Preparation method of double-channel nanofiltration membrane and double-channel nanofiltration membrane

Abstract

The application discloses a preparation method of a double-channel nanofiltration membrane and the double-channel nanofiltration membrane. The preparation method comprises the following steps: preparing a casting solution, wherein the casting solution comprises a metal salt precursor; preparing a base film by a non-solvent induced phase separation method, wherein the base film has a second channel formed by the metal salt precursor; obtaining hydrogen-bonded organic framework nanoparticles; uniformly dispersing the hydrogen-bonded organic framework nanoparticles in an aqueous solution, forming a polyamide separation layer on the surface of the base film through an interfacial polymerization reaction between the aqueous solution and an oil phase solution, and obtaining a precursor membrane, wherein the hydrogen-bonded organic framework nanoparticles are coated in the polyamide separation layer; and performing elution treatment on the precursor membrane by using an alkaline solution, eluting the hydrogen-bonded organic framework nanoparticles from the polyamide separation layer, forming a first channel, and obtaining a target double-channel nanofiltration membrane.

Description

Technical Field

[0001] This invention relates to the field of water treatment membrane technology, and in particular to a method for preparing a dual-channel nanofiltration membrane and the dual-channel nanofiltration membrane itself. Background Technology

[0002] Humanity is facing a global water shortage crisis. Membrane separation technology is a promising method for solving the freshwater shortage problem. Among them, nanofiltration (NF) occupies a strategic position, effectively bridging the gap between ultrafiltration (UF) and reverse osmosis (RO). The performance of NF mainly depends on its selective layer. Polyamide membrane composite membranes are prepared on porous substrates through interfacial polymerization (IP) of amine and acyl chloride monomers. Due to their excellent separation performance and mechanical stability, they have become the industry standard and a research hotspot for membranes.

[0003] However, traditional polyamide nanofiltration membranes face a "flux-selectivity trade-off": increasing the degree of crosslinking can enhance the rejection rate, but it leads to a significant decrease in water flux; conversely, decreasing the degree of crosslinking can increase the flux, but it sacrifices selectivity.

[0004] Therefore, there is an urgent need for a nanofiltration membrane that can achieve both high throughput and high rejection rate. Summary of the Invention

[0005] The technical problem to be solved by the embodiments of the present invention is to provide a method for preparing a dual-channel nanofiltration membrane and a dual-channel nanofiltration membrane that can achieve high throughput and high rejection rate.

[0006] To address the aforementioned technical problems, this invention provides a method for preparing a dual-channel nanofiltration membrane. The method includes the following steps: preparing a casting solution comprising a metal salt precursor; preparing a base membrane from the casting solution using a solvent-inducible phase separation method, wherein the base membrane has a second channel formed by the metal salt precursor; obtaining hydrogen-bonded organic framework nanoparticles; uniformly dispersing the hydrogen-bonded organic framework nanoparticles in an aqueous solution, and forming a polyamide separation layer on the surface of the base membrane through an interfacial polymerization reaction between the aqueous and oil phase solutions to obtain a precursor membrane, wherein the hydrogen-bonded organic framework nanoparticles are coated within the polyamide separation layer; eluting the precursor membrane with an alkaline solution to elute the hydrogen-bonded organic framework nanoparticles from the polyamide separation layer, forming a first channel, thereby obtaining the target dual-channel nanofiltration membrane.

[0007] In one feasible implementation, the operation of preparing the casting solution includes: dissolving the film-forming polymer polyethersulfone, the water-soluble polymeric porogen polyvinylpyrrolidone, and the metal salt precursor in an organic solvent, stirring at 80°C for 10 hours until the polyethersulfone is completely dissolved to obtain the target casting solution; wherein the metal salt precursor is a zinc salt, and the second channel is a ZIF-8 nanocrystalline layer.

[0008] In one feasible implementation, the film-forming polymer is selected from at least one of polyethersulfone, polysulfone, and polyvinylidene fluoride; In one feasible implementation, the water-soluble polymeric porogen is selected from at least one of polyvinylpyrrolidone and polyethylene glycol; In one feasible implementation, the mass percentage of the film-forming polymer in the casting solution ranges from 15wt% to 20wt%, and the mass percentage of the water-soluble polymeric porogen ranges from 1wt% to 5wt%. In one feasible implementation, the zinc salt is Zn(NO3)2·6H2O, and the mass percentage of the zinc salt ranges from 0.1% to 1%.

[0009] In one feasible implementation, the preparation of the base membrane by the non-solvent-induced phase separation method includes: scraping a film with a thickness of 100μm-200μm onto a nonwoven fabric using the casting solution, then immersing the film in a coagulation water bath containing 2-methylimidazole until phase separation is completed and the film is solidified to obtain the base membrane, which is then stored in deionized water for later use.

[0010] In one feasible implementation, the hydrogen-bonded organic framework nanoparticles are formed by the self-assembly of aromatic tricarboxylic acids and triazine nitrogen-containing compounds via hydrogen bonding; In one feasible implementation, the aromatic tricarboxylic acid is pyromellitic acid, and the triazine nitrogen-containing compound is melamine.

[0011] In one feasible implementation, the steps for obtaining hydrogen-bonded organic framework nanoparticles include the following operations: dissolving aromatic tricarboxylic acids and triazine nitrogen-containing compounds in methanol or ethanol solvent at a preset molar ratio, dispersing them evenly by ultrasonication, pouring the solution into a polytetrafluoroethylene reactor, and carrying out a solvothermal reaction at 70℃-170℃ for 8h-10h. After the reaction is completed, the nanoparticles are centrifuged, washed, and freeze-dried to obtain the target hydrogen-bonded organic framework nanoparticles.

[0012] In one feasible implementation, the aqueous solution further includes an aqueous monomer, a surfactant, and a cationic polymer; In one feasible implementation, the oil phase monomer in the oil phase solution is pyromellitic trimethylol chloride; In one feasible implementation, the organic solvent in the oil phase solution is n-heptane; In one feasible implementation, the aqueous monomer is m-phenylenediamine, the surfactant is sodium dodecyl sulfate, and the cationic polymer is a polyquaternary ammonium salt; In one feasible implementation, the mass percentage of intermediate-phenylenediamine in the aqueous solution ranges from 1% to 5%. In one feasible implementation, the mass percentage of sodium dodecyl sulfate in the aqueous solution ranges from 0.01% to 0.1%. In one feasible implementation, the mass percentage of hydrogen-bonded organic framework nanoparticles in the aqueous solution ranges from 0.1% to 1%.

[0013] In one feasible implementation, the interfacial polymerization reaction includes the following steps: fixing the base membrane in a membrane frame, pouring an aqueous solution onto the surface of the membrane for 30-60 seconds, and then pouring out the excess aqueous solution; then pouring an oil solution onto the membrane, keeping it for 1-5 minutes, and then removing the excess oil solution; and finally placing the membrane in an oven at 60-80°C for 6-10 minutes for heat treatment.

[0014] In one feasible implementation, the alkaline solution is a sodium carbonate solution with a pH of 10-11; In one feasible implementation, the elution conditions are: 30℃-50℃, and the soaking time is 30min-40min.

[0015] Accordingly, the present invention also provides a dual-channel nanofiltration membrane, prepared using any of the preparation methods described above.

[0016] Implementing this invention has the following beneficial effects: The method for preparing a dual-channel nanofiltration membrane provided in this application achieves a synergistic effect by constructing a second channel in situ formed by a metal salt precursor in the base membrane and introducing elutable hydrogen-bonded organic framework (HOF) nanoparticles into the polyamide separation layer to form a first channel. The second channel provides molecular sieving capability to maintain a high rejection rate, while the first channel significantly increases the free volume and hydrophilicity of the polyamide layer, effectively breaking the flux-selectivity trade-off bottleneck of traditional nanofiltration membranes and obtaining a nanofiltration membrane with high flux and high desalination rate. Attached Figure Description

[0017] Figure 1 This is a flowchart of the preparation method of the dual-channel nanofiltration membrane provided in the embodiments of this application. Detailed Implementation

[0018] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0020] Please see Figure 1 A method for preparing a dual-channel nanofiltration membrane according to an embodiment of the present invention includes the following steps S110-S150: S110. Prepare a casting solution, wherein the casting solution includes a metal salt precursor.

[0021] Among them, metal salt precursors are used to form metal-organic frameworks (MOFs) in the base film and on the surface as a second channel.

[0022] Furthermore, the casting solution is composed of a metal salt precursor, a film-forming polymer, a water-soluble polymeric pore-forming agent, and an organic solvent.

[0023] In one feasible implementation, the operation of preparing the casting solution includes: stirring the metal salt precursor, film-forming polymer, and water-soluble polymeric porogen in an organic solvent at 80°C for 10 hours until the polyethersulfone is completely dissolved to obtain the target casting solution.

[0024] Furthermore, the metal salt precursor is a zinc salt, and the second channel is a ZIF-8 nanocrystalline layer. The ZIF-8 nanocrystalline layer structure formed by the zinc salt has a pore size of ~3.4 Å, moderate hydrophilicity, good water stability, and low cost. It should be noted that this ZIF-8 nanocrystalline layer is not merely present on the surface of the base film, but rather is an intercalation structure embedded both inside and on the surface of the film.

[0025] Further, the film-forming polymer is selected from at least one of polyethersulfone, polysulfone, and polyvinylidene fluoride. Optionally or preferably, the film-forming polymer is polyethersulfone. The pore-forming agent is selected from at least one of polyvinylpyrrolidone and polyethylene glycol. Optionally or preferably, the water-soluble polymeric pore-forming agent is polyvinylpyrrolidone (PVP). PVP is a water-soluble polymer that is uniformly dispersed in the casting solution. When the film enters the water coagulation bath, PVP is extracted and eluted by water, leaving micropores or mesopores in the base film, thereby improving the porosity and permeability of the base film. At the same time, the PVP molecular chain contains a strongly polar pyrrolidone ring (lactam structure), which can form hydrogen bonds with water, making the base film surface more hydrophilic, enhancing the wettability of subsequent interfacial polymerization, and improving the antifouling ability of the final film. PVP can also regulate the viscosity and thermodynamic / kinetic stability of the casting solution, slow down the diffusion rate of non-solvent (water) into the film, inhibit the excessive growth of finger pores, promote the formation of a more uniform sponge-like pore structure, and improve the mechanical strength of the film. Furthermore, in zinc salt-containing casting solutions, the carbonyl oxygen of PVP can react with Zn. 2+ Weak coordination occurs, preventing the aggregation and precipitation of metal ions and facilitating the uniform in-situ crystallization of ZIF-8 on the surface of the base film.

[0026] In one feasible implementation, the mass percentage of the film-forming polymer in the casting solution ranges from 15wt% to 20wt%, and the mass percentage of the water-soluble polymeric porogen ranges from 1wt% to 5wt%. Optionally or preferably, the mass percentage of polyethersulfone in the casting solution ranges from 15wt% to 20wt%, and the mass percentage of polyvinylpyrrolidone ranges from 1wt% to 5wt%. Further, the mass percentage of polyethersulfone in the casting solution can be, but is not limited to, any specific value between 15wt%, 16wt%, 17wt%, 18wt%, 19wt%, 20wt%, or 15wt%-20wt%. The mass percentage of polyvinylpyrrolidone in the casting solution can be, but is not limited to, any specific value between 1wt%, 2wt%, 3wt%, 4wt%, -5wt%, or 1wt%-5wt%.

[0027] In one feasible implementation, the zinc salt is Zn(NO3)2·6H2O, and the mass percentage of the zinc salt ranges from 0.1% to 1%. Optionally or preferably, the mass percentage of the zinc salt can be, but is not limited to, any specific value between 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 0.1%-1%.

[0028] By limiting the proportions and composition of each component in the casting solution, the appropriate viscosity and film-forming properties of the casting solution are ensured, while avoiding excessive phase separation or excessive crystallization of ZIF-8 due to excessive zinc salt, which would block the pores of the base film. This allows for the controllable construction of the second channel of ZIF-8 while maintaining the permeability of the base film.

[0029] In one feasible implementation, the zinc salt may also be zinc acetate or zinc chloride.

[0030] In one feasible implementation, the metal salt precursor can also be other metal salts that can react with imidazole ligands to form a metal-organic framework in situ. For example, cobalt salts, such as cobalt nitrate hexahydrate or cobalt acetate.

[0031] In one feasible implementation, the organic solvent is selected from one or a mixture of several of N,N-dimethylacetamide (DMAC), N,N-dimethylformamide (DMF), and N-methylpyrrolidone (NMP).

[0032] S120. The casting solution is used to prepare a base film by a non-solvent-induced phase separation method, wherein the base film has a second channel formed by the metal salt precursor.

[0033] In one feasible implementation, the preparation of the base film using the solvent-inducing phase separation method includes: scraping a film of a predetermined thickness from the casting solution onto a nonwoven fabric, then immersing the film in a coagulation water bath containing 2-methylimidazole until phase separation is complete and the film solidifies, obtaining the base film, which is then stored in deionized water for later use. In this way, by scraping a 100–200 μm thick film of casting solution onto a nonwoven fabric and immersing it in a coagulation water bath containing 2-methylimidazole, solvent-inducing phase separation and ligand diffusion are carried out simultaneously, allowing ZIF-8 to crystallize in situ on the surface of the base film to form a dense and continuous nanocrystalline layer. This operation is simple and controllable, resulting in a smooth base film surface and uniformly distributed second channels, providing good support for subsequent interfacial polymerization.

[0034] Among them, zinc ions Zn 2+ It forms a metal-organic framework (MOF) with 2-methylimidazole (2-MIM). Specifically, it is ZIF-8. That is, during the preparation of polyethersulfone (PES)-based films, 2-methylimidazole is introduced into the coagulation bath via in-situ interfacial assembly, allowing Zn to form a metal-organic framework (MOF). 2+ It reacts with the base membrane to generate ZIF-8 in situ. The ZIF-8 layer remains on the surface of the base membrane, forming a second channel with molecular sieving capabilities.

[0035] Furthermore, the mass percentage of 2-methylimidazole in the coagulation bath ranges from 0.01% to 0.1%. Furthermore, the mass percentage of 2-methylimidazole in the coagulation bath can be, but is not limited to, any specific value between 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 1%, or 0.01%-0.1%.

[0036] In one feasible implementation, a film with a thickness of 100μm-200μm is scraped onto a nonwoven fabric. Further, the thickness can be, but is not limited to, 100μm, 110μm, 120μm, 130μm, 140μm, 150μm, 160μm, 170μm, 180μm, 190μm, or 200μm.

[0037] S130, obtaining hydrogen-bonded organic framework nanoparticles.

[0038] In one feasible implementation, the hydrogen-bonded organic framework (HOF) nanoparticles are formed by the self-assembly of an aromatic tricarboxylic acid and a triazine nitrogen-containing compound via hydrogen bonding. Further, the aromatic tricarboxylic acid is trimesolic acid, and the triazine nitrogen-containing compound is melamine. Thus, the HOF nanoparticles formed by the self-assembly of trimesolic acid (TMA) and melamine (MA) possess excellent hydrophilicity due to their rich carboxyl and amino groups, and the hydrogen-bonded structure reversibly dissociates under alkaline conditions. After being washed away as a sacrificial template, they leave highly hydrophilic nanopores, significantly improving the water molecule transport efficiency of the polyamide layer, while avoiding the use of non-degradable or toxic template materials.

[0039] In one feasible implementation, the steps for obtaining hydrogen-bonded organic framework nanoparticles include the following operations: Aromatic tricarboxylic acids and triazine nitrogen-containing compounds are dissolved in methanol or ethanol at a predetermined molar ratio. After ultrasonic dispersion, the solution is poured into a polytetrafluoroethylene (PTFE) reactor and subjected to a solvothermal reaction at 70℃-170℃ for 8-10 hours. After the reaction is complete, the nanoparticles are centrifuged, washed, and freeze-dried to obtain the target hydrogen-bonded organic framework nanoparticles. Thus, by synthesizing TMA-MA HOF nanoparticles at 70–170℃ via a solvothermal method, followed by centrifugation, washing, and freeze-drying, nanoparticles with uniform particle size, good dispersibility, and no agglomeration can be obtained. This method is mature, highly reproducible, and produces stable HOF particles that are easily dispersed uniformly in an aqueous phase, laying the foundation for the subsequent formation of regular first channels.

[0040] Furthermore, the preset molar ratio of pyromellitic acid to melamine is 1:1.

[0041] S140. The hydrogen-bonded organic framework nanoparticles are uniformly dispersed in an aqueous solution, and a polyamide separation layer is formed on the surface of the base film through an interfacial polymerization reaction between the aqueous solution and the oil solution to obtain a precursor film, wherein the hydrogen-bonded organic framework nanoparticles are coated in the polyamide separation layer.

[0042] In one feasible implementation, the aqueous solution further includes an aqueous monomer, a surfactant, and a cationic polymer.

[0043] Furthermore, the oil phase monomer in the oil phase solution is pyromellitic acid trimethylolpropionate chloride; the organic solvent in the oil phase solution is n-heptane.

[0044] Further, the aqueous monomer is m-phenylenediamine (MPD), the surfactant is sodium dodecyl sulfate (SDS), and the cationic polymer is a polyquaternary ammonium salt. Further, the cationic polymer includes triethylamine (TEA) and camphor sulfonic acid (CSA).

[0045] Further, the mass percentage of intermediate-phenylenediamine in the aqueous solution ranges from 1% to 5%. Optionally or preferably, the mass percentage of intermediate-phenylenediamine in the aqueous solution can be, but is not limited to, any specific value between 1%, 2%, 3%, 4%, 5%, or 1%-5%.

[0046] Further, the mass percentage of sodium dodecyl sulfate in the aqueous solution ranges from 0.01% to 0.1%. Optionally or preferably, the mass percentage of sodium dodecyl sulfate in the aqueous solution can be, but is not limited to, any specific value between 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 1%, or 0.01% to 0.1%.

[0047] Further, the mass percentage of hydrogen-bonded organic framework nanoparticles in the aqueous solution ranges from 0.1% to 1%. Optionally or preferably, the mass percentage of hydrogen-bonded organic framework nanoparticles in the aqueous solution can be, but is not limited to, any specific value between 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 0.1%-1%.

[0048] Further, the mass percentage of triethylamine in the aqueous solution ranges from 0.1% to 3%. Optionally or preferably, the mass percentage of triethylamine in the aqueous solution can be, but is not limited to, any specific value between 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3%, or 0.1%-3%.

[0049] Further, the mass percentage of camphor sulfonic acid in the aqueous solution ranges from 1% to 5%. Optionally or preferably, the mass percentage of camphor sulfonic acid in the aqueous solution can be, but is not limited to, any value between 1%, 2%, 3%, 4%, 5%, or 1%-5%.

[0050] In one feasible implementation, the interfacial polymerization reaction includes the following steps: fixing the base membrane in a membrane frame; pouring an aqueous solution onto the membrane surface for 30-60 seconds, then pouring out the excess aqueous solution; pouring an oil phase solution onto the membrane, holding it for 1-5 minutes, then removing the excess oil phase solution; and heat-treating the membrane in an oven at 60-80°C for 6-10 minutes. This ensures sufficient interfacial polymerization to form a complete polyamide network while avoiding excessive cross-linking that could lead to membrane embrittlement. Simultaneously, it ensures that the HOF nanoparticles are effectively encapsulated and not washed away, providing structural support for subsequent elution to form interconnected channels.

[0051] S150. The precursor membrane is eluted with an alkaline solution to elute the hydrogen-bonded organic framework nanoparticles from the polyamide separation layer, forming a first channel to obtain the target dual-channel nanofiltration membrane.

[0052] In one feasible implementation, the alkaline solution is a sodium carbonate solution with a pH of 10-11; the elution conditions are: 30℃-50℃, and the soaking time is 30min-40min. This allows for the selective breaking of hydrogen bonds between HOF particles under mild conditions, causing them to dissociate and dissolve without damaging the polyamide backbone or the ZIF-8 second channel structure. These conditions balance elution efficiency with membrane structural integrity, ensuring unobstructed first channel and stable overall membrane performance.

[0053] The method for preparing a dual-channel nanofiltration membrane provided in this application achieves a synergistic effect by constructing a second channel in situ formed by a metal salt precursor in the base membrane and introducing elutable hydrogen-bonded organic framework (HOF) nanoparticles into the polyamide separation layer to form a first channel. The second channel provides molecular sieving capability to maintain a high rejection rate, while the first channel significantly increases the free volume and hydrophilicity of the polyamide layer, effectively breaking the traditional nanofiltration membrane "flux-selectivity" trade-off bottleneck and obtaining a nanofiltration membrane with high flux and high desalination rate.

[0054] Accordingly, this application also provides a dual-channel nanofiltration membrane, which is prepared using any of the preparation methods described above.

[0055] The dual-channel nanofiltration membrane provided in this application has a second channel formed by a metal salt precursor and a first channel formed by elution of hydrogen-bonded organic framework nanoparticles (HOF). Thus, the high selective sieving capability of the second channel combined with the high hydrophilicity and high flux characteristics of the first channel gives the dual-channel nanofiltration membrane excellent water flux and high rejection rate, which is beneficial for its wide range of applications.

[0056] Referring to the above implementation details, in order to make the technical solution of this application more specific, clear, and easy to understand, examples of the technical solution of this application are given below. However, it should be noted that the content to be protected by this application is not limited to the following embodiments.

[0057] Example 1 18g of polyethersulfone, 2g of polyvinylpyrrolidone and 2g of Zn(NO3)2·6H2O were dissolved in 78g of N,N-dimethylformamide and stirred at 80℃ for 10h until the polyethersulfone was completely dissolved to obtain the casting solution. A 150μm thick thin film composite membrane (TFC) was scraped onto a nonwoven fabric. The membrane was then immersed in a 100ml coagulation water bath containing 5g of 2-methylimidazole. After the polyethersulfone membrane was completely cured, it was stored in deionized water for later use. In this process, zinc salt formed a ZIF-8 MOF layer on the surface of the polyethersulfone membrane, which is a second channel layer formed inside and on the surface of the polyethersulfone membrane, thus obtaining the base membrane. Trimethylbenzene acid (TMA) and melamine (MA) were dissolved in 100 ml of methanol solution at a 1:1 molar ratio. After ultrasonic dispersion for 30 min, the solvent was poured into a polytetrafluoroethylene reactor and reacted at 145 °C for 9 hours. After the reaction was completed, the mixture was centrifuged and washed three times with methanol solution, then washed with ethanol solution, and finally washed with deionized water. Finally, it was freeze-dried to obtain hydrogen-bonded organic framework nanoparticles. The base membrane was fixed in a membrane frame. A mixed aqueous solution of 2% m-phenylenediamine (MPD), 0.02% sodium dodecyl sulfate (SDS), 2% triethylamine (TEA), 3% camphor sulfonic acid (CSA), and 0.5% HOF was poured onto the membrane surface for 1 minute, and then the excess liquid was poured off. Next, a heptane solution of trimesoyl chloride (TMC) was poured onto the membrane and held for 1 minute. Finally, the membrane was heat-treated in an oven at 80°C for 10 minutes to obtain the precursor membrane. The precursor membrane was immersed in a sodium carbonate solution at pH=10 and soaked in the sodium carbonate solution at 40℃ for 30 min. Then, it was washed with a large amount of deionized water to remove HOF and obtain the target dual-channel nanofiltration membrane.

[0058] Example 2 The example is basically the same as in Example 1, except that in this example, 5g of Zn(NO3)2·6H2O is dissolved in 75g of N,N-dimethylformamide.

[0059] Example 3 The example is basically the same as in Example 1, except that in this example, 10g of Zn(NO3)2·6H2O is dissolved in 70g of N,N-dimethylformamide.

[0060] Example 4 The example is basically the same as Example 2, except that the mass percentage of hydrogen-bonded organic framework nanoparticles is 0.1% in this example.

[0061] Example 5 This example is essentially the same as Example 2, except that in this example, the hydrogen-bonded organic framework nanoparticles account for 1%.

[0062] Comparative Example 1 Dissolve 18g of polyethersulfone and 2g of polyvinylpyrrolidone in 80g of N,N-dimethylformamide and stir at 80℃ for 10h until the polyethersulfone is completely dissolved to obtain the casting solution. A 150μm thick thin film composite membrane (TFC) is scraped onto a nonwoven fabric, and then the membrane is immersed in a coagulation water bath. After the polyethersulfone membrane is completely cured, it is stored in deionized water for later use to obtain the base membrane. The base membrane was fixed in a membrane frame. An aqueous solution of 2% m-phenylenediamine (MPD), 0.02% sodium dodecyl sulfate (SDS), 2% triethylamine (TEA), and 3% camphor sulfonic acid (CSA) was poured onto the membrane surface for 1 minute, after which excess liquid was poured off. Then, a solution of 0.2% trimesoyl chloride (TMC) in n-heptane was poured onto the membrane and held for 1 minute. Finally, the membrane was heat-treated in an oven at 80°C for 10 minutes to obtain the precursor membrane. Comparative Example 2 Dissolve 18g of polyethersulfone and 2g of polyvinylpyrrolidone in 80g of N,N-dimethylformamide, and stir at 80℃ for 10h until the polyethersulfone is completely dissolved to obtain the casting solution. A 150μm thick thin film composite membrane (TFC) is scraped onto a nonwoven fabric, and then the membrane is immersed in a coagulation water bath. After the polyethersulfone membrane is completely cured, it is stored in deionized water for later use to obtain the base membrane. Trimethylbenzene acid (TMA) and melamine (MA) were dissolved in 100 ml of methanol solution at a 1:1 molar ratio. After ultrasonic dispersion for 30 min, the solvent was poured into a polytetrafluoroethylene reactor and reacted at 145 °C for 9 hours. After the reaction was completed, the mixture was centrifuged and washed three times with methanol solution, then washed with ethanol solution, and finally washed with deionized water. Finally, it was freeze-dried to obtain hydrogen-bonded organic framework nanoparticles. The base membrane was fixed in a membrane frame. A mixed aqueous solution of 2% m-phenylenediamine (MPD), 0.02% sodium dodecyl sulfate (SDS), 2% triethylamine (TEA), 3% camphor sulfonic acid (CSA), and 0.5% HOF was poured onto the membrane surface for 1 minute, and then the excess liquid was poured off. Next, a heptane solution of 0.2% trimesoyl chloride (TMC) was poured onto the membrane and held for 1 minute. Finally, the membrane was heat-treated in an oven at 80°C for 10 minutes to obtain the precursor membrane. The precursor membrane was immersed in a sodium carbonate solution at pH=10 and soaked in the sodium carbonate solution at 40℃ for 30 min. Then, it was washed with a large amount of deionized water to remove HOF and obtain the target dual-channel nanofiltration membrane.

[0063] Comparative Example 3 18g of polyethersulfone, 2g of polyvinylpyrrolidone and 5g of Zn(NO3)2·6H2O were dissolved in 78g of N,N-dimethylformamide and stirred at 80℃ for 10h until the polyethersulfone was completely dissolved to obtain the casting solution. A 150μm thick thin film composite membrane (TFC) was scraped onto a nonwoven fabric. The membrane was then immersed in a 100ml coagulation water bath containing 5g of 2-methylimidazole. After the polyethersulfone membrane was completely cured, it was stored in deionized water for later use. In this process, zinc salt formed a ZIF-8 MOF layer on the surface of the polyethersulfone membrane, which is a second channel layer formed inside and on the surface of the polyethersulfone membrane, thus obtaining the base membrane. The base membrane was fixed in a membrane frame. An aqueous solution of 2% m-phenylenediamine (MPD), 0.02% sodium dodecyl sulfate (SDS), 2% triethylamine (TEA), and 3% camphor sulfonic acid (CSA) was poured onto the membrane surface for 1 minute, after which excess liquid was poured off. Then, a solution of 0.2% trimesoyl chloride (TMC) in n-heptane was poured onto the membrane and held for 1 minute. Finally, the membrane was heat-treated in an oven at 80°C for 10 minutes to obtain the precursor membrane. Performance testing: Flux and retention tests were performed on the membrane. The test pressure was 15 bar, the solution temperature was 25°C, the NaCl concentration was 2000 ppm, and the membrane was run in the cross-flow device for 30 minutes to measure the flux and retention. The test results are shown in Table 1. As shown in the table above, the flux of the dual-channel nanofiltration membranes prepared in Examples 1-5 is significantly higher than that of Comparative Examples 1-3. Specifically, the dual-channel nanofiltration membranes prepared in Examples 1-5 have better flux and retention rates than those in Comparative Examples 1-3. Although the retention rates of the dual-channel nanofiltration membranes prepared in Examples 4 and 5 are slightly lower, their flux is significantly higher than that of the comparative examples. The dual-channel nanofiltration membrane prepared in Example 5 has the highest flux of 110 LMH, while its retention rate is lower than that of Examples 1-4. This is because in the preparation process of Example 5, the concentration of elutable hydrogen-bonded organic framework nanoparticles (HOF) introduced into the polyamide separation layer is at most 1%. The first channel formed by the eluted HOF significantly increases the free volume and hydrophilicity of the polyamide layer, improving the flux, while also having some impact on the retention rate.

[0064] As can be seen from the table above, the flux of the dual-channel nanofiltration membrane prepared in Example 2 is significantly improved compared with the comparative example, and the rejection rate is as high as 99.4%. It effectively breaks through the traditional nanofiltration membrane "flux-selectivity" trade-off bottleneck, and has high flux and high desalination rate. Its comprehensive performance is optimal and it is conducive to wide application.

[0065] The basic concepts have been described above. Obviously, for those skilled in the art, the detailed disclosure above is merely illustrative and does not constitute a limitation of this specification. Although not explicitly stated herein, those skilled in the art may make various modifications, improvements, and corrections to this specification. Such modifications, improvements, and corrections are suggested in this specification and therefore remain within the spirit and scope of the exemplary embodiments described herein.

[0066] Furthermore, this specification uses specific terms to describe embodiments thereof. For example, "an embodiment," "one embodiment," and / or "some embodiments" refer to a particular feature, structure, or characteristic associated with at least one embodiment of this specification. Therefore, it should be emphasized and noted that references to "an embodiment," "one embodiment," or "an alternative embodiment" in different locations throughout this specification do not necessarily refer to the same embodiment. Moreover, certain features, structures, or characteristics in one or more embodiments of this specification can be appropriately combined.

[0067] Furthermore, unless expressly stated in the claims, the order of processing elements and sequences, the use of numbers and letters, or other names described in this specification are not intended to limit the order of the processes and methods described herein. Although some inventive embodiments that are currently considered useful have been discussed by way of various examples in the foregoing disclosure, it should be understood that such details are for illustrative purposes only, and the appended claims are not limited to the disclosed embodiments. Rather, the claims are intended to cover all modifications and equivalent combinations that conform to the substance and scope of the embodiments described herein.

[0068] Similarly, it should be noted that, in order to simplify the description disclosed herein and thus aid in the understanding of one or more embodiments of the invention, the foregoing description of embodiments in this specification may sometimes combine multiple features into a single embodiment, drawing, or description thereof. However, this method of disclosure does not imply that the subject matter of this specification requires more features than those mentioned in the claims. In fact, the embodiments contain fewer features than all the features of a single embodiment disclosed above.

[0069] In some embodiments, numbers describing the quantity of components and attributes are used. It should be understood that such numbers used in the description of embodiments are modified in some examples with the terms "approximately," "approximately," or "generally." Unless otherwise stated, "approximately," "approximately," or "generally" indicates that the numbers are allowed to vary by ±20%. Accordingly, in some embodiments, the numerical parameters used in the specification and claims are approximate values, which may be changed depending on the characteristics required by individual embodiments. In some embodiments, numerical parameters should take into account specified significant digits and employ a general method of digit reservation. Although the numerical ranges and parameters used to confirm their breadth of range in some embodiments of this specification are approximate values, in specific embodiments, such values ​​are set as precisely as feasible.

[0070] For each patent, patent application, patent application publication, and other material such as articles, books, specifications, publications, and documents referenced in this specification, the entire contents of which are incorporated herein by reference. This excludes historical application documents that are inconsistent with or conflict with the content of this specification, as well as documents that limit the broadest scope of the claims in this specification (currently or subsequently appended to this specification). It should be noted that in the event of any inconsistency or conflict between the descriptions, definitions, and / or terminology used in the supplementary materials to this specification and the content of this specification, the descriptions, definitions, and / or terminology used in this specification shall prevail.

[0071] Finally, it should be understood that the embodiments described in this specification are merely illustrative of the principles of the embodiments described herein. Other variations may also fall within the scope of this specification. Therefore, alternative configurations of the embodiments described herein are intended to be illustrative rather than limiting, and are considered consistent with the teachings of this specification. Accordingly, the embodiments described herein are not limited to those explicitly introduced and described herein.

Claims

1. A method for preparing a dual-channel nanofiltration membrane, characterized in that, The preparation method includes the following steps: A casting solution is prepared, wherein the casting solution includes a metal salt precursor; The casting solution is used to prepare a base film by a solvent-inducible phase separation method, wherein the base film has a second channel formed by the metal salt precursor; Obtaining hydrogen-bonded organic framework nanoparticles; The hydrogen-bonded organic framework nanoparticles are uniformly dispersed in an aqueous solution, and a polyamide separation layer is formed on the surface of the base film through an interfacial polymerization reaction between the aqueous solution and the oil solution to obtain a precursor film, wherein the hydrogen-bonded organic framework nanoparticles are coated in the polyamide separation layer. The precursor membrane is eluted with an alkaline solution to remove the hydrogen-bonded organic framework nanoparticles from the polyamide separation layer, forming a first channel, thus obtaining the target dual-channel nanofiltration membrane.

2. The method for preparing the dual-channel nanofiltration membrane according to claim 1, characterized in that, The operation of preparing the casting solution includes: The film-forming polymer polyethersulfone, the water-soluble polymeric porogen polyvinylpyrrolidone, and the metal salt precursor were dissolved in an organic solvent and stirred at 80°C for 10 hours until the polyethersulfone was completely dissolved to obtain the target casting solution. The metal salt precursor is a zinc salt, and the second channel is a ZIF-8 nanocrystal layer.

3. The method for preparing a dual-channel nanofiltration membrane according to claim 2, characterized in that, The film-forming polymer is selected from at least one of polyethersulfone, polysulfone, and polyvinylidene fluoride; The water-soluble polymeric porogen is selected from at least one of polyvinylpyrrolidone and polyethylene glycol; The mass percentage of the film-forming polymer in the casting solution ranges from 15wt% to 20wt%, and the mass percentage of the water-soluble polymeric porogen ranges from 1wt% to 5wt%. The zinc salt is Zn(NO3)2·6H2O, and the mass percentage of the zinc salt ranges from 0.1% to 1%.

4. The method for preparing a dual-channel nanofiltration membrane according to claim 1, characterized in that, The steps for preparing the base film using the solvent-inducible phase separation method include: The casting solution is scraped onto a nonwoven fabric to form a film with a thickness of 100μm-200μm. The film is then immersed in a coagulation water bath containing 2-methylimidazole until phase separation is completed and the film is solidified to obtain a base film, which is then stored in deionized water for later use.

5. The method for preparing a dual-channel nanofiltration membrane according to claim 1, characterized in that, The hydrogen-bonded organic framework nanoparticles are formed by the self-assembly of aromatic tricarboxylic acids and triazine nitrogen-containing compounds through hydrogen bonding. The aromatic tricarboxylic acid is pyromellitic acid, and the triazine nitrogen-containing compound is melamine.

6. The method for preparing a dual-channel nanofiltration membrane according to claim 5, characterized in that, The steps to obtain hydrogen-bonded organic framework nanoparticles include the following operations: Aromatic tricarboxylic acids and triazine nitrogen-containing compounds were dissolved in methanol or ethanol at a predetermined molar ratio. After being ultrasonically dispersed, the solution was poured into a polytetrafluoroethylene reactor and subjected to a solvothermal reaction at 70℃-170℃ for 8-10 hours. After the reaction was completed, the nanoparticles were obtained by centrifugation, washing, and freeze-drying.

7. The method for preparing a dual-channel nanofiltration membrane according to claim 1, characterized in that, The aqueous solution also includes aqueous monomers, surfactants, and cationic polymers; The oil phase monomer in the oil phase solution is pyromellitic trimethylol chloride; The organic solvent in the oil phase solution is n-heptane; The aqueous monomer is m-phenylenediamine, the surfactant is sodium dodecyl sulfate, and the cationic polymer is a polyquaternary ammonium salt; The mass percentage of intermediate-phenylenediamine in the aqueous solution ranges from 1% to 5%. The mass percentage of sodium dodecyl sulfate in the aqueous solution ranges from 0.01% to 0.1%. The mass percentage of hydrogen-bonded organic framework nanoparticles in the aqueous solution ranges from 0.1% to 1%.

8. The method for preparing a dual-channel nanofiltration membrane according to claim 1, characterized in that, The operation of the interface polymerization reaction includes the following steps: The base membrane is fixed in the membrane frame, and the aqueous solution is poured onto the surface of the membrane for 30-60 seconds. Then, the excess aqueous solution is poured out. Pour the oil phase solution onto the membrane, keep it for 1-5 minutes, and then remove the excess oil phase solution. Place the membrane in an oven at 60℃-80℃ for 6-10 minutes for heat treatment.

9. The method for preparing a dual-channel nanofiltration membrane according to claim 1, characterized in that, The alkaline solution is a sodium carbonate solution with a pH of 10-11; The elution conditions are: 30℃-50℃, soaking time is 30min-40min.

10. A dual-channel nanofiltration membrane, characterized in that, Prepared using the preparation method described in any one of claims 1-9.

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