Monovalent ion selective cation exchange membrane as well as preparation method and application thereof

Abstract

The invention relates to a monovalent ion selective cation exchange membrane and a preparation method and application thereof, and the preparation method comprises the following steps: mixing polyethyleneimine and a cross-linking agent to obtain a pre-polymerized solution; coating the surface of a cationic base membrane with the pre-polymerized solution, and then heating to carry out a cross-linking reaction, so as to obtain a base membrane after surface cross-linking; and soaking the base membrane subjected to surface crosslinking in an oil phase solution of acyl chloride monomers, and carrying out interfacial polymerization reaction to obtain the monovalent ion selective cation exchange membrane. Through the synergistic effect of a three-dimensional network structure constructed through cross-linking reaction and a selective separation layer constructed through interfacial polymerization, the Li < + > / Mg < 2 + > separation coefficient can be increased, and long-acting and stable operation is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of cation exchange membranes, and in particular to a monovalent ion selective cation exchange membrane and a preparation method and application thereof. Background Art

[0002] Ion exchange membranes are a class of functional polymer materials that selectively transmit specific ions. Their performance directly impacts 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 monovalent / divalent cation selectivity has been increasing.

[0003] Although the traditional homogeneous cation exchange membrane has high ion conductivity, it is not very effective for monovalent cations (such as Na + 、Li + ) and divalent cations (such as Mg 2+ , Ca 2+ ) have limited selectivity differences, making it difficult to meet high-precision separation requirements. Although heterogeneous ion exchange membranes can improve selectivity by doping with inorganic fillers, there are problems with decreased mechanical strength after doping and insufficient long-term operational stability. For example, CN109157991A discloses a side-chain quaternized polyaniline monovalent selective cation exchange membrane and a preparation method thereof. The method uses quaternized polyphenylene ether to prepare a cation exchange membrane, but its cross-linking process is complex and its acid and alkali resistance is insufficient. In addition, although organic-inorganic hybrid membranes (such as MOF-doped membranes) can improve selectivity through pore size screening effects, interface defects are prone to occur during the preparation process, resulting in a significant increase in membrane resistance and a decrease in selectivity.

[0004] In summary, the problems with existing cation exchange membranes include: (1) insufficient selectivity for monovalent / divalent cations (separation factor < 10), which makes it difficult to meet the needs 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; (3) the complexity of existing preparation processes (such as multi-step cross-linking and high-temperature sulfonation) restricts large-scale production applications.

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

[0006] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a monovalent ion selective cation exchange membrane and its preparation method and application. Compared with the existing technology, the present invention can improve the Li+ / Li-ion selective cation exchange membrane under the premise of maintaining low membrane resistance by synergistic effect of the three-dimensional network structure constructed by cross-linking reaction and the selective separation layer constructed by interfacial polymerization. + / Mg2+ The separation coefficient is high and the structure is stable, which enables long-term stable operation.

[0007] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

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

[0009] mixing polyethyleneimine and a cross-linking agent to obtain a prepolymer solution;

[0010] The prepolymer solution is coated on the surface of the cationic base membrane, and then heated to perform a cross-linking reaction to obtain a surface-crosslinked base membrane;

[0011] The surface-crosslinked base membrane is immersed in an oil phase solution of an acyl chloride monomer to carry out an interfacial polymerization reaction to obtain the monovalent ion selective cation exchange membrane.

[0012] In the preparation method provided by the present invention, a three-dimensional network structure is constructed by polyethyleneimine and a cross-linking agent, and ion channels are further regulated by interfacial polymerization, so that the structure and performance optimization of the membrane material can be achieved. Among them, polyethyleneimine and the cross-linking agent form a primary cross-linking skeleton, and the acyl chloride undergoes interfacial polymerization to form a selective functional layer. The two work synergistically to achieve an organic combination of supporting network and selective separation at the molecular level. The primary cross-linking skeleton provides the basis for mechanical stability, and the selective functional layer achieves selective ion transmission by precisely controlling the surface charge density and pore size distribution. The preparation method provided by the present invention simplifies the traditional multi-step cross-linking process into a simple coating-immersion process, and the reaction conditions are mild and controllable, which significantly reduces energy consumption and production costs.

[0013] In the present invention, the coating method may be any coating method commonly used in the art for modifying the surface of a film, such as spray coating or blade coating.

[0014] In the present invention, the preparation method of the prepolymer solution is a conventional method in the art, for example, using water as a solvent, dissolving polyethyleneimine and a cross-linking agent in deionized water, and stirring for 30-60 minutes.

[0015] In the present 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 a polysulfone base membrane, a polyethersulfone base membrane, a polyacrylonitrile base membrane, etc.

[0016] Preferably, the cross-linking agent includes at least one of an epoxy cross-linking agent, a polyacrylamide derivative cross-linking agent or a polyphenol cross-linking agent, and is preferably an epoxy cross-linking agent.

[0017] In the present invention, the epoxy crosslinking agent, polyacrylamide derivative crosslinking agent or polyphenol crosslinking agent can be crosslinking agents commonly used in the art. For example, the epoxy crosslinking agent can specifically be epichlorohydrin.

[0018] In the present invention, the crosslinking agent can not only enable polyethyleneimine to form a three-dimensional network structure, but also stably fix the amino groups in polyethyleneimine on the membrane surface, thereby preventing the modified layer, ie, the three-dimensional network structure, from falling off and becoming ineffective.

[0019] Preferably, the mass percentage of the cross-linking 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, and other unlisted values ​​within the numerical range are also applicable.

[0020] In the present invention, by preferably controlling the mass percentage ratio of polyethyleneimine and the cross-linking agent, the amino group of polyethyleneimine can be controlled to be appropriately excessive relative to the cross-linking agent, so that a part of the amino group of polyethyleneimine undergoes a cross-linking reaction with the cross-linking agent to form a three-dimensional network structure, and the other part of the amino group undergoes interfacial polymerization with the acyl chloride to form a selective separation layer.

[0021] In the present 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, excessive crosslinking occurs, resulting in loss of membrane 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, and other values ​​not listed within the numerical range are also applicable.

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

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

[0025] Preferably, the polyethyleneimine comprises a combination of two polyethyleneimines having 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 the present invention preferably uses two different molecular weight polyethyleneimine in combination, which can use the low molecular weight polyethyleneimine to penetrate into the microscopic pores of the membrane material and effectively block the non-selective defect channels; at the same time, the high molecular weight polyethyleneimine is used to form a stable three-dimensional network structure through long chain entanglement. The two work together to construct a transmission channel with a pore size gradient, so that Li + (hydration radius is 0.38nm) and Mg 2+ The difference in diffusion rates of the hydrated ions (with a hydration radius of 0.43 nm) is maximized, which is beneficial to the selective separation of monovalent ions.

[0027] Preferably, the cationic-based membrane is pretreated before coating.

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

[0029] In the present invention, the cationic membrane is pretreated with ultrasonic cleaning to remove surface impurities, debris, and unreacted monomers. Deionized water or ethanol can be used as the washing solution. Soaking in sodium chloride solution can effectively remove residual inorganic salt ions on the membrane surface. The mass ratio of sodium chloride solution is generally 5-10%, and the soaking time is generally 24-48 hours.

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

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

[0032] In the present invention, by preferably controlling the temperature and time of the cross-linking reaction, the reaction conditions, especially the control of the reaction temperature, can be further optimized. When the reaction temperature is too low, the activation energy of the ring-opening reaction of the cross-linking agent is insufficient. When the reaction temperature is too high, it is easy to cause the molecular chain of poly(ethylene amide) to break.

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

[0034] Preferably, the oil phase in the oil phase solution of the acyl chloride monomer comprises 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, and other values ​​not listed within the numerical range are also applicable.

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

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

[0038] In the present invention, after the polymerization reaction, the monovalent ion-selective cation exchange membrane is generally post-treated. The post-treatment includes soaking the monovalent ion-selective cation exchange membrane in a sodium chloride solution, then rinsing with deionized water, and then preserving the membrane to maintain moisture. The sodium chloride solution generally has a mass concentration of 5-10%, and the soaking time is generally 24-48 hours.

[0039] As a preferred technical solution of the first aspect of the present invention, the preparation method comprises the following steps:

[0040] Mixing polyethyleneimine and a cross-linking agent to obtain a prepolymerization solution, wherein the weight percentage of the cross-linking agent in the prepolymerization solution is 0.6-0.8%, the weight percentage of the polyethyleneimine is 0.1-0.8%, and the polyethyleneimine comprises a combination of two polyethyleneimines having a number average molecular weight of 10 kDa and a number average molecular weight of 70 kDa;

[0041] The cationic base membrane is sequentially subjected to ultrasonic cleaning, soaking in a sodium chloride solution, and rinsing with deionized water, and then the prepolymer solution is coated on the surface of the cationic base membrane, and then a cross-linking reaction is carried out at a temperature of 75-85° C. for 30-60 minutes to obtain a surface-crosslinked base membrane;

[0042] The surface-crosslinked base membrane is immersed in a n-hexane solution of trimesoyl chloride with a mass percentage of 0.1-1.0%, and an interfacial polymerization reaction is carried out for 20-60 seconds to obtain the monovalent ion selective cation exchange membrane.

[0043] In a second aspect, the present invention provides a monovalent ion selective cation exchange membrane, which is obtained by the method for preparing 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 the present invention has excellent mechanical properties and good ion screening ability and operates stably.

[0045] In a third aspect, 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 the present invention is suitable for the separation of various monovalent / divalent cations, especially for Li + / Mg 2+ The high separation coefficient makes 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 lithium extraction from electrolysis, electrodialysis of high-salt wastewater, and ion distillation.

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

[0048] (1) The present invention forms a three-dimensional network structure using poly(ethylene amide) and a cross-linking agent, significantly improving its anti-swelling properties and mechanical strength. It also maintains stability in strong acid and alkali environments (pH 2-12) and high-pressure environments. Specifically, the wet anti-swelling rate reaches 2-4%, a 60% reduction compared to traditional membranes, with a flux attenuation rate of less than 5% and a cycle life of more than 20 hours.

[0049] (2) The present invention can construct an ultra-thin selective separation layer by using acyl chloride monomers for interfacial polymerization, and can achieve precise control of the pore size of the selective separation layer, so that monovalent ions (such as Li + ) selectivity is significantly improved compared to traditional cation exchange membranes. Specifically, under optimal conditions, Li + / Mg 2+ The separation coefficient reaches above 11.77, and can reach 12.55 under better conditions, which is 60% higher than that of traditional membranes. Under better conditions, the lithium flux can reach 1.482 mol / (m 2 h) and above, and under better conditions it can reach 1.791 mol / (m 2 h), 50% higher than traditional membranes.

[0050] (3) The present invention replaces the step-by-step modification with the 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 reducing the replacement frequency and operation and maintenance costs of the selective ion exchange membrane in industrial applications. It can simplify the process by more than 50%, has no solvent emissions, and reduces 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 various substrates. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0053] Figure 2 is a SEM image of the monovalent ion selective cation exchange membrane provided in Example 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 Example 1 of the present invention at a magnification of 100,000X. DETAILED DESCRIPTION

[0055] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[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) Two polyethyleneimines with number average molecular weights of 10 kDa and 70 kDa (mass ratio of 1:1) were mixed in deionized water, stirred until a transparent solution was obtained, and a crosslinking agent, epichlorohydrin, with a purity of ≥99.9%, was added to obtain a prepolymerization solution, wherein the mass percentage of the crosslinking agent in the prepolymerization solution was 0.7% and the mass percentage of the polyethyleneimines was 1.0%;

[0059] (2) Soaking the cationic base membrane (blue polyolefin substrate) in deionized water for ultrasonic cleaning, then soaking it in a sodium chloride solution with a mass concentration of 10% for 24 hours, then rinsing it with deionized water, and then coating the prepolymer solution on the surface of the cationic base membrane, and then performing a cross-linking reaction at a temperature of 80° C. for 30 minutes to obtain a surface-crosslinked base membrane;

[0060] (3) immersing the surface-crosslinked base membrane in a 0.1% by weight n-hexane solution of trimesoyl chloride to perform interfacial polymerization for 60 seconds to obtain the monovalent ion selective cation exchange membrane;

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

[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, it can be seen that the obtained monovalent ion selective cation exchange membrane forms a dense and uniform modified layer, indicating that the modified layer is 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) Two polyethyleneimines with molecular weights of 10 kDa and 70 kDa (mass ratio of 1:1.5) were mixed in deionized water, stirred until a transparent solution was obtained, and a crosslinking agent, epichlorohydrin, with a purity of ≥99.9%, was added to obtain a prepolymer solution, wherein the mass percentage of the crosslinking agent in the prepolymer solution was 0.7% and the mass percentage of the polyethyleneimines was 0.8%;

[0066] (2) Soaking the cationic base membrane (blue polyolefin substrate) in deionized water for ultrasonic cleaning, then soaking it in a sodium chloride solution with a mass concentration of 10% for 24 hours, then rinsing it with deionized water, and then coating the prepolymer solution on the surface of the cationic base membrane, and then performing a cross-linking reaction at a temperature of 85° C. for 40 minutes to obtain a surface-crosslinked base membrane;

[0067] (3) immersing the surface-crosslinked base membrane in a 0.5% by weight n-hexane solution of trimesoyl chloride to perform interfacial polymerization for 50 seconds to obtain the monovalent ion selective cation exchange membrane;

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

[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) Two polyethyleneimines with molecular weights of 10 kDa and 70 kDa (mass ratio of 1:1.7) were mixed in deionized water, stirred until a transparent solution was obtained, and a crosslinking agent, epichlorohydrin, with a purity of ≥99.9% was added to obtain a prepolymerization solution, wherein the mass percentage of the crosslinking agent in the prepolymerization solution was 0.8%, and the mass percentage of the polyethyleneimines was 0.8%;

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

[0073] (3) immersing the surface-crosslinked base membrane in a 0.8% by weight n-hexane solution of trimesoyl chloride to perform interfacial polymerization for 40 seconds to obtain the monovalent ion selective cation exchange membrane;

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

[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), the polyethyleneimine used is polyethyleneimine with a number average molecular weight of 10 kDa.

[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), the polyethyleneimine used is polyethyleneimine with a number average molecular weight of 70 kDa.

[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 cross-linking agent added in step (1) is adjusted so that the mass percentage of the cross-linking 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 cross-linking agent added in step (1) is adjusted so that the mass percentage of the cross-linking 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 cross-linking agent in step (1) is replaced by 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 temperature of the cross-linking reaction 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 temperature of the cross-linking reaction in step (2) is 90°C.

[0093] Comparative Example 1

[0094] This comparative example provides a method for preparing a selective cation exchange membrane. The only difference from Example 1 is that no cross-linking agent is added to the prepolymerization 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 compared with 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 salt lake brine solution, in which the lithium content was 0.6 g / L and the magnesium content was 12 g / L.

[0099] (1) Lithium flux and magnesium flux: The two electrodes of the electrochemical device use Lanran's heterogeneous cathode membrane, and the test membrane is placed in the middle to ensure that the effective membrane area is 19.6 cm 2 , at a constant current density of 100A / m 2 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 in a cycle, and the results of each cycle are shown in Table 2.

[0102] Table 1

[0103]

[0104]

[0105] From the data in Table 1 we can see that:

[0106] (1) From the data of Examples 1-3, it can be seen that under optimal conditions, the lithium flux of the monovalent ion selective cation exchange membrane provided by the present invention can reach 1.482 mol / (m 2 ·h), the magnesium flux can reach 0.816mol / (m 2 h) below, Li + / Mg 2+ The separation coefficient can reach above 11.77.

[0107] (2) From the data of Example 1 and Example 4-5, it can be seen that the difference between Example 4-5 and Example 1 is that only one molecular weight polyethylene amide is used, and its selective separation coefficient is significantly lower than that of Example 1. This is because in Example 1, not only the low molecular weight (10kDa) polyethyleneimine is used to penetrate into the microscopic pores of the membrane material and effectively block the non-selective defect channels, but also the high molecular weight (70kDa) polyethyleneimine is used to form a stable three-dimensional network structure through long chain entanglement. The two work together to construct a transmission channel with a pore size gradient, which makes Li + (hydration radius is 0.38nm) and Mg 2+ The difference in diffusion rates of the hydrated ions (with a hydration radius of 0.43 nm) is maximized, which is beneficial to the selective separation of monovalent ions.

[0108] (3) From the data of Examples 1 and 6-7, it can be seen that the difference between Examples 6-7 and Example 1 is that the mass percentage of polyethyleneimine is not within the preferred range of the present invention. In Example 6, the content of polyethyleneimine is too low, which easily leads to incomplete reaction in the interfacial polymerization step and the appearance of pinhole defects visible to the naked eye on the membrane surface. In Example 7, when the concentration of polyethyleneimine is too high, it is easy to cause excessive solution viscosity to introduce uneven coating, resulting in inconsistent thickness of the surface coating layer and causing local stress concentration. In contrast, in Example 1, by preferably controlling the mass percentage of polyethyleneimine, it is possible to promote film formation continuity and thickness uniformity, thereby achieving good flux and separation coefficient.

[0109] (4) From the data of Example 1 and Example 8-9, it can be seen that the difference between Example 8-9 and Example 1 is that the mass percentage of the crosslinking agent is not within the preferred range of the present invention. The concentration of the crosslinking agent in Example 8 is too low, resulting in the unreacted active sites causing the modified layer to swell and fall off in water. The concentration of the crosslinking agent in Example 9 is too high, resulting in excessive crosslinking and loss of membrane flexibility. In Example 1, by preferably controlling the mass percentage of the crosslinking agent, good comprehensive performance can be achieved, improving lithium flux and Li + / Mg 2+ Separation coefficient.

[0110] (5) The data from Examples 1 and 10 show that the covalently cross-linked network formed by the reaction of polyethyleneimine (PEI) and epichlorohydrin (ECH) has 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 residual GA poses a toxicity risk. Experiments have shown that the PEI-ECH system is significantly superior to the PEI-GA system in terms of long-term stability, safety, and scope of application, making it more suitable for practical applications.

[0111] (6) From the data of Example 1 and Example 11-12, it can be seen that the only difference between Example 11-12 and Example 1 is that the temperature of the cross-linking reaction is not within the preferred range of the present invention. In Example 11, the reaction temperature is too low, and the activation energy of the ring-opening reaction of the cross-linking agent is insufficient. In Example 12, the reaction temperature is too high, which easily causes the molecular chain of polyacrylamide to break. In Example 1, by preferably controlling the temperature of the cross-linking reaction, good comprehensive performance can be achieved, and the lithium flux and Li + / Mg 2+ Separation coefficient.

[0112] (7) The data from Example 1 and Comparative Examples 1-2 indicate that the polyelectrolyte composite membranes formed by the reaction of polyethyleneimine (PEI) and trimesoyl chloride (TMC) exhibit similar ion selectivity, primarily due to the similar effects of the amide bond structure generated by the reaction on ion screening. However, if TMC is omitted and only the electrostatic interaction between PEI and epichlorohydrin's amino groups is relied upon, the membrane structure becomes loose and the pore size distribution becomes broad, significantly reducing the screening ability for specific ions.

[0113] Table 2

[0114]

[0115] As can be seen from Table 2, when no cross-linking agent is used, a three-dimensional network structure cannot be formed. The poly(vinyl amide) in the resulting cation exchange membrane completely falls off after four cycles of testing, indicating that its attachment mode is physical adsorption and that it cannot be stably present, and its separation coefficient decreases each time. In Example 1, however, a stable three-dimensional network structure is formed by a cross-linking agent. After four cycles of testing, the lithium flux remains above 90% of the initial value, and the separation coefficient is significantly improved by the interfacial polymerization reaction of the acyl chloride.

[0116] In summary, the present invention can improve the Li + / Mg 2+ Separation factor, while long-term stable operation.

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

Claims

1. A method for preparing a monovalent ion selective cation exchange membrane, characterized in that: The preparation method comprises the following steps: mixing polyethyleneimine and a cross-linking agent to obtain a prepolymer solution; The prepolymer solution is coated on the surface of the cationic base membrane, and then heated to perform a cross-linking reaction to obtain a surface-crosslinked base membrane; The surface-crosslinked base membrane is immersed in an oil phase solution of an acyl chloride monomer to carry out an interfacial polymerization reaction to obtain the monovalent ion selective cation exchange membrane.

2. The preparation method according to claim 1, characterized in that The cross-linking agent includes at least one of an epoxy cross-linking agent, a polyacrylamide derivative cross-linking agent or a polyphenol cross-linking agent, and is preferably an epoxy cross-linking agent.

3. The preparation method according to claim 1 or 2, characterized in that The mass percentage of the cross-linking agent in the prepolymer solution is 0.6-0.8%; Preferably, the mass percentage of polyethyleneimine in the prepolymerization solution is 0.1-0.8%.

4. The preparation method according to any one of claims 1 to 3, characterized in that The number average molecular weight of the polyethyleneimine is 1.8-70 kDa; Preferably, the polyethyleneimine comprises a combination of two polyethyleneimines having a number average molecular weight of 10 kDa and a number average molecular weight of 70 kDa.

5. The preparation method according to any one of claims 1 to 4, characterized in that The cationic base membrane is pretreated before coating; Preferably, the pretreatment comprises: sequentially subjecting the cationic-based membrane to ultrasonic cleaning, soaking in sodium chloride solution, and rinsing with deionized water.

6. The preparation method according to any one of claims 1 to 5, characterized in that The temperature of the cross-linking reaction is 75-85°C; Preferably, the cross-linking reaction time is 30-60 minutes.

7. The preparation method according to any one of claims 1 to 6, characterized in that The monomers in the oil phase solution of the acyl chloride monomers include trimesoyl chloride; Preferably, the oil phase in the oil phase solution of the acyl chloride monomer comprises n-hexane; Preferably, the mass percentage of the monomer in the oil phase solution of the acyl chloride monomer is 0.1-1.0%; Preferably, the interfacial polymerization reaction time is 20-60s.

8. The preparation method according to any one of claims 1 to 7, characterized in that The preparation method comprises the following steps: Mixing polyethyleneimine and a cross-linking agent to obtain a prepolymerization solution, wherein the weight percentage of the cross-linking agent in the prepolymerization solution is 0.6-0.8%, the weight percentage of the polyethyleneimine is 0.1-0.8%, and the polyethyleneimine comprises a combination of two polyethyleneimines having a number average molecular weight of 10 kDa and a number average molecular weight of 70 kDa; The cationic base membrane is sequentially subjected to ultrasonic cleaning, soaking in a sodium chloride solution, and rinsing with deionized water, and then the prepolymer solution is coated on the surface of the cationic base membrane, and then a cross-linking reaction is carried out at a temperature of 75-85° C. for 30-60 minutes to obtain a surface-crosslinked base membrane; The surface-crosslinked base membrane is immersed in a n-hexane solution of trimesoyl chloride with a mass percentage of 0.1-1.0%, and an interfacial polymerization reaction is carried out for 20-60 seconds to obtain the monovalent ion selective cation exchange membrane.

9. 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 to 8.

10. Use of the monovalent ion selective cation exchange membrane according to claim 9, characterized in that: The monovalent ion selective cation exchange membrane is used for selective lithium extraction.

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