A composite solvent-resistant nanofiltration membrane doped with modified carbon-based nanofiller and its preparation method and application

Abstract

The present invention provides a composite solvent-resistant nanofiltration membrane doped with modified carbon-based nanofillers, the nanofiltration membrane comprising a support layer, an intermediate layer and a selective separation layer; the support layer is an amino-containing polyimide base membrane cross-linked with ethylene glycol diamine; the intermediate layer is obtained by depositing multi-walled carbon nanotubes coated with polydopamine and piperazine monomers on the surface of the base membrane; the selective separation layer is a polyamide film obtained by polymerizing piperazine monomers and acyl chloride monomers on the surface of the intermediate layer. The composite solvent-resistant nanofiltration membrane doped with modified carbon-based nanofillers prepared in the present application shows good separation effects in different solution systems, has good permeation flux for different organic solvents, and has good tolerance in various organic solvents, meeting the requirements of various solution separation scenarios while maintaining a good service life.

Description

Technical Field

[0001] The invention belongs to the technical field of nanofiltration membrane preparation, and specifically relates to a composite solvent-resistant nanofiltration membrane doped with modified carbon-based nanofillers, and a preparation method and application thereof. Background Art

[0002] Organic solvent nanofiltration separation technology has become a highly-regarded energy-saving solvent-solute separation, purification / concentration technology due to its advantages such as low energy consumption, near-zero solid waste generation, low carbon emissions, strong tolerance (maintaining stability in acidic, alkaline, high temperature, high concentration solvent, etc.) and mild operating conditions (room temperature, low pressure, etc.). Organic solvent nanofiltration membrane separation is driven by pressure and can effectively intercept solutes with a particle size range of 0.5-2nm.

[0003] In the early 1960s, Sourirajan and Loeb first realized the application of asymmetric cellulose acetate polymer membrane in non-aqueous systems and successfully separated high-purity hydrocarbon solvents. In recent decades, polyamide thin film composite membranes constructed by interfacial polymerization have become the focus of nanofiltration and reverse osmosis membrane research. These thin film composite membranes have a dense polyamide selective separation layer on the surface of the base membrane, showing good separation performance and stable structure. However, there is a "seesaw" effect between the permeability and separation selectivity of traditional thin film composite nanofiltration membranes, that is, a certain amount of permeability is usually sacrificed to ensure its separation selectivity. This trade-off limits the widespread application of polyamide thin film composite membranes. In addition, the affinity between the supporting layer base membrane and the selective separation layer of most polyamide thin film composite membranes is poor, resulting in swelling and deconstruction of the base membrane, separation and shedding of the base membrane and the polyamide film when used in a long-term organic solvent environment, which in turn leads to a decrease in separation selectivity or failure.

[0004] Therefore, there is an urgent need to provide a simple and effective method to prepare composite membranes with controllable surface properties, making them perfectly suitable for the separation of organic solvents and having strong tolerance. Summary of the invention

[0005] In view of this, the present invention provides a defect-free thin film nanocomposite solvent-resistant nanofiltration membrane prepared by unsupported interfacial polymerization using multi-walled carbon nanotubes modified with polydopamine as an intermediate layer.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A composite solvent-resistant nanofiltration membrane doped with modified carbon-based nanofillers, the nanofiltration membrane comprising a support layer, an intermediate layer and a selective separation layer;

[0008] The support layer is an amino-containing polyimide-based film cross-linked with ethylene glycol diamine;

[0009] The intermediate layer is obtained by depositing multi-walled carbon nanotubes coated with polydopamine and piperazine monomers on the surface of the base film;

[0010] The selective separation layer is a polyamide film obtained by polymerizing piperazine monomers and acyl chloride monomers on the surface of the intermediate layer.

[0011] Furthermore, the preparation method of the polydopamine-coated multi-walled carbon nanotubes is as follows:

[0012] S11, adding multi-walled carbon nanotubes into ultrapure water, stirring and ultrasonicating to obtain a multi-walled carbon nanotube aqueous solution;

[0013] S12, adding a buffer to the multi-walled carbon nanotube aqueous solution to adjust the pH to obtain a multi-walled carbon nanotube buffer;

[0014] S13, adding polydopamine to the multi-walled carbon nanotube buffer, stirring for reaction, filtering through a microporous filter membrane after the reaction, rinsing, and drying to obtain the multi-walled carbon nanotubes coated with polydopamine.

[0015] Furthermore, in step S11, the mass ratio of ultrapure water to multi-walled carbon nanotubes is 1:(0.001-0.5); stirring conditions: 10000-20000 rpm, time 8-12 min; ultrasonic conditions: 80-200 W, time 25-35 min.

[0016] Furthermore, in step S12, the buffer solution is 0.01-0.1 M Tris buffer solution; and the pH value is 8.0-9.0.

[0017] Furthermore, in step S13, the mass ratio of multi-walled carbon nanotubes to polydopamine is 1: (1.5-2.5);

[0018] The pore size of the microporous filter membrane is 0.2 μm;

[0019] The stirring reaction conditions are: stirring at a speed of 1000-5000 rpm for 10-15 hours under light-shielding conditions;

[0020] The drying conditions are as follows: temperature 55-65° C., time 2-5 min.

[0021] The preparation method of the composite solvent-resistant nanofiltration membrane comprises the following steps:

[0022] S21, dissolving polyimide and polyethylene glycol in N-methylpyrrolidone to prepare a casting solution, then applying the casting solution on a non-woven fabric, performing a phase inversion process in deionized water to obtain a membrane, and then immersing the membrane in a polyethylene glycol diamine aqueous solution to obtain an amino-containing polyimide-based membrane cross-linked with polyethylene glycol diamine;

[0023] S22. Add polydopamine-coated multi-walled carbon nanotubes and piperazine monomers into ultrapure water and perform ultrasound to obtain aqueous solution I. Then place the basement membrane in step S21 in aqueous solution I, let it stand, take it out and dry it, so that the intermediate layer is deposited on the surface of the basement membrane.

[0024] S23. Place the base film in step S22 in an aqueous solution II containing only piperazine monomers, add an oil phase solution containing acyl chloride monomers, and perform unsupported interfacial polymerization to obtain a polyamide film. Finally, open the valve at the bottom of the container to discharge the liquid, allowing the polyamide film to fall on the middle layer, take it out and dry it, so as to obtain a composite solvent-resistant nanofiltration membrane doped with modified carbon-based nanofillers.

[0025] Furthermore, in step S21, the concentration of polyimide in the casting solution is 10-40 wt%, the concentration of polyethylene glycol is 0.5-15 wt%, and the concentration of polyethylene glycol diamine in the polyethylene glycol diamine aqueous solution is 3-7 wt%.

[0026] Furthermore, the concentration of the polydopamine-coated multi-walled carbon nanotubes in the aqueous solution I of step S22 is 0.0025-0.1 wt %, and the concentration of the piperazine monomers is 0.1-2.0 wt %; and the standing time in step S22 is 0.5-5 min.

[0027] Furthermore, in step S23, the concentration of piperazine monomers in aqueous solution II is 0.1-2.0 wt %; the concentration of acyl chloride monomers in the oil phase solution is 0.01-0.5 wt %; and the drying conditions in step S23 are: temperature 55-65° C., time 2-5 min.

[0028] Application of the above composite solvent-resistant nanofiltration membrane in the separation of organic solution systems.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) The present invention uses polydopamine to modify multi-walled carbon nanotubes. By utilizing the large number of active groups such as phenolic hydroxyl groups and amino groups contained in polydopamine, these functional groups are linked to the side walls of the multi-walled carbon nanotubes, thereby greatly improving the dispersibility of the multi-walled carbon nanotubes.

[0031] (2) The present invention proposes a method for preparing a composite solvent-resistant nanofiltration membrane by using polydopamine-coated multi-walled carbon nanotubes as an intermediate layer through unsupported interfacial polymerization. The polydopamine-coated multi-walled carbon nanotubes serve as an intermediate layer between the supporting base membrane and the polyamide film, which increases the surface roughness of the base membrane, improves the affinity and bonding performance between the base membrane and the selective separation layer, and provides a transmission channel for non-polar solvent molecules, thereby reducing surface defects of the composite membrane and improving the retention rate of the composite membrane.

[0032] (3) Compared with conventional nanofiltration membranes, the nanofiltration membrane prepared in this application significantly enhances the flux and retention performance of the membrane; among them, the ethanol flux is increased by more than 50%, the retention rate of Congo red in ethanol is increased from 92% to 97%, and the flux of non-polar solvent n-hexane is increased from 0 to ; After being immersed in various organic solvents such as ethanol, DMF, acetone and n-hexane for 14 days, there was no shedding or stratification, and the tolerance was good. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a SEM cross-sectional view of the nanofiltration membrane prepared in the present invention;

[0034] Figure 2 This is a SEM surface image of the nanofiltration membrane prepared in the present invention;

[0035] Figure 3 AFM image of the nanofiltration membrane prepared in Comparative Example 1;

[0036] Figure 4 AFM image of the nanofiltration membrane prepared in Comparative Example 2;

[0037] Figure 5 This is an AFM image of the nanofiltration membrane prepared in the present invention. DETAILED DESCRIPTION

[0038] The present invention is further described in detail below in conjunction with specific embodiments so that those skilled in the art can understand the present invention more clearly.

[0039] Sources and physical and chemical parameters of key test materials:

[0040] Multi-walled carbon nanotubes were purchased from Aladdin Reagents (Shanghai) Co., Ltd.

[0041] Polydopamine, polyimide and polyethylene glycol were purchased from Shanghai MacLean Biochemical Technology Co., Ltd.

[0042] Example 1

[0043] This embodiment provides a composite solvent-resistant nanofiltration membrane doped with modified carbon-based nanofillers, and the preparation method is as follows:

[0044] S1. Add multi-walled carbon nanotubes to ultrapure water at a volume-mass ratio of 1:0.1, stir magnetically at room temperature for 10 minutes, and then use 150 W ultrasonic treatment for 30 minutes to obtain a multi-walled carbon nanotube aqueous solution; add 0.1 M Tris buffer to the multi-walled carbon nanotube aqueous solution, and adjust the pH to 8.5 with dilute hydrochloric acid to obtain a multi-walled carbon nanotube buffer; add polydopamine to the multi-walled carbon nanotube buffer at a mass ratio of multi-walled carbon nanotubes to polydopamine of 1:2, stir at a speed of 2000 rpm for 12 hours under light-shielding conditions, filter through a microporous filter membrane with a pore size of 0.2 μm after the reaction, then rinse with ultrapure water until neutral, and dry at a temperature of 60°C for 3 minutes to obtain polydopamine-coated multi-walled carbon nanotubes.

[0045] S2. Dissolve polyimide and polyethylene glycol in N-methylpyrrolidone, heat in a water bath, stir and degas to obtain a casting solution; use a 200µm scraper to evenly apply the casting solution on a non-woven fabric at a speed of 0.78cm / s, let it stand for a period of time, immerse the membrane in deionized water for phase inversion to obtain a membrane, and then immerse the membrane in a 5wt% polyethylene glycol diamine aqueous solution, and replace the deionized water every other day. After two changes, store the base membrane in deionized water to obtain an ethylene glycol diamine cross-linked amino-containing polyimide base membrane.

[0046] The concentration of polyimide in the casting solution is 22 wt%, and the concentration of polyethylene glycol is 5 wt%.

[0047] S3. Add polydopamine-coated multi-walled carbon nanotubes and piperazine monomers (hexahydropyrazine) into ultrapure water, and treat with 150 W ultrasound for 2 hours to obtain aqueous solution I; then place the basement membrane in step S2 in aqueous solution I, let it stand for 2 minutes, take it out and dry it, so that the intermediate layer is deposited on the surface of the basement membrane.

[0048] The concentration of the polydopamine-coated multi-walled carbon nanotubes in the aqueous solution I is 0.02 wt %, and the concentration of the piperazine monomers is 0.75 wt %.

[0049] S4. Place the base membrane in step S3 in an aqueous solution II (hexahydropyrazine concentration is 0.75wt%) containing only piperazine monomers (hexahydropyrazine), and then slowly pour in an oil phase solution containing trimesoyl chloride (trimesoyl chloride concentration is 0.015wt%, solvent is n-hexane); allow the oil phase solution and the aqueous solution II to form a complete, defect-free polyamide layer at the junction. After reacting for 1 minute, open the bottom water valve to allow the polyamide layer to slowly fall on the middle layer. After taking it out, place it at 60°C and dry it for 3 minutes to obtain a composite solvent-resistant nanofiltration membrane doped with modified carbon-based nanofillers.

[0050] Example 2

[0051] This embodiment provides a composite solvent-resistant nanofiltration membrane doped with modified carbon-based nanofillers. The preparation method is basically the same as that of Example 1, except that: the concentration of multi-walled carbon nanotubes coated with polydopamine in the aqueous phase solution I of step S3 is 0.01wt%, and the rest remains unchanged.

[0052] Example 3

[0053] This embodiment provides a composite solvent-resistant nanofiltration membrane doped with modified carbon-based nanofillers. The preparation method is basically the same as that of Example 2, except that in step S3, the base membrane in step S2 is placed in aqueous solution I and allowed to stand for 1.5 minutes, while the rest remains unchanged.

[0054] Comparative Example 1

[0055] This comparative example provides a nanofiltration membrane, and its preparation method is basically the same as that of Example 1, except that the preparation of polydopamine-coated multi-walled carbon nanotubes in step S1 is omitted, and the aqueous phase solution I in step S3 contains only piperazine monomers (hexahydropyrazine), and the rest remain unchanged.

[0056] Comparative Example 2

[0057] This comparative example provides a nanofiltration membrane, and its preparation method is basically the same as that of Example 1, except that: the preparation of multi-walled carbon nanotubes coated with polydopamine in step S1 is omitted, and the aqueous phase solution I in step S3 is prepared using uncoated multi-walled carbon nanotubes and piperazine monomers (hexahydropyrazine), and the rest remains unchanged.

[0058] Comparative Example 3

[0059] This comparative example provides a nanofiltration membrane, and its preparation method is basically the same as that of Example 1, except that the preparation of the intermediate layer in step S3 is omitted, and the aqueous phase solution II in step S4 is replaced by the aqueous phase solution I, and the rest remains unchanged.

[0060] Comparative Example 4

[0061] This comparative example provides a nanofiltration membrane, and its preparation method is basically the same as that of Example 1, except that: the base membrane is prepared from commercial polysulfone with a molecular weight cutoff of 20,000 Da (purchased from Guochu Technology (Xiamen) Co., Ltd.), and the rest remains unchanged.

[0062] In order to further understand the filtration effects of the nanofiltration membranes prepared above in different organic solvents, the following tests were also conducted:

[0063] Separation performance under different solution systems:

[0064] The separation performance results at 25°C and a transmembrane pressure difference of 1.0 MPa are shown in Table 1. The solution system details are: 1000 mg / L sodium sulfate solution-water solution, 50 mg / L Congo red (negatively charged, Mw=600 Da)-dimethylformamide solution, 50 mg / L Congo red (negatively charged, Mw=600 Da)-ethanol solution.

[0065] Table 1 Separation performance of various nanofiltration membranes in different solution systems

[0066]

[0067] It can be seen from the above table:

[0068] The nanofiltration membrane prepared in Examples 1-3 has a retention rate of 96.2-99.5% for Na2SO4 in a sodium sulfate solution-water solution system, a retention rate of 90.6-96.3% for Congo red in a Congo red-ethanol solution system, a retention rate of 93.2-97.1% for Congo red in a Congo red-ethanol solution system, and a retention rate of 90.6-96.3% for Congo red in a Congo red-dimethylformamide solution system. The nanofiltration membranes prepared in Examples 1-3 have good retention rates for solutes in different systems. Among them, by comparison, it can be seen that the interception rate increases with the reduction of the multi-walled carbon nanotube dosage, and the interception rate decreases with the reduction of the multi-walled carbon nanotube intermediate layer deposition time; comparing the interception rate of Congo red in the Congo red-ethanol system with that in the Congo red-dimethylformamide system, the interception rate of Congo red in Example 1 in the ethanol and dimethylformamide solvent system is basically the same, and the interception rate of Example 2 and Example 3 decreases significantly, indicating that with the increase of the multi-walled carbon nanotube dosage and the multi-walled carbon nanotube intermediate layer deposition time, the weather resistance of the nanofiltration membrane in the dimethylformamide solvent system is improved. It shows that the multi-walled carbon nanotube content and deposition time jointly affect the interception and weather resistance of the nanofiltration membrane; when the dosage is 0.01wt% and the intermediate layer deposition time is 2min, the interception is the largest and the weather resistance is the best.

[0069] Permeation flux performance of different solvents:

[0070] The permeation flux of the membrane for different solvents was tested at 25°C and a transmembrane pressure difference of 1.0 MPa. The results are shown in Table 2.

[0071] Table 2 Permeation flux of each nanofiltration membrane for different solutions

[0072]

[0073] It can be seen from the above table:

[0074] The flux of the nanofiltration membrane prepared in Example 1-3 for ethanol is 79.3~96.6 L·h -1 m-2 , the flux of acetone is 49.92~58.1 L·h -1 m -2 , the flux of dimethylformamide is 146.39~170.23 L·h -1 m -2 , the flux for n-hexane is 39.6~46.87 L·h -1 m -2 The nanofiltration membranes prepared in Examples 1-3 have higher fluxes for solvents in different systems, and their overall permeation fluxes are significantly increased.

[0075] By comparing with Comparative Example 1, it was found that after adding multi-walled carbon nanotubes, the permeation flux of the composite membrane to the non-polar solvent n-hexane and the protic solvent dimethylformamide was significantly improved. This may be because the incorporation of multi-walled carbon nanotubes significantly improved the affinity of the membrane surface to n-hexane and dimethylformamide.

[0076] By comparing with Comparative Examples 2 and 3, it is found that removing the preparation of polydopamine-coated multi-walled carbon nanotubes in step S1, removing the preparation of the intermediate layer in step S3, or replacing the aqueous phase solution II with the aqueous phase solution I in step S4 all have a significant impact on the permeation flux of the prepared nanofiltration membrane.

[0077] By comparing with Comparative Example 4, it is found that the composite membrane using the polyimide-based membrane has better tolerance.

[0078] Furthermore, in order to understand the stability of each nanofiltration membrane, the following tests were also conducted:

[0079] Each nanofiltration membrane was immersed in ethanol, dimethylformamide, acetone and n-hexane solvent at 25°C for 14 days to observe whether delamination or peeling occurred. The results are shown in Table 3.

[0080] Table 3 The results of nanofiltration membranes soaked for 14 days under different conditions

[0081]

[0082] It can be seen from the above table:

[0083] The nanofiltration membrane prepared in this embodiment has good stability and remains intact after being immersed in different solvents for 14 days at room temperature (25°C), and its service life is guaranteed; while the nanofiltration membrane prepared using commercial polysulfone-based membrane in Comparative Example 4 shows partial stratification and peeling in dimethylformamide and acetone, which cannot meet the requirements for strong solvent operations.

[0084] From the above tests, it can be seen that the composite solvent-resistant nanofiltration membrane of doped modified carbon-based nanofillers prepared in this application shows good separation effect in different solution systems, has good permeation flux to different organic solvents, and has good tolerance in various organic solvents. It can meet the needs of various solution separation scenarios while maintaining a good service life.

[0085] At the same time, according to the AFM images of the composite films prepared in Comparative Example 1, Comparative Example 2 and Example 3, it can be seen that the average roughness of the thin film nanocomposite film doped with a multi-walled carbon nanotube intermediate layer is significantly improved, but the average roughness of the thin film nanocomposite film doped with a polydopamine-coated multi-walled carbon nanotube intermediate layer is significantly lower than that of Comparative Example 2, indicating that a smooth separation cortex is obtained after coating. At the same time, the water contact angle of the composite film prepared in Example 3 is also significantly reduced, indicating that it has good pollution resistance.

[0086] The specific raw materials not described in the present invention are all existing materials and can be directly purchased from the market.

[0087] The above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A composite solvent-resistant nanofiltration membrane doped with modified carbon-based nanofillers, characterized in that: The nanofiltration membrane comprises a support layer, an intermediate layer and a selective separation layer; The support layer is an amino-containing polyimide-based film cross-linked with ethylene glycol diamine; The intermediate layer is obtained by depositing multi-walled carbon nanotubes and piperazine monomers coated with polydopamine on the surface of the base film; The selective separation layer is a polyamide film obtained by polymerizing piperazine monomers and acyl chloride monomers on the surface of the intermediate layer; The preparation method of the composite solvent-resistant nanofiltration membrane comprises the following steps: S21, dissolving polyimide and polyethylene glycol in N-methylpyrrolidone to prepare a casting solution, then applying the casting solution on a non-woven fabric, performing a phase inversion process in deionized water to obtain a membrane, and then immersing the membrane in a polyethylene glycol diamine aqueous solution to obtain an amino-containing polyimide-based membrane cross-linked with polyethylene glycol diamine; S22, adding polydopamine-coated multi-walled carbon nanotubes and piperazine monomers to ultrapure water, and ultrasonicating to obtain aqueous solution I, and then placing the basement membrane in step S21 in aqueous solution I, letting it stand, taking it out and drying it, so as to deposit an intermediate layer on the surface of the basement membrane; S23, placing the intermediate layer obtained by deposition on the surface film of the base film in step S22 in an aqueous solution II containing only piperazine monomers, adding an oil phase solution containing acyl chloride monomers, performing unsupported interfacial polymerization to obtain a polyamide film, and finally opening the valve at the bottom of the container to discharge the liquid, allowing the polyamide film to fall on the intermediate layer, taking it out and drying it, thereby obtaining a composite solvent-resistant nanofiltration membrane doped with modified carbon-based nanofillers; The concentration of the polydopamine-coated multi-walled carbon nanotubes in the aqueous solution I of step S22 is 0.0025-0.1 wt %, and the concentration of the piperazine monomer is 0.1-2.0 wt %; the standing time in step S22 is 0.5-5 min; Application of the composite solvent-resistant nanofiltration membrane in the separation of an organic solution system.

2. The composite solvent-resistant nanofiltration membrane according to claim 1, characterized in that: The preparation method of the polydopamine-coated multi-walled carbon nanotubes is as follows: S11, adding multi-walled carbon nanotubes into ultrapure water, stirring and ultrasonicating to obtain a multi-walled carbon nanotube aqueous solution; S12, adding a buffer to the multi-walled carbon nanotube aqueous solution to adjust the pH to obtain a multi-walled carbon nanotube buffer; S13, adding polydopamine to the multi-walled carbon nanotube buffer, stirring for reaction, filtering through a microporous filter membrane after the reaction, rinsing, and drying to obtain the multi-walled carbon nanotubes coated with polydopamine.

3. The composite solvent-resistant nanofiltration membrane according to claim 2, characterized in that: In step S11, the mass ratio of ultrapure water to multi-walled carbon nanotubes is 1: (0.001-0.5); stirring conditions: 10000-20000 rpm, time 8-12 min; ultrasonic conditions: 80-200 W, time 25-35 min.

4. The composite solvent-resistant nanofiltration membrane according to claim 2, characterized in that: The buffer in step S12 is 0.01-0.1 M Tris buffer; the pH value is 8.0-9.

0.

5. The composite solvent-resistant nanofiltration membrane according to claim 2, characterized in that: In step S13, the mass ratio of multi-walled carbon nanotubes to polydopamine is 1: (1.5-2.5); The pore size of the microporous filter membrane is 0.2 μm; The stirring reaction conditions are: stirring at a speed of 1000-5000 rpm for 10-15 hours under light-shielding conditions; The drying conditions are as follows: temperature 55-65° C., time 2-5 min.

6. The composite solvent-resistant nanofiltration membrane according to claim 1, characterized in that: In step S21, the concentration of polyimide in the casting solution is 10-40 wt%, the concentration of polyethylene glycol is 0.5-15 wt%, and the concentration of polyethylene glycol diamine in the polyethylene glycol diamine aqueous solution is 3-7 wt%.

7. The composite solvent-resistant nanofiltration membrane according to claim 1, characterized in that: The concentration of piperazine monomers in the aqueous phase solution II of step S23 is 0.1-2.0wt%; the concentration of acyl chloride monomers in the oil phase solution is 0.01-0.5wt%; the drying conditions in step S23 are: temperature 55-65°C, time 2-5min.

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