A polyimide-based carbon molecular sieve membrane with block hydroxyl monomer and preparation method thereof

Abstract

The present invention discloses a polyimide-based carbon molecular sieve membrane with a block hydroxyl monomer and a preparation method thereof. By block copolymerization, a low-content block hydroxyl monomer structure is introduced into the polyimide main chain, and the cross-linking effect of the hydroxyl group is used to directionally fine-tune the microstructure and properties of the polyimide, ultimately achieving the purpose of improving the gas separation performance of the carbon molecular sieve membrane. At the same time, the present invention involves utilizing the thermal rearrangement effect of the ortho-hydroxyl group at high temperature to form a thermally unstable rigid oxazole ring structure, which is beneficial to the directionally regulating the gas permeability of the carbon membrane. Under the synergistic influence of the hydroxyl cross-linking effect and the thermal rearrangement effect, the purpose of simultaneously directionally regulating the gas permeability and selectivity of the carbon membrane can be achieved. The advantages of the present invention are that it is easy to scale up, has a simple process, good repeatability, excellent performance, and high cost performance, and can be used in the research and manufacturing fields of precision instruments.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas separation, and in particular relates to a polyimide-based carbon molecular sieve membrane having a block hydroxyl monomer and a preparation method thereof. Background Art

[0002] Gas separation technology is widely used in a wide range of applications, including hydrogen recovery from refinery gas, carbon capture from boiler flue gas, hydrocarbon gas separation, air oxygen / nitrogen enrichment, helium extraction, food preservation, welding, protective atmospheres, chemical production, mechanical processing, and numerous other fields of production and daily life, including medicine, electronics, and aerospace. Currently, traditional gas purification and separation technologies, such as cryogenic separation and pressure swing adsorption, suffer from drawbacks such as high energy consumption, high costs, complex operating procedures, and bulky equipment. Membrane separation technology not only overcomes these shortcomings but also enables efficient production of high-purity gases and separation of mixed gases. While organic polymer membranes have been used in some gas separation applications, their inherent limitations, such as poor heat resistance, easy plasticization, and low permeability and selectivity, have hindered their widespread adoption. Therefore, the research and development of high-performance membrane materials directly determines the economic and social benefits of membrane separation technology.

[0003] Carbon molecular sieve membranes, as an excellent inorganic porous gas separation membrane material, not only offer superior heat resistance, corrosion resistance, solvent resistance, and plasticization resistance compared to organic polymer membranes, but also boast ultra-high gas permeability and selectivity. Currently, carbon molecular sieve membranes are primarily prepared using a commercially available precursor, pyromellitic dianhydride-4,4'-diaminodiphenyl ether polyimide. Through high-temperature pyrolysis, a porous carbon structure is formed within the membrane matrix via molecular chain decomposition, polycondensation, and rearrangement reactions. Gas separation is achieved through the pore molecular sieving mechanism. However, the carbon membranes prepared by this method lack a single pore structure, making it difficult to precisely tailor the pore structure to suit different separation systems and requirements. Some researchers have attempted to manipulate the microporous structure of carbon membranes by blending porous fillers. However, porous fillers often have poor dispersibility and compatibility with the carbon membrane matrix, easily creating non-selective defects within the membrane. Furthermore, some fillers can undergo structural damage or induce membrane embrittlement during the carbonization process, resulting in poor reproducibility in the preparation of carbon molecular sieve membranes and preventing their widespread application. Therefore, even under the same preparation conditions, the carbon membranes produced by the blending modification method often have large differences in microstructure and separation performance, and cannot achieve directional micro-control according to the tiny molecular size differences between different gases in the separation system.

[0004] Therefore, the most ideal solution is to conduct targeted micro-control of the stacking mode, stacking density, pore structure characteristics, etc. of the microcrystalline layer in the carbon membrane based on the molecular structure and properties of the precursor at the molecular level, so as to achieve the purpose of optimizing the control of the carbon membrane for permanent gases with extremely small molecular size differences, and rely on the molecular screening mechanism to achieve effective separation. Summary of the Invention

[0005] In order to solve the technical problems faced by the above-mentioned carbon molecular sieve membrane in preparation and precursor structure and performance optimization, such as the inability to perform directional micro-control according to the tiny molecular scale between different gases in the system to be separated, the present invention provides a polyimide-based carbon molecular sieve membrane with a block hydroxyl monomer and a preparation method thereof, which belongs to the field of chemical engineering gas separation technology.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] The invention provides a preparation method of a polyimide-based carbon molecular sieve membrane with a block hydroxyl monomer, comprising the following steps: adding a diamine monomer and a block hydroxyl monomer into an organic solvent, stirring to completely dissolve them; adding a dianhydride monomer, stirring and reacting to obtain a blocked polyamic acid slurry; pouring the blocked polyamic acid slurry into a clean flat-bottomed culture dish, and using a tape casting film forming method to naturally cover the bottom of the culture dish; placing the culture dish in a vacuum oven for vacuum degassing treatment; taking the culture dish out of the vacuum oven and placing it on a film casting platform for constant temperature drying; transferring the culture dish from the film casting platform to an oven for solvent removal and imidization treatment; taking the culture dish out of the oven and naturally cooling it to obtain a polyimide precursor membrane with the block hydroxyl monomer; placing the polyimide precursor membrane with the block hydroxyl monomer in a carbonization furnace for high-temperature pyrolysis; and naturally cooling it to room temperature to obtain a polyimide-based carbon molecular sieve membrane with the block hydroxyl monomer.

[0008] Furthermore, the diamine monomer is 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline]; the blocked hydroxyl monomer is one of the following hydroxyl-containing diamine monomers, that is, the blocked diamine monomer is 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 2,5-diaminophenol or 4,4'-methylenebis(2-aminophenol); the organic solvent is one of NN dimethylacetamide, NN dimethylformamide or N-methylpyrrolidone; and the dianhydride monomer is cyclobutanetetracarboxylic dianhydride.

[0009] Furthermore, the molar ratio of the sum of the dianhydride monomer and the diamine monomer to the blocked hydroxyl monomer is 1:1; the molar ratio of the blocked hydroxyl monomer to the diamine monomer is 1:99 to 9:91; the solid content of the reaction system consisting of the dianhydride monomer, the diamine monomer, the blocked hydroxyl monomer and the reaction solvent is 20 to 30 wt%; and the content of the blocked hydroxyl monomer is greater than or equal to 1% and less than or equal to 9%.

[0010] Furthermore, the temperature conditions for dissolving the diamine monomer and the block hydroxyl monomer are 0-10°C; the dissolution stirring time is 40-60 minutes; the reaction temperature is controlled at 0-10°C when adding the dianhydride monomer; the reaction stirring time is 6-10 hours; and the reaction stirring rate is 50-100 rpm / min.

[0011] Furthermore, the temperature condition for vacuum degassing of the polyamic acid slurry is 10 to 20° C., and the time for vacuum degassing is 1 to 3 hours.

[0012] Furthermore, the drying temperature on the film casting platform is 60-120°C, and the drying time is 8-18 hours; when in the oven, the temperatures are kept at 80°C, 120°C, 150°C, and the final imidization temperature for 12-24 hours respectively; when in the carbonization furnace, the nitrogen flow rate is 100-250 mL / min; the temperature is raised from room temperature to 400°C at a heating rate of 2°C / min, and the temperature is kept constant at 100°C, 200°C, 300°C, and 400°C for 30 minutes respectively; then the temperature is continued to be raised from 400°C to 650-850°C at a heating rate of 0.5-2°C / min, and the holding time is 30-80 minutes.

[0013] Furthermore, the final temperature of the imidization is 180°C to 300°C.

[0014] Furthermore, the structural formula of cyclobutanetetracarboxylic dianhydride is:

[0015]

[0016] The structural formula of 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] is:

[0017]

[0018] The structural formula of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane is:

[0019]

[0020] The structural formula of 2,5-diaminophenol is:

[0021]

[0022] The structural formula of 4,4'-methylenebis(2-aminophenol) is:

[0023]

[0024] The structural formula of the selected polyimide is:

[0025]

[0026] Where: n and m are both positive integers;

[0027] A1 and A2 are

[0028] B1 is

[0029] B2 is

[0030]

[0031] The present invention also provides a polyimide-based carbon molecular sieve membrane having a block hydroxyl monomer, which is prepared by the above-mentioned preparation method.

[0032] Furthermore, the polyimide-based carbon molecular sieve membrane with block hydroxyl monomers is used for separation of CO2 / N2, H2 / N2, or O2 / N2 gases.

[0033] Based on extensive experiments, this invention proposes using multi-component polymerization to embed low levels of specific structural monomers into the carbon molecular sieve membrane precursor polyimide molecules, thereby fine-tuning the molecular chain structure and achieving directional control of the carbon molecular sieve membrane's pore structure and gas separation performance. This technology not only eliminates uncontrollable factors during the membrane production process, such as the inability to directionally fine-tune the tiny molecular scales between different gas types in the separation system, but also improves the reproducibility and separation performance of the carbon molecular sieve membrane. It also can meet the customization requirements of carbon molecular sieve membranes with specific gas separation precision requirements.

[0034] The present invention provides a method for introducing a low content of hydroxyl-containing monomers into the polyimide main chain structure by ternary copolymerization, and fine-tuning the thermal stability, stacking mode, and bulk density of the polyimide main chain, thereby achieving directional fine-tuning of the microstructure and properties of the carbon molecular sieve membrane, and ultimately improving the gas separation performance of the carbon molecular sieve membrane. The main chain polyimide structure selected in the present invention is cyclobutane tetracarboxylic dianhydride / 4,4'-[1,4-phenylbis(oxy)]bis(dio)-1 ...

[0035] [3-(Trifluoromethyl)aniline]-type polyimide. The low-content monomers with specific structures introduced include: 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 2,5-diaminophenol, and 4,4'-methylenebis(2-aminophenol). By block copolymerization, low-content of these monomer structures are introduced into the polyimide backbone, and the cross-linking effect of hydroxyl groups is utilized to fine-tune the microstructure and properties of the polyimide, ultimately achieving the goal of improving the gas separation performance of carbon molecular sieve membranes. The advantages of the present invention are easy scale-up, simple process, good repeatability, excellent performance, and high cost-effectiveness. It can be used in the research and manufacturing of precision instruments.

[0036] It should be noted that in the present application, 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] is abbreviated as: 6FAPB; N-dimethylacetamide is abbreviated as: DMAC; cyclobutanetetracarboxylic dianhydride is abbreviated as: CBDA; 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane is abbreviated as: 6FAP; N-dimethylformamide is abbreviated as: DMF; and N-methylpyrrolidone is abbreviated as: NMP.

[0037] The polyimide chains of the CBDA-6FAPB system are segmented through a ternary copolymerization method, while ensuring the introduction of a low content of hydroxyl diamine monomers. This is done to achieve targeted fine-tuning of the stacking pattern, packing density, pore structure, and other characteristics closely related to separation performance within the final carbon membrane, based on the unique physical and chemical properties of the structure. This allows for optimized control of the carbon membrane's ability to effectively separate permanent gases with minimal molecular size differences, relying on molecular sieving mechanisms. This technology has significant application prospects in the research and manufacture of precision instruments with high gas flow rates and composition ratios. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 These are scanning electron microscope images of the carbon molecular sieve membranes in Examples 1, 2, and 3, wherein (I), (II), and (III) are the surfaces of B2, D2, and F2; and (IV), (V), and (VI) are the cross-sections of B2, D2, and F2.

[0039] Figure 2 1 is the infrared spectrum of the precursor and the carbon molecular sieve membrane in Example 1, wherein (I) is the precursor; (II) is the carbon molecular sieve membrane.

[0040] Figure 3 These are the X-ray diffraction patterns of the precursor and carbon molecular sieve membrane in Example 2; wherein (I) is the precursor; and (II) is the carbon molecular sieve membrane.

[0041] Figure 4 This is the DSC curve of the precursor in Example 3. DETAILED DESCRIPTION

[0042] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0043] Some specific embodiments are listed below to clarify the specific technical solutions of the present invention. The protection scope of the present invention includes but is not limited to the listed embodiments:

[0044] Comparative Example:

[0045] 1) Preparation of casting solution: 11.43 g of 6FAPB was weighed and added to a 500 mL three-necked flask. 50 g of DMAC was also added to the flask. N₂ was introduced, and the reaction temperature was maintained at 5°C. Stirring was performed at 70 rpm for 50 min. 5.23 g of CBDA was then added. After the addition of the ingredients, the reaction was continued for 18 h, maintaining the stirring speed and temperature, to obtain a transparent, viscous polyamic acid slurry.

[0046] 2) Pour the polyamic acid slurry prepared above into a clean flat-bottomed Petri dish and cast it into a film, then place the Petri dish in a vacuum oven and vacuum degas at 15°C for 2 hours; take out the Petri dish and place it on an 80°C casting platform to dry for 8 hours; transfer it to an oven and keep it at 80°C, 120°C, 150°C, 180°C, and 200°C for 12 hours each, and then cool it naturally to obtain a polyimide precursor film material.

[0047] 3) The precursor film prepared above was placed in a carbonization furnace and pyrolyzed at high temperature in a 150 mL / min N2 atmosphere. The temperature was first raised to 100°C at a heating rate of 2°C / min and held for 30 minutes. The temperature was then raised to 200°C, 300°C, and 400°C at a heating rate of 2°C / min, each held for 30 minutes. Finally, the temperature was raised to 650°C at a heating rate of 1°C / min and held for 1 hour. The temperature was then naturally cooled to room temperature to obtain a carbon molecular sieve membrane.

[0048] 4) The thermal stability and microstructure tests of this type of precursor membrane and carbon membrane were conducted, and the results are shown in Table 1. The gas separation performance was tested using the traditional constant pressure variable volume method, and the test results are shown in Table 2.

[0049] Table 1 Structural and performance parameters of homopolymeric polyimide-based carbon molecular sieve membranes

[0050]

[0051] Table 2 Gas separation performance of binary homopolymer polyimide-based carbon membranes

[0052]

[0053] Note: Test conditions are 0.01Mpa and 30℃

[0054] Example 1

[0055] 1) Preparation of casting solution: Appropriate amounts of 6FAPB and 6FAP were weighed and added to a 500 mL three-necked flask. 50 g of DMAC was added and N2 was introduced. The mixture was stirred at a rate of 70 rap / min for 50 min at a fixed reaction temperature. CBDA was then added. After the addition of the materials, the stirring speed, temperature, and time were maintained to obtain a transparent, viscous polyamic acid slurry containing 6FAP blocks. The polymerization process parameters are shown in Table 3, numbered A X -Y%, A represents the block hydroxyl monomer 6FAP, X represents the number under this polymerization process parameter, and Y represents the content of the block hydroxyl monomer.

[0056] 2) The polyamic acid slurry prepared above was poured into a clean flat-bottomed Petri dish and cast into a film, and then placed in a vacuum oven and vacuumed and degassed at 15°C for 2 hours; the Petri dish was removed and placed on a film casting platform for constant temperature drying for 8 hours; it was transferred to an oven and imidized at 80°C, 120°C, 150°C, and the final imidization temperature for 12 hours respectively; then, the temperature was continued to be increased for post-treatment; after natural cooling, a polyimide precursor film material with a 6FAP block was obtained, and the preparation parameters are shown in Table 3.

[0057] 3) The precursor film prepared above was placed in a carbonization furnace and pyrolyzed at high temperature in a 150 mL / min N2 atmosphere. First, the temperature was raised to 100°C at a rate of 2°C / min and kept warm for 30 min; then the temperature was raised to 200°C, 300°C, and 400°C at a rate of 2°C / min, and kept warm for 30 min respectively; finally, the temperature was raised to the final pyrolysis temperature at a rate of 1°C / min and kept warm. Finally, it was naturally cooled to room temperature to obtain polyimide-based carbon molecular sieve membranes modified with different 6FAP block contents. The preparation parameters are shown in Table 4, where B X The polyamic acid A prepared in Table 3 is shown X -Y% is the number of carbon molecular sieve membranes prepared under these preparation parameters.

[0058] 4) The thermal stability and microstructure tests of this type of precursor membrane and carbon membrane were conducted, and the results are shown in Table 5. The gas separation performance was tested using the traditional constant pressure variable volume method, and the results are shown in Table 6.

[0059] Table 3 Polymerization process parameters of 6FAP block-modified polyimide

[0060]

[0061]

[0062] Table 4 Preparation parameters of 6FAP block-modified polyimide-based carbon molecular sieve membrane

[0063]

[0064]

[0065] Table 5 Structure and performance parameters of 6FAP block-modified polyimide-based carbon molecular sieve membrane

[0066]

[0067] Table 6 Gas separation performance of 6FAP block-modified polyimide-based carbon membranes

[0068]

[0069] Note: Test conditions are 0.01Mpa and 30℃.

[0070] Example 2

[0071] 1) Preparation of casting solution: Weigh appropriate amount of 6FAPB and 2,5-diaminophenol into a 500mL three-necked flask, add 50g DMF, introduce N2, stir at a fixed reaction temperature, 100 rap / min rate for 40min, then add CBDA. After the addition is completed, maintain the stirring speed, temperature and time to obtain a transparent and viscous polyamic acid slurry blocked with 2,5-diaminophenol. The polymerization process parameters are shown in Table 7, numbered C X -Y%, C represents the block hydroxyl monomer 2,5-diaminophenol, X represents the number under this polymerization process parameter, and Y represents the content of the block hydroxyl monomer.

[0072] 2) The polyamic acid slurry prepared above was poured into a clean flat-bottomed Petri dish and cast into a film, and then placed in a vacuum oven and vacuumed and degassed at 10°C for 3 hours; the Petri dish was taken out and placed on a film casting platform for constant temperature drying for 12 hours; it was transferred to an oven and imidized at 80°C, 120°C, 150°C, and the final imidization temperature for 12 hours respectively; then, the temperature was continued to be increased for post-treatment; after natural cooling, a polyimide precursor film material with a 2,5-diaminophenol block was obtained, and the preparation parameters are shown in Table 8.

[0073] 3) The precursor film prepared above was placed in a carbonization furnace and pyrolyzed at high temperature in a 250 mL / min N2 atmosphere. First, the temperature was raised to 100°C at a rate of 2°C / min and kept at this temperature for 30 min; then, the temperature was raised to 200°C, 300°C, and 400°C at a rate of 2°C / min, and kept at this temperature for 30 min respectively; finally, the temperature was raised to the final pyrolysis temperature at a rate of 2°C / min and kept at this temperature. Finally, the temperature was naturally cooled to room temperature to obtain polyimide-based carbon molecular sieve membranes modified with different 2,5-diaminophenol block contents. The preparation parameters are shown in Table 8, where D X The polyamic acid C prepared in Table 7 is shown X-Y% is the number of carbon molecular sieve membranes prepared under these preparation parameters.

[0074] 4) The thermal stability and microstructure tests of this type of precursor membrane and carbon membrane were conducted, and the results are shown in Table 9. The gas separation performance was tested using the traditional constant pressure variable volume method, and the results are shown in Table 10.

[0075] Table 7 Polymerization process parameters of 2,5-diaminophenol block-modified polyimide

[0076]

[0077]

[0078] Table 8 Process parameters for preparing 2,5-diaminophenol block-modified polyimide-based carbon molecular sieve membranes

[0079]

[0080]

[0081] Table 9 Structure and performance parameters of 2,5-diaminophenol block-modified polyimide-based carbon molecular sieve membrane

[0082]

[0083] Table 10 Gas separation performance of 2,5-diaminophenol polyimide-based carbon molecular sieve membrane

[0084]

[0085] Note: Test conditions are 0.01Mpa and 30℃

[0086] Example 3

[0087] 1) Preparation of casting solution: Weigh appropriate amount of 6FAPB and 4,4'-methylenebis(2-aminophenol) into a 500mL three-necked flask, add 50g NMP, introduce N2, stir at a fixed reaction temperature, 50rap / min rate for 60min, then add CBDA. After the addition is completed, maintain the stirring speed, temperature and time to obtain a transparent and viscous polyamic acid slurry with 4,4'-methylenebis(2-aminophenol) block. The polymerization process parameters are shown in Table 11, numbered E X -Y%, E represents the block hydroxyl monomer 4,4'-methylenebis(2-aminophenol), X represents the number under this polymerization process parameter, and Y represents the content of the block hydroxyl monomer.

[0088] 2) The polyamic acid slurry prepared above was poured into a clean flat-bottomed Petri dish and cast into a film, and then placed in a vacuum oven and vacuumed and degassed at 20°C for 1 hour; the Petri dish was taken out and placed on a film casting platform for constant temperature drying for 18 hours; it was transferred to an oven and imidized at 80°C, 120°C, 150°C, and the final imidization temperature for 24 hours respectively; then, the temperature was continued to be increased for post-treatment; after natural cooling, a polyimide precursor film material with 4,4'-methylenebis(2-aminophenol) blocks was obtained, and the preparation parameters are shown in Table 12.

[0089] 3) The precursor film prepared above was placed in a carbonization furnace and pyrolyzed at high temperature in a N2 atmosphere of 100 mL / min. First, the temperature was raised to 100°C at a rate of 2°C / min and kept warm for 30 min; then the temperature was raised to 200°C, 300°C, and 400°C at a rate of 2°C / min, and kept warm for 30 min respectively; finally, the temperature was raised to the final pyrolysis temperature at a rate of 0.5°C / min and kept warm. Finally, the temperature was naturally cooled to room temperature to obtain polyimide-based carbon molecular sieve membranes modified with different 4,4'-methylenebis(2-aminophenol) block contents. The preparation parameters are shown in Table 12, where F X The polyamic acid E prepared in Table 11 is shown X -Y% is the number of carbon molecular sieve membranes prepared under these preparation parameters.

[0090] 4) The thermal stability and microstructure tests of this type of precursor membrane and carbon membrane were conducted, and the results are shown in Table 13. The gas separation performance was tested using the traditional constant pressure variable volume method, and the results are shown in Table 14.

[0091] Table 11 Polymerization process parameters of 4,4'-methylenebis(2-aminophenol) block-modified polyimide

[0092]

[0093]

[0094] Table 12 Process parameters for preparing 4,4'-methylenebis(2-aminophenol) block-modified polyimide-based carbon molecular sieve membranes

[0095]

[0096]

[0097] Table 13 Structure and performance parameters of 4,4'-methylenebis(2-aminophenol) block-modified polyimide-based carbon molecular sieve membranes

[0098]

[0099] Table 14 Gas separation performance of 4,4'-methylenebis(2-aminophenol) block-modified polyimide-based molecular sieve membranes

[0100]

[0101] Note: Test conditions are 0.01Mpa and 30℃

[0102] In the examples presented in this patent, the block monomer content is low, ranging from 1 to 9%, to ensure the basic properties of the main chain molecules. Excessively high content can lead to significant variations in the properties of the main chain, making it impossible to precisely fine-tune the final carbon membrane performance. During the polymerization process, excessively high temperatures or low solids content can hinder the polymerization reaction, while too low a molecular weight can prevent membrane formation. Conversely, the resulting slurry viscosity can be too high, requiring extended reaction times or the molecular weight can be too low. Furthermore, too low a reaction temperature can terminate the reaction. During the membrane formation process, too low a temperature can slow solvent evaporation, preventing membrane formation; too high a temperature can accelerate solvent evaporation and lead to bubbling and defects in the membrane material. Too low an imidization temperature can result in incomplete or ineffective imidization, while too high a temperature can lead to oxidative crosslinking of the membrane material, making it difficult to achieve the desired control of the final carbon membrane performance. Too low a carbonization temperature can result in incomplete carbonization and suboptimal gas permeability. Too high a carbonization temperature can exacerbate molecular chain dehydrogenation or graphitization, leading to a sudden drop in gas permeability and, again, ineffective control of the carbon membrane's gas separation performance. Too low a holding time can also result in incomplete carbonization, while too long can easily introduce defects into the membrane. Post-treatment is intended to promote thermal rearrangement of the hydroxyl groups of the block hydroxyl monomers to form oxazole ring structures. If the post-treatment temperature is too low or the rearrangement holding time is too short, the thermal rearrangement reaction will not occur or will be incomplete. Conversely, the membrane material will be damaged by excessive oxidation.

[0103] In combination with the polymerization process, film making process and gas separation performance in each embodiment, it can be seen that by adjusting the relevant parameters, the final carbon membrane gas separation performance can be effectively fine-tuned, so that the gas permeability and selectivity of the carbon membrane can be controlled accordingly. Such as comparing the gas separation performance of the comparative example and the embodiment, it can be seen that after blocking different trace amounts of hydroxyl-containing monomers, the gas permeability of the carbon membrane shows a gradient slight decrease, while the gas selectivity shows a gradient slight increase, achieving the directional controllability of the carbon membrane gas separation. Comparing Example 1 with Example 2, since diaminophenol is significantly more rigid than 6FAP, and its own volume is small, resulting in a higher molecular chain packing density, so its carbon membrane has low permeability and high selectivity. Comparing Example 1 with Example 3, due to the presence of a large amount of fluoromethyl in Example 1, the conjugated structure of the molecular chain is interrupted, causing the electron migration in the molecular chain to be blocked, and the 6FAP modified polyimide molecular chain packing density is reduced, so that the permeability of the carbon membrane is significantly improved, but the selectivity is slightly reduced.

[0104] like Figure 1As shown, the surface and cross-section morphologies of the carbon molecular sieve membrane prepared according to the provided technical solution are very uniform, dense, and free of any defects. Figure 2 As shown in the figure, a hydroxyl-containing block diamine monomer was successfully introduced into the polyimide main chain; in addition, most of the organic functional groups on the original precursor were decomposed through thermal decomposition, thermal polycondensation and thermal rearrangement reactions at high temperature, while a graphitized amorphous carbon structure and rich pore structure were formed in the carbon molecular sieve membrane matrix. Figure 3 As shown in the figure, the type and amount of hydroxyl-containing block diamine monomers introduced from the precursor polyimide can achieve changes in the derivative peak shape and peak position of the precursor film and carbon film, indicating that their microstructural parameters such as molecular spacing and carbon layer spacing have changed significantly, thus confirming the effectiveness of this method in directional fine-tuning carbon molecular sieve membranes. Figure 4 As shown in the figure, after modification, the molecular structure of the precursor polyimide changes, causing the corresponding melting temperature and thermal decomposition temperature to change, which will inevitably affect the final thermal decomposition behavior and microporous structure formation, and ultimately achieve directional fine-tuning of pores for gas separation performance.

[0105] This application first adopts a ternary copolymerization method to use cyclobutanetetracarboxylic acid dianhydride, 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] and a trace amount of block hydroxyl monomer to copolymerize and prepare a polyamic acid slurry, and then prepare a precursor film material by tape casting and solvent evaporation, and finally prepare a carbon molecular sieve membrane by high-temperature pyrolysis under an inert gas atmosphere. The present invention blocks a trace amount of hydroxyl diamine monomers with specific functional structures into the CBDA-6FAPB type polyimide main chain structure by copolymerization in different proportions to prepare a precursor casting liquid. The membrane liquid is then poured into a flat-bottomed glass culture dish, and a precursor membrane material is prepared by tape casting and solvent evaporation. Finally, it is pyrolyzed at high temperature under an inert gas atmosphere to prepare a block-modified polyimide-based carbon molecular sieve membrane for gas separation. The selected block hydroxydiamine monomers contain special chemical groups and structures, possessing unique physical and chemical properties. They can effectively improve the thermal stability of the precursor material, as well as the stacking pattern and density of the molecular chains, thereby affecting the pyrolysis process during carbonization and the pore structure within the membrane material. The low content of block, by leveraging the structure-activity relationship between the precursor and the carbon molecular sieve membrane at the molecular level, allows for precise and directional micro-control of gas separation performance, ultimately achieving the goal of controlling the carbon molecular sieve membrane pore structure and improving gas separation performance.

[0106] The carbon molecular sieve membrane preparation method described in this invention offers the following advantages over existing technologies: Using CBDA-6FAPB as the primary molecular chain structure, the method incorporates low-content monomer units with specific structures within the molecular chain through copolymerization. The crosslinking properties of the hydroxyl groups in the monomers and the high-temperature rearrangement reaction of the oxazole rings influence the thermal stability of the membrane material. Subsequent thermal decomposition of the oxazole rings creates a regular pore structure within the membrane, thereby improving membrane permeability. The crosslinked hydroxyl groups increase the packing density of the molecular chains within the membrane material, thereby enhancing membrane selectivity. Compared with traditional blending and modification methods, the polymer block modification process is simple and eliminates the need for fillers, significantly reducing uncontrollable factors during the preparation process and improving reproducibility. The low-content block allows for targeted fine-tuning of the gas separation performance of the carbon molecular sieve membrane. Furthermore, the preparation process reduces the number of raw materials and process steps, making it relatively simple and inexpensive, while also offering superior separation performance. This provides a basis for the large-scale production of carbon molecular sieve membranes for gas separation. Furthermore, microblock modification ensures that the microscopic pore structure of the main polyimide-based carbon molecular sieve membrane is at the gas molecular scale, leveraging the molecular sieving mechanism to achieve targeted fine-tuning of separation performance, thus achieving a certain degree of controllability and precision. This technology holds great promise for the development and manufacturing of precision instruments with stringent requirements for gas flow rates and gas content ratios.

Claims

1. A method for preparing a polyimide-based carbon molecular sieve membrane having a block hydroxyl monomer, comprising the following steps: Add the diamine monomer and the block hydroxyl monomer into the organic solvent and stir to completely dissolve them; Adding dianhydride monomer, stirring and reacting to obtain blocked polyamic acid slurry; The blocked polyamic acid slurry is poured into a clean flat-bottomed culture dish and naturally spread over the bottom of the culture dish by using a film casting method; the culture dish is placed in a vacuum oven for vacuum degassing; the culture dish is removed from the vacuum oven and placed on a film casting platform for constant temperature drying; the culture dish is transferred from the film casting platform to an oven for solvent removal and imidization treatment; the culture dish is removed from the oven and naturally cooled to obtain a polyimide precursor film having a blocked hydroxyl monomer; The polyimide precursor film having block hydroxyl monomers is placed in a carbonization furnace for high-temperature pyrolysis; the film is naturally cooled to room temperature to obtain a polyimide-based carbon molecular sieve film having block hydroxyl monomers. The diamine monomer is 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline]; The block hydroxyl monomer is one of the following hydroxyl-containing diamine monomers, that is, the block diamine monomer is 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 2,5-diaminophenol or 4,4'-methylenebis(2-aminophenol); The organic solvent is one of N-dimethylacetamide, N-dimethylformamide or N-methylpyrrolidone; The dianhydride monomer is cyclobutanetetracarboxylic dianhydride.

2. The preparation method according to claim 1, wherein: The molar ratio of the dianhydride monomer to the sum of the diamine monomer and the block hydroxyl monomer is 1:1; The molar ratio of the blocked hydroxyl monomer to the diamine monomer is 1:99 to 9:91; The solid content of the reaction system consisting of the dianhydride monomer, the diamine monomer, the blocked hydroxyl monomer and the organic solvent is 20 to 30 wt %; The content of the block hydroxyl monomer is greater than or equal to 1% and less than or equal to 9%.

3. The preparation method according to claim 1, wherein: The temperature condition for dissolving the diamine monomer and the blocked hydroxyl monomer is 0 to 10° C.; The dissolution and stirring time is 40 to 60 minutes; The reaction temperature is controlled to be 0-10° C. when the dianhydride monomer is added; The reaction stirring time is 6 to 10 hours; The reaction stirring rate is 50-100 rpm / min.

4. The preparation method according to claim 1, wherein: The temperature condition for vacuum degassing of the polyamic acid slurry is 10-20°C; The vacuum degassing time is 1 to 3 hours.

5. The preparation method according to claim 1, wherein: The drying temperature on the film casting platform is 60 to 120° C., and the drying time is 8 to 18 hours; In the oven, the temperature is kept at 80° C., 120° C., 150° C., and the final imidization temperature for 12 to 24 hours respectively; In the carbonization furnace, the flow rate of nitrogen is 100-250 mL / min; the temperature is raised from room temperature to 400°C at a heating rate of 2°C / min, and the temperature is kept at 100°C, 200°C, 300°C, and 400°C for 30 minutes respectively; then the temperature is continued to be raised from 400°C to 650-850°C at a heating rate of 0.5-2°C / min, and the holding time is 30-80 minutes.

6. The preparation method according to claim 5, characterized in that: The final temperature of the imidization is 180°C to 300°C.

7. The preparation method according to claim 1, wherein: The structural formula of the cyclobutane tetracarboxylic dianhydride is: The structural formula of the 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] is: The structural formula of the 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane is: The structural formula of the 2,5-diaminophenol is: The structural formula of the 4,4'-methylenebis(2-aminophenol) is: The structural formula of polyimide is: Where: n and m are both positive integers; A1 and A2 are B1 is B2 is 8. A polyimide-based carbon molecular sieve membrane having a block hydroxyl monomer, characterized in that: The preparation method according to any one of claims 1 to 7 is used.

9. The polyimide-based carbon molecular sieve membrane having block hydroxyl monomers according to claim 8, characterized in that: The polyimide-based carbon molecular sieve membrane with block hydroxyl monomers is used for separation of CO2 / N2, H2 / N2, or O2 / N2 gases.

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