Preparation method of acid-resistant nanofiltration membrane

Abstract

The application discloses a preparation method of acid-resistant nanofiltration membrane. The method comprises the following steps: immersing a PAN base membrane which has been soaked in an isopropyl alcohol solution and heated and soaked in a NaOH solution into an aqueous phase solution, pouring off the excess solution, and removing the residual solution on the surface; then pouring an oil phase solution on the surface of the PAN base membrane to perform an interfacial polymerization reaction; then placing the PAN base membrane in an oven to perform heat treatment, washing with deionized water, and then storing in deionized water at low temperature. The aqueous phase solution is obtained by dissolving a hydrophilic substance modified from cyanuric chloride in water, adding a polyamine-based polymer aqueous solution, and then mixing, stirring, heating and cooling. The acid-resistant nanofiltration membrane prepared by the method has a high rejection rate, and has excellent acid and alkali resistance and solvent stability.

Description

Technical Field

[0001] This invention relates to the field of membranes, and more particularly to a method for preparing an acid-resistant nanofiltration membrane. Background Technology

[0002] Nanofiltration membrane technology is widely used in wastewater treatment, water softening, food processing, and pharmaceutical industries, many of which involve the separation and purification of acidic solutions. Traditional commercial nanofiltration membrane materials are mainly polyamides, but polyamides are prone to hydrolysis in acidic environments, leading to a significant decline in membrane performance and limiting their application in the separation and purification of acidic wastewater. Therefore, acid-resistant nanofiltration membrane materials have emerged. Currently, commercially available acid-resistant membrane materials include polysulfonamides, sulfonated polymers, polyelectrolytes, and triazine ring polymers. Among them, triazine ring polymers are composed of carbon and nitrogen atoms linked by alternating single and double bonds, and the nitrogen atom has three pairs of uncoordinated electrons, thus exhibiting excellent acid resistance. Using monomers containing triazine rings (usually cyanuric chloride) instead of traditional acyl chloride monomers, and employing interfacial polymerization to prepare nanofiltration membranes, can remove the acid-sensitive C=O double bonds, endowing the membrane material with excellent acid resistance.

[0003] However, when using cyanuric chloride as the oil-phase monomer for membrane preparation, although the first chlorine atom on the cyanuric chloride has high reactivity, the electron density of the carbon atoms on the triazine ring increases after electron-donating monomers such as amines replace the first chlorine atom, thus reducing the reactivity of the remaining chlorine and making the substitution reaction of the remaining chlorine increasingly difficult. Therefore, nanofiltration membranes prepared by this method have poor retention capacity for divalent salts. Summary of the Invention

[0004] To address the problems of low reactivity and poor retention performance in the preparation of acid-resistant nanofiltration membranes using cyanuric chloride as the oil phase monomer, this invention provides a method for preparing an acid-resistant nanofiltration membrane, characterized by comprising the following steps:

[0005] Interfacial polymerization reaction: After immersing the treated PAN substrate in the aqueous solution, the excess solution is poured off and the residual solution on the surface is removed; then the oil phase solution is poured onto the surface of the PAN substrate to carry out the interfacial polymerization reaction.

[0006] The treated PAN substrate is prepared by immersing the PAN substrate in isopropanol solution, removing it, then placing it in NaOH solution and heating it to 85°C before removing it.

[0007] The aqueous solution is prepared by modifying cyanuric chloride into a hydrophilic substance, dissolving it in water, adding an aqueous solution of a polyamine polymer, mixing and stirring, heating and cooling to obtain the aqueous solution.

[0008] The oil phase solution is prepared by dissolving the acyl chloride monomer P in an organic solvent;

[0009] Post-processing: The PAN substrate film after the above reaction is placed in an oven for heat treatment. After treatment, the PAN substrate film is rinsed with deionized water to remove residual reactants and solvents, and then stored in deionized water at low temperature.

[0010] Furthermore, in the interfacial polymerization reaction, the aqueous phase solution C is an aqueous phase solution C diluted with water, with a dilution factor of 1-5 times and an immersion time of 2-10 min; the interfacial polymerization reaction time is 10-25 min.

[0011] Furthermore, the treated PAN substrate is prepared by immersing the PAN substrate in a 50-100wt% isopropanol solution for 30 minutes, then removing it and placing it in a NaOH solution at 70℃ and pH 11.0-12.0, followed by heating for 1-1.5 hours until the temperature reaches 85℃, which is then maintained at that temperature for at least 0.5 hours.

[0012] Furthermore, in the aqueous solution, the modification of cyanuric chloride into a hydrophilic substance is carried out by slowly adding the modified solution to the cyanuric chloride solution while continuously stirring to carry out the reaction. Preferably, the reaction conditions are: stirring rate of 30-40 rpm; reaction temperature of 0-6℃; reaction time of not less than 6 hours; and solution pH of 1.5-3.

[0013] After the reaction is complete, adjust the pH to 6.8-7.3, and filter the solution with acetone. Preferably, filter at least twice to remove unreacted substances. The white powder obtained after filtration is the hydrophilic substance.

[0014] The modified solution is prepared by dissolving compound II in an aqueous solution containing NaOH. Preferably, the molar ratio of compound II, NaOH and water is 916:19:16.

[0015] The cyanuric chloride solution is prepared by dissolving cyanuric chloride in an acetone solution containing water, wherein the mass fraction of cyanuric chloride is 17% and the volume ratio of acetone to water is 4:1.

[0016] Furthermore, the aqueous solution is prepared by dissolving a hydrophilic substance in deionized water, stirring for the first time, and then cooling to obtain solution A; dissolving a polyamine polymer in deionized water, stirring for the second time, and then cooling to obtain solution B; slowly adding solution B to solution A, mixing, stirring for the third time, heating, and cooling to room temperature to obtain aqueous solution C.

[0017] Preferably, the concentration of solution A is 5-20 wt%, the first stirring speed is 100-200 rpm, the time is 30-60 min, and the temperature is cooled to 5-20℃; the polyamine compound is one or more of PEI and PAH, with a molecular weight between 100-1000 kDa; the concentration of solution B is 0.5-3 wt%, the second stirring speed is 100-200 rpm, the time is 10-30 min, and the temperature is 5-15℃; the third stirring speed is 200-300 rpm, the time is 10-30 min, and the temperature is raised to 40-60℃.

[0018] Furthermore, the oil phase solution is wherein the acyl chloride monomer P is a compound containing two or more acyl chloride functional groups; preferably, it is one or more of pyromellitic trimethylolpropionate chloride, isophthaloyl chloride, and terephthaloyl chloride; the organic solvent is an alkane solvent; preferably, it is one or more of n-hexane, IsoPar, IsoL-C, and IsoL-G; and the concentration of the oil phase solution D is 0.05-0.5 wt%.

[0019] Furthermore, in the post-processing, the heat treatment temperature is 60-100℃ and the time is 2-20 min; the low-temperature storage temperature is 0-4℃.

[0020] The present invention also protects the acid-resistant nanofiltration membrane prepared by the method of preparing the acid-resistant nanofiltration membrane.

[0021] This invention first modifies cyanuric chloride, which is insoluble in water, into a hydrophilic substance through a chemical reaction. Then, the hydrophilic substance containing a triazine ring and polyamine groups are used as a pre-crosslinked polymer product as an aqueous phase, and interfacial polymerization is carried out with isobenzoic acid chloride, which has higher reactivity. This improves the retention rate while giving it excellent acid and alkali resistance and solvent resistance.

[0022] The S0-1 aqueous solution preparation step of this invention mainly aims to modify cyanuric chloride, which is originally insoluble in the aqueous phase, into a hydrophilic substance and introduce it into the aqueous phase, thus avoiding the disadvantage of poor membrane retention performance due to its low reactivity as an oil-phase monomer. Furthermore, the pre-crosslinking reaction further increases the molecular weight of the aqueous monomer and regulates the membrane pore structure, thereby ensuring simultaneous improvement in both permeate flux and retention performance.

[0023] The main purpose of S1 PAN substrate pretreatment is to hydrolyze the cyano groups in the PAN substrate into carboxyl groups. The substrate with abundant carboxyl groups has greatly enhanced hydrophilicity, which is conducive to the subsequent interfacial polymerization reaction. On the other hand, the presence of carboxyl groups allows the substrate and the aqueous solution to be connected through certain chemical reactions, which is beneficial to improving the stability of the membrane.

[0024] Controlling the monomer concentration and time in the S2 IP reaction is a crucial step in forming the acid-resistant nanofiltration membrane desalination layer. By controlling the concentration of monomers in the aqueous and oil phases and the reaction time, the degree of crosslinking of the desalination layer can be adjusted, thereby obtaining an acid-resistant nanofiltration membrane with high flux and good desalination performance.

[0025] The control of temperature and time in the S3 post-treatment process is a crucial step in determining the final membrane performance. By controlling the temperature and time in the post-treatment process, the solvent can be fully evaporated and the desalination layer formed on the membrane surface can be further cross-linked, thereby obtaining an acid-resistant nanofiltration membrane with high flux and good desalination performance. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the acid resistance stability test data of the acid-resistant nanofiltration membrane of the present invention.

[0027] Figure 2 This is a schematic diagram of the solvent resistance stability test data of the acid-resistant nanofiltration membrane of the present invention. Detailed Implementation

[0028] The embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0029] Preparation of an acid-resistant nanofiltration membrane:

[0030] Preparation of S0-1 aqueous solution

[0031] a. Hydrophilic modification of cyanuric chloride: Cyanuric chloride is reacted with a modification solution under certain conditions to obtain reaction product M. The reaction principle is as follows:

[0032]

[0033] Among the R1-R6 groups of compound II, at least one —NH2 is present, and at least one hydrophilic functional group is present, which may be one or more of —COOH, —OH, —SO3H, and —NH2.

[0034] The specific reaction steps are as follows: Slowly add the modified solution to the cyanuric chloride solution while continuously stirring at a speed of 30-40 rpm. Control the reaction temperature at 0-6℃ and the reaction time at least 6 hours (the reaction is considered complete after stirring at 40 rpm for at least 30 minutes after all the modified solution has been added). Control the pH of the solution at 1.5-3 (the pH can be adjusted by adding NaOH).

[0035] After the above reaction is completed, adjust the pH to 6.8-7.3 with NaOH. Filter the solution with acetone at least twice to remove unreacted substances. After filtration, a white powder M is obtained.

[0036] The modified solution is prepared by dissolving compound II in an aqueous solution containing NaOH, wherein the molar ratio of compound II, NaOH and water is 916:19:16.

[0037] Preparation of cyanuric chloride solution: Dissolve cyanuric chloride in an acetone solution containing water, wherein the mass fraction of cyanuric chloride is 17% and the volume ratio of acetone to water is 4:1.

[0038] b. Pre-crosslinking reaction: Dissolve the above reaction product M in deionized water, stir for the first time, and then cool to obtain solution A; dissolve the polyamine polymer N in deionized water, stir for the second time, and then cool to obtain solution B; slowly add solution B to solution A, mix, stir for the third time, and heat. After cooling to room temperature, obtain aqueous solution C. The reaction principle is as follows:

[0039]

[0040] Solution A has a concentration of 5-20 wt%, the first stirring speed is 100-200 rpm for 30-60 min, and the temperature is lowered to 5-20℃; the polyamine N can be one or more of PEI and PAH, with a molecular weight between 100-1000 kDa; solution B has a concentration of 0.5-3 wt%, the second stirring speed is 100-200 rpm for 10-30 min, and the temperature is 5-15℃; the third stirring speed is 200-300 rpm for 10-30 min, and the temperature is raised to 40-60℃.

[0041] Preparation of S0-2 oil phase solution

[0042] An oil phase solution D is prepared by dissolving an acyl chloride monomer P in an organic solvent. The acyl chloride monomer P is typically a compound containing two or more acyl chloride functional groups, and can be one or more of pyromellitic trimethylolpropionate (PMT), isophthaloyl chloride, and terephthaloyl chloride. The organic solvent is typically an alkane solvent, and can be one or more of n-hexane, IsoPar, IsoL-C, and IsoL-G. The concentration of the oil phase solution D is 0.05-0.5 wt%.

[0043] S1 PAN bottom film pretreatment

[0044] a. Prepare an isopropanol solution with a concentration of 50-100 wt%, and immerse the PAN substrate in the solution for 30 minutes;

[0045] b. Prepare a NaOH solution with a pH of 11.0-12.0, heat it to 70°C, place the PAN substrate that has been soaked in isopropanol into the solution and continue heating for 1-1.5 hours until the temperature reaches 85°C, then maintain this temperature for at least 0.5 hours.

[0046] The reaction principle is

[0047] R-CN + NaOH + H₂O → RCONa + NH₃

[0048] R-CN represents organic compounds containing cyano functional groups.

[0049] S2 Interface Polymerization (IP) Reaction

[0050] The PAN substrate film treated in S1 is immersed in diluted aqueous solution C. After a certain period of time, excess solution is poured off and residual solution on the surface is removed. Then, the oil phase solution D prepared in S0-2 is poured onto the film surface for interfacial polymerization. The dilution factor of aqueous solution C is 1-5 times, and the immersion time is 2-10 min. Oil phase solution D is prepared in step S0-2 and does not require further dilution. The interfacial polymerization reaction time is 10-25 min.

[0051] S3 Post-processing

[0052] After the S2 reaction, the membrane was placed in an oven for heat treatment. After treatment, the membrane was rinsed with deionized water to remove residual reactants and solvents, and then stored in deionized water at low temperature. The heat treatment temperature was 60-100℃, and the heat treatment time was 2-20 min; the membrane storage temperature was 0-4℃.

[0053] Example 1: Preparation of acid-resistant nanofiltration membrane

[0054] The preparation process is described above. The reaction parameters for steps S2 and S3 are shown in Table 1.

[0055] Effect verification: 2000ppm magnesium sulfate was used as the standard test solution.

[0056] Test conditions: Flux – 25℃, pure water pH=7, flow rate 10.5 L / min; 7.5 bar

[0057] Retention – 25℃, 2000ppm MgSO4 solution, pH=7, flow rate 10.5L / min; 7.5bar.

[0058] Retention rate (%) = (1-Cp / Cm)*100%, where Cp is the permeation conductivity and Cm is the raw material conductivity.

[0059] The results are shown in Table 1.

[0060] Examples 2-23: Preparation of acid-resistant nanofiltration membranes

[0061] The preparation process is described above. The reaction parameters for steps S2 and S3 are shown in Table 1.

[0062] Effect verification: 2000ppm magnesium sulfate was used as the standard test solution.

[0063] Test conditions: Same as in Example 1. Results are shown in Table 1.

[0064] Table 1. Reaction parameters and effect data of Examples 1-23

[0065]

[0066] Note: The aqueous solution C was diluted during the S2 reaction. 100% in the table means no dilution, 50% means dilution by half, and so on.

[0067] The membrane prepared in Example 19 was immersed in a 20wt% H2SO4 solution at 70°C for 30 days (720 h) to test its acid resistance. The test results are as follows: Figure 1 As shown, the membrane flux is slightly reduced, but the rejection rate remains stably above 96%. This result is roughly equivalent to stability in a 20 wt% H2SO4 solution at 20°C for 960 days (32040 h). In contrast, the comparative document CN202110880867.0 describes membrane immersion in a 5% H2SO4 solution at 25°C for 30 days. Both the sulfuric acid concentration and immersion temperature are significantly lower than those of this invention, yet the rejection rate decreases more significantly. This demonstrates that the membrane prepared in this invention possesses exceptionally excellent acid resistance.

[0068] The membrane prepared in Example 15 was immersed in a 99wt% NMP solution at 70°C for 123 days to test its solvent resistance stability. The test results are as follows: Figure 2As shown, the membrane maintains relatively stable flux and retention performance under these conditions for 53 days, demonstrating its excellent solvent resistance. After 123 days, although the membrane performance changes significantly, the retention rate for divalent salts remains above 90%.

[0069] Retention performance comparison: The comparative document CN201310026899.X mentions using a method of gradually increasing the treatment temperature to activate the functional groups in the reactant monomers. The S1 temperature is 50-90℃, the S2 temperature is 100-150℃, and the treatment time is 0.5-10 hours. This method requires high overall temperature and long treatment times. The retention performance of 1 g / L MgSO4 is not less than 90%. In contrast, the present invention can be carried out at room temperature during the interfacial polymerization reaction, requiring only a post-treatment step to solidify the cross-linked structure after the reaction. Furthermore, the membrane prepared by this invention achieves a MgSO4 retention rate of over 90%.

[0070] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention without departing from the principles and spirit of the present invention.

Claims

1. A method for preparing an acid-resistant nanofiltration membrane, characterized in that, Includes the following steps, Interfacial polymerization reaction: After immersing the treated PAN substrate in the aqueous solution, the excess solution is poured off and the residual solution on the surface is removed; then the oil phase solution is poured onto the surface of the PAN substrate to carry out the interfacial polymerization reaction. The treated PAN substrate is prepared by immersing the PAN substrate in isopropanol solution, removing it, then placing it in NaOH solution and heating it to 85°C before removing it. The aqueous solution is prepared by dissolving cyanuric chloride in an acetone solution containing water to obtain a cyanuric chloride solution. The modified solution is slowly added to the cyanuric chloride solution and stirred continuously to carry out the reaction. After the reaction is completed, the pH is adjusted to 6.8-7.

3. The solution after the reaction is completed is filtered with acetone to remove unreacted substances. After filtration, a white powder is obtained, which is the cyanuric chloride modified hydrophilic substance. The obtained cyanuric chloride modified hydrophilic substance is dissolved in water, and then an aqueous solution of polyamine polymer is added. The mixture is stirred, heated and cooled to obtain an aqueous solution. The modified solution is prepared by dissolving compound II in an aqueous solution containing NaOH. Compound II has the structure shown in formula (1). The polyamine polymer is one or more of PEI and PAH. Equation (1), In formula (1), at least one of R1, R2, R3, R4, R5 and R6 is -NH2 and at least one is a hydrophilic functional group; The oil phase solution is prepared by dissolving the acyl chloride monomer P in an organic solvent; Post-processing: The PAN substrate film after the above reaction is placed in an oven for heat treatment. After treatment, the PAN substrate film is rinsed with deionized water to remove residual reactants and solvents, and then stored in deionized water at low temperature.

2. The method for preparing the acid-resistant nanofiltration membrane as described in claim 1, characterized in that, In the interfacial polymerization reaction, the immersion time of the treated PAN substrate in the aqueous solution is 2-10 min; the interfacial polymerization reaction time is 10-25 min.

3. The method for preparing the acid-resistant nanofiltration membrane as described in claim 1, characterized in that, The treated PAN substrate is prepared by immersing the PAN substrate in a 50-100wt% isopropanol solution for 30 minutes, then removing it and placing it in a NaOH solution at 70℃ and pH 11.0-12.

0. The substrate is then heated for 1-1.5 hours until it reaches 85℃, and then maintained at that temperature for at least 0.5 hours.

4. The method for preparing the acid-resistant nanofiltration membrane as described in claim 1, characterized in that, In the preparation of the cyanuric chloride-modified hydrophilic substance, the reaction conditions are as follows: stirring speed 30-40 rpm; reaction temperature controlled at 0-6℃; reaction time not less than 6 hours; and solution pH controlled at 1.5-3. After the reaction is complete, adjust the pH to 6.8-7.3, and filter the solution with acetone to remove unreacted substances. The white powder obtained after filtration is the hydrophilic substance. The molar ratio of compound II, NaOH, and water in the modified solution is 916:19:16; The cyanuric chloride solution is prepared by dissolving cyanuric chloride in an acetone solution containing water, wherein the mass fraction of cyanuric chloride is 17% and the volume ratio of acetone to water is 4:

1.

5. The method for preparing the acid-resistant nanofiltration membrane as described in claim 1, characterized in that, The aqueous solution is aqueous solution C, which is prepared by the following method: dissolving a hydrophilic substance in deionized water, stirring for the first time, and then cooling to obtain solution A; The polyamine polymer was dissolved in deionized water, stirred a second time, and then cooled to obtain solution B. Solution B was slowly added to solution A and mixed. The mixture was stirred a third time and heated. After cooling to room temperature, aqueous solution C was obtained.

6. The method for preparing the acid-resistant nanofiltration membrane as described in claim 5, characterized in that, Solution A has a concentration of 5-20 wt%. The first stirring is performed at a speed of 100-200 rpm for 30-60 min, followed by cooling to 5-20℃. The molecular weight of the polyamine compound is between 100-1000 kDa. Solution B has a concentration of 0.5-3 wt%. The second stirring is performed at a speed of 100-200 rpm for 10-30 min, followed by cooling to 5-15℃. The third stirring is performed at a speed of 200-300 rpm for 10-30 min, followed by heating to 40-60℃.

7. The method for preparing the acid-resistant nanofiltration membrane as described in claim 1, characterized in that, The acyl chloride monomer P is a compound containing two or more acyl chloride functional groups.

8. The method for preparing the acid-resistant nanofiltration membrane as described in claim 1, characterized in that, The acyl chloride monomer P is one or more of pyromellitic methyl methacrylate (PMMA), isophthaloyl chloride (IMMA), and terephthaloyl chloride (TBA).

9. The method for preparing the acid-resistant nanofiltration membrane as described in claim 1, characterized in that, The organic solvent is an alkane solvent.

10. The method for preparing the acid-resistant nanofiltration membrane as described in claim 1, characterized in that, The organic solvent is one or more of n-hexane, IsoPar, IsoL-C, and IsoL-G.

11. The method for preparing the acid-resistant nanofiltration membrane as described in claim 1, characterized in that, The concentration of the oil phase solution D is 0.05-0.5 wt%.

12. The method for preparing the acid-resistant nanofiltration membrane as described in claim 1, characterized in that, In the post-processing, the heat treatment temperature is 60-100℃ and the time is 2-20 min; the low-temperature storage temperature is 0-4℃.

13. The acid-resistant nanofiltration membrane prepared by the method of any one of claims 1-12.

Images