Preparation method of polyimide organic solvent nanofiltration membrane

Abstract

The invention relates to the technical field of organic solvent nanofiltration, in particular to a preparation method of a polyimide organic solvent nanofiltration membrane. The preparation method comprises the following steps: preparing polyamide acid resin slurry as a membrane casting solution, and curing the membrane casting solution on a non-woven fabric to form a membrane; immersing the obtained membrane in deionized water; immersing the obtained membrane in an imidization reagent for reaction to generate polyimide, immersing the obtained membrane in deionized water, and immersing the polyimide membrane in a solution containing a cross-linking agent; and washing to obtain the polyimide organic solvent nanofiltration membrane. The organic solvent nanofiltration membrane has high flux and interception performance in a polar organic solvent, not only has long-term stability in common organic solvents such as alcohols, ketones, esters, hydrocarbons and the like, but also has long-term stability in strong polar aprotic solvents such as DMF (Dimethyl Formamide), DMAc (Dimethyl Acrylate), NMP (N-Methyl Pyrrolidone), DMSO (Dimethyl Sulfoxide) and the like; the film has potential application value in the fields of chemical industry, pharmacy, food and the like.

Description

Technical Field

[0001] This invention relates to the field of organic solvent nanofiltration technology, and specifically to a method for preparing a polyimide organic solvent nanofiltration membrane. Background Technology

[0002] Organic solvent nanofiltration (OSN), also known as solvent-resistant nanofiltration (SRNF), can effectively sieve small molecule compounds with molecular weights of 200-1000 Da in organic solvents. Compared with traditional distillation, adsorption, extraction and other operations, the advantages of OSN are mainly reflected in the following aspects: (1) low energy consumption; (2) less solid waste generation (compared to the solid waste generated by silica gel and adsorbents used in chromatographic analysis); (3) mild operating conditions, without the need for extreme temperatures and pressures; (4) clear scale-up process path, easy to achieve large-scale expansion; (5) strong stability in harsh chemical environments, and can be flexibly operated within a wide range of pH values, temperatures and solvent selection; (6) can achieve convenient switching between low-boiling-point and high-boiling-point solvents.

[0003] Organic solvent nanofiltration (OSN) membranes are the core of organic solvent nanofiltration (OSN). An ideal OSN membrane should maintain good chemical, thermal, and mechanical stability across a wide range of organic solvents, while exhibiting excellent solvent permeation flux and solute rejection. Currently, commercially available OSN membranes are mainly divided into two categories: ceramic membranes and polymer membranes. Ceramic membranes have high mechanical strength, good chemical and thermal stability, and do not swell in organic solvents. They also have advantages such as high pressure resistance and easy cleaning. However, ceramic membranes are brittle, difficult to process and mold, and expensive, which is not conducive to large-scale production and application. In contrast, polymer membranes have advantages such as good flexibility, easy processing and molding, and low cost, making them a more attractive OSN membrane material.

[0004] Currently, the main polymer materials used to prepare OSN membranes include polyimide (PI), polyacrylonitrile (PAN), polyetheretherketone (PEEK), polydimethylsiloxane (PDMS), polyamide (PA), polyvinyl alcohol (PVA), sulfonated polysulfone (SPS), sulfonated polyethersulfone (SPES), cellulose acetate (CA), and their derivatives. Among these, PI has gradually become one of the most studied membrane materials in the OSN field due to its diverse structures, excellent mechanical properties, and generally good solvent resistance. PI is classified into soluble PI (such as P84 and Matrimid) and insoluble PI (such as PMDA-based PI and BTDA-based PI). Currently, commercially available PI-based OSN membranes are all prepared from soluble PI, such as the PuraMem membrane from Evonik-MET and the MetMem membrane from DuPont. However, currently only a very few companies in Europe and the United States can produce soluble PI, making the raw material very expensive and its source limited.

[0005] Compared to soluble polyimide (PI), most PIs are insoluble, with dozens of varieties, wider availability, and lower cost. Due to their poor solubility, insoluble PIs are typically first prepared as soluble polyamic acid, then phase-inverted to form a film before imidization. Cao Bing and Li Yuan et al. used Kapton, one of the most commonly used insoluble PIs, as an example. They first spun polyamic acid hollow fibers, then prepared hollow fiber membranes suitable for OSNs through chemical imidization. These membranes had a molecular weight cutoff of approximately 800 Da in DMF and a flux of 2.51 Lm⁻²h⁻¹bar⁻¹ (Journal of Membrane Science, 2017, 544, 1-11; Applied Surface Science, 2019, 473, 1038-1048). However, this method is complex, requiring pore-forming agents and a dual coagulation bath, increasing the difficulty of actual production. Furthermore, as a glassy polymer, OSN films prepared by Kapton lack resistance to swelling and long-term stability.

[0006] Patent CN 104817707 A discloses a method for preparing a high-flux polyimide nanofiltration membrane. This method involves altering the polyamic acid membrane structure by crystallizing polyamic acid at low temperatures or incorporating inorganic nanoparticles, followed by gradual heating to 350 °C to induce thermal imidization of the polyamic acid membrane, ultimately yielding a high-flux, high-rejection-rate polyimide nanofiltration membrane. However, this method is complex and time-consuming, requiring at least several hours. Furthermore, the poor compatibility between inorganic nanoparticles and organic polymer solutions makes it difficult to obtain a uniform nanofiltration membrane.

[0007] In addition, patents with publication numbers CN 115505123 B, CN 106883431 B, and CN 113019136 A disclose methods for generating polyimide membranes by imidizing polyamic acid. However, the polyimide membranes obtained by these patents are not cross-linked, resulting in unsatisfactory long-term stability. Directly imidizing and cross-linking polyamic acid after film formation is more challenging than using soluble polyimide as a raw material because the dense skin formed after polyamic acid film formation hinders the penetration of the cross-linking agent, making it difficult to obtain a polymer with a high degree of cross-linking. Therefore, researching a low-cost, widely available, high-performance, and durable nanofiltration membrane remains of great significance. Summary of the Invention

[0008] To achieve the above-mentioned technical objectives, the present invention provides a method for preparing a polyimide organic solvent nanofiltration membrane.

[0009] The technical solution of the present invention: A method for preparing a polyimide organic solvent nanofiltration membrane: preparing a polyamic acid resin slurry as a casting solution, and curing the casting solution on a nonwoven fabric to form a membrane; immersing the obtained membrane in deionized water; immersing the obtained membrane in an imidizing agent to react and generate polyimide, then immersing the obtained membrane in deionized water, immersing the polyimide membrane in a solution containing a crosslinking agent; and obtaining a polyimide organic solvent nanofiltration membrane after washing.

[0010] Furthermore, a method for preparing a polyimide organic solvent nanofiltration membrane includes the following steps: (1) The polyamic acid resin solution is scraped onto the nonwoven fabric and then immediately immersed in the coagulation bath to solidify into a film. The concentration of the polyamic acid is 10-30 wt.%, the scraping speed is 100-300 mm / s, the coagulation bath is a mixed bath of water and DMAc, the concentration of water in the coagulation bath is 70-99 wt.%, the temperature of the coagulation bath is 0-30 ºC, and the coagulation time is 10-30 minutes. (2) The solidified polyamic acid film is immersed in an imidizing agent to obtain a polyimide film; the immersion temperature is 0-100 ºC and the immersion time is 2-10 minutes. (3) The polyimide membrane is immersed in a solution containing a crosslinking agent to obtain a polyimide organic solvent nanofiltration membrane. The concentration of the crosslinking agent is 1-20 wt.%, the crosslinking reaction temperature is 20-60 ºC, and the crosslinking reaction time is 3-20 minutes.

[0011] The polyamic acid resin solution is obtained by copolymerizing diamine monomers and dianhydride monomers in a polar solvent; the diamine monomers are selected from p-phenylenediamine, o-phenylenediamine, m-phenylenediamine, 1,3-cyclohexanediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, 2,2'-bis[4-(4-aminophenoxyphenyl)]propane, biphenylenediamine, 4,4'-diaminodiphenyl sulfide, 4-phenyl-2,6-bis(4-amino)propane, biphenylenediamine, 4,4'-diaminodiphenyl sulfide, and 4-phenyl-2,6-bis(4-amino)propane. The diamine monomer is selected from one or more of phenylpyridine; the dianhydride monomer is selected from one or more of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic acid dianhydride, 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride, bisphenol A dianhydride, and bisphenol F dianhydride, preferably, the diamine monomer is 4,4'-diaminodiphenylmethane, and the dianhydride monomer is pyromellitic dianhydride; the polar solvent is one or more of DMF, DMAc, NMP, and DMSO.

[0012] The viscosity of the polyamic acid is 5000-25000 cp; preferably, the viscosity of the polyamic acid is 15000-20000 cp. The imidizing agent consists of a dehydrating agent and an accelerator. The dehydrating agent is selected from one or more of trifluoroacetic anhydride, acetic anhydride, propionic anhydride, aromatic monocarboxylic acid anhydride, and acetyl chloride. Preferably, the dehydrating agent is acetic anhydride or propionic anhydride. The accelerator is selected from one or more of pyridine, p-pyrroline, dimethylpyridine, trimethylpyridine, quinoline, isoquinoline, triethylamine, and N,N-dimethylethanolamine. Preferably, the accelerator is pyridine or triethylamine. The molar ratio of the dehydrating agent to the accelerator is 9:1 to 5:5. Preferably, the molar ratio of the dehydrating agent to the accelerator is 8:2.

[0013] Preferably, in step (2), the soaking temperature is 30-60 ºC. The crosslinking agent is selected from one or more of ethylenediamine, 1,2-propanediamine, 1,3-propanediamine, 1,4-butanediamine, 1,5-pentanediamine, 1,6-hexanediamine, 1,8-octanediamine, 1,9-nonanediamine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, m-phenylenediamine, p-phenylenediamine, furanylenediamine, 1,5-naphthylenediamine, 2,6-naphthylenediamine, anthracene-2,6-diamine, and diethylenetriamine; the solvent of the crosslinking agent solution is one or more of water, methanol, ethanol, and isopropanol.

[0014] Preferably, the crosslinking agent is 1,6-hexanediamine or p-phenylenediamine.

[0015] The beneficial effects of this invention are: (1) The present invention uses poorly soluble polyimide as raw material to prepare organic solvent nanofiltration membrane, avoiding the high cost and limited raw material source problems caused by using soluble polyimide; (2) The present invention can effectively control the flux and desalination rate of the finished membrane. By controlling the synthesis monomer of polyamic acid, it can effectively control the polymer structure, thereby achieving effective control of the membrane's molecular weight cutoff and permeation flux; (3) The membrane material of the present invention has long-term stability: the solvent resistance of the membrane material is further improved by simple cross-linking, while enhancing the long-term stability of the membrane in organic solvents.

[0016] In summary, the asymmetric nanofiltration membrane prepared by the method of this invention exhibits high flux and retention performance in polar organic solvents. This membrane not only demonstrates long-term stability in common organic solvents such as alcohols, ketones, esters, and hydrocarbons, but also in strongly polar aprotic solvents such as DMF, DMAc, NMP, and DMSO. The manufacturing cost of this membrane is significantly lower than that of commercially available solvent-resistant polyimide membranes, and it has promising applications in the chemical, pharmaceutical, and food industries. Detailed Implementation

[0017] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0018] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0019] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0020] For the polyimide asymmetric nanofiltration membranes obtained below, the rejection rate for dyes of different molecular weights and the flux in organic solvents are evaluated as follows. The tests are conducted using a stainless steel high-pressure cross-flow membrane testing apparatus, following these steps: (1) Preparation of test solutions: Prepare 3 L of methanol solutions of methyl red, bromothymol blue and Bengal rose red with a mass concentration of 50 ppm respectively; (2) Flux Test: The membrane was installed in the test cell and sealed with a stainless steel cover. The prepared DMAc solution of the dye was added to the collection tank of the test device, and then the polyimide asymmetric nanofiltration membrane was subjected to cross-flow filtration under a pressure difference of 1 bar and a temperature of 25 °C. After 30 minutes, the volume of the collected filtrate was measured, and the flux was calculated. The flux calculation formula is: P = V / (A × t ×Δ P) Where: P—solvent permeation flux (Lm) -2 .h -1 .bar -1 V—time t The volume of solution permeating through the membrane (L); A—the effective area of ​​the filter membrane (m²). 2 ); t — Filtering time (h); Δ P — Operating pressure (bar).

[0021] Retention rate test: The retention rate test used three dye labels with different molecular weights: methyl red, bromophenol blue, and Bengal rose red, with molecular weights of 269.3 Da, 670.0 Da, and 1017.6 Da, respectively. Methanol solutions of methyl red, bromophenol blue, and Bengal rose red with mass concentrations of 5 ppm, 25 ppm, 50 ppm, 100 ppm, and 250 ppm were prepared. The absorbance values ​​of the methanol solutions of each dye at each mass concentration were measured using a UV-Vis spectrophotometer at the maximum absorption wavelengths (methyl red: 415 nm; bromophenol blue: 422 nm; Bengal rose red: 500 nm), and a standard curve of mass concentration versus absorbance value was obtained. By measuring the absorbance value of the dye in the collected filtrate, the dye concentration in the filtrate was calculated, and thus the retention rate of the membrane for the dye was calculated. The formula for calculating the retention rate is: R% = (1-C p / C f ) × 100% In the formula, R% is the retention rate, and C p C represents the concentration of dye in the permeate. f C represents the concentration of dye in the raw material solution. p and C f The absorbance of the dye at its maximum absorption wavelength was measured using a UV-Vis spectrophotometer and calculated.

[0022] The polyamic acid slurry used in the examples was provided by Zhuzhou Times New Material Technology Co., Ltd., and the other reagents were purchased from Aladdin Reagent (Shanghai) Co., Ltd.

[0023] Example 1 A method for preparing a polyimide organic solvent nanofiltration membrane includes the following steps: (1) Polymerize pyromellitic anhydride with 4,4'-diaminodiphenylmethane to obtain polyamic acid slurry, and uniformly coat the polyamic acid slurry onto the surface of PP nonwoven fabric at a speed of 200 mm / s, and then quickly immerse it in a mixture of deionized water and DMAc, wherein the concentration of DMAc is 5 wt.%, and after phase inversion at 25 ºC for 5 minutes, the membrane is transferred to another pure deionized water and washed for 2 minutes; the concentration of the polyamic acid slurry is 20 wt.%, and the viscosity is 15000 cp; (2) Blow away the water on the surface of the membrane, then immerse it in an imidizing agent composed of propionic anhydride and pyridine, and react at 40 ºC for 5 minutes to obtain a membrane containing polyimide; wash the polyimide membrane in deionized water for 5 minutes and then transfer it to another pure deionized water for 2 minutes; the molar ratio of propionic anhydride and pyridine in the imidizing agent is 8:2; (3) After blowing away the moisture on the surface of the polyimide membrane, place it in an isopropanol solution of p-phenylenediamine, react at 50 °C for 5 minutes and then take it out to obtain an OSN membrane containing cross-linked polyimide; wash the membrane with deionized water and then soak it in DMAc for later use; the concentration of p-phenylenediamine is 10 wt..

[0024] Example 2 Example 2 is basically the same as Example 1, except that the raw materials in Example 1 are replaced with polyamic acid slurry (20 wt.%, 20000 cp) obtained by polymerizing 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride and 2,2'-bis[4-(4-aminophenoxyphenyl)]propane.

[0025] Example 3 Example 3 is basically the same as Example 1, except that the imidizing agent in Example 1 is replaced with an imidizing agent composed of acetic anhydride and triethylamine; the molar ratio of acetic anhydride and triethylamine is 8:2.

[0026] Example 4 Example 4 is basically the same as Example 1, except that the aqueous solution of p-phenylenediamine is replaced with an aqueous solution of hexamethylenediamine, wherein the concentration of hexamethylenediamine is 10 wt.%.

[0027] Comparative Example 1 Comparative Example 1 is basically the same as Example 1, except that the imidization temperature is replaced with 20 ºC.

[0028] Comparative Example 2 Comparative Example 2 is basically the same as Example 1, except that the imidization temperature is replaced with 70 ºC.

[0029] The asymmetric nanofiltration membranes obtained in Examples 1-4 and Comparative Examples 1-2 were immersed in organic solvents such as methanol, chloroform, acetone, ethyl acetate, DMF, NMP, DMSO, DMAc, and tetrahydrofuran for 30 days. The results showed that all asymmetric nanofiltration membranes were stable in the above solvents and their performance remained basically unchanged.

[0030] The DMAc flux and the rejection rates of different dyes (methyl red, bromothymol blue, and Bengal rose red) of the asymmetric nanofiltration membranes obtained in Examples 1-4 and Comparative Examples 1-2 were tested using a stainless steel high-pressure cross-flow membrane testing device. The results are shown in Table 1 below.

[0031] Table 1

[0032] In summary, the polyimide organic solvent nanofiltration membrane prepared by this invention not only maintains long-term stability in strongly polar aprotic solvents, but also allows for changes in membrane separation performance by adjusting membrane preparation conditions, thereby obtaining organic solvent nanofiltration membranes with different rejection rates and fluxes for solutes of different molecular weights.

[0033] It should be noted that although the technical solution of the present invention has been described with specific examples, those skilled in the art will understand that the present invention should not be limited thereto.

[0034] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A method for preparing a polyimide organic solvent nanofiltration membrane, characterized in that, Includes the following steps: (1) The polyamic acid resin solution is scraped onto the nonwoven fabric and then immediately immersed in the coagulation bath to solidify into a film. The scraping rate is 100-300 mm / s and the coagulation bath is a mixed bath of water and DMAc. (2) The solidified polyamic acid film is immersed in an imidizing agent to obtain a polyimide film; the immersion temperature is 0-100 ºC and the immersion time is 2-10 minutes. (3) The polyimide membrane is immersed in a solution containing a crosslinking agent to obtain a polyimide organic solvent nanofiltration membrane. The concentration of the crosslinking agent is 1-20 wt.%, the crosslinking reaction temperature is 20-60 ºC, and the crosslinking reaction time is 3-20 minutes.

2. The method for preparing a polyimide organic solvent nanofiltration membrane as described in claim 1, characterized in that, The polyamic acid resin solution is obtained by copolymerizing diamine monomers and dianhydride monomers in a polar solvent; the diamine monomers are selected from p-phenylenediamine, o-phenylenediamine, m-phenylenediamine, 1,3-cyclohexanediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, 2,2'-bis[4-(4-aminophenoxyphenyl)]propane, biphenylenediamine, 4,4'-diaminodiphenyl sulfide, 4-phenyl-2,6-bis(4-amino)propane, biphenylenediamine, 4,4'-diaminodiphenyl sulfide, and 4-phenyl-2,6-bis(4-amino)propane. The diamine monomer is selected from one or more of phenylpyridine; the dianhydride monomer is selected from one or more of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic acid dianhydride, 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride, bisphenol A dianhydride, and bisphenol F dianhydride, preferably, the diamine monomer is 4,4'-diaminodiphenylmethane, and the dianhydride monomer is pyromellitic dianhydride; the polar solvent is one or more of DMF, DMAc, NMP, and DMSO.

3. The method for preparing a polyimide organic solvent nanofiltration membrane as described in claim 1, characterized in that, The concentration of the polyamic acid is 10-30 wt.%, and the viscosity of the polyamic acid is 5000-25000 cp.

4. The method for preparing a polyimide organic solvent nanofiltration membrane as described in claim 2, characterized in that, The viscosity of the polyamic acid is 15,000-20,000 cp.

5. The method for preparing a polyimide organic solvent nanofiltration membrane as described in claim 2, characterized in that, The concentration of water in the coagulation bath is 70-99 wt.%, the temperature of the coagulation bath is 0-30 ºC, and the coagulation time is 10-30 minutes.

6. The method for preparing a polyimide organic solvent nanofiltration membrane as described in claim 1, characterized in that, The imidizing agent is composed of a dehydrating agent and an accelerator in a molar ratio of dehydrating agent: accelerator = 9:1 to 5:5; the dehydrating agent is selected from one or more of trifluoroacetic anhydride, acetic anhydride, propionic anhydride, aromatic monocarboxylic acid anhydride, and acetyl chloride; the accelerator is selected from one or more of pyridine, p-pyrroline, dimethylpyridine, trimethylpyridine, quinoline, isoquinoline, triethylamine, and N,N-dimethylethanolamine.

7. The method for preparing a polyimide organic solvent nanofiltration membrane as described in claim 1, characterized in that, The dehydrating agent is acetic anhydride or propionic anhydride; the accelerator is pyridine or triethylamine; the molar ratio of the dehydrating agent to the accelerator is 8:

2.

8. The method for preparing a polyimide organic solvent nanofiltration membrane as described in claim 1, characterized in that, In step (2), the soaking temperature is 30-60 ºC.

9. The method for preparing a polyimide organic solvent nanofiltration membrane as described in claim 1, characterized in that, The crosslinking agent is selected from one or more of ethylenediamine, 1,2-propanediamine, 1,3-propanediamine, 1,4-butanediamine, 1,5-pentanediamine, 1,6-hexanediamine, 1,8-octanediamine, 1,9-nonanediamine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, m-phenylenediamine, p-phenylenediamine, furanylenediamine, 1,5-naphthylenediamine, 2,6-naphthylenediamine, anthracene-2,6-diamine, and diethylenetriamine; the solvent of the crosslinking agent solution is one or more of water, methanol, ethanol, and isopropanol.

10. A method for preparing a polyimide organic solvent nanofiltration membrane as described in claim 1 or 8, characterized in that, The crosslinking agent is 1,6-hexanediamine or p-phenylenediamine.

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