Preparation method of fluorinated MOFs (Metal-Organic Frameworks) mixed matrix membrane for efficiently extracting hydrogen / helium from natural gas

Abstract

The invention belongs to the technical field of gas membrane separation, and discloses a preparation method of a fluorinated MOFs (Metal-Organic Frameworks) mixed matrix membrane for efficiently extracting hydrogen / helium from natural gas. CF3 is introduced into an MOFs framework to prepare fluorinated MOFs, and introduction of CF3 reduces the aperture of MOFs, enhances the molecular sieving effect, and improves the diffusion selectivity of H2 (He) / CH4. In addition,-CF3 has an inhibition effect on CH4 adsorption, and the dissolution selectivity of H2 (He) / CH4 separation is improved. The existence of-CF3 in the fluorinated MOFs can induce the MOFs and the polymer to form dipole-dipole interaction (C-F... O = C), so that the filler and the polymer have good interfacial compatibility, the improvement of the filler loading capacity is facilitated, and meanwhile, the generation of non-selective defects at an interface is avoided. The mixed matrix membrane prepared by the method has important significance in the field of high-efficiency hydrogen / helium extraction of natural gas.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas membrane separation, and mainly designs a high-efficiency molecular sieving fluorinated MOFs mixed matrix membrane, reduces the pore size of MOFs molecular sieving, inhibits methane adsorption, improves the interface compatibility of the mixed matrix membrane, and increases the H 2 (He) Permeability and gas selectivity. Background Art

[0002] With the rapid development of science and technology, the application of helium and hydrogen in many fields is becoming more and more extensive and indispensable. In the electronic semiconductor industry, helium, as an extremely important coolant and protective gas, provides a stable and low-temperature environment for fine processes such as chip manufacturing, ensuring the high precision and high quality of the production process. Hydrogen has emerged in the field of new energy and is regarded as a key component of future clean energy. The application of its fuel cell technology in transportation tools such as electric vehicles has gradually become a mainstream trend, greatly reducing carbon emissions and improving energy efficiency. However, the current global supply of helium and hydrogen faces many challenges. The distribution of helium resources is extremely uneven, mainly concentrated in a few countries and regions, such as the United States, and its extraction process is complicated and costly. Although hydrogen can be produced through a variety of channels, large-scale, efficient and low-cost production technology still needs further breakthroughs. Traditional hydrogen production methods, such as water electrolysis, are often limited by energy costs and resource distribution. In this context, extracting helium and hydrogen from natural gas has become a very promising research and development direction. However, due to H 2 (He) and CH 4 The properties of H 2 (He) remains challenging due to H 2 He and CH 4 The dynamic diameter of the membrane-based gas separation technology is different for the separation of H 2 (He) / CH 4Mixtures offer huge advantages. Membrane separation technology is a new and efficient separation technology used in industrial production. Compared with traditional separation methods, it has the advantages of high efficiency and easy coupling. It requires relatively low operating energy during the separation process. It is a very competitive separation method and has attracted much attention. Its core is to prepare separation membrane materials with high separation performance. Conventional polymer membrane materials have an obvious "trade-off" phenomenon, and permeability and selectivity cannot be taken into account at the same time; while inorganic membranes usually have good separation performance and high stability, but are expensive. Mixed matrix membranes are prepared by embedding inorganic fillers into polymer matrices to fully utilize the advantages of polymers and nanoporous crystalline materials. Inorganic fillers provide additional gas adsorption sites and preferential diffusion paths, and in theory, high permeability and high selectivity can be achieved simultaneously. Metal-organic framework compounds (MOFs) are a type of inorganic-organic hybrid materials with porous structures formed by coordination self-assembly of metal ions and organic ligands. They are a new type of filler for preparing mixed matrix membranes. They have the advantages of developed pore structure, high specific surface area, adjustable pore size, and easy functionalization, which have attracted widespread attention. In theory, the porous structure of MOFs in mixed matrix membranes can enhance gas adsorption and diffusion, thereby improving gas permeability. Secondly, the small pore size of MOFs ensures molecular sieving, which helps to improve gas selectivity. The organic properties of some MOFs can improve the interfacial compatibility between polymers and MOFs, ensuring reasonable gas selectivity. Therefore, MOFs-based mixed matrix membranes have a good performance in H 2 (He) / CH 4 Separation has been widely used.

[0003] In the preparation of H 2 (He) / CH 4 The polymers used in the separation mixed matrix membranes have large -CF 3 The hexafluorodianhydride (6FDA) based polyimide (PI) has good H 2 (He) / CH 4 The rigid non-coplanar structure of the 6FDA-based PI polymer chain increases the steric hindrance of the polymer chain, reduces the intermolecular forces and the packing density of the molecular chains, has a large free volume, and can significantly improve the H 2 and He penetration. In addition, -CF 3 The strong electron-withdrawing property of the group weakens the electron cloud density of the strongly negatively charged benzene ring, causing the fluorine-containing fragment to bind to the positively charged CH 4 The binding energy between the polymer and CH 4 The interaction between CH 4 The molecules dissolve and adsorb in the polymer, thus further improving the selectivity. 3It is more difficult for the chain segments with side groups to overcome the rotation energy barrier, making the CH 4 The diffusion movement of fluorinated 6FDA-based PI mixed matrix membranes was further suppressed. 2 / CH 4 There is still a certain trade-off effect in separation performance. The researchers attributed it to two reasons: First, the interfacial compatibility between MOFs and polymers leads to insufficient MOF filler loading, making it difficult to improve performance, and weak interfacial compatibility easily leads to non-selective defects at the interface between MOF and polymer. Second, the "breathing effect" caused by the flexibility of the MOFs framework structure makes the pore size of MOFs much larger than the theoretical sieving pore size. Therefore, how to improve the interfacial compatibility between MOFs and polymers and suppress the "breathing effect" of MOFs is crucial to improving their gas permeability and selectivity.

[0004] At present, the core methods to improve the interfacial compatibility between MOFs and polymers are surface modification and in situ synthesis. Metal ion replacement, ligand post-replacement, and rational design and functionalization of ligands can effectively adjust the pore size of MOFs. Ding Baisuo proposed a method to improve the interfacial compatibility between MOFs and polymers by carboxylating PI (COOH-PI) and amino functionalizing ZIF-8 (NH 2 -PI) to prepare COOH-PI / NH 2 -ZIF-8 mixed matrix membrane method (CN202310874036.1), which improves the interfacial compatibility of the mixed matrix membrane, but due to NH 2 -ZIF-8 has a relatively large pore size, and the He / CH 4 The selectivity is not high. Wang Shaofei (CN202410955428.5) proposed a method for improving H 2 A Pd@MOF gel (Pd@MOF gel) mixed matrix membrane with high transmission separation efficiency and its preparation method. The membrane is formed by evenly distributing Pd on ZIF-67gel. The combination of Pd and ZIF-67gel effectively reduces the pore size of ZIF-67gel, which can more effectively screen hydrogen molecules and inhibit the passage of methane. Subsequently, Pd@ZIF-67gel was introduced into polyimide (P84) and self-polymerized microporous polymer (PIM-1) to prepare a mixed matrix membrane. However, the permeability of the prepared Pd@ZIF-67gel / P84 mixed matrix membrane is low, and the selectivity of the Pd@ZIF-67gel / PIM-1 mixed matrix membrane is low.

[0005] In order to solve the above-mentioned shortcomings and deficiencies, the purpose of the present invention is to design a method for preparing a highly efficient molecular sieving fluorinated MOFs mixed matrix membrane. Summary of the invention

[0006] The purpose of the present invention is to design a method for preparing a high-efficiency molecular sieving fluorinated MOFs mixed matrix membrane by introducing -CF 3 The group is used to inhibit the rotation of the ligand to reduce the pore size of MOFs and inhibit the CH 4 The diffusion rate of H 2 (He) / CH 4 Diffusion selectivity, and -CF 3 The introduction of CH 4 The adsorption of H 2 (He) / CH 4 Dissolution selectivity. At the same time -CF 3 Induce dipole-dipole interaction between filler and polymer, improve interfacial compatibility, and further enhance the permeability and selectivity of mixed matrix membranes.

[0007] The technical solution of the present invention:

[0008] A method for preparing a natural gas efficient hydrogen / helium extraction fluorinated MOFs mixed matrix membrane, the steps are as follows: firstly, -CF 3 Fluorinated MOFs with different fluorination degrees were prepared by introducing them into the MOFs framework. 3 The introduction of NH4+ reduces the pore size of MOFs, enhances the molecular sieving effect, and increases the H 2 / CH 4 Diffusion selectivity, in addition -CF 3 CH 4 Adsorption has a certain inhibitory effect, further increasing the H 2 / CH 4 The fluorinated MOFs were then doped into 6FDA-based PI polymers at different ratios to prepare mixed matrix membranes. 3 The presence of can induce the formation of dipole-dipole interaction (CF) between MOFs and polymers. … O=C), so that the filler and the polymer have good interface compatibility, which is conducive to the increase of filler loading, while avoiding the generation of non-selective defects at the interface, ensuring the permeability of the mixed matrix membrane and improving its selectivity. The specific steps include:

[0009] Step 1, dissolving imidazole and trifluoromethyl-containing imidazole as ligands in methanol, then adding triethylamine thereto, stirring at room temperature until all dissolved, to prepare a mixed solution A; dissolving a metal salt in methanol, stirring at room temperature until dissolved, to prepare a mixed solution B; adding the mixed solution B to the mixed solution A, stirring for a certain period of time, transferring to a reactor for a certain period of time, centrifuging, washing, and drying to obtain a fluorinated MOFs powder;

[0010] Step 2, dissolving and dispersing the dried PI polymer in a mixed solution of DMAC and THF, and stirring for a certain time to prepare a uniform PI solution; dispersing the fluorinated MOFs powder in a mixed solution of DMAC and THF, and stirring for a certain time to prepare a uniform MOFs suspension;

[0011] Step 3: Then, the MOFs suspension is added to the PI solution to prepare a mixed solution, the mixed solution is ultrasonically treated, and then stirred at room temperature for a certain period of time to obtain a membrane solution; finally, the membrane solution is cast on a polytetrafluoroethylene plate, and the membrane solution is dried to obtain a fluorinated MOFs mixed matrix membrane.

[0012] In step 1, the imidazole is 2'-methylimidazole or / and benzimidazole, the trifluoromethyl-containing imidazole is 2-(trifluoromethyl)-1H-imidazole or / and 2-(trifluoromethyl)benzimidazole, and the metal salt is cobalt nitrate hexahydrate or / and zinc nitrate hexahydrate; the molar ratio of imidazole to trifluoromethyl-containing imidazole is 3:1 to 0:4, and the molar ratio of ligand to metal salt is 4:1.

[0013] In step 1, the mixed solution B is added dropwise to the mixed solution A, stirred at room temperature for 6 hours, and then transferred to a reactor for reaction for 12 hours; the solvent used for washing is methanol, and the centrifugal speed during washing is 10000 rpm; the drying time is ≥12 hours, and the drying conditions are 80°C under vacuum.

[0014] In step 2, the PI polymer is a 6FDA-based PI polymer, which is one or a mixture of two or more of 6FDA-TFMB, 6FDA-DAM, 6FDA-DABA, 6FDA-BD, and 6FDA-FDA. The drying time is 12 hours, and the drying condition is 120° C. under vacuum. The concentration of the PI solution is 3.55wt%, the mass ratio of DMAC to THF is 8:2 to 6:4, and the PI solution is stirred at 50° C. for 4 hours.

[0015] In step 3, the mass fraction of fluorinated MOFs in the mixed solution is 10-30 wt %, the ultrasonic time is 10 min, and the membrane solution drying temperature is 50° C.

[0016] Beneficial effects of the present invention: The present invention designs a method for preparing a high-efficiency molecular sieving fluorinated MOFs mixed matrix membrane, -CF 3 The introduction of NH4+ reduces the pore size of MOF, enhances the molecular sieving effect, and increases the H 2 (He) / CH 4 Diffusion selectivity; in addition -CF 3 CH 4 Adsorption and dissolution have a certain inhibitory effect, further increasing the H 2 (He) / CH 4 Dissolution selectivity of isolated -CF in fluorinated MOFs3 The presence of can induce the formation of dipole-dipole interaction (CF) between MOFs and polymers. … O=C), so that the filler and the polymer have good interface compatibility, which is beneficial to the increase of filler loading, while avoiding the generation of non-selective defects at the interface, ensuring the permeability of the mixed matrix membrane and improving its selectivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is the pore size distribution diagram of MOFs before and after fluorination.

[0018] Figure 2 Scanning electron microscope images of MOFs mixed matrix membranes before and after fluorination, where (a) is the MOFs mixed matrix membrane before fluorination, and (b) is the MOFs mixed matrix membrane after fluorination.

[0019] Figure 3 Figure 2. MOFs mixed matrix membranes before and after fluorination. 2 (He) / CH 4 Gas separation performance diagram. DETAILED DESCRIPTION

[0020] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0021] Embodiment 1:

[0022] A method for preparing a high-efficiency molecular sieving fluorinated MOFs mixed matrix membrane, the specific process includes the following steps:

[0023] Step 1, 0.3712g of a mixture of benzimidazole and 2-(trifluoromethyl)benzimidazole with a molar ratio of 0:4 was dissolved in 15ml of methanol solution as a ligand, and then 15μl of triethylamine was added thereto, and stirred at room temperature until all dissolved to prepare a mixed solution A. 0.14g of cobalt nitrate hexahydrate was dissolved in 25ml of methanol solution, and stirred at room temperature until dissolved to prepare a mixed solution B. Solution B was added dropwise to solution A, stirred at room temperature for 6h, transferred to a reactor, stirred for 12h at 120°C, centrifuged and washed, the washing solvent was methanol, and the centrifugal rate during washing was 10000rpm. It was then dried to prepare a fluorinated ZIF-9 powder (FZIF-9), the drying time was ≥12h, and the drying condition was 80°C under vacuum.

[0024] Step 2: Dry the 6FDA-TFMB polymer at 120°C in vacuum for 12 h, then dissolve 0.8595 g of the 6FDA-TFMB polymer in 3.5 ml of a mixed solution of DMAC and THF in a mass ratio of 8:2, and stir at 50°C for 4 h to prepare a 6FDA-TFMB polymer solution with a mass fraction of 3.55 wt%, weigh 0.043 g of FZIF-9 particles and disperse them in 2.5 ml of a mixed solution of DMAC and THF in a mass ratio of 8:2, and stir to obtain a uniformly dispersed MOFs suspension.

[0025] Step 3, the obtained MOFs suspension was dispersed in the 6FDA-TFMB polymer solution, the mixed solution was ultrasonically treated, and then stirred at room temperature for 12 hours. Finally, the above membrane solution was cast on a polytetrafluoroethylene plate, and the membrane solution was dried at 50°C to obtain a fluorinated MOF-based mixed matrix membrane (20wt% FZIF / PI) with a FZIF-9 mass fraction of 20wt%.

[0026] Embodiment 2:

[0027] A method for preparing a high-efficiency molecular sieving fluorinated MOFs mixed matrix membrane, the specific process includes the following steps:

[0028] Step 1, 0.3712g of a mixture of benzimidazole and 2-(trifluoromethyl)benzimidazole with a molar ratio of 0:4 was dissolved in 15ml of methanol solution as a ligand, and then 15μl of triethylamine was added thereto, and stirred at room temperature until all dissolved to prepare a mixed solution A. 0.14g of cobalt nitrate hexahydrate was dissolved in 25ml of methanol solution, and stirred at room temperature until dissolved to prepare a mixed solution B. Solution B was added dropwise to solution A, stirred at room temperature for 6h, transferred to a reactor, stirred at 120°C for 12h, centrifuged and washed, the washing solvent was methanol, and the centrifugal rate during washing was 10000rpm. It was then dried to prepare fluorinated ZIF-9 powder (FZIF-9), the drying time was ≥12h, and the drying condition was 80°C under vacuum.

[0029] Step 2: Dry the 6FDA-TFMB polymer at 120°C in vacuum for 12 h, then dissolve 0.8595 g of the 6FDA-TFMB polymer in 3.5 ml of a mixed solution of DMAC and THF in a mass ratio of 8:2, and stir at 50°C for 4 h to prepare a 6FDA-TFMB polymer solution with a mass fraction of 3.55 wt%, weigh 0.057 g of FZIF-9 particles and disperse them in 2.5 ml of a mixed solution of DMAC and THF in a mass ratio of 8:2, and stir to obtain a uniformly dispersed MOFs suspension.

[0030] Step 3, the obtained MOFs suspension was dispersed in the 6FDA-TFMB polymer solution, the mixed solution was ultrasonically treated, and then stirred at room temperature for 12 hours. Finally, the above membrane solution was cast on a polytetrafluoroethylene plate, and the membrane solution was dried at 50°C to obtain a fluorinated MOF-based mixed matrix membrane (25wt% FZIF / PI) with a FZIF-9 mass fraction of 25wt%.

[0031] Embodiment 3:

[0032] A method for preparing a high-efficiency molecular sieving fluorinated MOFs mixed matrix membrane, the specific process includes the following steps:

[0033] Step 1. Dissolve 0.3712g of a mixture of benzimidazole and 2-(trifluoromethyl)benzimidazole in a molar ratio of 2:2 as a ligand in 15ml of methanol solution, then add 15μl of triethylamine, stir at room temperature until completely dissolved, and prepare a mixed solution A. Dissolve 0.14g of cobalt nitrate hexahydrate in 25ml of methanol solution, stir at room temperature until dissolved, and prepare a mixed solution B. Add solution B dropwise to solution A, stir at room temperature for 6h, transfer to a reactor, stir and react at 120°C for 12h, centrifuge and wash, the washing solvent is methanol, and the centrifugal rate during washing is 10000rpm. Then dry it to prepare a fluorinated ZIF-9 powder (F 2 ZIF-9), drying time ≥ 12h, drying conditions are 80℃ under vacuum.

[0034] Step 2: Dry the 6FDA-TFMB polymer at 120°C in vacuum for 12 h, then dissolve 0.8595 g of the 6FDA-TFMB polymer in 3.5 ml of a mixed solution of DMAC and THF in a mass ratio of 8:2, and stir at 50°C for 4 h to prepare a 6FDA-TFMB polymer solution with a mass fraction of 3.55 wt%, weigh 0.043 g of FZIF-9 particles and disperse them in 2.5 ml of a mixed solution of DMAC and THF in a mass ratio of 8:2, and stir to obtain a uniformly dispersed MOFs suspension.

[0035] Step 3: Disperse the obtained MOFs suspension in the 6FDA-TFMB polymer solution, ultrasonically treat the mixed solution, and then stir it at room temperature for 12 hours. Finally, cast the above membrane solution on a PTFE plate and dry the membrane solution at 50°C to obtain a fluorinated MOF-based mixed matrix membrane with a mass fraction of 20 wt% FZIF-9 (20 wt% F 2 ZIF / PI).

[0036] Comparative Example 1:

[0037] A method for preparing a high-efficiency molecular sieving MOFs mixed matrix membrane, the specific process includes the following steps:

[0038] Step 1, 0.3712g of a mixture of benzimidazole and 2-(trifluoromethyl)benzimidazole in a molar ratio of 4:0 was dissolved in 15ml of methanol solution as a ligand, and then 15μl of triethylamine was added thereto, and stirred at room temperature until all dissolved to prepare a mixed solution A. 0.14g of cobalt nitrate hexahydrate was dissolved in 25ml of methanol solution, and stirred at room temperature until dissolved to prepare a mixed solution B. Solution B was added dropwise to solution A, stirred at room temperature for 6h, transferred to a reactor, stirred for 12h at 120°C, centrifuged and washed, the washing solvent was methanol, and the centrifugal rate during washing was 10000rpm. It was then dried to prepare ZIF-9 powder, the drying time was ≥12h, and the drying condition was 80°C under vacuum.

[0039] Step 2: Dry the 6FDA-TFMB polymer at 120°C in vacuum for 12 h, then dissolve 0.8595 g of the 6FDA-TFMB polymer in 3.5 ml of a mixed solution of DMAC and THF in a mass ratio of 8:2, and stir at 50°C for 4 h to prepare a 6FDA-TFMB polymer solution with a mass fraction of 3.55 wt%, weigh 0.043 g of ZIF-9 particles and disperse them in 2.5 ml of a mixed solution of DMAC and THF in a mass ratio of 8:2, and stir to obtain a uniformly dispersed MOFs suspension.

[0040] Step 3, the obtained MOFs suspension was dispersed in the 6FDA-TFMB polymer solution, the mixed solution was ultrasonically treated, and then stirred at room temperature for 12 hours. Finally, the above membrane solution was cast on a polytetrafluoroethylene plate, and the membrane solution was dried at 50°C to obtain a fluorinated MOF-based mixed matrix membrane (20wt% ZIF / PI) with a ZIF-9 mass fraction of 20wt%.

[0041] The pore sizes of the nanoparticles before and after fluorination were characterized by Figure 1 It can be seen that -CF 3 The introduction of reduced the pore size of FZIF-9 by 11% compared with ZIF-9. It can be seen from the scanning electron microscopy that obvious interfacial voids were observed in the 20wt% ZIF / PI membrane prepared in Comparative Example 1, which means that the interfacial compatibility is poor. In contrast, no voids were found on the interface of the 20wt% FZIF / PI prepared in Example 1, which indicates that -CF 3 The introduction of greatly changes the interface compatibility. Figure 3 As shown, compared with the 20 wt % ZIF / PI mixed matrix membrane prepared in Comparative Example 1, the 20 wt % FZIF / PI mixed matrix membrane prepared in Example 1 has better H 2 (He) / CH 4 Separation performance, and H2 (He) / CH 4 The selectivity is much higher than that of the 20 wt% ZIF / PI mixed matrix membrane prepared in Comparative Example 1. This is due to the 3 The introduction of inhibits the diffusion and dissolution of methane and increases the H 2 (He) / CH 4 Selective.

Claims

1. A method for preparing a natural gas high-efficiency hydrogen / helium extraction fluorinated MOFs mixed matrix membrane, characterized in that: Here are the steps: Step 1, dissolving imidazole and trifluoromethyl-containing imidazole as ligands in methanol, then adding triethylamine thereto, stirring at room temperature until all dissolved, to prepare a mixed solution A; dissolving a metal salt in methanol, stirring at room temperature until dissolved, to prepare a mixed solution B; adding the mixed solution B to the mixed solution A, stirring for a certain period of time, transferring to a reactor for a certain period of time, centrifuging, washing, and drying to obtain a fluorinated MOFs powder; Step 2, dissolving and dispersing the dried PI polymer in a mixed solution of DMAC and THF, and stirring for a certain time to prepare a uniform PI solution; dispersing the fluorinated MOFs powder in a mixed solution of DMAC and THF, and stirring for a certain time to prepare a uniform MOFs suspension; Step 3: Then, the MOFs suspension is added to the PI solution to prepare a mixed solution, the mixed solution is ultrasonically treated, and then stirred at room temperature for a certain period of time to obtain a membrane solution; finally, the membrane solution is cast on a polytetrafluoroethylene plate, and the membrane solution is dried to obtain a fluorinated MOFs mixed matrix membrane.

2. The preparation method according to claim 1, characterized in that: In step 1, the imidazole is 2'-methylimidazole or / and benzimidazole, the trifluoromethyl-containing imidazole is 2-(trifluoromethyl)-1H-imidazole or / and 2-(trifluoromethyl)benzimidazole, and the metal salt is cobalt nitrate hexahydrate or / and zinc nitrate hexahydrate.

3. The preparation method according to claim 1, characterized in that: In step 1, the molar ratio of imidazole to trifluoromethyl-containing imidazole is 3:1 to 0:4, and the molar ratio of ligand to metal salt is 4:

1.

4. The preparation method according to claim 1, characterized in that: In step 1, the mixed solution B was added dropwise into the mixed solution A, stirred at room temperature for 6 hours, and then transferred to a reactor to react for 12 hours.

5. The preparation method according to claim 1, characterized in that: In step 1, the solvent used for washing is methanol, the centrifugal speed during washing is 10000 rpm; the drying time is ≥12 h, and the drying condition is 80° C. under vacuum.

6. The preparation method according to claim 1, characterized in that: In step 2, the PI polymer is a 6FDA-based PI polymer, which is one or a mixture of two or more of 6FDA-TFMB, 6FDA-DAM, 6FDA-DABA, 6FDA-BD, and 6FDA-FDA. The drying time is 12 hours, and the drying conditions are 120° C. under vacuum.

7. The preparation method according to claim 1, characterized in that: In step 2, the concentration of the PI solution is 3.55 wt %, the mass ratio of DMAC to THF is 8:2 to 6:4, and the PI solution is stirred at 50° C. for 4 h.

8. The preparation method according to claim 1, characterized in that: In step 3, the mass fraction of fluorinated MOFs in the mixed solution is 10-30 wt %, the ultrasonic time is 10 min, and the membrane solution drying temperature is 50° C.

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