Preparation method of nanofiltration membrane and nanofiltration membrane prepared therefrom

Abstract

This invention relates to a method for preparing nanofiltration membranes and nanofiltration membranes prepared therefrom. The preparation method includes the following steps: oxidizing a polypropylene microporous membrane to hydroxylate its surface; sequentially contacting the surface-hydroxylated polypropylene microporous membrane with a compound or polymer containing multiple hydroxyl groups and / or multiple amino groups, and a crosslinking agent for hydrophilic modification; applying nanomaterials containing multiple hydroxyl groups and / or multiple amino groups to the hydrophilic modified polypropylene microporous membrane; then sequentially contacting it with an aqueous phase solution and an organic phase solution for interfacial polymerization to form a polyamide separation layer; and finally, obtaining a nanofiltration membrane through post-treatment, wherein the post-treatment includes treatment with a carboxyl activator. The nanofiltration membrane provided by this invention has a thinner thickness, resulting in a higher effective filtration area per membrane element, high flux and excellent separation performance (separation of salt ions, etc.) in water treatment, and stable performance during long-term operation.

Description

Technical Field

[0001] This invention relates to the technical field of water treatment membranes, and more particularly to a method for preparing nanofiltration membranes and nanofiltration membranes prepared therefrom. Background Technology

[0002] Water resources are an essential natural resource for human life and production today.

[0003] Current solutions mainly include wastewater treatment and reuse, seawater desalination, and inter-regional water transfer. In wastewater treatment, the process of treating polluted water to create reclaimed usable water has been proven feasible in many countries and regions around the world, and it can alleviate the water supply and demand imbalance to some extent.

[0004] Membrane separation technology is widely used in wastewater treatment due to its high separation efficiency and low energy consumption. It is applied in water treatment, pharmaceuticals, food, biological purification, energy, and chemical industries. Membrane separation technologies primarily utilize pressure-driven membranes, including microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. Nanofiltration membranes possess unique separation properties, effectively retaining inorganic salts and small organic molecules in solutions. However, compared to reverse osmosis membranes, nanofiltration membranes have lower retention rates for monovalent ions. Therefore, they are suitable for processes such as drinking water treatment to ensure the retention of some beneficial elements. Combined with their low energy consumption, nanofiltration membranes have seen significant development in certain water treatment fields.

[0005] Reducing the cost of water treatment is a key aspect of the practical application of nanofiltration membranes. Current research focuses on improving the water flux and antifouling properties of nanofiltration membranes. Currently, reducing the thickness of nanofiltration membranes and preparing porous polyamide layer structures are considered methods to improve their flux. On the one hand, reducing thickness decreases the permeation resistance of water molecules; on the other hand, nanofiltration membranes with porous polyamide layer structures exhibit increased permeability due to their relatively high porosity.

[0006] Furthermore, current nanofiltration membranes mainly adopt a three-layer structure, namely polyester nonwoven fabric, polysulfone layer and polyamide separation layer, with an overall thickness mostly concentrated in the range of 100-150 micrometers. Some researchers have begun to focus on preparing nanofiltration membranes with a thickness of less than 100 micrometers or even less than 50 micrometers. Thinner nanofiltration membranes can provide a larger packing area for a single membrane element and improve the permeation performance of a single membrane element. That is, under the same water production conditions, the number of membrane elements can be reduced, which further reduces the cost of membrane-based water production, thereby reducing the cost of using nanofiltration membranes for water treatment.

[0007] Polypropylene microporous membranes for lithium batteries are a type of separator used in lithium-ion batteries. They possess high porosity, high tear strength, good acid and alkali resistance, and good elasticity. Common thicknesses range from 20 to 60 micrometers, and pore sizes range from 30 to 120 nanometers. Therefore, if polypropylene microporous membranes can be used as base membranes to prepare polyamide nanofiltration membranes with excellent comprehensive performance, and combined with their thinness, nanofiltration membrane elements with high packing area can be obtained, significantly improving the permeation performance of a single membrane element. However, due to the hydrophobic properties of the polypropylene microporous membrane surface, it cannot be directly used in the interfacial polymerization process to prepare polyamide nanofiltration membranes. Summary of the Invention

[0008] The problem the invention aims to solve

[0009] To address the aforementioned problems in the prior art, the present invention aims to provide a method for preparing a nanofiltration membrane using a polypropylene microporous membrane as the base membrane, and the nanofiltration membrane prepared therefrom. This method is simple, easy to operate, requires no modification to existing equipment, has effective operating costs, and the nanofiltration membrane prepared therefrom has a thinner thickness, which can result in a higher effective filtration area per membrane element. In water treatment, it exhibits high flux and excellent separation performance (separation of salt ions, etc.).

[0010] Solution for solving the problem

[0011] To solve the above-mentioned problems, the inventors of this invention conducted in-depth research and creatively conceived the following technical solution: A polypropylene microporous membrane is oxidized to hydroxylate its surface, and then a compound or polymer containing multiple hydroxyl groups and / or multiple amino groups is applied, achieving hydrophilic modification under the action of a crosslinking agent; subsequently, nanomaterials containing multiple hydroxyl groups and / or multiple amino groups are applied to the hydrophilically modified polypropylene microporous membrane, thereby providing a favorable reaction platform for subsequent interfacial polymerization; in the interfacial polymerization reaction, an alcohol additive is added to the aqueous solution to promote the diffusion of the nanomaterials and enable them to participate in the interfacial polymerization reaction, constructing a polyamide layer containing nanochannels; in the post-treatment, by using a carboxyl activator, the nanomaterials can be stably present in the polyamide layer, thereby ensuring the performance stability of the nanofiltration membrane during long-term operation.

[0012] This invention provides a method for preparing a nanofiltration membrane, characterized by comprising the following steps:

[0013] The surface of the polypropylene microporous membrane is hydroxylated by an oxidation treatment, wherein the oxidation treatment is carried out by an aqueous solution containing a persulfate compound;

[0014] The surface-hydroxylated polypropylene microporous membrane is sequentially contacted with a compound or polymer containing multiple hydroxyl groups and / or multiple amino groups, and a crosslinking agent to perform hydrophilic modification.

[0015] A nanomaterial containing multiple hydroxyl groups and / or multiple amino groups is applied to a hydrophilically modified polypropylene microporous membrane, and then sequentially contacted with an aqueous phase solution and an organic phase solution to carry out an interfacial polymerization reaction to form a polyamide separation layer, wherein the aqueous phase solution contains alcohol additives and amine compounds as aqueous phase monomers, and the organic phase solution contains acyl chloride compounds as organic phase monomers.

[0016] The nanofiltration membrane is obtained through post-processing, wherein the post-processing includes treatment with a carboxyl activator.

[0017] According to the preparation method of the present invention, the persulfate compound is at least one selected from potassium persulfate, sodium persulfate, and ammonium persulfate.

[0018] According to the preparation method of the present invention, the compound or polymer containing multiple hydroxyl groups and / or multiple amino groups is selected from at least one of polyvinyl alcohol, polyethylene glycol, tannic acid, polyethyleneimine, carboxylated chitosan, sorbitol, polyacrylic acid polyol, polyamino polyether, and polyacrylamide.

[0019] According to the preparation method of the present invention, the crosslinking agent is at least one selected from aliphatic dialdehyde, N,N-methylenebisacrylamide, glycidyl methacrylate, epichlorohydrin, and polyethylene glycol glycidyl ether, which have 2 to 6 carbon atoms.

[0020] According to the preparation method of the present invention, the nanomaterial containing multiple hydroxyl groups and / or multiple amino groups is selected from at least one of carboxylated graphene oxide, aminated graphene oxide, aminated graphene, aminated multi-walled carbon nanotubes, aminated single-walled carbon nanotubes, carboxylated single-walled carbon nanotubes, amino-functionalized metal-organic frameworks (MOFs), and aminated nanocrystalline cellulose.

[0021] According to the preparation method of the present invention, the alcohol additive is at least one selected from aliphatic alcohols having 1 to 6 carbon atoms and aromatic alcohols having 7 to 12 carbon atoms.

[0022] According to the preparation method of the present invention, the amine compound is at least one selected from piperazine, homopiperazine, 2-methylpiperazine, m-phenylenediamine, p-phenylenediamine, o-phenylenediamine, mesitylenetriamine and polyethyleneimine.

[0023] According to the preparation method of the present invention, the acyl chloride compound is at least one selected from isophthaloyl chloride, terephthaloyl chloride, o-phthaloyl chloride, trimesoyl chloride, biphenyltetramethyl chloride, cyanuric chloride, dansyl chloride, and benzenesulfonyl chloride.

[0024] According to the preparation method of the present invention, the carboxyl activator is a combination of N-hydroxysuccinimide and at least one selected from 1,3-dicyclohexylcarbodiimide, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride, diethylcyanomethyl phosphate, 4-dimethylaminopyridine, and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride.

[0025] The present invention also provides a nanofiltration membrane prepared by the preparation method described in the present invention.

[0026] The effects of the invention

[0027] The nanofiltration membrane preparation method provided by this invention is simple and easy to operate, requires no modification to existing equipment, and has effective operating costs. The nanofiltration membrane prepared by this method has a thinner thickness, which can result in a higher effective filtration area per membrane element. It has high flux and excellent separation performance (separation of salt ions, etc.) in water treatment, and its performance is stable during long-term operation. Attached Figure Description

[0028] Figure 1 The results of stability tests on the nanofiltration membranes obtained in Examples 1 to 4 (referred to as Examples 1, 2, 3, and 4) are shown. Detailed Implementation

[0029] Various exemplary embodiments, features, and aspects of the present invention will be described in detail below. The term "exemplary" as used herein means "serving as an example, embodiment, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as superior to or better than other embodiments.

[0030] Furthermore, to better illustrate the present invention, numerous specific details are set forth in the following detailed embodiments. Those skilled in the art should understand that the present invention can be practiced without certain specific details. In other instances, methods, means, apparatus, and steps well known to those skilled in the art have not been described in detail in order to highlight the spirit of the present invention.

[0031] Unless otherwise stated, all units used in this specification are international standard units, and all numerical values ​​and ranges appearing in this invention should be understood to include systematic errors that are unavoidable in industrial production.

[0032] In this specification, the word "may" has two meanings: to perform a certain process and not to perform a certain process.

[0033] In this specification, references to "some specific / preferred embodiments," "other specific / preferred embodiments," "implementation," etc., refer to specific elements (e.g., features, structures, properties, and / or characteristics) related to that embodiment, which are included in at least one of the embodiments described herein and may or may not be present in other embodiments. Furthermore, it should be understood that these elements may be combined in any suitable manner in various embodiments.

[0034] In this specification, the range of values ​​referred to as "value A to value B" refers to the range including the endpoint values ​​A and B.

[0035] In this instruction manual, when "room temperature" or "room temperature" is used, the temperature can be between 10-40℃.

[0036] This invention provides a method for preparing a nanofiltration membrane, which includes the following steps:

[0037] The surface of the polypropylene microporous membrane is hydroxylated by an oxidation treatment, wherein the oxidation treatment is carried out by an aqueous solution containing a persulfate compound;

[0038] The surface-hydroxylated polypropylene microporous membrane is sequentially contacted with a compound or polymer containing multiple hydroxyl groups and / or multiple amino groups, and a crosslinking agent to perform hydrophilic modification.

[0039] A nanomaterial containing multiple hydroxyl groups and / or multiple amino groups is applied to a hydrophilically modified polypropylene microporous membrane, and then sequentially contacted with an aqueous phase solution and an organic phase solution to carry out an interfacial polymerization reaction to form a polyamide separation layer, wherein the aqueous phase solution contains alcohol additives and amine compounds as aqueous phase monomers, and the organic phase solution contains acyl chloride compounds as organic phase monomers.

[0040] The nanofiltration membrane is obtained through post-processing, wherein the post-processing includes treatment with a carboxyl activator.

[0041] The technical concept of this invention is as follows: A polypropylene microporous membrane is oxidized to hydroxylate its surface, and then a compound or polymer containing multiple hydroxyl groups and / or multiple amino groups is applied, achieving hydrophilic modification under the action of a crosslinking agent; subsequently, nanomaterials containing multiple hydroxyl groups and / or multiple amino groups are applied to the hydrophilically modified polypropylene microporous membrane, thereby providing a favorable reaction platform for subsequent interfacial polymerization; in the interfacial polymerization reaction, an alcohol additive is added to the aqueous solution to promote the diffusion of the nanomaterials and enable them to participate in the interfacial polymerization reaction, constructing a polyamide layer containing nanochannels; in the post-treatment, treatment with a carboxyl activator ensures that the nanomaterials are stably present in the polyamide layer, thereby ensuring the performance stability of the nanofiltration membrane during long-term operation.

[0042] Preferably, the polypropylene microporous membrane used in this invention has a thickness of 20–40 micrometers and a pore size of 30–100 nanometers. If the polypropylene microporous membrane is too thick, it will negatively impact the membrane's permeability; if it is too thin, the prepared nanofiltration membrane will have insufficient mechanical properties. A pore size within the aforementioned range helps ensure the density and uniformity of the prepared polyamide layer.

[0043] In the preparation method of the present invention, the persulfate compound is at least one selected from potassium persulfate, sodium persulfate, and ammonium persulfate.

[0044] Preferably, the oxidation treatment is carried out as follows: using an aqueous solution of potassium persulfate with a concentration of 5–15 wt%, the treatment is carried out for 30–60 minutes at a temperature of 60–80°C. The reaction conditions can be adjusted as needed to provide different degrees of oxidation.

[0045] The preparation method of the present invention, wherein the compound or polymer containing multiple hydroxyl groups and / or multiple amino groups is selected from at least one of polyvinyl alcohol, polyethylene glycol, tannic acid, polyethyleneimine, carboxylated chitosan, sorbitol, polyacrylic acid polyol, polyamino polyether, and polyacrylamide.

[0046] Preferably, the surface-hydroxylated polypropylene microporous membrane is immersed in an aqueous solution containing a compound or polymer containing multiple hydroxyl groups and / or multiple amino groups, ensuring that the polypropylene microporous membrane (including the membrane surface and membrane pores) is wetted by the aqueous solution containing the compound or polymer containing multiple hydroxyl groups and / or multiple amino groups; after wetting, it is immersed in an aqueous solution containing a crosslinking agent, and the hydrophilic modification of the polypropylene microporous membrane is achieved through self-crosslinking between hydroxyl groups and crosslinking of hydroxyl groups introduced on the polypropylene chain through the above oxidation treatment, thereby constructing a hydrophilic membrane surface and pore channels.

[0047] Preferably, the concentration of the compound or polymer containing multiple hydroxyl groups and / or multiple amino groups in the aqueous solution is 5-10 wt%, the temperature of the aqueous solution is 30-50°C, and the soaking time is 20-40 minutes.

[0048] In the preparation method of the present invention, the crosslinking agent is at least one selected from aliphatic dialdehyde, N,N-methylenebisacrylamide, glycidyl methacrylate, epichlorohydrin, and polyethylene glycol glycidyl ether, which have 2 to 6 carbon atoms.

[0049] Preferably, the crosslinking agent is at least one selected from glyoxal, malondialdehyde, succinaldehyde, glutaraldehyde, and N,N-methylenebisacrylamide.

[0050] Preferably, the content of the crosslinking agent is 0.1–1 wt% based on the total weight of the aqueous solution containing the crosslinking agent; the immersion time in the aqueous solution containing the crosslinking agent is preferably 5–10 minutes, and the temperature of the aqueous solution containing the crosslinking agent is 20–40°C. The main purpose of this process is to ensure the stability of compounds or polymers containing multiple hydroxyl groups and / or multiple amino groups in polypropylene microporous membranes through chemical bonds and intermolecular entanglement.

[0051] The preparation method of the present invention, wherein the nanomaterial containing multiple hydroxyl groups and / or multiple amino groups is selected from at least one of carboxylated graphene oxide, aminated graphene oxide, aminated graphene, aminated multi-walled carbon nanotubes, aminated single-walled carbon nanotubes, carboxylated single-walled carbon nanotubes, amino-functionalized metal-organic frameworks (MOFs), and aminated nanocrystalline cellulose.

[0052] Preferably, the nanomaterial containing multiple hydroxyl groups and / or multiple amino groups is applied by spraying a dispersion containing the nanomaterial onto a hydrophilically modified polypropylene microporous membrane to construct a platform for subsequent interfacial polymerization reactions, thereby improving the reaction process of the interfacial polymerization reaction. The nanomaterial participates in the construction of water molecule channels, which helps to improve the permeation performance of the membrane and ensure the separation performance of the membrane.

[0053] The nanomaterial has a certain number of hydroxyl and / or amino groups, which can be fixed to the polyamide layer during interfacial polymerization or subsequent processing; preferably, the concentration of the nanomaterial in the dispersion is 0.01 to 1 wt%.

[0054] Preferably, a hydrophilically modified polypropylene microporous membrane with nanomaterials containing multiple hydroxyl and / or multiple amino groups is sequentially immersed in an aqueous solution containing amine compounds and an organic solution containing acyl chloride compounds to undergo interfacial polymerization to form an initial polyamide layer. During this process, the nanomaterials participate in the interfacial polymerization along with the diffusion of the amine compounds, thereby forming nano-water molecule channels to improve the permeation and separation performance of the polyamide layer. Furthermore, an alcohol additive is added to the aqueous solution containing the amine compounds; this additive promotes the diffusion of the nanomaterials, thereby promoting their participation in the interfacial polymerization reaction.

[0055] In the preparation method of the present invention, the amine compound is at least one selected from piperazine, homopiperazine, 2-methylpiperazine, m-phenylenediamine, p-phenylenediamine, o-phenylenediamine, mesitylenetriamine and polyethyleneimine.

[0056] Preferably, the concentration of the amine compound in the aqueous solution is 0.5–1.5 wt%.

[0057] In the preparation method of the present invention, the alcohol additive is at least one selected from aliphatic alcohols having 1 to 6 carbon atoms and aromatic alcohols having 7 to 12 carbon atoms.

[0058] The alcohol additives include, but are not limited to, methanol, ethanol, isopropanol, benzyl alcohol, glycerol, ethylene glycol, and n-butanol. Preferably, the concentration of the alcohol additives in the aqueous solution is 3-5 wt%.

[0059] Preferably, the aqueous solution further includes a pH adjuster to regulate the pH value of the solution, promoting the formation of an alkaline environment to absorb hydrogen chloride generated during the interfacial polymerization process, thereby promoting the reaction. The pH adjuster is preferably at least one selected from sodium hydroxide, potassium hydroxide, sodium bicarbonate, sodium hydrogen phosphate, potassium hydrogen phosphate, sodium carbonate, ammonium bicarbonate, triethanolamine, and a triethylamine / camphor sulfonic acid composite solution. Preferably, the pH value of the aqueous solution is adjusted to 10-11 using the pH adjuster.

[0060] Preferably, the contact time with the aqueous solution is 30 to 60 seconds; there is no limitation on the contact temperature, which can be in the range of 15 to 45°C.

[0061] The preparation method of the present invention wherein the acyl chloride compound is selected from at least one of isophthaloyl chloride, terephthaloyl chloride, o-phthaloyl chloride, trimesoyl chloride, biphenyltetramethyl chloride, cyanuric chloride, dansyl chloride, and benzenesulfonyl chloride.

[0062] Preferably, the content of the acyl chloride compound is 0.1 to 0.5 wt% based on the total weight of the organic phase solution.

[0063] Preferably, the organic phase solution further comprises an organic solvent, which is at least one selected from n-hexane, n-heptane, cyclohexane, Isopar M, Isopar H, Isopar L, Isopar E and Isopar G.

[0064] Preferably, the contact time with the organic phase solution is 30 to 60 seconds. There is no limitation on the contact temperature, which can be in the range of 15 to 45°C.

[0065] Preferably, after the interfacial polymerization reaction is completed, the nanofiltration membrane is initially heated to promote further cross-linking of the polyamide layer, thereby obtaining the initial nanofiltration membrane.

[0066] Preferably, the heating temperature range is 25–40°C, and the heating time is 1–5 minutes; the purpose of heating is to promote the evaporation of the solvent and the further cross-linking and curing of the polyamide layer, thereby promoting the formation of the nanofiltration membrane.

[0067] Preferably, the initial nanofiltration membrane is immersed in an aqueous solution containing a carboxyl activator to crosslink the residual material between the polyamide layers of the initial nanofiltration membrane, thereby ensuring the stability of the water channels formed by the nanomaterials.

[0068] In this step, the immersion treatment with a carboxyl activator promotes the chemical reaction between the amino groups on the nanomaterials and the carboxyl groups hydrolyzed from the acyl chloride groups on the polyamide layer. This promotes the fixation of the nanomaterials in the polyamide layer, ensuring the stable construction of the nano water channels and preventing the nanomaterials from detaching during subsequent water treatment processes, which could lead to changes in membrane performance.

[0069] In the preparation method of the present invention, the carboxyl activator is a combination of N-hydroxysuccinimide (NHS) and at least one selected from 1,3-dicyclohexylcarbodiimide (DCC), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDAC), diethylcyanomethyl phosphate (DEPC), 4-dimethylaminopyridine (DMAP), and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC).

[0070] Preferably, the content of the carboxyl activator is 0.5 to 2 wt% based on the total weight of the aqueous solution containing the carboxyl activator; the immersion time is preferably 1 to 3 minutes, and the immersion temperature is preferably 40 to 60°C.

[0071] The preparation method of the present invention preferably includes post-treatment further including washing with pure water, pore preservation treatment and drying treatment.

[0072] The purpose of rinsing with pure water is to remove unreacted amine compounds, acyl chloride compounds, and unfixed compounds or polymers containing multiple hydroxyl groups and / or multiple amino groups, crosslinking agents, and nanomaterials.

[0073] Preferably, the device is immersed in two sections of pure water for washing, with temperatures of 60–80°C and 20–30°C respectively, primarily to remove any residues from the aforementioned process; preferably, the washing time is 10–20 minutes, including 5 minutes of hot water washing.

[0074] Furthermore, the nanofiltration membrane prepared by this invention is immersed in an aqueous solution containing a pore-retaining agent. Since the nanofiltration membrane is relatively thin, the membrane that has not undergone pore-retaining treatment shrinks severely during the subsequent drying process, affecting the permeation performance of the nanofiltration membrane. Therefore, pore-retaining treatment is necessary to ensure the stability of the membrane pores during the subsequent drying process and to ensure the performance of the membrane. Preferably, the pore-retaining agent is one or more of glycerol and sorbitol. Preferably, the concentration of the pore-retaining agent is 10-15 wt%, the temperature of the aqueous solution containing the pore-retaining agent is 20-30°C, and the treatment time is 5-10 minutes.

[0075] Furthermore, a heating and drying process is carried out to obtain the final nanofiltration membrane, which is a moisture-free nanofiltration membrane, facilitating the subsequent membrane module winding.

[0076] Preferably, the heating and drying temperature range is 50–90°C, and the drying time is 1–3 minutes.

[0077] This invention also provides a nanofiltration membrane prepared according to the method described herein. The nanofiltration membrane has a relatively thin thickness, resulting in a higher effective filtration area per membrane element. The membrane possesses nanochannels, thus exhibiting high flux and excellent separation performance (separation of salt ions, etc.) during water treatment, and its performance remains stable during long-term operation. The nanofiltration membrane can be applied to separation and concentration technologies in water treatment, dyes, biochemicals, food, and environmental protection.

[0078] Example

[0079] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0080] Preparation Example 1

[0081] Prepare a polypropylene microporous membrane with a thickness of 30±5 micrometers and an average pore size of 40 nanometers. Immerse it in an aqueous solution of potassium persulfate with a concentration of 10wt%, heat it to 80℃ for an oxidation reaction for 40 minutes to introduce hydroxyl groups on the membrane surface, and wash it with water to remove residual substances.

[0082] A polypropylene microporous membrane with hydroxylated surface after oxidation treatment was immersed in an aqueous solution containing polyvinyl alcohol as a polymer containing multiple hydroxyl groups at a concentration of 8 wt% for 30 minutes at 40°C to ensure that the membrane was wetted by the aqueous solution. Then, it was immersed in an aqueous solution containing glutaraldehyde as a crosslinking agent (the concentration of the crosslinking agent was 0.5 wt%, and the pH of the aqueous solution was adjusted to 2.5-3.5 with sulfuric acid) for crosslinking reaction and reacted at 30°C for 8 minutes to obtain a hydrophilically modified polypropylene microporous membrane as polypropylene microporous base membrane 1.

[0083] Preparation Example 2

[0084] Except that polyethylene glycol was used instead of polyvinyl alcohol, polypropylene microporous membrane 2 was obtained in the same manner as in Preparation Example 1.

[0085] Preparation Example 3

[0086] Except that tannic acid was used instead of polyvinyl alcohol, polypropylene microporous membrane 3 was obtained in the same manner as in Preparation Example 1.

[0087] Preparation Example 4

[0088] Except that polyethyleneimine was used instead of polyvinyl alcohol, polypropylene microporous membrane 4 was obtained in the same manner as in Preparation Example 1.

[0089] Preparation Example 5

[0090] An aqueous dispersion of nanomaterials was prepared, wherein aminated graphene oxide was used as the nanomaterial at a concentration of 0.05 wt%.

[0091] The dispersion was sprayed onto the polypropylene microporous base membrane 1 obtained in Preparation Example 1 to obtain polypropylene microporous base membrane 1-1.

[0092] Preparation Example 6

[0093] Except that the dispersion was sprayed onto the polypropylene microporous base membrane 2 obtained in Preparation Example 2, the polypropylene microporous base membrane 2-1 was obtained in the same manner as in Preparation Example 5.

[0094] Preparation Example 7

[0095] Except that the dispersion was sprayed onto the polypropylene microporous base membrane 3 obtained in Preparation Example 3, the polypropylene microporous base membrane 3-1 was obtained in the same manner as in Preparation Example 5.

[0096] Preparation Example 8

[0097] Except that the dispersion was sprayed onto the polypropylene microporous base membrane 4 obtained in Preparation Example 4, the polypropylene microporous base membrane 4-1 was obtained in the same manner as in Preparation Example 5.

[0098] Preparation Example 9

[0099] Except that carboxylated graphene oxide was used instead of aminated graphene oxide as the nanomaterial, polypropylene microporous membranes 1-2 were obtained in the same manner as in Preparation Example 5.

[0100] Preparation Example 10

[0101] Except for using an amino-functionalized metal-organic framework (MOF) instead of amino-functionalized graphene oxide as the nanomaterial, polypropylene microporous membranes 1-3 were obtained in the same manner as in Preparation Example 5.

[0102] Preparation Example 11

[0103] Except for using aminated nanocrystalline cellulose instead of aminated graphene oxide as the nanomaterial, polypropylene microporous membranes 1-4 were obtained in the same manner as in Preparation Example 5.

[0104] Preparation Example 12

[0105] Except that aminated multi-walled carbon nanotubes were used instead of aminated graphene oxide as nanomaterials, polypropylene microporous membranes 1-5 were obtained in the same manner as in Preparation Example 5.

[0106] For ease of comparison, the following comparative examples and embodiments all use the aqueous phase solution, organic phase solution, and aqueous solution containing a carboxyl activator, and the contact time and temperature with the aqueous phase solution and organic phase solution are as follows:

[0107] Aqueous solution: Piperazine is used as the aqueous monomer at a concentration of 1 wt%. Sodium hydrogen phosphate is added as a pH adjuster to adjust the pH value to 10-11. Isopropanol is added as an alcohol additive at a concentration of 4 wt%. The immersion temperature in the aqueous solution is 20°C, and the contact time with the aqueous solution is 40 seconds.

[0108] Organic phase solution: Tristyroyl chloride was used as the organic phase monomer at a concentration of 0.2 wt%, and n-heptane was used as the solvent. The reaction temperature in the organic phase solution was 25 °C, and the reaction time was 40 seconds.

[0109] An aqueous solution containing a carboxyl activator: 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) in a weight ratio of 10:7, with a total concentration of 1.2 wt%. The pH of the solution is adjusted to 5-6 by adding hydrochloric acid dropwise.

[0110] Comparative Example 1

[0111] The polypropylene microporous membrane 1 obtained in Preparation Example 1 was sequentially immersed in an aqueous phase solution and an organic phase solution for interfacial polymerization. After removal, it was heated in an oven at 30°C for 3 minutes. Then, it was immersed in an aqueous solution containing a carboxyl activator for 2 minutes. However, it was then immersed in hot water at 65°C for 5 minutes and cold water at 25°C for 10 minutes to wash away residual chemicals on the membrane. It was then immersed in an aqueous solution with a glycerol concentration of 12 wt% at 25°C for 8 minutes to ensure that the membrane was wetted by the glycerol aqueous solution and that the membrane did not develop shrinkage pores in subsequent processes. Finally, it was dried in an oven at 85°C for 2 minutes to obtain the final nanofiltration membrane.

[0112] Comparative Example 2

[0113] Comparative Example 2 was carried out in the same manner as Comparative Example 1, except that the polypropylene microporous base membrane 2 obtained in Preparation Example 2 was used.

[0114] Comparative Example 3

[0115] Comparative Example 3 was carried out in the same manner as Comparative Example 1, except that the polypropylene microporous base membrane 3 obtained in Preparation Example 3 was used.

[0116] Comparative Example 4

[0117] Comparative Example 4 was carried out in the same manner as Comparative Example 1, except that the polypropylene microporous base membrane 4 obtained in Preparation Example 4 was used.

[0118] Example 1

[0119] Example 1 was carried out in the same manner as Comparative Example 1, except that the polypropylene microporous base membrane 1-1 obtained in Preparation Example 5 was used.

[0120] Example 2

[0121] Example 2 was carried out in the same manner as Comparative Example 1, except that the polypropylene microporous base membrane 2-1 obtained in Preparation Example 6 was used.

[0122] Example 3

[0123] Example 3 was carried out in the same manner as Comparative Example 1, except that the polypropylene microporous base membrane 3-1 obtained in Preparation Example 7 was used.

[0124] Example 4

[0125] Example 4 was carried out in the same manner as Comparative Example 1, except that the polypropylene microporous base membrane 4-1 obtained in Preparation Example 8 was used.

[0126] Example 5

[0127] Example 5 was carried out in the same manner as Example 1, except that the polypropylene microporous base membranes 1-2 obtained in Preparation Example 9 were used.

[0128] Example 6

[0129] Example 6 was carried out in the same manner as Example 1, except that the polypropylene microporous base membranes 1-3 obtained in Preparation Example 10 were used.

[0130] Example 7

[0131] Example 7 was carried out in the same manner as Example 1, except that the polypropylene microporous base membranes 1-4 obtained in Preparation Example 11 were used.

[0132] Example 8

[0133] Example 8 was carried out in the same manner as Example 1, except that the polypropylene microporous base membranes 1-5 obtained in Preparation Example 12 were used.

[0134] Comparative Example 5

[0135] Comparative Example 5 was carried out in the same manner as Example 1, except that the treatment using an aqueous solution containing a carboxyl activator was not performed.

[0136] Performance Tests and Results

[0137] (1) Permeation separation performance test

[0138] The nanofiltration membranes obtained in the above comparative examples and embodiments will be subjected to the following tests.

[0139] Membrane testing was conducted using a cross-flow membrane testing station. The test solution was a 2000 mg / L magnesium sulfate aqueous solution. The test pressure was 70 psi, the temperature was 25 °C, the pH was 7, and the stabilization time was 40 minutes. The test results are shown in Table 1 below:

[0140] Table 1

[0141]

[0142] Based on the results of Comparative Examples 1 to 4, by modifying the surface of polypropylene microporous membranes with hydroxylation and hydrophilicity, interfacial polymerization can be carried out directly. The resulting nanofiltration membranes also have certain permeability, but in terms of their desalination performance, they still cannot meet the high requirements for the desalination performance of nanofiltration membranes. The desalination rate of the obtained nanofiltration membranes is at most 90.36%.

[0143] By comparing Comparative Example 1 and Comparative Example 5, it can be seen that the flux of the membrane obtained in Comparative Example 5 is higher than that of the membrane obtained in Comparative Example 1. This is because the sprayed nanomaterials can act as an intermediate layer. On the one hand, the presence of the intermediate layer can make the prepared polyamide layer more uniform. On the other hand, it diffuses and participates in the water molecule transport channel constructed by the interfacial polymerization reaction. Therefore, it brings better permeability to the prepared nanofiltration membrane and improves the flux.

[0144] Comparing Comparative Example 5 with Examples 1 to 8 reveals that the desalination rate of the membrane obtained in Comparative Example 5 is lower than that of the membranes obtained in Examples 1 to 8. This is because the carboxyl activator treatment was not performed in Comparative Example 5, resulting in an unstable membrane. During operation, some nanoparticles were carried away by the water flow, leading to the loss of the constructed water channels, defects on the membrane surface, and reduced membrane density. Consequently, the desalination performance was lower than that of the membranes obtained in Examples 1 to 8. The flux of the membranes obtained in Examples 1 to 8 was slightly lower than that of the membrane obtained in Comparative Example 5. This is because the carboxyl activator treatment was further performed in Examples 1 to 8, resulting in further cross-linking of the membrane surface, making it more stable and dense. The increased density enhanced the pore size effect of the membrane, thus increasing the desalination performance, which was superior to that of the membrane obtained in Comparative Example 5. However, the permeation performance of the membrane was slightly lower than that of the membrane in Comparative Example 5.

[0145] Based on the results of Examples 1 to 8, since the nanofiltration membranes were further modified by surface hydroxylation and hydrophilicity by applying nanomaterials containing multiple hydroxyl groups and / or multiple amino groups and by using carboxyl activators, the resulting nanofiltration membranes have excellent overall performance, with good permeation and desalination performance. The flux is comparable to that of the nanofiltration membranes obtained in Comparative Examples 1 to 4, and the desalination rate is significantly higher than that of the nanofiltration membranes obtained in Comparative Examples 1 to 4.

[0146] (2) Stability test

[0147] The nanofiltration membranes obtained in Examples 1 to 4 (referred to as Examples 1, 2, 3, and 4) were subjected to stability tests under the following conditions: the test solution was a 2000 mg / L magnesium sulfate aqueous solution, the test pressure was 70 psi, the temperature was 25 °C, and the pH value was 7.

[0148] The results are as follows Figure 1 As shown, from Figure 1 As can be seen, the prepared membrane exhibited good stability during the 7-day operation. The flux and desalination rate only decreased slightly in the initial stage and remained stable during the subsequent long-term operation without significant decline.

[0149] It should be noted that although the technical solution of the present invention has been described with specific examples, those skilled in the art will understand that the present invention should not be limited thereto.

[0150] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

[0151] Industrial availability

[0152] The nanofiltration membrane preparation method provided by this invention is simple, easy to operate, requires no modification to existing equipment, and has effective operating costs. The nanofiltration membrane prepared by this method has a thinner thickness, resulting in a higher effective filtration area per membrane element. The membrane possesses nanochannels, thus exhibiting high flux and excellent separation performance (separation of salt ions, etc.) in water treatment processes, and its performance remains stable during long-term operation. This nanofiltration membrane can be applied to separation and concentration technologies in fields such as water treatment, dyes, biochemicals, food, and environmental protection.

Claims

1. A method for preparing a nanofiltration membrane, characterized in that, Includes the following steps: The surface of the polypropylene microporous membrane is hydroxylated by an oxidation treatment, wherein the oxidation treatment is carried out by an aqueous solution containing a persulfate compound, wherein the persulfate compound is at least one selected from potassium persulfate, sodium persulfate, and ammonium persulfate. The surface-hydroxylated polypropylene microporous membrane is sequentially contacted with a compound or polymer containing multiple hydroxyl groups and / or multiple amino groups, and a crosslinking agent to perform hydrophilic modification. The compound or polymer containing multiple hydroxyl groups and / or multiple amino groups is selected from at least one of polyvinyl alcohol, polyethylene glycol, tannic acid, polyethyleneimine, carboxylated chitosan, sorbitol, polyacrylic acid polyol, polyamino polyether, and polyacrylamide. The crosslinking agent is selected from at least one of aliphatic dialdehyde with 2 to 6 carbon atoms, N,N-methylenebisacrylamide, glycidyl methacrylate, epichlorohydrin, and polyethylene glycol glycidyl ether. A nanomaterial containing multiple hydroxyl and / or multiple amino groups is applied to a hydrophilically modified polypropylene microporous membrane, followed by sequential contact with an aqueous phase solution and an organic phase solution to perform an interfacial polymerization reaction to form a polyamide separation layer. The aqueous phase solution is formed from an alcohol additive, a pH adjuster, an amine compound as an aqueous phase monomer, and water. The organic phase solution contains an acyl chloride compound as an organic phase monomer. The nanomaterial containing multiple hydroxyl and / or multiple amino groups is at least one selected from carboxylated graphene oxide, aminated graphene oxide, aminated graphene, aminated multi-walled carbon nanotubes, aminated single-walled carbon nanotubes, carboxylated single-walled carbon nanotubes, amino-functionalized metal-organic frameworks (MOFs), and aminated nanocrystalline cellulose. The alcohol additive is at least one selected from aliphatic alcohols with 1 to 6 carbon atoms and aromatic alcohols with 7 to 12 carbon atoms. The pH adjuster is at least one selected from sodium hydroxide, potassium hydroxide, sodium bicarbonate, sodium hydrogen phosphate, potassium hydrogen phosphate, sodium carbonate, ammonium bicarbonate, triethanolamine, and a triethylamine / camphor sulfonic acid composite solution. The nanofiltration membrane is obtained by post-processing, wherein the post-processing includes treatment with a carboxyl activator, the carboxyl activator being a combination of N-hydroxysuccinimide and at least one selected from 1,3-dicyclohexylcarbodiimide, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride, diethylcyanomethyl phosphate, 4-dimethylaminopyridine, and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride.

2. The preparation method according to claim 1, wherein the oxidation treatment is carried out using an aqueous solution of potassium persulfate with a concentration of 5-15 wt%.

3. The preparation method according to claim 1 or 2, wherein the concentration of the compound or polymer containing multiple hydroxyl groups and / or multiple amino groups in the aqueous solution containing the compound or polymer containing multiple hydroxyl groups and / or multiple amino groups is 5 to 10 wt%.

4. The preparation method according to claim 1 or 2, wherein the content of the crosslinking agent is 0.1 to 1 wt% based on the total weight of the aqueous solution containing the crosslinking agent.

5. The preparation method according to claim 1 or 2, wherein the concentration of the nanomaterial containing multiple hydroxyl groups and / or multiple amino groups in the dispersion containing the nanomaterial is 0.01 to 1 wt%.

6. The preparation method according to claim 1 or 2, wherein the concentration of the alcohol additive in the aqueous solution is 3-5 wt%.

7. The preparation method according to claim 1 or 2, wherein the amine compound is at least one selected from piperazine, homopiperazine, 2-methylpiperazine, m-phenylenediamine, p-phenylenediamine, o-phenylenediamine, mesitylenetriamine and polyethyleneimine.

8. The preparation method according to claim 1 or 2, wherein the acyl chloride compound is at least one selected from isophthaloyl chloride, terephthaloyl chloride, orthophthaloyl chloride, trimesoyl chloride, biphenyltetramethyl chloride, cyanuric chloride, dansyl chloride, and benzenesulfonyl chloride.

9. The preparation method according to claim 1 or 2, wherein the content of the carboxyl activator is 0.5 to 2 wt% based on the total weight of the aqueous solution containing the carboxyl activator.

10. A nanofiltration membrane prepared by the preparation method according to any one of claims 1 to 9.

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