Covalent organic framework membrane for recycling lithium ion resources

Abstract

The invention belongs to the technical field of lithium ion resource recovery, and particularly relates to a covalent organic framework membrane for lithium ion resource recovery. The covalent organic framework membrane is formed by self-assembly of a water-phase monomer and an organic-phase monomer through double-activation interface polymerization, and is a self-supporting covalent organic framework membrane with an interface orientation asymmetric structure. According to the invention, the variety and density of functional groups in a COF framework are regulated and controlled, and the charge environment in a nano channel is finely regulated, so that the film realizes preferential transmission of lithium ions under the synergistic effect of size screening and charge regulation and control, migration of multivalent metal ions is effectively inhibited, and the separation selectivity of the lithium ions in a complex system is improved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion resource recycling technology, specifically relating to a covalent organic framework membrane for lithium-ion resource recycling. Background Technology

[0002] With the rapid development of electric vehicles, large-scale energy storage systems, and portable electronic devices, lithium resources have become an indispensable key material in modern energy technologies. However, the current rate of lithium resource extraction and supply is insufficient to meet the ever-increasing global demand, leading to a persistent shortage of lithium and severely restricting the sustainable development of energy technologies. Recovering lithium ions (Li-ion batteries) from spent lithium-ion batteries, salt lake brine, and industrial wastewater is a crucial step in this process. + It is considered an important strategy for alleviating resource shortages and reducing environmental pollution.

[0003] However, actual wastewater systems are complex in composition and typically contain a large number of metal ions with hydration radii similar to lithium ions, such as cobalt (Co). 2+ ), manganese (Mn) 2+ ), nickel (Ni 2+ ), iron (Fe) 2+ / Fe 3+ ), etc. The hydration radii of these coexisting ions are similar to those of Li. + The high degree of overlap significantly weakens the size sieving effect; simultaneously, the high acidity / salt and large pH fluctuations of the waste liquid easily induce ion hydrolysis precipitation or complexation, interfering with the separation process. The combination of these factors results in severe mutual interference between lithium ions and multivalent metal ions during the recovery process, making efficient and selective separation of lithium ions extremely challenging.

[0004] Currently, the mainstream technologies for recovering lithium ions from waste liquids in industry mainly include traditional methods such as precipitation, solvent extraction, and electrochemical deposition. However, these methods generally suffer from problems such as complex processes, high energy consumption, easy generation of secondary pollution, or unsatisfactory selectivity, making it difficult to meet the requirements of green, low-carbon, and sustainable development. In contrast, membrane separation technology is considered a highly promising alternative due to its environmental friendliness, ease of operation, and low energy consumption. Among them, layered membranes constructed based on two-dimensional (2D) materials such as graphene, molybdenum disulfide (MoS2), and MXene show potential in the field of ion selective separation due to their unique atomic-level thickness and tunable interlayer channels.

[0005] However, traditional non-porous two-dimensional layered membranes have inherent technical bottlenecks: their ion transport mainly relies on highly tortuous two-dimensional nanochannels formed between layers. This lengthy and tortuous transport path severely limits the ion migration rate, resulting in generally low membrane flux. More importantly, such membrane structures struggle to overcome the long-standing "permeability-selectivity trade-off" problem in membrane separation, where high selectivity often comes at the cost of reduced permeate flux, and vice versa. This significantly limits their application in efficient lithium recovery.

[0006] To overcome the aforementioned bottlenecks, covalent organic frameworks (COFs) with highly ordered porous crystal structures have attracted widespread attention in recent years. COFs possess well-defined one-dimensional through-pores, large specific surface areas, and customizable chemical functions, making them ideal membrane materials for achieving high-flux, highly selective ion transport. In existing technologies, to improve the selectivity of COF membranes for specific ions, researchers typically employ physical blocking (such as polymer coatings) or chemical post-modification methods to reduce their pore size, primarily relying on steric hindrance (size sieving) to block larger ions. However, for ions with similar sizes (such as Li), the pore size remains limited. + With Co 2+ Mn 2+ (etc.), relying solely on size to eliminate resistance often makes it difficult to achieve precise separation.

[0007] In natural biological systems, nanochannels can efficiently and selectively transport specific ions by precisely controlling the electrostatic environment on the pore surface, revealing the core role of electrostatic interactions in ion-selective transport. Unlike simple size sieving, electrostatic control can achieve more precise differentiation based on ion valence state, hydration radius, and other characteristics. Inspired by this, existing research has attempted to introduce charged functional groups such as carboxylic acid groups (-COOH), quaternary ammonium groups, and sulfonic acid groups into COF channels to regulate the electrostatic environment within the channels. However, precise and controllable regulation of the surface charge of nanopores remains a significant challenge in the preparation and application of COF membranes. On the one hand, chemical modification may disrupt the regularity of the COF crystal structure, clogging the channels and reducing flux; on the other hand, the intrinsic relationship between charge distribution, electrostatic potential gradient, and ion transport behavior within COF channels is currently unclear, lacking a clear "structure-performance" guideline. Summary of the Invention

[0008] The purpose of this invention is to provide a covalent organic framework membrane (COF) for lithium-ion resource recovery, thereby overcoming the shortcomings of existing technologies. By introducing sulfonic acid groups (-SO3H) into the COF framework and controlling the density and distribution of these groups, the charge environment within the nanopores can be precisely adjusted, thus optimizing ion sieving performance. This strategy not only provides an efficient and controllable lithium recovery method but also offers valuable insights into understanding the mechanism of ion-selective transport at the molecular level.

[0009] To achieve the above objectives, the technical solution of the present invention is as follows:

[0010] In a first aspect, the present invention provides a self-supporting covalent organic framework membrane, wherein the covalent organic framework membrane is self-assembled by the polymerization of aqueous phase monomers and organic phase monomers through a dual-activated interface, and is a self-supporting covalent organic framework membrane with an interface orientation asymmetric structure. The aqueous monomer includes one or more of 4,4'-diamino-[1,1'-biphenyl]-3,3'-disulfonic acid and 2,5-diaminobenzenesulfonic acid; The organic phase monomers include one or more of 1,3,5-tricarboxyloyl-phloroglucinol, 1,3,5-benzenetriformaldehyde, 2,5-dimethoxy-terephthalaldehyde, 2,5-dihydroxy-terephthalaldehyde, 2,6-pyridinediformaldehyde, 2,4,6-tricarboxy-1,3,5-triazine, and pyrazindiformaldehyde.

[0011] This self-supporting covalent organic framework membrane, with its unique bifacial heterogeneous structure and pore size gradient distribution characteristics, achieves synergistic optimization of efficient ion transport and precise sieving: its loose aqueous phase interface acts as an ion enrichment layer, which can efficiently capture target ions and reduce interfacial resistance; while the dense organic phase interface acts as a precision sieving layer, which utilizes steric hindrance and electrostatic interactions (such as the negatively charged environment provided by sulfonic acid groups) to achieve highly selective separation of ions of similar size, thereby breaking through the mutual constraint between permeability and selectivity in traditional membrane materials at the molecular scale.

[0012] In some other embodiments, the inner walls of the pores of the interface-functionalized covalent organic framework membrane are modified with sulfonic acid groups.

[0013] By precisely modifying the inner wall of the pores of a covalent organic framework membrane with sulfonic acid groups, a nano-confined space with strong electronegativity was successfully constructed. This structure can utilize electrostatic repulsion to contain multivalent metal ions (such as Co²⁺). + Mn² + This creates strong steric hindrance while allowing monovalent lithium ions (Li) to... + This allows for efficient passage, enabling highly selective sieving of ions of similar size in complex systems without significantly sacrificing membrane flux, thus providing an effective material basis for solving key separation problems in lithium resource recovery.

[0014] In some other embodiments, the self-supporting covalent organic framework membrane has a main pore size of 1.70-1.80 nm, a contact angle of 39-71°, and a surface electrostatic potential of -0.5-0.03 V.

[0015] The pore size of the self-supporting covalent organic framework membrane is sufficient to ensure high-flux transport of ions. At the same time, the hydrophilic surface promotes the rapid diffusion of ions, and the surface negative charge generates electrostatic repulsion against multivalent cations. Thus, through the synergy of steric hindrance, hydrophilicity and electrostatic interaction, efficient and highly selective separation of lithium ions is achieved.

[0016] In a second aspect, the present invention provides a method for preparing a self-supporting covalent organic framework membrane as described in the first aspect, comprising the following steps: An aqueous solution is prepared by mixing an aqueous monomer, a catalyst, and an aqueous solvent; an organic solution is prepared by mixing an organic monomer and an organic solvent; the organic solution is added dropwise to the aqueous solution, and the mixture is allowed to stand at room temperature for reaction. After washing and drying, a heterostructured two-dimensional covalent organic framework membrane is obtained.

[0017] This preparation method employs an innovative dual-activation interface polymerization strategy, activating monomers in both the aqueous and organic phases to promote efficient and controllable condensation reactions at the interface. This allows for the one-step in-situ construction of covalent organic framework membranes with pore size gradient distribution and surface functionalization. This not only achieves controllable preparation of membrane structures but also endows the membranes with excellent ion selectivity by introducing charged functional groups (such as sulfonic acid groups), providing a new approach for the preparation of efficient and green membrane materials.

[0018] Specifically, the aqueous solution uses deionized water as the solvent, and the aqueous phase monomers are 2,5-diaminobenzenesulfonic acid (Pa-SO3H) and 4,4'-diamino-[1,1'-biphenyl]-3,3'-disulfonic acid (BD-(SO3H)2), with appropriate amounts of sodium formate or acetonitrile added as catalysts for the imine condensation reaction. The organic phase monomer is 1,3,5-triformylphloroglucinol (Tp), with octanoic acid and / or 1,3,5-trimethylbenzene added as organic solvents.

[0019] In some other embodiments, the molar ratio of the aqueous phase monomer to the organic phase monomer is (1-2):1.

[0020] In some other embodiments, the organic phase monomer is 1,3,5-tricarboxymethylphloroglucinol; The aqueous solvent is deionized water, and the catalyst is one or more of sodium formate and acetonitrile. The organic solvent is one or more of octanoic acid and 1,3,5-trimethylbenzene.

[0021] In some other embodiments, the static reaction is carried out for 40-50 hours.

[0022] The study found that the catalysts added during the dual-activated interfacial polymerization process have a significant impact on the structure of the product. Among them, sodium formate, with its weak base catalytic properties, can neutralize by-products, lower the reaction energy barrier, promote the reversible reaction in the forward direction, and induce the ordered arrangement of monomers. Acetonitrile, through its polar solvent effect, regulates monomer diffusion and the reaction interface, forming a reaction gradient, and ultimately constructs a bifacial heterostructure in the interfacial polymerization, achieving precise control of pore size and function.

[0023] Meanwhile, the molar ratio of aqueous monomers to organic monomers and the standing reaction time directly affect the structural regularity, pore size distribution, crystallinity, thickness and mechanical strength of the prepared covalent organic framework membrane, thus determining its performance in ion sieving applications.

[0024] Specifically, the ratio of aqueous to organic monomers directly affects the stoichiometric equilibrium of the interfacial polymerization reaction. When the molar ratio deviates from the ideal ratio, it can easily lead to insufficient reaction sites or monomer residues, causing structural defects (such as unclosed pores and uneven crosslinking), resulting in a wider membrane pore size distribution and decreased selectivity. Conversely, a suitable molar ratio can promote the construction of an ordered covalent network, improving the crystallinity and structural stability of the membrane. If the static reaction time is too short, the interfacial polymerization reaction will be incomplete, resulting in a thin and loosely structured membrane with insufficient mechanical strength. If the time is too long, it may lead to excessive crosslinking, causing pore size shrinkage, increased mass transfer resistance, and even membrane peeling or cracking.

[0025] Thirdly, the present invention provides the application of the self-supporting covalent organic framework membrane of the first aspect in lithium-ion resource recycling.

[0026] Fourthly, the present invention provides a method for recovering lithium-ion resources, which uses a self-supporting covalent organic framework membrane as described in the first aspect to separate a solution containing lithium ions and multivalent metal ions.

[0027] This self-supporting covalent organic framework membrane can efficiently separate solutions containing lithium ions and multivalent metal ions. Its core mechanism lies in the Schiff base reaction between aldehyde monomers Tp and ionic amine monomers such as Pa-SO3H and BD-(SO3H)2, constructing a COF structure with angstrom-scale pores, highly ordered nanochannels, and a high density of sulfonic acid groups (-SO3H). The sulfonic acid groups form a strongly negatively charged environment within the channels, significantly inhibiting the separation of multivalent metal ions (such as Co) through an electrostatic repulsion effect (Donnan effect). 2+ Mn 2 + Ni 2+ Fe 2+ / 3+ The penetration of lithium ions (Li)+ Due to its low charge density, ionic COF membranes can efficiently transport Li through a synergistic effect of size sieving and weak electrostatic attraction; in contrast, neutral COF membranes (such as TpPa) rely solely on size sieving, resulting in significantly reduced selectivity. This synergistic mechanism of "size sieving-electrostatic repulsion" enables ionic COF membranes to maintain high flux while achieving Li-... + Highly selective separation of multivalent ions.

[0028] In some other embodiments, the multivalent metal ions include one or more of cobalt, manganese, nickel, and iron.

[0029] The beneficial effects of this invention are: By introducing sulfonic acid groups (-SO3H) into the COF framework, the electronegativity of the channels is enhanced, thereby modulating the electrostatic interactions between ions and functional groups. Multivalent metal ions (such as Co...) 2+ Mn 2+ Ni 2+ The affinity of sulfonic acid groups for sulfonic acid groups is significantly higher than that for Li. + Combined with the narrow and uniform structure of the membrane channels, Li + This invention maintains high flux and selectivity even in multi-metal mixed systems. By controlling the types and density of functional groups in the COF framework, the charge environment within the nanochannels is finely tuned. This allows the membrane to preferentially transport lithium ions through the synergistic effect of size sieving and charge regulation, effectively suppressing the migration of multivalent metal ions and improving the separation selectivity of lithium ions in complex systems. Attached Figure Description

[0030] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0031] Figure 1 The Fourier transform infrared (FT-IR) spectra of the TpPa film, TpPa-SO3H film and TpBD-(SO3H)2 film prepared in Comparative Example 1 and Examples 1-2 of the present invention are shown, where a is the TpPa film, b is the TpPa-SO3H film and c is the TpBD-(SO3H)2 film. Figure 2 The C 1s XPS spectra of the TpPa membrane, TpPa-SO3H membrane and TpBD-(SO3H)2 membrane prepared in Comparative Example 1 and Examples 1-2 of this invention are shown, where a is the TpPa membrane, b is the TpPa-SO3H membrane and c is the TpBD-(SO3H)2 membrane. Figure 3The images show the XRD patterns of the TpPa membrane, TpPa-SO3H membrane, and TpBD-(SO3H)2 membrane prepared in Comparative Example 1 and Examples 1-2 of this invention, where a is the TpPa membrane, b is the TpPa-SO3H membrane, and c is the TpBD-(SO3H)2 membrane. Figure 4 The following is an inset diagram showing the nitrogen adsorption-desorption isotherms and pore size distribution of the TpPa membrane, TpPa-SO3H membrane, and TpBD-(SO3H)2 membrane prepared in Comparative Example 1 and Examples 1-2 of this invention, where a is the TpPa membrane, b is the TpPa-SO3H membrane, and c is the TpBD-(SO3H)2 membrane. Figure 5 The images are scanning electron microscope (SEM) images of the TpPa membrane, TpPa-SO3H membrane and TpBD-(SO3H)2 membrane prepared in Comparative Example 1 and Examples 1-2 of the present invention, wherein a is the TpPa membrane, b is the TpPa-SO3H membrane and c is the TpBD-(SO3H)2 membrane; Figure 6 The images show the dynamic contact angle (WCA) of the TpPa membrane, TpPa-SO3H membrane, and TpBD-(SO3H)2 membrane prepared in Comparative Example 1 and Examples 1-2 of this invention, where a is the TpPa membrane, b is the TpPa-SO3H membrane, and c is the TpBD-(SO3H)2 membrane. Figure 7 The above are surface electrostatic potential diagrams of the TpPa film, TpPa-SO3H film and TpBD-(SO3H)2 film prepared in Comparative Example 1 and Examples 1-2 of the present invention, wherein a is the TpPa film, b is the TpPa-SO3H film and c is the TpBD-(SO3H)2 film. Figure 8 The results of zeta potential tests for different COF films prepared in Comparative Example 1 and Examples 1-2 of this invention, and SO3 2- The diagram shows the binding energy between a group or C=O group and an ion. In this diagram, a represents the zeta potential test results for different COF films, and the error bars indicate the standard deviation (n=5). b represents SO3. 2- The binding energy between a group and an ion; c is the binding energy between a C=O group and an ion; Figure 9 To illustrate the permeability and retention performance of TpPa membranes, TpPa-SO3H membranes, and TpBD-(SO3H)2 membranes prepared in Comparative Example 1 and Examples 1-2 of this invention for different ions, error bars represent standard deviations (n=5), where a is the TpPa membrane, b is the TpPa-SO3H membrane, and c is the TpBD-(SO3H)2 membrane; Figure 10This diagram shows the 800-minute permeation curves of the TpBD-(SO3H)2 membranes prepared in Comparative Example 1 and Examples 1-2 of this invention, the permeation rates of the TpPa, TpPa-SO3H, and TpBD-(SO3H)2 membranes in mixed ionic solutions, and the retention performance of various membranes for different ions in the mixed solution. In the diagram, a represents the 800-minute permeation curve of the TpBD-(SO3H)2 membrane; b represents the permeation rates of the TpPa, TpPa-SO3H, and TpBD-(SO3H)2 membranes in the mixed ionic solution. Error bars represent standard deviations (n=5); c represents the retention performance of various membranes for different ions in the mixed solution. Error bars represent standard deviations (n=5). Figure 11 Li₂ of different COF films prepared in Comparative Example 1 and Examples 1-2 of this invention + / M 2+ Selectivity, TpBD-(SO3H)2 membrane after six cycles of Li + Permeation rate and divalent ion rejection rate plot, where a represents the Li permeation rate of different COF membranes. + / M 2+ Selectivity, error bars represent standard deviation (n=5); b is the Li after six cycles of the TpBD-(SO3H)2 film. + Permeation rate, error bars represent standard deviation (n=5); c is the divalent ion rejection rate of the TpBD-(SO3H)2 membrane after six cycles. Detailed Implementation

[0032] Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be construed as limiting the scope of the invention. Specific conditions not specified in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Components whose manufacturers are not specified are all commercially available conventional products.

[0033] As mentioned earlier, this addresses the issue of lithium ions and various coexisting metal ions (such as Co) present in existing lithium resource recycling processes. 2 + Mn 2+ Ni 2+ To address the problems of similar physicochemical properties, low separation selectivity, complex processes, and high energy consumption in lithium-ion resource recovery, this invention provides an interface-functionalized covalent organic framework (COF) membrane and its application in lithium-ion resource recovery. This membrane constructs ordered transport channels with a controllable charge environment by introducing sulfonic acid groups of adjustable density into COF nanochannels, achieving high selectivity for lithium ions and thus efficiently recovering lithium resources in complex multi-ion systems.

[0034] The technical solution adopted in this invention is as follows: the desired COF film is obtained through a dual-activation interface polymerization method. Dual activation refers to the regulation of the monomers or reaction environment participating in the reaction in both the organic and aqueous phases, so that both types of monomers are simultaneously in an activated state conducive to the condensation reaction at the interface. The monomers in the organic phase include 1,3,5-tricarboxymethyl phloroglucinol (Tp), 1,3,5-benzenetriformaldehyde (TFB), 2,5-dimethoxy-terephthalaldehyde (DMDD), 2,5-dihydroxy-terephthalaldehyde (OPA), 2,6-pyridinediformaldehyde (DFP), 2,4,6-tricarboxy-1,3,5-triazine (TFT), pyrazinediformaldehyde, etc. (DFPz); the monomers in the aqueous phase include 4,4'-diamino-[1,1'-biphenyl]-3,3'-disulfonic acid (BD-(SO3H)2), 2,5-diaminobenzenesulfonic acid (Pa-SO3H), etc.

[0035] More specifically, deionized water is used as the solvent in the aqueous phase, and the monomers selected for the aqueous phase are 2,5-diaminobenzenesulfonic acid (Pa-SO3H) and 4,4'-diamino-[1,1'-biphenyl]-3,3'-disulfonic acid (BD-(SO3H)2), with an appropriate amount of sodium formate / acetonitrile added as a catalyst for the imine condensation reaction. The monomer selected for the organic phase is 1,3,5-triformylphloroglucinol (Tp), with n-octanoic acid / 1,3,5-trimethylbenzene added as an organic solvent.

[0036] The above-prepared interface-functionalized covalent organic framework membrane is used for lithium-ion resource recovery.

[0037] The solution of the present invention will be described below with reference to specific embodiments: Example 1 This embodiment provides a method for preparing an interface-functionalized covalent organic framework membrane TpBD-(SO3H)2, as follows: (1) Organic phase: Weigh 0.042 g (0.2 mmol) of 1,3,5-tricarboxymethyl phloroglucinol (Tp) at room temperature, and use a pipette to extract 15 mL of octanoic acid and 5 mL of 1,3,5-trimethylbenzene. Dissolve the weighed 1,3,5-tricarboxymethyl phloroglucinol (Tp) in a mixed solvent composed of octanoic acid and 1,3,5-trimethylbenzene.

[0038] (2) Place the weighed substance into a reagent bottle and sonicate for 30 minutes until there is no obvious precipitate in the reagent bottle.

[0039] (3) Aqueous phase: Weigh 0.103 g of 4,4′-diamino-[1,1′-biphenyl]-3,3′-disulfonic acid (BD-(SO3H)2) at room temperature and place it in a clean beaker, and add an appropriate amount of sodium formate as a catalyst for the imine condensation reaction.

[0040] (4) Place the weighed substance into a reagent bottle and sonicate for 20 minutes until there is no obvious precipitate in the reagent bottle.

[0041] (5) Transfer the lower phase aqueous solution to the reaction vessel and slowly add the Tp organic solution dropwise along the vessel wall to form a clear liquid-liquid interface. Let the reaction stand at room temperature for 48 hours. A self-supporting thin film COF material is gradually generated at the interface and labeled as TpBD-(SO3H)2 film.

[0042] Example 2 This embodiment provides the following steps for preparing an interface-functionalized covalent organic framework membrane, TpPa-SO3H: (1) Organic phase: Weigh 0.042 g (0.2 mmol) of 1,3,5-tricarboxymethyl phloroglucinol (Tp) at room temperature, pipette 20 mL of octanoic acid, and dissolve the weighed 1,3,5-tricarboxymethyl phloroglucinol (Tp) in 20 mL of octanoic acid.

[0043] (2) Place the weighed substance into a reagent bottle and sonicate for 20 minutes until there is no obvious precipitate in the reagent bottle.

[0044] (3) Aqueous phase: Weigh 0.0564 g of 2,5-diaminobenzenesulfonic acid (Pa-SO3H) at room temperature and place it in a clean beaker, and add an appropriate amount of sodium formate as a catalyst for the imine condensation reaction.

[0045] (4) Place the weighed substance into a reagent bottle and sonicate for 15 minutes until there is no obvious precipitate in the reagent bottle.

[0046] (5) Transfer the lower phase aqueous solution to the reaction vessel and slowly add the Tp organic solution dropwise along the vessel wall to form a clear liquid-liquid interface. Let the reaction stand at room temperature for 48 hours. A self-supporting thin film COF material is gradually generated at the interface and labeled as TpPa-SO3H film.

[0047] Comparative Example 1 This comparative example provides a method for preparing an interface-functionalized covalent organic framework membrane, TpPa, as follows: (1) Organic phase: Weigh 0.2 mmol of 1,3,5-tricarboxymethyl phloroglucinol (Tp) at room temperature and dissolve it in a mixed solvent composed of octanoic acid and 1,3,5-trimethylbenzene.

[0048] (2) Place the weighed substance into a reagent bottle and sonicate for 30 minutes until there is no obvious precipitate in the reagent bottle.

[0049] (3) Aqueous phase: Weigh an aqueous solution of p-phenylenediamine (Pa) containing 16 wt% acetonitrile at room temperature, wherein the amount of Pa is 0.3 mmol.

[0050] (4) Place the weighed substance into a reagent bottle and sonicate for 15 minutes until there is no obvious precipitate in the reagent bottle.

[0051] (5) Transfer the lower phase aqueous solution to the reaction vessel and slowly add the Tp organic solution dropwise along the vessel wall to form a clear liquid-liquid interface. Let the reaction stand at room temperature for 48 hours. A self-supporting thin film COF material is gradually generated at the interface and labeled as TpPa film.

[0052] Comparative Example 2 Unlike Example 1, sodium formate is not added in step (3), but the other preparation methods are the same as in Example 1.

[0053] Studies have found that without the addition of sodium formate, the Schiff base reaction rate of the TpBD-(SO3H)2 membrane is significantly reduced, leading to decreased structural regularity, crystallinity, and sulfonic acid group functionalization efficiency. This manifests as disordered nanochannels, wider pore size distribution, and insufficient negative charge density, thereby weakening the synergistic effect of size sieving and electrostatic repulsion, ultimately causing Li... + The reduced selectivity of multivalent ions, coupled with decreased mechanical strength and long-term stability, makes it difficult to meet the requirements for efficient ion separation.

[0054] Comparative Example 3 Unlike Example 1, the reaction time in step (5) is 24 h, while the other preparation methods are the same as in Example 1.

[0055] Studies have found that when the reaction time is too short, the Schiff base reaction of the TpBD-(SO3H)2 membrane is insufficient, leading to inadequate covalent network cross-linking and a loose structure. This manifests as incomplete formation of angstrom-scale pore channels and incomplete functionalization of sulfonic acid groups. This not only weakens the synergistic effect of size sieving and electrostatic repulsion, but also causes Li... + The selectivity of multivalent ions decreases significantly, and structural defects also cause problems such as low mechanical strength and easy swelling and collapse, making it difficult to maintain stable ion separation performance.

[0056] Comparative Example 4 Unlike Example 1, the reaction temperature in step (5) is 60 °C, while the other preparation methods are the same as in Example 1.

[0057] Studies have found that when the reaction temperature increases from room temperature to 60 °C, the Schiff base reaction rate of the TpBD-(SO3H)2 membrane becomes too fast, leading to an imbalance between monomer diffusion and interfacial polymerization. This easily results in the formation of a disordered structure with localized over-crosslinking, which not only causes uneven distribution of angstrom-scale pore channels and a reduction in effective sieving area, but may also cause partial decomposition of sulfonic acid groups due to high temperature, weakening the electrostatic repulsion effect and ultimately causing Li... + The reduced selectivity of multivalent ions and increased membrane brittleness, coupled with the increased solvent evaporation at high temperatures which easily leads to macroscopic cracks, severely affect the mechanical strength and separation stability of the membrane.

[0058] Performance testing (1) FT-IR testing: Before testing, the COF film samples of Comparative Example 1 and Examples 1-2 were thoroughly dried to remove residual solvent and moisture from the surface. The wavelength range was 4000-4000 cm⁻¹ using a Bruker ALPHA II spectrometer. -1 .

[0059] from Figure 1 FT-IR analysis revealed that the characteristic absorption peaks of the precursor monomers in the COF membranes of Comparative Example 1 and Examples 1-2 disappeared, while new skeletal characteristic absorption peaks appeared, indicating that the membranes mainly exist in the form of keto tautomers. In the TpPa-SO3H membrane of Example 2 and the TpBD-(SO3H)2 membrane of Example 1, characteristic absorption peaks of corresponding functional groups were also observed, proving that the introduced functional groups remained intact in the membranes, demonstrating the successful preparation of the COF membranes.

[0060] (2) XPS test: During the test, the COF membranes prepared in Comparative Example 1 and Examples 1-2 were vacuum dried and then tested with XPS.

[0061] from Figure 2 The results show that characteristic signals corresponding to different carbon bonding environments can be distinguished in the spectra, proving that a stable β-ketoenamine framework structure has been formed in the films of Comparative Example 1 and Examples 1-2. This result further confirms the successful construction of the COF film framework structure at the chemical state level and is consistent with the infrared spectroscopy results.

[0062] (3) XRD test: obtained by Rigaku SmartLab 9 kW X-ray diffractometer (Cu Kα radiation), with a scanning range of 2θ = 2-40° and a scanning speed of 20°·min. -1 .

[0063] from Figure 3It can be seen that the COF films of Comparative Example 1 and Examples 1-2 all exhibit sharp and relatively obvious diffraction peaks at 2θ≈4.4°. These characteristic diffraction peaks correspond to the (100) crystal plane in the COF crystal structure, indicating that the three synthesized COF film materials all have good crystallinity, providing a regular pore structure basis for ion transport. In addition, the TpPa film of Comparative Example 1 also exhibits clear diffraction peaks at 8.3°, 12.7°, and 27.0°, corresponding to the (110), (210), and (001) crystal planes, respectively. To further clarify the crystal stacking mode of the COF materials, crystal models were constructed and structural simulations were performed using Materials Studio software. The results show that the experimental PXRD patterns of the three materials and the simulated patterns of the AA-type stacking mode show good agreement, and the weighted residual factor (Rwp) is less than 10%, indicating that the three COF films are mainly stacked in the AA-type mode.

[0064] (4) Nitrogen adsorption-desorption isotherm test: The isotherm was tested at 77 K using a multi-station extended specific surface area and porosity analyzer (ASAP 2460-4MP). The corresponding pore size distribution was calculated using NLDFT theory, and the existence of large-scale pores was confirmed using BJH analysis.

[0065] from Figure 4 As can be seen, in the lower relative pressure range P / P0, the COF membranes of Comparative Example 1 and Examples 1-2 exhibit typical porous material adsorption characteristics, confirming that the materials possess a regular porous structure and excellent gas adsorption performance. Specifically, the main pore size peaks of the TpPa membrane in Comparative Example 1, the TpPa-SO3H membrane in Example 2, and the TpBD-(SO3H)2 membrane in Example 1 are concentrated at 1.80 nm, 1.73 nm, and 1.71 nm, respectively, showing a gradually decreasing trend in pore size. This trend can be attributed to the introduction of sulfonic acid groups (─SO3H). ─SO3H has a certain spatial volume; after being grafted onto the COF framework, it occupies part of the internal space of the pores, leading to a reduction in the effective pore size, thus providing a more favorable size repulsion environment for ion sieving.

[0066] (5) Scanning electron microscopy (SEM) testing: The covalent organic framework (COF) films prepared in Comparative Example 1 and Examples 1-2 were removed from the interface by a pull-out method. The films were then cleaned with ethanol and deionized water, respectively, to remove residual impurities on the surface, and then dried. Before SEM testing, the COF film samples were treated with platinum spraying to improve conductivity. The spraying voltage was 3 kV and the spraying time was 60 s. Subsequently, using a tungsten filament as the electron source, the film material was characterized by scanning electron microscopy at an accelerating voltage of 10 kV, and surface morphology images at different magnifications were obtained.

[0067] from Figure 5 It can be seen that the COF membranes of Comparative Example 1 and Examples 1-2 all exhibit typical bifacial heterogeneous structure characteristics. The organic phase side exhibits a smooth, dense membrane surface without obvious defects. This morphological feature can effectively reduce the interfacial resistance during ion transport on the membrane surface and ensure the continuity of the ion conduction path. In contrast, the aqueous phase side exhibits a relatively loose porous structure, which can provide sufficient channels for the rapid diffusion of ions inside the membrane.

[0068] (6) WCA test: The test was conducted using a JC2000C contact angle measuring instrument (POWEREACH®, China).

[0069] from Figure 6 As can be seen from the results, the contact angle of the TpPa membrane in Comparative Example 1 is 71°, and water droplets can spontaneously permeate and be completely absorbed by the membrane within approximately 1.48 seconds, indicating that the membrane has a certain degree of hydrophilicity, but the water absorption rate is relatively slow. In contrast, the contact angle of the functionalized TpPa-SO3H membrane in Example 2 is reduced to 59°, and water droplets can be absorbed within approximately 1.08 seconds, showing stronger hydrophilicity and a faster water permeation rate. This trend is even more pronounced in the TpBD-(SO3H)2 membrane of Example 1, where the contact angle further decreases to 39°, and water droplets can be completely absorbed by the membrane within approximately 0.12 seconds, exhibiting the best hydrophilicity and the fastest water permeation rate. The above results indicate that with the introduction of sulfonic acid groups, the hydrophilicity and water permeation rate of the membrane are significantly improved.

[0070] (7) The surface electrostatic potential of different COF films in Comparative Example 1 and Examples 1-2 was obtained by simulation calculation.

[0071] from Figure 7 It can be seen that, compared with the TpPa membrane of Comparative Example 1, the TpPa-SO3H membrane of Example 2 and the TpBD-(SO3H)2 membrane of Example 1, which are functionalized with sulfonic acid groups, exhibit significantly enhanced negative potential distribution regions within the nanopores. This result indicates that the introduction of sulfonic acid groups not only significantly improves the overall electronegativity level of the COF framework but also constructs more localized negative potential energy wells within the pores, thereby enhancing the electrostatic attraction and selective trapping ability for high-valence metal ions, thus providing an important charge basis for achieving ion-selective transport.

[0072] (8) Zeta potential testing method: The Zeta potentials of the films in Comparative Example 1 and Examples 1-2 were measured using a solid surface Zeta potential analyzer (SurPASS Anton Paar). The SO3 content in Comparative Example 1 and Examples 1-2 was calculated through simulation. 2- The binding energy between functional groups, C=O groups and ions.

[0073] from Figure 8 The results show that the theoretical predictions are consistent with the zeta potential test results, indicating that the introduction of sulfonic acid groups can significantly modulate the surface charge properties of the COF film. In Comparative Example 1, the surface potential of the unfunctionalized COF film is close to electroneutrality, while after the introduction of sulfonic acid groups, the film surfaces of Examples 1-2 exhibit obvious negative charge characteristics, indicating the formation of a stable negatively charged electric field environment within the nanopores. This fixed negative charge enhances the electrostatic interaction between the inner wall of the pores and the metal cations. Further theoretical analysis shows that the sulfonic acid groups and the polar functional groups in the COF framework have stronger interactions with divalent metal ions, while the interaction with lithium ions is relatively weaker. This differentiated interaction makes divalent metal ions more easily confined by the energy barrier within the pores, while lithium ions can achieve relatively rapid migration within the pores.

[0074] (9) Ion permeation test: In the H-type electrolytic cell, a concentration of 0.01 mol·L⁻¹ was added to the feed side. -1 The solutions were prepared using metal sulfate solutions (Li₂SO₄, CoSO₄, NiSO₄, and MnSO₄), while an equal volume of deionized water was added to the permeate side. The solutions on both sides were separated by the target COF membranes of Comparative Example 1 and Examples 1-2. It should be noted that the selected salts have the same anion (SO₄²⁻). 2- This effectively eliminates the influence of anion type on the overall transport process. Therefore, the permeation differences of different salts in the membrane are mainly determined by the intrinsic properties of cations.

[0075] from Figure 9 It can be seen that the permeation rate of metal ions through the three COF membranes all follow the Li... + Ni 2+ >Co 2+ >Mn 2+ The order, compared to Li in Example 1 + Ni 2+ Co 2+ Mn 2+ The rejection rates were only 25.2%, 45.0%, 45.5%, and 46.0%, respectively, with little difference in ion rejection, making it impossible to effectively screen monovalent and divalent cations. In contrast, the TpPa-SO3H membrane of Example 2 and the TpBD-(SO3H)2 membrane of Example 1, which introduced sulfonic acid groups, showed significant advantages in the rejection of divalent metal ions: for Mn 2+ The retention rates of Ni increased to 84.8% and 93.7% respectively, far exceeding the 46.0% of the comparative example 1 membrane; 2+ Co 2+ The retention performance also showed a similar increasing trend.

[0076] This significant improvement can be attributed to the following three points: First, the hydrophilic sulfonic acid groups in the TpPa-SO3H and TpBD-(SO3H)2 membranes can form strong hydrogen bonds with water molecules, generating a stable hydrated shell, thereby effectively reducing the effective channel size of the pores, enhancing the steric sieving effect, and restricting the passage of large hydrated ions; Second, the negatively charged surfaces of the TpPa-SO3H and TpBD-(SO3H)2 membranes repel anions through electrostatic repulsion. To maintain the electroneutrality of the solution, an equal number of cations are also simultaneously repelled, thereby improving the salt retention efficiency; Third, -SO3 - The binding affinity between functional groups and divalent metal ions is much higher than that of Li. + This induces frequent adsorption-desorption processes within the pores, significantly slowing down the migration rate of divalent ions; conversely, Li... + It has a weak interaction with sulfonic acid groups, allowing it to pass through membrane pores relatively easily and achieve rapid migration.

[0077] (10) The permeation rate and rejection rate of the mixed ions in Comparative Example 1 and Examples 1-2 were calculated. The concentration of each cation on the permeate side was determined using inductively coupled plasma optical emission spectrometry (ICP-OES). The ion permeation rate of the membrane ( Ji Calculate using the following formula:

[0078] in, C (μg·mL -1 () represents the cation concentration on the osmotic side. V (mL) represents the effective volume of the osmotic solution (V=17 mL in this work). Mr (g·mol -1 () represents the relative molecular mass of the cation. A (cm 2 () represents the effective penetration area. Δt (h) represents the infiltration time.

[0079] Membrane salt rejection rate ( R Calculate using the following formula:

[0080] in, Cp and Cf These represent the ion concentrations on the permeation side and the feed side, respectively.

[0081] from Figure 10It can be seen that the time-dependent permeation curve of monovalent cations in Example 1 maintains a good linear relationship during long-term permeation testing, indicating that lithium ions have stable and continuous transport behavior within the membrane channels. This also demonstrates that the membrane possesses good structural integrity and mechanical stability under long-term permeation conditions. In mixed systems closer to practical applications, the membrane's retention performance for divalent cations typically decreases significantly due to competitive adsorption of coexisting ions and charge shielding effects. However, in this study, the sulfonic acid-functionalized COF membrane in Example 1 maintained stable retention performance for divalent metal ions with only slight changes, and its overall selectivity remained stable. These results indicate that increasing the negative charge density within the nanopores can effectively suppress competitive interference between coexisting ions and enhance the membrane's selective control ability for ions of different valence states. Under these conditions, the membrane can not only stably suppress the migration of divalent metal ions but also further promote the preferential transport of lithium ions, thereby achieving efficient and stable recovery of lithium ions in mixed ion systems.

[0082] (11) Li + / M 2+ Selectivity, Li + Permeation rate and divalent ion rejection test: The permeation selectivity between the two ions is calculated by the following formula:

[0083] in, C M 2+ and C Li + M, a divalent metal ion in the feed side 2+ and Li + concentration, J M 2+ and J Li + M respectively 2+ and Li + The rate of penetration.

[0084] from Figure 11 It can be seen that: Example 1 affects Li + / M 2+ The selectivity of Example 1 was optimal, reaching 29.5, approximately three times that of Comparative Example 1. With increasing cycle number, the selectivity of the film in Example 1 for Li... + The permeation rate of ions only decreased slightly, remaining at a high level overall, while the permeation rate of divalent ions showed a gradual upward trend. Nevertheless, the overall retention performance of the membrane in Example 1 for divalent ions remained stable. For example, Mn 2+The rejection rate only decreased slightly from 93.7% at the beginning to 89.8%, and remained at a high level throughout the cycle test.

[0085] In summary, the densely distributed sulfonic acid groups on the inner wall of the pores of the COF membrane prepared by this invention provide the membrane material with a stable and strong electrostatic repulsion effect and specific ion-functional group interactions, effectively inhibiting the migration of divalent ions. This synergistic effect of structural advantages and chemical functions allows the membrane to maintain excellent retention performance for divalent metal ions without sacrificing flux, demonstrating its promising application prospects for lithium recovery in mixed ion systems.

[0086] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A self-supporting covalent organic framework membrane, characterized in that, The covalent organic framework membrane is self-assembled by the polymerization of aqueous monomers and organic monomers through a dual-activated interface, and is a self-supporting covalent organic framework membrane with an asymmetric structure of interface orientation. The aqueous monomer includes one or more of 4,4'-diamino-[1,1'-biphenyl]-3,3'-disulfonic acid and 2,5-diaminobenzenesulfonic acid; The organic phase monomers include one or more of 1,3,5-tricarboxyloyl-phloroglucinol, 1,3,5-benzenetriformaldehyde, 2,5-dimethoxy-terephthalaldehyde, 2,5-dihydroxy-terephthalaldehyde, 2,6-pyridinediformaldehyde, 2,4,6-tricarboxy-1,3,5-triazine, and pyrazindiformaldehyde.

2. The self-supporting covalent organic framework membrane according to claim 1, characterized in that, The pore walls of the interface-functionalized covalent organic framework membrane are modified with sulfonic acid groups.

3. The self-supporting covalent organic framework membrane according to claim 1, characterized in that, The interface-functionalized covalent organic framework membrane has a main pore size of 1.70-1.80 nm, a contact angle of 39-71°, and a surface electrostatic potential of -0.5 to -0.03 V.

4. A method for preparing a self-supporting covalent organic framework membrane according to any one of claims 1-3, characterized in that, An aqueous solution is prepared by mixing an aqueous monomer, a catalyst, and an aqueous solvent; an organic solution is prepared by mixing an organic monomer and an organic solvent; the organic solution is added dropwise to the aqueous solution, and the mixture is allowed to stand at room temperature for reaction. After washing and drying, a heterostructured two-dimensional covalent organic framework membrane is obtained.

5. The method for preparing a self-supporting covalent organic framework membrane according to claim 4, characterized in that, The molar ratio of the aqueous phase monomer to the organic phase monomer is (1-2):

1.

6. The method for preparing a self-supporting covalent organic framework membrane according to claim 4, characterized in that, The organic phase monomer is 1,3,5-tricarboxymethylphloroglucinol; The aqueous solvent is deionized water, and the catalyst is one or more of sodium formate and acetonitrile. The organic solvent is one or more of octanoic acid and 1,3,5-trimethylbenzene.

7. The method for preparing a self-supporting covalent organic framework membrane according to claim 4, characterized in that, The static reaction is allowed to proceed for 40-50 hours.

8. The application of a self-supporting covalent organic framework membrane according to any one of claims 1-3 in lithium-ion resource recovery.

9. A method for lithium-ion resource recovery, characterized in that, The self-supporting covalent organic framework membrane according to any one of claims 1-3 is used to separate solutions containing lithium ions and multivalent metal ions.

10. The method for lithium-ion resource recovery according to claim 9, characterized in that, The multivalent metal ions include one or more of cobalt, manganese, nickel, and iron.

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