Positively charged nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention relates to the technical field of membrane separation, and discloses a positively charged nanofiltration membrane as well as a preparation method and application thereof. The positively charged nanofiltration membrane sequentially comprises a bottom layer, a porous middle layer and a polypiperazine amide separation layer, the polypiperazine amide separation layer is grafted with a quaternary ammonium salt structure as shown in a formula I through N atoms; wherein R < 1 >, R < 2 > and R < 3 > are respectively and independently alkylidene of C1-C3; n1 and n2 are each independently an integer from 0 to 3; r < 4 >, R < 5 >, R < 6 >, R < 7 >, R < 8 > and R < 9 > are respectively and independently H, C1-C4 alkyl or phenyl; x is F, Cl or Br. The positively charged nanofiltration membrane comprises a polypiperazine amide separation layer, and a specific quaternary ammonium salt structure is grafted on the polypiperazine amide separation layer, so that the surface of the polypiperazine amide composite nanofiltration membrane is positively charged, and the composite nanofiltration membrane has high rejection rate on divalent / high-valent metal ions and keeps high water flux; the requirement of extracting lithium from a salt lake with a high magnesium-lithium ratio can be met.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, specifically to a positively charged nanofiltration membrane, its preparation method, and its application. Background Technology

[0002] Lithium is widely used in batteries, alloy manufacturing, and new energy fields, and is considered a strategic resource. Although China possesses abundant lithium resources, accounting for approximately 16% of global reserves, the vast majority (86.5%) are in the form of salt lake resources, mainly distributed in regions such as Qinghai and Tibet.

[0003] Currently available nanofiltration membranes are not specifically designed for lithium extraction from salt lakes, and their separation efficiency for magnesium and lithium ions is relatively low, failing to meet the demands of lithium extraction from salt lakes. Research indicates that increasing the positive charge density on the surface of the nanofiltration membrane separation layer, under the Donnan effect, can significantly improve the retention rate of magnesium chloride by the nanofiltration membrane, thereby effectively enhancing the magnesium-lithium separation efficiency.

[0004] However, current surface-modified positively charged magnesium-lithium nanofiltration membrane aqueous phase systems mostly employ polyamines such as PEI, which provide both positive charge and modified active sites. However, these nanofiltration membranes have a dense structure, thick separation layer, and low water flux. Furthermore, nanofiltration membranes prepared by PEI interfacial polymerization contain active hydrogen (-CO-NH-), making them susceptible to attack by active chlorine (such as sodium hypochlorite), resulting in less than ideal chlorine resistance.

[0005] Furthermore, commercially available nanofiltration membranes generally use piperazine as the monomer in the aqueous phase reaction. Due to the lack of amino groups as active sites, surface modification of this type of membrane is challenging, and the membrane material obtained after hydrolysis of acyl chlorides has a negative surface charge, making it unsuitable for lithium extraction from salt lakes. Therefore, a simple and efficient technology for directly modifying piperazine-based nanofiltration membranes to prepare membrane materials for magnesium-lithium separation has broad application prospects. Summary of the Invention

[0006] The purpose of this invention is to overcome the limitations of existing polypiperazine amide nanofiltration membranes, which have an electronegative surface and are sensitive to divalent Mg. 2+ To address the problem of low rejection rates that fail to meet the requirements for efficient lithium extraction from salt lakes, a positively charged nanofiltration membrane, its preparation method, and its application are provided. This positively charged nanofiltration membrane contains a polypiperazine amide separation layer, and a specific quaternary ammonium salt structure is grafted onto the polypiperazine amide separation layer. This results in a positively charged surface on the polypiperazine amide composite nanofiltration membrane, enabling it to have a high rejection rate for divalent / high-valent metal ions while maintaining a high water flux, thus meeting the requirements for lithium extraction from salt lakes with high magnesium-to-lithium ratios.

[0007] To achieve the above objectives, the first aspect of the present invention provides a positively charged nanofiltration membrane, wherein the positively charged nanofiltration membrane comprises, in sequence, a bottom layer, a porous intermediate layer, and a polypiperazine amide separation layer;

[0008] The polypiperazine amide separation layer is a quaternary ammonium salt structure represented by N-atom grafting formula I;

[0009]

[0010] Wherein, R1, R2, and R3 are each independently C1-C3 alkylene groups; n1 and n2 are each independently integers from 0 to 3; R4, R5, R6, R7, R8, and R9 are each independently H, C1-C4 alkyl or phenyl groups; and X is F, Cl, or Br.

[0011] A second aspect of the present invention provides a method for preparing a positively charged nanofiltration membrane, characterized in that the preparation method includes the following steps:

[0012] S1. A porous intermediate layer and a polypiperazine amide separation layer are sequentially prepared on the bottom layer to obtain a composite membrane;

[0013] S2. The composite membrane is contacted with an aqueous solution of the quaternary ammonium salt represented by Formula II and dried to obtain the positively charged nanofiltration membrane;

[0014]

[0015] Wherein, R1, R2, and R3 are each independently C1-C3 alkylene groups; n1 and n2 are each independently integers from 0 to 3; R4, R5, R6, R7, R8, and R9 are each independently H, C1-C4 alkyl or phenyl groups; and X is F, Cl, or Br.

[0016] A third aspect of the present invention provides a positively charged nanofiltration membrane prepared by the above-described preparation method.

[0017] The fourth aspect of the present invention provides an application of the above-mentioned positively charged nanofiltration membrane in lithium extraction from salt lakes.

[0018] Through the above technical solutions, the positively charged nanofiltration membrane and its preparation method provided by the present invention achieve the following beneficial effects:

[0019] In this invention, the polypiperazine amide separation layer of the positively charged nanofiltration membrane is grafted with quaternary ammonium salt groups of a specific structure, which significantly increases the surface positive charge density of the composite nanofiltration membrane. The Donnan effect between charges significantly improves the retention effect of the nanofiltration membrane on divalent magnesium ions, thereby ensuring that the nanofiltration membrane has high separation efficiency for monovalent and divalent metal ions and high flux, thus ensuring high efficiency of the separation process.

[0020] In the method for preparing a positively charged nanofiltration membrane provided by this invention, a quaternary ammonium salt compound with a specific structure reacts chemically with the acyl chloride groups in the separation layer of the composite nanofiltration membrane, grafting the positively charged quaternary ammonium salt groups onto the surface of the separation layer of the nanofiltration membrane. This significantly increases the surface positive charge density of the composite nanofiltration membrane, and the repulsion effect between charges significantly improves the retention effect of the nanofiltration membrane for divalent / high-valent metal ions. Thus, while ensuring that the nanofiltration membrane has a high water flux, the separation efficiency of lithium ions and magnesium ions is improved, meeting the needs of efficient lithium extraction from salt lakes. Attached Figure Description

[0021] Figure 1 This is the infrared spectrum of the quaternary ammonium salt prepared in the preparation example.

[0022] Figure 2 These are the surface infrared spectra of the nanofiltration membranes prepared in Example 1 and Comparative Example 1.

[0023] Figure 3 These are the fine XPS spectra of surface nitrogen elements in the nanofiltration membranes prepared in Example 1 and Comparative Example 1. Detailed Implementation

[0024] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0025] The first aspect of the present invention provides a positively charged nanofiltration membrane, wherein the positively charged nanofiltration membrane comprises, in sequence, a bottom layer, a porous intermediate layer and a polypiperazine amide separation layer;

[0026] The polypiperazine amide separation layer is a quaternary ammonium salt structure represented by N-atom grafting formula I;

[0027]

[0028] Wherein, R1, R2, and R3 are each independently C1-C3 alkylene groups; n1 and n2 are each independently integers from 0 to 3; R4, R5, R6, R7, R8, and R9 are each independently H, C1-C4 alkyl or phenyl groups; and X is F, Cl, or Br.

[0029] In this invention, the polypiperazine amide separation layer of the positively charged nanofiltration membrane is grafted with quaternary ammonium salt groups of a specific structure, which significantly increases the surface positive charge density of the composite nanofiltration membrane. The Donnan effect between charges significantly improves the retention effect of the nanofiltration membrane on divalent magnesium ions, thereby ensuring that the nanofiltration membrane has high separation efficiency for monovalent and divalent metal ions and has a high flux.

[0030] In this invention, * refers to the formation of amide bonds between the quaternary ammonium salt structure and the acyl chloride groups in the polypiperazine amide separation layer. The location.

[0031] Furthermore, R1, R2, and R3 are ethylene.

[0032] Furthermore, n1 and n2 are both 1.

[0033] Furthermore, R4, R5, R6, R7, R8, and R9 are each independently an H or C1-C4 alkyl group.

[0034] Furthermore, X is either Cl or Br.

[0035] According to the present invention, the nitrogen atom content in the quaternary ammonium salt group of the composite nanofiltration membrane is 0.7-5 at.%.

[0036] In this invention, when the nitrogen atom content in the quaternary ammonium salt group of the positively charged nanofiltration membrane meets the above-mentioned range, it indicates that the quaternary ammonium salt has been successfully introduced into the polypiperazine amide composite nanofiltration membrane. Furthermore, the introduced quaternary ammonium salt enables the surface of the positively charged nanofiltration membrane to carry a positive charge, overcoming the limitation of traditional polypiperazine amide composite nanofiltration membranes on divalent magnesium atoms due to their electronegativity. 2+ The low retention rate fails to meet the demand for efficient lithium extraction from salt lakes.

[0037] Furthermore, the nitrogen atom content in the quaternary ammonium salt groups of the composite nanofiltration membrane is 1.5-4 at.%.

[0038] According to the present invention, the surface zeta potential of the composite nanofiltration membrane is 0-35mV.

[0039] In this invention, when the surface Zeta potential of the positively charged nanofiltration membrane meets the above-mentioned range, it indicates that the composite nanofiltration membrane has a high surface electrode potential. When used for lithium-magnesium separation, it can better repel divalent magnesium ions, making it difficult for magnesium ions in the liquid to pass through the separation membrane, while allowing monovalent lithium ions to pass through as much as possible, thereby obtaining a high magnesium-lithium separation efficiency.

[0040] In this invention, the surface Zeta potential of the nanofiltration membrane refers to the surface Zeta potential at pH=7.

[0041] Furthermore, the surface zeta potential of the composite nanofiltration membrane is 5-30mV.

[0042] According to the present invention, the average pore size of the positively charged nanofiltration membrane is 0.1-0.4 nm.

[0043] In this invention, when the average pore size of the positively charged nanofiltration membrane meets the above-mentioned range, the pore size of the separation layer of the composite nanofiltration membrane is small and the pore size distribution is narrow, which can significantly improve the retention capacity of the composite nanofiltration membrane for divalent / high-valent metal ions.

[0044] Furthermore, the average pore size of the positively charged nanofiltration membrane is 0.15-0.3 nm.

[0045] According to the present invention, the contact angle of the positively charged nanofiltration membrane is 30-80°.

[0046] In this invention, when the contact angle of the positively charged nanofiltration membrane meets the above-mentioned range, the composite nanofiltration membrane has strong surface hydrophilicity and good water wetting ability, which can significantly improve the water flux of the composite nanofiltration membrane.

[0047] Furthermore, the contact angle of the positively charged nanofiltration membrane is 30-60°.

[0048] In this invention, the bottom layer and the porous support layer are not specifically limited and can be made of various existing materials with certain strength that can be used for nanofiltration and reverse osmosis membranes.

[0049] In this invention, the bottom layer is a non-woven fabric material, preferably polyester and / or polyethylene.

[0050] In this invention, the porous support layer material can be at least one of polyethersulfone, polysulfone, polyaryl ether, polybenzimidazole, polyetherketone, polyetheretherketone, polyacrylonitrile, polyvinylidene fluoride, and polyaryl etherketone.

[0051] According to the present invention, the thickness of the bottom layer, the porous intermediate layer, and the polypiperazine amide separation layer is not particularly limited, and can be a conventional choice in the art. However, in order to enable these three layers to play a better synergistic role and enable the resulting composite nanofiltration membrane to better combine high water flux and desalination rate, preferably, the thickness of the bottom layer is 30-150 μm, preferably 50-120 μm; the thickness of the porous intermediate layer is 10-100 μm, preferably 30-60 μm; and the thickness of the polyamide separation layer is 10-500 nm, preferably 50-300 nm.

[0052] In this invention, the polypiperazine amide separation layer is grafted with a specific bisquaternary ammonium salt group as shown in Formula I. Specifically, the specific bisquaternary ammonium salt group shown in Formula I is obtained by reacting the terminal amino bisquaternary ammonium salt shown in Formula II with the acyl chloride group remaining in the separation layer. This results in a significant increase in the surface positive charge density of the composite nanofiltration membrane. The repulsion effect between positive charges enhances the retention effect of the nanofiltration membrane on divalent metal ions (magnesium ions). Thus, while ensuring that the nanofiltration membrane has a high water flux, the separation efficiency of the nanofiltration membrane for divalent metal ions (e.g., magnesium ions) and monovalent metal ions (e.g., lithium ions) is improved.

[0053] In this invention, the polypiperazine amide separation layer is obtained by interfacial polymerization of a polyamine containing a piperazine structure with a polyacrylamide (e.g., 1,3,5-benzenetricarboxyl chloride or / and terephthaloyl chloride).

[0054] A second aspect of the present invention provides a method for preparing a positively charged nanofiltration membrane, characterized in that the preparation method includes the following steps:

[0055] S1. A porous intermediate layer and a polypiperazine amide separation layer are sequentially prepared on the bottom layer to obtain a composite membrane;

[0056] S2. The composite membrane is contacted with an aqueous solution of the quaternary ammonium salt represented by Formula II and dried to obtain the positively charged nanofiltration membrane;

[0057]

[0058] Wherein, R1, R2, and R3 are each independently C1-C3 alkylene groups; n1 and n2 are each independently integers from 0 to 3; R4, R5, R6, R7, R8, and R9 are each independently H, C1-C4 alkyl or phenyl groups; and X is F, Cl, or Br.

[0059] In existing technologies, most positively charged nanofiltration membrane surface modification strategies utilize active amino groups on the membrane surface. However, polypiperazine amide nanofiltration membranes lack amino groups, making surface modification difficult. In this invention, a quaternary ammonium salt compound with a specific structure reacts chemically with acyl chloride groups in the separation layer of the composite nanofiltration membrane, grafting positively charged quaternary ammonium salt groups onto the separation layer surface of the nanofiltration membrane. This significantly increases the surface positive charge density of the composite nanofiltration membrane. The repulsion effect between charges significantly improves the retention efficiency of the nanofiltration membrane for divalent / high-valent metal ions, thereby increasing the separation efficiency of lithium and magnesium ions while maintaining high water flux, thus meeting the requirements for efficient lithium extraction from salt lakes.

[0060] In a preferred embodiment of the present invention, R1, R2, and R3 are ethylene.

[0061] In a preferred embodiment of the present invention, n1 and n2 are both 1.

[0062] In a preferred embodiment of the present invention, R4, R5, R6, R7, R8, and R9 are each independently H or C1-C4 alkyl groups.

[0063] In a preferred embodiment of the present invention, X is Cl or Br.

[0064] In this invention, there is no particular limitation on the method for preparing the porous intermediate layer on the bottom layer. Conventional methods in the art can be used for preparation, with phase inversion method preferred. Specifically, a polymer solution of porous support layer material is coated on one surface of the bottom layer, and the porous intermediate layer is obtained through phase inversion.

[0065] In this invention, the phase inversion method is preferably as follows: dissolving the support layer polymer material in a solvent to obtain a polymer solution with a concentration of 10-20% by weight, degassing at 20-40°C for 10-180 min; then coating the polymer solution onto the bottom layer to obtain an initial film, and then immersing it in water at a temperature of 10-30°C for 10-60 min, thus forming the support layer polymer porous membrane through the phase inversion layer.

[0066] The solvent may be N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, etc.

[0067] According to the present invention, in step S2, the concentration of the aqueous solution containing the quaternary ammonium salt represented by Formula II is 0.1-3 wt%.

[0068] In this invention, controlling the concentration of the aqueous solution containing the quaternary ammonium salt of Formula II within the aforementioned range ensures sufficient reaction between the quaternary ammonium salt of Formula II and the residual acyl chloride groups in the separation layer. This significantly increases the surface positive charge density of the nanofiltration membrane, enhancing its retention of divalent metal ions (magnesium ions) through the repulsive effect between positive charges, while simultaneously ensuring high water flux. If the concentration of the aqueous solution containing the quaternary ammonium salt of Formula II is too low, the reaction will be incomplete; if the concentration is too high, the water flux of the nanofiltration membrane will decrease.

[0069] Further, in step S2, the concentration of the aqueous solution containing the quaternary ammonium salt represented by Formula II is 0.5-2.5 wt%.

[0070] In this invention, in step S2, the ratio of the mass of the aqueous solution containing the quaternary ammonium salt of formula II to the membrane area of ​​the composite membrane obtained in step S1 is 0.1-1 g / cm³. 2 The preferred concentration is 0.2-0.8 g / cm³. 2 .

[0071] According to the present invention, the contact conditions include a contact time of 5s-600s, preferably 10s-120s.

[0072] According to the present invention, the drying temperature is 40-70°C, preferably 50-60°C; the drying time is 0.5-10 minutes, preferably 3-5 minutes.

[0073] In this invention, there are no particular limitations on the method for preparing the polypiperazine amide separation layer on the porous intermediate layer; conventional methods in the art can be used.

[0074] In a preferred embodiment of the present invention, the composite membrane is prepared according to the following steps:

[0075] (1) Prepare a porous intermediate layer on the bottom layer;

[0076] (2) The membrane obtained in step (1) is brought into first contact with an aqueous phase containing polyamines;

[0077] (3) The membrane obtained in step (2) is brought into a second contact with an organic solution containing polyacryl chloride, and after heat treatment, the composite membrane is obtained;

[0078] The polyamine is selected from at least one of piperazine, hyperpiperazine, and (R)-piperazine-2-carboxylic acid, preferably piperazine.

[0079] According to the present invention, the concentration of the polyamine in the aqueous phase containing the polyamine is 0.05-5 wt%, preferably 0.1-2.5 wt%.

[0080] In one specific embodiment of the present invention, the polyamine is selected from two of piperazine, homopiperazine, and (R)-piperazine-2-carboxylic acid, preferably piperazine and homopiperazine, or piperazine and (R)-piperazine-2-carboxylic acid. Specifically, the mass ratio of homopiperazine or (R)-piperazine-2-carboxylic acid to piperazine is 0.1-10:1.

[0081] According to the present invention, the concentration of polyacryl chloride in the organic solution containing polyacryl chloride is 0.025wt%-1wt%, preferably 0.05wt%-0.5wt%.

[0082] According to the present invention, the amounts of the aqueous phase containing polyamine and the organic phase containing polyacryl chloride are such that the mass ratio of polyamine to polyacryl chloride is 0.1-50:1.

[0083] In this invention, controlling the mass ratio of polyamine to polyacrylamide to meet the above-mentioned range can ensure sufficient interfacial polymerization reaction between polyamine and polyacrylamide, thereby increasing the crosslinking density of the obtained polypiperazine amide separation layer.

[0084] Furthermore, the amounts of the aqueous phase containing polyamine and the organic phase containing polyacrylamide chloride are such that the mass ratio of polyamine to polyacrylamide chloride is 0.5-20:1.

[0085] According to the present invention, the polyacryl chloride is selected from 1,3,5-benzenetricarboxyl chloride and / or terephthaloyl chloride.

[0086] In this invention, there is no particular limitation on the type of solvent in the organic phase containing polyacryl chloride, as long as it can dissolve the polyacryl chloride. Preferably, the solvent of the organic phase is one or more of n-hexane, dodecane, n-heptane, and alkane solvent oils (Isopar E, Isopar G, Isopar H, Isopar L and Isopar M).

[0087] According to the present invention, there are no particular limitations on the interfacial polymerization conditions of polyamines and polyacrylamide chlorides, and they can be carried out according to conventional conditions in the art. However, in order to enable the three layers to play a better synergistic role and to enable the resulting composite nanofiltration membrane to better combine excellent desalination rate and high water flux, preferably, the first contact time is 5-120s, preferably 10-60s; the second contact time is 5-120s, preferably 10-60s; and the heat treatment conditions include: heat treatment temperature of 40-60℃, preferably 45-55℃; and heat treatment time of 0.5-20min, preferably 1-10min.

[0088] In this invention, in step (2), the ratio of the volume of the aqueous phase containing the polyamine to the membrane area of ​​the membrane obtained in step (1) is 0.05-1 mL / cm². 2 Preferably, it is 0.1-0.5 mL / cm 2 .

[0089] In this invention, in step (3), the ratio of the volume of the organic phase containing polyacrylamide chloride to the membrane area of ​​the membrane obtained in step (2) is 0.01-0.5 mL / cm². 2 Preferably, the concentration is 0.05-0.25 mL / cm³. 2 .

[0090] According to the present invention, the quaternary ammonium salt represented by Formula II is prepared according to the following steps:

[0091] The compound shown in Formula 1, at least one compound shown in Formula 2, and a solvent are mixed and reacted in the presence of a supported catalyst. After drying, the quaternary ammonium salt shown in Formula II is obtained; the reaction time is 2-24 h.

[0092]

[0093] Wherein, R1, R2, and R3 are each independently a C1-C3 alkylene group; n is an integer from 0 to 3; R 10 R 11 R 12 Each is independently H, a C1-C4 alkyl or phenyl group; X is F, Cl or Br.

[0094] In this invention, the molar ratio of the compound shown in Formula 1 to the compound described in Formula 2 is 1:1.9-2.1.

[0095] In this invention, the molar ratio of the catalyst to the compound shown in Formula 1 in the supported catalyst is 1:40-220, preferably 1:70-170.

[0096] In this invention, preferably, the reaction time is 3-16 hours.

[0097] In this invention, the reaction temperature is 15-35℃, preferably 25-35℃.

[0098] In this invention, the solvent is selected from dichloromethane and / or toluene.

[0099] In one specific embodiment of the present invention, the reaction is carried out in a self-priming high-pressure reactor.

[0100] In this invention, carrying out the reaction in a self-priming high-pressure reactor ensures that the protective atmosphere during the reaction process is maintained within a predetermined pressure range, thereby reducing the occurrence of side reactions.

[0101] In one specific embodiment of the present invention, the atmosphere of the self-priming high-pressure reactor is a protective atmosphere with a pressure of 0.2-1.5 bar. Preferably, the protective atmosphere is nitrogen with a pressure of 0.5-1 bar.

[0102] In this invention, the supported catalyst is a catalyst supported on Merrified peptide resin modified with terminal amino hyperbranched polyamide, wherein the catalyst is selected from at least one of scandium trifluoromethanesulfonate, lanthanum trifluoromethanesulfonate and barium trifluoromethanesulfonate, preferably scandium trifluoromethanesulfonate.

[0103] In this invention, based on the total amount of the supported catalyst, the supported amount of the catalyst is 4-25 wt%, preferably 6-17 wt%.

[0104] In this invention, the Merrifieid peptide resin was purchased from Merck, and can be, for example, Merrifieid peptide resin 522740, Merrifieid peptide resin 474517, Merrifieid peptide resin 497061, Merrifieid peptide resin 449113, Merrifieid peptide resin 577901, or Merrifieid peptide resin 22153. Preferably, the Merrifieid peptide resin is Merrifieid peptide resin 449113 (200-400 mesh, loaded with 2-2.5 mmol / g Cl). - 2% crosslinking).

[0105] In this invention, the terminal amino hyperbranched polyamide was purchased from Shanghai Ziyi Reagent Factory, with the grade N101 and an amino number of 350-370 g / mol.

[0106] In this invention, based on the weight of Merrified polypeptide resin, the content of terminal amino hyperbranched polyamide is 35-70 wt%, preferably 40-60 wt%.

[0107] In one specific embodiment of the present invention, the catalyst immobilized on Merrifiid polypeptide resin modified with terminal amino hyperbranched polyamide is prepared according to the following steps:

[0108] (1) Merrified peptide resin, terminal amino hyperbranched polyamide and first organic solvent are mixed and reacted in a shaker at 30-60℃ for 0.5-3 days. After solid-liquid separation and drying, resin carrier is obtained.

[0109] (2) In the presence of a second organic solvent, the catalyst is subjected to a coordination reaction with the resin support, and after filtration and drying, the solid-supported catalyst is obtained.

[0110] In this invention, in step (1), based on the weight of Merrified polypeptide resin, the amount of terminal amino hyperbranched polyamide is 40-60 wt%.

[0111] In this invention, the type of organic solvent in step (1) is not particularly limited and can be any conventional organic solvent in the art, such as NMP. The amount of organic solvent used is also not particularly limited, as long as it ensures that the Merrified polypeptide resin and the terminal amino hyperbranched polyamide are thoroughly and uniformly mixed.

[0112] In this invention, preferably, step (1) further includes washing the solid product after solid-liquid separation. Preferably, anhydrous dichloromethane is used to wash the solid product.

[0113] In this invention, there are no particular limitations on the drying conditions; drying can be carried out according to conventional conditions in the art.

[0114] In this invention, in step (2), the amount of catalyst used is 5-30 wt%, preferably 7-20 wt%, based on the weight of the resin support.

[0115] In this invention, the type of the second organic solvent is not particularly limited, and it can be a conventional organic solvent in the art, such as dichloromethane. The amount of the second organic solvent is also not particularly limited, as long as it ensures that the carrier resin and the catalyst are thoroughly and uniformly mixed.

[0116] In this invention, there are no particular limitations on the conditions for the coordination reaction, as long as the carrier resin and the catalyst can react fully, for example, reacting on a shaker for 3-10 hours.

[0117] In a preferred embodiment of the present invention, the method includes:

[0118] S1. Mix the compound shown in Formula 1 with a solvent, then add the compound shown in Formula 2 and a supported catalyst to carry out the reaction;

[0119] S2. The product of the reaction in step S1 is subjected to solid-liquid separation. After liquid-liquid separation, it is purified to obtain the terminal amino bis-quaternary ammonium salt compound.

[0120] In this invention, the terminal amino bisquaternary ammonium salt compound is a pale yellow oily substance.

[0121] In this invention, gas chromatography is used to determine the purity of the product. Specifically, the gas chromatogram is normalized, and the purity of the product is calculated by the area of ​​the chromatogram.

[0122] In this invention, the yield of the terminal amino bis-quaternary ammonium salt compound is calculated using the following formula:

[0123] Yield, % = m1 / m0 × 100%, where m1 is the mass of the terminal amino bis-quaternary ammonium salt compound in g, and m0 is the mass corresponding to the molar amount of the product calculated according to theory.

[0124] In this invention, there is no particular limitation on the method of solid-liquid separation; conventional solid-liquid separation methods in the art, such as filtration, can be used. In this invention, the supported catalyst is removed through solid-liquid separation.

[0125] In this invention, there is no particular limitation on the liquid-liquid separation method; conventional liquid-liquid separation methods in the art, such as rotary evaporation, can be used. In this invention, solvents, such as dichloromethane, are removed from the liquid phase obtained from solid-liquid separation through liquid-liquid separation.

[0126] According to the present invention, the purification is carried out in a chromatography column.

[0127] According to the present invention, the eluent for purification is a mixture of methanol and hydrocarbon solvent, wherein the volume ratio of methanol to hydrocarbon solvent is 0.1-0.4:1.

[0128] In this invention, the hydrocarbon solvent used in purification is the same as the hydrocarbon solvent used in the reaction process. Preferably, the hydrocarbon solvent is selected from dichloromethane and / or toluene.

[0129] A third aspect of the present invention provides a positively charged nanofiltration membrane prepared by the above-described preparation method.

[0130] The fourth aspect of this invention provides the application of the above-mentioned positively charged nanofiltration membrane in the field of lithium extraction from salt lakes.

[0131] The present invention will be described in detail below through embodiments.

[0132] In the following embodiments and comparative examples:

[0133] (1) The water flux of the composite nanofiltration membrane was obtained by testing as follows: The nanofiltration membrane was loaded into the membrane tank and pre-pressurized at 0.5 MPa for 0.5 h. Then, the water permeation rate of the separation membrane was measured at 0.5 MPa and 25 °C for a certain period of time, and the water flux was calculated by the following formula:

[0134] J = Q / (A·t), where J is the water flux, Q is the volume of water permeated (L), and A is the effective filtration membrane area (m²). 2 ), where t is time (h).

[0135] (2) The desalination rate of the composite nanofiltration membrane was tested using the following method: The nanofiltration membrane was loaded into a membrane tank, and the original aqueous solution was 2000 ppm magnesium chloride or 2000 ppm lithium chloride. After pre-pressurization at 0.5 MPa for 0.5 h, the permeate was obtained at a pressure of 0.5 MPa. The concentrations of magnesium chloride or lithium chloride in the original aqueous solution and the permeate were measured by conductivity method, and the desalination rate was calculated using the following formula:

[0136] R = (Cf - Cp) / Cf × 100%, where R is the desalination rate, Cf is the concentration of magnesium chloride or lithium chloride in the original aqueous solution (measured by conductivity method), and Cp is the concentration of magnesium chloride or lithium chloride in the permeate (measured by conductivity method).

[0137] (3) The nitrogen atom content in the quaternary ammonium salt structure of the positively charged nanofiltration membrane was determined by the following method:

[0138] Before measurement, the nanofiltration membrane was dried to constant weight in an oven, and the elemental composition of the nanofiltration membrane surface was determined using a SigmaProbe X-ray photoelectron spectroscopy system manufactured by Thermo VG, UK. The nitrogen atom content in the quaternary ammonium salt structure of the polyamide separation layer was calculated using the following formula:

[0139]

[0140] Among them, A N + The peak area of ​​the nitrogen atom in the quaternary ammonium salt group in the XPS spectrum ( Figure 3 (peak area at the middle bond energy of 403 eV); A N The peak area of ​​nitrogen atoms in the separated layer in the XPS spectrum represents ( Figure 3 (The sum of the peak areas at 403 eV and 399 eV); S N This represents the atomic content (at.%) of nitrogen atoms in the polyamide separator layer.

[0141] (4) Average pore size test of positively charged nanofiltration membrane: The average pore size was measured using the PEG solute transfer method, and the detailed steps are as follows:

[0142] (i) Test the retention rate of nanofiltration membranes for PEG of different molecular sizes;

[0143] (ii) Linearly fit the PEG size and the rejection rate in a log-probability coordinate system. The PEG size corresponding to a 50% rejection rate is the average pore size of the separation membrane.

[0144] (5) Surface Zeta potential test of positively charged nanofiltration membrane: The test was performed using a Surpass electric analyzer (Anton Paar), with the circulating solution being a dilute aqueous solution of KCl and the pH of the test solution being 7.

[0145] (6) Contact angle of positively charged nanofiltration membrane: The surface contact angle of the composite membrane sample was tested by the static drop method using a DSA100 surface contact angle meter manufactured by KRUSS GmbH, Germany. Before the test, the sample was dried in a vacuum oven at 60℃ for 30 minutes to remove surface and internal moisture. Then, the dried membrane was attached to a flat glass slide with double-sided tape. The volume of water droplets was 2μL each time during the test. The water droplets were dropped onto the membrane surface for 3 seconds and the test was performed immediately. The final contact angle was determined by taking the average value after multiple measurements.

[0146] (7) The thickness of each layer in the positively charged nanofiltration membrane was determined by a micrometer and a scanning electron microscope.

[0147] Additionally, in the following embodiments and comparative examples:

[0148] Piperazine, high piperazine, 1,3,5-benzenetricarboxylic acid chloride, and terephthaloyl chloride were all purchased from Bailingwei Technology Co., Ltd., and other chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd.

[0149] The support layer is prepared using a phase transformation method, and the specific steps are as follows:

[0150] A certain amount of polyethersulfone (number average molecular weight of 80,000 g / mol) was dissolved in N,N-dimethylformamide to prepare a polysulfone solution with a concentration of 18 wt%. The solution was degassed at 25 °C for 120 min. Then, the polyethersulfone solution was coated onto a polyester nonwoven fabric (75 μm thick) using a doctor blade to obtain an initial film. The film was then immersed in water at 25 °C for 60 min, which allowed the polysulfone layer on the surface of the polyester nonwoven fabric to undergo phase transformation into a porous film. Finally, after three water washes, a bottom layer and a porous intermediate layer with a total thickness of 115 μm were obtained.

[0151] Preparation Example

[0152] The quaternary ammonium salt shown in Formula II is prepared by the following method:

[0153] (i) In a 250 mL self-priming high-pressure reactor, add 150 mL of dichloromethane, 34 mmol of tris(2-aminoethyl)amine, 68 mmol of 2,3-epoxypropyltrimethylammonium chloride, and 1.65 g of supported catalyst (containing 0.34 mmol of scandium trifluoromethanesulfonate). Control the reaction temperature at 35 °C and stir for 12 h, during which nitrogen gas is purged and the pressure is controlled at 0.5 bar. The molar ratio of the catalyst to the tris(2-aminoethyl)amine in the supported catalyst is 1:100.

[0154] (ii) After stopping the reaction, filter to remove the catalyst, remove dichloromethane by rotary evaporation at 40°C, purify by column chromatography, using a mixture of methanol and dichloromethane as the eluent, wherein the volume ratio of methanol to dichloromethane is 0.25:1, and obtain a pale yellow oily substance after rotary evaporation at 40°C with a yield of 91%, and the purity is determined to be 93% by gas chromatography with internal standard method.

[0155] Figure 1 This is the infrared spectrum of the product, at 1450 cm⁻¹. -1 -1600cm -1 and 3000cm -1 Characteristic peaks of quaternary ammonium salts appeared nearby, at 3000 cm⁻¹. -1 -3500cm -1 The presence of characteristic amino peaks within the range, combined with the feed amount, indicates that tris(2-aminoethyl)amine and 2,3-epoxypropyltrimethylammonium chloride have successfully reacted to obtain the quaternary ammonium salt shown in Formula II.

[0156] In the preparation example, the supported catalyst was prepared according to the following steps:

[0157] (1) Mix 5g of Merrifieid polypeptide resin, 3g of terminal amino hyperbranched polyamide and 200mL of NMP, and react in a shaker at 50°C for 1 day to obtain a resin carrier. The amount of terminal amino hyperbranched polyamide is 60wt% based on the weight of Merrifieid polypeptide resin.

[0158] (2) 100 mL of dichloromethane, 0.5 g of scandium trifluoromethanesulfonate, and a Merrifield polypeptide resin carrier modified with terminal amino hyperbranched polyamide were added to a 250 mL reaction flask. After reacting on a shaker for 6 h, the mixture was filtered and dried in a vacuum oven at room temperature to obtain the supported catalyst. The amount of scandium trifluoromethanesulfonate used was 11 wt% based on the weight of the solid catalyst. Testing showed that the actual supported amount of scandium trifluoromethanesulfonate in the solid catalyst was 10 wt%.

[0159] Example 1

[0160] (1) 400cm 2 The upper surface of the polyethersulfone support layer was contacted with 100 mL of an aqueous solution containing 0.5 wt% piperazine at 25 °C for 60 s, after which the solution was drained. The residual aqueous phase on the surface was then removed using a rubber roller. Next, the upper surface of the support layer was contacted with 50 mL of IsoparE solution containing 0.1 wt% 1,3,5-benzenetricarboxylic acid chloride at 25 °C for 60 s, after which the solution was drained. The membrane was then placed in an oven and heat-treated at 50 °C for 1 min to obtain the composite membrane. The mass ratio of piperazine to 1,3,5-benzenetricarboxylic acid chloride was 10:1.

[0161] (2) At 30°C, the above composite membrane was immersed in an aqueous solution containing 150g of 1.5wt% terminal amino quaternary ammonium salt. After 60s, it was taken out and dried at 50°C for 5 minutes to obtain the positively charged nanofiltration membrane N1 modified with terminal amino quaternary ammonium salt.

[0162] Figure 2 These are the surface infrared spectra of the nanofiltration membranes prepared in Example 1 and Comparative Example 1. Figure 2 It can be seen that, compared with Comparative Example 1, the membrane obtained in Example 1 has a thickness of 966.5 cm⁻¹. -1 The presence of a characteristic peak for quaternary ammonium nitrogen indicates successful modification of the membrane surface with the quaternary ammonium salt. Simultaneously, the enhanced hydroxyl characteristic peak in the membrane obtained in Example 1 further demonstrates the successful modification of the membrane surface with the amino-terminated bis-quaternary ammonium salt.

[0163] Figure 3 These are the fine XPS spectra of surface nitrogen elements in the composite nanofiltration membranes prepared in Example 1 and Comparative Example 1. Figure 3As can be seen, compared with Comparative Example 1, the characteristic peaks corresponding to amide bonds in the fine nitrogen spectrum of the membrane surface obtained in Example 1 are significantly enhanced. This is due to the reaction of amino groups with acyl chloride groups on the membrane surface. On the other hand, characteristic peaks corresponding to quaternary ammonium nitrogen can be observed, further demonstrating the successful modification of the membrane surface by the quaternary ammonium salt.

[0164] Example 2

[0165] (1) 400cm 2 The upper surface of the polyethersulfone support layer was contacted with 100 mL of an aqueous solution containing 0.5 wt% piperazine at 25 °C for 60 s, after which the solution was drained. The residual aqueous phase on the surface was then removed using a rubber roller. Next, the upper surface of the support layer was contacted with 50 mL of IsoparE solution containing 0.1 wt% 1,3,5-benzenetricarboxylic acid chloride at 25 °C for 60 s, after which the solution was drained. The membrane was then placed in an oven and heat-treated at 50 °C for 1 min to obtain the composite membrane. The mass ratio of piperazine to 1,3,5-benzenetricarboxylic acid chloride was 10:1.

[0166] (2) At 30°C, the above composite membrane was immersed in an aqueous solution containing 150g of 2.5wt% terminal amino quaternary ammonium salt. After 60s, it was taken out and dried at 50°C for 5 minutes to obtain the positively charged nanofiltration membrane N2 modified with terminal amino quaternary ammonium salt.

[0167] Example 3

[0168] (1) 400cm 2 The upper surface of the polyethersulfone support layer was contacted with 100 mL of an aqueous solution containing 0.5 wt% piperazine at 25 °C for 60 s, after which the solution was drained. The residual aqueous phase on the surface was then removed using a rubber roller. Next, the upper surface of the support layer was contacted with 50 mL of IsoparE solution containing 0.1 wt% 1,3,5-benzenetricarboxylic acid chloride at 25 °C for 60 s, after which the solution was drained. The membrane was then placed in an oven and heat-treated at 50 °C for 1 min to obtain the composite membrane. The mass ratio of piperazine to 1,3,5-benzenetricarboxylic acid chloride was 10:1.

[0169] (2) At 30°C, the above composite membrane was immersed in an aqueous solution containing 150g of 0.8wt% terminal amino quaternary ammonium salt. After 60s, it was taken out and dried at 50°C for 5 minutes to obtain the positively charged nanofiltration membrane N3 modified with terminal amino quaternary ammonium salt.

[0170] Example 4

[0171] (1) 400cm 2The upper surface of the polyethersulfone support layer was contacted with 100 mL of an aqueous solution containing 0.5 wt% piperazine at 25 °C for 60 s, after which the solution was drained. The residual aqueous phase on the surface was then removed using a rubber roller. Next, the upper surface of the support layer was contacted with 50 mL of IsoparE solution containing 0.1 wt% 1,3,5-benzenetricarboxylic acid chloride at 25 °C for 60 s, after which the solution was drained. The membrane was then placed in an oven and heat-treated at 50 °C for 1 min to obtain the composite membrane. The mass ratio of piperazine to 1,3,5-benzenetricarboxylic acid chloride was 10:1.

[0172] (2) At 30°C, the above composite membrane was immersed in an aqueous solution containing 150g of 1.5wt% terminal amino quaternary ammonium salt. After 60s, it was taken out and dried at 50°C for 5 minutes to obtain the positively charged nanofiltration membrane N4 modified with terminal amino quaternary ammonium salt.

[0173] Example 5

[0174] (1) 400cm 2 The upper surface of the polyethersulfone support layer was contacted with 100 mL of a binary aqueous solution containing 0.25 wt% piperazine and 0.25% homopiperazine at 25 °C for 60 s, after which the solution was drained. The residual aqueous phase on the surface was then removed using a rubber roller. Next, the upper surface of the support layer was contacted with 50 mL of IsoparE solution containing 0.1 wt% 1,3,5-benzenetricarboxylic chloride at 25 °C for 60 s, after which the solution was drained. The membrane was then placed in an oven and heat-treated at 50 °C for 1 min to obtain the composite membrane. The mass ratio of the sum of piperazine and homopiperazine to the mass of 1,3,5-benzenetricarboxylic chloride was 10:1.

[0175] (2) At 30°C, the above composite membrane was immersed in an aqueous solution containing 150g of 1.5wt% terminal amino quaternary ammonium salt. After 60s, it was taken out and dried at 50°C for 5 minutes to obtain the positively charged nanofiltration membrane N5 modified with terminal amino quaternary ammonium salt.

[0176] Example 6

[0177] (1) 400cm 2 The upper surface of the polyethersulfone support layer was contacted with 100 mL of an aqueous solution containing 0.5 wt% piperazine at 25°C for 60 s, after which the solution was drained. The residual aqueous phase on the surface was then removed using a rubber roller. Next, the upper surface of the support layer was contacted with 50 mL of IsoparE solution containing 0.1 wt% terephthaloyl chloride at 25°C for 60 s, after which the solution was drained. The membrane was then placed in an oven and heat-treated at 50°C for 1 min to obtain the composite membrane. The mass ratio of piperazine to terephthaloyl chloride was 10:1.

[0178] (2) At 30°C, the above composite membrane was immersed in an aqueous solution containing 150g of 1.5wt% terminal amino quaternary ammonium salt. After 60s, it was taken out and dried at 50°C for 5 minutes to obtain the positively charged nanofiltration membrane N6 modified with terminal amino quaternary ammonium salt.

[0179] Example 7

[0180] (1) 400cm 2 The upper surface of the polyethersulfone support layer was contacted with 100 mL of an aqueous solution containing 0.5 wt% piperazine at 25°C for 60 s, after which the solution was drained. The residual aqueous phase on the surface was then removed using a rubber roller. Next, the upper surface of the support layer was contacted with 50 mL of IsoparE solution containing 0.05 wt% 1,3,5-benzenetriacyl chloride and 0.05 wt% terephthaloyl chloride at 25°C for 60 s, after which the solution was drained. The membrane was then placed in an oven and heat-treated at 50°C for 1 min to obtain the composite membrane. The mass ratio of piperazine to the sum of the masses of 1,3,5-benzenetriacyl chloride and terephthaloyl chloride was 10:1.

[0181] (2) At 30°C, the above composite membrane was immersed in an aqueous solution containing 150g of 1.5wt% terminal amino quaternary ammonium salt. After 60s, it was taken out and dried at 50°C for 5 minutes to obtain the positively charged nanofiltration membrane N7 modified with terminal amino quaternary ammonium salt.

[0182] Example 8

[0183] (1) 400cm 2 The upper surface of the polyethersulfone support layer was contacted with 100 mL of an aqueous solution containing 0.5 wt% piperazine at 25 °C for 60 s, after which the solution was drained. The residual aqueous phase on the surface was then removed using a rubber roller. Next, the upper surface of the support layer was contacted with 50 mL of IsoparE solution containing 0.1 wt% 1,3,5-benzenetricarboxylic acid chloride at 25 °C for 60 s, after which the solution was drained. The membrane was then placed in an oven and heat-treated at 50 °C for 1 min to obtain the composite membrane. The mass ratio of piperazine to 1,3,5-benzenetricarboxylic acid chloride was 10:1.

[0184] (2) At 30°C, the above composite membrane was immersed in an aqueous solution containing 150g of 1.5wt% terminal amino quaternary ammonium salt. After 30s, it was taken out and dried at 50°C for 5 minutes to obtain the positively charged nanofiltration membrane N8 modified with terminal amino quaternary ammonium salt.

[0185] Example 9

[0186] (1) 400cm 2The upper surface of the polyethersulfone support layer was contacted with 100 mL of an aqueous solution containing 0.5 wt% piperazine at 25 °C for 60 s, after which the solution was drained. The residual aqueous phase on the surface was then removed using a rubber roller. Next, the upper surface of the support layer was contacted with 50 mL of IsoparE solution containing 0.1 wt% 1,3,5-benzenetricarboxylic acid chloride at 25 °C for 60 s, after which the solution was drained. The membrane was then placed in an oven and heat-treated at 50 °C for 1 min to obtain the composite membrane. The mass ratio of piperazine to 1,3,5-benzenetricarboxylic acid chloride was 10:1.

[0187] (2) At 30°C, the above composite membrane was immersed in an aqueous solution containing 150g of 1.5wt% terminal amino quaternary ammonium salt. After 100s, it was taken out and dried at 50°C for 5 minutes to obtain the positively charged nanofiltration membrane N9 modified with terminal amino quaternary ammonium salt.

[0188] Example 10

[0189] (1) 400cm 2 The upper surface of the polyethersulfone support layer was contacted with 100 mL of an aqueous solution containing 0.5 wt% piperazine at 25 °C for 60 s, after which the solution was drained. The residual aqueous phase on the surface was then removed using a rubber roller. Next, the upper surface of the support layer was contacted with 50 mL of IsoparE solution containing 0.2 wt% 1,3,5-benzenetricarboxylic acid chloride at 25 °C for 60 s, after which the solution was drained. The membrane was then placed in an oven and heat-treated at 50 °C for 1 min to obtain the composite membrane. The mass ratio of piperazine to 1,3,5-benzenetricarboxylic acid chloride was 5:1.

[0190] (2) At 30°C, the above composite membrane was immersed in an aqueous solution containing 150g of 1.5wt% terminal amino quaternary ammonium salt. After 60s, it was taken out and dried at 50°C for 5 minutes to obtain the positively charged nanofiltration membrane N10 modified with terminal amino quaternary ammonium salt.

[0191] Example 11

[0192] (1) 400cm 2 The upper surface of the polyethersulfone support layer was contacted with 100 mL of an aqueous solution containing 0.5 wt% piperazine at 25 °C for 60 s, after which the solution was drained. The residual aqueous phase on the surface was then removed using a rubber roller. Next, the upper surface of the support layer was contacted with 50 mL of IsoparE solution containing 0.1 wt% 1,3,5-benzenetricarboxylic acid chloride at 25 °C for 60 s, after which the solution was drained. The membrane was then placed in an oven and heat-treated at 50 °C for 1 min to obtain the composite membrane N11. The mass ratio of piperazine to 1,3,5-benzenetricarboxylic acid chloride was 10:1.

[0193] (2) At 30°C, the above composite membrane was immersed in an aqueous solution containing 150g of 1.5wt% terminal amino quaternary ammonium salt. After 60s, it was taken out and dried at 60°C for 5 minutes to obtain the positively charged nanofiltration membrane N11 modified with terminal amino quaternary ammonium salt.

[0194] Example 12

[0195] (1) 400cm 2 The upper surface of the polyethersulfone support layer was contacted with 100 mL of an aqueous solution containing 0.5 wt% piperazine at 25 °C for 60 s, after which the solution was drained. The residual aqueous phase on the surface was then removed using a rubber roller. Next, the upper surface of the support layer was contacted with 50 mL of IsoparE solution containing 0.1 wt% 1,3,5-benzenetricarboxylic acid chloride at 25 °C for 60 s, after which the solution was drained. The membrane was then placed in an oven and heat-treated at 50 °C for 1 min to obtain the composite membrane. The mass ratio of piperazine to 1,3,5-benzenetricarboxylic acid chloride was 10:1.

[0196] (2) At 30°C, the above composite membrane was immersed in an aqueous solution containing 150g of 5wt% terminal amino quaternary ammonium salt. After 60s, it was taken out and dried at 50°C for 5 minutes to obtain the positively charged nanofiltration membrane N12 modified with terminal amino quaternary ammonium salt.

[0197] Comparative Example 1

[0198] 400cm 2 The upper surface of the polyethersulfone support layer was contacted with 100 mL of an aqueous solution containing 0.5 wt% piperazine at 25 °C for 60 s, after which the solution was drained. The residual aqueous phase on the surface was then removed using a rubber roller. Next, the upper surface of the support layer was contacted with 50 mL of IsoparE solution containing 0.1 wt% 1,3,5-benzenetricarboxylic acid chloride at 25 °C for 60 s, after which the solution was drained. The membrane was then placed in an oven and heat-treated at 50 °C for 1 min to obtain composite membrane D1. The mass ratio of piperazine to 1,3,5-benzenetricarboxylic acid chloride was 10:1.

[0199] Comparative Example 2

[0200] (1) 400cm 2 The upper surface of the polyethersulfone support layer was contacted with 100 mL of an aqueous solution containing 0.5 wt% piperazine at 25 °C for 60 s, after which the solution was drained. The residual aqueous phase on the surface was then removed using a rubber roller. Next, the upper surface of the support layer was contacted with 50 mL of IsoparE solution containing 0.1 wt% 1,3,5-benzenetricarboxylic acid chloride at 25 °C for 60 s, after which the solution was drained. The membrane was then placed in an oven and heat-treated at 50 °C for 1 min to obtain the composite membrane. The mass ratio of piperazine to 1,3,5-benzenetricarboxylic acid chloride was 10:1.

[0201] (2) At 30°C, the above composite membrane was immersed in an aqueous solution containing 150g of 1.5wt% tetrabutylammonium bromide. After 60s, it was taken out and dried at 50°C for 5 minutes to obtain composite membrane D2.

[0202] The thickness, surface Zeta potential, average pore size, nitrogen atom content in the quaternary ammonium salt structure of the polypiperazine amide separation layer, and contact angle of each layer of the composite membrane in the examples and comparative examples are shown in Table 1.

[0203] Table 1

[0204]

[0205] a This refers to the nitrogen atom content in the quaternary ammonium salt structure of the polypiperazine amide separation layer.

[0206] Table 2

[0207]

[0208]

[0209] As shown in Table 1-2, terminal amino bis-quaternary ammonium salts were successfully grafted onto the surface of the polypiperazine amide composite nanofiltration membrane through reaction with the acyl chloride functional groups. The modified composite nanofiltration membrane exhibited a significantly increased zeta potential, a reduced average pore size, a decreased contact angle, and demonstrated high rejection rate and high water flux for divalent (magnesium) metal ions.

[0210] As can be seen from Examples 1-3 and Example 12, different concentrations of terminal amino bis-quaternary ammonium salts, after successful chemical grafting on the membrane surface, significantly improved the magnesium ion retention capacity of the composite nanofiltration membrane.

[0211] In further research of this invention, the concentration of quaternary ammonium salts during the grafting process was analyzed in Examples 1-3. Examples 1-3 show that terminal amino-terminated bis-quaternary ammonium salts can be chemically grafted onto the membrane surface within a certain concentration range, increasing the positive charge density on the nanofiltration membrane surface, thereby improving the rejection rate of divalent and high-valent metal ions by the nanofiltration membrane, while also exhibiting high water flux. Example 12 shows that higher concentrations of quaternary ammonium salts cause a decrease in the flux of the composite membrane.

[0212] Examples 1, 6, and 7 illustrate that different acyl chloride monomers can provide active reaction sites to achieve chemical grafting of terminal amino bis-quaternary ammonium salts.

[0213] Examples 8, 9, and 11 demonstrate that the present invention has high process tolerance and strong prospects for industrial scale-up.

[0214] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A positively charged nanofiltration membrane, characterized in that, The positively charged nanofiltration membrane comprises, in sequence, a bottom layer, a porous intermediate layer, and a polypiperazine amide separation layer; The polypiperazine amide separation layer is a quaternary ammonium salt structure represented by N-atom grafting formula I; Wherein, R1, R2, and R3 are each independently C1-C3 alkylene groups; n1 and n2 are each independently integers from 0 to 3; R4, R5, R6, R7, R8, and R9 are each independently H, C1-C4 alkyl or phenyl groups; and X is F, Cl or Br. * indicates the position where the quaternary ammonium salt structure forms an amide bond with the acyl chloride group in the polypiperazine amide separation layer.

2. The positively charged nanofiltration membrane according to claim 1, wherein, R1, R2, and R3 are ethylene compounds; And / or, n1 and n2 are 1; And / or, R4, R5, R6, R7, R8, and R9 are each independently H or C1-C4 alkyl groups; And / or, X is Cl or Br.

3. The composite nanofiltration membrane according to claim 1 or 2, wherein, The nitrogen atom content in the quaternary ammonium salt groups of the composite nanofiltration membrane is 0.7-5 at.%, preferably 1.5-4 at.%.

4. The composite nanofiltration membrane according to any one of claims 1-3, wherein, The surface zeta potential of the composite nanofiltration membrane is 0-35mV, preferably 5-30mV.

5. The composite nanofiltration membrane according to any one of claims 1-4, wherein, The average pore size of the composite nanofiltration membrane is 0.1-0.4 nm, preferably 0.15-0.3 nm.

6. The composite nanofiltration membrane according to any one of claims 1-5, wherein, The contact angle of the composite nanofiltration membrane is 30-80°, preferably 30-60°.

7. A method for preparing a positively charged nanofiltration membrane, characterized in that, The preparation method includes the following steps: S1. A porous intermediate layer and a polypiperazine amide separation layer are sequentially prepared on the bottom layer to obtain a composite membrane; S2. The composite membrane is contacted with an aqueous solution of the quaternary ammonium salt represented by Formula II and dried to obtain the positively charged nanofiltration membrane; Wherein, R1, R2, and R3 are each independently C1-C3 alkylene groups; n1 and n2 are each independently integers from 0 to 3; R4, R5, R6, R7, R8, and R9 are each independently H, C1-C4 alkyl or phenyl groups; and X is F, Cl, or Br.

8. The preparation method according to claim 7, wherein, In step S2, the concentration of the aqueous solution containing the quaternary ammonium salt of Formula II is 0.1-3 wt%, preferably 0.5-2.5 wt%. Preferably, the contact conditions include a contact time of 5s-600s, more preferably 10s-120s; Preferably, the drying temperature is 40-70℃, and more preferably 50-60℃.

9. The preparation method according to claim 7 or 8, wherein, In step S1, the composite membrane is prepared according to the following steps: (1) Prepare a porous intermediate layer on the bottom layer; (2) The membrane obtained in step (1) is brought into first contact with an aqueous phase containing polyamines; (3) The membrane obtained in step (2) is brought into a second contact with an organic solution containing polyacryl chloride, and after heat treatment, the composite membrane is obtained; The polyamine is selected from at least one of piperazine, hyperpiperazine, and (R)-piperazine-2-carboxylic acid, preferably piperazine.

10. The preparation method according to claim 9, wherein, The concentration of polyamines in the aqueous phase containing polyamines is 0.05-5 wt%, preferably 0.1-2.5 wt%. Preferably, the first contact time is 5s-120s, more preferably 10s-60s; Preferably, the concentration of the polyacryl chloride in the organic solution containing polyacryl chloride is 0.025wt%-1wt%, more preferably 0.05wt%-0.5wt%. Preferably, the polyacryl chloride is selected from 1,3,5-benzotriacryl chloride and / or terephthaloyl chloride; Preferably, the second contact time is 5s-120s, more preferably 10s-60s.

11. A positively charged nanofiltration membrane prepared by the preparation method according to any one of claims 7-10.

12. The application of the positively charged nanofiltration membrane according to any one of claims 1-6 and 11 in lithium extraction from salt lakes.

Images