Polyimide multilayer composite membrane, preparation method thereof and application of polyimide multilayer composite membrane in gas separation

Abstract

The invention provides a polyimide multilayer composite membrane, a preparation method thereof and application of the polyimide multilayer composite membrane in gas separation, and relates to the technical field of gas separation. The preparation method provided by the invention comprises the following steps: carrying out electrostatic spinning on a polyamide acid glue solution, and then carrying out imidization to obtain a fiber membrane; carrying out in-situ MOF growth based on the fiber membrane, infiltrating the fiber membrane in a PDMS solution, and drying to obtain a composite membrane; and coating the surface of the composite film with a polyimide glue solution containing MOF particles, and curing to obtain the polyimide multilayer composite film. The compactness of the fiber membrane can be effectively improved, so that the pore permeation phenomenon is improved, the thickness of a separation layer of the multi-layer composite membrane is effectively reduced, and the gas transmission resistance is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of gas separation, and particularly relates to a polyimide multi-layer composite membrane, a preparation method thereof, and an application in gas separation. Background Art

[0002] With the adjustment of the global energy structure and the promotion of the carbon emission reduction strategy, gas separation technology has become a key technical support for achieving the carbon neutral goal. In industrial scenarios such as petrochemical industry, natural gas purification, and biogas utilization, although common processes such as cryogenic distillation and pressure swing adsorption are relatively mature, defects such as high energy consumption, large equipment floor area, and easy secondary pollution are becoming increasingly prominent. With the continuous development of material chemistry, membrane separation technology, relying on its unique "molecular sieve" mechanism, realizes the selective permeation of gas molecules through the microporous structure on the surface of the membrane material, and there is no phase change conversion during the separation process, and the energy consumption is significantly reduced compared with traditional processes. Therefore, it has been widely used in the separation and recovery of hydrogen, the separation and recovery of carbon dioxide, air separation (oxygen enrichment, nitrogen enrichment), and the separation of acidic corrosive gases.

[0003] Polyimide is a type of high molecular compound containing imide groups in its molecular structure. The imide ring contained in its molecular main chain is formed by the polycondensation of diamine and dianhydride compounds in an aprotic polar solvent. It has excellent thermal stability and good electrical insulation performance. Therefore, it has a good theoretical basis in gas separation. At the same time, in order to meet the application requirements in the gas separation field, inorganic materials are usually added to the polyimide matrix to prepare a mixed matrix membrane to improve its gas separation performance. However, the content of the filler has a significant impact on the performance of the polymer, and too much or too little filler content may have a negative impact on the performance of the polyimide film. For example, too little filler cannot form a gas transmission path, thus unable to significantly improve the gas permeation and separation performance of the gas separation membrane, while too much filler may cause the polymer to become too brittle and affect its mechanical properties. Therefore, there is an urgent need to provide a solution to improve the above problems. Summary of the Invention

[0004] The purpose of the present invention is to provide a polyimide multi-layer composite membrane, a preparation method thereof, and an application in gas separation, which can effectively improve the denseness of the fiber membrane, thus improving the pore penetration phenomenon, and further effectively reducing the thickness of the separation layer of the multi-layer composite membrane and reducing the gas transmission resistance.

[0005] In the first aspect, a preparation method of a polyimide multi-layer composite membrane provided by the present invention includes: electrospinning a polyamic acid solution and then imidizing it to obtain a fiber membrane; growing MOF in-situ based on the fiber membrane, then infiltrating PDMS solution and drying it to obtain a composite membrane; coating a polyimide solution containing MOF particles on the surface of the composite membrane and curing it to obtain a polyimide multi-layer composite membrane.

[0006] The preparation method provided by the present invention can form a fiber membrane with intertwined fibers through electrospinning. After in-situ growth of MOF on its surface, it can promote the directional alignment of MOF along the fiber filaments. At the same time, using PDMS as the intermediate layer can effectively prevent the penetration of the layer solution, optimize the gas flow distribution. Meanwhile, the polyimide mixed matrix membrane containing MOF particles formed on the surface can be used as the selective layer, providing molecular sieving function to effectively improve gas permeability and selectivity, and at the same time can improve the mechanical properties, hydrophobicity and thermal stability of the multi-layer composite membrane.

[0007] Optionally, electrospinning is carried out at 25°C - 50°C.

[0008] Optionally, electrospinning is carried out at a humidity of 25RH% - 50RH%.

[0009] Optionally, the solid content of the polyamic acid colloidal solution is 5% - 25%.

[0010] Optionally, pre-imideization is carried out at 60°C - 120°C after electrospinning.

[0011] Optionally, after electrospinning, the temperature is raised to 380°C - 420°C for imideization in a gradient manner.

[0012] Optionally, the imideization treatment is carried out for 10 min - 60 min after electrospinning.

[0013] Optionally, when in-situ MOF growth is carried out on the fiber membrane, it includes: circulating and impregnating the fiber membrane in the organic ligand solution and the metal salt solution and then drying.

[0014] Optionally, the circulating impregnation is carried out 1 - 10 times.

[0015] Optionally, the organic ligand in the organic ligand solution includes 2-methylimidazole, trimesic acid or terephthalic acid.

[0016] Optionally, the metal salt in the metal salt solution includes cobalt salt, zinc salt, copper salt or iron salt.

[0017] Optionally, the fiber membrane is impregnated in the organic ligand solution for 5 min - 20 min each time.

[0018] Optionally, the fiber membrane is impregnated in the metal salt solution for 1 h - 4 h each time.

[0019] Optionally, it is dried under vacuum after circulating impregnation.

[0020] Optionally, it is dried at 50°C - 80°C after circulating impregnation.

[0021] Optionally, the concentration of PDMS in the PDMS solution is 10wt% - 50wt%.

[0022] Optionally, the solvent of the PDMS solution includes one of n-heptane, n-hexane, cyclohexane, toluene, and chloroform.

[0023] Optionally, a crosslinking agent is also dissolved in the PDMS solution.

[0024] Optionally, the AAA includes one of tetraethyl orthosilicate, phenyltrimethoxysilane, octyltrimethoxysilane, γ-aminopropyltriethoxysilane, and vinyltrimethoxysilane.

[0025] Optionally, a catalyst is also dissolved in the PDMS solution.

[0026] Optionally, it includes one of dibutyltin dilaurate and dimethylcyclohexylamine.

[0027] Optionally, infiltrate the PDMS solution and dry it. After swelling by infiltration in the modified solution, separate and dry to obtain a composite membrane; wherein, a surfactant is dissolved in the modified solution.

[0028] Optionally, the surfactant is a non-ionic surfactant.

[0029] Optionally, the non-ionic surfactant includes one of polyethylene glycol surfactants, Tween surfactants, Span surfactants, and fatty alcohol surfactants.

[0030] Optionally, the content of the MOF particles in the polyimide glue solution is 1 wt% - 20 wt%.

[0031] Optionally, the particle size of the MOF particles is 0.1 μm - 5 μm.

[0032] Optionally, the type of the MOF particles is independent of the MOF grown in-situ on the fiber membrane.

[0033] Optionally, the solid content of the polyimide glue solution is 10% - 35%.

[0034] Optionally, the polyamic acid glue solution is formed by polycondensation of a dianhydride and a diamine monomer in a polar aprotic solvent.

[0035] Optionally, the dianhydride includes pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-oxydiphthalic anhydride, 3,3,4,4-diphenylsulfone tetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, or 2,3,3',4'-biphenyltetracarboxylic dianhydride.

[0036] Optionally, the diamine monomer includes 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.

[0037] Optionally, the polar aprotic solvent includes one of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone.

[0038] In a second aspect, the present invention also provides a polyimide multilayer composite film prepared by any of the above optional preparation methods.

[0039] In a third aspect, the present invention also provides the application of the polyimide multilayer composite film prepared by any of the above optional preparation methods in gas separation. Description of the Drawings

[0040] Figure 1 is a flowchart of a preparation method of a polyimide multilayer composite film provided by the present invention;

[0041] Figure 2 is an SEM characterization diagram of the fiber membrane after in-situ growth of MOF particles in step S2 of Example 1 of the present invention;

[0042] Figure 3 is an SEM characterization diagram of the composite film prepared in step S2 of Example 1 of the present invention;

[0043] Figure 4 is an SEM characterization diagram of the multilayer composite film prepared in Example 1 of the present invention;

[0044] Figure 5 is a stress-strain curve diagram of the multilayer composite film prepared in Example 1 of the present invention;

[0045] Figure 6 is a stress-strain curve diagram of the multilayer composite film prepared in Comparative Example 1 of the present invention. Detailed Embodiments

[0046] 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. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein shall have the ordinary meanings understood by those of ordinary skill in the art to which the present invention pertains.

[0047] See Figure 1, the present invention provides a method for preparing a polyimide multi-layer composite film, comprising the following steps:

[0048] S1. Electrospinning the polyamic acid solution and then imidizing to obtain a fibrous membrane;

[0049] S2. Based on the fibrous membrane, in-situ growth of MOF is carried out, and then infiltrated with PDMS solution and dried to obtain a composite membrane;

[0050] S3. Coating a polyimide solution containing MOF particles on the surface of the composite membrane and curing to obtain a polyimide multi-layer composite film.

[0051] In fact, by electrospinning the polyamic acid solution in the present invention, it is beneficial to reduce the size of the fiber filaments in the fibrous membrane and promote the interweaving of the fiber filaments. After in-situ growth of MOF, the MOF particles can grow in-situ along the cellulose direction. In addition, after infiltrating with PDMS solution, PDMS penetrates into the interior of the fibrous membrane and encapsulates the in-situ grown MOF particles, and at the same time can improve the denseness of the fibrous membrane. Finally, after curing with a polyimide solution containing MOF particles on the surface, a selective layer with molecular sieve function can be formed on the surface.

[0052] Thus, the present invention uses the fibrous membrane after in-situ growth of MOF particles as the support layer, PDMS as the intermediate layer, and the polyimide membrane containing MOF particles as the separation layer, which can effectively improve the denseness of the fibrous membrane, thus improving the pore seepage phenomenon, and further effectively reducing the overall thickness of the multi-layer composite membrane and reducing the gas transmission resistance.

[0053] Specifically, the polyamic acid solution used in step S1 can be prepared by polycondensation of a dianhydride and a diamine monomer in a polar aprotic solvent to obtain a polyamic acid solution with a solid content of 5%-25%, and in an environment of 25°C-50°C and 25RH%-50RH%, electrospinning equipment is used for fiber spinning. Specifically, the size of the fiber filaments in the fiber material obtained after fiber spinning is 0.05μm-1μm.

[0054] In some embodiments, the dianhydrides used in preparing the polyamic acid solution may include pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-oxydiphthalic anhydride, 3,3,4,4-diphenylsulfone tetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride or 2,3,3',4'-biphenyltetracarboxylic dianhydride; the diamine monomers used may include 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; and the polar aprotic solvents used include one of N,N-dimethylacetamide, N,N-dimethylformamide and N-methylpyrrolidone.

[0055] Actually, when performing step S1, the fiber material prepared by electrospinning is preimidized at 60°C - 120°C and then placed at 380°C - 420°C for complete imidization treatment, thereby obtaining the polyimide fiber membrane. In some embodiments, the fiber material can be subjected to heat preservation preimidization treatment at 60°C - 120°C with multiple gradually increasing temperature values. Specifically, the heat preservation time for each temperature value is independently 1 min - 30 min. In addition, when performing the complete imidization treatment, the preimidized fiber material can be fixed in the needle plate frame clamp, which is beneficial to forming a membrane structure after the complete imidization treatment, and at the same time, it can be heat-preserved at 380°C - 420°C for 10 min - 60 min for complete imidization treatment.

[0056] Specifically, the fiber membrane prepared after performing step S1 is one of the polyimide material nanofiber membranes in the original fluffy state, the nanofiber membranes in the physical cross-linked state, and the nanofiber membranes in the chemical cross-linked state. Actually, the nanofiber membranes in the physical cross-linked state are the nanofiber membranes in which the fiber filaments overlap with each other after mechanical rolling, and the nanofiber membranes in the chemical cross-linked state are the nanofiber membranes in which the contact points of the fiber filaments penetrate each other, and the ultra-long polyimide molecular chains jump out of the fiber filaments and are pinned into another fiber filament.

[0057] In some embodiments, when performing step S2, the fiber membrane can be surface-treated in advance to improve the efficiency and quality of in-situ MOF growth on the surface. Actually, the fiber membrane can be soaked in an alkali solution for surface treatment, such as soaking in inorganic alkali solutions such as sodium hydroxide solution and potassium hydroxide solution. In addition, the concentration of the alkali solute in the alkali solution can be 0.2 mol / L - 2 mol / L.

[0058] In some embodiments, when performing in-situ MOF growth based on the fiber membrane in step S2, the fiber membrane can be cyclically impregnated in the organic ligand solution and the metal salt solution and then dried. In fact, when the fiber membrane is impregnated in the organic ligand solution, the organic ligand can penetrate into the interior and surface of the fiber filaments. Thus, when impregnated in the metal salt solution, metal ions can coordinate with the organic ligand to in-situ generate metal-organic frameworks (MOFs) on the surface of the fiber filaments.

[0059] Specifically, when performing step S2, the fiber membrane is cyclically impregnated in the organic ligand and metal salt solution 1-10 times. This is beneficial for adjusting the loading amount and morphology of the MOF in-situ grown on the surface of the fiber filaments. In addition, through cyclic impregnation, the MOF on the surface of the fiber filaments can grow in a stacked manner, which is beneficial for improving the compactness of the MOF on the surface of the fiber membrane.

[0060] In some embodiments, the organic ligands in the organic ligand solution used when performing step S2 include 2-methylimidazole, trimesic acid or terephthalic acid, and the metal salts in the metal salt solution used include cobalt salts, zinc salts, copper salts or iron salts. In fact, when the fiber membrane is cyclically impregnated in the 2-methylimidazole solution and the cobalt salt solution, ZIF-67 can be in-situ generated on the surface of the fiber membrane. Specifically, by selecting different organic ligands and metal salts, the types of the generated MOFs can be adjusted, such as UiO-66, UiO-66-NH2, ZIF-8, MIL-101(Fe), MIL-101(Cr) or HKUST-1.

[0061] In some embodiments, the concentration of the organic ligand solution and the concentration of the metal salt solution used when performing step S2 are independently 0.1 mol / L - 4 mol / L. In addition, the types of solvents used in the organic ligand solution and the metal salt solution are independent of each other. Specifically, the fiber membrane can be cyclically impregnated in the organic ligand methanol solution and the metal salt methanol solution.

[0062] In some embodiments, the single impregnation duration of the fiber membrane in the organic ligand solution when performing step S2 is 5 min - 20 min to allow the organic ligand solution to fully infiltrate the fiber filaments of the fiber membrane. In addition, the single impregnation duration of the fiber membrane in the metal salt solution is 1 h - 4 h to allow metal ions to fully react with the organic ligand to generate metal-organic frameworks (MOFs).

[0063] Specifically, after performing step S2 and cyclically impregnating the fiber membrane, drying treatment is performed in a vacuum environment at 50°C - 80°C. This can improve the positional stability of the metal-organic frameworks (MOFs) attached to the surface of the fiber filaments, and thus is beneficial for subsequent potting. In addition, after performing in-situ MOF growth based on the fiber membrane in step S2, the loading amount of MOF on the fiber membrane is 5 wt% - 40 wt%.

[0064] In fact, after performing in-situ MOF growth on the surface of the fiber membrane in step S2, when infiltrated in the PDMS solution, the PDMS solution penetrates into the interior of the fiber membrane and encapsulates the in-situ grown MOF particles. At the same time, it can improve the denseness of the fiber membrane, which is beneficial to preventing solution penetration and maintaining the stability of the membrane structure. Meanwhile, the rubber film formed after the PDMS solution solidifies can optimize the gas diffusion path and reduce the transmission resistance.

[0065] In some embodiments, the concentration of PDMS (polydimethylsiloxane) in the PDMS solution used when performing step S2 is 10 wt% - 50 wt%. In addition, the solvent used in the PDMS solution includes one of n-heptane, n-hexane, cyclohexane, toluene, and chloroform. Specifically, the solvent used in the PDMS solution can be a commonly used solvent in the art that can dissolve PDMS and does not react with polyimide.

[0066] In some embodiments, a crosslinking agent and a catalyst are also dissolved in the PDMS solution used when performing step S2. Specifically, the crosslinking agent includes one of tetraethyl orthosilicate (TEOS), phenyltrimethoxysilane, octyltrimethoxysilane, γ-aminopropyltriethoxysilane, and vinyltrimethoxysilane, and the catalyst includes one of dibutyltin dilaurate (DBTDL) and dimethylcyclohexylamine. Further, the concentration of the crosslinking agent in the PDMS solution is 5% - 20%, and the concentration of the catalyst is 0.1% - 2%.

[0067] In fact, after infiltrating and drying the fiber membrane in the PDMS solution, a PDMS rubber film can be formed on the surface of the fiber membrane. After infiltrating in the modification solution, the PDMS rubber film and the fiber membrane swell, thereby improving the hydrophilicity of the composite membrane, enhancing the interfacial compatibility with the mixed matrix membrane, and enabling a complete and defect-free separation layer to be formed on the surface of the composite membrane. Further, a surfactant is dissolved in the modification solution, and the surfactant is a non-ionic surfactant, including one of polyethylene glycol surfactants, Tween surfactants, Span surfactants, and fatty alcohol surfactants.

[0068] In some embodiments, the polyimide adhesive solution used in step S3 includes a polyimide base adhesive solution and MOF particles dispersed in the polyimide base adhesive solution, and the content of the MOF particles in the polyimide adhesive solution is 1 wt% - 20 wt%. In fact, the polyimide base adhesive solution used can be synthesized by polycondensation of dianhydride and diamine in a non-polar proton solvent, or can be prepared by stirring and dissolving molding powder in N,N-dimethylacetamide. The molding powder used includes Vespel SP-1, Ultem 1000, Torlon 4203, Upimol, Aurum PL450c. Specifically, the solid content of the polyimide adhesive solution is 10% - 30%.

[0069] In some embodiments, the type of MOF particles used in step S3 is independent of the type of MOF in-situ generated on the surface of the fiber membrane. 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), HKUST-1. In addition, the MOF particles can be commercially available conventional products or can be pre-synthesized. At the same time, the particle size of the MOF particles used is 0.1 μm - 5 μm.

[0070] In some embodiments, when performing step S3, a polyimide adhesive solution containing MOF particles is coated on one surface of the composite membrane and cured. In fact, a selective layer can be formed on the surface of the composite membrane after curing, and the arranged MOF particles can increase the gas transmission path and perform gas screening. Specifically, the curing process can be drying in a vacuum drying oven at 60 °C for 12 h.

[0071] Example 1

[0072] This Example 1 provides a method for preparing a polyimide multi-layer composite membrane, including the following steps:

[0073] S1. Mix pyromellitic dianhydride (PMDA) and 4,4'-diaminodiphenyl ether (ODA) at a molar ratio of 1.01:1 and put them into N,N-dimethylacetamide (DMAc). A polyamic acid solution with a solid content of 10% is prepared by solution polycondensation and adjusting the solid content. Electrospinning is carried out at 25 °C and 30 RH% using an electrospinning device to obtain a fiber material; the fiber material is gradually kept at 60 °C, 80 °C, and 100 °C for 15 min for pre-imideization, and then the fiber material is fixed on a needle plate frame clip and gradually heated to 380 °C in an imideization furnace and kept at this temperature for 20 min for complete imideization treatment to obtain a fiber membrane;

[0074] S2. Immerse the fiber membrane in a sodium hydroxide solution with a concentration of 1 mol / L for 30 min, then wash it and immerse it in a 2-methylimidazole methanol solution with a concentration of 1 mol / L for 10 min. Then, place it in a cobalt acetate methanol solution with a concentration of 1.5 mol / L and immerse it for 3 h. After one cycle of impregnation, dry the fiber membrane in a vacuum environment at 60 °C for 12 h, then take it out after being completely immersed in a 30 wt% PDMS heptane solution (PDMS:TEOS:DBTDL mass ratio is 100:10:1) and let it air dry naturally, and cure it in an oven at 60 °C for 24 h to obtain a composite membrane;

[0075] S3. Uniformly brush a polyimide glue solution on one surface of the composite membrane (prepared by dissolving molding powder Vespel SP-1 in N,N-dimethylacetamide to obtain a 25 wt% glue solution, and containing 5 wt% ZIF-67 with a particle size of 500 nm), then dry it in a vacuum environment at 60 °C for 12 h to obtain a polyimide multi-layer composite membrane.

[0076] Example 2

[0077] This Example 2 provides a method for preparing a polyimide multi-layer composite membrane. The difference from Example 1 is that in step S2, after drying the fiber membrane in a vacuum environment at 60 °C for 24 h, immerse it in a 5 wt% Tween20 n-hexane solution at 40 °C for 3 min, then take it out and rinse it 3 times with absolute ethanol, dry it in an oven at 60 °C for 2 h, and then place it in a 30 wt% PDMS heptane solution (PDMS:TEOS:DBTDL mass ratio is 100:10:1) and let it air dry naturally after being completely immersed, and cure it in an oven at 60 °C for 24 h to obtain a composite membrane.

[0078] Comparative Example 1

[0079] This Comparative Example 1 provides a method for preparing a polyimide gas separation membrane. The difference from Example 1 is that in step S2, after drying the fiber membrane in a vacuum environment at 60 °C for 12 h, place it in a polyimide-filled glue solution (solid content 20%, prepared by dissolving molding powder Vespel SP-1 in N,N-dimethylacetamide, containing 5 wt% ZIF-67 with a particle size of 500 nm), and seal it at 0.5 MPa for 0.5 h, then dry it in a vacuum environment at 60 °C for 12 h to obtain a polyimide gas separation membrane.

[0080] Structure Characterization

[0081] Perform SEM characterization on the fiber membrane after in-situ growth of MOF particles in step S2 of Example 1 as Figure 2 shown, perform SEM characterization on the composite membrane prepared in step S2 of Example 1 as Figure 3 shown, perform SEM characterization on the multi-layer composite membrane prepared in Example 1 asFigure 4 As shown in Figure 2 and Figure 3 , it can be seen that the MOF particles can adhere tightly and densely to the surface of the filaments of the fiber membrane. At the same time, after PDMS encapsulation, the pores existing between the filaments can be effectively filled. As can be seen from Figure 4 , after PDMS encapsulation, a flat, dense and defect-free membrane structure can be formed on the surface.

[0082] Performance Testing

[0083] Using the tensile strength test method described in ASTM D822 standard, the multi-layer composite membranes prepared in Examples 1 to 2 and the gas separation membrane prepared in Comparative Example 1 were tested. The results are shown in Table 1 below, and the stress-strain curves of Example 1 and Comparative Example 1 are respectively as shown in Figure 5 and Figure 6 ; Using the method described in GB / T1038 - 2000 standard, the multi-layer composite membranes prepared in Examples 1 to 2 and the gas separation membrane prepared in Comparative Example 1 were tested for gas separation and permeation performance. The results are shown in Table 1 below.

[0084] Table 1 Test Results of Mechanical Properties, Gas Separation Properties and Permeation Properties

[0085]

[0086] Combined with Table 1, Figure 5 and Figure 6 , and combined with Example 1 and Comparative Example 1, it can be seen that since there is no PDMS intermediate layer in the gas separation membrane in Comparative Example 1, a large amount of the selective layer solution permeates into the pores of the fiber membrane. And because the polyimide solution infiltrates into the fiber membrane to form a stronger force, this improves the mechanical properties of the gas separation membrane in Comparative Example 1, but this also increases the thickness of the separation layer and the gas transmission resistance. Therefore, the gas separation and permeation performance of Comparative Example 1 both decrease significantly, while the multi-layer composite membrane in Example 1 can significantly enhance the permeation performance during gas separation by constructing a PDMS intermediate layer to prevent pore penetration.

[0087] Combined with Example 1 and Example 2, it can be seen that after modifying and swelling PDMS in Example 2, the hydrophilicity of the composite membrane (fiber membrane + PDMS) can be improved, so that a separation layer with a complete and defect-free surface can be formed, which is beneficial to improving the gas separation and permeation performance and slightly improving the mechanical properties.

[0088] In summary, the multi-layer composite film provided by the present invention uses a polyimide fiber film as the basis, and loads MOFs by in-situ growth method as the support layer, which can construct a complex three-dimensional network structure for gas transmission. It can not only enhance the overall mechanical strength of the film, but also provide rich transmission channels for gases. In fact, due to the different kinetic diameter differences of different gas molecules, their transmission rates in the transmission channels are different, thus achieving efficient selective permeation of specific gases.

[0089] In addition, the multi-layer composite film provided by the present invention uses PDMS as the intermediate layer. It not only has good chemical stability, thermal stability and gas flux, but also can effectively prevent the selective layer solution from excessively penetrating into the pores of the support layer in the system, effectively avoiding the increase in gas transmission resistance caused by the penetration of the selective layer solution, and promoting the formation of the rigid structure of the composite film, effectively ensuring the stability of the film structure and the smoothness of gas transmission. At the same time, PDMS can also improve the smoothness of the intermediate layer surface, which is beneficial to the uniform preparation of the selective layer and further optimizes the overall performance.

[0090] Furthermore, Examples 1 to 2 use polyimide mixed with ZIF-67 as the selective layer, which can utilize the molecular sieve mechanism (there is a strong interaction between ZIF-67 and CO2 molecules, which can limit the adsorption and diffusion of CO2 molecules in the selective layer, while the kinetic diameter of H2 molecules is smaller and can diffuse relatively freely) to screen according to the size and properties of gas molecules. Therefore, the present invention synergizes from the molecular sieve and adsorption-diffusion mechanisms of the selective layer to the diffusion selection mechanisms of the intermediate layer and the support layer, effectively improving the gas screening ability of the multi-layer composite film.

[0091] Although the embodiments of the present invention have been described in detail above, it is obvious to those skilled in the art that various modifications and changes can be made to these embodiments. However, it should be understood that such modifications and changes are all within the scope and spirit of the present invention described in the claims. Moreover, the present invention described herein can have other embodiments and can be implemented or realized in various ways.

Claims

1. A method for preparing a polyimide multilayer composite film, characterized in that, Including: Electrospinning a polyamic acid solution and then imidizing to obtain a fiber membrane; in-situ growing MOF on the fiber membrane, then infiltrating with a PDMS solution and drying to obtain a composite membrane; Coating a polyimide solution containing MOF particles on the surface of the composite membrane and curing to obtain a polyimide multi-layer composite membrane.

2. The preparation method according to claim 1, wherein Electrospinning at 25°C - 50°C; and / or electrospinning at a humidity of 25RH% - 50RH%; and / or the solid content of the polyamic acid solution is 5% - 25%; and / or pre-imidizing at 60°C - 120°C after electrospinning; and / or gradually heating to 380°C - 420°C for imidization after electrospinning; and / or imidizing for 10 min - 60 min after electrospinning.

3. The preparation method according to claim 1, characterized in that, When in-situ growing MOF on the fiber membrane, it includes: circulatingly impregnating the fiber membrane in an organic ligand solution and a metal salt solution and then drying.

4. The preparation method according to claim 3, characterized in that, Circulating impregnation is carried out 1 - 10 times; and / or the organic ligand in the organic ligand solution includes 2-methylimidazole, trimesic acid or terephthalic acid; and / or the metal salt in the metal salt solution includes cobalt salt, zinc salt, copper salt or iron salt; and / or the fiber membrane is singly impregnated in the organic ligand solution for 5 min - 20 min; and / or the fiber membrane is singly impregnated in the metal salt solution for 1 h - 4 h; and / or drying under vacuum after circulating impregnation; and / or drying at 50°C - 80°C after circulating impregnation.

5. The preparation method according to claim 1, characterized in that, The concentration of PDMS in the PDMS solution is 10wt% - 50wt%; and / or the solvent of the PDMS solution includes one of n-heptane, n-hexane, cyclohexane, toluene, chloroform; and / or a crosslinking agent is also dissolved in the PDMS solution, preferably the crosslinking agent includes one of tetraethyl orthosilicate, phenyltrimethoxysilane, octyltrimethoxysilane, γ-aminopropyltriethoxysilane, vinyltrimethoxysilane; and / or a catalyst is also dissolved in the PDMS solution, preferably the catalyst includes one of dibutyl dodecyl tin dilaurate, dimethylcyclohexylamine.

6. The preparation method according to claim 1, wherein Infiltrating with the PDMS solution and drying, infiltrating and swelling in a modification solution and then separating and drying to obtain a composite membrane; wherein, a surfactant is dissolved in the modification solution, preferably the surfactant is a non-ionic surfactant, more preferably the non-ionic surfactant includes one of polyethylene glycol surfactants, Tween surfactants, Span surfactants, fatty alcohol surfactants.

7. The preparation method according to claim 1, characterized in that, The content of the MOF particles in the polyimide solution is 1wt% - 20wt%; and / or the particle size of the MOF particles is 0.1μm - 5μm; and / or the type of the MOF particles is independent of the MOF in-situ grown on the fiber membrane; and / or the solid content of the polyimide solution is 10% - 35%; and / or coating a polyimide solution containing MOF particles on one surface of the composite membrane and curing.

8. The preparation method according to claim 1, wherein The polyamic acid solution is formed by polycondensation of a dianhydride and a diamine monomer in a polar aprotic solvent; preferably: the dianhydride includes pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-oxydiphthalic anhydride, 3,3,4,4-diphenylsulfone tetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride or 2,3,3',4'-biphenyltetracarboxylic dianhydride; and / or, the diamine monomer includes 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; and / or, the polar aprotic solvent includes one of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone.

9. A polyimide multilayer composite film prepared by the preparation method according to any one of claims 1 to 8.

10. Use of a polyimide multilayer composite film prepared by the preparation method according to any one of claims 1 to 8 in gas separation.

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