Helium-enriched gas separation membrane and preparation method thereof

Abstract

The invention discloses a helium-enriched gas separation membrane and a preparation method thereof, and the method comprises the following steps: (1) adding a diamine monomer and a dianhydride monomer into a first solvent, then heating to 60-90 DEG C, adding a catalyst, reacting for 2-4 hours, heating to 100-130 DEG C, reacting for 1-3 hours, finally heating to 160-200 DEG C, reacting for 5-8 hours, and carrying out polymerization reaction to obtain a reaction solution; a mixed solution containing a polyimide polymer is obtained; (2) cooling the mixed solution obtained in the step (1) to room temperature, washing for multiple times by using absolute ethyl alcohol and a second solvent to obtain a fibrous polyimide polymer, and drying the fibrous polyimide polymer; and (3) dissolving the polyimide polymer obtained in the step (2) by using a third solvent, then defoaming by using a hydrophobic PTFE filter, slowly pouring a clear solution into a culture dish, controlling the solution to slowly evaporate, and then stripping the polyimide film from the culture dish to obtain the transparent polyimide film. The separation membrane prepared by the preparation method disclosed by the invention has relatively high permeability and relatively high He / CH4 separation performance at the same time.

Description

Technical Field

[0001] The present invention relates to the technical field of gas separation membranes, and in particular to a helium-enriched gas separation membrane and a preparation method thereof. Background Art

[0002] As an important strategic resource, helium is widely used in fields such as low-temperature superconductors, semiconductor manufacturing, and aerospace. However, the helium content in natural gas is generally low (usually less than 1%), and its efficient separation and enrichment has always been a technical difficulty in the industry. Traditional helium extraction technology is mainly based on cryogenic separation, which requires multi-stage compression and cryogenic distillation to separate helium from other gases (such as methane and nitrogen). Although this method can achieve the separation and recovery of high-purity helium, its energy consumption is extremely high, especially when processing natural gas with low helium concentration. The liquefaction and separation process of non-helium components (such as methane) leads to significant energy waste and poor economic efficiency.

[0003] In recent years, gas separation membrane technology has become a research hotspot due to its advantages such as low energy consumption and simple operation. Gas separation membrane technology achieves separation through the difference in the permeation rate of gas molecules in the membrane material, but existing membrane materials still face multiple challenges in the application of helium extraction from natural gas. For example, there is the trade-off between selectivity and permeability. Most membrane materials cannot simultaneously meet high helium selectivity and high permeation flux, resulting in limited separation efficiency. Generally, when the permeability of the membrane material is high, the selectivity will decrease. In particular, for the separation of helium from natural gas with low helium concentration, if the selectivity decreases, it will greatly affect the helium separation effect, and ultimately the purity of the helium. Currently, although Chinese patent application CN113996193A discloses a copolyimide membrane, a preparation method, and its application in helium purification, which produces a membrane with high permeability and selectivity, the preparation method is complex and requires multiple steps such as low-temperature polymerization (273–283K), high-temperature imidization (400-500K), and thermal rearrangement (650–800K). In particular, the thermal rearrangement temperature is as high as 800K (i.e., 527°C), which not only requires extremely high high-temperature resistance of the equipment, but also has high production energy consumption and difficult process control. Summary of the Invention

[0004] To solve the above technical problems, the present invention proposes a helium-enriched gas separation membrane and a preparation method thereof. The preparation method is relatively simple, and the prepared membrane can have high He / CH4 separation performance while maintaining a high permeability.

[0005] The present invention specifically adopts the following technical solutions:

[0006] A helium-enriched gas separation membrane, the chemical structure of the gas separation membrane is shown in the following formula:

[0007]

[0008] Wherein, R is -CH3 or -Cl or -F, and n is the number of repeating units.

[0009] A method for preparing a helium-enriched gas separation membrane comprises the following steps:

[0010] (1) adding a diamine monomer and a dianhydride monomer to a first solvent, stirring uniformly under a nitrogen atmosphere, then heating to 60-90° C., adding a catalyst and reacting for 2-4 hours, then heating to 100-130° C. and reacting for 1-3 hours, and finally heating to 160-200° C. and reacting for 5-8 hours to cause a polymerization reaction, thereby obtaining a mixed solution containing a polyimide polymer;

[0011] (2) cooling the mixed solution of step (1) to room temperature, washing it multiple times with anhydrous ethanol and a second solvent to obtain a fibrous polyimide polymer, and drying it;

[0012] (3) The polyimide polymer obtained in step (2) is dissolved in a third solvent, and then defoamed using a hydrophobic PTFE filter. The clarified solution is slowly poured into a culture dish, and the solution is controlled to evaporate slowly. Then, the polyimide film is peeled off from the culture dish with the assistance of deionized water, and dried to obtain a transparent polyimide film.

[0013] Furthermore, the diamine monomer in step (1) is one of 9,9-bis(3-methyl-4-aminophenyl)fluorene, 9,9-bis(4-amino-3-chlorophenyl)fluorene and 9,9-bis(4-amino-3-fluorophenyl)fluorene.

[0014] Furthermore, the dianhydride monomer in step (1) is hexafluorodianhydride.

[0015] Furthermore, in step (1), the molar ratio of the diamine monomer to the dianhydride monomer is 1:1 to 1.5.

[0016] Furthermore, in the step (1), the catalyst is at least one of isoquinoline, triethylamine, and β-pyridine, and the mass concentration of the catalyst is 0.1 to 10%.

[0017] Furthermore, the step (2) is specifically as follows: after the mixed solution of step (1) is cooled to room temperature, it is slowly poured into anhydrous ethanol and continuously stirred with a glass rod to precipitate the solution to form a fibrous polymer, which is then soaked and washed in ethanol for 2 to 4 hours and then separated; the precipitate is dissolved with a second solvent, and then the solution is poured into anhydrous ethanol again, and the dissolution and precipitation are repeated multiple times to remove residual substances; and then the fibrous polymer is placed in a vacuum drying oven at 140 to 180° C. and dried for 48 to 72 hours.

[0018] Furthermore, in step (1), the first solvent is at least one of toluene, o-xylene, m-xylene, p-xylene, o-cresol, m-cresol, and p-cresol.

[0019] Furthermore, in step (2), the second solvent is at least one of acetone, acetonitrile, chloroform, tetrahydrofuran, and dichloromethane.

[0020] Furthermore, in step (3), the third solvent is at least one of acetone, acetonitrile, chloroform, tetrahydrofuran, and dichloromethane.

[0021] The present invention has the following beneficial effects:

[0022] The present invention provides a helium-enriched gas separation membrane with a relatively simple preparation method. The membrane is prepared by polycondensation reaction of hexafluorodianhydride and diamine monomers containing different substituent groups, and has excellent film-forming properties. In addition, the present invention selects diamine monomers containing different substituent groups as reaction raw materials, that is, introduces substituent groups into the polyimide chain generated by the polymerization reaction of the diamine monomer and the dianhydride monomer, thereby adjusting the microporous structure of the polyimide membrane, thereby adjusting the permeability and selectivity of the polyimide membrane to helium. In particular, for the double-fluorinated separation membrane, due to the presence of more F elements that have specific recognition for He, the winning rate of He in the competition between He and CH4 is increased, so that the polyimide separation membrane has a high He / CH4 selectivity while maintaining a high permeability, thereby effectively improving the He / CH4 separation performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the polymerization process of the present invention;

[0024] Figure 2 These are atomic force microscope and scanning electron microscope photographs of the separation membranes prepared in Examples 1-3 of the present invention and Comparative Example 1, wherein (a1), (b1), (c1) (d1) are atomic force microscope photographs of the surfaces of the separation membranes prepared in Examples 1-3 and Comparative Example 1, respectively; (a2), (b2), (c2) (d2) are scanning electron microscope photographs of the surfaces of the separation membranes prepared in Examples 1-3 and Comparative Example 1, respectively; (a3), (b3), (c3) (d3) are scanning electron microscope photographs of the cross-sections of the separation membranes prepared in Examples 1-3 and Comparative Example 1, respectively; and (a4), (b4), (c4) (d4) are enlarged views of the scanning electron microscope photographs of the cross-sections of the separation membranes prepared in Examples 1-3 and Comparative Example 1, respectively.

[0025] Figure 3 Graph showing the hydrophobic performance of the separation membranes prepared in Examples 1-3 and Comparative Example 1 of the present invention.

[0026] Figure 4This is a long-term stability test diagram of the separation membrane prepared in Example 3 of the present invention. DETAILED DESCRIPTION

[0027] The specific implementation of the present invention will be further described below with reference to the accompanying drawings and specific examples.

[0028] Reference Figure 1 The present invention provides a method for preparing a helium-enriched gas separation membrane, comprising the steps of:

[0029] (1) adding a diamine monomer and a dianhydride monomer to a first solvent, stirring uniformly under a nitrogen atmosphere, then heating to 60-90° C., adding a catalyst and reacting for 2-4 hours, then heating to 100-130° C. and reacting for 1-3 hours, and finally heating to 160-200° C. and reacting for 5-8 hours to cause a polymerization reaction, thereby obtaining a mixed solution containing a polyimide polymer;

[0030] (2) cooling the mixed solution of step (1) to room temperature, washing it multiple times with anhydrous ethanol and a second solvent to obtain a fibrous polyimide polymer, and drying it;

[0031] (3) The polyimide polymer obtained in step (2) is dissolved in a third solvent, and then defoamed using a hydrophobic PTFE filter. The clarified solution is slowly poured into a culture dish, and the solution is controlled to slowly evaporate at room temperature. The polyimide film is then peeled off from the culture dish with the assistance of deionized water and dried to obtain an isotropic transparent polyimide film.

[0032] In a preferred embodiment of the present invention, the diamine monomer in step (1) is one of 9,9-bis(3-methyl-4-aminophenyl)fluorene, 9,9-bis(4-amino-3-chlorophenyl)fluorene and 9,9-bis(4-amino-3-fluorophenyl)fluorene; the dianhydride monomer in step (1) is hexafluorodianhydride; and the diamine monomer and the dianhydride monomer can be purified and dried in advance by recrystallization before use.

[0033] In a preferred embodiment of the present invention, the molar ratio of the diamine monomer to the dianhydride monomer in step (1) is 1:1 to 1.5.

[0034] In a preferred embodiment of the present invention, the catalyst in step (1) is at least one of isoquinoline, triethylamine, and β-pyridine, and the mass concentration of the catalyst is 0.1 to 10% of the total mass of the mixed solution of the diamine monomer, the dianhydride monomer, the first solvent, and the catalyst.

[0035] In a preferred embodiment of the present invention, step (2) is specifically as follows: after the mixed solution of step (1) is cooled to room temperature, it is slowly poured into anhydrous ethanol and continuously stirred with a glass rod to precipitate the solution to form a fibrous polymer, which is then soaked and washed in ethanol for 2 to 4 hours and then separated; the precipitate is dissolved with a second solvent, and then the solution is poured into anhydrous ethanol again, and the dissolution and precipitation are repeated multiple times to remove residual substances; and then the fibrous polymer is placed in a vacuum drying oven at 140 to 180°C and dried for 48 to 72 hours.

[0036] In a preferred embodiment of the present invention, the first solvent in step (1) is at least one of toluene, o-xylene, m-xylene, p-xylene, o-cresol, m-cresol, and p-cresol.

[0037] In a preferred embodiment of the present invention, the second solvent in step (2) is at least one of acetone, acetonitrile, chloroform, tetrahydrofuran, and dichloromethane.

[0038] In a preferred embodiment of the present invention, the third solvent in step (3) is at least one of acetone, acetonitrile, chloroform, tetrahydrofuran, and dichloromethane.

[0039] The general chemical structure of the helium-enriched gas separation membrane obtained based on the above preparation method is shown below:

[0040]

[0041] Wherein, R is -CH3 or -Cl or -F, and n is the number of repeating units.

[0042] In the above technical solution, the present invention selects a diamine monomer containing a -CH3, -Cl, or -F substituent as a reaction raw material in the process of preparing a helium-enriched gas separation membrane, that is, a -CH3, -Cl, or -F group is introduced into the polyimide membrane generated by the polymerization reaction of the diamine monomer and the dianhydride monomer, which can adjust the microporous structure of the polyimide membrane, thereby adjusting the permeability and selectivity of the polyimide membrane to helium, so that the polyimide membrane has a high He / CH4 selectivity while maintaining a high permeability.

[0043] Example 1

[0044] This embodiment provides a helium-enriched gas separation membrane, and the preparation process is as follows:

[0045] (1) Hexafluorodianhydride monomer and 9,9-bis(3-methyl-4-aminophenyl)fluorene monomer were purified by recrystallization and dried in a vacuum drying oven at 60°C for 16 hours;

[0046] (2) 1.604 g of 9,9-bis(3-methyl-4-aminophenyl)fluorene monomer and 15 mL of m-cresol were added to a three-necked round-bottom flask (the three-necked round-bottom flask was placed in an oil bath), purged with flowing N2, and stirred at 35°C for 50 min. After the 4,4'-(9-fluorenyl)diphenylamine monomer was completely dissolved, 1.893 g of hexafluorodianhydride monomer was added and stirred for 1.5 h to completely dissolve the hexafluorodianhydride monomer. The oil bath was then heated to 75°C, 0.3 ml of isoquinoline was added, and the mixture was reacted for 3 h. The oil bath was then heated to 115°C and the mixture was reacted for 2 h. Finally, the oil bath was heated to 180°C and the mixture was reacted for 7 h to obtain a mixed solution containing a polyimide polymer.

[0047] (3) After the reaction is completed, the mixed solution containing the polyimide polymer in step (2) is cooled to room temperature, washed several times with anhydrous ethanol and a solvent to obtain a fibrous polyimide polymer and dried; specifically, the mixed solution in step (2) is cooled to room temperature, and the solution is slowly poured into 50 mL of anhydrous ethanol. During this period, a glass rod needs to be continuously stirred to precipitate the solution to form a fibrous polymer, which is then separated after being soaked and washed in ethanol for 3 hours; the precipitate is dissolved with 30 mL of chloroform, and the solution is poured into 100 mL of anhydrous ethanol again, and the dissolution and precipitation are repeated three times to remove residual substances; and the fibrous polyimide polymer is then placed in a vacuum drying oven at 160° C. and dried for 60 hours to constant weight;

[0048] (4) 0.15 g of the polyimide obtained in step (3) was dissolved in 5 mL of chloroform and stirred at room temperature for 4 h to dissolve it. The mixture was then filtered and defoamed using a 0.45 μm hydrophobic PTFE filter. The solution was slowly poured into a culture dish and the solution was controlled to evaporate slowly. The polyimide film was peeled off from the culture dish with the assistance of deionized water to obtain an isotropic transparent polyimide film with a thickness of about 30 to 40 μm. The film was dried naturally and stored in a vacuum.

[0049] The chemical structure of the gas separation membrane prepared in this embodiment is shown below:

[0050]

[0051] Example 2

[0052] This embodiment provides a helium-enriched gas separation membrane, and the preparation process is as follows:

[0053] (1) Hexafluorodianhydride monomer and 9,9-bis(4-amino-3-chlorophenyl)fluorene monomer were purified by recrystallization and dried in a vacuum drying oven at 60°C for 16 hours;

[0054] (2) 1.611 g of 9,9-bis(4-amino-3-chlorophenyl)fluorene monomer and 15 mL of m-cresol were added to a three-necked round-bottom flask (the three-necked round-bottom flask was placed in an oil bath), purged with flowing N2, and stirred at 35°C for 50 min. After the 4,4'-(9-fluorenyl)diphenylamine monomer was completely dissolved, 1.715 g of hexafluorodianhydride monomer was added and stirred for 1.5 h to completely dissolve the hexafluorodianhydride monomer. The oil bath was then heated to 75°C, 0.3 ml of isoquinoline was added, and the mixture was reacted for 3 h. The oil bath was then heated to 115°C and the mixture was reacted for 2 h. Finally, the oil bath was heated to 180°C and the mixture was reacted for 7 h to obtain a mixed solution containing a polyimide polymer.

[0055] (3) After the reaction is completed, the mixed solution containing the polyimide polymer in step (2) is cooled to room temperature, washed several times with anhydrous ethanol and a solvent to obtain a fibrous polyimide polymer and dried; specifically, the mixed solution in step (2) is cooled to room temperature, and the solution is slowly poured into 50 mL of anhydrous ethanol. During this period, a glass rod needs to be continuously stirred to precipitate the solution to form a fibrous polymer, which is then separated after being soaked and washed in ethanol for 3 hours; the precipitate is dissolved with 30 mL of chloroform, and the solution is poured into 100 mL of anhydrous ethanol again, and the dissolution and precipitation are repeated three times to remove residual substances; and the fibrous polyimide polymer is then placed in a vacuum drying oven at 160° C. and dried for 60 hours to constant weight;

[0056] (4) 0.15 g of the polyimide obtained in step (3) was dissolved in 5 mL of chloroform and stirred at room temperature for 4 h to dissolve it. The mixture was then filtered and defoamed using a 0.45 μm hydrophobic PTFE filter. The solution was slowly poured into a culture dish and the solution was controlled to evaporate slowly. The polyimide film was peeled off from the culture dish with the assistance of deionized water to obtain an isotropic transparent polyimide film with a thickness of about 30 to 40 μm. The film was dried naturally and stored in a vacuum.

[0057] The chemical structure of the gas separation membrane prepared in this embodiment is shown below:

[0058]

[0059] Example 3

[0060] This embodiment provides a helium-enriched gas separation membrane, and the preparation process is as follows:

[0061] (1) Hexafluorodianhydride monomer and 9,9-bis(4-amino-3-fluorophenyl)fluorene monomer were purified by recrystallization and dried in a vacuum drying oven at 60°C for 16 hours;

[0062] (2) 1.646 g of 9,9-bis(4-amino-3-fluorophenyl)fluorene monomer and 15 mL of m-cresol were added to a three-necked round-bottom flask (the three-necked round-bottom flask was placed in an oil bath), purged with flowing N2, and stirred at 35°C for 50 min. After the 4,4'-(9-fluorenyl)diphenylamine monomer was completely dissolved, 1.905 g of hexafluorodianhydride monomer was added and stirred for 1.5 h to completely dissolve the hexafluorodianhydride monomer. The oil bath was then heated to 75°C, 0.3 ml of isoquinoline was added, and the mixture was reacted for 3 h. The oil bath was then heated to 115°C and the mixture was reacted for 2 h. Finally, the oil bath was heated to 180°C and the mixture was reacted for 7 h to obtain a mixed solution containing a polyimide polymer.

[0063] (3) After the reaction is completed, the mixed solution containing the polyimide polymer in step (2) is cooled to room temperature, washed several times with anhydrous ethanol and a solvent to obtain a fibrous polyimide polymer and dried; specifically, the mixed solution in step (2) is cooled to room temperature, and the solution is slowly poured into 50 mL of anhydrous ethanol. During this period, a glass rod needs to be continuously stirred to precipitate the solution to form a fibrous polymer, which is then separated after being soaked and washed in ethanol for 3 hours; the precipitate is dissolved with 30 mL of chloroform, and the solution is poured into 100 mL of anhydrous ethanol again, and the dissolution and precipitation are repeated three times to remove residual substances; and the fibrous polyimide polymer is then placed in a vacuum drying oven at 160° C. and dried for 60 hours to constant weight;

[0064] (4) 0.15 g of the polyimide obtained in step (3) was dissolved in 5 mL of chloroform and stirred at room temperature for 4 h to dissolve it. The mixture was then filtered and defoamed using a 0.45 μm hydrophobic PTFE filter. The solution was slowly poured into a culture dish and the solution was controlled to evaporate slowly. The polyimide film was peeled off from the culture dish with the assistance of deionized water to obtain an isotropic transparent polyimide film with a thickness of about 30 to 40 μm. The film was dried naturally and stored in a vacuum.

[0065] The chemical structure of the gas separation membrane prepared in this embodiment is shown below:

[0066]

[0067] Example 4

[0068] This embodiment provides a helium-enriched gas separation membrane, and the preparation process is as follows:

[0069] (1) Hexafluorodianhydride monomer and 9,9-bis(4-amino-3-fluorophenyl)fluorene monomer were purified by recrystallization and dried in a vacuum drying oven at 60°C for 16 hours;

[0070] (2) 1.646 g of 9,9-bis(4-amino-3-fluorophenyl)fluorene monomer and 30 mL of o-xylene were added to a three-necked round-bottom flask (the three-necked round-bottom flask was placed in an oil bath), purged with flowing N2, and stirred at 30°C for 60 min. After the 9,9-bis(4-amino-3-fluorophenyl)fluorene monomer was completely dissolved, 1.905 g of hexafluorodianhydride monomer was added and stirred for 2 h to completely dissolve the hexafluorodianhydride monomer; then the oil bath was heated to 90°C, 0.25 ml of β-pyridine was added, and the mixture was reacted for 2 h, then the oil bath was heated to 130°C, the mixture was reacted for 1 h, and finally the oil bath was heated to 200°C, the mixture was reacted for 5 h, and a mixed solution containing a polyimide polymer was obtained;

[0071] (3) After the reaction is completed, the mixed solution containing the polyimide polymer in step (2) is cooled to room temperature, washed several times with anhydrous ethanol and a solvent to obtain a fibrous polyimide polymer and dried; specifically, the mixed solution in step (2) is cooled to room temperature, and the solution is slowly poured into 50 mL of anhydrous ethanol. During this period, it is necessary to continuously stir with a glass rod to precipitate the solution to form a fibrous polymer, and then soak and wash in ethanol for 3 hours and then separate; the precipitate is dissolved with 30 mL of acetonitrile, and the solution is poured into 100 mL of anhydrous ethanol again, and the dissolution and precipitation are repeated three times to remove residual substances; and then the fibrous polyimide polymer is placed in a vacuum drying oven at 180°C and dried for 50 hours to constant weight;

[0072] (4) 0.15 g of the polyimide obtained in step (3) was dissolved in 5 mL of acetonitrile and stirred at room temperature for 3 h to dissolve it. The mixture was then filtered and defoamed using a 0.45 μm hydrophobic PTFE filter. The solution was slowly poured into a culture dish and the solution was controlled to evaporate slowly. The polyimide film was peeled off from the culture dish with the assistance of deionized water to obtain an isotropic transparent polyimide film with a thickness of about 30 to 40 μm. The film was dried naturally and stored in a vacuum.

[0073] The chemical structure of the gas separation membrane prepared in this embodiment is shown below:

[0074]

[0075] Comparative Example 1

[0076] This comparative example provides a helium-enriched gas separation membrane, and the preparation process is as follows:

[0077] (1) The hexafluorodianhydride monomer and the 4,4'-(9-fluorenylene)diphenylamine monomer were purified by recrystallization and dried in a vacuum drying oven at 60°C for 16 hours;

[0078] (2) 1.370 g of 4,4'-(9-fluorenyl)diphenylamine monomer and 15 mL of m-cresol were added to a three-necked round-bottom flask (the three-necked round-bottom flask was placed in an oil bath), purged with flowing N2, and stirred at 35°C for 50 min. After the 4,4'-(9-fluorenyl)diphenylamine monomer was completely dissolved, 1.747 g of hexafluorodianhydride monomer was added and stirred for 1.5 h to completely dissolve the hexafluorodianhydride monomer. The oil bath was then heated to 75°C, isoquinoline was added, and the mixture was reacted for 3 h. The oil bath was then heated to 115°C, the mixture was reacted for 2 h, and the mixture was finally heated to 180°C, the mixture was reacted for 7 h, to obtain a mixed solution containing a polyimide polymer.

[0079] (3) After the reaction is completed, the mixed solution containing the polyimide polymer in step (2) is cooled to room temperature, washed several times with anhydrous ethanol and a solvent to obtain a fibrous polyimide polymer and dried; specifically, the mixed solution in step (2) is cooled to room temperature, and the solution is slowly poured into 50 mL of anhydrous ethanol. During this period, a glass rod needs to be continuously stirred to precipitate the solution to form a fibrous polymer, which is then separated after being soaked and washed in ethanol for 3 hours; the precipitate is dissolved with 30 mL of chloroform, and the solution is poured into 100 mL of anhydrous ethanol again, and the dissolution and precipitation are repeated three times to remove residual substances; and the fibrous polyimide polymer is then placed in a vacuum drying oven at 160° C. and dried for 60 hours to constant weight;

[0080] (4) 0.15 g of the polyimide obtained in step (3) was dissolved in 5 mL of chloroform and stirred at room temperature for 4 h to dissolve it. The mixture was then filtered and defoamed using a 0.45 μm hydrophobic PTFE filter. The solution was slowly poured into a culture dish and the solution was controlled to evaporate slowly. The polyimide film was peeled off from the culture dish with the assistance of deionized water to obtain an isotropic transparent polyimide film with a thickness of about 30 to 40 μm. The film was dried and stored in a vacuum.

[0081] The chemical structure of the gas separation membrane prepared in this embodiment is shown below:

[0082]

[0083] The surface and cross-sectional morphologies of the separation membranes prepared in Examples 1-3 and Comparative Example 1 were analyzed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Figure 2 As shown. Figure 2 It can be seen that the morphologies of the four polyimide films are different. Figure 2 It can be seen from the atomic force microscope photos in that the molecular chains of the separation membrane of Comparative Example 1 have high rigidity, and the surface roughness is moderate due to local accumulation during the film formation process; the Ra value of the separation membrane of Example 1 is 0.936nm, which is due to the topological fluctuation caused by methyl agglomeration, such as Figure 2(a2) is shown as the white dot-shaped area; the introduction of chlorine in Example 2 significantly increases the roughness. This is because the van der Waals radius of chlorine atoms is large, and the introduction will destroy the tight packing of molecular chains, forming a loose or twisted chain conformation. This disordered arrangement appears as an uneven surface morphology; Example 3 achieves an ultra-low Ra value of 0.325nm. This is because double fluorination significantly enhances the regularity of the molecular chains. When a crystalline region is formed, surface defects (such as undulations in the amorphous region) are reduced, and the macroscopic appearance is a smoother surface. Figure 2 It can be found from the scanning electron microscope photos that the cross-sections of the separation membranes prepared in Example 1, Example 2 and Comparative Example 1 are relatively regular and smooth, while the cross-section of the separation membrane prepared in Example 3 is rougher. Combined with the corresponding magnified images, it can be seen that the pores of the separation membrane prepared in Example 3 are denser. This is because the low surface energy of fluorine promotes the accumulation of molecular chains, and the high electronegativity enhances the interaction between molecules, thereby forming denser pores, which provides a basis for high selectivity.

[0084] The hydrophobic properties of the separation membranes prepared in Examples 1-3 and Comparative Example 1 were tested. Figure 3 shown. Figure 3 It shows that the incorporation of substituents enhances the non-polar properties of the polymer molecules, thereby improving the hydrophobic properties of the separation membrane. According to Wenzel's equation, when the water contact angle on the solid surface exceeds 90°, the increase in surface roughness will lead to an increase in the contact angle. Consistent with this principle, the separation membranes prepared in Examples 1 and 2 have a large roughness, thereby presenting a larger contact angle than that of Comparative Example 1; and although the fluorine substituents introduced in Example 3 and the chlorine substituents in Example 2 have similar chemical properties, fluorine has superior electron-withdrawing ability and stronger CF bond energy, so the membrane of Example 3 exhibits obvious hydrophobicity. These characteristics make the double fluorinated membrane have lower polarity and lower surface tension, ultimately achieving superior hydrophobic properties.

[0085] The separation membranes prepared in Examples 1-3 and Comparative Example 1 were tested for mechanical properties, glass transition temperature, permeability and selectivity.

[0086] Mechanical Properties Testing Method: Films (30-40 μm thick) were cut into 10 mm × 5 mm rectangular specimens and stretched at a rate of 5 mm / min at 25°C using a HY-0580 microcomputer-controlled electronic universal materials testing machine in accordance with GB / T 1040.3-2006. The stress-strain curves were recorded, and the tensile strength was calculated.

[0087] Glass transition temperature test method: Differential scanning calorimetry (DSC, Shimadzu DSC-60 plus) was used to analyze the thermal stability of the film at 25-500°C at a heating rate of 5°C / min under a nitrogen atmosphere.

[0088] Permeability and selectivity test methods: Gas permeability tests were conducted using the Wicke-Kallenbach method for a helium / methane mixture (V / V = 50 / 50). When testing helium-enriched gas separation membranes, the membranes were cut to appropriate size and taped to the center of the permeation cell with aluminum foil tape. The effective membrane area tested was 0.1256 cm 2 The temperature was 30°C, the test pressure was 0.3 MPa, the feed flow rate was set to 50 mL / min, and argon was used as the purge gas at a flow rate of 25 mL / min. Each gas was tested for a sufficient time to ensure that the system reached equilibrium. The permeability coefficient and mixture selectivity are defined as follows:

[0089] The calculation formula of permeability coefficient is:

[0090]

[0091] Among them, N i is the steady-state flux of component i (cm 3 (STP)cm -2 s -1 ), l is the film thickness (cm), ΔP i is the partial pressure difference of component i (cmHg), P i is the permeability coefficient of component i, expressed in Barrer (1Barrer=10 -10 cm 3 (STP)·cm / (cm 2 ·s·cmHg));

[0092] Selectivity (α ij ) is calculated as follows:

[0093]

[0094] Among them, P i 、P j are the permeability coefficients of components i and j, respectively.

[0095] The test results of various properties of the separation membranes prepared in Examples 1-3 and Comparative Example 1 are shown in Table 1 below.

[0096] Table 1 Performance test data of separation membranes prepared in Examples 1-3 and Comparative Example 1

[0097]

[0098]

[0099] The performance test data of the separation membranes prepared in Examples 1-3 and Comparative Example 1 in Table 1 above were analyzed:

[0100] Compared with Comparative Example 1, the permeability of the separation membranes prepared in Examples 1 and 2 to He increased, but at the same time, the permeability of the separation membranes prepared in Examples 1 and 2 to CH4 also increased accordingly, resulting in He / CH4 selectivity that is not much different from Comparative Example 1, but both are relatively low. This is because the addition of Cl increases the permeability of the membrane to gas; the introduction of methyl groups can increase the solubility of the membrane to gas, ensure the diffusion, and thus increase the permeability of the membrane to gas. The permeability of the separation membrane prepared in Example 3 also increased, and while maintaining a relatively high permeability, the separation membrane prepared in Example 3 also had a higher selectivity. This is because the double fluorinated separation membrane prepared in Example 3 has more F elements that specifically recognize He, which increases the winning rate of He in the competition between He and CH4, thereby improving the He / CH4 separation performance of the separation membrane.

[0101] In addition, compared with Comparative Example 1, the fracture strength of the separation membranes prepared in Examples 2 and 3 increased, while the fracture strength of the separation membrane prepared in Example 1 decreased; the glass transition temperatures of the separation membranes prepared in Examples 1-3 all decreased, but were all above 200°C. The operating temperature of the separation membranes is generally 50-150°C, which meets the use requirements.

[0102] In addition, a long-term stability test was conducted on Example 3, and the results were as follows: Figure 4 After sufficient aging and compaction, the stable He permeability is about 1438 Barrer, and the He / CH4 selectivity can reach about 25. After 240 hours of operation, the high permeability and selectivity are still maintained.

[0103] It should be noted that the parts not described in the present invention can be implemented by adopting or drawing on existing technologies.

[0104] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the scope of protection of the present invention.

Claims

1. A helium-enriched gas separation membrane, characterized in that: The chemical structure of the gas separation membrane is shown below: Wherein, R is -CH3 or -Cl or -F, and n is the number of repeating units.

2. The method for preparing a helium-enriched gas separation membrane according to claim 1, wherein: Including steps: (1) adding a diamine monomer and a dianhydride monomer to a first solvent, stirring uniformly under a nitrogen atmosphere, then heating to 60-90° C., adding a catalyst and reacting for 2-4 hours, then heating to 100-130° C. and reacting for 1-3 hours, and finally heating to 160-200° C. and reacting for 5-8 hours to cause a polymerization reaction, thereby obtaining a mixed solution containing a polyimide polymer; (2) cooling the mixed solution of step (1) to room temperature, washing it multiple times with anhydrous ethanol and a second solvent to obtain a fibrous polyimide polymer, and drying it; (3) dissolving the polyimide polymer obtained in step (2) in a third solvent, defoaming the solution using a hydrophobic PTFE filter, slowly pouring the clarified solution into a culture dish, controlling the solution to evaporate slowly, and then using deionized water to assist in peeling the polyimide film from the culture dish, and drying the film to obtain a polyimide film.

3. The method for preparing a helium-enriched gas separation membrane according to claim 2, wherein: The diamine monomer in step (1) is one of 9,9-bis(3-methyl-4-aminophenyl)fluorene, 9,9-bis(4-amino-3-chlorophenyl)fluorene and 9,9-bis(4-amino-3-fluorophenyl)fluorene.

4. The method for preparing a helium-enriched gas separation membrane according to claim 2, wherein: In the step (1), the dianhydride monomer is hexafluorodianhydride.

5. The method for preparing a helium-enriched gas separation membrane according to claim 2, wherein: The molar ratio of the diamine monomer to the dianhydride monomer in step (1) is 1:1 to 1.

5.

6. The method for preparing a helium-enriched gas separation membrane according to claim 2, wherein: The catalyst in step (1) is at least one of isoquinoline, triethylamine and β-pyridine, and the mass concentration of the catalyst is 0.1-10%.

7. The method for preparing a helium-enriched gas separation membrane according to claim 2, wherein: The step (2) specifically comprises: after the mixed solution of step (1) is cooled to room temperature, slowly pouring it into anhydrous ethanol and continuously stirring it with a glass rod to precipitate the solution to form a fibrous polymer, soaking and washing it in ethanol for 2 to 4 hours and then separating it; dissolving the precipitate with a second solvent, and then pouring the solution into anhydrous ethanol again, repeating the dissolution and precipitation multiple times to remove residual substances; and then placing the fibrous polymer in a vacuum drying oven at 140 to 180° C. and drying it for 48 to 72 hours.

8. The method for preparing a helium-enriched gas separation membrane according to claim 2, wherein: In the step (1), the first solvent is at least one of toluene, o-xylene, m-xylene, p-xylene, o-cresol, m-cresol, and p-cresol.

9. The method for preparing a helium-enriched gas separation membrane according to claim 2, wherein: The second solvent in step (2) is at least one of acetone, acetonitrile, chloroform, tetrahydrofuran, and dichloromethane.

10. The method for preparing a helium-enriched gas separation membrane according to claim 2, characterized in that: The third solvent in step (3) is at least one of acetone, acetonitrile, chloroform, tetrahydrofuran, and dichloromethane.

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