Preparation method of MOFs composite nanofiltration membrane assembled by confined interfacial polymerization and application thereof

Abstract

The application discloses a preparation method of a limited interface polymerization assembled MOFs composite nanofiltration membrane and application thereof, and belongs to the field of preparation and modification of nanofiltration membranes. Amino-modified MOFs nanosheets and free amine monomers are deposited on the surface of a porous base film, the ratio of the nanosheets and the amine monomers is regulated, and the limited interface polymerization reaction between MOFs layers is realized by pouring into an organic solvent containing TMC monomers, so that a two-dimensional MOFs composite nanofiltration membrane with a small amount of polyamide cross-linking is prepared. The limited interface polymerization method fills the defects of the nanosheet disordered assembly, enhances the compactness, stability and surface negative charge density of the MOFs nanofiltration membrane, guarantees the high flux characteristic of the two-dimensional membrane, realizes a significant reduction of the molecular weight cut-off, and improves the separation efficiency of the membrane to divalent anion salts. The limited interface polymerization assembled two-dimensional MOFs composite nanofiltration membrane provides a new way for developing advanced preparation and modification membranes, and has good application potential.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nanofiltration membrane preparation, and particularly relates to a preparation method of a confined interfacial polymerization assembled MOFs composite nanofiltration membrane and application thereof. BACKGROUND

[0002] The rapid development of industrialization and the rapid growth of population have caused the shortage of drinking water resources and environmental pollution. The membrane separation technology has the advantages of high efficiency and energy saving, and has great potential in seawater desalination and water resource recovery. Among them, the nanofiltration membrane, as a kind of membrane between the reverse osmosis membrane and the ultrafiltration membrane, can realize the separation of different valence inorganic salts through the electrostatic effect and the steric hindrance. Due to the bottleneck of membrane technology, the traditional polyamide nanofiltration membrane has the problems of small water permeability and poor selectivity of different valence salts. The use of nanomaterials can introduce rich nanocapillary channels into the membrane, which can improve the permeation flux and selectivity of the membrane, and provides an opportunity for the development of a new generation of nanofiltration membranes.

[0003] In view of the shortcomings of polyamide, a large number of nanomaterials are used to prepare modified polyamide nanofiltration membranes. Among them, the emerging two-dimensional metal organic framework (MOFs) material can significantly improve the permeability of the membrane due to its rich pores and high organic-inorganic compatibility. In the prior art, the preparation methods of two-dimensional material modified membranes are as follows: material interlayer modification, addition of modification in water phase or oil phase, self-assembly process, etc. The conventional interfacial polymerization modification method is to directly introduce the two-dimensional nanomaterial as an interlayer or an additive into the preparation process of the polyamide nanofiltration membrane, but the porous characteristics of the material cannot be fully utilized, and a large amount of material is embedded under the PA layer, thereby causing waste. Another common preparation method is to stack the two-dimensional nanosheet material to form a layered membrane, which has the characteristics of high permeation flux and easy control of interlayer structure. However, the two-dimensional layered structure formed by the conventional assembly process has many defects, the molecular weight cut-off is large, and the salt separation effect is not ideal, which affects the effect of the material modified membrane. SUMMARY

[0004] In order to solve the problems of two-dimensional MOFs in water treatment applications, the purpose of the present application is to provide a preparation method of a confined interfacial polymerization assembled two-dimensional MOFs composite nanofiltration membrane and application thereof, so as to solve the problems of poor selectivity of two-dimensional MOFs membranes and low water flux of existing nanofiltration membranes. The method is simple, efficient and easy to control the membrane structure. The confined interfacial polymerization method aims to further and limit the interfacial polymerization reaction space in the defect pores between the two-dimensional layers, instead of polymerizing only on the surface of the two-dimensional membrane. The two-dimensional MOFs nanofiltration membrane prepared by the method can ensure a high water flux and realize high-efficiency selective separation of divalent and monovalent anion salts in water treatment without affecting the separation mechanism of the two-dimensional membrane.

[0005] The rejection of neutral organic pollutants and inorganic charged particles by nanofiltration membranes is influenced by steric hindrance, Donnan effect and dielectric effect. Generally, the smaller the membrane pore size, the higher the rejection rate of neutral organic pollutants. The dense selective layer on the surface of polyamide nanofiltration membranes is negatively charged, and the charge repulsion effect on anions is obvious. Therefore, to improve the performance of MOFs nanofiltration membranes, structure regulation and charge modification of the membranes are particularly crucial. The structure of the nanofiltration membrane assembled by two-dimensional MOFs nanosheet membranes is loose, and the surface and surface defects caused by irregular stacking between layers are more. In addition, the surface charge effect is significant, which leads to poor selectivity of salt ions. The mainstream method of nanomaterial modification in the existing technology is to introduce two-dimensional nanomaterials directly as an intermediate layer or additive into the preparation process of polyamide nanofiltration membranes, which cannot take advantage of the porous characteristics of the material, and a large amount of material is embedded in the PA layer, thereby causing waste. In addition, the excessive stacking of the intermediate layer material will increase the additional mass transfer resistance and reduce the permeation flux of the nanofiltration membrane. Therefore, there is an urgent need for a regulation method that can anchor a small amount of local polyamide network structure to the defects of two-dimensional membranes while retaining the effective permeation path of two-dimensional membranes.

[0006] Specifically, the application first provides a preparation method of a two-dimensional MOFs composite nanofiltration membrane assembled by limited interfacial polymerization, which comprises: taking a porous support layer as a base film, synchronously depositing amino-modified MOFs two-dimensional nanosheets and free amine monomers on the substrate, removing excess water to form a two-dimensional layered membrane, immersing the surface of the two-dimensional layered membrane in an organic monomer solution to perform interfacial polymerization limited between two-dimensional layers, then removing excess solution, and obtaining the two-dimensional MOFs composite nanofiltration membrane assembled by limited interfacial polymerization after heat treatment.

[0007] In an embodiment of the application, the method specifically comprises: taking a porous support film as a base film, synchronously depositing amino-modified MOFs two-dimensional nanosheets and a small amount of free amine monomers on the porous substrate, optimizing the content of the nanosheets and the free amine monomers to regulate the properties of the two-dimensional membrane, removing excess water by vacuum filtration to realize self-assembly of the two-dimensional nanomaterials to form a two-dimensional layered membrane. Then, the surface of the two-dimensional layered membrane is immersed in an organic monomer solution to perform limited interfacial polymerization, and trimesoyl chloride and piperazine react with the edge imino group of the modified MOFs nanosheets. A small amount of local polyamide network generated in the reaction fills the defects caused by the disordered assembly of the nanosheets, without embedding the entire two-dimensional layered structure, thereby realizing limited interfacial polymerization. Then, excess organic solvent is removed, and the two-dimensional MOFs composite nanofiltration membrane with high permeation flux assembled by limited interfacial polymerization is obtained after the membrane is immersed in water after heat treatment.

[0008] In an embodiment of the application, the porous support film is a polyether sulfone microporous film.

[0009] In one embodiment of the present invention, the amino-based substances used to modify MOFs include piperazine, m-phenylenediamine, and polyethyleneimine (600 Da, 1800 Da), preferably piperazine.

[0010] In one embodiment of the present invention, the method for preparing the amino-modified MOF two-dimensional nanosheets includes: preparing an EDC / NHS buffer solution, wherein the concentrations of the two solutions are 0.1-0.5 wt / % and 0.01-0.5 wt / %, respectively, preferably 0.1 wt% and 0.05 wt%, respectively; adding the mixture to a MOF nanosheet suspension, then adding an amino substance solution, stirring and reacting for 4-24 h, and washing after solid-liquid separation to obtain the amino-modified MOF two-dimensional nanosheets; further, the concentration ratio of the amino substance to the MOF two-dimensional nanosheets is 0.05-0.5 wt% / 0.05-0.5 wt%.

[0011] In one embodiment of the present invention, the two-dimensional MOF nanosheets are carboxyl ligand MOFs such as Cu-TCPP, Al-TCPP, Cr-TCPP, and Zr-BTB.

[0012] In one embodiment of the present invention, the preparation method of the MOFs two-dimensional nanosheets is as follows: metal salt ligands (hydrated copper nitrate or aluminum nitrate, chromium nitrate), formic acid and polyvinylpyrrolidone are dispersed in a dimethylformamide-ethanol (V:V = 1 to 3:1) mixed solvent. Then, TCPP dissolved in the dimethylformamide-ethanol mixture is stirred and added to the metal salt ligand solvent. The mixture is heated at 40-90°C for 1-24 hours and washed with ethanol.

[0013] In one embodiment of the present invention, the molar ratio of the metal salt ligand to TCPP is 0.1 mmol / 0.05 mmol.

[0014] In one embodiment of the present invention, the MOFs and amino-modified MOFs two-dimensional nanosheets have a sheet thickness of 1-10 nm and a sheet diameter of 0.1 μm-10 μm.

[0015] In one embodiment of the present invention, the deposition mass of the modified MOF nanosheets is 0.000452 mg / cm³. 2 ~0.0679mg / cm 2 The preferred concentration is 0.0113 mg / cm³. 2 ~0.0452mg / cm 2 .

[0016] In one embodiment of the present invention, the free amine monomer is piperazine, and the mass concentration of the piperazine solution is 0.01wt%-0.15wt%, preferably 0.01wt%-0.04wt%.

[0017] In one embodiment of the present invention, the mass concentration ratio of the amino-modified MOF two-dimensional nanosheets to the free amine monomer is 0.001-0.1 wt% / 0.01-0.5 wt%, preferably 0.01 wt% / 0.02 wt%.

[0018] In one embodiment of the present invention, before preparation, the surface of the porous PES base membrane is immersed in a polyvinyl alcohol solution (0.1-5.0 wt%) for 0.5-10 minutes to enhance the adhesion between the nanosheets and the scaffold.

[0019] In one embodiment of the present invention, the organic phase monomer solution is a hexane solution of trimesoyl chloride, and the concentration of trimesoyl chloride is 0.05wt%-0.3wt%, preferably 0.1wt%-0.15wt%.

[0020] In one embodiment of the present invention, the interfacial polymerization reaction time is 30-300s, preferably 30-90s; the heat treatment time is 2-6min, preferably 3-5min, and the heat treatment temperature is 40-70℃.

[0021] The present invention also provides a two-dimensional MOFs composite nanofiltration membrane prepared by the above preparation method.

[0022] The present invention also provides a water treatment method, wherein the method uses the above-mentioned two-dimensional MOFs composite nanofiltration membrane to treat water.

[0023] The present invention also provides the application of the above-mentioned high-permeability nanofiltration membrane in water treatment processes.

[0024] The beneficial effects achieved by this invention are as follows:

[0025] This invention involves pre-embedding a small amount of free amine monomers within the two-dimensional nanosheets during the filtration process of amino-modified MOF nanosheets to form a two-dimensional nanosheet layered membrane. Subsequently, when an organic solvent comes into contact with the surface of the two-dimensional membrane, the free amine monomers diffuse to the defects formed by the irregular stacking. Tristyrene chloride in the organic phase system interacts with the small amount of free amine monomers at the defects and the amino groups at the nanosheet edges to form a small amount of localized polyamide network. This targeted, confined crosslinking occurs at the edges and defects of the two-dimensional membrane, resulting in the preparation of the target composite nanofiltration membrane. This invention induces the formation of a polyamide network at pore defects and nanosheet edges during confined interfacial polymerization, compensating for the defects caused by the irregular stacking of the two-dimensional MOF layered membrane. It also ensures sufficient layered nanochannels and enhances the negative charge on the nanofiltration membrane surface, facilitating the simultaneous preparation of high-flux and high-selectivity two-dimensional MOF nanofiltration membranes.

[0026] The two-dimensional MOF nanofiltration membrane prepared by this invention exhibits high permeation flux while significantly reducing the molecular weight cutoff and improving the rejection rate of divalent anions. The covalent bonds formed by the local polyamide network between nanosheet edges and at defects enhance the cross-current stability of the two-dimensional MOF membrane structure. This invention provides an effective method for developing highly efficient and stable two-dimensional MOF nanofiltration membranes, which can significantly reduce energy consumption in practical water treatment engineering and has good application potential. Attached Figure Description

[0027] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate exemplary embodiments of the invention and, together with their descriptions, serve to explain the invention and do not constitute an undue limitation thereof. In the drawings:

[0028] Figure 1 The water flux is for Example 1 and Comparative Examples 1-3.

[0029] Figure 2 The salt rejection rates are those of Example 1 and Comparative Examples 1-3.

[0030] Figure 3 The images are scanning electron microscope (SEM) images of Example 1 and Comparative Examples 1-3.

[0031] Figure 4 The molecular weight cutoff diagrams are for Example 1 and Comparative Examples 1-3.

[0032] Figure 5 The water flux and sodium sulfate rejection rate of the nanofiltration membrane prepared by using amino-modified Al-TCPP and Cr-TCPP MOF nanosheets as the base membrane in Example 1 are shown. Detailed Implementation

[0033] To make the technical solutions and advantages of the embodiments of the present invention clearer, the exemplary embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0034] The amino-modified two-dimensional MOF nanosheets in the embodiments and comparative examples of this invention were prepared by the following method:

[0035] Preparation method of two-dimensional MOFs (Cu-TCPP) nanosheets: 0.01 mmol of copper nitrate, 400 μL of formic acid, and 50 mg of polyvinylpyrrolidone were dispersed in 120 mL of dimethylformamide-ethanol (V:V = 3:1) mixed solvent. Then, 0.005 mmol of TCPP was added to the above solvent containing 0.01 mmol of copper ligand with stirring, and the mixture was heated at 80 °C for 12 h. After washing with ethanol, carboxylated MOF nanosheets were obtained. The concentration of the carboxylated MOF nanosheet aqueous dispersion was 1.0 mg / mL.

[0036] Synthesis of piperazine-grafted MOF nanosheets: Preparation of EDC / NHS solution: EDC and NHS solvents were added sequentially to deionized water at mass concentrations of 0.1 wt% and 0.05 wt%, respectively. Then, MES buffer solution (mass concentration 0.1 wt%) was added. The mixed solution was then added to Cu-TCPP suspension to activate the edge carboxyl groups. PIP solution (1.5 mg / mL) was added, and the mixture was stirred for 8 hours. After washing three times with deionized water and centrifuging, the piperazine-grafted suspension with a concentration of 0.75 mg / mL was obtained.

[0037] Using the same activation method, monomers to be grafted, such as m-phenylenediamine and PEI, are added to the activated MOF dispersion to obtain the MOF nanosheets grafted with the amino groups of m-phenylenediamine and PEI.

[0038] Using the same activation method, when the metal salt ligands are replaced with aluminum nitrate or chromium nitrate, amino-modified Al-TCPP and Cr-TCPP MOF nanosheets can be prepared.

[0039] Example 1

[0040] The surface of the porous PES-based membrane (22.1 cm²) 2 The nanosheets were immersed in a 0.2 wt% polyvinyl alcohol solution for 3 minutes to enhance the adhesion between the nanosheets and the scaffold. Then, a piperazine solution was added as a free amine monomer to 50 mL of a 10 mg / L piperazine-grafted MOF dispersion, wherein the piperazine concentration was 0.02 wt%. The nanosheet solution was assembled onto a PES microporous substrate using a vacuum filtration device, with a nanosheet loading of 0.0226 mg / cm³. 2A layered piperazine-grafted two-dimensional MOF membrane containing free piperazine monomers was obtained. Based on the two-dimensional MOF membrane, a confined interfacial polymerization was used to prepare a MOF nanofiltration membrane. First, the upper surface of the two-dimensional membrane was immersed in 10 mL of a hexane solution containing trimesoyl chloride (TMC, 0.15 wt%) and reacted at room temperature for 2 min. Then, excess solution on the membrane surface was discarded, and the membrane surface was rinsed three times with hexane solution to remove unreacted monomers. After heat treatment in an oven at 60 °C for 3 min, a two-dimensional MOF composite membrane was prepared.

[0041] The membrane was immersed in deionized water, and then its filtration performance was tested. Using a CF016 membrane module, the water flux and inorganic salt ion rejection capacity of the composite membrane prepared in the above embodiment were tested under cross-flow filtration conditions at an operating pressure of 4 bar and a temperature of 25°C. The test results are shown below. Figure 2 and Figure 3 .

[0042] Comparative Example 1

[0043] Comparative Example 1 used a vacuum-assisted filtration method to assemble 0.5 mg of ungrafted modified MOF nanosheets and a 0.02 wt% piperazine mixed solution into nanosheets with a loading of 0.0226 mg / cm³. 2 The two-dimensional MOF membrane was not subsequently reacted with the TMC / n-hexane solution. Comparative Example 1 was dried and then immersed in water, and its filtration performance was tested in the same manner as in Example 1.

[0044] Comparative Example 2

[0045] Comparative Example 2 used a vacuum-assisted filtration method to assemble 0.5 mg of piperazine-grafted MOF nanosheets with a mixed solution of 0.02 wt% piperazine to form nanosheets with a loading of 0.0226 mg / cm³. 2 The two-dimensional MOFs film was obtained, and no confined interface polymerization process was subsequently performed; otherwise, it was the same as in Example 1.

[0046] Comparative Example 3

[0047] Comparative Example 3 used a vacuum-assisted filtration method to assemble 0.5 mg of piperazine-grafted MOF nanosheets onto a PES microporous substrate, obtaining a two-dimensional MOF membrane containing layered piperazine grafts with a nanosheet loading of 0.0226 mg / cm³. 2The upper surface of the two-dimensional membrane was immersed in 10 mL of a hexane solution containing TMC (0.15 wt%) and reacted at room temperature for 2 min. Then, excess solution was discarded from the membrane surface, and the membrane surface was rinsed three times with hexane solution to remove unreacted monomers. After heat treatment in an oven at 60°C for 3 min, a two-dimensional MOFs composite membrane was prepared. The preparation conditions for Comparative Example 3 were the same as in Example 1, except that no free amine monomer was added during the assembly of the two-dimensional membrane. The filtration performance was tested in the same manner as in Example 1, and the results are shown below. Figure 2 and Figure 3 .

[0048] Comparative Example 4

[0049] Comparative Example 4 uses a conventional nanofiltration membrane preparation method with nanomaterials as the intermediate layer. First, a 20 kDa PES ultrafiltration membrane is used as the base membrane. Then, an aqueous solution of 0.5 mg of MOF nanosheets is deposited onto the base membrane using a vacuum-assisted filtration method, assembling a membrane with a nanosheet loading of 0.0226 mg / cm³. 2 A layered MOF membrane was then constructed. 30 mL of PIP solution (0.15 wt%) was carefully poured onto the membrane surface, and the solution was removed from the membrane surface using a vacuum filtration device after 1 minute. 10 mL of a 0.15 wt% TMC / n-hexane solution was then poured onto the membrane surface, and the reaction was allowed to proceed at room temperature for 2 minutes before removing excess organic solvent. The remaining post-treatment procedures were the same as in Example 1, and the filtration performance was tested in the same manner as in Example 1. The results are shown in [Figure 1]. Figure 5 .

[0050] Table 1. Performance test results of nanofiltration membranes prepared with different concentrations of free PIPs and MOFs.

[0051]

[0052]

[0053] The pure water permeation flux of Example 1 and Comparative Examples 1-3 is shown in the figure. Figure 1It can be seen that the pure water flux of Comparative Example 1 and Comparative Example 2, assembled from nanosheets, is 141.70 LMH / bar and 202.30 LMH / bar, respectively, indicating that the two-dimensional layered membrane has excellent water permeability. As shown in Table 1, Comparative Example 1 and Comparative Example 2 have low rejection rates for divalent salt sodium sulfate (40.7% and 37.8%, respectively), which cannot meet the requirements for nanofiltration membrane removal of divalent salts. This is attributed to defects formed by the stacking of two-dimensional membranes and the lack of surface charge effect. Example 1 is based on the process of Comparative Example 2, but introduces a confined interfacial polymerization process. By adding 10 mL of TMC / n-hexane solution to react on the surface of the two-dimensional membrane, a very small amount of free PIP molecules (0.02 wt%) at the defects of the two-dimensional system and amino groups at the edges of the nanosheets will form a local polyamide network structure with TMC. It can be seen that as the confined polyamide structure gradually forms at the defects, the pure water flux of Example 1 gradually decreases from Comparative Example 1 (141.70 LMH / bar) to (37.40 LMH / bar). This is due to the increased membrane compactness and the disappearance of surface defects. Figure 3 The molecular weight cutoff was significantly reduced. Figure 4 The concentration of organic pollutants (NO3) was reduced from 704.6 Da to 305.6 Da, which can improve the retention performance of MOF membranes for neutral organic pollutants.

[0054] The difference between Example 1 and Comparative Example 3 is that the two-dimensional membrane system of Comparative Example 3 does not contain free amine monomers; only the groups at the edges of the nanosheets react with TMC. This results in the inability to form a polyamide network structure at the defects in Comparative Example 3, with only a small number of amide groups forming between the layers. Its water flux is as high as 63.4 LMH / bar, but the sodium sulfate rejection rate is 88.2%, which does not meet the general requirements of nanofiltration membranes. Therefore, the PIP free molecules pre-embedded at the defects play a crucial role in the confined interfacial polymerization method, determining whether the defects in the two-dimensional membrane pores can be effectively compensated.

[0055] The inorganic salt rejection rates of Examples 1 and 1-3 are as follows: Figure 2 The inorganic salt solution concentration was 1 g / L, and the pH was 7.2. It can be seen that as the pure water flux of the example decreased, the rejection rate for divalent sulfate ions significantly increased, while the rejection rate for sodium chloride remained at a low level. This is attributed to the significantly enhanced negative charge on the membrane surface. The selectivity S(NaCl / Na2SO4) for sodium sulfate and sodium chloride was calculated using the formula... The MOF nanofiltration membrane prepared by confined interface polymerization maintains high permeation flux while also exhibiting high S (NaCl / Na2SO4) selectivity. As shown in Table 2, with the improvement of the confined interface polymerization method, the S of the composite nanofiltration membrane in Example 1 increased significantly, and its permeation performance was also good. This indicates that the pore structure and surface charge of the MOF nanofiltration membrane have reached nanofiltration levels. High efficiency removal of divalent anions was achieved under high water permeation flux, and the confined interface polymerization method provides a reference for controlling two-dimensional membranes to achieve nanofiltration separation.

[0056] Table 2 Selectivity S of Example 1 and Comparative Examples 1-3

[0057]

[0058] Therefore, the confined interfacial polymerization method can effectively compensate for the defect space of two-dimensional layered MOF membranes, and the resulting composite membrane can significantly improve the sulfate rejection rate. The presence of pre-embedded free amine monomers in the interlayer plays a crucial role in subsequent targeted polyamide polymerization, which can significantly reduce two-dimensional membrane defects. It maintains a high permeation flux and exhibits good selectivity for both divalent and monovalent salts.

[0059] The water flux and inorganic salt rejection of Example 1 and Comparative Example 4 are shown in Table 1. Comparative Example 4 used a conventional interfacial polymerization process with a two-dimensional membrane as the intermediate layer material. It can be seen that, under the same nanosheet loading, the polyamide nanofiltration membrane prepared by the conventional interfacial polymerization method has a selectivity for sodium sulfate as high as 98.5%, but the water flux of Comparative Example 4 is only 9.1 LMH / bar. This indicates that using a large amount of two-dimensional MOF material as the intermediate layer in nanofiltration membrane preparation increases the thickness of the selective separation layer, leading to an increase in membrane mass transfer resistance and a rapid decrease in water permeability, making it difficult to demonstrate the material's advantages. This highlights the advantage of the confined interfacial polymerization method used in Example 1, where a small amount of polyamide network structure is used to compensate for the defects of the two-dimensional layer, increasing the membrane's selectivity while inheriting the high permeability of the two-dimensional membrane layer.

[0060] When amino-modified Al-TCPP and Cr-TCPP MOF nanosheets are used to replace the amino-modified Cu-TCPP MOF nanosheets in Example 1, the remaining methods are the same as in Example 1, such as... Figure 5 As shown, the prepared nanofiltration membrane can also achieve a water flux of 30-45 LMH / bar, a sodium sulfate rejection rate of 96-99%, and a selectivity S of 35-45.

[0061] Examples 2-4

[0062] The deposition quality of nanosheets can control the thickness of two-dimensional nanosheet layered membranes, directly affecting membrane selectivity and permeate flux. In confined interfacial polymerization, the performance of a small amount of polyamide network structure influences the compensation of membrane defects and the improvement of membrane performance. Controlling the content of free amine monomers can affect the interfacial polymerization reaction rate and the compactness of the local polyamide network structure. By adjusting the loading mass of two-dimensional nanosheets and the concentration of free piperazine, the permeate separation performance of MOFs-composite polyamide nanofiltration membranes can be optimized to achieve different target separation requirements.

[0063] Examples 2-4 were prepared using the same confined interfacial polymerization method as Example 1. Except for the loading mass of the two-dimensional nanosheets and the mass concentration of free PIP molecules, all other aspects were the same as in Example 1. Filtration performance tests were conducted in the same manner as in Example 1. The preparation conditions and membrane performance test results are shown in Table 1. Specifically, two-dimensional MOF composite membranes were prepared using the confined interfacial polymerization method, with a deposition density of 0.0226 mg / cm³. 2 0.0113 mg / cm 2 Orthogonal experiments were conducted to fabricate PIP-MOF nanosheets and a solution containing 0.02 wt% and 0.04 wt% piperazine to form a film.

[0064] The results showed that Examples 2-4 all met the requirements for selective retention of divalent salts by nanofiltration membranes and exhibited high water flux, indicating that the pore size of the two-dimensional MOFs membrane was appropriate, interlayer defects were effectively compensated, the charge density was high, and the filtration performance was good. Comparatively, Examples 1 and 2 showed better permeate flux than Examples 3 and 4; Examples 1 and 3 had higher sodium sulfate rejection rates. This indicates that the concentration of free PIP has a significant impact on the membrane permeate flux. Increased PIP concentration at layer defects increases the cross-linking degree of the local polyamide network structure and the coverage area of ​​the local polyamide structure, which is detrimental to the rapid permeation of water through the two-dimensional interlayer channels. The loading of PIP-MOFs nanosheets in Examples 1 and 3 was 0.0113 mg / cm³ compared to Examples 2 and 5. 2 Increased to 0.0226 mg / cm³ 2 The retention rates of sodium sulfate were all above 98%, meeting the standards for commercial nanofiltration membranes. This indicates that in the confined interfacial polymerization method, increasing the thickness of the two-dimensional layer can achieve a high retention rate of sodium sulfate, satisfying the requirement for high retention rates of divalent salts.

[0065] Comparative Example 5

[0066] In addition to preparing two-dimensional membranes using piperazine-grafted MOF nanosheets, we also investigated the preparation of MOF-based composite nanofiltration membranes using m-phenylenediamine and polyethyleneimine (600 Da, 1800 Da)-grafted MOF nanosheets as building blocks via confined interfacial polymerization. The results showed that the water permeation flux decreased in the nanofiltration membrane prepared using m-phenylenediamine and polyethyleneimine-grafted MOF nanosheets due to increased crosslinking degree. The nanofiltration membrane prepared using polyethyleneimine-grafted MOFs exhibited a slightly reduced rejection rate for divalent sulfates and a significantly increased rejection rate for divalent cations due to decreased surface negative charge and increased positive charge, which is detrimental to achieving selective separation of divalent sulfates and high permeation flux.

[0067] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, and for those of ordinary skill in the art, various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. Therefore, the present invention is not limited to the specific details without departing from the general concept defined by the claims and their equivalents.

Claims

1. A method for preparing a confined interface polymerization assembly of two-dimensional MOFs composite nanofiltration membrane, characterized in that, The preparation method includes: using a porous support layer as a substrate, simultaneously depositing amino-modified MOFs two-dimensional nanosheets and free amine monomers on the substrate, removing excess water to form a two-dimensional layered membrane, then immersing the surface of the two-dimensional layered membrane in an organic monomer solution to carry out a two-dimensional interlayer confined interfacial polymerization reaction, subsequently removing excess solution, and obtaining the two-dimensional MOFs composite nanofiltration membrane assembled by the confined interfacial polymerization method after heat treatment; the amino substances used to modify MOFs include piperazine, m-phenylenediamine, and polyethyleneimine; the two-dimensional MOFs nanosheets are Cu-TCPP, Al-TCPP, Cr-TCPP, and Zr-BTB carboxyl ligand MOFs; the free amine monomer is piperazine, and the mass concentration of the piperazine solution is 0.01 wt%-0.15 wt%.

2. The preparation method according to claim 1, characterized in that: The amino group used to modify MOFs is piperazine.

3. The preparation method according to claim 1, characterized in that: The mass concentration of the piperazine solution is 0.01 wt% to 0.04 wt%.

4. The preparation method according to claim 1, characterized in that, The preparation method of the amino-modified MOFs two-dimensional nanosheets includes: preparing an EDC / NHS buffer solution, adding the buffer solution to the MOFs nanosheet suspension, then adding an amino substance solution, stirring and reacting for 4-24 h, and washing after solid-liquid separation to obtain amino-modified MOFs two-dimensional nanosheets.

5. The preparation method according to claim 1, characterized in that, The deposition mass of the modified MOF two-dimensional nanosheets was 0.000452 mg / cm³. 2 ~0.0679 mg / cm³ 2 .

6. The preparation method according to claim 5, characterized in that: The deposition mass of the modified MOF two-dimensional nanosheets was 0.0113 mg / cm³. 2 ~0.0452 mg / cm 2 .

7. The preparation method according to claim 1, characterized in that, The interfacial polymerization reaction takes 30-300 seconds; the heat treatment takes 2-6 minutes and the heat treatment temperature is 40-70 °C.

8. The preparation method according to claim 7, characterized in that, The interfacial polymerization reaction takes 30-90 seconds, and the heat treatment takes 3-5 minutes.

9. The two-dimensional MOFs composite nanofiltration membrane prepared by the preparation method according to any one of claims 1 to 8.

10. A water treatment method, characterized in that, The method utilizes the two-dimensional MOFs composite nanofiltration membrane described in claim 9 to treat water.

11. The application of the two-dimensional MOFs composite nanofiltration membrane according to claim 9 in water treatment processes.

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