A polyimide multilayer composite film material, its preparation method and application

Abstract

The application provides a polyimide multilayer composite membrane material and a preparation method and application thereof, and belongs to the technical field of gas separation. The method solves the problems of pore penetration of a support layer, MOF agglomeration and weak interface combination in existing composite membrane materials by synergistically designing a functional support layer, a gradient MOF layer and an ultrathin selective layer structure. First, polyamide acid glue is electrospun, and then dopamine is introduced simultaneously by using a chemical imidization method to obtain a functional polyimide fiber membrane containing hydroxyl and amine groups on the surface, so that the mechanical properties of the support layer and the MOF anchoring capacity are significantly improved. Then, by adjusting the ligand / metal salt concentration gradient and the reaction time, an MOF layer with different particle sizes is grown on the surface of the fiber membrane, and after each growth step, silane coupling agent passivation is performed to avoid heterogeneous nucleation, so as to construct a gradient structure. After the MOF gradient layer is pretreated by Ar plasma to introduce hydroxyl groups, an ultrathin polyamide selective layer is formed by the interfacial polymerization of fluorine-containing acyl chloride and a macrocyclic amine monomer, and the ultrathin polyamide selective layer is suitable for gas separation.

Description

Technical Field

[0001] This invention relates to the field of gas separation technology, and in particular to a polyimide multilayer composite membrane material, its preparation method, and its application. Background Technology

[0002] While traditional separation processes such as cryogenic distillation and pressure swing adsorption are technically mature, they suffer from high energy consumption, redundant equipment footprint, and the risk of secondary pollution, making them unsuitable for the development demands of green chemical engineering. Membrane separation technology, with its unique "molecular sieving" mechanism, achieves selective permeation of gas molecules through micropores, requiring no phase change throughout the process and significantly reducing energy consumption. It is rapidly being adopted in fields such as hydrogen recovery, carbon dioxide capture, air separation (oxygen enrichment), and removal of acidic corrosive gases (natural gas desulfurization).

[0003] Polyimide, with its main molecular chain containing an imide ring (formed by the condensation polymerization of diamine and dianhydride in aprotic polar solvents), possesses excellent thermal stability and electrical insulation, providing a solid materials science foundation for gas separation membranes. Electrospinning technology offers high processability, endowing fibers with high specific surface area and structural designability. Combining these two technologies with a metal-organic framework (MOF) can theoretically construct a synergistic separation system of "high throughput and high selectivity." However, existing technologies still suffer from systemic bottlenecks: the high porosity of the electrospun fiber support layer leads to a pore-permeation effect that hinders the formation of subsequent separation layers; when MOF is loaded, particles easily agglomerate and deposit in the fiber interstices, blocking mass transfer channels and preventing the formation of continuous functional layers. More importantly, the interfacial bonding between single-size MOFs and fibers is insufficient, weakening structural stability and resulting in low separation efficiency due to a single mass transfer path. Therefore, it is necessary to develop a high-performance, structurally stable polyimide multilayer composite membrane that overcomes the challenges of supporting layer porosity control, MOF dispersion and gradient loading, and enhanced interfacial bonding for efficient gas separation. Summary of the Invention

[0004] The purpose of this invention is to provide a polyimide multilayer composite film material, its preparation method, and its application.

[0005] To address the bottlenecks in existing technologies, such as excessively high porosity of the support layer, easy aggregation of MOFs, weak interfacial bonding between MOFs and the support layer, and poor stability in acidic environments, this paper proposes a breakthrough in high mechanical strength, high gas flux, high selectivity, and high stability by simultaneously functionalizing the support layer through chemical imidization, synergistic design of MOF particle size gradient loading and interfacial polymerization covalently selective layer, significantly improving the gas separation performance of the composite membrane.

[0006] In a first aspect, the present invention provides a method for preparing a polyimide multilayer composite membrane material, comprising: electrospinning a polyamic acid solution to obtain a polyamic acid fiber membrane; subsequently, immersing the polyamic acid fiber membrane in a mixed solution containing a chemical imidizing agent and dopamine to simultaneously achieve chemical imidization and dopamine functionalization, thereby obtaining a functionalized fiber membrane with hydroxyl (-OH) and amino (-NH2) groups on its surface; based on the functionalized fiber membrane, by controlling the concentration gradient of organic ligands / metal salts and the reaction time, cyclically impregnating and growing MOF layers of different particle sizes on the functionalized fiber membrane, and after each round of cyclic impregnation, growth, and drying, passivating treatment with a silane coupling agent is performed to form a MOF gradient layer with a gradient MOF structure; after pretreating the MOF gradient layer with Ar plasma, interfacial polymerization is performed to construct an aqueous-oil phase reaction system, and a polyamide selective layer is formed on its surface through interfacial polymerization, and the polyimide multilayer composite membrane material is obtained by curing.

[0007] Optionally, the core is a "three-step synergistic process":

[0008] Preparation of functionalized polyimide support layer: A polyamic acid solution with a solid content of 5wt%-25wt% was electrospun at 25℃-50℃ and 25%RH-50%RH to obtain a polyamic acid fiber membrane. This membrane was then immersed in a mixture of chemical imidizing agent and dopamine (e.g., a mixture of acetic anhydride and isoquinoline, dopamine, with a molar ratio of acetic anhydride to isoquinoline of 2:1 and containing 0.1wt%~1wt% dopamine. At this concentration, the polydopamine coating has a uniform thickness (±10 nm), avoiding fiber swelling due to excessive dopamine while ensuring the active site density meets the coordination requirements for MOF gradient growth. Dopamine, in conjunction with the chemical imidizing agent, enables simultaneous polyamic acid cyclization and dopamine self-polymerization, providing active sites for MOF coordination anchoring). The reaction was carried out at 40℃-80℃ for 1h-4h, simultaneously achieving chemical imidization and dopamine functionalization. Optionally, polydopamine is formed through dopamine self-polymerization, introducing -OH / -NH2 groups simultaneously with polyamic acid cyclization (generating polyimide), thereby improving the hydrophilicity of the fiber membrane surface and the MOF anchoring ability. This allows for a polyamic acid cyclization degree greater than 95%, forming a cross-linked structure in the fiber, and increasing the tensile strength from 8 MPa after thermal imidization to ≥69 MPa, thus solving the problem of severe porosity in the support layer. The polydopamine coating uniformly covers the fiber surface, introducing hydroxyl (-OH) and amino (-NH2) groups, providing strong binding sites for subsequent MOF coordination and anchoring.

[0009] MOF gradient layer preparation: In-situ gradient growth of MOFs was achieved using a particle size gradient loading method and silane passivation. Based on functionalized fiber membranes, MOF layers of different particle sizes were grown by cyclic impregnation by controlling the concentration gradient of organic ligands / metal salts and the reaction time, with silane coupling agent passivation performed after each growth step. Preferably, one MOF layer was generated per impregnation cycle. The first layer was loaded with small-particle-size MOFs as an inner layer, which helps to enhance the binding force between MOFs and the fiber membrane and inhibit detachment; the second layer increased the solution concentration and extended the reaction time to prepare a medium-particle-size MOF layer; and the third layer was loaded with large-particle-size MOFs, which helps to increase gas flux. After each growth cycle, the functionalized fiber membrane is passivated for 5-10 minutes with a 1wt%-3wt% silane coupling agent solution (to seal the active sites of surface metal ions and prevent heterogeneous nucleation), and then dried at 60℃-80℃. This forces the new particle size MOF to preferentially grow in the interlayer gap, forming a gradient structure with dense anchoring in the inner layer, transition in the middle layer, and high porosity and low resistance in the outer layer, thus solving the problems of low separation efficiency and poor anchoring effect of single MOF.

[0010] Preparation of ultrathin polyamide selective layers: MOF gradient layers were pretreated with Ar plasma (treatment conditions: plasma power 100W~300W, treatment time 1min~5min, treatment spacing 5mm~10mm, ambient humidity 40%RH~60%). The reaction process involves RH (where the treatment spacing is the distance between the MOF layer and the plasma nozzle, and the ambient humidity is used to regulate and promote the uniform generation of hydroxyl groups). Hydroxyl groups (-OH) are introduced through etching to enhance their reactivity with subsequent monomers. Subsequently, interfacial polymerization is performed to construct an aqueous-oil phase reaction system. The aqueous phase is a 0.5wt%~2wt% aqueous solution of macrocyclic amines (such as piperazine derivatives) (immersed for 1min-2min, adsorbed at -OH sites), and the oil phase is a 0.1wt%~0.5wt% n-hexane solution of perfluoroterephthaloyl chloride (bifunctional group) (immersed for 1min-3min, condensed with -OH / -NH2 by acyl chloride). This forms an ultrathin selective layer of 100nm-200nm in situ, which is covalently bonded to the outer layer of the MOF through amide bonds. The fluorinated groups endow the selective layer with hydrophobic and corrosion-resistant properties. The macrocyclic structure precisely traps CH4 (kinetic diameter 0.38nm) through size sieving and preferentially allows CO2 to pass through (kinetic diameter 0.33nm), solving the problems of thick selective layer and poor stability.

[0011] The preparation method provided by this invention utilizes electrospinning to construct a fiber-interwoven network, and chemical imidization to simultaneously achieve mechanical strengthening and functionalization of the support layer, solving the problems of porosity and anchoring. Gradient MOF optimizes the gas diffusion path through increasing particle size (improving flux and selectivity). Plasma pretreatment and interfacial polymerization form an ultrathin selective layer, enhancing stability and sieving efficiency. The synergistic effect of these three elements results in a composite membrane with a CO2 / CH4 selectivity ≥16 and a CO2 permeability coefficient of 131 Barrer at 25℃ and 0.1 MPa, significantly superior to existing technologies.

[0012] Optionally, electrospinning can be performed at 25℃-50℃.

[0013] Optionally, electrospinning can be performed at a humidity level of 25%-50%RH.

[0014] Optionally, the solid content of the polyamic acid liquid is 5%-25%.

[0015] Optionally, after electrospinning, a pre-reaction of chemical imidization and dopamine surface functionalization is carried out for 10 min-60 min; then, the main reaction is carried out at 40℃-80℃ for 1 h-4 h, so as to simultaneously achieve chemical imidization and dopamine functionalization.

[0016] Optionally, when growing MOF layers of different particle sizes based on the cyclic impregnation of functionalized fiber membranes, the process includes: cyclically impregnating the functionalized fiber membrane in an organic ligand solution and a metal salt solution, followed by drying.

[0017] Optionally, the immersion can be repeated 3 times.

[0018] Optionally, the process involves the following steps: First cycle: Immerse the fiber membrane in a 0.1M-0.2M metal salt solution for 30-60 minutes, then transfer it to a 0.4M-0.8M ligand solution for 5-10 minutes; Second cycle: Immerse the fiber membrane in a 0.3M-0.4M metal salt solution for 60-120 minutes, then transfer it to a 1.2M-1.6M ligand solution for 10-15 minutes; Third cycle: Immerse the fiber membrane in a 0.5M-0.6M metal salt solution for 120-240 minutes, then transfer it to a 2.0M-2.4M ligand solution for 15-20 minutes. After each cycle, the fiber membrane is passivated by immersing it in a 1wt%-3wt% silane coupling agent solution for 5-10 minutes, then dried at 60℃-80℃ to seal the active sites on the MOF surface (such as Zn in ZIF-8). 2+ ), to avoid heterogeneous nucleation.

[0019] Optionally, the organic ligand in the organic ligand solution includes 2-methylimidazole, trimesic acid, or terephthalic acid.

[0020] Optionally, the metal salt in the metal salt solution includes cobalt salt, zinc salt, copper salt, or iron salt.

[0021] Optionally, the silane coupling agent is any one or a combination of KH-550, KH-560, KH-570, and KH-590. These silane coupling agents can promote the bonding between the functionalized fiber membrane and the MOF (metal-organic framework) by interacting with the surface active groups (such as hydroxyl and amino groups introduced by dopamine) of the functionalized fiber membrane and the MOF components through their own functional groups.

[0022] Optionally, the fiber membrane is immersed in the organic ligand solution for a single time for 5-20 minutes.

[0023] Optionally, the fiber membrane is immersed in the metal salt solution for 0.5h-4h in a single immersion.

[0024] Optionally, the product is dried under vacuum after cyclic impregnation.

[0025] Optionally, after each in-situ MOF growth, the fiber membrane is immersed in a silane coupling agent solution for passivation, which seals the active sites on the MOF surface and prevents heterogeneous nucleation.

[0026] Optionally, the concentration of the silane coupling agent solution is 1wt%-3wt%.

[0027] Optionally, the passivation time is 5 min-10 min.

[0028] Optionally, the sample is passivated by immersion in a silane coupling agent solution and then dried at 60°C-80°C.

[0029] Optionally, the particle size of the MOF particles is 0.1 μm-5 μm.

[0030] Optionally, the type of MOF particles is independent of the MOFs grown in situ on the functionalized fiber membrane.

[0031] Optionally, the polyamic acid liquid is formed by the condensation polymerization of diacid anhydride and diamine monomer in a polar aprotic solvent.

[0032] Optionally, the dicarboxylic anhydride includes one or more of pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-oxobisphthalic anhydride, 3,3,4,4-diphenylsulfone tetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, or 2,3,3',4'-biphenyltetracarboxylic dianhydride.

[0033] Optionally, the diamine monomer includes one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, 4,4-diaminobiphenyl, 3,4'-diaminodiphenyl ether, m-phenylenediamine, 4,4'-diaminodiphenyl sulfone, 1,4-bis(4-aminophenoxy)benzene, or 1,3-bis(3-aminophenoxy)benzene.

[0034] Optionally, the polar aprotic solvent includes one of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone.

[0035] Optionally, the chemical imidizing agent is any one or a combination of acetic anhydride, propionic anhydride, butyric anhydride, benzoic anhydride, chloroacetic anhydride, bromoadipic anhydride, trifluoroacetic anhydride, pyridine and its derivatives, methylpyridine and its derivatives, dimethylpyridine, N,N-dimethylaminopyridine, quinoline, and isoquinoline. The solvent for the mixture of the chemical imidizing agent and dopamine is any one of N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), or N-methylpyrrolidone (NMP).

[0036] Optionally, when pretreating the MOF gradient layer with Ar plasma, the processing power is 100W-300W.

[0037] Optionally, the Ar plasma pretreatment time is 1 min to 5 min to ensure the introduction of sufficient hydroxyl groups and avoid damage to the MOF structure.

[0038] Optionally, during Ar plasma pretreatment, the distance between the MOF layer and the plasma nozzle is 5mm-10mm.

[0039] Optionally, the ambient humidity for Ar plasma pretreatment is 40%-60%RH.

[0040] Optionally, the aqueous phase for interfacial polymerization is an aqueous solution containing macrocyclic amine monomers, wherein the macrocyclic amines include one or a combination of piperazine derivatives, cyclodextrin derivatives, and calixarene amines.

[0041] Optionally, the concentration of macrocyclic amines in the aqueous phase is 0.5wt%-2wt%.

[0042] Optionally, the immersion time of the fiber membrane in the aqueous phase is 1 min to 2 min.

[0043] Optionally, the oil phase of the interfacial polymerization is an organic solvent solution of a fluorinated acyl chloride monomer, wherein the fluorinated acyl chloride is a bifunctional monomer, including one of perfluoroterephthaloyl chloride and perfluoroisophthaloyl chloride.

[0044] Optionally, the concentration of fluorinated acyl chloride in the oil phase is 0.1wt%-0.5wt%.

[0045] Optionally, the oil phase solvent is one of n-hexane, cyclohexane, and petroleum ether.

[0046] Optionally, the immersion time of the fiber membrane in the oil phase is 1 min to 3 min.

[0047] Optionally, the interface is cured at 60℃-80℃ for 1h-3h after polymerization.

[0048] Optionally, the thickness of the polyamide selective layer is 100nm-200nm. The polyamide selective layer formed by interfacial polymerization has the following characteristics: a thickness of 100nm-200nm; covalent bonding with the MOF gradient layer via amide bonds; and a 30%-50% increase in selectivity for polar gases such as CO2 after the introduction of fluorinated monomers (compared to the fluorine-free monomer system).

[0049] In a second aspect, the present invention also provides a polyimide multilayer composite film material prepared by any of the above-mentioned optional preparation methods.

[0050] Thirdly, the present invention also provides the application of polyimide multilayer composite membrane materials prepared by any of the above-mentioned optional preparation methods in gas separation. Attached Figure Description

[0051] Figure 1 A flowchart illustrating a method for preparing a polyimide multilayer composite film provided by the present invention;

[0052] Figure 2 SEM image of the polyimide fiber membrane subjected to chemical imidization and dopamine modification functionalization in Example 1 of this invention;

[0053] Figure 3 SEM image of the polyimide fiber membrane subjected to thermal imidization treatment in Comparative Example 1 provided by the present invention;

[0054] Figure 4 The stress-strain curve of the composite membrane in Example 1 of this invention;

[0055] Figure 5 The stress-strain curve of the composite membrane in Comparative Example 1 provided by the present invention;

[0056] Figure 6 This is a SEM image of the 130nm ZIF-8 particles grown using gradient growth in Example 1.

[0057] Figure 7 This is a SEM image of 400nm ZIF-8 particles grown using a gradient in Example 1.

[0058] Figure 8 This is a SEM image of the 750nm ZIF-8 particles grown using gradient growth in Example 1. Detailed Implementation

[0059] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art to which this invention pertains.

[0060] See Figure 1 This invention provides a method for preparing polyimide multilayer composite film materials, the core of which is a "three-step synergistic process", including the following steps:

[0061] S1. After electrospinning the polyamic acid solution, the fiber membrane is immersed in a mixture of chemical imidizing agent and dopamine (such as a mixture of acetic anhydride and isoquinoline and dopamine, with a molar ratio of acetic anhydride to isoquinoline of 2:1, containing 0.1wt%~1wt% dopamine, and the system is kept at pH=8~9). Chemical imidization and dopamine functionalization are carried out simultaneously. The reaction is carried out at 40℃-80℃ for 1h-4h to directly obtain a functionalized cross-linked polyimide fiber membrane with hydroxyl (-OH) and amine (-NH2) groups on the surface, thus obtaining the functionalized fiber membrane.

[0062] S2. Based on functionalized fiber membranes, 130nm, 400nm, and 750nm MOF layers (such as ZIF-8 layers) are grown by cyclic impregnation through low, medium, and high concentration organic ligands / metal salt gradient regulation. After each cycle of impregnation and drying, the layers are passivated with 1wt%-3wt% silane coupling agent solution to construct a gradient structure.

[0063] S3. After pretreating the MOF gradient layer with Ar plasma (100W-300W, 1-5min), the fluorinated acyl chloride and macrocyclic amine are interfacially polymerized to form an ultrathin selective layer of 100nm-200nm, which is then cured at 60℃~80℃ to obtain a composite film.

[0064] This invention innovatively proposes a simultaneous chemical imidization-dopamine modification: Through a mixed system of chemical imidizing reagents and dopamine (such as a mixture of acetic anhydride and isoquinoline, dopamine, with a molar ratio of acetic anhydride to isoquinoline of 2:1, containing 0.1wt%~1wt% dopamine, and maintaining a weakly alkaline environment at pH 8~9), two reactions are simultaneously driven: polyamic acid (PAA) cyclization to polyimide (PI, cyclization degree ≥95%) and dopamine self-polymerization to form a polydopamine (PDA) coating. This synergistic effect causes the fibers to crosslink, reducing porosity and increasing tensile strength from 8MPa to 69MPa; simultaneously, high-density hydroxyl (-OH) and amino (-NH2) groups are introduced onto the PI surface. The -OH groups strengthen MOF anchorage through coordination, while the -NH2 groups enhance surface hydrophilicity, laying the foundation for subsequent MOF growth.

[0065] The low porosity of functionalized fiber membranes avoids the deposition of MOF particles in the fiber interstices in traditional processes, allowing MOF particles (such as ZIF-8 (zeolite imidazolium ester framework-8)) to grow directionally on the fiber surface. A gradient design of a 130nm inner layer, a 400nm middle layer, and a 750nm outer layer achieves triple synergy: ① The small particle size in the inner layer strongly binds to the -OH groups on the PI surface through coordination bonds, preventing detachment; ② The large particle size in the outer layer constructs high-porosity channels, reducing gas transport resistance; ③ After each growth cycle, metal ions (such as Zn) are passivated and blocked by a silane coupling agent solution (such as KH-550). 2+ Active sites are created to force new particle sizes (such as ZIF-8) to grow in the interlayer gaps, avoiding particle size disorder caused by heterogeneous nucleation. Subsequent Ar plasma etching (1 min~5 min) introduces hydroxyl groups (-OH), further enhancing the covalent bonding with the selected layer.

[0066] Therefore, this invention uses a fiber membrane prepared by chemical imidization and dopamine synergistic modification as the support layer substrate, in-situ growth of MOF particles of different sizes to achieve gradient loading, and a polyamide membrane prepared by interfacial polymerization as the separation layer. The synergistic process enables the composite membrane to achieve a balance between mechanics, separation and stability, which can effectively improve the gas molecule sieving ability.

[0067] Specifically, the polyamic acid solution used in step S1 can be prepared by polycondensation of a diacid anhydride and a diamine monomer in a polar aprotic solvent to obtain a polyamic acid solution with a solid content of 5%-25%, and then spun into fibers using an electrospinning device in an environment of 25℃-50℃ and 25RH%-50RH%. Specifically, in some embodiments, the fiber material obtained after fiber spinning has a fiber size of 0.05μm-1μm.

[0068] In some embodiments, the diacid anhydride used in preparing the polyamic acid adhesive may include one or more of pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-oxophthalic anhydride, 3,3,4,4-diphenyl sulfone tetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, or 2,3,3',4'-biphenyltetracarboxylic dianhydride. The diamine monomer used may include one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, 4,4-diaminobiphenyl, 3,4'-diaminodiphenyl ether, m-phenylenediamine, 4,4'-diaminodiphenyl sulfone, 1,4-bis(4-aminophenoxy)benzene, or 1,3-bis(3-aminophenoxy)benzene. The polar aprotic solvent used includes one of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone.

[0069] In some embodiments, when performing step S2 for in-situ MOF growth based on the fiber membrane, the fiber membrane can be cyclically immersed in an organic ligand solution and a metal salt solution, followed by drying. In fact, when the fiber membrane is immersed in the metal salt solution, metal ions can penetrate into the interior and surface of the fiber filaments. Therefore, when immersed in the organic ligand solution, the metal ions can coordinate with the organic ligands, thereby generating a metal-organic framework (MOF) in situ on the fiber filament surface.

[0070] Specifically, when performing step S2, it is preferable to perform three cycles of impregnation: the first cycle constructs the inner layer (load capacity 20wt%~30wt%), the second cycle constructs the middle layer (total load capacity 40wt%~50wt%), and the third cycle constructs the outer layer (total load capacity 60wt%~70wt%).

[0071] In some embodiments, the organic ligands in the organic ligand solution used in step S2 include 2-methylimidazole, trimesic acid, or terephthalic acid, and the metal salts in the metal salt solution include cobalt, zinc, copper, or iron salts. In fact, when the fiber membrane is cyclically impregnated in a 2-methylimidazole solution and a zinc salt solution, ZIF-8 can be generated in situ on the fiber membrane surface. Specifically, by selecting different organic ligands and metal salts, the type of MOF generated can be adjusted, such as UiO-66, UiO-66-NH2, ZIF-67, MIL-101(Fe), MIL-101(Cr), or HKUST-1.

[0072] In some embodiments, the concentration of the organic ligand solution used in step S2 is independently between 0.1 mol / L and 4 mol / L, and the types of solvents used in the organic ligand solution and the metal salt solution are independent of each other. The solvents for both the organic ligand solution and the metal salt solution can be any one of methanol, ethanol, N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), or water. Specifically, the fiber membrane can be cyclically impregnated in both the organic ligand methanol solution and the metal salt methanol solution.

[0073] In some embodiments, during step S2, the single immersion time of the fiber membrane in the organic ligand solution is 5 min to 20 min, so that the organic ligand solution can fully wet the fiber filaments of the fiber membrane. In addition, the single immersion time of the fiber membrane in the metal salt solution is 0.5 h to 4 h, so that the metal ions can fully react with the organic ligand to generate a metal-organic framework (MOF).

[0074] Specifically, after performing step S2 by cyclically impregnating the fiber membrane, it is dried in a vacuum environment at 60°C-80°C. This improves the positional stability of the metal-organic framework (MOF) attached to the fiber surface, thereby facilitating subsequent bonding with the selective layer. Furthermore, after performing step S2 for in-situ MOF growth based on the fiber membrane, the MOF loading on the fiber membrane is 10wt%-80wt%.

[0075] In practice, after each round of MOF growth, the fiber membrane needs to be passivated for 5-10 minutes by immersing it in a 1wt%-3wt% silane coupling agent solution (such as KH-550 ethanol solution) and then dried at 60℃-80℃. For example, the amino groups of KH-550 react with the Zn exposed on the ZIF-8 surface. 2+ Coordination and sealing of active sites ensure that the new ZIF-8 particles grow only in the interlayer gaps, thus ensuring the orderly gradient structure.

[0076] In some embodiments, the MOF particles used in step S2 are independent of the type of MOF generated in situ on the fiber membrane surface. Specifically, the type of MOF particles can be one of UiO-66, UiO-66-NH2, ZIF-8, ZIF-67, MIL-101(Fe), MIL-101(Cr), and HKUST-1. Furthermore, the MOF particles can be commercially available conventional products or synthesized in advance. Meanwhile, the particle size of the MOF particles used is 0.1 μm-5 μm.

[0077] In some embodiments, the concentration of the silane coupling agent solution used in step S2 is 1wt%-3wt%. Furthermore, the solvent used in the silane coupling agent solution includes one of ethanol, isopropanol, or an ethanol-water mixture. Specifically, the solvent used in the silane coupling agent solution can be a solvent commonly used in the art that does not react with polyimide.

[0078] In some embodiments, the Ar plasma pretreatment in step S3 needs to meet the following requirements: power 100W~300W, treatment time 1min~5min, and distance between MOF layer and plasma nozzle 5mm~10mm. Hydroxyl groups (-OH) are introduced on the MOF surface through etching. The ambient humidity is simultaneously controlled to 40%RH~60%RH to promote the uniform generation of hydroxyl groups by using water vapor and enhance the chemical reaction activity of subsequent interfacial polymerization.

[0079] In some embodiments, the monomer system for interfacial polymerization during step S3 is designed as follows: an aqueous phase containing macrocyclic amine monomers (such as β-cyclodextrin derivatives, piperazine-trimethylammonium benzoate derivatives) at a concentration of 0.5wt%~2wt%, and an oil phase containing fluorinated acyl chloride monomers (such as perfluoroterephthaloyl chloride, fluorinated trimellitic chloride) at a concentration of 0.1wt%~0.5wt%. The introduction of fluorine atoms imparts hydrophobic selectivity, and the oil phase solvent is selected as n-hexane, cyclohexane, or petroleum ether. Due to its hydrophobic properties, it avoids miscibility with the aqueous phase and maintains the interfacial stability of the two phases in the interfacial polymerization. After the two phases react in contact on the surface of the MOF gradient layer for 1min~5min, they are cured at 60℃~80℃ for 1h~3h. The acyl chloride groups of the fluorinated acyl chloride undergo a condensation reaction with the hydroxyl groups on the MOF surface and the amino groups of the macrocyclic amine to form an ultrathin polyamide selective layer of 100nm~200nm covalently linked by amide bonds.

[0080] Example 1

[0081] This embodiment 1 provides a method for preparing a polyimide multilayer composite film material, including the following steps:

[0082] S1. Pyromellitic dianhydride (PMDA) and 4,4'-diaminodiphenyl ether (ODA) are mixed at a molar ratio of 1.01:1 and added to N,N-dimethylacetamide (DMAc). A polyamic acid (PAA) solution with a solid content of 10% is obtained through solution polycondensation and adjustment of the solid content. PAA fiber material is then produced by electrospinning at 25°C and 30% RH using an electrospinning device. The PAA fibers are then directly immersed in a mixture of a chemical imidizing agent and dopamine (composed of acetic anhydride:isoquinoline). A chemical imidizing agent with a 2:1 ratio, in conjunction with 0.5 wt% dopamine (maintaining the system pH=8), utilizes the weakly alkaline environment provided by isoquinoline (system pH=8) to drive the cyclization of PAA to form PI and the self-polymerization of dopamine to form polydopamine (PDA) simultaneously. The reaction temperature is controlled at 60℃, and the reaction time is maintained at 4 h. The catechol hydroxyl groups (-OH) of dopamine form hydrogen bonds with the PI molecular chain, simultaneously introducing free amino groups (-NH2), which are pre-constructed active sites for subsequent MOF coordination anchoring. Functionalized PI fiber membranes possessing both mechanical strength and surface activity are directly obtained after chemical imidization treatment, thus yielding the functionalized fiber membrane.

[0083] S2. A gradient impregnation and silane coupling agent passivation process was used to grow ZIF-8 layers of different particle sizes on the surface of functionalized PI fiber membranes: the fiber membrane was first immersed in 0.15 mol / L zinc acetate methanol solution for 45 min, and then transferred to 0.6 mol / L 2-methylimidazolium methanol solution for 8 min to complete the 130 nm layer growth. Dense anchoring growth of the inner layer of ZIF-8 was achieved. Subsequently, the system concentration was increased to 0.35 mol / L zinc acetate + 1.4 mol / L 2-methylimidazole, with zinc acetate soaking for 90 min and ligand solution soaking for 12 min to achieve the growth of a 400 nm ZIF-8 middle layer. The system was further adjusted to 0.55 mol / L zinc acetate + 2.2 mol / L 2-methylimidazole, with zinc acetate soaking for 180 min and ligand solution soaking for 18 min to induce the growth of a 750 nm ZIF-8 outer layer. After each cycle, the fiber membrane was immersed in 2 wt% KH-550 ethanol solution for 8 min and dried at 70 °C to seal the active sites on the ZIF-8 surface, avoid heterogeneous nucleation, and ensure that the new ZIF-8 particles preferentially grow in the interlayer gaps. After 3 cycles, a ZIF-8 functional layer with a particle size gradient distribution of 130 nm, 400 nm, and 750 nm was obtained.

[0084] S3. Ar plasma pretreatment of ZIF-8 gradient functional layers: power set at 200W, treatment time at 4min, and distance between functional layer and plasma nozzle at 8mm. Ambient humidity adjusted to 50% RH. Hydroxyl groups (-OH) were introduced into the ZIF-8 surface by etching. Subsequently, an interfacial polymerization system was constructed: the aqueous phase was an aqueous solution containing 2wt% cyclodextrin derivative (or piperazine macrocyclic amine), and the oil phase was a hexane solution containing 0.5wt% perfluoroterephthaloyl chloride. The pretreated sample was first immersed in the aqueous phase for 2min (to allow the macrocyclic amine to adsorb at the -OH sites), drained, and then contacted with the oil phase for 3min to drive the condensation reaction between acyl chloride groups and amino / hydroxyl groups, forming an initial polyamide film in situ on the MOF surface. Finally, it was transferred to a 70℃ oven for curing for 2h. Utilizing the hydrophobic properties of perfluoroacyl chloride and the size sieving of the macrocyclic structure, an ultrathin selective layer with a thickness of 200nm was constructed and covalently linked to the MOF layer through amide bonds, thus obtaining the polyimide multilayer composite film material.

[0085] Comparative Example 1

[0086] Comparative Example 1 provides a method for preparing a polyimide gas separation membrane to compare the technical advantages of Example 1. Steps S2 and S3 of Comparative Example 1 are exactly the same as those of Example 1, except for S1, as follows:

[0087] S1. The preparation of the polyamic acid (PAA) solution and the electrospinning parameters were the same as in Example 1 (10% solid content, spinning at 25°C and 30% RH). Instead of chemical imidization and dopamine modification, a traditional thermal imidization process was used: the PAA fiber membrane was first pre-imidized at 60°C, 80°C, and 100°C (15 min each), then heated to 350°C in a high-temperature imidization furnace at a rate of 2°C / min and held for 30 min to complete cyclization. After cyclization, a thermally imidized PI fiber membrane was obtained. Testing showed that the tensile strength of this fiber membrane was only 8 MPa (compared to 69 MPa in Example 1).

[0088] The steps of S2, ZIF-8 growth and S3, and the preparation of the selective layer are completely consistent with those in Example 1 (ensuring that only S1 is a variable). However, due to the structural defects of S1, the high porosity during MOF growth causes a large number of ZIF-8 particles to be deposited in the fiber gaps, which cannot be oriented and anchored on the fiber surface and cannot form a gradient structure.

[0089] Comparative Example 2

[0090] Comparative Example 2 provides a method for preparing a polyimide multilayer composite film material to compare the process advantages of "simultaneous chemical imidization and dopamine modification" in Example 1. Steps S2 and S3 of Comparative Example 2 are exactly the same as those in Example 1, except that S1 adopts a step-by-step process of "chemical imidization followed by dopamine functionalization", as detailed below:

[0091] S1. The preparation of the polyamic acid solution and electrospinning were completely consistent with Example 1. Pyromellitic dianhydride (PMDA) and 4,4'-diaminodiphenyl ether (ODA) were mixed at a molar ratio of 1.01:1 and added to N,N-dimethylacetamide (DMAc). A polyamic acid (PAA) solution with a solid content of 10% was obtained through solution polycondensation. PAA fiber material was obtained by electrospinning at 25°C and 30% RH. Without adding dopamine, the PAA fiber membrane was immersed in a chemical imidizing reagent (same formulation as in Example 1: acetic anhydride:isoquinoline = 2:1, solvent: DMAc), and reacted at 60°C for 4 hours to complete the chemical imidization, thus obtaining a dopamine-free functionalized polyimide (PI) fiber membrane. The chemically imidized PI fiber membrane was immersed in a 0.5 wt% dopamine aqueous solution and reacted at 60°C for 2 hours to achieve dopamine self-polymerization. After the reaction, the fiber membrane was removed, rinsed three times with deionized water, and dried at 70°C to prepare a stepwise functionalized PI fiber membrane. The thickness deviation of the polydopamine (PDA) coating on the surface of the fiber membrane was found to be ±35 nm (±10 nm in the synchronous process of Example 1), and local aggregation of PDA occurred due to the small number of active sites on the PI surface.

[0092] The steps of S2, ZIF-8 growth, and S3, and the preparation of the selective layer are completely consistent with those in Example 1. However, due to the defects in the stepwise process of S1, the PDA coating on the surface of the stepwise functionalized PI fiber membrane is uneven and the density of active sites is low, resulting in the inability of ZIF-8 to grow directionally on the fiber surface. The loading of ZIF-8 in the inner 130nm layer is only 15wt%, while that in Example 1 is 20wt%-30wt%, and MOF vacancies appear locally due to PDA agglomeration. In addition, the stepwise operation is time-consuming and inefficient. In the synchronous process, the catechol structure of dopamine can form a hydrogen bond network with the PI molecular chain in the weakly alkaline environment of imidization. At the same time, the polydopamine generated by dopamine self-polymerization will be anchored in situ on the PI surface, naturally introducing a large number of -OH / -NH2. However, in the stepwise process, the PI molecular chain formed first is already highly cross-linked and dense, and the surface lacks anchoring sites that can bind to dopamine.

[0093] Comparative Example 3

[0094] Comparative Example 3 provides a method for preparing a polyimide multilayer composite film material to compare the advantages of KH-550 passivation after each cycle in Example 1. Steps S1 and S3 of Comparative Example 3 are exactly the same as those in Example 1, except that step S2 removes the KH-550 passivation treatment, as detailed below:

[0095] S1 is completely consistent with Example 1.

[0096] S2. ZIF-8 Growth without Passivation: The same concentration gradient cyclic immersion parameters as in Example 1 were used, but the KH-550 passivation step after each cycle was removed. First cycle: The fiber membrane was immersed in 0.15 mol / L zinc acetate methanol solution for 45 min, then transferred to 0.6 mol / L 2-methylimidazole methanol solution for 8 min, completing the growth of the 130 nm ZIF-8 inner layer; no KH-550 passivation was performed, and the next cycle was directly initiated. Second cycle: The concentration was increased to 0.35 mol / L zinc acetate + 1.4 mol / L 2-methylimidazole, with zinc acetate immersion for 90 min and ligand solution immersion for 12 min. Third cycle: The concentration was adjusted to 0.55 mol / L zinc acetate + 2.2 mol / L 2-methylimidazole, with zinc acetate immersion for 180 min and ligand solution immersion for 18 min; no passivation treatment was performed throughout the process, ultimately obtaining the ZIF-8 functional layer.

[0097] S3 is completely consistent with Example 1. However, due to structural defects in the ZIF-8 layer, the membrane performance is significantly reduced. The CO2 permeability coefficient is only 58.3 Barrer, due to the surge in gas diffusion resistance caused by agglomerate blockage; the CO2 / CH4 selectivity drops to 8.7, due to the formation of non-selective permeation channels between agglomerates; the tensile strength drops to 55 MPa, due to interfacial stress concentration caused by ZIF-8 agglomeration, making it prone to fracture during bending. Due to the lack of a passivation process, the active sites on the ZIF-8 surface are not sealed, and a large amount of uncoordinated Zn remains on the ZIF-8 surface. 2+ This becomes the heterogeneous nucleation site for subsequent growth. Ultimately, the ZIF-8 particle size distribution is chaotic, with no gradient stratification.

[0098] Comparative Example 4

[0099] Comparative Example 4 provides a method for preparing a polyimide multilayer composite membrane material. The core objective is to compare the technical advantages of the "cyclodextrin derivative / piperazine macrocyclic amine aqueous phase + perfluoroterephthaloyl chloride oil phase" interfacial polymerization system in Example 1. The S1 functionalized PI fiber membrane preparation and S2 ZIF-8 gradient functional layer growth steps are completely consistent with Example 1, except that the S3 interfacial polymerization step uses a traditional system. The specific preparation process is as follows:

[0100] S1, Same as in Example 1.

[0101] S2, Same as Example 1.

[0102] S3. The interfacial polymerization step adopts a traditional system. The only difference from Example 1 is the composition of the aqueous phase and oil phase and the choice of solvent. The specific process is as follows: First, the ZIF-8 gradient functional layer is pretreated with Ar plasma in the same way as in Example 1 (power 200W, treatment time 4min, distance between functional layer and nozzle 8mm, ambient humidity 50%RH) to ensure that sufficient hydroxyl groups are introduced on the surface. Then, a traditional interfacial polymerization system is constructed, in which the aqueous phase is replaced with an aqueous solution containing 2wt% piperazine and the oil phase is replaced with a hexane solution containing 0.5wt% pyromellitic chloride. The interfacial polymerization process is consistent with Example 1. That is, the pretreated sample is first immersed in the aqueous phase for 2min. After the macrocyclic amine is fully adsorbed at the -OH site, it is drained and then immersed in the oil phase for 3min to drive the condensation reaction between the acyl chloride group and the amino / hydroxyl group to form an initial polyamide film on the MOF surface. Finally, it is transferred to a 70℃ oven for curing for 2h to obtain the traditional polyamide selective layer and the final polyimide multilayer composite film material.

[0103] Structural characterization

[0104] The polyimide fiber membrane of Example 1, which underwent chemical imidization and functionalization treatment in step S1, was characterized by SEM as follows: Figure 2 As shown, the electrospun polyimide fiber membrane in Comparative Example 1 was characterized by SEM. Figure 3 As shown, the stress-strain curve of the polyimide multilayer composite film in Example 1 is as follows. Figure 4 As shown, the stress-strain curve of the polyimide multilayer composite film in Comparative Example 1 is as follows: Figure 5 As shown, the ZIF-8 particles with an average particle size of 130 nm prepared in Example 1 are as follows: Figure 6 As shown, the ZIF-8 particles with an average particle size of 400 nm prepared in Example 1 are as follows: Figure 7 As shown, the ZIF-8 particles with an average particle size of 750 nm prepared in Example 1 are as follows: Figure 8 As shown.

[0105] from Figure 2 and Figure 3 As can be seen, chemical imidization and dopamine functionalization can significantly reduce the porosity of the fiber membrane, making the fibers tightly cross-linked. Figure 4 and Figure 5 As can be seen, chemical imidization and dopamine functionalization significantly improved the tensile strength of the composite membrane.

[0106] Performance testing

[0107] The tensile strength test method described in ASTM D822 was used to test the multilayer composite membrane prepared in Example 1 and the gas separation membranes prepared in Comparative Examples 1-4. The results are shown in Table 1 below, and the stress-strain curves of Example 1 and Comparative Example 1 are shown in Table 1 below. Figure 4 and Figure 5 As shown in Table 1, the gas separation and permeation performance of the multilayer composite membranes prepared in Example 1 and Comparative Examples 1-4 were tested using the methods described in GB / T 1038-2000.

[0108] Table 1. Test results of mechanical properties, gas separation performance, and permeability.

[0109]

[0110] While embodiments of the present invention have been described in detail above, it will be apparent to those skilled in the art that various modifications and variations can be made to these embodiments. However, it should be understood that such modifications and variations fall within the scope and spirit of the invention as set forth in the claims. Furthermore, the invention described herein may have other embodiments and can be implemented or carried out in various ways.

Claims

1. A method for producing a polyimide multilayer composite film material, characterized by, The polyamide acid fiber membrane is obtained by electrospinning the polyamide acid glue solution, then the polyamide acid fiber membrane is immersed in a mixed solution containing a chemical imidization reagent and dopamine, and the chemical imidization and dopamine functionalization are simultaneously realized, so as to obtain a functionalized fiber membrane containing hydroxyl and amine groups on the surface; based on the functionalized fiber membrane, by adjusting the concentration gradient and reaction time of the organic ligand solution and the metal salt solution, the functionalized fiber membrane is cyclically immersed to grow MOF layers with different particle sizes, and after each cycle of cyclic immersion growth and drying, passivation treatment is performed by a silane coupling agent, thereby forming a MOF gradient layer with a gradient MOF structure; after the MOF gradient layer is pretreated by Ar plasma, interfacial polymerization is performed, a water-oil reaction system is constructed, a polyamide selective layer is formed on the surface thereof by interfacial polymerization, and a polyimide multilayer composite membrane material is obtained after solidification; the organic ligand in the organic ligand solution includes 2-methyl imidazole, benzene-1, 3, 5-tricarboxylic acid or terephthalic acid; the metal salt in the metal salt solution includes a cobalt salt, a zinc salt, a copper salt or an iron salt. The electrospinning is performed at 25-50 DEG C; and / or, the electrospinning is performed at a humidity of 25-50 RH %; and / or, the solid content of the polyamide acid glue solution is 5-25 %; and / or, the concentration of dopamine in the mixed solution of the chemical imidization reagent and dopamine is 0.1-1 wt %, and / or, the chemical imidization reagent is any one or combination of acetic anhydride, propionic anhydride, butyric anhydride, benzoic anhydride, chloroacetic anhydride, bromo-adipic anhydride, trifluoroacetic anhydride, pyridine and its derivatives, methylpyridine and its derivatives, dimethylpyridine, N, N-dimethylaminopyridine, quinoline, isoquinoline; and / or, the reaction is performed at 40-80 DEG C for 1-4 h, and the chemical imidization and dopamine functionalization are simultaneously realized.

2. The production method according to claim 1, characterized by, When the functionalized fiber membrane is cyclically immersed to grow MOF layers with different particle sizes, the functionalized fiber membrane is cyclically immersed in the metal salt solution and the organic ligand solution and then dried, and after each cycle of cyclic immersion and drying, the functionalized fiber membrane is immersed in a 1-3 wt % silane coupling agent solution for passivation for 5-10 min and vacuum dried at 60-80 DEG C.

3. The preparation method according to claim 1, characterized in that, The functionalized fiber membrane is immersed in the organic ligand solution for 5-20 min at a time; and / or, the functionalized fiber membrane is immersed in the metal salt solution for 0.5-4 h at a time; 4. The production method according to claim 3, characterized by, And / or, the silane coupling agent is any one or combination of KH-550, KH-560, KH-570 and KH-590; ​ Or, cyclically immersed for 3 times, the first cycle is that the functionalized fiber membrane is immersed in 0.1M-0.2M metal salt solution for 30min-60min, and then transferred into 0.4M-0.8M ligand solution for 5min-10min; the second cycle is that the functionalized fiber membrane is immersed in 0.3M-0.4M metal salt solution for 60min-120min, and then transferred into 1.2M-1.6M ligand solution for 10min-15min; the third cycle is that the functionalized fiber membrane is immersed in 0.5M-0.6M metal salt solution for 120min-240min, and then transferred into 2.0M-2.4M ligand solution for 15min-20min.

5. The preparation method according to claim 1, characterized in that, When the MOF gradient layer is subjected to Ar plasma pretreatment, hydroxyl groups are introduced on the surface of the MOF gradient layer by etching, and the reactivity of the amide bond in subsequent interfacial polymerization is enhanced, wherein the plasma power is 100W-300W, the treatment time is 1min-5min, the treatment distance is 5mm-10mm, and the environmental humidity is 40%RH-60%RH.

6. The method of claim 1, wherein, Based on interfacial polymerization, a water phase-oil phase reaction system is constructed, and a polyamide selective layer is formed by interfacial polymerization on the surface thereof, and when the polyimide multilayer composite membrane material is prepared by solidification, the water phase for interfacial polymerization is an aqueous solution containing a macrocyclic amine monomer, the macrocyclic amine includes one or a combination of a piperazine derivative, a cyclodextrin derivative, and a calixarene amine, the concentration of the macrocyclic amine in the water phase is 0.5wt%-2wt%, and the immersion time of the functionalized fiber membrane with the MOF gradient layer in the water phase is 1min-2min; and / or, the oil phase for interfacial polymerization is an organic solvent solution containing a fluorine-containing acyl chloride monomer, the fluorine-containing acyl chloride is a bifunctional monomer, including one of perfluorophthaloyl chloride and perfluorophthaloyl chloride, the concentration of the fluorine-containing acyl chloride in the oil phase is 0.1wt%-0.5wt%, the organic solvent of the oil phase is one of n-hexane, cyclohexane, and petroleum ether, the immersion time of the functionalized fiber membrane with the MOF gradient layer in the oil phase is 1min-3min; and, after interfacial polymerization, solidification is performed at 60℃-80℃ for 1h-3h.

7. The preparation method according to claim 1, characterized in that, The polyamic acid glue solution is formed by condensation polymerization of a binary anhydride and a binary amine monomer in a polar aprotic solvent; the binary anhydride includes one or more of pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-oxybisphthalic anhydride, 3,3,4,4-diphenyl sulfone tetracarboxylic dianhydride, 3,3',4,4'-biphenyl tetracarboxylic dianhydride, or 2,3,3',4'-biphenyl tetracarboxylic dianhydride; and / or, the binary amine monomer includes one or more of 4,4'-oxydianiline, p-phenylenediamine, 4,4-diaminobiphenyl, 3,4'-oxydianiline, m-phenylenediamine, 4,4'-oxydianiline, 1,4-bis(4-aminophenoxy)benzene, or 1,3-bis(3-aminophenoxy)benzene; and / or, the polar aprotic solvent includes one of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone.

8. A polyimide multilayer composite film material produced by the production method according to any one of claims 1 to 7.

9. Use of a polyimide multilayer composite film material produced by the production method according to any one of claims 1 to 7 in gas separation.

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