Low nucleophilic organic base assisted compact polymeric nanofiltration membranes and methods of making the same

Abstract

Low nucleophilic organic base assisted dense polyester nanofiltration membrane and preparation method thereof belong to the field of water treatment. The present application is aimed at the problems of low reactivity of hydroxyl monomers, difficulty in efficient polymerization with acyl chloride monomers, and easy initiation of ester bond hydrolysis by traditional inorganic strong base catalysts, which leads to the decline of membrane structure stability in the preparation process of existing polyester nanofiltration membrane. The present application adopts strong alkaline low nucleophilic organic base to regulate the activation process of hydroxyl monomers. The specific preparation method is as follows: dissolving polyol monomers in an aprotic solvent to prepare an aqueous solution, and adding 1,8-diazabicyclo[5.4.0]undec-7-ene to regulate the deprotonation process of hydroxyl groups, thereby improving the reactivity of hydroxyl groups with 1,3,5-benzene triformyl chloride; dissolving TMC in n-hexane to form an organic phase solution; sequentially immersing the polyethersulfone-based membrane in the aqueous solution and the organic phase solution, so that the acyl chloride groups of the polyol hydroxyl groups and TMC occur interface polymerization reaction at the oil-water interface to form a dense polyester nanofiltration membrane.

Description

Technical Field

[0001] This invention belongs to the field of water treatment, specifically, it relates to a dense polyester nanofiltration membrane assisted by a low nucleophilic organic base and its preparation method; the polyester nanofiltration membrane is used for salt retention. Background Technology

[0002] Nanofiltration membrane separation technology has been widely used in drinking water purification, industrial wastewater treatment, resource recovery, and the separation of high-value-added substances due to its advantages of low operating pressure, high solute selectivity, and low energy consumption. Currently, commercial nanofiltration membranes are mainly made of polyamide materials, typically prepared through interfacial polymerization between aqueous amine monomers and organic acyl chloride monomers. However, the amide bonds in the polyamide membrane structure are susceptible to oxidative attack by free chlorine, leading to membrane structure damage and decreased separation performance, thus limiting its long-term application in chlorinated water treatment environments.

[0003] Compared to polyamide materials, polyester membrane materials possess higher chemical stability and excellent resistance to chlorine oxidation. The ester bonds in their molecular structure are not easily damaged by chlorine oxidation, making them an important candidate material for constructing highly chlorine-resistant nanofiltration membranes. However, traditional polyester nanofiltration membranes are typically prepared by interfacial polymerization of hydroxyl-containing monomers and acyl chloride monomers. Because the nucleophilic reactivity of hydroxyl functional groups is significantly lower than that of amine functional groups, they are difficult to directly attack acyl chloride groups. This results in a slow polymerization rate between hydroxyl monomers and 1,3,5-benzenetricarboxylic acid chloride (TMC), making it difficult to form a dense polyester selective layer with good separation properties. Therefore, improving the reactivity of hydroxyl monomers is key to achieving high-performance polyester nanofiltration membranes.

[0004] To address the aforementioned issues, existing technologies typically involve adding an alkaline catalyst to the aqueous solution to promote the deprotonation of hydroxyl monomers, generating highly nucleophilic alkoxide anions. This enhances the nucleophilic reaction between hydroxyl and acyl chloride groups, accelerating the interfacial polymerization process. Inorganic strong bases (such as NaOH) can completely ionize to generate hydroxide ions, thereby promoting the deprotonation of hydroxyl monomers through ionization equilibrium. However, during interfacial polymerization, while the inorganic strong base catalyst achieves hydroxide ionization, excess or residual highly nucleophilic hydroxide ions may also nucleophilically attack the already formed ester bonds, initiating ester bond hydrolysis reactions. This leads to the destruction of the polyester network structure and a decrease in film stability.

[0005] To mitigate the adverse effects of strong base catalysis on polyester structures, employing organic base catalysts with high basicity and low nucleophilicity is a potential solution. Organic bases with significant steric hindrance can effectively capture protons to promote hydroxyl activation. Simultaneously, due to the steric limitation of their nucleophilic attack ability, they can reduce side reactions to already formed ester bonds, thus balancing the activation efficiency of hydroxyl monomer reactions and the stability of the polyester membrane structure. In the field of organic synthesis, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), as a strong basic and low nucleophilic organic base, has been widely used in the deprotonation activation process of alcohols. In existing technologies, DBU abstracts protons from the alcohol hydroxyl group through its sp²-hybridized imine nitrogen atom to generate the corresponding alkoxide anion. This process utilizes the resonant delocalization effect of the positive charge in its conjugate acid cation between the two nitrogen atoms, thus possessing high thermodynamic driving force. Simultaneously, the low nucleophilicity of DBU can effectively reduce the risk of non-specific side reactions to electrophilic groups such as ester groups in the system.

[0006] Therefore, developing a method for preparing dense polyester nanofiltration membranes based on the regulation of interfacial polymerization by a strongly alkaline, low-nucleophilic organic base is of great significance for constructing green nanofiltration membrane materials with high chlorine resistance and high stability, and for achieving a balance between reaction activation and structural protection. Summary of the Invention

[0007] This invention addresses the problems of low reactivity of hydroxyl monomers and difficulty in efficient polymerization with acyl chloride monomers in the preparation of existing polyester nanofiltration membranes, as well as the tendency of traditional inorganic strong base catalysts to induce ester bond hydrolysis, leading to a decrease in membrane structural stability. It provides a dense polyester nanofiltration membrane assisted by a low nucleophilic organic base and its preparation method.

[0008] To address the aforementioned technical problems, the present invention adopts the following technical solution: The purpose of this invention is to provide a method for preparing a dense polyester nanofiltration membrane assisted by a low nucleophilic organic base, comprising the following steps: Step 1: Wash the polyethersulfone-based membrane with ultrapure water, air dry it at room temperature, and then fix it in the mold; Step 2: Dissolve the polyol monomer in 1-ethyl-3-methylimidazolium chloride ionic liquid to obtain an ionic solution, and then adjust the pH value to above 12 with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to obtain an aqueous solution; Step 3: Dissolve 1,3,5-benzenetricarboxylic acid chloride (TMC) in n-hexane to obtain an organic phase solution; Step 4: Immerse the base membrane treated in Step 1 into an aqueous solution, remove the surface liquid and air dry at room temperature, then immerse it in an organic solution and carry out an interfacial polymerization reaction at room temperature and pressure. After the reaction is complete, wash it with n-hexane and ultrapure water in sequence; thermal crosslinking is performed to obtain the polyester nanofiltration membrane.

[0009] Further specifying, in step 1, the molecular weight cutoff of the polyethersulfone-based membrane is 150 kDa-200 kDa.

[0010] Further specifying, in step 2, the polyol monomers are erythritol, 1,2,4-butanetriol, or 1,4-butanediol.

[0011] Further specifying, in step 2, the mass fraction of the aqueous solution is 6.5%-8.0%.

[0012] To be further specified, in step 2, the mass fraction of DBU is 0.05% to 0.2%.

[0013] Further specifying, in step 3, the mass fraction of the organic phase solution is 0.05%-0.15%.

[0014] Further specifying, in step 4, the immersion time in the aqueous solution is 2 min-3 min.

[0015] Further specifying, in step 4, the interfacial polymerization reaction time is at least 2 min-40 min.

[0016] Further specifying, in step 4, thermal crosslinking is performed at 50℃-65℃.

[0017] The polyester nanofiltration membrane prepared according to the present invention is stored in ultrapure water.

[0018] Another object of the present invention is to provide a polyester nanofiltration membrane prepared by any of the above methods.

[0019] Compared with the prior art, the present invention has the following beneficial effects: This invention selects 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as an aqueous alkaline catalyst to promote the deprotonation process of the hydroxyl groups of polyols, improve their nucleophilic reactivity, and enable them to undergo interfacial polymerization with 1,3,5-benzenetricarboxyl chloride (TMC) under mild conditions (room temperature and pressure), thereby realizing the preparation of polyester nanofiltration membranes based on polyhydroxy monomers.

[0020] This invention utilizes DBU, a sterically hindered organic base with high basicity and low nucleophilicity, which can promote the activation of hydroxyl monomers through proton transfer. Simultaneously, due to the significant steric hindrance of DBU, its own nucleophilic attack ability is limited, reducing the side reactions and hydrolytic damage to already formed ester bonds caused by residual basic catalysts. This helps maintain the integrity of the polyester crosslinking network structure and improves the stability of the membrane structure.

[0021] This invention uses the aprotic solvent 1-ethyl-3-methylimidazolium chloride ionic liquid as the aqueous phase solvent to avoid the formation of nucleophilic hydroxyl ions between DBU and water, which would damage the ester bond.

[0022] The raw materials used in this invention are widely available and readily available. The preparation process does not require complex equipment, the process flow is simple, and the reaction conditions are mild. It has the advantages of convenient operation and easy scale-up production, which is conducive to the large-scale preparation of polyester nanofiltration membranes.

[0023] For a deeper understanding of the features and technical content of this invention, please refer to the accompanying detailed description and drawings. It should be noted that the drawings are provided for illustrative purposes only and are not intended to limit the scope of the invention. Attached Figure Description

[0024] Figure 1 The images show scanning electron microscope (SEM) images of the surface morphology of the original polyethersulfone (PES) membrane and polyester nanofiltration membranes prepared using polyol monomers such as erythritol, 1,2,4-butanetriol, or 1,4-butanediol as aqueous monomers, respectively. Image a is an SEM image of the morphology of the PES membrane; image b is an SEM image of the surface morphology of the polyester nanofiltration membrane prepared using erythritol as an aqueous monomer; image c is an SEM image of the surface morphology of the polyester nanofiltration membrane prepared using 1,2,4-butanetriol as an aqueous monomer; and image d is an SEM image of the surface morphology of the polyester nanofiltration membrane prepared using 1,4-butanediol as an aqueous monomer. Figure 2 X-ray photoelectron spectroscopy (XPS) results for the original polyethersulfone-based membrane and polyester nanofiltration membranes prepared using polyol monomers such as erythritol, 1,2,4-butanetriol, or 1,4-butanediol as aqueous monomers, respectively. Figure 3 The molecular weight cutoff values ​​are the molecular weight cutoff values ​​of polyester films prepared using erythritol as a monomer at different DBU concentrations.

[0025] Figure 4 The water permeability of polyester membranes prepared using erythritol as a monomer at different DBU concentrations is given.

[0026] Figure 5 The salt rejection rate of polyester films prepared using erythritol as a monomer at different DBU concentrations is given. Detailed Implementation

[0027] The present invention will be described in detail below with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but should not be considered as limiting the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0028] Example 1: This embodiment provides a method for preparing a dense polyester nanofiltration membrane controlled by a strongly basic, low-nucleophilic organic base. The steps are as follows: S1. Base film preparation: The polyethersulfone base film is washed three times with ultrapure water, dried at room temperature, and then fixed in a 12 cm × 12 cm plastic mold. S2. Preparation of aqueous solution: Erythritol is dissolved in 1-ethyl-3-methylimidazolium chloride ionic liquid to prepare a solution with a mass fraction of 2.5%-7.5%. Then, the pH of the solution is adjusted by using DBU with a mass fraction of 0.05%-0.2%, that is, the DBU is mixed with the solution in equal volume and stirred at room temperature for 1 min to obtain an aqueous solution. S3. Preparation of organic phase solution: Dissolve TMC in n-hexane to prepare an organic phase solution with a mass fraction of 0.1%; S4. Interfacial polymerization reaction: The base membrane was immersed in 20 mL of the above aqueous solution for 2 min, the surface liquid was removed and then dried at room temperature; the base membrane was then immersed in 20 mL of the above organic solution for 40 min to carry out the interfacial polymerization reaction; the polyester nanofiltration membrane was obtained after washing three times with n-hexane and ultrapure water respectively. S5. Post-processing: Heat the polyester film prepared above in an oven at 60°C for 2 minutes to perform thermal cross-linking.

[0029] The surface morphology of the material was characterized using scanning electron microscopy (SEM), such as... Figure 1 As shown, the pores on the surface of the polyethersulfone base film are completely covered, indicating that the polyester separation layer has been successfully formed on the surface of the base film.

[0030] The elemental composition of the material was determined using X-ray photoelectron spectroscopy (XPS), such as... Figure 2 As shown, after the interfacial polymerization reaction, the intensity of the sulfur characteristic peak on the surface of the polyethersulfone base film decreased significantly, while the oxygen content increased significantly, indicating that oxygen-containing functional groups have been introduced into the base film surface, thus confirming that the polyester separation layer has been successfully synthesized.

[0031] The following experiments were used to verify the effectiveness of the invention. Experiment 1: Effect of DBU concentration on the pore structure of the polyester separator layer. The molecular weight cutoff (MWCO) test included the following steps: using 100 mg / L glycerol (92 Da), glucose (180 Da), sucrose (342 Da), raffinose (504 Da), and polyethylene glycol (800 Da) as feed water, the membrane's rejection rate (R, %) for different molecular weight substances was tested. Specifically, the total organic carbon (TOC) concentrations in the influent and effluent were determined using a total organic carbon (TOC) analyzer to determine the rejection rate, molecular weight cutoff (MWCO), and pore size.

[0032] like Figure 3As shown, with increasing DBU concentration, the molecular weight cutoff of the prepared polyester membrane first decreased from 731.5 Da to about 330 Da and remained stable. Within a suitable concentration range, DBU activates hydroxyl groups through direct proton exchange, thereby increasing its reactivity and successfully reacting with TMC to form a dense polyester separation layer, gradually reducing the membrane pore size.

[0033] Experiment 2: Effect of DBU concentration on water flux of polyester separation layer; The water flux test includes the following steps, and the experiment uses an effective area of ​​27.8 cm². 2 The performance of the nanofiltration membrane was tested using a filtration device. Before testing, the membrane was pre-pressurized with 5 bar of pure water for 3 hours to reach a stable state. Then, the pure water permeability was tested at a pressure of 4 bar. The water permeability is expressed as: J = V / (A·Δt·ΔP), where V is the volume of permeated water, A is the effective area, Δt is the time interval, and ΔP is the applied pressure.

[0034] like Figure 4 As shown, the water flux of the prepared polyester membrane first decreases and then stabilizes with increasing DBU concentration. Specifically, the water flux decreases from 23.7 L·m⁻¹ at 0.05 wt% DBU. -2 ·h -1 ·bar -1 17.1 L·m when reduced to 0.15 wt%. -2 ·h -1 ·bar -1 The results indicate that an appropriate amount of DBU can effectively promote the deprotonation of erythritol hydroxyl groups, increase the interfacial polymerization rate between erythritol and TMC, and allow more hydroxyl groups to participate in the construction of the polyester network, thereby forming a dense separation layer with a higher degree of crosslinking and smaller pore size, increasing transport resistance and thus reducing water flux. Subsequently, as the reaction reaches the self-limiting stage, the middle membrane structure is relatively dense and changes little, so the resistance to solvent transport also changes little, and the water flux stabilizes.

[0035] Experiment 3: Effect of DBU concentration on salt rejection rate of polyester separation layer; The salt rejection test includes the following steps: The salt rejection performance of polyester nanofiltration membranes prepared under different DBU concentrations was tested using different salt solutions (sodium sulfate (Na₂SO₄), magnesium sulfate (MgSO₄), and sodium chloride (NaCl)) at a concentration of 1000 ppm. The salt rejection rate was calculated as follows: R = (1 - C p / C f )×100%, where C f and C p These are the conductivity values ​​of the permeate solution and the feed solution, respectively, measured by a conductivity meter.

[0036] With increasing DBU concentration, the salt rejection rate of the polyester nanofiltration membrane showed a trend of first increasing and then stabilizing. Specifically, when the DBU concentration increased from 0.05 wt% to 0.15 wt%, the Na2SO4 rejection rate increased from 81.7% to 92.4%, indicating that an appropriate amount of DBU can promote the deprotonation of hydroxyl monomers, enhance their interfacial polymerization reaction with TMC, improve the crosslinking degree of the polyester separation layer, and thus enhance the membrane's ion sieving capacity. When the DBU concentration further increased to 0.2 wt%, the Na2SO4 rejection rate reached 93.5%.

[0037] Furthermore, the salt rejection rate follows the order Na₂SO₄ > MgSO₄ > NaCl, which is mainly attributed to the surface charge repulsion and ion hydration effect of the polyester separation layer. Divalent sulfate ions are subject to stronger electrostatic repulsion, while NaCl, due to its lower valence state and smaller hydrated size, is more likely to permeate through the polyester separation layer.

[0038] DBU concentration has a significant impact on the salt retention performance of polyester nanofiltration membranes. An appropriate amount of DBU can abstract hydrogen from alcohol hydroxyl groups, thereby deprotonating the hydroxyl groups and enhancing their reactivity with TMC, thus forming a dense polyester layer.

[0039] Example 2: This example differs from Example 1 in that 1,2,4-butanetriol is used instead of erythritol. The remaining steps are the same as in Example 1.

[0040] The scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) results of the polyester nanofiltration membrane obtained in this embodiment are highly consistent with those of Example 1, both showing the characteristics of a continuous and complete polyester separation layer. This result fully demonstrates that DBU is also applicable to the interfacial polymerization system of 1,2,4-butanetriol and TMC, and can effectively promote the polymerization reaction between the two to form a dense polyester layer.

[0041] Example 3: This example differs from Example 1 in that 1,4-butanediol is used. The remaining steps are the same as in Example 1.

[0042] The polyester nanofiltration membrane prepared in this embodiment forms a continuous and complete polyester separation layer. Its scanning electron microscope (SEM) morphology and X-ray photoelectron spectroscopy (XPS) chemical composition analysis results are basically consistent with those of Example 1, indicating that DBU is also suitable for the interfacial polymerization system of 1,4-butanediol and TMC, and can effectively promote the polymerization reaction between the two to form a dense polyester layer.

[0043] The specific embodiments of the present invention have been described in detail above. It should be noted that the present invention is not limited to the specific embodiments described above. Various modifications or alterations can be made by those skilled in the art without departing from the scope of protection defined by the claims, and all such modifications or alterations fall within the scope of the present invention.

Claims

1. A method for preparing a dense polyester nanofiltration membrane assisted by a low nucleophilic organic base, characterized in that, Includes the following steps: Step 1: Wash the polyethersulfone-based membrane with ultrapure water, air dry it at room temperature, and then fix it in the mold; Step 2: Dissolve the polyol monomer in 1-ethyl-3-methylimidazolium chloride ionic liquid to obtain an ionic solution, and then adjust the pH value to above 12 with 1,8-diazabicyclo[5.4.0]undec-7-ene to obtain an aqueous solution; Step 3: Dissolve 1,3,5-benzenetricarboxyl chloride in n-hexane to obtain an organic phase solution; Step 4: Immerse the base film treated in Step 1 into an aqueous solution to remove surface liquid and air dry at room temperature. Then immerse it in an organic solution and carry out interfacial polymerization reaction at room temperature and pressure. After the reaction is complete, wash it with n-hexane and ultrapure water in sequence. Thermal crosslinking yields the polyester nanofiltration membrane.

2. The method according to claim 1, characterized in that, The molecular weight cutoff of polyethersulfone-based membranes is 150 kDa-200 kDa.

3. The method according to claim 1, characterized in that, The polyol monomers are erythritol, 1,2,4-butanetriol, or 1,4-butanediol.

4. The method according to claim 1, characterized in that, The mass fraction of the aqueous solution is 6.5%-8.0%.

5. The method according to claim 1, characterized in that, The mass fraction of 1,8-diazabicyclo[5.4.0]undec-7-ene is 0.05%~0.2%.

6. The method according to claim 1, characterized in that, The mass fraction of the organic phase solution is 0.05%-0.15%.

7. The method according to claim 1, characterized in that, The immersion time in the aqueous solution is 2 min-3 min.

8. The method according to claim 1, characterized in that, The interfacial polymerization reaction time is 2 min-40 min.

9. The method according to claim 1, characterized in that, Thermal crosslinking at 50℃-65℃.

10. A polyester nanofiltration membrane prepared by the method according to any one of claims 1-8.

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