In-situ small molecule encapsulated COF composite membranes for ion selective separation, their preparation methods and applications

Abstract

This invention discloses an in-situ small-molecule encapsulated COF composite membrane for ion-selective separation, its preparation method, and its application. The preparation method includes: immersing a porous membrane in an organic phase containing trialdehyde compounds, then removing it and contacting it with an aqueous phase containing COF monomers and functional small molecules to undergo an interfacial reaction; or, immersing the porous membrane in an organic phase containing trialdehyde compounds and COF monomers, then removing it and contacting it with an aqueous phase containing functional small molecules to undergo an interfacial reaction, thereby obtaining the in-situ small-molecule encapsulated COF composite membrane for ion-selective separation. This invention proposes a "one-step in-situ interfacial polymerization + functional small molecule encapsulation" strategy for constructing COF composite membranes. This technical solution utilizes the pore formation window during the COF film formation process to achieve in-situ simultaneous encapsulation of ion-affinity functional small molecules, controlling ion migration paths, thereby achieving a balance between high selectivity and high throughput in ion separation.

Description

Technical Field

[0001] This invention belongs to the field of COF composite membrane technology, specifically relating to an in-situ small molecule encapsulated COF composite membrane for ion selective separation, its preparation method, and its application. Background Technology

[0002] Lithium is an indispensable metallic element in the modern energy field, widely used in key industries such as power batteries, electric vehicles, and energy storage systems. With the rapid development of the new energy market, global demand for lithium resources has risen sharply. Compared to traditional ore extraction methods, salt lake brines are gradually becoming the mainstream lithium source due to their large reserves, low cost, and low pollution. However, the Li in salt lake brines... + The content is extremely low (generally less than 0.2 mol / L), accompanied by a large amount of Na. + K + Mg 2+ Competing ions, especially Na + Concentrations often reach as high as Li + The concentration of Na is tens to hundreds of times higher than that of Na in the background. Given this context, how can we obtain high background concentrations of Na... + Medium-efficiency and selective extraction of Li + This has become a core technological bottleneck in lithium resource development.

[0003] Membrane separation, as a green, continuous, and pollution-free separation method, is gradually showing its potential in the field of lithium extraction. Its advantages include low energy consumption, simple operation, and controllable process, making it particularly suitable for low-concentration Li. + The efficient enrichment of resources. However, due to Li + and Na + The Li₂O₃ membrane is very similar in terms of charge, mass, and hydration radius, leading to the similarity between conventional membrane materials (such as nanofiltration membranes and ion exchange membranes) and Li₂O₃. + / Na + Selectivity during separation is generally poor, making it difficult to meet the industrial-grade demand for high-purity lithium salts. Therefore, the development of membrane materials with precise ion recognition capabilities has become a current research focus, especially novel membrane systems with tunable structures and high levels of functionalization (such as covalent organic framework membranes and two-dimensional material membranes).

[0004] Currently used for Li + / Na + The membrane materials used for separation are mainly classified into the following categories, but all of them have obvious limitations: (1) Polymer-based nanofiltration membranes rely on the repulsion of membrane pore size and electrostatic action to sieve ions; the membrane pore size is usually >1 nm, which is much larger than that of Li + / Na + Hydrated size, poor selectivity; poor stability in high-salt environments; lacks directional recognition function and is easily affected by Na+. +(2) Cation and anion exchange membranes use fixed charges on the membrane to generate an electrostatic sieving effect on cations; they cannot distinguish monovalent ions of the same valence (such as Li). + and Na + Competing ions (Na+) under electric field driving + Low migration resistance makes it easy for Li to migrate. + Low flux; (3) Two-dimensional material membranes (such as GO membranes, MXene membranes) sieve ions by controlling the spacing of two-dimensional channels; poor chemical stability of materials, prone to interlayer expansion; channels are not adjustable, selectivity is limited; redox / crosslinking stability enhancement is required; (4) Covalent organic framework (COF) membranes have ordered crystal structures and uniform pores, which are suitable for constructing highly selective ion channels; simple framework COFs are difficult to distinguish Li + / Na + Functional groups need to be introduced, such as –SO3H and COOH, but the post-modification steps are complicated and inefficient; the static group recognition ability is insufficient and it is difficult to adapt to complex dynamic mass transfer environments; although existing COF membranes have made progress in the field of ion sieving, they still have the following shortcomings: (1) It is difficult to balance selectivity and flux: many membrane materials often cause Li to be reduced in order to improve selectivity. + Flux decreases, or selectivity is sacrificed at high flux rates. For example, conventional sulfonic acid-based COF membranes suffer from reduced selectivity due to the strong binding of Na+. + Increase Na + / Li + Selectivity, but at the same time it also hinders Li + (2) Limited channel function regulation: Existing technologies mostly use immobilized groups (such as –SO3H) to modify the COF framework, and the coordination environment in the channel is relatively rigid and lacks dynamic adjustment ability; (3) Complex preparation process: Traditional COF membrane preparation requires multiple steps or layer-by-layer assembly, and functionalized molecules are usually introduced after film formation by impregnation or post-treatment, which is cumbersome and difficult to accurately control the doping amount; (4) Insufficient application scenarios: Some studies only focus on seawater or single salt systems, and the application scenarios for Na-rich systems are limited. + Its adaptability to the complex brine conditions of salt lakes is still unclear. Summary of the Invention

[0005] The main objective of this invention is to provide an in-situ small molecule encapsulated COF composite membrane for ion selective separation, its preparation method and application, in order to overcome the shortcomings of the prior art.

[0006] To achieve the aforementioned objectives, the technical solution adopted by this invention includes:

[0007] This invention provides a method for preparing an in-situ small molecule encapsulated COF composite membrane for ion selective separation, comprising:

[0008] Provide a porous membrane as a support layer;

[0009] The porous membrane is immersed in an organic phase containing trialdehyde compounds, and then removed and contacted with an aqueous phase containing COF monomers and functional small molecules to undergo an interfacial reaction; or, the porous membrane is immersed in an organic phase containing trialdehyde compounds and COF monomers, and then removed and contacted with an aqueous phase containing functional small molecules to undergo an interfacial reaction, thereby obtaining an in-situ small molecule encapsulated COF composite membrane for ion selective separation.

[0010] The COF monomer includes 2,5-diaminophenyl-1,4-disulfonic acid and / or tris(4-aminophenyl)amine; the functional small molecule includes any one or more combinations of sulfonic acid small molecules, β-diketone small molecules, ketone / carbonyl small molecules, amide small molecules, and phosphate ester small molecules.

[0011] The present invention also provides an in-situ small molecule encapsulated COF composite membrane for ion selective separation prepared by the aforementioned preparation method. The in-situ small molecule encapsulated COF composite membrane includes a support layer and a covalent organic framework layer on its surface encapsulated with functional small molecules. The functional small molecules are uniformly embedded in the COF framework. The functional small molecules include any one or more combinations of sulfonic acid small molecules, β-diketone small molecules, ketone / carbonyl small molecules, amide small molecules, and phosphate ester small molecules.

[0012] This invention also provides the aforementioned in-situ small molecule encapsulated COF composite membrane for ion selective separation in Li + / Na + Applications in selective separation.

[0013] This invention also provides a method for selectively separating lithium ions, comprising:

[0014] Provides the aforementioned in-situ small molecule encapsulated COF composite membrane for ion selective separation;

[0015] Furthermore, under the influence of concentration difference or electric field, the in-situ small molecule encapsulated COF composite membrane is used to selectively separate lithium ions from salt lake brine.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] (1) Extremely high Li + / Na + Selectivity and flux: The membrane of this invention achieves Li through pore functionalization and small molecule synergy. + / Na + The selectivity far exceeds that of existing membrane technologies. Taking the HTTA-TpTap membrane as an example, Li + / Na + Selectivity up to 260, Li+ The flux can reach 145.8 mmol·m -2 ·h -1 ;TpPa-(SO3H)2-MOPS membrane Li + / Na + Selectivity can reach 185-215, flux 65.7 mmol·m -2 ·h -1 While maintaining high selectivity, the membrane of this invention retains excellent flux performance, which is significantly better than similar membrane materials reported to date.

[0018] (2) Simplified preparation of integrated film formation process: Unlike traditional COF membranes which require multiple layers to be stacked or subsequently functionalized, this invention only requires one step of interfacial polymerization to simultaneously form a COF network and encapsulate small molecules. The process is simpler, the preparation time and cost are reduced, and it is easy to achieve large-scale production.

[0019] (3) Adjustable pore environment: By changing the type and amount of encapsulating molecules, the chemical hydrophilicity and hydrophobicity, charge density and coordination site layout in the pores can be flexibly adjusted to achieve fine design of membrane properties. This "pore within a pore" strategy provides richer control means than single group modification and can optimize membrane structure for different ion separation tasks.

[0020] (4) High stability and recyclability: The membrane material has excellent chemical stability (COF itself is resistant to high salt and corrosion) and exhibits long-term stability in the electrodialysis experiment of multi-ion mixed solution (the performance remains basically unchanged after 10 cycles). Compared with some adsorbents that require chemical regeneration, the membrane of this invention can work continuously by only needing an electric field, without the need for frequent replacement or regeneration, and has low operating costs.

[0021] (5) Adaptable to complex brine applications: The membrane of this invention is designed in an electric field driven mode, which can be used in complex brine applications. + K + Competing ions and Mg 2+ Ca 2+ Selective migration of Li ions under complex brine conditions in salt lakes + This greatly simplifies the lithium extraction process, and its high selectivity and high throughput make direct electrodialysis or similar systems more efficient and economical in the treatment of low-concentration lithium resources. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of a typical embodiment of the present invention, showing a one-step in-situ COF encapsulation process for small molecules.

[0024] Figure 2 This is a schematic diagram of the concentration gradient permeation process in a typical embodiment of the present invention;

[0025] Figure 3 This is a schematic diagram of a membrane stack simulating an electrodialysis process in a typical embodiment of the present invention. Detailed Implementation

[0026] In view of the deficiencies of existing technologies, the inventors of this invention, through long-term research and extensive practice, have proposed the technical solution of this invention. This invention proposes a one-step in-situ interfacial polymerization method for preparing functionalized COF separation membranes. This method can simultaneously dope specific small molecule functional materials (such as molecules containing –SO3H or other coordinating groups) into COF nanopores, thereby precisely controlling the chemical and spatial environment of the pores to achieve Li + with Na + Highly selective separation. The membrane of this invention has the following characteristics: it can combine thermodynamic and kinetic mechanisms in parallel, utilizing small molecule coordination to enhance Li... + The binding / debinding transition capability within the channel, while hindering Na + Migration, balancing selectivity and throughput; the preparation process employs single-step interfacial polymerization, completing framework formation and functional molecule encapsulation in one step, simplifying the process; suitable for Na-rich environments. + An electric field separation system in a salt lake brine environment is expected to significantly improve the efficiency of direct lithium extraction.

[0027] For those containing Na + High-efficiency extraction of Li from abundant salt lake brine + To meet the needs of Li, this invention proposes a novel COF membrane material and its preparation method, aiming to achieve Li + with Na + The ultra-high selectivity separation. Traditional lithium extraction processes from salt lakes rely on multi-stage evaporation and precipitation processes, Li + In Na + Crystallization and K + The precipitation stage results in significant losses (Li + Losses often exceed 50%, making efficient recovery of lithium resources impossible. Meanwhile, Li... + and Na + Since monovalent metal ions have the same charge and similar water-binding radii, conventional membrane separation methods (such as nanofiltration, exchange membranes, and electrodialysis) struggle to effectively distinguish them. Therefore, there is an urgent need for a membrane material with an tunable pore environment to achieve efficient Li-type separation while ensuring flux. + / Na + Extremely high selectivity separation.

[0028] This invention targets substances containing high concentrations of Na. + High-efficiency extraction of Li from salt lake brine with background ions + The core challenge is to propose a one-step in-situ interfacial polymerization technique for preparing functionalized covalent organic framework (COF) composite films, which encapsulates Li... + Affinity-dependent functional small molecules are incorporated into the COF channels to precisely regulate the chemical and spatial environment of the channels, thereby enabling the control of Li. + / Na + The goal is to improve the lithium flux, selectivity, and long-term stability of membrane separation systems under actual electric field conditions, thereby solving the problem of balancing flux and selectivity in current membrane separation lithium extraction processes and providing a feasible path for efficient membrane-based lithium extraction.

[0029] The common problems solved by this invention are as follows: (1) Li + with Na + With highly similar physicochemical properties, traditional membrane separation mechanisms exhibit poor selectivity, and Li + with Na + Both are monovalent cations with similar hydration radii (Li + It is 3.82 Å, Na + The charge density and dehydration energy are similar (3.58 Å), making it difficult for membranes relying on electrostatic repulsion or size sieving mechanisms (such as nanofiltration membranes and cation exchange membranes) to effectively distinguish between different types of sodium, especially at high concentrations of Na+. + (2) Conventional COF membranes have bottlenecks such as high structural rigidity, single function, and complex preparation. Conventional COF materials have highly ordered channels and good thermochemical stability, but their channel environment is relatively "static". Fixed functional groups (such as -SO3H) are limited in improving selectivity. Moreover, the functionalization steps mostly depend on post-modification or multi-stage processes, making it difficult to accurately control the type and distribution of functional groups, which affects reproducibility and industrial scale-up. (3) The introduction of functional materials lacks an in-situ synergistic mechanism, which leads to structural instability or functional loss. Some studies have tried to introduce coordination molecules or auxiliary migration factors (such as chelating agents and crown ethers) into the membrane structure, but most of them are physical impregnation methods, which have problems such as easy loss, pore blockage, and functional instability, and cannot achieve long-term controllable Li + Identification and transport behavior; (4) In actual separation performance, there is a problem that selectivity and flux are difficult to balance. High-selectivity membranes often sacrifice mass transfer rate (such as strong ion binding leading to a decrease in flux), while high-flux membranes have reduced selectivity due to insufficient ion dehydration or scarce recognition sites. In practical applications (such as electrodialysis), lithium extraction efficiency and energy consumption control cannot be balanced; (5) Many membrane materials have insufficient durability in complex salt lake environments and low reusability. Salt lake brine often contains multiple competing cations (Na+, Na ...+ K + Mg 2+ Membrane materials need to operate stably for a long time without being contaminated, swollen, or structurally damaged, as they contain organic impurities such as pollutants and organic impurities. However, the performance of many membranes (especially adsorption type) deteriorates severely during the cycling process, limiting their engineering applications.

[0030] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Specifically, as one aspect of the technical solution of this invention, a method for preparing an in-situ small molecule encapsulated COF composite membrane for ion selective separation includes:

[0032] Provide a porous membrane as a support layer;

[0033] The porous membrane is immersed in an organic phase containing trialdehyde compounds, and then removed and contacted with an aqueous phase containing COF monomers and functional small molecules to undergo an interfacial reaction; or, the porous membrane is immersed in an organic phase containing trialdehyde compounds and COF monomers, and then removed and contacted with an aqueous phase containing functional small molecules to undergo an interfacial reaction, thereby obtaining an in-situ small molecule encapsulated COF composite membrane for ion selective separation.

[0034] The COF monomer includes 2,5-diaminophenyl-1,4-disulfonic acid and / or tris(4-aminophenyl)amine; the functional small molecule includes any one or more combinations of sulfonic acid small molecules, β-diketone small molecules, ketone / carbonyl small molecules, amide small molecules, and phosphate ester small molecules.

[0035] In some preferred embodiments, the preparation method specifically includes: immersing the porous membrane in an organic phase containing trialdehyde compounds, then taking it out and contacting it with an aqueous phase containing COF monomers and functional small molecules to undergo an interfacial reaction, thereby obtaining an in-situ small molecule encapsulated COF composite membrane for ion selective separation; wherein the COF monomer includes 2,5-diaminobenzene-1,4-disulfonic acid; and the functional small molecules include sulfonic acid-based small molecules.

[0036] Furthermore, the temperature of the interfacial reaction is 50-70 °C, and the time is 12-24 h.

[0037] Furthermore, the sulfonic acid group-based small molecule has any one of the following structures:

[0038] .

[0039] Furthermore, the sulfonic acid group small molecules include, but are not limited to, 3-morpholinopropanesulfonic acid (MOPS) and / or 1,4-piperazine dipropanesulfonic acid (PIPPS).

[0040] In some preferred embodiments, the preparation method specifically includes: immersing the porous membrane in an organic phase containing a trialdehyde compound and a COF monomer, then removing it and contacting it with an aqueous phase containing functional small molecules to undergo an interfacial reaction, thereby obtaining an in-situ small molecule encapsulated COF composite membrane for ion selective separation; wherein the COF monomer includes tris(4-aminophenyl)amine; and the functional small molecules include any one or a combination of β-diketone small molecules, ketone / carbonyl small molecules, amide small molecules, and phosphate ester small molecules.

[0041] Furthermore, the interface reaction is carried out at room temperature for 15-30 minutes.

[0042] Furthermore, the β-diketone small molecule has any one of the following structures:

[0043] .

[0044] Furthermore, the ketone / carbonyl small molecules have any one of the following structures:

[0045] .

[0046] Furthermore, the β-diketone small molecules and ketone / carbonyl small molecules include, but are not limited to, 2-thiophenecarboxyltrifluoroacetone (HTTA) and / or methyl isobutyl ketone (MIBK).

[0047] Furthermore, the amide-type small molecule has any one of the following structures:

[0048] .

[0049] Furthermore, the phosphate ester small molecule has any one of the following structures:

[0050] .

[0051] Furthermore, the amide small molecules include, but are not limited to, N,N-bis(2-ethylhexyl)acetamide (NB2EHOTA).

[0052] Furthermore, the phosphate ester small molecules include, but are not limited to, tributyl phosphate (TBP).

[0053] In some preferred embodiments, the trialdehyde compound includes, but is not limited to, any one or more combinations of trialdehyde-resorcinol, 2,4,6-tris(4-aldehydephenyl)-1,3,5-triazine, and 1,3,5-trialdehyde-1,3,5-triazine.

[0054] In some preferred embodiments, the porous membrane includes, but is not limited to, a porous polyacrylonitrile membrane.

[0055] Furthermore, the porous polyacrylonitrile membrane has a pore size of 0.15-0.30 μm and a thickness of 50-200 μm.

[0056] In some preferred embodiments, the preparation method further includes: pretreating the porous membrane.

[0057] Furthermore, the pretreatment includes sequentially subjecting the porous membrane to alkali immersion and acid immersion treatments.

[0058] In some preferred embodiments, the preparation method further includes: after the interfacial reaction is completed, immersing the obtained film in an acetic acid solution for soaking, and then washing it.

[0059] This invention proposes a COF membrane construction strategy of "one-step in-situ interfacial polymerization + functional small molecule encapsulation". This technical solution utilizes the pore formation window during the COF film formation process to achieve in-situ simultaneous encapsulation of ion-affinity functional small molecules, which are uniformly and stably embedded in the COF framework, thereby regulating the ion migration path and achieving both high selectivity and high throughput in ion separation.

[0060] (1) In-situ encapsulation path

[0061] This invention employs organic solvent / aqueous phase interfacial polymerization technology to simultaneously encapsulate small molecules during COF film formation, avoiding the functional loss, uneven doping, and leakage problems associated with traditional post-modification or physical doping. Specifically, it includes:

[0062] Integrated encapsulation process: COF backbone monomers and functional small molecules are dissolved separately in a two-phase system, and the small molecules are gradually embedded as the COF network crystallizes during the film formation process;

[0063] Site co-construction mechanism: Functional groups such as sulfonic acid groups, carbonyl groups, hydroxyl groups, and amide bonds contained in small molecules serve as new ion binding sites in the membrane, working in conjunction with skeletal sites to form dynamic ion migration pathways;

[0064] It is suitable for various functional molecules, including: sulfonic acid groups such as 3-morpholinepropanesulfonic acid (MOPS) and 1,4-piperazinedipropanesulfonic acid (PIPPS); β-diketones, ketones / carbonyl groups such as 2-thenoyltrifluoroacetone (HTTA) and methyl isobutyl ketone (MIBK); and amides and phosphate esters such as N,N-di(2-ethylhexyl)acetamide (NB2EHOTA) and tributyl phosphate (TBP).

[0065] The aforementioned molecules not only possess ion affinity, but also have advantages such as structural stability, good solvent compatibility, and high coordination flexibility, making them suitable for forming a stable pore environment in synergy with the COF framework.

[0066] (2) Multi-solution adaptation and substitutability

[0067] Different types of small molecules can combine with the COF backbone to form a variety of membrane structures, for example:

[0068] Sulfonic acid molecules synergistically enhance charge sieving with the –SO3H skeleton;

[0069] β-Diketone molecules endow highly selective ion-trapping coordination sites;

[0070] Amides / phosphate esters can introduce a weak acid / hydrogen bonding mechanism, enhancing reversible debonding dynamics.

[0071] The following are the specific encapsulation methods for three of the small molecule molecules: MOPS, PIPPS, and HTTA:

[0072] (a) Sulfonic acid-based (e.g., MOPS) encapsulated COF films

[0073] Based on a COF framework containing an appropriate amount of –SO3H groups, such as TpPa-(SO3H)2, a porous polyacrylonitrile (PAN) membrane with a pore size of 0.22 μm and a thickness of approximately 100 μm was selected as the support layer. The PAN membrane was cut into 5 cm × 5 cm pieces and pretreated: first, it was immersed in 1.5 mol / L NaOH solution at 40 °C for 60 min, then rinsed with plenty of deionized water until neutral, and then transferred to 0.1 mol / L HCl solution and immersed at room temperature for 12 h. Finally, it was thoroughly rinsed with deionized water for use. The pretreated PAN membrane was fixed in a polytetrafluoroethylene mold. 0.09 mmol of trialdehyde phloroglucinol (Tp) was dissolved in 4 mL of dichloromethane and magnetically stirred for 10 min to form a homogeneous organic phase solution. This solution was evenly poured onto the surface of the PAN membrane, allowed to stand for adsorption for 20 min, then poured off and the residual solvent was dried. Aqueous solution was then prepared: 0.135 mmol Pa-(SO3H)2 was dissolved in 4 mL of 3 mol / L acetic acid solution, and 0.045 mmol MOPS molecules were added simultaneously. The mixture was stirred thoroughly until completely dissolved. This aqueous phase was slowly added to the mold, allowing it to contact the organic phase interface. The reaction was carried out at a constant temperature of 60 °C for 12 h, thereby forming a TpPa-(SO3H)2 covalent organic framework layer on the PAN surface, which binds MOPS within the pores. After the reaction, the membrane was removed, rinsed with deionized water, and immersed in 3 mol / L acetic acid solution at 60 °C for 48 h to enhance structural stability. It was then rinsed thoroughly with deionized water and stored.

[0074] The resulting TpPa-(SO3H)2-MOPS / PAN membrane has internal pores with both fixed –SO3H adsorption sites and introduced MOPS chains as temporary Li. + Coordination / transition sites. The Li-coated membrane... + After partial surface dehydration, it migrates at high speed through the film layer via a "binding-release" process between –SO3H groups and MOPS coordination centers; while Na… + The binding with –SO3H is stronger, hindering movement within the pores. Electrodialysis results show that the TpPa-(SO3H)2-MOPS membrane in a mixed LiCl / NaCl solution (30 mM) exhibits better adhesion to LiCl. + The flux reached 65.6 mmol·m -2 ·h -1 Li + / Na + Selectivity reaches approximately 190; under concentration dialysis conditions, Li + Flux approximately 40 mmol·m -2 ·h -1The selectivity can reach approximately 215. Structural characterization (SEM / XRD / FTIR) confirmed that the crystal framework of the membrane system was intact, and MOPS molecules were uniformly distributed within the channels, bonded to the COF framework via hydrogen bonds (as shown by charge density calculations, there is strong hydrogen bonding between MOPS and SO3H). The membrane exhibited stable performance during 10 cycles of electrodialysis (Li). + (The flux and selectivity do not change much), demonstrating good reusability and water chemical stability.

[0075] (b) β-Diketone, ketone / carbonyl (e.g., HTTA) small molecule encapsulation COF membrane

[0076] HTTA1-TpTapa or HTTA2-TpTapa composite membranes were grown on porous PAN substrates using interfacial polymerization. A porous polyacrylonitrile (PAN) membrane with a pore size of 0.22 μm and a thickness of approximately 100 μm was selected as the support layer. The PAN membranes were cut to 5 cm × 5 cm pieces and immersed in 1.5 mol / L NaOH solution at 40 °C for 60 min. They were then rinsed thoroughly with deionized water until neutral, transferred to 0.1 mol / L HCl solution, and immersed at room temperature for 12 h. Finally, they were rinsed thoroughly with deionized water and stored for later use. 0.09 mmol of trialdehyde phloroglucinol (Tp) and 0.09 or 0.18 mmol of 2-thiophenecarboxylic acid trifluoroacetone (HTTA) were dissolved in 4 mL of dichloromethane and magnetically stirred for 10 min to form a homogeneous organic phase solution. The pretreated PAN membrane was fixed in a polytetrafluoroethylene mold. The organic phase was uniformly poured onto the membrane surface, and after standing for 10 min for adsorption, excess solution was poured off. Then, 0.09 mmol of tris(4-aminophenyl)amine (Tapa) was weighed and dissolved in 4 mL of deionized water, and glacial acetic acid was added to adjust the solution concentration to 3 mM. The solution was ultrasonically dissolved for 5 min to form an aqueous phase. The aqueous phase was slowly added to the mold to contact the interface with the organic phase, and the reaction was carried out at room temperature (25±2℃) for 20 min. After the reaction, the membrane was removed, rinsed with deionized water, and immersed in 1 mol / L acetic acid solution for 36 h. It was then washed three times each with tetrahydrofuran, ethanol, and deionized water, and finally stored in deionized water to obtain the HTTA-TpTapa / PAN membrane.

[0077] In some preferred embodiments, a schematic diagram of the one-step in-situ encapsulation process of small molecules using COF is shown below. Figure 1 As shown.

[0078] HTTA molecules have strong lithium affinity, allowing them to react with Li via carbonyl groups within the pores. + Coordination bonds are formed. The ion transport mechanism of this HTTA encapsulated film is "binding-transition": Li +Upon entering the pores, the carbon groups of the HTTA are first specifically captured and partially dehydrated (binding stage), then "jump" from one HTTA coordination site to the next, achieving accelerated migration; while Na + The binding with HTTA is weak, and the migration is slow. This unique mechanism enables the HTTA-TpTapa / PAN membrane to achieve extremely superior separation performance under electrodialysis conditions: Li + / Na + Selectivity exceeds 320 times, Li + The flux was approximately 142.9 mmol·m⁻². -2 ·h -1 Microstructure analysis of the membrane revealed that HTTA was completely and uniformly encapsulated within the COF nanopores (SEM-EDS elemental mapping confirmed the uniform distribution of HTTA within the COF layer), and the COF framework maintained high crystallinity (XRD, TEM, and other results were consistent). The membrane exhibited stable separation performance during multiple electrodialysis cycles (no significant attenuation of indicators after 10 cycles), validating its recyclability and structural robustness.

[0079] (c) Amide and phosphate ester (e.g., TBP) small molecule encapsulation COF film

[0080] A TpTapa covalent organic framework composite membrane encapsulating amide or phosphate ester small molecules was grown in situ on a porous PAN substrate using interfacial polymerization. A porous polyacrylonitrile (PAN) membrane with a pore size of 0.22 μm and a thickness of approximately 100 μm was selected as the support layer. The PAN membrane was cut into 5 cm × 5 cm pieces and immersed in a 1.5 mol / L NaOH solution at 40 °C for 60 min. It was then rinsed thoroughly with deionized water until neutral, transferred to a 0.1 mol / L HCl solution, and immersed at room temperature for 12 h. Finally, it was thoroughly rinsed with deionized water and stored for later use. 0.09 mmol of trialdehyde phloroglucinol (Tp) and 0.09 mmol of amide small molecules (e.g., acrylamide, p-aminobenzamide) or phosphate small molecules (e.g., tributyl phosphate) were dissolved in 4 mL of dichloromethane and magnetically stirred for 10 min to form a homogeneous organic phase solution. The pretreated PAN membrane was fixed in a polytetrafluoroethylene mold. The organic phase was uniformly poured onto the membrane surface, and after standing for 10 min for adsorption, excess solution was poured off. Then, 0.09 mmol of tris(4-aminophenyl)amine (Tapa) was weighed and dissolved in 4 mL of deionized water, and glacial acetic acid was added to adjust the solution concentration to 3 mM. The solution was ultrasonically dissolved for 5 min to form an aqueous solution. The aqueous solution was slowly added to the mold to contact the interface with the organic phase, and the reaction was carried out at room temperature (25±2℃) for 20 min. After the reaction, the membrane was removed, rinsed with deionized water, and immersed in 1 mol / L acetic acid solution for 36 h. It was then washed three times each with tetrahydrofuran, ethanol, and deionized water, and finally stored in deionized water to obtain amide or phosphate ester small molecule encapsulated TpTapa / PAN membranes.

[0081] In this system, amide molecules interact with Li through the carbonyl oxygen atom. + Reversible coordination is formed, and its amino or amide hydrogens can participate in hydrogen bonding to regulate the pore microenvironment, thereby enhancing Li + Selective migration ability within pores; the P=O bond and P–O–C structure in phosphate ester small molecules can interact with Li. + Formation of multi-site coordination interactions, enhancing Li + The combination and directional transmission capabilities of Na + Due to its weak coordination ability, migration within the pores is inhibited. Electrodialysis test results show that this type of membrane material exhibits good performance in 30 mM equimolar Li₂. + / Na + Li exhibits good separation performance under mixed solution conditions. + The flux can reach 90–120 mmol·m -2 ·h -1 Li + / Na +The selectivity reaches 130–180, and the performance remains stable during multiple cycle tests, indicating that the small molecule achieves stable confined encapsulation within the COF channels without significant leakage.

[0082] The above implementation scheme demonstrates that by employing a one-step in-situ confinement encapsulation strategy during interfacial polymerization, small molecules with ion-affinity functional groups can be simultaneously introduced into the pores of the COF membrane. This allows for precise control of the pore microenvironment without disrupting the COF crystal framework structure. These small molecules include, but are not limited to, sulfonic acid groups, β-diketones, ketones / carbonyl groups, amides, and phosphate esters, all characterized by containing groups capable of reacting with Li. + Functional groups that undergo reversible coordination or weak interactions (such as –SO3H, C=O, CONH–, P=O, etc.). Through in-situ encapsulation, these small molecules form synergistic ion migration channels within the COF nanopores, enabling Li… + Directional transport is achieved within the channel via a "binding-release" or multi-site hopping mechanism, while Na... + Due to weak coordination ability or differences in binding kinetics, migration within the pores is restricted, thus significantly improving the efficiency of Li. + / Na + Separation selectivity and maintaining high Li + Flux. This technical approach has good versatility and scalability. Functional regulation can be achieved by replacing different types of small molecules, forming an ion-selective separation platform that can be adapted to different salt lake brine systems.

[0083] Another aspect of the present invention provides an in-situ small molecule encapsulated COF composite membrane for ion selective separation prepared by the aforementioned preparation method. The in-situ small molecule encapsulated COF composite membrane includes a support layer and a covalent organic framework layer on the surface of which functional small molecules are encapsulated. The functional small molecules are uniformly embedded in the COF framework. The functional small molecules include any one or more combinations of sulfonic acid small molecules, β-diketone small molecules, ketone / carbonyl small molecules, amide small molecules, and phosphate ester small molecules.

[0084] Another aspect of the present invention provides the aforementioned in-situ small molecule encapsulated COF composite membrane for ion selective separation in Li + / Na + Applications in selective separation.

[0085] Another aspect of the present invention provides a method for selectively separating lithium ions, comprising:

[0086] Provides the aforementioned in-situ small molecule encapsulated COF composite membrane for ion selective separation;

[0087] Furthermore, under the influence of concentration difference or electric field, the in-situ small molecule encapsulated COF composite membrane is used to selectively separate lithium ions from salt lake brine.

[0088] The technical solution of the present invention will be further described in detail below with reference to several preferred embodiments and accompanying drawings. This embodiment is implemented on the premise of the technical solution of the invention, and provides detailed implementation methods and specific operation processes. However, the protection scope of the present invention is not limited to the following embodiments.

[0089] Unless otherwise specified, the experimental materials used in the examples below can be purchased from conventional biochemical reagent companies.

[0090] Example 1: Achieving 30 mM equimolar Li3 by using β-diketone HTTAP1-TpTapa / PAN membranes + / Na + Li in binary solution under electrodialysis conditions + Separation and extraction

[0091] A porous polyacrylonitrile (PAN) membrane with a pore size of 0.22 μm and a thickness of approximately 100 μm was selected as the support layer. The PAN membrane was cut into 5 cm × 5 cm pieces and immersed in a 1.5 mol / L NaOH solution at 40°C for 60 min. It was then rinsed thoroughly with deionized water until neutral, transferred to a 0.1 mol / L HCl solution, and immersed at room temperature for 12 h. Finally, it was rinsed thoroughly with deionized water and stored for later use. 0.09 mmol of trialdehyde phloroglucinol (Tp) and 0.09 mmol of 2-thiophenecarboxylic acid trifluoroacetone (HTTA) were dissolved in 4 mL of dichloromethane and magnetically stirred for 10 min to form a homogeneous organic phase solution. The pretreated PAN membrane was fixed in a polytetrafluoroethylene mold, and the above organic phase was uniformly poured onto the membrane surface. After standing for 10 min for adsorption, excess solution was poured off. Subsequently, 0.09 mmol of tris(4-aminophenyl)amine (Tapa) was dissolved in 4 mL of deionized water, and glacial acetic acid was added to adjust the solution concentration to 3 mM. The solution was then sonicated for 5 min to form an aqueous phase. The aqueous phase was slowly added to the mold, ensuring contact with the organic phase interface, and the reaction was carried out at room temperature (25±2℃) for 20 min. After the reaction, the membrane was removed, rinsed with deionized water, and immersed in 1 mol / L acetic acid solution for 36 h. It was then washed three times each with tetrahydrofuran, ethanol, and deionized water, and finally stored in deionized water, yielding a membrane with an area of ​​7.065 cm². 2 The HTTA1-TpTapa / PAN membrane employs a four-chamber device simulating an electrodialysis membrane stack, with an applied current density of 10 mA / cm². 2 Under a constant current electric field, the prepared film was used for Li +Separation and extraction, with an effective membrane area of ​​2 cm². 2 The circulating solution used was 0.3 M Na₂SO₄, and the feed solution was 30 mM equimolar Li₂. + / Na + Binary solution, Li + Concentration 207.29 mg·L -1 Na + Concentration 668.87 mg·L -1 The receiving solution used is 5mM HCl. A schematic diagram of the membrane stack simulating the electrodialysis process is shown below. Figure 3 As shown, after running for 2 hours, the receiving solution was collected for analysis, and the Li in the receiving solution was detected by ICP-OES. + / Na + Ion concentration. Performance indicators: Li + The flux was 131.98 mmol m -2 h -1 Li + / Na + The selectivity is 225.33.

[0092] Example 2: Achieving 30 mM equimolar Li3 by using β-diketone HTTAP1-TpTapa / PAN membranes + / Na + Li in binary solution under concentration conditions + Separation and extraction

[0093] The HTTAP1-TpTapa / PAN membrane obtained based on the preparation method of Example 1 was used for Li [material name missing] without electrodialysis, instead relying on the concentration gradient as the driving force for ion transport. + Separation and extraction, with an effective membrane area of ​​2 cm². 2 The feed solution is 30 mM equimolar Li + / Na + Binary solution, Li + Concentration 206.76 mg·L -1 Na + Concentration 687.95 mg·L -1 The receiving solution is deionized water. A schematic diagram of the concentration gradient osmosis process is shown below. Figure 2 As shown, after running for 1 hour, the receiving solution was collected for analysis, and the Li in the receiving solution was detected by ICP-OES. + / Na + Ion concentration. Performance indicators: Li + The flux was 33.70 mmol m -2 h -1 Li + / Na + The selectivity is 259.44.

[0094] Example 3: Achieving 30 mM equimolar Li3 using β-diketone HTTA2-TpTapa / PAN membranes + / Na + Li in binary solution under electrodialysis conditions + Separation and extraction

[0095] A PAN membrane of the same specifications was selected as the support layer and pretreated according to the following steps: It was soaked in 1.5 mol / L NaOH solution at 40 ℃ for 60 min, rinsed until neutral, and then soaked in 0.1 mol / L HCl solution for 12 h, followed by rinsing with deionized water. 0.09 mmol of trialdehyde phloroglucinol (Tp) and 0.18 mmol of 2-thiophenecarboxylic acid trifluoroacetone (HTTA) were dissolved in 4 mL of dichloromethane to form an organic phase solution. This solution was poured onto the fixed PAN membrane surface and allowed to stand for 10 min before pouring off the excess solution. Subsequently, 0.09 mmol of tris(4-aminophenyl)amine (Tapa) was dissolved in 4 mL of deionized water containing 3 mM glacial acetic acid to form an aqueous phase solution. This solution was slowly added to the mold and reacted at room temperature for 20 min. After the reaction was completed, the mixture was sequentially rinsed with deionized water, soaked in 1 mol / L acetic acid for 36 h, and then washed with tetrahydrofuran, ethanol, and deionized water to obtain a sample with an area of ​​7.065 cm². 2 The HTTA2-TpTapa / PAN membrane employs a four-chamber device simulating an electrodialysis membrane stack, with an applied current density of 10 mA / cm². 2 Under a constant current electric field, the prepared film was used for Li + Separation and extraction, with an effective membrane area of ​​2 cm². 2 The circulating solution used was 0.3 M Na₂SO₄, and the feed solution was 30 mM equimolar Li₂. + / Na + Binary solution, Li + Concentration 206.47 mg·L -1 Na + Concentration 670.38 mg·L -1 The receiving solution used is 5mM HCl. A schematic diagram of the membrane stack simulating the electrodialysis process is shown below. Figure 3 As shown, after running for 2 hours, the receiving solution was collected for analysis, and the Li in the receiving solution was detected by ICP-OES. + / Na + Ion concentration. Performance indicators: Li + The flux was 145.79 mmol m -2 h -1 Li + / Na + The selectivity is 120.06.

[0096] Example 4: Achieving 30 mM equimolar Li3 by using sulfonic acid-based TpPa-(SO3H)2-MOPS / PAN membranes + / Na + Li in binary solution under electrodialysis conditions + Separation and extraction

[0097] A 5 cm × 5 cm PAN membrane was immersed in 1.5 mol / L NaOH solution at 40℃ for 60 min, rinsed, and then immersed in 0.1 mol / L HCl solution for 12 h. After rinsing with deionized water, it was ready for use. 0.09 mmol of trialdehyde phloroglucinol (Tp) was dissolved in 4 mL of dichloromethane to form an organic phase solution. 0.09 mmol of p-phenylenediamine sulfonic acid monomer Pa-(SO3H)2 and 0.03 mmol of MOPS were added to the aqueous phase, and glacial acetic acid was added to make the solution concentration 3 mM, with a total volume of 4 mL. The organic phase was inverted onto the PAN membrane surface for adsorption for 10 min, then decanted, and the aqueous phase was slowly added. The reaction was allowed to proceed for 20 min at room temperature. Subsequently, the membrane was rinsed with deionized water, immersed in 1 mol / L acetic acid for 36 h, and subjected to multi-solvent cleaning steps to obtain a membrane with an area of ​​7.065 cm². 2 The TpPa-(SO3H)2-MOPS / PAN membrane, using a four-chamber device simulating an electrodialysis membrane stack, is tested with an applied current density of 10 mA / cm². 2 Under a constant current electric field, the prepared film was used for Li + Separation and extraction, with an effective membrane area of ​​2 cm². 2 The feed solution is 30 mM equimolar Li + / Na + Binary solution, Li + Concentration 207.23 mg·L -1 Na + Concentration 689.17 mg·L -1 The receiving solution is deionized water. A schematic diagram of the membrane stack simulating the electrodialysis process is shown below. Figure 3 As shown, after running for 1 hour, the receiving solution was collected for analysis, and the Li in the receiving solution was detected by ICP-OES. + / Na + Ion concentration. Performance indicators: Li + The flux was 39.5 mmol m -2 h -1 Li + / Na + The selectivity is 212.42.

[0098] Example 5: Achieving 30 mM equimolar Li3 by using sulfonic acid-based TpPa-(SO3H)2-PIPPS / PAN membranes + / Na + Li in binary solution under electrodialysis conditions + Separation and extraction

[0099] The PAN membrane pretreatment procedure is the same as described above. 0.09 mmol Tp was dissolved in 4 mL of dichloromethane as the organic phase. 0.09 mmol Pa-(SO3H)2 and 0.03 mmol PIPPS were added to the aqueous phase, and glacial acetic acid was added to adjust the concentration to 3 mM, for a total volume of 4 mL. After reacting for 20 min according to the interfacial polymerization procedure, acetic acid post-treatment and multi-solvent washing were performed to obtain a membrane with an area of ​​7.065 cm². 2 The TpPa-(SO3H)2-PIPPS / PAN membrane, using a four-chamber device simulating an electrodialysis membrane stack, is tested with an applied current density of 10 mA / cm². 2 Under a constant current electric field, the prepared film was used for Li + Separation and extraction, with an effective membrane area of ​​2 cm². 2 The circulating solution used was 0.3 M Na₂SO₄, and the feed solution was 30 mM equimolar Li₂. + / Na + Binary solution, Li + Concentration 207.38 mg·L -1 Na + Concentration 679.23 mg·L -1 The receiving solution used is 5mM HCl. A schematic diagram of the membrane stack simulating the electrodialysis process is shown below. Figure 3 As shown, after running for 1 hour, the receiving solution was collected for analysis, and the Li in the receiving solution was detected by ICP-OES. + / Na + Ion concentration. Performance indicators: Li + The flux was 65.7 mmol m -2 h -1 Li + / Na + The selectivity is 188.71.

[0100] Example 6: Achieving 30 mM equimolar Li3 by using ketone-based MIBK-TpTapa / PAN membranes + / Na + Li in binary solution under electrodialysis conditions + Separation and extraction

[0101] A porous polyacrylonitrile (PAN) membrane with a pore size of 0.22 μm and a thickness of approximately 100 μm was selected as the support layer. The PAN membrane was cut into 5 cm × 5 cm pieces and immersed in a 1.5 mol / L NaOH solution at 40°C for 60 min. It was then rinsed thoroughly with deionized water until neutral, transferred to a 0.1 mol / L HCl solution, and immersed at room temperature for 12 h. Finally, it was rinsed thoroughly with deionized water and stored for later use. 0.09 mmol of trialdehyde phloroglucinol (Tp) and 0.09 mmol of methyl isobutyl ketone (MIBK) were dissolved in 4 mL of dichloromethane and magnetically stirred for 10 min to form a homogeneous organic phase solution. The pretreated PAN membrane was fixed in a polytetrafluoroethylene mold, and the organic phase was evenly poured onto the membrane surface. After standing for 10 min for adsorption, excess solution was poured off. Subsequently, 0.09 mmol of tris(4-aminophenyl)amine (Tapa) was dissolved in 4 mL of deionized water, and glacial acetic acid was added to adjust the solution concentration to 3 mM. The solution was then sonicated for 5 min to form an aqueous phase. The aqueous phase was slowly added to the mold, ensuring contact with the organic phase interface, and reacted at room temperature (25±2℃) for 20 min. After the reaction, the membrane was removed, rinsed with deionized water, and immersed in 1 mol / L acetic acid solution for 36 h. It was then washed three times each with tetrahydrofuran, ethanol, and deionized water, and finally stored in deionized water to obtain the MIBK-TpTapa / PAN membrane. The membrane was then subjected to 30 mM equimolar Li + / Na + Under the electrodialysis test conditions of the mixed solution, the Li membrane + Flux approximately 105 mmol·m -2 ·h -1 Li + / Na + With a selectivity of approximately 145, it exhibits excellent ion-selective separation performance.

[0102] Example 7: Achieving equimolar Li30 mM using amide-based NB2EHOTA-TpTapa / PAN membranes + / Na + Li in binary solution under electrodialysis conditions + Separation and extraction

[0103] A porous PAN membrane with a pore size of 0.22 μm and a thickness of approximately 100 μm was selected as the support layer. After cutting, the PAN membrane was immersed in 1.5 mol / L NaOH solution at 40℃ for 60 min, rinsed until neutral, and then immersed in 0.1 mol / L HCl solution for 12 h. Finally, it was rinsed with deionized water for later use. 0.09 mmol of trialdehyde phloroglucinol (Tp) and 0.09 mmol of N,N-di(2-ethylhexyl)acetamide were dissolved in 4 mL of dichloromethane and stirred for 10 min to form a homogeneous organic phase solution. The organic phase was inverted onto the surface of the PAN membrane, allowed to stand for 10 min, and then the excess solution was poured off. Subsequently, 0.09 mmol of tris(4-aminophenyl)amine was dissolved in 4 mL of deionized water containing 3 mM glacial acetic acid to form an aqueous phase solution, which was slowly added to the mold and reacted at room temperature for 20 min. After the reaction was complete, the membrane was removed and subjected to a series of steps including rinsing with deionized water, immersion in 1 mol / L acetic acid for 36 h, and multi-solvent cleaning. Finally, it was soaked and stored to obtain the NB2EHOTA encapsulated TpTapa / PAN membrane. Electrodialysis test results showed that in 30 mM equimolar Li... + / Na + Under mixed solution conditions, the Li membrane + Flux approximately 112 mmol·m -2 ·h -1 Li + / Na + With a selectivity of approximately 165, it exhibits strong lithium-ion affinity and stable separation performance.

[0104] Example 8: Achieving 30 mM equimolar Li3 by using phosphate ester-based TBP-TpTapa / PAN membranes + / Na + Li in binary solution under electrodialysis conditions + Separation and extraction

[0105] A porous PAN membrane with a pore size of 0.22 μm and a thickness of approximately 100 μm was selected as the support layer and pretreated as follows: It was soaked in 1.5 mol / L NaOH solution at 40 ℃ for 60 min, rinsed, and then soaked in 0.1 mol / L HCl solution for 12 h. Finally, it was rinsed with deionized water for later use. 0.09 mmol of trialdehyde phloroglucinol (Tp) and 0.09 mmol of tributyl phosphate (TBP) were dissolved in 4 mL of dichloromethane and magnetically stirred for 10 min to form a homogeneous organic phase solution. This organic phase was uniformly poured onto the surface of the PAN membrane, allowed to stand for 10 min for adsorption, and then the excess solution was poured off. Subsequently, 0.09 mmol of tris(4-aminophenyl)amine was dissolved in 4 mL of deionized water containing 3 mM glacial acetic acid to form an aqueous phase solution, which was slowly added to the mold and reacted at room temperature for 20 min. After the reaction was complete, the membrane was removed, rinsed with deionized water, and soaked in 1 mol / L acetic acid solution for 36 h. It was then washed sequentially with tetrahydrofuran, ethanol, and deionized water to obtain the TBP-encapsulated TpTapa / PAN membrane. Testing was performed under electrodialysis conditions, using 30 mM equimolar Li... + / Na + The Li membrane in the system + Flux approximately 118 mmol·m -2 ·h -1 Li + / Na + The selectivity of approximately 175 indicates that phosphate ester molecules can be Li-type molecules within the channels. + It provides multi-site coordination channels, thereby improving separation performance.

[0106] In addition, the inventors of this case also conducted experiments with other raw materials, process operations, and process conditions described in this specification, referring to the aforementioned embodiments, and obtained relatively ideal results in all cases.

[0107] It should be understood that the technical solutions of the present invention are not limited to the specific embodiments described above. Any technical modifications made to the technical solutions of the present invention without departing from the spirit and scope of the claims are within the scope of protection of the present invention.

Claims

1. A method for preparing an in-situ small molecule encapsulated COF composite membrane for ion selective separation, characterized in that, include: Provide a porous membrane as a support layer; The porous membrane is immersed in an organic phase containing trialdehyde compounds, and then removed and contacted with an aqueous phase containing COF monomers and functional small molecules to undergo an interfacial reaction; or, the porous membrane is immersed in an organic phase containing trialdehyde compounds and COF monomers, and then removed and contacted with an aqueous phase containing functional small molecules to undergo an interfacial reaction, thereby obtaining an in-situ small molecule encapsulated COF composite membrane for ion selective separation. The COF monomer includes 2,5-diaminophenyl-1,4-disulfonic acid and / or tris(4-aminophenyl)amine; the functional small molecule includes any one or more combinations of sulfonic acid small molecules, β-diketone small molecules, ketone / carbonyl small molecules, amide small molecules, and phosphate ester small molecules.

2. The preparation method according to claim 1, characterized in that, Specifically, it includes: The porous membrane is immersed in an organic phase containing trialdehyde compounds, and then taken out and contacted with an aqueous phase containing COF monomers and functional small molecules to undergo an interfacial reaction, thereby obtaining an in-situ small molecule encapsulated COF composite membrane for ion selective separation; wherein, the COF monomer includes 2,5-diaminobenzene-1,4-disulfonic acid; and the functional small molecules include sulfonic acid-based small molecules. Preferably, the temperature of the interfacial reaction is 50-70 °C and the time is 12-24 h.

3. The preparation method according to claim 2, characterized in that: The sulfonic acid group-type small molecule has any one of the following structures: ； And / or, the sulfonic acid group small molecules include 3-morpholinopropanesulfonic acid and / or 1,4-piperazine dipropanesulfonic acid.

4. The preparation method according to claim 1, characterized in that, Specifically, it includes: The porous membrane is immersed in an organic phase containing trialdehyde compounds and COF monomers, and then removed and contacted with an aqueous phase containing functional small molecules to undergo an interfacial reaction, thereby obtaining an in-situ small molecule encapsulated COF composite membrane for ion selective separation; wherein, the COF monomer includes tris(4-aminophenyl)amine; the functional small molecules include any one or more combinations of β-diketone small molecules, ketone / carbonyl small molecules, amide small molecules, and phosphate ester small molecules; Preferably, the interface reaction is carried out at room temperature for 15-30 minutes.

5. The preparation method according to claim 1, characterized in that: The β-diketone small molecule has any one of the following structures: ； And / or, the ketone / carbonyl small molecules have any one of the following structures: ； And / or, the β-diketone small molecules, ketone / carbonyl small molecules include 2-thiophenecarboxyltrifluoroacetone and / or methyl isobutyl ketone; And / or, the amide small molecule has any one of the following structures: ； And / or, the phosphate ester small molecule has any one of the following structures: ； And / or, the amide small molecule includes N,N-bis(2-ethylhexyl)acetamide; And / or, the phosphate ester small molecules include tributyl phosphate.

6. The preparation method according to claim 1, characterized in that: The trialdehyde compounds include any one or more combinations of trialdehyde-resorcinol, 2,4,6-tris(4-aldehydephenyl)-1,3,5-triazine, and 1,3,5-trialdehyde-1,3,5-triazine; And / or, the porous membrane comprises a porous polyacrylonitrile membrane; preferably, the porous polyacrylonitrile membrane has a pore size of 0.15-0.30 μm and a thickness of 50-200 μm.

7. The preparation method according to claim 1, characterized in that, Also includes: First, the porous membrane is pretreated; Preferably, the pretreatment includes: sequentially subjecting the porous membrane to alkali immersion and acid immersion treatment; And / or, the preparation method further includes: after the interfacial reaction is completed, immersing the obtained film in an acetic acid solution for soaking, and then washing it.

8. The in-situ small molecule encapsulated COF composite membrane for ion selective separation prepared by the preparation method according to any one of claims 1-7, characterized in that: The in-situ small molecule encapsulated COF composite film includes a support layer and a covalent organic framework layer on its surface encapsulated with functional small molecules. The functional small molecules are uniformly embedded in the COF framework. The functional small molecules include any one or more combinations of sulfonic acid small molecules, β-diketone small molecules, ketone / carbonyl small molecules, amide small molecules, and phosphate ester small molecules.

9. The in-situ small molecule encapsulated COF composite membrane for ion selective separation as described in claim 8 in Li + / Na + Applications in selective separation.

10. A method for selectively separating lithium ions, characterized in that, include: Provides the in-situ small molecule encapsulated COF composite membrane for ion selective separation as described in claim 8; Furthermore, under the influence of concentration difference or electric field, the in-situ small molecule encapsulated COF composite membrane is used to selectively separate lithium ions from salt lake brine.

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