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

Abstract

The present invention discloses a polyimide-based carbon molecular sieve membrane containing a block fluorene-based monomer and a method for preparing the same. By introducing a low-content fluorene-based diamine monomer structure into the polyimide backbone through block copolymerization, the bulky structural effect of the fluorene group is utilized to reduce the packing density of the precursor molecular chain and the permeation resistance of gases through the carbon molecular sieve membrane. This results in a directional gradient that improves the gas permeability and selectivity of the carbon molecular sieve membrane, achieving a gradient-based improvement in the gas separation performance of the carbon molecular sieve membrane. The advantages of the present invention are ease of scale-up, simple process, good reproducibility, excellent performance, and high cost-effectiveness, making it suitable for the research and manufacture 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 fluorene-based monomer and a preparation method thereof. Background Art

[0002] With the continuous development of industry and technology, the use of fossil fuels has increased dramatically, producing large amounts of toxic and harmful gases such as CO2 and NO2. These gases are deteriorating the climate and environment, seriously threatening the survival of all life on Earth. Therefore, it is imperative to develop viable environmentally friendly technologies to curb the excessive emission of these polluting gases. Furthermore, emerging industries such as semiconductors, new pharmaceuticals, aerospace, and new energy are in urgent need of high-purity gases. Therefore, the development of novel gas separation and purification technologies is crucial for both environmental protection and meeting production needs.

[0003] Traditional gas separation technologies include pressure swing adsorption and cryogenic separation, but they face disadvantages such as high cost, high energy consumption, and complex production processes. In recent years, gas membrane separation technology, which offers low cost, low energy consumption, and safe and simple operation, has attracted widespread attention. After decades of development, polymer gas separation membranes have been used in industrial production. However, organic polymer membrane materials have also exposed many shortcomings in application, such as low permeability, easy plasticization, and poor heat resistance. Therefore, the development of new high-performance membrane materials is the core key to the large-scale promotion and application of membrane separation technology, which will greatly help promote the large-scale application of this technology in the field of gas separation.

[0004] Carbon molecular sieve membranes are obtained by pyrolyzing organic polymer precursor membranes and possess excellent properties such as high temperature resistance, plasticization resistance, and solvent resistance. In particular, their gas permeability is much higher than that of organic membranes. However, the carbon membrane precursors currently used are all produced by pyrolyzing ordinary polysulfone, homophenyl, or biphenyl polyimide membranes. Due to the single raw material of the membrane material, it is impossible to achieve targeted optimization of the separation performance of specific mixed gases by adjusting the preparation process. Therefore, it is necessary to develop highly controllable raw materials based on the molecular structure characteristics of the precursor. In this way, important parameters such as the molecular stacking pattern and stacking density within the carbon molecular sieve membrane material, the distribution density and size of the pore structure, etc. can be adjusted in a targeted manner to achieve the purpose of optimizing and controlling the carbon membrane's resistance to permanent gases with extremely small molecular size differences, relying on the molecular sieving mechanism to achieve efficient separation. Summary of the Invention

[0005] In order to solve the technical problems encountered in the preparation and precursor structure and performance optimization of the above-mentioned carbon molecular sieve membrane, such as the inability to perform directional micro-control according to the tiny molecular scale between different gas types in the system to be separated, the present invention provides a polyimide-based carbon molecular sieve membrane with a block fluorene 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 fluorene-based monomer. The method comprises the following steps: adding a diamine monomer and a block fluorene-based monomer into an organic solvent, stirring to completely dissolve the monomer; adding a dianhydride monomer, stirring and reacting to obtain a block polyamic acid slurry; pouring the block polyamic acid slurry into a clean flat-bottomed culture dish, and naturally spreading the block polyamic acid slurry over the bottom of the culture dish by a tape casting film forming method; placing the culture dish in a vacuum oven for vacuum degassing; 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 fluorene-based monomer; placing the polyimide precursor membrane with the block fluorene-based monomer in a carbonization furnace for high-temperature pyrolysis; and naturally cooling it to room temperature to obtain the polyimide-based carbon molecular sieve membrane with the block fluorene-based monomer.

[0008] Furthermore, the diamine monomer is 4,4′-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline]; the blocked fluorenyl monomer is one of the following fluorenyl-containing diamine monomers, namely 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene, 2,7-diamino-9,9-spirobifluorene, 9,9-bis(4-amino-3-tolyl)fluorene, 9,9-bis(4-aminophenyl)fluorene; 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 block fluorene monomer is 1:1; the molar ratio of the block fluorene 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 block fluorene monomer and the reaction solvent is 20 to 30 wt%; and the content of the block fluorene 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 fluorene 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] The structural formula of 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene is:

[0018] The structural formula of 2,7-diamino-9,9-spirobifluorene is:

[0019]

[0020] The structural formula of 9,9-bis(4-amino-3-methylphenyl)fluorene is:

[0021]

[0022] The structural formula of 9,9-bis(4-aminophenyl)fluorene 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 B1 is B2 is or or or

[0028]

[0029] The present invention also provides a polyimide-based carbon molecular sieve membrane having a block fluorene-based monomer, which is prepared by the above-mentioned preparation method.

[0030] Furthermore, the polyimide-based carbon molecular sieve membrane with block fluorene monomers is used for the separation of CO2 / N2, H2 / N2, or O2 / N2 gases.

[0031] Based on a large number of experiments, the present invention proposes to use a multi-polymerization method to block a low content of monomers containing a relatively large volume of fluorenyl structures in the carbon molecular sieve membrane precursor polyimide molecules, thereby fine-tuning the precursor molecular chain structure. By controlling the content of the fluorenyl diamine monomer, the gas permeability of the carbon molecular sieve membrane is gradually increased, thereby achieving the regulation of gas separation performance. This technology not only eliminates the occurrence of uncontrollable factors such as the inability to perform directional fine-tuning according to the molecular size differences between different gases in the system to be separated during the membrane preparation process, but also achieves a gradient micro-enhancement of the gas permeability of the carbon molecular sieve membrane. At the same time, the use of homogeneous precursor materials effectively improves the reproducibility and separation performance of the carbon molecular sieve membrane, and can also meet the preparation of carbon molecular sieve membranes with special precision requirements for gas separation.

[0032] The present invention introduces a low content of bulky fluorene-based monomers into the polyimide backbone structure through ternary copolymerization, thereby fine-tuning the thermal stability, stacking pattern, and bulk density of the polyimide backbone, thereby achieving directional fine-tuning of the microstructure and properties of the polyimide-based carbon molecular sieve membrane, and ultimately improving the gas separation performance of the carbon molecular sieve membrane in a gradient manner. The main chain polyimide structure selected in the present invention is a cyclobutanetetracarboxylic dianhydride / 4,4′-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] type polyimide. The low content of monomers with specific structures introduced include: 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene, 2,7-diamino-9,9-spirobifluorene, 9,9-bis(4-amino-3-methylphenyl)fluorene, and 9,9-bis(4-aminophenyl)fluorene. By introducing low levels of these monomer structures into the polyimide backbone through block copolymerization, the volumetric structural effect of the fluorene group is exploited to fine-tune the polyimide's microstructure and properties, ultimately improving the gas separation performance of carbon molecular sieve membranes. This invention offers advantages such as easy scale-up, simple process, good reproducibility, excellent performance, and high cost-effectiveness, making it suitable for the research and manufacture of precision instruments.

[0033] It should be noted that in this 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; N-dimethylformamide is abbreviated as: DMF; N-methylpyrrolidone is abbreviated as: NMP; 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene is abbreviated as: BAOFL; 9,9-bis(4-aminophenyl)fluorene is abbreviated as: FDA.

[0034] The CBDA-6FAPB system's polyimide molecular chains are segmented through a ternary copolymerization process, while ensuring a low content of the fluorenyl diamine monomer. This is done to achieve targeted fine-tuning of key parameters within the final carbon membrane, such as the molecular stacking pattern and density, and the distribution density and size of the pore structure, based on the unique physical and chemical activity of the structure. This optimizes the membrane's separation performance for gases with minimal molecular size differences, achieving effective separation through molecular sieving mechanisms. This technology has significant application prospects in the research and manufacture of precision instruments with high gas flow rates and component ratios. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] 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 N2, P2, and R5; and (IV), (V), and (VI) are the cross-sections of N2, P2, and R5.

[0036] Figure 2 1 is the infrared spectrum of the polyimide and carbon molecular sieve membrane in Example 1, wherein (I) is polyimide; (II) is carbon molecular sieve membrane.

[0037] Figure 3 1 is the X-ray diffraction pattern of the polyimide and carbon molecular sieve membrane in Example 2, wherein (I) is polyimide; (II) is carbon molecular sieve membrane.

[0038] Figure 4 This is the DSC curve of the polyimide in Example 4. DETAILED DESCRIPTION

[0039] 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.

[0040] In the embodiments listed in this patent, the block monomer content ratio is low (0-10%) to ensure the basic performance of the main chain molecule. If the content is too high, the properties of the main molecule chain will change too much, making it difficult to achieve directional gradient fine-tuning of the final carbon membrane performance. At the same time, when the content of the block fluorene-containing monomer is too high, it is very easy to cause the final carbon membrane to significantly lose gas separation selectivity. In the polymerization process, if the polymerization temperature is too high or too low, or the polymerization time is too long or too short, it is not conducive to the product polyimide having sufficient molecular weight. The carbon membrane prepared with it will not only have defects and lose separation performance, but also be too brittle and unusable. Therefore, the polymerization temperature is preferably 0-10°C, and the polymerization time is preferably 6-10 hours. Similarly, if the solid content of the polymerization solution is too high or too low, the viscosity of the final polyimide will be too high or too low, which is not conducive to subsequent membrane formation. Therefore, the solid content is preferably 20-30wt%. During the membrane formation process, if the temperature is too low, solvent evaporation will be too slow, reducing membrane formation efficiency. Conversely, if the temperature is too high, bubbles will form within the membrane, resulting in defects. If the imidization temperature is too low, imidization will be incomplete or unreacted, resulting in poor membrane toughness and strength. If the temperature is too high, the membrane will denature due to oxidative crosslinking, making it impossible to control the final carbon membrane properties. If the carbonization temperature is too low, carbonization will be incomplete, resulting in low gas permeability and impractical performance. If the carbonization temperature is too high, molecular chain dehydrogenation and graphitization will be intensified, causing a sharp drop in gas permeability. If the holding time is too short, carbonization will also be incomplete, while if it is too long, defects will form within the membrane due to excessive thermal polycondensation, all of which are detrimental to permeability. Therefore, the final membrane formation temperature is selected to be 60-120°C, the imidization temperature is selected to be 180-300°C, the carbonization temperature is selected to be 650-800°C, and the holding time is selected to be 30-80 minutes.

[0041] 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:

[0042] Comparative Example:

[0043] 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. N was purged, and the reaction temperature was maintained at 5°C. Stirring was performed at 70 rpm for 50 min. 5.23 g of CBDA was 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.

[0044] 2) The polyamic acid slurry 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 an 80°C casting platform to dry for 8 hours; the film was transferred to an oven and kept at 80°C, 120°C, 150°C, 180°C, and 200°C for 12 hours each, and then naturally cooled to obtain a polyimide precursor film material.

[0045] 3) The precursor film prepared above was placed in a carbonization furnace and pyrolyzed 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 min. 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 min. Finally, the temperature was raised to 650°C at a heating rate of 1°C / min and held for 1 h. The temperature was then naturally cooled to room temperature to obtain a carbon molecular sieve membrane.

[0046] 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.

[0047] Table 1 Structural and performance parameters of homopolymeric polyimide-based carbon molecular sieve membranes

[0048]

[0049] Table 2 Gas separation performance of binary homopolymer polyimide-based carbon membranes

[0050]

[0051] Note: Test conditions are 0.01Mpa and 30℃

[0052] Example 1

[0053] 1) Preparation of casting solution: Weigh appropriate amounts of 6FAPB and BAOFL and add them to a 500 mL three-necked flask, add 50 g of DMAC to the three-necked flask, introduce N2, and stir for 50 min at a fixed reaction temperature and a stirring rate of 70 rap / min, then add CBDA. After the addition is completed, maintain the stirring speed, temperature, and time to obtain a transparent and viscous polyamic acid slurry of BAOFL block. The polymerization process parameters are shown in Table 3, numbered M X -Y%, M represents the block monomer BAOFL, X represents the number under this polymerization process parameter, and Y represents the content of the block monomer.

[0054] 2) The polyamic acid slurry was poured into a clean flat-bottomed Petri dish and cast into a film, which was then placed in a vacuum oven at 15°C for 2 hours for vacuum degassing; the Petri dish was removed and placed on a film casting platform for constant temperature drying for 8 hours; the dish was transferred to an oven and imidized at 80°C, 120°C, 150°C, and the final imidization temperature for 12 hours, respectively. After natural cooling, a polyimide precursor film material with BAOFL was obtained. The preparation parameters are shown in Table 4.

[0055] 3) The precursor film prepared above was placed in a carbonization furnace and subjected to high-temperature pyrolysis in a 150 mL / min N2 atmosphere. First, the temperature was raised to 100°C at a heating rate of 2°C / min and kept warm for 30min; then the temperature was raised to 200°C, 300°C, and 400°C at a heating rate of 2°C / min, and kept warm for 30min respectively; finally, the temperature was raised to the final pyrolysis temperature at a heating rate of 1°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 BAOFL block contents. The preparation parameters are shown in Table 4, where N X The polyamic acid M prepared in Table 3 is shown X -Y% is the number of carbon molecular sieve membranes prepared under these preparation parameters.

[0056] 4) The thermal stability and microstructure tests of this type of precursor membrane and carbon molecular sieve membrane were carried out, 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.

[0057] Table 3 Polymerization process parameters of BAOFL block-modified polyimide

[0058]

[0059] Table 4. Process parameters for preparing BAOFL block-modified polyimide-based carbon molecular sieve membranes

[0060]

[0061]

[0062] Table 5 Structure and performance parameters of BAOFL block-modified polyimide-based carbon molecular sieve membrane

[0063]

[0064] Table 6 Gas separation performance of BAOFL block-modified polyimide-based carbon molecular sieve membrane

[0065]

[0066] Note: Test conditions are 0.01Mpa and 30℃

[0067] Example 2

[0068] 1) Preparation of casting solution: Weigh appropriate amount of 6FAPB and 2,7-diamino-9,9-spirobifluorene and add them to a 500mL three-necked flask, add 50g DMF to the three-necked flask, pass through N2, and stir for 40min at a stirring rate of 100rap / min at a fixed reaction temperature, then add CBDA. After the feeding is completed, the stirring speed, temperature and time are maintained to obtain a transparent and viscous polyamic acid slurry of 2,7-diamino-9,9-spirobifluorene block. The polymerization process parameters are shown in Table 7, numbered 0 X -Y%, where O represents the block monomer 2,7-diamino-9,9-spirobifluorene, X represents the number under this polymerization process parameter, and Y represents the content of the block monomer.

[0069] 2) The polyamic acid slurry 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 and dried at a constant temperature for 12 hours; the film was transferred to an oven and imidized at 80°C, 120°C, 150°C, and the final imidization temperature for 12 hours respectively. After natural cooling, a polyimide precursor film material having 2,7-diamino-9,9-spirobifluorene was obtained. The preparation parameters are shown in Table 8.

[0070] 3) The precursor film prepared above was placed in a carbonization furnace and subjected to high-temperature pyrolysis in a 250 mL / min N2 atmosphere. First, the temperature was raised to 100°C at a heating 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 heating rate of 2°C / min, and kept warm for 30 min respectively; finally, the temperature was raised to the final pyrolysis temperature at a heating rate of 2°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 2,7-diamino-9,9-spirobifluorene block contents. The preparation parameters are shown in Table 8, where P X The polyamic acid prepared in Table 7 is shown in FIG. X -Y% is the number of carbon molecular sieve membranes prepared under these preparation parameters.

[0071] 4) The thermal stability and microstructure tests of this type of precursor membrane and carbon molecular sieve membrane were performed, 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.

[0072] Table 7 Polymerization process parameters of 2,7-diamino-9,9-spirobifluorene block-modified polyimide

[0073]

[0074]

[0075] Table 8. Process parameters for preparing polyimide-based carbon molecular sieve membranes modified with 2,7-diamino-9,9-spirobifluorene blocks.

[0076]

[0077] Table 9 Structure and performance parameters of 2,7-diamino-9,9-spirobifluorene block-modified polyimide-based carbon molecular sieve membrane

[0078]

[0079] Table 10 Gas separation performance of 2,7-diamino-9,9-spirobifluorene block-modified polyimide-based carbon molecular sieve membranes

[0080]

[0081] Note: Test conditions are 0.01Mpa and 30℃

[0082] Example 3

[0083] 1) Preparation of casting solution: Weigh appropriate amount of 6FAPB and 9,9-bis(4-amino-3-methylphenyl)fluorene and add them to a 500mL three-necked flask, add 50g NMP to the three-necked flask, introduce N2, and stir for 60min at a fixed reaction temperature and a stirring rate of 50rap / min, then add CBDA. After the addition is completed, maintain the stirring speed, temperature and time to obtain a transparent and viscous polyamic acid slurry of 9,9-bis(4-amino-3-methylphenyl)fluorene block. The polymerization process parameters are shown in Table 11, numbered Q X -Y%, Q represents the block monomer 9,9-di(4-amino-3-methylphenyl)fluorene, X represents the number under this polymerization process parameter, and Y represents the content of the block monomer.

[0084] 2) The polyamic acid slurry 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 18 hours respectively. After natural cooling, a polyimide precursor film material having 9,9-di(4-amino-3-tolyl)fluorene was obtained. The preparation parameters are shown in Table 12.

[0085] 3) The precursor film was placed in a carbonization furnace and pyrolyzed at high temperature in a 100mL / min N2 atmosphere. First, the temperature was raised to 100°C at a heating rate of 2°C / min and kept warm for 30min; then the temperature was raised to 200°C, 300°C, and 400°C at a heating rate of 2°C / min, and kept warm for 30min respectively; finally, the temperature was raised to the final pyrolysis temperature at a heating 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 9,9-di(4-amino-3-tolyl)fluorene block contents. The preparation parameters are shown in Table 12, where R X The polyamic acid Q prepared in Table 11 is shown X -Y% is the number of carbon molecular sieve membranes prepared under these preparation parameters.

[0086] 4) The thermal stability and microstructure tests of this type of precursor membrane and carbon molecular sieve membrane were performed, 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.

[0087] Table 11 Polymerization process parameters of 9,9-bis(4-amino-3-methylphenyl)fluorene block-modified polyimide

[0088]

[0089] Table 12. Membrane preparation process parameters of 9,9-bis(4-amino-3-methylphenyl)fluorene block-modified polyimide-based carbon molecular sieve membrane

[0090]

[0091]

[0092] Table 13 Structure and performance parameters of 9,9-bis(4-amino-3-methylphenyl)fluorene block-modified polyimide-based carbon molecular sieve membrane

[0093]

[0094] Table 14 Gas separation performance of 9,9-bis(4-amino-3-methylphenyl)fluorene block-modified polyimide-based carbon molecular sieve membrane

[0095]

[0096]

[0097] Note: Test conditions are 0.01Mpa and 30℃

[0098] Example 4

[0099] 1) Preparation of casting solution: Weigh appropriate amount of 6FAPB and 4,4'-methylenebis(2-aminophenol) and add them to a 500mL three-necked flask, add 50g of DMAC to the three-necked flask, introduce N2, and stir for 50min at a fixed reaction temperature and a stirring rate of 70rap / min, 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 15, numbered S X -Y%, S represents the block monomer 4,4'-methylenebis(2-aminophenol), X represents the number under this polymerization process parameter, and Y represents the content of the block monomer.

[0100] 2) The polyamic acid slurry was poured into a clean flat-bottomed Petri dish and cast into a film, which was 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. The film was transferred to an oven and imidized at 80°C, 120°C, 150°C, and the final imidization temperature for 24 hours. After natural cooling, a polyimide precursor film material with FDA was obtained. The preparation parameters are shown in Table 16.

[0101] 3) The precursor film prepared above was placed in a carbonization furnace and subjected to high-temperature pyrolysis in a 150 mL / min N2 atmosphere. First, the temperature was raised to 100°C at a heating rate of 2°C / min and kept warm for 30 minutes; then the temperature was raised to 200°C, 300°C, and 400°C at a heating rate of 2°C / min, and kept warm for 30 minutes respectively; finally, the temperature was raised to the final pyrolysis temperature at a heating rate of 1°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 FDA block contents. The preparation parameters are shown in Table 16, where T X The polyamic acid S prepared in Table 15 is shown X -Y% is the number of carbon molecular sieve membranes prepared under these preparation parameters.

[0102] 4) The thermal stability and microstructure tests of this type of precursor membrane and carbon molecular sieve membrane were performed, and the results are shown in Table 17. The gas separation performance was tested using the traditional constant pressure variable volume method, and the results are shown in Table 18.

[0103] Table 15 Polymerization process parameters of FDA block modified polyimide

[0104]

[0105] Table 16 Membrane preparation parameters of FDA block-modified polyimide-based carbon molecular sieve membrane

[0106]

[0107]

[0108] Table 17 Structure and performance parameters of FDA block-modified polyimide-based carbon molecular sieve membrane

[0109]

[0110] Table 18 Gas separation performance of FDA block-modified polyimide-based carbon molecular sieve membrane

[0111]

[0112]

[0113] Note: Test conditions are 0.01Mpa and 30℃.

[0114] It can be seen from the comparative examples and various examples that the glass transition temperature and 10% mass loss temperature of the polyimide are significantly reduced by the modification of the block fluorene-containing monomer, and the range of variation of the final residual carbon content and the carbon layer spacing tends to expand. This shows that the method effectively changes the pyrolysis process of the precursor and the microstructure and properties of the carbon membrane matrix. It increases the permeability of H2, CO2, and O2 gases, and greatly improves the selectivity of H2 / N2, CO2 / N2, and O2 / N2. The present invention can effectively achieve gradient fine-tuning of the gas separation performance of the final carbon membrane by adjusting the polymerization process parameters and the membrane preparation process parameters. In particular, as the block content of the fluorene-containing monomer increases, the gradient of the increase in the gas permeability of the carbon membrane increases, while the gas selectivity does not decrease significantly, achieving the purpose of directional improvement of the gas separation performance of the carbon membrane. In addition, a comparison of the various examples shows that due to the differences in rigidity and volume effect of the fluorene-containing monomers with different structures, the degree of regulation of the gas separation performance of the carbon membrane is different. As shown in Example 1, when the block rigidity is small BAOFL monomer, the entanglement or stacking between its flexible molecular chains is tighter, which reduces the interlayer spacing of the carbon membrane and makes the gas permeability of the prepared carbon membrane lower. In Example 4, when the block rigidity is high FDA modified monomer, the entanglement between the precursor molecular chains is limited, the molecular chain stacking density is greatly reduced, and the final carbon molecular sieve membrane has a higher gas permeability. In Example 3, the block side chain group contains a methyl group, which further increases the volume of the monomer, further reduces the cohesive energy of the precursor molecules, reduces the density of the membrane matrix, and greatly increases the gas permeability of the carbon membrane.

[0115] like Figure 1 As 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 fluorene-containing block diamine monomer was successfully introduced into the polyimide main chain; in addition, after high-temperature pyrolysis, the organic functional groups on the original precursor in the carbon molecular sieve membrane matrix prepared from it have been basically decomposed, leaving a rich pore structure in the matrix. Figure 3As shown in the figure, the number of fluorenyl-containing block diamine monomers introduced from the precursor polyimide can change the shape and position of the derivative peak, indicating that the inter-layer spacing of the microstructural parameters of the carbon molecular sieve membrane has changed significantly. The fluorenyl monomer structure with a condensed ring structure promotes the formation of the carbon structure, thus confirming the effectiveness of this method for directional fine-tuning. Figure 4 As shown, after modification by this technology, the melting temperature and thermal decomposition temperature of the precursor can be effectively regulated, which also shows that the molecular structure has changed, thereby affecting the thermal decomposition behavior and microporous structure formation, and ultimately achieving directional fine-tuning of pores for gas separation performance.

[0116] This application first adopts a ternary copolymerization method to use cyclobutanetetracarboxylic dianhydride, 4,4′-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline] and a trace amount of block fluorene monomer to copolymerize and prepare a polyamic acid slurry, and then prepares a precursor membrane material by tape casting and solvent evaporation, and finally prepares a carbon molecular sieve membrane by high-temperature pyrolysis under an inert gas atmosphere. The present invention blocks a trace amount of fluorene-containing diamine monomers with specific functional structures into the CBDA-6FAPB type polyimide main chain by copolymerization in different proportions to prepare a block-modified precursor casting liquid. The membrane liquid is then poured into a flat-bottomed glass culture dish, and a variety of block-modified precursor membrane materials with specific structures are prepared by tape casting and solvent evaporation. Finally, the precursor membrane material is subjected to high-temperature pyrolysis under an inert gas atmosphere to prepare a block-modified polyimide-based carbon molecular sieve membrane for gas separation. The selected block-modified diamine monomers contain special chemical groups and structures, possessing unique physical and chemical properties. They can effectively improve the thermal stability of the precursor material and the stacking pattern and density of the internal polyimide molecular chains, thereby affecting the pyrolysis process during carbonization and the pore structure within the membrane material. The low content of block, by virtue of the structure-activity relationship between the precursor and the carbon molecular sieve membrane at the molecular level, precisely and directionally fine-tunes the gas separation performance, ultimately achieving the goal of controlling the carbon molecular sieve membrane pore structure and improving gas separation performance.

[0117] Compared with the prior art, the method for preparing a carbon molecular sieve membrane of the present invention has the following advantages: the present invention uses CBDA-6FAPB as the main molecular chain structure, and blocks a low content of monomer units with a specific structure inside the molecular chain by copolymerization. The large-volume fluorene group in the monomer is used to increase the free volume of the molecular chain, thereby reducing the stacking density of the molecular chain and achieving the purpose of effectively improving the gas permeability of the carbon molecular sieve membrane. Compared with the traditional blending modification method, the polymerization block modification process is simple and does not require the introduction of fillers, which greatly reduces the uncontrollable factors in the preparation process and improves the repeatability of the preparation process. The use of low-content blocks can achieve directional fine-tuning of the gas separation performance of the carbon molecular sieve membrane. At the same time, the raw materials and process steps in the preparation process are reduced, making the preparation process relatively simple and cheap, and the separation performance is excellent, providing a basis for the large-scale preparation 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 fluorene monomer, comprising the following steps: Add the diamine monomer and the block fluorene 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 fluorene monomer; placing the polyimide precursor film having block fluorene monomers in a carbonization furnace for high-temperature pyrolysis; and naturally cooling the film to room temperature to obtain a polyimide-based carbon molecular sieve film having block fluorene monomers; The diamine monomer is 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline]; The block fluorenyl monomer is one of the following fluorenyl-containing diamine monomers, namely, 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene, 2,7-diamino-9,9-spirobifluorene, 9,9-bis(4-amino-3-methylphenyl)fluorene, and 9,9-bis(4-aminophenyl)fluorene; 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 fluorene monomer is 1:1; The molar ratio of the block fluorene 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 block fluorene monomer and the organic solvent is 20-30 wt %; The content of the block fluorene 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 block fluorene monomer is 0-10°C; The dissolution and stirring time is 40~60min; The reaction temperature is controlled to be 0-10° C. when the dianhydride monomer is added; The reaction stirring time is 6~10h; 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 1~3h.

5. The preparation method according to claim 1, wherein: The drying temperature on the film casting platform is 60-120° C., and the drying time is 8-18 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 100~250mL / min; the temperature is raised from room temperature to 400℃ at a heating rate of 2℃ / min, and the temperature is kept at 100℃, 200℃, 300℃, and 400℃ for 30min respectively; then the temperature is continued to be raised from 400℃ to 650~850℃ at a heating rate of 0.5~2℃ / min, and the holding time is 30~80min.

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 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene is: ； The structural formula of the 2,7-diamino-9,9-spirobifluorene is: ; The structural formula of the 9,9-bis(4-amino-3-methylphenyl)fluorene is: ; The structural formula of the 9,9-bis(4-aminophenyl)fluorene is: ; The structural formula of polyimide is: ; Where: n and m are both positive integers; A1 and A2 are ; B1 is ; B2 is or or or .

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

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