Acid-resistant nanofiltration membrane and preparation method thereof

Abstract

The invention relates to an acid-resistant nanofiltration membrane and a preparation method thereof, the acid-resistant nanofiltration membrane comprises a porous support layer and an active separation layer formed on the surface of the porous support layer; the active separation layer is a polyamide network layer formed by carrying out interfacial polymerization reaction on a water-phase solution containing a phosphoric acid esterification piperazine derivative and an oil-phase solution containing trimesoyl chloride on the surface of the porous support layer; the phosphate esterification piperazine derivative is a compound in which a phosphate ester group is covalently connected to a nitrogen atom of a piperazine ring through a connecting arm, and the polyamide network layer has reversible hydrogen bond interaction sites introduced by the phosphate esterification piperazine derivative. The membrane has the beneficial effects that a molecular switch which is dynamically adjusted along with the pH is constructed in the membrane, so that the separation performance of the membrane can realize intelligent response along with the change of the environmental pH.

Description

Technical Field

[0001] This invention relates to the field of high-performance separation membrane technology, and in particular to an acid-resistant nanofiltration membrane and its preparation method. Background Technology

[0002] Nanofiltration membrane technology, as a pressure-driven membrane separation process between reverse osmosis and ultrafiltration, has wide applications in water treatment, material separation and concentration, and resource recovery. Especially in strongly acidic environments such as acidic industrial wastewater treatment, metal ion recovery, and acid purification, the demand for acid-resistant nanofiltration membranes is increasingly urgent. However, acidic environments pose a severe challenge to the chemical stability of membrane materials. The amide bonds in traditional polyamide nanofiltration membranes are prone to hydrolytic breakage under strong acid hydrothermal conditions, leading to rapid degradation or even failure of membrane performance.

[0003] To improve the acid resistance of nanofiltration membranes, existing technologies mainly focus on three directions: main chain structure modification, functionalized monomer copolymerization, and interface regulation and post-treatment. However, each approach has limitations to varying degrees. Regarding main chain structure modification, systems such as polysulfonamides, polyureas, and polyarylethers can construct intrinsically stable polymer main chains, which improve acid resistance, but often face a trade-off between flux and selectivity. In terms of functionalized monomer copolymerization, methods such as copolymerizing acid-resistant structural units containing sulfonamide groups or quaternary ammonium salts with piperazine polyamides, or increasing covalent bonds through ultraviolet irradiation, can introduce functional groups, but the process is complex and it's difficult to impart intelligent responsive characteristics. Regarding interface regulation and post-treatment, methods such as introducing an acrylic interlayer to enhance interfacial stability or using a secondary crosslinking process increase production complexity and cost due to the multi-step process. Overall, existing technologies generally suffer from common defects such as insufficient functional durability, static performance rigidity, and difficulty in synergistically improving acid resistance and functionality, making it difficult to simultaneously meet the dual requirements of long-term stable operation and intelligent separation under extreme environments.

[0004] For example, patent CN113509840A discloses a composite nanofiltration membrane and its preparation method. This technology involves adding polyols containing phosphate or sulfonic acid groups and polyamines to an aqueous phase for interfacial polymerization. However, the phosphorus-containing functional components are only dispersed in the polymer matrix through physical blending and do not form stable chemical bonds with the polymer network. During long-term operation in strong acid, the functional groups are at risk of leaching, leading to irreversible degradation of membrane performance over time. Currently, post-crosslinking modification technology first prepares a polyamide nascent membrane through conventional interfacial polymerization, and then immerses it in an acidic crosslinking agent solution for additional post-treatment. This not only increases the difficulty of production control and has poor compatibility with the continuous processes of existing spiral wound membrane production lines, but also suggests that the introduced crosslinked structure may have interfacial weaknesses with the native network, which can easily become the breakthrough point for performance degradation during long-term operation. Functionalized piperazine derivative technology uses piperazine derivatives containing cyclic ethers or hydroxyl groups (such as hydroxyethyl piperazine) as aqueous monomers. While utilizing the hydrophilicity of hydroxyl groups to increase throughput, hydroxyl groups lack reversible ionization properties, failing to achieve pH-responsive adaptive separation. This stems from the fact that the selection of functional groups is limited to conventional hydrophilic modification, failing to design a synergistic mechanism of acid resistance and intelligent response at the molecular level. Furthermore, related research has used acyl chloride monomers containing -PO(Cl)2 groups to introduce phosphorus into polyamide networks. However, the -PO(Cl)2 group has high hydrolytic activity, easily hydrolyzing to form short-chain oligomers during interfacial polymerization, resulting in weak film-forming ability and numerous network structural defects. Using this monomer alone is insufficient to form a complete active separation layer; it must be compounded with highly active acyl chloride monomers, resulting in an extremely narrow process window and significant controllability. Moreover, the phosphorus atom is located in the polymer backbone, and the long-term stability of its PO / C bond under strong acid conditions has not been verified, posing a risk of network degradation.

[0005] While the aforementioned existing technical solutions have made beneficial explorations in the field of acid-resistant nanofiltration membranes, they suffer from the following core common problems: First, the introduction of functional groups mostly adopts physical doping or post-treatment, failing to become covalent components of the polymer network. This leads to the easy leaching or migration of functional groups under harsh environments of strong acid and high flow rates over a long period, resulting in irreversible degradation of membrane performance over time and an inability to guarantee the long-term stability required for industrial applications. Second, the separation performance design exhibits static and rigid characteristics. Once the membrane is manufactured, its separation characteristics cannot be dynamically adjusted according to changes in the chemical conditions of the feed liquid. When faced with complex components and pH fluctuations in actual industrial wastewater, the separation efficiency is limited, lacking the intelligence and adaptability to cope with complex systems. Third, acid resistance and functionality are difficult to coordinate. Existing technologies often sacrifice other functions or simplicity in pursuit of acid resistance. This fragmented design approach makes it difficult to prepare membrane products that simultaneously meet the dual requirements of long-term stable operation in extreme environments and intelligent separation. Summary of the Invention

[0006] (a) Technical problems to be solved

[0007] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention aims to solve the above problems and provide an acid-resistant nanofiltration membrane and its preparation method that introduces phosphate ester groups through covalent bonding, has both ultra-long acid resistance and pH intelligent response, and whose preparation process is compatible with existing spiral wound membrane production lines. It solves the technical problems in the prior art where functional groups are easily lost due to physical doping or post-processing, resulting in insufficient long-term acid resistance and static rigidity of membrane separation performance, which cannot intelligently respond to changes in environmental pH, and it is difficult to balance acid resistance and functionality.

[0008] (II) Technical Solution

[0009] To achieve the above objectives, the main technical solutions adopted by the present invention include:

[0010] In a first aspect, embodiments of the present invention provide an acid-resistant nanofiltration membrane, comprising a porous support layer and an active separation layer formed on the surface of the porous support layer;

[0011] The active separation layer is a polyamide network layer formed by interfacial polymerization of an aqueous solution containing a phosphorylated piperazine derivative and an oil solution containing pyromellitic trichomeoyl chloride on the surface of a porous support layer.

[0012] The phosphorylated piperazine derivative is a compound in which a phosphate group is covalently linked to a nitrogen atom of a piperazine ring via a linker arm, and the polyamide network layer has reversible hydrogen bond interaction sites introduced by the phosphorylated piperazine derivative.

[0013] In a preferred embodiment of the present invention, the phosphorylated piperazine derivative has the following general structural formula:

[0014] (I);

[0015] Wherein, L is a C1-C6 alkylene group, and one or more carbon atoms in the alkylene chain are optionally replaced by -O-, -NH-, -C(O)NH- or -C(O)O-;

[0016] R1 is a hydrogen atom, a C1-C4 alkyl group, or a C1-C4 hydroxyalkyl group;

[0017] R2 and R3 are each independently -OH, -O (C1-C4 alkyl), -O (C1-C4 hydroxyalkyl), or -O - M + M + It is an alkali metal ion or an ammonium ion.

[0018] In a preferred embodiment of the present invention, the L in the phosphated piperazine derivative is a C2-C4 alkylene group, and one carbon atom in the alkylene chain is replaced by -O-.

[0019] In a preferred embodiment of the present invention, the phosphorylated piperazine derivative is N-(2-phosphoethyl)piperazine, with the structural formula as follows:

[0020] (II).

[0021] In a preferred embodiment of the present invention, the porous support layer is an acid-resistant ultrafiltration membrane, and the material is selected from at least one of polysulfone, polyethersulfone, polyacrylonitrile, and polyvinylidene fluoride.

[0022] Secondly, embodiments of the present invention provide a method for preparing an acid-resistant nanofiltration membrane, comprising the following steps:

[0023] S1 provides phosphorylated piperazine derivatives;

[0024] S2. Dissolve the phosphorylated piperazine derivative from step S1 in water to prepare an aqueous solution; dissolve trimesoyl chloride in an organic solvent to prepare an oil solution.

[0025] S3, Interfacial polymerization: The aqueous solution and the oil solution undergo an interfacial polymerization reaction on the surface of the porous support layer to obtain a nascent composite membrane;

[0026] S4, Acid-catalyzed thermosetting: The nascent composite film obtained in step S3 is heat-treated under acidic conditions to further crosslink the phosphate groups in the phosphate-esterified piperazine derivative with the polyamide network, thereby constructing reversible hydrogen bond interaction sites introduced by the phosphate-esterified piperazine derivative in the polyamide network.

[0027] S5. The membrane obtained in step S4 is post-processed to obtain the acid-resistant nanofiltration membrane.

[0028] In a preferred embodiment of the present invention, in step S1, the preparation process of the phosphate-esterified piperazine derivative is as follows: under a protective gas, piperazine or its derivative is reacted with an epoxide to generate a hydroxyalkyl-containing piperazine intermediate; the obtained intermediate is reacted with a phosphorylation reagent, and after hydrolysis, the phosphate-esterified piperazine derivative is obtained; the structure of the phosphate-esterified piperazine derivative includes a piperazine ring, a linking arm, and a phosphate ester group, wherein the linking arm is -L-, L is a C1-C6 alkylene group, and one or more carbon atoms in the alkylene chain are optionally replaced by -O-, -NH-, -C(O)NH-, or -C(O)O-.

[0029] In a preferred embodiment of the present invention, in step S2, the concentration of the phosphorylated piperazine derivative in the aqueous solution is 0.1wt%-5.0wt%; and the concentration of the trimesoyl chloride in the oil solution is 0.05wt%-0.35wt%.

[0030] In a preferred embodiment of the present invention, in step S2, the concentration of the phosphorylated piperazine derivative in the aqueous solution is 1.0wt%-2.0wt%; and the concentration of the trimesoyl chloride in the oil solution is 0.1wt%-0.2wt%.

[0031] In a preferred embodiment of the present invention, in step S2, the organic solvent is selected from n-hexane, cyclohexane, n-heptane, or Isopar G-type isoalkanes.

[0032] In a preferred embodiment of the present invention, in step S3, the aqueous phase solution is poured onto the surface of the porous support layer and allowed to stand for 60s-120s to soak. Excess aqueous phase droplets on the membrane surface are scraped or blown off with a rubber roller or nitrogen knife to form a uniform adsorption liquid film. The oil phase solution is then poured onto the membrane surface immediately and reacted for 10s-60s.

[0033] In a preferred embodiment of the present invention, in step S4, the temperature of the heat treatment is 50°C-90°C and the heat treatment time is 1 min-60 min.

[0034] In a preferred embodiment of the present invention, in step S4, the acidic condition is to immerse the nascent composite membrane in an acid solution; the acid solution is an aqueous solution of an inorganic acid or an organic acid with a pH value of 0-3.

[0035] In a preferred embodiment of the present invention, in step S4, the acid solution is hydrochloric acid, phosphoric acid, nitric acid, p-toluenesulfonic acid, or citric acid.

[0036] In a preferred embodiment of the present invention, in step S4, the concentration of the acid solution is 0.1M-5.0M.

[0037] In a preferred embodiment of the present invention, step S5 includes the following post-processing steps: cooling the membrane to room temperature, rinsing it with deionized water, and storing it in deionized water.

[0038] (III) Beneficial Effects

[0039] The beneficial effects of this invention are as follows: The acid-resistant nanofiltration membrane and its preparation method utilize a phosphorylated piperazine derivative with a specific structure as the polymer monomer. Through the interfacial polymerization reaction between the reactive groups on its piperazine ring and trimesoyl chloride, the phosphate groups are directly anchored to the polyamide backbone via covalent bonds, becoming an inherent component of the network structure. This fundamentally eliminates the risk of functional group loss under harsh environments such as strong acids and high flow rates. During the interfacial polymerization process, the phosphorylated piperazine derivative dissolved in the aqueous phase and the trimesoyl chloride dissolved in the oil phase rapidly undergo interfacial condensation polymerization on the surface of the porous support layer, forming a nascent polyamide network. In this network, the covalently anchored phosphate groups (such as -PO(OH)2) can form reversible intramolecular or intermolecular hydrogen bond interactions with the amide bonds (-CO-NH-) in the polymer chain. The membrane of this invention exhibits excellent long-term stability under extremely harsh conditions: after continuous immersion in a strong acid environment (50°C, 15wt% H2SO4) for 21 days, the nanofiltration membrane of this invention exhibits excellent long-term stability against Mg2+. 2+ The retention rate remains at an excellent level of 99.0%.

[0040] By covalently anchoring phosphate groups to a polyamide network and activating reversible hydrogen bonding interactions between them and neighboring amide groups in subsequent processes, a pH-dynamically adjustable molecular switch is constructed within the membrane. This switching mechanism enables the membrane's separation performance to intelligently respond to changes in ambient pH: In a strongly acidic environment (low pH), the phosphate groups (-PO(OH)2) are protonated and become positively charged, enhancing intermolecular / intramolecular hydrogen bonding with the NH groups in the amide bond. This leads to network contraction and increased positive charge on the membrane surface, facilitating efficient retention of multivalent cations through a combination of steric hindrance and electrostatic repulsion. In a neutral or alkaline environment (high pH), the phosphate groups deprotonate to form -PO3. 2- With a negative charge, hydrogen bonding weakens, the polyamide network expands, and the negative charge on the membrane surface increases. At this point, the membrane separation mechanism relies on size sieving and electrostatic interactions, thus exhibiting differentiated ion selectivity. Mg 2+ / Na + The separation factor α increased sharply to 65.8 as the pH value increased from 2.0 to 4.0, and then decreased as the pH value increased again, exhibiting typical switching characteristics.

[0041] When the linker arm L of a phosphate-esterified piperazine derivative is a C2-C4 alkylene group (especially ethylene or butylene), it provides an appropriate spatial distance between the piperazine ring and the phosphate group. This distance avoids the phosphate group being too close to the piperazine ring, which could interfere with the polymerization reactivity of the piperazine ring and acyl chloride, while also preventing excessive free rotation of functional groups due to an overly long linker arm, which could affect the regularity of the hydrogen bond network. When the carbon atom in the alkylene chain is replaced by heteroatoms such as -O- or -NH-, additional hydrogen bond acceptor or donor sites can be introduced, further enhancing the hydrogen bond interaction density in the polyamide network, thereby improving the sensitivity and adjustable range of the pH response.

[0042] The in-situ acid-catalyzed thermosetting process of this invention involves heat treatment at 50-90°C under acidic conditions after the formation of a nascent polyamide network through interfacial polymerization. This process achieves integrated network strengthening and functional activation through a triple synergistic mechanism: First, under acidic heating conditions, residual organic solvents (n-hexane) and moisture in the nascent polyamide network are further removed, while unreacted groups undergo deep crosslinking, significantly increasing the overall crosslinking density and compactness of the network, thereby greatly enhancing the resistance of the active separation layer to strong acid hydrolysis. Second, the acidic environment, combined with thermal energy, induces in-situ hydrolysis of covalently anchored phosphate groups, transforming them into pH-responsive phosphate groups (-PO(OH)2 or their ionic form). These newly generated phosphate groups form stable, reversible hydrogen bond interactions with neighboring amide groups in the network, constructing a pH-dynamically adjustable molecular switch and firmly integrating it into the polyamide network. Third, film formation and functionalization are combined, enabling network strengthening and intelligence to be synergistically achieved in the same key step. Compared to existing post-crosslinking modification technologies, which involve secondary crosslinking with an external crosslinking agent on the basis of an already formed polyamide nascent film, attempting to compensate for the defects of the original network with the additional crosslinked structure, the acid-catalyzed thermosetting process of this invention requires no external crosslinking agent. It utilizes only the unreacted functional groups and covalently bonded phosphate groups in the nascent film to achieve a synergistic integration of network strengthening and functional activation under acidic heating conditions.

[0043] The preparation method of this invention is highly compatible with existing spiral wound membrane production lines, achieving the aforementioned performance breakthroughs without complex post-processing steps or special equipment. The resulting membrane product exhibits both long-term stability and intelligent separation characteristics in strongly acidic environments, making it widely applicable to harsh scenarios such as acidic industrial wastewater treatment, metal ion recovery, hydrometallurgy, and acid purification, demonstrating significant industrial application value and economic benefits.

[0044] In summary, this invention achieves a triple leap from physical addition to chemical bonding, from static separation to intelligent response, and from process superposition to in-situ integration, successfully constructing a high-performance nanofiltration membrane that combines ultra-long acid resistance stability, intelligent pH response, and industrial feasibility. Detailed Implementation

[0045] To better explain and facilitate understanding of the invention, specific embodiments are described in detail below.

[0047] Example 1:

[0048] This embodiment provides a method for preparing an acid-resistant nanofiltration membrane, including the following steps:

[0049] (1) Preparation of phosphate-esterified piperazine derivative: Dissolve piperazine (1 mol) in an appropriate amount of deionized water (about 500 mL), stir and cool to 15-20℃, and slowly pass ethylene oxide (1.1 mol) under nitrogen protection, controlling the rate of introduction so that the reaction temperature does not exceed 35℃. After the passage is complete, stir and react at 30-35℃ for 2 hours, then heat to distill off 90% of the original amount of water, cool to below 15℃, filter to remove unreacted piperazine, and perform vacuum distillation on the filtrate, collecting the fraction at 145-155℃ / 5.33 kPa to obtain N-(2-hydroxyethyl)piperazine intermediate; dissolve the obtained intermediate (1 mol) in anhydrous ethanol (300 mL), add an alkaline catalyst (0.05 mol potassium tert-butoxide), and slowly add diphosphate phosphate dropwise. Ethyl phosphate (1.05 mol) was added, and the dropping rate was controlled to keep the reaction temperature below 40°C (potassium tert-butoxide was used as a basic catalyst to activate the hydroxyl groups of the piperazine intermediate and promote the esterification reaction between the hydroxyl groups and diethyl phosphate). After the addition was complete, the mixture was heated to 70°C and refluxed for 5 hours. After the reaction was completed, ethanol and excess diethyl phosphate were removed by vacuum distillation to obtain N-(2-(diethylphosphooxy)ethyl)piperazine. The crude product was dissolved in 2M hydrochloric acid and heated to 50°C for 2 hours to hydrolyze the product, converting diethyl phosphate to monoester (-PO(OH)2). The solution was neutralized to pH≈7 with saturated NaHCO3 solution, extracted with dichloromethane, and the organic phases were combined, dried over anhydrous sodium sulfate, and the solvent was removed by vacuum distillation to obtain the target product N-(2-ethyl phosphate)piperazine, whose structural formula is:

[0050] (II).

[0051] (2) Preparation of aqueous solution: Dissolve the phosphorylated piperazine derivative synthesized in step (1) in deionized water, stir evenly, and prepare a 1.0 wt% aqueous solution.

[0052] (3) Preparation of oil phase solution: Dissolve trimesoyl chloride (TMC) in n-hexane, stir evenly, and prepare a 0.15wt% oil phase solution.

[0053] (4) Provide a porous support base membrane: Use a polysulfone ultrafiltration membrane (molecular weight cutoff 50kDa) as a porous support base membrane, and after thoroughly washing with deionized water, fix it on a flat substrate.

[0054] (5) Interfacial polymerization: Pour the aqueous solution prepared in step (2) onto the surface of the porous support base membrane, ensuring that the solution completely covers the membrane surface, let it stand for 60 seconds, scrape off the excess aqueous droplets on the membrane surface with a rubber roller to form a uniform adsorption liquid membrane; immediately pour the oil phase solution prepared in step (3) onto the membrane surface at room temperature to completely cover it, let it stand for 30 seconds, during which the interfacial polymerization reaction occurs rapidly to obtain the nascent composite membrane.

[0055] (6) Acid-catalyzed thermosetting: Pour off the oil phase solution on the membrane surface, immerse the nascent composite membrane in a 1.0 M H2SO4 solution, and heat-treat at 70 °C for 5 minutes. During this process, the phosphate groups in the phosphate-esterified piperazine derivative undergo further cross-linking with the polyamide network, constructing reversible hydrogen bond interaction sites in the network.

[0056] (7) Post-treatment and storage: Remove the heat-treated membrane, cool it to room temperature, and rinse it with plenty of deionized water to remove residual reagents. Finally, immerse the membrane in deionized water and store it at 4°C for later use.

[0057] The resulting acid-resistant nanofiltration membrane is designated as E-1.

[0058] Example 2

[0059] This embodiment provides an acid-resistant nanofiltration membrane. The difference between this embodiment and Example 1 is that: Step (1) Preparation of phosphorylated piperazine derivative: The same preparation method as in Example 1 is used, except that ethylene oxide is replaced with epichlorohydrin. The target product is N-(2-hydroxy-3-phosphate propionic acid)piperazine, and its structural formula is:

[0060] (III).

[0061] The remaining steps are the same, denoted as E-2.

[0062] Example 3

[0063] This embodiment provides an acid-resistant nanofiltration membrane. The difference between this embodiment and Example 1 is that: Step (1) Preparation of phosphate-esterified piperazine derivative: The same preparation method as in Example 1 is used, except that ethylene oxide is replaced with 1,2-epoxybutane. The target product is N-(2-butyl phosphate)piperazine, and its structural formula is:

[0064] (IV).

[0065] The remaining steps are the same, denoted as E-3.

[0066] Example 4

[0067] This embodiment provides an acid-resistant nanofiltration membrane. The difference between this embodiment and Example 1 is that: Step (1) Preparation of phosphorylated piperazine derivative: The same preparation method as in Example 1 is used, except that ethylene oxide is replaced with 1,2-epoxypentane. The target product is N-(2-pentyl phosphate)piperazine, and its structural formula is:

[0068] (V).

[0069] The remaining steps are the same, denoted as E-4.

[0070] Example 5

[0071] This embodiment provides an acid-resistant nanofiltration membrane. The difference between this embodiment and Example 1 is that: Step (1) Preparation of phosphate-esterified piperazine derivative: The same preparation method as in Example 1 is used, except that piperazine is replaced with N-methylpiperazine. The target product is N-methyl-N'-(2-phosphate ethyl)piperazine (R1=CH3), and its structural formula is:

[0072] (VI).

[0073] The remaining steps are the same, denoted as E-5.

[0074] Example 6

[0075] This embodiment provides an acid-resistant nanofiltration membrane. The difference between this embodiment and Example 1 is that: Step (1) Preparation of phosphorylated piperazine derivative: The same preparation method as in Example 1 is used, except that piperazine is replaced with N-hydroxyethyl piperazine. The target product is N-hydroxyethyl-N'-(2-phosphoethyl)piperazine (R1=CH2OH), and its structural formula is:

[0076] (VII).

[0077] The remaining steps are the same, denoted as E-6.

[0078] Example 7

[0079] This embodiment provides an acid-resistant nanofiltration membrane. The difference between this embodiment and Embodiment 1 is that the phosphate-esterified piperazine derivative in step (1) is different. It is N-(2-(diethylphosphooxy)ethyl)piperazine, with the structure shown in VIII. Its preparation method is as follows: piperazine (1 mol) is dissolved in an appropriate amount of deionized water (about 500 mL), stirred and cooled to 15-20°C. Under nitrogen protection, ethylene oxide (1.1 mol) is slowly introduced, and the introduction rate is controlled so that the reaction temperature does not exceed 35°C. After the introduction is completed, the reaction is stirred at 30-35°C for 2 hours. Then, 90% of the original amount of water is evaporated by heating, and cooled to 15°C. Unreacted piperazine was removed by filtration, and the filtrate was distilled under reduced pressure. The fraction collected at 145-155℃ / 5.33kPa was used to obtain N-(2-hydroxyethyl)piperazine intermediate. The obtained intermediate (1 mol) was dissolved in anhydrous ethanol (300 mL), and an alkaline catalyst (0.05 mol potassium tert-butoxide) was added. Diethyl phosphate (1.05 mol) was slowly added dropwise, and the dropping rate was controlled so that the reaction temperature did not exceed 40℃. After the addition was complete, the mixture was heated to 70℃ and refluxed for 5 hours. After the reaction was completed, ethanol and excess diethyl phosphate were removed by vacuum distillation to obtain N-(2-(diethylphosphooxy)ethyl)piperazine.

[0080] (VIII).

[0081] The remaining steps are the same, denoted as E-7.

[0082] Example 8

[0083] This embodiment provides an acid-resistant nanofiltration membrane. The difference between this embodiment and embodiment 1 is that: step (6) acid-catalyzed thermal curing: H2SO4 is replaced with HCl, and the rest of the steps are the same, denoted as E-8.

[0084] Example 9

[0085] This embodiment provides an acid-resistant nanofiltration membrane. The difference between this embodiment and embodiment 1 is that: step (6) acid-catalyzed thermal curing: H2SO4 is replaced with H3PO4, and the remaining steps are the same, denoted as E-9.

[0086] Example 10

[0087] This embodiment provides an acid-resistant nanofiltration membrane. The difference between this embodiment and embodiment 1 is that: step (6) acid-catalyzed thermal curing: H2SO4 is replaced with p-toluenesulfonic acid, and the remaining steps are the same, denoted as E-10.

[0088] Example 11

[0089] This embodiment provides an acid-resistant nanofiltration membrane. The difference between this embodiment and embodiment 1 is that: step (6) acid-catalyzed thermal curing: the heat treatment time is 1 minute, and the other steps are the same, denoted as E-11.

[0090] Example 12

[0091] This embodiment provides an acid-resistant nanofiltration membrane. The difference between this embodiment and embodiment 1 is that: step (6) acid-catalyzed thermal curing: the heat treatment time is 15 minutes, and the other steps are the same, denoted as E-12.

[0092] Example 13

[0093] This embodiment provides an acid-resistant nanofiltration membrane. The difference between this embodiment and embodiment 1 is that: step (6) acid-catalyzed thermal curing: the heat treatment time is 30 minutes, and the other steps are the same, denoted as E-13.

[0094] Example 14

[0095] This embodiment provides an acid-resistant nanofiltration membrane. The difference between this embodiment and embodiment 1 is that: step (6) acid-catalyzed thermal curing: the heat treatment time is 60 minutes, and the other steps are the same, denoted as E-14.

[0096] Comparative Example 1

[0097] This comparative example provides an acid-resistant nanofiltration membrane. The difference between this comparative example and Example 1 is that the phosphate-esterified piperazine derivative is not added and piperazine is not used, and acid catalytic curing is not performed. Specifically, step (2) prepares an aqueous solution: dissolve piperazine in deionized water, stir evenly, and prepare a 0.5wt% aqueous solution.

[0098] Step (6) Heat treatment: Pour off the oil phase solution on the surface of the membrane, put the membrane into an oven, and heat treat it at 70°C for 5 minutes.

[0099] The remaining steps are the same, denoted as C-1.

[0100] Comparative Example 2

[0101] This comparative example provides an acid-resistant nanofiltration membrane. The difference between this comparative example and Example 1 is that acid catalytic curing is not performed. Specifically, step (6) heat treatment: pour off the oil phase solution on the membrane surface, put the membrane into an oven, and heat treat it at 70°C for 5 minutes.

[0102] The remaining steps are the same, denoted as C-2.

[0103] Comparative Example 3

[0104] This comparative example provides an acid-resistant nanofiltration membrane. The difference between this comparative example and Example 1 is that the membrane is physically doped and not acid-catalyzed for curing. Specifically, step (2) is to prepare an aqueous solution: piperazine and glycerophosphate (as a phosphorus-containing additive) are dissolved together in deionized water and stirred evenly to prepare an aqueous solution. The concentration of piperazine in the aqueous solution is 1.0 wt%, and the amount of glycerophosphate added is equal to the number of moles of phosphorus in the phosphoric acid esterified piperazine derivative in Example 1. The glycerophosphate used in this example does not have a piperazine ring structure and cannot undergo interfacial polymerization with trimesoyl chloride. It is only dispersed in the polyamide network through physical action, so it is easily lost in a strong acid environment and cannot form reversible hydrogen bond interaction sites.

[0105] Step (6) Heat treatment: Pour off the oil phase solution on the surface of the membrane, put the membrane into an oven, and heat treat it at 70°C for 5 minutes.

[0106] The remaining steps are the same, and are denoted as C-3.

[0107] Verification Example 1

[0108] The acid-resistant nanofiltration membranes prepared in Examples 1-14 and Comparative Examples 1-3 were subjected to systematic performance tests. The specific test methods are as follows:

[0109] (1) Long-term acid resistance test: The membrane samples were immersed in a 50℃, 15wt% H2SO4 solution for accelerated aging test. The samples were taken out periodically (up to 21 days) and the acid resistance of each sample to MgSO4 was measured under standard conditions (1.0MPa, feed solution 2000ppm MgSO4, pH≈7). 2+ Retention rate is used to evaluate the long-term acid resistance stability of the membrane.

[0110] (2) Extreme strong acid instantaneous separation performance test: Under conditions of 3.0 MPa and 25℃, the feed solution was a mixed solution containing 15 wt% H2SO4 and 2000 ppm MgSO4. The Mg content of the membrane was measured. 2+ Retention rate, acid permeability and water flux (LMH).

[0111] (3) pH-responsive intelligent separation characteristic test: Equimolar MgCl2 / NaCl mixed solutions (500 ppm each) under different pH conditions were used to test the MgCl2 content of each membrane sample at different pH values ​​under 1.0 MPa. 2+ / Na + Separation factor α (Mg 2+ / Na + To evaluate the pH-responsive intelligent separation characteristics of the membrane.

[0112] The test results of the membranes prepared in Examples 1-14 and Comparative Examples 1-3 are shown in Tables 1 to 4.

[0113] Table 1:

[0114]

[0115] The peak pH response values ​​of E-11 to E-14 were all between 60 and 65, which was not significantly different from E-1. The heat treatment time had no significant effect on pH responsiveness, but only affected the degree of network crosslinking and water flux.

[0116] Table 2: Test Results of pH-Response Intelligent Separation Characteristics

[0117]

[0118] Table 3: Results of long-term acid resistance test after immersion in 50℃, 15wt% H2SO4 solution

[0119]

[0120] Table 4: Test results of instantaneous separation performance of extreme strong acid at 3.0 MPa and 25℃

[0121]

[0122] Verification Example 2

[0123] The chemical structure of the acid-resistant nanofiltration membranes prepared in Example 1, Comparative Example 1 and Comparative Example 2 was characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) to verify the introduction of functional monomers, hydrolysis and transformation of ester groups and the formation of hydrogen bond interactions. The test results are shown in Table 5.

[0124] Table 5:

[0125]

[0126] Verification Example 3

[0127] This verification example uses a small-scale continuous coating line to simulate the industrial spiral wound membrane production process, verifying the compatibility of the preparation method of this invention with existing membrane production lines and the feasibility of process scale-up. The specific steps are as follows:

[0128] (1) Continuous production line setup: A laboratory-scale continuous coating equipment was adopted, which includes the following stations: unwinding unit, aqueous phase coating unit, hot air drying unit, oil phase coating unit, reaction zone, acid curing bath, water washing tank, drying unit, and rewinding unit. The polysulfone supported film (molecular weight cutoff 50kDa) was operated in continuous roll form with a linear speed set at 0.5m / min. This linear speed is consistent with the conventional linear speed of existing industrial roll film production lines, demonstrating the industrial compatibility of the process of this invention.

[0129] (2) Aqueous phase coating: The phosphorylated piperazine derivative synthesized according to step (1) of Example 1 was used as the functional monomer to prepare a 1.0 wt% aqueous phase solution. The polysulfone support membrane was continuously passed through the aqueous phase coating tank, and the aqueous phase solution was uniformly adsorbed on the membrane surface by immersion coating, with a residence time of about 30 seconds. Then, excess droplets on the surface were removed by squeezing with a rubber roller, and the membrane was put into the hot air drying zone (60°C, residence time 20 seconds) to form a uniform aqueous phase liquid film.

[0130] (3) Oil phase coating and interfacial polymerization: Trimethylbenzene chloride (TMC) was dissolved in n-hexane to prepare a 0.15 wt% oil phase solution. The membrane after aqueous phase coating was continuously passed through an oil phase coating tank to ensure that the oil phase solution uniformly covered the membrane surface. The length of the reaction zone was controlled so that the residence time of the membrane in the oil phase was 30 seconds, during which a nascent polyamide active layer was formed.

[0131] (4) Acid-catalyzed thermal curing: After interfacial polymerization, the membrane is continuously passed through an acid curing bath containing a 1.0M sulfuric acid solution, with the temperature controlled at 70℃. The heat treatment time of the membrane in the acid solution is controlled to be 5 minutes by adjusting the length of the bath and the linear speed of the membrane.

[0132] (5) Post-treatment and winding: After acid curing, the membrane is continuously passed through a water washing tank and thoroughly rinsed with deionized water to remove residual reagents from the membrane surface. Then it enters the hot air drying zone (50℃, residence time 30 seconds) for drying, and finally is wound up by the winding unit. Three batches were prepared continuously under the same process conditions, and the resulting acid-resistant nanofiltration membranes were designated as Y-1, Y-2 and Y-3, respectively.

[0133] (6) Verification of process stability and performance repeatability: The above three batches of membrane samples (Y-1, Y-2, Y-3) were tested for performance, including: initial Mg²⁺ rejection rate, Mg²⁺ rejection rate after 21 days, and Mg²⁺ / Na⁺ separation factor α under pH 4.0 conditions. The test results are shown in Table 6.

[0134] Meanwhile, the performance of continuously prepared membranes Y-1 to Y-3 was compared with that of laboratory-scale membrane E-1 to evaluate the performance retention during process scale-up.

[0135] Table 6:

[0136]

[0137] Referring to the performance test results in Tables 1 to 4, the following analytical conclusions can be drawn:

[0138] A comparison of E-1, E-2, E-3 and E-4 shows that the length of the connecting arm chain significantly affects the pH response sensitivity. E-1 (L=C2H4O) and E-3 (L=C4H8O) have the best performance, especially E-3 with a peak selectivity of up to 70.2.

[0139] A comparison of E-1, E-5, and E-6 shows that when R1 is hydroxymethyl, hydrophilicity is enhanced, flux increases to 68 LMH, and the pH response peak remains at 62.1, with only a slight decrease. However, when R1 is methyl, due to increased hydrophobicity, both flux and pH response decrease significantly (peak value 58.3). This demonstrates that hydrophilic groups (such as hydroxymethyl) can be preferred for R1 to optimize permeation performance while maintaining excellent pH response characteristics.

[0140] A comparison of E-1 and E-7 shows that E-7 and E-1 are highly similar in key performance indicators. Initial retention rates: E-7 (99.2%) and E-1 (99.4%) are comparable; retention rates after 21 days of aging: E-7 (98.8%) and E-1 (99.0%) are comparable. The phosphate groups (-PO(OEt)2) in E-7 are also firmly anchored in the network without loss; pH-responsive separation factors: E-7 (64.5) and E-1 (65.8) are highly similar, proving that E-7 constructs a complete hydrogen bond network and molecular switch. It is evident that the phosphate-esterified piperazine derivative of this invention, regardless of whether it is introduced in acidic form (such as -PO(OH)2) or ester form (-PO(OEt)2), can be completely hydrolyzed into the active form after acid-catalyzed thermosetting process, resulting in a highly consistent microstructure and macroscopic properties of the final membrane.

[0141] A comparison of E-1, E-8, E-9 and E-10 shows that both inorganic and organic acids can achieve effective curing, with a retention rate of >97.5%, indicating that the acid-catalyzed thermal curing process has universality.

[0142] A comparison of E-1, E-11, E-12, E-13, and E-14 shows that when the heat treatment time is ≥5 minutes, the membrane retention rate remains above 99.0% after 21 days of strong acid aging, with a degradation rate ≤0.4%. When the heat treatment time is extended from 1 minute to 5 minutes, the initial retention rate increases from 98.8% to 99.4%, while the water flux decreases from 55 to 48. This is a normal trade-off effect caused by the increased network cross-linking degree. A heat treatment time of 5-30 minutes can achieve a good balance between stability and water flux.

[0143] A comparison between E-1 and C-1 shows that: E-1 has Mg 2+ / Na + The separation factor α increased sharply to 65.8 as the pH value increased from 2.0 to 4.0, and then decreased again with further pH increases, exhibiting typical intelligent switching characteristics. In contrast, the α of comparative example C-1 remained below 5 throughout. This demonstrates that the present invention achieves a leap from static to intelligent switching. After soaking in 50℃ and 15% H2SO4 for 21 days, the Mg of E-1... 2+The retention rate remained at 99.0%, while the C-1 of conventional piperazine membranes without phosphate functional monomers had decreased to 19.0%, demonstrating an order-of-magnitude improvement in stability. This invention fundamentally solves the problem of functional group loss through a covalent bonding strategy.

[0144] A comparison of E-1 (acid-cured) and C-2 (acid-free cured) shows that E-1 maintained a retention rate of 99.0% after 21 days of aging, significantly higher than C-2's 89.0%, demonstrating that acid curing substantially improved the network crosslinking density and structural stability. Furthermore, only E-1 exhibited significant pH responsiveness (peak α = 65.8), while C-2 lacked this characteristic, proving that acid curing is a necessary condition for activating and stabilizing the reversible hydrogen bond molecular switching between phosphate ester groups and amide groups. This demonstrates that the present invention synergistically solves two major challenges through a single key process.

[0145] A comparison between E-1 and C-3 demonstrates the fundamental advantage of chemical bonding over physical doping, improving stability by an order of magnitude (99.0% vs 63.5% after 21 days). Furthermore, only chemically bonded structures can achieve pH-intelligent response (peak value of 65.8 vs no peak value). The physically doped film C-3 exhibits separation factors below 7.0 at all pH levels and shows no significant peak value, proving its lack of pH-intelligent response characteristics. This contrasts sharply with the peak selectivity of the present invention's film E-1, which reaches 65.8 at pH 4.0, further confirming that chemical bonding, rather than physical doping, is a necessary condition for achieving intelligent response.

[0146] Based on the ATR-FTIR test results in Table 5, the following conclusions can be drawn:

[0147] Acid curing treatment enhanced the FTIR characteristic peaks of the phosphate ester groups and broadened and strengthened the absorption peaks in the hydrogen bonding region, indicating that acid curing promoted the formation and stabilization of hydrogen bonds between the phosphate ester groups and the amide groups in the network. These structural changes provided direct spectroscopic evidence for the molecular switching mechanism, confirming that the acid-catalyzed thermocuring step not only strengthened the network structure but also activated and locked the reversible hydrogen bond network between the phosphate ester and the amide groups in situ.

[0148] Referring to the performance test results in Table 6, the following analytical conclusions can be drawn:

[0149] Three batches of membranes Y-1~Y-3 prepared continuously were subjected to 21 days of strong acid aging, and Mg... 2+ The retention rates remained above 98.7%, with batch-to-batch fluctuations less than ±0.3%, demonstrating the good stability and controllability of the preparation process of this invention. The three batches of membranes under pH 4.0 conditions showed Mg... 2+ / Na +The separation factor α was consistently between 62.9 and 64.1, demonstrating that the pH-responsive molecular switch constructed in this invention can be stably reproduced in continuous production. The average performance of the continuously prepared membranes Y-1 to Y-3 was highly similar to that of the laboratory-scale membrane E-1, proving that the preparation method of this invention is highly compatible with existing spiral wound membrane production lines and can meet the requirements of large-scale industrial production.

[0150] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An acid-resistant nanofiltration membrane, characterized in that, It includes a porous support layer and an active separation layer formed on the surface of the porous support layer; The active separation layer is a polyamide network layer formed by interfacial polymerization of an aqueous solution containing a phosphorylated piperazine derivative and an oil solution containing pyromellitic trichomeoyl chloride on the surface of a porous support layer. The phosphorylated piperazine derivative is a compound in which a phosphate group is covalently linked to a nitrogen atom of a piperazine ring via a linker arm, and the polyamide network layer has reversible hydrogen bond interaction sites introduced by the phosphorylated piperazine derivative.

2. The acid-resistant nanofiltration membrane as described in claim 1, characterized in that, The phosphorylated piperazine derivative has the following general structural formula: （I）； Wherein, L is a C1-C6 alkylene group, and one or more carbon atoms in the alkylene chain are optionally replaced by -O-, -NH-, -C(O)NH- or -C(O)O-; R1 is a hydrogen atom, a C1-C4 alkyl group, or a C1-C4 hydroxyalkyl group; R2 and R3 are each independently -OH, -O (C1-C4 alkyl), -O (C1-C4 hydroxyalkyl), or -O - M + M + It is an alkali metal ion or an ammonium ion.

3. The acid-resistant nanofiltration membrane as described in claim 2, characterized in that, The L in the phosphated piperazine derivative is a C2-C4 alkylene group, and one carbon atom in the alkylene chain is replaced by -O-.

4. The acid-resistant nanofiltration membrane as described in claim 3, characterized in that, The phosphorylated piperazine derivative is N-(2-phosphoethyl)piperazine, with the following structural formula: (II)。 5. The acid-resistant nanofiltration membrane as described in claim 2, characterized in that, The porous support layer is an acid-resistant ultrafiltration membrane, and the material is selected from at least one of polysulfone, polyethersulfone, polyacrylonitrile, and polyvinylidene fluoride.

6. A method for preparing an acid-resistant nanofiltration membrane, characterized in that, Includes the following steps: S1 provides phosphorylated piperazine derivatives; S2. Dissolve the phosphorylated piperazine derivative from step S1 in water to prepare an aqueous solution; dissolve trimesoyl chloride in an organic solvent to prepare an oil solution. S3, Interfacial polymerization: The aqueous solution and the oil solution undergo an interfacial polymerization reaction on the surface of the porous support layer to obtain a nascent composite membrane; S4, Acid-catalyzed thermosetting: The nascent composite film obtained in step S3 is heat-treated under acidic conditions to further crosslink the phosphate groups in the phosphate-esterified piperazine derivative with the polyamide network, thereby constructing reversible hydrogen bond interaction sites introduced by the phosphate-esterified piperazine derivative in the polyamide network. S5. The membrane obtained in step S4 is post-processed to obtain the acid-resistant nanofiltration membrane.

7. The preparation method according to claim 6, characterized in that, In step S1, the preparation process of the phosphorylated piperazine derivative is as follows: under a protective gas, piperazine or its derivative is reacted with an epoxide to generate a hydroxyalkyl-containing piperazine intermediate, the obtained intermediate is reacted with a phosphorylation reagent, and after hydrolysis, the phosphorylated piperazine derivative is obtained; the structure of the phosphorylated piperazine derivative includes a piperazine ring, a linking arm and a phosphate ester group, wherein the linking arm is -L-, L is a C1-C6 alkylene group, and one or more carbon atoms in the alkylene chain are optionally replaced by -O-, -NH-, -C(O)NH- or -C(O)O-.

8. The preparation method according to claim 6, characterized in that, In step S2, the concentration of the phosphorylated piperazine derivative in the aqueous solution is 0.1wt%-5.0wt%; the concentration of the trimesoyl chloride in the oil solution is 0.05wt%-0.35wt%.

9. The preparation method according to claim 6, characterized in that, In step S4, the heat treatment temperature is 50℃-90℃, and the heat treatment time is 1min-60min.

10. The preparation method according to claim 6, characterized in that, In step S4, the acidic condition is to immerse the nascent composite membrane in an acid solution; the acid solution is an aqueous solution of inorganic or organic acid with a pH value of 0-3.

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