Method for preparing nanofiltration membrane by using metal-induced microporous polymer as middle layer

Abstract

The invention relates to a method for preparing a nanofiltration membrane by using a metal-induced microporous polymer as a middle layer. Metal-induced microporous polymer nanoparticles are synthesized by adopting a polymer directional synthesis method, and the pore diameter of the prepared nanoparticles is suitable for water molecule transfer. Then depositing a metal-induced microporous polymer on a support body by using polydopamine-assisted deposition to prepare an intermediate layer; the preparation method comprises the following steps: preparing a water-phase solution of piperazine and an n-heptane solution containing trimesoyl chloride, preparing a polyamide separation layer on a support body with a middle layer through an interfacial polymerization reaction, and placing the support body with the polyamide separation layer in an air dry oven for heat treatment to prepare the polyamide nanofiltration membrane. The water permeability of the prepared polyamide nanofiltration membrane is 20.93 Lm < 2 > h < 1 > bar < 1 >, and the separation factor of NaCl / Na2SO4 is 62.88. The preparation method is easy to implement, the selective permeability of the prepared polyamide nanofiltration membrane is greatly improved, and the polyamide nanofiltration membrane has a wide application prospect.

Description

[0001] The invention relates to the field of polyamide nanofiltration membrane preparation, and in particular to a method for preparing a nanofiltration membrane using a metal-induced microporous polymer as an intermediate layer. Background Art

[0002] Nanofiltration membrane separation technology is used in the separation of monovalent and divalent salts and seawater desalination due to its advantages such as high efficiency, low energy consumption, simple operation, high integration, and suitable pore size. However, the selective permeability of current polyamide nanofiltration membranes needs to be improved.

[0003] Strategically placing an interlayer between a porous support and a polyamide separation layer has become a practical approach to enhance the permselectivity of polyamide nanofiltration membranes. An "interlayer" refers to a layer of material deposited on the support prior to forming the polyamide separation layer via interfacial polymerization. Because it lies between the support and the polyamide separation layer, it is referred to as an "interlayer." On the one hand, the interlayer can mitigate the effects of the support structure and properties on the interfacial polymerization process. On the other hand, the properties of the interlayer can be exploited to regulate the interfacial polymerization process, thereby producing polyamide nanofiltration membranes with high permselectivity. Niu et al. (B. Tian, ​​P. Hu, S. Zhao, M. Wang, Y. Hou, QJ Niu, P. Li, Nanofiltration membrane combining environmentally-friendly polycarboxylic interlayer prepared from catechol for enhanced desalination performance, Desalination 512 (2021)115118.) prepared a polycarboxylic acid interlayer using a catechol / sodium periodate mixture. The introduced carboxyl groups impart hydrophilicity to the support, enabling better retention of piperazine monomers, thereby forming a thinner and denser polyamide separation layer. Compared with the membrane without the added intermediate layer, the water permeability of the modified membrane is 15.4±1 L⋅m −2 ⋅h −1 bar −1), with a sodium sulfate rejection rate of 98.42±1.2%. However, the hydrophilic polymer material as the intermediate layer does not provide additional channels for the transmembrane transfer of water molecules, so the water permeability of the prepared nanofiltration membrane is limited. Currently, some researchers use nanomaterials as the intermediate layer in the hope of further improving the water permeability of the prepared polyamide nanofiltration membrane. Lienhard et al. (S. Cao, A. Deshmukh, L. Wang, Q.Han, Y. Shu, HY Ng, Z. Wang, JH Lienhard, Enhancing the Permselectivityof Thin-Film Composite Membranes Interlayered with MoS(2) Nanosheets viaPrecise Thickness Control, Environ Sci Technol 56 (2022) 8807-8818.) constructed a molybdenum disulfide intermediate layer on a polyethersulfone support layer, and then prepared a high-performance polyamide nanofiltration membrane by interfacial polymerization. The surface roughness and cross-linking degree of the resulting polyamide nanofiltration membrane were improved due to the enhanced restraint effect of the molybdenum disulfide intermediate layer on interfacial gas release and its adsorption of amine monomers. The optimal membrane had a sodium sulfate rejection rate of 96.8% and a water permeability of 15.9 L·m −2 ·h −1 bar −1 However, the interfacial compatibility of porous materials as the intermediate layer with the polyamide separation layer is relatively poor, resulting in low selectivity of the resulting polyamide nanofiltration membrane. Therefore, the selective permeability of polyamide nanofiltration membranes prepared by placing an intermediate layer in the prior art needs to be improved. Summary of the Invention

[0004] To solve the problem of low selective permeability of polyamide nanofiltration membranes made of existing intermediate layer materials, we developed a new type of high-valent metal-induced microporous polymer, which contains metal ions (Cr 3+), and a polymer backbone (polyethyleneimine);selective metal-induced microporous polymer construction of the interlayer, its free amino group gives the interlayer high hydrophilicity, help to regulate the interface polymerization process. In addition, its pore structure (pore size of 0.56 nm) can provide additional water transport channels, while maintaining the selectivity of the prepared membrane to improve water permeation flux. However, the direct deposition of metal-induced microporous polymer construction of the interlayer between the support layer and the polyamide separation layer has weak binding force, which may lead to instability of the polyamide nanofiltration membrane. In order to enhance the binding force between the interlayer, the support layer and the polyamide separation layer, the method of depositing metal-induced microporous polymer assisted by polydopamine is used to construct the interlayer. Polydopamine shows strong binding force to the support layer and the polyamide separation layer, thereby improving the stability of the prepared membrane. Polydopamine can react with the free amino group of the metal-induced microporous polymer through Michael addition / shiff base reaction, thereby depositing the metal-induced microporous polymer on the support layer and constructing a stable interlayer. In addition, polydopamine has good hydrophilicity, which can improve the hydrophilicity of the constructed interlayer, and help to regulate the interface polymerization process.

[0005] The application adopts the method of depositing metal-induced microporous polymer assisted by polydopamine to construct the interlayer on the support layer. Subsequently, a high selective permeation performance polyamide nanofiltration membrane is prepared by interfacial polymerization reaction of piperazine and trimesoyl chloride.

[0006] The technical scheme adopted by the application to achieve the above-mentioned purposes is as follows:

[0007] The application provides a method for preparing a nanofiltration membrane by using a metal-induced microporous polymer as an interlayer, comprising the following steps:

[0008] 1) Synthesis of metal-induced microporous polymer nanoparticles

[0009] The polymer directional synthesis method is adopted, Cr(NO3)3·9H2O is dissolved in deionized water, and 5-nitroisophthalic acid is dissolved in N,N-dimethylformamide; after mixing the two solutions, stirring is carried out; in addition, polyethyleneimine is dissolved in deionized water, and the obtained polyethyleneimine solution is added to the above-mentioned mixed solution under continuous stirring; the three mixed solutions are transferred to a hydrothermal reaction kettle, and reaction is carried out at 180~220 ℃ for 10~14 hours; the obtained solid nanoparticles are subjected to washing and centrifugal treatment; after being sequentially washed with deionized water, N,N-dimethylformamide and methanol, the solid nanoparticles are placed in a vacuum oven for drying, and finally metal-induced microporous polymer nanoparticles are obtained;

[0010] 2) A tris aqueous solution is prepared, hydrochloric acid is added to adjust the pH to 8~10, and a hydrochloric acid-tris solution is obtained;

[0011] 3) adding the metal-induced microporous polymer nanoparticles synthesized in step 1) to the hydrochloric acid-tris buffer solution prepared in step 2) and ultrasonically dispersing the nanoparticles to obtain a hydrochloric acid-tris solution of the metal-induced microporous polymer;

[0012] 4) adding dopamine to the hydrochloric acid-tris solution of the metal-induced microporous polymer prepared in step 3), and pouring the hydrochloric acid-tris solution of the metal-induced microporous polymer with dopamine added onto the surface of the support layer, and performing static deposition to obtain a support having an intermediate layer;

[0013] 5) preparing an aqueous solution of piperazine and a n-heptane solution of trimesoyl chloride;

[0014] 6) Preparation of a polyamide nanofiltration membrane by interfacial polymerization. The support with the intermediate layer prepared in step 4) is fully immersed in an aqueous solution containing piperazine, and then the excess aqueous solution on the surface is removed. The support with the intermediate layer is then immersed in an n-heptane solution containing trimesoyl chloride to form a polyamide separation layer by interfacial polymerization. The support with the polyamide separation layer is placed in a forced air drying oven for heat treatment to obtain a polyamide nanofiltration membrane; Figure 1 shown.

[0015] In the step 1), the molar concentration of Cr(NO3)3·9H2O after being dissolved in deionized water is 0.08-0.15 mol / L.

[0016] In the step 1), the molar concentration of 5-nitroisophthalic acid after being dissolved in N,N-dimethylformamide is 0.08-0.15 mol / L, and the molar ratio of 5-nitroisophthalic acid to Cr(NO3)3·9H2O is 0.53-1.88:1.

[0017] In step 1), the molar concentration of polyethyleneimine after being dissolved in deionized water is 0.08-0.15 mol / L, and the molar ratio of polyethyleneimine to Cr(NO3)3·9H2O is 0.53-1.88:1.

[0018] In step 3), the concentration of the metal-induced microporous polymer in the solution is 0.1-1.0 g / L.

[0019] In step 4), after adding dopamine to the hydrochloric acid-tris solution of the metal-induced microporous polymer, the concentration of dopamine is 1.0-2.0 g / L.

[0020] The piperazine solution prepared in step 5) has a mass volume fraction of 0.3% to 1.0%.

[0021] The mass volume fraction of trimesoyl chloride prepared in step 5) is 0.05% to 0.15%.

[0022] The immersion time in the aqueous solution in step 6) is 60 to 120 seconds.

[0023] In step 6), the immersion time in the n-heptane solution containing trimesoyl chloride is 120-180 seconds.

[0024] The heat treatment temperature in step 6) is 70-90°C.

[0025] Adopting the above technical scheme, metal-induced microporous polymer was synthesized by polymer directional synthesis, the intermediate layer was prepared by polydopamine-assisted deposition of metal-induced microporous polymer, and then a polyamide nanofiltration membrane with high selective permeability was prepared by interfacial polymerization. Figure 2 As shown in the figure, by analyzing the pore size distribution of the metal-induced microporous polymer, the pores of the metal-induced microporous polymer have a most probable pore size of 0.56 nm. These pores can provide additional channels for the transmembrane transfer of water molecules, which can significantly increase the water permeability of the membrane. Due to the presence of the polymer skeleton, the metal-induced microporous polymer has good compatibility with the interface between the separation layer and will not destroy the selectivity of the prepared membrane. Since the metal-induced microporous polymer nanoparticles in the intermediate layer can affect the distribution of the aqueous solution, such as Figure 3 As shown, the surface of the prepared nanofiltration membrane exhibits a wrinkled structure and a rough surface. This increases the contact area between water molecules and the membrane surface, helping to improve the water permeability of the polyamide nanofiltration membrane. The hydrophilic surface of the prepared intermediate layer can load more aqueous solution. Interfacial polymerization on its surface can produce a thinner and denser polyamide separation layer. A thinner separation layer helps to increase the water permeability of the prepared membrane, while a denser separation layer helps to improve the selectivity of the membrane.

[0026] The selective permeability of polyamide nanofiltration membranes prepared with different concentrations of metal-induced microporous polymer as the intermediate layer was tested. The water permeability and sodium sulfate rejection of the polyamide nanofiltration membrane prepared with a metal-induced microporous polymer concentration of 0.5 g / L were 20.93 L∙m −2 ∙h −1 ∙bar −1 The polyamide nanofiltration membrane exhibited excellent permselectivity and a promising application prospect. The membrane was tested with a 2000 mg / L sodium chloride aqueous solution and demonstrated a sodium chloride rejection of 30.65% and a separation factor of 62.88 between sodium chloride and sodium sulfate. Compared to a polyamide nanofiltration membrane without an intermediate layer, the membrane exhibited a 193.53% increase in water permeability and a 91.42% increase in the separation factor between sodium chloride and sodium sulfate.

[0027] The advantages of the present invention are:

[0028] 1) The present application can realize fine regulation of the performance of the prepared polyamide nanofiltration membrane by changing the concentration of the metal-induced microporous polymer in the intermediate layer.

[0029] 2) The polyamide nanofiltration membrane prepared by the present application has greatly improved selective permeation performance. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 A schematic diagram for preparing a polyamide nanofiltration membrane;

[0031] Figure 2 A pore size distribution diagram of a metal-induced microporous polymer;

[0032] Figure 3 A SEM diagram of the surface of the prepared polyamide nanofiltration membrane;

[0033] Figure 4 A TEM diagram of the cross section of the prepared polyamide nanofiltration membrane. DETAILED DESCRIPTION

[0034] Example 1

[0035] Cr(NO3)3·9H2O was dissolved in deionized water with a molar concentration of 0.15 mol / L, and 5-nitroisophthalic acid was dissolved in N,N-dimethylformamide with a molar concentration of 0.08 mol / L. The two solutions were mixed and stirred, and the molar ratio of 5-nitroisophthalic acid to Cr(NO3)3·9H2O was 0.53:1. Polyethyleneimine was dissolved in deionized water with a molar concentration of 0.08 mol / L, and the obtained polyethyleneimine solution was added to the above mixed solution under continuous stirring, and the molar ratio of polyethyleneimine to Cr(NO3)3·9H2O was 0.53:1. The mixed solution was transferred to a hydrothermal reaction kettle and reacted at 180 ℃ for 14 hours. After the reaction was completed, the obtained solid nanoparticles were washed and centrifuged. After being washed with deionized water, N,N-dimethylformamide and methanol in turn, the solid nanoparticles were placed in a vacuum oven for drying, and finally metal-induced microporous polymer nanoparticles were obtained.

[0036] A tris (hydroxymethyl)aminomethane (TRIS) aqueous solution was prepared and hydrochloric acid was added to adjust the pH to 9 to obtain a HCl-TRIS solution. The synthesized metal-induced microporous polymer was added to the HCl-TRIS buffer and ultrasonically dispersed to obtain a 0.1 g / L HCl-TRIS solution of the metal-induced microporous polymer. Dopamine was added to the HCl-TRIS solution to a dopamine concentration of 1.0 g / L. The mixed solution was poured onto the surface of a polysulfone ultrafiltration membrane and subjected to static deposition to obtain a support with an intermediate layer. A 0.3% (by volume) piperazine aqueous solution was prepared. The support with the intermediate layer was immersed in the aqueous solution for 60 seconds, after which the excess aqueous solution was removed. After the membrane surface dried, a 0.05% (by volume) n-heptane solution containing trimesoyl chloride was poured in for interfacial polymerization for 150 seconds. The excess oil phase solution was then removed to produce a nanofiltration membrane. Finally, the membrane was placed in a forced air drying oven and heat treated at 70 °C for 5 minutes to obtain a polyamide nanofiltration membrane.

[0037] The flux and sodium sulfate rejection of the polyesteramide composite nanofiltration membrane were 17.80 L∙m-1 at 0.6 MPa and 25°C using a 2000 mg / L sodium sulfate aqueous solution. −2 ∙h −1 ∙bar −1 and 98.66%. The sodium chloride retention rate of the polyester amide composite nanofiltration membrane was tested with a 2000 mg / L sodium chloride aqueous solution and was 29.35%, and the separation factor of sodium chloride and sodium sulfate was 52.98.

[0038] Example 2

[0039] Cr(NO3)3·9H2O was dissolved in deionized water with a molar concentration of 0.10 mol / L, and 5-nitroisophthalic acid was dissolved in N,N-dimethylformamide with a molar concentration of 0.10 mol / L. The two solutions were mixed and stirred, wherein the molar ratio of 5-nitroisophthalic acid to Cr(NO3)3·9H2O was 1:1. Polyethyleneimine was also dissolved in deionized water with a molar concentration of 0.10 mol / L, and the obtained polyethyleneimine solution was added to the above-mentioned mixed solution under continuous stirring, wherein the molar ratio of polyethyleneimine to Cr(NO3)3·9H2O was 1:1. The mixed solution was transferred to a hydrothermal reactor and reacted at 220°C for 12 hours. After the reaction was completed, the obtained solid nanoparticles were washed and centrifuged. After washing with deionized water, N,N-dimethylformamide and methanol in sequence, the solid nanoparticles were placed in a vacuum oven to dry, and finally metal-induced microporous polymer nanoparticles were obtained. As Figure 2 As shown in FIG, the pore size distribution diagram of the metal-induced microporous polymer was analyzed, and the most probable pore size of the prepared metal-induced microporous polymer was 0.56 nm.

[0040] A tris (hydroxymethyl)aminomethane (TRIS) aqueous solution was prepared and hydrochloric acid was added to adjust the pH to 8 to obtain a hydrochloric acid-TRIS solution. The synthesized metal-induced microporous polymer was added to the hydrochloric acid-TRIS buffer and ultrasonically dispersed to obtain a 0.5 g / L hydrochloric acid-TRIS solution of the metal-induced microporous polymer. Dopamine was added to the hydrochloric acid-TRIS solution of the metal-induced microporous polymer at a concentration of 2.0 g / L. The mixed solution was poured onto the surface of a polysulfone ultrafiltration membrane and subjected to static deposition to obtain a support with an intermediate layer. A 0.5% by volume fraction of piperazine was prepared in an aqueous phase solution. The support with the intermediate layer was immersed in the aqueous phase solution for 120 seconds, after which the excess aqueous phase solution was removed. After the membrane surface dried, a 0.1% by volume fraction of trimesoyl chloride in n-heptane was poured in for interfacial polymerization for 180 seconds. The excess oil phase solution was then removed to produce a nanofiltration membrane. Finally, the membrane was placed in a blast drying oven and heat treated at 80 °C for 5 minutes to obtain a polyamide nanofiltration membrane. Figure 3 As shown in the figure, the surface is wrinkled and rough. Figure 4 As shown, the prepared nanofiltration membrane includes a support, an intermediate layer and a polyamide separation layer; the polyamide separation layer is relatively thin, with thicknesses of 29.02 nm, 29.12 nm, 37.41 nm, 25.16 nm and 29.41 nm at different positions, and an average thickness of 30.02 ± 4.48 nm.

[0041] The flux and sodium sulfate rejection of the polyesteramide composite nanofiltration membrane were 20.93 L∙m −2 ∙h −1 ∙bar −1 and 98.90%, and the sodium chloride rejection rate of the polyester amide composite nanofiltration membrane was 30.65% using a 2000 mg / L sodium chloride aqueous solution test, and the separation factor of sodium chloride and sodium sulfate was 62.88.

[0042] Example 3

[0043] Cr(NO3)3·9H2O was dissolved in deionized water with a molar concentration of 0.08 mol / L, and 5-nitroisophthalic acid was dissolved in N,N-dimethylformamide with a molar concentration of 0.15 mol / L. The two solutions were mixed and stirred, and the molar ratio of 5-nitroisophthalic acid to Cr(NO3)3·9H2O was 1.88:1. Polyethyleneimine was dissolved in deionized water with a molar concentration of 0.15 mol / L, and the obtained polyethyleneimine solution was added to the above mixed solution under continuous stirring, and the molar ratio of polyethyleneimine to Cr(NO3)3·9H2O was 1.88:1. The mixed solution was transferred to a hydrothermal reaction kettle and reacted at 200 ℃ for 10 hours. After the reaction was completed, the obtained solid nanoparticles were washed and centrifuged. After being washed with deionized water, N,N-dimethylformamide and methanol in turn, the solid nanoparticles were placed in a vacuum oven for drying, and finally metal-induced microporous polymer nanoparticles were obtained.

[0044] A tris aqueous solution was prepared, and hydrochloric acid was added to adjust the pH to 10 to obtain a hydrochloric acid-tris solution. The synthesized metal-induced microporous polymer was ultrasonically dispersed in the hydrochloric acid-tris buffer solution to obtain a 1.0 g / L metal-induced microporous polymer hydrochloric acid-tris solution. Dopamine was added to the metal-induced microporous polymer hydrochloric acid-tris solution, and the concentration of dopamine was 1.5 g / L. The above mixed solution was poured onto the surface of a polysulfone ultrafiltration membrane, and after static deposition, a support with an intermediate layer was obtained. A 1.0% (mass / volume) aqueous piperazine solution was prepared. The support with the intermediate layer was immersed in the aqueous solution, and after sufficient immersion for 90 seconds, the excess aqueous solution was removed. After the membrane surface was dried, a 0.15% (mass / volume) uniform triformyl chloride n-heptane solution was poured, and interfacial polymerization was carried out for 120 seconds. Then, the excess oil phase solution was removed to prepare a nanofiltration membrane. Finally, the membrane was placed in a blast drying oven and heat treated at 90 ℃ for 5 minutes to prepare a polyamide nanofiltration membrane.

[0045] The flux and sodium sulfate rejection rate of the polyamide composite nanofiltration membrane were tested using a 2000 mg / L sodium sulfate aqueous solution at 0.6 MPa and 25 ℃, and the flux and sodium sulfate rejection rate were 14.81 L·m −2 ∙h −1 ∙bar −1 and 99.10%, respectively. The sodium chloride rejection rate of the polyamide composite nanofiltration membrane was 38.70% using a 2000 mg / L sodium chloride aqueous solution, and the separation factor of sodium chloride and sodium sulfate was 68.47.

[0046] Comparative Example

[0047] The comparative example is the NF270 commercial nanofiltration membrane from Dow Chemical. The selective permeability of the membrane tested at 0.60 MPa and 25°C is shown in Table 1.

[0048] Table 1

[0049]

[0050] As can be seen from Table 1, the water permeability of the polyamide nanofiltration membranes prepared in Examples 1-3 is much greater than that of the comparative example, and the separation factors of sodium chloride and sodium sulfate are also significantly higher than those of the comparative example. The above results show that dopamine-assisted deposition of metal-induced microporous polymers as an intermediate layer can greatly improve the selective permeability of the prepared polyamide nanofiltration membrane. This is because the polyamide nanofiltration membrane prepared by dopamine-assisted deposition of metal-induced microporous polymers as an intermediate layer is thinner and denser. In addition, the metal-induced microporous polymer can also provide additional channels for the transmembrane transfer of water molecules. The membrane preparation method of the present invention enriches the preparation method of nanofiltration membranes with high selective permeability and provides a new idea for the preparation of nanofiltration membranes with high selective permeability.

[0051] The present invention relates to a method for preparing a nanofiltration membrane using a metal-induced microporous polymer as an intermediate layer. Metal-induced microporous polymer nanoparticles are synthesized using a polymer-directed synthesis method, and the pore size of the prepared nanoparticles is suitable for the transmission of water molecules. The metal-induced microporous polymer is then deposited on a support using polydopamine-assisted deposition to prepare an intermediate layer. An aqueous solution of piperazine and an n-heptane solution containing trimesoyl chloride are prepared, and a polyamide separation layer is prepared on the support having the intermediate layer through an interfacial polymerization reaction. The support forming the polyamide separation layer is placed in a forced air drying oven for heat treatment to produce a polyamide nanofiltration membrane. The prepared polyamide nanofiltration membrane has a water permeability of 20.93 L∙m −2 ∙h −1 ∙bar −1 The NaCl / Na2SO4 separation factor is 62.88. The present invention is easy to implement, and the selective permeability of the prepared polyamide nanofiltration membrane is greatly improved, and has broad application prospects.

[0052] The technical solutions disclosed and proposed by the present invention can be implemented by those skilled in the art by drawing on the content of this document and appropriately changing the conditions, routes, and other aspects. Although the methods and preparation techniques of the present invention have been described through preferred embodiments, it is obvious that those skilled in the art can modify or recombine the methods and technical routes described herein without departing from the content, spirit, and scope of the present invention to achieve the ultimate preparation technology. It is particularly important to point out that all similar substitutions and modifications that are obvious to those skilled in the art are considered to be included in the spirit, scope, and content of the present invention.

Claims

1. A method for preparing a nanofiltration membrane using a metal-induced microporous polymer as an intermediate layer, characterized in that: The following steps are involved: 1) Synthesis of metal-induced microporous polymer nanoparticles Using a polymer-directed synthesis method, Cr(NO₃)₃·9H₂O was dissolved in deionized water, and 5-nitroisophthalic acid was dissolved in N,N-dimethylformamide. The two solutions were mixed and stirred. Polyethyleneimine was also dissolved in deionized water, and the resulting polyethyleneimine solution was added to the mixture under continuous stirring. The three mixed solutions were transferred to a hydrothermal reactor and reacted at 180-220°C for 10-14 hours. The resulting solid nanoparticles were washed with deionized water, N,N-dimethylformamide, and methanol, respectively, and then dried in a vacuum oven to obtain metal-induced microporous polymer nanoparticles. 2) Prepare a tris aqueous solution of tris(hydroxymethyl)aminomethane (Tris), add hydrochloric acid to adjust the pH to 8~10 to obtain a hydrochloric acid-tris solution; 3) The metal-induced microporous polymer nanoparticles synthesized in step 1) were added to the hydrochloric acid-tris buffer solution prepared in step 2) and ultrasonically dispersed to obtain a hydrochloric acid-tris solution of the metal-induced microporous polymer; 4) adding dopamine to the hydrochloric acid-tris solution of the metal-induced microporous polymer in step 3), pouring the hydrochloric acid-tris solution of the metal-induced microporous polymer with dopamine on the surface of the support layer, and after static deposition, obtaining a support having an intermediate layer; 5) preparing an aqueous solution of piperazine and a n-heptane solution of trimesoyl chloride; 6) The support with the intermediate layer prepared in step 4) is fully immersed in an aqueous solution containing piperazine, and then excess aqueous solution on the surface is removed. The support with the intermediate layer is then immersed in an n-heptane solution containing trimesoyl chloride to form a polyamide separation layer via interfacial polymerization. The support with the polyamide separation layer is then placed in a forced air drying oven for heat treatment to produce a polyamide nanofiltration membrane.

2. The method for preparing a nanofiltration membrane using a metal-induced microporous polymer as an intermediate layer as claimed in claim 1, wherein: In the step 1), the molar concentration of Cr(NO3)3·9H2O after being dissolved in deionized water is 0.08-0.15 mol / L; the molar concentration of 5-nitroisophthalic acid after being dissolved in N,N-dimethylformamide is 0.08-0.15 mol / L; the molar concentration of polyethyleneimine after being dissolved in deionized water is 0.08-0.15 mol / L; the molar ratio of 5-nitroisophthalic acid to Cr(NO3)3·9H2O is 0.53-1.88:1; and the molar ratio of polyethyleneimine to Cr(NO3)3·9H2O is 0.53-1.88:

1.

3. The method for preparing a nanofiltration membrane using a metal-induced microporous polymer as an intermediate layer as claimed in claim 1, wherein: In step 3), the concentration of the metal-induced microporous polymer in the solution is 0.1-1.0 g / L.

4. The method for preparing a nanofiltration membrane using a metal-induced microporous polymer as an intermediate layer according to claim 1, wherein in step 4) after adding dopamine to the hydrochloric acid-tris solution of the metal-induced microporous polymer, the concentration of dopamine is 1.0-2.0 g / L.

5. The method for preparing a nanofiltration membrane using a metal-induced microporous polymer as an intermediate layer according to claim 1, wherein the piperazine solution prepared in step 5) has a mass volume fraction of 0.3% to 1.0%.

6. The method for preparing a nanofiltration membrane using a metal-induced microporous polymer as an intermediate layer according to claim 1, wherein the mass volume fraction of trimesoyl chloride prepared in step 5) is 0.05% to 0.15%.

7. The method for preparing a nanofiltration membrane using a metal-induced microporous polymer as an intermediate layer according to claim 1, wherein the immersion time in the aqueous solution in step 6) is 60 to 120 seconds.

8. The method for preparing a nanofiltration membrane using a metal-induced microporous polymer as an intermediate layer according to claim 1, wherein the immersion time in the n-heptane solution containing trimesoyl chloride in step 6) is 120 to 180 seconds.

9. The method for preparing a nanofiltration membrane using a metal-induced microporous polymer as an intermediate layer according to claim 1, wherein the heat treatment temperature in step 6) is 70-90°C.

10. The method for preparing a nanofiltration membrane using a metal-induced microporous polymer as an intermediate layer according to claim 1, wherein the polyamide nanofiltration membrane prepared in step 6) is washed with deionized water.

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