Zif-7-8@polyimide mixed matrix membrane for helium extraction from natural gas and preparation method and application thereof

Abstract

This invention relates to the field of composite material technology, and particularly to a ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas, its preparation method, and its application. The preparation method of this invention uses 2-methylimidazole and benzimidazole as dual ligands and zinc acetate dihydrate as the metal source to prepare ZIF-7-8. ZIF-7-8 is dispersed in N-methylpyrrolidone, polyimide is added, and after stirring, ultrasonication, and degassing, a film is formed by casting. The resulting ZIF-7-8@polyimide membrane is obtained by vacuum drying. Through layer-by-layer self-assembly technology, using 1,3,5,7-tetra(4-aminophenyl)adamantane and terephthalaldehyde as monomers, a COF separation layer is constructed on the activated substrate surface via a Schiff base reaction in anhydrous trimethylbenzene-1,4-dioxane mixed solvent. Post-treatment yields the hybrid matrix membrane. The hybrid matrix membrane of this invention exhibits high permeation flux and selectivity for helium.

Description

Technical Field

[0001] This invention relates to the field of composite material technology, and in particular to a ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas, its preparation method and application. Background Technology

[0002] Helium (He), with its lowest boiling point (-269℃), is an irreplaceable resource in high-end fields such as semiconductor lithography, low-temperature superconducting magnets, and aerospace propellants. With the development of cutting-edge technologies such as quantum computing and nuclear fusion, the demand for ultrapure helium (6N, ≥99.9999%) has surged, and its purity directly affects the performance of precision equipment. Currently, the main method of helium production is extraction from helium-rich natural gas (>0.5%).

[0003] Extensive research has been conducted on helium extraction from natural gas, resulting in the development of various technologies, including cryogenic separation, membrane separation, adsorption, and absorption methods. Cryogenic separation and membrane separation technologies are among the most widely used. Traditional cryogenic distillation processes require significant energy and financial investment for helium recovery units. The low helium concentration and the need for cryogenic operations contribute to this high energy consumption. Membrane separation technology is gradually replacing more traditional gas separation technologies, such as cryogenic distillation and absorption, and its current applications include hydrogen separation, nitrogen recovery, oxygen and nitrogen enrichment, and natural gas purification.

[0004] Metal-organic frameworks (MOFs) have become a common alternative to traditional membrane materials due to their wide pore size range, high permanent porosity, and large surface area. Their selectivity offers significant advantages in reducing separation costs and meeting stringent purity requirements. Hybrid matrix membranes prepared using MOF materials are composed of a blend of dispersed porous packing materials and a continuous polymer matrix. For example, the literature (J. Membr. Sci., 2015, 490: 364-379) embedded modified nanoporous Y-type sodium zeolite particles into a Matrimid 5218 matrix to obtain a novel hybrid matrix membrane that increased the CO2 permeability coefficient from 8.34 Barrer to 9.70 Barrer (approximately 16%). The literature (Sep. Purif. Technol., 2011, 77, 1: 128-136) prepared a TiO2 / Matrimid 5218 hybrid matrix membrane, and the test results showed that the membrane achieved a CO2 permeability coefficient of 10.54 Barrer and a CO2 / CH4 selectivity of 13.69, which is an improvement over pure Matrimid 5218. There are few reports on mixed matrix membranes used for helium extraction from natural gas. The complex impurities in natural gas can affect the stability of mixed matrix membranes, causing them to undergo physical aging, degradation, and plasticization. Therefore, improving the long-term stability of membranes is a problem that urgently needs to be solved for membrane materials used in natural gas decarbonization and helium extraction. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas, its preparation method and application, wherein the hybrid matrix membrane has excellent permeability and selectivity for helium and can be used in helium separation materials.

[0006] The present invention solves the above-mentioned technical problems through the following technical means:

[0007] In a first aspect, embodiments of the present invention provide a method for preparing a ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas, comprising the following steps:

[0008] To prepare a ZIF-7-8@polyimide film, ZIF-7-8 was dispersed in N-methylpyrrolidone at room temperature and mixed to obtain a MOF suspension. Polyimide was added in batches and stirred, ultrasonicated, and allowed to stand at room temperature for 40-50 h. The film was then coated and dried under vacuum at 90-110 °C for 40-50 h to obtain the ZIF-7-8@polyimide film.

[0009] To construct the COF separation layer, 1,3,5,7-tetra(4-aminophenyl)adamantane was dissolved in a mixed solvent of mesitylene / 1,4-dioxane, and then acetic acid was added and stirred to obtain an amine monomer solution. Terephthalaldehyde was dissolved in a mixed solvent of mesitylene / 1,4-dioxane to obtain an aldehyde monomer solution. The ZIF-7-8@polyimide membrane was activated by plasma treatment for 60–120 s, then completely immersed in the amine monomer solution, and allowed to stand at room temperature for 2–4 h before being removed. The membrane was rinsed 2–4 times with a mixed solvent of mesitylene / 1,4-dioxane and purged with nitrogen until no droplets were visible on the membrane surface. The membrane was then laid flat and an aldehyde monomer solution was poured on it to form a liquid film 1–2 mm thick on the membrane surface. The membrane was then kept at 40–50 °C for 12–24 h. The resulting membrane was then immersed in anhydrous N,N-dimethylformamide and methanol for 12–36 h each, and then vacuum dried at 115–130 °C for 10–14 h to obtain the ZIF-7-8@polyimide mixed matrix membrane.

[0010] In some embodiments, the mass fraction of ZIF-7-8 in the MOF suspension is 5-30%, and after adding N-methylpyrrolidone to the ZIF-7-8, the mixture is stirred for 2-4 hours and then sonicated for 20-30 minutes to obtain the MOF suspension.

[0011] In some embodiments, the mass ratio of ZIF-7-8 to polyimide is (0.05-0.45):1, and the ZIF-7-8 and polyimide need to be dried in a vacuum environment at 80-110°C before use.

[0012] In some embodiments, the ZIF-7-8 is prepared as follows:

[0013] Weigh 2-methylimidazole and benzimidazole and dissolve them in anhydrous N,N-dimethylformamide to obtain an imidazole solution; weigh zinc acetate dihydrate and dissolve it in anhydrous N,N-dimethylformamide to obtain a metal salt solution; mix the metal salt solution with the imidazole solution and stir at room temperature for 2-4 hours; centrifuge the solid obtained and wash it with methanol, then disperse it in methanol for solvent exchange for 40-50 hours; after solid-liquid separation, place the solid product obtained in the solid-liquid solution under vacuum at 140-160℃ to obtain ZIF-7-8.

[0014] In some embodiments, the mass ratio of 2-methylimidazole to benzimidazole is 1:0.4, and the molar ratio of the total molar amount of 2-methylimidazole and benzimidazole to the molar amount of zinc acetate dihydrate is (2.0~4.0):1.

[0015] In some embodiments, the mesitylene / 1,4-dioxane mixed solvent is prepared by mixing anhydrous mesitylene and 1,4-dioxane in a volume ratio of 1:1.

[0016] In some embodiments, the concentration of 1,3,5,7-tetra(4-aminophenyl)adamantane in the amine monomer solution is 0.5 to 1 mol / L, and the concentration of terephthalaldehyde in the aldehyde monomer solution is 0.5 to 1 mol / L.

[0017] In some embodiments, the ZIF-7-8@polyimide film needs to be ultrasonically cleaned in methanol and acetone for 10-20 minutes each before plasma treatment, and then vacuum dried at 50-70°C for 1-3 hours.

[0018] In a second aspect, embodiments of the present invention provide a ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas, wherein the hybrid matrix membrane is prepared by the preparation method described in any one of claims 1-8.

[0019] Thirdly, embodiments of the present invention provide the application of the hybrid matrix membrane described in the second aspect in the preparation of helium separation materials.

[0020] One or more technical solutions provided in the embodiments of the present invention have at least the following technical effects or advantages:

[0021] In the ZIF-7-8@polyimide with a COF surface layer of the present invention, on the one hand, ZIF-7-8 adopts a dual-ligand design of 2-methylimidazole and benzimidazole, which is more compatible with polyimide in terms of hydrophobicity and molecular structure, avoiding the aggregation of inorganic fillers and interface defects. Plasma activation further enhances the bonding force between the bottom membrane and the COF layer, and the membrane density and mechanical stability are significantly better than those of traditional mixed matrix membranes. On the other hand, the dual-ligand ZIF-7-8 provides a fast molecular transport channel, and the surface COF layer achieves precise sieving. The two work together to significantly improve the separation selectivity of the membrane while maintaining a high permeation flux, breaking through the "permeability-selectivity trade-off" bottleneck of traditional polymer membranes. Compared with the single-component Matrimid 5218 membrane, the permeability of helium is improved by 6-11%, the He / N2 selectivity is improved by 12-26%, and the He / CH4 selectivity is improved by 23-34%. Attached Figure Description

[0022] Figure 1 This is the 1H NMR spectrum of the ZIF-7-8 nanoparticles prepared in Example 1;

[0023] Figure 2 This is a comparison of the ZIF-7-8@Matrimid5218 prepared in Example 1 and the simulated XRD pattern;

[0024] Figure 3 The graph shows the anti-plasticization performance test results of the ZIF-7-8@polyimide hybrid matrix membrane prepared in Example 1 for helium extraction from natural gas. Detailed Implementation

[0025] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0026] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0027] The preparation method of the ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas according to the present invention is as follows:

[0028] (1) Preparation of ZIF-7-8

[0029] Weigh out 2-methylimidazole and benzimidazole in a mass ratio of 1:0.4 and dissolve them in anhydrous N,N-dimethylformamide. Place the solution in a single-necked flask at room temperature and stir until both 2-methylimidazole and benzimidazole are completely dissolved to obtain an imidazole solution. Dissolve zinc acetate dihydrate in anhydrous N,N-dimethylformamide to obtain a metal salt solution. Mix the metal salt solution and the imidazole solution according to a molar ratio of the total molar amount of 2-methylimidazole and benzimidazole to the molar amount of zinc acetate dihydrate of (2.0–4.0):1. Stir at room temperature for 2–4 h at a speed of 7000–9000 r·min. -1 Centrifuge for 15–25 min, remove the supernatant to obtain a white solid powder, wash with methanol 3–5 times, then redisperse the solid powder in methanol for solvent exchange for 40–50 h, and finally place the solid product obtained from solid-liquid separation into a vacuum dryer at 140–160 °C to obtain ZIF-7-8 nanoparticles.

[0030] In this step, zinc acetate dihydrate dissociates to produce Zn. 2+ It forms coordinate bonds with the N atom of the imidazole ring in 2-methylimidazole and benzimidazole, and self-assembles into a porous crystal with a zeolite-like topology. The dual ligands optimize the pore size (0.3-0.5 nm) and surface hydrophobicity of ZIF-7-8 by regulating the ligand spatial structure and electronic effects, thereby improving its subsequent interfacial compatibility with Matrimid5218.

[0031] (2) Preparation of ZIF-7-8@polyimide film

[0032] The polyimide used in this invention is Matrimid 5218. Matrimid 5218 and ZIF-7-8 powders were dried overnight in a vacuum environment at 80–110°C for later use. The dried ZIF-7-8 was dispersed in N-methylpyrrolidone at room temperature, stirred for 2–4 hours, and then sonicated for 20–30 minutes to obtain a MOF suspension with a ZIF-7-8 mass fraction of 5–30%. Matrimid 5218 was added to the MOF suspension in multiple portions according to a mass ratio of ZIF-7-8 to polyimide of (0.05–0.45):1. After each addition, the mixture was stirred for 2–4 hours and ultrasonically treated for 30–60 minutes. This process was repeated until Matrimid 5218 was completely added. The mixture was then ultrasonically treated for 20–40 minutes and allowed to stand at room temperature for 40–50 hours. The resulting membrane solution was then poured onto a flat glass plate using a casting applicator. Finally, the resulting film was vacuum dried at 90–110°C for 40–50 hours to obtain the ZIF-7-8@polyimide membrane.

[0033] In this step, N-methylpyrrolidone is a good solvent for Matrimid 5218, which can fully dissolve the polymer chains; the combined effect of stirring and ultrasound makes ZIF-7-8 nanoparticles uniformly dispersed in the polymer matrix, avoiding particle agglomeration; during vacuum drying, N-methylpyrrolidone slowly volatilizes, and the polymer chains crosslink to form a dense and defect-free bottom film. The porous structure of ZIF-7-8 provides a fast channel for molecular transport.

[0034] (3) Constructing the COF separation layer

[0035] Weigh out 1,3,5,7-tetra(4-aminophenyl)adamantane and dissolve it in a mixed solvent of mesitylene / 1,4-dioxane. Add acetic acid and stir until homogeneous to obtain an amine monomer solution with a concentration of 0.5–1 mol / L for 1,3,5,7-tetra(4-aminophenyl)adamantane. Weigh out terephthalaldehyde and dissolve it in a mixed solvent of mesitylene / 1,4-dioxane to obtain an aldehyde monomer solution with a concentration of 0.5–1 mol / L for terephthalaldehyde. The mesitylene / 1,4-dioxane mixed solvent was prepared by mixing anhydrous mesitylene and 1,4-dioxane in a volume ratio of 1:1.

[0036] The ZIF-7-8@polyimide membrane was ultrasonically cleaned in methanol and acetone for 10–20 min each, then vacuum dried at 50–70 °C for 1–3 h. After drying, the ZIF-7-8@polyimide membrane was activated in a flat-panel plasma cleaner for 60–120 s, and then completely immersed in an amine monomer solution. After standing at room temperature for 2–4 h, the membrane was removed and rinsed 2–4 times with a mixture of mesitylene / 1,4-dioxane solvent. Nitrogen gas was then used to purge the membrane surface until no droplets remained. The membrane was then laid flat in a petri dish, and an aldehyde monomer solution was poured on it to form a 1–2 mm thick liquid film on the membrane surface. The membrane was kept at 40–50 °C for 12–24 h, and the surface liquid was poured off. The resulting membrane was then immersed in anhydrous N,N-dimethylformamide and methanol for 12–36 h each, and then vacuum dried at 115–130 °C for 10–14 h to obtain the ZIF-7-8@polyimide mixed matrix membrane.

[0037] In this step, plasma activation generates polar groups such as -OH and -C=O on the substrate surface, enhancing the interaction with the amine monomer. After the amine monomer (1,3,5,7-tetra(4-aminophenyl)adamantane) is adsorbed onto the substrate surface, it undergoes a Schiff base reaction with the aldehyde monomer (terephthalaldehyde) (-NH2 + -CHO → -CH=N- + H2O), forming a regular porous COF surface layer through covalent bonding. The pore size (0.2–0.4 nm) of the COF surface layer allows for precise molecular sieving, while its chemical stability improves the membrane's antifouling and solvent resistance.

[0038] In the specific implementation:

[0039] Example 1

[0040] The preparation method of the ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas in this embodiment is as follows:

[0041] (1) Preparation of ZIF-7-8

[0042] Weigh 0.821 g of 2-methylimidazole and 0.3328 g of benzimidazole and add them to 20 mL of anhydrous N,N-dimethylformamide. Place the solution in a single-necked flask at room temperature and stir until the 2-methylimidazole and benzimidazole are completely dissolved to obtain an imidazole solution. Weigh 1.0975 g of zinc acetate dihydrate and dissolve it in 10 mL of anhydrous N,N-dimethylformamide to obtain a metal salt solution. Mix the metal salt solution with the imidazole solution and stir at room temperature for 3 h. Then, heat the mixture at 8000 r·min. -1Centrifuge for 20 min, remove the supernatant to obtain a white solid powder, wash four times with methanol, then redisperse the solid powder in 100 mL of methanol for solvent exchange for 48 h. The solid product obtained after solid-liquid separation is then vacuum dried overnight at 150 °C to obtain ZIF-7-8 nanoparticles. The 1H NMR spectrum of the ZIF-7-8 nanoparticles in this embodiment is shown below. Figure 1 As stated above.

[0043] (2) Preparation of ZIF-7-8@polyimide film

[0044] The polyimide used in this invention is Matrimid 5218. 9.5g of Matrimid 5218 and 0.5g of ZIF-7-8 powder were weighed and dried overnight in a vacuum environment at 100℃ for later use. At room temperature, 0.5g of the dried ZIF-7-8 was dispersed in N-methylpyrrolidone, stirred for 3 hours, and then sonicated for 30 minutes to obtain a MOF suspension with a ZIF-7-8 mass fraction of 5%. Matrimid 5218 was added to the MOF suspension in three portions: 3g, 3g, and 3.5g, with stirring for 3 hours after each addition and sonication for 30 minutes, until Matrimid 5218 was completely added. After sonication for another 30 minutes, the mixture was allowed to stand at room temperature for 48 hours. The resulting film solution was poured onto a flat glass plate using a casting applicator. Finally, the film was dried under vacuum at 100℃ for 48 hours to obtain a ZIF-7-8@polyimide film, i.e., ZIF-7-8@Matrimid 5218. A comparison of the ZIF-7-8@Matrimid5218 in this embodiment with the simulated XRD pattern is shown below. Figure 2 As shown.

[0045] (3) Constructing the COF separation layer

[0046] Weigh 3.5 g of 1,3,5,7-tetra(4-aminophenyl)adamantane and dissolve it in 5 mL of anhydrous trimethylbenzene and 5 mL of a mixed solvent of 1,4-dioxane. Then add 0.33 mL of acetic acid and stir until well mixed to obtain an amine monomer solution. Weigh 1.34 g of terephthalaldehyde and dissolve it in 5 mL of anhydrous trimethylbenzene and 5 mL of a mixed solvent of 1,4-dioxane to obtain an aldehyde monomer solution.

[0047] The ZIF-7-8@polyimide membrane was ultrasonically cleaned in methanol and acetone for 15 min each, then vacuum dried at 60℃ for 2 h. After drying, the ZIF-7-8@polyimide membrane was placed in a flat plasma cleaner and activated with an oxygen flow rate of 50 sccm for 80 s. Then, it was completely immersed in an amine monomer solution and allowed to stand at room temperature for 3 h. The membrane was then removed and rinsed three times with a mixed solvent of mesitylene / 1,4-dioxane. Nitrogen gas was used to purge the membrane until there were no droplets on the surface. The membrane was then laid flat in a petri dish, and an aldehyde monomer solution was poured on it to form a 1.5 mm thick liquid film on the membrane surface. The membrane was kept at 45℃ for 18 h, and the surface liquid was poured off. The resulting membrane was then immersed in anhydrous N,N-dimethylformamide and methanol for 24 h each, and then vacuum dried at 120℃ for 12 h to obtain the ZIF-7-8@polyimide mixed matrix membrane for helium extraction from natural gas.

[0048] The anti-plasticization performance test results of the ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas in this embodiment are as follows: Figure 3 As shown. From Figure 3 It can be seen that the ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas in this embodiment can maintain the stability of selectivity for α(He / CH4)α(He / N2) and N2 permeability, and has good anti-plasticization properties.

[0049] Example 2

[0050] The preparation method of the ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas in this embodiment is as follows:

[0051] (1) Preparation of ZIF-7-8

[0052] Weigh 1.642 g of 2-methylimidazole and 0.6656 g of benzimidazole and add them to 50 mL of anhydrous N,N-dimethylformamide. Place the solution in a single-necked flask at room temperature and stir until the 2-methylimidazole and benzimidazole are completely dissolved to obtain an imidazole solution. Weigh 2.195 g of zinc acetate dihydrate and dissolve it in 20 mL of anhydrous N,N-dimethylformamide to obtain a metal salt solution. Mix the metal salt solution with the imidazole solution and stir at room temperature for 3 h. Then, heat the mixture at 8000 r·min. -1 After centrifugation for 20 min, the supernatant was removed and the resulting white solid powder was washed four times with methanol. Then, the solid powder was redispersed in 200 mL of methanol for solvent exchange for 48 h. The solid product obtained from the solid-liquid separation was dried under vacuum at 150 °C overnight to obtain ZIF-7-8 nanoparticles.

[0053] (2) Preparation of ZIF-7-8@polyimide film

[0054] The polyimide used in this invention is Matrimid 5218. 9.0 g of Matrimid 5218 and 1.0 g of ZIF-7-8 powder were weighed and dried overnight in a vacuum environment at 100°C for later use. 1 g of the dried ZIF-7-8 was dispersed in N-methylpyrrolidone at room temperature, stirred for 3 hours, and then sonicated for 30 minutes to obtain a MOF suspension with a ZIF-7-8 mass fraction of 10%. Matrimid 5218 was added to the MOF suspension in three portions of 3 g, 3 g, and 3 g, with stirring for 3 hours after each addition and sonication for 30 minutes, until Matrimid 5218 was completely added. After sonication for another 30 minutes, the mixture was allowed to stand at room temperature for 48 hours. The resulting film solution was poured onto a flat glass plate using a casting applicator. Finally, the film was dried under vacuum at 100°C for 48 hours to obtain a ZIF-7-8@polyimide film.

[0055] (3) Constructing the COF separation layer

[0056] Weigh 3.5 g of 1,3,5,7-tetra(4-aminophenyl)adamantane and dissolve it in 5 mL of anhydrous trimethylbenzene and 5 mL of a mixed solvent of 1,4-dioxane. Then add 0.33 mL of acetic acid and stir until well mixed to obtain an amine monomer solution. Weigh 1.34 g of terephthalaldehyde and dissolve it in 5 mL of anhydrous trimethylbenzene and 5 mL of a mixed solvent of 1,4-dioxane to obtain an aldehyde monomer solution.

[0057] The ZIF-7-8@polyimide membrane was ultrasonically cleaned in methanol and acetone for 15 min each, then vacuum dried at 60℃ for 2 h. After drying, the ZIF-7-8@polyimide membrane was placed in a flat plasma cleaner and activated with an oxygen flow rate of 50 sccm for 80 s. Then, it was completely immersed in an amine monomer solution and allowed to stand at room temperature for 3 h. The membrane was then removed and rinsed three times with a mixed solvent of mesitylene / 1,4-dioxane. Nitrogen gas was used to purge the membrane until there were no droplets on the surface. The membrane was then laid flat in a petri dish, and an aldehyde monomer solution was poured on it to form a 1.5 mm thick liquid film on the membrane surface. The membrane was kept at 45℃ for 18 h, and the surface liquid was poured off. The resulting membrane was then immersed in anhydrous N,N-dimethylformamide and methanol for 24 h each, and then vacuum dried at 120℃ for 12 h to obtain the ZIF-7-8@polyimide mixed matrix membrane for helium extraction from natural gas.

[0058] Example 3

[0059] The preparation method of the ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas in this embodiment is as follows:

[0060] (1) Preparation of ZIF-7-8

[0061] Weigh 1.642 g of 2-methylimidazole and 0.6656 g of benzimidazole and add them to 50 mL of anhydrous N,N-dimethylformamide. Place the solution in a single-necked flask at room temperature and stir until the 2-methylimidazole and benzimidazole are completely dissolved to obtain an imidazole solution. Weigh 2.195 g of zinc acetate dihydrate and dissolve it in 20 mL of anhydrous N,N-dimethylformamide to obtain a metal salt solution. Mix the metal salt solution with the imidazole solution and stir at room temperature for 3 h. Then, heat the mixture at 8000 r·min. -1 After centrifugation for 20 min, the supernatant was removed and the resulting white solid powder was washed four times with methanol. Then, the solid powder was redispersed in 200 mL of methanol for solvent exchange for 48 h. The solid product obtained from the solid-liquid separation was dried under vacuum at 150 °C overnight to obtain ZIF-7-8 nanoparticles.

[0062] (2) Preparation of ZIF-7-8@polyimide film

[0063] The polyimide used in this invention is Matrimid 5218. 8.0 g of Matrimid 5218 and 2.0 g of ZIF-7-8 powder were weighed and dried overnight in a vacuum environment at 100°C for later use. 2 g of the dried ZIF-7-8 was dispersed in N-methylpyrrolidone at room temperature, stirred for 3 hours, and then sonicated for 30 minutes to obtain a MOF suspension with a ZIF-7-8 mass fraction of 20%. Matrimid 5218 was added to the MOF suspension in three portions: 3 g, 3 g, and 2 g, with stirring for 3 hours after each addition and sonication for 30 minutes, until all Matrimid 5218 was added. After sonication for another 30 minutes, the mixture was allowed to stand at room temperature for 48 hours. The resulting film solution was poured onto a flat glass plate using a casting applicator. Finally, the film was dried under vacuum at 100°C for 48 hours to obtain a ZIF-7-8@polyimide film.

[0064] (3) Constructing the COF separation layer

[0065] Weigh 3.5 g of 1,3,5,7-tetra(4-aminophenyl)adamantane and dissolve it in 5 mL of anhydrous trimethylbenzene and 5 mL of a mixed solvent of 1,4-dioxane. Then add 0.33 mL of acetic acid and stir until well mixed to obtain an amine monomer solution. Weigh 1.34 g of terephthalaldehyde and dissolve it in 5 mL of anhydrous trimethylbenzene and 5 mL of a mixed solvent of 1,4-dioxane to obtain an aldehyde monomer solution.

[0066] The ZIF-7-8@polyimide membrane was ultrasonically cleaned in methanol and acetone for 15 min each, then vacuum dried at 60℃ for 2 h. After drying, the ZIF-7-8@polyimide membrane was placed in a flat plasma cleaner and activated with an oxygen flow rate of 50 sccm for 80 s. Then, it was completely immersed in an amine monomer solution and allowed to stand at room temperature for 3 h. The membrane was then removed and rinsed three times with a mixed solvent of mesitylene / 1,4-dioxane. Nitrogen gas was used to purge the membrane until there were no droplets on the surface. The membrane was then laid flat in a petri dish, and an aldehyde monomer solution was poured on it to form a 1.5 mm thick liquid film on the membrane surface. The membrane was kept at 45℃ for 18 h, and the surface liquid was poured off. The resulting membrane was then immersed in anhydrous N,N-dimethylformamide and methanol for 24 h each, and then vacuum dried at 120℃ for 12 h to obtain the ZIF-7-8@polyimide mixed matrix membrane for helium extraction from natural gas.

[0067] Example 4

[0068] The preparation method of the ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas in this embodiment is as follows:

[0069] (1) Preparation of ZIF-7-8

[0070] Weigh 2.463 g of 2-methylimidazole and 0.9984 g of benzimidazole and add them to 70 mL of anhydrous N,N-dimethylformamide. Place the solution in a single-necked flask at room temperature and stir until the 2-methylimidazole and benzimidazole are completely dissolved to obtain an imidazole solution. Weigh 3.2925 g of zinc acetate dihydrate and dissolve it in 50 mL of anhydrous N,N-dimethylformamide to obtain a metal salt solution. Mix the metal salt solution with the imidazole solution and stir at room temperature for 3 h. Then, heat the mixture at 8000 r·min. -1 After centrifugation for 20 min, the supernatant was removed and the resulting white solid powder was washed four times with methanol. Then, the solid powder was redispersed in 300 mL of methanol for solvent exchange for 48 h. The solid product obtained from the solid-liquid separation was dried under vacuum at 150 °C overnight to obtain ZIF-7-8 nanoparticles.

[0071] (2) Preparation of ZIF-7-8@polyimide film

[0072] The polyimide used in this invention is Matrimid 5218. 7.0 g of Matrimid 5218 and 3.0 g of ZIF-7-8 powder were weighed and dried overnight in a vacuum environment at 100°C for later use. 3 g of the dried ZIF-7-8 was dispersed in N-methylpyrrolidone at room temperature, stirred for 3 hours, and then sonicated for 30 minutes to obtain a MOF suspension with a ZIF-7-8 mass fraction of 30%. Matrimid 5218 was added to the MOF suspension in three portions: 3 g, 3 g, and 1 g, with stirring for 3 hours after each addition and sonication for 30 minutes, until all Matrimid 5218 was added. The mixture was then sonicated for another 30 minutes and allowed to stand at room temperature for 48 hours. The resulting film solution was poured onto a flat glass plate using a casting applicator. Finally, the film was dried under vacuum at 100°C for 48 hours to obtain a ZIF-7-8@polyimide film.

[0073] (3) Constructing the COF separation layer

[0074] Weigh 3.5 g of 1,3,5,7-tetra(4-aminophenyl)adamantane and dissolve it in 5 mL of anhydrous trimethylbenzene and 5 mL of a mixed solvent of 1,4-dioxane. Then add 0.33 mL of acetic acid and stir until well mixed to obtain an amine monomer solution. Weigh 1.34 g of terephthalaldehyde and dissolve it in 5 mL of anhydrous trimethylbenzene and 5 mL of a mixed solvent of 1,4-dioxane to obtain an aldehyde monomer solution.

[0075] The ZIF-7-8@polyimide membrane was ultrasonically cleaned in methanol and acetone for 15 min each, then vacuum dried at 60℃ for 2 h. After drying, the ZIF-7-8@polyimide membrane was placed in a flat plasma cleaner and activated with an oxygen flow rate of 50 sccm for 80 s. Then, it was completely immersed in an amine monomer solution and allowed to stand at room temperature for 3 h. The membrane was then removed and rinsed three times with a mixed solvent of mesitylene / 1,4-dioxane. Nitrogen gas was used to purge the membrane until there were no droplets on the surface. The membrane was then laid flat in a petri dish, and an aldehyde monomer solution was poured on it to form a 1.5 mm thick liquid film on the membrane surface. The membrane was kept at 45℃ for 18 h, and the surface liquid was poured off. The resulting membrane was then immersed in anhydrous N,N-dimethylformamide and methanol for 24 h each, and then vacuum dried at 120℃ for 12 h to obtain the ZIF-7-8@polyimide mixed matrix membrane for helium extraction from natural gas.

[0076] Example 5

[0077] The preparation method of the ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas in this embodiment is as follows:

[0078] (1) Preparation of ZIF-7-8

[0079] Weigh 1.44 g of 2-methylimidazole and 0.584 g of benzimidazole and add them to 50 mL of anhydrous N,N-dimethylformamide. Place the solution in a single-necked flask at room temperature and stir until the 2-methylimidazole and benzimidazole are completely dissolved to obtain an imidazole solution. Weigh 2.195 g of zinc acetate dihydrate and dissolve it in 20 mL of anhydrous N,N-dimethylformamide to obtain a metal salt solution. Mix the metal salt solution with the imidazole solution and stir at room temperature for 2 h. Then, heat the mixture at 7000 r·min. -1 After centrifugation for 25 min, the supernatant was removed and the resulting white solid powder was washed three times with methanol. Then, the solid powder was redispersed in 100 mL of methanol for solvent exchange for 40 h. The solid product obtained from solid-liquid separation was dried under vacuum at 140 °C overnight to obtain ZIF-7-8 nanoparticles.

[0080] (2) Preparation of ZIF-7-8@polyimide film

[0081] The polyimide used in this invention is Matrimid 5218. 9.0 g of Matrimid 5218 and 1.0 g of ZIF-7-8 powder were weighed and dried overnight in a vacuum environment at 80°C for later use. 1 g of the dried ZIF-7-8 was dispersed in N-methylpyrrolidone at room temperature, stirred for 2 hours, and then sonicated for 20 minutes to obtain a MOF suspension with a ZIF-7-8 mass fraction of 10%. Matrimid 5218 was added to the MOF suspension in three portions of 3 g, 3 g, and 3 g, with stirring for 2 hours after each addition and sonication for 40 minutes, until Matrimid 5218 was completely added. After sonication for another 20 minutes, the mixture was allowed to stand at room temperature for 40 hours. The resulting film solution was poured onto a flat glass plate using a casting applicator. Finally, the film was dried under vacuum at 90°C for 40 hours to obtain a ZIF-7-8@polyimide film.

[0082] (3) Constructing the COF separation layer

[0083] Weigh 2.5 g of 1,3,5,7-tetra(4-aminophenyl)adamantane and dissolve it in 5 mL of anhydrous trimethylbenzene and 5 mL of a mixed solvent of 1,4-dioxane. Then add 0.28 mL of acetic acid and stir to mix well to obtain an amine monomer solution. Weigh 0.671 g of terephthalaldehyde and dissolve it in 5 mL of anhydrous trimethylbenzene and 5 mL of a mixed solvent of 1,4-dioxane to obtain an aldehyde monomer solution.

[0084] The ZIF-7-8@polyimide membrane was ultrasonically cleaned in methanol and acetone for 10 min each, then vacuum dried at 50℃ for 3 h. After drying, the ZIF-7-8@polyimide membrane was placed in a flat plasma cleaner and activated with an oxygen flow rate of 50 sccm for 60 s. Then, it was completely immersed in an amine monomer solution and allowed to stand at room temperature for 2 h. The membrane was then removed and rinsed twice with a mixed solvent of mesitylene / 1,4-dioxane. Nitrogen gas was used to purge the membrane until there were no droplets on the surface. The membrane was then laid flat in a petri dish, and an aldehyde monomer solution was poured on it to form a 1 mm thick liquid film on the surface. The membrane was kept at 40℃ for 24 h, and the surface liquid was poured off. The resulting membrane was then soaked in anhydrous N,N-dimethylformamide and methanol for 12 h each, and then vacuum dried at 115℃ for 14 h to obtain the ZIF-7-8@polyimide mixed matrix membrane for helium extraction from natural gas.

[0085] Example 6

[0086] The preparation method of the ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas in this embodiment is as follows:

[0087] (1) Preparation of ZIF-7-8

[0088] Weigh 2.89 g of 2-methylimidazole and 1.16 g of benzimidazole and add them to 60 mL of anhydrous N,N-dimethylformamide. Place the solution in a single-necked flask at room temperature and stir until the 2-methylimidazole and benzimidazole are completely dissolved to obtain an imidazole solution. Weigh 2.195 g of zinc acetate dihydrate and dissolve it in 20 mL of anhydrous N,N-dimethylformamide to obtain a metal salt solution. Mix the metal salt solution with the imidazole solution and stir at room temperature for 4 h at a speed of 9000 r·min. -1 After centrifugation for 15 min, the supernatant was removed and the resulting white solid powder was washed 5 times with methanol. Then, the solid powder was redispersed in 200 mL of methanol for solvent exchange for 50 h. The solid product obtained by solid-liquid separation was placed at 160 °C and vacuum dried overnight to obtain ZIF-7-8 nanoparticles.

[0089] (2) Preparation of ZIF-7-8@polyimide film

[0090] The polyimide used in this invention is Matrimid 5218. 9.0 g of Matrimid 5218 and 1.0 g of ZIF-7-8 powder were weighed and dried overnight in a vacuum environment at 110°C for later use. 1 g of the dried ZIF-7-8 was dispersed in N-methylpyrrolidone at room temperature, stirred for 4 hours, and then sonicated for 25 minutes to obtain a MOF suspension with a ZIF-7-8 mass fraction of 10%. Matrimid 5218 was added to the MOF suspension in three portions of 3 g, 3 g, and 3 g, with stirring for 4 hours after each addition and sonication for 60 minutes, until Matrimid 5218 was completely added. The mixture was then sonicated for another 40 minutes and allowed to stand at room temperature for 50 hours. The resulting film solution was poured onto a flat glass plate using a casting applicator. Finally, the film was vacuum dried at 110°C for 50 hours to obtain a ZIF-7-8@polyimide film.

[0091] (3) Constructing the COF separation layer

[0092] Weigh 5.01 g of 1,3,5,7-tetra(4-aminophenyl)adamantane and dissolve it in 5 mL of anhydrous trimethylbenzene and 5 mL of a mixed solvent of 1,4-dioxane. Then add 0.50 mL of acetic acid and stir to mix well to obtain an amine monomer solution. Weigh 1.34 g of terephthalaldehyde and dissolve it in 5 mL of anhydrous trimethylbenzene and 5 mL of a mixed solvent of 1,4-dioxane to obtain an aldehyde monomer solution.

[0093] The ZIF-7-8@polyimide membrane was ultrasonically cleaned in methanol and acetone for 20 min each, then vacuum dried at 70℃ for 1 h. After drying, the ZIF-7-8@polyimide membrane was placed in a flat plasma cleaner and activated with an oxygen flow rate of 50 sccm for 120 s. Then, it was completely immersed in an amine monomer solution and allowed to stand at room temperature for 4 h. The membrane was then removed and rinsed four times with a mixed solvent of mesitylene / 1,4-dioxane. Nitrogen gas was used to purge the membrane until there were no droplets on the surface. The membrane was then laid flat in a petri dish, and an aldehyde monomer solution was poured on it to form a 2 mm thick liquid film on the membrane surface. The membrane was kept at 50℃ for 12 h, and the surface liquid was poured off. The resulting membrane was then soaked in anhydrous N,N-dimethylformamide and methanol for 36 h each, and then vacuum dried at 130℃ for 10 h to obtain the ZIF-7-8@polyimide mixed matrix membrane for helium extraction from natural gas.

[0094] Comparative Example 1

[0095] This comparative example uses a Matrimi5218 membrane.

[0096] Comparative Example 2

[0097] The difference between this comparative example and Example 3 is that step (3) is omitted in constructing the COF separation layer, which is a ZIF-7-8@Matrimid5218 monolayer membrane.

[0098] The membranes prepared in Examples 1-4 and Comparative Examples 1-2 were used as samples. They were encapsulated in a gas permeation / separation tester with epoxy resin. Their performance was tested under the conditions of a pressure of 0.5 bar and a temperature of room temperature. The test gases were CH4, He, and N2. The test results are shown in Table 1.

[0099] Table 1 Performance test results of each sample membrane

[0100]

[0101] The data in Table 1 show that the helium flux and selectivity of a single Matrimid 5218 membrane without MOF particles and a COF layer are limited, while the addition of ZIF-7-8 and a COF layer significantly improves helium flux and selectivity, especially for He / CH4. The sample membrane prepared in Example 3 achieved the highest permeability at 355.7 barrer, and the sample membrane prepared in Example 3 also showed the highest selectivity for He / CH4 and He / N2, at 433.5 and 387.7, respectively. The increase in permeability can be attributed to the increased polymer volume in the presence of MOF particles and the improved membrane integrity by eliminating interfacial defects after the introduction of the COF shell. Furthermore, the data in Table 1 also show that the ZIF-7-8@polyimide mixed matrix membrane for helium extraction from natural gas of the present invention improves He permeability by 6-11%, He / N2 selectivity by 12-26%, and He / CH4 selectivity by 23-34% compared to the single-component Matrimid 5218 membrane.

[0102] In summary, the ZIF-7-8@polyimide hybrid matrix membrane for helium extraction from natural gas of the present invention exhibits excellent permeability and selectivity for helium and can be used in helium separation materials.

[0103] The foregoing description includes examples from this specification. Of course, for the purposes of describing this specification, it is impossible to describe every conceivable combination of components or methods; however, those skilled in the art will understand that many other combinations and arrangements are possible. Therefore, this specification is intended to cover all such changes, modifications, and variations that fall within the spirit and scope of the appended claims. Furthermore, with regard to the use of the term "comprising" in the detailed description or claims, the term is intended to be inclusive in a manner similar to the term "including," as interpreted when "comprising" is used as a transitional word in the claims.

[0104] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing ZIF-7-8@polyimide mixed matrix membranes for helium extraction from natural gas, characterized in that, Includes the following steps: To prepare a ZIF-7-8@polyimide film, ZIF-7-8 was dispersed in N-methylpyrrolidone at room temperature and mixed to obtain a MOF suspension. Polyimide was added in batches and stirred, ultrasonicated, and allowed to stand at room temperature for 40-50 h. The film was then coated and dried under vacuum at 90-110 °C for 40-50 h to obtain the ZIF-7-8@polyimide film. To construct the COF separation layer, 1,3,5,7-tetra(4-aminophenyl)adamantane was dissolved in a mixed solvent of mesitylene / 1,4-dioxane, and then acetic acid was added and stirred to obtain an amine monomer solution. Terephthalaldehyde was dissolved in a mixed solvent of mesitylene / 1,4-dioxane to obtain an aldehyde monomer solution. The ZIF-7-8@polyimide membrane was activated by plasma treatment for 60–120 s, then completely immersed in the amine monomer solution, and allowed to stand at room temperature for 2–4 h before being removed. The membrane was rinsed 2–4 times with a mixed solvent of mesitylene / 1,4-dioxane and purged with nitrogen until no droplets were visible on the membrane surface. The membrane was then laid flat and an aldehyde monomer solution was poured on it to form a liquid film 1–2 mm thick on the membrane surface. The membrane was then kept at 40–50 °C for 12–24 h. The resulting membrane was then immersed in anhydrous N,N-dimethylformamide and methanol for 12–36 h each, and then vacuum dried at 115–130 °C for 10–14 h to obtain the ZIF-7-8@polyimide mixed matrix membrane.

2. The production method according to claim 1, characterized by, The MOF suspension contains 5-30% ZIF-7-8 by mass. After adding N-methylpyrrolidone to ZIF-7-8, the mixture is stirred for 2-4 hours and then sonicated for 20-30 minutes to obtain the MOF suspension.

3. The preparation method according to claim 1, characterized in that, The mass ratio of ZIF-7-8 to polyimide is (0.05~0.45):

1. ZIF-7-8 and polyimide need to be dried in a vacuum environment at 80~110℃ before use.

4. The production method according to claim 1, characterized by, The preparation method of ZIF-7-8 is as follows: Weigh 2-methylimidazole and benzimidazole and dissolve them in anhydrous N,N-dimethylformamide to obtain an imidazole solution; weigh zinc acetate dihydrate and dissolve it in anhydrous N,N-dimethylformamide to obtain a metal salt solution; mix the metal salt solution with the imidazole solution and stir at room temperature for 2-4 hours; centrifuge the solid obtained and wash it with methanol, then disperse it in methanol for solvent exchange for 40-50 hours; after solid-liquid separation, place the solid product obtained in the solid-liquid solution under vacuum at 140-160℃ to obtain ZIF-7-8.

5. The preparation method according to claim 4, characterized in that, The mass ratio of 2-methylimidazole to benzimidazole is 1:0.4, and the molar ratio of the total molar amount of 2-methylimidazole and benzimidazole to the molar amount of zinc acetate dihydrate is (2.0~4.0):

1.

6. The method of claim 1, wherein, The mesitylene / 1,4-dioxane mixed solvent was prepared by mixing anhydrous mesitylene and 1,4-dioxane in a volume ratio of 1:

1.

7. The production method according to claim 6, characterized by, The concentration of 1,3,5,7-tetra(4-aminophenyl)adamantane in the amine monomer solution is 0.5–1 mol / L, and the concentration of terephthalaldehyde in the aldehyde monomer solution is 0.5–1 mol / L.

8. The method of claim 1, wherein, Before plasma treatment, the ZIF-7-8@polyimide film needs to be ultrasonically cleaned in methanol and acetone for 10-20 minutes each, and then vacuum dried at 50-70°C for 1-3 hours.

9. ZIF-7-8@polyimide mixed matrix membranes for natural gas- based helium extraction, characterized in that, The hybrid matrix membrane is prepared using the preparation method described in any one of claims 1-8.

10. The application of the hybrid matrix membrane according to claim 9 in the preparation of helium separation materials.

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