Photo-responsive nanochannel membrane, preparation method and application thereof

Abstract

The application discloses a kind of photoresponsive nanochannel membranes and its preparation method and application.The preparation method is: photoresponsive molecule of sulfuryl fluoride is dissolved in acetonitrile solution containing catalyst, grafting solution is obtained;Covalent organic framework composite membrane is soaked in grafting solution and placed to react, and photoresponsive nanochannel membrane is obtained.Covalent organic framework composite membrane is obtained by adding equal volume of amino monomer aqueous solution and aldehyde monomer organic phase solution on both sides of porous substrate, and placed to react.The application is first grafted to the covalent organic framework membrane pore by SuFEx reaction, photoresponsive molecule, so that the nanochannel membrane prepared has photoresponsive, and the pore size is less than the hydration diameter of ion to be separated in salt lake brine;Lithium ion can be precisely and efficiently filtered, and is expected to be popularized and applied in salt lake lithium extraction.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, specifically to a photoresponsive nanochannel membrane, its preparation method, and its application. Background Technology

[0002] Lithium metal plays a crucial role in green energy storage. To achieve its "dual carbon" goals, my country's lithium demand is growing at an average annual rate of nearly 30%. As of 2020, China's proven lithium reserves accounted for approximately 6.3% of the global total, but these reserves are mainly distributed in salt lake brines due to their low lithium-ion concentration (0.003-0.157 wt.%) and the presence of ions with similar properties (such as Mg). 2+ Ca 2+ Na + K + The presence of various ions (such as carbon dioxide, sulfur dioxide, and nitrogen dioxide) makes lithium extraction from salt lakes difficult. Existing lithium extraction technologies from salt lakes mainly include evaporation, adsorption, solvent extraction, and membrane separation. Membrane separation, due to its simplicity, reagent-free operation, energy efficiency, and environmental friendliness, has gradually replaced traditional lithium extraction processes in recent years. However, the complex composition of salt lake brine often necessitates the coupling of multiple processes to separate lithium ions from other ions, increasing operating costs. Therefore, developing lithium-ion selective membrane materials is crucial for achieving efficient lithium extraction using membrane methods.

[0003] Membrane pores interact with ions through steric hindrance, van der Waals interactions, and electrostatic interactions; the strength of these interactions affects ion transport behavior. By modulating pore properties and enhancing the interaction between the pores and specific ions, the selectivity for target ions can be improved. In recent years, inspired by ion transport phenomena in biological cell membranes, biomimetic membrane materials with stimulus-responsive nanochannels have been successfully prepared, and their responsive pores show great promise for improving the ion selectivity of membrane materials. However, existing stimulus-responsive nanochannel membrane preparation methods are cumbersome, costly, and require harsh operating conditions, severely hindering the application of such membrane materials in lithium extraction from salt lakes.

[0004] Covalent organic frameworks (COFs) have broad application prospects in the construction of biomimetic nanochannel membranes due to their well-defined pore structures, tunable pore properties, and stable pore structures. For example, patent application number 202111618234.9 discloses a COF composite membrane and its in-situ polymerization preparation method. This method involves contacting an acidified support membrane with a two-component organic solution containing amine and aldehyde monomers, causing the amine and aldehyde monomers to undergo in-situ polymerization on the surface of the acidified support membrane. This composite membrane is then used for the removal and desalination of small molecules such as dyes and drugs. However, the inherent pore size of most existing COFs is larger than 1 nm (larger than the hydrated diameter of the ions to be separated in brine), making it difficult to efficiently sieve the target ions. Furthermore, the lack of functional groups on the pore walls that interact with the target ions weakens the selectivity. In addition, the pores lack stimulus-responsiveness, preventing the dynamic gradient separation of ions. Therefore, the application of COF membrane materials in lithium extraction from brine is limited. Introducing stimulus-responsive molecules into the nanopores holds promise for overcoming the inherent limitations of COFs. Currently, pre-modification and post-synthetic modification methods have been developed to prepare porous materials with stimulus-responsive nanochannels. However, most existing materials are in powder form, which differs significantly from the pore regulation mechanism of membrane materials and cannot be directly used for the preparation of stimulus-responsive nanochannel membrane materials. Therefore, it is necessary to design a novel nanochannel membrane containing covalent organic framework materials to achieve precise and efficient filtration of lithium ions, thereby improving the efficiency of lithium extraction from salt lakes and reducing costs. Summary of the Invention

[0005] To address the aforementioned limitations of existing technologies, the present invention aims to provide a photoresponsive nanochannel membrane, its preparation method, and its applications. This invention, for the first time, utilizes a hexavalent sulfur-fluorine exchange (SuFEx) click reaction to precisely graft photoresponsive molecules into the pores of a covalent organic framework membrane, resulting in a photoresponsive nanochannel membrane with a pore size smaller than the hydrated diameter of the ions to be separated in brine. This enables precise and efficient filtration of lithium ions and holds promise for widespread application in lithium extraction from brine lakes.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing a photoresponsive nanochannel membrane, the method comprising:

[0008] The photoresponsive sulfonyl fluoride molecule was dissolved in an acetonitrile solution containing a catalyst to obtain a grafting solution; the covalent organic framework composite membrane was immersed in the grafting solution and allowed to react statically to obtain a photoresponsive nanochannel membrane.

[0009] Preferably, the thioyl fluoride photoresponsive molecule is selected from 4-thioyl fluoride azobenzene, 4-((4-thioyl fluoride phenyl)diazeninyl)benzoic acid, or 4-[(4-thioyl fluoride phenyl)diazeninyl]benzenesulfonic acid; the catalyst comprises 2-tert-butyl-1,1,3,3-tetramethylguanidine and hexamethyldisilazane; the ratio of the amount of 2-tert-butyl-1,1,3,3-tetramethylguanidine to hexamethyldisilazane added is (0.23-2.3) μL:(0.45-1.8) mg.

[0010] Preferably, the concentration of the thioyl fluoride photoresponsive molecule in the grafting solution is ≤0.112 mM; the concentration of 2-tert-butyl-1,1,3,3-tetramethylguanidine in the grafting solution is 0.11-1.14 mM, and the concentration of hexamethyldisilazane is 0.28-1.12 mM.

[0011] Preferably, the covalent organic framework composite membrane is prepared by the following method:

[0012] Equal volumes of an aqueous solution of an amino monomer and an organic solution of an aldehyde monomer were added to both sides of a porous substrate, and the mixture was allowed to stand to react, resulting in a covalent organic framework composite membrane.

[0013] Preferably, the porous substrate is a polyacrylonitrile ultrafiltration membrane with a molecular weight cutoff of 10-100 kDa; the amino monomer is selected from 1,2,3-triaminoguanidine, 1,3,5-tris(4-aminophenyl)benzene, tris(4-aminophenyl)amine or 2,4,6-tris(4-aminophenyl)-1,3,5-triazine; and the aldehyde monomer is 2,5-dihydroxyterephthalaldehyde and / or 2,5-dimethoxyterephthalaldehyde.

[0014] More preferably, the molar ratio of 2,5-dihydroxyterephthalaldehyde to 2,5-dimethoxyterephthalaldehyde is (0-3):(0-3).

[0015] Preferably, the concentration of amino monomer in the aqueous phase solution is 0.025-0.25 mM; the concentration of aldehyde monomer in the organic phase solution is 0.037-0.37 mM; the temperature of the static reaction is 30°C; and the time of the static reaction is 96 h.

[0016] More preferably, the solvent in the aqueous phase solution is an acetic acid solution with a concentration of 1-6M; the solvent in the organic phase solution is an ethyl acetate / trimethylbenzene mixture with a volume ratio of ethyl acetate to trimethylbenzene of (1-3):1.

[0017] Preferably, the molar ratio of the 2,5-dihydroxyterephthalaldehyde and the thioyl fluoride photoresponsive molecule is 1:1.

[0018] Preferably, the temperature of the static reaction is 30°C, and the time of the static reaction is 5-60 minutes.

[0019] In a second aspect, the present invention provides a photoresponsive nanochannel membrane obtained by the above preparation method, wherein the pore size of the photoresponsive nanochannel membrane increases under ultraviolet light irradiation and decreases under visible light irradiation.

[0020] A third aspect of the present invention provides the application of a photoresponsive nanochannel membrane in any of the following 1) to 2):

[0021] 1) Gradient separation of mixed salt solutions;

[0022] 2) Lithium extraction from salt lakes.

[0023] The beneficial effects of this invention are:

[0024] (1) This invention is the first to use SuFEx click reaction to precisely graft photoresponsive molecules into the pores of a covalent organic framework membrane, so that the prepared nanochannel membrane has photoresponsiveness and the pore size is smaller than the hydration diameter of the ions to be separated in the brine of the salt lake; it can achieve precise and efficient filtration of lithium ions and is expected to be widely used in lithium extraction from salt lakes.

[0025] (2) The preparation method of this invention is simple, and efficient grafting can be achieved in just 5-60 minutes. The membrane material preparation process is simple, requiring only room temperature and normal pressure. The raw materials are readily available and have strong versatility. The separation layer pores obtained by this invention have photoresponsiveness, and the pore size can be controlled at the sub-nanometer level (0.74-0.9 nm). When this membrane material is used for permeation separation of mixed salt solutions, gradient separation of mixed ions can be achieved by adjusting the light stimulation of different wavelengths, and it has long-term operational stability. Attached Figure Description

[0026] Figure 1 This is a scanning electron microscope image of the covalent organic framework composite membrane prepared in the examples;

[0027] Figure 2 This is a microscopic molecular structure diagram of the covalent organic framework composite membrane prepared in the examples;

[0028] Figure 3 This is a scanning electron microscope image of the photoresponsive nanochannel membrane prepared in the example;

[0029] Figure 4 These are microscopic molecular structure diagrams of the photoresponsive nanochannel membranes prepared in the examples under irradiation with light of different wavelengths;

[0030] Figure 5 These are water contact angle test images of the photoresponsive nanochannel membranes prepared in the examples under light irradiation of different wavelengths;

[0031] Figure 6 The graphs show the ion separation performance of the membranes prepared in the control example and the example under different wavelengths of light irradiation. Detailed Implementation

[0032] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0033] As described in the background section, most existing covalent organic framework materials have inherent pore sizes larger than 1 nm (larger than the hydrated diameter of the ions to be separated in brine), making it difficult to efficiently sieve the target ions by size. Furthermore, the lack of functional groups on the pore walls that interact with the target ions weakens their selectivity. In addition, the pores lack stimulus-responsiveness, preventing the realization of gradient separation of ions through dynamic pore channels. Therefore, the application of covalent organic framework membrane materials in lithium extraction from brine lakes is limited.

[0034] Based on this, the purpose of this invention is to provide a photoresponsive nanochannel membrane, its preparation method, and its applications. This invention utilizes a SuFEx click reaction, where fluoride ions in the thiofluoride group interact closely with silicon in the catalyst to form thermodynamically favorable Si-F bonds. This breaks the strongly covalently bonded thiofluoride group, achieving SO bond connection, which can be further accelerated under organic base catalysis. Photoresponsive molecules are precisely grafted into the pores of a covalent organic framework membrane. Through stimulation with different wavelengths of light, the photoresponsive molecules undergo molecular isomerization under different wavelengths of light irradiation. When irradiated with ultraviolet light, the grafted photoresponsive molecules change to a cis configuration, contracting and opening the pores; when irradiated with visible light, the grafted photoresponsive molecules change from a cis configuration back to a trans configuration, extending and closing the pores. This achieves controllable adjustment of the nanochannels under different wavelengths of light irradiation. Traditional membrane-based lithium extraction processes often require coupling with other processes to achieve the separation of lithium ions from multivalent mixed ions. The membrane material with tunable channels proposed in this invention can separate lithium ions / other monovalent ions and lithium ions / multivalent ions into a single membrane, achieving gradient separation of multivalent ions (e.g., calcium and magnesium ions), lithium ions, and other monovalent ions (e.g., sodium and potassium ions) through illumination of different wavelengths. Under different light irradiation, the hydrophilicity of the surface of the photoresponsive nanochannel membrane of this invention changes significantly, which is due to changes in the photoresponsive molecular configuration. Under ultraviolet light irradiation, its hydrophilicity decreases. This phenomenon may be due to the grafted photoresponsive molecule changing to a cis configuration under ultraviolet light, with the hydrophilic sulfonic acid group being embedded and the relatively hydrophobic azo group being exposed. The pores become larger, but this does not affect the passage of lithium ions. At the same time, it can block the passage of divalent ions (such as calcium and magnesium ions). Under visible light irradiation, its hydrophilicity is restored, indicating that the grafted photoresponsive molecule reverts to a trans configuration under visible light irradiation. The more hydrophilic sulfonic acid group is exposed again, and the pores become smaller, allowing other monovalent ions (such as potassium ions) to pass through, while lithium ions cannot pass through, resulting in lithium ion enrichment.

[0035] To enable those skilled in the art to better understand the technical solution of this application, the technical solution of this application will be described in detail below with reference to specific embodiments.

[0036] The test materials used in the embodiments of this invention are all conventional test materials in the art and can be purchased through commercial channels.

[0037] Example

[0038] (1) Preparation of covalent organic framework composite membranes:

[0039] A polyacrylonitrile ultrafiltration membrane with a molecular weight cutoff of 100 kDa was used as a porous substrate. A circular porous substrate with a diameter of 2 cm was fixed in the middle of the diffusion cell, thus dividing the cell into two small compartments, each with a volume of 30 mL. 26.4 mg of 1,3,5-tris(4-aminophenyl)benzene was dissolved in 30 mL of the solution. The mixture was sonicated in 3M acetic acid solution for 10 min and then added to the left chamber of the diffusion cell. 9.3 mg of 2,5-dihydroxyterephthalaldehyde and 10.9 mg of 2,5-dimethoxyterephthalaldehyde (phenolic hydroxyl content 50 mol%) were dissolved in a mixed solution of 22.5 mL ethyl acetate and 7.5 mL mesitylene (volume ratio 3:1), sonicated for 10 min, and then added to the right chamber of the diffusion cell. The reaction system was allowed to stand at 30 °C for 96 h to form an orange-yellow covalent organic framework separation layer on a porous substrate. The composite membrane was removed and rinsed sequentially with 500 mL acetonitrile, methanol, and distilled water to obtain the covalent organic framework composite membrane.

[0040] The prepared covalent organic framework membrane was observed using a scanning electron microscope, such as... Figure 1 As shown in the figure, the covalent organic framework layer is continuous and has a smooth, flat surface, with a thickness of approximately 0.4 μm. The microscopic molecular structure of the separation layer is as follows: Figure 2 As shown.

[0041] (2) Preparation of photoresponsive nanochannel membranes:

[0042] 20.2 mg of 4-[(4-thioylfluorophenyl)diazepine]benzenesulfonic acid was dissolved in 10 mL of acetonitrile solution containing 0.23 μL of 2-tert-butyl-1,1,3,3-tetramethylguanidine (1.0 mol%) and 9.0 mg of hexamethyldisilazane (1 stoichiometric amount). The solution was sonicated for 10 min to obtain a grafting solution. The covalent organic framework composite membrane prepared in step (1) was immersed in the grafting solution and allowed to stand at 30 °C for 5 min to achieve photoresponsive modification of the covalent organic framework membrane pores. The membrane was removed from the solution and rinsed sequentially with 500 mL of acetonitrile, methanol, and distilled water to obtain a photoresponsive nanochannel membrane (grafting rate of 50%).

[0043] The membrane was observed using a scanning electron microscope, such as Figure 3 As shown in the figure, the morphology of the membrane obtained in step (1) is not significantly different, indicating that grafting mainly occurs in the pores. The microscopic molecular structure of the separation layer after grafting under different wavelengths of light is shown in the figure. Figure 4 As shown: Under ultraviolet light irradiation, the effective pore size increases from 0.74 nm to 0.8 nm, which allows for efficient separation of lithium ions and multivalent ions. When irradiated with visible light, the effective pore size returns to 0.74 nm, facilitating the passage of other monovalent ions and achieving lithium ion enrichment. The water contact angle of the membrane under different wavelengths of light was measured, as shown... Figure 5 As shown, the hydrophilicity of the membrane material changes with different wavelengths of light irradiation, indicating that the grafted membrane pores have photoresponsiveness.

[0044] Application examples

[0045] The membrane materials prepared in the examples were subjected to lithium extraction tests by permeation with mixed salt solutions. The covalent organic framework composite membrane prepared in step (1) of the examples without photoresponsive molecular modification was used as a control example. The operation method is as follows:

[0046] The membrane material prepared in the example was installed on a diffusion cell, dividing the cell into left and right sections. A mixed salt solution of KCl / MgCl2 / LiCl with a concentration of 0.1M was prepared to simulate brine from a salt lake. This salt solution was placed on the side of the diffusion cell facing the membrane separation layer, while distilled water was added to the other side. The membrane surface was alternately irradiated with light sources at wavelengths of 365nm and 455nm, and the Li content of the membrane material was measured under both conditions. + / Mg 2+ Selectivity and K + / Li + Selectivity, such as Figure 6 As shown, the Kc of the ungrafted covalent organic framework membrane in the control example + / Li + Selectivity and Li + / Mg 2+ The selectivity values ​​all approached 1, indicating that the unmodified channels lacked ion selectivity due to their excessively large pore size (greater than the hydration diameter of the aforementioned ions). When photoresponsive molecules were grafted onto the channels, the pore size was effectively reduced. In a solution environment under UV irradiation, the effective pore size of the membrane channels was 0.8 nm, a size precisely between the diameters of lithium and magnesium hydrated ions. This allowed for the rapid passage of lithium ions and the efficient retention of magnesium ions. Figure 6 As shown, Li + / Mg 2+ The selectivity is as high as approximately 20; by irradiating the membrane material with visible light, the effective pore size of the membrane channels will be further reduced to 0.74 nm. This size falls between the diameters of potassium and lithium hydrate ions, enabling rapid passage of potassium ions and effective enrichment of lithium ions. Figure 6 As shown, K + / Li + The selectivity is approximately 3. This indicates that the ungrafted covalent organic framework composite membrane in the control example lacks ion selectivity; while the photoresponsive nanochannel membrane material prepared in the examples can simultaneously achieve efficient separation of divalent / monovalent ions and lithium ions / other monovalent ions, and the separation performance remains stable after alternating irradiation with different wavelengths of light. Therefore, the photoresponsive nanochannel membrane preparation method proposed in this invention is simple, efficient, and has broad application potential.

[0047] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing a light-responsive nanochannel membrane, characterized in that, The preparation method is: The sulfuryl fluoride photoresponsive molecule is dissolved in an acetonitrile solution containing a catalyst to obtain a grafting solution; the covalent organic framework composite membrane is soaked in the grafting solution and left to react to obtain a photoresponsive nanochannel membrane; the sulfuryl fluoride photoresponsive molecule is selected from 4-sulfuryl fluoride azobenzene, 4-[(4-sulfuryl fluoride phenyl) diazenyl] benzoic acid or 4-[(4-sulfuryl fluoride phenyl) diazenyl] benzene sulfonic acid; and the catalyst comprises 2-tert-butyl-1,1,3,3-tetramethyl guanidine and hexamethyl disilazane.

2. The production method according to claim 1, characterized by, The ratio of the addition amount of the 2-tert-butyl-1,1,3,3-tetramethyl guanidine to the hexamethyl disilazane is (0.23-2.3) μL:(0.45-1.8) mg.

3. The production method according to claim 1, characterized by, The concentration of the sulfuryl fluoride photoresponsive molecule in the grafting solution is ≤0.112 mM; and the concentrations of the 2-tert-butyl-1,1,3,3-tetramethyl guanidine and the hexamethyl disilazane in the grafting solution are 0.11-1.14 mM and 0.28-1.12 mM, respectively.

4. The preparation method according to claim 1, characterized in that, The covalent organic framework composite membrane is prepared by the following method: An equal volume of an aqueous solution of an amino monomer and an organic phase solution of an aldehyde monomer are added to both sides of a porous substrate, and left to react to obtain a covalent organic framework composite membrane.

5. The preparation method according to claim 4, characterized in that, The porous substrate is a polyacrylonitrile ultrafiltration membrane with a molecular weight cut-off of 10-100 kDa; the amino monomer is selected from 1,2,3-triamino guanidine, 1,3,5-tris(4-aminophenyl) benzene, tris(4-aminophenyl) amine or 2,4,6-tris(4-aminophenyl)-1,3,5-triazine; and the aldehyde monomer is 2,5-dihydroxy terephthaldehyde and / or 2,5-dimethoxy terephthaldehyde.

6. The preparation method according to claim 4, characterized in that, The concentration of the amino monomer in the aqueous solution is 0.025-0.25 mM; the concentration of the aldehyde monomer in the organic phase solution is 0.037-0.37 mM; the temperature of the left-to-react is 30°C; and the left-to-react time is 96 h.

7. The preparation method according to claim 5, characterized in that, The molar ratio of the 2,5-dihydroxy terephthaldehyde to the sulfuryl fluoride photoresponsive molecule is 1:

1.

8. The method of claim 1, wherein, The temperature of the left-to-react is 30°C; and the left-to-react time is 5-60 min.

9. The light-responsive nanochannel membrane prepared by the method of any one of claims 1 to 8, characterized in that, Under ultraviolet light irradiation, the pore size of the photoresponsive nanochannel membrane changes from small to large; and under visible light irradiation, the pore size of the photoresponsive nanochannel membrane changes from large to small.

10. The photoresponsive nanochannel membrane of claim 9 is used in any one of the following 1) to 2): 1) gradient separation of mixed salt solutions; 2) lithium extraction from salt lakes.

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