Helium-enriched gas separation membrane and method of making same

Abstract

The application discloses a helium-enriched gas separation membrane and a preparation method thereof, and the method comprises the following steps: (1) adding diamine monomers and dianhydride monomers into a first solvent, then adding a catalyst and reacting for 2-4 hours after being heated to 60-90 DEG C, then reacting for 1-3 hours after being heated to 100-130 DEG C, and finally reacting for 5-8 hours after being heated to 160-200 DEG C, so that a polymerization reaction is generated to obtain a mixed solution containing a polyimide polymer; (2) cooling the mixed solution in the step (1) to room temperature, washing with anhydrous ethanol and a second solvent for multiple times to obtain a fibrous polyimide polymer and dry the fibrous polyimide polymer; (3) dissolving the polyimide polymer obtained in the step (2) with a third solvent, defoaming by using a hydrophobic PTFE filter, slowly pouring the clear solution into a culture dish, controlling slow evaporation of the solution, and then peeling the polyimide film from the culture dish to obtain a transparent polyimide film. The prepared separation membrane has high He / CH4 separation performance while maintaining high permeability.

Description

Technical Field

[0001] This invention relates to the field of gas separation membrane technology, specifically to a helium-enriched gas separation membrane and its preparation method. Background Technology

[0002] Helium, as an important strategic resource, is widely used in fields such as cryogenic superconductivity, semiconductor manufacturing, and aerospace. However, the helium content in natural gas is generally low (usually below 1%), and its efficient separation and enrichment has always been a technical challenge in the industry. Traditional helium extraction technology mainly relies 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 high-purity helium separation and recovery, it is extremely energy-intensive, especially when processing natural gas with low helium concentrations. The liquefaction and separation processes of non-helium components (such as methane) result in 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 ease of operation. Gas separation membrane technology achieves separation by utilizing the difference in permeation rates of gas molecules within the membrane material. However, 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 satisfy high helium selectivity and high permeation flux, resulting in limited separation efficiency. Typically, when the permeability of the membrane material is high, the selectivity decreases. Especially for the separation of helium from low-helium-concentration natural gas, a decrease in selectivity will significantly affect the helium separation effect, ultimately impacting the purity of the helium. Currently, although Chinese patent application CN113996193A discloses a copolyimide membrane, its preparation method, and its application in helium purification, which yields 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 reaches up to 800K (i.e., 527℃), which not only requires extremely high equipment temperature resistance but also results in high production energy consumption and difficulty in process control. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a helium-enriched gas separation membrane and its preparation method. The preparation method is relatively simple, and the prepared membrane can maintain high permeability while exhibiting high He / CH4 separation performance.

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

[0006] A helium-enriched gas separation membrane, wherein the general chemical structural formula of the gas separation membrane is shown below:

[0007]

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

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

[0010] (1) Add the diamine monomer and dianhydride monomer to the first solvent, stir evenly under a nitrogen atmosphere, then heat to 60-90℃, add the catalyst and react for 2-4 hours, then heat to 100-130℃ and react for 1-3 hours, and finally heat to 160-200℃ and react for 5-8 hours to produce a polymerization reaction and obtain a mixed solution containing polyimide polymer.

[0011] (2) Cool the mixed solution from step (1) to room temperature, wash it multiple times with anhydrous ethanol and a second solvent to obtain fibrous polyimide polymer and dry it.

[0012] (3) Dissolve the polyimide polymer obtained in step (2) using a third solvent, then use a hydrophobic PTFE filter to defoam, slowly pour the clear solution into a petri dish, control the slow evaporation of the solution, then use deionized water to help peel the polyimide film off the petri dish and dry it to obtain a transparent polyimide film.

[0013] Further, in step (1), the diamine monomer 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, in step (1), the dianhydride monomer is hexafluorodianhydride.

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

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

[0017] Further, step (2) specifically involves: after cooling the mixed solution from step (1) to room temperature, slowly pouring it into anhydrous ethanol and stirring continuously with a glass rod to precipitate the solution and form a fibrous polymer. After soaking and washing in ethanol for 2-4 hours, the polymer is separated. The precipitate is dissolved in a second solvent, and then the solution is poured into anhydrous ethanol again. The process of dissolving and precipitating is repeated multiple times to remove residual substances. Then, the fibrous polymer is placed in a vacuum drying oven at 140-180°C and dried for 48-72 hours.

[0018] Further, 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] Further, 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] This invention provides a helium-enriched gas separation membrane with a relatively simple preparation method. It is prepared by polycondensation reaction of hexafluorodianhydride and diamine monomers containing different substituent groups, exhibiting excellent film-forming properties. Furthermore, by selecting diamine monomers containing different substituent groups as reaction raw materials, this invention introduces substituent groups into the polyimide chain generated by the polymerization reaction of diamine monomers and dianhydride monomers. This allows for adjustment of the microporous structure of the polyimide membrane, thereby adjusting the permeability and selectivity of the polyimide membrane for helium. Especially for the double-fluorinated separation membrane, due to the presence of more F elements that specifically recognize He, the success rate of He in the competition between He and CH4 is increased. This allows the polyimide separation membrane to maintain high permeability while also exhibiting high He / CH4 selectivity, effectively improving the He / CH4 separation performance. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the polymerization reaction process of the present invention;

[0024] Figure 2 The images shown are atomic force microscope (AFM) and scanning electron microscope (SEM) images of the separation membranes prepared in Examples 1-3 and Comparative Example 1 of the present invention. Specifically, (a1), (b1), (c1), and (d1) are AFM images of the surface of the separation membranes prepared in Examples 1-3 and Comparative Example 1, respectively; (a2), (b2), (c2), and (d2) are SEM images of the surface of the separation membranes prepared in Examples 1-3 and Comparative Example 1, respectively; (a3), (b3), (c3), and (d3) are SEM images of the cross-section of the separation membranes prepared in Examples 1-3 and Comparative Example 1, respectively; and (a4), (b4), (c4), and (d4) are magnified views of the SEM images of the cross-section of the separation membranes prepared in Examples 1-3 and Comparative Example 1, respectively.

[0025] Figure 3 The figures show the hydrophobic properties of the separation membranes prepared in Examples 1-3 and Comparative Example 1 of this invention.

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

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

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

[0029] (1) Add the diamine monomer and dianhydride monomer to the first solvent, stir evenly under a nitrogen atmosphere, then heat to 60-90℃, add the catalyst and react for 2-4 hours, then heat to 100-130℃ and react for 1-3 hours, and finally heat to 160-200℃ and react for 5-8 hours to produce a polymerization reaction and obtain a mixed solution containing polyimide polymer.

[0030] (2) Cool the mixed solution from step (1) to room temperature, wash it multiple times with anhydrous ethanol and a second solvent to obtain fibrous polyimide polymer and dry it.

[0031] (3) Dissolve the polyimide polymer obtained in step (2) using a third solvent, then use a hydrophobic PTFE filter to defoam, slowly pour the clear solution into a petri dish, control the slow evaporation of the solution at room temperature, then use deionized water to help peel the polyimide film off the petri dish and dry it 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-di(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 by recrystallization before use.

[0033] In a preferred embodiment of the present invention, the molar ratio of diamine monomer to 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 diamine monomer, dianhydride monomer, first solvent, and catalyst.

[0035] In a preferred embodiment of the present invention, step (2) specifically involves: after the mixed solution from 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 and form a fibrous polymer. After soaking and washing in ethanol for 2-4 hours, the polymer is separated. The precipitate is dissolved in a second solvent, and then the solution is poured into anhydrous ethanol again. The process of dissolving and precipitating is repeated multiple times to remove residual substances. The fibrous polymer is then placed in a vacuum drying oven at 140-180°C and dried for 48-72 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 helium-enriched gas separation membrane prepared by the above method has the following general chemical structure formula:

[0040]

[0041] Where R is -CH3, -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 -CH3, -Cl, or -F substituents as a reaction raw material in the process of preparing a helium-enriched gas separation membrane. That is, -CH3, -Cl, or -F groups are introduced into the polyimide membrane generated by the polymerization reaction of the diamine monomer and the dianhydride monomer. This 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 can maintain a high permeability while also having a high He / CH4 selectivity.

[0043] Example 1

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

[0045] (1) The hexafluorodianhydride monomer and 9,9-di(3-methyl-4-aminophenyl)fluorene monomer were purified by recrystallization and dried in a vacuum drying oven at 60°C for 16 h.

[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-pyridyl)diphenylamine monomer was completely dissolved, 1.893 g of hexafluorodianhydride monomer was added and stirred for 1.5 h until the hexafluorodianhydride monomer was completely dissolved. Then the oil bath was heated to 75 °C, 0.3 mL of isoquinoline was added and reacted for 3 h. The oil bath was then heated to 115 °C and reacted for 2 h. Finally, the oil bath was heated to 180 °C and reacted for 7 h to obtain a mixed solution containing polyimide polymer.

[0047] (3) After the reaction is complete, the mixed solution containing the polyimide polymer in step (2) is cooled to room temperature, and washed multiple times with anhydrous ethanol and solvent to obtain fibrous polyimide polymer and dry it; 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 needs to be stirred continuously with a glass rod to make the solution precipitate and form fibrous polymer. After soaking and washing in ethanol for 3 hours, it is separated; the precipitate is dissolved in 30 mL of chloroform, and the solution is poured into 100 mL of anhydrous ethanol again. The dissolution and precipitation are repeated three times to remove residual substances; then the fibrous polyimide polymer is placed in a vacuum drying oven at 160℃ and dried for 60 hours to constant weight;

[0048] (4) Dissolve 0.15g of the polyimide obtained in step (3) in 5mL of chloroform and stir at room temperature for 4h to dissolve it. Then filter and defoam using a 0.45μm hydrophobic PTFE filter. Slowly pour the solution into a petri dish and control the solution to evaporate slowly. Use deionized water to help peel the polyimide film off the petri dish to obtain an isotropic transparent polyimide film with a thickness of about 30-40μm. Let it dry naturally and store it under vacuum.

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

[0050]

[0051] Example 2

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

[0053] (1) The 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 h.

[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-pyridyl)diphenylamine monomer was completely dissolved, 1.715 g of hexafluorodianhydride monomer was added and stirred for 1.5 h until the hexafluorodianhydride monomer was completely dissolved. Then the oil bath was heated to 75 °C, 0.3 mL of isoquinoline was added and reacted for 3 h. The oil bath was then heated to 115 °C and reacted for 2 h. Finally, the oil bath was heated to 180 °C and reacted for 7 h to obtain a mixed solution containing polyimide polymer.

[0055] (3) After the reaction is complete, the mixed solution containing the polyimide polymer in step (2) is cooled to room temperature, and washed multiple times with anhydrous ethanol and solvent to obtain fibrous polyimide polymer and dry it; 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 needs to be stirred continuously with a glass rod to make the solution precipitate and form fibrous polymer. After soaking and washing in ethanol for 3 hours, it is separated; the precipitate is dissolved in 30 mL of chloroform, and the solution is poured into 100 mL of anhydrous ethanol again. The dissolution and precipitation are repeated three times to remove residual substances; then the fibrous polyimide polymer is placed in a vacuum drying oven at 160℃ and dried for 60 hours to constant weight;

[0056] (4) Dissolve 0.15g of the polyimide obtained in step (3) in 5mL of chloroform and stir at room temperature for 4h to dissolve it. Then filter and defoam using a 0.45μm hydrophobic PTFE filter. Slowly pour the solution into a petri dish and control the solution to evaporate slowly. Use deionized water to help peel the polyimide film off the petri dish to obtain an isotropic transparent polyimide film with a thickness of about 30-40μm. Let it dry naturally and store it under vacuum.

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

[0058]

[0059] Example 3

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

[0061] (1) The 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 h.

[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-pyridyl)diphenylamine monomer was completely dissolved, 1.905 g of hexafluorodianhydride monomer was added and stirred for 1.5 h until the hexafluorodianhydride monomer was completely dissolved. Then the oil bath was heated to 75 °C, 0.3 mL of isoquinoline was added and reacted for 3 h. The oil bath was then heated to 115 °C and reacted for 2 h. Finally, the oil bath was heated to 180 °C and reacted for 7 h to obtain a mixed solution containing polyimide polymer.

[0063] (3) After the reaction is complete, the mixed solution containing the polyimide polymer in step (2) is cooled to room temperature, and washed multiple times with anhydrous ethanol and solvent to obtain fibrous polyimide polymer and dry it; 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 needs to be stirred continuously with a glass rod to make the solution precipitate and form fibrous polymer. After soaking and washing in ethanol for 3 hours, it is separated; the precipitate is dissolved in 30 mL of chloroform, and the solution is poured into 100 mL of anhydrous ethanol again. The dissolution and precipitation are repeated three times to remove residual substances; then the fibrous polyimide polymer is placed in a vacuum drying oven at 160℃ and dried for 60 hours to constant weight;

[0064] (4) Dissolve 0.15g of the polyimide obtained in step (3) in 5mL of chloroform and stir at room temperature for 4h to dissolve it. Then filter and defoam using a 0.45μm hydrophobic PTFE filter. Slowly pour the solution into a petri dish and control the solution to evaporate slowly. Use deionized water to help peel the polyimide film off the petri dish to obtain an isotropic transparent polyimide film with a thickness of about 30-40μm. Let it dry naturally and store it under vacuum.

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

[0066]

[0067] Example 4

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

[0069] (1) The 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 h.

[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 until the hexafluorodianhydride monomer was completely dissolved. Then the oil bath was heated to 90 °C, 0.25 mL of β-pyridine was added and reacted for 2 h. The oil bath was then heated to 130 °C and reacted for 1 h. Finally, the oil bath was heated to 200 °C and reacted for 5 h to obtain a mixed solution containing polyimide polymer.

[0071] (3) After the reaction is complete, the mixed solution containing the polyimide polymer in step (2) is cooled to room temperature, and washed multiple times with anhydrous ethanol and solvent to obtain fibrous polyimide polymer and dry it; 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 needs to be stirred continuously with a glass rod to precipitate the solution and form fibrous polymer. After soaking and washing in ethanol for 3 hours, it is separated; the precipitate is dissolved in 30 mL of acetonitrile, and the solution is poured into 100 mL of anhydrous ethanol again. The dissolution and precipitation are repeated three times to remove residual substances; then the fibrous polyimide polymer is placed in a vacuum drying oven at 180℃ and dried for 50 hours to constant weight;

[0072] (4) Dissolve 0.15g of the polyimide obtained in step (3) in 5mL of acetonitrile and stir at room temperature for 3h to dissolve it. Then filter and defoam using a 0.45μm hydrophobic PTFE filter. Slowly pour the solution into a petri dish and control the solution to evaporate slowly. Use deionized water to help peel the polyimide film off the petri dish to obtain an isotropic transparent polyimide film with a thickness of about 30-40μm. Let it dry naturally and store it under vacuum.

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

[0074]

[0075] Comparative Example 1

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

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

[0078] (2) 1.370 g of 4,4'-(9-pyridyl)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-pyridyl)diphenylamine monomer was completely dissolved, 1.747 g of hexafluorodianhydride monomer was added and stirred for 1.5 h until the hexafluorodianhydride monomer was completely dissolved. Then the oil bath was heated to 75 °C, isoquinoline was added and reacted for 3 h. The oil bath was then heated to 115 °C and reacted for 2 h. Finally, the oil bath was heated to 180 °C and reacted for 7 h to obtain a mixed solution containing polyimide polymer.

[0079] (3) After the reaction is complete, the mixed solution containing the polyimide polymer in step (2) is cooled to room temperature, and washed multiple times with anhydrous ethanol and solvent to obtain fibrous polyimide polymer and dry it; 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 needs to be stirred continuously with a glass rod to make the solution precipitate and form fibrous polymer. After soaking and washing in ethanol for 3 hours, it is separated; the precipitate is dissolved in 30 mL of chloroform, and the solution is poured into 100 mL of anhydrous ethanol again. The dissolution and precipitation are repeated three times to remove residual substances; then the fibrous polyimide polymer is placed in a vacuum drying oven at 160℃ and dried for 60 hours to constant weight;

[0080] (4) Dissolve 0.15g of the polyimide obtained in step (3) in 5mL of chloroform and stir at room temperature for 4h to dissolve it. Then filter and defoam using a 0.45μm hydrophobic PTFE filter. Slowly pour the solution into a petri dish and control the solution to evaporate slowly. Use deionized water to help peel the polyimide film off the petri dish to obtain an isotropic transparent polyimide film with a thickness of about 30-40μm. Dry and store under vacuum.

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

[0082]

[0083] The surface and cross-sectional morphology of the separation membranes prepared in Examples 1-3 and Comparative Example 1 were analyzed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). Figure 2 As shown. From Figure 2 As can be seen, the four types of polyimide films have different morphologies. Specifically, from... Figure 2 The atomic force microscopy images show that the molecular chains of the separation membrane in Comparative Example 1 have high rigidity, and the surface roughness is moderate due to localized accumulation during film formation. The Ra value of the separation membrane in Example 1 is 0.936 nm, which is due to topological fluctuations caused by methyl aggregation. Figure 2The white dotted areas in (a2) are shown; the introduction of chlorine in Example 2 significantly increased the roughness because the van der Waals radius of chlorine atoms is large, and its introduction disrupts the tight packing of molecular chains, forming a loose or twisted chain conformation. This disordered arrangement manifests as an uneven morphology on the surface; Example 3 achieved an ultra-low Ra value of 0.325 nm, which is due to the fact that double fluorination significantly enhances the regularity of the molecular chains. When crystalline regions are formed, surface defects (such as undulations in amorphous regions) are reduced, resulting in a smoother surface macroscopically. From Figure 2 The scanning electron microscope images show that the separation membranes prepared in Examples 1, 2, and Comparative Example 1 have relatively regular and smooth cross-sections, while the separation membrane prepared in Example 3 has a rougher cross-section. Combined with the corresponding magnified images, it can be seen that the separation membrane prepared in Example 3 has denser pores. This is because the low surface energy of fluorine promotes the stacking of molecular chains, and the high electronegativity enhances the intermolecular interactions, 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, such as... Figure 3 As shown. Figure 3 This indicates that the incorporation of substituents enhances the nonpolar 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 a solid surface exceeds 90°, an increase in surface roughness leads to an increase in the contact angle. Consistent with this principle, the separation membranes prepared in Examples 1 and 2 have high roughness, thus exhibiting a larger contact angle than Comparative Example 1. Although the fluorine substituent introduced in Example 3 and the chlorine substituent in Example 2 have similar chemical properties, the membrane in Example 3 exhibits significant hydrophobicity due to the superior electron-withdrawing ability and stronger CF bond energy of fluorine. These characteristics give the double-fluorinated membrane lower polarity and lower surface tension, ultimately achieving superior hydrophobic properties.

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

[0086] Mechanical property testing method: The film (thickness 30-40 μm) was cut into rectangular specimens of 10 mm × 5 mm. Using a HY-0580 microcomputer-controlled electronic universal testing machine, the specimens were stretched at 25℃ according to GB / T 1040.3-2006 standard at a rate of 5 mm / min. The stress-strain curves were recorded, and the tensile strength was calculated.

[0087] Glass transition temperature test method: The thermal stability of the film in the range of 25 to 500℃ was analyzed using a differential scanning calorimeter (DSC, Shimadzu DSC-60plush) under a nitrogen atmosphere at a heating rate of 5℃ / min.

[0088] Permeability and selectivity testing methods: The Wicke-Kallenbach method was used to test the gas permeability of a helium / methane mixture (V / V = 50 / 50). For the helium-enriched gas separation membrane test, the membrane was cut to an appropriate size and adhered to the center of the permeation cell using aluminum foil tape; the effective membrane area tested was 0.1256 cm². 2 The temperature was 30℃, the test pressure was 0.3MPa, the feed flow rate was set to 50mL / min, and argon was used as the purge gas at a flow rate of 25mL / min. Each gas was tested for a sufficient duration to ensure the system reached equilibrium. The definitions of permeability coefficient and mixture selectivity are as follows:

[0089] The formula for calculating the permeability coefficient is:

[0090]

[0091] Where, N i The steady-state flux (cm) of component i 3 (STP)cm -2 s -1 ), l is the film thickness (cm), ΔP i P is the partial pressure difference (cmHg) of component i. i Let be the permeability coefficient of component i, denoted by Barrer (1 Barrer = 10⁻⁶). -10 cm 3 (STP)·cm / (cm 2 ·s·cmHg));

[0092] Selectivity (α) ij The formula for calculating ) is:

[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 the 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 to Comparative Example 1, the permeability of the separation membranes prepared in Examples 1 and 2 increased, but the permeability of the separation membranes prepared in Examples 1 and 2 also increased accordingly, resulting in He / CH4 selectivity that was not much different from Comparative Example 1, but both were relatively low. This is because the addition of Cl increased the membrane's gas permeability; the introduction of methyl groups can increase the membrane's gas solubility, ensuring diffusion and thus increasing the membrane's gas permeability. The separation membrane prepared in Example 3 also showed increased He permeability, and while maintaining a relatively high permeability, the separation membrane prepared in Example 3 also had higher selectivity. This is because the double-fluorinated separation membrane prepared in Example 3 has more F elements that specifically recognize He, increasing the win 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 tensile strength of the separation membranes prepared in Examples 2 and 3 increased, while the tensile strength of the separation membrane prepared in Example 1 decreased. The glass transition temperature of the separation membranes prepared in Examples 1-3 decreased, but remained above 200°C. The operating temperature of these separation membranes is typically between 50-150°C, which meets the requirements for use.

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

[0103] It should be noted that any parts not mentioned in this invention can be achieved by using or referencing existing technologies.

[0104] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for preparing a helium-enriched gas separation membrane, characterized in that, The gas separation membrane is used to separate He / CH4; the general chemical formula of the gas separation membrane is shown below: Where R is -F, and n is the number of repeating units; The method for preparing the gas separation membrane includes the following steps: (1) Add the diamine monomer and dianhydride monomer to the first solvent, stir evenly under a nitrogen atmosphere, then heat to 60~90℃, add the catalyst and react for 2~4h, then heat to 100~130℃ and react for 1~3h, and finally heat to 160~200℃ and react for 5~8h to produce a polymerization reaction and obtain a mixed solution containing polyimide polymer; (2) Cool the mixed solution from step (1) to room temperature, wash it multiple times with anhydrous ethanol and a second solvent to obtain fibrous polyimide polymer and dry it; (3) Dissolve the polyimide polymer obtained in step (2) using a third solvent, then use a hydrophobic PTFE filter to defoam, slowly pour the clear solution into a petri dish, control the slow evaporation of the solution, then use deionized water to help peel the polyimide film off the petri dish, and dry it to obtain a polyimide film. In step (1), the diamine monomer is 9,9-bis(4-amino-3-fluorophenyl)fluorene; In step (1), the dianhydride monomer is hexafluorodianhydride.

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

5.

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

4. The method for preparing a helium-enriched gas separation membrane according to claim 1, characterized in that, The specific steps (2) are as follows: After the mixed solution in step (1) is cooled to room temperature, it is slowly poured into anhydrous ethanol and stirred continuously with a glass rod to precipitate the solution and form a fibrous polymer. After soaking and washing in ethanol for 2-4 hours, the polymer is separated. The precipitate is dissolved in a second solvent, and then the solution is poured into anhydrous ethanol again. The process of dissolving and precipitating is repeated multiple times to remove residual substances. Then the fibrous polymer is placed in a vacuum drying oven at 140-180℃ and dried for 48-72 hours.

5. The method for preparing a helium-enriched gas separation membrane according to claim 1, characterized in that, 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.

6. The method for preparing a helium-enriched gas separation membrane according to claim 1, characterized in that, In step (2), the second solvent is at least one of acetone, acetonitrile, chloroform, tetrahydrofuran, and dichloromethane.

7. The method for preparing a helium-enriched gas separation membrane according to claim 1, characterized in that, The third solvent in step (3) is at least one of acetone, acetonitrile, chloroform, tetrahydrofuran, and dichloromethane.

Images