A monovalent ion-selective cation exchange membrane, its preparation method and application

Abstract

The present application relates to a kind of monovalent ion selective cation exchange membrane and its preparation method and application, the preparation method includes the following steps: mixing polyethylene imine and crosslinking agent, obtain pre-polymer solution;The pre-polymer solution is coated on the surface of cation base film, then heated and crosslinked to obtain the base film after surface crosslinking;The base film after surface crosslinking is soaked in the oil phase solution of acyl chloride monomer, and interface polymerization is carried out to obtain the monovalent ion selective cation exchange membrane.The three-dimensional network structure constructed by crosslinking reaction and the selective separation layer constructed by interface polymerization synergistic effect can improve Li + / Mg 2+ Separation coefficient, and long-term stable operation.

Description

Technical Field

[0001] This invention relates to the field of cation exchange membrane technology, specifically to a monovalent ion selective cation exchange membrane, its preparation method, and its application. Background Technology

[0002] Ion exchange membranes are a class of functional polymer materials capable of selectively permeating specific ions, and their performance directly affects separation efficiency and energy consumption. In recent years, with the rapid development of lithium resource extraction, industrial wastewater treatment, and clean energy technologies, the demand for ion exchange membranes with high selectivity for monovalent / divalent cations has been increasing.

[0003] Traditional homogeneous cation exchange membranes, while possessing high ion conductivity, are limited in their ability to conduct monovalent cations (such as Na+). + Li + ) and divalent cations (such as Mg) 2+ Ca 2+ The selectivity of heterogeneous ion exchange membranes is limited, making it difficult to meet the requirements of high-precision separation. While the selectivity of heterogeneous ion exchange membranes can be improved by doping with inorganic fillers, problems such as decreased mechanical strength and insufficient long-term operational stability arise after doping. For example, CN109157991A discloses a side-chain quaternized polyaniline monovalent selective cation exchange membrane and its preparation method. This method uses quaternized polyaniline to prepare the cation exchange membrane, but its crosslinking process is complex and lacks acid and alkali resistance. In addition, although organic-inorganic hybrid membranes (such as MOF-doped membranes) can improve selectivity through pore size sieving effect, interface defects are prone to occur during the preparation process, leading to a significant increase in membrane resistance and a decrease in selectivity.

[0004] In summary, the existing problems with cation exchange membranes include: (1) insufficient selectivity of monovalent / divalent cations (separation factor <10), which makes it difficult to meet the requirements of selective extraction of lithium from salt lake brine and battery leachate and wastewater treatment; (2) existing modification methods (such as surface grafting and inorganic doping) often lead to increased membrane resistance or deterioration of mechanical properties; and (3) the existing preparation process is complex (such as multi-step crosslinking and high-temperature sulfonation), which restricts large-scale production and application.

[0005] Therefore, providing a monovalent ion-selective cation exchange membrane and its preparation method is a technical problem that needs to be solved in this field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a monovalent ion-selective cation exchange membrane, its preparation method, and its applications. Compared with existing technologies, the present invention, through the synergistic effect of the three-dimensional network structure constructed by cross-linking reaction and the selective separation layer constructed by interfacial polymerization, can improve the Li-ion exchange capacity while maintaining a low membrane resistance. + / Mg2+ It has a high separation coefficient and a stable structure, enabling long-term stable operation.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a monovalent ion-selective cation exchange membrane, the method comprising the following steps:

[0009] A prepolymer solution is obtained by mixing polyethyleneimine and a crosslinking agent;

[0010] The prepolymer solution is coated onto the surface of a cationic base film, and then heated to carry out a crosslinking reaction to obtain a crosslinked base film.

[0011] The cross-linked base film is immersed in an oil phase solution of acyl chloride monomers to carry out an interfacial polymerization reaction, thereby obtaining the monovalent ion selective cation exchange membrane.

[0012] The preparation method provided by this invention constructs a three-dimensional network structure using polyethyleneimine and a crosslinking agent, and further modulates ion channels through interfacial polymerization, thereby optimizing the structure and performance of the membrane material. Specifically, polyethyleneimine and the crosslinking agent form a primary crosslinking framework, while acyl chloride undergoes interfacial polymerization to form a selective functional layer. The two work synergistically to achieve an organic combination of a supporting network and selective separation at the molecular level. The primary crosslinking framework provides the basis for mechanical stability, while the selective functional layer achieves selective ion transport by precisely controlling the surface charge density and pore size distribution. The preparation method provided by this invention simplifies the traditional multi-step crosslinking process to a simple coating-immersion process, with mild and controllable reaction conditions, significantly reducing energy consumption and production costs.

[0013] In this invention, the coating method can be any coating method commonly used in the art for film surface modification, such as spraying or scraping.

[0014] In this invention, the preparation method of the prepolymer solution is a conventional method in the art, such as using water as a solvent to dissolve polyethyleneimine and crosslinking agent in deionized water and stirring for 30-60 minutes.

[0015] In this invention, the cationic base membrane is a homogeneous cationic base membrane, and its specific composition is not particularly limited. Any cationic base membrane commonly used in the art can be used, such as polysulfone base membrane, polyethersulfone base membrane, polyacrylonitrile base membrane, etc.

[0016] Preferably, the crosslinking agent includes at least one of epoxy crosslinking agents, polyacrylamide derivative crosslinking agents, or polyphenol crosslinking agents, and is preferably an epoxy crosslinking agent.

[0017] In this invention, the epoxy crosslinking agent, polyacrylamide derivative crosslinking agent, or polyphenol crosslinking agent can be a commonly used crosslinking agent in the art, such as epichlorohydrin.

[0018] In this invention, the crosslinking agent not only enables polyethyleneimine to form a three-dimensional network structure, but also stably fixes the amino groups in polyethyleneimine to the film surface, thus preventing the modified layer, i.e., the three-dimensional network structure, from falling off and failing.

[0019] Preferably, the mass percentage of the crosslinking agent in the prepolymer solution is 0.6-0.8%, for example, it can be 0.6%, 0.62%, 0.64%, 0.66%, 0.68%, 0.7%, 0.72%, 0.74%, 0.76%, 0.78% or 0.8%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0020] In this invention, by optimally controlling the mass percentage ratio of polyethyleneimine and crosslinking agent, the amino group of polyethyleneimine can be appropriately over-represented by the crosslinking agent, so that part of the amino group of polyethyleneimine undergoes a crosslinking reaction with the crosslinking agent to form a three-dimensional network structure, while the other part of the amino group undergoes interfacial polymerization with acyl chloride to form a selective separation layer.

[0021] In this invention, when the concentration of the crosslinking agent is too low, the unreacted active sites cause the modified layer to swell and fall off in water. When the concentration of the crosslinking agent is too high, the crosslinking is too high, resulting in the loss of the membrane's flexibility.

[0022] Preferably, the mass percentage of polyethyleneimine in the prepolymer solution is 0.1-0.8%, for example, it can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7% or 0.8%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0023] In this invention, when the content of polyethyleneimine is too low, the reaction in the interfacial polymerization step is incomplete, and pinhole defects visible to the naked eye appear on the film surface. When the concentration of polyethyleneimine is too high, the solution viscosity is too high, which leads to uneven coating and inconsistent thickness of the surface coating layer, resulting in local stress concentration.

[0024] Preferably, the number average molecular weight of the polyethyleneimine is 1.8-70 kDa, for example, it can be 1.8 kDa, 10 kDa, 25 kDa or 70 kDa, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] Preferably, the polyethyleneimine comprises a combination of two polyethyleneimines with a number average molecular weight of 10 kDa and a number average molecular weight of 70 kDa, and preferably the mass ratio of the two is 1:(1-2).

[0026] It is worth noting that this invention preferably uses a combination of two polyethyleneimines with different molecular weights. The low molecular weight polyethyleneimine can penetrate into the micropores of the membrane material, effectively blocking non-selective defect channels; simultaneously, the high molecular weight polyethyleneimine forms a stable three-dimensional network structure through long-chain entanglement. The synergistic effect of these two materials constructs transport channels with pore size gradients, enabling Li... + (Hydration radius is 0.38 nm) and Mg 2+ Maximizing the difference in diffusion rates (with a hydration radius of 0.43 nm) is beneficial for achieving selective separation of monovalent ions.

[0027] Preferably, the cationic base film is pretreated before coating.

[0028] Preferably, the pretreatment includes: sequentially subjecting the cationic membrane to ultrasonic cleaning, immersion in sodium chloride solution, and rinsing with deionized water.

[0029] In this invention, the cationic membrane is pretreated by ultrasonic cleaning, which removes surface impurities, debris, and unreacted monomers through vibration. The cleaning solution can be deionized water or ethanol. Immersion in a sodium chloride solution can effectively remove residual inorganic salt ions from the membrane surface. The mass ratio of sodium chloride solution is generally 5-10%, and the immersion time is generally 24-48 hours.

[0030] Preferably, the temperature of the crosslinking reaction is 75-85℃, for example, it can be 75℃, 76℃, 77℃, 78℃, 79℃, 80℃, 81℃, 82℃, 83℃, 84℃ or 85℃, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0031] Preferably, the crosslinking reaction time is 30-60 min, for example, 30 min, 32 min, 35 min, 38 min, 40 min, 50 min or 60 min, but not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0032] In this invention, by optimizing the temperature and time of the crosslinking reaction, the reaction conditions can be further optimized, especially the control of the reaction temperature. When the reaction temperature is too low, the activation energy of the ring-opening reaction of the crosslinking agent is insufficient, and when the reaction temperature is too high, it is easy to cause the molecular chain of polyvinylamide to break.

[0033] Preferably, the monomer in the oil phase solution of the acyl chloride monomer includes pyromellitic trimethylolpropionate chloride.

[0034] Preferably, the oil phase of the acyl chloride monomer oil phase solution includes n-hexane.

[0035] Preferably, the mass percentage of the monomer in the oil phase solution of the acyl chloride monomer is 0.1-1.0%, for example, it can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1.0%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0036] In this invention, the method for preparing the oil phase solution of the acyl chloride monomer is a conventional method in the art, such as mixing trimesoyl chloride and n-hexane and stirring for 30-60 minutes.

[0037] Preferably, the time for the interfacial polymerization reaction is 20-60 seconds, for example, 20 seconds, 30 seconds, 40 seconds, 50 seconds or 60 seconds, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0038] In this invention, the monovalent ion-selective cation exchange membrane generally requires post-treatment after the polymerization reaction. This post-treatment includes: immersing the monovalent ion-selective cation exchange membrane in a sodium chloride solution, rinsing it with deionized water, and then storing it while maintaining moisture. The mass concentration of the sodium chloride solution is generally 5-10%, and the immersion time is generally 24-48 hours.

[0039] As a preferred embodiment of the first aspect of the present invention, the preparation method includes the following steps:

[0040] A prepolymer solution is obtained by mixing polyethyleneimine and a crosslinking agent, wherein the mass percentage of the crosslinking agent in the prepolymer solution is 0.6-0.8%, and the mass percentage of the polyethyleneimine is 0.1-0.8%, wherein the polyethyleneimine comprises a combination of two polyethyleneimines with a number average molecular weight of 10 kDa and a number average molecular weight of 70 kDa;

[0041] The cationic base membrane was sequentially subjected to ultrasonic cleaning, immersion in sodium chloride solution and rinsing with deionized water. Then, the prepolymer solution was coated on the surface of the cationic base membrane, and a crosslinking reaction was carried out at a temperature of 75-85℃ for 30-60 minutes to obtain a surface-crosslinked base membrane.

[0042] The cross-linked base film is immersed in a hexane solution of 0.1-1.0% (w / w) of trimesoyl chloride to carry out interfacial polymerization for 20-60 s to obtain the monovalent ion selective cation exchange membrane.

[0043] Secondly, the present invention provides a monovalent ion selective cation exchange membrane, wherein the monovalent ion selective cation exchange membrane is obtained by the preparation method of the monovalent ion selective cation exchange membrane described in the first aspect of the present invention.

[0044] The monovalent ion-selective cation exchange membrane provided by this invention has excellent mechanical properties and good ion sieving ability, and operates stably.

[0045] Thirdly, the present invention provides an application of the monovalent ion selective cation exchange membrane as described in the second aspect of the present invention, wherein the monovalent ion selective cation exchange membrane is used for selective lithium extraction.

[0046] The monovalent ion-selective cation exchange membrane provided by this invention is suitable for the separation of various monovalent / divalent cations, especially for Li. + / Mg 2+ It exhibits a high separation coefficient, making it suitable for high-value ion separation applications such as lithium extraction from salt lakes. Furthermore, it demonstrates industrial-grade stability and scalable production feasibility in applications such as electrolytic lithium extraction, electrodialysis of high-salinity wastewater, and ion distillation.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] (1) This invention forms a three-dimensional network structure using polyvinylamide and a crosslinking agent, which significantly improves the anti-swelling performance and mechanical strength, and maintains stable performance under strong acid and alkali (pH 2-12) and high pressure environments. Specifically, the wet anti-swelling rate reaches 2-4%, which is 60% lower than that of traditional membranes, the flux decay rate is <5%, and the cycle life is >20h.

[0049] (2) This invention utilizes acyl chloride monomers for interfacial polymerization to construct an ultrathin selective separation layer, and enables precise control of the pore size of the selective separation layer, allowing for the separation of monovalent ions (such as Li) to be separated. + The selectivity of Li is significantly improved compared to traditional cation exchange membranes. Specifically, under optimal conditions, Li + / Mg 2+ The separation coefficient reaches over 11.77, and can reach 12.55 under optimal conditions, representing a 60% improvement over traditional membranes. Under optimal conditions, the lithium flux can reach 1.482 mol / (m²). 2 ·h) or higher, and under even better conditions, it can reach 1.791 mol / (m 2 •h), which is 50% higher than traditional membranes.

[0050] (3) The present invention replaces the step-by-step modification with a coating-immersion step, which can reduce the energy consumption and time cost of film modification. At the same time, the cross-linked structure can extend the service life of the membrane, greatly reduce the replacement frequency and maintenance cost of selective ion exchange membranes in industrial applications, simplify the process by more than 50%, eliminate solvent emissions, and reduce energy consumption by more than 30%.

[0051] (4) The preparation method provided by the present invention has high compatibility and can be applied to the surface modification of a variety of substrates. Attached Figure Description

[0052] Figure 1 This is a SEM image of the monovalent ion selective cation exchange membrane provided in Embodiment 1 of the present invention at a magnification of 1000X;

[0053] Figure 2 This is a SEM image of the monovalent ion selective cation exchange membrane provided in Embodiment 1 of the present invention at a magnification of 10000X;

[0054] Figure 3 This is a SEM image of the monovalent ion selective cation exchange membrane provided in Embodiment 1 of the present invention at a magnification of 100,000X. Detailed Implementation

[0055] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0056] Example 1

[0057] This embodiment provides a method for preparing a monovalent ion-selective cation exchange membrane, the preparation method comprising the following steps:

[0058] (1) Mix two types of polyethyleneimine with a number average molecular weight of 10kDa and 70kDa (mass ratio of 1:1) in deionized water, stir until a transparent solution is obtained, add epichlorohydrin with a purity of ≥99.9% to obtain a prepolymer solution, wherein the mass percentage of the crosslinking agent in the prepolymer solution is 0.7% and the mass percentage of the polyethyleneimine is 1.0%;

[0059] (2) The cationic base membrane (Lanran polyolefin substrate) was immersed in deionized water for ultrasonic cleaning, then immersed in a 10% sodium chloride solution for 24 hours, then rinsed with deionized water, and then the prepolymer solution was coated on the surface of the cationic base membrane. Then, a crosslinking reaction was carried out at a temperature of 80°C for 30 minutes to obtain the base membrane after surface crosslinking.

[0060] (3) The cross-linked base film is immersed in a hexane solution of 0.1% trimesoyl chloride by mass and subjected to interfacial polymerization for 60s to obtain the monovalent ion selective cation exchange membrane.

[0061] (4) The monovalent ion selective cation exchange membrane is soaked in a 10% sodium chloride solution for 24 hours, then rinsed with deionized water, and then stored in a moisturizing environment.

[0062] Taking Example 1 as an example, the surface SEM images of the monovalent ion-selective cation exchange membrane obtained at different magnifications are as follows: Figure 1 , Figure 2 and Figure 3 As shown, the obtained monovalent ion selective cation exchange membrane forms a dense and uniform modified layer, indicating that the modified layer has been successfully reacted on the base membrane.

[0063] Example 2

[0064] This embodiment provides a method for preparing a monovalent ion-selective cation exchange membrane, the preparation method comprising the following steps:

[0065] (1) Mix two types of polyethyleneimine with molecular weights of 10kDa and 70kDa (mass ratio of 1:1.5) in deionized water, stir until a transparent solution is obtained, add epichlorohydrin with a purity of ≥99.9% to obtain a prepolymer solution, wherein the mass percentage of the crosslinking agent in the prepolymer solution is 0.7% and the mass percentage of the polyethyleneimine is 0.8%;

[0066] (2) The cationic base membrane (Lanran polyolefin substrate) was immersed in deionized water for ultrasonic cleaning, then immersed in a 10% sodium chloride solution for 24 hours, then rinsed with deionized water, and then the prepolymer solution was coated on the surface of the cationic base membrane. Then, a crosslinking reaction was carried out at a temperature of 85°C for 40 minutes to obtain the surface crosslinked base membrane.

[0067] (3) The cross-linked base film is immersed in a hexane solution of 0.5% pyromellitic chloride by mass and subjected to interfacial polymerization for 50s to obtain the monovalent ion selective cation exchange membrane.

[0068] (4) The monovalent ion selective cation exchange membrane is soaked in a 10% sodium chloride solution for 24 hours, then rinsed with deionized water, and then stored in a moisturizing environment.

[0069] Example 3

[0070] This embodiment provides a method for preparing a monovalent ion-selective cation exchange membrane, the preparation method comprising the following steps:

[0071] (1) Mix two types of polyethyleneimine with molecular weights of 10kDa and 70kDa (mass ratio of 1:1.7) in deionized water, stir until a transparent solution is obtained, add epichlorohydrin with a purity of ≥99.9% to obtain a prepolymer solution, wherein the mass percentage of the crosslinking agent in the prepolymer solution is 0.8% and the mass percentage of the polyethyleneimine is 0.8%;

[0072] (2) The cationic base membrane (Lanran polyolefin substrate) was immersed in deionized water for ultrasonic cleaning, then immersed in a 10% sodium chloride solution for 24 hours, then rinsed with deionized water, and then the prepolymer solution was coated on the surface of the cationic base membrane. Then, a crosslinking reaction was carried out at a temperature of 75°C for 60 minutes to obtain the base membrane after surface crosslinking.

[0073] (3) The cross-linked base film is immersed in a hexane solution of 0.8% trimesoyl chloride by mass and subjected to interfacial polymerization for 40s to obtain the monovalent ion selective cation exchange membrane.

[0074] (4) The monovalent ion selective cation exchange membrane is soaked in a 10% sodium chloride solution for 24 hours, then rinsed with deionized water, and then stored in a moisturizing environment.

[0075] Example 4

[0076] This embodiment provides a method for preparing a monovalent ion selective cation exchange membrane. The only difference from Example 1 is that in step (1), only polyethyleneimine with a number average molecular weight of 10 kDa is used.

[0077] Example 5

[0078] This embodiment provides a method for preparing a monovalent ion selective cation exchange membrane. The only difference from Example 1 is that in step (1), only polyethyleneimine with a number average molecular weight of 70 kDa is used.

[0079] Example 6

[0080] This embodiment provides a method for preparing a monovalent ion selective cation exchange membrane. The only difference from Example 1 is that the amount of polyethyleneimine added in step (1) is adjusted so that the mass percentage of polyethyleneimine in the prepolymer solution is 0.05%.

[0081] Example 7

[0082] This embodiment provides a method for preparing a monovalent ion selective cation exchange membrane. The only difference from Example 1 is that the amount of polyethyleneimine added in step (1) is adjusted so that the mass percentage of polyethyleneimine in the prepolymer solution is 1.0%.

[0083] Example 8

[0084] This embodiment provides a method for preparing a monovalent ion selective cation exchange membrane. The only difference from Example 1 is that the amount of crosslinking agent added in step (1) is adjusted so that the mass percentage of crosslinking agent in the prepolymer solution is 0.5%.

[0085] Example 9

[0086] This embodiment provides a method for preparing a monovalent ion selective cation exchange membrane. The only difference from Example 1 is that the amount of crosslinking agent added in step (1) is adjusted so that the mass percentage of crosslinking agent in the prepolymer solution is 1.0%.

[0087] Example 10

[0088] This embodiment provides a method for preparing a monovalent ion selective cation exchange membrane. The only difference from Example 1 is that the crosslinking agent in step (1) is replaced with glutaraldehyde.

[0089] Example 11

[0090] This embodiment provides a method for preparing a monovalent ion selective cation exchange membrane. The only difference from Example 1 is that the crosslinking reaction temperature in step (2) is 60°C.

[0091] Example 12

[0092] This embodiment provides a method for preparing a monovalent ion selective cation exchange membrane. The only difference from Example 1 is that the crosslinking reaction temperature in step (2) is 90°C.

[0093] Comparative Example 1

[0094] This comparative example provides a method for preparing a selective cation exchange membrane, which differs from Example 1 only in that no crosslinking agent is added to the prepolymer solution in step (1).

[0095] Comparative Example 2

[0096] This comparative example provides a method for preparing a selective cation exchange membrane. The only difference from Example 1 is that step (3) is not performed, that is, the base membrane after surface cross-linking is the obtained selective cation exchange membrane.

[0097] Performance testing:

[0098] The test was conducted using a simulated brine solution from an old salt lake, in which lithium was 0.6 g / L and magnesium was 12 g / L.

[0099] (1) Lithium flux and magnesium flux: Blueran heterogeneous anion exchange membranes were used at both electrodes of the electrochemical device, with a test membrane placed in the middle to ensure an effective membrane area of ​​19.6 cm². 2 At a constant current density of 100 A / m 2 The test was conducted for 2 hours under the specified conditions, and the results are shown in Table 1.

[0100] (2)Li + / Mg 2+ The separation coefficients are shown in Table 1.

[0101] Taking Example 1 and Comparative Example 1 as examples, the obtained selective cation exchange membrane was used to perform the above performance test four times, and the results of each test are shown in Table 2.

[0102] Table 1

[0103]

[0104]

[0105] As can be seen from the data in Table 1:

[0106] (1) As can be seen from the data in Examples 1-3, the monovalent ion-selective cation exchange membrane provided by the present invention can achieve a lithium flux of 1.482 mol / (m²) under optimal conditions. 2 With a flux of 0.816 mol / (m·h) or higher, the magnesium flux can reach 0.816 mol / (m·h). 2 ·h) and below, Li + / Mg 2+ The separation coefficient can reach over 11.77.

[0107] (2) As can be seen from the data of Examples 1 and 4-5, the difference between Examples 4-5 and Example 1 is that only one molecular weight polyvinylamide was used, and its selective separation coefficient was significantly lower than that of Example 1. This is because Example 1 not only utilized low molecular weight (10kDa) polyethyleneimine to penetrate into the micropores of the membrane material, effectively blocking non-selective defect channels; but also utilized high molecular weight (70kDa) polyethyleneimine to form a stable three-dimensional network structure through long chain entanglement. The two worked synergistically to construct a transport channel with a pore size gradient, enabling Li + (Hydration radius is 0.38 nm) and Mg 2+ Maximizing the difference in diffusion rates (with a hydration radius of 0.43 nm) is beneficial for achieving selective separation of monovalent ions.

[0108] (3) As can be seen from the data of Examples 1 and 6-7, the difference between Examples 6-7 and Example 1 is that the mass percentage of polyethyleneimine in Examples 6-7 is not within the preferred range of this invention. In Example 6, the content of polyethyleneimine is too low, which easily leads to incomplete reaction in the interfacial polymerization step, resulting in visible pinhole defects on the film surface. In Example 7, when the concentration of polyethyleneimine is too high, it easily leads to excessively high solution viscosity, resulting in uneven coating and inconsistent thickness of the surface coating layer, causing local stress concentration. In contrast, in Example 1, by optimally controlling the mass percentage of polyethyleneimine, the continuity and thickness uniformity of film formation can be promoted, thereby achieving good throughput and separation coefficient.

[0109] (4) As can be seen from the data of Examples 1 and 8-9, the difference between Examples 8-9 and Example 1 is that the mass percentage content of the crosslinking agent is not within the preferred range of this invention. In Example 8, the concentration of the crosslinking agent was too low, causing unreacted active sites to cause the modified layer to swell and detach in water. In Example 9, the concentration of the crosslinking agent was too high, resulting in excessive crosslinking and loss of membrane flexibility. In contrast, Example 1, by optimally controlling the mass percentage content of the crosslinking agent, achieved good overall performance and improved lithium flux and Li + / Mg 2+ Separation coefficient.

[0110] (5) Data from Examples 1 and 10 show that the covalently cross-linked network formed by the reaction of polyethyleneimine (PEI) and epichlorohydrin (ECH) exhibits excellent mechanical strength and stability, making it suitable for high-performance materials. In contrast, the dynamic imine bonds formed by the reaction of PEI and glutaraldehyde (GA) are easily hydrolyzed, and GA residues pose a toxic risk. Experiments demonstrate that the PEI-ECH system is significantly superior to the PEI-GA system in terms of long-term stability, safety, and applicability, making it more suitable for practical applications.

[0111] (6) As can be seen from the data of Examples 1 and 11-12, the only difference between Examples 11-12 and Example 1 is that the temperature of the crosslinking reaction is not within the preferred range of the present invention. In Example 11, the reaction temperature is too low, resulting in insufficient activation energy for the ring-opening reaction of the crosslinking agent. In Example 12, the reaction temperature is too high, which easily leads to the breakage of the polyvinylamide molecular chain. However, in Example 1, by optimally controlling the temperature of the crosslinking reaction, good comprehensive performance can be achieved, improving lithium flux and Li + / Mg 2+ Separation coefficient.

[0112] (7) Data from Example 1 and Comparative Examples 1-2 show that the polyelectrolyte composite membranes formed by the reaction of polyethyleneimine (PEI) and trimesoyl chloride (TMC) exhibit similar ion selectivity. This is mainly attributed to the similar influence of the amide bond structure generated by the reaction on the ion sieving effect. However, if TMC is not added at all, relying solely on the amino electrostatic interaction between PEI and epichlorohydrin will result in a loose membrane structure, a wider pore size distribution, and a significant reduction in the sieving capacity for specific ions.

[0113] Table 2

[0114]

[0115] As shown in Table 2, without the use of a crosslinking agent, a three-dimensional network structure could not be formed. The polyvinylamide in the resulting cation exchange membrane completely detached after four cycles, indicating that its adhesion was by physical adsorption and it could not exist stably, resulting in a progressively decreasing separation coefficient. In contrast, in Example 1, a stable three-dimensional network structure was formed using a crosslinking agent. After four cycles, the lithium flux remained above 90% of its initial value, and the separation coefficient was significantly improved through the interfacial polymerization reaction of acyl chlorides.

[0116] In summary, the synergistic effect of the three-dimensional network structure constructed by cross-linking reaction and the selective separation layer constructed by interfacial polymerization in this invention can improve the efficiency of Li... + / Mg 2+ Separation coefficient, while maintaining long-term stable operation.

[0117] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a monovalent ion-selective cation exchange membrane, characterized in that, The preparation method includes the following steps: A prepolymer solution is obtained by mixing polyethyleneimine and a crosslinking agent; the mass percentage of the crosslinking agent in the prepolymer solution is 0.6-0.8%, and the mass percentage of polyethyleneimine is 0.1-0.8%, wherein the polyethyleneimine comprises a combination of two types of polyethyleneimine with a number average molecular weight of 10 kDa and a number average molecular weight of 70 kDa; the mass percentage ratio of polyethyleneimine and crosslinking agent is controlled, and the amino group of polyethyleneimine is adjusted to be in excess relative to the crosslinking agent, so that part of the amino group of polyethyleneimine undergoes a crosslinking reaction with the crosslinking agent to form a three-dimensional network structure, and the other part of the amino group undergoes interfacial polymerization with acyl chloride to form a selective separation layer; The prepolymer solution is coated on the surface of a cationic base film, and then a crosslinking reaction is carried out at a temperature of 75-85℃ for 30-60 min to obtain a crosslinked base film. The cross-linked base film is immersed in a hexane solution of 0.1-1.0% (w / w) of trimesoyl chloride to carry out interfacial polymerization for 20-60 s to obtain the monovalent ion selective cation exchange membrane.

2. The preparation method according to claim 1, characterized in that, The crosslinking agent includes at least one of epoxy crosslinking agents, polyacrylamide derivative crosslinking agents, or polyphenol crosslinking agents.

3. The preparation method according to claim 2, characterized in that, The crosslinking agent is an epoxy crosslinking agent.

4. The preparation method according to claim 1, characterized in that, The cationic base film is pretreated before coating.

5. The preparation method according to claim 4, characterized in that, The pretreatment includes: sequentially subjecting the cationic membrane to ultrasonic cleaning, immersion in sodium chloride solution, and rinsing with deionized water.

6. A monovalent ion-selective cation exchange membrane, characterized in that, The monovalent ion selective cation exchange membrane is obtained by the preparation method of the monovalent ion selective cation exchange membrane according to any one of claims 1-5.

7. An application of the monovalent ion-selective cation exchange membrane as described in claim 6, characterized in that, The monovalent ion selective cation exchange membrane is used for selective lithium extraction.

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