Method for preparing two-dimensional covalent organic framework composite membrane for lithium-magnesium ion separation

Abstract

The application discloses a preparation method of a two-dimensional covalent organic framework composite film for lithium-magnesium ion separation, and comprises a nucleation solution and a crystal nucleus growth solution; a base film is sequentially soaked in the nucleation solution and the crystal nucleus growth solution, and finally a complete COFs composite film is obtained; wherein the concentration of the crystal nucleus growth solution is less than that of the nucleation solution; the preparation methods of the nucleation solution and the crystal nucleus growth solution are as follows: 1,3,5-tri(4-aminophenyl)benzene is dissolved in a mixed solvent of ethyl acetate and mesitylene, and then 2,5-dimethoxybenzene-1,4-diformaldehyde or 2,5-bis(triethylene glycol dimethyl ether)-1,4-p-xylylene glycol is added. According to the characteristics of the nucleation-growth process of the crystal material, the nucleation-growth process of the covalent organic framework layer on the surface of the porous base is finely controlled by adjusting the monomer concentration in stages.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of separation membranes, and particularly relates to a preparation method of a two-dimensional covalent organic framework composite membrane for lithium-magnesium ion separation. BACKGROUND

[0002] Two-dimensional covalent organic frameworks (2DCOFs) are crystalline porous materials with regular channels formed by polymerization of monomers. The adjustable nanopores and stable chemical links of the materials endow the materials with application capabilities in various fields, such as gas separation, ion sieving, sensing and energy storage.

[0003] The COFs synthesis process requires a high temperature and a sealed environment, and the reactants are all put into the reactor at one time before the reaction, and then the COFs are finally formed through a long reaction-crystallization process. At present, the method for preparing the COFs material is mostly one-time feeding reaction molding. This method is extremely easy to cause the rapid precipitation of amorphous (amorphous) polymers in the initial stage of the reaction. Although the chemical connection in the COFs structure has a certain error correction ability, the process of “self-correction-structural rearrangement-crystallization” is slow, and it needs sufficient time (more than 2 days) to complete the crystallization process. In addition, the mechanical strength of the COFs thin film is low, and the self-supporting ability is poor, and the COFs thin film must rely on the carrier capable of enhancing the mechanical strength to exist. The growth of the crystal material on the surface of the carrier also has a higher requirement: firstly, the concentration condition of the crystal nucleation on the surface is met to ensure that the crystal nucleus can be formed on the surface of the carrier; and then the process of the crystal nucleus growth and connection into a complete thin film. According to the classical crystal nucleation-growth theory (Wang, FD.;Richards, VN.;Shields, SP.;Buhro, WE. Kinetics and Mechanics of Aggregative Nanocrystal Growth[J]. Chem. Mater., 2014, 26(1): 5-21), the above two processes have different requirements for the monomer concentration: the critical nucleation concentration is higher than the growth concentration. If the initial concentration is lower than the critical nucleation concentration, nucleation cannot be achieved. On the contrary, if the monomer concentration is too high, excessive nucleation will occur in the solution, which affects the nucleation-growth process on the surface. Figure 1 Figure 2 ​, Ring, TA. Fundamental s of Ceramic Powder Process ing and Synthesi s. San Di ego:Academic Press, 1996). Therefore, how to control the concentration of monomers during the reaction to ensure the nucleation and growth of COFs on the surface of the carrier is a difficult problem. In summary, the film forming conditions of COFs are harsh, the process is long and the control is complex, which seriously limits the preparation and application of COF thin films. SUMMARY

[0004] The present application aims at the complex preparation process and poor separation performance of COF thin films in the prior art. According to the characteristics of the nucleation and growth process of crystal materials, the present application controls the nucleation and growth process of the covalent organic framework layer on the surface of the porous substrate by adjusting the monomer concentration in stages.

[0005] To solve the above technical problems, one technical scheme adopted by the present application is as follows: the present application provides a preparation method of a two-dimensional covalent organic framework composite film for lithium-magnesium ion separation, comprising the following steps:

[0006] The nucleation solution and the crystal nucleus growth solution are configured respectively: the base film is immersed in the nucleation solution and the crystal nucleus growth solution in turn, and finally a complete COF composite film is obtained;

[0007] The concentration of the crystal nucleus growth solution is less than that of the nucleation solution;

[0008] The preparation methods of the nucleation solution and the crystal nucleus growth solution are as follows: 1,3,5-tris(4-aminophenyl)benzene is dissolved in a mixed solvent of ethyl acetate and mesitylene, and then 2,5-dimethoxybenzene-1,4-diformaldehyde or 2,5-bis(triethylene glycol dimethyl ether)-1,4-toluene dimethyl formaldehyde is added.

[0009] Further, the concentration of 1,3,5-tris(4-aminophenyl)benzene in the nucleation solution is 0.68-2.73mM, and the concentration of 2,5-dimethoxybenzene-1,4-diformaldehyde or 2,5-bis(triethylene glycol dimethyl ether)-1,4-toluene dimethyl formaldehyde is 1.0-4.0mM;

[0010] The concentration of 1,3,5-tris(4-aminophenyl)benzene in the crystal nucleus growth solution is 0.11-0.34mM, and the concentration of 2,5-dimethoxybenzene-1,4-diformaldehyde or 2,5-bis(triethylene glycol dimethyl ether)-1,4-toluene dimethyl formaldehyde is 0.17-0.50mM.

[0011] Further, the nucleation solution is configured as follows: 1.2-4.8 mg of 1,3,5-tris(4-aminophenyl)benzene is dissolved in 5.0 mL of a mixed solvent of ethyl acetate and mesitylene, and ultrasonic treatment is performed until the monomer is completely dissolved; 1.0-4.0 mg of 2,5-dimethoxybenzene-1,4-diformaldehyde or 2.3-9.2 mg of 2,5-bis(triethylene glycol dimethyl ether)-1,4-benzene dicarboxaldehyde is added to the above solution, and shaking is performed to obtain a solution in which the covalent organic framework is nucleated on the surface of the substrate.

[0012] Further, the crystal nucleus growth solution is configured as follows: 0.2-0.6 mg of 1,3,5-tris(4-aminophenyl)benzene is dissolved in 5.0 mL of a mixed solvent of ethyl acetate and mesitylene, and ultrasonic treatment is performed until the monomer is completely dissolved; 0.16-0.5 mg of 2,5-dimethoxybenzene-1,4-diformaldehyde or 0.38-1.16 mg of 2,5-bis(triethylene glycol dimethyl ether)-1,4-benzene dicarboxaldehyde is added to the above solution, and shaking is performed to obtain a solution for growing a covalent organic framework crystal nucleus on the surface of the substrate.

[0013] Further, the volume ratio of the mixed solvent of ethyl acetate and mesitylene is 3:1.

[0014] Further, the deposition and growth of the COFs layer on the surface of the porous substrate is as follows: the substrate film is placed in a glass bottle, the prepared nucleation solution is added to the glass bottle, the substrate film is slightly shaken to fully contact the reaction solution, and then acetic acid is added as a catalyst, the bottle cap is tightly screwed after thorough mixing, and the reaction is allowed to stand at room temperature, at which time a layer of COFs crystal nucleus is grown on the surface of the substrate film; the prepared crystal nucleus growth solution is slowly and uniformly injected into the above glass bottle using a syringe pump, and a complete COFs composite film is finally obtained.

[0015] The present application has the following advantages:

[0016] In the preparation method of the present application, a reaction solution with a relatively high monomer concentration is first used to initiate the nucleation of COFs on the surface of the substrate, and then a reaction solution with a relatively low monomer concentration is used to promote the growth of the COFs crystal nucleus on the surface of the substrate into a film, and to inhibit the excessive nucleation of COFs. According to the surface nucleation mechanism, the crystal nucleus is preferentially formed on the surface of the carrier; then, the use of a low-concentration monomer solution in the crystal nucleus growth stage can avoid excessive nucleation on the surface of the substrate or in the solution, thereby ensuring the continuous growth of the crystal nucleus on the surface of the substrate into a film.

[0017] The preparation method provided by the application is simple, the film forming time is short, and the obtained 2D COFs film has a significant layered morphology. The layered morphology shows that the 2D COFs film has good crystallinity, that is, has unobstructed and uniform size channels, which can ensure the performance of the composite film for ion separation. The composite film has good screening performance in the separation process of lithium ions and magnesium ions, and the preparation method provides a reference for designing high-crystallinity and high-performance covalent organic framework composite films.

[0018] The above description is only a summary of the technical solutions of the application. In order to more clearly understand the technical means of the application and to implement the content of the description, the following will be described in detail with reference to the preferred embodiments of the application and in conjunction with the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a classical crystal nucleation-growth theory diagram;

[0020] Figure 2 is a schematic diagram of the nucleation-growth process;

[0021] Figure 3 is a COFs composite film morphology diagram (taken by a scanning electron microscope (SEM));

[0022] Figure 4 is a schematic diagram of a diffusion dialysis test device;

[0023] Figure 5 is a molecular structural formula of 1,3,5-tris(4-aminophenyl)benzene;

[0024] Figure 6 is a molecular structural formula of 2,5-dimethoxybenzene-1,4-diformaldehyde;

[0025] Figure 7 is a molecular structural formula of 2,5-bistriglycol dimethyl ether-1,4-p-xylylene formaldehyde. DETAILED DESCRIPTION

[0026] The specific embodiments of the application are described below through specific specific embodiments, and those skilled in the art can easily understand the advantages and effects of the application from the content disclosed in the specification. The application can also be implemented in other different ways, that is, different modifications and changes can be made without departing from the scope disclosed by the application.

[0027] Example 1: A preparation method of a two-dimensional covalent organic framework composite film for lithium-magnesium ion separation:

[0028] 1) Configure a nucleation solution: weigh 1.2 mg of 1,3,5-tris(4-aminophenyl)benzene (molecular structural formula as shown in Figure 5The monomer was dissolved in 5.0 mL of ethyl acetate / mesitylene (3:1, v / v) mixed solvent, and ultrasonicated until the monomer was completely dissolved; 1.0 mg of 2,5-dimethoxybenzene-1,4-diformaldehyde was added to the above solution, and after rapid shaking, ultrasonication was performed for 10 seconds to obtain a solution in which the covalent organic framework was nucleated on the surface of the substrate.

[0029] 2) Preparation of crystal nucleus growth solution: 0.6 mg of 1,3,5-tris(4-aminophenyl)benzene was weighed and dissolved in 5.0 mL of ethyl acetate / mesitylene (3:1, v / v) mixed solvent, and ultrasonicated until the monomer was completely dissolved; 0.5 mg of 2,5-dimethoxybenzene-1,4-diformaldehyde (molecular structure as shown) was added to the above solution, and after rapid shaking, ultrasonication was performed for 10 seconds, and the obtained solution was used for the growth of the covalent organic framework crystal nucleus on the surface of the substrate. Figure 6

[0030] 3) Deposition and growth of COFs layer on the surface of porous substrate: a substrate film (5 mm x 5 mm) was placed in a glass bottle with a volume of 4.0 mL, 1.0 mL of the nucleation solution prepared in step (1) was added to the glass bottle, the substrate film was fully contacted with the reaction solution by slightly shaking, then 0.02 mL of acetic acid was added as a catalyst, the bottle cap was tightly closed after thorough mixing, and the reaction was carried out at room temperature for 1.5 hours, at which time a layer of COFs crystal nucleus was grown on the surface of the substrate film. A syringe pump was used to slowly inject 3 mL of the fresh solution prepared in step (2) into the above glass bottle at a constant speed, and the injection was completed in a total of 6 hours, finally obtaining a complete COFs composite membrane.

[0031] 4) Test of ion diffusion dialysis performance of the composite membrane:

[0032] The device for evaluating the ion diffusion dialysis performance of the COFs composite membrane is as shown in Figure 4 The COFs composite membrane was clamped between two glass diffusion cells using a clamp, 7.0 mL of salt solution was added to the membrane front side of the diffusion cell as the feed liquid, and 7.0 mL of ultrapure water was added to the other side, and magnetic bars were placed in the two solutions for stirring. Due to the difference in salt concentration between the two sides of the membrane, salt ions will diffuse from the feed side with high concentration to the permeate side with low concentration, and the concentration of salt ions in the permeate after 1 hour of diffusion is detected by an ion chromatograph, and the transmembrane diffusion performance of various ions can be calculated. The feed liquid is a binary salt solution of LiCl and MgCl2 (both with a concentration of 0.1 M, M is molar concentration indicating Mol / L).

[0033] In the diffusion dialysis experiment, the ion concentrations of the feed liquid and the permeate were determined by an LC-20A ion chromatograph (Shimadzu, Japan). 0.1 mL of the permeate was taken from the permeate side of the diffusion cell, diluted 10 times with ultrapure water, and then tested. Chromatographic column: Shodex IC YS-50 (4.6 mm I.D. x 125 mm). ​

[0034] In the diffusion dialysis test of ions, the ion concentration in the permeate can be calculated by measuring the peak area of the cation in the permeate and according to the linear relationship equation described above. Determination of the standard curve: a series of concentration gradient LiCl or MgCl2 standard solutions were prepared, and the peak area of the cation at each concentration was measured by ion chromatography to prepare a standard curve of concentration-peak area, and a linear relationship equation was fitted.

[0035] Example 2

[0036] The same as Example 1, except that the concentrations of 1,3,5-tris(4-aminophenyl)benzene and 2,5-dimethoxybenzene-1,4-diformaldehyde in step (2) were both reduced to 1 / 2.

[0037] Example 3

[0038] The same as Example 1, except that the concentrations of 1,3,5-tris(4-aminophenyl)benzene and 2,5-dimethoxybenzene-1,4-diformaldehyde in step (2) were both reduced to 1 / 3.

[0039] Example 4

[0040] 1) Preparation of nucleation solution: 1.2 mg of 1,3,5-tris(4-aminophenyl)benzene was dissolved in 5.0 mL of a mixed solvent of ethyl acetate and mesitylene (3:1, v / v) by ultrasonic treatment until the monomer was completely dissolved; 2.3 mg of 2,5-bis-triethylene glycol dimethyl ether-1,4-p-xylylene formaldehyde was added to the above solution, shaken quickly and ultrasonic treated for 10 seconds to obtain a solution for nucleation of covalent organic frameworks on the surface of the substrate.

[0041] 2) Preparation of crystal nucleus growth solution: 0.6 mg of 1,3,5-tris(4-aminophenyl)benzene was dissolved in 5.0 mL of a mixed solvent of ethyl acetate and mesitylene (3:1, v / v) by ultrasonic treatment until the monomer was completely dissolved; 1.16 mg of 2,5-bis-triethylene glycol dimethyl ether-1,4-p-xylylene formaldehyde (molecular structure as shown in Figure 7 ) was added to the above solution, shaken quickly and ultrasonic treated for 10 seconds to obtain a solution for crystal nucleus growth of covalent organic frameworks on the surface of the substrate.

[0042] 3) Deposition growth of COFs layer on porous substrate surface: Put the substrate film (5 mm x 5 mm) in a glass bottle with a volume of 4.0 mL, take 1.0 mL of the nucleation solution prepared in step (1) and add it to the glass bottle, slightly shake to make the substrate film fully contact the reaction solution, then add 0.02 mL of acetic acid as catalyst, mix well, then tighten the bottle cap, and react at room temperature for 1.5 hours, at this time a layer of COFs crystal nucleus is grown on the surface of the substrate film. Use a syringe pump to slowly inject 3 mL of fresh solution prepared in step (2) into the above glass bottle at a constant speed, a total of 6 h is completed, and finally a complete COFs composite film is obtained.

[0043] Comparative Example 1

[0044] (1) Preparation of nucleation solution: 1.2 mg of 1,3,5-tris (4-aminophenyl) benzene was dissolved in 5.0 mL of ethyl acetate / mesitylene (3:1, v / v) mixed solvent, and ultrasonic was used until the monomer was completely dissolved; 1.0 mg of 2,5-dimethoxybenzene-1,4-diformaldehyde was added to the above solution, shaken quickly, and then ultrasonic was used for 10 seconds to obtain a solution for nucleation of COFs on the surface of the substrate.

[0045] (2) Deposition growth of COFs layer on porous substrate surface: Put the substrate film (5 mm x 5 mm) in a glass bottle with a volume of 4.0 mL, take 1.0 mL of the nucleation solution prepared in step (1) and add it to the glass bottle, slightly shake to make the substrate film fully contact the reaction solution, then add 0.02 mL of acetic acid as catalyst, mix well, then tighten the bottle cap, and react at room temperature for 1.5 hours, at this time a layer of COFs crystal nucleus is grown on the surface of the substrate film. Use a syringe pump to slowly inject 3 mL of fresh solution prepared in step (1) into the above glass bottle at a constant speed, a total of 6 h is completed, and finally a COFs composite film is obtained.

[0046] Comparative Example 2

[0047] (1) Preparation of solution: 1.2 mg of 1,3,5-tris (4-aminophenyl) benzene was dissolved in 5.0 mL of ethyl acetate / mesitylene (3:1, v / v) mixed solvent, and ultrasonic was used until the monomer was completely dissolved; 1.0 mg of 2,5-dimethoxybenzene-1,4-diformaldehyde was added to the above solution, shaken quickly, and then ultrasonic was used for 10 seconds to obtain a solution for nucleation of COFs on the surface of the substrate.

[0048] (2) Deposition growth of COFs layer on porous substrate surface: Put the substrate film (5 mm x 5 mm) in a glass bottle with a volume of 4.0 mL, take 4.0 mL of the nucleation solution prepared in step (1) and add it to the glass bottle, slightly shake to make the substrate film fully contact the reaction solution, then add 0.02 mL of acetic acid as catalyst, mix well, then tighten the bottle cap, and react at room temperature for 7.5 hours, and finally a COFs composite film is obtained.

[0049] Performance data table of examples and comparative examples

[0050]

[0051]

[0052] Note: mM is 0.001 mol / L, read as millimoles per liter.

[0053] As Figure 3 shown, the surface morphology of the composite membrane and the lithium-magnesium ion separation performance can be seen, the COFs layer of the composite membrane obtained by comparative example 1 is relatively thin and a large number of COFs particles are deposited on the surface, during the performance test, the transmembrane rate of lithium-magnesium ions is low and there is no rate difference, indicating that the composite membrane has no separation selectivity; the surface of comparative example 2 does not form a complete COFs membrane and a large number of COFs particles are deposited on the surface, during the performance test, lithium-magnesium ions quickly and without difference pass through the defects of COFs; the surfaces of examples 1-3 have no COFs particle deposition and the COFs layer has a clear layered structure, the three membranes have good selective permeability to lithium-magnesium ions. The above shows that controlling (reducing) monomers (monomers refer to 1,3,5-tris(4-aminophenyl)benzene, 2,5-dimethoxybenzene-1,4-diformaldehyde or 2,5-bis-triethylene glycol dimethyl ether-1,4-p-xylylene formaldehyde, which is called monomer for convenience of expression) in the crystal nucleus growth stage is conducive to the growth process of the COFs layer and can effectively inhibit further nucleation. In addition, as can be seen from examples 1-3, the higher the monomer concentration in the solution, the greater the thickness of the obtained COFs thin film, indicating that when the monomer concentration is lower than the critical nucleation concentration in the crystal nucleus growth stage, increasing the monomer concentration can obtain a thick film. Example 4 uses COFs containing long ether side chains to prepare a separation membrane with higher lithium-magnesium ion selectivity, indicating that adjusting the chemical environment inside the pore can change the transmission rate of various ions.

[0054] The above only describes the embodiments of the present application, and does not limit the patent scope of the present application, and any equivalent structure made by using the content of the specification and drawings of the present application, or directly or indirectly used in other related technical fields, is also included in the patent protection scope of the present application.

[0055] The above only describes the embodiments of the present application, and does not limit the patent scope of the present application, and any equivalent structure made by using the content of the specification and drawings of the present application, or directly or indirectly used in other related technical fields, is also included in the patent protection scope of the present application.

Claims

1. A method for the preparation of a two-dimensional covalent organic framework composite membrane for lithium-magnesium ion separation, characterized by: The method comprises the following steps: The base film is sequentially soaked in the nucleation solution and the crystal nucleus growth solution to obtain a complete COFs composite film; The concentration of the crystal nucleus growth solution is less than that of the nucleation solution; The preparation method of the nucleation solution and the crystal nucleus growth solution is as follows: 1,3,5-tris(4-aminophenyl)benzene is dissolved in a mixed solvent of ethyl acetate and mesitylene, and then 2,5-dimethoxybenzene-1,4-diformaldehyde or 2,5-bis-triethylene glycol dimethyl ether-1,4-p-xylylene formaldehyde is added; the concentration of 1,3,5-tris(4-aminophenyl)benzene in the nucleation solution is 0.68-2.73 mM, and the concentration of 2,5-dimethoxybenzene-1,4-diformaldehyde or 2,5-bis-triethylene glycol dimethyl ether-1,4-p-xylylene formaldehyde is 1.0-4.0 mM; The concentration of 1,3,5-tris(4-aminophenyl)benzene in the crystal nucleus growth solution is 0.11-0.34 mM, and the concentration of 2,5-dimethoxybenzene-1,4-diformaldehyde or 2,5-bis-triethylene glycol dimethyl ether-1,4-p-xylylene formaldehyde is 0.17-0.50 mM; The volume ratio of ethyl acetate to mesitylene in the mixed solvent is 3:1; The specific method for depositing and growing the COFs layer on the surface of the porous substrate is as follows: the base film is placed in a glass bottle, the prepared nucleation solution is added to the glass bottle, the base film is fully contacted with the reaction solution by slightly shaking, and then acetic acid is added as a catalyst, the bottle cap is tightly screwed after fully mixing, and the reaction is statically carried out at room temperature, at this time, a layer of COFs crystal nucleus is grown on the surface of the base film; the prepared crystal nucleus growth solution is slowly and uniformly injected into the above glass bottle by using a syringe pump, and finally a complete COFs composite film is obtained.

2. The method for preparing a two-dimensional covalent organic framework composite membrane for lithium-magnesium ion separation according to claim 1, characterized in that: The specific method for preparing the nucleation solution is as follows: 1.2-4.8 mg of 1,3,5-tris(4-aminophenyl)benzene is dissolved in 5.0 mL of a mixed solvent of ethyl acetate and mesitylene, and ultrasonic treatment is performed until the monomers are completely dissolved; 1.0-4.0 mg of 2,5-dimethoxybenzene-1,4-diformaldehyde or 2.3-9.2 mg of 2,5-bis-triethylene glycol dimethyl ether-1,4-p-xylylene formaldehyde is added to the above solution, and the mixture is shaken to obtain a solution for nucleation of the covalent organic framework on the surface of the substrate.

3. The method for preparing a two-dimensional covalent organic framework composite membrane for lithium-magnesium ion separation according to claim 1, characterized in that: The specific method for preparing the crystal nucleus growth solution is as follows: 0.2-0.6 mg of 1,3,5-tris(4-aminophenyl)benzene is dissolved in 5.0 mL of a mixed solvent of ethyl acetate and mesitylene, and ultrasonic treatment is performed until the monomers are completely dissolved; 0.16-0.5 mg of 2,5-dimethoxybenzene-1,4-diformaldehyde or 0.38-1.16 mg of 2,5-bis-triethylene glycol dimethyl ether-1,4-p-xylylene formaldehyde is added to the above solution, the mixture is quickly shaken, and then ultrasonic treatment is performed for 10 seconds, and the obtained solution is used for growing a film of the covalent organic framework on the surface of the substrate.

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