COF membrane, preparation method thereof and application of COF membrane in cesium-sodium separation

Abstract

The invention discloses a COF membrane, a preparation method thereof and application of the COF membrane in cesium-sodium separation. The preparation method of the COF membrane comprises the following steps: preparing carboxyl COF nanosheet water-phase colloid by a three-phase method; activating the carboxyl COF nanosheet water-phase colloid by EDC / NHSs to prepare a crown ether modified COF nanosheet solution; and preparing the crown ether modified COF membrane on the surface of the substrate membrane through vacuum-assisted self-assembly suction filtration. The method has the advantages that the introduction of the crown ether effectively reduces the aperture of the COF to prevent all hydrated ions from passing through, and meanwhile, the crown ether selectively recognizes sodium ions to ensure that the sodium ions pass through a crown ether cavity in a bare ion form and then pass through a membrane while cesium ions are intercepted.

Description

A COF membrane, its preparation method, and its application in cesium sodium separation. Technical Field

[0001] This invention relates to the field of membrane technology, and in particular to a COF membrane, its preparation method, and its application in the separation of cesium and sodium. Background Technology

[0002] Cesium (Cs), as a strategic resource, plays a crucial role in catalyst development, aerospace technology, and battery production. Salt lake brines are rich in Cesium resources. + The concentration was around 1-9 mg / L, and cesium was further enriched in the tail brine after lithium extraction. Therefore, the recovery of Cs from natural brine resources is possible. + It is imperative to develop efficient cesium extraction and separation technologies, which are of great significance to national resource security and the development of high-end manufacturing industries.

[0003] Implement Cs + The key to extraction lies in the high-purity separation of these cations from brine rich in coexisting ions. The monovalent cations possess identical valence states, sub-nanometer ion sizes, and size differences of less than 2 Å, making the separation of mixtures of monovalent cations one of the most challenging tasks. In salt lake brine, Na... + With an absolute numerical advantage (compared to Cs) + The difference in concentration (3-6 orders of magnitude higher) has become the primary bottleneck affecting separation efficiency and economy. Existing technologies struggle to achieve precise differentiation between cesium and sodium ions. Precipitation and solvent extraction methods have limited selectivity and are susceptible to high concentrations of sodium. + Interference, high reagent consumption, and potential secondary pollution are issues. Ion exchange and adsorption methods are less effective in high Na+ environments. + In this context, the limited number of active sites are rapidly saturated, affecting Cs. + The dynamic adsorption capacity is low, and Cs + / Na +The selectivity factor is insufficient to meet economical separation requirements. Membrane separation technology has advantages such as high selectivity, high efficiency and energy saving, continuous operation, and environmental friendliness. For cesium extraction, the key lies in the selection of membrane materials, thereby achieving the separation of monovalent ions. The amorphous structure of polymer membranes leads to a trade-off between permeability and selectivity, resulting in a performance upper limit. Furthermore, their chemical structure is a randomly cross-linked network, which determines that polymer membranes lack precise specific recognition sites. The development of emerging materials is an effective way to overcome performance limitations, such as graphene oxide and MXene. Graphene membranes have been reported to be used in nanofiltration, reverse osmosis, and salt separation, but their application in monovalent cation separation is limited. The main reason is that their abundant hydrophilic groups swell, significantly increasing the interlayer spacing and leading to a decrease in selectivity. Currently, highly selective metal-organic framework (MOF) membranes have been reported, but MOFs are prone to hydrolysis, exhibiting low stability, which limits their application in separation processes. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a COF membrane, its preparation method, and its application in cesium-sodium separation. This invention utilizes the reaction between the carboxyl groups within the pores of the COF and the amino groups of the crown ether (CE), confining the crown ether within the COF pores with matching pore size, thus obtaining a crown ether-modified COF membrane for efficient sieve separation of cesium and sodium. The advantage of this method lies in the effective reduction of the COF pore size through the introduction of the crown ether, preventing all hydrated ions from passing through. Simultaneously, the crown ether selectively recognizes sodium ions, ensuring that sodium ions pass through the CE cavity as bare ions and then through the membrane, while cesium ions are retained.

[0005] This invention provides a COF membrane, its preparation method, and its application in cesium sodium separation. The technical solution is as follows: In a first aspect, this invention provides a method for preparing a COF membrane, comprising the following steps: Step 1): Using 2,5-diaminobenzoic acid solution as the upper phase, sodium hydroxide solution as the intermediate buffer phase, and 1,3,5-tricarboxymethyl phloroglucinol solution as the base phase, the reaction is carried out under static conditions. The upper phase is removed, and the intermediate buffer phase solution is collected using a dropper. The resulting aqueous intermediate buffer phase solution is purified by a first dialysis to obtain a carboxyl-modified COF nanosheet aqueous colloid; Step 2): EDC / NHSs solution is mixed with the carboxyl-modified COF nanosheet aqueous colloid obtained in Step 1) to activate the carboxyl groups; then CE solution is added, and a crosslinking reaction is performed. A second dialysis purification is then performed to obtain a CE-modified COF nanosheet solution; Step 3): The crown ether-modified COF nanosheet solution obtained in Step 2) is vacuum-assisted self-assembly filtration onto the surface of a substrate membrane to prepare a CE-modified COF membrane.

[0006] Preferably, in step 1), the solvent for the 2,5-diaminobenzoic acid solution is one or more of DMF, DMSO, acetonitrile, methanol, or ethanol; the solvent for the sodium hydroxide solution is water; and the solvent for the 1,3,5-tricarboxymethyl phloroglucinol solution is one or more of dichloromethane, n-hexane, ethyl acetate, toluene, mesitylene, anisole, or dioxane.

[0007] Preferably, in step 1), the concentration of the 2,5-diaminobenzoic acid solution is 0.001~0.01 mmol / mL; the concentration of the sodium hydroxide solution is 0.01~0.1 mol / L; and the concentration of the 1,3,5-tricarboxymethyl phloroglucinol solution is 0.001~0.01 mmol / mL.

[0008] Preferably, in step 1), the static reaction temperature is 18~80℃; the static reaction time is 1~5 days.

[0009] Preferably, in step 1), the first dialysis purification uses a dialysis bag with a capacity of 30,000 to 100,000 Da.

[0010] Preferably, in step 2), the EDC / NHSs solution is prepared by the following method: EDC·HCl and NHSs in a mass ratio of 1:1~10 are dissolved in PBS buffer solution and stirred for 10~30 min to ensure complete dissolution and prevent deactivation of the coupling agent, thereby obtaining the EDC / NHSs solution.

[0011] Among them, EDC·HCl (N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride, CAS No. 25952-53-8) and NHSs (N-hydroxysuccinimide sulfonate sodium salt, CAS No. 106627-54-7).

[0012] Preferably, in step 2), the molar ratio of carboxyl COF nanosheets to CE is 1:0.5~3; CE is one or more of 2-aminomethyl-15-crown-5, 4'-aminobenzo-15-crown-5-ether, 4'-aminobenzo-18-crown-6, 4'-aminodibenzo-18-crown-6, and 1-aza-12-crown-4-ether; the activation time of the carboxyl group is 10~60 min, and the activation temperature of the carboxyl group is 0~25℃; the crosslinking reaction time is 6~12 h; the crosslinking reaction temperature is 0~25℃; the crosslinking reaction rotation speed is 100~200 r; the second dialysis purification is carried out using a 30000~100000 Da dialysis bag for 1~3 days.

[0013] Preferably, in step 3), the base membrane is a PAN membrane with a molecular weight cutoff of 10,000 to 1,500,000 Da or a PES membrane with a pore size of 0.1 to 0.22 μm.

[0014] In a second aspect, the present invention provides a COF membrane prepared by the above-described method for preparing a COF membrane.

[0015] A third aspect of the present invention provides an application of the above-described COF membrane in the separation of cesium and sodium.

[0016] Compared with existing technologies, this invention has the following advantages: This invention prepares a crown ether-modified COF membrane with a pore size of 14 Å, providing sufficiently narrow ion-selective channels to achieve selective separation of cesium ions. First, the pores of the COF membrane possess order, regularity, and uniformity, enabling precise ion screening. Simultaneously, it combines covalent bonding and crystal structure characteristics, ensuring stability under harsh operating conditions. Second, the crown ether is introduced into the pores of the COF through a precise post-modification strategy, enhancing the selective recognition of monovalent ions under confined conditions, thus achieving selective separation of cesium ions. This invention improves the separation efficiency of cesium ions by modifying the nanosheets to achieve the highest possible modification rate. Attached Figure Description

[0017] Figure 1 is a SEM image of the COF nanosheets obtained in Example 1 of the present invention; Figure 2 is a SEM image of the COF membrane obtained in Example 1 of the present invention; Figure 3 is an ATR-FTIR image of the COF membrane obtained in Example 1 of the present invention; Figure 4 is the separation factor of the PAN and CE modified COF membranes of the present invention. Detailed Implementation

[0018] The present invention will be further described below with reference to the accompanying drawings and specific embodiments to make the invention more apparent and understandable, but the embodiments are not intended to limit the invention.

[0019] This invention provides a COF membrane, its preparation method, and its application in cesium-sodium separation. The COF membrane preparation method includes: preparing a carboxyl-modified COF nanosheet aqueous colloid using a three-phase method; activating the carboxyl-modified COF nanosheet aqueous colloid with EDC / NHSs to prepare a crown ether-modified COF nanosheet solution; and preparing the crown ether-modified COF membrane on the substrate membrane surface via vacuum-assisted self-assembly filtration. The advantage of this invention is that the introduction of the crown ether effectively reduces the pore size of the COF, preventing all hydrated ions from passing through. Simultaneously, the crown ether selectively recognizes sodium ions, ensuring that sodium ions pass through the crown ether cavity as bare ions and then through the membrane, while cesium ions are retained.

[0020] Example 1 This invention provides a method for preparing a COF membrane, comprising the following steps: I. Preparation of carboxyl COF nanosheet aqueous colloid by three-phase method: 50 mL of 0.003 mmol / mL 2,5-diaminobenzoic acid in DMF is used as the upper phase, 20 mL of 0.05 mol / L NaOH solution is used as the intermediate buffer phase (aqueous phase), and 80 mL of 0.00125 mmol / mL 1,3,5-tricarboxymethyl phloroglucinol in dichloromethane is used as the bottom phase; subsequently, the reaction is carried out at 25°C under static conditions for 3 days; after the reaction is completed, the organic upper phase is removed, the intermediate buffer aqueous phase is collected with a dropper, and the aqueous solution is purified by the first dialysis using a 30000 Da dialysis bag. After dialysis and purification with deionized water for 3 days, a transparent carboxyl COF nanosheet aqueous colloid is obtained.

[0021] II. Preparation of Crown Ether-Modified COF Nanosheets: The amino groups in the crown ether molecules undergo a coupling reaction with carboxyl COF nanosheets activated by EDC / NHSs chemical reagents. 28 mg EDC·HCl and 28 mg NHSs were dissolved in 2 mL PBS and stirred for 10 min to ensure complete dissolution and prevent deactivation of the coupling agent, yielding an EDC / NHSs solution. 20 mL of the carboxyl COF nanosheet aqueous colloid obtained in step I was taken, and then the prepared EDC / NHSs solution was added. The carboxyl groups were activated at 25 °C for 30 min. After activation, 0.15 mmol of 2-aminomethyl-15-crown-5 (CAS No. 83585-56-2, 37.5 mg of 2-aminomethyl-15-crown-5 dispersed in 3 mL of water) was added, and the mixture was crosslinked at 25 °C for 12 h on a shaker at 120 rpm. After the reaction, the mixture was purified for 1 day using a 30000 Da dialysis bag to obtain a crown ether-modified COF nanosheet solution.

[0022] III. Preparation of crown ether modified COF membrane: Take 30 mL (the thickness of the COF membrane can be controlled by the volume of the solution during filtration) of an aqueous solution of crown ether modified COF nanosheets with a concentration of 1.1 mg / mL, and prepare the crown ether modified COF membrane on the surface of a PAN membrane with a molecular weight cutoff of 100,000 Da by vacuum-assisted self-assembly filtration.

[0023] The morphology of the obtained crown ether modified COF nanosheets was verified by scanning electron microscopy (SEM). As shown in Figure 1, the nanosheets exhibited a sheet-like morphology with a lateral size of 5-10 μm and a good two-dimensional conformation, proving the successful synthesis of the nanosheets.

[0024] The obtained crown ether-modified COF membrane was characterized by SEM and ATR-FTIR. As shown in Figure 2, the COF membrane surface is smooth and the thickness is about 1.4 μm; as shown in Figure 3, the ATR-FTIR results show that 1,3,5-tricarboxymethyl phloroglucinol (Tp) is at 2890 cm⁻¹. -1The -CHO peak at wavelength and the 2,5-diaminobenzoic acid (Pa-COOH) peak at 3292 cm⁻¹ -1 The -NH2 peak disappears at wavelength, and the COF film at 1575 cm⁻¹... -1 Typical C=C stretching vibrations were observed at 1254 cm. -1 The CN stretching vibration at the site proves the success of the Schiff base reaction and the successful synthesis of the COF membrane.

[0025] The separation factor of cesium sodium was tested using a diffusion cell: The experimental setup was a U-shaped diffusion cell with a volume of 200 mL. Stirring was required to prevent concentration polarization. A crown ether-modified COF membrane was fixed in the middle of the diffusion cell. A mixed salt solution containing 0.1 M NaCl and 0.1 M CsCl was added to the feed side, and an equal volume of deionized water was added to the permeate side. Data collection: ICP MS / OES was performed on the permeate side at specific time points to obtain the ion concentration. The ion permeability was calculated based on the change in ion concentration on the permeate side using the following formula:

[0026] Where C (g / L) is the osmotic ion concentration, V (L) is the osmotic solution volume, and A (m 2 The effective membrane area is given by Δt(h), Mr (g / mol) is the relative molecular mass of the ion, and Δt(h) is the permeation time. The membrane selectivity (separation factor) is calculated using the following formula:

[0027] Among them, P i (mol·h) -1 ·m -2 ) and P j (mol·h) -1 ·m -2 ) represent the permeability of ions i and j, respectively, ΔC i and ΔC j Let i and j represent the concentration gradients of ion i and ion j, respectively.

[0028] As shown in Figure 4, the separation factor of PAN substrate membrane for sodium cesium ions is 1.13±0.07, while the separation factor of crown ether modified COF membrane (CE-COF) is 1.49±0.02. Compared with PAN membrane, crown ether modified COF membrane has a separation factor that is about 32% higher, thus achieving separation and purification of cesium ions.

[0029] Example 2 This invention provides a method for preparing a COF membrane, comprising the following steps: Same as Example 1, except that: in step two, the crown ether is 4'-aminobenzo-15-crown-5-ether.

[0030] Example 3 This invention provides a method for preparing a COF membrane, comprising the following steps: Same as Example 1, except that: in step two, the crown ether is 4'-aminobenzo-18-crown ether-6.

[0031] Example 4 This invention provides a method for preparing a COF membrane, comprising the following steps: Same as Example 1, except that: in step three, the base membrane is a PES membrane with a pore size of 0.22 μm.

[0032] Obviously, the above embodiments are merely for illustrating the present invention and are not intended to limit the implementation. Those skilled in the art can make other variations or modifications based on the above description, and these variations or modifications also fall within the scope of the present invention.

Claims

1. A method for preparing a COF membrane, characterized in that, Includes the following steps: Step 1): Using 2,5-diaminobenzoic acid solution as the upper phase, sodium hydroxide solution as the intermediate buffer phase, and 1,3,5-tricarboxymethyl phloroglucinol solution as the base phase, the reaction was carried out under static conditions. The upper phase was removed, and the intermediate buffer phase solution was collected with a dropper. The resulting aqueous intermediate buffer phase solution was purified by a first dialysis to obtain carboxyl-modified COF nanosheet aqueous colloid. Step 2): EDC / NHSs solution was mixed with the carboxyl-modified COF nanosheet aqueous colloid obtained in Step 1) to activate the carboxyl groups. Then, crown ether solution was added, and a crosslinking reaction was carried out. A second dialysis purification was performed to obtain crown ether-modified COF nanosheet solution. Step 3): The crown ether-modified COF nanosheet solution obtained in Step 2) was vacuum-assisted self-assembly filtration onto the surface of a substrate membrane to prepare a crown ether-modified COF membrane.

2. The method for preparing a COF membrane according to claim 1, characterized in that, In step 1), the solvent for the 2,5-diaminobenzoic acid solution is DMF; the solvent for the sodium hydroxide solution is water; and the solvent for the 1,3,5-tricarboxymethyl phloroglucinol solution is dichloromethane.

3. The method for preparing a COF membrane according to claim 1, characterized in that, In step 1), the concentration of the 2,5-diaminobenzoic acid solution is 0.001~0.01 mmol / mL; the concentration of the sodium hydroxide solution is 0.01~0.1 mol / L; and the concentration of the 1,3,5-tricarboxymethylphloroglucinol solution is 0.001~0.01 mmol / mL.

4. The method for preparing a COF membrane according to claim 1, characterized in that, In step 1), the static reaction temperature is 18~80℃; the static reaction time is 1~5 days.

5. The method for preparing a COF membrane according to claim 1, characterized in that, In step 1), the first dialysis purification uses a dialysis bag with a capacity of 30,000 to 100,000 Da.

6. The method for preparing a COF membrane according to claim 1, characterized in that, In step 2), the EDC / NHSs solution is prepared by the following method: EDC·HCl and NHSs in a mass ratio of 1:1~10 are dissolved in PBS buffer solution and stirred for 10~30 min to ensure complete dissolution and prevent deactivation of the coupling agent, thereby obtaining the EDC / NHSs solution.

7. The method for preparing a COF membrane according to claim 1, characterized in that, In step 2), the molar ratio of carboxyl COF nanosheets to crown ether is 1:0.5~3; the crown ether is 2-aminomethyl-15-crown-5, 4'-aminobenzo-15-crown-5-ether, or 4'-aminobenzo-18-crown-ether-6; the activation time of carboxyl groups is 10~60 min; the activation temperature of carboxyl groups is 0~25℃; the crosslinking reaction time is 6~12 h; the crosslinking reaction temperature is 0~25℃; the crosslinking reaction rotation speed is 100~200 r; the second dialysis purification is carried out using a 30000~100000 Da dialysis bag for 1~3 days.

8. The method for preparing a COF membrane according to claim 1, characterized in that, In step 3), the base membrane is a PAN membrane with a molecular weight cutoff of 10,000 to 1,500,000 Da or a PES membrane with a pore size of 0.1 to 0.22 μm.

9. A COF membrane, characterized in that, The COF membrane is prepared by the method described in claims 1 to 8.

10. The application of the COF membrane as described in claim 9 in the separation of cesium and sodium.

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