A metal-organic framework-based separation membrane, its preparation method, and lithium extraction device.

Abstract

This invention discloses a metal-organic framework (MOF)-based separation membrane, its preparation method, and a lithium extraction device, belonging to the field of separation membrane technology. The MOF-based separation membrane includes a base membrane and a functional coating on the surface of the base membrane; the functional coating includes a bulk crosslinked polymer and a MOF. The bulk crosslinkable polymer can improve the adhesion strength between the MOF and the base membrane in the separation membrane, thereby improving the membrane's service life and long-term performance. The MOF-based separation membrane prepared by the method provided in this invention exhibits good ion flux and lithium selective permeability.

Description

Technical Field

[0001] This invention relates to the field of separation membrane technology, and more specifically, to a metal-organic framework-based separation membrane, its preparation method, and a lithium extraction apparatus. Background Technology

[0002] Compared to traditional lithium extraction processes from salt lakes, membrane separation methods offer advantages such as high efficiency, energy saving, environmental friendliness, small footprint, and high operational flexibility. Nanofiltration (NF) and electrodialysis (ED) technologies are used for lithium extraction from brine. + Purification of Li: Forward osmosis (FO), reverse osmosis (RO), and membrane distillation (MD) are used for Li + Concentration. In practical applications, multi-process coupling technology is usually used to achieve higher magnesium-lithium separation requirements. At present, although membrane separation technology has a promising future, it still has problems such as membrane loss, membrane fouling, poor stability, and optimization of high and low valence ion rejection rates.

[0003] Metal-organic framework (MOF)-based separation membranes are widely used in magnesium-lithium separation due to their precise pore size, high porosity, unique pore structure, and abundant molecular / ion-specific functional groups. However, the instability and processing difficulties of MOFs limit their practical application in membrane materials. Therefore, they are generally composited with base membranes to prepare MOF-based heterostructures. MOF-based heterostructures are mainly classified into polycrystalline MOF membranes formed by nucleation and growth of MOFs on a substrate, MOF polyamide membranes prepared by embedding MOFs in a polyamide layer, and channel membranes.

[0004] In polycrystalline MOF films, multiphase crystallization forms continuous MOF films, while homogeneous crystallization may generate film defects. Substrate modification is beneficial for heterogeneous nucleation of MOFs, promoting the formation of defect-free films. Xu (https: / / doi.org / 10.1016 / j.memsci.2021.119101) disclosed a method for heterogeneous nucleation of MOF polycrystalline films. First, the substrate film was modified using a tannic acid-Zn complex, and then the ligand imidazole was added to enhance the Zn content in the tannic acid-Zn complex. 2+ Partially converted to a metal-organic framework ZIF (a Zn-imidazole complex); due to Zn 2+ The partial conversion of tannins results in residual tannins in the membrane. Because tannins have a low molecular weight and good water solubility, they are easy to migrate in water, causing unstable membrane flux. When they are released, they also cause water pollution, leading to microbial membrane fouling and consequently, a decline in membrane performance over the long term.

[0005] Therefore, this invention is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a metal-organic framework-based separation membrane, its preparation method, and lithium extraction device, aiming to improve the bonding strength between the metal-organic framework and the base membrane in the separation membrane, thereby improving the membrane's service life and long-term performance.

[0007] This invention can be implemented as follows:

[0008] In a first aspect, the present invention provides a separation membrane based on a metal-organic framework, the separation membrane based on the metal-organic framework comprising a base membrane and a functional coating on the surface of the base membrane; the functional coating comprising a bulk crosslinked polymer and a metal-organic framework.

[0009] In some embodiments of the present invention, the metal-organic framework-based separation membrane includes at least one of the following features:

[0010] Feature 1: The bulk crosslinked polymer is obtained by bulk crosslinking of a polymer under oxygen-containing conditions, and the polymer is polyethyleneimine grafted with catechol;

[0011] Feature 2: The base film comprises one of polysulfone, polyethersulfone, polyvinylidene fluoride, polypropylene, polyethylene, polyacrylonitrile, polyimide, or polytetrafluoroethylene; preferably, the base film is one of polypropylene, polyethersulfone, or polyvinylidene fluoride.

[0012] Feature 3: The metal ion of the metal-organic framework is a zinc ion, and the ligand is an imidazole compound;

[0013] Feature 4: The functional coating covers both sides of the base film, and the thickness of the functional coating on one side is 80-150 nm;

[0014] Feature 5: The thickness of the base film is 90–150 μm;

[0015] Feature 6: The average pore size of the base film is 100-1000 nm, preferably 400-800 nm.

[0016] In some embodiments of the present invention, the metal-organic framework-based separation membrane further includes at least one of the following features:

[0017] Feature 7: The polymer is prepared by acylation reaction of polyethyleneimine and catechol monomer; the catechol monomer includes at least one selected from 3,4-dihydroxyphenylpropionic acid, 3,4-dihydroxyphenylbutyric acid, 3-(2,3-dihydroxyphenyl)propionic acid, α-methyl-3,4-dihydroxyphenylpropionic acid, and 5-(3,4-dihydroxyphenyl)valerate; preferably, the catechol monomer is 3,4-dihydroxyphenylpropionic acid;

[0018] Feature 8: The number-average molecular weight of the polyethyleneimine is 600-10000 g / mol; preferably, the number-average molecular weight of the polyethyleneimine is 10000 g / mol.

[0019] Feature 9: The amine value of the polyethyleneimine is 18–22 mmol / g;

[0020] Feature 10: The molar ratio of amine groups to catechol groups in the polymer is (2:3) to (5:1);

[0021] Feature 11: The imidazole compounds include at least one of benzimidazole, 2-methylimidazole, 4-methylimidazole, and 2-aminobenzimidazole.

[0022] In some embodiments of the present invention, the metal-organic framework-based separation membrane further includes at least one of the following features:

[0023] Feature 12: The lithium-ion flux of the metal-organic framework-based separation membrane during its first 1-hour operation is no less than 0.3 mol·h⁻¹. -1 ·m -2 ;

[0024] Feature 13: The lithium-ion selective permeability of the metal-organic framework-based separation membrane is not less than 8.4;

[0025] Feature 14: The lithium-ion flux retention rate of the metal-organic framework-based separation membrane is not less than 95.4% after 24 hours of continuous operation.

[0026] Secondly, the present invention provides a method for preparing a separation membrane based on a metal-organic framework, comprising the following steps:

[0027] Step 1: Polyethyleneimine is acylated with catechol monomer to prepare the polymer;

[0028] Step 2: Prepare polymer solution and zinc salt solution respectively, and apply the polymer solution and zinc salt solution to the surface of the base film in sequence to form a complex coating modified base film;

[0029] Step 3: Prepare a ligand solution and apply it to the surface of the complex coating modified base membrane. Heat and dry it under oxygen-containing conditions to obtain a metal-organic framework-based separation membrane.

[0030] In some embodiments of the present invention, step one includes at least one of the following features:

[0031] Feature 1: The acylation reaction is carried out with the catalyst 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride;

[0032] Feature 2: The acylation reaction uses water and / or alcohol as solvents;

[0033] Feature 3: The acylation reaction is carried out in an acidic environment;

[0034] Feature 4: The ratio of the molar amount of the catechol monomer to the total molar amount of primary and secondary amines in polyethyleneimine is 0.2 to 0.6:1.

[0035] In some embodiments of the present invention, step two includes at least one of the following features:

[0036] Feature 5: The polymer solution has a mass percentage concentration of 2% to 6%;

[0037] Feature 6: The zinc salt solution has a mass percentage concentration of 0.5% to 1.5%;

[0038] Feature 7: The zinc salt includes at least one of zinc acetate, zinc chloride, and zinc nitrate;

[0039] Feature 8: The complex coating covers both sides of the base film, and the thickness of the complex coating on one side of the modified base film is 80-160 nm.

[0040] In some embodiments of the present invention, step three includes at least one of the following features:

[0041] Feature 9: The ligand comprises at least one of benzimidazole, 2-methylimidazole, 4-methylimidazole, and 2-aminobenzimidazole;

[0042] Feature 10: The molar concentration of the ligand solution is 2–4 M;

[0043] Feature 11: The solvent of the ligand solution includes one of ethanol, methanol, and water.

[0044] In some embodiments of the present invention, heating and drying under oxygen-containing conditions is equivalent to heating and drying under air purging.

[0045] Thirdly, the present invention provides a lithium extraction device, comprising the metal-organic framework-based separation membrane described in the first aspect or the metal-organic framework-based separation membrane prepared by the preparation method of the second aspect.

[0046] The beneficial effects of this invention include:

[0047] This invention provides a metal-organic framework-based separation membrane containing a functional coating comprising a bulk crosslinked polymer and a metal-organic framework. The bulk crosslinked polymer can improve the adhesion strength between the metal-organic framework and the base membrane in the separation membrane, thereby improving the membrane's service life and long-term performance. The metal-organic framework-based separation membrane prepared by the method provided in this invention exhibits good ion flux and lithium selective permeability. Attached Figure Description

[0048] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0049] Figure 1 The XRD patterns of the metal-organic framework-based separation membrane (1), the complex-coated modified base membrane (2), and ZIF-8 and polypropylene (PP) prepared in Example 1 are shown.

[0050] Figure 2 Cross-sectional SEM images of the metal-organic framework-based separation membrane (c), the complex-coated modified base membrane (b), and the polypropylene base membrane (a) prepared in Example 1;

[0051] Figure 3 This is a lithium extraction device. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0053] The following is a detailed description of the metal-organic framework-based separation membrane, its preparation method, and the lithium extraction device provided by this invention.

[0054] The present invention provides a separation membrane based on a metal-organic framework, the separation membrane comprising a base membrane and a functional coating on the surface of the base membrane; the functional coating comprising a bulk crosslinked polymer and a metal-organic framework.

[0055] The present invention provides a separation membrane based on a metal-organic framework containing a functional coating comprising a bulk crosslinked polymer and a metal-organic framework. The polymer capable of bulk crosslinking can improve the adhesion strength between the metal-organic framework and the base membrane in the separation membrane, thereby improving the service life and long-term performance of the membrane.

[0056] The aforementioned bulk crosslinked polymers are obtained by bulk crosslinking of polymers under oxygen-containing conditions. That is, under natural air conditions, no additional treatment is required for the polymers to undergo intermolecular and / or intramolecular crosslinking, and no additional crosslinking agent is needed. Compared with linear polymer adhesives, bulk crosslinkable polymers can improve the bonding strength between the metal-organic framework and the base membrane in the separation membrane, thereby improving the membrane's service life and long-term performance.

[0057] In some embodiments of the present invention, the polymer in the bulk crosslinking polymer is polyethyleneimine grafted with catechol, which is prepared by acylation reaction of polyethyleneimine and catechol monomer.

[0058] In some alternative embodiments, the catechol monomer includes at least one of 3,4-dihydroxyphenylpropionic acid, 3,4-dihydroxyphenylbutyric acid, 3-(2,3-dihydroxyphenyl)propionic acid, α-methyl-3,4-dihydroxyphenylpropionic acid, and 5-(3,4-dihydroxyphenyl)valerate. The alkyl chain length in the catechol monomer affects the water solubility of the polymer, and the position of the phenolic hydroxyl group in the catechol monomer affects the acylation reaction rate, thereby affecting the grafting rate, crosslinking density, and ultimately the adhesion strength between the metal-organic framework and the base membrane in the separation membrane. 3,4-dihydroxyphenylpropionic acid is a preferred embodiment of the present invention.

[0059] In some alternative embodiments, the number-average molecular weight of the polyethyleneimine is 600–10000 g / mol. The number-average molecular weight of polyethyleneimine affects the adhesive strength of the formed bulk crosslinked polymer. The larger the molecular weight, the more entanglement between molecules and the stronger the adhesive strength. However, if the molecular weight is too large, the polymer viscosity will be too high, making it difficult to process and resulting in poor dispersibility on the base film. Preferably, the number-average molecular weight of the polyethyleneimine is 10000 g / mol.

[0060] In some alternative embodiments, the amine value of the polyethyleneimine is 18–22 mmol / g; the amine value of polyethyleneimine affects the grafting rate of catechol groups in the polymer, the bulk crosslinking density, and ultimately the adhesion strength between the metal-organic framework and the base membrane in the separation membrane.

[0061] In some alternative embodiments, the molar ratio of amine groups to catechol groups in the polymer is (2:3) to (5:1); the bulk crosslinking described in this invention refers to the crosslinking achieved by the chemical reaction of phenolic hydroxyl groups and amine groups according to Formula I and / or Formula II. It can be seen that the ratio of the two affects the degree of bulk crosslinking of the polymer, thereby affecting the bonding strength, hardness and lithium separation selectivity between the metal-organic framework and the base membrane in the separation membrane.

[0062] Formula I:

[0063]

[0064] Formula II:

[0065]

[0066] In some embodiments of the present invention, the base membrane includes one of polysulfone, polyethersulfone, polyvinylidene fluoride, polypropylene, polyethylene, polyacrylonitrile, polyimide, or polytetrafluoroethylene; the type, thickness, and pore size of the base membrane affect the ion flux, lithium selective permeability, and durability of the separation membrane; preferably, the base membrane is one of polypropylene, polyethersulfone, or polyvinylidene fluoride.

[0067] In some alternative embodiments, the thickness of the base film is 90–150 μm.

[0068] In some alternative embodiments, the average pore size of the base film is 100-1000 nm, preferably 400-800 nm.

[0069] In some embodiments of the present invention, the metal ion of the metal-organic framework is a zinc ion, and the ligand is an imidazole compound; the imidazole compound includes at least one of benzimidazole, 2-methylimidazolium, 4-methylimidazolium, and 2-aminobenzimidazole.

[0070] In some embodiments of the present invention, the functional coating covers the outer surfaces of both sides of the base membrane, where both sides refer to the two planes with the largest area formed by the length and width directions of the membrane; the thickness of the functional coating on one side is 80-150 nm; the thickness affects the ion flux and lithium selective permeability of the separation membrane.

[0071] In some embodiments of the present invention, the lithium-ion flux of the metal-organic framework-based separation membrane during its first 1 hour of operation is not less than 0.3 mol·h. -1 ·m -2 .

[0072] In some embodiments of the present invention, the lithium-ion selective permeability of the metal-organic framework-based separation membrane is not less than 8.4.

[0073] In some embodiments of the present invention, the lithium-ion flux retention rate of the metal-organic framework-based separation membrane is not less than 95.4% after 24 hours of continuous operation.

[0074] This invention provides a method for preparing a separation membrane based on a metal-organic framework, comprising the following steps:

[0075] Step 1: Polyethyleneimine is acylated with catechol monomer to prepare the polymer;

[0076] Step 2: Prepare polymer solution and zinc salt solution respectively, and apply the polymer solution and zinc salt solution to the surface of the base film in sequence to form a complex coating modified base film;

[0077] Step 3: Prepare a ligand solution and apply it to the surface of the complex coating modified base membrane. Heat and dry it under oxygen-containing conditions to obtain a metal-organic framework-based separation membrane.

[0078] The polymer provided by this invention is grafted with catechol groups, and catechol is effective against Zn. 2+ It has a strong complexing effect, so the polymer is first applied to the surface of the base film, and then Zn is applied. 2+ This makes Zn 2+ Due to complexation, it is fixed on the surface of the base film; when ligands are added, the ligands react with Zn... 2+ The coordination effect is stronger and it can form an ordered arrangement, thus making Zn 2+ The metal centers, converted into metal-organic frameworks, form metal-organic framework crystals with separation activity on the surface of the base membrane. When the polymer is subsequently heated and dried under oxygen-containing conditions, it undergoes bulk cross-linking, thus transforming into an adhesive between the metal-organic framework and the base membrane. This adhesive method greatly improves the bonding strength between the two, thereby improving the service life and long-term performance of the membrane.

[0079] In some embodiments of the present invention, step one specifically includes the following steps: dissolving polyethyleneimine in a solvent, then adding acid to adjust the pH value, adding catechol monomer and catalyst, stirring at room temperature to carry out an acylation reaction, and obtaining the polymer after purification and drying.

[0080] In some alternative approaches, the catalyst is 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride.

[0081] In some alternative approaches, the solvent is water and / or alcohol.

[0082] In some alternative embodiments, the pH value is 4 to 6; the acid is selected from at least one of hydrochloric acid, acetic acid, sulfuric acid, and nitric acid.

[0083] In some alternative methods, the molar ratio of the catechol monomer to the total molar ratio of primary and secondary amines in polyethyleneimine is 0.2–0.6:1. The molar ratio of catechol monomer to amine groups affects the grafting ratio of catechol groups onto polyethyleneimine. On the one hand, a higher amount of catechol groups results in more active sites for coordination with zinc ions, leading to a greater amount of subsequent metal-organic framework conversion and thus improved lithium-ion selective permeability. On the other hand, since catechol groups and amine groups undergo subsequent bulk crosslinking reactions, an excessive amount of catechol groups results in less remaining amine groups in the polymer, thus affecting the crosslinking density of the subsequent polymer and consequently impacting adhesive strength and lithium-ion selective permeability.

[0084] The purpose of step two of this invention is to form a uniform complex coating on the surface of the base film. Therefore, the method of applying the polymer solution and zinc salt solution to the surface of the base film is not limited, and may include, but is not limited to, immersion, spraying, scraping, casting, etc.

[0085] In some embodiments of the present invention, step two specifically includes the following steps: preparing a polymer solution and a zinc salt solution respectively; firstly immersing the base film in the polymer solution, allowing it to stand for a first time, and then removing it; then immersing it in the zinc salt solution, allowing it to stand for a second time, and then removing it; rinsing with water and drying; repeating the above steps until the target thickness is reached to obtain the complex coating modified base film.

[0086] In some alternative embodiments, the mass percentage concentration of the polymer solution is 2% to 6%.

[0087] In some alternative embodiments, the zinc salt solution has a mass percentage concentration of 0.5% to 1.5%.

[0088] In some alternative embodiments, the zinc salt includes at least one of zinc acetate, zinc chloride, and zinc nitrate.

[0089] In some alternative approaches, the target thickness refers to the thickness of the complex coating on one side of the base film surface, and the target thickness is 80–160 nm.

[0090] In some alternative methods, the first settling time is 5 to 10 minutes; the second settling time is 8 to 12 minutes.

[0091] The purpose of step three of this invention is to uniformly apply ligands to the surface of the complex coating modified base film. Therefore, the method of applying the ligand solution to the base film surface is not limited, and may include, but is not limited to, soaking, spraying, scraping, casting, etc.

[0092] In some embodiments of the present invention, step three specifically includes the following steps: dissolving the ligand in a solvent to form a ligand solution, immersing the modified base membrane in the above ligand solution, allowing it to stand for a third time, removing it and rinsing it with water, and then drying it under air purging and heating to obtain a separation membrane.

[0093] In some alternative embodiments, the ligand is an imidazole compound; the imidazole compound includes at least one of benzimidazole, 2-methylimidazolium, 4-methylimidazolium, and 2-aminobenzimidazole.

[0094] In some alternative embodiments, the molar concentration of the ligand solution is 2–4 M.

[0095] In some alternative embodiments, the solvent for the ligand solution includes one of ethanol, methanol, or water.

[0096] In some alternative methods, the third settling time is 30 to 60 minutes.

[0097] In some alternative methods, the drying under air purging and heating means drying in a blower box at 60–80°C for 12–24 hours.

[0098] In addition, the present invention also provides a lithium extraction device, including a metal-organic framework-based separation membrane according to any of the foregoing embodiments.

[0099] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0100] Example 1

[0101] This embodiment provides a separation membrane based on a metal-organic framework, and the preparation method is as follows:

[0102] (1) Preparation of polymers

[0103] Polyethyleneimine (PEI) was weighed and dissolved in deionized water. Hydrochloric acid was added to adjust the pH to 5. Then, 3,4-dihydroxyphenylpropionic acid and 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) were added. The mixture was stirred at room temperature for 6 hours to carry out the acylation reaction. After the reaction was completed, the mixture was purified by dialysis and then dried at 120°C to obtain the polymer.

[0104] The PEI has a number-average molecular weight of 10,000 g / mol and an amine value of 18 mmol / g.

[0105] The molar ratio of the 3,4-dihydroxyphenylpropionic acid to the total molar ratio of primary and secondary amines in PEI is 0.4:1.

[0106] The molar amount of EDC is twice the molar amount of catechol monomer.

[0107] The proton peaks of the prepared polymer in the 1H NMR spectrum are as follows, with D2O as the solvent:

[0108] 1 1H-NMR (400MHz, D2O), (ppm): 2.42 (t, Ar-CH2), 2.4 (t, O=C-CH2), 2.69 (m, PEI-CH2), 6.55 (d, Ar-H), 6.67 (s, Ar-H), 6.72 (d, Ar-H), 8.22 (s, O=C-NH), 9.58 (s, Ar-OH). The presence of amide proton peaks in the 1H NMR spectrum indicates successful grafting of the catechol group onto polyethyleneimine. Peak area analysis of the proton peaks in the 1H NMR spectrum reveals a molar ratio of amine groups (primary and secondary amines) to catechol groups in the polymer of 5:1.5.

[0109] (2) Complex coating modified base film

[0110] The polymer was dissolved in deionized water to obtain a polymer solution with a concentration of 4 wt%, and zinc acetate was dissolved in deionized water to obtain a zinc salt solution with a concentration of 1 wt%. First, the polypropylene base film was immersed in the polymer solution and allowed to stand for 5 minutes. After that, it was taken out and immersed in the zinc salt solution and allowed to stand for 10 minutes. Then, it was taken out, rinsed with deionized water 5 times, and dried by blowing with nitrogen. The above steps were repeated until the complex coating on the surface of one side of the base film reached the target thickness of 160 nm, thus obtaining the complex coating modified base film.

[0111] The polypropylene base film has a thickness of 120 μm and an average pore size of 600 nm.

[0112] (3) Preparation of metal-organic framework-based separation membranes

[0113] Benzimidazole was dissolved in ethanol to form a ligand solution with a concentration of 3M. The complex-coated modified base membrane was immersed in the above ligand solution, allowed to stand for 45 minutes, and then taken out and rinsed 3 times with deionized water. It was then dried in a 60°C forced-air oven for 12 hours to obtain a metal-organic framework-based separation membrane.

[0114] Example 2

[0115] The difference from Example 1 is that the molar ratio of the 3,4-dihydroxyphenylpropionic acid in step (1) to the total molar ratio of primary and secondary amines in PEI is 0.2:1.

[0116] The molar ratio of amine groups (primary and secondary amines) to catechol groups in the polymer is 5:1.

[0117] Example 3

[0118] The difference from Example 1 is that the molar ratio of the 3,4-dihydroxyphenylpropionic acid in step (1) to the total molar ratio of primary and secondary amines in PEI is 0.6:1.

[0119] The molar ratio of amine groups (primary and secondary amines) to catechol groups in the polymer is 5:2.6.

[0120] Example 4

[0121] The difference from Example 1 is that the target thickness in step (2) is 80 nm.

[0122] Example 5

[0123] The difference from Example 1 is that the target thickness in step (2) is 120 nm.

[0124] Example 6

[0125] The difference from Example 1 is that step (3) uses an aqueous solution of 2-methylimidazole instead of an ethanol solution of benzimidazole as the ligand solution.

[0126] Example 7

[0127] This embodiment provides a separation membrane based on a metal-organic framework, and the preparation method is as follows:

[0128] (1) Preparation of polymers

[0129] Polyethyleneimine (PEI) was weighed and dissolved in deionized water. Hydrochloric acid was added to adjust the pH to 4. Then, 3,4-dihydroxyphenylbutyric acid and 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) were added. The mixture was stirred at room temperature for 6 hours to carry out the acylation reaction. After the reaction was completed, the mixture was purified by dialysis and then dried at 120°C to obtain the polymer.

[0130] The PEI has a number-average molecular weight of 600 g / mol and an amine value of 20 mmol / g.

[0131] The molar ratio of the 3,4-dihydroxyphenylbutyric acid to the total molar ratio of primary and secondary amines in PEI is 0.4:1.

[0132] The molar amount of EDC is twice the molar amount of catechol monomer.

[0133] The molar ratio of amine groups (primary and secondary amines) to catechol groups in the polymer is 5:1.3.

[0134] (2) Complex coating modified base film

[0135] The polymer was dissolved in deionized water to obtain a polymer solution with a concentration of 6 wt%, and zinc chloride was dissolved in deionized water to obtain a zinc salt solution with a concentration of 0.5 wt%. The polyethersulfone base film was first immersed in the polymer solution and allowed to stand for 8 minutes. After being removed, it was immersed in the zinc salt solution and allowed to stand for another 8 minutes. Then, it was removed, rinsed 5 times with deionized water, and dried by blowing with nitrogen. The above steps were repeated until the complex coating on one side of the base film surface reached the target thickness of 160 nm, thus obtaining the complex coating modified base film.

[0136] The polyethersulfone-based film has a thickness of 150 μm and an average pore size of 1000 nm.

[0137] (3) Preparation of metal-organic framework-based separation membranes

[0138] 4-Methylimidazole was dissolved in deionized water to form a ligand solution with a concentration of 2M. The complex-coated modified base membrane was immersed in the above ligand solution, left to stand for 30 minutes, then removed and rinsed 3 times with deionized water. It was then dried in an 80℃ forced-air oven for 12 hours to obtain a metal-organic framework-based separation membrane.

[0139] Example 8

[0140] This embodiment provides a separation membrane based on a metal-organic framework, and the preparation method is as follows:

[0141] (1) Preparation of polymers

[0142] Polyethyleneimine (PEI) was weighed and dissolved in deionized water. Hydrochloric acid was added to adjust the pH to 6. Then, 5-(3,4-dihydroxyphenyl)valeric acid and 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) were added. The mixture was stirred at room temperature for 6 hours to carry out the acylation reaction. After the reaction was completed, the mixture was purified by dialysis and then dried at 120°C to obtain the polymer.

[0143] The PEI has a number-average molecular weight of 1800 g / mol and an amine value of 19 mmol / g.

[0144] The molar ratio of the 5-(3,4-dihydroxyphenyl)valerate to the total molar ratio of primary and secondary amines in PEI is 0.4:1.

[0145] The molar amount of EDC is twice the molar amount of catechol monomer.

[0146] The molar ratio of amine groups (primary and secondary amines) to catechol groups in the polymer is 5:1.

[0147] (2) Complex coating modified base film

[0148] The polymer was dissolved in deionized water to obtain a polymer solution with a concentration of 2 wt%, and zinc nitrate was dissolved in deionized water to obtain a zinc salt solution with a concentration of 1.5 wt%. First, the polyvinylidene fluoride membrane was immersed in the polymer solution and allowed to stand for 10 min. After that, it was taken out and immersed in the zinc salt solution and allowed to stand for another 12 min. Then, it was taken out, rinsed with deionized water 5 times, and dried by blowing with nitrogen. The above steps were repeated until the complex coating on one side of the base film surface reached the target thickness of 160 nm, thus obtaining the complex coating modified base film.

[0149] The polyvinylidene fluoride (PVDF) film has a thickness of 90 μm and an average pore size of 400 nm.

[0150] (3) Preparation of metal-organic framework-based separation membranes

[0151] 2-Aminobenzimidazole was dissolved in ethanol to form a ligand solution with a concentration of 4M. The complex-coated modified base membrane was immersed in the above ligand solution, allowed to stand for 60 min, and then taken out and rinsed 3 times with deionized water. It was then dried in a 60℃ forced-air oven for 24 h to obtain a metal-organic framework-based separation membrane.

[0152] Comparative Example 1

[0153] The difference from Example 1 is that step (2) uses an aqueous solution of tannic acid instead of an aqueous solution of the polymer.

[0154] Comparative Example 2

[0155] This comparative example prepares a separation membrane, and the preparation method is as follows:

[0156] A polypropylene membrane (120 μm thick, average pore size 600 nm) was placed at the bottom of a mold, and then the precursor solution was poured in. After standing at room temperature for 24 hours, the membrane was removed, rinsed five times with deionized water, and dried to obtain the separation membrane. The precursor solution consisted of an aqueous solution containing 2.7 mg / mL zinc acetate and 57 mg / mL 2-methylimidazole. The area ratio of the polypropylene membrane to the volume of the precursor solution was 1 cm². 2 : 0.5mL.

[0157] Comparative Example 3

[0158] The difference from Example 1 is that the ratio of the molar amount of 3,4-dihydroxyphenylpropionic acid in step (1) to the total molar amount of primary and secondary amines in PEI is 0.1:1.

[0159] The molar ratio of amine groups (primary and secondary amines) to catechol groups in the polymer is 10:1.

[0160] Comparative Example 4

[0161] The difference from Example 1 is that the ratio of the molar amount of 3,4-dihydroxyphenylpropionic acid in step (1) to the total molar amount of primary and secondary amines in PEI is 0.7:1.

[0162] The molar ratio of amine groups (primary and secondary amines) to catechol groups in the polymer is 5:3.2.

[0163] Test case

[0164] (1) The metal-organic framework-based separation membrane (1) and the complex-coated modified base membrane (2) prepared in Example 1 were characterized by X-ray diffraction. The obtained XRD patterns are shown below. Figure 1 As shown.

[0165] Depend on Figure 1 It can be seen that the XRD spectra of the separation membrane based on metal-organic framework all include the diffraction peaks of pure metal-organic framework (ZIF-8) and the peaks of polypropylene base membrane (PP), indicating that the metal-organic framework was successfully grown on the base membrane.

[0166] (2) The metal-organic framework-based separation membrane (c), the complex-coated modified base membrane (b), and the polypropylene base membrane (a) prepared in Example 1 were observed by scanning electron microscopy. The obtained SEM images are shown below. Figure 2 As shown.

[0167] contrast Figure 2 As can be seen from (a) and (b), the complex coating successfully modified the base film; comparing (b) and (c), it can be seen that crystals grew on the film surface, the roughness increased significantly, and the polymer layer thickness decreased, indicating that the polymer underwent bulk polymerization.

[0168] (3) The lithium extraction performance of the separation membranes prepared in the examples and comparative examples was tested. The separation membranes were assembled into an electrodialysis lithium extraction device, and the ion flux (J) and lithium ion selective permeability (P) were tested.

[0169] Electrodialysis lithium extraction equipment as shown in the attached document Figure 3 As shown, it consists of a polarization chamber (including a cathode polarization chamber and an anodic polarization chamber), a concentration chamber, and a desalination chamber, wherein the electrodes are titanium electrodes coated with ruthenium oxide. The polarization chamber is filled with a 0.3M sodium sulfate solution, and the desalination chamber is filled with a Li₂ solution with a metal ion concentration of 0.1M. + and Mg 2+ The mixture consisted of a concentration chamber containing 0.01 M KCl solution; the electrodialysis membrane assembly comprised a CEM (AMX, purchased from Asahi Kasei Corporation, Japan), the sample membrane, and the CEM. Each compartment contained 100 mL of solution, and the pump flow rate was 20 mL / min. A constant current mode was used, and the concentration of Li in the concentration chamber was determined by ICP-OES. + Mg 2+The concentration of lithium ions is determined, and the ion flux J and lithium ion selectivity P are calculated using the following formulas:

[0170]

[0171] In the formula, J is the ion flux (mol·h⁻¹). -1 ·m -2 ), C0 and C t denoted as ion concentrations (mol / L) at time 0 and time t in the concentration chamber, respectively; V is the liquid volume in the concentration chamber (L); and A is the effective area of ​​the membrane (m²). 2 ).

[0172]

[0173] In the formula, P represents lithium-ion selectivity, and J... Li and J Mg The ion fluxes of lithium ions and magnesium ions at time t are C and C, respectively. Mg and C Li t represents the concentrations of lithium ions and magnesium ions in the desalination chamber at time t.

[0174] Table 1 shows the lithium-ion flux (J) after the first 1 hour of operation (i.e., t = 1 hour). Li The lithium-ion selective permeability (P) and the lithium-ion flux after 24 hours of continuous operation (i.e., t=24h) relative to the J after the first 1 hour of operation. Li Retention rate.

[0175] Table 1

[0176]

[0177] As shown in Table 1, the metal-organic framework-based separation membrane prepared in the embodiments of the present invention exhibits superior long-term performance and lithium selective permeability compared to the comparative examples. Specifically, Comparative Example 1 uses tannic acid as a complexing ligand. Due to the small molecular weight of tannic acid and its inability to undergo bulk cross-linking, it exhibits almost no adhesive effect and will dissolve into water during use, causing water pollution; therefore, its long-term performance is poor. Comparing Examples 1-3 and Comparative Examples 3-4, it is evident that the grafting ratio of catechol groups in the polymer has a significant impact on the separation membrane performance. As the grafting ratio increases, the J of the separation membrane... LiThe decrease in retention rate and P both showed an initial increase followed by a decrease. This is because: firstly, the higher the grafting ratio of catechol groups, the more active sites there are for coordination with zinc ions, resulting in a greater amount of subsequent conversion into a metal-organic framework, thus reducing ion permeability; secondly, catechol groups are grafted onto the PEI side chains through acylation reactions between catechol monomers and amino groups on the PEI chain, so the higher the grafting ratio of catechol groups, the fewer remaining amino groups. The ratio of catechol groups to amino groups affects the degree of bulk cross-linking reaction between the two. The higher the degree of cross-linking reaction, the greater the adhesive strength. Long-term performance shows an initial increase followed by a decrease, because excessive cross-linking leads to a lower pore size, which easily causes blockage. Similarly, the pore size decreases continuously with the increase of the polymer bulk cross-linking degree, so the selective permeability for lithium shows an initial increase followed by a decrease. Too low a pore size leads to too low lithium ion flux, thus worsening the selective permeability for lithium. Compared to Example 1, Comparative Example 2 has a higher lithium-ion flux, but its long-term performance is poor and its lithium-ion selective permeability is low. This is because there is no cross-linked polymer as an adhesive medium between the metal-organic framework on the membrane prepared in Comparative Example 2 and the base membrane, resulting in low adhesion strength of the metal-organic framework, which is easy to fall off during use, leading to poor long-term performance. In addition, because the metal-organic framework cannot be uniformly dispersed and fully cover the surface of the base membrane under this preparation method, the local pore size is too large, resulting in low lithium selective permeability.

[0178] 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 separation membrane based on a metal-organic framework, characterized in that, The metal-organic framework-based separation membrane includes a base membrane and a functional coating on the surface of the base membrane; the functional coating includes a bulk crosslinked polymer and a metal-organic framework. The metal-organic framework-based separation membrane has the following characteristics: Feature 1: The bulk crosslinked polymer is obtained by bulk crosslinking of a polymer under oxygen-containing conditions, and the polymer is polyethyleneimine grafted with catechol; Feature 2: The base film includes one of polysulfone, polyethersulfone, polyvinylidene fluoride, polypropylene, polyethylene, polyacrylonitrile, polyimide, or polytetrafluoroethylene; Feature 3: The metal ion of the metal-organic framework is a zinc ion, and the ligand is an imidazole compound; Feature 4: The functional coating covers both sides of the base film, and the thickness of the functional coating on one side is 80~150nm; Feature 5: The thickness of the base film is 90~150μm; Feature 6: The average pore size of the base film is 100~1000nm.

2. The separation membrane based on a metal-organic framework according to claim 1, characterized in that, The metal-organic framework-based separation membrane further includes at least one of the following features: Feature 7: The polymer is prepared by acylation reaction of polyethyleneimine and catechol monomer; the catechol monomer includes at least one of 3,4-dihydroxyphenylpropionic acid, 3,4-dihydroxyphenylbutyric acid, 3-(2,3-dihydroxyphenyl)propionic acid, α-methyl-3,4-dihydroxyphenylpropionic acid, and 5-(3,4-dihydroxyphenyl)valerate. Feature 8: The number-average molecular weight of the polyethyleneimine is 600~10000 g / mol; Feature 9: The amine value of the polyethyleneimine is 18~22 mmol / g; Feature 10: The molar ratio of amine groups to catechol groups in the polymer is (2:3) to (5:1); Feature 11: The imidazole compounds include at least one of benzimidazole, 2-methylimidazole, 4-methylimidazole, and 2-aminobenzimidazole.

3. The separation membrane based on a metal-organic framework according to any one of claims 1 to 2, characterized in that, The metal-organic framework-based separation membrane further includes at least one of the following features: Feature 12: The lithium-ion flux of the metal-organic framework-based separation membrane during its first 1-hour operation is no less than 0.3 mol·h⁻¹. -1 ·m -2 ; Feature 13: The lithium-ion selective permeability of the metal-organic framework-based separation membrane is not less than 8.4; Feature 14: The lithium-ion flux retention rate of the metal-organic framework-based separation membrane is not less than 95.4% after 24 hours of continuous operation.

4. A method for preparing a metal-organic framework-based separation membrane as described in any one of claims 1 to 3, characterized in that, Includes the following steps: Step 1: Polyethyleneimine is acylated with catechol monomer to prepare the polymer; Step 2: Prepare polymer solution and zinc salt solution respectively, and apply the polymer solution and zinc salt solution to the surface of the base film in sequence to form a complex coating modified base film; Step 3: Prepare a ligand solution and apply it to the surface of the complex coating modified base membrane. Heat and dry it under oxygen-containing conditions to obtain a metal-organic framework-based separation membrane.

5. The preparation method according to claim 4, characterized in that, Step one includes at least one of the following features: Feature 1: The acylation reaction is carried out with the catalyst 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride; Feature 2: The acylation reaction uses water and / or alcohol as solvents; Feature 3: The acylation reaction is carried out in an acidic environment; Feature 4: The ratio of the molar amount of the catechol monomer to the total molar amount of primary and secondary amines in the polyethyleneimine is 0.2~0.6:

1.

6. The preparation method according to claim 4, characterized in that, Step two includes at least one of the following features: Feature 5: The polymer solution has a mass percentage concentration of 2% to 6%; Feature 6: The zinc salt solution has a mass percentage concentration of 0.5% to 1.5%; Feature 7: The zinc salt includes at least one of zinc acetate, zinc chloride, and zinc nitrate; Feature 8: The complex coating covers both sides of the base film, and the thickness of the complex coating on one side of the modified base film is 80~160nm.

7. The preparation method according to claim 4, characterized in that, Step three includes at least one of the following features: Feature 9: The ligand comprises at least one of benzimidazole, 2-methylimidazole, 4-methylimidazole, and 2-aminobenzimidazole; Feature 10: The molar concentration of the ligand solution is 2~4M; Feature 11: The solvent of the ligand solution includes one of ethanol, methanol, and water.

8. The preparation method according to claim 4, characterized in that, The heating and drying under oxygen-containing conditions refers to heating and drying under air purging.

9. A lithium extraction device, characterized in that, It includes the metal-organic framework-based separation membrane as described in any one of claims 1 to 3, or the metal-organic framework-based separation membrane prepared by the preparation method as described in any one of claims 4 to 8.

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