Dual-crosslinked helium separation membrane, and preparation method therefor and use thereof

Abstract

The present invention relates to the field of gas separation membranes, and relates to a dual-crosslinked helium separation membrane, and a preparation method therefor and a use thereof. The present invention provides an innovative research idea and preparation method, comprising the following experimental steps: (1) molecular design and synthesis of a diamine monomer containing alkynyl and a ligand; (2) synthesis of a crosslinkable TB polymer and preparation of a separation membrane; and (3) construction of a dual-network helium separation membrane by means of a "covalent bonding-metal coordination" step-growth crosslinking strategy. The present invention prepares a membrane material integrating good permeability, relatively high selectivity and excellent anti-aging performance. The permeability coefficient of He is larger than or equal to 50 Barrer, the selection coefficient of He / N2 and He / CH4 is larger than or equal to 150, the permeability coefficient of gas is improved, and high selectivity is kept, an anti-trade-off effect is achieved, and in a 50-day aging data test, the helium flux of the covalent bonding-metal coordination dual-crosslinked separation membrane is only reduced by 3.5%, and has good stability.

Description

A double cross-linking type helium separation membrane and a preparation method and application thereof

[0001] The application relates to a high-performance double cross-linking type gas separation membrane for separating helium and a preparation method and application thereof, and belongs to the field of membrane separation. BACKGROUND

[0002] Helium plays an irreplaceable role in the fields of nuclear magnetic resonance scanners, aerospace, industrial leak detection systems, electronic and optical fiber manufacturing, welding and nuclear research facilities due to its unique properties such as low boiling point, low density, low solubility, high thermal conductivity and high ionization energy. The demand for helium in China is extremely large, ranking second in the world, but more than 95% of helium is dependent on imports, so helium is a serious 'neck' gas. As a scarce non-renewable resource, with the rapid improvement of China's scientific and technological strength, the demand for He resources also presents a sharp growth trend. In order to meet this demand, we urgently need to develop and apply advanced natural gas helium extraction technology to optimize the helium production process and improve production efficiency.

[0003] At present, natural gas is the main source of economically obtained helium, and natural gas helium extraction technologies mainly include cryogenic separation technology, adsorption purification technology, membrane permeation separation technology and the like. Cryogenic separation technology is the most mature helium purification technology at present, and is the only large-scale industrialized technology for separating helium from natural gas at present, but its investment and energy consumption are very high, and it is difficult to obtain high-purity helium products; pressure swing adsorption (PSA) is a new technology for removing a small amount of impurities from gas. After primary purification, crude helium needs to further remove residual impurities, and gas membrane separation technology uses the difference in permeation rate of different gas molecules through the membrane material to realize separation of different components, with the pressure difference of gas on both sides of the membrane as the mass transfer driving force. Membrane separation technology has the advantages of small occupied area, simple operation and maintenance and no pollution, and shows broad application prospects in the field of natural gas helium extraction. Membrane separation method mainly depends on synthetic polymer materials with customizable gas transmission properties. Polymer membranes usually have good flexibility, good film-forming property and excellent gas screening performance, and are commonly used membrane materials in the field of membrane separation. However, the existing helium separation membrane has the disadvantages of poor selectivity, low separation efficiency and easy aging. Although gas separation membrane technology has been widely applied in many fields, mature technical solutions have not yet been formed in the aspect of efficiently separating helium and nitrogen and the like. In industrial practice, in order to efficiently extract helium from natural gas, traditional technologies such as pressure swing adsorption and cryogenic method are often combined, which are effective but have high energy consumption, so that the whole helium extraction process is not satisfactory in economy. In view of this situation, we need to develop a new type of helium separation membrane with high selectivity and high permeability.

[0004] SUMMARY

[0005] Polyimide (PI) is mainly composed of imide ring as the main structural unit. The material has excellent mechanical properties, high temperature resistance, chemical stability and high free volume, and has good gas screening ability, and has been widely used in the field of gas separation. Troger base (TB) contains a rigid V-shaped bridged bicyclic connecting group, which enhances the rigidity of the polymer and hinders the excellent function of chain packing, so as to improve the gas separation performance. The polymer synthesized in the present application is similar to polyimide and has a TB rigid structure, so it also has corresponding gas separation performance. The present application designs and synthesizes a cyano and alkyne-containing diamine monomer, polymerizes the cyano and alkyne-containing diamine monomer to generate a TB polymer, and performs a carboxylation reaction in an alkaline solution to prepare a cross-linkable TB polymer. A double cross-linked helium separation membrane is constructed by a "covalent bonding-metal coordination" step-by-step cross-linking strategy. The polymer unit contains eight benzene rings and a rigid V-shaped bridge to form a large free volume in the interior of the polymer membrane. At the same time, the fully aromatic reticular polymer formed by thermal cross-linking can inhibit the spontaneous creep and relaxation of the polymer chain over time to achieve the purpose of anti-aging, and can also expand the pore to improve the permeability and selectivity of the gas. The coordination network formed by metal coordination cross-linking can improve the permeability and selectivity of the gas, and realize the anti "trade-off" effect.

[0006] In view of the shortcomings of the current helium separation membrane technology, the present application provides a preparation method and industrial application of a double cross-linked helium separation membrane. The double cross-linked helium separation membrane of the present application has high selectivity in the application systems of He / CH4 and He / N2, and also has good permeability for helium. In the 50-day aging data test, the helium flux of the non-thermal cross-linked membrane decreased from 15.6 Barrer to 9.5 Barrer, with a flux reduction of 39.1%; the helium flux of the thermal cross-linked membrane decreased from 36.5 Barrer to 32 Barrer, with a flux reduction of 12.33%; the helium flux of the metal coordination cross-linked membrane decreased from 28.6 Barrer to 24.1 Barrer, with a flux reduction of 15.73%; and the helium flux of the double cross-linked separation membrane decreased from 50 Barrer to 48.3 Barrer, with a flux reduction of 3.5%, which has good stability.

[0007] To solve the problems of low separation efficiency, poor selectivity and easy aging in the membrane separation method for helium extraction from natural gas, the present application provides a double cross-linked helium separation membrane, which is obtained by "covalent bonding-metal coordination" from a TB polymer containing alkyne and coordination groups.

[0008] The polymer structure is as shown in formula 1:

[0009] wherein n is 40-70, and the number average molecular weight of the polymer is 4000-7000 g / mol.

[0010] The double cross-linking type helium separation membrane of the present application is shown in Figure 1.

[0011] The second object of the present application is to provide a preparation method of the double cross-linking type helium separation membrane, and the "covalent bonding-metal coordination" step-by-step cross-linking strategy is used to construct the double cross-linking type helium separation membrane, which comprises the following specific synthesis steps:

[0012] Step 1, the cross-linkable TB polymer membrane is placed in a tube furnace and heated for thermal cross-linking reaction under nitrogen atmosphere to obtain a thermal cross-linking polymer membrane;

[0013] Step 2, a metal coordination cross-linking reaction is carried out on the surface of the thermal cross-linking polymer membrane to obtain the double cross-linking type helium separation membrane.

[0014] In one embodiment, the synthesis of the cross-linkable TB polymer and the preparation method of the separation membrane comprise the following steps:

[0015] Step 1-1, the diamine monomer containing alkynyl and cyano groups and formaldehyde dimethyl acetal are reacted at a molar ratio of 1:1-6 with trifluoroacetic acid as the solvent, and the reaction is carried out for 24-72 h to generate a polymer containing Troger's Base (TB structure);

[0016] Step 1-2, the TB polymer is reacted in a NaOH solution at 60℃ for 2-4 h, and after the reaction is completed, the TB polymer is washed with deionized water until the pH reaches about 7.0, and then dried to obtain a TB polymer containing alkynyl and coordination groups;

[0017] Step 1-3, the TB polymer containing alkynyl and coordination groups is dissolved in an organic solvent, impurities are removed by filtration to obtain a casting solution, and insoluble substances are removed by centrifugation, and the casting solution is uniformly coated on a PTFE base film by a flow casting method, and after the solvent is volatilized, a cross-linkable TB polymer membrane is obtained.

[0018] In one embodiment, the synthesis method of the diamine monomer containing alkynyl and cyano groups comprises the following specific steps:

[0019] Step 3-1, N-methyl pyrrolidone is added to 3-methyl, 4-aminophenol and 3, 5-dichlorobenzonitrile, and after being fully dissolved, toluene and anhydrous potassium carbonate powder are added, and the mixture is heated and refluxed at 120-190℃ under nitrogen protection for about 3-12 h, and the product is discharged into ice water, and after separation and purification, the diamine monomer containing cyano groups is obtained;

[0020] Step 3-2, adding N-methyl pyrrolidone into the diamine monomer containing cyano and 4, 4'-(ethyne-1, 2-diyl) diphthalic anhydride, after complete dissolution, adding toluene, mixing uniformly, heating to reflux at 130℃ under nitrogen protection for about 3h to remove water, after complete dehydration, increasing temperature to 140-190℃, reacting for 6-12h, discharging into ice water, after separation and purification, obtaining the diamine monomer containing cyano and acetylene group.

[0021] In an embodiment, 3-methyl, 4-aminophenol and 3, 5-dichlorobenzonitrile are in a molar ratio of 2-2.5:1;

[0022] The diamine monomer containing cyano and 4, 4'-(ethyne-1, 2-diyl) diphthalic anhydride are in a molar ratio of 2.5-4:1.

[0023] In an embodiment, the solid content of the casting solution is 0.5-5wt.%

[0024] The organic solvent includes at least one of N-methyl pyrrolidone, tetrahydrofuran, N-ethyl pyrrolidone and N, N-dimethylformamide; the pore size of the base film is 0.1-1μm.

[0025] In an embodiment, the obtained polymer film is dried in a vacuum oven at 60-100℃ for 48-72h by volatilizing solvent, and the thickness of the obtained polymer film is 5-15μm.

[0026] In an embodiment, the target temperature of thermal crosslinking is 130-200℃, and the crosslinking reaction time is 10-120min.

[0027] In an embodiment, the target temperature of coordination is 25-50℃, the coordination time is 2-48h, and the coordinated metal ions include Cr 3+ , Fe 3+ , etc.

[0028] A third object of the present application is to provide the application of the above-mentioned separation membrane or the separation membrane prepared according to the above-mentioned method to the separation of He and N2 and the separation of He and CH4.

[0029] The gas with the volume fraction of helium in natural gas being 1-60% and the volume fraction of nitrogen being 0-30% is separated by the double crosslinking type membrane in the present application, and the selectivity of He / N2 is ≥150. The gas with the volume fraction of helium in natural gas being 1-60% and the volume fraction of methane being 0-30% is separated by the double crosslinking type membrane in the present application, and the selectivity of He / CH4 is ≥150. Beneficial effects:

[0030] 1. All the reaction processes of the present application are simple, high in yield and low in cost.

[0031] 2. The double cross-linked helium separation membrane prepared by the present application has high separation selectivity and high gas permeability, wherein the permeation coefficient of He is ≥50 Barrer, and the selectivity coefficients of He / N2 and He / CH4 are ≥150.

[0032] 3. The introduction of the alkynyl group in the present application enables the Diels-Alder addition reaction of the membrane under heating conditions, forming a special network structure of full aromaticity which can inhibit the creep of the polymer chain and play an anti-aging role, thereby improving the service life of the membrane. Meanwhile, the thermal cross-linking also plays a role in hole expansion, which can improve the permeability and selectivity of the polymer membrane. The introduction of the carboxyl group in the present application enables the coordination reaction of the membrane at room temperature, forming a coordination cross-linking network, and the mechanical properties of the membrane are good, which has been preliminarily industrialized. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 is a schematic diagram of polymer cross-linking reaction; A. is thermal cross-linking; B. is coordination cross-linking; C. is double cross-linking.

[0034] Figure 2 is a gas permeation performance testing device.

[0035] Figure 3 is an optical photograph of part of the helium separation membrane; a1 is the front view of the uncross-linked polymer membrane in Comparative Example 1, a2 is the back view of the uncross-linked polymer membrane in Comparative Example 1; b1 is the front view of the double cross-linked helium separation membrane-1 in Example 1, b2 is the back view of the double cross-linked helium separation membrane-1 in Example 1; c1 is the front view of the double cross-linked helium separation membrane-2 in Example 2, c2 is the back view of the double cross-linked helium separation membrane-2 in Example 2. DETAILED DESCRIPTION

[0036] The present application will be further described below by combining with the implementation examples, which are not limitations to the present application unless specifically indicated. The raw materials used in the present examples are all purchased from the market.

[0037] The test method used in the present application is the constant volume pressure change method, and the gas permeability of the membrane is measured by the gas permeation performance testing device prepared in the laboratory. The test gas is He, N2 and CH4. The specific operation steps are as follows:

[0038] (1) According to the actual size of the sample pool, the membrane sample is cut into a proper size of a circular disc shape. Then, the thickness of each area of the membrane sample is measured and the average value is calculated. After completing these preparation steps, the sample loading operation is performed, the necessary sealing ring is added, and it is ensured that the interface is tightly screwed.

[0039] (2) According to the specific requirements of the experiment, set the temperature of the low-temperature constant-temperature tank. Then, start the circulating pump to maintain the constant temperature required during the test. At the same time, turn on the other power switches and valves of the device to ensure that the entire system is in normal working condition. Connect the gas inlet to the steel cylinder of the gas to be tested, open the cylinder valve, and allow the gas to flow. This process lasts about 10 minutes. After the purging is completed, close valves 1 and 7, and start the vacuum pump to perform the vacuuming operation on the device, which lasts about 10 minutes. After the vacuuming is completed, first close valve 4, and then close the vacuum pump. Finally, check the air tightness of the entire device.

[0040] (3) While closing valves 2, 3, and 5, open valve 1 and the cylinder master valve. This step is to fill the pressure tank with one atmosphere of the gas to be tested. After the filling is completed, quickly close the cylinder master valve and valve 1, and detect whether the membrane sample has defects. Slowly open valve 2 and closely observe the change in the downstream side vacuum gauge reading of the membrane cell. Detect whether the sample has defects. After the preliminary detection is completed, close valve 2. Then open the cylinder master valve and valve 1 again, and continue to fill the pressure tank with the gas to be tested until the pressure reaches 0.2 MPa, and then close the valve.

[0041] (4) First, start the vacuum pump and sequentially open valves 4, 3, and 5 to perform a 2h vacuum treatment on the membrane sample. After the treatment is completed, first close valve 4, then close the vacuum pump, and finally close valves 3 and 5. To verify the system error caused by air tightness, after closing all valves, start recording the change in the vacuum reading, which lasts for 30 minutes. This error value needs to be deducted during subsequent data processing.

[0042] (5) After verifying the system error, open valve 2 to allow the gas to be tested to flow from the pressure tank to the upstream side of the membrane cell, and use the pressure gauge software to record the downstream side vacuum degree change over time data. The test time for He is one hour, and the test time for N2 and CH4 is two hours.

[0043] (6) After the test is completed, ensure that the exhaust gas treatment device is used to completely discharge the gas in the device. Then, turn off the power of the device, and take out the test completed sample.

[0044] 1. The gas permeability parameter calculation method of the present application:

[0045] In the formula: P is the gas permeability coefficient (Barrer [1 Barrer = 1 x 10 -10 cm 3 (STP) x cm / (cm 2 x s x cmHg]);Q is the cumulative amount of gas permeation from the beginning to t (cm 3) ; A is the area of the membrane (cm 2 ) ; l is the thickness of the membrane (cm) ; ΔP is the pressure difference between the upstream side and the downstream side (MPa).

[0046] 2. The formula for calculating the gas selectivity of the present application is:

[0047] wherein: α i / j represents the ideal selectivity of the membrane to the gas components He and CH4, N2; P i represents the permeability coefficient of He (Barrer) ; Pj represents the permeability coefficient of the gas components CH4, N2 (Barrer).

[0048] Example 1 Preparation of a dual cross-linked helium separation membrane - 1 flow as follows:

[0049] 1. Preparation of a diamine monomer containing a coordination group and an alkyne group:

[0050] Step 1, add N-methyl pyrrolidone (50 mL) to 3-methyl, 4-aminophenol (3.075 g, 25.0 mmol) and 3, 5-dichlorobenzonitrile (2.15 g, 12.5 mmol), after complete dissolution, add toluene (10 mL), anhydrous potassium carbonate powder (4.15 g, 30.0 mmol), heat to reflux at 180°C under nitrogen protection for about 6 h, discharge into ice water, separate and purify to obtain a diamine monomer containing a cyano group;

[0051] Step 2, add N-methyl pyrrolidone (50 mL) to the diamine monomer containing a cyano group (3.95 g, 10 mmol) and 4, 4'-(ethynyl-1, 2-diyl) di-phthalic anhydride (1.27 g, 4 mmol), after complete dissolution, add toluene (20 mL), mix well, heat to reflux at 130°C under nitrogen protection for about 3 h to remove water. After complete dehydration, heat to 180°C, react for 8 h, discharge into ice water, separate and purify to obtain a diamine monomer containing a cyano group and an alkyne group.

[0052] 2. Synthesis of TB polymer and preparation of separation membrane:

[0053] Step 1, take 1 g of diamine monomer containing a cyano group and an alkyne group, place it in an ice water bath, add 1 mL of formaldehyde dimethyl acetal to the system, add 8 mL of trifluoroacetic acid at a rate of 1 drop per second, remove the ice water bath after 1 h of reaction, react at room temperature for 24 h, then terminate the reaction with 2.5% ammonia solution, wash and dry to obtain a polymer containing a cyano group and an alkyne group;

[0054] Step 2, the polymer containing cyano group and alkyne group is hydrolyzed in NaOH solution at 60°C for 2h. After reaction, rinse with deionized water until pH reaches about 7.0, dry to obtain a new polymer which can realize double crosslinking;

[0055] The polymer structure containing coordination group and alkyne group is shown as formula 1:

[0056] Wherein, n is 38, and the number average molecular weight of the polymer is 42824 g / mol.

[0057] Step 3, the polymer is dissolved in NMP with a solid content of 5 wt.%, centrifuged to remove insoluble substances, coated on a PTFE base film with a pore size of 1 μm by a flow casting method, and dried in a vacuum oven at 60°C for 24h to obtain a composite film with a selected layer thickness of about 10 μm.

[0058] 3. The "covalent bond-metal coordination" step-by-step crosslinking strategy constructs a double network type helium separation membrane:

[0059] The composite film is cut into a rectangle with a size of 6 cm x 4 cm and placed in a tube furnace. By nitrogen protection, the temperature is raised at a rate of 0.5°C / min, the target is 160°C, and the temperature is kept for 15 minutes. Then the temperature is lowered to room temperature at a rate of 10°C / min to obtain a heat crosslinked polymer film. The heat crosslinked polymer film is clamped between two glass plates, one of which is a glass plate with a hole in the middle. 10 mL of CrCl3(1.0 x 10 -2 mol / L) solution is dropped into the hole, and the coordination time is 24h. Wash with methanol and dry in a vacuum oven at 60°C for 2h.

[0060] Example 2 Preparation of a double crosslinked helium separation membrane-2 flow as follows:

[0061] 1. Preparation of coordination group and alkyne group containing diamine monomer:

[0062] Step 1, add N-methyl pyrrolidone (100 mL) to 3-methyl, 4-aminophenol (7.32 g, 60.0 mmol) and 3,5-dichlorobenzonitrile (4.3 g, 25.0 mmol), and after fully dissolving, add toluene (20 mL), anhydrous potassium carbonate powder (8.3 g, 60.0 mmol), heat under nitrogen protection at 190°C for about 5h, and discharge into ice water. After separation and purification, a cyano-containing diamine monomer is obtained;

[0063] Step 2, add N-methyl pyrrolidone (100 mL) into the diamin monomer containing cyano group (7.9 g, 20 mmol) and 4,4'-(ethyne-1,2-diyl)diphthalic anhydride (2.54 g, 8 mmol), after complete dissolution, add toluene (40 mL), mix well, heat to reflux under nitrogen protection at 130 °C for about 3 h to remove water. After complete dehydration, heat to 190 °C, react for 7 h, discharge into ice water, separate and purify to obtain diamin monomer containing cyano group and alkyne group.

[0064] 2. Synthesis of TB polymer and preparation of separation membrane:

[0065] Step 1, take 1 g of diamin monomer containing cyano group and alkyne group, place in ice water bath, add 1 mL of formaldehyde dimethyl acetal to the system, add 8 mL of trifluoroacetic acid at a rate of 1 drop per second, remove the ice water bath after 2 h of reaction, react at room temperature for 60 h, then terminate the reaction with 2.5% ammonia water solution, wash and dry to obtain polymer containing cyano group and alkyne group;

[0066] Step 2, hydrolyze the polymer containing cyano group and alkyne group in NaOH solution at 60 °C for 2 h. After the reaction is completed, rinse with deionized water until the pH reaches about 7.0, and dry to obtain a new polymer that can achieve double crosslinking.

[0067] Polymer structure as shown in formula 2:

[0068] Wherein, n is 53, and the number average molecular weight of the polymer is 59731 g / mol.

[0069] Step 3, dissolve the polymer in NMP, the solid content is 5 wt.%, centrifuge to remove insoluble matter, coat on a PTFE base film with a pore size of 1 μm by a casting method, dry in a vacuum oven at 60 °C for 24 h to obtain a composite membrane with a thickness of about 10 μm.

[0070] 3. "Covalent bond-metal coordination" step-by-step crosslinking strategy to construct double network type helium separation membrane:

[0071] Cut the composite membrane into a rectangle of 6 cm x 4 cm size and place it in a tube furnace, heat to 160 °C at a rate of 0.5 °C / min under nitrogen protection, keep for 15 min, then cool to room temperature at a rate of 10 °C / min to obtain a thermally crosslinked polymer membrane, sandwich the thermally crosslinked polymer membrane between two glass plates, one of which has a hole in the middle, and drop 10 mL of FeCl3(1.0 x 10 -2 mol / L) solution into the hole, coordinate for 24 h, wash with methanol, and dry in a vacuum oven at 60 °C for 2 h.

[0072] Example 3 Preparation of a dual crosslinkable helium separation membrane - Scheme 3:

[0073] 1. Preparation of a diamine monomer containing a coordinating group and an alkyne group:

[0074] Step 1. To 3-methyl, 4-aminophenol (7.32 g, 60.0 mmol) and 3,5-dichlorobenzonitrile (4.3 g, 25.0 mmol) was added N-methyl pyrrolidinone (100 mL) and after complete dissolution, toluene (20 mL) and anhydrous potassium carbonate powder (8.3 g, 60.0 mmol) was added. The reaction was heated to reflux at 190 °C under nitrogen for about 5 h. The product was discharged into ice water and after isolation and purification, a diamine monomer containing a cyano group was obtained.

[0075] Step 2. To the diamine monomer containing a cyano group (7.9 g, 20 mmol) and 4,4'-(ethyn-l,2-diyl)diphthalic anhydride (2.54 g, 8 mmol) was added N-methyl pyrrolidinone (100 mL) and after complete dissolution, toluene (40 mL) was added. The reaction was heated to reflux at 130 °C under nitrogen for about 3 h to remove water. After complete dehydration, the reaction was heated to 190 °C and reacted for 7 h. The product was discharged into ice water and after isolation and purification, a diamine monomer containing a cyano group and an alkyne group was obtained.

[0076] 2. Synthesis of TB polymer and preparation of a separation membrane:

[0077] Step 1. To 1 g of the diamine monomer containing a cyano group and an alkyne group was added 1 mL of formaldehyde dimethyl acetal and 8 mL of trifluoroacetic acid was added dropwise at a rate of 1 drop per second under ice water bath conditions. After 2 h of reaction, the ice water bath was removed and the reaction was allowed to proceed at room temperature for 72 h. The reaction was terminated using 2.5% aqueous ammonia solution and the product was washed and dried to obtain a polymer containing a cyano group and an alkyne group.

[0078] Step 2. The polymer containing a cyano group and an alkyne group was hydrolyzed in a NaOH solution at 60 °C for 2 h. After the reaction was completed, the reaction was washed with deionized water until the pH reached about 7.0 and the product was dried to obtain a novel polymer that can be dual crosslinked.

[0079] Polymer structure as shown in Formula 3:

[0080] wherein n is 63 and the number average molecular weight of the polymer is 71001 g / mol.

[0081] Step 3. The polymer was dissolved in NMP at a solid content of 5 wt.%. Insoluble materials were removed by centrifugation and the solution was cast on a PTFE base membrane having a pore size of 1 μm. The membrane was dried in a vacuum oven at 60 °C for 24 h to obtain a composite membrane having a thickness of about 10 μm.

[0082] 3. Double-network type helium separation membrane constructed by "covalent bonding-metal coordination" step-by-step crosslinking strategy:

[0083] The composite membrane was cut into a rectangle of 6 cm x 4 cm in size and placed in a tube furnace, protected by nitrogen, heated at a rate of 0.5°C / min, the target temperature was 160°C, and the temperature was kept for 15 min, then cooled to room temperature at a rate of 10°C / min to obtain a heat crosslinked polymer membrane. The heat crosslinked polymer membrane was sandwiched between two glass plates, one of which had a hole in the middle, and 10 mL of AlCl3(1.0 x 10 -2 mol / L) solution was dropped into the hole, and the coordination time was 24 h. The membrane was washed with methanol and dried in a vacuum oven at 60°C for 2 h.

[0084] Preparation of uncrosslinked polymer membrane of Comparative Example 1

[0085] 1. Preparation of monomer containing coordination group and alkyne group diamine:

[0086] Step 1: To 3-methyl, 4-aminophenol (3.075 g, 25.0 mmol) and 3, 5-dichlorobenzonitrile (2.15 g, 12.5 mmol), N-methyl pyrrolidone (50 mL) was added, after complete dissolution, toluene (10 mL), anhydrous potassium carbonate powder (4.15 g, 30.0 mmol) was added, and heated to reflux at 180°C under nitrogen protection for about 6 h. The product was discharged into ice water, and after separation and purification, a diamine monomer containing a cyano group was obtained;

[0087] Step 2: To the diamine monomer containing a cyano group (3.95 g, 10 mmol) and 4, 4'-(ethynyl-1, 2-diyl)diphthalic anhydride (1.27 g, 4 mmol), N-methyl pyrrolidone (50 mL) was added, after complete dissolution, toluene (20 mL) was added, and mixed uniformly, and dehydrated at 130°C under nitrogen protection for about 3 h. After complete dehydration, heated to 180°C, and reacted for 8 h. The product was discharged into ice water, and after separation and purification, a diamine monomer containing a cyano group and an alkyne group was obtained.

[0088] 2. Synthesis of TB polymer and preparation of separation membrane:

[0089] Step 1: 1 g of diamine monomer containing a cyano group and an alkyne group was placed in an ice water bath, 1 mL of formaldehyde dimethyl acetal was added to the system, 8 mL of trifluoroacetic acid was added at a rate of 1 drop per second, the ice water bath was removed after 1 h of reaction, and the reaction was carried out at room temperature for 24 h. The reaction was terminated with 2.5% ammonia solution, washed and dried to obtain a polymer containing a cyano group and an alkyne group;

[0090] Step 2, the polymer containing cyano group and alkynyl group is hydrolyzed in NaOH solution at 60°C for 2h. After reaction, rinse with deionized water until pH reaches about 7.0, dry to obtain new polymer which can realize double crosslinking;

[0091] Polymer containing coordination group and alkynyl group has structure as shown in formula 1:

[0092] Wherein, n is 38, and the number average molecular weight of the polymer is 42824 g / mol.

[0093] Step 3, the polymer is dissolved in NMP with solid content of 5 wt.%, insoluble substances are removed by centrifugation, and the composite film with a selected layer thickness of about 10 μm is obtained by coating on a PTFE base film with a pore size of 1 μm and drying in a vacuum oven at 60°C for 24 h.

[0094] Preparation of heat crosslinking polymer film of comparative example 2

[0095] 1. Preparation of diamine monomer containing coordination group and alkynyl group:

[0096] Step 1, N-methyl pyrrolidone (50 mL) is added to 3-methyl, 4-aminophenol (3.075 g, 25.0 mmol) and 3, 5-dichlorobenzonitrile (2.15 g, 12.5 mmol), after complete dissolution, toluene (10 mL) and anhydrous potassium carbonate powder (4.15 g, 30.0 mmol) are added, and the mixture is heated to reflux at 180°C under nitrogen protection for about 6 h, and the product is discharged into ice water, and after separation and purification, the diamine monomer containing cyano group is obtained;

[0097] Step 2, N-methyl pyrrolidone (50 mL) is added to the diamine monomer containing cyano group (3.95 g, 10 mmol) and 4, 4'-(ethynyl-1, 2-diyl)diphthalic anhydride (1.27 g, 4 mmol), after complete dissolution, toluene (20 mL) is added, and the mixture is heated to reflux at 130°C under nitrogen protection for about 3 h to remove water. After complete dehydration, the mixture is heated to 180°C, and the reaction is carried out for 8 h. The product is discharged into ice water, and after separation and purification, the diamine monomer containing cyano group and alkynyl group is obtained.

[0098] 2. Synthesis of TB polymer and preparation of separation film:

[0099] Step 1, 1 g of the diamine monomer containing cyano group and alkynyl group is placed in an ice water bath, 1 mL of formaldehyde dimethyl acetal is added to the system, 8 mL of trifluoroacetic acid is added dropwise at a rate of 1 drop per second, the ice water bath is removed after 1 h of reaction, and the reaction is carried out at room temperature for 24 h. The reaction is terminated by using 2.5% ammonia water solution, and the polymer containing cyano group and alkynyl group is obtained after washing and drying;

[0100] Step 2, the polymer containing cyano group and alkynyl group is hydrolyzed in NaOH solution at 60℃ for 2h. After reaction, rinse with deionized water until pH reaches about 7.0, dry to obtain new polymer which can realize double crosslinking;

[0101] The polymer structure containing coordination group and alkynyl group is shown as formula 1:

[0102] Wherein, n is 38, the number average molecular weight of the polymer is 42824 g / mol.

[0103] Step 3, the polymer is dissolved in NMP with solid content of 5 wt.%, insoluble matter is removed by centrifugation, and the composite membrane with a selected layer thickness of about 10 μm is obtained by coating on the PTFE base film with a pore size of 1 μm by flow casting method and drying in a vacuum oven at 60℃ for 24h.

[0104] 3. Preparation of heat crosslinking type helium separation membrane:

[0105] The composite membrane is cut into a rectangle with a size of 6 cm x 4 cm and placed in a tube furnace, and the heat crosslinking polymer membrane is obtained by heating at a rate of 0.5℃ / min, target temperature of 160℃ for 15 min, and then cooling to room temperature at a rate of 10℃ / min under nitrogen protection.

[0106] Preparation of coordination crosslinking polymer membrane of comparative example 3

[0107] 1. Preparation of coordination group and alkynyl diamine monomer:

[0108] Step 1, N-methyl pyrrolidone (50 mL) is added to 3-methyl, 4-aminophenol (3.075 g, 25.0 mmol) and 3, 5-dichlorobenzonitrile (2.15 g, 12.5 mmol), after complete dissolution, toluene (10 mL) and anhydrous potassium carbonate powder (4.15 g, 30.0 mmol) are added, and heated to reflux at 180℃ for about 6h under nitrogen protection, and the product is discharged into ice water, separated and purified to obtain a cyano-containing diamine monomer;

[0109] Step 2, N-methyl pyrrolidone (50 mL) is added to the cyano-containing diamine monomer (3.95 g, 10 mmol) and 4, 4'-(ethynyl-1, 2-diyl) di-phthalic anhydride (1.27 g, 4 mmol), after complete dissolution, toluene (20 mL) is added, mixed uniformly, and dehydrated at 130℃ for about 3h under nitrogen protection. After complete dehydration, heat to 180℃ and react for 8h, discharge into ice water, separate and purify to obtain a diamine monomer containing cyano group and alkynyl group.

[0110] 2. Synthesis of TB polymer and preparation of separation membrane:

[0111] Step 1, 1 g of the monomer containing cyano and alkyne groups was placed in an ice water bath, 1 mL of formaldehyde dimethyl acetal was added to the system, 8 mL of trifluoroacetic acid was added at a rate of 1 drop per second, after 1 h of reaction, the ice water bath was removed, after 24 h of reaction at room temperature, the reaction was terminated with 2.5% ammonia water solution, washed and dried to obtain a polymer containing cyano and alkyne groups;

[0112] Step 2, the polymer containing cyano and alkyne groups was hydrolyzed in NaOH solution at 60°C for 2 h. After the reaction was completed, it was washed with deionized water until the pH reached about 7.0, and dried to obtain a novel polymer that can realize double crosslinking;

[0113] The structure of the polymer containing coordination groups and alkyne groups is shown in Formula 1:

[0114] Wherein, n is 38, and the number average molecular weight of the polymer is 42824 g / mol.

[0115] Step 3, the polymer was dissolved in NMP with a solid content of 5 wt.%, insoluble substances were removed by centrifugation, and the composite membrane was coated on a PTFE base film with a pore size of 1 μm by a flow casting method, and dried in a vacuum oven at 60°C for 24 h to obtain a composite membrane with a selective layer thickness of about 10 μm.

[0116] 3. Preparation of metal coordination crosslinked helium separation membrane:

[0117] The composite membrane was cut into a rectangle with a size of 6 cm x 4 cm, and the membrane was clamped between two glass plates, one of which was a glass plate with a hole in the middle, 10 mL of CrCl3(1.0 x 10 -2 mol / L) solution was dropped into the hole, the coordination time was 24 h, washed with methanol, and dried in a vacuum oven at 60°C for 2 h.

[0118] Result analysis:

[0119] From Table 1, compared with Comparative Example 1, both thermal crosslinking and metal coordination crosslinking have higher He / N2, He / CH4 selectivity, breaking the "trade-off" effect, improving the gas permeation coefficient while maintaining high selectivity. Compared with Comparative Example 2 and Comparative Example 3, the importance of coupling thermal crosslinking and coordination crosslinking strategies is illustrated. The dual crosslinking network structure synergistically enhances the role of He mass transfer separation and physical aging resistance. From the long-term stability test data in Table 2, in the 50-day aging data test, the helium flux of the non-thermal crosslinking membrane decreased from 15.6 Barrer to 9.5 Barrer, with a flux reduction of 39.1%; the helium flux of the thermal crosslinking membrane decreased from 36.5 Barrer to 32 Barrer, with a flux reduction of 12.33%; the helium flux of the metal coordination crosslinking membrane decreased from 28.6 Barrer to 24.1 Barrer, with a flux reduction of 15.73%; and the helium flux of the dual crosslinking type separation membrane decreased from 50 Barrer to 48.3 Barrer, with a flux reduction of 3.5%, showing good stability.

[0120] Table 1: Single component gas permeation performance data

[0121] Table 2: Aging data

Claims

1. A dual-crosslinked helium separation membrane, mainly characterized in that the dual-crosslinked helium separation membrane is constructed by a "covalent bonding-metal coordination" step-by-step crosslinking strategy using a polymer containing an alkyne group and a coordination group. Polymer structure as Formula 1: Wherein n is 40-70, and the number average molecular weight of the polymer is 4000-7000 g / mol.

2. The method of claim 1, wherein the method is characterized by: The "covalent bonding-metal coordination" step-by-step crosslinking strategy for constructing the dual-crosslinked helium separation membrane comprises the following specific synthesis steps: Step 1, the crosslinkable TB polymer membrane is placed in a tube furnace and heated for thermal crosslinking reaction under nitrogen atmosphere to obtain a thermally crosslinked polymer membrane; Step 2, a metal coordination crosslinking reaction is carried out on the surface of the thermally crosslinked polymer membrane to obtain the dual-crosslinked helium separation membrane.

3. The method of claim 2, wherein the method further comprises the step of: The synthesis of the crosslinkable TB polymer and the preparation method of the separation membrane comprise the following steps: ​ Step 1-1, a diamine monomer containing an alkyne group and a cyano group is reacted with formaldehyde dimethyl acetal in a molar ratio of 1:1-6 using trifluoroacetic acid as a solvent, and the reaction is carried out for 24-72 h to obtain a polymer containing Troger's Base, i.e., a TB polymer; Step 1-2, the TB polymer is reacted in a NaOH solution at 60℃ for 2-4 h, and after the reaction is completed, the solution is washed with deionized water until the pH reaches about 7.0, and then dried to obtain a TB polymer containing an alkyne group and a coordination group; Step 1-3, the TB polymer containing an alkyne group and a coordination group is dissolved in an organic solvent, filtered to remove impurities to obtain a casting solution, and the insoluble substances are removed by centrifugation, and the casting solution is uniformly coated on a PTFE base film by a casting method, and after the solvent is volatilized, a crosslinkable TB polymer membrane is obtained.

4. The method of claim 3, wherein the method is characterized by: The synthesis method of the diamine monomer containing an alkyne group and a cyano group comprises the following specific steps: Step 3-1, N-methyl pyrrolidone is added to 3-methyl, 4-aminophenol and 3, 5-dichlorobenzonitrile, and after being fully dissolved, toluene and anhydrous potassium carbonate powder are added, and the mixture is heated to reflux at 120-190℃ under nitrogen protection for about 3-12 h, and the product is discharged into ice water, and after separation and purification, a diamine monomer containing a cyano group is obtained; Step 3-2, N-methyl pyrrolidone is added to the diamine monomer containing a cyano group and 4, 4'-(ethyne-1, 2-diyl) di-phthalic anhydride, and after being completely dissolved, toluene is added, and the mixture is uniformly mixed, and then dehydrated by heating to reflux at 130℃ under nitrogen protection for about 3 h, and after complete dehydration, the temperature is increased to 140-190℃, and the reaction is carried out for 6-12 h, and the product is discharged into ice water, and after separation and purification, a diamine monomer containing a cyano group and an alkyne group is obtained.

5. The method of claim 4, wherein the method further comprises the step of: The molar ratio of 3-methyl, 4-aminophenol and 3, 5-dichlorobenzonitrile is 2-2.5:1; ​ The molar ratio of the diamine monomer containing a cyano group to 4, 4'-(ethyne-1, 2-diyl) di-phthalic anhydride is 2.5-4:

1.

6. The method of claim 3, wherein the method is characterized by: The solid content of the casting solution is 0.5-5 wt.% The organic solvent includes at least one of N-methyl pyrrolidone, tetrahydrofuran, N-ethyl pyrrolidone and N, N-dimethylformamide; The pore size of the base film is 0.1-1 μm.

7. The method of claim 3, wherein the method further comprises the step of: 7-1) adding a cross-linking agent to the solution of step 6-1) to form a cross-linked solution. The obtained polymer membrane has a thickness of 5-15 μm by drying in a vacuum oven at 60-100℃ for 48-72 h by volatilizing the solvent.

8. The method of claim 2, wherein the method is characterized by: The target temperature for thermal crosslinking is 130-200 DEG C, and the crosslinking reaction time is 10-120 min.

9. The method for preparing a thermally cross-linked helium separation membrane according to claim 2, characterized in that, The target temperature for coordination is 25-50℃, the coordination time is 2-48h, and the metal ions for coordination include Cr 3+ , Fe 3+ , etc.

10. Use of the separation membrane according to claim 1 or prepared according to the method of claims 2-9 for the separation of He from N2, He from CH4.

Images