mixed acid solution at a ratio of 1g:(90-100)mL, sonicate for 1-2 hours, wash until neutral and vacuum dry to obtain carboxylated nanosheets;

[0023] S1.4: Disperse the above carboxylated nanosheets in deionized water at a ratio of 1g:(200-300)mL, and sonicate for 30-40min to obtain a carboxylated nanosheet suspension;

[0024] S1.5: Add N,N-dimethylaminoethyl methacrylate to the above carboxylated nanosheet suspension, stir and mix thoroughly, then add 5% ammonium persulfate solution, and heat and react at 60-70℃ for 8-10h under nitrogen protection. After cooling, centrifuge, wash and dry to obtain modified porous nanosheets.

[0025] Furthermore, S2 specifically includes the following steps:

[0026] S2.1: Add zirconium tetrachloride and terephthalic acid to N,N-dimethylformamide at a ratio of (1.2-1.3) g: 1 g: (230-250) mL. After ultrasonic dispersion for 10-20 min, add formic acid and stir evenly. Then transfer to a high-pressure reactor and react at 120-150℃ for 20-24 h. After cooling, centrifuge, wash, and then disperse in anhydrous ethanol. Reflux at 70-80℃ for 10-12 h and then vacuum dry to obtain Zr-based metal-organic frameworks.

[0027] S2.2: Disperse the above Zr-based metal-organic framework in toluene at a ratio of 1g:(180-200)mL, sonicate for 15-25min to obtain a metal-organic framework suspension, then add perfluorooctyltriethoxysilane, and heat and stir under nitrogen protection at 70-80℃ for 10-12h. After cooling, centrifuge, wash and vacuum dry to obtain the modified Zr-based metal-organic framework.

[0028] Furthermore, the molar ratio of p-toluenesulfonic acid to p-phenylenediamine is (1.9-2.1):1, and the volume ratio of N,N-dimethylaminoethyl methacrylate to the carboxylated nanosheet suspension is 1:(15-20).

[0029] Furthermore, the mixed acid solution is prepared by mixing concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1.

[0030] Furthermore, the amount of ammonium persulfate solution added is 8-10% of the volume of the carboxylated nanosheet suspension.

[0031] Furthermore, the amount of formic acid added is 2% of the volume of N,N-dimethylformamide, and the amount of perfluorooctyltriethoxysilane added is 5% of the volume of the metal-organic framework suspension.

[0032] Furthermore, by mass percentage, the raw material composition of the casting solution is: 10-15% polyvinylidene fluoride, 0.06-0.1% modified porous nanosheets, 6-8% modified Zr-based metal-organic framework, 4-8% pore-forming agent, and the balance being solvent. The pore-forming agent is any one of polyethylene glycol, glycerol, propylene glycol, and Tween-80, and the solvent is any one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0033] Furthermore, the volume ratio of butanone to glycidyl methacrylate solution is 1:(8-10).

[0034] An oxidation-resistant polyvinylidene fluoride microfiltration membrane, which is prepared by the preparation process of an oxidation-resistant polyvinylidene fluoride microfiltration membrane as described in any one of the above claims.

[0035] The present invention has the following advantages:

[0036] 1. In this invention, after alkalizing the composite base membrane, glycidyl methacrylate is first grafted onto its surface to provide reaction sites for subsequent crosslinking. Then, it is mixed with hyperbranched polyethyleneimine and polyethylene glycol diamine to undergo a crosslinking reaction, forming a hydrophilic crosslinking network. The amino groups of the hyperbranched polyethyleneimine and polyethylene glycol diamine undergo a ring-opening reaction with the epoxy groups on the glycidyl methacrylate graft layer, firmly introducing a hydrophilic polymer network rich in amino groups and ether oxygen bonds onto the membrane surface. This forms a stable hydration layer on the membrane surface, which is beneficial for the formation and maintenance of water molecule channels. Simultaneously, it greatly reduces the interfacial energy between water molecules and membrane materials, reducing the resistance of water molecules passing through membrane pores and significantly increasing the permeation flux of water molecules. In addition, the hydrophilic cross-linked network can form a dense hydration layer, making it difficult for pollutants to penetrate or replace the tightly bound water molecules to directly contact the membrane surface, thereby greatly reducing the tendency of pollutants to be adsorbed and deposited on the membrane surface. Furthermore, the high-density amine groups of hyperbranched polyethyleneimine impart a positive charge to the surface, preventing the adsorption of negatively charged pollutants through electrostatic repulsion, thus effectively improving the antifouling performance of polyvinylidene fluoride microfiltration membranes.

[0037] 2. In this invention, porous nanosheets are first prepared using p-phenylenediamine and p-toluenesulfonic acid as raw materials, and then carboxylated using a mixed acid. Following this modification, N,N-dimethylaminoethyl methacrylate is grafted onto the surface of the porous nanosheets under the initiation of an initiator. This modification effectively improves the dispersibility of the porous nanosheets in polyvinylidene fluoride (PVDF) and prevents agglomeration. When the modified porous nanosheets are mixed with PVDF to form a composite base membrane, the modified porous nanosheets can synergistically work with the microporous structure of the base membrane to optimize the membrane's porosity and pore size distribution, reducing the flow resistance of water within the membrane and thus increasing the water flux of the PVDF microfiltration membrane. Furthermore, the tertiary amine groups in the molecular chain of the N,N-dimethylaminoethyl methacrylate grafted onto the porous nanosheet surface have electron-donating properties, preferentially reacting with oxidants in redox reactions to consume the oxidants, thereby improving the oxidation resistance of the PVDF microfiltration membrane.

[0038] 3. In this invention, a Zr-based metal-organic framework is prepared using zirconium tetrachloride and terephthalic acid as raw materials, and then its surface is modified with perfluorooctyltriethoxysilane to form a modified Zr-based metal-organic framework. After being mixed with polyvinylidene fluoride and other materials to form a composite membrane, the modified Zr-based metal-organic framework can serve as a stabilizing reinforcing phase in the membrane matrix, reducing the degradation of the overall membrane structure in an oxidizing environment. The modification effect of perfluorooctyltriethoxysilane can not only improve the dispersibility of the Zr-based metal-organic framework in the composite membrane, but also consume free radicals and block the chain attack of free radicals generated during oxidation cleaning on the composite membrane, thereby further improving the oxidation resistance of the polyvinylidene fluoride microfiltration membrane. Attached Figure Description

[0039] Figure 1 This is a flowchart illustrating the preparation process of the oxidation-resistant polyvinylidene fluoride microfiltration membrane used in an embodiment of the present invention. Detailed Implementation

[0040] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of this invention.

[0041] Example 1: A preparation process for an oxidation-resistant polyvinylidene fluoride microfiltration membrane, such as... Figure 1 As shown, it includes the following steps:

[0042] S1: Preparation of modified porous nanosheets

[0043] S1.1: Dissolve p-phenylenediamine in methanol at a ratio of 1g:6mL to obtain a p-phenylenediamine solution, then dissolve p-toluenesulfonic acid in methanol to prepare a p-toluenesulfonic acid solution with a mass fraction of 20%.

[0044] S1.2: The above p-phenylenediamine solution was added to the above p-toluenesulfonic acid solution, and the reaction was stirred for 1 hour under nitrogen protection. After filtration, washing and vacuum drying, the mixture was carbonized at 650°C for 2 hours under nitrogen atmosphere, and then activated with potassium hydroxide at 700°C under nitrogen atmosphere for 1 hour. After washing and drying, porous nanosheets were obtained, wherein the molar ratio of p-toluenesulfonic acid to p-phenylenediamine was 1.9:1.

[0045] S1.3: The above porous nanosheets were dispersed in a mixed acid solution at a ratio of 1g:90mL, ultrasonicated for 1h, washed until neutral and vacuum dried to obtain carboxylated nanosheets. The mixed acid solution was prepared by mixing concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1.

[0046] S1.4: Disperse the above carboxylated nanosheets in deionized water at a ratio of 1g:200mL, and sonicate for 30min to obtain a carboxylated nanosheet suspension;

[0047] S1.5: Add N,N-dimethylaminoethyl methacrylate to the above carboxylated nanosheet suspension, stir and mix thoroughly, then add 5% (w / w) ammonium persulfate solution, and heat at 60°C for 8 hours under nitrogen protection. After cooling, centrifuge, wash and dry to obtain modified porous nanosheets. The volume ratio of N,N-dimethylaminoethyl methacrylate to carboxylated nanosheet suspension is 1:15, and the amount of ammonium persulfate solution added is 8% of the volume of carboxylated nanosheet suspension.

[0048] S2: Preparation of modified Zr-based metal-organic frameworks

[0049] S2.1: Zirconium tetrachloride and terephthalic acid were added to N,N-dimethylformamide at a ratio of 1.2g:1g:230mL. After ultrasonic dispersion for 10-20min, formic acid was added and stirred evenly. The mixture was then transferred to a high-pressure reactor and reacted at 120℃ for 20h. After cooling, the mixture was centrifuged, washed, and then dispersed in anhydrous ethanol. It was refluxed at 70℃ for 10h and then vacuum dried to obtain a Zr-based metal-organic framework. The amount of formic acid added was 2% of the volume of N,N-dimethylformamide.

[0050] S2.2: The above Zr-based metal-organic framework was dispersed in toluene at a ratio of 1g:180mL, and ultrasonicated for 15min to obtain a metal-organic framework suspension. Then, perfluorooctyltriethoxysilane was added, and the mixture was heated and refluxed at 70℃ for 10h under nitrogen protection. After cooling, the mixture was centrifuged, washed, and vacuum dried to obtain the modified Zr-based metal-organic framework. The amount of perfluorooctyltriethoxysilane added was 5% of the volume of the metal-organic framework suspension.

[0051] S3: Preparation of composite base film

[0052] Polyvinylidene fluoride, the modified porous nanosheets, the modified Zr-based metal-organic framework, and polyethylene glycol were added to N,N-dimethylformamide and heated and stirred at 90°C to obtain a casting solution. The casting solution was then allowed to stand to remove bubbles and uniformly coated onto a nonwoven fabric support layer. After immersion in deionized water and undergoing a phase inversion process, a solid membrane was formed. The membrane was then soaked in deionized water for 22 hours and dried to obtain a composite base membrane. The raw material composition of the casting solution, by mass percentage, was: 10% polyvinylidene fluoride, 0.06% modified porous nanosheets, 6% modified Zr-based metal-organic framework, 4% polyethylene glycol, and the balance being N,N-dimethylformamide.

[0053] S4: Preparation of polyvinylidene fluoride microfiltration membrane

[0054] S4.1: The above composite base membrane is immersed in 1 mol / L sodium hydroxide solution for 3 hours, and then washed with deionized water until neutral to obtain an alkalized composite base membrane;

[0055] S4.2: Dissolve glycidyl methacrylate in anhydrous ethanol to prepare a 20wt% glycidyl methacrylate solution. Dissolve benzoyl peroxide in butanone at a ratio of 1g:20mL, and then add it to the glycidyl methacrylate solution. Stir and mix evenly to obtain a reaction solution. The volume ratio of butanone to glycidyl methacrylate solution is 1:8.

[0056] S4.3: The above-mentioned alkalized composite base film is added to the above reaction solution, and after being deoxygenated by nitrogen for 10 min, it is heated and refluxed at 70°C for 5 h. After cooling, it is washed and dried to obtain the primary modified base film.

[0057] S4.4: Hyperbranched polyethyleneimine and polyethylene glycol diamine were added to anhydrous ethanol at a ratio of 1g:1g:90mL. After thorough stirring and dissolution, deionized water of equal volume to anhydrous ethanol was added, and the pH was adjusted to 8 with 0.1mol / L hydrochloric acid solution. Then, the above-mentioned primary modified base membrane was added, and crosslinked at 50℃ for 5h. After washing with deionized water and vacuum drying, polyvinylidene fluoride microfiltration membrane was obtained.

[0058] Example 2: A preparation process for an oxidation-resistant polyvinylidene fluoride microfiltration membrane, as follows... Figure 1 As shown, it includes the following steps:

[0059] S1: Preparation of modified porous nanosheets

[0060] S1.1: Dissolve p-phenylenediamine in methanol at a ratio of 1g:7mL to obtain a p-phenylenediamine solution, then dissolve p-toluenesulfonic acid in methanol to prepare a p-toluenesulfonic acid solution with a mass fraction of 22.5%;

[0061] S1.2: The above p-phenylenediamine solution was added to the above p-toluenesulfonic acid solution, and the reaction was stirred for 1.5 h under nitrogen protection. After filtration, washing and vacuum drying, the mixture was carbonized at 675 °C for 2.5 h under nitrogen atmosphere, and then activated with potassium hydroxide at 725 °C under nitrogen atmosphere for 1.5 h. After washing and drying, porous nanosheets were obtained, wherein the molar ratio of p-toluenesulfonic acid to p-phenylenediamine was 2:1.

[0062] S1.3: The above porous nanosheets were dispersed in a mixed acid solution at a ratio of 1g:95mL, ultrasonicated for 1.5h, washed until neutral and vacuum dried to obtain carboxylated nanosheets. The mixed acid solution was prepared by mixing concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1.

[0063] S1.4: Disperse the above carboxylated nanosheets in deionized water at a ratio of 1g:250mL, and sonicate for 35min to obtain a carboxylated nanosheet suspension;

[0064] S1.5: N,N-dimethylaminoethyl methacrylate was added to the above carboxylated nanosheet suspension. After thorough mixing, a 5% (w / w) ammonium persulfate solution was added, and the mixture was heated at 65°C for 9 hours under nitrogen protection. After cooling, the mixture was centrifuged, washed, and dried to obtain modified porous nanosheets. The volume ratio of N,N-dimethylaminoethyl methacrylate to the carboxylated nanosheet suspension was 1:17.5, and the amount of ammonium persulfate solution added was 9% of the volume of the carboxylated nanosheet suspension.

[0065] S2: Preparation of modified Zr-based metal-organic frameworks

[0066] S2.1: Zirconium tetrachloride and terephthalic acid were added to N,N-dimethylformamide at a ratio of 1.25g:1g:240mL. After ultrasonic dispersion for 15min, formic acid was added and stirred evenly. Then, the mixture was transferred to a high-pressure reactor and reacted at 135℃ for 22h. After cooling, the mixture was centrifuged, washed, and then dispersed in anhydrous ethanol. It was refluxed at 75℃ for 11h and then vacuum dried to obtain a Zr-based metal-organic framework. The amount of formic acid added was 2% of the volume of N,N-dimethylformamide.

[0067] S2.2: The above Zr-based metal-organic framework was dispersed in toluene at a ratio of 1g:190mL, and ultrasonicated for 20min to obtain a metal-organic framework suspension. Then, perfluorooctyltriethoxysilane was added, and the mixture was heated and refluxed at 75℃ for 11h under nitrogen protection. After cooling, the mixture was centrifuged, washed, and vacuum dried to obtain the modified Zr-based metal-organic framework. The amount of perfluorooctyltriethoxysilane added was 5% of the volume of the metal-organic framework suspension.

[0068] S3: Preparation of composite base film

[0069] Polyvinylidene fluoride, the modified porous nanosheets, the modified Zr-based metal-organic framework, and glycerol were added to N,N-dimethylacetamide and heated and stirred at 95°C to obtain a casting solution. The casting solution was then allowed to stand to remove bubbles and uniformly coated onto a nonwoven fabric support layer. After immersion in deionized water and undergoing a phase inversion process, a solid membrane was formed. The membrane was then soaked in deionized water for 23 hours and dried to obtain a composite base membrane. The raw material composition of the casting solution, by mass percentage, was: 12.5% ​​polyvinylidene fluoride, 0.08% modified porous nanosheets, 7% modified Zr-based metal-organic framework, 6% glycerol, and the balance being N,N-dimethylacetamide.

[0070] S4: Preparation of polyvinylidene fluoride microfiltration membrane

[0071] S4.1: Immerse the above composite base membrane in a 1 mol / L sodium hydroxide solution for 3.5 h, and then wash it with deionized water until neutral to obtain an alkalized composite base membrane;

[0072] S4.2: Dissolve glycidyl methacrylate in anhydrous ethanol to prepare a 20wt% glycidyl methacrylate solution. Dissolve benzoyl peroxide in butanone at a ratio of 1g:25mL, and then add it to the glycidyl methacrylate solution. Stir and mix evenly to obtain a reaction solution. The volume ratio of butanone to glycidyl methacrylate solution is 1:9.

[0073] S4.3: The above-mentioned alkalized composite base film is added to the above reaction solution, and after being deoxygenated by nitrogen for 15 min, it is heated and refluxed at 75°C for 5.5 h. After cooling, it is washed and dried to obtain the primary modified base film.

[0074] S4.4: Hyperbranched polyethyleneimine and polyethylene glycol diamine were added to anhydrous ethanol at a ratio of 1g:1g:95mL. After thorough stirring and dissolution, deionized water of equal volume to anhydrous ethanol was added, and the pH was adjusted to 8.5 with 0.1mol / L hydrochloric acid solution. Then, the above-mentioned primary modified base membrane was added, and crosslinking was carried out at 55℃ for 5.5h. After washing with deionized water and vacuum drying, polyvinylidene fluoride microfiltration membrane was obtained.

[0075] Example 3: A preparation process for an oxidation-resistant polyvinylidene fluoride microfiltration membrane, as follows... Figure 1 As shown, it includes the following steps:

[0076] S1: Preparation of modified porous nanosheets

[0077] S1.1: Dissolve p-phenylenediamine in methanol at a ratio of 1g:8mL to obtain a p-phenylenediamine solution, then dissolve p-toluenesulfonic acid in methanol to prepare a p-toluenesulfonic acid solution with a mass fraction of 25%.

[0078] S1.2: The above p-phenylenediamine solution was added to the above p-toluenesulfonic acid solution, and the reaction was stirred for 2 hours under nitrogen protection. After filtration, washing and vacuum drying, the mixture was carbonized at 700°C for 3 hours under nitrogen atmosphere, and then activated with potassium hydroxide at 750°C under nitrogen atmosphere for 2 hours. After washing and drying, porous nanosheets were obtained, wherein the molar ratio of p-toluenesulfonic acid to p-phenylenediamine was 2.1:1.

[0079] S1.3: The above porous nanosheets were dispersed in a mixed acid solution at a ratio of 1g:100mL, ultrasonicated for 2h, washed until neutral and vacuum dried to obtain carboxylated nanosheets. The mixed acid solution was prepared by mixing concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1.

[0080] S1.4: Disperse the above carboxylated nanosheets in deionized water at a ratio of 1g:300mL, and sonicate for 40min to obtain a carboxylated nanosheet suspension;

[0081] S1.5: Add N,N-dimethylaminoethyl methacrylate to the above carboxylated nanosheet suspension, stir and mix thoroughly, then add 5% (w / w) ammonium persulfate solution, and heat at 70°C for 10 h under nitrogen protection. After cooling, centrifuge, wash and dry to obtain modified porous nanosheets. The volume ratio of N,N-dimethylaminoethyl methacrylate to carboxylated nanosheet suspension is 1:20, and the amount of ammonium persulfate solution added is 10% of the volume of carboxylated nanosheet suspension.

[0082] S2: Preparation of modified Zr-based metal-organic frameworks

[0083] S2.1: Zirconium tetrachloride and terephthalic acid were added to N,N-dimethylformamide at a ratio of 1.3g:1g:250mL. After ultrasonic dispersion for 20min, formic acid was added and stirred evenly. Then, the mixture was transferred to a high-pressure reactor and reacted at 150℃ for 24h. After cooling, the mixture was centrifuged, washed, and then dispersed in anhydrous ethanol. It was refluxed at 80℃ for 12h and then vacuum dried to obtain a Zr-based metal-organic framework. The amount of formic acid added was 2% of the volume of N,N-dimethylformamide.

[0084] S2.2: The above Zr-based metal-organic framework was dispersed in toluene at a ratio of 1 g: 200 mL, and ultrasonicated for 25 min to obtain a metal-organic framework suspension. Then, perfluorooctyltriethoxysilane was added, and the mixture was heated and refluxed at 80 °C for 12 h under nitrogen protection. After cooling, the mixture was centrifuged, washed, and vacuum dried to obtain the modified Zr-based metal-organic framework. The amount of perfluorooctyltriethoxysilane added was 5% of the volume of the metal-organic framework suspension.

[0085] S3: Preparation of composite base film

[0086] Polyvinylidene fluoride, the modified porous nanosheets, the modified Zr-based metal-organic framework, and propylene glycol were added to N-methylpyrrolidone and heated and stirred at 100°C to obtain a casting solution. The casting solution was then allowed to stand to remove bubbles and uniformly coated onto a nonwoven fabric support layer. After immersion in deionized water and undergoing a phase inversion process, a solid membrane was formed. The membrane was then soaked in deionized water for 24 hours and dried to obtain a composite base membrane. The raw material composition of the casting solution, by mass percentage, was: 15% polyvinylidene fluoride, 0.1% modified porous nanosheets, 8% modified Zr-based metal-organic framework, 8% propylene glycol, and the balance being N-methylpyrrolidone.

[0087] S4: Preparation of polyvinylidene fluoride microfiltration membrane

[0088] S4.1: The above composite base film is immersed in 1 mol / L sodium hydroxide solution for 4 hours, and then washed with deionized water until neutral to obtain an alkalized composite base film;

[0089] S4.2: Dissolve glycidyl methacrylate in anhydrous ethanol to prepare a 20wt% glycidyl methacrylate solution. Dissolve benzoyl peroxide in butanone at a ratio of 1g:30mL, and then add it to the glycidyl methacrylate solution. Stir and mix evenly to obtain a reaction solution. The volume ratio of butanone to glycidyl methacrylate solution is 1:10.

[0090] S4.3: The above-mentioned alkalized composite base film is added to the above reaction solution, and after being deoxygenated by nitrogen for 20 minutes, it is heated and refluxed at 80°C for 6 hours. After cooling, it is washed and dried to obtain the primary modified base film.

[0091] S4.4: Hyperbranched polyethyleneimine and polyethylene glycol diamine were added to anhydrous ethanol at a ratio of 1g:1g:100mL. After thorough stirring and dissolution, deionized water of equal volume to anhydrous ethanol was added, and the pH was adjusted to 9 with 0.1mol / L hydrochloric acid solution. Then, the above-mentioned primary modified base membrane was added, and crosslinked at 60℃ for 6h. After washing with deionized water and vacuum drying, polyvinylidene fluoride microfiltration membrane was obtained.

[0092] Comparative Example 1 differs from Example 1 in that step S4 is removed, i.e., the composite base film is not subjected to surface modification treatment.

[0093] Comparative Example 2 differs from Example 1 in that steps S1.3-S1.4 are removed, and the modified porous nanosheets in step S3 are replaced with an equal amount of carboxylated nanosheets.

[0094] Comparative Example 3 differs from Example 1 in that the modified Zr-based metal-organic framework in step S3 is removed.

[0095] Comparative Example 4 differs from Example 1 in that step S2.2 is removed, and the modified Zr-based metal-organic framework in step S3 is replaced with an equal amount of Zr-based metal-organic framework.

[0096] Test example:

[0097] Test 1: The water flux of the membranes prepared in Examples 1-3 and Comparative Examples 1-2 was tested, and the results are shown in Table 1.

[0098] Table 1: Water Flux Test Results of Microfiltration Membrane

[0099]

[0100] As shown in Table 1 above, the water flux of the composite membrane prepared in Comparative Example 1 without surface modification is significantly lower than that in Example 1. This indicates that by alkalizing the composite membrane and then grafting glycidyl methacrylate onto its surface to provide reaction sites for subsequent crosslinking, and then mixing it with hyperbranched polyethyleneimine and polyethylene glycol diamine to carry out a crosslinking reaction to form a hydrophilic crosslinking network, a stable hydration layer can be formed on the membrane surface, which is beneficial to the formation and maintenance of water molecule channels. At the same time, it greatly reduces the interfacial energy between water molecules and membrane materials, reduces the resistance of water molecules passing through the membrane pores, and significantly increases the permeation flux of water molecules.

[0101] Furthermore, in Comparative Example 2, the water flux of the microfiltration membrane obtained without modifying the porous nanosheets with N,N-dimethylaminoethyl methacrylate was lower than that in Example 1. This indicates that by first preparing porous nanosheets using p-phenylenediamine and p-toluenesulfonic acid as raw materials, then carboxylating them with a mixed acid, and finally grafting N,N-dimethylaminoethyl methacrylate onto the surface of the porous nanosheets under the initiation of an initiator, the dispersibility of the porous nanosheets in the polyvinylidene fluoride matrix can be effectively improved, preventing agglomeration. This allows the modified porous nanosheets to synergize with the microporous structure of the base membrane, optimizing the membrane's porosity and pore size distribution, reducing the flow resistance of water within the membrane, and thus increasing the water flux of the polyvinylidene fluoride microfiltration membrane.

[0102] Test 2: Using sodium alginate at a mass concentration of 100 mg / L as a model pollutant, the flux recovery rate of the membranes prepared in Examples 1-3 and Comparative Example 1 was determined to characterize the antifouling performance of the membranes.

[0103] Flux recovery rate = Clean water flux after simple physical cleaning of the membrane after it has been fouled by sodium alginate / Clean water flux before the membrane was fouled. The results are shown in Table 2.

[0104] Table 2: Results of Microfiltration Membrane Antifouling Test

[0105]

[0106] As shown in Table 2 above, in Comparative Example 1, without surface modification of the composite base membrane, the water flux recovery rate of the membrane after being contaminated by pollutants and then simply cleaned was much lower than that in Example 1. This indicates that by alkalizing the composite base membrane and then grafting glycidyl methacrylate onto its surface to provide reaction sites for subsequent crosslinking, and then mixing it with hyperbranched polyethyleneimine and polyethylene glycol diamine to carry out a crosslinking reaction to form a hydrophilic crosslinking network, the antifouling performance of the polyvinylidene fluoride microfiltration membrane can be effectively improved.

[0107] Test 3: The membranes prepared in Examples 1-3 and Comparative Examples 2-4 were immersed in a 200 mg / L NaClO solution for 96 h, and the membrane flux retention rate was then measured to characterize the membrane's oxidation resistance.

[0108] Flux retention rate = water flux after membrane soaking in NaClO solution for 96 h / water flux before membrane soaking. The results are shown in Table 3.

[0109] Table 3: Results of Oxidation Resistance Test of Microfiltration Membranes

[0110]

[0111] As shown in Table 3 above, in Comparative Example 2, the flux retention rate of the microfiltration membrane obtained after soaking in NaClO solution was lower than that in Example 1 because the porous nanosheets were not modified with N,N-dimethylaminoethyl methacrylate. This indicates that by first preparing porous nanosheets using p-phenylenediamine and p-toluenesulfonic acid as raw materials, then carboxylating them with a mixed acid, and then grafting N,N-dimethylaminoethyl methacrylate onto the surface of the porous nanosheets under the initiation of an initiator, the oxidation resistance of the polyvinylidene fluoride microfiltration membrane can be improved.

[0112] Furthermore, in Comparative Example 3, without the addition of a modified Zr-based metal-organic framework, the flux retention rate of the microfiltration membrane after immersion in NaClO solution was significantly lower than that in Example 1. This indicates that by preparing a Zr-based metal-organic framework using zirconium tetrachloride and terephthalic acid as raw materials, and then modifying its surface with perfluorooctyltriethoxysilane to form a modified Zr-based metal-organic framework, and then mixing it with polyvinylidene fluoride to form a composite base membrane, the modified Zr-based metal-organic framework can serve as a stabilizing reinforcing phase in the membrane matrix, reducing the degradation of the overall membrane structure in an oxidizing environment. In contrast, in Comparative Example 4, without the modification of the Zr-based metal-organic framework with perfluorooctyltriethoxysilane, the flux retention rate of the microfiltration membrane after immersion in NaClO solution was higher than that of Comparative Example 3, but still lower than that of Example 1. Therefore, the modification effect of perfluorooctyltriethoxysilane can further improve the oxidation resistance of the polyvinylidene fluoride microfiltration membrane.

[0113] It should be understood that those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims. Parts not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A process for preparing an oxidation-resistant polyvinylidene fluoride microfiltration membrane, characterized in that, Includes the following steps: S1: Preparation of modified porous nanosheets p-phenylenediamine and p-toluenesulfonic acid were dissolved separately, then mixed and reacted. After carbonization and activation, porous nanosheets were prepared. These nanosheets were then carboxylated and reacted with dimethylaminoethyl N,N-methacrylate to obtain modified porous nanosheets. S2: Preparation of modified Zr-based metal-organic frameworks Zr-based metal-organic frameworks were first prepared using zirconium tetrachloride and terephthalic acid as raw materials, then dispersed in toluene, and perfluorooctyltriethoxysilane was added to carry out the reaction to obtain modified Zr-based metal-organic frameworks. S3: Preparation of composite base film Polyvinylidene fluoride, the above-mentioned modified porous nanosheets, the above-mentioned modified Zr-based metal-organic framework and pore-forming agent are added to a solvent and heated and stirred at 90-100℃ to obtain a casting solution. The casting solution is then allowed to stand to degas and uniformly coated onto a non-woven fabric support layer. It is then immersed in deionized water. After a phase inversion process, a solid membrane is formed. The membrane is then immersed in deionized water for 22-24 hours and dried to obtain a composite base membrane. S4: Preparation of polyvinylidene fluoride microfiltration membrane S4.1: Immerse the above composite base membrane in a 1 mol / L sodium hydroxide solution for 3-4 hours, then wash with deionized water until neutral to obtain an alkalized composite base membrane; S4.2: Dissolve glycidyl methacrylate in anhydrous ethanol to prepare a 20wt% glycidyl methacrylate solution. Dissolve benzoyl peroxide in butanone at a ratio of 1g:(20-30)mL, and then add it to the glycidyl methacrylate solution. Stir and mix evenly to obtain the reaction solution. S4.3: Add the above-mentioned alkalized composite base film to the above-mentioned reaction solution, deoxygenate with nitrogen for 10-20 min, heat and reflux at 70-80℃ for 5-6 h, cool, and then wash and dry to obtain the primary modified base film. S4.4: Add hyperbranched polyethyleneimine and polyethylene glycol diamine to anhydrous ethanol at a ratio of 1g:1g:(90-100)mL. After stirring and dissolving thoroughly, add an equal volume of deionized water to the anhydrous ethanol and adjust the pH to 8-9 with 0.1mol / L hydrochloric acid solution. Then add the above-mentioned primary modified base membrane and crosslink at 50-60℃ for 5-6h. After washing with deionized water and vacuum drying, polyvinylidene fluoride microfiltration membrane is obtained.

2. The preparation process of an oxidation-resistant polyvinylidene fluoride microfiltration membrane according to claim 1, characterized in that, S1 specifically includes the following steps: S1.1: Dissolve p-phenylenediamine in methanol at a ratio of 1g:(6-8)mL to obtain a p-phenylenediamine solution, then dissolve p-toluenesulfonic acid in methanol to prepare a p-toluenesulfonic acid solution with a mass fraction of 20-25%. S1.2: Add the above p-phenylenediamine solution to the above p-toluenesulfonic acid solution, stir and react for 1-2 hours under nitrogen protection, filter, wash and vacuum dry, carbonize at 650-700℃ for 2-3 hours under nitrogen atmosphere, and then activate with potassium hydroxide at 700-750℃ under nitrogen atmosphere for 1-2 hours. After washing and drying, porous nanosheets are obtained. S1.3: Disperse the above porous nanosheets in a mixed acid solution at a ratio of 1g:(90-100)mL, sonicate for 1-2 hours, wash until neutral and vacuum dry to obtain carboxylated nanosheets; S1.4: Disperse the above carboxylated nanosheets in deionized water at a ratio of 1g:(200-300)mL, and sonicate for 30-40min to obtain a carboxylated nanosheet suspension; S1.5: Add N,N-dimethylaminoethyl methacrylate to the above carboxylated nanosheet suspension, stir and mix thoroughly, then add 5% ammonium persulfate solution, and heat and react at 60-70℃ for 8-10h under nitrogen protection. After cooling, centrifuge, wash and dry to obtain modified porous nanosheets.

3. The preparation process of an oxidation-resistant polyvinylidene fluoride microfiltration membrane according to claim 2, characterized in that, S2 specifically includes the following steps: S2.1: Add zirconium tetrachloride and terephthalic acid to N,N-dimethylformamide at a ratio of (1.2-1.3) g: 1 g: (230-250) mL. After ultrasonic dispersion for 10-20 min, add formic acid and stir evenly. Then transfer to a high-pressure reactor and react at 120-150℃ for 20-24 h. After cooling, centrifuge, wash, and then disperse in anhydrous ethanol. Reflux at 70-80℃ for 10-12 h and then vacuum dry to obtain Zr-based metal-organic frameworks. S2.2: Disperse the above Zr-based metal-organic framework in toluene at a ratio of 1g:(180-200)mL, sonicate for 15-25min to obtain a metal-organic framework suspension, then add perfluorooctyltriethoxysilane, and heat and stir under nitrogen protection at 70-80℃ for 10-12h. After cooling, centrifuge, wash and vacuum dry to obtain the modified Zr-based metal-organic framework.

4. The preparation process of an oxidation-resistant polyvinylidene fluoride microfiltration membrane according to claim 2, characterized in that, The molar ratio of p-toluenesulfonic acid to p-phenylenediamine is (1.9-2.1):1, and the volume ratio of N,N-methacrylate dimethylaminoethyl ester to carboxylated nanosheet suspension is 1:(15-20).

5. The preparation process of an oxidation-resistant polyvinylidene fluoride microfiltration membrane according to claim 2, characterized in that, The mixed acid solution is prepared by mixing concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:

1.

6. The preparation process of an oxidation-resistant polyvinylidene fluoride microfiltration membrane according to claim 2, characterized in that, The amount of ammonium persulfate solution added is 8-10% of the volume of the carboxylated nanosheet suspension.

7. The preparation process of an oxidation-resistant polyvinylidene fluoride microfiltration membrane according to claim 3, characterized in that, The amount of formic acid added is 2% of the volume of N,N-dimethylformamide, and the amount of perfluorooctyltriethoxysilane added is 5% of the volume of the metal-organic framework suspension.

8. The preparation process of an oxidation-resistant polyvinylidene fluoride microfiltration membrane according to claim 1, characterized in that, The casting solution comprises, by mass percentage: 10-15% polyvinylidene fluoride, 0.06-0.1% modified porous nanosheets, 6-8% modified Zr-based metal-organic frameworks, 4-8% pore-forming agent, and the balance being solvent. The pore-forming agent is any one of polyethylene glycol, glycerol, propylene glycol, and Tween-80, and the solvent is any one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

9. The preparation process of an oxidation-resistant polyvinylidene fluoride microfiltration membrane according to claim 1, characterized in that, The volume ratio of butanone to glycidyl methacrylate solution is 1:(8-10).

10. An oxidation-resistant polyvinylidene fluoride microfiltration membrane, characterized in that, It is prepared by the preparation process of an oxidation-resistant polyvinylidene fluoride microfiltration membrane as described in any one of claims 1-9.

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