Novel polymer blended carbon molecular sieve membrane for separating butadiene / butane and preparation method of novel polymer blended carbon molecular sieve membrane

Abstract

The invention belongs to the technical field of gas separation membranes, and discloses a novel polymer blended carbon molecular sieve membrane for butadiene / butane separation and a preparation method thereof. The polymer blended carbon molecular sieve membrane is prepared by an improved method which comprises the following steps: firstly, introducing a fluorenyl structure into polyimide to prepare high-carbon-content and high-rigidity copolyimide so as to prevent serious pore channel collapse in a carbonization process, and then blending with a polymer with good pore-forming capability according to a certain proportion. The obtained polymer is blended to prepare a flat and uniform flat sheet membrane through a solvent evaporation method, and the flat sheet membrane is carbonized through an improved pyrolysis procedure, so that the prepared carbon molecular sieve membrane has a micropore-ultramicropore structure with uniform size distribution and good mechanical properties; the catalyst shows excellent gas separation performance in butadiene / n-butane, butadiene / iso-butane and other systems, and can effectively solve the industrial problem of high energy consumption of C4 fraction separation.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas separation membranes and relates to a polymer blended carbon molecular sieve membrane with excellent butadiene / butane separation performance, a preparation method and application thereof in the separation of C4 fractions. Background Art

[0002] 1,3-Butadiene (abbreviated as butadiene, molecular dynamics diameter 4.31 Å) is a fundamental and important raw material in the petrochemical industry. Its conjugated double bonds in its molecular structure give it excellent polymerization activity, making it an irreplaceable raw material in the production of synthetic rubber (butadiene rubber), synthetic resin (ABS resin), and other high-value-added chemicals (adiponitrile). Butadiene is primarily obtained by separating C4 hydrocarbon mixtures from petroleum cracking. These mixed fractions contain a large number of other C4 components, including 1-butene (4.46 Å), isobutylene (4.84 Å), n-butane (4.69 Å), and isobutane (5.28 Å). Therefore, separating butadiene from these other C4 hydrocarbons is essential.

[0003] The boiling point difference between butadiene and components like 1-butene and isobutylene is less than 5°C, resulting in low relative volatility, making traditional distillation techniques difficult to effectively separate. Furthermore, the presence of alkanes can cause coordination competition with the Ziegler-Natta catalyst, leading to kinetic instability in the polymerization reaction. Therefore, to produce high-purity butadiene, current industrial plants commonly use N-methylpyrrolidone (NMP) extractive distillation, which uses polar solvents to alter the relative volatilities of the components. While this process can achieve butadiene purities exceeding 99.5%, it faces several significant technical limitations. The process requires a multi-stage distillation column (extraction column, stripping column, solvent recovery column) with over 100 stages and operating temperatures of 120-150°C. This leads to high energy consumption, a large equipment footprint, and high equipment investment (accounting for over 55% of the total investment in a C4 separation unit). Furthermore, high temperatures accelerate the dimerization of butadiene to vinylcyclohexene, necessitating the continuous addition of a polymerization inhibitor such as tert-butylcatechol (TBC), which increases production costs by approximately 15%. At the same time, the extraction solvent circulation rate is 5-8 times the feed rate, resulting in an energy intensity of 2.5-3.5GJ / ton of product. In addition, the nitrogen-containing organic matter produced by solvent degradation adds to the downstream wastewater treatment load. Therefore, the development of new separation technologies is imperative.

[0004] According to statistics, membrane separation saves approximately 90% of energy compared to energy-intensive distillation processes. Furthermore, compared to extractive distillation, which consumes large amounts of solvent and pollutes the environment, membrane separation is environmentally friendly and pollution-free. These advantages have made membrane separation a key separation technology today. Shimazu et al. used a glassy polyimide membrane with excellent thermal and chemical stability to separate butadiene and n-butane. The 6FDA-based polyimide exhibited high selectivity, ranging from 220 to 6770, but the butadiene permeability was only 0.07 to 2.14 Barrer, making it unsuitable for C4 fraction separation (A. Shimazu, et al., J. Polym. Sci. Part B: Polym. Phys. 1999, 37, 2941). Okamoto et al. reported that the 6FDA-TrMPD polyimide achieved a butadiene permeability of 110 barrer and a selectivity of 67, demonstrating excellent separation performance. However, under a 50 / 50 mol% butadiene / n-butane mixture, the selectivity dropped by approximately one-third and gradually decreased with time, with a significantly prolonged time to steady state. This is attributed to butadiene-induced plasticization (K. Okamoto, et al., J. Membr. Sci. 1997, 134, 171). Therefore, suppressing plasticization has become a bottleneck in butadiene separation. Although Koros et al. demonstrated that cross-linking can effectively suppress plasticization (WJ Koros et al., MACROMOLECULES. 2011, 44, 15), cross-linking can also lead to a decrease in separation performance. Therefore, to obtain membrane materials that combine high separation performance with plasticization resistance, the preparation of carbon molecular sieve membranes by high-temperature pyrolysis of thermally stable polymer precursors will become the most effective future technology path. Carbon molecular sieve membranes have a microporous to ultramicroporous pore size distribution, overcoming trade-off limitations with their excellent size-sieving performance. Furthermore, the introduction of a non-thermally stable polymer into the precursor membrane acts as a pore-forming agent during the carbonization process, regulating the pore size distribution and enabling applications in the separation of larger C4 gases.

[0005] At present, there is still a gap in the research on the separation of carbon molecular sieve membranes in butadiene / n-butane and butadiene / isobutane systems. Given the key role of this material in the separation of C4 isomers, it is of great scientific value to study the regulation mechanism of pore-forming agents on the butadiene permeation selectivity of carbon molecular sieve membranes. Summary of the Invention

[0006] To address the trade-off between permeability and selectivity of traditional polyimides, as well as the severe plasticization effect induced by butadiene, the present invention provides a novel polymer-blended carbon molecular sieve membrane for butadiene / butane separation and its preparation method by blending a thermally stable polyimide with a non-thermally stable polymer to prepare a blend precursor membrane, which is then pyrolyzed at high temperature. By selecting a highly aromatic and rigid fluorene-based polyimide precursor and blending it with a non-thermally stable polymer as a pore-forming agent, the present invention explores the effects of different structures and pore-forming agents on the separation performance of carbon molecular sieve membranes at different pyrolysis temperatures. The resulting novel polymer-blended carbon molecular sieve membrane exhibits high butadiene / n-butane and butadiene / isobutane selectivity, as well as high butadiene permeability.

[0007] The present invention primarily synthesizes a novel polymer blend precursor membrane comprising polymers A and B. Polymer A is a thermally stable fluorenyl polyimide: dianhydrides selected from the highly aromatic 6FDA, PBD, and BTDA; diamines ① selected from the highly rigid and side-group-rich Durene, DAM, and mPDA; and diamines ② selected from the high-carbon-content and highly aromatic fluorenyl monomers FFDA and BAMF. Polymer B is a thermally unstable polymer selected from polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and polyvinyl alcohol (PVA). The blend precursor membranes of varying compositions and proportions are then carbonized. The pore size and pore distribution are adjusted by adjusting the pore-forming agent content and carbonization temperature to yield a carbon molecular sieve membrane with excellent butadiene / butane separation performance.

[0008] The technical solution of the present invention:

[0009] A novel polymer blend carbon molecular sieve membrane for butadiene / butane separation. The polymer blend precursor of the novel polymer blend carbon molecular sieve membrane consists of polymer A and polymer B, with the mass ratio of polymer A to polymer B being 95:5 to 50:50; wherein polymer A contains the following repeating unit structure:

[0010]

[0011] Wherein, x, y are the degree of polymerization of polymer A, x, y=500-1000;

[0012] Wherein, the dianhydride is any one of the following structures, namely 6FDA, PBD, and BTDA:

[0013] ;

[0014] Diamine ① is any one of the following highly rigid structures, namely Durene, DAM, and mPDA:

[0015]

[0016] Diamine ② is any one of the following fluorene-containing structures, namely FFDA and BAMF:

[0017]

[0018] Polymer B is any one of the following pore-forming agents, namely polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG), and polyvinyl alcohol (PVA):

[0019] .

[0020] A method for preparing a novel polymer blended carbon molecular sieve membrane for butadiene / butane separation comprises the following steps:

[0021] (1) Preparation of polymer A (polyimide): Diamine ① and diamine ② were mixed in a three-necked flask filled with nitrogen, and m-cresol was added and stirred to dissolve the mixed monomers until the solution was clear. Subsequently, dianhydride was added and reacted until the solution was clear. A catalyst was added and the reaction system was heated for a period of time before stopping the heating. After cooling to room temperature, the product was precipitated in methanol to obtain a white filamentous product. After washing several times, the product was placed in a vacuum oven at a certain temperature and dried for a certain period of time to obtain polymer A.

[0022] The molar ratio of diamine ① to diamine ② is 3:1 to 1:3;

[0023] The total concentration of diamine ①, diamine ② and dianhydride in the reaction system is 5% to 30% (w / w);

[0024] The amount of dianhydride added is the sum of the moles of diamine ① and diamine ②;

[0025] The reaction catalyst is isoquinoline;

[0026] The reaction system is heated to a temperature of 160-200°C;

[0027] The reaction time is 6 to 10 hours;

[0028] The vacuum drying temperature is 120-190°C;

[0029] The vacuum drying time is 24 to 48 hours.

[0030] (2) Preparation of a novel polymer blend precursor membrane: polymer A and polymer B are mixed in a casting solvent to prepare a casting solution; after stirring and completely dissolving, the mixture is filtered through a filter membrane and cast into a culture dish of a certain size, and placed in a constant temperature drying oven at a certain temperature to evaporate until a membrane is formed. The membrane is then transferred to a vacuum drying oven at a certain temperature for drying to obtain a novel polymer blend precursor membrane.

[0031] The mass ratio of polymer A to polymer B is 95:5 to 50:50;

[0032] The casting solution solvent is one of THF, DMAc and DMF;

[0033] The solute concentration in the casting solution is 0.5% to 5% (w / w);

[0034] The constant temperature drying temperature is 50-100°C;

[0035] The vacuum drying temperature is 120-150°C;

[0036] The vacuum drying time is 48 to 96 hours;

[0037] The thickness of the novel polymer blend precursor film is 30-60 μm.

[0038] (3) Preparation of a novel polymer blend carbon molecular sieve membrane: The novel polymer blend precursor membrane is placed between two smooth square corundum plates. Corundum flakes of a certain thickness are placed between the two square corundum plates at the four corners to prevent the plates from crushing the membrane. After pyrolysis in an argon atmosphere according to a certain procedure, a novel polymer blend carbon molecular sieve membrane with a smooth and uniform surface is obtained.

[0039] The thickness of the corundum flakes is 1 to 3 mm;

[0040] The pyrolysis temperature of the carbonization process is 500-1000°C;

[0041] The pyrolysis time of the carbonization process is 1 to 4 hours.

[0042] The present invention has the following beneficial effects: By blending a high-carbon-content, high-rigidity, highly aromatic polyimide with a pore-forming polymer, the present invention effectively prevents the introduction of the pore-forming agent from degrading the mechanical properties of the carbon membrane. By adjusting the pore-forming agent ratio and carbonization temperature, a series of uniform and smooth carbon molecular sieve membranes were prepared. The effects of different pore-forming agent ratios on the butadiene / butane separation performance during the carbonization process were investigated, as were the effects of different pyrolysis temperatures on the gas separation performance of the carbon molecular sieve membranes. By adjusting these parameters, a novel polymer-blended carbon molecular sieve membrane with excellent butadiene / butane separation performance was prepared. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a comparison of the performance and separation upper limit of the gas separation membranes prepared in the comparative examples and examples. DETAILED DESCRIPTION

[0044] The specific implementation of the present invention is further described below in conjunction with the technical solution.

[0045] Example 1

[0046] A novel polymer blend carbon molecular sieve membrane is prepared by mixing polyimide 6FDA-FFDA / DAM (1:1) and polyvinylpyrrolidone (PVP) in a mass ratio of 90:10 and pyrolyzing the mixture at 500°C to prepare the polymer blend carbon molecular sieve membrane.

[0047] This embodiment provides a novel polymer blended carbon molecular sieve membrane, which specifically includes the following steps:

[0048] Before the reaction, the three-necked flask was vacuum-dried at 100°C for 12 hours. High-purity nitrogen was then purged at 200 mL / min to remove residual water and air from the reactor. After purging for ten minutes, the nitrogen flow rate was reduced to 100 mL / min and used as a protective gas until the reaction was complete. Subsequently, 10 mmol of FFDA and 10 mmol of DAM were added, along with 50 mL of m-cresol. Mechanical stirring was used to dissolve the diamine. Once the solution clarified, 20 mmol of 6FDA was added. The reaction was continued at room temperature until the solution became clear. Then, 1.2 mL of isoquinoline was added, and the system was heated to 200°C for 16 hours. After the reaction was completed, the mixture was cooled to room temperature and precipitated in methanol to obtain a white, filamentous product. The product was washed three times with methanol for 24 hours each time and dried in a vacuum oven at 120°C for 48 hours to remove any residual solvent.

[0049] 0.18 g of the prepared polyimide and 0.02 g of PVP were mixed in 10 g of DMAc and completely dissolved and blended using magnetic stirring to prepare a 2% (w / w) casting solution. The casting solution was filtered through a filter membrane and cast onto a smooth watch glass. The film was evaporated in an 80°C constant temperature drying oven and then transferred to a 120°C vacuum drying oven to dry for 48 hours.

[0050] The resulting membrane was placed between two sufficiently large, smooth corundum plates. Small 2 mm-thick corundum flakes were placed at each of the four corners between the plates to prevent the plates from crushing the carbon membrane. The membrane was then transferred to a tube furnace under continuous argon purge, heated to 500°C for 2 hours, and then cooled naturally to obtain a 25 μm-thick 6FDA-FFDA / DAM (1:1) / PVP-90:10-CMS500 carbon molecular sieve membrane.

[0051] The obtained carbon molecular sieve membrane was tested for gas separation performance at 50°C and 1 bar, where P(butadiene) = 2801.6 Barrer, P(n-butane) = 116.7 Barrer, P(isobutane) = 13.2 Barrer; selectivity α(butadiene / n-butane) = 24.0, α(butadiene / isobutane) = 212.2.

[0052] Compared to Comparative Example 1, this film incorporates 10 wt% PVP as a pore-forming agent in the precursor membrane. PVP has a thermal decomposition temperature between 400°C and 500°C, so a carbonization temperature of 500°C allows for complete pyrolysis of PVP. The gas molecules produced during pyrolysis open interchain cavities, generating numerous micropores and ultramicropores. Consequently, compared to Comparative Example 1, butadiene permeability increased by 24.4%, while the butadiene / n-butane selectivity remained virtually unchanged.

[0053] Example 2

[0054] A novel polymer blend carbon molecular sieve membrane is provided. The polymer blend membrane is prepared by mixing polyimide 6FDA-FFDA / DAM (1:1) and polyvinylpyrrolidone (PVP) in a mass ratio of 90:10 and pyrolyzing the mixture at 525°C to prepare the polymer blend carbon molecular sieve membrane.

[0055] This embodiment provides a novel polymer blended carbon molecular sieve membrane, which specifically includes the following steps:

[0056] Before the reaction, the three-necked flask was vacuum-dried at 100°C for 12 hours. High-purity nitrogen was then purged at 200 mL / min to remove residual water and air from the reactor. After purging for ten minutes, the nitrogen flow rate was reduced to 100 mL / min and used as a protective gas until the reaction was complete. Subsequently, 10 mmol of FFDA and 10 mmol of DAM were added, along with 50 mL of m-cresol. Mechanical stirring was used to dissolve the diamine. Once the solution clarified, 20 mmol of 6FDA was added. The reaction was continued at room temperature until the solution became clear. Then, 1.2 mL of isoquinoline was added, and the system was heated to 200°C for 16 hours. After the reaction was completed, the mixture was cooled to room temperature and precipitated in methanol to obtain a white, filamentous product. The product was washed three times with methanol for 24 hours each time and dried in a vacuum oven at 120°C for 48 hours to remove any residual solvent.

[0057] 0.18 g of the prepared polyimide and 0.02 g of PVP were mixed in 10 g of DMAc and completely dissolved and blended using magnetic stirring to prepare a 2% (w / w) casting solution. The casting solution was filtered through a filter membrane and cast onto a smooth watch glass. The film was evaporated in an 80°C constant temperature drying oven and then transferred to a 120°C vacuum drying oven to dry for 48 hours.

[0058] The resulting membrane was placed between two sufficiently large, smooth corundum plates. Small 2 mm thick corundum flakes were placed at each of the four corners between the plates to prevent the plates from crushing the carbon membrane. The membrane was then transferred to a tube furnace under continuous argon purge, heated to 525°C for 2 hours, and then cooled naturally to obtain a 6FDA-FFDA / DAM (1:1) / PVP-90:10-CMS525 carbon molecular sieve membrane with a thickness of 23 μm.

[0059] The obtained carbon molecular sieve membrane was tested for gas separation performance at 50°C and 1 bar, where P(butadiene) = 3965.2 Barrer, P(n-butane) = 88.7 Barrer, P(isobutane) = 13.3 Barrer; selectivity α(butadiene / n-butane) = 44.7, α(butadiene / isobutane) = 299.6.

[0060] Compared to Comparative Example 2, the precursor membrane contains 10 wt% PVP as a pore-forming agent. At a carbonization temperature of 525°C, the addition of the pore-forming agent also exerts a positive effect, increasing permeability by 8.5% compared to Comparative Example 2, which does not incorporate a pore-forming agent, while maintaining selectivity. However, this improvement is significantly lower than that observed in Example 1 and Comparative Example 1, suggesting that further increases in temperature may lead to other effects.

[0061] Example 3

[0062] A novel polymer blend carbon molecular sieve membrane is prepared by mixing polyimide 6FDA-FFDA / DAM (1:1) and polyvinylpyrrolidone (PVP) in a mass ratio of 90:10 and pyrolyzing the mixture at 600°C to prepare the polymer blend carbon molecular sieve membrane.

[0063] This embodiment provides a novel polymer blended carbon molecular sieve membrane, which specifically includes the following steps:

[0064] Before the reaction, the three-necked flask was vacuum-dried at 100°C for 12 hours. High-purity nitrogen was then purged at 200 mL / min to remove residual water and air from the reactor. After purging for ten minutes, the nitrogen flow rate was reduced to 100 mL / min and used as a protective gas until the reaction was complete. Subsequently, 10 mmol of FFDA and 10 mmol of DAM were added, along with 50 mL of m-cresol. Mechanical stirring was used to dissolve the diamine. Once the solution clarified, 20 mmol of 6FDA was added. The reaction was continued at room temperature until the solution became clear. Then, 1.2 mL of isoquinoline was added, and the system was heated to 200°C for 16 hours. After the reaction was completed, the mixture was cooled to room temperature and precipitated in methanol to obtain a white, filamentous product. The product was washed three times with methanol for 24 hours each time and dried in a vacuum oven at 120°C for 48 hours to remove any residual solvent.

[0065] 0.18 g of the prepared polyimide and 0.02 g of PVP were mixed in 10 g of DMAc and completely dissolved and blended using magnetic stirring to prepare a 2% (w / w) casting solution. The casting solution was filtered through a filter membrane and cast onto a smooth watch glass. The film was evaporated in an 80°C constant temperature drying oven and then transferred to a 120°C vacuum drying oven to dry for 48 hours.

[0066] The resulting membrane was placed between two sufficiently large, smooth corundum plates. Small 2 mm thick corundum flakes were placed at each of the four corners between the plates to prevent the plates from crushing the carbon membrane. The membrane was then transferred to a tube furnace under continuous argon purge, heated to 600°C for 2 hours, and then cooled naturally to obtain a 26 μm thick 6FDA-FFDA / DAM (1:1) / PVP-90:10-CMS600 carbon molecular sieve membrane.

[0067] The obtained carbon molecular sieve membrane was tested for gas separation performance at 50°C and 1 bar, where P(butadiene) = 4023.7 Barrer, P(n-butane) = 27.9 Barrer, P(isobutane) = 6.1 Barrer; selectivity α(butadiene / n-butane) = 144.4, α(butadiene / isobutane) = 663.1.

[0068] Compared to Comparative Example 3, the difference lies in the addition of 10 wt% PVP as a pore-forming agent to the precursor membrane. After carbonization at 700°C, permeability only increased by 9.3%, but butadiene / n-butane selectivity increased significantly by approximately 72%. This increase in selectivity relies on the enhanced size-sieving capability of ultramicropores. The micropores produced by the pore-forming agent gradually transform into an ultramicroporous structure, resulting in a more uniform pore distribution. More ultramicropores have sizes between the kinetic diameters of butadiene (4.31 Å) and n-butane (4.69 Å). Compared to Examples 1 and 2, the pyrolysis temperature of 600°C is superior to that of 500 and 525°C, making it more suitable for C4 fraction separation applications.

[0069] Example 4

[0070] A novel polymer blend carbon molecular sieve membrane is provided. The polymer blend membrane is prepared by mixing polyimide 6FDA-FFDA / DAM (1:1) and polyvinylpyrrolidone (PVP) in a mass ratio of 90:10 and pyrolyzing the mixture at 700°C to prepare the polymer blend carbon molecular sieve membrane.

[0071] This embodiment provides a novel polymer blended carbon molecular sieve membrane, which specifically includes the following steps:

[0072] Before the reaction, the three-necked flask was vacuum-dried at 100°C for 12 hours. High-purity nitrogen was then purged at 200 mL / min to remove residual water and air from the reactor. After purging for ten minutes, the nitrogen flow rate was reduced to 100 mL / min and used as a protective gas until the reaction was complete. Subsequently, 10 mmol of FFDA and 10 mmol of DAM were added, along with 50 mL of m-cresol. Mechanical stirring was used to dissolve the diamine. Once the solution clarified, 20 mmol of 6FDA was added. The reaction was continued at room temperature until the solution became clear. Then, 1.2 mL of isoquinoline was added, and the system was heated to 200°C for 16 hours. After the reaction was completed, the mixture was cooled to room temperature and precipitated in methanol to obtain a white, filamentous product. The product was washed three times with methanol for 24 hours each time and dried in a vacuum oven at 120°C for 48 hours to remove any residual solvent.

[0073] 0.18 g of the prepared polyimide and 0.02 g of PVP were mixed in 10 g of DMAc and completely dissolved and blended using magnetic stirring to prepare a 2% (w / w) casting solution. The casting solution was filtered through a filter membrane and cast onto a smooth watch glass. The film was evaporated in an 80°C constant temperature drying oven and then transferred to a 120°C vacuum drying oven to dry for 48 hours.

[0074] The resulting membrane was placed between two sufficiently large, smooth corundum plates. Small 2 mm thick corundum flakes were placed at each of the four corners between the plates to prevent the plates from crushing the carbon membrane. The membrane was then transferred to a tube furnace under continuous argon purge, heated to 700°C for 2 hours, and then cooled naturally to obtain a 6FDA-FFDA / DAM (1:1) / PVP-90:10-CMS700 carbon molecular sieve membrane with a thickness of 20 μm.

[0075] The obtained carbon molecular sieve membrane was tested for gas separation performance at 50°C and 1 bar, where P(butadiene) = 1114.7 Barrer, P(n-butane) = 7.0 Barrer, P(isobutane) = 1.5 Barrer; selectivity α(butadiene / n-butane) = 160.3, α(butadiene / isobutane) = 733.1.

[0076] Compared to Comparative Example 4, the difference lies in the addition of 10 wt% PVP as a pore-forming agent to the precursor membrane. When the pyrolysis temperature rises to 700°C, the performance deteriorates significantly compared to the undoped Comparative Example 4, with butadiene permeability decreasing by 30.1% and butadiene / n-butane selectivity decreasing by 55.6%. This is due to the collapse of a large number of micropores and some ultramicropores at high temperatures, which clogs the pores and increases the density of chain accumulation, ultimately leading to a decrease in both permeability and selectivity. Therefore, pyrolysis temperatures of 700°C and above are not suitable for carbon molecular sieve membranes doped with pore-forming agents.

[0077] Example 5

[0078] A novel polymer blend carbon molecular sieve membrane is provided. The polymer blend membrane is prepared by mixing polyimide 6FDA-FFDA / DAM (1:1) and polyvinylpyrrolidone (PVP) in a mass ratio of 95:5 and pyrolyzing the mixture at 600°C to prepare the polymer blend carbon molecular sieve membrane.

[0079] This embodiment provides a novel polymer blended carbon molecular sieve membrane, which specifically includes the following steps:

[0080] Before the reaction, the three-necked flask was vacuum-dried at 100°C for 12 hours. High-purity nitrogen was then purged at 200 mL / min to remove residual water and air from the reactor. After purging for ten minutes, the nitrogen flow rate was reduced to 100 mL / min and used as a protective gas until the reaction was complete. Subsequently, 10 mmol of FFDA and 10 mmol of DAM were added, along with 50 mL of m-cresol. Mechanical stirring was used to dissolve the diamine. Once the solution clarified, 20 mmol of 6FDA was added. The reaction was continued at room temperature until the solution became clear. Then, 1.2 mL of isoquinoline was added, and the system was heated to 200°C for 16 hours. After the reaction was completed, the mixture was cooled to room temperature and precipitated in methanol to obtain a white, filamentous product. The product was washed three times with methanol for 24 hours each time and dried in a vacuum oven at 120°C for 48 hours to remove any residual solvent.

[0081] 0.19 g of the prepared polyimide and 0.01 g of PVP were mixed in 10 g of DMAc and completely dissolved and blended using magnetic stirring to prepare a 2% (w / w) casting solution. The casting solution was filtered through a filter membrane and cast onto a smooth watch glass. The film was evaporated in an 80°C constant temperature drying oven and then transferred to a 120°C vacuum drying oven to dry for 48 hours.

[0082] The resulting membrane was placed between two sufficiently large, smooth corundum plates. Small 2 mm thick corundum flakes were placed at each of the four corners between the plates to prevent the plates from crushing the carbon membrane. The membrane was then transferred to a tube furnace continuously purged with argon and pyrolyzed at 600°C for 2 hours, followed by natural cooling. A 28 μm thick 6FDA-FFDA / DAM (1:1) / PVP-95:5-CMS600 carbon molecular sieve membrane was obtained.

[0083] The obtained carbon molecular sieve membrane was tested for gas separation performance at 50°C and 1 bar, where P(butadiene) = 3146.2 Barrer, P(n-butane) = 53.9 Barrer, P(isobutane) = 10.1 Barrer; selectivity α(butadiene / n-butane) = 58.4, α(butadiene / isobutane) = 312.0.

[0084] Compared to Example 3, this example differs in that 5 wt% PVP was added to the precursor membrane as a pore-forming agent. At the optimal carbonization temperature of 600°C, when the pore-forming agent content was reduced from 10% to 5%, the permeability decreased significantly by 21.8%, while the butadiene / n-butane selectivity also dropped by 59.6%. This is because a decrease in pore-forming agent content reduces the number of micropores generated and the number of ultramicropores converted from micropores. Therefore, the following examples will attempt to increase the pore-forming agent content.

[0085] Example 6

[0086] A novel polymer blend carbon molecular sieve membrane is provided. The polymer blend membrane is prepared by mixing polyimide 6FDA-FFDA / DAM (1:1) and polyvinylpyrrolidone (PVP) in a mass ratio of 85:15, and pyrolysis is performed at 600°C to prepare the polymer blend carbon molecular sieve membrane.

[0087] This embodiment provides a novel polymer blended carbon molecular sieve membrane, which specifically includes the following steps:

[0088] Before the reaction, the three-necked flask was vacuum-dried at 100°C for 12 hours. High-purity nitrogen was then purged at 200 mL / min to remove residual water and air from the reactor. After purging for ten minutes, the nitrogen flow rate was reduced to 100 mL / min and used as a protective gas until the reaction was complete. Subsequently, 10 mmol of FFDA and 10 mmol of DAM were added, along with 50 mL of m-cresol. Mechanical stirring was used to dissolve the diamine. Once the solution clarified, 20 mmol of 6FDA was added. The reaction was continued at room temperature until the solution became clear. Then, 1.2 mL of isoquinoline was added, and the system was heated to 200°C for 16 hours. After the reaction was completed, the mixture was cooled to room temperature and precipitated in methanol to obtain a white, filamentous product. The product was washed three times with methanol for 24 hours each time and dried in a vacuum oven at 120°C for 48 hours to remove any residual solvent.

[0089] 0.17g of the prepared polyimide and 0.03g of PVP were mixed in 10g of DMAc and completely dissolved and blended using magnetic stirring to prepare a 2% (w / w) casting solution. The casting solution was filtered through a filter membrane and cast onto a smooth watch glass. The film was evaporated in an 80°C constant temperature drying oven and then transferred to a 120°C vacuum drying oven to dry for 48 hours.

[0090] The resulting membrane was placed between two sufficiently large, smooth corundum plates. Small 2 mm thick corundum flakes were placed at each of the four corners between the plates to prevent the plates from crushing the carbon membrane. The membrane was then transferred to a tube furnace under continuous argon purge, heated to 600°C for 2 hours, and then cooled naturally to obtain a 26 μm thick 6FDA-FFDA / DAM (1:1) / PVP-85:15-CMS600 carbon molecular sieve membrane.

[0091] The obtained carbon molecular sieve membrane was tested for gas separation performance at 50°C and 1 bar, with P(butadiene) = 4769.2 Barrer, P(n-butane) = 44.9 Barrer, P(isobutane) = 10.1 Barrer; selectivity α(butadiene / n-butane) = 106.1, α(butadiene / isobutane) = 473.0.

[0092] Compared to Examples 3 and 5, the difference lies in the addition of 15 wt% PVP as a pore-forming agent to the precursor membrane. After carbonization at 600°C, the permeability changes were consistent with predictions, showing a significant improvement. Compared to the 10% doping, butadiene permeability increased by 18.5%, but the selectivity decreased by 26.5%. This is because increasing the pore-forming agent content significantly increases the number of micropores, which in turn reduces the ultramicroporosity and selectivity. It is foreseeable that with further increases in the pore-forming agent content, the selectivity will continue to decline, making it unsuitable for industrial separation of C4 fractions.

[0093] Comparative Example 1

[0094] This comparative example provides a polyimide-derived carbon molecular sieve membrane, which differs from Example 1 only in that the precursor membrane does not contain the pore-forming agent PVP. The other raw materials, dosages, and implementation steps are the same as those in Example 1. Finally, the precursor membrane is carbonized at 500°C. The carbon molecular sieve membrane is: 6FDA-FFDA / DAM (1:1)-CMS500.

[0095] The obtained carbon molecular sieve membrane was tested for gas separation performance at 50°C and 1 bar, where P(butadiene) = 2252.3 Barrer, P(n-butane) = 81.9 Barrer, P(isobutane) = 10.5 Barrer; selectivity α(butadiene / n-butane) = 27.5, α(butadiene / isobutane) = 215.1.

[0096] Comparative Example 2

[0097] This comparative example provides a polyimide-derived carbon molecular sieve membrane, which differs from Example 2 only in that the precursor membrane does not contain the pore-forming agent PVP. The other raw materials, dosages, and implementation steps are the same as those in Example 2. Finally, the precursor membrane is carbonized at 525°C. The carbon molecular sieve membrane is: 6FDA-FFDA / DAM (1:1)-CMS525.

[0098] The obtained carbon molecular sieve membrane was tested for gas separation performance at 50°C and 1 bar, with P(butadiene) = 3655.6 Barrer, P(n-butane) = 76.2 Barrer, P(isobutane) = 13.4 Barrer; selectivity α(butadiene / n-butane) = 47.9, α(butadiene / isobutane) = 273.7.

[0099] Comparative Example 3

[0100] This comparative example provides a polyimide-derived carbon molecular sieve membrane, which differs from Example 3 only in that the precursor membrane does not contain the pore-forming agent PVP. The other raw materials, dosages, and implementation steps are the same as those in Example 3. Finally, the precursor membrane is carbonized at 600°C. The carbon molecular sieve membrane is: 6FDA-FFDA / DAM (1:1)-CMS600.

[0101] The obtained carbon molecular sieve membrane was tested for gas separation performance at 50°C and 1 bar, where P(butadiene) = 3681.3 Barrer, P(n-butane) = 43.8 Barrer, P(isobutane) = 4.8 Barrer; selectivity α(butadiene / n-butane) = 84.0, α(butadiene / isobutane) = 775.7.

[0102] Comparative Example 4

[0103] This comparative example provides a polyimide-derived carbon molecular sieve membrane, which differs from Example 4 only in that the precursor membrane does not contain the pore-forming agent PVP. The other raw materials, dosages, and implementation steps are the same as those in Example 4. Finally, the precursor membrane is carbonized at 700°C. The carbon molecular sieve membrane is: 6FDA-FFDA / DAM (1:1)-CMS700.

[0104] The obtained carbon molecular sieve membrane was tested for gas separation performance at 50°C and 1 bar, where P(butadiene) = 1596.2 Barrer, P(n-butane) = 4.4 Barrer, P(isobutane) = 0.3 Barrer; selectivity α(butadiene / n-butane) = 361.0, α(butadiene / isobutane) = 4977.1.

[0105] Table 1 Gas permeability and selectivity of gas separation membranes prepared in comparative examples and examples

[0106]

[0107] The performance of blended membranes doped with different pore-forming agent ratios after carbonization at different temperatures was compared. As shown in Table 1, when the carbonization temperature is below 700°C, the addition of the pore-forming agent effectively increases the microporosity and permeability of the carbon membrane. In particular, at 600°C, the pore-forming agent completely decomposes and transforms into micropores, with some micropores transforming into ultramicropores, thereby increasing butadiene / butane selectivity. Carbonization temperatures of 700°C or higher cause micropore collapse, severely blocking the pores and reducing both permeability and selectivity. Therefore, this study specifically selected 600°C as the optimal carbonization temperature. The effects of different pore-forming agent ratios on separation performance were compared at 600°C. When the pore-forming agent content is less than 10%, fewer micropores are generated, and consequently, fewer micropores are converted into ultramicropores. Consequently, both permeability and selectivity increase with increasing pore-forming agent ratios. When the pore-forming agent content is greater than 10%, a large number of micropores will be generated. The micropores will not only promote the penetration of butadiene, but also accelerate the penetration of large molecular butane. Therefore, as the proportion of pore-forming agent increases, the permeability increases and the selectivity decreases. In summary, the best performance carbon molecular sieve membrane 6FDA-FFDA / DAM (1:1) / PVP-90:10-CMS600 is obtained. Figure 1 It can be seen that its separation performance far exceeds the upper limit of butadiene / n-butane separation, and has good development prospects in the field of C4 fraction separation.

Claims

1. A novel polymer blended carbon molecular sieve membrane for butadiene / butane separation, characterized in that: The polymer blend precursor of the novel polymer blend carbon molecular sieve membrane consists of polymer A and polymer B, with the mass ratio of polymer A to polymer B being 95:5 to 50:50; wherein polymer A contains the following repeating unit structure: , where x, y are the degree of polymerization of polymer A, x, y = 500-1000; Wherein, the dianhydride is any one of the following structures, namely 6FDA, PBD, and BTDA: ； Diamine ① is any one of the following highly rigid structures, namely Durene, DAM, and mPDA: , diamine ② is any one of the following fluorene-containing structures, namely FFDA, BAMF: , polymer B is any one of the following pore-forming agents, namely polyvinyl pyrrolidone, polyethylene glycol, and polyvinyl alcohol: 。 2. A method for preparing the novel polymer blended carbon molecular sieve membrane for butadiene / butane separation according to claim 1, characterized in that: Here are the steps: (1) Preparation of polymer A: Diamine ① and diamine ② were mixed in a three-necked flask filled with nitrogen, and m-cresol was added and stirred to dissolve the mixed monomers until the solution was clear; then dianhydride was added and reacted until the solution was clear, a catalyst was added and the reaction system was heated for a period of time, then heating was stopped, and the reaction system was cooled naturally to room temperature and precipitated in methanol to obtain a white filamentous product; after washing several times, the product was placed in a vacuum oven at a certain temperature and dried for a certain period of time to obtain polymer A; (2) Preparation of a novel polymer blend precursor membrane: polymer A and polymer B are mixed in a casting solvent to prepare a casting solution; after stirring and completely dissolving, the mixture is filtered through a filter membrane and cast into a culture dish, and placed in a constant temperature drying oven at a certain temperature to evaporate until a membrane is formed. The membrane is then transferred to a vacuum drying oven at a certain temperature for drying to obtain a novel polymer blend precursor membrane; (3) Preparation of a novel polymer blend carbon molecular sieve membrane: The novel polymer blend precursor membrane is placed between two square corundum plates with smooth surfaces. Corundum sheets of a certain thickness are placed at the four corners between the two square corundum plates to prevent the corundum plates from crushing the membrane. After pyrolysis according to a certain procedure under an argon atmosphere, a novel polymer blend carbon molecular sieve membrane with a smooth and uniform surface is obtained.

3. The preparation method according to claim 2, characterized in that In step (1), The molar ratio of diamine ① to diamine ② is 3:1 to 1:3; The total mass percentage concentration of diamine ①, diamine ② and dianhydride in the reaction system is 5% to 30%; The amount of dianhydride added is the sum of the moles of diamine ① and diamine ②; The reaction catalyst is isoquinoline; The reaction system is heated to a temperature of 160-200°C; The reaction time is 6 to 10 hours; The vacuum drying temperature is 120-190°C; The vacuum drying time is 24 to 48 hours.

4. The preparation method according to claim 2, characterized in that In step (2), The mass ratio of polymer A to polymer B is 95:5 to 50:50; The casting solution solvent is one of THF, DMAc and DMF; The mass percentage concentration of the solute in the casting solution is 0.5% to 5%; The constant temperature drying temperature is 50-100°C; The vacuum drying temperature is 120-150°C; The vacuum drying time is 48 to 96 hours; The thickness of the novel polymer blend precursor film is 30-60 μm.

5. The preparation method according to claim 2, characterized in that In step (3), The thickness of the corundum flakes is 1 to 3 mm; The pyrolysis temperature of the carbonization process is 500-1000°C; The pyrolysis time of the carbonization process is 1 to 4 hours.

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