A modified MOF-based mixed matrix membrane and a preparation method and application thereof

Abstract

The application relates to the technical field of gas separation membrane materials, in particular to a modified MOF-based mixed matrix membrane and a preparation method and application thereof. Zn(NO3)2*6H2O is stirred and dissolved in a PVA solution, slowly poured on the surface of a PI membrane, soaked for 3-5 hours, the soaked PI membrane is taken out and dried in a vacuum oven, a PVA-Zn interface layer is formed on the PI membrane layer; Zn(NO3)2*6H2O and silver nitrate are dissolved in deionized water and coated on the PVA-Zn interface layer, dried to obtain an organic interface layer riveted by metal ions; 2-methyl imidazole is dispersed and dissolved in deionized water to obtain a ligand reaction solution, the organic interface layer is completely soaked in the ligand reaction solution, reacts for 4-8 hours in dark conditions, a modified ZIF-8 membrane is obtained, dried, and the modified ZIF-8 membrane is washed with anhydrous methanol and dried at 30-35 DEG C for 12-24 hours. The modified MOF-based mixed matrix membrane has the advantages of simple preparation method, easily obtained raw materials, high selectivity permeability to helium, and the like, is used for the separation and purification of helium, and can effectively and quickly separate out helium.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of gas separation membrane materials, and particularly relates to a modified MOF-based mixed matrix membrane and a preparation method and application thereof. BACKGROUND

[0002] As a kind of rare inert gas, helium cannot be replaced by other substances in the application of nuclear energy, medical treatment and space industry due to its unique physical and chemical properties, and is regarded as one of the indispensable strategic materials for the development of national defense, military industry and technology. At present, the main raw material for industrial production of helium is helium-containing natural gas. In the existing industrial production technology of helium, the production of helium is divided into three stages, namely helium recovery, helium upgrading and helium purification stage. In the recovery stage of helium, the traditional low-temperature distillation technology is used to recover helium from the outlet gas of the denitrogenation unit in the liquefaction process of helium-containing natural gas, so that the concentration of helium is increased from 1-3vol% to 50-70vol%. However, due to the need of low-temperature distillation technology to consume a large amount of energy, it becomes one of the main expenditures of helium production cost.

[0003] In recent years, with the development of separation membrane technology, membrane separation technology with the advantages of high separation efficiency, low energy consumption and large operation flexibility has been gradually applied to the experimental research process of natural gas helium extraction, showing great application potential. Organic polymer membrane has the advantages of corrosion resistance, easy processing and molding, high gas selectivity, etc., and is the most commonly used and effective method in the field of gas separation. Due to the poor selectivity and low separation efficiency of the current helium separation membrane, there is no separate use of gas separation membrane to realize the separation of helium, methane and nitrogen, and the industry needs to be combined with pressure swing adsorption, cryogenic method, etc. to realize efficient helium extraction. The energy consumption of these methods is very large, and it is not economical and effective to realize natural gas helium extraction. Therefore, it is necessary to develop a helium separation membrane with high selectivity and high separation efficiency to realize low-energy and high-efficiency helium separation and concentration. SUMMARY

[0004] Therefore, the purpose of the present application is to provide a modified MOF-based mixed matrix membrane and a preparation method and application thereof, to solve the problems of low selectivity and low separation efficiency in the existing helium membrane separation technology.

[0005] The present application solves the above technical problems through the following technical means:

[0006] The first aspect of the present application provides a preparation method of a modified MOF-based mixed matrix membrane, comprising the following steps:

[0007] Preparation of the PVA-Zn interface layer, Zn(NO3)2·6H2O is stirred and dissolved in the PVA solution to form a PVA-Zn modified solution, which is slowly poured on the surface of the PI film, soaked for 3-5 h, and then the soaked PI film is taken out and dried in a vacuum oven at 60-80°C for 12-24 h to form a PVA-Zn interface layer on the PI film layer;

[0008] Preparation of the organic interface layer, Zn(NO3)2·6H2O and silver nitrate are dissolved in deionized water to obtain a composite metal ion modified solution, the completely dissolved composite metal ion modified solution is coated on the PVA-Zn interface layer, and then dried in a constant temperature room at 30-50°C for 6-10 h to obtain a metal ion riveted organic interface layer;

[0009] Preparation of the modified ZIF-8 film, 2-methylimidazole is dispersed and dissolved in deionized water to obtain a ligand reaction solution, the metal ion riveted organic interface layer is completely soaked in the ligand reaction solution, and the reaction is carried out in the dark for 4-8 h to obtain a modified ZIF-8 film, which is dried at 30-50°C for 6-10 h, and then the modified ZIF-8 film is washed with anhydrous methanol, and then dried in a constant temperature room at 30-35°C for 12-24 h.

[0010] In combination with the first aspect, in some embodiments, the PVA solution is prepared as follows:

[0011] 0.065-0.1 parts by mass of polyvinyl alcohol is added to 98.5-120 parts by mass of deionized water, and the mixture is stirred at 80-100°C to obtain a PVA solution.

[0012] In combination with the first aspect, in some embodiments, the PI film is prepared as follows: 4.5-8.7 parts by mass of polyimide pellets are poured into 25-40 parts by volume of chloroform, and the mixture is stirred at room temperature for 12-48 h to obtain a film forming solution, which is then coated into a film using a film coating machine, and the obtained film material is dried in a vacuum oven at 120-200°C overnight to obtain a PI film.

[0013] In combination with the first aspect, in some embodiments, the mass fraction of silver nitrate in the composite metal ion modified solution is 1-4 wt%.

[0014] In combination with the first aspect, in some embodiments, the mass fraction of Zn(NO3)2·6H2O in the composite metal ion modified solution is 3 wt%.

[0015] In combination with the first aspect, in some embodiments, the mass fraction of 2-methylimidazole in the ligand reaction solution is 3.2-5.4 wt%.

[0016] The second aspect of the present application is to improve a modified MOF-based mixed matrix membrane prepared by the above preparation method.

[0017] The third aspect of the present application is to provide the use of the above modified MOF-based mixed matrix membrane in separating helium.

[0018] The modified MOF-based mixed matrix membrane preparation method is simple, raw materials are easy to obtain, has high selectivity for helium, and can be used for the separation and purification of helium, so that helium can be effectively and quickly separated out. The gas test data show that, compared with the PI interface layer coated on the unmodified ZIF-8, the helium permeability of the PI mixed matrix membrane prepared by the modified ZIF-8 interface layer is increased by 26.9-40.1%, the selectivity of He / CH4 is increased by 19.7-30.2%, and the selectivity of He / N2 is increased by 21.5-29.7%, DETAILED DESCRIPTION

[0019] The technical solutions in the embodiments of the present application will be clearly and completely described below in combination with the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0020] In the following examples, the specific conditions not specified are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The raw materials, equipment or instruments used are not specified by the manufacturer, and are all conventional products that can be obtained by purchase.

[0021] In the present application, the island-shaped layered porous ZIF-8 film doped with silver ions is etched by salt-containing template chemistry, which is used for efficient helium separation in natural gas. Under silver nitrate solution etching, separated mesopores are formed in situ during the growth of the MOF membrane. In the ZIF-8 membrane, the separated mesopores can act as island structures, and micropores can also be obtained from the MOF crystals. Both macropores and micropores play a crucial role in improving the separation performance of the membrane. The preparation method of the modified MOF-based mixed matrix membrane includes the following steps:

[0022] The PVA-Zn interface layer is prepared by stirring and dissolving Zn(NO3)2·6H2O in the PVA solution to form a PVA-Zn modified solution, slowly pouring the PVA-Zn modified solution on the surface of the PI membrane, soaking the PI membrane for 3-5 hours, taking out the soaked PI membrane and drying it in a vacuum oven at 60-80℃ for 12-24 hours to form a PVA-Zn interface layer on the PI membrane layer;

[0023] An organic interface layer is prepared by dissolving Zn(NO3)2·6H2O and silver nitrate in deionized water to obtain a composite metal ion modified solution, and the completely dissolved composite metal ion modified solution is coated on the PVA-Zn interface layer, and then dried in a constant temperature room at 30-50°C for 6-10h to obtain a metal ion riveted organic interface layer;

[0024] A modified ZIF-8 film is prepared by dispersing and dissolving 2-methylimidazole in deionized water to obtain a ligand reaction solution, and the metal ion riveted organic interface layer is completely immersed in the ligand reaction solution, and reacted in the dark for 4-8h to obtain a modified ZIF-8 film, which is dried at 30-50°C for 6-10h, and then the modified ZIF-8 film is washed with anhydrous methanol, and then dried in a constant temperature room at 30-35°C for 12-24h.

[0025] The modified MOF-based mixed matrix membrane of the present application is described in detail by Examples 1-4 as follows:

[0026] Example 1

[0027] The preparation of the modified MOF-based mixed matrix membrane of this example is as follows:

[0028] (1) Preparation of polyimide (PI) film

[0029] 4.8g of Matrimid@5218 raw material is poured into 30ml of chloroform, and stirred at room temperature for 12h to obtain a film-forming solution, which is then coated into a film using a film coating machine. The obtained film material is placed in a vacuum oven at 150°C and dried overnight to obtain a polyimide (PI) film.

[0030] (2) Preparation of modified ZIF-8 coated film material

[0031] First, 0.085g of polyvinyl alcohol (PVA) and 98.5g of deionized water are mixed and stirred at 80°C using a magnetic stirrer to obtain a PVA solution, which is then placed at room temperature. 0.6g of Zn(NO3)2·6H2O is dissolved in the above PVA solution to form a PVA-Zn modified solution. The uniformly stirred PVA-Zn modified solution is slowly poured onto the surface of the PI film and soaked for 3h. Then the excess PVA-Zn modified solution is poured off, and the film is placed in a vacuum oven at 60°C and dried for 12h to form a PVA-Zn interface layer on the film layer.

[0032] A composite metal ion modified solution was prepared by dissolving 3 wt% (3 g) of Zn(N03)2*6H20 and 1.5 wt% of silver nitrate in 100 ml of deionized water. The preparation of the composite metal ion modified solution must be carried out in a brown transparent bottle with an aluminum foil wrap because silver nitrate is easily decomposed under light. The fully dissolved composite metal ion modified solution was coated on the PVA-Zn interfacial layer using a wet coating machine with adjustable thickness. The treated membrane was dried in a constant temperature room at 30 °C for 6 h to obtain the metal ion riveted organic interfacial layer.

[0033] A ligand reaction solution with a mass fraction of 3.6 wt% was prepared by dispersing and dissolving 2-methylimidazole in 80 ml of deionized water. The metal ion riveted organic interfacial layer was fully immersed in the ligand reaction solution and reacted for 5 h in the dark. The modified ZIF-8 membrane obtained was dried in a constant temperature laboratory at 30 °C for 6 h. The modified ZIF-8 membrane was then washed with clean anhydrous methanol and 3 times to remove the residual metal salt and ligand in the membrane structure, and finally the membrane material was dried in a constant temperature room at 30 °C for 12 h to obtain the modified MOF-based mixed matrix membrane.

[0034] Comparative Example 1

[0035] The MOF-based mixed matrix membrane of this example was prepared as follows:

[0036] (1) Preparation of a polyimide (PI) membrane

[0037] 4.8 g of Matrimid®5218 raw material was poured into 30 ml of chloroform, and stirred magnetically at room temperature for 12 h to obtain a film-forming solution, which was then coated into a film using a film coating machine. The obtained membrane material was placed in a vacuum oven at 150 °C and dried overnight to obtain a polyimide (PI) membrane.

[0038] (2) Preparation of a modified ZIF-8 coated membrane material

[0039] First, 0.085 g of polyvinyl alcohol (PVA) and 98.5 g of deionized water were mixed and stirred at 80 °C using a magnetic stirrer to obtain a PVA solution, which was then placed at room temperature. 0.6 g of Zn(N03)2*6H20 was dissolved in the above PVA solution to form a PVA-Zn modified solution. The uniformly stirred PVA-Zn modified solution was slowly poured onto the surface of the PI membrane and soaked for 3 h. Then the excess PVA-Zn modified solution was poured off, and the membrane was placed in a vacuum oven at 60 °C and dried for 12 h to form a PVA-Zn interfacial layer on the membrane layer.

[0040] A 3 wt% (3 g) Zn(N03)2*6H20 solution was prepared by dissolving Zn(N03)2*6H20 in 100 ml of deionized water to form a metal ion-modified solution. The completely dissolved metal ion-modified solution was coated on the PVA-Zn interfacial layer using a wet coating machine with adjustable thickness. The treated membrane was dried in a constant temperature room at 30 °C for 6 h to obtain the organic interfacial layer.

[0041] A 3.6 wt% ligand reaction solution was prepared by dispersing and dissolving 2-methylimidazole in 80 ml of deionized water. The organic interfacial layer was completely immersed in the ligand reaction solution and reacted for 5 h in the dark. The obtained ZIF-8 membrane was dried in a constant temperature laboratory at 30 °C for 6 h. The prepared ZIF-8 membrane was then washed with clean anhydrous methanol and 3 times to remove the residual metal salt and ligand in the membrane structure. Finally, the membrane material was dried in a constant temperature room at 30 °C for 12 h to obtain the MOF-based mixed matrix membrane.

[0042] Example 2

[0043] The modified MOF-based mixed matrix membrane of this example was prepared as follows:

[0044] (1) Preparation of polyimide (PI) membrane

[0045] 5.9 g of Matrimid® 5218 raw material was poured into 35 ml of chloroform, and magnetic stirring was performed at room temperature for 24 h to obtain a film-forming solution. The film-forming solution was then coated into a film using a film coating machine, and the obtained membrane material was placed in a vacuum oven at 180 °C and dried overnight.

[0046] (2) Preparation of modified ZIF-8 coated membrane material

[0047] First, 0.089 g of polyvinyl alcohol (PVA) and 110 g of deionized water were mixed and stirred at 90 °C using a magnetic stirrer to obtain a PVA solution, which was then placed at room temperature. 1.2 g of Zn(N03)2*6H20 was dissolved in the above PVA solution to form a PVA-Zn modified solution. The uniformly stirred PVA-Zn modified solution was slowly poured onto the surface of the PI membrane and soaked for 4 h. Then, the excess PVA-Zn modified solution was poured off, and the membrane was placed in a vacuum oven at 80 °C and dried for 24 h to form a PVA-Zn interfacial layer on the membrane layer.

[0048] A 3 wt% (3 g) Zn(N03)2*6H20 and 2 wt% silver nitrate solution was prepared by dissolving Zn(N03)2*6H20 and silver nitrate in 100 ml of deionized water to form a composite metal ion-modified solution. This process must be carried out in a brown transparent bottle with an aluminum foil wrap because silver nitrate is easily decomposed under light. The completely dissolved composite metal ion-modified solution was coated on the PVA-Zn interfacial layer using a wet coating machine with adjustable thickness. The treated membrane was dried in a constant temperature room at 40 °C for 8 h to obtain a metal ion riveted organic interfacial layer.

[0049] A ligand reaction solution with a mass fraction of 4.4 wt% was prepared by dispersing and dissolving 2-methylimidazole in 100 ml of deionized water. The organic interface layer of the metal ion riveted layer was completely immersed in the ligand reaction solution and reacted for 8 h in the dark. The modified ZIF-8 membrane obtained was dried in a constant temperature laboratory at 40 °C for 8 h. The modified ZIF-8 membrane was then washed with clean anhydrous methanol three times to remove the residual metal salt and ligand in the membrane structure, and finally the membrane material was dried in a constant temperature room at 30 °C for 24 h to obtain a modified MOF-based mixed matrix membrane.

[0050] Comparative Example 2

[0051] The MOF-based mixed matrix membrane of this example was prepared as follows:

[0052] (1) Preparation of polyimide (PI) membrane

[0053] 5.9 g of Matrimid®5218 raw material was poured into 35 ml of chloroform, and stirred magnetically at room temperature for 24 h to obtain a film-forming solution, which was then coated into a film using a film coating machine. The obtained membrane material was placed in a vacuum oven at 180 °C and dried overnight.

[0054] (2) Preparation of modified ZIF-8 coated membrane material

[0055] First, 0.089 g of polyvinyl alcohol (PVA) and 110 g of deionized water were mixed and stirred at 90 °C using a magnetic stirrer to obtain a PVA solution, which was then placed at room temperature. 1.2 g of Zn(NO3)2·6H2O was dissolved in the above PVA solution to form a PVA-Zn modified solution. The uniformly stirred PVA-Zn modified solution was slowly poured onto the surface of the PI membrane and soaked for 4 h. Then the excess PVA-Zn modified solution was poured off, and the membrane was placed in a vacuum oven at 80 °C and dried for 24 h to form a PVA-Zn interface layer on the membrane layer.

[0056] 3 wt% (3 g) of Zn(NO3)2·6H2O was dissolved in 100 ml of deionized water to form a metal ion modified solution. A wet coating machine with adjustable thickness was used to coat the completely dissolved metal ion modified solution on the PVA-Zn interface layer. The treated membrane was dried in a constant temperature room at 40 °C for 8 h to obtain an organic interface layer.

[0057] A 2-methylimidazole ligand solution was prepared by dispersing and dissolving 2-methylimidazole in 100 ml of deionized water to obtain a mass fraction of 4.4 wt%. The organic interface layer surface was completely immersed in the ligand solution, and reacted for 8 h in the dark. The obtained ZIF-8 membrane was dried in a constant temperature laboratory at 40 °C for 8 h. The prepared ZIF-8 membrane was then washed with clean anhydrous methanol three times to remove residual metal salts and ligands in the membrane structure, and finally the membrane material was dried in a constant temperature room at 30 °C for 24 h to obtain a MOF-based mixed matrix membrane.

[0058] Example 3

[0059] The preparation of the modified MOF-based mixed matrix membrane of this example is as follows:

[0060] (1) Preparation of a polyimide (PI) membrane

[0061] 4.5 g of Matrimid®5218 raw material was poured into 25 ml of chloroform, and stirred magnetically at room temperature for 20 h to obtain a film-forming solution, which was then coated into a film using a film coating machine. The obtained membrane material was placed in a vacuum oven at 120 °C and dried overnight.

[0062] (2) Preparation of a modified ZIF-8 coated membrane material

[0063] First, 0.065 g of polyvinyl alcohol (PVA) and 100 g of deionized water were mixed and stirred at 90 °C using a magnetic stirrer to obtain a PVA solution, which was then placed at room temperature. 0.5 g of Zn(NO3)2·6H2O was dissolved in the above PVA solution to form a PVA-Zn modified solution. The uniformly stirred PVA-Zn modified solution was slowly poured onto the surface of the PI membrane and soaked for 5 h. Then the excess PVA-Zn modified solution was poured off, and the membrane was placed in a vacuum oven at 70 °C and dried for 20 h to form a PVA-Zn interface layer on the membrane layer.

[0064] 3 wt% (3 g) of Zn(NO3)2·6H2O and 1 wt% of silver nitrate were dissolved in 100 ml of deionized water to form a composite metal ion modified solution. This process must be carried out in a brown transparent bottle with an aluminum foil wrap, because silver nitrate is easily decomposed under light. A wet coating machine with adjustable thickness was used to coat the completely dissolved composite metal ion modified solution on the PVA-Zn interface layer. The treated membrane was dried in a constant temperature room at 50 °C for 8 h to obtain a metal ion riveted organic interface layer.

[0065] A 2-methylimidazole ligand solution was prepared by dispersing and dissolving 2-methylimidazole in 100 ml of deionized water to obtain a 3.2 wt% ligand solution. The metal ion pinning organic interfacial layer of the ZIF-8 membrane was completely immersed in the ligand solution and reacted for 4 h in the dark. The modified ZIF-8 membrane obtained was dried in a constant temperature laboratory at 50°C for 6 h. The modified ZIF-8 membrane was then washed with clean anhydrous methanol three times to remove residual metal salts and ligands in the membrane structure, and finally the membrane material was dried in a constant temperature room at 30°C for 20 h to obtain a modified MOF-based mixed matrix membrane.

[0066] Comparative Example 3

[0067] The MOF-based mixed matrix membrane of this example was prepared as follows:

[0068] (1) Preparation of a polyimide (PI) membrane

[0069] 4.5 g of Matrimid® 5218 raw material was poured into 25 ml of chloroform, and stirred at room temperature for 20 h to obtain a membrane solution, which was then coated into a membrane using a film coating machine. The obtained membrane material was placed in a vacuum oven at 120°C and dried overnight.

[0070] (2) Preparation of a modified ZIF-8 coated membrane material

[0071] First, 0.065 g of polyvinyl alcohol (PVA) and 100 g of deionized water were mixed and stirred at 90°C using a magnetic stirrer to obtain a PVA solution, which was then placed at room temperature. 0.5 g of Zn(NO3)2·6H2O was dissolved in the above PVA solution to form a PVA-Zn modified solution. The uniformly stirred PVA-Zn modified solution was slowly poured onto the surface of the PI membrane and soaked for 5 h. Then the excess PVA-Zn modified solution was poured off, and the membrane was placed in a vacuum oven at 70°C and dried for 20 h to form a PVA-Zn interfacial layer on the membrane layer.

[0072] 3 wt% (3 g) of Zn(NO3)2·6H2O was dissolved in 100 ml of deionized water to form a metal ion modified solution. A wet coating machine with adjustable thickness was used to coat the completely dissolved metal ion modified solution onto the PVA-Zn interfacial layer. The treated membrane was dried in a constant temperature room at 50°C for 8 h to obtain an organic interfacial layer.

[0073] A 3.2 wt% ligand reaction solution was prepared by dispersing and dissolving 2-methylimidazole in 100 ml of deionized water. The organic interfacial layer was completely immersed in the ligand reaction solution and reacted for 4 h in the dark. The obtained ZIF-8 membrane was dried in a constant temperature laboratory at 50 °C for 6 h. The prepared ZIF-8 membrane was then washed with clean anhydrous methanol three times to remove residual metal salts and ligands in the membrane structure, and finally the membrane material was dried in a constant temperature room at 30 °C for 20 h to obtain a MOF-based mixed matrix membrane.

[0074] Example 4

[0075] The preparation of the modified MOF-based mixed matrix membrane of this example is as follows:

[0076] (1) Preparation of polyimide (PI) membrane

[0077] 8.7 g of Matrimid® 5218 raw material was poured into 40 ml of chloroform, and stirred magnetically at room temperature for 48 h to obtain a film-forming solution, which was then coated into a film using a film coating machine. The obtained membrane material was placed in a vacuum oven at 200 °C and dried overnight.

[0078] (2) Preparation of modified ZIF-8 coated membrane material

[0079] First, 0.1 g of polyvinyl alcohol (PVA) and 120 g of deionized water were mixed and stirred at 100 °C using a magnetic stirrer to obtain a PVA solution, which was then placed at room temperature. 1.7 g of Zn(NO3)2·6H2O was dissolved in the above PVA solution to form a PVA-Zn modified solution. The uniformly stirred PVA-Zn modified solution was slowly poured onto the surface of the PI membrane and soaked for 4 h. Then the excess PVA-Zn modified solution was poured off, and the membrane was placed in a vacuum oven at 80 °C and dried for 20 h to form a PVA-Zn interfacial layer on the membrane layer.

[0080] 3 wt% (3 g) of Zn(NO3)2·6H2O and 4 wt% of silver nitrate were dissolved in 100 ml of deionized water to form a composite metal ion modified solution. This process must be carried out in a brown transparent bottle with an aluminum foil wrap, because silver nitrate is easily decomposed under light. A wet coating machine with adjustable thickness was used to coat the completely dissolved composite metal ion modified solution on the PVA-Zn interfacial layer. The treated membrane was dried in a constant temperature room at 50 °C for 10 h to obtain a metal ion riveted organic interfacial layer.

[0081] A 2-methylimidazole ligand solution was prepared by dispersing and dissolving 2-methylimidazole in 100 ml of deionized water to obtain a 5.4 wt% ligand solution. The metal ion pinning organic interface layer of the ZIF-8 membrane was completely immersed in the ligand solution and reacted for 5 h in the dark. The modified ZIF-8 membrane obtained was dried in a constant temperature laboratory at 50°C for 10 h. The modified ZIF-8 membrane was then washed with clean anhydrous methanol three times to remove residual metal salts and ligands in the membrane structure, and finally the membrane material was dried in a constant temperature room at 30°C for 20 h to obtain a modified MOF-based mixed matrix membrane.

[0082] Comparative Example 4

[0083] The MOF-based mixed matrix membrane of this example was prepared as follows:

[0084] (1) Preparation of a polyimide (PI) membrane

[0085] 8.7 g of Matrimid® 5218 raw material was poured into 40 ml of chloroform, and stirred magnetically at room temperature for 48 h to obtain a membrane solution, which was then coated into a membrane using a film coating machine. The obtained membrane material was placed in a vacuum oven at 200°C and dried overnight.

[0086] (2) Preparation of a modified ZIF-8 coated membrane material

[0087] First, 0.1 g of polyvinyl alcohol (PVA) and 120 g of deionized water were mixed and stirred at 100°C using a magnetic stirrer to obtain a PVA solution, which was then placed at room temperature. 1.7 g of Zn(NO3)2·6H2O was dissolved in the above PVA solution to form a PVA-Zn modified solution. The uniformly stirred PVA-Zn modified solution was slowly poured onto the surface of the PI membrane and soaked for 4 h. Then the excess PVA-Zn modified solution was poured off, and the membrane was placed in a vacuum oven at 80°C and dried for 20 h to form a PVA-Zn interface layer on the membrane layer.

[0088] 3 wt% (3 g) of Zn(NO3)2·6H2O was dissolved in 100 ml of deionized water to form a metal ion modified solution. A wet coating machine with adjustable thickness was used to coat the completely dissolved metal ion modified solution on the PVA-Zn interface layer. The treated membrane was dried in a constant temperature room at 50°C for 10 h to obtain an organic interface layer.

[0089] A 2-methylimidazole ligand solution with a mass fraction of 5.4wt% was prepared by dispersing and dissolving 2-methylimidazole in 100ml deionized water. The organic interfacial layer was completely immersed in the ligand solution and reacted for 5h in the dark. The obtained ZIF-8 membrane was dried in a constant temperature laboratory at 50℃ for 10h. The prepared ZIF-8 membrane was then cleaned with dry anhydrous methanol for 3 times to remove the residual metal salt and ligand in the membrane structure, and finally the membrane material was dried in a constant temperature room at 30℃ for 20h to obtain a MOF-based mixed matrix membrane.

[0090] The gas permeation and separation performance of the membrane materials prepared in Examples 1-4 and Comparative Examples 1-4 were evaluated at room temperature by using the constant volume pressure swing method, and the test gas pair was He, CH4 and N2. The test results are shown in Table 1:

[0091] Table 1

[0092]

[0093]

[0094] The data in Table 1 show that compared with the PI mixed matrix membrane prepared by using the unmodified ZIF-8 interfacial layer, the PI mixed matrix membrane prepared by using the modified ZIF-8 interfacial layer has a helium permeability increased by 26.9-40.1%, a He / CH4 selectivity increased by 19.7-30.2%, and a He / N2 selectivity increased by 21.5-29.7%. Therefore, the modified MOF-based mixed matrix membrane has a high selective permeability to helium, and can be applied in the separation and purification of helium.

[0095] The above examples are only used to illustrate the technical solutions of the present application and not limit the present application. Although the present application has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the present application, and all of them should be covered in the scope of the claims of the present application. The technical, shape and structure parts not described in detail in the present application are well-known technologies.

Claims

1. A method for preparing a modified MOF-based mixed matrix membrane, characterized in that, The method comprises the following steps: Preparation of the PVA-Zn interface layer, Zn(NO3)2·6H2O is stirred and dissolved in the PVA solution to form a PVA-Zn modified solution, which is slowly poured on the surface of the PI film, and the PI film after soaking for 3-5 h is taken out and dried in a vacuum oven at 60-80℃ for 12-24 h to form the PVA-Zn interface layer on the PI film layer; Preparation of the organic interface layer, Zn(NO3)2·6H2O and silver nitrate are dissolved in deionized water to obtain a composite metal ion modified solution, the completely dissolved composite metal ion modified solution is coated on the PVA-Zn interface layer, and then dried in a constant-temperature room at 30-50℃ for 6-10 h to obtain a metal ion riveted organic interface layer; Preparation of the modified ZIF-8 film, 2-methyl imidazole is dispersed and dissolved in deionized water to obtain a ligand reaction solution, the metal ion riveted organic interface layer is completely soaked in the ligand reaction solution, and the obtained modified ZIF-8 film is dried at 30-50℃ for 6-10 h in a constant-temperature room, and then washed with anhydrous methanol, and then dried at 30-35℃ for 12-24 h in a constant-temperature room.

2. The method of claim 1, wherein the modified MOF-based mixed matrix membrane is prepared by the steps of: The PVA solution is prepared as follows: 0.065-0.1 parts by mass of polyvinyl alcohol is added to 98.5-120 parts by mass of deionized water, and the mixture is stirred and mixed at 80-100℃ to obtain the PVA solution.

3. The method of claim 1, wherein the modified MOF-based mixed matrix membrane is prepared by the steps of: The PI film is prepared as follows: 4.5-8.7 parts by mass of polyimide pellets are poured into 25-40 parts by volume of chloroform, and the mixture is stirred at room temperature for 12-48 h to obtain a film-forming solution, which is coated into a film by using a film coating machine, and the obtained film material is dried in a vacuum oven at 120-200℃ overnight to obtain the PI film.

4. The method of claim 1, wherein the modified MOF-based mixed matrix membrane is prepared by the steps of: The mass fraction of silver nitrate in the composite metal ion modified solution is 1-4wt%.

5. The method of claim 4, wherein the modified MOF-based mixed matrix membrane is prepared by a method comprising: The mass fraction of Zn(NO3)2·6H2O in the composite metal ion modified solution is 3wt%.

6. The method of claim 1, wherein the modified MOF-based mixed matrix membrane is prepared by a method comprising: The mass fraction of 2-methyl imidazole in the ligand reaction solution is 3.2-5.4wt%.

7. A modified MOF-based mixed matrix membrane characterized in that, The mixed matrix membrane is prepared by the preparation method of any one of claims 1-6.

8. Use of the modified MOF-based mixed matrix membrane according to claim 7 in the separation of helium.

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