Crosslinked polyimide nanofiltration membrane and method for preparing the same

Abstract

The application relates to the technical field of nanofiltration separation, and discloses a cross-linked polyimide nanofiltration membrane and a preparation method thereof. The preparation method comprises the following steps: (1) a support layer is contacted with a solution containing an amine group monomer to perform pretreatment, and a modified support layer is obtained; (2) an organic phase solution is added dropwise to the surface of the modified support layer to perform an interfacial polymerization reaction, and a polyamide acid membrane is obtained; wherein the organic phase solution comprises one or more of a monomer containing an acyl chloride group, a cross-linking agent and an organic solvent; and (3) the polyamide acid membrane is contacted with a chemical imidization solution to perform an imidization reaction, and a cross-linked polyimide nanofiltration membrane is obtained. The cross-linked polyimide nanofiltration membrane can efficiently separate a salt solution, and the water flux is improved while the retention rate is maintained.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration separation technology, and more specifically, to a cross-linked polyimide nanofiltration membrane and its preparation method. Background Technology

[0002] Nanofiltration (NF) is a novel membrane separation technology. Compared to reverse osmosis, it features lower energy consumption, lower rejection rate for monovalent ions, and higher rejection rate for small organic molecules and polyvalent ions. It is commonly used in oil refining and degumming, catalyst recovery, polymer fractionation, and seawater desalination processes. Particularly in water softening, nanofiltration membranes can remove pesticides, synthetic detergents, soluble organic matter, and calcium carbonate from aqueous solutions. 2+ and Mg 2+ Hardness component. However, nanofiltration membranes suffer from trade-offs between water flux and retention rate in practical applications, as well as insufficient antifouling performance, leading to reduced membrane lifespan. Therefore, selecting suitable polymers as membrane materials has become a key focus for nanofiltration membranes. Currently, the main polymers preferred for manufacturing nanofiltration membranes include sulfonated polysulfone (PSF), polyester (PET), polyimide (PI), and polyamide (PA). These polymers, with their aromatic backbone or imide bonds, provide basic solvent resistance and antifouling properties. Polyimide, due to its good heat resistance, mechanical properties, and solvent resistance, has become a preferred material for nanofiltration membranes.

[0003] Initially, polyimide membranes were frequently used for gas separation due to their dense chain structure and controllable chain spacing. Subsequently, due to the excellent dehydration properties of dense polyimide membranes to organic solvents such as acetone and isopropanol, polyimide membranes have been successfully applied to pervaporation. Since then, polyimide membranes have gradually developed towards porous membrane fabrication and have become one of the key materials for nanofiltration membranes.

[0004] Currently, the main methods for preparing polyimide nanofiltration membranes include phase inversion (LS), interfacial polymerization (IP), and blending. Polyimide nanofiltration membranes prepared by the traditional LS method have relatively low water flux due to the thickness of the separation layer, compact structure, and high membrane resistance. To meet the design requirements of lighter, thinner, and more reliable products, interfacial polymerization is used to obtain a thinner polyimide (PI) active layer. However, the linear polyimide structure leads to chain phase compression, resulting in a reduction in water flux. Generally, modification methods such as thermal annealing, blending, and crosslinking can improve the performance of polyimide nanofiltration membranes to some extent, but the first two methods require high temperatures or the use of expensive monomers. In comparison, using a crosslinking structure is a simple method to improve the plasticity and chemical resistance of polyimide nanofiltration membranes.

[0005] Therefore, it is of great significance to research and develop a new method for preparing polyimide nanofiltration membranes. Summary of the Invention

[0006] The purpose of this invention is to overcome the problem of low water flux of polyimide nanofiltration membranes prepared by existing technologies, and to provide a cross-linked polyimide nanofiltration membrane and its preparation method. This cross-linked polyimide nanofiltration membrane can efficiently separate salt solutions and improve water flux while maintaining a rejection rate of ≥92%.

[0007] To achieve the above objectives, a first aspect of the present invention provides a method for preparing a cross-linked polyimide nanofiltration membrane, wherein the preparation method includes:

[0008] (1) The support layer is pretreated by contacting a solution containing an amine group with the support layer to obtain a modified support layer;

[0009] (2) An organic phase solution is dropped onto the surface of the modified support layer to carry out an interfacial polymerization reaction to obtain a polyamic acid film; wherein, the organic phase solution includes one or more of a monomer containing acyl chloride groups, a crosslinking agent and an organic solvent;

[0010] (3) The polyamic acid membrane is contacted with a chemical imidization solution to undergo an imidization reaction to obtain a cross-linked polyimide nanofiltration membrane.

[0011] A second aspect of the present invention provides a cross-linked polyimide nanofiltration membrane prepared by the preparation method described above.

[0012] The technical solution provided by the present invention has the following advantages through the above-described technical solution:

[0013] (1) When the cross-linked polyimide nanofiltration membrane provided by the present invention is used for brine separation, the water flux is 9 L / (m²). -2 ·h -1 ·bar);

[0014] (2) The cross-linked polyimide nanofiltration membrane provided by the present invention has a rejection rate of 94% for salt solutions, such as CaSO4, and maintains a high rejection rate for salt solutions.

[0015] (3) The cross-linked polyimide nanofiltration membrane provided by the present invention has a CaSO4 rejection rate of 93% after being soaked in organic solvents such as tetrahydrofuran (THF), N,N-dimethylacetamide (DMAc), and N,N-dimethylformamide (DMF) for a long time, and has good solvent resistance.

[0016] (4) In addition, the preparation method of the present invention reduces the salt / water system separation steps, saving equipment and operating costs. Attached Figure Description

[0017] Figure 1 It is PTFE 0.1 H-PTFE0.22 and H-PTFE 0.45 The thickness of the polytetrafluoroethylene support layer;

[0018] Figure 2 This is the infrared spectrum of the cross-linked polyimide nanofiltration membrane prepared in Example 1;

[0019] Figure 3 These are SEM images of the surface and cross-section of the cross-linked polyimide nanofiltration membrane prepared in Example 4;

[0020] Figure 4 These are magnified SEM images of the surface and cross-section of the cross-linked polyimide nanofiltration membrane prepared in Example 4.

[0021] Figure 5 These are SEM images of the surface and cross-section of the cross-linked polyimide nanofiltration membrane prepared in Example 5;

[0022] Figure 6 This is a magnified SEM image of the surface and cross-section of the cross-linked polyimide nanofiltration membrane prepared in Example 5. Detailed Implementation

[0023] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0024] The first aspect of this invention provides a method for preparing a cross-linked polyimide nanofiltration membrane, wherein the preparation method includes:

[0025] (1) The support layer is pretreated by contacting a solution containing an amine group with the support layer to obtain a modified support layer;

[0026] (2) An organic phase solution is dropped onto the surface of the modified support layer to carry out an interfacial polymerization reaction to obtain a polyamic acid film; wherein, the organic phase solution includes one or more of a monomer containing acyl chloride groups, a crosslinking agent and an organic solvent;

[0027] (3) The polyamic acid membrane is contacted with a chemical imidization solution to undergo an imidization reaction to obtain a cross-linked polyimide nanofiltration membrane.

[0028] The inventors of this invention unexpectedly discovered that, using the preparation method of this invention, contacting a "solution containing amine-group monomers" with an "organic phase solution including monomers containing acyl chloride groups" during the interfacial polymerization reaction is beneficial for generating a regularly arranged cross-linked polyimide nanofiltration membrane. Furthermore, preferably, using a cross-linking agent containing acyl chloride groups increases the number of acyl chloride groups bonded to the amine groups in the "solution containing amine-group monomers," increasing the degree of imidization and thus increasing the degree of cross-linking of the resulting polyimide nanofiltration membrane, further facilitating the formation of a regularly arranged cross-linked polyimide nanofiltration membrane. Moreover, this regularly arranged cross-linked polyimide nanofiltration membrane has a high degree of cross-linking, which, while ensuring the retention rate of CaSO4 solution, can effectively improve the water flux. Simultaneously, this regularly arranged cross-linked polyimide nanofiltration membrane retains its separation performance after immersion in an organic solvent.

[0029] According to the present invention, in step (1), preferably, the support layer is pretreated by immersing it in an aqueous solution containing an amine group. In the present invention, "immersion" in the solution allows for more thorough contact between the support layer and the aqueous solution containing the amine group, resulting in better performance.

[0030] According to the present invention, the monomer containing an amino group is selected from one or more of m-phenylenediamine, propylenediamine and hexamethylenediamine; preferably, the monomer containing an amino group is m-phenylenediamine.

[0031] According to the present invention, the concentration of the solution containing the amino group is 2-8 wt%, preferably, the concentration of the solution containing the amino group is 4-6 wt%; more preferably, the solution containing the amino group is an aqueous solution of m-phenylenediamine.

[0032] According to the present invention, the crosslinking agent is selected from one or more of pyromellitic chloride, ethylenediamine and terephthaloyl chloride; preferably, the crosslinking agent is pyromellitic chloride.

[0033] In this invention, preferably, pyromellitic methyl chloride is used as a crosslinking agent to obtain a regularly arranged polyimide separation layer; furthermore, interfacial polymerization technology is used to perform an interfacial polymerization reaction on the polytetrafluoroethylene support layer to generate a polyamic acid membrane, and subsequent chemical imidization treatment is used to enhance the separation performance and solvent resistance of the membrane, thereby preparing a crosslinked polyimide separation layer with high throughput, high rejection rate and excellent antifouling performance.

[0034] According to the present invention, the organic solvent is selected from one or more of n-hexane, ethanol and carbon tetrachloride; preferably, the organic solvent is n-hexane.

[0035] According to the present invention, based on the total weight of the organic phase solution, the content of the monomer containing acyl chloride groups is 0.1-0.8 wt%, and the content of the crosslinking agent is 0.04-0.2 wt%; preferably, based on the total weight of the organic phase solution, the content of the monomer containing acyl chloride groups is 0.2-0.4 wt%, and the content of the crosslinking agent is 0.08-0.12 wt%. In the present invention, limiting the content of the monomer containing acyl chloride groups and the content of the crosslinking agent to the aforementioned ranges has the advantage that the resulting polyimide nanofiltration membrane has higher separation performance.

[0036] According to the present invention, the chemical imidization solution contains one or more of a catalyst, a dehydrating agent, and a solvent.

[0037] According to the present invention, the catalyst is triethylamine and / or pyridine; preferably, the catalyst is triethylamine.

[0038] According to the present invention, the dehydrating agent is selected from one or more of acetic anhydride, trifluoroacetic anhydride and propionic anhydride; preferably, the dehydrating agent is acetic anhydride.

[0039] According to the present invention, the solvent is acetone and / or toluene; preferably, the solvent is acetone.

[0040] According to the present invention, the volume ratio of the catalyst, the dehydrating agent, and the solvent is (1-3):(1-3):(5-10); preferably (2-3):1:(8-10). In this invention, limiting the amounts of the catalyst, the dehydrating agent, and the solvent to the aforementioned range has the advantage of resulting in a polyimide nanofiltration membrane with a higher degree of imidization.

[0041] According to the present invention, the pretreatment conditions include a time of 5-25 min, preferably 5-10 min. In the present invention, after pretreatment by contacting the support layer with a solution containing an amine group in step (1), the pretreated support layer is further removed and allowed to air dry naturally to remove excess aqueous solution from the surface.

[0042] According to the present invention, the conditions for the dripping include: a dripping rate of 2-5 ml / 1-10 s, that is, a dripping rate of 2-5 ml within 1-10 s; preferably 4-5 ml / s.

[0043] According to the present invention, the conditions for the interfacial polymerization reaction include: a temperature of 10-35°C and a time of 10-150 s; preferably, the temperature is 20-30°C and the time is 60-90 s. In the present invention, after the interfacial polymerization reaction, the surface of the product obtained after the interfacial polymerization reaction is rinsed 2-3 times with a hexane solution to remove excess organic monomers and prepare a polyamic acid film.

[0044] In this invention, the pretreatment, the drop acceleration rate, and the conditions of the interfacial polymerization reaction are specifically limited to the aforementioned range, which enables the prepared cross-linked polyimide nanofiltration membrane to have a better thickness, thereby obtaining better mechanical properties, while also having high throughput and high rejection rate.

[0045] According to the present invention, the imidization reaction conditions include a time of 4-24 hours, preferably 8-12 hours. In the present invention, the chemical imidization solution immersion treatment can make the polymer chains of the cross-linked polyimide separation layer more compact, reduce the free volume, and improve the solvent resistance.

[0046] According to the present invention, the support layer is a polytetrafluoroethylene support layer; preferably, the pore size of the support layer is 0.22-0.30 μm. In the present invention, by using the aforementioned support layer with a specifically defined pore size, a cross-linked polyimide nanofiltration membrane with high separation performance can be obtained.

[0047] According to the present invention, in step (3), the preparation method further includes: imidizing the support layer obtained by the interfacial polymerization reaction and the cross-linked polyamic acid separation layer stacked on the surface of the support layer to obtain the cross-linked polyimide nanofiltration membrane.

[0048] According to the present invention, in step (3), preferably, the polyamic acid membrane is immersed in a chemical imidization solution to carry out an imidization reaction, thereby obtaining a cross-linked polyimide nanofiltration membrane. In the present invention, "immersion" in the solution allows the polyamic acid membrane to have more sufficient contact with the chemical imidization solution, resulting in better performance.

[0049] According to the present invention, the thickness of the cross-linked polyimide nanofiltration membrane is less than 1 μm, preferably 50-800 nm. This thickness ensures that the prepared polyimide nanofiltration membrane has a high rejection rate while also having a large flux and good mechanical properties.

[0050] A second aspect of the present invention provides a cross-linked polyimide nanofiltration membrane prepared by the preparation method described above.

[0051] According to a particularly preferred embodiment of the present invention, a method for preparing a cross-linked polyimide nanofiltration membrane includes:

[0052] (1) Immerse the polytetrafluoroethylene support layer in an aqueous solution of m-phenylenediamine for 5-10 minutes, and then air dry it naturally to remove excess aqueous solution from the surface; wherein the concentration of the aqueous solution of m-phenylenediamine is 4-6 wt%.

[0053] (2) Then, 5-10 ml of a solution of pyromellitic tetracarboxylate organic phase (n-hexane) containing a crosslinking agent (pyromellitic tetracarboxylate) is added dropwise at a dropping rate of 4-5 ml / s to carry out an interfacial polymerization reaction. The interfacial polymerization time is 60-90 s. Subsequently, the surface is rinsed 2-3 times with n-hexane solution to remove excess organic phase monomers and prepare a polyamic acid film. The content of the monomer containing acyl chloride groups is 0.2-0.4 wt%, and the content of the crosslinking agent is 0.08-0.12 wt%, based on the total weight of the organic phase solution.

[0054] (3) The polyamic acid membrane obtained in step (2) is immersed in a chemical imidization solvent for 8-12 hours. The chemical imidization solution includes a catalyst, a dehydrating agent and a solvent. The dehydrating agent is acetic anhydride, the catalyst is triethylamine, and the solvent is acetone. The volume ratio of acetic anhydride, triethylamine and acetone in the chemical imidization solution is (2-3):1:(8-10). As a result, a cross-linked polyimide nanofiltration membrane is obtained.

[0055] The present invention will be described in detail below through embodiments.

[0056] In the following examples and comparative examples:

[0057] The surface chemical structure of the polyimide nanofiltration membrane was determined by infrared spectroscopy using an IS50 infrared spectrometer purchased from Nicolet, USA. Surface morphology parameters were determined by scanning electron microscopy (SEM) using a SUPRA55 SEM purchased from Carl Zeiss, Germany. Surface chemical composition parameters were determined by XPS (X-ray photoelectron spectroscopy) using an ESCALAB250Xi XPS purchased from Thermo Fisher Scientific, USA.

[0058] The m-phenylenediamine raw material is a commercially available product of Sinopharm Chemical Reagent Co., Ltd.; the pyromellitic trimethylol chloride raw material is a commercially available product of Aladdin Chemical Reagent Co., Ltd.

[0059] Example 1

[0060] This embodiment illustrates a cross-linked polyimide nanofiltration membrane prepared using the preparation method of the present invention.

[0061] (1) Preparation of solution

[0062] (1-1) m-Phenylenediamine aqueous solution: Dissolve m-phenylenediamine in 100 mL of water to prepare a 5 wt% m-phenylenediamine aqueous solution.

[0063] (1-2) Pyromellitic tetracarboxylate organic phase solution: Pyromellitic tetracarboxylate crystals and crosslinking agent pyromellitic tricarboxylate chloride were dissolved in 76 mL of n-hexane to prepare a 0.2 wt% pyromellitic tetracarboxylate chloride n-hexane solution, wherein the content of crosslinking agent was 0.08 wt%.

[0064] (1-3) Chemical imidization solution, wherein the volume ratio of the dehydrating agent acetic anhydride, the catalyst triethylamine, and the solvent acetone is 3:1:10.

[0065] (2) Preparation of cross-linked polyimide nanofiltration membrane

[0066] (2-1) PTFE with a pore size of 0.1 μm 0.1 ) and hydrophilic pore size of 0.22 μm (H-PTFE) 0.22 ), 0.45μm (H-PTFE) 0.45 The polytetrafluoroethylene (PTFE) porous support layer was immersed in the m-phenylenediamine solution prepared in step (1-1) for 5 minutes, and then removed and air-dried to remove excess aqueous phase solution from the surface.

[0067] (2-2) After that, the modified polytetrafluoroethylene support layer of step (2-1) was placed in the self-made interfacial polymerization reaction device, and 6 mL of pyromellitic tetramethyl chloride organic phase solution was evenly dropped onto its surface. The interfacial polymerization reaction temperature was 25℃ and the time was 60s. After the reaction was completed, the membrane surface was rinsed with n-hexane 2-3 times to remove the excess organic phase monomers on the membrane surface and obtain a polyamic acid membrane.

[0068] (2-3) The polyamic acid membrane prepared in step (2-2) was then immersed in a chemical imidization solution for 8 hours. After the reaction was completed, the membrane surface was repeatedly rinsed to remove excess chemical imidization solution, and a cross-linked polyimide nanofiltration membrane was obtained, labeled as S1.

[0069] The cross-linked polyimide nanofiltration membrane S1 was placed in distilled water for testing and characterization.

[0070] The separation performance of the prepared cross-linked polyimide nanofiltration membrane S1 was tested at 25℃. The solution used was 0.5 g / L CaSO4 solution, and the operating pressure was 4 bar. The test results are shown in Table 1.

[0071] Table 1

[0072] support layer PINF membrane <![CDATA[Water flux (L / (m -2 ·h -1 ·bar))]]> Retention rate (%) <![CDATA[PTFE 0.1 ]]> <![CDATA[PTFE 0.1 / PI]]> 2.6 21 <![CDATA[H-PTFE 0.22 ]]> <![CDATA[H-PTFE 0.22 / PI]]> 9.1 94 <![CDATA[H-PTFE 0.45 ]]> <![CDATA[H-PTFE 0.45 / PI]]> 18.9 22

[0073] Table 1 shows that the hydrophilic pore size is 0.22 μm (H-PTFE). 0.22The porous polytetrafluoroethylene (PTFE) support layer, with a retention rate exceeding 92% (i.e., reaching 94%), exhibits a water flux of 9.1 L / (m²). -2 ·h -1 ·bar), the effect is best.

[0074] in addition, Figure 1 It is PTFE 0.1 H-PTFE 0.22 and H-PTFE 0.45 The thickness of the polytetrafluoroethylene support layer, from Figure 1 It can be concluded that: PTFE 0.1 The support layer is approximately 0.13 mm thick, H-PTFE. 0.22 The support layer is approximately 0.16 mm thick, H-PTFE. 0.45 The support layer is the thickest due to its rough surface, measuring approximately 0.31 mm.

[0075] in addition, Figure 2 This is the infrared spectrum of the cross-linked polyimide nanofiltration membrane prepared in Example 1. Figure 2 It can be concluded that at 1748cm -1 The peak at 1687 cm⁻¹ is the C=O antisymmetric tensile vibration peak; -1 The peak at 1375 cm⁻¹ is a C=O symmetrical tensile vibration peak; -1 The nearby peak is a CN tensile vibration peak; 708cm -1 The peak at the specified location corresponds to the tensile vibration peak of the imide ring. These results confirm that interfacial polymerization and subsequent imidization treatment successfully formed PI separation layers on PTFE support layers with three different pore sizes.

[0076] Example 2

[0077] This embodiment illustrates a cross-linked polyimide nanofiltration membrane prepared using the preparation method of the present invention.

[0078] (1) Preparation of solution

[0079] (1-1) m-Phenylenediamine aqueous solution: Dissolve m-phenylenediamine in 100 mL of water to prepare a 5 wt% m-phenylenediamine aqueous solution.

[0080] (1-2) Pyromellitic tetracarboxylate organic phase solution: Pyromellitic tetracarboxylate crystals and crosslinking agent pyromellitic tricarboxylate chloride were dissolved in 76 mL of n-hexane to prepare a 0.2 wt% pyromellitic tetracarboxylate chloride n-hexane solution, wherein the crosslinking agent content was 0.05 wt%.

[0081] (1-3) Chemical imidization solution, wherein the volume ratio of the dehydrating agent acetic anhydride, the catalyst triethylamine, and the solvent acetone is 3:1:10.

[0082] (2) Preparation of cross-linked polyimide nanofiltration membrane

[0083] (2-1) Hydrophilic PTFE with a pore size of 0.22 μm 0.22 The polytetrafluoroethylene (PTFE) porous support layer was immersed in the m-phenylenediamine solution prepared in step (1-1) for 5 minutes, and then removed and air-dried to remove excess aqueous phase solution from the surface.

[0084] (2-2) After that, the modified polytetrafluoroethylene support layer of step (2-1) was placed in the self-made interfacial polymerization reaction device, and 6 mL of pyromellitic tetramethyl chloride organic phase solution was evenly dropped onto its surface. The interfacial polymerization reaction temperature was 25℃ and the time was 60s. After the reaction was completed, the membrane surface was rinsed with n-hexane 2-3 times to remove the excess organic phase monomers on the membrane surface and obtain a polyamic acid membrane.

[0085] (2-3) The polyamic acid membrane prepared in step (2-2) was then immersed in a chemical imidization solution for 8 hours. After the reaction was completed, the membrane surface was repeatedly rinsed to remove excess chemical imidization solution, and a cross-linked polyimide nanofiltration membrane was obtained, labeled as S2.

[0086] The cross-linked polyimide nanofiltration membrane S2 was placed in distilled water for testing and characterization.

[0087] The separation performance of the prepared cross-linked polyimide nanofiltration membrane S2 was tested at 25℃ using a 0.5 g / L CaSO4 solution and an operating pressure of 4 bar. The test result showed a water flux of 6.2 L / (m³). -2 ·h -1 (·bar), with a CaSO4 retention rate of 92%.

[0088] Example 3

[0089] This embodiment illustrates a cross-linked polyimide nanofiltration membrane prepared using the preparation method of the present invention.

[0090] Crosslinked polyimide nanofiltration membranes were prepared using the same method as in Example 2, except that in steps (1-2), the crosslinking agent content in the pyromellitic tetracarboxylate organic phase solution was 0.16 wt%.

[0091] The separation performance of the prepared cross-linked polyimide nanofiltration membrane S3 was tested at 25℃. The solution used was 0.5 g / L CaSO4 solution, the operating pressure was 4 bar, and the water flux was 6.4 L / (m³). -2 ·h -1 (·bar), with a CaSO4 retention rate of 92%.

[0092] Example 4

[0093] This embodiment illustrates a cross-linked polyimide nanofiltration membrane prepared using the preparation method of the present invention.

[0094] Crosslinked polyimide nanofiltration membranes were prepared using the same method as in Example 2, except that in steps (1-2), the crosslinking agent content in the pyromellitic tetracarboxylate organic phase solution was 0.20 wt%.

[0095] The separation performance of the prepared cross-linked polyimide nanofiltration membrane S4 was tested at 25℃. The solution used was 0.5 g / L CaSO4 solution, the operating pressure was 4 bar, and the water flux was 6.6 L / (m³). -2 ·h -1 (·bar), with a CaSO4 retention rate of 93%.

[0096] in addition, Figure 3 These are SEM images of the surface and cross-section of the cross-linked polyimide nanofiltration membrane prepared in Example 4. Figure 4 These are magnified SEM images of the surface and cross-section of the cross-linked polyimide nanofiltration membrane prepared in Example 4. Figure 3 and Figure 4 It can be concluded that the membrane surface exhibits a dense porous network structure, which is due to the fact that the MPDA monomer structure is an aromatic diamine. The PI separation layer can be clearly seen, and the membrane thickness is about 1 μm.

[0097] Example 5

[0098] This embodiment illustrates a cross-linked polyimide nanofiltration membrane prepared using the preparation method of the present invention.

[0099] Crosslinked polyimide nanofiltration membranes were prepared using the same method as in Example 2, except that in steps (1-2), the crosslinking agent content in the pyromellitic tetracarboxylate organic phase solution was 0.08 wt%.

[0100] The solvent resistance of the prepared cross-linked polyimide nanofiltration membrane S5 was tested at 25℃. After soaking in tetrahydrofuran, N,N-dimethylacetamide, and methanol for 48 h, the water flux was 9.5 L / (m³). -2 ·h -1 (bar). The retention rate of CaSO4 was 93%.

[0101] Figure 5 These are SEM images of the surface and cross-section of the cross-linked polyimide nanofiltration membrane prepared in Example 5. Figure 6 These are magnified SEM images of the surface and cross-section of the cross-linked polyimide nanofiltration membrane prepared in Example 5. Figure 5 and Figure 6The membrane surface exhibits a dense porous network structure, which is due to the aromatic diamine structure of the MPDA monomer. A PI separation layer is clearly visible, and the membrane thickness is approximately 1 μm. Figure 3 In terms of film thickness, PI TMC-0.08 <PI TMC-0.20 This is due to the increased content of crosslinking agent, which increases the degree of crosslinking and also increases the film thickness.

[0102] Example 6

[0103] Crosslinked polyimide nanofiltration membranes were prepared using the same preparation method as in Example 5, except that in steps (1-2), the crosslinking agent content in the pyromellitic tetracarboxylate organic phase solution was 0.12 wt%.

[0104] The separation performance of the prepared cross-linked polyimide nanofiltration membrane S6 was tested at 25℃ using a 0.5 g / L CaSO4 solution and an operating pressure of 4 bar. The test result showed a water flux of 8.7 L / (m³). -2 ·h -1 (·bar), with a CaSO4 retention rate of 94%.

[0105] Example 7

[0106] Crosslinked polyimide nanofiltration membranes were prepared using the same preparation method as in Example 5, except that in step (2-1), the phrase "chemical imidization solution, wherein the volume ratio of dehydrating agent acetic anhydride, catalyst triethylamine, and solvent acetone is 3:1:10" was modified to "chemical imidization solution, wherein the volume ratio of dehydrating agent acetic anhydride, catalyst triethylamine, and solvent acetone is 1:1:10".

[0107] The separation performance of the prepared cross-linked polyimide nanofiltration membrane S7 was tested at 25℃ using a 0.5 g / L CaSO4 solution and an operating pressure of 4 bar. The test result showed a water flux of 7.9 L / (m³). -2 ·h -1 (·bar), with a CaSO4 retention rate of 93%.

[0108] Example 8

[0109] Crosslinked polyimide nanofiltration membranes were prepared using the same preparation method as in Example 5, except that in steps (1-2), the "crosslinking agent pyromellitic chloride" in the pyromellitic tetracarboxylate organic phase solution was changed to "crosslinking agent terephthaloyl chloride".

[0110] The separation performance of the prepared cross-linked polyimide nanofiltration membrane S8 was tested at 25℃ using a 0.5 g / L CaSO4 solution and an operating pressure of 4 bar. The test result showed a water flux of 8.4 L / (m³). -2 ·h -1 (·bar), with a CaSO4 retention rate of 93%.

[0111] Comparative Example 1

[0112] Crosslinked polyimide nanofiltration membranes were prepared using the same preparation method as in Example 2, except that in steps (1-2), the crosslinking agent content in the pyromellitic tetracarboxylate organic phase solution was 0.03 wt%.

[0113] The separation performance of the prepared cross-linked polyimide nanofiltration membrane DS1 was tested at 25℃ using a 0.5 g / L CaSO4 solution and an operating pressure of 4 bar. The test result showed a water flux of 5.7 L / (m³). -2 ·h -1 (·bar), with a CaSO4 retention rate of 91%.

[0114] Comparative Example 2

[0115] Crosslinked polyimide nanofiltration membranes were prepared using the same preparation method as in Example 5, except that in steps (1-2), the "crosslinking agent trimethylolpropionate chloride" in the pyromellitic tetracarboxylate organic phase solution was changed to "crosslinking agent hexamethylenediamine".

[0116] The separation performance of the prepared cross-linked polyimide nanofiltration membrane DS2 was tested at 25℃ using a 0.5 g / L CaSO4 solution and an operating pressure of 4 bar. The test result showed a water flux of 6.1 L / (m²). -2 ·h -1 (·bar), with a CaSO4 retention rate of 91%.

[0117] Comparative Example 3

[0118] Crosslinked polyimide nanofiltration membranes were prepared using the same preparation method as in Example 3, except that in steps (1-2), the content of the crosslinking agent in the pyromellitic tetracarboxylate organic phase solution was changed from "0.04 wt%" to "0 wt%".

[0119] The separation performance of the prepared cross-linked polyimide nanofiltration membrane DS3 was tested at 25℃ using a 0.5 g / L CaSO4 solution and an operating pressure of 4 bar. The test result showed a water flux of 5.9 L / (m³). -2 ·h -1 (·bar), with a CaSO4 retention rate of 91%.

[0120] Comparative Example 4

[0121] Crosslinked polyimide nanofiltration membranes were prepared using the same preparation method as in Example 5, except that in step (2-1), "hydrophilic pore size 0.22 μm (H-PTFE)" was used. 0.22 The phrase "polytetrafluoroethylene (PTFE) porous support layer" has been revised to "hydrophilic pore size of 0.1 μm (H-PTFE)". 0.1 "Porous support layer of polytetrafluoroethylene (PTFE)".

[0122] The separation performance of the prepared cross-linked polyimide nanofiltration membrane DS4 was tested at 25℃ using a 0.5 g / L CaSO4 solution and an operating pressure of 4 bar. The test result showed a water flux of 6.0 L / (m³). -2 ·h -1 (·bar), with a CaSO4 retention rate of 91%.

[0123] Comparative Example 5

[0124] Crosslinked polyimide nanofiltration membranes were prepared using the same preparation method as in Example 2, except that in steps (1-3), the phrase "chemical imidization solution, wherein the volume ratio of dehydrating agent acetic anhydride, catalyst triethylamine, and solvent acetone is 3:1:10" was modified to "chemical imidization solution, wherein the volume ratio of dehydrating agent acetic anhydride, catalyst triethylamine, and solvent acetone is 1:3:7".

[0125] The separation performance of the prepared cross-linked polyimide nanofiltration membrane DS5 was tested at 25℃ using a 0.5 g / L CaSO4 solution and an operating pressure of 4 bar. The test result showed a water flux of 5.8 L / (m³). -2 ·h -1 (·bar), with a CaSO4 retention rate of 89%.

[0126] Comparative Example 6

[0127] Crosslinked polyimide nanofiltration membranes were prepared using the same preparation method as in Example 5, except that in step (2-1), "hydrophilic pore size 0.22 μm (H-PTFE)" was used. 0.22 The phrase "polytetrafluoroethylene (PTFE) porous support layer" has been revised to "hydrophilic pore size of 0.45 μm (H-PTFE)". 0.22 "Porous support layer of polytetrafluoroethylene (PTFE)".

[0128] The separation performance of the prepared cross-linked polyimide nanofiltration membrane DS6 was tested at 25℃ using a 0.5 g / L CaSO4 solution and an operating pressure of 4 bar. The test result showed a water flux of 13.0 L / (m³). -2 ·h -1(·bar), with a CaSO4 retention rate of 29%.

[0129] Test case

[0130] The cross-linked polyimide nanofiltration membranes (0.22 μm) prepared in Examples 1-8 and Comparative Examples 1-6 were tested using X-ray photoelectron spectroscopy (XPS), and the results are shown in Table 2.

[0131] Table 2

[0132]

[0133]

[0134] As can be seen from the results in Table 2, the pore size of H-PTFE in Examples 1-8 is 0.22 μm. 0.22 Polyimide nanofiltration membranes prepared on porous polytetrafluoroethylene (PTFE) support layers. According to X-ray photoelectron spectroscopy (XPS) data, during the interfacial polymerization process, amine groups react with acyl chloride groups to form -N-CO groups, and unreacted acyl chloride groups hydrolyze to form -COOH. Therefore, the more -COOH generated, the more acyl chlorides do not react with amine groups, and the lower the degree of crosslinking of the resulting polyimide nanofiltration membrane.

[0135] As can be seen from Table 2, in Examples 1-8, the ratio of -N-CO groups to -COOH groups was greater after the addition of the crosslinking agent than in Comparative Example 1 without the addition of the crosslinking agent. Therefore, the addition of the crosslinking agent can improve the degree of crosslinking of the polyimide nanofiltration membrane.

[0136] Comparative Example 2 showed poor cross-linking effect due to changes in the type of cross-linking agent.

[0137] In Comparative Example 3, the degree of cross-linking decreased and the throughput was not high because the cross-linking agent content was 0 wt%.

[0138] In Comparative Example 4, the flux and rejection rate were not high due to the smaller pore size of the support layer.

[0139] Comparative Example 5 showed lower throughput and rejection rate due to changes in the chemical imine solution reagent ratio.

[0140] In Comparative Example 6, although the flux was higher due to the larger pore size of the support layer, the retention effect was very poor. In addition, although the flux of Comparative Example 6 was as high as 13, the retention rate was only 29%.

[0141] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing a cross-linked polyimide nanofiltration membrane, characterized in that, The preparation method includes: (1) A modified support layer is obtained by contacting the support layer with a solution containing an amine group for pretreatment; wherein the support layer is a polytetrafluoroethylene support layer; the pore size of the support layer is 0.22-0.3 μm; the amine group-containing monomer is selected from one or more of m-phenylenediamine, propylenediamine and hexamethylenediamine; the concentration of the amine group-containing monomer solution is 2-8 wt%; (2) An organic phase solution is dropped onto the surface of the modified support layer to carry out an interfacial polymerization reaction to obtain a polyamic acid film; wherein, the organic phase solution includes one or more of a monomer containing acyl chloride groups, a crosslinking agent, and an organic solvent; wherein, the crosslinking agent is selected from trimesoyl chloride and / or terephthaloyl chloride; the organic solvent is selected from one or more of n-hexane, carbon tetrachloride, and ethanol; the monomer containing acyl chloride groups is trimesoyl chloride; based on the total weight of the organic phase solution, the content of the monomer containing acyl chloride groups is 0.1-0.8 wt%, and the content of the crosslinking agent is 0.04-0.2 wt%; the dropping conditions include: a dropping rate of 2-5 ml / 1-10 s; the interfacial polymerization reaction conditions include: a temperature of 10-35 °C and a time of 10-150 s; (3) The polyamic acid membrane is contacted with a chemical imidization solution to carry out an imidization reaction, wherein the chemical imidization solution contains one or more of a catalyst, a dehydrating agent and a solvent; the catalyst is triethylamine and / or pyridine, the dehydrating agent is selected from one or more of acetic anhydride, trifluoroacetic anhydride and propionic anhydride, and the solvent is acetone and / or toluene; a cross-linked polyimide nanofiltration membrane is obtained.

2. The preparation method according to claim 1, wherein, The concentration of the solution containing the amine group is 4-6 wt%.

3. The preparation method according to claim 1, wherein, The volume ratio of the catalyst, the dehydrating agent, and the solvent is (1-3):(1-3):(5-10).

4. The preparation method according to claim 1, wherein, The pretreatment conditions include a time of 5-25 minutes.

5. The preparation method according to claim 1, wherein, The conditions for the imidization reaction include a time of 4-24 hours.

6. The preparation method according to any one of claims 1-5, wherein, In step (3), the preparation method further includes: imidizing the support layer and the cross-linked polyamic acid separation layer stacked on the surface of the support layer to obtain the cross-linked polyimide nanofiltration membrane.

7. The preparation method according to claim 1, wherein, The thickness of the cross-linked polyimide nanofiltration membrane is less than 1 μm.

8. The preparation method according to claim 7, wherein, The thickness of the cross-linked polyimide nanofiltration membrane is 50-800 nm.

9. A cross-linked polyimide nanofiltration membrane prepared by the preparation method according to any one of claims 1-8.

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