Metal organic framework composite membrane for separating helium and methane mixed gas as well as preparation method and application of metal organic framework composite membrane

Abstract

The invention discloses a metal organic framework composite membrane for helium and methane mixed gas separation and a preparation method and application thereof. The metal organic framework composite membrane comprises a porous base membrane and a metal organic framework ZIF-8 membrane layer arranged on at least one surface of the porous base membrane, the metal organic framework ZIF-8 film layer is formed by self-assembling metal ions and imidazole ligands on the surface of the porous base film, and the imidazole ligands comprise 2-methylimidazole and other imidazole ligands; the other imidazole ligands are benzimidazole substituted by alkyl and / or halogen. The invention further provides a method for preparing the composite membrane through diffusion growth. The metal organic framework ZIF-8 prepared through the method is continuous and compact in membrane layer, stable in structure and high in permeation capacity. The metal organic framework composite membrane is used for separating helium and methane mixed gas, the helium permeation flux can reach 700 * 10 <-9 > mol m <-2 > s <-1 > Pa <-1 > or above, the separation selectivity reaches 280, and meanwhile the helium permeation flux is not lower than 100 * 10 <-9 > mol m <-2 > s <-1 > Pa <-1 >.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane separation, and in particular to a metal organic framework composite membrane for separating helium-methane mixed gas, and a preparation method and application thereof. Background Art

[0002] The rare gas helium (He) is widely used in various fields such as magnetic resonance imaging, balloon flight, leak detection, welding, etc. At the same time, helium is also one of the special gases required for the manufacture of semiconductors and electronic devices. However, helium in nature usually exists in low concentrations in the atmosphere, natural gas, synthetic ammonia tail gas and geothermal water. At present, the production of helium mainly relies on extraction from natural gas through low-temperature distillation and pressure swing adsorption, which greatly limits the purification and utilization of helium. Compared with traditional gas separation methods, membrane gas separation technology has the characteristics of low energy consumption, small footprint, and environmental friendliness, and is regarded as a very promising green separation technology. Commercial polyimide hollow fiber membranes have been used to recover helium from natural gas, but existing polymer membranes usually encounter a trade-off between permeability and selectivity, and suffer from plasticization and swelling, resulting in low separation efficiency.

[0003] In the past decade, Metal-Organic Frameworks (MOFs) have become ideal materials for gas storage and separation due to their high specific surface area, high pore volume, and pore structure that can be designed and adjusted. Similarly, by preparing them into membrane materials, selective separation of gas molecules can be achieved by adjusting pore size and functional modification, and they have high permeability separation performance, which will greatly reduce the cost and energy consumption of gas separation and purification. However, MOF membranes have been reported to show very low He / CH4 separation factors and He permeation fluxes, with a separation factor of about 10 and a He permeation flux of only 70×10 -9 mol m -2 s -1 Pa -1 About (reference J.Am.Chem.Soc.2023,145,27,14793–14801), which seriously limits the application of MOF membranes in He / CH4 separation.

[0004] Based on this, there is an urgent need for a membrane material that can efficiently separate He / CH4 so as to be used for low-cost and high-efficiency recovery of helium from natural gas. Summary of the invention

[0005] To solve the above problems, the present invention provides a metal organic framework composite membrane for separation of helium and methane mixed gas, and a preparation method and application thereof. The present invention uses a porous membrane material as a substrate, and adopts a mixed imidazole ligand and a metal ion to form a continuous, defect-free and dense metal organic framework ZIF-8 membrane layer on the porous substrate by diffusion growth. The metal organic framework ZIF-8 membrane layer with functionalized functional groups and suitable pore sizes is obtained by regulating the type and proportion of imidazole ligands in the mixed imidazole ligands, so that the prepared metal organic framework composite membrane can 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 separation of 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 layer is formed by self-assembly of metal ions and imidazole ligands on the surface of the porous base membrane; the imidazole ligands include 2-methylimidazole and other imidazole ligands, and the other imidazole ligands are benzimidazoles substituted with hydrocarbon groups and / or halogens.

[0009] Furthermore, the material of the porous base membrane includes one or more of porous organic polymer materials, porous metal oxides, and porous non-metallic 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-metallic oxide is selected from one or more of porous silicon oxide, 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-Phenylbenzimidazole 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 .

[0013] Furthermore, the other imidazole ligand is a benzimidazole containing a halogen substituent, and 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 the benzimidazole.

[0014] Furthermore, 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] The present invention forms a ZIF-8 membrane with a metal organic skeleton by introducing different imidazole ligands and metal ions. The molar ratio of 2-methylimidazole to other imidazole ligands in the imidazole ligands will affect the formation of the metal organic skeleton and the pore size of the formed metal organic skeleton ZIF-8 membrane layer; wherein, the amount of other imidazole ligands introduced should not be too small, otherwise the pore size cannot be effectively regulated, thereby failing to effectively improve the He / CH4 separation factor; meanwhile, the amount of other imidazole ligands introduced should not be too large, otherwise too much other imidazole ligands and relatively small amount of 2-methylimidazole cannot form a metal organic skeleton with metal ions; therefore, in order to ensure the formation of the metal organic skeleton ZIF-8 membrane layer structure and to effectively separate He / CH4, the molar ratio of 2-methylimidazole to other imidazole ligands in the imidazole ligands needs to be controlled in the range of 1:(0.05-1), more preferably 1:(0.3-0.6).

[0016] The second aspect of the present invention provides a method for preparing the metal organic framework composite membrane according to the first aspect, comprising the following steps:

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

[0018] (2) placing the first metal source solution and the mixed solution on both sides of an H-type diffusion cell respectively, fixing a porous base membrane in the middle of the H-type diffusion cell, performing a diffusion reaction, forming a metal organic framework ZIF-8 membrane layer on the surface of the porous base membrane, and obtaining the metal organic framework composite membrane.

[0019] Furthermore, 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 coordinated with the imidazole ligands.

[0020] Furthermore, 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] Furthermore, in step (2), the diffusion reaction temperature is 0-60°C and the time is 1-72h; more preferably, the diffusion reaction is carried out at a temperature of 0-30°C for 12-48h to form a complete and dense metal organic framework ZIF-8 film layer.

[0022] Furthermore, the preparation method further includes the step of pre-treating 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, taking out the porous base membrane to remove the metal source solution remaining on the surface, and obtaining a pre-treated 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 pre-treating 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 membrane.

[0023] The third aspect of the present invention provides an 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 separating He / CH4 mixed gas.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The present invention provides a metal organic framework composite membrane for separation of helium-methane mixed gas, which is formed by self-assembly of different types of imidazole ligands and metal ions on the surface of a porous base membrane. A metal organic framework ZIF-8 membrane layer with a suitable pore size is formed on the porous base membrane by introducing an appropriate amount of imidazole ligands containing functionalized functional groups, 2-methylimidazole and metal ion ligands. The interaction between the introduced functionalized functional groups and methane gas molecules is utilized to coordinate the regulation of the pore size, thereby effectively improving the selective permeability of the metal organic framework membrane to helium and the permeation flux.

[0026] 2. The present invention also provides a method for preparing the above-mentioned metal organic framework composite membrane, in which 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 the present invention is used for separation of helium-methane mixed gas, and can have both high separation factor and high permeation flux. When the helium permeation flux is greater than 100×10 -9 mol m -2 s -1 Pa -1 Under the conditions of , the separation factor can be as high as 280, which is much better than the helium / methane separation effect of MOF membrane reported in the prior art, and can effectively promote the efficiency of recovering helium from natural gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a SEM of the metal organic framework composite membrane prepared in Example 1, wherein a is the front side and b is the cross section;

[0029] Figure 2 is the XRD pattern of the metal organic framework composite film prepared in Example 1;

[0030] Figure 3 This is a SEM of the metal organic framework composite membrane prepared in Example 7, wherein a is the front side and b is the cross section;

[0031] Figure 4 This is the XRD pattern of the metal organic framework composite film prepared in Example 7. DETAILED DESCRIPTION

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art to which the present invention belongs. The terms used herein in the specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items. The term "includes" or "comprising" described in the present invention means that it may include or contain other components in addition to the components described. The term "includes" or "comprising" described in the present invention may also be replaced by the closed form "for" or "consisting of..."

[0033] The present invention is further described below in conjunction with specific embodiments and drawings so that those skilled in the art can better understand the present invention and implement it, but the embodiments 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, which is specifically as follows:

[0036] Preparation of porous base membrane:

[0037] Add 0.02 mol of aniline and 0.01 mol of ammonium persulfate to 50 mL of perchloric acid solution (1 mol / L) respectively and stir evenly. Then cool the above solution to 0 degrees Celsius. Under magnetic stirring, add the ammonium persulfate solution to the aniline solution, put a polyethersulfone porous carrier with a pore size of 0.1 micron and a diameter of 4.7 cm into the mixed solution, and react for 6 hours under ice-water bath conditions. After the reaction is completed, take out the membrane and rinse it with deionized water; then soak the porous carrier in ammonium hydroxide solution (0.1 mol / L) for 30 minutes to neutralize the residual acid. After taking it out, rinse it with deionized water, and finally dry it for use to obtain a polyaniline-modified polyethersulfone porous base membrane;

[0038] Preparation of metal organic framework composite membranes:

[0039] (1) Soak the polyaniline-modified polyethersulfone porous support in a 0.02 mol / L zinc nitrate methanol solution for 30 minutes, then wipe it dry to keep it in a semi-wet state for later use;

[0040] A 0.01 mol / L methanol solution of zinc nitrate and a 0.05 mol / L methanol solution of imidazole mixed ligands were prepared respectively, wherein the molar ratio of 2-methylimidazole to 2-chloromethylbenzimidazole in the imidazole mixed ligands was 1:0.1;

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

[0042] The metal organic framework composite film prepared in this example was characterized by SEM and XRD, and the results were as follows:

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

[0044] Example 2

[0045] This embodiment relates to the preparation of a metal organic framework composite membrane, and the only difference from Embodiment 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 remaining 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, and the only difference from Embodiment 1 is: the preparation of the metal organic framework composite membrane, and the other operations are the same, as follows:

[0048] (1) Soak the polyaniline-modified polyethersulfone porous support in a 0.03 mol / L zinc nitrate methanol solution for 30 minutes, then wipe it dry to keep it in a semi-wet state for later use;

[0049] (2) preparing a 0.01 mol / L methanol solution of zinc nitrate and a 0.06 mol / L methanol solution of an imidazole mixed ligand respectively, wherein the molar ratio of 2-methylimidazole to 2-methylbenzimidazole in the imidazole mixed ligand is 1:0.08;

[0050] The polyaniline-modified polyethersulfone porous carrier pretreated in step (1) is placed in the middle of an H-type diffusion cell and fixed; the zinc nitrate solution and the 2-methylbenzimidazole mixed solution are then placed on both sides of the H-type diffusion cell respectively; the solutions on both sides diffuse and react to grow at 25 degrees Celsius for 48 hours to form a dense 2-methylbenzimidazole mixed ligand ZIF-8 membrane on the surface of the porous carrier, which is taken out, washed and dried to obtain a metal organic framework composite membrane.

[0051] Example 4

[0052] This embodiment relates to the preparation of a metal organic framework composite membrane, and the only difference from Embodiment 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 remaining 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, and the only difference from Embodiment 1 is: the preparation of the metal organic framework composite membrane, and the other operations are the same, as follows:

[0055] (1) Soak the polyaniline-modified polyethersulfone porous support in a 0.05 mol / L zinc nitrate methanol solution for 30 minutes, then wipe it dry to keep it in a semi-wet state for later use;

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

[0057] The polyaniline-modified polyethersulfone porous carrier pretreated in step (1) is placed in the middle of an H-type diffusion cell and fixed; the zinc nitrate solution and the 2-ethylbenzimidazole mixed solution are then placed on both sides of the H-type diffusion cell respectively; the solutions on both sides diffuse and react to grow at 25 degrees Celsius for 48 hours to form a dense 2-ethylbenzimidazole mixed ligand ZIF-8 membrane on the surface of the porous carrier, which is taken out, washed and dried to obtain a metal organic framework composite membrane.

[0058] Example 6

[0059] This embodiment relates to the preparation of a metal organic framework composite membrane, and the only difference from Embodiment 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 remaining 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, and the only difference from Embodiment 1 is: the preparation of the metal organic framework composite membrane, and the other operations are the same, as follows:

[0062] (1) Soak the polyaniline-modified polyethersulfone porous support in a 0.02 mol / L zinc nitrate methanol solution for 30 minutes, then wipe it dry to keep it in a semi-wet state for later use;

[0063] (2) preparing a 0.01 mol / L methanol solution of zinc nitrate and a 0.05 mol / L methanol solution of an imidazole mixed ligand respectively, wherein the molar ratio of 2-methylimidazole to 5-chloro-2-methylbenzimidazole in the imidazole mixed ligand is 1:0.1;

[0064] The polyaniline-modified polyethersulfone porous carrier pretreated in step (1) is placed in the middle of an H-type diffusion cell and fixed; the zinc nitrate solution and the 5-chloro-2-methylbenzimidazole mixed solution are then placed on both sides of the H-type diffusion cell respectively; the solutions on both sides diffuse and react to grow 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 carrier, which is taken out, washed and dried to obtain a metal organic framework composite membrane.

[0065] The metal organic framework composite film prepared in this example was characterized by SEM and XRD, and the results were as follows:

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

[0067] Example 8

[0068] This embodiment relates to the preparation of a metal organic framework composite membrane, and 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 remaining 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, and the only difference from Embodiment 1 is: the preparation of the metal organic framework composite membrane, and the other operations are the same, as follows:

[0071] (1) Soak the polyaniline-modified polyethersulfone porous support in a 0.02 mol / L zinc nitrate methanol solution for 30 minutes, then wipe it dry to keep it in a semi-wet state for later use;

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

[0073] The polyaniline-modified polyethersulfone porous carrier pretreated in step (1) is placed in the middle of an H-type diffusion cell and fixed; the zinc nitrate solution and the 5-bromo-2-methylbenzimidazole mixed solution are then placed on both sides of the H-type diffusion cell respectively; the solutions on both sides diffuse and react to grow 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 carrier, which is taken out, washed and dried to obtain a metal organic framework composite membrane.

[0074] Example 10

[0075] This embodiment relates to the preparation of a metal organic framework composite membrane, and the only difference from Embodiment 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 remaining operations are the same, and the corresponding metal organic framework composite membrane is prepared.

[0076] Embodiment 11

[0077] This embodiment relates to the preparation of a metal organic framework composite membrane, and the only difference from Embodiment 1 is: the preparation of the metal organic framework composite membrane, and the other operations are the same, as follows:

[0078] (1) Soak the polyaniline-modified polyethersulfone porous support in a 0.02 mol / L zinc nitrate methanol solution for 30 minutes, then wipe it dry to keep it in a semi-wet state for later use;

[0079] (2) preparing a 0.01 mol / L methanol solution of zinc nitrate and a 0.05 mol / L methanol solution of an imidazole mixed ligand respectively, wherein the molar ratio of 2-methylimidazole to 5-fluoro-2-methylbenzimidazole in the imidazole mixed ligand is 1:0.2;

[0080] The polyaniline-modified polyethersulfone porous carrier pretreated in step (1) is placed in the middle of an H-type diffusion cell and fixed; the zinc nitrate solution and the 5-fluoro-2-methylbenzimidazole mixed solution are then placed on both sides of the H-type diffusion cell respectively; the solutions on both sides diffuse and react to grow at 25 degrees Celsius for 48 hours to obtain a dense 5-fluoro-2-methylbenzimidazole mixed ligand ZIF-8 membrane, which is then taken out, washed, and dried to obtain a metal organic framework composite membrane.

[0081] Example 12

[0082] This embodiment relates to the preparation of a metal organic framework composite membrane, and the only difference from Embodiment 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 remaining operations are the same, and the corresponding metal organic framework composite membrane is prepared.

[0083] Embodiment 13

[0084] This embodiment relates to the preparation of a metal organic framework composite membrane, and the only difference from Embodiment 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 remaining 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, and the only difference from Embodiment 1 is that 2-chloromethylbenzimidazole is replaced by an equal molar amount of 2-methylimidazole; the rest of the operations are the same, and a ZIF-8 membrane is prepared.

[0087] The molar amount of zinc nitrate used in the zinc nitrate methanol solution prepared in the above examples and comparative examples for preparing the metal organic framework composite membrane is consistent, and the total molar amount of imidazole ligands in the prepared imidazole mixed ligand methanol solution is also consistent.

[0088] Application and performance testing

[0089] The metal organic framework composite membrane prepared in the above examples and comparative examples was used for He / CH4 separation, and the specific operation was as follows:

[0090] Referring to the Wicke-Kallenbach method, gas chromatography was used to perform mixed gas separation tests at room temperature. The pressure difference on both sides of the membrane was 1 bar, the mixed gas was fed in equimolar amounts, the purge gas was argon, and the composite membrane to be tested was glued to an open-pored aluminum foil. The He and CH4 mixed gas passed through one side of the membrane, while argon countercurrently passed through the other side of the membrane. The purge gas component content was analyzed by gas chromatograph, and the permeation flux of each gas was calculated. Separation factor = He permeation flux / CH4 permeation flux.

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

[0092] Table 1

[0093]

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

[0095] In addition, it can be seen from the above Examples 1-13 that 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 permeation capacity of the prepared metal organic framework composite membrane for He is not less than 100×10 -9 mol m -2 s -1 Pa -1 The He / CH4 separation factor is not less than 80. In Example 12, when 5-fluoro-2-methylbenzimidazole is used as the other ligand and the molar ratio of 2-methylimidazole to 5-fluoro-2-methylbenzimidazole is controlled to be 1:0.6, the He / CH4 separation factor can be as high as 280, and the He permeation capacity can be as high as 184×10 - 9 mol m -2 s -1 Pa -1 When the content of 5-fluoro-2-methylbenzimidazole continued to increase, for example, when the ratio to 2-methylimidazole 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 was not significantly improved.

[0096] The above-described embodiments are only preferred embodiments for fully illustrating the present invention, and the protection scope of the present invention is not limited thereto. Equivalent substitutions or changes made by those skilled in the art based on the present invention are within the protection scope of the present invention. The protection scope of the present invention shall be subject to the claims.

Claims

1. A metal organic framework composite membrane for separation of helium-methane mixed gas, characterized in that: The metal organic framework composite membrane comprises a porous base membrane and a metal organic framework ZIF-8 membrane layer arranged on at least one side of the porous base membrane; The metal organic framework ZIF-8 membrane layer is formed by self-assembly of metal ions and imidazole ligands on the surface of the porous base membrane; the imidazole ligands include 2-methylimidazole and other imidazole ligands, and the other imidazole ligands are benzimidazoles substituted with hydrocarbon groups and / or halogens.

2. The metal organic framework composite membrane according to claim 1, characterized in that: The material of the porous base film includes one or more of porous organic polymer materials, porous metal oxides, and porous non-metallic oxides; Preferably, the porous organic polymer material is selected from one or more of polyethersulfone, polyaniline and polyimide; Preferably, the porous metal oxide is porous alumina; Preferably, the porous non-metallic oxide is selected from one or more of porous silicon oxide, porous silicon carbide and porous glass.

3. The metal organic framework composite membrane according to claim 1, characterized in that: The metal ions include zinc ions; The hydrocarbon group includes one or more of methyl, ethyl, propyl and phenyl; Preferably, the other imidazole ligands are selected from one or more of 2-methylbenzimidazole, 2-ethylbenzimidazole, 5-methyl-2-methylbenzimidazole, 2-phenylbenzimidazole, 2-chloromethylbenzimidazole, 5-fluoro-2-methylbenzimidazole, 5-chloro-2-methylbenzimidazole, 5-bromo-2-methylbenzimidazole, 5,6-dichloro-2-methylbenzimidazole and 2-trifluoromethylbenzimidazole.

4. The metal organic framework composite membrane according to claim 1 or 3, characterized in that: The molar ratio of 2-methylimidazole to other imidazole ligands in the imidazole ligand is 1:(0.05-1).

5. The metal organic framework composite membrane according to claim 4, characterized in that: The other imidazole ligand is a benzimidazole containing a halogen substituent, and the molar ratio of 2-methylimidazole to the other imidazole ligand in the imidazole ligand is 1:(0.3-0.6).

6. A method for preparing a metal organic framework composite membrane according to any one of claims 1 to 5, characterized in that: The following steps are involved: (1) providing a first metal source solution and a mixed solution containing 2-methylimidazole and other imidazole ligands; (2) placing the first metal source solution and the mixed solution on both sides of an H-type diffusion cell respectively, fixing a porous base membrane in the middle of the H-type diffusion cell, performing a diffusion reaction, forming a metal organic framework ZIF-8 membrane layer on the surface of the porous base membrane, and obtaining the metal organic framework composite membrane.

7. The preparation method according to claim 6, 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).

8. The preparation method according to claim 6, characterized in that: In step (2), the temperature of the diffusion reaction is 0-60°C and the time is 1-72h.

9. The preparation method according to claim 6, characterized in that: The preparation method further comprises the step of pre-treating 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, taking out the porous base membrane to remove the metal source solution remaining on the surface, and obtaining a pre-treated porous base membrane; Preferably, the metal ions in the second metal source solution are zinc ions; Preferably, the concentration of metal ions in the second metal source solution is 0.02-0.05 mol / L.

10. Use of the metal organic framework composite membrane according to any one of claims 1 to 5 or the metal organic framework composite membrane prepared by the preparation method according to any one of claims 6 to 9 in separating He / CH4 mixed gas.

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