Thermally cross-linkable polyimide gas separation membranes containing spirobifluorene structures and methods of making the same

Abstract

The present application relates to a kind of thermal oxygen crosslinking polyimide gas separation membrane comprising spirobifluorene structure and its preparation method.The present application uses the anhydride containing spirobifluorene (SBF) structure and the aromatic diamine comprising short-chain alkyl substituent as monomer, polyimide precursor film is obtained by polymerization, and the chain packing state and membrane pore structure are optimized by "thermal oxygen crosslinking" post-processing process, while significantly improving gas selectivity by moderately sacrificing permeation flux, surpassing the Robeson upper limit.The present application provides diversified design ideas for the molecular structure design, efficient preparation and separation performance optimization of polyimide gas separation membrane, and helps to promote its application in the field of separation and purification.

Description

Technical Field

[0001] This invention belongs to the field of gas separation and purification technology, specifically relating to a thermo-oxidative crosslinked polyimide gas separation membrane whose molecular backbone contains a spirobisfluorene structure and its preparation method. Background Technology

[0002] In gas separation technology, compared with traditional separation technologies such as cryogenic separation, pressure swing adsorption, and amine absorption, membrane separation technology has gradually gained attention and demonstrated great application potential and development prospects due to its advantages such as no need for complex equipment, low energy consumption, controllable cost, small footprint, flexible operation, and environmental friendliness. Polyimide (PI) has excellent comprehensive physical properties and can withstand complex working conditions such as high pressure and high temperature, and has received much attention in the field of gas separation membranes in recent years.

[0003] However, for a long time, there has been a trade-off between the permeation flux and selectivity of polymer gas separation membranes, meaning that high gas permeation flux is often accompanied by low selectivity, which severely limits the practical application of polymer gas separation membranes. Research has found that introducing functional units with rigid, tortuous structures into the polymer backbone can effectively suppress the close packing between polymer chain segments, forming internally connected and irregularly shaped free volume units. FVE s) (Reference) Advanced Materials (2004, 16, 456-459.). These units are similar in nature to micropores with a size of less than 2 nanometers, which significantly improves the gas separation performance of the material and breaks through the bottleneck problem of flux and selectivity being mutually constrained by traditional separation membranes. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a thermo-oxidative crosslinked polyimide gas separation membrane containing a spirobisfluorene structure and its preparation method. This invention introduces a rigid, twisted SBF structure and an aromatic structure containing short-chain alkyl substituents into the polyimide molecular chain, resulting in a polyimide film with excellent gas separation performance.

[0005] First, this invention provides a thermo-oxidative crosslinked polyimide gas separation membrane containing a spirobisfluorene structure, wherein the molecular structure of the gas separation membrane precursor before thermo-oxidative crosslinking has the following general formula:

[0006] (I) The molar fraction of the dicarboxylic acid anhydride A residue containing the spirobisfluorene (SBF) structure is 50%, the molar fraction of the aromatic diamine B residue containing the short-chain alkyl substituent is 50%, and m is an integer greater than 1.

[0007] According to one embodiment of the present invention, the dicarboxylic acid anhydride A containing the spirobisfluorene (SBF) structure is any one or a combination of the following compounds: .

[0008] According to one embodiment of the present invention, the aromatic diamine B containing a short-chain alkyl substituent is any one or a combination of the following compounds: .

[0009] The present invention also provides a method for preparing a thermo-oxidative crosslinked polyimide gas separation membrane containing a spirobisfluorene structure, wherein the preparation method comprises the following steps: Step 1: The dicarboxylic acid anhydride A containing the spirobisfluorene (SBF) structure and the aromatic diamine B containing short-chain alkyl substituents are vacuum treated at 80~150 °C for 6~12 h; Step 2: Under inert gas protection, add the diacid anhydride A containing the spirobisfluorene (SBF) structure treated in Step 1, the aromatic diamine B containing short-chain alkyl substituents, the catalyst C, and the high-boiling solvent D to the reactor in sequence. Stir and heat the reaction system to 50~100 °C and stir for 3~8 h. The total mass fraction of the dicarboxylic acid anhydride A containing a spirobisfluorene (SBF) structure and the aromatic diamine B containing short-chain alkyl substituents in the reaction system is 10 wt% to 30 wt%. The molar ratio of the dicarboxylic anhydride A containing the spirobisfluorene (SBF) structure to the aromatic diamine B containing short-chain alkyl substituents is 1:1; The molar ratio of the dicarboxylic anhydride A containing the spirobisfluorene (SBF) structure to the catalyst C is 1:1.5~4.0; Step 3: Under inert gas protection, add catalyst E and solubilizer F, heat the reaction system to 160~220℃, and continue stirring the reaction for 8~20 h; The amount of catalyst E is 5 wt% to 10 wt.% of the mass of the dicarboxylic anhydride A containing the spirobisfluorene (SBF) structure. The amount of solubilizer F is 50 wt% to 300 wt% of the mass of the dicarboxylic anhydride A containing the spirobisfluorene (SBF) structure. Step 4: When the system is cooled to 15~35 ℃, the reaction solution is poured into alcohol G to precipitate fibrous solid product. The product is filtered and the filter cake is washed with alcohol G 2~4 times. Then the filter cake is extracted with alcohol G for 8~24 h. Finally, the filter cake is vacuum dried at 80~160 ℃ for 12~24 h to obtain polymer H. Step 5: Dissolve polymer H in solvent J, with the solid content controlled at 0.5 wt%~15 wt%, filter, and then pour onto a clean glass plate that has been pre-leveled; place the glass plate in a vacuum oven at 25~80 ℃ to evaporate the solvent for 10~72 h, then raise the temperature to 80~160 ℃ and vacuum dry for 6~12 h; after the temperature drops to 20~35 ℃, remove the film to obtain the gas separation membrane precursor; Step 6: Place the gas separation membrane precursor in a tubular furnace equipped with a gas flow meter and an oxygen concentration sensor. Purge the tubular furnace continuously with an oxygen / nitrogen mixture for 10-30 min to stabilize the oxygen concentration (volume fraction) in the tubular furnace at a certain value. Close the gas flow valve and heat the tubular furnace to 300-450 ℃ at a heating rate of 1-10 ℃ / min, and hold at that temperature for 0.5-5 h. After the temperature drops to 20-35 ℃, remove the membrane to obtain the thermo-oxygen crosslinked SBF-PI-TC gas separation membrane material.

[0010] According to one embodiment of the present invention, the catalyst C in the preparation method is any one or a combination of benzoic acid, terephthalic acid, isophthalic acid, phthalic acid, p-methylbenzoic acid, m-methylbenzoic acid, o-methylbenzoic acid, p-ethylbenzoic acid, m-ethylbenzoic acid, o-ethylbenzoic acid, p-isopropylbenzoic acid, m-isopropylbenzoic acid, o-isopropylbenzoic acid, p-hydroxybenzoic acid, m-hydroxybenzoic acid, o-hydroxybenzoic acid, naphtholic acid, 4-phenylbenzoic acid, and 3-phenylbenzoic acid. According to one embodiment of the present invention, the high-boiling solvent D in the preparation method is any one or a combination of phenol, p-cresol, m-cresol, o-cresol, p-ethylphenol, m-ethylphenol, o-ethylphenol, 2,4-dimethylphenol, 2,3-dimethylphenol, 2,6-dimethylphenol, 2,5-dimethylphenol, 3,5-dimethylphenol, and 3,4-dimethylphenol. According to one embodiment of the present invention, the catalyst E in the preparation method is any one or a combination of tert-butylpyridine, quinoline, 6-methylquinoline, 4-methylquinoline, isoquinoline, and 8-methylisoquinoline; According to one embodiment of the present invention, the solubilizer F in the preparation method is any one or a combination of calcium chloride, lithium chloride, and lithium bromide; According to one embodiment of the present invention, alcohol G in the preparation method is any one or a combination of methanol, ethanol, and isopropanol; According to one embodiment of the present invention, the solvent J in the preparation method is any one or a combination of chloroform, dichloromethane, tetrahydrofuran, dioxane, acetonitrile, ethyl acetate, acetone, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, and γ-butyrolactone. According to one embodiment of the present invention, after the oxygen / nitrogen mixed gas stream purges the tube furnace in the preparation method, the oxygen volume fraction in the tube furnace is stabilized at 5%~10%.

[0011] Beneficial effects This invention, from the perspective of molecular structure design, develops a technical method for introducing the SBF structure along with short-chain alkyl substituents into the PI molecular backbone. Spirobifluorene (SBF) possesses a relatively rigid and twisted molecular structure, which is beneficial for reducing close packing between molecular chains and increasing micropores and ultramicropores (pore size <7 Å). By selecting anhydrides containing the SBF structure and introducing the spirobifluorene structure into the PI molecular chain, the gas permeability and selectivity of the membrane can be improved. Simultaneously, the thermo-oxidative crosslinking reaction of alkyl groups under aerobic conditions further optimizes the inter-chain packing state and membrane pore structure, significantly improving the overall gas separation performance of the PI membrane, exceeding the 2008 Robeson limit. The PI gas separation membrane prepared by this invention possesses the advantages of high permeation flux and high selectivity. The technical route proposed in this invention can provide diverse design ideas for the molecular structure design, efficient preparation, and separation performance optimization of PI gas separation membranes, contributing to the widespread application of PI gas separation membranes in the field of separation and purification.

[0012] This invention achieves precise control over the free volume and pore structure of the heat-treated separation membrane through fine optimization of the post-processing technology. The aim is to significantly improve gas selectivity by moderately sacrificing gas permeation flux within an acceptable range, thereby optimizing the overall separation performance and providing an effective technical solution to overcome the "trade-off" effect. Attached Figure Description

[0013] Figure 1 These are (a) thermogravimetric curves and (b) loss factor curves of uncrosslinked polyimide films prepared in several embodiments in an argon atmosphere. Figure 2 These are (a) stress-strain curves and (b) XRD patterns of uncrosslinked polyimide films prepared in several examples; Figure 3 Here are some images of the gas separation membranes prepared by thermo-oxidative crosslinking in several examples: (a) Example 9, (b) Example 10, (c) Example 11; Figure 4 These are (a) nitrogen adsorption-desorption curves and (b) pore size distribution diagrams of uncrosslinked polyimide powders prepared in several examples; Figure 5The following are comparisons of the CO2 / CH4 separation performance of gas separation membranes prepared in several examples with the upper limit of Robeson's algorithm: (a) Examples 1, 2, 3, 4; (b) Examples 5, 6, 7, 8; (c) Examples 9, 10, 11; (d) Examples 12, 13, 14. Detailed Implementation

[0014] The present invention is described in detail through the following embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection of the present invention.

[0015] Example 1 As a comparative example, this embodiment provides a method for preparing a polyimide film containing 6FDA and diamine 3, wherein the 6FDA and diamine 3 have the following structures: ,

[0016] Specifically, the following steps are included: 1.33 g of 6-FDA and 0.45 g of diamine 3 were vacuum-treated at 80 °C for 6 h. After cooling to room temperature, they were added sequentially to a 100 mL three-necked flask under argon protection, along with 1.25 g of terephthalic acid and 7.5 g of m-cresol. The reaction system was stirred and heated to 80 °C for 4 h. Subsequently, 0.1 g of 6-methylquinoline and 1.0 g of lithium bromide were added, and the reaction system was heated to 180 °C and stirred for another 16 h. When the system cooled to 30 °C, it was poured into ethanol to precipitate a fibrous solid product. The product was filtered, and the filter cake was washed three times with ethanol. The filter cake was then extracted with ethanol for 12 h, and finally dried under vacuum at 80 °C for 12 h to obtain 1.5 g of polymer (yield 84%). 0.6 g of the polymer was dissolved in 11.4 g of chloroform (5 w.t.% solids), filtered, and dripped onto a clean, leveled glass plate. The glass plate was placed in a vacuum oven at 30 °C to evaporate the solvent for 18 h, then the temperature was raised to 80 °C and vacuum dried for 6 h. After the temperature dropped to 25 °C, a polyimide precursor film was obtained. The gas separation membrane precursor was placed in a tube furnace equipped with a gas flow meter and an oxygen concentration sensor. The tube furnace was continuously purged with an oxygen / nitrogen mixture for 20 min to stabilize the oxygen concentration (volume fraction) at 5%. The gas flow valve was then closed, and the tube furnace was heated to 350 °C at a rate of 5 °C / min and held at that temperature for 1 h. After the temperature dropped to 25 °C, the film was removed to obtain the thermo-oxygen crosslinked gas separation membrane material. The gas separation performance of the separation membrane material prepared in this embodiment is shown in Table 1. Figure 5 As shown.

[0017] Example 2 As a comparative example, this embodiment provides a method for preparing a polyimide film containing BPAF and diamine 3, wherein BPAF and diamine 3 have the following structures: ,

[0018] Specifically, the following steps are included: 1.38 g BPAF and 0.45 g diamine 3 were vacuum-treated at 80 °C for 6 h. After cooling to room temperature, they were added sequentially to a 100 mL three-necked flask under argon protection, along with 1.25 g terephthalic acid and 7.5 g m-cresol. The reaction system was stirred and heated to 80 °C for 4 h. Subsequently, 0.1 g 6-methylquinoline and 1.0 g lithium bromide were added, and the reaction system was heated to 180 °C and stirred for another 16 h. When the system cooled to 30 °C, it was poured into ethanol to precipitate a fibrous solid product. The product was filtered, and the filter cake was washed three times with ethanol. The filter cake was then extracted with ethanol for 12 h, and finally dried under vacuum at 80 °C for 12 h to obtain 1.5 g of polymer (82% yield). 0.6 g of the polymer was dissolved in 11.4 g chloroform (5 w.t.% solids), filtered, and dripped onto a clean, leveled glass plate. The glass plate was placed in a vacuum oven at 30 °C to evaporate the solvent for 18 h, then the temperature was raised to 80 °C and vacuum dried for 6 h. After the temperature dropped to 25 °C, a polyimide precursor film was obtained. The gas separation membrane precursor was placed in a tube furnace equipped with a gas flow meter and an oxygen concentration sensor. The tube furnace was continuously purged with an oxygen / nitrogen mixture for 20 min to stabilize the oxygen concentration (volume fraction) at 5%. The gas flow valve was then closed, and the tube furnace was heated to 350 °C at a rate of 5 °C / min and held at that temperature for 1 h. After the temperature dropped to 25 °C, the film was removed to obtain the thermo-oxygen crosslinked gas separation membrane material. The gas separation performance of the separation membrane material prepared in this embodiment is shown in Table 1. Figure 5 As shown.

[0019] Example 3 This embodiment provides a method for preparing a polyimide film containing SBF-5 and diamine 3, wherein SBF-5 and diamine 3 have the following structures: ,

[0020] Specifically, the following steps are included: 1.47 g SBF-5 and 0.45 g diamine 3 were vacuum-treated at 80 °C for 6 h. After cooling to room temperature, they were added sequentially to a 100 mL three-necked flask under argon protection, along with 1.25 g terephthalic acid and 7.5 g m-cresol. The reaction system was stirred and heated to 80 °C for 4 h. Subsequently, 0.1 g 6-methylquinoline and 2.0 g lithium bromide were added, and the reaction system was heated to 180 °C and stirred for another 16 h. When the system cooled to 30 °C, it was poured into ethanol to precipitate a fibrous solid product. The product was filtered, and the filter cake was washed three times with ethanol. The filter cake was then extracted with ethanol for 12 h, and finally dried under vacuum at 80 °C for 12 h to obtain 1.7 g of polymer (88% yield). 0.6 g of the polymer was dissolved in 11.4 g chloroform (5 w.t.% solids), filtered, and dripped onto a clean, leveled glass plate. The glass plate was placed in a vacuum oven at 30 °C to evaporate the solvent for 18 h, then the temperature was raised to 80 °C and vacuum dried for 6 h. After the temperature dropped to 25 °C, a polyimide precursor film was obtained. The gas separation membrane precursor was placed in a tube furnace equipped with a gas flow meter and an oxygen concentration sensor. The tube furnace was continuously purged with an oxygen / nitrogen mixture for 20 min to stabilize the oxygen concentration (volume fraction) at 5%. The gas flow valve was then closed, and the tube furnace was heated to 350 °C at a rate of 5 °C / min and held at that temperature for 1 h. After the temperature dropped to 25 °C, the film was removed to obtain the thermo-oxygen crosslinked gas separation membrane material. The gas separation performance of the separation membrane material prepared in this embodiment is shown in Table 1. Figure 5 As shown.

[0021] Example 4 This embodiment provides a method for preparing a polyimide film containing SBF-2 and diamine 3, wherein SBF-2 and diamine 3 have the following structures: ,

[0022] Specifically, the following steps are included: 1.37 g SBF-2 and 0.45 g diamine 3 were vacuum-treated at 80 °C for 6 h. After cooling to room temperature, they were added sequentially to a 100 mL three-necked flask under argon protection, along with 1.25 g terephthalic acid and 7.5 g m-cresol. The reaction system was stirred and heated to 80 °C for 4 h. Subsequently, 0.1 g 6-methylquinoline and 2.0 g lithium bromide were added, and the reaction system was heated to 180 °C and stirred for another 16 h. When the system cooled to 30 °C, it was poured into ethanol to precipitate a fibrous solid product. The product was filtered, and the filter cake was washed three times with ethanol. The filter cake was then extracted with ethanol for 12 h, and finally dried under vacuum at 80 °C for 12 h to obtain 1.6 g of polymer (88% yield). 0.6 g of polymer was dissolved in 11.4 g chloroform (5 w.t.% solids), filtered, and dripped onto a clean, leveled glass plate. The glass plate was placed in a vacuum oven at 30 °C to evaporate the solvent for 18 h, then the temperature was raised to 80 °C and vacuum dried for 6 h. After the temperature dropped to 25 °C, a polyimide precursor film was obtained. The gas separation membrane precursor was placed in a tube furnace equipped with a gas flow meter and an oxygen concentration sensor. The tube furnace was continuously purged with an oxygen / nitrogen mixture for 20 min to stabilize the oxygen concentration (volume fraction) at 5%. The gas flow valve was then closed, and the tube furnace was heated to 350 °C at a rate of 5 °C / min and held at that temperature for 1 h. After the temperature dropped to 25 °C, the film was removed to obtain the thermo-oxygen crosslinked gas separation membrane material. The gas separation performance of the separation membrane material prepared in this embodiment is shown in Table 1. Figure 5 As shown.

[0023] Table 1

[0024] The results show that, compared with conventional 6FDA and BPAF, the membrane containing SBF structure has higher CO2 / CH4 selectivity and excellent overall gas separation performance.

[0025] Example 5 As a comparative example, this embodiment provides a method for preparing a polyimide film containing SBF-2 and diamine 17, wherein SBF-2 and diamine 17 have the following structures: ,

[0026] Specifically, the following steps are included: 1.37 g SBF-2 and 0.72 g diamine 17 were vacuum-treated at 80 °C for 6 h. After cooling to room temperature, they were added sequentially to a 100 mL three-necked flask under argon protection, along with 0.92 g benzoic acid and 7.5 g m-ethylphenol. The reaction system was stirred and heated to 80 °C for 4 h. Subsequently, 0.1 g quinoline and 2.0 g lithium bromide were added, and the reaction system was heated to 180 °C and stirred for another 16 h. When the system cooled to 30 °C, it was poured into methanol to precipitate a fibrous solid product. The product was filtered, and the filter cake was washed three times with methanol. The filter cake was then extracted with methanol for 12 h, and finally dried under vacuum at 80 °C for 12 h to obtain 1.8 g of polymer (86% yield). 0.6 g of the polymer was dissolved in 11.4 g chloroform (5 w.t.% solids), filtered, and dripped onto a clean, leveled glass plate. The glass plate was placed in a vacuum oven at 30 °C to evaporate the solvent for 18 h, and then the temperature was raised to 80 °C for vacuum drying for 6 h. After the temperature dropped to 25 °C, a polyimide film was obtained.

[0027] Example 6 As a comparative example, this embodiment provides a method for preparing a polyimide film containing SBF-2 and diamine 17, wherein SBF-2 and diamine 17 have the following structures: ,

[0028] Specifically, the following steps are included: 1.37 g SBF-2 and 0.72 g diamine 17 were vacuum-treated at 80 °C for 6 h. After cooling to room temperature, they were added sequentially to a 100 mL three-necked flask under argon protection, along with 0.92 g benzoic acid and 7.5 g m-ethylphenol. The reaction system was stirred and heated to 80 °C for 4 h. Subsequently, 0.1 g quinoline and 2.0 g lithium bromide were added, and the reaction system was heated to 180 °C and stirred for another 16 h. When the system cooled to 30 °C, it was poured into methanol to precipitate a fibrous solid product. The product was filtered, and the filter cake was washed three times with methanol. The filter cake was then extracted with methanol for 12 h, and finally dried under vacuum at 80 °C for 12 h to obtain 1.8 g of polymer (86% yield). 0.6 g of the polymer was dissolved in 11.4 g chloroform (5 w.t.% solids), filtered, and dripped onto a clean, leveled glass plate. The glass plate was placed in a vacuum oven at 30 °C to evaporate the solvent for 18 h, and then the temperature was raised to 80 °C for vacuum drying for 6 h. After the temperature dropped to 25 °C, a polyimide precursor film was obtained. The gas separation membrane precursor was placed in a tube furnace equipped with a gas flow meter and an oxygen concentration sensor. The tube furnace was continuously purged with an oxygen / nitrogen mixture for 20 min to stabilize the oxygen concentration (volume fraction) in the tube furnace at 20%. The gas flow valve was closed, and the temperature of the tube furnace was raised to 350 °C at a rate of 5 °C / min and held for 1 h. After the temperature dropped to 25 °C, the film was removed to obtain the thermo-oxygen crosslinked gas separation membrane material.

[0029] Example 7 This embodiment provides a method for preparing a polyimide film containing SBF-2 and diamine 17, the structures of which are shown in Example 6. Specifically, the method includes the following steps: 1.37 g SBF-2 and 0.72 g diamine 17 were vacuum-treated at 80 °C for 6 h. After cooling to room temperature, they were added sequentially to a 100 mL three-necked flask under argon protection, along with 0.92 g benzoic acid and 7.5 g m-ethylphenol. The reaction system was stirred and heated to 80 °C for 4 h. Subsequently, 0.1 g quinoline and 2.0 g lithium bromide were added, and the reaction system was heated to 180 °C and stirred for another 16 h. When the system cooled to 30 °C, it was poured into methanol to precipitate a fibrous solid product. The product was filtered, and the filter cake was washed three times with methanol. The filter cake was then extracted with methanol for 12 h, and finally dried under vacuum at 80 °C for 12 h to obtain 1.8 g of polymer (86% yield). 0.6 g of the polymer was dissolved in 11.4 g chloroform (5 w.t.% solids), filtered, and dripped onto a clean, leveled glass plate. The glass plate was placed in a vacuum oven at 30 °C to evaporate the solvent for 18 h, and then the temperature was raised to 80 °C for vacuum drying for 6 h. After the temperature dropped to 25 °C, a polyimide precursor film was obtained. The gas separation membrane precursor was placed in a tube furnace equipped with a gas flow meter and an oxygen concentration sensor. The tube furnace was continuously purged with an oxygen / nitrogen mixture for 20 min to stabilize the oxygen concentration (volume fraction) in the tube furnace at 5%. The gas flow valve was closed, and the temperature of the tube furnace was raised to 350 °C at a rate of 5 °C / min and held for 1 h. After the temperature dropped to 25 °C, the film was removed to obtain the thermo-oxygen crosslinked gas separation membrane material.

[0030] Example 8 As a comparative example, this embodiment provides a method for preparing a polyimide film containing SBF-2 and diamine 17, wherein the structures of SBF-2 and diamine 17 are as shown in Example 6. Specifically, the method includes the following steps: 1.37 g SBF-2 and 0.72 g diamine 17 were vacuum-treated at 80 °C for 6 h. After cooling to room temperature, they were added sequentially to a 100 mL three-necked flask under argon protection, along with 0.92 g benzoic acid and 7.5 g m-ethylphenol. The reaction system was stirred and heated to 80 °C for 4 h. Subsequently, 0.1 g quinoline and 2.0 g lithium bromide were added, and the reaction system was heated to 180 °C and stirred for another 16 h. When the system cooled to 30 °C, it was poured into methanol to precipitate a fibrous solid product. The product was filtered, and the filter cake was washed three times with methanol. The filter cake was then extracted with methanol for 12 h, and finally dried under vacuum at 80 °C for 12 h to obtain 1.8 g of polymer (86% yield). 0.6 g of the polymer was dissolved in 11.4 g chloroform (5 w.t.% solids), filtered, and dripped onto a clean, leveled glass plate. The glass plate was placed in a vacuum oven at 30 °C to evaporate the solvent for 18 h, and then the temperature was raised to 80 °C for vacuum drying for 6 h. After the temperature dropped to 25 °C, a polyimide precursor film was obtained. The gas separation membrane precursor was placed in a tube furnace equipped with a gas flow meter and an oxygen concentration sensor. The tube furnace was continuously purged with an oxygen / nitrogen mixture for 20 min to stabilize the oxygen concentration (volume fraction) in the tube furnace at 1%. The gas flow valve was closed, and the temperature of the tube furnace was raised to 350 °C at a rate of 5 °C / min and held for 1 h. After the temperature dropped to 25 °C, the film was removed to obtain the thermo-oxygen crosslinked gas separation membrane material.

[0031] Example 9 This embodiment provides a method for preparing a polyimide film containing SBF-1 and diamine 4, wherein SBF-1 and diamine 4 have the following structures: ,

[0032] Specifically, the following steps are included: 1.37 g SBF-1 and 0.49 g diamine 4 were vacuum-treated at 80 °C for 6 h. After cooling to room temperature, they were added sequentially to a 100 mL three-necked flask under argon protection, along with 1.02 g p-methylbenzoic acid and 8.0 g m-cresol. The reaction system was stirred and heated to 80 °C for 4 h. Subsequently, 0.1 g isoquinoline and 2.0 g calcium chloride were added, and the reaction system was heated to 180 °C and stirred for another 16 h. When the system cooled to 30 °C, it was poured into methanol to precipitate a fibrous solid product. The product was filtered, and the filter cake was washed three times with methanol. The filter cake was then extracted with methanol for 12 h, and finally dried under vacuum at 80 °C for 12 h to obtain 1.6 g of polymer (86% yield). 0.6 g of polymer was dissolved in 11.4 g chloroform (5 w.t.% solids), filtered, and dripped onto a clean, leveled glass plate. The glass plate was placed in a vacuum oven at 30 °C to evaporate the solvent for 18 h, and then the temperature was raised to 80 °C for vacuum drying for 6 h. After the temperature dropped to 25 °C, a polyimide precursor film was obtained. The gas separation membrane precursor was placed in a tube furnace equipped with a gas flow meter and an oxygen concentration sensor. The tube furnace was continuously purged with an oxygen / nitrogen mixture for 20 min to stabilize the oxygen concentration (volume fraction) in the tube furnace at 5%. The gas flow valve was closed, and the tube furnace was heated to 350 °C at a heating rate of 5 °C / min and held at that temperature for 1 h. After the temperature dropped to 25 °C, the film was removed to obtain the thermo-oxidative crosslinked gas separation membrane material. The polyimide powder prepared in this example was treated with the same thermo-oxidative crosslinking process. The BET specific surface area and pore volume of the treated polymer powder are shown in Table 2.

[0033] Example 10 This embodiment provides a method for preparing a polyimide film containing SBF-1 and diamine 4, wherein the structures of SBF-1 and diamine 4 are as shown in Example 9. Specifically, the method includes the following steps: 1.37 g SBF-1 and 0.49 g diamine 4 were vacuum-treated at 80 °C for 6 h. After cooling to room temperature, they were added sequentially to a 100 mL three-necked flask under argon protection, along with 1.02 g p-methylbenzoic acid and 8.0 g m-cresol. The reaction system was stirred and heated to 80 °C for 4 h. Subsequently, 0.1 g isoquinoline and 2.0 g calcium chloride were added, and the reaction system was heated to 180 °C and stirred for another 16 h. When the system cooled to 30 °C, it was poured into methanol to precipitate a fibrous solid product. The product was filtered, and the filter cake was washed three times with methanol. The filter cake was then extracted with methanol for 12 h, and finally dried under vacuum at 80 °C for 12 h to obtain 1.6 g of polymer (86% yield). 0.6 g of polymer was dissolved in 11.4 g chloroform (5 w.t.% solids), filtered, and dripped onto a clean, leveled glass plate. The glass plate was placed in a vacuum oven at 30 °C to evaporate the solvent for 18 h, and then the temperature was raised to 80 °C for vacuum drying for 6 h. After the temperature dropped to 25 °C, a polyimide precursor film was obtained. The gas separation membrane precursor was placed in a tube furnace equipped with a gas flow meter and an oxygen concentration sensor. The tube furnace was continuously purged with an oxygen / nitrogen mixture for 20 min to stabilize the oxygen concentration (volume fraction) in the tube furnace at 5%. The gas flow valve was closed, and the tube furnace was heated to 350 °C at a heating rate of 5 °C / min and held at that temperature for 2 h. After the temperature dropped to 25 °C, the film was removed to obtain the thermo-oxidative crosslinked gas separation membrane material. The polyimide powder prepared in this example was treated with the same thermo-oxidative crosslinking process. The BET specific surface area and pore volume of the treated polymer powder are shown in Table 2.

[0034] Example 11 This embodiment provides a method for preparing a polyimide film containing SBF-1 and diamine 4, wherein the structures of SBF-1 and diamine 4 are as shown in Example 9. Specifically, the method includes the following steps: 1.37 g SBF-1 and 0.49 g diamine 4 were vacuum-treated at 80 °C for 6 h. After cooling to room temperature, they were added sequentially to a 100 mL three-necked flask under argon protection, along with 1.02 g p-methylbenzoic acid and 8.0 g m-cresol. The reaction system was stirred and heated to 80 °C for 4 h. Subsequently, 0.1 g isoquinoline and 2.0 g calcium chloride were added, and the reaction system was heated to 180 °C and stirred for another 16 h. When the system cooled to 30 °C, it was poured into methanol to precipitate a fibrous solid product. The product was filtered, and the filter cake was washed three times with methanol. The filter cake was then extracted with methanol for 12 h, and finally dried under vacuum at 80 °C for 12 h to obtain 1.6 g of polymer (86% yield). 0.6 g of polymer was dissolved in 11.4 g chloroform (5 w.t.% solids), filtered, and dripped onto a clean, leveled glass plate. The glass plate was placed in a vacuum oven at 30 °C to evaporate the solvent for 18 h, and then the temperature was raised to 80 °C for vacuum drying for 6 h. After the temperature dropped to 25 °C, a polyimide precursor film was obtained. The gas separation membrane precursor was placed in a tube furnace equipped with a gas flow meter and an oxygen concentration sensor. The tube furnace was continuously purged with an oxygen / nitrogen mixture for 20 min to stabilize the oxygen concentration (volume fraction) in the tube furnace at 5%. The gas flow valve was closed, and the tube furnace was heated to 350 °C at a heating rate of 5 °C / min and held at that temperature for 5 h. After the temperature dropped to 25 °C, the film was removed to obtain the thermo-oxidative crosslinked gas separation membrane material. The polyimide powder prepared in this example was treated with the same thermo-oxidative crosslinking process. The BET specific surface area and pore volume of the treated polymer powder are shown in Table 2.

[0035] Table 2

[0036] Example 12 This embodiment provides a method for preparing a polyimide film containing SBF-1 and diamine 19, wherein SBF-1 and diamine 19 have the following structures: ,

[0037] Specifically, the following steps are included: 1.37 g SBF-1 and 0.72 g diamine 19 were vacuum-treated at 80 °C for 6 h. After cooling to room temperature, they were added sequentially to a 100 mL three-necked flask under argon protection, along with 1.02 g p-methylbenzoic acid and 8.0 g m-cresol. The reaction system was stirred and heated to 80 °C for 4 h. Subsequently, 0.1 g isoquinoline and 2.0 g calcium chloride were added, and the reaction system was heated to 180 °C and stirred for another 16 h. When the system cooled to 30 °C, it was poured into methanol to precipitate a fibrous solid product. The product was filtered, and the filter cake was washed three times with methanol. The filter cake was then extracted with methanol for 12 h, and finally dried under vacuum at 80 °C for 12 h to obtain 1.7 g of polymer (yield 81%). 0.6 g of the polymer was dissolved in 11.4 g chloroform (5 w.t.% solids), filtered, and dripped onto a clean, leveled glass plate. The glass plate was placed in a vacuum oven at 30 °C to evaporate the solvent for 18 h, and then the temperature was raised to 80 °C for vacuum drying for 6 h. After the temperature dropped to 25 °C, a polyimide precursor film was obtained. The gas separation membrane precursor was placed in a tube furnace equipped with a gas flow meter and an oxygen concentration sensor. The tube furnace was continuously purged with an oxygen / nitrogen mixture for 20 min to stabilize the oxygen concentration (volume fraction) in the tube furnace at 5%. The gas flow valve was closed, and the temperature of the tube furnace was raised to 350 °C at a rate of 5 °C / min and held for 1 h. After the temperature dropped to 25 °C, the film was removed to obtain the thermo-oxygen crosslinked gas separation membrane material.

[0038] Example 13 This embodiment provides a method for preparing a polyimide film containing SBF-6 and diamine 26, wherein SBF-6 and diamine 26 have the following structures: ,

[0039] Specifically, the following steps are included: 1.61 g of SBF-6 and 0.95 g of diamine 26 were treated under vacuum at 80 °C for 6 h. After cooling to room temperature, they were added sequentially to a 100 mL three-necked flask under argon protection, along with 1.02 g of p-methylbenzoic acid and 10.0 g of m-cresol. The reaction system was stirred and heated to 80 °C for 4 h. Subsequently, 0.1 g of isoquinoline and 2.0 g of calcium chloride were added, and the reaction system was heated to 180 °C and stirred for another 16 h. When the system cooled to 30 °C, it was poured into methanol to precipitate a fibrous solid product. The product was filtered, and the filter cake was washed three times with methanol. The filter cake was then extracted with methanol for 12 h, and finally dried under vacuum at 80 °C for 12 h to obtain 2.1 g of polymer (yield 82%). 0.6 g of the polymer was dissolved in 11.4 g of chloroform (5 w.t.% solids), filtered, and dripped onto a clean, leveled glass plate. The glass plate was placed in a vacuum oven at 30 °C to evaporate the solvent for 18 h, and then the temperature was raised to 80 °C for vacuum drying for 6 h. After the temperature dropped to 25 °C, a polyimide precursor film was obtained. The gas separation membrane precursor was placed in a tube furnace equipped with a gas flow meter and an oxygen concentration sensor. The tube furnace was continuously purged with an oxygen / nitrogen mixture for 20 min to stabilize the oxygen concentration (volume fraction) in the tube furnace at 10%. The gas flow valve was closed, and the temperature of the tube furnace was raised to 350 °C at a rate of 5 °C / min and held for 1 h. After the temperature dropped to 25 °C, the film was removed to obtain the thermo-oxygen crosslinked gas separation membrane material.

[0040] Example 14 This embodiment provides a method for preparing a polyimide film containing SBF-6 and diamine 27, wherein SBF-6 and diamine 27 have the following structures: ,

[0041] Specifically, the following steps are included: 1.61 g of SBF-6 and 1.03 g of diamine 27 were treated under vacuum at 80 °C for 6 h. After cooling to room temperature, they were added sequentially to a 100 mL three-necked flask under argon protection, along with 1.02 g of p-methylbenzoic acid and 10.0 g of m-cresol. The reaction system was stirred and heated to 80 °C for 4 h. Subsequently, 0.1 g of isoquinoline and 2.0 g of lithium chloride were added, and the reaction system was heated to 180 °C and stirred for another 16 h. When the system cooled to 30 °C, it was poured into methanol to precipitate a fibrous solid product. The product was filtered, and the filter cake was washed three times with methanol. The filter cake was then extracted with methanol for 12 h, and finally dried under vacuum at 80 °C for 12 h to obtain 2.2 g of polymer (yield 83%). 0.6 g of the polymer was dissolved in 11.4 g of chloroform (5 w.t.% solids), filtered, and dripped onto a clean, leveled glass plate. The glass plate was placed in a vacuum oven at 30 °C to evaporate the solvent for 18 h, and then the temperature was raised to 80 °C for vacuum drying for 6 h. After the temperature dropped to 25 °C, a polyimide precursor film was obtained. The gas separation membrane precursor was placed in a tube furnace equipped with a gas flow meter and an oxygen concentration sensor. The tube furnace was continuously purged with an oxygen / nitrogen mixture for 20 min to stabilize the oxygen concentration (volume fraction) in the tube furnace at 10%. The gas flow valve was closed, and the temperature of the tube furnace was raised to 350 °C at a rate of 5 °C / min and held for 1 h. After the temperature dropped to 25 °C, the film was removed to obtain the thermo-oxygen crosslinked gas separation membrane material.

[0042] Extensive experimental testing has confirmed that, compared to conventional 6FDA and BPAF, polyimides incorporating anhydrides with SBF structures exhibit stronger pore-forming ability and superior pore structure. After thermo-oxidative crosslinking, the SBF-containing membrane demonstrates higher CO2 permeability and CO2 / CH4 selectivity, resulting in excellent overall gas separation performance. Oxygen concentration significantly impacts the crosslinking effect; higher oxygen concentrations decrease gas permeability and increase selectivity, but do not significantly improve overall gas separation performance. Through extensive research and process control, this invention has found that crosslinking significantly improves the membrane's gas separation performance at an oxygen concentration of 5%–10%, meeting the standards for practical industrial applications.

Claims

1. A thermo-oxidative crosslinked polyimide gas separation membrane containing a spirobisfluorene structure, characterized in that... The molecular structure of the gas separation membrane precursor before thermo-oxidative crosslinking has the following general formula: (Ⅰ) The molar fraction of the dicarboxylic acid anhydride A residue containing the spirobisfluorene (SBF) structure is 50%, the molar fraction of the aromatic diamine B residue containing the short-chain alkyl substituent is 50%, and m is an integer greater than 1.

2. The thermo-oxidative crosslinked polyimide gas separation membrane containing a spirobisfluorene structure according to claim 1, wherein, The dicarboxylic acid anhydride A containing the spirobisfluorene (SBF) structure is any one or a combination of the following compounds, with the following chemical structural formula: 。 3. The thermo-oxidative crosslinked polyimide gas separation membrane containing a spirobisfluorene structure according to claim 1, wherein, The aromatic diamine B containing short-chain alkyl substituents is any one or a combination of the following compounds, and its chemical structural formula is: 。 4. A method for preparing a thermo-oxidative crosslinked polyimide gas separation membrane containing a spirobisfluorene structure as described in any one of claims 1-3, characterized in that, The preparation method includes the following steps: Step 1: The dicarboxylic acid anhydride A containing the spirobisfluorene (SBF) structure and the aromatic diamine B containing short-chain alkyl substituents are vacuum treated at 80~150 °C for 6~12 h; Step 2: Under inert gas protection, add the diacid anhydride A containing the spirobisfluorene (SBF) structure treated in Step 1, the aromatic diamine B containing short-chain alkyl substituents, the catalyst C, and the high-boiling solvent D to the reactor in sequence. Stir and heat the reaction system to 50~100 °C and stir for 3~8 h. The total mass fraction of the dicarboxylic acid anhydride A containing a spirobisfluorene (SBF) structure and the aromatic diamine B containing short-chain alkyl substituents in the reaction system is 10 wt% to 30 wt%. The molar ratio of the dicarboxylic anhydride A containing the spirobisfluorene (SBF) structure to the aromatic diamine B containing short-chain alkyl substituents is 1:1; The molar ratio of the dicarboxylic anhydride A containing the spirobisfluorene (SBF) structure to the catalyst C is 1:1.5~4.0; Step 3: Under inert gas protection, add catalyst E and solubilizer F, heat the reaction system to 160~220 ℃, and continue stirring the reaction for 8~20 h; The amount of catalyst E is 5 wt% to 10 wt.% of the mass of the dicarboxylic anhydride A containing the spirobisfluorene (SBF) structure. The amount of solubilizer F is 50 wt% to 300 wt.% of the mass of the dicarboxylic anhydride A containing the spirobisfluorene (SBF) structure. Step 4: When the system is cooled to 15~35 ℃, the reaction solution is poured into alcohol G to precipitate fibrous solid product. The product is filtered and the filter cake is washed with alcohol G 2~4 times. Then the filter cake is extracted with alcohol G for 8~24 h. Finally, the filter cake is vacuum dried at 80~160 ℃ for 12~24 h to obtain polymer H. Step 5: Dissolve polymer H in solvent J, with the solid content controlled at 0.5 wt%~15 wt%, filter, and then pour onto a clean glass plate that has been pre-leveled; place the glass plate in a vacuum oven at 25~80 ℃ to evaporate the solvent for 10~72 h, then raise the temperature to 80~160 ℃ and vacuum dry for 6~12 h; after the temperature drops to 20~35 ℃, remove the film to obtain the gas separation membrane precursor; Step 6: Place the gas separation membrane precursor in a tubular furnace equipped with a gas flow meter and an oxygen concentration sensor. Purge the tubular furnace continuously with an oxygen / nitrogen mixture for 10-30 min to stabilize the oxygen concentration (volume fraction) in the tubular furnace at a certain value. Close the gas flow valve and heat the tubular furnace to 300-450 ℃ at a heating rate of 1-10 ℃ / min, and hold at that temperature for 0.5-5 h. After the temperature drops to 20-35 ℃, remove the membrane to obtain the thermo-oxygen crosslinked SBF-PI-TC gas separation membrane material.

5. The method for preparing a thermo-oxidative crosslinked polyimide gas separation membrane containing a spirobisfluorene structure according to claim 4, wherein, In step 2, catalyst C is any one or a combination of benzoic acid, terephthalic acid, isophthalic acid, phthalic acid, p-methylbenzoic acid, m-methylbenzoic acid, o-methylbenzoic acid, p-ethylbenzoic acid, m-ethylbenzoic acid, o-ethylbenzoic acid, p-isopropylbenzoic acid, m-isopropylbenzoic acid, o-isopropylbenzoic acid, p-hydroxybenzoic acid, m-hydroxybenzoic acid, o-hydroxybenzoic acid, naphtholic acid, 4-phenylbenzoic acid, and 3-phenylbenzoic acid. The high-boiling-point solvent D is any one or a combination of phenol, p-cresol, m-cresol, o-cresol, p-ethylphenol, m-ethylphenol, o-ethylphenol, 2,4-dimethylphenol, 2,3-dimethylphenol, 2,6-dimethylphenol, 2,5-dimethylphenol, 3,5-dimethylphenol, and 3,4-dimethylphenol.

6. The method for preparing a thermo-oxidative crosslinked polyimide gas separation membrane containing a spirobisfluorene structure according to claim 4, wherein, In step 3, the catalyst E is any one or a combination of tert-butylpyridine, quinoline, 6-methylquinoline, 4-methylquinoline, isoquinoline, and 8-methylisoquinoline. The solubilizer F is any one of calcium chloride, lithium chloride, lithium bromide, or a combination thereof.

7. The method for preparing a thermo-oxidative crosslinked polyimide gas separation membrane containing a spirobisfluorene structure according to claim 4, wherein, In step 4, alcohol G is any one of methanol, ethanol, isopropanol, or a combination thereof.

8. The method for preparing a thermo-oxidative crosslinked polyimide gas separation membrane containing a spirobisfluorene structure according to claim 4, wherein, In step 5, the solvent J is any one or a combination of chloroform, dichloromethane, tetrahydrofuran, dioxane, acetonitrile, ethyl acetate, acetone, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, and γ-butyrolactone.

9. The method for preparing a thermo-oxidative crosslinked polyimide gas separation membrane containing a spirobisfluorene structure according to claim 4, wherein, In step 6, after the oxygen / nitrogen mixed gas stream purges the tubular furnace, the oxygen volume fraction inside the tubular furnace stabilizes at 5%~10%.

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