Polyimide for oxygen-nitrogen gas separation membrane, method for producing the same, and oxygen-nitrogen gas separation membrane

Abstract

This invention provides a polyimide for oxygen and nitrogen gas separation membranes and its preparation method, comprising the following steps: a tetracarboxylic dianhydride monomer and a diisocyanate monomer undergo a polycondensation reaction in an aprotic polar solvent in the presence of a catalyst to obtain the polyimide; the molar ratio of the tetracarboxylic dianhydride monomer to the diisocyanate monomer is 95:100 to 100:95. The tetracarboxylic dianhydride monomer is a mixture selected from one or more of aromatic tetracarboxylic dianhydrides, aliphatic tetracarboxylic dianhydrides, and heterocyclic tetracarboxylic dianhydrides; the diisocyanate monomer is a mixture selected from one or more of aromatic diisocyanates and aliphatic diisocyanates. The resulting polyimide membrane material possesses both a high free volume fraction (for increasing O2 permeation flux) and suitable interchain spacing (for achieving the O2 / N2 molecular sieving effect), thereby overcoming the bottleneck of the mutual constraint between permeability and selectivity in traditional polyimide membranes.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, and relates to a polyimide, specifically to a polyimide for oxygen and nitrogen gas separation membrane, its preparation method, and the oxygen and nitrogen gas separation membrane. Background Technology

[0002] Gas separation technology is one of the core technologies in modern chemical, energy, environmental protection, and defense fields. Traditional gas separation methods mainly include cryogenic distillation, pressure swing adsorption, and chemical absorption. Although these technologies are mature and widely used, they generally suffer from drawbacks such as high energy consumption, large equipment size, and complex operation. Membrane separation technology, as an emerging and highly efficient separation method, has significant advantages such as low energy consumption, simple operation, no phase change, small footprint, and easy scale-up, showing broad application prospects in the field of gas separation.

[0003] Air separation (oxygen / nitrogen separation) is one of the important applications of gas membrane separation technology. Oxygen-enriched air (O2 content 21%~40%) can be widely used in medical oxygen supply, oxygen-enriched combustion, wastewater treatment, metallurgy and chemical industries; while nitrogen-enriched air (N2 content 95%~99.9%) has important application value in inert protective atmosphere scenarios such as food preservation, electronic packaging, metal heat treatment, and oil and gas storage and transportation. Compared with traditional cryogenic air separation (i.e., air separation) and pressure swing adsorption technology, membrane air separation has outstanding advantages such as fast start-up, low energy consumption, compact equipment, and simple maintenance, and is especially suitable for small and medium-scale applications with medium to low purity requirements.

[0004] The performance of gas separation membranes is primarily determined by two core indicators: permeate flux and separation selectivity. Developing membrane materials that combine high permeability and high selectivity is a core technological challenge in the field of gas separation membranes. Currently, the most widely studied gas separation membrane materials include polysulfone, polyethersulfone, polycarbonate, cellulose acetate, polyimide, and polyetheretherketone. Among these, polyimide (PI) has become a research hotspot in the field of high-performance gas separation membranes due to its unique molecular structure and excellent overall performance.

[0005] Polyimide is a high-performance polymer containing an imide ring structure, generated by the polycondensation reaction of diamine and dianhydride monomers. As a gas separation membrane material, polyimide has the following outstanding advantages: (1) Good film-forming properties: Polyimide has good solubility and processability, and can be used to prepare dense membranes, asymmetric membranes and hollow fiber membranes by various processes such as solution casting, non-solvent induced phase separation (NIPS), and dry-wet spinning; (2) Flexible structural designability: By selecting dianhydride and diamine monomers with different structures, the segment rigidity and packing density of polyimide can be controlled at the molecular level, thereby optimizing its gas separation performance. For example, introducing polar groups (such as -OH, -SO3H) can enhance inter-chain interactions and improve selectivity. However, existing polyimide membrane materials still have shortcomings in terms of synergistic optimization of permeability and selectivity, processability, and long-term operational stability, and there is an urgent need to develop new polyimide membrane materials and membrane preparation processes. Existing polyimide materials are commonly prepared by reacting dianhydrides and diamine monomers. The resulting polyimide materials are not easily or insoluble in organic solvents, have high melting points, and are difficult to dissolve, which means they cannot be processed into the required separation membranes, especially hollow fiber polyimide separation membranes. Alternatively, polyimide materials prepared by this method have low mechanical strength, low efficiency, and short lifespan when applied to gas separation membranes. Summary of the Invention

[0006] In order to solve the above-mentioned technical problems, the purpose of this invention is to provide a soluble high-strength polyimide for oxygen and nitrogen gas separation membranes.

[0007] To achieve the above-mentioned technical objectives, the present invention provides a method for preparing polyimide for oxygen and nitrogen gas separation membranes, comprising the following steps: The polyimide is obtained by polycondensation of tetracarboxylic dianhydride monomer and diisocyanate monomer in an aprotic polar solvent, with or without a catalyst. The molar ratio of the tetracarboxylic dianhydride monomer to the diisocyanate monomer is 95:100 to 100:95; The tetracarboxylic dianhydride monomer is a mixture of one or more of aromatic tetracarboxylic dianhydrides, aliphatic tetracarboxylic dianhydrides, and heterocyclic tetracarboxylic dianhydrides; the diisocyanate monomer is a mixture of one or more of aromatic diisocyanates and aliphatic diisocyanates.

[0008] Optimally, the aromatic tetracarboxylic dianhydride includes 4,4′-biphenyl ether dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, pyromellitic dianhydride, 4,4′-(acetylene-1,2-diyl)phthalic anhydride, 3,3′,4,4′-biphenyltetracarboxylic dianhydride, 2,3,3′,4′-biphenyltetracarboxylic dianhydride, 3,3,4,4-benzophenone tetracarboxylic dianhydride, hexafluoro dianhydride, perylene-3,4,9,10-tetracarboxylic dianhydride, 3,3′,4,4′-triphenylbisether tetracarboxylic dianhydride, 3,3′,4,4′-diphenylsulfonyl tetracarboxylic dianhydride, 4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride), and 4,4′-bis(3,4-dicarboxyphenylthio)diphenylsulfide dianhydride; The aliphatic tetracarboxylic dianhydrides include cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, methylcyclohexenetetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, pentacyclo[8.2.1.1]tetradecane-5,6,11,12-tetracarboxylic dianhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, ethylenediaminetetraacetic acid dianhydride, and dicyclohexyl-3,4,3',4'-tetracarboxylic dianhydride. The heterocyclic tetracarboxylic dianhydrides include 5,6-dihydro-1,4-dithiane-2,3-dicarboxylic anhydride and pyrimidine tetracarboxylic dianhydride.

[0009] Optimally, the aromatic diisocyanate includes toluene diisocyanate, diphenylmethane diisocyanate, naphthalene-1,5-diisocyanate, and terephthalic diisocyanate. The aliphatic diisocyanates include hexamethylene diisocyanate, isophorone diisocyanate, tetramethyl dimethylphenyl diisocyanate, trimethyl hexamethylene diisocyanate, dicyclohexylmethane diisocyanate, methylcyclohexane diisocyanate, phenylenediamine diisocyanate, hydrogenated phenylenediamine diisocyanate, 1,4-cyclohexane diisocyanate, and norbornene diisocyanate.

[0010] Preferably, the catalyst is an alkaline catalyst or an acidic catalyst; The alkaline catalyst is selected from one or more of organic amine compounds, nitrogen-containing heterocyclic compounds, inorganic bases, and organometallic compounds; The acidic catalyst is selected from one or more of inorganic acids, organic acids, aqueous solutions of protic acids, and organic alcohol solutions; The amount of catalyst used is 0 to 2.5% of the total molar number of monomers.

[0011] Optimally, the aprotic polar solvent is a mixture of one or more of the following: acetone, dimethyl sulfoxide, hexamethylphosphoric triamine, N,N-dimethylformamide, dimethylacetamide, acetonitrile, dimethylvinylurea, N-methylpyrrolidone, tetrahydrofuran, and dichloromethane.

[0012] Ideally, when a catalyst is present, the polycondensation reaction is carried out under an inert gas atmosphere, with the reaction temperature first raised from room temperature to 60-80°C and then held at 60-150°C, so that the number average molecular weight of the polyimide is 10,000-150,000. When no catalyst is added, the polycondensation reaction is carried out at 100-150°C under inert gas protection.

[0013] Optimally, the tetracarboxylic dianhydride monomer is an aromatic tetracarboxylic dianhydride or a mixture of aromatic tetracarboxylic dianhydride and aliphatic tetracarboxylic dianhydride, wherein the aliphatic tetracarboxylic dianhydride accounts for 0-10% of the total molar number of the tetracarboxylic dianhydride monomer; When the tetracarboxylic dianhydride monomer is only an aromatic tetracarboxylic dianhydride, at least a portion of the aromatic tetracarboxylic dianhydride contains a large side chain group or a flexible group; the aromatic tetracarboxylic dianhydride containing the large side chain group or the flexible group is 40 to 100% of the total molar number of the tetracarboxylic dianhydride monomer.

[0014] Ideally, it also includes end-base control steps: An isocyanate-terminated polyimide is prepared by adding an excess of the diisocyanate monomer; or After the polycondensation reaction is completed, a diamine monomer or a diol monomer is added to obtain an amino-terminated or hydroxyl-terminated polyimide. The diamine monomer is selected from one or more of 4,4′-diaminodiphenyl ether, ethylenediamine and 4,4′-diaminodiphenyl sulfone; The diol monomer is selected from one or more of 1,4-butanediol, pentanediol, hexanediol, and polyether diols.

[0015] Another object of the present invention is to provide a polyimide for an oxygen-nitrogen gas separation membrane, which is prepared by the above-described preparation method.

[0016] A third objective of the present invention is to provide an oxygen-nitrogen gas separation membrane comprising the aforementioned polyimide.

[0017] Compared with the prior art, the present invention has the following advantages: 1. Excellent gas separation performance This invention effectively controls the rigidity and packing density of polyimide segments at the molecular level by rationally selecting the types and ratios of dianhydrides and diisocyanate monomers, particularly by introducing fluorinated monomers (such as hexafluorodianhydride), monomers with large side chains or flexible groups (such as BPADA and MCTC). The resulting polyimide membrane material is soluble and processable, possessing both a high free volume fraction (for increasing O2 permeation flux) and suitable interchain spacing (for achieving the O2 / N2 molecular sieving effect), thus overcoming the bottleneck of the traditional polyimide membrane's mutual constraint between permeability and selectivity. Simultaneously, the resulting polyimide membrane material exhibits excellent mechanical strength. Experiments show that the polyimide membrane prepared by this invention reaches or exceeds the Robeson upper limit in terms of both O2 permeation flux and O2 / N2 separation coefficient.

[0018] 2. Broadly tunable molecular weight and end-group structure This invention employs a polycondensation route involving dianhydride and diisocyanate. By precisely controlling the monomer molar ratio (95:100 to 100:95, preferably 98:100 to 100:98) and the degree of reaction, the number-average molecular weight can be precisely controlled within the range of 10,000 to 150,000 (preferably 20,000 to 90,000). Furthermore, by adding an excess of diisocyanate or subsequently adding diamine / diol monomers, isocyanate-terminated, amino-terminated, or hydroxyl-terminated polyimides can be obtained, respectively. These active end groups provide reaction sites for subsequent crosslinking modification, blending modification, or membrane surface functionalization, which is beneficial for further improving the long-term stability and anti-plasticization ability of the membrane.

[0019] 3. Good solvent processing performance This invention effectively reduces the regularity of the molecular chain and interchain interactions by introducing large side-chain groups (such as trifluoromethyl groups in 6FDA) and flexible bridging groups (i.e., flexible groups, such as ether bonds and sulfone groups) into the polyimide backbone, resulting in polyimides with good solubility in common aprotic polar solvents (NMP, DMAc, DMF, etc.). This facilitates the preparation of asymmetric membranes or hollow fiber membranes using industrially mature membrane fabrication processes such as non-solvent-induced phase separation (NIPS) and dry spinning, avoiding the use of highly toxic or difficult-to-handle solvents. Simultaneously, by appropriately introducing flexible segments and large side groups to regulate free volume relaxation behavior, the common physical aging problems of high free volume polyimide membranes are effectively alleviated, enabling the membrane to maintain stable gas permeation flux and separation selectivity during long-term continuous operation. Detailed Implementation

[0020] The present invention discloses a method for preparing polyimide for oxygen and nitrogen gas separation membranes, comprising the following steps: performing a polycondensation reaction of tetracarboxylic dianhydride monomer and diisocyanate monomer in the presence of a catalyst or without a catalyst in an aprotic polar solvent to obtain the polyimide; wherein the molar ratio of the tetracarboxylic dianhydride monomer to the diisocyanate monomer is 95:100 to 100:95. The tetracarboxylic dianhydride monomer is a mixture selected from one or more of aromatic tetracarboxylic dianhydrides, aliphatic tetracarboxylic dianhydrides, and heterocyclic tetracarboxylic dianhydrides; the diisocyanate monomer is a mixture selected from one or more of aromatic diisocyanates and aliphatic diisocyanates.

[0021] This invention effectively controls the rigidity and packing density of polyimide segments at the molecular level by rationally selecting the types and ratios of dianhydride and diisocyanate monomers, particularly by introducing fluorinated monomers (such as hexafluorodianhydride) and monomers with large side groups or flexible groups (such as BPADA and MCTC). The resulting polyimide membrane material possesses both a high free volume fraction (for increasing O2 permeation flux) and suitable interchain spacing (for achieving the O2 / N2 molecular sieving effect), thus overcoming the bottleneck of the mutual constraint between permeability and selectivity in traditional polyimide membranes. Experiments show that the polyimide membrane prepared by this invention reaches or exceeds the Robeson upper limit in terms of both O2 permeation flux and O2 / N2 separation coefficient. By precisely controlling the monomer molar ratio (95:100 to 100:95, preferably 98:100 to 100:98) and the degree of reaction, precise control of the number-average molecular weight within the range of 10,000 to 150,000 (preferably 20,000 to 90,000) can be achieved. Furthermore, by adding an excess of diisocyanate or subsequently adding diamine / diol monomers, isocyanate-terminated, amino-terminated, or hydroxyl-terminated polyimides can be obtained, respectively. These active end groups provide reaction sites for subsequent crosslinking modification, blending modification, or membrane surface functionalization, which is beneficial for further improving the long-term stability and anti-plasticization ability of the membrane. By introducing large side chain groups (such as trifluoromethyl in 6FDA) and flexible bridging groups (such as ether bonds and sulfone groups) into the polyimide backbone, the regularity of the molecular chain and interchain interactions are effectively reduced, resulting in polyimides with good solubility in conventional aprotic polar solvents (NMP, DMAc, DMF, etc.). This facilitates the preparation of asymmetric membranes or hollow fiber membranes using industrially mature membrane-making processes such as non-solvent-induced phase separation (NIPS) and dry spinning, avoiding the use of highly toxic or difficult-to-handle solvents. Meanwhile, by appropriately introducing flexible segments and large side groups to regulate free volume relaxation behavior, the common physical aging problem of high free volume polyimide membranes is effectively alleviated, enabling the membrane to maintain stable gas permeation flux and separation selectivity during long-term continuous operation.

[0022] The aromatic tetracarboxylic dianhydrides mentioned include, but are not limited to, 4,4′-biphenyl ether dianhydride (ODPA), 2,3,6,7-naphthalenetetracarboxylic dianhydride, pyromellitic dianhydride (PMDA), 4,4'-(acetylene-1,2-diyl)phthalic anhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 2,3,3',4'-biphenyltetracarboxylic dianhydride, 3,3,4,4-benzophenone tetracarboxylic dianhydride (BTDA), and hexafluorodicarboxylic acid dianhydride. Anhydride (6FDA), perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA), 3,3',4,4'-triphenyl diether tetracarboxylic dianhydride (HQDPA), 3,3',4,4'-diphenylsulfonyl tetracarboxylic dianhydride, 4,4′-(4,4′-isopropylidene diphenoxy)bis(phthalic anhydride) (BPADA), and 4,4'-bis(3,4-dicarboxyphenylthio)diphenyl sulfide dianhydride (3SDEA), etc. The aliphatic tetracarboxylic dianhydrides include, but are not limited to, cyclobutanetetracarboxylic dianhydride (CBDA), 1,2,3,4-cyclopentanetetracarboxylic dianhydride, methylcyclohexenetetracarboxylic dianhydride (MCTC), 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA), pentacyclic [8.2.1.1]tetradecane-5,6,11,12-tetracarboxylic dianhydride, bicyclic [2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, ethylenediaminetetraacetic dianhydride (EDTA dianhydride), and dicyclohexyl-3,4,3',4'-tetracarboxylic dianhydride. The heterocyclic tetracarboxylic dianhydrides include, but are not limited to, 5,6-dihydro-1,4-dithiaene-2,3-dicarboxylic dianhydride and pyrimidinetetracarboxylic dianhydride.

[0023] The aromatic diisocyanates include, but are not limited to, toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), naphthalene-1,5-diisocyanate (NDI), and terephthalic diisocyanate (PPDI). The aliphatic diisocyanates include, but are not limited to, hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), tetramethyl dimethylphenyl diisocyanate (TMXDI), trimethyl hexamethylene diisocyanate (TMDI), dicyclohexylmethane diisocyanate (HMDI), methylcyclohexane diisocyanate (HTDI), phenylenediamine diisocyanate (XDI), hydrogenated phenylenediamine diisocyanate (H6XDI), 1,4-cyclohexane diisocyanate (CHDI), and norbornene diisocyanate (NBDI).

[0024] The catalyst is either an alkaline catalyst or an acidic catalyst; The alkaline catalyst is selected from one or more of organic amine compounds, nitrogen-containing heterocyclic compounds, inorganic bases, and organometallic compounds; including but not limited to tertiary amine compounds, 4-methoxypyridine, pyrrole, pyridine, alkali metal hydroxides (NaOH, KOH, LiOH), and organometallic alcohol compounds (potassium tert-butoxide, NaOC2H5, sodium phenolate), etc. The acidic catalyst is selected from one or more of inorganic acids, organic acids, aqueous solutions of protic acids, and organic alcohol solutions, with acetic acid being commonly used; the amount of the catalyst is 0-2.5% of the total molar amount of the monomer (preferably 0.1-1%, more preferably 0.2-0.5%).

[0025] The aprotic polar solvent is a mixture of one or more selected from acetone, dimethyl sulfoxide (DMSO), hexamethylphosphoric triamine (HMPA), N,N-dimethylformamide (DMF), dimethylacetamide (DMAC), acetonitrile, dimethylvinylurea (DMI), N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), tetrahydrofuran (THF), and dichloromethane, preferably DMAc, NMP, or DMF.

[0026] When a catalyst is present, the polycondensation reaction is carried out under inert gas protection, with the reaction temperature raised from room temperature to 60-80°C and then held at 60-150°C (preferably 80-100°C) to make the number average molecular weight of the polyimide 10,000-150,000. When no catalyst is added, the polycondensation reaction is carried out at 100-150°C under inert gas protection.

[0027] The tetracarboxylic dianhydride monomer is an aromatic tetracarboxylic dianhydride or a mixture of aromatic tetracarboxylic dianhydride and aliphatic tetracarboxylic dianhydride, wherein the aliphatic tetracarboxylic dianhydride accounts for 0 to 10% of the total molar number of the tetracarboxylic dianhydride monomer; when the tetracarboxylic dianhydride monomer is only an aromatic tetracarboxylic dianhydride, at least a portion of the aromatic tetracarboxylic dianhydride contains a large side chain group or a flexible group; the aromatic tetracarboxylic dianhydride containing the large side chain group or the flexible group accounts for 40 to 100% of the total molar number of the tetracarboxylic dianhydride monomer. Tetracarboxylic dianhydride monomers (hereinafter referred to as dianhydrides) can be a single monomer, or a mixture of two or more dianhydride monomers, including but not limited to 4,4'-terephthalodioxydiphthalic anhydride (HQDPA), or benzophenone tetracarboxylic anhydride (BTDA), or 4,4'-oxydiphthalic anhydride (ODPA, i.e., 4,4′-biphenyl ether dianhydride), or hexafluorodianhydride (6FDA), or bisphenol A diether dianhydride (BPADA), or a mixture of PCTDA and BTDA, or a mixture of MCTC and BTDA, or a mixture of MCTC and PMDA, or a mixture of MCTC and 6FDA, or a mixture of PMDA and BTDA, or a mixture of PMDA and CBDA, or a mixture of PMDA, BTDA and 6FDA, or a mixture of MCTC, PMDA and BTDA, etc.

[0028] Diisocyanate monomers can be a single monomer, or a mixture of two or more diisocyanate monomers, including but not limited to MDI, TDI, HMDI, IPDI, HDI, XDI, a mixture of HDI and MDI, a mixture of HDI and HMDI, a mixture of TDI and HMDI, a mixture of TDI and MDI, a mixture of NBDI and HMDI, a mixture of TMXDI and MDI, a mixture of HDI, MDI and TDI, or a mixture of HDI, HMDI, MDI and TDI, etc.

[0029] The diisocyanate monomer required for the reaction is preferably an aromatic monomer, containing a small amount of aromatic or aliphatic diisocyanates with large side-chain groups or flexible groups. The aromatic or aliphatic diisocyanate with large side-chain groups or flexible groups constitutes 0-10% (preferably 1-3%, more preferably 2.5%) of the total molar amount of the diisocyanate monomer. The diisocyanate monomer includes, but is not limited to, those described above. HMDI, isophorone diisocyanate (IPDI), and thiophene diisocyanate compounds (2,5-bis(4-isocyanatophenyl)-3,4-diphenyl-thiophene, etc.)

[0030] The above-mentioned method for preparing polyimide for oxygen and nitrogen gas separation membranes further includes an end-group control step: An isocyanate-terminated polyimide is prepared by adding an excess of the diisocyanate monomer; or After the polycondensation reaction is completed, a diamine monomer or a diol monomer is added to obtain an amino-terminated or hydroxyl-terminated polyimide; the diamine monomer is one or more selected from 4,4′-diaminodiphenyl ether, ethylenediamine and 4,4′-diaminodiphenyl sulfone; the diol monomer is one or more selected from 1,4-butanediol, pentanediol, hexanediol and polyether diol.

[0031] The feeding sequence in the preparation method of polyimide for oxygen and nitrogen gas separation membrane is as follows: dianhydride monomers are uniformly mixed together, and diisocyanate monomers are uniformly mixed together. During the reaction, the dianhydride monomer mixture is slowly added to the diisocyanate monomer mixture, or the diisocyanate monomer is slowly added to the dianhydride monomer, and the reaction is carried out to obtain a random polyimide material; or, the dianhydride monomers are uniformly mixed together, and then one of the diisocyanate monomers is added to the excess dianhydride monomer. After reacting for a period of time, the remaining diisocyanate monomer is added, and the reaction continues until the reaction is completed, to obtain a polyimide material with alternating chain segments; or, the dianhydride monomers are uniformly mixed together, and first a portion of one of the diisocyanate monomers is added, and then the remaining diisocyanate monomer mixture is added; or, the diisocyanate monomers are uniformly mixed together, and then one of the dianhydride monomers is added to the excess diisocyanate monomer. After reacting for a period of time, the remaining diisocyanate monomer is added, and the reaction continues until the reaction is completed, to obtain a polyimide material with alternating chain segments.

[0032] The preferred embodiments of the present invention will now be described in detail.

[0033] Example 1 This embodiment provides a polyimide (random copolymer polyimide) for oxygen and nitrogen gas separation membranes and its preparation method, as detailed below: In a dry three-necked flask, under nitrogen protection, 100 mL of anhydrous NMP solvent was added, and 0.05 mol of pyromellitic dianhydride (PMDA) and 0.05 mol of 3,3,4,4-benzophenone tetracarboxylic acid dianhydride (BTDA) were added with stirring. Then, 0.002 mol of pyridine catalyst was added, and the mixture was heated to 50 °C and stirred until fully dissolved to obtain a dianhydride solution. Separately weigh 0.096 mol toluene diisocyanate (TDI) and 0.004 mol hexamethylene diisocyanate (HDI) and dissolve them in 40 mL of anhydrous NMP solvent, and mix them thoroughly; add the mixed diisocyanate monomers to the above dianhydride solution, and control the addition rate so that the reaction temperature does not exceed 60 °C; The reaction was carried out at 60°C for 6 hours, then at 80°C for 4 hours. After the reaction, the product was slowly poured into methanol to precipitate, filtered, washed, and dried to obtain a polyimide product (the number average molecular weight of the polyimide was approximately 22,200). The obtained polyimide was dissolved in DMAc to prepare a 10 wt% casting solution. After coating, the film was dried at 80°C for 15 hours, then vacuum dried at 80°C for 6 hours, and finally vacuum dried at 150°C for 10 hours to obtain a dense film.

[0034] Gas permeation testing: Dense membranes were tested using a Labthink Instruments C101B instrument conforming to ISO 15105-1 (each dense membrane was divided into 5 samples for 5 tests), and the average of the 5 test results was taken as the gas flux of the reaction product. O2 / N2 selectivity was obtained by calculating the ratio of the permeation coefficients of O2 and N2 to a specific membrane sample. Tensile strength and Young's modulus tests were conducted according to GB / T 1040.3 test standard: the test sample width was 25 mm, the sample membrane thickness was 0.015 mm, the total length was 180 mm, the gauge length was 50 mm, the initial distance between the clamps was 110 mm, and the tensile rate was 5 mm / min; 5 samples of each reaction product were prepared and tested 5 times, and the average of the 5 test results was taken as the gas flux of the reaction product.

[0035] The measured O2 permeation flux was 3.5 Barrer, the O2 / N2 selectivity was 3.1, the tensile strength was 58 MPa, and the Young's modulus was 1422 MPa.

[0036] Example 2

[0037] This embodiment provides a polyimide for oxygen and nitrogen gas separation membranes and its preparation method, which is basically the same as that in Example 1, except that 0.091 mol of toluene diisocyanate (TDI) and 0.004 mol of hexamethylene diisocyanate (HDI) are weighed and dissolved in 40 mL of anhydrous NMP solvent. The number average molecular weight of the polyimide is approximately 16,800.

[0038] The measured O2 permeation flux was 5.2 Barrer, the O2 / N2 selectivity was 2.8, the tensile strength was 40 MPa, and the Young's modulus was 1180 MPa.

[0039] Example 3

[0040] This embodiment provides a polyimide for oxygen and nitrogen gas separation membranes and its preparation method, which is basically the same as that in Example 1, except that 0.101 mol of toluene diisocyanate (TDI) and 0.004 mol of hexamethylene diisocyanate (HDI) are weighed and dissolved in 40 mL of anhydrous NMP solvent. The number average molecular weight of the polyimide is approximately 19,000.

[0041] The measured O2 permeation flux was 4.1 Barrer, the O2 / N2 selectivity was 3.05, the tensile strength was 52 MPa, and the Young's modulus was 1450 MPa.

[0042] Example 4

[0043] This embodiment provides a polyimide for oxygen and nitrogen gas separation membranes and its preparation method, which is basically the same as that in Example 1, except for the composition of the dianhydride monomer: 0.05 mol PMDA and 0.05 mol hexafluorodianhydride (6FDA). The resulting polyimide has a number-average molecular weight of approximately 34,300.

[0044] The measured O2 permeation flux was 4.6 Barrer, the O2 / N2 selectivity was 5.2, the tensile strength was 72 MPa, and the Young's modulus was 1407 MPa.

[0045] Example 5

[0046] This embodiment provides a polyimide for oxygen and nitrogen gas separation membranes and its preparation method, which is basically the same as that in Example 1, except that no catalyst is added after the feed is completed, and the temperature needs to be raised to 120°C for 10 hours. Finally, the number-average molecular weight of the obtained polyimide is approximately 21400.

[0047] The membrane was prepared according to the method of Example 1, and the measured O2 permeation flux was 3.9 Barrer, the O2 / N2 selectivity was 3.0, the tensile strength was 50 MPa, and the Young's modulus was 1122 MPa.

[0048] Comparative Example 1 This example provides a polyimide for oxygen and nitrogen gas separation membranes and its preparation method, which is basically the same as in Example 1, except that 0.081 mol of toluene diisocyanate (TDI) and 0.004 mol of hexamethylene diisocyanate (HDI) are weighed and dissolved in 40 mL of anhydrous NMP solvent. The number average molecular weight of the resulting polyimide is 6800 (lower than 10000). Due to the limited degree of polymerization and insufficient molecular weight of the product, a complete self-supporting membrane cannot be formed, thus gas permeation and mechanical property tests cannot be performed. The number average molecular weight of the obtained polyimide is only about 6800, which is lower than the lower limit (10000) required by this invention. This product cracks severely during the membrane formation process and cannot form a complete self-supporting membrane, thus gas permeation and mechanical property tests cannot be performed.

[0049] Comparative Example 2 This example provides a polyimide for oxygen and nitrogen gas separation membranes and its preparation method, which is basically the same as that in Example 1, except that: 0.121 mol of toluene diisocyanate (TDI) and 0.004 mol of hexamethylene diisocyanate (HDI) are weighed and dissolved in 40 mL of anhydrous NMP solvent. The number average molecular weight of the obtained polyimide is 14000.

[0050] The measured O2 permeation flux was 6.5 Barrer, the O2 / N2 selectivity was 2.5, the tensile strength was 36 MPa, and the Young's modulus was 1300 MPa.

[0051] Comparative Example 3 This example provides a polyimide for oxygen and nitrogen gas separation membranes and its preparation method, which is basically the same as that in Example 1, except that the reaction temperature is controlled at 50°C and maintained for 10 hours. The system viscosity is still low, and the number-average molecular weight of the obtained polyimide is approximately 9800. Membranes prepared according to the method in Example 1 are brittle and hard, making it impossible to effectively test gas separation performance (the membrane easily breaks under test pressure). This indicates that when the reaction temperature is too low, the polycondensation reaction is insufficient, and high molecular weight polyimides cannot be obtained.

[0052] Comparative Example 4 This example provides a polyimide for oxygen and nitrogen gas separation membranes and its preparation method, which is basically the same as that in Example 5, except that the temperature is raised to 80°C and the reaction is carried out for 10 hours. After 10 hours of reaction, the viscosity of the system did not increase significantly. The number average molecular weight of the obtained polyimide is approximately 9200. The membrane prepared according to the method of Example 1 is brittle and hard, and the gas separation performance cannot be effectively tested (the membrane is easily broken under the test pressure). This indicates that without a catalyst at this reaction temperature, the polycondensation reaction is incomplete, and high molecular weight polyimide cannot be obtained.

[0053] Comparative Example 5 This example provides a polyimide for oxygen and nitrogen gas separation membranes and its preparation method, which is basically the same as that in Example 1, except that 0.1 mol PMDA is used, and BTDA or other dianhydride monomers containing large side groups / flexible groups are not included. The resulting polyimide has a number-average molecular weight of 14700, which has poor solubility in DMAc and NMP, and begins to precipitate after reacting to a certain molecular weight, which is not conducive to solution-based membrane preparation.

[0054] The film was prepared according to the method in Example 1 (the casting solution needs to be heated for preparation). The measured O2 permeation flux was 2.5 Barrer, the O2 / N2 selectivity was 3.8, the tensile strength was 51 MPa, and the Young's modulus was 1522 MPa. This indicates that without flexible or large side group groups, the solubility of polyimide decreases, the processing window narrows, and the gas permeation flux is lower.

[0055] Comparative Example 6 This example provides a polyimide for oxygen and nitrogen gas separation membranes and its preparation method, which is basically the same as that in Example 1, except that the dianhydride monomers are 0.05 mol PMDA and 0.05 mol methylcyclohexenetetracarboxylic dianhydride (MCTC). Finally, the number-average molecular weight of the obtained polyimide is approximately 29,100.

[0056] The measured O2 permeation flux was 6.4 Barrer, the O2 / N2 selectivity was 1.8, the tensile strength was 30 MPa, and the Young's modulus was 878 MPa. The introduction of MCTC provided too much flexible cyclohexene structure, resulting in excessive interchain spacing, which weakened the size sieving effect and thus significantly reduced selectivity.

[0057] Comparative Example 7 This example provides a polyimide for oxygen and nitrogen gas separation membranes and its preparation method, which is basically the same as that in Example 1, except that: the dianhydride monomer is 0.10 mol BTDA; the diisocyanate monomers are 0.055 mol toluene diisocyanate (TDI) and 0.045 mol isophorone diisocyanate (IPDI). Finally, the number-average molecular weight of the obtained polyimide is approximately 29400.

[0058] The measured O2 permeation flux was 7.5 Barrer, the O2 / N2 selectivity was 1.4, the tensile strength was 32 MPa, and the Young's modulus was 734 MPa. The synergistic effect of the cyclic structure of IPDI and the flexible ether bonds of the BTDA monomer resulted in a looser chain stacking, which improved flux but sacrificed selectivity.

[0059] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a polyimide for an oxygen-nitrogen gas separation membrane, characterized in that, Includes the following steps: The polyimide is obtained by polycondensation of tetracarboxylic dianhydride monomer and diisocyanate monomer in an aprotic polar solvent, with or without a catalyst. The molar ratio of the tetracarboxylic dianhydride monomer to the diisocyanate monomer is 95:100 to 100:95; The tetracarboxylic dianhydride monomer is a mixture of one or more of aromatic tetracarboxylic dianhydrides, aliphatic tetracarboxylic dianhydrides, and heterocyclic tetracarboxylic dianhydrides; the diisocyanate monomer is a mixture of one or more of aromatic diisocyanates and aliphatic diisocyanates.

2. The method for preparing polyimide for oxygen and nitrogen gas separation membrane according to claim 1, characterized in that: The aromatic tetracarboxylic dianhydrides include 4,4′-biphenyl ether dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, pyromellitic dianhydride, 4,4'-(acetylene-1,2-diyl)phthalic anhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4'-biphenyltetracarboxylic dianhydride, 3,3,4,4-benzophenone tetracarboxylic dianhydride, hexafluoro dianhydride, perylene-3,4,9,10-tetracarboxylic dianhydride, 3,3',4,4'-triphenylbisether tetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfonyl tetracarboxylic dianhydride, 4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride), and 4,4'-bis(3,4-dicarboxyphenylthio)diphenylsulfide dianhydride; The aliphatic tetracarboxylic dianhydrides include cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, methylcyclohexenetetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, pentacyclo[8.2.1.1]tetradecane-5,6,11,12-tetracarboxylic dianhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, ethylenediaminetetraacetic acid dianhydride, and dicyclohexyl-3,4,3',4'-tetracarboxylic dianhydride. The heterocyclic tetracarboxylic dianhydrides include 5,6-dihydro-1,4-dithiane-2,3-dicarboxylic anhydride and pyrimidine tetracarboxylic dianhydride.

3. The method for preparing polyimide for oxygen and nitrogen gas separation membrane according to claim 1, characterized in that: The aromatic diisocyanates include toluene diisocyanate, diphenylmethane diisocyanate, naphthalene-1,5-diisocyanate, and terephthalic diisocyanate. The aliphatic diisocyanates include hexamethylene diisocyanate, isophorone diisocyanate, tetramethyl dimethylphenyl diisocyanate, trimethyl hexamethylene diisocyanate, dicyclohexylmethane diisocyanate, methylcyclohexane diisocyanate, phenylenediamine diisocyanate, hydrogenated phenylenediamine diisocyanate, 1,4-cyclohexane diisocyanate, and norbornene diisocyanate.

4. The method for preparing polyimide for oxygen and nitrogen gas separation membrane according to claim 1, characterized in that: The catalyst is either an alkaline catalyst or an acidic catalyst; The alkaline catalyst is selected from one or more of organic amine compounds, nitrogen-containing heterocyclic compounds, inorganic bases, and organometallic compounds; The acidic catalyst is selected from one or more of inorganic acids, organic acids, aqueous solutions of protic acids, and organic alcohol solutions; The amount of catalyst used is 0 to 2.5% of the total molar number of monomers.

5. The method for preparing polyimide for oxygen and nitrogen gas separation membrane according to claim 1, characterized in that: The aprotic polar solvent is a mixture of one or more of the following: acetone, dimethyl sulfoxide, hexamethylphosphoric triamine, N,N-dimethylformamide, dimethylacetamide, acetonitrile, dimethylvinylurea, N-methylpyrrolidone, tetrahydrofuran, and dichloromethane.

6. The method for preparing polyimide for oxygen and nitrogen gas separation membrane according to claim 1, characterized in that: When a catalyst is present, the polycondensation reaction is carried out under inert gas protection, with the reaction temperature first raised from room temperature to 60-80°C and then held at 60-150°C, so that the number average molecular weight of the polyimide is 10,000-150,000. When no catalyst is added, the polycondensation reaction is carried out at 100-150°C under inert gas protection.

7. The method for preparing polyimide for oxygen and nitrogen gas separation membrane according to claim 1, characterized in that: The tetracarboxylic dianhydride monomer is an aromatic tetracarboxylic dianhydride or a mixture of aromatic tetracarboxylic dianhydride and aliphatic tetracarboxylic dianhydride, wherein the aliphatic tetracarboxylic dianhydride accounts for 0 to 10% of the total molar number of the tetracarboxylic dianhydride monomer; When the tetracarboxylic dianhydride monomer is only an aromatic tetracarboxylic dianhydride, at least a portion of the aromatic tetracarboxylic dianhydride contains a large side chain group or a flexible group; the aromatic tetracarboxylic dianhydride containing the large side chain group or the flexible group is 40 to 100% of the total molar number of the tetracarboxylic dianhydride monomer.

8. The method for preparing polyimide for oxygen and nitrogen gas separation membrane according to claim 1, characterized in that, It also includes end-base control steps: An isocyanate-terminated polyimide is prepared by adding an excess of the diisocyanate monomer; or After the polycondensation reaction is completed, a diamine monomer or a diol monomer is added to obtain an amino-terminated or hydroxyl-terminated polyimide. The diamine monomer is selected from one or more of 4,4′-diaminodiphenyl ether, ethylenediamine and 4,4′-diaminodiphenyl sulfone; The diol monomer is selected from one or more of 1,4-butanediol, pentanediol, hexanediol, and polyether diols.

9. A polyimide for oxygen and nitrogen gas separation membrane, characterized in that, It is prepared by any one of claims 1-8.

10. An oxygen-nitrogen gas separation membrane, characterized in that, It comprises the polyimide of claim 9.