COF heterostructure separation membrane, preparation method and application in monovalent cation separation
By designing a heterogeneous structure of stacked sulfonic acid and methyl COF nanosheets on a porous support substrate, the problem of balancing flux and selectivity in the separation of monovalent cations in COF membranes was solved, achieving efficient and stable separation of monovalent cations.
Patent Information
- Application Number
- CN202610053172.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-01-15
- Publication Date
- 2026-03-20
AI Technical Summary
Existing COF membranes struggle to balance high throughput and high selectivity in the separation of monovalent cations. Nanosheets assembled into membranes are prone to non-selective defects, have a simple structure and limited surface charge control capabilities, and the preparation process is complex and lacks reproducibility.
A heterogeneous structure design is adopted, in which a first COF layer and a second COF layer are stacked on a porous support substrate. The first COF layer is assembled from COF nanosheets containing sulfonic acid groups, and the second COF layer is assembled from COF nanosheets containing alkyl substituents. A dense film layer is constructed through oil-water-oil three-phase interface reaction and vacuum filtration technology, thereby controlling the pore structure and surface charge characteristics.
It achieves highly selective separation of monovalent cations, with a dense and continuous membrane layer, stable structure, reliable operation, adaptability to a wide pH range, reduced long-term performance degradation, and good potential for reproducibility and large-scale application.
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Figure CN121695709A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of membrane separation technology, specifically to a method for preparing a sulfonic acid / methyl covalent organic framework (COF) heterostructure separation membrane for highly selective separation of monovalent cations, and the application of the separation membrane in the selective separation of monovalent cations. Background Technology
[0002] With the continuous development of new energy development, resource recycling, and chemical separation processes, efficient and precise separation and purification of solutions containing multiple monovalent cations is of significant scientific and engineering value. Monovalent cations such as lithium, sodium, and potassium ions are widely present in salt lake brines, seawater desalination concentrates, battery recovery solutions, and chemical process fluids. Their efficient separation plays a crucial role in the recovery of key metal resources, the preparation of high-purity salts, and process intensification. However, different monovalent cations typically have the same charge number, and their hydration radii and ion sizes are relatively similar, resulting in highly similar migration behaviors in solution. This makes the highly selective separation of monovalent cations a significant challenge.
[0003] Currently, methods for separating monovalent cations mainly include precipitation, solvent extraction, ion exchange, adsorption, and membrane separation. Among these, membrane separation technology is considered a promising separation method due to its advantages such as low energy consumption, continuous operation, and ease of integration and scale-up. Existing membrane materials mainly include polymer membranes, inorganic membranes, and their composite membranes. Although polymer membranes have advantages such as mature preparation processes and low cost, their pore size distribution is usually wide, and they are prone to swelling or structural aging under high salt, strong acid, or strong alkali conditions, making it difficult to simultaneously achieve high throughput and high selectivity. Inorganic membranes, on the other hand, have high chemical stability, but they generally suffer from problems such as thick membrane layers, harsh preparation conditions, and difficulty in controlling interface defects, which limit their further application in the field of fine ion separation.
[0004] Covalent organic frameworks (COFs) are a class of crystalline porous materials formed by organic structural units linked by covalent bonds. They possess characteristics such as designable pore structures, tunable chemical compositions, and good structural stability. The regular and ordered pore structure and functionalizable framework of COF materials provide a new material platform for ion-selective transport, and have gradually attracted attention in the field of membrane separation in recent years. In particular, two-dimensional COF nanosheets have high long-range order and short mass transfer paths, which are theoretically beneficial for constructing ultrathin, high-throughput separation membranes. However, in practical applications, two-dimensional COF membranes still face several challenges. On the one hand, the controllable preparation and stable dispersion of two-dimensional COF nanosheets are difficult, and the nanosheets are prone to agglomeration in solution, affecting the subsequent film quality. On the other hand, during the assembly of nanosheets into a film, non-selective defects such as pores and cracks are prone to appear in the membrane layer, making it difficult to form a continuous and dense functional layer. In addition, COF membranes with single structures or single surface chemical properties have limitations in pore size control or surface charge control, making it difficult to meet the requirements for fine sieving of monovalent cations. Therefore, how to achieve synergistic regulation of membrane pore structure and surface chemical properties while ensuring the integrity and stability of the membrane structure is an urgent problem to be solved in the current application of COF membranes for the separation of monovalent cations.
[0005] To address the aforementioned issues, existing research has attempted to improve the separation performance of COF membranes by introducing functional group modifications, adjusting film-forming conditions, or constructing composite structures. However, these methods generally suffer from drawbacks such as complex preparation processes, insufficient reproducibility, or limited structural control. Therefore, developing a COF-based separation membrane with a controllable preparation process, stable structure, and the ability to achieve highly selective separation of monovalent cations through rational structural design remains of significant research importance and application value. Summary of the Invention
[0006] To address the challenges of separating monovalent cations in existing technologies, the difficulty of using conventional membrane materials to achieve both high throughput and high selectivity, and the shortcomings of existing COF-based separation membranes such as simple structure, limited ability to control pore and surface charge, easy generation of non-selective defects during nanosheet assembly, insufficient membrane continuity and stability, and limited adjustability and reproducibility of separation performance, this invention aims to provide a method for preparing a sulfonic acid / methyl COF heterostructure separation membrane for highly selective separation of monovalent cations.
[0007] A covalent organic framework (COF) heterostructure separation membrane for highly selective separation of monovalent cations is characterized by comprising a porous support substrate and a COF selective layer disposed on the porous support substrate, wherein the COF selective layer comprises a first COF layer and a second COF layer stacked on each other; wherein the first COF layer is assembled from COF nanosheets containing sulfonic acid groups or their salt groups, and the second COF layer is assembled from COF nanosheets containing alkyl substituents; the framework of the COF nanosheets contains imine bonds.
[0008] The first COF layer and / or the second COF layer are formed by stacking two-dimensional COF nanosheets.
[0009] The first COF layer is located between the porous support substrate and the second COF layer.
[0010] The sulfonic acid group is selected from -SO3H and -SO3−M+, wherein M+ is selected from at least one of H+, Li+, Na+, and K+.
[0011] The alkyl group is a C1-C6 alkyl group.
[0012] The alkyl group is -CH3.
[0013] The COF nanosheets are formed by a Schiff base condensation reaction of a multifunctional aldehyde monomer and an aromatic diamine monomer, wherein the multifunctional aldehyde monomer is an aromatic trialdehyde monomer.
[0014] The aromatic trialdehyde monomer is 2,4,6-tricarboxymethyl phloroglucinol.
[0015] The aromatic diamine monomer forming the first COF layer is an aromatic diamine monomer containing a sulfonic acid group, and the aromatic diamine monomer forming the second COF layer is an aromatic diamine monomer containing an alkyl substituent.
[0016] The alkyl-substituted aromatic diamine monomer is at least one of 2,5-diaminotoluene and / or its positional isomers, and the sulfonic acid-containing aromatic diamine monomer is at least one of 2,5-diaminobenzenesulfonic acid and / or its positional isomers.
[0017] The mass ratio of the first COF layer to the second COF layer is 0.05:1 to 1:1.
[0018] The total thickness of the COF selective layer is 50-800 nm, preferably 100-400 nm.
[0019] The method for preparing the separation membrane includes the following steps:
[0020] (1) COF nanosheet dispersions containing sulfonic acid groups or their salt groups and COF nanosheet dispersions containing alkyl substituents were prepared respectively.
[0021] (2) The COF nanosheet dispersion containing sulfonic acid group or its salt group and the COF nanosheet dispersion containing alkyl substituent are respectively diluted in an aqueous medium and dispersed to obtain a uniform and stable nanosheet dispersion system.
[0022] (3) The COF nanosheet dispersion system containing sulfonic acid groups or their salt groups is deposited on a porous support substrate by vacuum filtration to form a first COF layer, and then cured.
[0023] (4) The COF nanosheet dispersion system containing alkyl substituents is deposited on the surface of the first COF layer by vacuum filtration to form a second COF layer, and then dried to obtain the COF heterostructure separation membrane.
[0024] The COF nanosheets in step (1) are prepared by an oil-water-oil three-phase interface reaction. The bottom oil phase is an organic phase containing dissolved aldehyde monomers, the middle water phase is an acidic aqueous solution, and the top oil phase is an organic phase containing dissolved amine monomers. The aldehyde monomers and the amine monomers undergo a Schiff base condensation reaction at the interface to obtain the COF nanosheets.
[0025] The aldehyde monomer is an aromatic trialdehyde monomer, preferably 2,4,6-tricarboxymethylphloroglucinol; the amount of the aldehyde monomer used is 0.05-0.20 mmol.
[0026] The bottom oil phase solvent is selected from at least one of halogenated hydrocarbon solvents, preferably from at least one of dichloromethane and chloroform; the volume of the bottom oil phase solvent is 50-100 mL.
[0027] The top oil phase solvent is selected from at least one of polar aprotic organic solvents, preferably from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide; the amount of the amine monomer is 0.08-0.30 mmol, and the volume of the top oil phase solvent is 20-80 mL.
[0028] The acidic aqueous solution is an aqueous solution of an organic acid, preferably an aqueous solution of acetic acid; the concentration of the organic acid is 0.5-8.0 mol / L, preferably 1.0-6.0 mol / L.
[0029] The Schiff base condensation reaction is carried out at a temperature of 10-50°C, preferably 20-30°C, and for a reaction time of 1-10 days, preferably 2-7 days.
[0030] In step (1), the COF nanosheets are purified by dialysis for 1-7 days, preferably 2-5 days.
[0031] The dispersion process in step (2) includes ultrasonic dispersion, with an ultrasonic dispersion time of 10-240 min, preferably 30-180 min.
[0032] The curing temperature in step (3) is 40-100℃ and the curing time is 5-60min, preferably 55-65℃ and 10-30min; the drying time in step (4) is 12-72h, preferably 24-60h.
[0033] The application of the separation membrane in the selective separation of monovalent cations.
[0034] The monovalent cation includes Li + Na+, K + 、Rb + Cs + At least two of them.
[0035] The separation process operates at a pH of 3-10.
[0036] The monovalent cation source solution is selected from at least one of salt lake brine, seawater desalination concentrate, battery recovery solution, and chemical process liquid.
[0037] This invention provides a sulfonic acid / methyl COF heterostructure separation membrane for highly selective separation of monovalent cations and its preparation method. Compared with existing ion separation membrane materials and traditional membrane fabrication processes, it has the following advantages:
[0038] 1. Sulfonic acid-based and methyl COF nanosheets were prepared by oil-water-oil three-phase interfacial reaction, and heterogeneous separation membranes were constructed by combining ultrasonic dispersion and layered vacuum filtration. This enabled continuous construction and controllable structure of the membrane, which is conducive to the formation of a dense and stable functional layer, thereby improving separation efficiency while ensuring the integrity of the membrane.
[0039] 2. By assembling sulfonic acid-based COF nanosheets and methyl COF nanosheets with different surface chemical properties in a stepwise manner and regulating their ratio, a heterogeneous membrane structure with synergistic regulation is constructed, thereby enhancing the selective separation ability of monovalent cations and improving separation selectivity and tunability.
[0040] 3. The sulfonic acid / methyl COF heterostructure separation membrane is formed by covalent bonds to form a stable framework structure. The membrane layer is dense and continuous and is firmly bonded to the porous support substrate. It has good structural stability and operational reliability, can work stably in a wide pH range, reduces performance degradation during long-term operation, and has good reproducibility and potential for large-scale preparation and practical application. Attached Figure Description
[0041] Figure 1 This is a scanning electron microscope (SEM) image of the sulfonic acid / methyl COF heterostructure separation membrane prepared in Example 1 of the present invention.
[0042] Figure 2 The images show the X-ray diffraction (XRD) patterns of the methyl COF nanosheets prepared in Examples 1–6 and Comparative Example 2 of this invention.
[0043] Figure 3 The Fourier transform infrared (FT-IR) spectrum of the sulfonic acid / methyl COF heterostructure separation membrane prepared in Example 1 of this invention. Detailed Implementation
[0044] This invention relates to a sulfonic acid / methyl covalent organic framework (COF) heterostructure separation membrane for highly selective separation of monovalent cations, its preparation method, and its application. The preparation method includes: firstly, using an oil-water-oil three-phase interface reaction, 2,4,6-tricarboxymethyl phloroglucinol undergoes a Schiff base condensation reaction with an aromatic diamine monomer containing sulfonic acid or methyl substituents under acidic aqueous phase catalysis to prepare sulfonic acid COF nanosheets and methyl COF nanosheets containing imine bonds, respectively; subsequently, the obtained nanosheets are dispersed in an aqueous medium and ultrasonically treated to form a stable dispersion system; further, sulfonic acid COF nanosheets and methyl COF nanosheets are sequentially deposited onto a porous support substrate using vacuum filtration in a predetermined order to construct a COF heterostructure separation membrane with a well-defined hierarchical structure. The heterostructure membrane achieves synergistic regulation of ion transport behavior through the differences in pore structure and surface charge characteristics of different functional layers. This invention features a simple preparation process, controllable film formation, and can obtain a dense, thickness-adjustable COF heterostructure membrane. The resulting separation membrane exhibits high selectivity and good operational stability during the separation of monovalent cations, which can effectively improve separation efficiency and reduce energy consumption, providing a new membrane material construction strategy for the efficient and selective separation of monovalent cations.
[0045] In some typical implementations, the technical solutions of this patent are described in detail below:
[0046] A method for preparing a sulfonic acid / methyl COF heterostructure separation membrane for highly selective separation of monovalent cations, the specific method being as follows:
[0047] S1: Two-dimensional COF nanosheets were prepared using an oil-water-oil three-phase interface reaction. An aldehyde monomer was dissolved in dichloromethane to form the bottom oil phase, an acidic aqueous solution was used as the intermediate aqueous phase, and an amine monomer was dissolved in N,N-dimethylformamide to form the top oil phase. At the oil-water-oil three-phase interface, the aldehyde monomer and the amine monomer underwent a Schiff base condensation reaction to prepare COF nanosheets containing sulfonic acid groups (–SO3H) and COF nanosheets containing methyl groups (–CH3), respectively.
[0048] Further, in step S1, the aldehyde monomer in the bottom oil phase is 2,4,6-tricarboxymethyl phloroglucinol (Tp), the amount of Tp is 0.05–0.20 mmol, and the volume of dichloromethane is 50–100 mL.
[0049] Furthermore, the top oil phase mentioned in step S1 is formed by dissolving an amine monomer in N,N-dimethylformamide (DMF), with the amount of amine monomer being 0.08–0.30 mmol and the volume of DMF being 20–80 mL.
[0050] Further, the acidic aqueous phase in step S1 is an aqueous solution of acetic acid with a concentration of 1.0–6.0 mol·L⁻¹.
[0051] Furthermore, in step S1, the amine monomer used to prepare methyl COF nanosheets is 2,5-diaminotoluene, and the amine monomer used to prepare sulfonic acid COF nanosheets is 2,5-diaminobenzenesulfonic acid. The reaction temperature is 25 °C, and the reaction time is 2–7 days.
[0052] S2: The sulfonic acid-based COF nanosheets and methyl COF nanosheets obtained in step S1 are collected and purified by dialysis to obtain the corresponding COF nanosheet dispersions. Further, the dialysis purification time is 2–5 days.
[0053] S3: The sulfonic acid-based COF nanosheet dispersion and the methyl COF nanosheet dispersion were respectively diluted in an aqueous medium and subjected to ultrasonic dispersion treatment to obtain a uniform and stable nanosheet dispersion system. Further, the ultrasonic dispersion time was 30–180 min.
[0054] S4: The sulfonic acid-based COF nanosheet dispersion system obtained in step S3 is deposited on a porous support substrate by vacuum filtration to form a sulfonic acid-based membrane, and then cured. Subsequently, the methyl COF nanosheet dispersion system is deposited on the surface of the sulfonic acid-based membrane by vacuum filtration, and after drying, a sulfonic acid / methyl COF heterostructure separation membrane is obtained.
[0055] Furthermore, after the sulfonic acid-based membrane is filtered in step S4, it is cured at 60 °C for 20 min, and after the heterostructure membrane is filtered, it is dried at room temperature for 48 h.
[0056] Furthermore, heterogeneous separation membranes with different ratios were prepared by changing the amount of sulfonic acid COF nanosheets and methyl COF nanosheets added, wherein the amount of sulfonic acid COF nanosheets added was 0.1–1.0 mg, the amount of methyl COF nanosheets added was 1–10 mg, and the mass ratio of the two was 0.05:1–1:1.
[0057] Furthermore, the porous support substrate mentioned in step S4 is one of polytetrafluoroethylene microporous membrane, polyvinylidene fluoride microporous membrane, or polyacrylonitrile porous substrate.
[0058] Furthermore, the thickness of the sulfonic acid / methyl COF heterostructure separation membrane obtained in step S4 is approximately 200 nm.
[0059] The sulfonic acid / methyl COF heterostructure separation membrane described above can be used for the selective separation of monovalent cations.
[0060] Furthermore, the monovalent cation includes Li + Na + K + 、Rb + Cs + Two of them, the separation process operates at pH 3–10.
[0061] Example 1
[0062] A sulfonic acid / methyl COF heterostructure separation membrane for highly selective separation of monovalent cations and its preparation method thereof, the method comprising the following steps:
[0063] S1: Sulfonic acid-based COF nanosheets and methyl COF nanosheets were prepared by oil-water-oil three-phase interface reaction.
[0064] Taking the preparation of methyl COF nanosheets as an example: 2,4,6-tricarboxymethyl phloroglucinol (Tp) was dissolved in dichloromethane to form a bottom oil phase, prepared as 80 mL CH2Cl2 and 0.10 mmol Tp; 20 mL of 3 M acetic acid aqueous solution was used as the intermediate aqueous phase; 2,5-diaminotoluene was dissolved in N,N-dimethylformamide to form a top oil phase, prepared as 50 mL DMF and 0.15 mmol amine monomer. The top oil phase was slowly pushed into the system consisting of the bottom oil phase and the intermediate aqueous phase using a booster. Schiff base condensation reaction occurred at the interface, and the reaction was carried out at 25 °C for 2–7 days to generate methyl COF nanosheets.
[0065] The preparation method of sulfonic acid COF nanosheets is the same as the above process, except that the amine monomer is replaced with an aromatic diamine monomer containing sulfonic acid groups.
[0066] S2: Collect the sulfonic acid-based COF nanosheets and methyl COF nanosheets obtained in step S1 separately, place them in dialysis bags, and dialyze them in deionized water for 2–5 days to obtain the corresponding COF nanosheet dispersions.
[0067] S3: Take the sulfonic acid-based COF nanosheet dispersion and the methyl COF nanosheet dispersion obtained in step S2 and dilute them in an aqueous medium. Taking the sulfonic acid-based COF nanosheets as an example, take 0.8 mg of nanosheets and add them to a 250 mL beaker, dilute with deionized water to 200 mL, and place in an ultrasonic cleaner for ultrasonic dispersion for 90 min to obtain a uniform and stable nanosheet dispersion system; the methyl COF nanosheet dispersion system is prepared in the same way, wherein the amount of methyl COF nanosheets added is 3.0 mg.
[0068] S4: The sulfonic acid-based COF nanosheet dispersion system obtained in step S3 is deposited on a porous support substrate by vacuum filtration to form a sulfonic acid-based membrane; after filtration, it is cured at 60 °C for 20 min. Subsequently, the methyl COF nanosheet dispersion system is deposited on the surface of the sulfonic acid-based membrane by vacuum filtration. After filtration, it is dried at room temperature for 48 h to obtain a sulfonic acid / methyl COF heterostructure separation membrane, denoted as SO3H / CH3–COF–1 membrane.
[0069] Example 2
[0070] A sulfonic acid / methyl COF heterostructure separation membrane for highly selective separation of monovalent cations and its preparation method are disclosed. Steps S1–S4 are the same as in Example 1, except that the amount of sulfonic acid-based COF nanosheets added in step S3 is different, wherein the amount of sulfonic acid-based COF nanosheets added is 0.6 mg. The remaining steps are the same as in Example 1, resulting in a sulfonic acid / methyl COF heterostructure separation membrane, denoted as SO3H / CH3–COF–2 membrane.
[0071] Example 3
[0072] A sulfonic acid / methyl COF heterostructure separation membrane for highly selective separation of monovalent cations and its preparation method are disclosed. Steps S1–S4 are the same as in Example 1, except that the amount of sulfonic acid-based COF nanosheets added in step S3 is different, wherein the amount of sulfonic acid-based COF nanosheets added is 1.0 mg. The remaining steps are the same as in Example 1, resulting in a sulfonic acid / methyl COF heterostructure separation membrane, denoted as SO3H / CH3–COF–3 membrane.
[0073] Example 4
[0074] A sulfonic acid / methyl COF heterostructure separation membrane for highly selective separation of monovalent cations and its preparation method are disclosed. Steps S1–S4 are the same as in Example 1, except that the amount of methyl COF nanosheets added in step S3 is different; the amount of methyl COF nanosheets added is 1.0 mg. The remaining steps are the same as in Example 1, resulting in a sulfonic acid / methyl COF heterostructure separation membrane, denoted as SO3H / CH3–COF–4 membrane.
[0075] Example 5
[0076] A sulfonic acid / methyl COF heterostructure separation membrane for highly selective separation of monovalent cations and its preparation method are disclosed. Steps S1–S4 are the same as in Example 1, except that the amount of methyl COF nanosheets added in step S3 is different; the amount of methyl COF nanosheets added is 2.0 mg. The remaining steps are the same as in Example 1, resulting in a sulfonic acid / methyl COF heterostructure separation membrane, denoted as SO3H / CH3–COF–5 membrane.
[0077] Example 6
[0078] A sulfonic acid / methyl COF heterostructure separation membrane for highly selective separation of monovalent cations and its preparation method are disclosed. Steps S1–S4 are the same as in Example 1, except that the amount of methyl COF nanosheets added in step S3 is different; the amount of methyl COF nanosheets added is 4.0 mg. The remaining steps are the same as in Example 1, resulting in a sulfonic acid / methyl COF heterostructure separation membrane, denoted as SO3H / CH3–COF–6 membrane.
[0079] Furthermore, this invention also applies the sulfonic acid / methyl COF heterostructure separation membranes prepared in Examples 1–6 above to the selective separation of monovalent cations. This invention employs concentration-driven ion separation testing, and the apparatus consists of a feed cell, a membrane, and a permeation cell. The COF membrane is sandwiched in the middle of an H-type diffusion cell (effective permeation radius of 1.5 cm). 0.1 mol / L Li + and 0.1 mol / LK + A salt mixture solution was added to the feed side, while 200 mL of deionized water was added to the permeate side. To eliminate concentration polarization and maintain uniform experimental conditions, magnetic stir bar was used on both sides for stirring, and the separation test lasted for 24 h. The separation test was conducted at room temperature (approximately 25 °C), and at least four COF membranes prepared under the same conditions were tested to obtain the average value. The ion concentrations measured by permeation were analyzed to evaluate the selective separation performance of the sulfonic acid / methyl COF heterostructure separation membrane for monovalent cations. The test results show that the above-mentioned sulfonic acid / methyl COF heterostructure separation membrane can be used for the selective separation of monovalent cations, and exhibits good selective separation capability and operational stability.
[0080] Comparative Example 1
[0081] A sulfonic acid-based COF membrane for selective separation of monovalent cations and a method for preparing the same, the method comprising the following steps:
[0082] S1: Sulfonic acid-based COF nanosheets were prepared by an oil-water-oil three-phase interface reaction. 2,4,6-tricarboxymethyl phloroglucinol (Tp) and an aromatic diamine monomer containing sulfonic acid groups underwent a Schiff base condensation reaction at the oil-water-oil three-phase interface and reacted at 25 °C for 2–7 days to generate sulfonic acid-based COF nanosheets.
[0083] S2: Collect the sulfonic acid COF nanosheets obtained in step S1, place them in a dialysis bag, and dialyze them in deionized water for 2–5 days to obtain a sulfonic acid COF nanosheet dispersion.
[0084] S3: Take the sulfonic acid-based COF nanosheet dispersion obtained in step S2 and dilute it in an aqueous medium. Taking sulfonic acid-based COF nanosheets as an example, take 0.8 mg of nanosheets and add them to a 250 mL beaker, add deionized water to dilute to 200 mL, and ultrasonically disperse in an ultrasonic cleaner for 90 min to obtain a uniform and stable nanosheet dispersion system.
[0085] S4: The sulfonic acid-based COF nanosheet dispersion system obtained in step S3 is deposited on a porous support substrate by vacuum filtration to form a sulfonic acid-based COF monolayer membrane. After filtration, it is cured at 60 °C for 20 min and then dried at room temperature for 48 h to obtain a sulfonic acid-based COF single-component separation membrane, denoted as SO3H–COF–single-component membrane.
[0086] Comparative Example 2
[0087] A methyl COF membrane for selective separation of monovalent cations and a method for preparing the same, the method comprising the following steps:
[0088] S1: Methyl COF nanosheets were prepared by an oil-water-oil three-phase interface reaction. 2,4,6-tricarboxymethyl phloroglucinol (Tp) and 2,5-diaminotoluene underwent a Schiff base condensation reaction at the oil-water-oil three-phase interface and reacted at 25 °C for 2–7 days to generate methyl COF nanosheets.
[0089] S2: Collect the methyl COF nanosheets obtained in step S1, place them in a dialysis bag, and dialyze them in deionized water for 2–5 days to obtain a methyl COF nanosheet dispersion.
[0090] S3: Take the methyl COF nanosheet dispersion obtained in step S2 and dilute it in an aqueous medium. Taking methyl COF nanosheets as an example, take 3.0 mg of nanosheets and add them to a 250 mL beaker, add deionized water to dilute to 200 mL, and ultrasonically disperse in an ultrasonic cleaner for 90 min to obtain a uniform and stable nanosheet dispersion system.
[0091] S4: The methyl COF nanosheet dispersion system obtained in step S3 is deposited on a porous support substrate by vacuum filtration to form a methyl COF membrane; after filtration, it is cured at 60 ℃ for 20 min and then dried at room temperature for 48 h to obtain a methyl COF single-component separation membrane, denoted as CH3–COF–single-component membrane.
[0092] Table 1 Test Performance Table
[0093]
[0094] Table 1 shows that the resolution of Examples 1-6 ranged from 2.54 to 4.98, all higher than that of Comparative Examples 1 and 2. In particular, Example 1 achieved a resolution of 4.98, approximately 2.42 times higher than Comparative Example 1 and approximately 6.30 times higher than Comparative Example 2. This result indicates that it is difficult to achieve high resolution using only a single functional layer of COF, while constructing a heterostructure with stacked sulfonic acid COF layers and methyl COF layers can significantly improve the selective separation ability of monovalent cations. From the flux data, the heterostructure membrane at K... + Flux remained between 271.6 and 364.83 mmol / m³. 2 ·h, at the same time, for Li + The flux exhibits a stronger suppression trend; conversely, Comparative Example 2, although possessing higher Li... + Flux and a certain K + While the flux was high, the resolution was only 0.79, indicating insufficient ability to distinguish between different monovalent cations. Therefore, the heterostructure of this invention does not simply pursue increased flux, but rather significantly improves the key indicator of resolution while maintaining usable flux through structural design. Table 1 further shows that the performance of the heterostructure membrane can be adjusted by adding the amounts of two types of COF nanosheets, exhibiting an optimal ratio range. When the amount of sulfonic acid COF nanosheets remained constant, increasing the amount of methyl COF nanosheets from 1.0 mg to 2.0 mg and 3.0 mg increased the resolution from 2.54 to 3.06 and further to 4.98. When the amount of methyl COF nanosheets increased to 4.0 mg, the resolution dropped back to 3.50, indicating the existence of an optimal methyl layer loading range. With the amount of methyl COF nanosheets remaining at 3.0 mg, changing the amount of sulfonic acid COF nanosheets resulted in resolutions of 3.65 and 3.46, respectively, also demonstrating that the separation performance is adjustable with changes in the layer ratio.
Claims
1. A covalent organic framework (COF) heterostructure separation membrane for highly selective separation of monovalent cations, characterized in that, The invention includes a porous support substrate and a COF selective layer disposed on the porous support substrate. The COF selective layer includes a first COF layer and a second COF layer stacked on top of each other. The first COF layer is assembled from COF nanosheets containing sulfonic acid groups or their salt groups, and the second COF layer is assembled from COF nanosheets containing alkyl substituents. The framework of the COF nanosheets contains imine bonds.
2. The separation membrane according to claim 1, characterized in that, The first COF layer and / or the second COF layer are formed by stacking two-dimensional COF nanosheets, and the first COF layer is located between the porous support substrate and the second COF layer; the sulfonic acid group is selected from -SO3H and -SO3. − M + M + Selected from H + Li + Na + K + At least one of the following; the alkyl group is a C1-C6 alkyl group, preferably -CH3.
3. The separation membrane according to claim 1, characterized in that, The COF nanosheets are formed by a Schiff base condensation reaction of a multifunctional aldehyde monomer and an aromatic diamine monomer, wherein the multifunctional aldehyde monomer is an aromatic trialdehyde monomer, preferably 2,4,6-tricarboxymethyl phloroglucinol; the aromatic diamine monomer forming the first COF layer is an aromatic diamine monomer containing a sulfonic acid group, and the aromatic diamine monomer forming the second COF layer is an aromatic diamine monomer containing an alkyl substituent; preferably, the aromatic diamine monomer containing an alkyl substituent is at least one of 2,5-diaminotoluene and / or its positional isomers, and the aromatic diamine monomer containing a sulfonic acid group is at least one of 2,5-diaminobenzenesulfonic acid and / or its positional isomers.
4. The separation membrane according to claim 1, characterized in that, The mass ratio of the first COF layer to the second COF layer is 0.05:1-1:1; the total thickness of the COF selective layer is 50-800 nm, preferably 100-400 nm.
5. A method for preparing the separation membrane according to any one of claims 1-4, characterized in that, The process includes the following steps: (1) preparing COF nanosheet dispersions containing sulfonic acid groups or their salt groups and COF nanosheet dispersions containing alkyl substituents, respectively; (2) diluting the COF nanosheet dispersions containing sulfonic acid groups or their salt groups and the COF nanosheet dispersions containing alkyl substituents in an aqueous medium and dispersing them to obtain a uniform and stable nanosheet dispersion system; (3) depositing the COF nanosheet dispersion system containing sulfonic acid groups or their salt groups onto a porous support substrate by vacuum filtration to form a first COF layer, and then performing a curing treatment; (4) depositing the COF nanosheet dispersion system containing alkyl substituents onto the surface of the first COF layer by vacuum filtration to form a second COF layer, and then performing a drying treatment to obtain the COF heterostructure separation membrane.
6. The method according to claim 5, characterized in that, The COF nanosheets in step (1) are prepared by an oil-water-oil three-phase interface reaction. The bottom oil phase is an organic phase containing dissolved aldehyde monomers, the middle water phase is an acidic aqueous solution, and the top oil phase is an organic phase containing dissolved amine monomers. The aldehyde monomers and the amine monomers undergo a Schiff base condensation reaction at the interface to obtain the COF nanosheets.
7. The method according to claim 6, characterized in that, The aldehyde monomer is an aromatic trialdehyde monomer, preferably 2,4,6-tricarboxymethyl phloroglucinol, and the amount of the aldehyde monomer used is 0.05-0.20 mmol; the bottom oil phase solvent is selected from at least one of halogenated hydrocarbon solvents, preferably at least one of dichloromethane and chloroform, and the volume of the bottom oil phase solvent is 50-100 mL; the top oil phase solvent is selected from at least one of polar aprotic organic solvents, preferably at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide, and the amount of the amine monomer used is 0.08-0.30 mmol, and the volume of the top oil phase solvent is 20-80 mL.
8. The method according to claim 6, characterized in that, The acidic aqueous solution is an organic acid aqueous solution, preferably an acetic acid aqueous solution; the concentration of the organic acid is 0.5-8.0 mol / L, preferably 1.0-6.0 mol / L; the reaction temperature of the Schiff base condensation reaction is 10-50℃, preferably 20-30℃, and the reaction time is 1-10 days, preferably 2-7 days; in step (1), the COF nanosheets are purified by dialysis, and the dialysis purification time is 1-7 days, preferably 2-5 days.
9. The method according to claim 5, characterized in that, The dispersion treatment in step (2) includes ultrasonic dispersion, with an ultrasonic dispersion time of 10-240 min, preferably 30-180 min; the curing treatment temperature in step (3) is 40-100℃ and the curing time is 5-60 min, preferably 55-65℃ and 10-30 min; the drying treatment time in step (4) is 12-72 h, preferably 24-60 h.
10. The application of the separation membrane according to any one of claims 1-4 in the selective separation of monovalent cations, wherein the monovalent cations include Li + Na + K + 、Rb + Cs + The separation process operates at a pH of 3-10, and the monovalent cation source solution is selected from at least one of salt lake brine, seawater desalination concentrate, battery recovery solution, and chemical process liquid.