A hexacyclic polyimide copolymer separation membrane, a method for preparing the same, and use thereof

Abstract

This invention belongs to the field of membrane material technology, and particularly relates to a six-membered ring polyimide copolymer separation membrane, its preparation method, and its applications. This invention utilizes naphthalene dianhydride copolymerized with rigid diamine monomers such as BAPF and TrMPD. The rigid structure of the naphthalene ring and fluorene ring on the main chain, with their high electron cloud density, improves the gas separation performance of the polymer membrane. The six-membered ring polyimide obtained through this method exhibits excellent gas separation performance for CO2. 2 / N 2 The separation effect of the mixed gas is better than the test results in the pure gas, indicating its broad application prospects in the field of exhaust gas treatment.

Description

A six-membered ring polyimide copolymer separation membrane, its preparation method and applications Technical Field

[0001] This invention belongs to the field of membrane material technology, and particularly relates to a six-membered ring polyimide copolymer separation membrane, its preparation method and application. Background Technology

[0002] Gas separation membrane technology has been widely used in fields such as air separation and purification, carbon dioxide recovery from combustion exhaust gases, and natural gas purification due to its outstanding advantages such as low energy consumption, convenient operation, and no environmental pollution. [1] As the core of gas separation membrane technology, the performance of gas separation membrane materials directly affects the preparation, separation performance, and long-term service performance of gas separation membranes. Based on material properties, gas separation membranes can generally be divided into three main categories: polymeric materials, inorganic materials, and metallic materials. Among these, polymers occupy an important position in membrane materials due to their unique dissolution-diffusion mass transfer mechanism, high selectivity, high flux, sufficiently high mechanical strength and stability, and relatively low cost. Aromatic polyimides are a class of polymers with imide rings in their main chain. Depending on the type of imide ring, they can be further divided into five-membered ring polyimides and six-membered ring polyimides. Aromatic polyimides possess excellent gas transport performance, thermal and chemical stability, high mechanical strength, and good film-forming properties, making them one of the most promising gas separation membrane materials.

[0003] Polyimides are typically formed by the polycondensation of dianhydrides and diamines. Currently, the vast majority of polyimide membranes used for gas separation are five-membered ring polyimides. [2] Six-membered ring sulfonated polyimide has been extensively studied as a proton conductive membrane for fuel cells since the beginning of this century. It is usually produced by the condensation polymerization of naphthalene dianhydride and diamine. [3] Due to the higher electron cloud density on the naphthalene ring of six-membered ring polyimides, the positive charge of the carbonyl group decreases, leading to reduced reactivity of the dianhydride. Compared to five-membered ring polyimides, they require higher polymerization temperatures and more stringent reaction conditions. Furthermore, the rigid structure and strong intermolecular interactions of six-membered ring polyimides result in decreased solubility, making the preparation of high-molecular-weight soluble six-membered ring polyimides different from that of five-membered ring polyimides. In recent years, with increasing demands for gas separation membrane performance, research on six-membered ring polyimides as gas separation membranes has increased compared to five-membered ring polyimides due to their superior hydrolytic stability, solvent resistance, and thermal stability. [4-6] The study on the relationship between the gas separation performance and structure of polyimide materials shows that high rigidity and large free volume are important guarantees for the good gas separation performance of polyimide gas separation membranes. [7-8] Therefore, those skilled in the art are dedicated to developing novel, high-performance six-membered ring polyimide films. Summary of the Invention

[0004] In a first aspect, the present invention provides a six-membered ring polyimide copolymer separation membrane, characterized in that the structure of the six-membered ring polyimide copolymer is as shown in Formula I:

[0005]

[0006] R1 is selected from R2 is selected from x=0-1, y=1-0, n=10-1000.

[0007] In a preferred embodiment of the present invention, R1 is

[0008] In a preferred embodiment of the present invention, x:y = 1 / 10 - 10 / 1.

[0009] In a second aspect, the present invention provides a method for preparing the six-membered ring polyimide copolymer separation membrane, comprising the following steps:

[0010] S1: Dissolve monomers R1 and R2 in the solvent m-cresol and magnetically stir the mixture at room temperature under a nitrogen stream to obtain the reaction mixture;

[0011] S2: After monomers R1 and R2 are completely dissolved, 1,4,5,8-naphthalenetetracarboxylic anhydride (NTDA) and benzoic acid are added. The mixture is heated at 80°C for 2-6 hours, then at 180°C for 10 hours. After the reaction solution is cooled to room temperature, isoquinoline is added all at once, and the reaction mixture is heated again at 180°C for 20 hours. After cooling to about 120°C, a polyimide solution is obtained, which is then poured into methanol.

[0012] S3: The precipitate collected by filtration was washed with methanol and dried under vacuum at 120°C for 10 hours to obtain a six-membered ring polyimide copolymer;

[0013] S4: The six-membered ring polyimide copolymer solution dissolved in dimethylacetamide (DMAc) at 5 w / v% was cast onto a glass plate and dried in an oven at 70°C for 8 hours. The cast film was then peeled off from the glass plate and soaked in a hot methanol solution for 24 hours to remove residual solvent.

[0014] S5: Remove the membrane and vacuum dry it at 120°C for 12 hours to obtain the six-membered ring polyimide copolymer separation membrane.

[0015] The R1 monomer is selected from: 9,9-bis(4-methylphenyl)-2,7-diaminofluorene (BMPDAF), 9,9-bis(3,4-dimethylphenyl)-2,7-diaminofluorene (BDMPDAF) or 9,9-bis(4-methoxyphenyl)-2,7-diaminofluorene (BMOPDAF);

[0016] The R2 monomer is selected from: 2,4,6-trimethyl-m-phenylenediamine (TrMPD), 9,9-bis(4-aminophenyl)fluorene (BAPF) or 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (BTFBz).

[0017] In a preferred embodiment of the present invention, in step S1, the feeding ratio of monomer R1 to monomer R2 is 1 / 10-10 / 1.

[0018] In a preferred embodiment of the present invention, in step S2, the amount of NTDA fed is the sum of the amounts of monomers R1 and R2.

[0019] In a preferred embodiment of the present invention, in step S2, the mixture is heated at 80°C for 4 hours and then at 180°C for 10 hours; after adding isoquinoline, the reaction mixture is heated again at 180°C for 20 hours.

[0020] In a preferred embodiment of the present invention, in step S2, the obtained polyimide solution is dark brown and highly viscous, so it is first diluted with 10 mL of m-cresol and then poured into methanol.

[0021] In a preferred embodiment of the present invention, in step S4, the hot methanol solution is at 40-50°C.

[0022] In a third aspect, the present invention provides the application of the six-membered ring polyimide copolymer separation membrane in gas separation.

[0023] In a preferred embodiment of the present invention, the mixed gas to be separated in the gas separation process contains CO2 / N2.

[0024] Technical effect

[0025] The reaction between six-membered ring naphthalene dianhydride and aromatic amines does not form a polyamic acid structure. Therefore, the polycondensation of six-membered ring naphthalene dianhydride often employs a high-temperature one-step polycondensation method rather than the two-step method commonly used for traditional five-membered ring acid anhydrides. In this invention, after the diamine monomer is fully dispersed in the reaction system, naphthalene dianhydride (NTDA) is added, and random copolymerization is carried out under the catalysis of benzoic acid and isoquinoline to successfully prepare a six-membered ring polyimide random copolymer. This invention selects novel rigid diamine monomers such as BMPDAF, BDMODAF, and BMOPDAF, resulting in a fluorene ring structure on the polymer backbone. Fluorene rings have a high electron cloud density and strong intermolecular forces, which can improve the selectivity of the gas separation membrane. At the same time, the rigid planar structure of the fluorene ring can improve the permeability of the gas separation membrane. These diamine monomers have rigid large substituents, which increases the interchain spacing and free volume of the polymer chain, thereby improving gas permeability. The resulting six-membered ring gas separation membrane exhibits excellent gas permeability and selectivity, and its separation effect for CO2 / N2 mixed gases exceeds the Robeson upper limit in 2008.

[0026] This invention employs copolymerization of naphthalene dianhydride with rigid diamine monomers such as BAPF, TrMPD, and BAPF. The rigid structures of the naphthalene ring and fluorene ring on the main chain, both with high electron cloud density, enhance the gas separation performance of the polymer membrane. The six-membered ring polyimide obtained through this method exhibits better separation performance for CO2 / N2 mixed gases than in pure gases, demonstrating its broad application prospects in the field of exhaust gas treatment. Attached Figure Description

[0027] Figure 1 is a schematic diagram of the six-membered ring polyimide structure of the present invention;

[0028] Figure 2 is the infrared spectrum of the six-membered ring polyimide prepared in Example 1 of the present invention;

[0029] Figure 3 shows the CO2 / N2 gas separation performance of the separation membrane in Example 3 of the present invention;

[0030] Figure 4 shows the separation performance of the separation membrane of Embodiment 3 of the present invention for CO2 / N2 mixed gas. Detailed Implementation

[0031] The following description, with reference to the accompanying drawings, illustrates several preferred embodiments of the present invention to make its technical content clearer and easier to understand. The present invention can be embodied in many different forms, and the scope of protection of the present invention is not limited to the embodiments mentioned herein.

[0032] Example 1: Preparation of a separation membrane made from a six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD (1 / 1).

[0033] S1: Add 1.1286 g (3.0 mmol) of 9,9-bis(4-methylphenyl)-2,7-diaminofluorene (BMPDAF), 0.4507 g (3.0 mmol) of 2,4,6-trimethyl-m-phenylenediamine (TrMPD) and 20 mL of m-cresol to a 100 mL dry three-necked flask, and magnetically stir the reaction mixture at room temperature under a nitrogen stream.

[0034] S2: After BMPDAF and TrMPD are completely dissolved, add 1.6091 g (6.0 mmol) NTDA and 1.465 g (12.0 mmol) benzoic acid. Heat the mixture at 80 °C for 4 hours, then at 180 °C for 10 hours. After cooling to room temperature, add 1.55 g (12 mmol) isoquinoline in one go, and heat the reaction mixture again at 180 °C for 20 hours. After cooling to approximately 120 °C, dilute the highly viscous, dark brown polyimide solution with 10 mL m-cresol, then pour it into methanol.

[0035] S3: The precipitate collected by filtration was thoroughly washed with methanol and dried under vacuum at 120°C for 10 hours to obtain the six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD(1 / 1).

[0036] S4: A solution of the six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD (1 / 1) dissolved in DMAc at 5 w / v% was cast onto a glass plate and dried in an oven at 70°C for 8 hours. The cast film was then peeled off the glass plate and immersed in a hot methanol solution (40–50°C) for 24 hours to remove residual solvent.

[0037] S5: Remove the membrane and vacuum dry it at 120°C for 12 hours to obtain the six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD(1 / 1) separation membrane.

[0038] The six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD(1 / 1) separation membrane prepared above was detected using infrared spectroscopy, as shown in Figure 2, at 1716 and 1675 cm⁻¹. -1 The strong absorption peaks nearby are the asymmetric stretching vibration peak and the symmetric stretching vibration peak of the six-membered ring carbonyl group, at 1331 cm⁻¹. -1 The absorption peaks at 1581 and 1512 cm⁻¹ are due to the stretching vibration of the CN bond in the imide ring. -1 The nearby absorption bands are characteristic peaks of the skeletal vibrations of naphthalene and benzene rings, respectively, indicating that the product prepared above is a six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD(1 / 1).

[0039] The aforementioned six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD (1 / 1) separation membrane was dissolved in NMP at a concentration of 0.5 wt% and its intrinsic viscosity was measured to be 0.87 dL / g. Due to the characteristics of polycondensation reactions, diamine and dianhydride must react in equal proportions to obtain high molecular weight polymers. Therefore, for successfully prepared polymers, the composition ratio is consistent with the feed ratio. The intrinsic viscosity measurement, coupled with the polymer's ability to form a film smoothly and its certain mechanical properties, confirms that a polymer with the required molecular weight has been obtained, i.e., the n-value meets the requirements. For polymers with the same structure, a higher intrinsic viscosity generally indicates a higher molecular weight.

[0040] Example 2: Preparation of a separation membrane made from a six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD (1 / 2)

[0041] The molar ratio of BMPDAF to TrMPD was controlled at 1:2, with a total of 6 mol. The remaining operation steps and conditions were similar to those in Example 1, and a six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD(1 / 2) separation membrane with a BMPDAF to TrMPD unit ratio of 1:2 was prepared.

[0042] The above-mentioned six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD(1 / 2) separation membrane was dissolved in NMP at a content of 0.5wt% and the intrinsic viscosity was detected. The intrinsic viscosity was found to be 1.4 dL / g.

[0043] Example 3: Preparation of a separation membrane made from a six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD (2 / 1)

[0044] The molar ratio of BMPDAF to TrMPD was controlled at 2:1, with a total of 6 mol. The remaining operation steps and conditions were similar to those in Example 1, and a six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD (2 / 1) separation membrane with a BMPDAF to TrMPD unit ratio of 2:1 was obtained.

[0045] The above-mentioned six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD(2 / 1) separation membrane was dissolved in NMP at a content of 0.5wt% and the intrinsic viscosity was detected. The intrinsic viscosity was found to be 4.32 dL / g.

[0046] Example 4: Preparation of a separation membrane made from a six-membered ring polyimide copolymer NTDA-BMPDAF / BAPF (1 / 1)

[0047] The molar ratio of BMPDAF to BAPF was controlled at 3:1, with a total of 6 mol. The remaining operation steps and conditions were similar to those in Example 1, and a six-membered ring polyimide copolymer NTDA-BMPDAF / BAPF (1 / 1) separation membrane with a BMPDAF to BAPF unit ratio of 1:1 was obtained.

[0048] The above-mentioned six-membered ring polyimide copolymer NTDA-BMPDAF / BAPF(1 / 1) separation membrane was dissolved in NMP at a content of 0.5wt% and the intrinsic viscosity was detected. The intrinsic viscosity was found to be 2.0 dL / g.

[0049] Example 5: Preparation of a separation membrane made from a six-membered ring polyimide copolymer NTDA-BDMPDAF / TrMPD (1 / 1).

[0050] S1: Add 1.212 g (3.0 mmol) of 9,9-bis(3,4-dimethylphenyl)-2,7-diaminofluorene (BDMPDAF), 0.4507 g (3.0 mmol) of 2,4,6-trimethyl-m-phenylenediamine (TrMPD) and 20 mL of m-cresol to a 100 mL dry three-necked flask, and magnetically stir the reaction mixture at room temperature under a nitrogen stream.

[0051] S2: After BMPDAF and TrMPD are completely dissolved, add 1.6091 g (6.0 mmol) NTDA and 1.465 g (12.0 mmol) benzoic acid. Heat the mixture at 80 °C for 4 hours, then at 180 °C for 10 hours. After cooling to room temperature, add 1.55 g (12 mmol) isoquinoline in one go, and heat the reaction mixture again at 180 °C for 20 hours. After cooling to approximately 120 °C, dilute the highly viscous, dark brown polyimide solution with 10 mL m-cresol, then pour it into methanol.

[0052] S3: The precipitate collected by filtration was thoroughly washed with methanol and dried under vacuum at 120°C for 10 hours to obtain the six-membered ring polyimide copolymer NTDA-BDMPDAF / TrMPD(1 / 1).

[0053] S4: A solution of the six-membered ring polyimide copolymer NTDA-BDMPDAF / TrMPD (1 / 1) dissolved in DMAc at 5 w / v% was cast onto a glass plate and dried in an oven at 70°C for 8 hours. The cast film was then peeled off the glass plate and immersed in a hot methanol solution (40–50°C) for 24 hours to remove residual solvent.

[0054] S5: Remove the membrane and vacuum dry it at 120°C for 12 hours to obtain the six-membered ring polyimide copolymer NTDA-BDMPDAF / TrMPD(1 / 1) separation membrane.

[0055] The above-mentioned six-membered ring polyimide copolymer NTDA-BDMPDAF / TrMPD(1 / 1) separation membrane was dissolved in NMP at a content of 0.5wt% and its intrinsic viscosity was measured to be 1.1 dL / g.

[0056] Example 6: Preparation of a separation membrane made from a six-membered ring polyimide copolymer NTDA-BMOPDAF / TrMPD (1 / 1).

[0057] S1: Add 1.224 g (3.0 mmol) of 9,9-bis(4-methoxyphenyl)-2,7-diaminofluorene (BMOPDAF), 0.4507 g (3.0 mmol) of 2,4,6-trimethyl-m-phenylenediamine (TrMPD) and 20 mL of m-cresol to a 100 mL dry three-necked flask, and magnetically stir the reaction mixture at room temperature under a nitrogen stream.

[0058] S2: After BMPDAF and TrMPD are completely dissolved, add 1.6091 g (6.0 mmol) NTDA and 1.465 g (12.0 mmol) benzoic acid. Heat the mixture at 80 °C for 4 hours, then at 180 °C for 10 hours. After cooling to room temperature, add 1.55 g (12 mmol) isoquinoline in one go, and heat the reaction mixture again at 180 °C for 20 hours. After cooling to approximately 120 °C, dilute the highly viscous, dark brown polyimide solution with 10 mL m-cresol, then pour it into methanol.

[0059] S3: The precipitate collected by filtration was thoroughly washed with methanol and dried under vacuum at 120°C for 10 hours to obtain the six-membered ring polyimide copolymer NTDA-BMOPDAF / TrMPD(1 / 1).

[0060] S4: A solution of the six-membered ring polyimide copolymer NTDA-BMOPDAF / TrMPD (1 / 1) dissolved in DMAc at 5 w / v% was cast onto a glass plate and dried in an oven at 70°C for 8 hours. The cast film was then peeled off the glass plate and immersed in a hot methanol solution (40–50°C) for 24 hours to remove residual solvent.

[0061] S5: Remove the membrane and vacuum dry it at 120°C for 12 hours to obtain the six-membered ring polyimide copolymer NTDA-BMOPDAF / TrMPD(1 / 1) separation membrane.

[0062] The above-mentioned six-membered ring polyimide copolymer NTDA-BMOPDAF / TrMPD(1 / 1) separation membrane was dissolved in NMP at a content of 0.5wt% and its intrinsic viscosity was measured to be 1.2 dL / g.

[0063] Comparative Example 1: Preparation of a 6FDA-BMPDAF / TrMPD (1 / 1) separation membrane, a five-membered ring polyimide copolymer membrane.

[0064] S1: Add 1.5123 g (4.0 mmol) of BMPDAF, 0.6009 g (4.0 mmol) of TrMPD and 24 mL of N-methylpyrrolidone (NMP) to a 100 mL dry three-necked flask, and magnetically stir the reaction mixture at room temperature under a nitrogen stream.

[0065] S2: After BMPDAF and TrMPD are completely dissolved, add 3.5539 g (8.0 mmol) of 4,4'-(hexafluoroisopropene)phthalic anhydride (6FDA) in several portions over 5 hours. After the complete addition of 6FDA, continue stirring the reaction mixture at room temperature for 1 hour. Heat the system to 180°C, and add 7.6 mL of toluene dropwise before the temperature reaches 120°C, followed by the dropwise addition of 11.3 mL of xylene to initiate a thermal imidization reaction, stirring for 10 hours. Cool the polyimide solution to 120°C and pour it into methanol, washing thoroughly with methanol.

[0066] S3: The precipitate was collected by filtration and dried under vacuum at 120°C for 10 hours to obtain the five-membered ring polyimide copolymer 6FDA-BMPDAF / TrMPD(1 / 1).

[0067] S4: A solution of 6FDA-BMPDAF / TrMPD (1 / 1) pentacyclic polyimide copolymer dissolved in DMAc at 5 w / v% was cast onto a glass plate and dried in an oven at 70°C for 8 hours. The cast film was then peeled off the glass plate and immersed in a hot methanol solution (40–50°C) for 10 hours to remove residual solvent.

[0068] S5: Remove the membrane and vacuum dry it at 120°C for 12 hours to obtain the five-membered ring polyimide copolymer 6FDA-BMPDAF / TrMPD(1 / 1) separation membrane.

[0069] The above-mentioned five-membered ring polyimide copolymer 6FDA-BMPDAF / TrMPD(1 / 1) separation membrane was dissolved in DMAc at a content of 0.5wt% and the intrinsic viscosity was measured to be 0.47dL / g.

[0070] Test Example 1: Pure Gas Permeation Test

[0071] The performance of the six-membered ring polyimide copolymer separation membranes prepared in Examples 1-6 and the five-membered ring polyimide copolymer separation membrane prepared in Comparative Example 1 was tested using a GTR-1ADFE gas adsorption coefficient and dispersion coefficient tester.

[0072] The testing conditions were: temperature 35℃, pressure 0.1-0.5 MPa; effective membrane area 15.2 cm². 2 ;

[0073] Detection parameters:

[0074] (1) The gas permeability coefficient P is determined by the stable permeation flux of gas during a lag time (θ) of 5 to 10 times.

[0075] (2) The diffusion coefficient D is calculated by the following formula:

[0076]

[0077] Where L is the thickness of the membrane;

[0078] (3) The solubility coefficient S is calculated by the following formula:

[0079]

[0080] (4) Ideal selectivity is defined as the ratio of the gas permeability coefficients P of gases A and B, which is consistent with the diffusion selectivity and solubility selectivity of the product, as shown in the following formula:

[0081]

[0082] Test results and analysis:

[0083] Table 1. Pure gas permeability and ideal selectivity of separation membranes with different six-membered ring polyimide copolymers.

[0084]

[0085] a.1Barrer=10 -10 cm 3 (STP)cm cm -2 s -1 cmHg -1

[0086] As shown in Table 1, the six-membered ring polyimide copolymer separation membranes prepared in Examples 1-4 of this invention all exhibit high gas permeability and good selectivity. The permeability of the gas separation membrane also increases with the increase in the proportion of BMPDAF in the polymer. Examples 5 and 6 also demonstrate excellent gas separation performance, but compared to Example 1, their gas permeability is lower, while their selectivity is comparable. This is because the para-methyl substituent in BMPDAF in Example 1 effectively supports the polymer chain, increasing the inter-chain spacing and free volume, which facilitates gas permeation. BMPDAF, on the other hand, introduces a methyl substituent again at its ortho position, making it difficult to provide further support to the polymer chain. Furthermore, due to the small size of the methyl group itself, it may block some of the free volume, leading to a decrease in gas permeability. BMPDAF, due to the introduction of flexible ether bonds, reduces the rigidity of the side groups. Simultaneously, the strong polarity of the oxygen atom increases the inter-chain forces, reducing the inter-chain spacing and free volume, thereby decreasing the gas permeability. In summary, the six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD (1 / 1) separation membrane prepared in Example 1 exhibits excellent gas separation performance, demonstrating the highest selectivity among gas separation membranes of similar composition while possessing high permeability.

[0087] Test Example 2: Mixed Gas Permeation Test

[0088] Table 2 shows the permeability and selectivity of the mixed gas (CO2 / N2 = 20 / 80) under different pressures in Example 3.

[0089]

[0090] The gas separation performance of membrane materials is typically assessed initially through pure gas permeation tests. However, the transport performance of membranes in practical applications is affected by other permeaters in the gas mixture, leading to permeation performance significantly different from that measured in pure gas. In particular, when adsorbable gases are separated in a gas mixture, gas permeation is affected due to competitive adsorption. Therefore, evaluating the mixed gas separation performance of membrane materials is also crucial.

[0091] Table 2 shows the gas permeability and selectivity of NTDA-BMPDAF / TrMPD (2 / 1) in a mixed gas (CO2 / N2 = 20 / 80) at different pressures and 35°C. This data exceeds the Robeson upper limit from 2008 (as shown in Figure 3). This is consistent with many other reported gas separation membranes. [9,10]Unlike in pure gas tests, NTDA-BMPDAF / TrMPD(2 / 1) exhibits higher CO2 permeability and better CO2 / N2 selectivity under mixed gas conditions. This is primarily because CO2 itself has higher permeability than N2. In the competitive environment of a mixed gas, CO2 will preferentially diffuse, resulting in higher separation efficiency. The dual improvement in gas permeability and gas selectivity in CO2 / N2 mixed gases indicates its significant application potential in exhaust gas treatment.

[0092] Table 2 also shows the interaction between feed pressure and the membrane's mixed gas permeability and selectivity. It can be seen that both CO2 permeability and CO2 / N2 selectivity decrease with increasing feed pressure. Notably, NTDA-BMPDAF / TrMPD(2 / 1) does not exhibit plasticization due to the high rigidity of the polymer chains, and its permeability does not show an increasing trend even at mixed gas feed pressures up to 20 atm.

[0093] Experiment Example 3: Gas Separation Effect Test

[0094] The separation performance of the six-membered ring polyimide copolymer separation membranes prepared in Examples 1-3 and the five-membered ring polyimide copolymer separation membrane prepared in Comparative Example 1 was monitored under the same conditions, as shown in Figure 4. Compared with the five-membered ring polyimide copolymer separation membrane of Comparative Example 1, the six-membered ring polyimide copolymer NTDA-BMPDAF / TrMPD(1 / 1) separation membrane of Example 1 of the present invention showed excellent CO2 / N2 separation performance, and its selectivity / permeability coefficient was close to the new Robeson upper limit (as shown in Figure 4). This indicates that the six-membered ring polyimide copolymer separation membrane of the present invention has better gas separation performance than the five-membered ring polyimide copolymer separation membrane.

[0095] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

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Claims

1. A six-membered ring polyimide copolymer separation membrane, characterized in that, The structure of the six-membered ring polyimide copolymer is shown in Formula I: (Equation I) Where R1 is selected from 、 or R2 is selected from 、 or ,x:y=1 / 10-10 / 1,n=10-1000。 2. The six-membered ring polyimide copolymer separation membrane according to claim 1, wherein, R1 is 。 3. A method for preparing the six-membered ring polyimide copolymer separation membrane according to claim 1 or 2, comprising the following steps: S1: dissolving monomers R1 and R2 in the solvent m-cresol, and magnetically stirring at room temperature under a nitrogen flow to obtain a reaction mixture; S2: after monomers R1 and R2 are completely dissolved, adding 1,4,5,8-naphthalenetetracarboxylic anhydride and benzoic acid, heating at 80°C for 2-6 hours, then heating at 180°C for 8-12 hours, cooling the reaction solution to room temperature, adding isoquinoline in one step, and heating the reaction mixture again at 180°C for 18-22 hours; cooling to approximately 120°C. S3: The obtained polyimide solution was then poured into methanol; S4: The precipitate was collected by filtration, washed with methanol, and vacuum dried at 120°C for 10 hours to obtain a six-membered ring polyimide copolymer; S5: The six-membered ring polyimide copolymer solution dissolved in dimethylacetamide at 5 w / v% was cast onto a glass plate and dried in an oven at 70°C for 8 hours. The cast membrane was then peeled off the glass plate and immersed in a hot methanol solution for 24 hours to remove residual solvent; S6: The membrane was then removed and vacuum dried at 120°C for 12 hours to obtain the six-membered ring polyimide copolymer separation membrane. The R1 monomer is selected from: 9,9-bis(4-methylphenyl)-2,7-diaminofluorene, 9,9-bis(3,4-dimethylphenyl)-2,7-diaminofluorene or 9,9-bis(4-methoxyphenyl)-2,7-diaminofluorene; the R2 monomer is selected from: 2,4,6-trimethyl-m-phenylenediamine, 9,9-bis(4-aminophenyl)fluorene or 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl.

4. The preparation method according to claim 3, wherein, In step S1, the feeding ratio of monomer R1 to monomer R2 is 1 / 10 to 10 / 1.

5. The preparation method according to claim 3, wherein, In step S2, the amount of 1,4,5,8-naphthalenetetracarboxylic anhydride added is the sum of the amounts of monomers R1 and R2.

6. The preparation method according to claim 3, wherein, In step S2, the mixture is heated at 80°C for 4 hours, and then at 180°C for 10 hours; after the addition of isoquinoline, the reaction mixture is heated again at 180°C for 20 hours.

7. The preparation method according to claim 3, wherein, In step S2, the obtained polyimide solution is first diluted with 10 mL of m-cresol, and then poured into methanol.

8. The application of the six-membered ring polyimide copolymer separation membrane according to claim 1 in gas separation.

9. The application according to claim 8, wherein, The gas mixture to be separated in the gas separation process contains CO2 / N2.

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