A method for preparing an organic solvent nanofiltration membrane

Abstract

The application discloses a preparation method of an organic solvent nanofiltration membrane, and steps are as follows: firstly, ZIF-8 nanoparticles are prepared through a solution method, and the ZIF-8 nanoparticles are used as a hard template to prepare polydopamine composite nanocapsules through self-polymerization of dopa monomers and removal of the hard template; then, after synthesizing a self-microporous polymer PIM-1, a polymer mixed solution with dispersed capsules is obtained through simple blending, and a thin layer with selective filtering performance is prepared on a solvent-resistant bottom membrane through a dip coating method or a spin coating method, so that the organic solvent-resistant membrane is formed. The organic solvent-resistant membrane prepared by the application has better swelling resistance and long-term service stability in organic solvents on the basis of ensuring the separation function.

Description

Technical Field

[0001] The present invention relates to the field of methods for preparing organic solvent nanofiltration membranes, and in particular to a method for preparing an organic solvent nanofiltration membrane. Background Art

[0002] Organic solvent nanofiltration is an emerging membrane separation technology that offers advantages over traditional separation and purification techniques such as distillation, including high efficiency, energy efficiency, and ease of operation. It can be used to efficiently separate small organic molecules with molecular weights ranging from 200 to 1000 g / mol in organic solutions, leading to widespread application in the chemical and environmental fields. Membranes used for organic solvent nanofiltration can be categorized by material as either inorganic-based or polymer-based. While inorganic materials offer excellent tolerance to organic solvents, they suffer from a broad pore size distribution, difficulty in controlling pore size distribution, and high production costs. Polymer membranes, however, are becoming the mainstream solvent-resistant nanofiltration membranes due to their ease of processing and low cost. However, membranes based on commonly used polymers such as polyacrylonitrile and polysulfone exhibit limited tolerance to organic solvents. They swell to varying degrees in solvents such as alkanes and alcohols, affecting the strength and performance of the polymer membranes. Therefore, the development of high-performance solvent-resistant nanofiltration membranes is crucial for the application and advancement of organic solvent nanofiltration.

[0003] The discovery of self-microporous polymers provides a more economical and efficient path for technologies such as hydrogen storage and gas separation. Unlike conventional linear polymers, the rigid twisted segment structure on the main chain of self-microporous polymers limits the rotation of chemical bonds and conformational changes to a certain extent, making it impossible for the segments to stack effectively, resulting in a very high free volume, and leading to the formation of interconnected pores between molecules inside the polymer, namely micropores. The micropore channel size of this type of polymer is <2nm, which matches the retention pore size of nanofiltration. The mostly hydrophobic properties are conducive to the permeation of organic molecules. They are only soluble in a few organic solvents such as tetrahydrofuran and chloroform, and can therefore be stably present in most organic systems. These advantages make them show great application potential in the preparation of high-performance organic solvent nanofiltration membranes.

[0004] The main limitation to their application is that naturally occurring microporous polymers are always in a thermodynamically nonequilibrium state with excess free volume, which can lead to physical aging. Furthermore, as polymers, they still swell to a certain extent in various organic solvents, resulting in insufficient long-term stability. In recent years, researchers at home and abroad have made numerous attempts to modify the readily prepared PIM-1, a leading example. Gao et al. spin-coated thioamide-modified PIM-1 membranes and cross-linked them with trimesoyl chloride, improving their solvent stability [Chemical Engineering Journal, 2018, 353:689–698]. Zhou et al. hydrolyzed PIM-1 and subjected it to an acyl chloride reaction to obtain PIM-COCl. They then dip-coated and cross-linked it with aminated polyacrylonitrile to create an organic solvent nanofiltration membrane. The resulting membrane exhibited long-term stability in ethanol [Journal of Membrane Science, 2019, 591:117-347]. In addition to chemical modification and crosslinking using the cyano groups of PIM-1, crosslinking or structural rearrangement can also be achieved through heat treatment, ultraviolet irradiation, and other methods, which can improve stability to a certain extent. However, crosslinking can also cause pore size reduction and alter performance. Therefore, further extensive and in-depth research is needed to improve the solvent resistance of inherently microporous polymers. To address the above-mentioned problems in the existing technology, the following proposes a solution. Summary of the Invention

[0005] The present invention aims to provide a method for preparing an organic solvent nanofiltration membrane, which combines the nanofiltration characteristics and film-forming properties of a self-microporous polymer, the stability and activity of a polydopamine material, and the interaction between the two, so that the membrane not only has the nanofiltration performance of a self-microporous polymer membrane, but also has excellent resistance to organic solvents.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions:

[0007] A method for preparing an organic solvent nanofiltration membrane, characterized in that it comprises the following steps:

[0008] S1: Preparation of solvent-resistant base film: Prepare a polymer solution at a certain temperature and stirring conditions, degas to obtain a uniform casting solution, then form a membrane using a non-solvent-induced phase inversion method, and cross-link under certain conditions to obtain a stable membrane. The stable membrane serves as the support layer of the thin composite membrane; S2: Obtain a self-contained microporous polymer PIM-1 through reaction;

[0009] S3: Based on the filler design of dopamine materials, the adhesive properties of dopamine and its derivatives were utilized to prepare polydopamine composite nanocapsules using a template method;

[0010] S4: Preparation of organic solvent-resistant composite membrane: disperse polydopamine composite nanocapsules in a chloroform solution of microporous polymer PIM-1 to obtain a uniform mixed solution, and use dip coating or spin coating to prepare a thin layer with selective filtration performance on the solvent-resistant base membrane to obtain an organic solvent-resistant membrane.

[0011] Preferably, the solvent-resistant base film described in S1 is a cross-linked polyimide film (XP84) prepared by the phase inversion method, the solute of the polymer solution in S1 is polyimide (P84), the mass fraction of the polyimide is 22-26wt%, the solvent of the polymer solution in S1 is N,N-dimethylformamide, the temperature when the polyimide solute is dissolved in the N,N-dimethylformamide solvent is 60°C, the scraper specification used for the phase inversion method in S1 to prepare the membrane is 250μm, the polymer solution is scraped on a polyethylene terephthalate non-woven fabric, the coagulation bath for the phase inversion method to prepare the membrane is pure water, the crosslinking agent for the phase inversion method to prepare the membrane is 20g / L 1,6-hexanediamine / isopropanol solution, the crosslinking time is 16-24h, and the membrane obtained after cross-linking is stored in ethanol.

[0012] Preferably, the hard template for preparing the polydopamine composite nanocapsules in S3 is ZIF-8, and the shell of the polydopamine composite nanocapsules in S3 is polydopamine and its derivatives.

[0013] Preferably, the ZIF-8 hard template is prepared by mixing and stirring an aqueous solution of zinc nitrate hexahydrate and 2-methylimidazole.

[0014] Preferably, the polydopamine shell is formed by reacting dopamine hydrochloride with dopamine hydrochloride, polyacetimide, N-3,4-dihydroxyphenethyl methacrylamide, and N-3,4-dihydroxyphenethyl methacrylamide with tris(2-aminoethyl)amine. The reaction is carried out in a tris(hydroxymethylaminomethane) aqueous solution in which ZIF-8 nanoparticles are dispersed. The pH during the reaction is 8.8. Upon completion of the reaction, polydopamine / ZIF-8 nanoparticles are obtained.

[0015] Preferably, the polydopamine / ZIF-8 nanoparticles are dispersed in water, the pH is adjusted to 7, and stirred for 1 hour to etch the ZIF-8 core to prepare composite nanocapsules.

[0016] Preferably, in S4, the mass fraction of the polydopamine composite nanocapsules added to the chloroform solution relative to the self-polymerized microporous polymer is 0-20 wt %, and the mass fraction of the self-polymerized microporous polymer relative to the total system is 2-4 wt %.

[0017] Preferably, the dip coating method in S4 is preceded by solvent rinsing and solvent volatilization, wherein the solvent rinsing and solvent volatilization comprise the following steps:

[0018] ①First, immerse the solvent-resistant base film in chloroform and rinse thoroughly.

[0019] ② Take out the membrane and dry it in the air. After the liquid on the membrane surface evaporates completely, immerse it in the dipping solution. The dipping method also includes a solvent annealing process, which includes the following steps: ① After dipping, preliminarily dry the membrane in a chloroform atmosphere for 0 to 3 minutes.

[0020] ②Take out and dry in the air;

[0021] The spin coating method in S4 has a spin coating speed of 1000 to 1500 rps and a spin coating time of 5 to 15 s.

[0022] The present invention provides a novel method for preparing an organic solvent nanofiltration membrane based on a self-microporous polymer. A polydopamine derivative, insoluble in any organic solvent, is introduced into the self-microporous polymer to provide hydrogen bonding, π-π stacking, and charge transfer, resulting in a nanofiltration membrane with excellent organic solvent resistance. The organic solvent-resistant membrane prepared by the present invention not only maintains separation performance but also exhibits improved swelling resistance and long-term service stability in organic solvents. DETAILED DESCRIPTION

[0023] The following description is only a preferred embodiment of the present invention, and the protection scope is not limited to this embodiment. All technical solutions under the concept of the present invention should fall within the protection scope of the present invention.

[0024] Example 1:

[0025] Preparation of polydopamine composite nanocapsules: 1.46g of zinc nitrate hexahydrate and 28.4g of 2-methylimidazole were dissolved in 10g and 100g of deionized water, respectively. The former was slowly added to the latter under vigorous stirring. The reaction was continued for 5 minutes. The product was washed, centrifuged, and then dissolved in 200mL of deionized water for storage. 0.36g of tris(hydroxymethylaminomethane) was then added, and the pH was adjusted to 8.8 with dilute hydrochloric acid to form a 15mmol / L tris(hydroxymethylaminomethane)-hydrochloric acid buffer system. 0.02g of dopamine hydrochloride was added, and the mixture was vigorously stirred at room temperature for 2 hours to form polydopamine@ZIF-8 nanoparticles. After washing and centrifugation, the mixture was dispersed in deionized water. The pH was adjusted to 7 with dilute hydrochloric acid. Under these conditions, ZIF-8 was etched for 1 hour to obtain polydopamine composite nanocapsules dispersed in water. Centrifugal exchange using ethanol as the intermediate solvent was performed to obtain polydopamine composite nanocapsules dispersed in chloroform, approximately 85nm in size.

[0026] Preparation of an organic solvent-resistant nanofiltration membrane: 26.4 g of polyimide (P84) was dissolved in N,N-dimethylformamide to prepare a 22 wt% casting solution. The solution was stirred at 60°C for 6 h until the P84 was fully dissolved. After vacuum degassing, phase inversion was performed using 30°C deionized water as a non-solvent to produce a P84 / non-woven fabric support membrane. The membrane was then immersed in a 20 g / L 1,6-hexanediamine / isopropanol solution for cross-linking for 16 h to obtain the support layer material (XP84) suitable for the preparation of thin-layer composite membranes. PIM-1 was dissolved in a chloroform solution containing dispersed nanocapsules and stirred uniformly to prepare a solution with a mass fraction of 2 wt% (relative to chloroform) and a nanocapsule content of 5 wt% (relative to PIM-1). The base film was rinsed with the chloroform solution and removed. After the chloroform on the surface had completely evaporated, the coating solution was dip-coated onto the membrane surface. After coating, the membrane was annealed in a chloroform atmosphere for 1 minute, removed, dried, and stored to obtain the organic solvent-resistant membrane of the present invention.

[0027] Membrane performance test results: Using cobalamin / ethanol solution as the probe molecule, the ethanol permeability of the membrane prepared in this example was measured to be 1.338 L / (m 2 h bar), and the cobalamin retention rate was 94.9%.

[0028] Example 2:

[0029] Synthesis of N-3,4-dihydroxyphenethyl methacrylamide: Add 4.6g of borax and 100mL of deionized water to a round-bottom flask. After bubbling with nitrogen for 30 minutes, transfer the flask to a glove box. First, add 2.3g of dopamine hydrochloride while stirring. Let it dissolve completely for 15 minutes, then add 1.3g of sodium carbonate. After sealing, remove the flask from the glove box and slowly add 1.2mL of methacryloyl chloride dropwise in an ice bath under a nitrogen atmosphere. After completion, cool the flask for 10 minutes before returning it to the glove box and allowing it to react overnight at room temperature. After the reaction is complete, acidify the flask with concentrated hydrochloric acid to a pH of less than 2. Extract the reaction mixture three times with 50mL of ethyl acetate. Pool the organic phases, dry them over magnesium sulfate for 2 hours, filter, spin dry, and vacuum dry to obtain a solid product.

[0030] Preparation of polydopamine composite nanocapsules: Keeping the synthesis and removal conditions of the ZIF-8 hard template unchanged, dopamine hydrochloride was replaced with N-3,4-dihydroxyphenethyl methacrylamide at a concentration of 1 mg / mL, and a small amount of tris(2-aminoethyl)amine was added to encapsulate and synthesize polydopamine composite nanocapsules in a tris(hydroxymethylaminomethane)-hydrochloric acid buffer solution at pH = 8.8. The size of the capsules was about 98 nm.

[0031] Preparation of an organic solvent-resistant nanofiltration membrane: Maintaining the same mass ratio of nanoparticles / polymer / solvent, a membrane solution was prepared and the organic solvent-resistant membrane described herein was prepared using spin coating. A predetermined amount of the membrane solution was dripped onto the center of a scraped XP84 base film and spin-coated at 1350 rpm for 10 seconds to obtain the organic solvent-resistant membrane described herein.

[0032] Membrane performance test results: Using cobalamin / ethanol solution as the probe molecule, the ethanol permeability of the membrane prepared in this example was measured to be 2.347 L / (m 2 h bar), and the cobalamin retention rate was 93.6%.

[0033] Example 3:

[0034] Preparation of polydopamine composite nanocapsules: Keeping the synthesis and removal conditions of the ZIF-8 hard template unchanged, dopamine hydrochloride and polyethyleneimine with a molecular weight of 600Da were dissolved in equimolar concentrations in tris(hydroxymethylaminomethane)-hydrochloric acid buffer solution at pH = 8.8 and reacted for 6 hours. The size of the coated synthesized polydopamine composite nanocapsules was about 125nm.

[0035] Preparation of organic solvent-resistant nanofiltration membrane: PIM-1 is dissolved in a chloroform solution in which nanocapsules are dispersed and stirred evenly to form a solution with a mass fraction of 4wt% (relative to chloroform) and a nanocapsule content of 5wt% (relative to PIM-1). The XP84 base membrane is rinsed with the chloroform solution and removed. After the chloroform on the surface has completely evaporated, the coating solution is dip-coated onto the membrane surface. After coating, the membrane is annealed in a chloroform atmosphere for 3 minutes, removed, dried, and stored to obtain the organic solvent-resistant membrane of the present invention.

[0036] Membrane performance test results: Using cobalamin / ethanol solution as the probe molecule, the ethanol permeability of the membrane prepared in this example was measured to be 1.058 L / (m 2 h bar), and the cobalamin retention rate was 93.4%.

[0037] Example 4:

[0038] Preparation of polydopamine composite nanocapsules: Keeping the synthesis and removal conditions of the ZIF-8 hard template unchanged, dopamine hydrochloride and the synthesized N-3,4-dihydroxyphenethyl methacrylamide were dissolved in a tris(hydroxymethylaminomethane)-hydrochloric acid buffer solution with a mass ratio of 1:2 and reacted for 2 hours. The size of the coated synthesized polydopamine composite nanocapsules was about 88 nm.

[0039] Preparation of an organic solvent-resistant nanofiltration membrane: A membrane solution was prepared, maintaining the same mass ratio of nanoparticles / polymer / solvent. The organic solvent-resistant membrane described herein was prepared using spin coating. A predetermined amount of the membrane solution was added dropwise to a scraped XP84 membrane and spin-coated at 1350 rps for 15 seconds to obtain the organic solvent-resistant membrane described herein.

[0040] Membrane performance test results: Using cobalamin / ethanol solution as the probe molecule, the ethanol permeability of the membrane prepared in this example was measured to be 1.623 L / (m 2 h bar), and the cobalamin retention rate was 95.2%.

[0041] Example 5:

[0042] Preparation of polydopamine composite nanocapsules: Keeping the synthesis and removal conditions of the ZIF-8 hard template unchanged, dopamine hydrochloride was replaced with N-3,4-dihydroxyphenylethyl methacrylamide at a concentration of 1 mg / mL, and a small amount of 1,6-hexanediamine was added. Polydopamine composite nanocapsules with a size of approximately 93 nm were synthesized by coating in a tris(hydroxymethyl)aminomethane-hydrochloric acid buffer solution at pH = 8.8.

[0043] Preparation of organic solvent-resistant nanofiltration membrane: PIM-1 is dissolved in a chloroform solution in which nanocapsules are dispersed and stirred evenly to prepare a solution with a mass fraction of 2 wt% (relative to chloroform) and a nanocapsule content of 1 wt% (relative to PIM-1). The XP84 base membrane is rinsed with the chloroform solution and removed. After the chloroform on the surface has completely evaporated, the coating solution is dip-coated onto the membrane surface. After coating, the membrane is annealed in a chloroform atmosphere for 1 minute, removed, dried, and stored to obtain the organic solvent-resistant membrane of the present invention.

[0044] Membrane performance test results: Using cobalamin / ethanol solution as the probe molecule, the ethanol permeability of the membrane prepared in this example was measured to be 1.868 L / (m 2 h bar), and the cobalamin retention rate was 94.7%.

[0045] In the above embodiment, the solvent-resistant base film as the support layer may be prepared using the above method or an existing cross-linked solvent-resistant porous film.

[0046] Table 1 Swelling ratio of solvent-resistant nanofiltration membrane in different organic solvents

[0047] Membrane type ethanol n-hexane acetone Ethyl acetate Undoped film 45％ 15％ 12％ 16％ Doped film 22％ 10％ 7％ 9％

[0048] Table 2 Changes in ethanol permeability of the solvent-resistant nanofiltration membrane of Example 1 after long-term immersion in ethanol

[0049] Membrane type 5 days 10 days 15 days 20 days 25 days 30 days 35 days Undoped film -4.3％ -7.3％ -10.5％ -14.1％ -16.7％ -18.9％ -23.2％ Doped film -1.1％ -2.6％ -3.1％ -3.8％ -4.2％ -4.5％ -5.1％

[0050] Table 3 Swelling ratio of solvent-resistant nanofiltration membrane in different organic solvents

[0051] Membrane type ethanol n-hexane acetone Ethyl acetate Undoped film 45％ 15％ 12％ 16％ Doped film 32％ 13％ 9％ 15％

[0052] Table 4 Changes in ethanol permeability of the solvent-resistant nanofiltration membrane of Example 2 after long-term immersion in ethanol

[0053] Membrane type 5 days 10 days 15 days 20 days 25 days 30 days 35 days Undoped film -4.3％ -7.3％ -10.5％ -14.1％ -16.7％ -18.9％ -23.2％ Doped film -2.1％ -3.6％ -5.1％ -6.8％ -8.2％ -10.5％ -13.1％

[0054] Table 5 Swelling ratio of solvent-resistant nanofiltration membrane in different organic solvents

[0055] Membrane type ethanol n-hexane acetone Ethyl acetate Undoped film 45％ 15％ 12％ 16％ Doped film 18％ 9％ 10％ 12％

[0056] Table 6 Changes in ethanol permeability of the solvent-resistant nanofiltration membrane of Example 3 after long-term immersion in ethanol

[0057] Membrane type 5 days 10 days 15 days 20 days 25 days 30 days 35 days Undoped film -4.3％ -7.3％ -10.5％ -14.1％ -16.7％ -18.9％ -23.2％ Doped film -1.7％ -2.9％ -3.7％ -4.1％ -4.8％ -6.8％ -7.5％

[0058] The specific embodiments described above further illustrate the technical problems, technical solutions and beneficial effects solved by the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing an organic solvent nanofiltration membrane, characterized in that: The steps include: S1: Preparation of solvent-resistant base film: preparing a polymer solution under certain temperature and stirring conditions, degassing to obtain a uniform casting solution, forming a membrane using a non-solvent-induced phase inversion method, and cross-linking under certain conditions to obtain a stable membrane, which is used as the support layer of the thin-layer composite membrane; S2: Self-microporous polymer PIM-1 is obtained by reaction; S3: Based on the filler design of dopamine materials, the adhesive properties of dopamine and its derivatives were utilized to prepare polydopamine composite nanocapsules using a template method; The polydopamine shell is formed by reacting dopamine hydrochloride with dopamine hydrochloride, polyacetimide, N-3,4-dihydroxyphenethyl methacrylamide, and N-3,4-dihydroxyphenethyl methacrylamide with tris(2-aminoethyl)amine. The reaction is carried out in a tris(hydroxymethyl)aminomethane aqueous solution in which ZIF-8 nanoparticles are dispersed. The pH during the reaction is 8.

8. Upon completion of the reaction, polydopamine / ZIF-8 nanoparticles are obtained. The polydopamine / ZIF-8 nanoparticles were dispersed in water, the pH was adjusted to 7, and stirred for 1 h to etch the ZIF-8 core to prepare composite nanocapsules; S4: Preparation of organic solvent-resistant composite membrane: disperse polydopamine composite nanocapsules in a chloroform solution of microporous polymer PIM-1 to obtain a uniform mixed solution, and use dip coating or spin coating to prepare a thin layer with selective filtration performance on the solvent-resistant base membrane to obtain an organic solvent-resistant membrane.

2. The method for preparing an organic solvent nanofiltration membrane according to claim 1, wherein: The solvent-resistant base film described in S1 is a cross-linked polyimide film (XP84) prepared by the phase inversion method. The solute of the polymer solution in S1 is polyimide (P84), the mass fraction of the polyimide is 22~26 wt%, the solvent of the polymer solution in S1 is N,N-dimethylformamide, the temperature when the polyimide solute is dissolved in the N,N-dimethylformamide solvent is 60°C, the scraper specification used in the phase inversion method for membrane preparation in S1 is 250 μm, the polymer solution is scraped on a polyethylene terephthalate non-woven fabric, the coagulation bath when the phase inversion method is used for membrane preparation is pure water, the crosslinking agent when the phase inversion method is used for membrane preparation is 20 g / L 1,6-hexanediamine / isopropanol solution, the crosslinking time is 16~24 h, and the membrane obtained after cross-linking is stored in ethanol.

3. The method for preparing an organic solvent nanofiltration membrane according to claim 2, wherein: The hard template of the polydopamine composite nanocapsule is ZIF-8, and the shell of the polydopamine composite nanocapsule is polydopamine and its derivatives.

4. The method for preparing an organic solvent nanofiltration membrane according to claim 3, characterized in that: The ZIF-8 hard template is prepared by mixing and stirring an aqueous solution of zinc nitrate hexahydrate and 2-methylimidazole.

5. The method for preparing an organic solvent nanofiltration membrane according to claim 1, wherein: In S4, the mass fraction of the polydopamine composite nanocapsules added to the chloroform solution relative to the self-polymerized microporous polymer is 0-20 wt %, and the mass fraction of the self-polymerized microporous polymer relative to the total system is 2-4 wt %.

6. The method for preparing an organic solvent nanofiltration membrane according to claim 5, characterized in that: The dip coating method in S4 is performed with solvent rinsing and solvent volatilization before dip coating, and the solvent rinsing and solvent volatilization include the following steps: ①First, immerse the solvent-resistant base film in chloroform and rinse thoroughly. ② Take out the membrane and dry it in the air. After the liquid on the membrane surface evaporates completely, immerse it in the dipping liquid. The dipping method also includes a solvent annealing process, which includes the following steps: ① After dipping, the film was initially dried in a chloroform atmosphere for 0 to 3 minutes. ②Take out and dry in the air; The spin coating method in S4 has a spin coating speed of 1000-1500 rps and a spin coating time of 5-15 s.