Metal organic framework composite membrane for separating helium-methane mixed gas and preparation method and application thereof

Abstract

This invention discloses a metal-organic framework (MOF) composite membrane for separating a helium-methane mixture, its preparation method, and its application. The MOF composite membrane comprises a porous base membrane and a ZIF-8 MOF membrane layer disposed on at least one side of the porous base membrane. The ZIF-8 MOF membrane layer is formed by the self-assembly of metal ions and imidazole ligands on the surface of the porous base membrane. The imidazole ligands include 2-methylimidazolium and other imidazole ligands; the other imidazole ligands are hydrocarbon-based and / or halogen-substituted benzimidazoles. This invention also provides a method for preparing the above-mentioned composite membrane by diffusion growth. The MOF ZIF-8 membrane layer prepared by this method is continuous, dense, structurally stable, and has high permeability. When the above-mentioned MOF composite membrane is used to separate a helium-methane mixture, the helium permeation flux can reach as high as 700 × 10⁻⁶. ‑9 mol m ‑2 s ‑1 Pa ‑1 The above parameters are achieved, with a separation selectivity as high as 280 while maintaining a helium permeation flux of no less than 100 × 10⁻⁶. ‑9 mol m ‑2 s ‑1 Pa ‑1 .

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, specifically to a metal-organic framework composite membrane for separating helium-methane mixed gases, its preparation method, and its application. Background Technology

[0002] Helium (He), a rare gas, is widely used in various fields such as magnetic resonance imaging, ballooning, leak detection, and welding. It is also one of the specialty gases required for manufacturing semiconductors and electronic devices. However, helium in nature typically exists in low concentrations in the atmosphere, natural gas, synthetic ammonia tail gas, and geothermal water. Currently, helium production mainly relies on extraction from natural gas through cryogenic distillation and pressure swing adsorption, which significantly limits the purification and utilization of helium. Membrane gas separation technology, compared to traditional gas separation methods, has advantages such as low energy consumption, small footprint, and environmental friendliness, and is considered a highly promising green separation technology. Commercially available polyimide hollow fiber membranes are used to recover helium from natural gas, but existing polymer membranes often face a trade-off between permeability and selectivity, and suffer from plasticization and swelling, leading to reduced separation efficiency.

[0003] In the past decade or so, metal-organic frameworks (MOFs) have become ideal materials for gas storage and separation due to their high specific surface area, high pore volume, and tunable pore structure. Similarly, by fabricating MOFs into membrane materials and adjusting pore size and functionalization, selective separation of gases and other molecules can be achieved with high permeation performance, which will significantly reduce the cost and energy consumption of gas separation and purification. However, current reports indicate that MOF membranes exhibit very low He / CH4 separation factors and He permeation fluxes, with a separation factor of approximately 10 and a He permeation flux of only 70 × 10⁻⁶. -9 mol m -2 s -1 Pa -1 The extremely limited range (see J. Am. Chem. Soc. 2023, 145, 27, 14793–14801) severely restricts the application of MOF membranes in He / CH4 separation.

[0004] Therefore, there is an urgent need for a membrane material that can efficiently separate He / CH4 for low-cost and efficient recovery of helium from natural gas. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a metal-organic framework (MOF) composite membrane for separating helium and methane mixed gases, along with its preparation method and application. This invention uses a porous membrane material as a substrate and employs a diffusion growth method involving mixed imidazole ligands and metal ions to form a continuous, defect-free, and dense MOF ZIF-8 membrane layer on the porous substrate. Furthermore, by controlling the type and proportion of imidazole ligands in the mixed imidazole ligands, a MOF ZIF-8 membrane layer with functionalized functional groups and suitable pore size is obtained, enabling the prepared MOF composite membrane to efficiently separate He / CH4.

[0006] Specifically, the present invention provides the following technical solutions:

[0007] The first aspect of the present invention provides a metal-organic framework composite membrane for separating a helium-methane mixed gas, the metal-organic framework composite membrane comprising a porous base membrane and a metal-organic framework ZIF-8 membrane layer disposed on at least one side of the porous base membrane;

[0008] The metal-organic framework ZIF-8 membrane is formed by the self-assembly of metal ions and imidazole ligands on the surface of the porous substrate membrane; the imidazole ligands include 2-methylimidazolium and other imidazole ligands, and the other imidazole ligands are benzimidazoles substituted with hydrocarbon groups and / or halogens.

[0009] Furthermore, the porous base membrane is made of one or more of porous organic polymer materials, porous metal oxides, and porous non-metal oxides; preferably, the porous organic polymer material is selected from one or more of polyethersulfone, polyaniline, and polyimide, the porous metal oxide is porous alumina, and the porous non-metal oxide is selected from one or more of porous silica, porous silicon carbide, and porous glass, such as a polyethersulfone porous base membrane modified with polyaniline.

[0010] Furthermore, the metal ions include zinc ions.

[0011] Furthermore, the hydrocarbon group includes one or more of methyl, ethyl, propyl, and phenyl.

[0012] Furthermore, the other imidazole ligands may be selected from 2-methylbenzimidazole. 2-Ethylbenzimidazole 5-Methyl-2-methylbenzimidazole 2-Phenylenimazole 2-Chloromethylbenzimidazole 5-Fluoro-2-methylbenzimidazole 5-Chloro-2-methylbenzimidazole 5-Bromo-2-methylbenzimidazole 5,6-Dichloro-2-methylbenzimidazole 2-Trifluoromethylbenzimidazole One or more of them.

[0013] Further, the other imidazole ligands are benzimidazoles containing halogen substituents, wherein the halogen is selected from one or more of F, Cl, and Br; more preferably, the halogen substituent is located on the benzene ring of benzimidazole.

[0014] Further, the molar ratio of 2-methylimidazole to other imidazole ligands in the imidazole ligand is 1:(0.05-1), for example 1:0.05, 1:0.06, 1:0.07, 1:0.08, 1:0.09, 1:0.1, etc., including but not limited to the molar ratios listed above; more preferably 1:(0.3-0.6).

[0015] This invention introduces different imidazole ligands to form a ZIF-8 membrane with a metal-organic framework (MOF) with metal ions. The molar ratio of 2-methylimidazolium to other imidazole ligands affects the formation of the MOF and the pore size of the resulting ZIF-8 membrane. The amount of other imidazole ligands introduced should not be too small, as this would prevent effective pore size control and thus hinder the improvement of the He / CH4 separation factor. Conversely, the amount of other imidazole ligands introduced should not be too large, as excessive amounts of other imidazole ligands and relatively low amounts of 2-methylimidazolium would prevent the formation of the MOF with metal ions. Therefore, to ensure the formation of the ZIF-8 membrane structure and effective separation of He / CH4, the molar ratio of 2-methylimidazolium to other imidazole ligands should be controlled within the range of 1:(0.05-1), more preferably 1:(0.3-0.6).

[0016] A second aspect of this invention provides a method for preparing the metal-organic framework composite membrane described in the first aspect, comprising the following steps:

[0017] (1) Provide a first metal source solution, and provide a mixed solution containing 2-methylimidazole and other imidazole ligands;

[0018] (2) The first metal source solution and the mixed solution are placed on both sides of the H-type diffusion cell, and the porous base membrane is fixed in the middle of the H-type diffusion cell to carry out the diffusion reaction, forming a metal-organic framework ZIF-8 membrane layer on the surface of the porous base membrane, thus obtaining the metal-organic framework composite membrane.

[0019] Further, in step (1), the first metal source is selected from one or more of zinc sulfate, zinc nitrate, zinc acetate, and zinc chloride, providing metal ions that coordinate with the imidazole ligand.

[0020] Further, in step (1), the concentration of metal ions in the first metal source solution is preferably 0.01-0.03 mol / L, and more preferably, the ratio of the molar concentration of metal ions in the first metal source solution to the total molar concentration of imidazole ligands in the mixed solution is 1:(3-8).

[0021] Further, in step (2), the diffusion reaction is carried out at a temperature of 0-60°C for 1-72 hours; more preferably, the diffusion is carried out at a temperature of 0-30°C for 12-48 hours to form a complete and dense metal-organic framework ZIF-8 film.

[0022] Furthermore, the preparation method further includes a pretreatment step for the porous base membrane, specifically: immersing the porous base membrane in a second metal source solution to allow metal ions to enter the pores of the porous base membrane; removing the porous base membrane to remove residual metal source solution from the surface, thus obtaining a pretreated porous base membrane; preferably, the metal ions in the second metal source solution are zinc ions; the concentration of the metal ions in the second metal source solution is 0.02-0.05 mol / L. By pretreating the porous base membrane, some metal ions enter the pores of the porous base membrane, promoting the subsequent formation of the surface metal-organic framework ZIF-8 film.

[0023] The third aspect of this invention provides the application of the metal-organic framework composite membrane described in the first aspect or the metal-organic framework composite membrane prepared by the preparation method described in the second aspect in the separation of He / CH4 mixed gas.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] 1. This invention provides a metal-organic framework composite membrane for separating helium and methane mixed gases. It is formed by the self-assembly of different types of imidazole ligands and metal ions on the surface of a porous base membrane. By introducing an appropriate amount of imidazole ligands with functionalized functional groups and 2-methylimidazolium and metal ion ligands, a metal-organic framework ZIF-8 membrane layer with suitable pore size is formed on the porous base membrane. By utilizing the interaction between the introduced functionalized functional groups and methane gas molecules, the pore size is synergistically controlled, effectively improving the selective permeation and permeation flux of the metal-organic framework membrane for helium.

[0026] 2. The present invention also provides a method for preparing the above-mentioned metal-organic framework composite membrane, wherein a continuous, defect-free and dense metal-organic framework ZIF-8 membrane layer is formed on a porous base membrane by diffusion growth of mixed imidazole ligands and metal ions. The method is simple to operate, has low energy consumption, good repeatability, and the prepared metal-organic framework composite membrane has good structural stability and is suitable for mass production.

[0027] 3. The metal-organic framework composite membrane provided by this invention, when used in the separation of helium and methane mixed gases, can possess both a high separation factor and a high permeation flux, with a helium permeation flux greater than 100 × 10⁻⁶. -9 mol m -2 s -1 Pa -1 Under these conditions, the separation factor can reach as high as 280, which is far superior to the separation effect of MOF membranes for helium / methane reported by existing technologies, and can effectively promote the efficiency of helium recovery from natural gas. Attached Figure Description

[0028] Figure 1 SEM image of the metal-organic framework composite film prepared in Example 1, where a is the front side and b is the cross-section;

[0029] Figure 2 The image shows the XRD pattern of the metal-organic framework composite membrane prepared in Example 1.

[0030] Figure 3 SEM images of the metal-organic framework composite film prepared in Example 7, where a is the front side and b is the cross-section;

[0031] Figure 4 The image shows the XRD pattern of the metal-organic framework composite membrane prepared in Example 7. Detailed Implementation

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items. "Comprising" or "containing" as used herein means that it may include or contain other components in addition to the stated components. "Comprising" or "containing" as used herein may also be replaced with the closed form "is" or "consisting of".

[0033] The present invention will be further described below with reference to specific embodiments and accompanying drawings, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0034] Example 1

[0035] This embodiment relates to the preparation of a metal-organic framework composite membrane, as detailed below:

[0036] Preparation of porous base membranes:

[0037] 0.02 mol of aniline and 0.01 mol of ammonium persulfate were added separately to 50 mL of perchloric acid solution (1 mol / L) and stirred until homogeneous. The solution was then cooled to 0°C. Under magnetic stirring, the ammonium persulfate solution was added to the aniline solution. A polyethersulfone porous support with a pore size of 0.1 μm and a diameter of 4.7 cm was placed into the mixed solution, and the reaction was carried out in an ice-water bath for 6 hours. After the reaction, the membrane was removed and rinsed thoroughly with deionized water. The porous support was then immersed in ammonium hydroxide solution (0.1 mol / L) for 30 minutes to neutralize any residual acid. After rinsing with deionized water, the membrane was dried to obtain the polyaniline-modified polyethersulfone porous membrane.

[0038] Preparation of metal-organic framework composite membranes:

[0039] (1) Immerse the polyaniline-modified polyethersulfone porous carrier in a 0.02 mol / L zinc nitrate methanol solution for 30 minutes, then wipe it slightly dry to keep it semi-moist, and set it aside for later use.

[0040] Prepare a 0.01 mol / L zinc nitrate methanol solution and a 0.05 mol / L imidazole mixed ligand methanol solution, wherein the molar ratio of 2-methylimidazolium to 2-chloromethylbenzimidazole in the imidazole mixed ligand is 1:0.1.

[0041] (2) The polyaniline-modified polyethersulfone porous support after step (1) is placed in the middle of the H-type diffusion cell and fixed; then the zinc nitrate solution and the 2-chloromethylbenzimidazole mixed solution are placed on both sides of the H-type diffusion cell respectively; the solutions on both sides diffuse and grow, and react at 25 degrees Celsius for 30 hours to form a dense 2-chloromethylbenzimidazole mixed ligand ZIF-8 membrane on the surface of the porous support. After taking it out, it is washed and dried to obtain a metal-organic framework composite membrane.

[0042] The metal-organic framework composite membrane prepared in this embodiment was characterized by SEM and XRD, and the results are as follows:

[0043] Figure 1 a and 1b are SEM images of the surface and cross-section of the metal-organic framework composite film, respectively. As shown in the images, a dense modification layer grows on the surface of the metal-organic framework composite film. Figure 2 It can be seen that the modified layer is a metal-organic framework ZIF-8 film with good crystallinity.

[0044] Example 2

[0045] This embodiment relates to the preparation of a metal-organic framework composite membrane. The only difference from Example 1 is that in step (1), the molar ratio of 2-methylimidazole to 2-chloromethylbenzimidazole in the imidazole mixed ligand is 1:0.3; the rest of the operations are the same, and the corresponding metal-organic framework composite membrane is prepared.

[0046] Example 3

[0047] This embodiment relates to the preparation of a metal-organic framework composite membrane. The only difference from Example 1 is the preparation of the metal-organic framework composite membrane; all other operations are the same, as detailed below:

[0048] (1) The polyaniline-modified polyethersulfone porous carrier was immersed in a 0.03 mol / L zinc nitrate methanol solution for 30 minutes, and then slightly dried to keep it semi-moist for later use.

[0049] (2) Prepare a methanol solution of 0.01 mol / L zinc nitrate and a methanol solution of 0.06 mol / L imidazole mixed ligands, wherein the molar ratio of 2-methylimidazolium to 2-methylbenzimidazole in the imidazole mixed ligands is 1:0.08.

[0050] The polyaniline-modified polyethersulfone porous support pretreated in step (1) was placed in the middle of an H-type diffusion cell and fixed. Then, the zinc nitrate solution and the 2-methylbenzimidazole mixed solution were placed on both sides of the H-type diffusion cell. The solution on both sides diffused and grew, and the reaction was carried out at 25 degrees Celsius for 48 hours to form a dense 2-methylbenzimidazole mixed ligand ZIF-8 membrane on the surface of the porous support. After being taken out, washed and dried, the metal-organic framework composite membrane was obtained.

[0051] Example 4

[0052] This embodiment relates to the preparation of a metal-organic framework composite membrane. The only difference from Example 3 is that in step (1), the molar ratio of 2-methylimidazole to 2-methylbenzimidazole in the imidazole mixed ligand is 1:0.3; the rest of the operations are the same, and the corresponding metal-organic framework composite membrane is prepared.

[0053] Example 5

[0054] This embodiment relates to the preparation of a metal-organic framework composite membrane. The only difference from Example 1 is the preparation of the metal-organic framework composite membrane; all other operations are the same, as detailed below:

[0055] (1) Immerse the polyaniline-modified polyethersulfone porous carrier in a 0.05 mol / L zinc nitrate methanol solution for 30 minutes, then wipe it slightly dry to keep it semi-moist for later use.

[0056] (2) Prepare a methanol solution of 0.01 mol / L zinc nitrate and a methanol solution of 0.04 mol / L imidazole mixed ligands, wherein the molar ratio of 2-methylimidazolium to 2-ethylbenzimidazole in the imidazole mixed ligands is 1:0.09.

[0057] The polyaniline-modified polyethersulfone porous support pretreated in step (1) was placed in the middle of an H-type diffusion cell and fixed. Then, the zinc nitrate solution and the 2-ethylbenzimidazole mixed solution were placed on both sides of the H-type diffusion cell. The solutions on both sides diffused and grew, and the reaction was carried out at 25 degrees Celsius for 48 hours to form a dense 2-ethylbenzimidazole mixed ligand ZIF-8 membrane on the surface of the porous support. After being taken out, washed and dried, the metal-organic framework composite membrane was obtained.

[0058] Example 6

[0059] This embodiment relates to the preparation of a metal-organic framework composite membrane. The only difference from Example 5 is that in step (1), the molar ratio of 2-methylimidazole to 2-ethylbenzimidazole in the imidazole mixed ligand is 1:0.18; the rest of the operations are the same, and the corresponding metal-organic framework composite membrane is prepared.

[0060] Example 7

[0061] This embodiment relates to the preparation of a metal-organic framework composite membrane. The only difference from Example 1 is the preparation of the metal-organic framework composite membrane; all other operations are the same, as detailed below:

[0062] (1) Immerse the polyaniline-modified polyethersulfone porous carrier in a 0.02 mol / L zinc nitrate methanol solution for 30 minutes, then wipe it slightly dry to keep it semi-moist, and set it aside for later use.

[0063] (2) Prepare a methanol solution of 0.01 mol / L zinc nitrate and a methanol solution of 0.05 mol / L imidazole mixed ligands, wherein the molar ratio of 2-methylimidazolium to 5-chloro-2-methylbenzimidazole in the imidazole mixed ligands is 1:0.1.

[0064] The polyaniline-modified polyethersulfone porous support pretreated in step (1) was placed in the middle of an H-type diffusion cell and fixed. Then, the zinc nitrate solution and the 5-chloro-2-methylbenzimidazole mixed solution were placed on both sides of the H-type diffusion cell. The solution on both sides diffused and grew. The reaction was carried out at 25 degrees Celsius for 48 hours to form a dense 5-chloro-2-methylbenzimidazole mixed ligand ZIF-8 membrane on the surface of the porous support. After being taken out, washed and dried, the metal-organic framework composite membrane was obtained.

[0065] The metal-organic framework composite membrane prepared in this embodiment was characterized by SEM and XRD, and the results are as follows:

[0066] Figure 3 a and 3b are SEM images of the surface and cross-section of the metal-organic framework composite film, respectively. As shown in the images, a dense modification layer grows on the surface of the metal-organic framework composite film. Figure 4 It can be seen that the modified layer is a metal-organic framework ZIF-8 film.

[0067] Example 8

[0068] This embodiment relates to the preparation of a metal-organic framework composite membrane. The only difference from Example 7 is that in step (1), the molar ratio of 2-methylimidazole to 5-chloro-2-methylbenzimidazole in the imidazole mixed ligand is 1:0.3; the rest of the operations are the same, and the corresponding metal-organic framework composite membrane is prepared.

[0069] Example 9

[0070] This embodiment relates to the preparation of a metal-organic framework composite membrane. The only difference from Example 1 is the preparation of the metal-organic framework composite membrane; all other operations are the same, as detailed below:

[0071] (1) Immerse the polyaniline-modified polyethersulfone porous carrier in a 0.02 mol / L zinc nitrate methanol solution for 30 minutes, then wipe it slightly dry to keep it semi-moist, and set it aside for later use.

[0072] (2) Prepare a methanol solution of 0.01 mol / L zinc nitrate and a methanol solution of 0.05 mol / L imidazole mixed ligands, wherein the molar ratio of 2-methylimidazolium to 5-bromo-2-methylbenzimidazole in the imidazole mixed ligands is 1:0.1;

[0073] The polyaniline-modified polyethersulfone porous support pretreated in step (1) was placed in the middle of an H-type diffusion cell and fixed. Then, the zinc nitrate solution and the 5-bromo-2-methylbenzimidazole mixed solution were placed on both sides of the H-type diffusion cell. The solutions on both sides diffused and grew, and the reaction was carried out at 25 degrees Celsius for 48 hours to form a dense 5-bromo-2-methylbenzimidazole mixed ligand ZIF-8 membrane on the surface of the porous support. After being removed, washed and dried, a metal-organic framework composite membrane was obtained.

[0074] Example 10

[0075] This embodiment relates to the preparation of a metal-organic framework composite membrane. The only difference from Example 9 is that in step (1), the molar ratio of 2-methylimidazole to 5-bromo-2-methylbenzimidazole in the imidazole mixed ligand is 1:0.2; the rest of the operations are the same, and the corresponding metal-organic framework composite membrane is prepared.

[0076] Example 11

[0077] This embodiment relates to the preparation of a metal-organic framework composite membrane. The only difference from Example 1 is the preparation of the metal-organic framework composite membrane; all other operations are the same, as detailed below:

[0078] (1) Immerse the polyaniline-modified polyethersulfone porous carrier in a 0.02 mol / L zinc nitrate methanol solution for 30 minutes, then wipe it slightly dry to keep it semi-moist, and set it aside for later use.

[0079] (2) Prepare a methanol solution of 0.01 mol / L zinc nitrate and a methanol solution of 0.05 mol / L imidazole mixed ligands, wherein the molar ratio of 2-methylimidazolium to 5-fluoro-2-methylbenzimidazole in the imidazole mixed ligands is 1:0.2.

[0080] The polyaniline-modified polyethersulfone porous support pretreated in step (1) was placed in the middle of the H-type diffusion cell and fixed. Then, the zinc nitrate solution and the 5-fluoro-2-methylbenzimidazole mixed solution were placed on both sides of the H-type diffusion cell. The solution on both sides diffused and grew. The reaction was carried out at 25 degrees Celsius for 48 hours to form a dense 5-fluoro-2-methylbenzimidazole mixed ligand ZIF-8 membrane. After being taken out, washed and dried, the metal-organic framework composite membrane was obtained.

[0081] Example 12

[0082] This embodiment relates to the preparation of a metal-organic framework composite membrane. The only difference from Example 9 is that in step (1), the molar ratio of 2-methylimidazole to 5-fluoro-2-methylbenzimidazole in the imidazole mixed ligand is 1:0.6; the rest of the operations are the same, and the corresponding metal-organic framework composite membrane is prepared.

[0083] Example 13

[0084] This embodiment relates to the preparation of a metal-organic framework composite membrane. The only difference from Example 9 is that in step (1), the molar ratio of 2-methylimidazole to 5-fluoro-2-methylbenzimidazole in the imidazole mixed ligand is 1:0.7; the rest of the operations are the same, and the corresponding metal-organic framework composite membrane is prepared.

[0085] Comparative Example 1

[0086] This embodiment relates to the preparation of a metal-organic framework composite membrane. The only difference from Example 1 is that 2-chloromethylbenzimidazole is replaced with an equimolar amount of 2-methylimidazole; all other operations are the same, and a ZIF-8 membrane is prepared.

[0087] The amount of zinc nitrate used in the methanol solution of zinc nitrate used in the preparation of the metal-organic framework composite membranes in the above embodiments and comparative examples is the same, as is the total amount of imidazole ligands in the methanol solution of the prepared imidazole mixed ligands.

[0088] Application and performance testing

[0089] The metal-organic framework composite membranes prepared in the above examples and comparative examples were used for He / CH4 separation. The specific operation is as follows:

[0090] The Wicke-Kallenbach method was used to perform gas chromatography separation tests at room temperature. The pressure difference across the membrane was 1 bar. The mixed gases were fed in equimolar amounts, and argon was used as the purge gas. The composite membrane to be tested was attached to an open aluminum foil. The He and CH4 mixture passed through one side of the membrane, while argon flowed countercurrently through the other side. The composition of the purge gas was analyzed by gas chromatography, and the permeation flux of each gas was calculated. The separation factor was calculated as He permeation flux / CH4 permeation flux.

[0091] The test results are shown in Table 1 below:

[0092] Table 1

[0093]

[0094] As shown in Table 1, compared with the metal-organic framework composite membrane prepared using only 2-methylimidazole as an organic ligand, the metal-organic framework composite membrane prepared in Examples 1-13 by introducing a certain amount of benzimidazole with specific hydrocarbon groups and / or halogen-substituted and 2-methylimidazole as mixed ligands, when used for separating He / CH4, maintains a high permeation capacity for He while significantly improving the separation factor.

[0095] Furthermore, as can be seen from Examples 1-13 above, when benzimidazole containing halogen atoms is introduced, and the molar ratio of 2-methylimidazole to other imidazole ligands is controlled within the range of 1:(0.3-0.6), the prepared metal-organic framework composite membrane can achieve a He permeation capacity of not less than 100 × 10⁻⁶. -9 mol m -2 s -1 Pa -1 Meanwhile, the He / CH4 separation factor is not less than 80. In Example 12, when 5-fluoro-2-methylbenzimidazole is used as another ligand and the molar ratio of 2-methylimidazole to 5-fluoro-2-methylbenzimidazole is controlled at 1:0.6, the He / CH4 separation factor can reach as high as 280, and the He permeation capacity can reach as high as 184 × 10⁻⁶. - 9 mol m -2 s -1 Pa -1 However, when the content of 5-fluoro-2-methylbenzimidazole was further increased, for example, when the ratio of 5-fluoro-2-methylbenzimidazole to 2-methylimidazole was increased to 0.7, the permeation capacity of the metal-organic framework composite membrane prepared in Example 13 for He decreased significantly, but the separation factor did not increase significantly.

[0096] The embodiments described above are merely preferred examples to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the claims.

Claims

1. A metal organic framework composite membrane for separating helium methane gas mixture, characterized in that, The metal-organic framework composite membrane includes a porous base membrane and a metal-organic framework ZIF-8 membrane layer disposed on at least one side of the porous base membrane; The metal-organic framework ZIF-8 membrane is formed by the self-assembly of metal ions and imidazole ligands on the surface of the porous substrate membrane; the imidazole ligands include 2-methylimidazolium and other imidazole ligands, and the other imidazole ligands are selected from one or more of 5-fluoro-2-methylbenzimidazole, 5-chloro-2-methylbenzimidazole, and 5-bromo-2-methylbenzimidazole. Furthermore, the molar ratio of 2-methylimidazole to other imidazole ligands in the imidazole ligand is 1:(0.3-0.6). The porous base membrane is a polyethersulfone porous base membrane modified with polyaniline; The preparation method of the metal-organic framework composite membrane includes the following steps: (1) Provide a first metal source solution, and provide a mixed solution containing 2-methylimidazole and other imidazole ligands; (2) The first metal source solution and the mixed solution are placed on both sides of the H-type diffusion cell, and the pretreated porous base membrane is fixed in the middle of the H-type diffusion cell to carry out the diffusion reaction, forming a metal-organic framework ZIF-8 membrane layer on the surface of the porous base membrane, and the metal-organic framework composite membrane is obtained. The pretreatment steps for the porous base membrane are as follows: immersing the porous base membrane in a second metal source solution to allow metal ions to enter the pores of the porous base membrane, removing the porous base membrane to remove residual metal source solution from the surface, and obtaining the pretreated porous base membrane.

2. A method of preparing the metal organic framework composite film of claim 1, characterized in that, Includes the following steps: (1) A first metal source solution is provided, and a mixed solution containing 2-methylimidazole and other imidazole ligands is provided; the other imidazole ligands are selected from one or more of 5-fluoro-2-methylbenzimidazole, 5-chloro-2-methylbenzimidazole, and 5-bromo-2-methylbenzimidazole; and the molar ratio of 2-methylimidazole to other imidazole ligands in the imidazole ligands is 1:(0.3-0.6); the porous base membrane is a polyethersulfone porous base membrane modified with polyaniline; (2) The first metal source solution and the mixed solution are placed on both sides of the H-type diffusion cell, and the pretreated porous base membrane is fixed in the middle of the H-type diffusion cell to carry out the diffusion reaction, forming a metal-organic framework ZIF-8 membrane layer on the surface of the porous base membrane, and the metal-organic framework composite membrane is obtained. The pretreatment steps for the porous base membrane are as follows: immersing the porous base membrane in a second metal source solution to allow metal ions to enter the pores of the porous base membrane, removing the porous base membrane to remove residual metal source solution from the surface, and obtaining the pretreated porous base membrane.

3. The preparation method according to claim 2, characterized in that, In step (1): the first metal source is selected from one or more of zinc sulfate, zinc nitrate, zinc acetate, and zinc chloride; The concentration of metal ions in the first metal source solution is 0.01-0.03 mol / L; The ratio of the molar concentration of metal ions in the first metal source solution to the total molar concentration of imidazole ligands in the mixed solution is 1:(3-8).

4. The production method according to claim 2, characterized by, In step (2), the temperature of the diffusion reaction is 0-60 °C and the time is 1-72 h.

5. The preparation method according to claim 2, characterized in that, The metal ions in the second metal source solution are zinc ions; The concentration of metal ions in the second metal source solution is 0.02-0.05 mol / L.

6. The application of the metal-organic framework composite membrane according to claim 1 or the metal-organic framework composite membrane prepared by any one of claims 2-5 in the separation of He / CH4 mixed gas.

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