A polyimide hollow fiber gas separation membrane for nitrogen and oxygen separation, its preparation method and its application

Abstract

This invention relates to the field of gas separation membrane technology, and discloses a polyimide hollow fiber gas separation membrane for nitrogen and oxygen separation, its preparation method, and its application. The membrane uses polyimide synthesized with 6FDA-TrMPD as a matrix, and introduces a specific proportion of octaaminophenyl POSS nanoparticles to form an 80-150 nm thick organic-inorganic hybrid gradient interlocking network skin on the fiber surface, with the POSS concentration decreasing from the outside to the inside. The preparation employs a precisely controlled dry-wet process, combined with an innovative two-step online displacement post-treatment process using ethanol and ionic liquid, reducing the post-treatment time from several days to within half an hour. This results in an O2 / N2 separation performance retention rate of over 95% for the membrane product, significantly improving the membrane's pressure resistance and operational stability. This invention solves the technical bottleneck of continuous and large-scale production of high-performance polyimide gas separation membranes, combining high permeability, high selectivity, and industrial preparability, and is suitable for the separation and enrichment of nitrogen and oxygen in air.

Description

Technical Field

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

[0002] The efficient separation of nitrogen and oxygen from air is of great significance in industrial applications (such as nitrogen enrichment, oxygen enrichment, and inert protection). Compared with traditional cryogenic separation and pressure swing adsorption technologies, membrane separation technology has advantages such as low energy consumption, compact equipment, and simple operation. Polyimide (PI) is considered an ideal gas separation membrane material due to its excellent thermal stability, mechanical properties, and adjustable gas separation performance.

[0003] However, existing technologies have the following drawbacks: Material performance bottleneck: Most commercial polyimides (such as Matrimid®) exhibit a “trade-off” effect in O2 / N2 separation, meaning that high selectivity is often accompanied by low permeability, and vice versa, which limits separation efficiency and economy.

[0004] The process is difficult to scale up continuously: the preparation of high-performance hollow fiber membranes typically involves complex dry-wet spinning processes, and obtaining a defect-free, dense, selective skin requires extremely stringent control of spinning conditions (such as air gap humidity and temperature). In particular, the post-processing stage traditionally requires prolonged (several days) immersion in solvents such as isopropanol to displace residual highly polar solvents (such as NMP and DMAc) within the membrane, preventing the membrane structure from collapsing during drying. This step is extremely time-consuming and is a key bottleneck hindering the large-scale continuous production of hollow fiber membranes.

[0005] The process parameters are vague: key process parameters such as humidity and length of air gaps are crucial to the formation and defect control of the skin, but the ranges given in existing patents and literature are too broad and lack guidance on the optimal range for specific polyimide formulations and O2 / N2 separation applications.

[0006] Therefore, developing a polyimide hollow fiber membrane that combines excellent O2 / N2 separation performance and structural strength, and can be efficiently and continuously prepared through optimized post-processing, has significant industrial value. Summary of the Invention

[0007] The present invention aims to provide a novel polyimide hollow fiber gas separation membrane, its preparation method, and its applications. This membrane, by constructing a gradient distribution structure of POSS nanoparticles, enhances segment rigidity while avoiding interfacial defects. Simultaneously, through an innovative online post-processing technique, the production cycle is significantly shortened with acceptable performance degradation, while improving the membrane's pressure resistance and operational stability, achieving a leap from intermittent laboratory preparation to continuous industrial production.

[0008] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows: On one hand, this invention provides a polyimide hollow fiber gas separation membrane for nitrogen and oxygen separation. The membrane comprises a polyimide copolymer matrix and cage-like polysilsesquioxane (POSS) nanoparticles gradient dispersed therein. The polyimide is prepared by a two-step method from a specific diamine monomer, 4,4'-(hexafluoroisopropene)phthalic anhydride (6FDA), and a dianhydride monomer, 2,4,6-trimethyl-1,3-phenylenediamine (TrMPD), with a number-average molecular weight (Mn) of 80,000-150,000 g / mol. The hollow fiber membrane has an asymmetric structure, with a surface layer of 80-150 nm thick organic-inorganic hybrid dense skin. The concentration of POSS nanoparticles in the skin layer decreases gradient from the surface to the interior along the membrane thickness direction, forming a gradient interlocking network structure.

[0009] On the other hand, the present invention provides a method for preparing the above-mentioned hollow fiber gas separation membrane, comprising the following steps: Step 1. Preparation of hybrid casting solution: Dissolve polyimide powder in N-methyl-2-pyrrolidone (NMP) or N,N-dimethylacetamide (DMAc) to prepare a homogeneous solution with a solid content of 20-30 wt%; ultrasonically disperse POSS nanoparticles in tetrahydrofuran, a volatile solvent accounting for 5-15% of the total solvent mass; then mix the dispersion with the polymer solution and add ethylene glycol, a non-solvent additive accounting for 1-5% of the total solvent mass. After stirring and degassing, a stable hybrid casting solution is obtained.

[0010] Step 2. Dry-wet spinning: Using a dual-channel spinneret, the hybrid casting solution obtained in Step 1 is used as the shell fluid, and the NMP / H2O mixture containing surfactants (such as polyvinylpyrrolidone, abbreviated as PVP) is used as the core fluid for spinning.

[0011] Step 3. Post-treatment: Soak the nascent fiber filaments in deionized water for 24 hours to displace residual solvent, then transfer them to ethanol or isopropanol (IPA) for 2-3 days. Finally, vacuum dry at 80°C for 6-12 hours to obtain the finished hollow fiber membrane.

[0012] Preferably, the present invention further selects a specific monomer to synthesize the polyimide, and the preparation method is as follows: Under nitrogen protection and ice-water bath conditions, TrMPD was dissolved in anhydrous NMP, and 6FDA solid powder in an equimolar ratio was slowly added. The reaction temperature was controlled below 10°C, and the reaction was stirred for 4-6 hours to obtain a high-viscosity polyamic acid (PAA) solution.

[0013] Add stoichiometric amounts of acetic anhydride as a dehydrating agent and pyridine as a catalyst to the above PAA solution, and carry out a chemical imidization reaction for 12-14 hours by stirring at room temperature to 80°C.

[0014] The reaction solution was poured into excess ethanol or methanol to precipitate the fibrous polymer, which was then filtered and repeatedly washed with ethanol. The polymer was then vacuum dried at 80-100℃ for 24-48 hours to obtain 6FDA-TrMPD polyimide powder.

[0015] Preferably, the POSS nanoparticles are octaaminophenyl POSS, and their mass fraction in the casting solution is 3-8% of the mass of polyimide.

[0016] Preferably, the flow rate of the shell fluid during spinning is 6 to 60 mL / min, and the flow rate ratio of the shell fluid to the core fluid is 1:0.5 to 1:1.2.

[0017] Preferably, after the hybrid casting solution is extruded, it passes through an air gap, the length of which is strictly controlled to be 3-6 cm; at the same time, the relative humidity of this air gap area needs to be precisely controlled at 55-65%, and the temperature at 25-30℃. Subsequently, the fiber is immersed in a water-based coagulation bath to complete the phase transformation.

[0018] Preferably, as an alternative to step 3, during the continuous winding spinning process, the washed nascent fibers are first guided through one or more online cleaning tanks containing ethanol or isopropanol, with a total soaking time of 10-20 minutes. Subsequently, the fibers are guided through a secondary treatment tank containing an ionic liquid solution, with a treatment time of 5-10 minutes, and finally enter an online drying device (such as a hot air channel) for drying and winding. This approach sacrifices less than 5% of separation performance (compared to the standard IPA post-treatment method) to reduce the post-treatment time from several days to less than half an hour, achieving not only efficient continuous production but also imparting superior pressure resistance to the membrane.

[0019] Preferably, a gas separation component includes the hollow fiber gas separation membrane described above.

[0020] Preferably, a gas separation method involves passing the air to be separated through the aforementioned gas separation assembly, where nitrogen or oxygen is separated and enriched under pressure.

[0021] Compared with the prior art, the present invention has the following beneficial effects: Material Advantages: Utilizing 6FDA-TrMPD type polyimide, the large -C(CF3)2- groups and twisted non-coplanar structure in its molecular chain effectively suppress chain segment stacking and improve gas permeability. Based on this, an innovative "interlocking network" skin layer with a POSS concentration gradient distribution is constructed. POSS particles act as "reinforcing bars," forming physical cross-linking points on the skin layer surface, effectively anchoring the polyimide molecular chains. This structure more effectively utilizes the reinforcing effect of nanoparticles than uniformly mixed hybrid films, while avoiding interfacial defects under high filler content, achieving a dual improvement in selectivity and pressure resistance.

[0022] Process advantages: By strictly limiting the air gap to 3-6cm and controlling the humidity at 55-65%, within this "window", combined with the effect of non-solvent additives, the hybrid casting solution stream undergoes moderate solvent evaporation and rapid gelation before entering the coagulation bath, "freezing" the gradient distribution state of POSS, which is conducive to forming a thin and defect-free dense hybrid skin.

[0023] Industrialization Advantages: An innovative "ethanol-ionic liquid two-step online cleaning" continuous solution is proposed. This solution not only utilizes ethanol for rapid solvent replacement but also introduces a new function of "ionic liquid pore preservation / drag reduction." Due to its extremely low volatility and affinity for CO2, the ionic liquid adsorbs a large amount of CO2, forming a gas-supported framework on the separation membrane before drying to counteract surface tension, maintain the original pore morphology, prevent pore structure collapse during drying, and form a pressure-resistant and drag-reducing layer during membrane use. This approach fundamentally differs from existing technologies that simply use ethanol or isopropanol for replacement, achieving minute-level continuous processing while significantly improving the membrane's pressure resistance and long-term stability. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0025] Figure 1 This is an enlarged cross-sectional view (2μm scale) of the outer surface (far from the central hole) of the hollow fiber membrane in this invention.

[0026] Figure 2 This is an enlarged cross-sectional view (500nm scale) of the outer surface (far from the central hole) of the hollow fiber membrane in this invention.

[0027] Figure 3 This is an enlarged cross-sectional view (1μm scale) of the inner surface (near the central hole) of the hollow fiber membrane in this invention. Detailed Implementation

[0028] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of the embodiments of the invention. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0029] The embodiments of the present invention will be described in detail below.

[0030] Example 1 PI Synthesis: Under nitrogen protection, 4.35 g (0.03 mol) TrMPD was dissolved in 60 mL of anhydrous NMP and cooled in an ice-water bath. 13.32 g (0.03 mol) 6FDA was added in portions, and the reaction was stirred for 5 hours while maintaining the temperature below 10 °C to obtain a viscous PAA solution. 6.12 g acetic anhydride and 4.74 g pyridine were added, and the mixture was stirred for 14 hours after reaching 80 °C from room temperature. The solution was poured into 500 mL of ethanol to precipitate the precipitate. The precipitate was filtered, washed three times with ethanol, and dried under vacuum at 90 °C for 36 hours to obtain a pale yellow PI powder. GPC analysis showed that Mn = 125000 g / mol.

[0031] Preparation of casting solution: Take 24g of the above PI powder and dissolve it in 68g of DMAc. Separately, take 1.2g of octaaminophenyl POSS (5% of the PI mass) and ultrasonically disperse it in a mixture of 6g THF and 2g anhydrous ethanol. Slowly add the POSS dispersion to the polymer solution, then add 1g of ethylene glycol, stir at 60℃ for 12 hours, and allow to stand to remove bubbles.

[0032] Spinning: The hybrid casting solution (shell layer) and core solution (NMP:H2O=70:30 w / w, containing 1% PVP K30) are metered and delivered separately. The shell fluid flow rate is 36 mL / min, and the core solution flow rate is 30 mL / min. The air gap is set to 5 cm, and the environment within the gap is controlled at a temperature of 28℃ and a relative humidity of 60%. The coagulation bath is tap water.

[0033] Post-treatment: After washing with water for 24 hours, the membrane was immersed in isopropanol for 3 days and then vacuum dried at 80°C for 10 hours to obtain the control membrane M1.

[0034] Example 2 The experiment was basically the same as in Example 1, except that the amount of octaaminophenyl POSS added was changed to 3% of the mass of PI, i.e., 0.72g.

[0035] Example 3 The process is basically the same as in Example 1, except that the spinning process parameters are changed: the air gap is 3 cm and the relative humidity is 65%.

[0036] Example 4 The process is essentially the same as in Example 1, except that a continuous process is used in the post-processing stage: the nascent fibers after washing are first passed through two online ethanol washing tanks connected in series, with a residence time of 8 minutes in each tank, for a total processing time of 16 minutes. They are then immediately passed through an ethanol solution containing 3% [EMIM][BF4], with a residence time of 8 minutes. Finally, they are passed through an 80°C hot air drying channel and wound online to obtain the membrane.

[0037] Comparative Example 1 The PI synthesized using commercially available polyimide Matrimid® instead of 6FDA-TrMPD was not modified with POSS; otherwise, it was the same as in Example 1.

[0038] Comparative Example 2 The process was essentially the same as in Example 1, but without the addition of POSS nanoparticles, to prepare a pure PI film.

[0039] Comparative Example 3 It is basically the same as Example 1, but the humidity of the air gap during spinning is controlled at 40%.

[0040] Performance testing experiment Performance testing: Using a single fiber membrane module, the permeation flux of pure gases O2 and N2 was tested at 25℃ and a pressure difference of 0.2MPa. The O2 permeability coefficient (GPU) and the O2 / N2 separation factor (α) were calculated. After running at a high pressure of 2.0MPa for 100 hours, the performance degradation rate was calculated.

[0041] Performance test results comparison table Examples / Comparative Examples <![CDATA[O2 Permeability Coefficient GPU]]> <![CDATA[O2 / N2 separation factor]]> Flux attenuation rate after high-voltage operation Selective attenuation rate after high voltage operation Example 1 38.5 4.92 4.50% 2.10% Example 2 42.1 4.65 6.20% 3.50% Example 3 40.3 4.71 5.10% 2.80% Example 4 37.2 4.83 3.80% 1.90% Comparative Example 1 12.5 4.46 15.30% 8.70% Comparative Example 2 45.2 4.97 12.10% 9.40% Comparative Example 3 58.6 3.42 - - *Note: 1 GPU = 10 -6 cm 3 (STP) / (cm 2 ·s·cmHg)* As can be seen from Examples 1-4, within the process parameters defined by the present invention, the hybrid films all exhibit excellent initial performance (α>4.6, O2 flux>37 GPU).

[0042] As can be seen from Comparative Examples 1 and 2, compared with commercially available Matrimid®, the PI separation membrane synthesized by the present invention using 6FDA-TrMPD improves selectivity and increases initial flux, solving the "trade-off" effect problem that exists in most commercial polyimides in O2 / N2 separation.

[0043] Compared with Comparative Example 2 (pure PI film without POSS), Examples 1-4 using the present invention, after adding POSS, show the following: Figure 1 , 2As shown, an organic-inorganic hybrid gradient interlocking network skin with a thickness of 80-150 nm is formed in situ on the fiber surface away from the central pore. The POSS concentration decreases gradually from the surface to the interior along the membrane thickness direction, providing support for the separation membrane. Figure 3 As shown, a thinner and slightly less dense skin layer is formed in situ on the fiber surface near the central hole, allowing for smoother gas flow. The separation membrane prepared using the method of this invention, although showing a slight decrease in initial flux, exhibits significantly improved stability after high-pressure operation. The flux attenuation rate decreases from 12.1% to below 6.2%, and the selective attenuation rate also decreases from 9.4% to below 3.5%, demonstrating the anti-compaction effect of the gradient interlocking network structure of the fiber membrane surface skin layer.

[0044] Comparative Example 3 shows that uncontrolled (too low) air gap humidity will lead to rapid water loss in the cortex, resulting in defects, severely deteriorating selectivity, and reducing the separation factor to 3.42, demonstrating the necessity of the critical control point of 55-65% humidity.

[0045] A comparison of Example 4 and Example 1 shows that the continuous online cleaning scheme using "ethanol-ionic liquid" achieves an initial separation performance retention rate of over 95%. More importantly, the membrane treated with the ionic liquid exhibits slightly better high-pressure operating stability than membranes soaked in traditional ethanol or isopropanol for extended periods, demonstrating the multiple positive effects of ionic liquid treatment in "pore preservation / drag reduction / pressure resistance." Simultaneously, the post-treatment time is reduced from 72 hours to 24 minutes, resulting in an efficiency improvement of over 180 times.

[0046] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. It should be noted that any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a polyimide hollow fiber gas separation membrane for nitrogen and oxygen separation, characterized in that: Specifically, the steps include the following: Step 1, Preparation of hybrid casting solution: Dissolve polyimide powder in N-methylpyrrolidone or N,N-dimethylacetamide to form a solution with a solid content of 20-30 wt%; ultrasonically disperse POSS nanoparticles in tetrahydrofuran, a volatile solvent accounting for 5-15% of the total solvent mass; then mix the dispersion with the polymer solution and add ethylene glycol, a non-solvent additive accounting for 1-5% of the total solvent mass. After stirring and degassing, a stable hybrid casting solution is obtained. Step 2, dry-wet spinning: using the hybrid casting solution obtained in Step 1 as the shell fluid and a solution containing 1% PVP K30 and NMP:H2O=70:30w / w as the core fluid for spinning, the fibers enter the coagulation bath after passing through the air gap. Step 3, post-processing: The nascent fiber filaments are sequentially washed with water and subjected to solvent replacement treatment, and finally dried and wound online; the solvent replacement treatment method is a continuous online treatment: the washed fiber filaments are first immersed in a primary cleaning tank containing ethanol or isopropanol, and the total cleaning and soaking time is 10-20 minutes. The fibers are then immersed in a secondary treatment tank containing an ionic liquid solution for 5-10 minutes. In step 2, the air gap is 3-6 cm and the humidity is 55-65%. An organic-inorganic hybrid gradient interlocking network skin with a thickness of 80-150 nm is formed in situ on the surface of the hollow fiber membrane, and the concentration of POSS nanoparticles in the skin decreases in a gradient from the membrane surface to the inside.

2. The method for preparing the separation membrane according to claim 1, characterized in that: In step 1, the polyimide is prepared through the following steps: Step 1.1: Under an inert atmosphere and at a temperature below 10°C, TrMPD is dissolved in a polar aprotic solvent, and 6FDA is slowly added in an equimolar ratio and reacted for 4-6 hours to obtain a polyamic acid solution; the polar aprotic solvent is N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, or dimethyl sulfoxide. Step 1.2: Add acetic anhydride and pyridine to the solution obtained in Step 1.

1. The amount of acetic anhydride is 2 to 3 times that of the polyamic acid structural unit, and the amount of pyridine is in an equimolar ratio with that of acetic anhydride. The reaction is carried out at room temperature to 80°C for 12 to 14 hours to complete the chemical imidization. Step 1.3: After the reaction is complete, pour the reaction solution into methanol or ethanol as a precipitant, filter, wash and dry to obtain polyimide powder.

3. The method for preparing the separation membrane according to claim 1, characterized in that: In step 1, POSS is octaaminophenyl POSS, and its mass fraction in the casting solution is 3-8% of the mass of polyimide.

4. The method for preparing the separation membrane according to claim 1, characterized in that: The ionic liquid solution in step 3 is an ethanol solution containing 3% [EMIM][BF4].

5. A polyimide hollow fiber gas separation membrane for nitrogen and oxygen separation prepared by the preparation method of any one of claims 1-4, characterized in that: The separation membrane comprises a polyimide matrix polymerized from 6FDA and TrMPD, and POSS nanoparticles; the polyimide has a number-average molecular weight Mn of 80,000-150,000 g / mol.

6. A gas separation assembly, characterized in that, It includes the hollow fiber gas separation membrane as described in claim 5.

7. A gas separation method, characterized in that, The air to be separated is passed through the gas separation assembly as described in claim 6, and nitrogen or oxygen is separated and enriched under pressure.

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