Crown etherized cof film based on ice crystal template and preparation method and application thereof

Abstract

The application discloses a crown etherized COF membrane based on ice crystal template induction and a preparation method and application thereof, and belongs to the technical field of membrane separation materials. The crown etherized COF membrane based on ice crystal template induction has a nano-pore channel structure which is continuously through and orderly arranged along the film thickness direction and is formed by directional freezing of an ice crystal template, and the nano-pore channel has a crown ether structure introduced therein. The preparation raw materials of the crown etherized COF membrane based on ice crystal template induction include reaction monomers and a water-containing polar solvent. The directional freezing structure strategy is adopted in the application, a nano-pore channel structure which is continuously through and orderly arranged along the film thickness direction is constructed in the covalent organic framework membrane, and thus a continuous directional ion transmission channel is formed. Compared with a traditional disordered porous membrane, the structure can significantly reduce the bending degree of an ion transmission path, improve the ion transmembrane transmission efficiency, and improve the lithium isotope separation effect.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation materials technology, specifically relating to crown etherified COF membranes induced by ice crystal templates, their preparation methods, and applications. Background Technology

[0002] The stable isotopes of lithium mainly include 6 Li and 7 Lithium, due to the significant differences in nuclear reaction properties between its two isotopes, has important applications in nuclear energy engineering, nuclear material preparation, and related high-end technology fields. 6 Li can be used as an important raw material for tritium breeding reactions and is widely used in tritium self-sustaining systems in fusion reactors; 7 Lithium (Li) is used in reactor coolants, chemical control systems, and related nuclear engineering fields due to its low neutron absorption cross-section. With the continuous development of nuclear energy technology, especially controlled nuclear fusion technology, the demand for high-abundance lithium isotope materials is constantly increasing, highlighting the growing importance of lithium isotope separation technology.

[0003] However, due to 6 Li and 7 The relatively small mass difference between lithium isotopes and their highly similar chemical properties result in extremely limited differences in their partitioning behavior in solution or solid-phase systems, making lithium isotope separation a technically challenging process. Existing lithium isotope separation technologies mainly include amalgamation, ion exchange, chemical exchange, and extraction separation methods based on coordinating molecules such as crown ethers.

[0004] Among them, the amalgam method has poor process safety and has been gradually restricted; ion exchange and chemical exchange methods usually have problems such as low separation coefficient, complex process and high energy consumption; although the separation method based on crown ether molecules has a certain selectivity for lithium ions, it mostly relies on liquid phase extraction system, has poor reusability, and is difficult to achieve continuous operation.

[0005] In recent years, membrane separation technology has attracted attention due to its advantages such as simplified process, low energy consumption, and ease of continuous operation. However, most existing polymer or inorganic membranes have disordered pore structures, and the transport of ions in the membrane mainly relies on random diffusion behavior, which makes it difficult to effectively amplify the small differences between lithium isotopes, thus limiting further improvement in separation efficiency.

[0006] On the other hand, covalent organic frameworks (COFs) possess strong structural designability, uniform pore size distribution, and good crystallinity, providing a new material basis for constructing ordered ion transport channels. However, existing technologies still lack a membrane material system capable of stably immobilizing lithium-ion recognition units within a membrane structure while simultaneously constructing a nanoporous structure that extends along the membrane thickness direction, thus limiting its application in continuous lithium isotope membrane separation.

[0007] Therefore, developing a separation membrane material that has both an ordered nanoporous structure and the ability to introduce lithium-ion selective recognition units inside the pores to form a continuous mass transfer channel along the membrane thickness direction is of great significance for improving lithium isotope separation efficiency and realizing continuous membrane separation. Summary of the Invention

[0008] One of the objectives of this invention is to provide a crown etherified COF membrane based on ice crystal template to solve the problem of low separation efficiency in existing membrane materials for lithium isotope separation.

[0009] The second objective of this invention is to provide a method for preparing the above-mentioned crown etherified COF membrane based on ice crystal template induction.

[0010] The third objective of this invention is the application of the crown etherified COF membrane induced by ice crystal templates.

[0011] To achieve the above objectives, the first aspect of the present invention provides a crown etherified COF membrane induced by ice crystal template. The crown etherified COF membrane induced by ice crystal template has a continuously interconnected and orderly arranged nanopore structure formed by directional freezing of ice crystal template along the membrane thickness direction, and a crown ether structure is introduced into the nanopore.

[0012] The raw materials for preparing crown etherified COF membranes based on ice crystal templates include reactive monomers and aqueous polar solvents.

[0013] The reaction monomers include crown ether functionalized monomers, aromatic aldehyde monomers, and aromatic amino monomers, with a molar ratio of 1:(1.5-7):(3-10.5).

[0014] Furthermore, the crown ether functionalized monomer contains amino and / or aldehyde groups.

[0015] Further, the crown ether functionalized monomer is at least one selected from 4'-formylbenzo-12-crown-4, 4'-aminobenzo-12-crown-4, 4'-formylbenzo-15-crown-5-ether, 4'-aminobenzo-15-crown-ether-5, 4'-formylbenzo-18-crown-6-ether, 4'-aminobenzo-18-crown-ether-6, and azabenzocrown-ether.

[0016] Further, the azirbenzacrown ether is at least one of azirbenza-12-crown-4 or a derivative thereof, azirbenza-15-crown-5 or a derivative thereof, azirbenza-18-crown-6 or a derivative thereof.

[0017] Furthermore, the aromatic aldehyde monomer is an aromatic compound containing at least two aldehyde functional groups.

[0018] Furthermore, the aromatic aldehyde monomer is at least one of benzene or a derivative thereof containing at least two aldehyde functional groups, biphenyl or a derivative thereof containing at least two aldehyde functional groups, or bipyridine or a derivative thereof containing at least two aldehyde functional groups.

[0019] Furthermore, the aromatic aldehyde monomer is at least one selected from terephthalaldehyde, biphenyl dimethylformaldehyde, pyromellitic pyrogallol, and trialdehyde-resorcinol.

[0020] Furthermore, the aromatic amino monomer is an aromatic compound containing at least two amino functional groups.

[0021] Furthermore, the aromatic amino monomer is at least one of benzene or a derivative thereof containing at least two amino functional groups, biphenyl or a derivative thereof containing at least two amino functional groups, or naphthalene or a derivative thereof containing at least two amino functional groups.

[0022] Further, the aromatic amino monomer is at least one selected from p-phenylenediamine, 1,3,5-triaminobenzene, benzidine, naphthylenediamine, 2,5-diaminobenzenesulfonic acid, 2,5-diaminobenzene-1,4-disulfonic acid, and 2,5-diaminobenzoic acid.

[0023] Furthermore, the aqueous polar solvent is a composite solvent composed of water and an organic solvent, wherein the volume ratio of water to organic solvent is (1-4):(1-4), more preferably, the volume ratio of water to organic solvent is (1-2):(1-2).

[0024] Furthermore, the organic solvent is at least one selected from dimethyl sulfoxide, anhydrous ethanol, methanol, and acetonitrile.

[0025] Furthermore, the thickness of the crown etherified COF film induced by the ice crystal template is 1 μm to 150 μm, more preferably, the thickness is 2 μm to 100 μm.

[0026] Furthermore, the crown etherified COF membrane induced by ice crystal template is a continuous self-supporting membrane structure or a composite membrane structure loaded on a substrate. It is preferably a continuous self-supporting membrane structure. Compared with a composite membrane loaded on a supporting substrate, the self-supporting membrane structure can maintain the continuity of nanopores along the membrane thickness direction, thereby reducing the influence of the supporting substrate on the transmembrane mass transfer process.

[0027] Furthermore, the substrate is a porous inorganic material or a polymer material.

[0028] A second aspect of this invention provides a method for preparing crown etherified COF membranes induced by ice crystal templates, comprising the following steps:

[0029] S1. Add the reactant monomer to an aqueous polar solvent and stir until homogeneous to obtain a COF precursor solution. Then add the catalyst and stir at 20-50°C for 4-24 hours. Degas under vacuum to obtain a gel-like precursor solution.

[0030] S2. The gel-like precursor solution is placed on a mold or substrate and then transferred to a cooling device for directional freezing along the vertical direction of the mold or substrate. The ice crystal template is then removed by freeze drying and finally heat-treated to obtain a crown etherified COF membrane induced by the ice crystal template.

[0031] Furthermore, the mass fraction of the reactant monomer in the COF precursor solution is 0.5–3.0%, preferably 1–2.5%; the molar ratio of the aromatic aldehyde monomer in the catalyst and reactant monomer is 0.5–8:1, preferably 1–6:1.

[0032] Further, the catalyst is at least one selected from acetic acid, trifluoroacetic acid, formic acid, propionic acid, oxalic acid, benzoic acid, p-toluenesulfonic acid, and methanesulfonic acid.

[0033] Furthermore, the vacuum degassing is carried out under reduced pressure to remove dissolved gases and bubbles from the solution; preferably, the vacuum degree is -0.06MPa to -0.10MPa, more preferably -0.08MPa to -0.095MPa; the degassing time is 3-30min, more preferably 5-15min.

[0034] Furthermore, the cooling device forms a stable temperature gradient from a low-temperature end to a high-temperature end in its spatial direction, wherein the low-temperature end is located on the side close to the mold or substrate.

[0035] During directional freezing, the system freezes gradually from the low-temperature end, with the freezing interface advancing unidirectionally away from the low-temperature end. This causes the gel-like precursor solution to solidify directionally in a direction perpendicular to the membrane surface (i.e., the membrane thickness direction). During this process, water in the solvent undergoes directional crystallization as the freezing interface advances, forming ice crystal templates aligned along the freezing direction, with the growth direction of these ice crystals consistent with the membrane thickness direction. After subsequent removal of the ice crystal templates, a continuous and orderly arranged nanoporous structure along the membrane thickness direction is formed within the membrane material.

[0036] Furthermore, the directional freezing temperature is -10℃ to -80℃, preferably -20℃ to -60℃, wherein the freezing rate is 0.1℃ / min to 10℃ / min, more preferably 0.5℃ / min to 5℃ / min; and the temperature gradient is 1℃ / cm to 20℃ / cm, more preferably 3℃ / cm to 10℃ / cm.

[0037] Furthermore, the freeze-drying is carried out at -60℃ to -10℃, and the freeze-drying time is 12 to 72 hours, more preferably 24 to 48 hours.

[0038] Furthermore, the heat treatment temperature is 60℃~120℃, and the heat treatment time is 12h~72h.

[0039] Furthermore, after the crown etherified COF membrane induced by ice crystal template is prepared, the obtained crown etherified COF separation membrane can be purified by organic solvent to remove unreacted monomers and low molecular weight impurities, thereby obtaining a structurally stable and reusable membrane material.

[0040] A third aspect of the present invention provides the application of crown etherified COF membranes induced by ice crystal templates in lithium isotope separation.

[0041] Furthermore, the crown etherified COF membrane based on ice crystal template is used for the selective separation or enrichment of lithium isotopes in lithium-containing solution systems.

[0042] Furthermore, the lithium-containing solution includes, but is not limited to, lithium chloride solution, lithium nitrate solution, and lithium carbonate solution.

[0043] Furthermore, the crown etherified COF film based on ice crystal template induced by lithium isotopes... 6 Li and 7 Applications in Li separation.

[0044] Furthermore, the separation process may employ one or more of the following membrane separation methods:

[0045] Pressure-driven membrane separation; electromigration membrane separation; diffusion permeation membrane separation; membrane electrodialysis separation.

[0046] The beneficial effects of this invention are:

[0047] 1. The crown etherified COF membrane induced by ice crystal templates in this invention possesses a directional, interconnected nanoporous structure. This invention utilizes an ice crystal template-based directional freezing construction strategy to build a continuous and ordered nanoporous structure along the membrane thickness within the covalent organic framework membrane, thereby forming a continuous, directional ion transport channel. Compared to traditional disordered porous membranes, this structure significantly reduces the tortuosity of the ion transport path, improves the efficiency of ion transmembrane transport, and enhances lithium isotope separation.

[0048] 2. This invention achieves a stable confined distribution of lithium-ion recognition units (crown ether structures) within nanopores using an ice crystal template-induced crown etherified COF membrane. This invention utilizes an in-situ polycondensation reaction to stably embed the crown ether recognition structure into the COF framework and the inner wall of the nanopores via covalent bonds, thereby forming a stable distribution of lithium-ion recognition sites within the pores. Compared to traditional physical loading or post-modification methods, this structure exhibits higher structural stability and a more uniform distribution of recognition sites, demonstrating superior lithium isotope separation performance.

[0049] 3. In this invention, an ion transport channel with both structure-guided transport and molecular recognition functions is constructed based on an ice crystal template-induced crown etherified COF membrane. By combining a directional, interconnected nanopore structure with crown ether recognition sites, this invention constructs lithium-ion complexation sites continuously distributed along the pore direction within the membrane, enabling continuous recognition and dissociation processes of lithium ions during transmembrane transport, thereby achieving effective regulation of lithium-ion migration behavior.

[0050] 4. The crown etherified COF membrane induced by ice crystal templates in this invention has potential for large-scale application. The ice crystal template-based directional freezing construction method proposed in this invention is simple in process and can achieve controllable adjustment of membrane pore structure and membrane thickness, providing a feasible technical route for the large-scale preparation of high-efficiency lithium isotope separation membrane materials. Attached Figure Description

[0051] The invention will now be further described with reference to the accompanying drawings.

[0052] Figure 1 This is a flowchart illustrating the preparation process of the crown etherified COF membrane induced by ice crystal template according to the present invention;

[0053] Figure 2 This is a schematic diagram of the structure of the crown etherified COF membrane induced by ice crystal template for lithium isotope transmembrane separation device of the present invention;

[0054] Figure 3 This is a comparison of the infrared spectra of the crown etherified COF membrane prepared based on ice crystal template in Example 1 of the present invention and the pure COF membrane prepared in Comparative Example 1.

[0055] Figure 4 This is a cross-sectional SEM image of the crown etherified COF membrane prepared in Example 1 of the present invention. Detailed Implementation

[0056] To make the objectives, technical solutions, and advantages of this application clearer, the following description and illustration are provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. All other embodiments obtained by those skilled in the art based on the embodiments provided in this application without inventive effort are within the scope of protection of this application.

[0057] Obviously, the following description is merely some examples or embodiments of this application. Those skilled in the art can apply this application to other similar scenarios without any inventive effort. Furthermore, it is understood that although the effort involved in such development may be complex and lengthy, for those skilled in the art related to the content disclosed in this application, any changes to design, manufacturing, or production based on the technical content disclosed in this application are merely conventional technical means and should not be construed as insufficient disclosure of the content of this application.

[0058] However, there may be instances where unnecessary detailed descriptions are omitted. For example, detailed descriptions of well-known matters or repetitive descriptions of essentially the same structure may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the following description is provided to enable those skilled in the art to fully understand this application and is not intended to limit the subject matter of the claims.

[0059] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions, and all technical features and optional technical features of this application can be combined to form new technical solutions.

[0060] To address the problem that existing polymer or inorganic membranes are mostly disordered porous structures, and the transport of ions in the membrane mainly relies on random diffusion, it is difficult to effectively amplify the small differences between lithium isotopes, resulting in low separation efficiency, the first aspect of this application proposes a crown etherified COF membrane based on ice crystal template. The crown etherified COF membrane has a continuous and orderly arranged nanopore structure formed by the directional freezing of ice crystal template along the membrane thickness direction, and a crown ether structure is introduced into the nanopore.

[0061] Constructing directional ion transport channels that run through the thickness of the membrane can significantly reduce the mass transfer tortuosity during ion migration within the membrane. Crown ether structures, acting as lithium ion recognition units, are confined within the nanopores and continuously arranged along the pore direction, thereby forming continuously distributed lithium ion complexation sites within the pores.

[0062] Through the above structural design, lithium ions can rapidly migrate along the through-pores during transmembrane transport and continuously coordinate with crown ether recognition sites distributed within the pores. This amplifies the kinetic differences in the migration of different lithium isotopes, achieving selective lithium ion transport and further enabling… 6 Li and 7 Efficient separation of Li isotopes.

[0063] In some embodiments, the raw materials for preparing the crown etherified COF membrane based on ice crystal templates include reactive monomers and aqueous polar solvents.

[0064] The reaction monomers include crown ether functionalized monomers, aromatic aldehyde monomers, and aromatic amino monomers, with a molar ratio of 1:(1.5-7):(3-10.5).

[0065] Crown ether functionalized monomers contain amino and / or aldehyde groups.

[0066] Crown ether compounds carrying amino and / or aldehyde groups are used as functionalized monomers. Through the condensation reaction between amino and aldehyde groups, the crown ether structure is introduced into the COF membrane. This allows the crown ether lithium ion recognition unit to covalently embed into the COF framework structure and / or the inner wall of the nanopores by participating in the COF condensation reaction in situ. This achieves a stable confined distribution of the crown ether lithium ion recognition unit in the ordered nanopores, which is beneficial for regulating the migration behavior of lithium ions in the nanopores.

[0067] In some specific embodiments, the crown ether functionalized monomer is at least one selected from 4'-formylbenzo-12-crown-4, 4'-aminobenzo-12-crown-4, 4'-formylbenzo-15-crown-5-ether, 4'-aminobenzo-15-crown-ether-5, 4'-formylbenzo-18-crown-6-ether, 4'-aminobenzo-18-crown-ether-6, and azabenzocrown-ether.

[0068] In some specific embodiments, the azirbenzocrown ether is at least one of azirbenzo-12-crown-4 or a derivative thereof, azirbenzo-15-crown-5 or a derivative thereof, azirbenzo-18-crown-6 or a derivative thereof.

[0069] In some embodiments, the aromatic aldehyde monomer is an aromatic compound containing at least two aldehyde functional groups.

[0070] In some specific embodiments, the aromatic aldehyde monomer is at least one of benzene or a derivative thereof containing at least two aldehyde functional groups, biphenyl or a derivative thereof containing at least two aldehyde functional groups, or bipyridine or a derivative thereof containing at least two aldehyde functional groups.

[0071] In some specific embodiments, the aromatic aldehyde monomer is at least one of terephthalaldehyde, biphenyl dimethylformaldehyde, pyromellitic pyrogallol and trialdehyde-resorcinol.

[0072] In some embodiments, the aromatic amino monomer is an aromatic compound containing at least two amino functional groups.

[0073] In some specific embodiments, the aromatic amino monomer is at least one of benzene or a derivative thereof containing at least two amino functional groups, biphenyl or a derivative thereof containing at least two amino functional groups, or naphthalene or a derivative thereof containing at least two amino functional groups.

[0074] In some specific embodiments, the aromatic amino monomer is at least one selected from p-phenylenediamine, 1,3,5-triaminobenzene, benzidine, naphthylenediamine, 2,5-diaminobenzenesulfonic acid, 2,5-diaminobenzene-1,4-disulfonic acid, and 2,5-diaminobenzoic acid.

[0075] In some embodiments, the aqueous polar solvent is a composite solvent composed of water and an organic solvent, wherein the volume ratio of water to organic solvent is (1-4):(1-4), more preferably, the volume ratio of water to organic solvent is (1-2):(1-2). By adjusting the ratio of water to organic solvent in the aqueous polar solvent, the size and growth behavior of ice crystals can be controlled, thereby further adjusting the size and arrangement structure of nanopores in the obtained COF.

[0076] In some specific embodiments, the organic solvent is at least one selected from dimethyl sulfoxide, anhydrous ethanol, methanol, and acetonitrile.

[0077] In some specific embodiments, the thickness of the crown etherified COF film induced by ice crystal template is 1 μm to 150 μm, more preferably 2 μm to 100 μm.

[0078] In some specific embodiments, the crown etherified COF membrane induced by ice crystal template is a continuous self-supporting membrane structure or a composite membrane structure loaded on a substrate. It is preferred to be a continuous self-supporting membrane structure. Compared with the composite membrane loaded on the supporting substrate, the self-supporting membrane structure can maintain the continuity of nanopores along the membrane thickness direction, thereby reducing the influence of the supporting substrate on the transmembrane mass transfer process.

[0079] In some specific embodiments, the substrate is a porous inorganic material or a polymer material.

[0080] The second aspect of this application provides a method for preparing crown etherified COF membranes induced by ice crystal templates, comprising the following steps:

[0081] S1. Add the reactant monomer to an aqueous polar solvent and stir until homogeneous to obtain a COF precursor solution. Then add the catalyst and stir at 20-50°C for 4-24 hours. Degas under vacuum to obtain a gel-like precursor solution.

[0082] S2. The gel-like precursor solution is placed on a mold or substrate and then transferred to a cooling device for directional freezing along the vertical direction of the mold or substrate. The ice crystal template is then removed by freeze drying and finally heat-treated to obtain a crown etherified COF membrane induced by the ice crystal template.

[0083] This application involves dissolving aromatic aldehyde monomers, aromatic amino monomers, and crown ether functionalized monomers for forming the COF framework structure in an aqueous polar solvent to form a precursor reaction solution. A gel-like precursor solution is prepared through a condensation reaction, followed by directional freezing under a controlled temperature gradient. During freezing, the aqueous polar solvent in the solution forms ice crystal structures that grow directionally along the temperature gradient direction. Subsequently, the ice crystal template is removed through a freeze-drying process, thereby replicating and forming a nanoporous structure arranged along the freezing direction and penetrating the thickness direction inside the membrane. Then, a heat treatment step is used to promote the full condensation and crystallization of the COF framework structure, thereby obtaining a crown ether functionalized COF separation membrane with a stable structure.

[0084] In some specific embodiments, the mass fraction of the reactant monomer in the COF precursor solution is 0.5-3.0%, preferably 1-2.5%; the molar ratio of the aromatic aldehyde monomer in the catalyst and reactant monomer is 0.5-8:1, preferably 1-6:1.

[0085] In some specific embodiments, the catalyst is at least one selected from acetic acid, trifluoroacetic acid, formic acid, propionic acid, oxalic acid, benzoic acid, p-toluenesulfonic acid, and methanesulfonic acid.

[0086] In some specific embodiments, the process is carried out under reduced pressure to remove dissolved gases and bubbles from the solution; preferably, the vacuum degree is -0.06MPa to -0.10MPa, more preferably -0.08MPa to -0.095MPa; the degassing time is 3-30 min, more preferably 5-15 min.

[0087] In some specific embodiments, the cooling device forms a stable temperature gradient from a low-temperature end to a high-temperature end in its spatial direction, wherein the low-temperature end is located on the side close to the mold or substrate.

[0088] During directional freezing, the system freezes gradually from the low-temperature end, with the freezing interface advancing unidirectionally away from the low-temperature end. This causes the gel-like precursor solution to solidify directionally in a direction perpendicular to the membrane surface (i.e., the membrane thickness direction). During this process, water in the solvent undergoes directional crystallization as the freezing interface advances, forming ice crystal templates aligned along the freezing direction, with the growth direction of these ice crystals consistent with the membrane thickness direction. After subsequent removal of the ice crystal templates, a continuous and orderly arranged nanoporous structure along the membrane thickness direction is formed within the membrane material.

[0089] In some specific embodiments, the directional freezing temperature is -10℃ to -80℃, preferably -20℃ to -60℃, wherein the freezing rate is 0.1℃ / min to 10℃ / min, more preferably 0.5℃ / min to 5℃ / min; and the temperature gradient is 1℃ / cm to 20℃ / cm, more preferably 3℃ / cm to 10℃ / cm.

[0090] In some specific embodiments, the freeze-drying temperature is -60°C to -10°C, and the freeze-drying time is 12 to 72 hours, more preferably 24 to 48 hours.

[0091] In some specific embodiments, the heat treatment temperature is 60℃~120℃, and the heat treatment time is 12h~72h.

[0092] In some specific embodiments, after the crown etherified COF membrane based on ice crystal template is prepared, the obtained crown etherified COF separation membrane can be purified by organic solvent to remove unreacted monomers and low molecular weight impurities, thereby obtaining a structurally stable and reusable membrane material.

[0093] The third aspect of this application provides the application of crown etherified COF membranes induced by ice crystal templates in lithium isotope separation.

[0094] In some specific embodiments, crown etherified COF membranes induced by ice crystal templates are used for the selective separation or enrichment of lithium isotopes in lithium-containing solution systems.

[0095] In some specific embodiments, the lithium-containing solution includes, but is not limited to, lithium chloride solution, lithium nitrate solution, and lithium carbonate solution.

[0096] In some specific embodiments, the application of ice crystal template-induced crown etherified COF membranes in the separation of lithium isotopes 6Li and 7Li involves the following process: During separation, the lithium-containing solution passes through the ice crystal template-induced crown etherified COF membrane under pressure-driven, electric field-driven, or concentration gradient-driven conditions, causing lithium ions to migrate across the membrane along the nanopore direction. Because the COF membrane has a directional nanopore structure extending along its thickness, and crown ether lithium ion recognition units are stably introduced into the inner walls of the nanopores, lithium ions can reversibly complex with the crown ether recognition sites during transmembrane transport, resulting in selective migration behavior. Furthermore, due to the differences in coordination kinetics and stability of different lithium isotopes during crown ether complexation,… 6 Li and 7 The migration rate of Li in nanopores varies, thereby enabling the selective separation and enrichment of lithium isotopes.

[0097] In some specific embodiments, the separation process may employ one or more of the following membrane separation methods:

[0098] Pressure-driven membrane separation; electromigration membrane separation; diffusion osmosis membrane separation; membrane electrodialysis separation.

[0099] The following description, in conjunction with specific embodiments, provides further details.

[0100] Example 1

[0101] For the preparation method of crown etherified COF membranes induced by ice crystal templates, please refer to [link / reference]. Figure 1 This includes the following steps:

[0102] S1. Add 0.23 mmol of trialdehyde phloroglucinol, 0.34 mmol of p-phenylenediamine and 0.033 mmol of 4'-aminobenzo-12-crown-4 to 6 mL of an aqueous polar solvent, which is prepared by mixing deionized water and dimethyl sulfoxide in a volume ratio of 2:1. After stirring evenly, add 100 μL of acetic acid, stir at room temperature for 24 h, and degas under vacuum of -0.08 MPa for 5 min to obtain a gel-like precursor solution.

[0103] S2. The gel-like precursor solution was placed on a polytetrafluoroethylene mold and then transferred to a cooling device for directional freezing along the vertical direction of the mold. Subsequently, it was freeze-dried at -40°C for 48 hours to remove the ice crystal template, and then heat-treated at 60°C for 24 hours. Finally, the obtained membrane material was washed with anhydrous ethanol and tetrahydrofuran in sequence to obtain the crown etherified COF membrane induced by the ice crystal template.

[0104] The cooling device forms a stable temperature gradient from a low temperature end to a high temperature end in its spatial direction. The low temperature end is located on the side close to the mold. The directional freezing treatment temperature is -40℃, the freezing rate is 1℃ / min, and the temperature gradient is 5℃ / cm.

[0105] The cross-sectional morphology of the crown-etherified COF membrane obtained in Example 1 was observed using a scanning electron microscope, and the results are as follows: Figure 4 As shown, by Figure 4 It can be seen that the thickness of the crown etherified COF film obtained in Example 1 is 35 μm.

[0106] Example 2

[0107] The preparation method of crown etherified COF membrane based on ice crystal template includes the following steps:

[0108] S1. Add 0.13 mmol of trialdehyde phloroglucinol, 0.185 mmol of 2,5-diaminobenzenesulfonic acid and 0.059 mmol of 4'-aminobenzo-15-crown ether-5 to 4 mL of an aqueous polar solvent, which is prepared by mixing deionized water and dimethyl sulfoxide in a volume ratio of 2:1. After stirring evenly, add 30 μL of acetic acid, stir at room temperature for 24 h, and degas under vacuum of -0.08 MPa for 5 min to obtain a gel-like precursor solution.

[0109] S2. The gel-like precursor solution was placed on a polytetrafluoroethylene mold and then transferred to a cooling device for directional freezing along the vertical direction of the mold. Subsequently, it was freeze-dried at -40°C for 48 hours to remove the ice crystal template, and then heat-treated at 60°C for 24 hours. Finally, the obtained membrane material was washed with anhydrous ethanol and tetrahydrofuran in sequence to obtain the crown etherified COF membrane induced by the ice crystal template.

[0110] The cooling device forms a stable temperature gradient from a low temperature end to a high temperature end in its spatial direction. The low temperature end is located on the side close to the mold. The directional freezing treatment temperature is -40℃, the freezing rate is 1℃ / min, and the temperature gradient is 5℃ / cm.

[0111] Example 3

[0112] The preparation method of crown etherified COF membrane based on ice crystal template includes the following steps:

[0113] S1. Add 0.21 mmol of pyromellitic aldehyde, 0.3 mmol of 2,5-diaminobenzoic acid and 0.03 mmol of 4'-formylbenzo-15-crown-5-ether to 5 mL of an aqueous polar solvent, which is prepared by mixing deionized water, dimethyl sulfoxide and anhydrous ethanol in a volume ratio of 1:1:1. After stirring evenly, add 10 μL of trifluoroacetic acid, stir at 50 °C for 4 h, and degas under vacuum of -0.08 MPa for 5 min to obtain a gel-like precursor solution.

[0114] S2. The gel-like precursor solution was placed on a polytetrafluoroethylene mold and then transferred to a cooling device for directional freezing along the vertical direction of the mold. Subsequently, it was freeze-dried at -35°C for 36 hours to remove the ice crystal template, and then heat-treated at 60°C for 48 hours. Finally, the obtained membrane material was washed with anhydrous ethanol and tetrahydrofuran in sequence to obtain the crown etherified COF membrane induced by the ice crystal template.

[0115] The cooling device forms a stable temperature gradient from a low temperature end to a high temperature end in its spatial direction. The low temperature end is located on the side close to the mold. The directional freezing treatment temperature is -45℃, the freezing rate is 1℃ / min, and the temperature gradient is 5℃ / cm.

[0116] Example 4

[0117] The preparation method of crown etherified COF membrane based on ice crystal template is the same as that in Example 1, except that "0.23 mmol trialdehyde phloroglucinol, 0.34 mmol p-phenylenediamine and 0.033 mmol 4'-aminobenzo-12-crown-4" in Example 1 is adjusted to "0.0495 mmol trialdehyde phloroglucinol, 0.099 mmol p-phenylenediamine and 0.033 mmol 4'-aminobenzo-12-crown-4", the amount of acetic acid is 10 μL, and the molar ratio of crown ether functionalized monomer, aromatic aldehyde monomer and aromatic amino monomer is 1:1.5:3.

[0118] Example 5

[0119] The preparation method of crown etherified COF membrane based on ice crystal template is the only difference from Example 1, except that "0.23 mmol trialdehyde phloroglucinol, 0.34 mmol p-phenylenediamine and 0.033 mmol 4'-aminobenzo-12-crown-4" in Example 1 is adjusted to "0.099 mmol trialdehyde phloroglucinol, 0.231 mmol p-phenylenediamine and 0.033 mmol 4'-aminobenzo-12-crown-4", corresponding to a molar ratio of crown ether functionalized monomer, aromatic aldehyde monomer and aromatic amino monomer of 1:3:7.

[0120] Comparative Example 1

[0121] The preparation method of COF membrane based on ice crystal template is different from that in Example 1 only in that 4'-aminobenzo-12-crown-4 is not introduced into the reaction system. Specifically, "0.34 mmol p-phenylenediamine" in Example 1 is replaced with "0.345 mmol p-phenylenediamine".

[0122] Comparative Example 2

[0123] The preparation method of crown-etherified COF membrane differs from that of Example 1 only in that the deionized water in the aqueous polar solvent in step S1 of Example 1 is replaced with an equal volume of N-methylpyrrolidone, and the obtained gel-like precursor solution is poured onto a glass plate and reacted at 60°C for 3 days. Then, unreacted monomers and low-molecular-weight impurities are removed with ethanol and tetrahydrofuran, and the membrane is demolded in water to obtain the crown-etherified COF membrane.

[0124] Comparative Example 3

[0125] The preparation method of crown ether polysulfone composite film is as follows:

[0126] S1. Using commercially available chloromethylated polysulfone (purchased from Sigma-Aldrich) as the polymer matrix, 3.0 g of the chloromethylated polysulfone was dissolved in 28.5 mL of N,N-dimethylformamide (DMF) to obtain a polymer solution; 1.21 g of 4-aminobenzo-15-crown-5 (AB15C5) and 0.59 g of potassium carbonate were added, and the reaction was carried out at 80 °C for 12 h under nitrogen protection, so that the amino group reacted with the chloromethyl group on the polysulfone backbone through nucleophilic substitution, thereby covalently grafting the crown ether molecule onto the polymer backbone. After the reaction was completed, the reaction solution was poured into deionized water to precipitate, and the mixture was repeatedly washed to remove unreacted monomers and impurities. After drying, the crown ether-grafted polysulfone material was obtained.

[0127] S2. Dissolve 1.5g of the obtained crown ether-grafted polysulfone material in 9.0mL of N,N-dimethylformamide (DMF) to prepare a homogeneous casting solution with a mass fraction of 15%. Degas under vacuum at -0.08MPa for 10min. Coat the casting solution evenly onto the surface of a glass plate (film thickness 200μm). Immediately immerse the plate in a deionized water coagulation bath for non-solvent-induced phase separation (NIPS) to form a porous membrane structure. After thorough washing with water, the crown ether polysulfone composite membrane is obtained.

[0128] The crown etherified COF membrane prepared in Example 1 based on ice crystal template and the COF membrane prepared in Comparative Example 1 were detected using infrared spectroscopy. The detection results are as follows: Figure 3 As shown, by Figure 3 It can be seen that the crown-etherified COF membrane of Example 1 has a performance of 1373 cm⁻¹ -1 A new vibrational peak appeared, corresponding to the stretching vibration of the carbon-oxygen-carbon single bond, proving that the crown ether was successfully grafted into the COF membrane.

[0129] The membrane materials prepared in Examples 1-5 and Comparative Examples 1-3 were used as separation membranes, and their lithium isotope separation performance was tested. The test process is as follows:

[0130] See Figure 2 The testing apparatus was assembled and tested. The support substrate was a 100 μm thick, 0.1 μm pore size polyethersulfone (PES) ultrafiltration membrane, which provided mechanical support and ensured the stability of the separation membrane during testing. Before the experiment, each group of membrane materials was thoroughly cleaned with ultrapure water to remove any possible impurities. Then, a membrane sample with an effective diameter of 35 mm was clamped in a self-made membrane cell. The lithium ion concentration was 100 mg·L⁻¹. -1 A lithium carbonate aqueous solution was used as the feed liquid, and the solution was pumped at a rate of 5 mL / min using a peristaltic pump. -1 The feed solution was driven to flow through the membrane sample at a certain flow rate, and the permeate was returned to the feed reservoir until the lithium isotope composition on the permeate side stabilized (running time was 6 hours). The operating temperature during the experiment was room temperature.

[0131] Finally, inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the lithium isotopes in the feed solution and permeate. 6 Li and 7 The abundance or isotope ratio of Li was determined, and the lithium isotope separation factor of each group of membrane samples was calculated based on the measured data.

[0132] The lithium isotope separation factor (α) is calculated according to the following formula:

[0133] .

[0134] The first test result is recorded as the first time. 6 Li and 7 The Li separation factor was determined, and the membrane sample was then eluted and regenerated using a 2% nitric acid solution. This process was repeated under the same conditions for lithium isotope separation, and after four consecutive repetitions, the fifth repetition was recorded. 6 Li and 7 Li separation factor, denoted as the number of times reused (5 times). 6 Li and 7 The Li separation factor was analyzed, and the detection results are summarized in Table 1.

[0135] Table 1. Test results of lithium isotope separation performance of membrane materials prepared in the examples and comparative examples.

[0136]

[0137] As can be seen from the data recorded in Table 1, the separation factor of lithium isotopes of the crown etherified COF membrane based on ice crystal template in Example 1 is 1.045. After being reused 4 times, its lithium isotope separation factor is still stable at 1.040, indicating that the membrane material prepared by the present invention has good lithium isotope separation performance and reusability.

[0138] Specifically, the test results of Example 1 and Comparative Example 1 show that Comparative Example 1, lacking the introduction of crown ether recognition units, lacks selective recognition capability, resulting in a separation factor close to 1. Although Comparative Example 2 contains crown ethers, the absence of ice crystal guidance and the lack of directional, interconnected pore structures lead to tortuous ion transport paths, significantly reducing separation efficiency. In Comparative Example 3, although the crown ether is grafted onto the polymer matrix, the disordered pore structure and discontinuous distribution of recognition sites also reduce separation performance. Furthermore, the membrane of the present invention maintains a high separation factor (α remains above 1.028) after five reuses, indicating good structural stability and reusability.

[0139] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0140] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A crown etherified COF membrane based on ice crystal template, characterized in that, The crown etherified COF membrane based on ice crystal template has a continuous and orderly arranged nanopore structure formed by directional freezing of ice crystal template along the membrane thickness direction, and a crown ether structure is introduced into the nanopore. The raw materials for preparing the crown etherified COF membrane based on ice crystal template induced by the method include reactive monomers and aqueous polar solvents. The reactive monomers include crown ether functionalized monomers, aromatic aldehyde monomers, and aromatic amino monomers. The molar ratio of crown ether functionalized monomers, aromatic aldehyde monomers, and aromatic amino monomers is 1:(1.5-7):(3-10.5). The crown ether functionalized monomers contain amino and / or aldehyde groups. The aromatic aldehyde monomer is an aromatic compound containing at least two aldehyde functional groups, and the aromatic amino monomer is an aromatic compound containing at least two amino functional groups. The aqueous polar solvent is a composite solvent composed of water and an organic solvent, wherein the volume ratio of water to organic solvent is (1-4):(1-4), and the organic solvent is at least one of dimethyl sulfoxide, anhydrous ethanol, methanol, and acetonitrile.

2. The crown etherified COF membrane based on ice crystal template induced according to claim 1, characterized in that, The crown etherified COF membrane induced by ice crystal template is a continuous self-supporting membrane structure or a composite membrane structure loaded on a substrate.

3. A method for preparing crown etherified COF membranes induced by ice crystal templates, characterized in that, The method for preparing the crown etherified COF membrane based on ice crystal template as described in any one of claims 1-2 comprises the following steps: S1. Add the reactant monomer to an aqueous polar solvent and stir until homogeneous to obtain a COF precursor solution. Then add the catalyst and stir at 20-50°C for 4-24 hours. Degas under vacuum to obtain a gel-like precursor solution. S2. The gel-like precursor solution is placed on a mold or substrate and then transferred to a cooling device for directional freezing along the vertical direction of the mold or substrate. The ice crystal template is then removed by freeze drying and finally heat-treated to obtain a crown etherified COF membrane induced by the ice crystal template.

4. The method for preparing crown etherified COF membranes based on ice crystal templates according to claim 3, characterized in that, The mass fraction of the reactant monomer in the COF precursor solution is 0.5–3.0%, and the molar ratio of aromatic aldehyde monomers in the catalyst and reactant monomers is 0.5–8:

1.

5. The method for preparing crown etherified COF membranes based on ice crystal templates according to claim 3, characterized in that, The directional freezing treatment temperature is -10℃ to -80℃, the freezing rate is 0.1℃ / min to 10℃ / min, and the temperature gradient is 1℃ / cm to 20℃ / cm.

6. The method for preparing a crown etherified COF membrane based on an ice crystal template according to claim 3, characterized in that, The heat treatment temperature is 60℃~120℃, and the heat treatment time is 12h~72h.

7. The application of the crown etherified COF membrane based on ice crystal template as described in any one of claims 1-2 in lithium isotope separation.

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