Functional group-regulated COF (Chip On Filter) membrane as well as preparation method and application thereof

Abstract

The invention relates to the technical field of membrane separation, in particular to a functional group regulated covalent organic framework (COF) membrane and a preparation method thereof, and relates to application of the membrane in selective separation of monovalent cations. The preparation method comprises the following steps: condensing a polyaldehyde monomer and aromatic diamine with different side chain substitutions on an interface through oil-water-oil three-phase interface micro-fluidic dripping to generate a two-dimensional COF nanosheet, carrying out ultrasonic dispersion, carrying out vacuum filtration, self-assembling on a porous substrate, and drying to obtain a continuous separation layer with the thickness of 50-150nm. The functional group can regulate and control pore channels and surface charges, flux and selectivity of monovalent cations such as Li < + > can be adjusted, effective separation is kept at the pH of 3-11, and the functional group is suitable for lithium salt extraction and electrolyte purification.

Description

[0001] Shao Feng, Cao Qian, Li Long Technical Field

[0002] This invention relates to the field of membrane separation technology, and more particularly to a functional group-regulated covalent organic framework (COF) membrane and its preparation method, and to the application of the membrane in the selective separation of monovalent cations. Background Technology

[0003] With the increasing demand for new energy, electrochemical energy storage, and resource separation, efficient separation and purification of solutions containing multiple monovalent cations (such as lithium, sodium, and potassium) is of great significance. Monovalent cations are widely found in salt lake brines, seawater desalination concentrates, metallurgical and chemical process fluids, and battery recycling. Their separation and purification have significant engineering value for high-purity salt preparation, key metal resource extraction, and process intensification. However, compared to multivalent ions, different monovalent cations usually have the same charge number and small differences in hydration radius, resulting in similar migration behaviors in solution, thus posing a challenge to highly selective separation.

[0004] Currently, monovalent cation separation technologies mainly include precipitation, solvent extraction, ion exchange, adsorption, and membrane separation. Among these, membrane separation technology has attracted widespread attention due to its advantages such as low energy consumption, continuous operation, ease of scaling, and process integrability. Existing membrane materials for ion separation include polymer membranes, inorganic membranes, and composite membranes. Polymer membranes are easy to process but are prone to swelling or aging under strong acid, alkali, and high salt environments, and their pore size distribution and charge control capabilities are limited. Inorganic membranes, while possessing high chemical stability, typically suffer from problems such as large membrane thickness, difficulty in controlling interface defects, and high preparation costs, making it difficult to simultaneously achieve high throughput and high selectivity.

[0005] Covalent organic frameworks (COFs), as crystalline porous materials formed by covalent bonds, possess advantages such as designable pore size, tunable structure, and good chemical stability. Their regular channels and functionalizable frameworks provide a new material platform for ion-selective transport. In particular, two-dimensional COF nanosheets have high long-range order and short mass transfer paths, which theoretically facilitate the construction of high-throughput, high-selectivity ultrathin separation membranes. However, in practical applications, COF membranes still face the following challenges: First, the controllable preparation and stable dispersion of two-dimensional COF nanosheets are difficult, and they are prone to aggregation; second, during the assembly of nanosheets into films, pores, cracks, and non-selective defects easily occur, making it difficult to form continuous and dense ultrathin films; third, the tunable range of membrane pore structure and surface charge is limited, making it difficult to achieve precise sieving of different monovalent cations; fourth, existing membrane preparation methods still have room for improvement in terms of film formation speed, repeatability, and large-scale preparation. Therefore, developing a COF membrane with a designable structure and separation performance that can be regulated by functional groups, and with a simple and controllable preparation process, is of great significance for achieving efficient and selective separation of monovalent cations. Summary of the Invention

[0006] To address the challenges of separating monovalent cations in existing technologies, the difficulty of achieving both high throughput and high selectivity with conventional membrane materials, and the shortcomings of existing COF membranes such as easy agglomeration of nanosheets, poor dispersion stability, non-selective defects such as pores / cracks during membrane assembly, difficulty in precisely controlling membrane continuity and thickness, insufficient adjustability of separation performance, and limited reproducibility and scalability of preparation, this invention aims to provide a method for preparing functional group-regulated covalent organic framework (COF) membranes.

[0007] A functional group-regulated covalent organic framework (COF) membrane for selective separation of monovalent cations is disclosed. The COF membrane includes a separation layer formed by the self-assembly of stacked two-dimensional COF nanosheets. The two-dimensional COF nanosheets are composed of a COF framework formed by the interfacial condensation reaction of an aldehyde-containing aromatic polyaldehyde monomer and an amino-containing aromatic diamine monomer. The aromatic diamine monomer has side-chain substituted functional groups to regulate the pore structure and / or surface charge characteristics of the separation layer, thereby achieving selective transport of different monovalent cations.

[0008] The aromatic polyaldehyde monomer is 2,4,6-tricarboxymethyl phloroglucinol.

[0009] The side chain substituted functional group is selected from at least one of C1-C6 alkyl, hydroxyl, C1-C4 alkoxy, halogen and nitro.

[0010] The side chain substituted functional group is selected from one of -CH3, -OH, -OCH3, -Cl, and -NO2.

[0011] The aromatic diamine monomer is selected from at least one of 2,5-diaminotoluene, 2,5-diaminophenol or its salts, 2-methoxyphenyl-1,4-diamine, 2-chloro-1,4-phenylenediamine, and nitro-substituted aromatic diamines.

[0012] The separation layer is a continuous ultrathin film layer.

[0013] The thickness of the separation layer is 50-150 nm.

[0014] The COF membrane is a composite membrane, and the separation layer is deposited on a porous support substrate.

[0015] The porous support substrate is one of polytetrafluoroethylene microporous membrane, polyvinylidene fluoride microporous membrane, or polyacrylonitrile microporous membrane.

[0016] The method for preparing the functional group-regulated COF membrane includes the following steps:

[0017] S1, Construct an oil-water-oil three-phase system: Dissolve aromatic polyaldehyde monomers in a first organic solvent as the bottom oil phase, dissolve aromatic diamine monomers in a second organic solvent as the upper oil phase, and use an acidic or alkaline aqueous solution as the intermediate aqueous phase;

[0018] S2, the upper oil phase is slowly added to the surface of the intermediate aqueous phase using a micro-injection pump, so that the aromatic polyaldehyde monomer and the aromatic diamine monomer undergo a condensation reaction at the interface to generate two-dimensional COF nanosheets.

[0019] S3, the two-dimensional COF nanosheets are dispersed in an aqueous medium to obtain a nanosheet dispersion, and the nanosheet dispersion is subjected to ultrasonic dispersion treatment to obtain a uniform and stable nanosheet dispersion system.

[0020] S4, the nanosheet dispersion system is deposited on a porous support substrate by vacuum filtration and dried to obtain the functional group-regulated COF membrane.

[0021] The first organic solvent is a halogenated hydrocarbon solvent.

[0022] The first organic solvent is dichloromethane.

[0023] The amount of aromatic polyaldehyde monomer used in step S1 is 0.05-0.20 mmol, and the volume of the first organic solvent is 50-150 mL.

[0024] The second organic solvent is a polar aprotic organic solvent.

[0025] The second organic solvent is N,N-dimethylformamide.

[0026] The amount of aromatic diamine monomer used in step S1 is 0.08-0.25 mmol, and the volume of the second organic solvent is 20-100 mL.

[0027] The intermediate aqueous phase in step S1 is an aqueous solution of acetic acid or an aqueous solution of sodium bicarbonate.

[0028] The concentration of the acetic acid aqueous solution is 1.0-5.0 mol / L, and / or the concentration of the sodium bicarbonate aqueous solution is 0.1-1.0 mol / L.

[0029] In step S3, the amount of two-dimensional COF nanosheets added is 1-10 mg, and the volume of the aqueous medium is 50-300 mL.

[0030] The ultrasonic dispersion treatment in step S3 takes 30-180 minutes.

[0031] The drying temperature in step S4 is 50-80℃.

[0032] The application of functional group-regulated COF membranes in the selective separation of monovalent cations.

[0033] The monovalent cation includes Li + Na + K + 、Rb + Cs + At least two of them.

[0034] The separation process operates at a pH of 3-11.

[0035] This invention provides a functional group-regulated covalent organic framework (COF) membrane for the 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:

[0036] 1. Two-dimensional COF nanosheets were prepared by oil-water-oil three-phase interface reaction, and then combined with ultrasonic dispersion and vacuum filtration to self-assemble into a membrane, so as to achieve continuous construction and controllable thickness (50-150 nm) of the super COF membrane, effectively shortening the ion transport path and reducing mass transfer resistance, thereby improving separation efficiency while ensuring structural integrity.

[0037] 2. The COF membrane framework can be controlled by different substituent functional groups, so that the pore structure, surface hydrophilicity and hydrophobicity and surface charge characteristics of the membrane can be designed and adjusted, thereby enhancing the recognition and selective separation ability of different monovalent cations and improving the separation selectivity and applicability.

[0038] 3. The COF 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. It can work stably within a certain pH range, reduce performance degradation during long-term operation, and has value for large-scale preparation and practical application. Attached Figure Description

[0039] Figure 1 This is a schematic diagram illustrating the preparation and structure of COF nanosheets based on different side chain groups according to the present invention.

[0040] Figure 2 The images show the X-ray diffraction (XRD) patterns of the functional group-modulated COF nanosheets prepared in Examples 1-5 and the comparative examples of this invention.

[0041] Figure 3 The images show the Zeta potentials of the functional group-regulated COF membranes prepared in Examples 1-5 and the comparative examples of this invention at pH 3-10.

[0042] Figure 4 The diagram shows the water contact angle (WCA) of the functional group-regulated COF membranes prepared in Examples 1-5 and the comparative examples of this invention. Detailed Implementation

[0043] This invention relates to a method for preparing a functional group-controlled covalent organic framework (COF) membrane and its application in the selective separation of monovalent cations. The preparation method includes: specifically, using aromatic diamines with different side-chain functional groups as amine monomers and 2,4,6-tricarboxymethyl phloroglucinol as an aldehyde monomer, and combining a microfluidic pump to precisely control the time of the amine monomers entering the reaction system, two-dimensional COF materials containing different functional groups were successfully synthesized. Subsequently, the nanosheets were dispersed in water and subjected to ultrasonic treatment to form a stable dispersion system; then, the nanosheets were stacked and self-assembled into a membrane by vacuum filtration, and after drying, a COF membrane with controllable thickness was obtained. The functional groups of the membrane include –CH3, –OH, –OCH3, –Cl, –NO2, etc. By controlling the stacking density, pore structure, and surface charge characteristics of the nanosheets through functional group regulation, the membrane separation performance can be designed. This invention has a simple preparation process, strong controllability, and can prepare continuous ultrathin films with a thickness of approximately 50–150 nm. The resulting COF membrane exhibits tunable wettability and surface charge. In monovalent cation separation applications, the COF membrane demonstrates high selectivity and good stability, significantly improving separation efficiency and reducing operating energy consumption. This invention provides an effective technical route for the construction and application of high-efficiency monovalent cation selective separation membrane materials.

[0044] In some typical implementations, the technical solutions of this patent are described in detail below:

[0045] A method for preparing a functional group-controlled COF membrane for selective separation of monovalent cations, the specific method being as follows:

[0046] S1: 2,4,6-Trimethylol-Resorcinol was dissolved in an organic solvent to form the bottom oil phase. A diamine monomer was added to another organic solvent to form the upper oil phase. Sodium bicarbonate aqueous solution and acetic acid aqueous solution were used as the intermediate layers. While keeping the lower oil phase stable, the intermediate aqueous phase was slowly poured in along the beaker wall. The upper oil phase was placed in a 50 ml syringe, which was then fixed to a laboratory microinjection pump. After both phases stabilized, the upper oil phase was slowly added dropwise onto the surface of the aqueous phase by pressing the start button. The three-phase system was allowed to react statically in a 25 ℃ biochemical incubator to generate COF nanosheets.

[0047] Furthermore, in step S1, the amount of Tp in the bottom oil phase is 0.05-0.20 mmol, and the volume of dichloromethane is 50-150 mL.

[0048] Furthermore, in step S1, the amount of the aromatic diamine monomer in the top oil phase is 0.08-0.25 mmol, and the volume of the N,N-dimethylformamide is 20-100 mL.

[0049] Furthermore, the substituent of the aromatic diamine monomer with substituents mentioned in step S1 is one of -CH3, OH, -OCH3, -Cl, and -NO2.

[0050] Further, the catalyst aqueous solution mentioned in step S1 is preferably an aqueous solution of acetic acid or an aqueous solution of sodium bicarbonate, with an acetic acid concentration of 1.0-5.0 mol / L. -1 The sodium bicarbonate concentration is 0.1-1.0 mol / L. -1 .

[0051] S2: The two-dimensional COF nanosheets obtained in step S1 are dispersed in an aqueous medium to obtain a COF nanosheet dispersion. Further, the amount of the two-dimensional COF nanosheets added is 1-10 mg, and the volume of the aqueous medium is 50-300 mL.

[0052] S3: The COF nanosheet dispersion obtained in step S2 is subjected to ultrasonic dispersion treatment to obtain a uniform and stable nanosheet dispersion system. Further, the ultrasonic dispersion time is 30-180 min.

[0053] S4: The nanosheet dispersion system obtained in step S3 is deposited onto a porous support substrate using vacuum filtration and then dried to obtain a functional group-controlled COF membrane for selective separation of monovalent cations. Further, the porous support substrate is one of a polytetrafluoroethylene microporous membrane, a polyvinylidene fluoride microporous membrane, or a polyacrylonitrile microporous membrane. Further, the drying temperature is 50-80 °C. Further, the thickness of the COF membrane obtained in step S4 is 50–150 nm.

[0054] The above-mentioned functional group regulation of COF membranes in the selective separation of monovalent cations.

[0055] Example 1

[0056] A functional group-regulated covalent organic framework (COF) membrane for selective separation of monovalent cations and its preparation method are disclosed below:

[0057] (1) 2,4,6-tricarboxymethyl phloroglucinol (Tp) was dissolved in dichloromethane (CH2Cl2) 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 (-CH3-substituted aromatic diamine) was dissolved in N,N-dimethylformamide (DMF) to form a top oil phase, prepared as 50 mL DMF and 0.15 mmol diamine monomer. The top oil phase was slowly pushed into the system consisting of the bottom oil phase / intermediate aqueous phase using a booster, so that Tp and the diamine monomer would undergo a Schiff base condensation reaction at the interface to generate two-dimensional COF nanosheets.

[0058] (2) The two-dimensional COF nanosheets obtained in step (1) are dispersed in an aqueous medium to obtain a COF nanosheet dispersion.

[0059] (3) Weigh 3 mg of two-dimensional COF nanosheets and add them to a 250 mL beaker. Add deionized water to dilute to 200 mL and place in an ultrasonic cleaner for ultrasonic dispersion for 90 min to obtain a uniform and stable nanosheet dispersion system.

[0060] (4) The nanosheet dispersion system obtained in step (3) is deposited on a porous support substrate by vacuum filtration. After filtration, it is taken out and dried to obtain a COF membrane for selective separation of monovalent cations, denoted as TpPa-CH3 membrane.

[0061] Example 2

[0062] A functional group-controlled COF membrane for selective separation of monovalent cations and its preparation method are disclosed, with the specific steps as follows:

[0063] The difference from Example 1 is that the aromatic diamine monomer used in step (1) is 2,5-diaminophenol dihydrochloride (-OH). The remaining steps are the same as in Example 1, and a COF membrane is obtained, which is denoted as TpPa-OH membrane.

[0064] Example 3

[0065] A functional group-controlled COF membrane for selective separation of monovalent cations and its preparation method are disclosed, with the specific steps as follows:

[0066] The difference from Example 1 is that the aromatic diamine monomer used in step (1) is 2-methoxyphenyl-1,4-diamine (-OCH3). The remaining steps are the same as in Example 1, and a COF membrane is obtained, which is denoted as TpPa-OCH3 membrane.

[0067] Example 4

[0068] A functional group-controlled COF membrane for selective separation of monovalent cations and its preparation method are disclosed, with the specific steps as follows:

[0069] The difference from Example 1 is that the aromatic diamine monomer used in step (1) is 2-chloro-1,4-phenylenediamine (-Cl), and the aqueous catalyst used is 20 mL of 1 M sodium bicarbonate aqueous solution. The remaining steps are the same as in Example 1, and a COF membrane is obtained, denoted as TpPa-Cl membrane.

[0070] Example 5

[0071] A functional group-controlled COF membrane for selective separation of monovalent cations and its preparation method are disclosed, with the specific steps as follows:

[0072] The difference from Example 1 is that the aromatic diamine monomer used in step (1) is an aromatic diamine monomer with a -NO2 substituent (-NO2). The remaining steps are the same as in Example 1, and a COF membrane is obtained, denoted as TpPa-NO2 membrane.

[0073] Comparative Example

[0074] A functional group-controlled COF membrane for selective separation of monovalent cations and its preparation method are disclosed, with the specific steps as follows:

[0075] The difference from Example 1 is that the aromatic diamine monomer used in step (1) is 1,4-p-phenylenediamine, which is not modified with functional groups. The remaining steps are the same as in Example 1, and a COF membrane is obtained, which is denoted as TpPa membrane.

[0076] Furthermore, the functional group-controlled COF membranes prepared in Examples 1-6 above are also applied to the selective separation of monovalent cations, and tested using a concentration-driven ion separation testing system. An H-type diffusion cell device consisting of a feed cell, a separation membrane, and a permeation cell is used. The target separation membrane is sandwiched between the two cells, and the effective permeation radius of the membrane is precisely controlled to be 1.5 cm. 200 ml of a prepared 0.1 mol / L Li₂ solution is injected into one side of the feed cell. + and 0.1 mol / LK + A salt mixture solution was prepared, and 200 ml of deionized water was added to the permeate tank. The concentration difference between the two solutions was used to construct the driving force for ion-driven migration. To eliminate concentration polarization, magnetic stir bar was placed in both the feed tank and the permeate tank and stirring was started simultaneously. The entire separation test was conducted stably at room temperature (approximately 25°C) for 24 hours. Feed solutions were prepared using deionized water and the pH was adjusted to obtain feed solutions under different acid-base conditions, with pH values ​​set to 3, 7, and 11. The test results in Table 1 were obtained at pH=7. After the test, solution samples were collected from the permeate tank, and the concentration of monovalent cations in the samples was determined using ion chromatography. Combined with the effective permeate area of ​​the membrane and the test duration, the ion flux and resolution were calculated to evaluate the membrane's selective separation performance for monovalent cations. The test results in Tables 2 and 3 show that the functional group-modulated COF membranes described above can all be used for the selective separation of monovalent cations, and their separation performance is adjustable under different pH conditions.

[0077] Table 1. Test Performance Table (pH=7)

[0078]

[0079] Table 2 Test Performance Table (pH=3)

[0080]

[0081] Table 3 Test Performance Table (pH=11)

[0082]

[0083] As shown in Table 1, there are significant differences in separation performance corresponding to different functional groups. In terms of ion flux, Li + The flux varied in the range of 125.47–434.24 mmol / m²·h, K +The flux varied from 128.46 to 625.42 mmol / m²·h, indicating that the ion transport rate can be adjusted by changing the side chain functional groups of the COF membrane framework. Regarding resolution, the resolution of Examples 1-5 varied from 0.69 to 0.97, demonstrating that under the membrane fabrication route of this invention, introducing different substituent functional groups into the COF membrane framework allows for adjustable changes in both flux and resolution, thereby enhancing the recognition and selective separation of different monovalent cations. This verifies the technical concept of functional group regulation to achieve designable separation performance. Tables 2 and 3 show that the membrane exhibits high ion selectivity under acidic conditions, possibly due to pore size contraction caused by the acidic environment, which enhances selectivity but reduces flux. Under alkaline conditions, ion selectivity decreases, possibly because imine bonds hydrolyze in an alkaline environment, leading to framework structure destruction and pore enlargement, thus reducing selectivity.

Claims

1. A functional group-regulated covalent organic framework (COF) membrane for the selective separation of monovalent cations, characterized in that, The COF membrane includes a separation layer formed by the self-assembly of stacked two-dimensional COF nanosheets. The two-dimensional COF nanosheets are composed of a COF framework formed by the interfacial condensation reaction of aromatic polyaldehyde monomers containing aldehyde groups and aromatic diamine monomers containing amino groups. The aromatic diamine monomers have side-chain substituted functional groups to regulate the pore structure and / or surface charge characteristics of the separation layer, thereby achieving selective transport of different monovalent cations.

2. The COF membrane according to claim 1, characterized in that, The aromatic polyaldehyde monomer is 2,4,6-tricarboxymethyl phloroglucinol; the side chain substituted functional group is selected from at least one of C1-C6 alkyl, hydroxyl, C1-C4 alkoxy, halogen and nitro.

3. The COF membrane according to claim 1, characterized in that, The side chain substituted functional group is selected from one of -CH3, -OH, -OCH3, -Cl, and -NO2.

4. The COF membrane according to claim 1, characterized in that, The aromatic diamine monomer is selected from at least one of 2,5-diaminotoluene, 2,5-diaminophenol or its salts, 2-methoxyphenyl-1,4-diamine, 2-chloro-1,4-phenylenediamine, and nitro-substituted aromatic diamines.

5. The COF membrane according to claim 1, characterized in that, The separation layer is a continuous ultrathin film layer; the thickness of the separation layer is 50-150 nm; the COF membrane is a composite membrane, and the separation layer is deposited on a porous support substrate; the porous support substrate is one of polytetrafluoroethylene microporous membrane, polyvinylidene fluoride microporous membrane, or polyacrylonitrile microporous membrane.

6. A method for preparing a functional group-regulated COF membrane according to any one of claims 1-5, characterized in that, Includes the following steps: S1, Construct an oil-water-oil three-phase system: Dissolve aromatic polyaldehyde monomers in a first organic solvent as the bottom oil phase, dissolve aromatic diamine monomers in a second organic solvent as the upper oil phase, and use an acidic or alkaline aqueous solution as the intermediate aqueous phase; S2, the upper oil phase is slowly added to the surface of the intermediate aqueous phase using a micro-injection pump, so that the aromatic polyaldehyde monomer and the aromatic diamine monomer undergo a condensation reaction at the interface to generate two-dimensional COF nanosheets. S3, the two-dimensional COF nanosheets are dispersed in an aqueous medium to obtain a nanosheet dispersion, and the nanosheet dispersion is subjected to ultrasonic dispersion treatment to obtain a uniform and stable nanosheet dispersion system. S4, the nanosheet dispersion system is deposited on a porous support substrate by vacuum filtration and dried to obtain the functional group-regulated COF membrane.

7. The preparation method according to claim 6, characterized in that, The first organic solvent is a halogenated hydrocarbon solvent; the first organic solvent is dichloromethane; the amount of the aromatic polyaldehyde monomer used in step S1 is 0.05-0.20 mmol, and the volume of the first organic solvent is 50-150 mL; the second organic solvent is a polar aprotic organic solvent; the second organic solvent is N,N-dimethylformamide; the amount of the aromatic diamine monomer used in step S1 is 0.08-0.25 mmol, and the volume of the second organic solvent is 20-100 mL; the intermediate aqueous phase in step S1 is an aqueous solution of acetic acid or an aqueous solution of sodium bicarbonate; the concentration of the aqueous solution of acetic acid is 1.0-5.0 mol / L, and / or the concentration of the aqueous solution of sodium bicarbonate is 0.1-1.0 mol / L.

8. The preparation method according to claim 6, characterized in that, The amount of two-dimensional COF nanosheets added in step S3 is 1-10 mg, and the volume of the aqueous medium is 50-300 mL; the ultrasonic dispersion treatment time in step S3 is 30-180 min; and the drying temperature in step S4 is 50-80 °C.

9. The application of the functional group-regulated COF membrane according to any one of claims 1-5 in the selective separation of monovalent cations.

10. The application according to claim 9, characterized in that, The monovalent cation includes Li + Na + K + 、Rb + Cs + At least two of them; the separation process is operated at pH 3-11.

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