A composite membrane for cation-selective separation, and a method of making and use thereof

Abstract

The application provides a composite membrane for cation selective separation and a preparation method and use thereof. The composite membrane comprises a support layer and a functional layer on the surface of the support layer, and the functional layer comprises a seed layer and a MOF layer on the surface of the seed layer; wherein the seed layer is in direct contact with the support layer, and the seed layer comprises an oxidized graphene sheet layer structure and a MOF material in-situ grown in the oxidized graphene sheet layer structure. When hydrated ions (cations) pass through the composite membrane structure provided by the application, dehydration occurs due to the size sieving effect, and appropriate pore sizes will promote the dehydration of ions, so that various cations can be separated, and the composite membrane can be effectively used in the separation and recovery process of resources in water bodies such as salt lakes.

Description

Technical Field

[0001] This invention belongs to the field of cation selective separation technology, and relates to a composite membrane for cation selective separation, its preparation method and application. Background Technology

[0002] Membrane separation processes are widely used in water treatment and seawater desalination. In recent years, much research has focused on improving the water flux of desalination membranes. However, compared to the more mature commercial nanofiltration membranes, the increased water flux from new research only slightly improves the energy efficiency of desalination. Furthermore, some membrane fabrication methods sacrifice certain desirable properties of existing nanofiltration membranes, posing challenges to the large-scale production and practical application of newly developed nanofiltration membranes. With the increasing demands for water treatment and the expansion of membrane separation applications in recent years, the design and fabrication of nanofiltration membranes with highly selective separation performance has become a key focus for researchers. In water treatment, selective separation membranes can specifically remove certain pollutants from wastewater, thereby reducing the harm of concentrated wastewater to environmental water bodies. When applied to drinking water treatment, selective separation membranes can reduce the remineralization process of effluent, avoiding the health hazards associated with long-term consumption of "pure water." More importantly, selective separation membranes can be used to extract valuable resources from natural water bodies, such as abundant lithium resources and other metal ions in salt lakes. The adoption of membrane technology will avoid the problems of adding chemical agents and high energy consumption associated with traditional techniques (such as precipitation and solvent extraction). Therefore, the development of related ion-selective separation technologies is of great significance for the improvement and expansion of the membrane field.

[0003] Currently, improvements in ion-selective separation performance often utilize the interaction between membrane pore materials and specific ions. For example, CN105646927A discloses a method for preparing a monovalent selective cation exchange membrane. This method involves reacting a cation exchange membrane with sulfonic acid groups with thionyl chloride to generate an ion exchange membrane with sulfonyl chloride. Polyquaternium-7 is then subjected to a Hoffman degradation reaction to remove the carbonyl groups. The degraded polyquaternium-7 is then reacted with the sulfonyl chloride-containing ion exchange membrane via a chemical reaction, fixing the positively charged degraded polyquaternium-7 onto the membrane surface, thus forming a thin layer of positive charge on the membrane surface. Another example is CN104001435A, which discloses a method for preparing a monovalent cation-selective separation membrane. This method uses a basic polymer and an acidic monomer as raw materials, partially or completely "acid-base pairing" the basic groups of the basic polymer to obtain a monovalent cation-selective separation membrane. This invention utilizes the ability of basic polymers and acidic monomers to form acid-base pairs, resulting in partial or complete acid-base pairing of the basic groups, thereby constructing a membrane capable of transporting only H+. +The channels enable the selective separation of monovalent and polyvalent cations. Other interactions, such as the coordination between carboxyl functional groups and divalent heavy metal ions, are also often used in the preparation of separation membranes. However, this selective separation performance is often only possible for a specific type of ion, and stronger interactions will increase the difficulty of ions detaching from the membrane pores.

[0004] Therefore, how to achieve effective selective separation of multiple cations (monovalent and polyvalent) for widespread use in water treatment and seawater desalination is a technical problem that urgently needs to be solved. Summary of the Invention

[0005] The purpose of this invention is to provide a composite membrane for selective cation separation, its preparation method, and its applications. When hydrated ions (cations) pass through the composite membrane structure provided by this invention, they undergo dehydration due to the size sieving effect. Appropriate pore sizes will promote ion dehydration, thereby enabling the separation of multiple cations. This can be effectively applied to the separation and recovery of resources in water bodies, such as lithium extraction from salt lakes.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a composite membrane for cation selective separation, the composite membrane comprising a support layer and a functional layer located on the surface of the support layer, the functional layer comprising a seed layer and an MOF layer located on the surface of the seed layer;

[0008] The seed layer is in direct contact with the support layer, and the seed layer includes a graphene oxide sheet structure and MOF material grown in situ in the graphene oxide sheet structure.

[0009] The composite membrane structure provided by this invention has a close contact between the MOF layer and the seed layer in the functional layer, and the pore size is uniform, both being the window size of MOF crystals. At the same time, some MOF crystals also grow in the membrane pores of the support layer, reducing the pore size. Hydrated ions can pass through these uniform pores in the sub-nanometer range (the confined pores of MOF), allowing ions to undergo dehydration by utilizing the size sieving effect during the process of entering the pore opening. Thus, based on the differences in the hydration energy of the ions and the different size changes after dehydration, ions can be selectively separated. This can be effectively applied to the separation and recovery of water resources such as lithium extraction from salt lakes.

[0010] In other words, this invention achieves the preparation of uniform confined channels and the size sieving of hydrated ions through the synergistic effect of the seed layer and the functional layer. If the functional layer does not contain a MOF layer, there will be certain defects, making it difficult to achieve the function of selective cation separation. If there is no seed layer, there will be a lack of interaction between the MOF layer and the base film, making it difficult for MOF to grow uniformly in situ on the base film. Moreover, the MOF material in the seed layer plays a key role in the subsequent nucleation and growth of MOF layer crystals.

[0011] Preferably, the support layer comprises a polyvinylidene fluoride (PVDF) ultrafiltration membrane.

[0012] Preferably, the window size of the MOF material in the seed layer is 0.3 to 0.6 nm, such as 0.3 nm, 0.35 nm, 0.4 nm, 0.45 nm, 0.5 nm, 0.55 nm or 0.6 nm.

[0013] In this invention, when the window size of the MOF material in the seed layer is in the range of 0.3 to 0.6 nm, it is located between the diameter of the hydrated cation and the diameter of the bare ion, and has good water stability and chemical stability. If it is not in this range, problems such as the bare ion being unable to pass through at all and the energy consumption being high (too small size) or the ion passing through not undergoing dehydration and being difficult to separate (too large size) are likely to occur.

[0014] Preferably, the MOF material includes ZIF type.

[0015] Preferably, the thickness of the seed layer is 20-30 μm, such as 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm or 30 μm.

[0016] Preferably, the thickness of the MOF layer is 30-40 μm, such as 30 μm, 31 μm, 32 μm, 33 μm, 34 μm, 35 μm, 36 μm, 37 μm, 38 μm, 39 μm or 40 μm.

[0017] In a second aspect, the present invention provides a method for preparing a composite membrane for cation selective separation as described in the first aspect, the method comprising the following steps:

[0018] (1) Mix graphene sheet dispersion, organic ligand solution and metal salt solution, react, and composite the reacted material onto the surface of the support layer to obtain a seed layer located on the surface of the support layer.

[0019] (2) The seed layer located on the surface of the support layer is vertically placed in an H-type electrolytic cell, with a metal salt solution and an organic ligand solution on both sides of the electrolytic cell, and the reaction is carried out to obtain the composite membrane for selective separation of cations.

[0020] The preparation method provided by this invention obtains MOF material in a graphene oxide sheet structure through in-situ growth, thereby effectively leveraging the size sieving effect of the composite membrane. Furthermore, the preparation of the MOF layer by applying an electric field makes the MOF layer more compact, fills the defects in the seed layer, and results in more uniform pore sizes, all of which are window sizes of MOF crystals. At the same time, some MOF crystals also grow in the membrane pores of the support layer, reducing the pore size. Through these uniform pores in the sub-nanometer range, ions can undergo dehydration during their entry into the pore opening, thereby selectively separating ions based on differences in their own hydration energy and the different size changes after dehydration.

[0021] If the preparation method provided by this invention does not use an electrolytic cell to prepare the MOF layer, the resulting functional layer will have certain defects and will not be able to form uniform channels on the membrane surface, resulting in a decrease in the selective separation performance of ions.

[0022] Preferably, the size of the graphene sheet in step (1) is >500nm, such as 550nm, 600nm, 650nm, 700nm, 750nm, 800nm, 850nm, 900nm, 950nm or 1μm.

[0023] Preferably, the mass concentration of the graphene sheet dispersion in step (1) is 0.5 to 1 mg / L, such as 0.5 mg / L, 0.6 mg / L, 0.7 mg / L, 0.8 mg / L, 0.9 mg / L or 1 mg / L.

[0024] Preferably, the molar ratio of the organic ligand to the metal salt in step (1) is (3-5):1, for example, 3:1, 3.5:1, 4:1, 4.5:1 or 5:1.

[0025] Preferably, the organic ligand in step (1) does not contain charged functional groups.

[0026] Preferably, in step (1), the mass ratio of the metal salt to the graphene sheet is (150-250):1, for example, 150:1, 160:1, 170:1, 180:1, 190:1, 200:1, 210:1, 220:1, 230:1, 240:1 or 250:1, etc.

[0027] In this invention, when the mass ratio of the metal salt to the graphene sheet is (150-250):1, the connection and in-situ growth of MOF on graphene can be guaranteed, while too much metal salt will cause some graphene to settle.

[0028] Preferably, the reaction in step (1) is carried out under closed conditions.

[0029] Preferably, the reaction time in step (1) is 20 to 30 hours, such as 20 hours, 21 hours, 22 hours, 23 hours, 24 hours, 25 hours, 26 hours, 27 hours, 28 hours, 29 hours, or 30 hours.

[0030] Preferably, the composite method in step (1) includes redispersing the reacted substances and filtering the redispersed solution on a support layer to obtain a seed layer.

[0031] Preferably, the mass concentration of the redispersed dispersion is 15-25 mg / L, such as 15 mg / L, 16 mg / L, 17 mg / L, 18 mg / L, 19 mg / L, 20 mg / L, 21 mg / L, 22 mg / L, 23 mg / L, 24 mg / L, or 25 mg / L.

[0032] Preferably, in step (2), the side of the H-type electrolytic cell where the seed layer is located is an organic ligand solution.

[0033] Preferably, the reaction temperature in step (2) is 80 to 120°C, such as 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C or 120°C.

[0034] If the reaction temperature in step (2) is too low, it will not be conducive to the rapid and uniform growth of MOF, and will easily cause MOF crystals to accumulate under the film for a long time. If the reaction temperature is too high, it will lead to a decrease in the stability of the basement film and cause the functional layer to fall off.

[0035] Preferably, the reaction time in step (2) is 20 to 30 hours, such as 20 hours, 21 hours, 22 hours, 23 hours, 24 hours, 25 hours, 26 hours, 27 hours, 28 hours, 29 hours, or 30 hours.

[0036] As a preferred technical solution, the preparation method includes the following steps:

[0037] (1) A graphene sheet dispersion with a mass concentration of 0.5-1 mg / L, an organic ligand solution, and a metal salt solution are mixed. The molar ratio of the organic ligand to the metal salt is (3-5):1, and the mass ratio of the metal salt to the graphene sheet is (150-250):1. The mixture is reacted under sealed conditions for 20-30 hours. The reacted material is then redispersed, and the redispersed solution is filtered on a support layer to obtain a seed layer located on the surface of the support layer.

[0038] (2) The seed layer located on the surface of the support layer is placed vertically in an H-type electrolytic cell. The two sides of the electrolytic cell are a metal salt solution and an organic ligand solution, respectively. The side where the seed layer is located is an organic ligand solution. The reaction is carried out at 80-120℃ for 20-30 hours to obtain the composite membrane used for cation selective separation.

[0039] Thirdly, the present invention also provides an use of the composite membrane as described in the first aspect, the use including using the composite membrane for cation-selective separation in an aqueous environment.

[0040] Preferably, an external field is applied to the composite membrane to achieve selective separation of cations.

[0041] Using the composite membrane provided by this invention, ions undergo transmembrane transport under the influence of an external field. Upon passing through the confined channels of the MOF (Metal-Oxide-Foil) pores, hydrated ions undergo dehydration due to a size sieving effect. Suitable channel sizes promote ion dehydration. Specifically, when the external field provides adequate energy, divalent cations with higher hydration energies are retained by the selective separation membrane due to insufficient external energy for dehydration. Monovalent cations with different hydration energies exhibit different transport diameters due to varying degrees of dehydration, resulting in selective separation within the channels due to differences in transport rates. Ultimately, the desired cations can be obtained on the transmembrane side, thus achieving selective separation of multiple types (monovalent or polyvalent) of cations in the aquatic environment. In this invention, without an external field, the ion separation and transport speed would be slow, leading to ion accumulation within the channels.

[0042] Preferably, the external field includes a pressure field and / or an electric field.

[0043] Preferably, the cation includes monovalent cations and / or polyvalent cations.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] The composite membrane structure provided by this invention has a close contact between the MOF layer and the seed layer in the functional layer, and the pore size is uniform, both being the window size of MOF crystals. At the same time, some MOF crystals also grow in the membrane pores of the support layer, reducing the pore size. Hydrated ions can pass through these uniform pores in the sub-nanometer range (the confined pores of MOF), allowing ions to undergo dehydration by utilizing the size sieving effect during the process of entering the pore opening. Thus, ions can be selectively separated based on the differences in their own hydration energy and the different size changes after dehydration. This can be effectively applied to the separation and recovery process of water resources such as lithium extraction from salt lakes and in the process of seawater desalination. The composite membrane structure provided by this invention, when selectively separating cations in an aqueous environment, achieves a selectivity coefficient of 123 or higher between monovalent and divalent cations and a selectivity coefficient of 0.89 or higher between monovalent and monovalent cations under the application of an external field. Furthermore, during the preparation of the composite membrane, the mass ratio of the metal salt to the graphene sheet is controlled at (150-250):1, and the reaction temperature in the electrolytic preparation of the MOF layer is 80-120°C. Under the application of an external field, the selectivity coefficient between monovalent and divalent cations can reach 18811 or higher, and the selectivity coefficient between monovalent and monovalent cations can reach 1.42 or higher. Attached Figure Description

[0046] Figure 1 This is a top SEM view of the composite membrane provided in Example 1.

[0047] Figure 2 The image shows a cross-sectional SEM view of the composite membrane provided in Example 1.

[0048] Figure 3 This is a comparison diagram of the selectivity between monovalent and divalent cations provided in Application Example 1, Comparative Application Example 3, and Comparative Application Example 4.

[0049] Figure 4 A comparison diagram of selectivity among monovalent cations provided in Application Example 1, Comparative Application Example 3, and Comparative Application Example 4.

[0050] Among them, 1 is the MOF layer and 2 is the support layer. Detailed Implementation

[0051] 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.

[0052] Example 1

[0053] This embodiment provides a composite membrane for cation selective separation, such as... Figure 1 and Figure 2As shown, the composite membrane includes a support layer 2 (PVDF ultrafiltration substrate membrane) and a functional layer located on the surface of the support layer. The functional layer includes a seed layer (thickness of 28 μm) and an MOF layer 1 (MOF material is ZIF-8, thickness of 37 μm, window size of 0.34 nm) located on the surface of the seed layer.

[0054] The seed layer is in direct contact with the support layer 2. The seed layer includes a graphene oxide sheet structure and MOF material (ZIF-8) grown in situ in the graphene oxide sheet structure. The support layer 2 also contains a portion of MOF crystal material.

[0055] The composite membrane is prepared as follows:

[0056] A graphene sheet dispersion (sheet size 800 nm, concentration 1 mg / L) was prepared and ultrasonically treated under constant temperature to maintain the dispersibility of graphene. Meanwhile, a methanol solution of organic ligand (2-methylimidazole) and a methanol solution of metal salt (zinc acetate) were slowly added to the graphene dispersion sequentially, ensuring that the molar ratio of organic ligand to metal salt was between 4 and the mass ratio of metal salt to graphene was 250:1. After reacting at room temperature for 30 h under sealed conditions, a milky white turbid liquid of MOF / GO composite material was obtained. The prepared composite material was centrifuged and washed three times to remove excess unreacted substances, and then redispersed with methanol at a concentration maintained at 25 mg / L. After that, it was filtered onto a polyvinylidene fluoride (PVDF) ultrafiltration membrane substrate with a molecular weight cutoff of 100 kDa to form the seed layer of MOF (the seed layer located on the surface of the support layer).

[0057] (2) The composite membrane with the seed layer was placed vertically in an H-type electrolytic cell, with a metal ion solution (zinc acetate) of the MOF material precursor and an organic ligand solution (2-methylimidazole, with the organic ligand solution on the seed layer side) on the other side. After reacting at 80°C for 30 h, the membrane was washed to obtain the composite membrane structure. Figure 3 and Figure 4 (ZIF-8 / GO / PVDF in the middle).

[0058] Example 2

[0059] This embodiment provides a composite membrane for cation selective separation. The composite membrane includes a support layer (PVDF ultrafiltration substrate membrane) and a functional layer located on the surface of the support layer. The functional layer includes a seed layer (22 μm thick) and an MOF layer (MOF material is ZIF-8, thickness is 34 μm, window size is 0.34 nm) located on the surface of the seed layer.

[0060] The seed layer is in direct contact with the support layer. The seed layer includes a graphene oxide sheet structure and MOF material (ZIF-8) grown in situ in the graphene oxide sheet structure. The support layer also contains a portion of MOF crystal material.

[0061] The composite membrane is prepared as follows:

[0062] A graphene sheet dispersion (sheet size 600 nm, concentration 0.5 mg / L) was prepared and ultrasonically treated under constant temperature to maintain the dispersibility of graphene. Meanwhile, a methanol solution of organic ligand (2-methylimidazole) and a methanol solution of metal salt (zinc acetate) were slowly added to the graphene dispersion sequentially, ensuring that the molar ratio of organic ligand to metal salt was between 3 and 1, and the mass ratio of metal salt to graphene was 150:1. After reacting at room temperature for 20 h under sealed conditions, a milky white turbid liquid of MOF / GO composite material was obtained. The prepared composite material was centrifuged and washed three times to remove excess unreacted substances, and then redispersed with methanol at a concentration of 15 mg / L. After that, it was filtered onto a polyvinylidene fluoride (PVDF) ultrafiltration membrane substrate with a molecular weight cutoff of 100 kDa to form the seed layer of MOF (the seed layer located on the surface of the support layer).

[0063] (2) The composite membrane with the seed layer is placed vertically in an H-type electrolytic cell, with the metal ion solution (zinc acetate) of the MOF material precursor and the organic ligand solution (2-methylimidazole, with the organic ligand solution on the seed layer side) on the other side. After reacting at 100°C for 25 h, the composite membrane structure is obtained by washing.

[0064] Example 3

[0065] This embodiment provides a composite membrane for cation selective separation. The composite membrane includes a support layer (PVDF ultrafiltration substrate membrane) and a functional layer located on the surface of the support layer. The functional layer includes a seed layer (26 μm thick) and an MOF layer (MOF material is ZIF-8, 35 μm thick, and window size is 0.34 nm) located on the surface of the seed layer.

[0066] The seed layer is in direct contact with the support layer. The seed layer includes a graphene oxide sheet structure and MOF material (ZIF-8) grown in situ in the graphene oxide sheet structure. The support layer also contains a portion of MOF crystal material.

[0067] The composite membrane is prepared as follows:

[0068] A graphene sheet dispersion (sheet size 550 nm, concentration 0.8 mg / L) was prepared and ultrasonically treated under constant temperature to maintain the dispersibility of graphene. Meanwhile, a methanol solution of organic ligand (2-methylimidazole) and a methanol solution of metal salt (zinc acetate) were slowly added to the graphene dispersion sequentially, ensuring that the molar ratio of organic ligand to metal salt was between 5 and that the mass ratio of metal salt to graphene was 200:1. After reacting at room temperature for 25 h under sealed conditions, a milky white turbid liquid of MOF / GO composite material was obtained. The prepared composite material was centrifuged and washed three times to remove excess unreacted substances and then redispersed with methanol at a concentration maintained at 20 mg / L. After that, it was filtered onto a polyvinylidene fluoride (PVDF) ultrafiltration membrane substrate with a molecular weight cutoff of 100 kDa to form the seed layer of MOF (the seed layer located on the surface of the support layer).

[0069] (2) The composite membrane with the seed layer is placed vertically in an H-type electrolytic cell, with the metal ion solution (zinc acetate) of the MOF material precursor and the organic ligand solution (2-methylimidazole, with the organic ligand solution on the seed layer side) on the other side. After reacting at 120°C for 20 h, the composite membrane structure is obtained by washing.

[0070] Example 4

[0071] The difference between this embodiment and embodiment 1 is that in step (1) of this embodiment, the mass ratio of metal salt to graphene is 300:1.

[0072] The remaining preparation methods and parameters are consistent with those in Example 1.

[0073] Example 5

[0074] The difference between this embodiment and embodiment 1 is that in step (1) of this embodiment, the mass ratio of metal salt to graphene is 100:1.

[0075] The remaining preparation methods and parameters are consistent with those in Example 1.

[0076] Example 6

[0077] The difference between this embodiment and embodiment 1 is that the reaction temperature in step (2) of this embodiment is 60°C.

[0078] The remaining preparation methods and parameters are consistent with those in Example 1.

[0079] Comparative Example 1

[0080] The difference between this comparative example and Example 1 is that the functional layer of this comparative example only contains a seed layer.

[0081] Step (2) is not performed in the preparation method.

[0082] The remaining preparation methods and parameters are consistent with those in Example 1.

[0083] Comparative Example 2

[0084] The difference between this comparative example and Example 1 is that the functional layer of this comparative example only contains a MOF layer.

[0085] In the preparation method, step (2) is performed directly on the surface of the PVDF ultrafiltration membrane.

[0086] The remaining preparation methods and parameters are consistent with those in Example 1.

[0087] Comparative Example 3

[0088] The difference between this comparative example and Example 1 is that the functional layer of this comparative example consists only of graphene oxide sheet structures. Figure 3 and Figure 4 (GO / PVDF in the middle).

[0089] In the preparation method, graphene oxide sheet structures are directly composited on the surface of the PVDF ultrafiltration membrane.

[0090] The remaining preparation methods and parameters are consistent with those in Example 1.

[0091] Comparative Example 4

[0092] The difference between this comparative example and Example 1 is that the composite film in this comparative example does not contain a functional layer. Figure 3 and Figure 4 PVDF in (the context of the text).

[0093] The composite membrane structures provided in Examples 1-6 and Comparative Examples 1-4 were used for cation-selective separation in an aqueous environment, serving as Application Examples 1-6 and Comparative Application Examples 1-4, respectively. Specifically, 20 mL of the solution to be separated was placed on one side of an H-type electrolytic cell, with a salt ion concentration of 0.1 M. 20 mL of ultrapure water was placed on the other side. An external electric field was applied to both sides through a silver-silver chloride reference electrode, powered by a picoammeter. The current value obtained during the process was monitored simultaneously. The voltage was set to 0.3 V, and the energizing time was approximately 25 minutes.

[0094] Application Example 7

[0095] This application example uses the composite membrane structure provided in Example 1 to perform selective cation separation, but no external field is applied during the separation process.

[0096] Figure 3 The diagram shows a comparison of selectivity between monovalent and divalent cations provided in Application Example 1, Comparative Application Example 3, and Comparative Application Example 4. Figure 3It can be seen that the introduction of confined channels by the MOF functional layer effectively hinders the passage of divalent cations. Therefore, the selectivity between monovalent and divalent cations can reach a high value. Due to their high hydration energy, divalent cations are difficult to enter the confined channels of MOF through dehydration under the action of an external electric field, and thus are difficult to pass through the composite membrane.

[0097] Figure 4 The diagram shows a comparison of selectivity among the monovalent cations provided in Application Example 1, Comparative Application Example 3, and Comparative Application Example 4. Figure 4 It can be seen that, compared with Comparative Application Example 3 and Comparative Application Example 4, which do not have confined uniform channels, the MOF confined channels in Application Example 1 increase the size difference of the monovalent cations themselves through dehydration caused by the size sieving effect, thereby resulting in different final transport speeds and improved final selective separation performance.

[0098] The selectivity results for monovalent and divalent cations in Application Examples 1-7 and Comparative Application Examples 1-4 are shown in Table 1 (expressed as selectivity coefficients).

[0099] Table 1

[0100]

[0101]

[0102] The selectivity results for monovalent cations in Application Examples 1-7 and Comparative Application Examples 1-4 are shown in Table 2 (expressed as selectivity coefficients).

[0103] Table 2

[0104]

[0105]

[0106] Combining the data from Tables 1 and 2, we can see that:

[0107] The data results from Application Examples 1, 4, and 5 show that in step (1), if the mass ratio of metal salt to graphene is too large, i.e., there is too much metal salt, it is not conducive to the uniform dispersion of graphene. Therefore, the uniformity of the seed layer formed on the film is poor, resulting in defects in the later synthesis process. If the mass ratio of metal salt to graphene is too small, i.e., there is too little metal salt, it will result in a smaller amount of MOF growing in situ on graphene, which is not conducive to the continued growth of MOF layer crystals in the later stage.

[0108] The data from Application Example 1 and Application Example 6 show that if the reaction temperature in step (2) is too low, it will affect the uniform and rapid growth of the MOF layer, causing the MOF layer to accumulate more under the membrane, resulting in an unevenness of the entire membrane surface.

[0109] The data from Application Example 1 and Comparative Application Examples 1 and 2 show that when only a seed layer is present and no MOF layer is present, it is difficult to achieve uniformity of the confined pore size. Defects will still exist on the membrane, causing ion short-flow and reducing selectivity. Conversely, when only a MOF layer is present and no seed layer is present, a composite membrane with a tight connection between the MOF and the substrate membrane cannot be obtained. In other words, the composite membrane structure of this invention requires synergistic effects between the layers to achieve uniform arrangement and growth of confined pores on the membrane, thereby enabling size sieving and dehydration of cations passing through the membrane.

[0110] The data from Application Example 1 and Comparative Application Examples 3 and 4 show that neither pure support layer structures nor graphene oxide with only a surface layer can solve the problem of selective sieving of cations with similar sizes and properties.

[0111] In summary, the composite membrane structure provided by this invention features a close contact between the MOF layer and the seed layer in the functional layer, with uniform pore sizes that correspond to the window size of MOF crystals. Simultaneously, some MOF crystals also grow within the membrane pores of the support layer, reducing the pore size. Hydrated ions, passing through these uniform, sub-nanometer-scale channels (the confined channels of the MOF), can undergo dehydration due to size sieving during their entry into the pores. This allows for selective separation of ions based on differences in their hydration energy and the resulting size changes after dehydration. This structure can be effectively applied to the separation and recovery of resources in water bodies, such as lithium extraction from salt lakes, and in seawater desalination. The composite membrane structure provided by this invention, when selectively separating cations in an aqueous environment, achieves a selectivity coefficient of 123 or higher between monovalent and divalent cations and a selectivity coefficient of 0.89 or higher between monovalent and monovalent cations under the application of an external field. Furthermore, during the preparation of the composite membrane, the mass ratio of the metal salt to the graphene sheet is controlled at (150-250):1, and the reaction temperature in the electrolytic preparation of the MOF layer is 80-120°C. Under the application of an external field, the selectivity coefficient between monovalent and divalent cations can reach 18811 or higher, and the selectivity coefficient between monovalent and monovalent cations can reach 1.42 or higher.

[0112] 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 composite membrane for selective cation separation, characterized in that, The composite membrane includes a support layer and a functional layer located on the surface of the support layer. The functional layer includes a seed layer and an MOF layer located on the surface of the seed layer. The seed layer is in direct contact with the support layer, and the seed layer includes a graphene oxide sheet structure and MOF material grown in situ in the graphene oxide sheet structure. The support layer includes a polyvinylidene fluoride ultrafiltration membrane; The composite membrane for cation selective separation is prepared by the following method, which includes the following steps: (1) Mix the graphene sheet dispersion, organic ligand solution and metal salt solution, react them, and then composite the reacted substances onto the surface of the support layer to obtain a seed layer located on the surface of the support layer. (2) The seed layer located on the surface of the support layer is vertically placed in an H-type electrolytic cell, with a metal salt solution and an organic ligand solution on both sides of the electrolytic cell, and the reaction is carried out to obtain the composite membrane for selective separation of cations. In step (1), the mass ratio of the metal salt to the graphene sheet is (150~250):1; The MOF material includes ZIF type.

2. The composite membrane for cation selective separation according to claim 1, characterized in that, The window size of the MOF material in the seed layer is 0.3~0.6 nm.

3. The composite membrane for cation selective separation according to claim 1, characterized in that, The thickness of the seed layer is 20~30μm.

4. The composite membrane for cation selective separation according to claim 1, characterized in that, The thickness of the MOF layer is 30~40μm.

5. A method for preparing a composite membrane for cation selective separation as described in any one of claims 1-4, characterized in that, The preparation method includes the following steps: (1) Mix the graphene sheet dispersion, organic ligand solution and metal salt solution, react them, and then composite the reacted substances onto the surface of the support layer to obtain a seed layer located on the surface of the support layer. (2) The seed layer located on the surface of the support layer is vertically placed in an H-type electrolytic cell, with a metal salt solution and an organic ligand solution on both sides of the electrolytic cell, and the reaction is carried out to obtain the composite membrane for selective separation of cations. In step (1), the mass ratio of the metal salt to the graphene sheet is (150~250):

1.

6. The method for preparing the composite membrane for cation selective separation according to claim 5, characterized in that, The size of the graphene sheet in step (1) is >500nm.

7. The method for preparing the composite membrane for cation selective separation according to claim 5, characterized in that, The mass concentration of the graphene sheet dispersion in step (1) is 0.5~1 mg / L.

8. The method for preparing the composite membrane for cation selective separation according to claim 5, characterized in that, The molar ratio of the organic ligand to the metal salt in step (1) is (3~5):

1.

9. The method for preparing the composite membrane for cation selective separation according to claim 5, characterized in that, The organic ligands described in step (1) do not contain charged functional groups.

10. The method for preparing the composite membrane for cation selective separation according to claim 5, characterized in that, The reaction described in step (1) is carried out under closed conditions.

11. The method for preparing the composite membrane for cation selective separation according to claim 5, characterized in that, The reaction time in step (1) is 20-30 hours.

12. The method for preparing the composite membrane for cation selective separation according to claim 5, characterized in that, The composite method in step (1) includes redispersing the reacted substances and filtering the redispersed solution on a support layer to obtain a seed layer.

13. The method for preparing the composite membrane for cation selective separation according to claim 12, characterized in that, The mass concentration of the redispersed dispersion is 15~25 mg / L.

14. The method for preparing the composite membrane for cation selective separation according to claim 5, characterized in that, In step (2), the side of the H-type electrolytic cell where the seed layer is located is an organic ligand solution.

15. The method for preparing the composite membrane for cation selective separation according to claim 5, characterized in that, The reaction temperature in step (2) is 80~120℃.

16. The method for preparing the composite membrane for cation selective separation according to claim 5, characterized in that, The reaction time in step (2) is 20-30 hours.

17. The method for preparing the composite membrane for cation selective separation according to claim 5, characterized in that, The preparation method includes the following steps: (1) A graphene sheet dispersion with a mass concentration of 0.5~1 mg / L, an organic ligand solution and a metal salt solution are mixed. The molar ratio of the organic ligand to the metal salt is (3~5):1, and the mass ratio of the metal salt to the graphene sheet is (150~250):

1. The mixture is reacted under closed conditions for 20~30 h. The reacted material is then dispersed again. The dispersed solution is then filtered on a support layer to obtain a seed layer on the surface of the support layer. (2) The seed layer located on the surface of the support layer is placed vertically in an H-type electrolytic cell. The two sides of the electrolytic cell are metal salt solution and organic ligand solution, respectively. The side where the seed layer is located is organic ligand solution. The reaction is carried out at 80~120℃ for 20~30h to obtain the composite membrane for selective separation of cations.

18. Use of the composite membrane as described in any one of claims 1-4, characterized in that, The applications include using the composite membrane for cation-selective separation in an aqueous environment.

19. The use of the composite membrane according to claim 18, characterized in that, An external field is applied to the composite membrane to achieve selective separation of cations.

20. The use of the composite membrane according to claim 19, characterized in that, The external field includes a pressure field and / or an electric field.

21. The use of the composite membrane according to claim 19, characterized in that, The cations include monovalent cations and / or polyvalent cations.

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