A triazine ring crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane selectivity and its preparation method

Abstract

This invention belongs to the field of gas separation membrane technology and discloses a triazine ring crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane selectivity and its preparation method. The triazine ring crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane uses a homogeneous blend of polyimides containing carboxyl groups as a precursor. A triazine ring diamine crosslinking agent is introduced into the polymer backbone through an amidation reaction to form a crosslinked polyimide with a uniform network structure. This is then converted into a nitrogen-rich carbon molecular sieve membrane through programmed pyrolysis. This method constructs a regular polymer network through crosslinking, forming a microporous-ultramicroporous structure with a concentrated pore size distribution during pyrolysis, thus improving size sieving capacity. Simultaneously, the introduction of triazine rings and amide bonds enriches the carbon membrane with active sites such as pyridine nitrogen and pyrrole nitrogen, which can enhance the selective adsorption of butadiene based on π-π interactions, thereby synergistically improving the membrane's selectivity for butadiene / butane, ultimately exhibiting separation performance far exceeding the upper limit.

Description

Technical Field

[0001] This invention belongs to the field of gas separation membrane technology, and relates to a triazine ring crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane selectivity and its preparation method. Background Technology

[0002] 1,3-Butadiene is an indispensable basic petrochemical raw material for the synthesis of high-performance rubbers such as nitrile rubber and styrene-butadiene rubber. It is widely used in the production of tires, automotive parts, and various rubber products. It is also a key precursor for the synthesis of high-performance polymer materials such as adipate esters. These downstream applications have extremely strict requirements for the purity of 1,3-butadiene, typically requiring a purity standard of over 99.5%.

[0003] However, the efficient purification of 1,3-butadiene from the C4 fraction, a byproduct of naphtha cracking, has been a long-standing technical challenge. This mixture fraction is complex, containing 1,3-butadiene, n-butane, isobutane, and various structurally similar olefin / alkane isomers such as 1-butene and cis / trans-2-butene. The extremely similar molecular sizes and boiling points of these components lead to bottlenecks in traditional extractive distillation methods based on relative volatility differences, including complex processes, high energy consumption, and extractant degradation and contamination. Therefore, developing a separation technology that combines high selectivity, low energy consumption, and simple process is crucial for the green upgrading of the butadiene industry.

[0004] Among numerous separation processes, membrane separation technology, especially gas separation membrane technology, stands out with its advantages of low energy consumption, simple operation, and easy modular integration, making it a potential solution to the aforementioned bottleneck problem in butadiene purification. Its core advantage lies in its ability to achieve molecular-scale sieving with lower energy consumption, avoiding the enormous energy consumption of traditional thermally integrated separation processes.

[0005] The core performance of membrane separation technology lies in the membrane material itself. Currently, industrially produced membrane materials are generally organic polymer membranes, such as polyethersulfone, polybenzimidazole, polyimide, and self-porous polymers. These polymer membranes exhibit good chemical and mechanical stability, are low-cost, and easy to mass-produce. However, their permeability and selectivity are often subject to a "trade-off" effect, making it difficult to achieve both high-throughput and high-precision separation in butadiene / butane systems. Furthermore, organic membranes are prone to plasticization, gradually losing selectivity. On the other hand, inorganic membranes, represented by molecular sieves, possess excellent sieving performance but are limited by high mechanical brittleness and difficulty in large-scale production. Addressing the characteristics of both organic and inorganic membranes, carbon molecular sieve membranes, derived from polymer precursors through pyrolysis, exhibit unique advantages. It organically combines the good processability of organic membrane precursors with the stability of inorganic carbon materials. Its micropore-ultramicropore pore size distribution can be finely controlled through precursor chemistry and pyrolysis processes, thus providing an ideal platform for separating C4 components with extremely similar sizes. It is regarded as one of the most promising membrane materials for overcoming this separation problem.

[0006] However, current reports on the separation of low-carbon olefins / alkanes using carbon molecular sieve membranes are limited to ethylene / ethane and propylene / propane. For the butadiene / butane system, only two articles have reported on this. In 1999, Okamoto et al. reported the use of sulfur-containing polyimide (DDBT-BPDA) as a precursor to prepare a carbon molecular sieve membrane for butadiene purification. This carbon membrane achieved a butadiene / n-butane selectivity of up to 51, confirming the feasibility of this application. However, its butadiene permeability was only 78 GPU (approximately 7.8 Barrer), indicating significant mass transfer resistance and limited flux, making it difficult to apply in practical production (Okamoto, et al., Ind. Eng. Chem. Res. 1999, 38, 4424.). In recent years, Ma et al. proposed a method for preparing carbon molecular sieve membranes through polymer blending: by blending a high-rigidity copolyimide with a fluorene group structure with a pore-forming polymer, and then carbonizing it through a specific pyrolysis procedure, a carbon membrane with a microporous-ultramicroporous structure was obtained, achieving a butadiene permeability of 3965.2 Barrer and a butadiene / n-butane selectivity of 44.7 (Chinese Invention Patent, Publication No.: CN120459818A). This blending approach effectively improves the problem of poor butadiene permeability of carbon membranes; however, due to problems such as uneven pore size distribution, its separation selectivity is low. Therefore, it is urgent to construct carbon molecular sieve membranes with high butadiene / butane selectivity through precursor design. Summary of the Invention

[0007] To address the poor selectivity of carbon molecular sieve membranes for butadiene / butane separation, a novel network polymer structure was constructed by introducing a triazine-containing diamine as a crosslinking agent into the precursor polymer. This diamine reacts with the carboxyl groups on the polyimide backbone. This network-crosslinked polyimide aims to optimize the pore structure of the final carbon membrane and enhance its molecular sieving capacity. Simultaneously, it significantly increases the content of pyridine and pyrrole nitrogen in the carbon membrane. These nitrogen sites can enhance the membrane's adsorption affinity for butadiene through specific intermolecular interactions (such as dipole-π and π-π interactions) with the π bonds in the 1,3-butadiene molecule, thereby greatly improving the adsorption selectivity of butadiene relative to butane at the molecular level. This fundamentally overcomes the bottleneck of insufficient selectivity of traditional membrane materials in this system.

[0008] The main content of this invention is to provide a nitrogen-rich carbon molecular sieve membrane for efficient separation of butadiene / butane and its preparation method. The core of the invention lies in: firstly, designing and synthesizing a novel carboxyl-containing homogeneous blended polyimide, where the carboxyl groups serve as uniformly distributed molecular anchoring sites; then, precisely introducing a specific triazine cyclodiamine crosslinking agent into the main chain via an amidation reaction to construct a stable network polymer structure; finally, transforming this precursor into a nitrogen-rich carbon molecular sieve membrane with a uniform microporous-ultramicroporous structure through a controllable carbonization process. This method aims to significantly improve the selective adsorption and separation capability of butadiene relative to butane by controlling the pore size and surface chemical properties of the carbon membrane through chemical structure design.

[0009] The technical solution of this invention: A triazine ring crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane selectivity is disclosed. The precursor of this triazine ring crosslinked polyimide membrane is composed of a uniformly blended polyimide A and a triazine cyclodiamine crosslinking agent B, wherein the uniformly blended polyimide A has the following repeating unit structure: ; Where m:n = 3:1 to 1:3, and the Ar structure is one of the following carboxyl diamine structures: ; Triazine cyclodiamine crosslinking agent B is any one of the following structures: ; The structure of the cross-linked triazine ring cross-linked polyimide is as follows: ; Where R is -H, -F or -N(CH3)2.

[0010] A method for preparing a triazine ring crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane selectivity, comprising the following steps: (1) Preparation of uniformly blended polyimide A: Diamine 9,9-bis(4-amino-3-fluorophenyl)fluorene was dissolved in m-cresol and transferred to a nitrogen-filled environment and stirred until completely dissolved; then hexafluorodianhydride was added, and after reacting in an ice bath for a certain time, diamine Ar was slowly added, the ice bath was removed, isoquinoline catalyst and azeotropic agent were added, and the reaction was raised to a certain temperature; after the reaction was completed, it was cooled to room temperature and collected in methanol to form a white filamentous product; after washing with methanol three times, it was vacuum dried at a certain temperature to obtain uniformly blended polyimide A; The molar ratio of the diamine 9,9-bis(4-amino-3-fluorophenyl)fluorene, the diamine Ar, and the hexafluorodianhydride is 3:1:4 to 1:3:4; The molar ratio of hexafluorodianhydride, isoquinoline, and azeotropic agent is 1:1:10; The ice bath reaction time is 12 hours; The azeotropic agent is any one of benzene, toluene, and o-xylene; The specified temperature is 150~185℃; The time period is 8 to 12 hours; The vacuum drying temperature is 120~160℃.

[0011] (2) Preparation of novel triazine cyclocrosslinked polyimide precursor membrane: The uniformly blended polyimide A and triazine cyclodiamine crosslinking agent B were mixed in a solvent and stirred until completely dissolved to prepare a casting solution of a certain concentration; then the casting solution system was transferred to an ice bath, a catalyst and additives were added, and after reacting for a certain time, it was cast into a smooth and flat petri dish and placed in a constant temperature drying room at a certain temperature to evaporate and form a film. After the film was formed, the film was immersed in a mixture of ethanol and water to wash away the side reactants. Finally, the washed film was vacuum dried to obtain the novel triazine cyclocrosslinked polyimide precursor membrane. The molar ratio of the uniformly blended polyimide A to the triazine cyclodiamine crosslinking agent B is 8:3 to 8:1; The solvent is DMAc or DMF; The total mass concentration of the casting solution is 5-10%; The catalyst and additives in the reaction system are 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl) and 1-hydroxybenzotriazole (HOBt), respectively. The molar ratio of carboxyl groups, catalyst, and additives in the uniformly blended polyimide A is 1:1.2:1.2; The specified time period is 8 to 10 hours; The temperature for constant temperature drying is 50~70℃; The vacuum drying temperature is 100~180℃; The thickness of the novel triazine cyclocrosslinked polyimide precursor film is 30~40 μm.

[0012] (3) Preparation of triazine ring cross-linked polyimide nitrogen-rich carbon molecular sieve membrane: The novel triazine ring cross-linked polyimide precursor membrane was placed between two smooth square corundum plates and pyrolyzed under an argon atmosphere according to a certain procedure to obtain a smooth and uniform triazine ring cross-linked polyimide nitrogen-rich carbon molecular sieve membrane.

[0013] The pyrolysis temperature is 500~900℃; The pyrolysis time is 2 hours.

[0014] The beneficial effects of this invention are as follows: This invention uses a uniform blending polymerization method to evenly distribute the "anchor" carboxyl groups within the polymer backbone. Through crosslinking, triazine cyclodiamine is introduced into the backbone to form a uniform network of polyimide precursors. During carbonization, the network of polyimide promotes the formation of a more uniformly distributed microporous structure, enhancing size sieving performance. Simultaneously, the nitrogen elements in the triazine ring and amide bond are converted into a large amount of pyridine nitrogen and pyrrole nitrogen during carbonization, enhancing the specific adsorption capacity for butadiene, ultimately improving the butadiene / butane separation selectivity. Furthermore, this invention investigates the effects of adjusting the anchor ratio, crosslinking agent ratio, nitrogen content, and carbonization temperature on the gas separation performance of the carbon molecular sieve membrane, and ultimately prepares a novel triazine cyclocrosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane separation selectivity. Attached Figure Description

[0015] Figure 1 This is a comparison of the butadiene / butane separation performance of the gas separation membranes prepared in the comparative examples and embodiments. Detailed Implementation

[0016] The specific embodiments of the present invention will be further described below in conjunction with the technical solution.

[0017] Example 1 A nitrogen-rich carbon molecular sieve membrane based on triazine cyclocrosslinked polyimide with high butadiene / butane selectivity is provided. The triazine cyclocrosslinked polyimide is prepared by uniformly blending polyimide PI-3,5-diaminobenzoic acid and triazine cyclodiamine crosslinking agent 2,4-diamino-1,3,5-triazine, crosslinking them at a molar ratio of 2:1 for carboxyl groups and triazine cyclodiamine crosslinking agent, and then pyrolyzing the membrane at 550°C.

[0018] This embodiment provides a triazine ring crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane selectivity, specifically including the following steps: Before the reaction began, the three-necked flask was purged with high-purity nitrogen for 30 minutes to remove water vapor and oxygen. Then, 5 mmol of 9,9-di(4-amino-3-fluorophenyl)fluorene diamine and 25 mL of m-cresol were added sequentially, and the mixture was stirred until dissolved and clear. Next, 10 mmol of hexafluorodianhydride was added, and the reaction was carried out in an ice bath for 12 hours. Then, 5 mmol of 3,5-diaminobenzoic acid was added, and the reaction was continued for 3 hours. Subsequently, 0.6 mL of isoquinoline catalyst was added, and the reaction system was heated to 185°C and maintained for 12 hours. After the reaction was complete, the mixture was allowed to cool naturally to room temperature, and the reaction solution was poured into methanol to precipitate a white filamentous product. This product was washed three times with methanol and dried in a vacuum oven at 150°C for 24 hours to completely remove residual solvent, finally yielding a white, uniformly blended polyimide filament.

[0019] 0.2 g of the synthesized white, uniformly blended filamentous polyimide was added to 6 g of DMAc solvent and stirred until completely dissolved. Then, 0.0084 g of 2,4-diamino-1,3,5-triazine was added and stirred until dissolved, followed by 0.0243 g of HOBt and stirred until dissolved. Under ice bath conditions, 0.0345 g of EDC·HCl was slowly added, and the reaction was carried out for 8 hours until viscous. The reaction system was filtered through a 0.45 μm filter membrane and cast into a smooth petri dish. The mixture was then placed in a 60°C vacuum drying oven to evaporate and form a film. After film formation, the film was washed with an ethanol / deionized water mixture for 24 hours, and then transferred to a 100°C oven for vacuum drying for 24 hours to obtain the novel triazine cyclic crosslinked polyimide precursor membrane PI-CL.

[0020] The obtained novel triazine cyclocrosslinked polyimide precursor membrane was placed between two sufficiently large, smooth corundum plates, and then placed in a tube furnace continuously purged with argon gas. The temperature was raised to 550℃ for 2 hours according to the program, and then cooled naturally to obtain a triazine cyclocrosslinked polyimide nitrogen-rich carbon molecular sieve membrane with a thickness of 27 μm, which is PI-CL-CMS550 carbon molecular sieve membrane.

[0021] The obtained PI-CL-CMS550 carbon molecular sieve membrane was tested for gas separation performance at 40℃ and 1.5 bar, where P(butadiene) = 4440.0 Barrer; selectivity α(butadiene / n-butane) = 121.2, α(butadiene / isobutane) = 1518.6.

[0022] Compared to Comparative Example 1, the difference lies in the addition of 2,4-diamino-1,3,5-triazine as a crosslinking agent to the precursor membrane. Crosslinking transforms the novel triazine cyclic crosslinked polyimide precursor membrane into a network structure, forming a more uniform ultraporous structure during carbonization. Therefore, compared to Comparative Example 1, the selectivity for butadiene / n-butane increased by 350%, while the selectivity for butadiene / isobutane increased by 194%. This example verifies that the synergistic modification method of optimizing the carbon membrane pore structure through crosslinking of the novel triazine cyclic crosslinked polyimide precursor membrane and introducing nitrogen-rich components to enhance the specific adsorption of butadiene can significantly improve the separation selectivity of carbon molecular sieve membranes for the butadiene / butane system.

[0023] Example 2 A triazine cyclocrosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane selectivity is provided. The triazine cyclocrosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane is prepared by uniformly blending polyimide PI-3,5-diaminobenzoic acid and triazine cyclodiamine crosslinking agent 2,4-diamino-1,3,5-triazine, crosslinking them at a molar ratio of carboxyl group to triazine cyclodiamine crosslinking agent of 2:1, and then pyrolyzing the membrane at 600°C.

[0024] This embodiment provides a triazine cyclocrosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane. The specific steps for uniformly blending polyimide and crosslinking triazine cyclodiamine are the same as in Example 1, except for the following carbonization steps: The obtained novel triazine cyclocrosslinked polyimide precursor membrane was placed between two sufficiently large, smooth corundum plates, and then placed in a tube furnace continuously purged with argon gas. The temperature was raised to 600℃ for 2 hours according to the program, and then allowed to cool naturally. Finally, a triazine cyclocrosslinked polyimide nitrogen-rich carbon molecular sieve membrane with a thickness of 28 μm was obtained, which is PI-CL-CMS600 carbon molecular sieve membrane.

[0025] The obtained triazine ring crosslinked polyimide nitrogen-rich carbon molecular sieve membrane was tested for gas separation performance at 40℃ and 1.5 bar, where P(butadiene) = 4353.0 Barrer; selectivity α(butadiene / n-butane) = 141.9, α(butadiene / isobutane) = 1817.2.

[0026] Compared to Comparative Example 2, the difference lies in the addition of 2,4-diamino-1,3,5-triazine as a crosslinking agent in the precursor membrane. Crosslinking transforms the novel triazine cyclic crosslinked polyimide precursor membrane into a network structure, forming a more uniform ultraporous structure during carbonization. Therefore, compared to Comparative Example 2, the butadiene / n-butane selectivity increased by 107%, while the butadiene / isobutane selectivity increased by 94%. Compared to Comparative Example 5, the butadiene / n-butane selectivity increased by 215%, and the butadiene / isobutane selectivity increased by 164%. These data validate the effectiveness of the modification method—introducing molecular anchors to construct a uniformly crosslinked network of novel triazine cyclic crosslinked polyimide precursor membranes—in improving the butadiene / butane selectivity of the carbon membrane. Compared to Comparative Example 6, butadiene permeability increased by 100%, and the butadiene / n-butane and butadiene / isobutane selectivities increased by 4% and 22%, respectively. This result confirms that the novel triazine cyclocrosslinked polyimide precursor membrane formed by crosslinking will produce denser channels after carbonization, which may limit the gas permeation rate. However, by introducing distorted large-volume rigid groups into the novel triazine cyclocrosslinked polyimide precursor membrane, the tight packing of molecular chains can be effectively inhibited and the gas transport channels can be widened, thereby significantly improving gas permeability while maintaining high selectivity.

[0027] Example 3 A triazine cyclocrosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane selectivity is provided. The triazine cyclocrosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane is prepared by uniformly blending polyimide PI-3,5-diaminobenzoic acid and triazine cyclodiamine crosslinking agent 2,4-diamino-1,3,5-triazine, crosslinking them at a molar ratio of carboxyl group to triazine cyclodiamine crosslinking agent of 2:1, and then pyrolyzing the membrane at 700°C.

[0028] This embodiment provides a triazine cyclocrosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane. The specific steps for uniformly blending polyimide and crosslinking triazine cyclodiamine are the same as in Example 1, except for the following carbonization steps: The obtained novel triazine cyclocrosslinked polyimide precursor membrane was placed between two sufficiently large, smooth corundum plates, and then placed in a tube furnace continuously purged with argon gas. The temperature was raised to 700℃ for pyrolysis for 2 hours according to the program, and then cooled naturally to finally obtain a 25μm thick triazine cyclocrosslinked polyimide nitrogen-rich carbon molecular sieve membrane, namely PI-CL-CMS700 carbon molecular sieve membrane.

[0029] The obtained triazine ring crosslinked polyimide nitrogen-rich carbon molecular sieve membrane was tested for gas separation performance at 40℃ and 1.5 bar, where P(butadiene) = 1598.8 Barrer; selectivity α(butadiene / n-butane) = 162.8, α(butadiene / isobutane) = 2084.0.

[0030] Compared to Comparative Example 3, the difference lies in the addition of 2,4-diamino-1,3,5-triazine as a crosslinking agent to the precursor membrane, thereby forming a more uniform ultraporous structure during carbonization. Therefore, compared to Comparative Example 3, the butadiene / n-butane selectivity increased by 61%, while the butadiene / isobutane selectivity increased by 25%. This example verifies that the synergistic modification method of optimizing the carbon membrane pore structure through crosslinking of a novel triazine cyclic crosslinked polyimide precursor membrane and introducing nitrogen-rich components to enhance the specific adsorption of butadiene can significantly improve the separation selectivity of a 700℃ carbon molecular sieve membrane for the butadiene / butane system.

[0031] Example 4 A triazine cyclocrosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane selectivity is provided. The triazine cyclocrosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane is prepared by uniformly blending polyimide PI-3,5-diaminobenzoic acid and triazine cyclodiamine crosslinking agent 2,4-diamino-1,3,5-triazine, crosslinking them at a molar ratio of carboxyl group to triazine cyclodiamine crosslinking agent of 2:1, and then pyrolyzing the membrane at 800°C.

[0032] This embodiment provides a triazine cyclocrosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane. The specific steps for uniformly blending polyimide and crosslinking triazine cyclodiamine are the same as in Example 1, except for the following carbonization steps: The obtained novel triazine cyclocrosslinked polyimide precursor membrane was placed between two sufficiently large, smooth corundum plates, and then placed in a tube furnace continuously purged with argon gas. The temperature was raised to 800℃ and pyrolyzed for 2 hours according to a program, and then cooled naturally to obtain a 26μm thick triazine cyclocrosslinked polyimide nitrogen-rich carbon molecular sieve membrane, which is PI-CL-CMS800 carbon molecular sieve membrane.

[0033] The obtained triazine ring crosslinked polyimide nitrogen-rich carbon molecular sieve membrane was tested for gas separation performance at 40℃ and 1.5 bar, where P(butadiene) = 167.4 Barrer; selectivity α(butadiene / n-butane) = 267.6, α(butadiene / isobutane) = 3451.4.

[0034] Compared to Comparative Example 4, the difference lies in the addition of 2,4-diamino-1,3,5-triazine as a crosslinking agent to the precursor membrane, thereby forming a more uniform ultraporous structure during carbonization. Therefore, compared to Comparative Example 4, the butadiene / n-butane selectivity increased by 130%, while the butadiene / isobutane selectivity increased by 52%. This example verifies that the synergistic modification method of optimizing the carbon membrane pore structure through crosslinking of a novel triazine cyclic crosslinked polyimide precursor membrane and introducing nitrogen-rich components to enhance the specific adsorption of butadiene can significantly improve the separation selectivity of the 800℃ carbon molecular sieve membrane for the butadiene / butane system.

[0035] Example 5 A triazine cyclocrosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane selectivity is disclosed. The membrane is prepared by uniformly blending polyimide PI-3,5-diaminobenzoic acid and triazine cyclodiamine crosslinking agent 2-fluoro-4,6-diamino-1,3,5-triazine, crosslinking them at a molar ratio of 2:1 for carboxyl groups and triazine cyclodiamine crosslinking agent, and then pyrolyzing the mixture at 600°C.

[0036] This embodiment provides a triazine ring crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane, specifically including the following steps: Before the reaction began, the three-necked flask was purged with high-purity nitrogen for 30 minutes to remove water vapor and oxygen. Then, 5 mmol of 9,9-di(4-amino-3-fluorophenyl)fluorene diamine and 25 mL of m-cresol were added sequentially, and the mixture was stirred until dissolved and clear. Next, 10 mmol of hexafluorodianhydride was added, and the reaction was carried out in an ice bath for 12 hours. Then, 5 mmol of 3,5-diaminobenzoic acid was added, and the reaction was continued for 3 hours. Subsequently, 0.6 mL of isoquinoline catalyst was added, and the reaction system was heated to 185°C and maintained for 12 hours. After the reaction was complete, the mixture was allowed to cool naturally to room temperature, and the reaction solution was poured into methanol to precipitate a white filamentous product. This product was washed three times with methanol and dried in a vacuum oven at 150°C for 24 hours to completely remove residual solvent, finally yielding a white, uniformly blended polyimide filament.

[0037] 0.2 g of the synthesized white filamentous homogeneous polyimide was added to 6 g of DMAc solvent and stirred until completely dissolved. Then, 0.0089 g of 2-fluoro-4,6-diamino-1,3,5-triazine was added and stirred until dissolved. Next, 0.0243 g of HOBt was added and stirred until dissolved. Under ice bath conditions, 0.0345 g of EDC·HCl was slowly added, and the reaction was carried out for 8 hours until viscous. The reaction system was filtered through a 0.45 μm filter membrane and cast into a smooth petri dish. The mixture was then placed in a 60 °C vacuum drying oven to evaporate and form a film. After film formation, the film was washed with an ethanol / deionized water mixture for 24 hours. Following washing, the film was transferred to a 100 °C oven and vacuum dried for 24 hours to obtain the novel triazine cyclic crosslinked polyimide precursor membrane PI-F-CL.

[0038] The obtained novel triazine cyclocrosslinked polyimide precursor membrane was placed between two sufficiently large, smooth corundum plates, and then placed in a tube furnace continuously purged with argon gas. The temperature was raised to 600℃ for 2 hours according to the program, and then cooled naturally to obtain a triazine cyclocrosslinked polyimide nitrogen-rich carbon molecular sieve membrane with a thickness of 31μm, which is PI-F-CL-CMS600 carbon molecular sieve membrane.

[0039] The obtained triazine ring crosslinked polyimide nitrogen-rich carbon molecular sieve membrane was tested for gas separation performance at 40℃ and 1.5 bar, where P(butadiene) = 4193.7 Barrer; selectivity α(butadiene / n-butane) = 127.3, α(butadiene / isobutane) = 1709.2.

[0040] Compared to Comparative Example 2, the difference lies in the addition of 2-fluoro-4,6-diamino-1,3,5-triazine as a crosslinking agent to the precursor membrane, resulting in a uniform ultraporous structure of the triazine-crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane. Therefore, compared to Comparative Example 2, the butadiene / n-butane selectivity increased by 85%, and the butadiene / isobutane selectivity increased by 83%. However, compared to Example 2 with 2,4-diamino-1,3,5-triazine crosslinking, butadiene permeability decreased by 4%, butadiene / n-butane selectivity decreased by 10%, and butadiene / isobutane selectivity decreased by 18%.

[0041] Example 6 A triazine cyclocrosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane selectivity is provided. The triazine cyclocrosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane is prepared by uniformly blending polyimide PI-3,5-diaminobenzoic acid and triazine cyclodiamine crosslinking agent 2,4-diamino-6-dimethylamino-1,3,5-triazine, crosslinking them at a molar ratio of carboxyl group to triazine cyclodiamine crosslinking agent of 2:1, and then pyrolyzing the membrane at 600°C.

[0042] This embodiment provides a novel triazine triazine ring-crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane, specifically including the following steps: Before the reaction began, the three-necked flask was purged with high-purity nitrogen for 30 minutes to remove water vapor and oxygen. Then, 5 mmol of 9,9-di(4-amino-3-fluorophenyl)fluorene diamine and 25 mL of m-cresol were added sequentially, and the mixture was stirred until dissolved and clear. Next, 10 mmol of hexafluorodianhydride was added, and the reaction was carried out in an ice bath for 12 hours. Then, 5 mmol of 3,5-diaminobenzoic acid was added, and the reaction was continued for 3 hours. Subsequently, 0.6 mL of isoquinoline catalyst was added, and the reaction system was heated to 185°C and maintained for 12 hours. After the reaction was complete, the mixture was allowed to cool naturally to room temperature, and the reaction solution was poured into methanol to precipitate a white filamentous product. This product was washed three times with methanol and dried in a vacuum oven at 150°C for 24 hours to completely remove residual solvent, finally yielding a white, uniformly blended polyimide filament.

[0043] 0.2 g of the synthesized white filamentous homogeneous polyimide was added to 6 g of DMAc solvent and stirred until completely dissolved. Then, 0.0097 g of 2,4-diamino-6-dimethylamino-1,3,5-triazine was added and stirred until dissolved. Next, 0.0243 g of HOBt was added and stirred until dissolved. Under ice bath conditions, 0.0345 g of EDC·HCl was slowly added, and the reaction was carried out for 8 hours under ice bath conditions until viscous. The reaction system was filtered through a 0.45 μm filter membrane and cast into a smooth petri dish. The mixture was then placed in a 60 °C vacuum drying oven to evaporate and form a film. After film formation, the film was washed with an ethanol / deionized water mixture for 24 hours. After washing, the film was transferred to a 100 °C oven and vacuum dried for 24 hours to obtain the novel triazine cyclic crosslinked polyimide precursor membrane PI-N-CL.

[0044] The obtained novel triazine cyclocrosslinked polyimide precursor membrane was placed between two sufficiently large, smooth corundum plates, and then placed in a tube furnace continuously purged with argon gas. The temperature was raised to 600℃ for pyrolysis for 2 hours according to the program, and then cooled naturally to finally obtain a 26μm thick triazine cyclocrosslinked polyimide nitrogen-rich carbon molecular sieve membrane, which is a PI-N-CL-CMS600 carbon molecular sieve membrane.

[0045] The obtained triazine ring crosslinked polyimide nitrogen-rich carbon molecular sieve membrane was tested for gas separation performance at 40℃ and 1.5 bar, where P(butadiene) = 3672.0 Barrer; selectivity α(butadiene / n-butane) = 143.8, α(butadiene / isobutane) = 1872.5.

[0046] Compared to Comparative Example 2, the difference lies in the addition of 2,4-diamino-6-dimethylamino-1,3,5-triazine as a crosslinking agent to the precursor membrane, resulting in a uniform ultraporous structure and higher nitrogen enrichment in the triazine cyclic crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane. Therefore, compared to Comparative Example 2, the butadiene / n-butane selectivity increased by 109%, while the butadiene / isobutane selectivity increased by 100%. However, compared to Example 2 with 2,4-diamino-1,3,5-triazine crosslinking, butadiene permeability decreased by 16%, butadiene / n-butane selectivity increased by 1%, and butadiene / isobutane selectivity increased by 3%, demonstrating that the higher nitrogen content of the novel triazine cyclic crosslinked polyimide precursor membrane can lead to higher butadiene / butane selectivity in the carbon membrane.

[0047] Example 7 A nitrogen-rich carbon molecular sieve membrane based on triazine cyclocrosslinked polyimide with high butadiene / butane selectivity is provided. The triazine cyclocrosslinked polyimide is prepared by uniformly blending polyimide PI-4,4'-diaminobiphenyl-2,2-dicarboxylic acid and triazine cyclodiamine crosslinking agent 2,4-diamino-1,3,5-triazine at a molar ratio of 4:1 for carboxyl groups and triazine cyclodiamine crosslinking agent, and then pyrolyzing the membrane at 600°C.

[0048] This embodiment provides a triazine ring crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane selectivity, specifically including the following steps: Before the reaction began, the three-necked flask was purged with high-purity nitrogen for 30 minutes to remove water vapor and oxygen. Then, 5 mmol of 9,9-di(4-amino-3-fluorophenyl)fluorene diamine and 25 mL of m-cresol were added sequentially, and the mixture was stirred until dissolved and clear. Next, 10 mmol of hexafluorodianhydride was added, and the reaction was carried out in an ice bath for 12 hours. Then, 5 mmol of 4,4'-diaminobiphenyl-2,2-dicarboxylic acid was added, and the reaction was continued for 3 hours. Subsequently, 0.6 mL of isoquinoline catalyst was added, and the reaction system was heated to 160 °C and maintained for 8 hours. After the reaction was complete, the mixture was allowed to cool naturally to room temperature, and the reaction solution was poured into methanol to precipitate a white filamentous product. This product was washed three times with methanol and dried in a vacuum oven at 120 °C for 24 hours to completely remove residual solvent, finally yielding a white, uniformly blended polyimide filament.

[0049] 0.2 g of the synthesized white filamentous homogeneous polyimide was added to 6 g of DMAc solvent and stirred until completely dissolved. Then, 0.0168 g of 2,4-diamino-1,3,5-triazine was added and stirred until dissolved, followed by 0.0486 g of HOBt and stirred until dissolved. Under ice bath conditions, 0.069 g of EDC·HCl was slowly added, and the reaction was carried out for 10 hours until viscous. The reaction system was filtered through a 0.45 μm filter membrane and cast into a smooth petri dish. The mixture was then placed in a 70°C vacuum drying oven to evaporate and form a film. After film formation, the film was washed with an ethanol / deionized water mixture for 24 hours, and then transferred to a 100°C oven for vacuum drying for 24 hours to obtain the novel triazine cyclic crosslinked polyimide precursor membrane PI-4,4'-diaminobiphenyl-2,2-dicarboxylic acid-CL.

[0050] The obtained novel triazine cyclocrosslinked polyimide precursor membrane was placed between two sufficiently large, smooth corundum plates, and then placed in a tube furnace continuously purged with argon gas. The temperature was raised to 600℃ for 2 hours according to the program, and then cooled naturally to obtain a triazine cyclocrosslinked polyimide nitrogen-rich carbon molecular sieve membrane with a thickness of 30 μm, which is PI-4,4'-diaminobiphenyl-2,2-dicarboxylic acid-CL-CMS600 carbon molecular sieve membrane.

[0051] The obtained PI-4,4'-diaminobiphenyl-2,2-dicarboxylic acid-CL-CMS600 carbon molecular sieve membrane was tested for gas separation performance at 40℃ and 1.5 bar, where P(butadiene) = 2870.0 Barrer; selectivity α(butadiene / n-butane) = 151.7, α(butadiene / isobutane) = 1681.5.

[0052] Example 8 A nitrogen-rich carbon molecular sieve membrane based on triazine cyclocrosslinked polyimide with high butadiene / butane selectivity is provided. The triazine cyclocrosslinked polyimide is prepared by uniformly blending polyimide PI-3-amino-5-(3-aminophenyl)benzoic acid and triazine cyclodiamine crosslinking agent 2,4-diamino-1,3,5-triazine, crosslinking them at a molar ratio of 2:1 for carboxyl groups and triazine cyclodiamine crosslinking agent, and then pyrolyzing the membrane at 600°C.

[0053] This embodiment provides a triazine ring crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane selectivity, specifically including the following steps: Before the reaction began, the three-necked flask was purged with high-purity nitrogen for 30 minutes to remove water vapor and oxygen. Then, 5 mmol of 9,9-di(4-amino-3-fluorophenyl)fluorene diamine and 25 mL of m-cresol were added sequentially, and the mixture was stirred until dissolved and clear. Next, 10 mmol of hexafluorodianhydride was added, and the reaction was carried out in an ice bath for 12 hours. Then, 5 mmol of 3-amino-5-(3-aminophenyl)benzoic acid was added, and the reaction was continued for 3 hours. Subsequently, 0.6 mL of isoquinoline catalyst was added, and the reaction system was heated to 190 °C and maintained for 10 hours. After the reaction was complete, the mixture was allowed to cool naturally to room temperature, and the reaction solution was poured into methanol to precipitate a white filamentous product. This product was washed three times with methanol and dried in a vacuum oven at 140 °C for 24 hours to completely remove residual solvent, finally yielding a white, uniformly blended polyimide filament.

[0054] 0.2 g of the synthesized white filamentous homogeneous polyimide was added to 6 g of DMAc solvent and stirred until completely dissolved. Then, 0.0084 g of 2,4-diamino-1,3,5-triazine was added and stirred until dissolved, followed by 0.0243 g of HOBt and stirred until dissolved. Under ice bath conditions, 0.0345 g of EDC·HCl was slowly added, and the reaction was carried out for 10 hours until viscous. The reaction system was filtered through a 0.45 μm filter membrane and cast into a smooth petri dish. The mixture was then placed in a 55°C vacuum drying oven to evaporate and form a film. After film formation, the film was washed with an ethanol / deionized water mixture for 24 hours, and then transferred to a 100°C oven for vacuum drying for 24 hours to obtain the novel triazine cyclic crosslinked polyimide precursor membrane PI-3-amino-5-(3-aminophenyl)benzoic acid-CL.

[0055] The obtained novel triazine cyclocrosslinked polyimide precursor membrane was placed between two sufficiently large, smooth corundum plates, and then placed in a tube furnace continuously purged with argon gas. The temperature was raised to 600℃ for 2 hours according to the program, and then cooled naturally to obtain a triazine cyclocrosslinked polyimide nitrogen-rich carbon molecular sieve membrane with a thickness of 25 μm, which is PI-3-amino-5-(3-aminophenyl)benzoic acid-CL-CMS600 carbon molecular sieve membrane.

[0056] The obtained PI-3-amino-5-(3-aminophenyl)benzoic acid-CL-CMS600 carbon molecular sieve membrane was tested for gas separation performance at 40℃ and 1.5 bar, where P(butadiene) = 3760.0 Barrer; selectivity α(butadiene / n-butane) = 113.5, α(butadiene / isobutane) = 1207.9.

[0057] Comparative Example 1 This comparative example provides a non-crosslinked polyimide-based carbon molecular sieve membrane, which differs from Example 1 only in that the precursor does not introduce 2,4-diamino-1,3,5-triazine crosslinking. The other raw materials, dosages, and implementation steps are the same as in Example 1. Finally, the precursor membrane is carbonized at 550°C to obtain the carbon molecular sieve membrane PI-CMS550.

[0058] The obtained carbon molecular sieve membrane was tested for gas separation performance at 40℃ and 1.5 bar, where P(butadiene) = 5334.5 Barrer; selectivity α(butadiene / n-butane) = 26.9, α(butadiene / isobutane) = 516.4.

[0059] Comparative Example 2 This comparative example provides a non-crosslinked polyimide-based carbon molecular sieve membrane, which differs from Example 2 only in that the precursor does not introduce 2,4-diamino-1,3,5-triazine crosslinking. The other raw materials, dosages, and implementation steps are the same as in Example 2. Finally, the precursor membrane is carbonized at 600°C to obtain a carbon molecular sieve membrane of PI-CMS600.

[0060] The obtained carbon molecular sieve membrane was tested for gas separation performance at 40℃ and 1.5 bar, where P(butadiene) = 5566.3 Barrer; selectivity α(butadiene / n-butane) = 26.9, α(butadiene / isobutane) = 516.4.

[0061] Comparative Example 3 This comparative example provides a non-crosslinked polyimide-based carbon molecular sieve membrane, which differs from Example 3 only in that the precursor does not introduce 2,4-diamino-1,3,5-triazine crosslinking. The other raw materials, dosages, and implementation steps are the same as in Example 3. Finally, the precursor membrane is carbonized at 700°C to obtain a carbon molecular sieve membrane of PI-CMS700.

[0062] The obtained carbon molecular sieve membrane was tested for gas separation performance at 40℃ and 1.5 bar, where P(butadiene) = 4541.8 Barrer; selectivity α(butadiene / n-butane) = 100.9, α(butadiene / isobutane) = 1673.1.

[0063] Comparative Example 4 This comparative example provides a non-crosslinked polyimide-based carbon molecular sieve membrane, which differs from Example 4 only in that the precursor does not introduce 2,4-diamino-1,3,5-triazine crosslinking. The other raw materials, dosages, and implementation steps are the same as in Example 4. Finally, the precursor membrane is carbonized at 800°C to obtain a carbon molecular sieve membrane of PI-CMS800.

[0064] The obtained carbon molecular sieve membrane was tested for gas separation performance at 40℃ and 1.5 bar, where P(butadiene) = 776.5 Barrer; selectivity α(butadiene / n-butane) = 116.3, α(butadiene / isobutane) = 2263.8.

[0065] Comparative Example 5 This comparative example provides a non-crosslinked polyimide-based carbon molecular sieve membrane, which differs from Example 2 only in that the precursor polyimide is 6FDA-FFDA without carboxyl groups. The other raw materials, dosages, and implementation steps are the same as in Example 2. Finally, the precursor membrane is carbonized at 600°C to obtain a carbon molecular sieve membrane of 6FDA-FFDA-CMS600.

[0066] The obtained carbon molecular sieve membrane was tested for gas separation performance at 40℃ and 1.5 bar, where P(butadiene) = 5977.2 Barrer; selectivity α(butadiene / n-butane) = 45.0, α(butadiene / isobutane) = 689.5.

[0067] Comparative Example 6 This comparative example provides a cross-linked polyimide-based carbon molecular sieve membrane, which differs from Example 2 only in that the precursor polyimide is 6FDA-DABA without FFDA. The other raw materials, dosages, and cross-linking steps are the same as in Example 2. Finally, the precursor membrane is carbonized at 600°C to obtain a carbon molecular sieve membrane of 6FDA-DABA-CL-CMS600.

[0068] The obtained carbon molecular sieve membrane was tested for gas separation performance at 40℃ and 1.5 bar, where P(butadiene) = 2177.3 Barrer; selectivity α(butadiene / n-butane) = 137.1, α(butadiene / isobutane) = 1492.9.

[0069] Table 1. C4 permeability and selectivity of gas separation membranes prepared in comparative examples and embodiments.

[0070] The above 12 sets of experimental data fully verify the improved separation performance of the "triazine ring crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane" proposed in this invention on the butadiene / butane system. At the same carbonization temperature, the crosslinked membranes (Examples 1-4) showed a significant improvement in butadiene / butane selectivity compared to the corresponding uncrosslinked samples (Comparative Examples 1-4). For example, at 600℃, Example 2 showed a 107% and 94% increase in selectivity for butadiene / n-butane and butadiene / isobutane, respectively, compared to Comparative Example 2. This is directly attributed to the enhanced molecular sieving ability resulting from the more uniform and regular ultraporous structure formed by the crosslinked network during pyrolysis. Simultaneously, the nitrogen-rich characteristics introduced by crosslinking significantly enhanced the specific adsorption of butadiene by the triazine ring crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane. Comparative Example 5, which lacks carboxyl anchor points and therefore cannot be crosslinked, showed a much lower selectivity than Example 2, confirming the contribution of this chemisorption mechanism. Furthermore, by adjusting the chemical structure of the precursor membrane, permeability can be optimized without sacrificing selectivity. Compared to Comparative Example 6, which does not contain rigid 9,9-bis(4-amino-3-fluorophenyl)fluorene units, Example 2 showed a significant 100% increase in butadiene permeability while maintaining similar selectivity, indicating that rigid twisted groups can effectively inhibit the tight packing of polymer chains and broaden transport channels. The performance of the triazine ring crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane also showed a regular change with increasing carbonization temperature. From 550℃ to 800℃, selectivity increased significantly and permeability decreased, demonstrating that separation performance can be controlled by adjusting the pyrolysis temperature. In summary, the modification strategy of this invention, through the synergistic effect of "crosslinking to optimize pores" and "nitrogen-enriched adsorption," fundamentally overcomes the permeability-selectivity trade-off bottleneck faced by traditional carbon membranes in butadiene / butane separation, providing a highly promising solution for low-energy and high-efficiency separation of C4 fractions.

Claims

1. A triazine ring-crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane having high butadiene / butane selectivity, characterized by, The precursor of the triazine cyclocrosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane, the triazine cyclocrosslinked polyimide, is composed of a uniformly blended polyimide A and a triazine cyclodiamine crosslinking agent B, wherein the uniformly blended polyimide A has the following repeating unit structure: ； Where m:n = 3:1 to 1:3, and the Ar structure is one of the following carboxyl diamine structures: ； Triazine cyclodiamine crosslinking agent B is any one of the following structures: ； The structure of the cross-linked triazine ring cross-linked polyimide is as follows: ； Where R is -H, -F or -N(CH3)2.

2. A method for preparing a triazine ring crosslinked polyimide-based nitrogen-rich carbon molecular sieve membrane with high butadiene / butane selectivity, characterized in that, The steps are as follows: (1) Preparation of uniformly blended polyimide A: Diamine 9,9-bis(4-amino-3-fluorophenyl)fluorene was dissolved in m-cresol and transferred to a nitrogen-filled environment and stirred until completely dissolved; then hexafluorodianhydride was added, and after reacting in an ice bath for a certain time, diamine Ar was slowly added, the ice bath was removed, isoquinoline catalyst and azeotropic agent were added, and the reaction was raised to a certain temperature; after the reaction was completed, it was cooled to room temperature and collected in methanol to form a white filamentous product; after washing with methanol three times, it was vacuum dried at a certain temperature to obtain uniformly blended polyimide A; (2) Preparation of novel triazine cyclocrosslinked polyimide precursor membrane: The uniformly blended polyimide A and triazine cyclodiamine crosslinking agent B were mixed in a solvent and stirred until completely dissolved to prepare a casting solution of a certain concentration; then the casting solution system was transferred to an ice bath, a catalyst and additives were added, and after reacting for a certain time, it was cast into a smooth and flat petri dish and placed in a constant temperature drying room at a certain temperature to evaporate and form a film. After the film was formed, the film was immersed in a mixture of ethanol and water to wash away the side reactants. Finally, the washed film was vacuum dried to obtain the novel triazine cyclocrosslinked polyimide precursor membrane. (3) Preparation of triazine ring cross-linked polyimide nitrogen-rich carbon molecular sieve membrane: The novel triazine ring cross-linked polyimide precursor membrane was placed between two smooth square corundum plates and pyrolyzed under an argon atmosphere according to a certain procedure to obtain a smooth and uniform triazine ring cross-linked polyimide nitrogen-rich carbon molecular sieve membrane.

3. The preparation method according to claim 2, characterized in that, In step (1), The molar ratio of the diamine 9,9-bis(4-amino-3-fluorophenyl)fluorene, the diamine Ar, and the hexafluorodianhydride is 3:1:4 to 1:3:4; The molar ratio of hexafluorodianhydride, isoquinoline, and azeotropic agent is 1:1:10; The ice bath reaction time is 12 hours; The azeotropic agent is any one of benzene, toluene, and o-xylene; The specified temperature is 150~185℃; The time period is 8 to 12 hours; The vacuum drying temperature is 120~160℃.

4. The production method according to claim 2, characterized by, In step (2), The molar ratio of the uniformly blended polyimide A to the triazine cyclodiamine crosslinking agent B is 8:3 to 8:1; The solvent is DMAc or DMF; The total mass concentration of the casting solution is 5-10%; The catalyst and additive in the reaction system are 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride and 1-hydroxybenzotriazole, respectively. The molar ratio of carboxyl groups, catalyst, and additives in the uniformly blended polyimide A is 1:1.2:1.2; The specified time period is 8 to 10 hours; The temperature for constant temperature drying is 50~70℃; The vacuum drying temperature is 100~180℃; The thickness of the novel triazine cyclocrosslinked polyimide precursor film is 30~40 μm.

5. The preparation method according to claim 2, characterized in that, In step (2), The pyrolysis temperature is 500~900℃; The pyrolysis time is 2 hours.

Images