Polyimide hollow fiber gas separation membrane and preparation method thereof

Abstract

The invention discloses a polyimide hollow fiber gas separation membrane and a preparation method thereof. The polyimide hollow fiber gas separation membrane is prepared from a mixture containing polyimide and an additive through a phase inversion method, the polyimide is prepared by using 2, 2 '-bis (3, 4-dicarboxyphenyl) hexafluoropropane dianhydride as a dianhydride monomer and 1, 3-phenylenediamine and 4, 4'-diaminodiphenyl ether as diamine monomers through solution polycondensation. The weight-average molecular weight of the polyimide is greater than or equal to 60000. The prepared polyimide hollow fiber gas separation membrane is suitable for being applied to multiple fields such as air separation, hydrogen separation, decarburization and rare gas concentration, and the application prospect is wide.

Description

Technical Field

[0001] The present application relates to a polyimide hollow fiber gas separation membrane and a preparation method thereof, belonging to the technical field of chemical separation. Background Art

[0002] As a polymer material, polyimide has excellent comprehensive properties, such as high glass transition temperature, superior mechanical properties, thermal stability and chemical resistance, and can be processed into various shapes by multiple methods. Therefore, it has a wide range of applications in aviation, aerospace, electrical, mechanical, chemical, microelectronics and other fields. Polyimide is also a good membrane-making material, and a gas separation membrane with a dense functional layer can be prepared in one step by the phase inversion method. Since the mid-1980s, polyimide has achieved good results in some separation systems with strong industrial backgrounds, such as H2 / N2, O2 / N2, He / CH4, CO2 / N2, CO2 / CH4, etc.

[0003] There are various types and forms of polyimides, and there are multiple synthetic routes, so they can be selected according to various application purposes. This synthetic flexibility is difficult to possess by other polymers. Polyimide is mainly an aromatic heterocyclic high polymer obtained by polycondensation of dianhydride and diamine. Compared with the monomers of many other heterocyclic polymers such as polybenzimidazole, polybenzoxazole, polybenzothiazole, polyquinoline, etc., the raw materials of these two monomers are widely available and the synthesis is relatively easy. There are many types of dianhydrides and diamines, and different combinations can obtain polyimides with different properties. 6FDA [2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride] type polyimide has both a high gas permeability coefficient and a separation coefficient because the hexafluoride monomer can significantly enhance the gas permeability of the polyimide membrane. In order to improve the gas permeability of polyimide, many scholars have studied the influence of substituents on the gas permeability of polyimide and found that 6FDA type polyimide has both high gas permeability and high gas permeation selectivity. After introducing fluorine atoms into the polyimide main chain, due to the strong mutual attraction between polar fluorine atoms in different chain segments, the polyimide polymer chains curl and entangle with each other, reducing the packing density of the chain segments and increasing the free volume; at the same time, due to the large attraction between fluorine atoms, the formed polyimide molecular chains are firm and rigid, so while improving the gas permeability, good permeation selectivity can be maintained. 6FDA-NDA (1,5-naphthalenediamine) is an example with the above structural characteristics. The CO2 permeability coefficient of 6FDA-NDA is 50.5 Barrer (1 Barrer = 1×10 -10 cm 3 (STP)·cm·cm -2 ·s -1 ·cmHg -1) The ideal separation coefficient of CO2 / CH4 is 144. Copolymerization is one of the most effective methods to regulate and improve the gas separation performance of polyimides. Introducing a third or fourth monomer into the main chain of polyimide molecules with a smaller free volume, poor gas permeability but high gas permeation selectivity can increase the free volume and improve their gas separation performance. From an economic perspective, copolymerizing a monomer with a lower price with an expensive monomer can significantly reduce costs and be more suitable for industrial applications if a membrane material with good separation performance can be obtained. Copolymerizing the monomers of polyimide with good permeation performance and the monomers of polyimide with good permeation selectivity can change the structure of the polymer molecular chain by changing the copolymer composition.

[0004] The separation efficiency of asymmetric hollow fiber membranes is higher than that of asymmetric flat membranes. To obtain a defect-free asymmetric polyimide membrane, a polyimide that can be dissolved in an organic solvent should be selected as the membrane material. A casting solution is prepared with a good solvent / additive mixture, and a defect-free asymmetric polyimide membrane can be obtained by the dry-wet phase inversion method. However, the surface skin layer of this membrane is often too thick, resulting in a low gas permeation rate. In the 1980s, Peinenann et al. prepared defect-free asymmetric polyetherimide (Ultem) membranes by the dry-wet phase inversion method using chlorinated hydrocarbons as solvents and acetone or toluene as coagulation baths, and their He / CH4 separation coefficient was higher than that of the homogeneous membrane. Chlorinated hydrocarbons are toxic and the viscosity of Ultem in chlorinated hydrocarbons is too low, which is not conducive to actual operation. Summary of the Invention

[0005] The purpose of this application is to provide a polyimide hollow fiber gas separation membrane, which is prepared by a phase inversion method from a mixture containing polyimide and an additive; the polyimide is prepared by solution polycondensation using 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride as the dianhydride monomer, 1,3-phenylenediamine and 4,4'-diaminodiphenyl ether as the diamine monomers; the weight average molecular weight of the polyimide ≥ 60000. The hollow fiber membrane prepared in this application is suitable for applications in multiple fields such as air separation, hydrogen separation, decarbonization, and noble gas concentration, and has broad application prospects.

[0006] The permeability and selectivity of a membrane are often a pair of contradictions. Generally, if the permeability of a membrane material is good, its selectivity is poor. This is because when the segmental motion in the polymer accelerates, not only will its diffusion rate increase exponentially, thereby improving the membrane's permeability, but also the segmental gaps in the polymer will increase and the void differences will become smaller, thus reducing its selectivity. Therefore, polymer materials must take both into account and have a relatively balanced permeability and selectivity for the components to be separated. From the perspective of membrane preparation, improving the permeation flux of the membrane has always been the goal pursued by researchers. This application uses 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) as the dianhydride monomer, and 1,3-phenylenediamine (mPDA) and 4,4'-diaminodiphenyl ether (ODA) as the diamine monomers to prepare a polyimide soluble in common membrane-making solvents through solution polycondensation. Furthermore, 6FDA-mPDA / ODA polyimide is used as the raw material, and salicylic acid is added as an additive and solvent, and a polyimide hollow fiber membrane with an ultrathin dense functional layer is prepared by the phase inversion method.

[0007] This application improves the permeation flux of the membrane from two aspects: membrane material design and hollow fiber membrane preparation. On the one hand, 6FDA is selected as the dianhydride monomer, and introducing -CF3- into the polyimide molecular chain can increase the permeability coefficient. The -C(CF3)2- group in the dianhydride residue has three functions: firstly, it reduces the mobility of the molecular segments and enhances the rigidity of the molecular main chain; secondly, it makes there be more "cavities" in the molecular segments, increasing the free volume; at the same time, -C(CF3)2- can improve the solubility of gases in the membrane. However, the homopolymer polyimides 6FDA-mPDA and 6FDA-ODA are difficult to obtain hollow fiber membranes by the phase inversion method due to their strong rigidity. By copolymerization means, 6FDA-mPDA / ODA type polyimide is prepared, which has good permeation separation performance, mechanical properties and solubility, and can be successfully made into hollow fiber membranes.

[0008] In the composition of the casting solution of the hollow fiber membrane of this application, salicylic acid is selected as the additive. It has a large molecule and a high boiling point, and it is easier to obtain high-flux defect-free hollow fiber membranes than other low-boiling point additives. The support layer of the hollow fiber membrane is a sponge-like structure, and the outer surface is the functional layer, and the thickness of the dense layer is very small; salicylic acid is safe, non-toxic, green and environmentally friendly, and is easily soluble in hot water, and can be removed without high-temperature post-treatment for the prepared hollow fiber.

[0009] According to the first aspect of this application, a polyimide hollow fiber gas separation membrane is provided, and the polyimide hollow fiber gas separation membrane is prepared by the phase inversion method from a mixture containing polyimide and an additive;

[0010] The polyimide is prepared by solution polycondensation using 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride as the dianhydride monomer, 1,3-phenylenediamine and 4,4'-diaminodiphenyl ether as the diamine monomers;

[0011] The weight-average molecular weight of the polyimide is ≥60,000.

[0012] Optionally, the polyimide is soluble in common aprotic organic solvents and is prepared by solution polycondensation using 6FDA as the dianhydride monomer, 1,3-mPDA and 4,4'-ODA as the diamine monomers. It has the following structural formula:

[0013]

[0014] The weight-average molecular weight of the polyimide is ≥60,000, where m and n are 50 - 200 and 200 - 50 respectively.

[0015] Optionally, the polyimide hollow fiber gas separation membrane in this application is prepared by the solution spinning method.

[0016] Optionally, the polyimide hollow fiber gas separation membrane has an asymmetric structure, with an ultrathin dense layer on the skin and a sponge-like pore structure in the support layer.

[0017] Optionally, the additive is salicylic acid.

[0018] According to the second aspect of this application, a preparation method of the above polyimide hollow fiber gas separation membrane is provided. The preparation method includes:

[0019] Step S1: Mix the polyimide, additive and solvent, and degas to obtain a casting solution;

[0020] Step S2: Feed the casting solution into a spinneret through a metering pump, and the core liquid is pumped into the spinneret by a liquid-phase pump. The casting solution is extruded in the spinneret and enters a gel bath. The outer part of the membrane filament is formed by the gel bath, and the inner part is formed by the core liquid gel to form a hollow fiber internal cavity;

[0021] Step S3: Pass the hollow fiber obtained in Step S2 through a washing tank, and the hollow fiber membrane filaments are collected on a reel and washed;

[0022] Step S4: Perform solvent exchange on the washed hollow fiber membrane, and after drying, coat it with silicone rubber to obtain the polyimide hollow fiber gas separation membrane.

[0023] As a specific embodiment, the preparation method includes:

[0024] a) Prepare a casting solution using 6FDA-mPDA / ODA copolymer polyimide as the raw material, add salicylic acid as the additive and a solvent, stir and dissolve in a stirring kettle, and degas;

[0025] b) Hollow fiber membrane forming The casting solution obtained in step a) enters the spinneret through a metering pump, and the core liquid is pumped into the spinneret by a liquid-phase pump. The casting solution is extruded in the spinneret and enters the gel bath through an air gap. The outer part of the membrane fiber is formed by the gel bath, and the inner part is formed by the core liquid gel to form a hollow cavity inside the hollow fiber;

[0026] c) Collecting the hollow fiber membrane The nascent hollow fiber passes through a washing tank, and then the hollow fiber membrane filaments are collected on a reel and further washed before post-treatment;

[0027] d) Post-treatment of the hollow fiber membrane After the hollow fiber membrane is washed in water for a period of time, solvent exchange is carried out to remove residual solvents and water;

[0028] e) Coating of the hollow fiber membrane The hollow fiber membrane obtained in step d) is dried and then coated with silicone rubber to obtain a high-flux polyimide hollow fiber gas separation membrane.

[0029] According to the above method, the preparation temperature of the casting solution is from room temperature to 100 °C; the solvent used for the spinning core liquid is N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, methanol, ethanol or an aqueous solution of several mixtures thereof, with a concentration of 5-100%; the external gel bath is water with a temperature of 0-50 °C; the extrusion temperature of the casting solution at the spinneret is from room temperature to 100 °C; dry-wet spinning is adopted, and the air gap distance is 0 to 20 cm; the washing time of the hollow fiber membrane in water is 3 to 7 days, and solvent exchange is carried out using methanol / ethanol, n-pentane / n-hexane for solvent exchange, 15 to 60 min for each solvent, for a total of three times.

[0030] In order to fill the micro-defects on the surface of the hollow fiber membrane and improve its gas separation effect, in this application, the hollow fiber membrane is vacuum-coated with a certain concentration of polydimethylsiloxane (PDMS) / crosslinking agent for 5 to 30 min. The ratio of PDMS to the crosslinking agent is 10:1, the concentration of the coating liquid is 0.5% to 5% (volume percentage), and the solvent of the coating liquid is one of n-hexane, n-pentane, isopentane, n-octane, isooctane, petroleum ether.

[0031] Optionally, in the casting solution, the mass content of each component is:

[0032] Polyimide 20 - 40%

[0033] Additive 5 - 35%

[0034] Solvent 40 - 65%.

[0035] Optionally, in step S1, the solvent is selected from one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, N-methylpiperidine.

[0036] Optionally, the temperature for preparing the casting solution is from room temperature to 100 °C.

[0037] Optionally, the core liquid is selected from aqueous solutions of Compound A; Compound A is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, methanol, and ethanol;

[0038] Preferably, in the core liquid, the mass concentration of Compound A is 5-100%;

[0039] Preferably, the gel bath is a water bath with a temperature of 0-50 °C.

[0040] Optionally, the extrusion temperature of the casting solution at the spinneret is from room temperature to 100 °C.

[0041] Optionally, in Step S2, the casting solution is extruded from the spinneret, passes through an air gap and enters the gel bath; the distance of the air gap is 0-20 cm.

[0042] The beneficial effects of the present invention include:

[0043] (1) High flux, no defects in the functional layer, and the separation coefficient is comparable to the intrinsic value of the material;

[0044] (2) Using salicylic acid as an additive, it is easier to prepare a high-flux defect-free hollow fiber membrane due to its large molecule and high boiling point compared to other low-boiling point additives;

[0045] (3) Salicylic acid is safe, non-toxic, environmentally friendly, and easily soluble in hot water, and can be removed without high-temperature post-treatment for the prepared hollow fibers;

[0046] (4) The 6FDA-mPDA / ODA polyimide hollow fiber membrane has better mechanical properties than homopolymers and is convenient for making membrane modules. The present invention has the advantages of simple process, convenient operation, and easy industrial production. The prepared hollow fiber membrane is suitable for applications in multiple fields such as air separation, hydrogen separation, decarbonization, and rare gas concentration, and has broad application prospects. Description of the Drawings

[0047] Figure 1 It is the infrared spectrum of 6FDA-mPDA / ODA polyimide;

[0048] Figure 2 It is the result of gel permeation chromatography analysis of 6FDA-mPDA / ODA polyimide;

[0049] Figure 3 It is the scanning electron microscope images of the 6FDA-mPDA / ODA polyimide hollow fiber membrane (prepared in Example 2) at different magnifications, (a) magnification is 200, (b) magnification is 2000. Detailed implementation manners

[0050] The present application will be described in detail below in conjunction with embodiments, but the present application is not limited to these embodiments.

[0051] Unless otherwise specified, the raw materials in the embodiments of the present application are all purchased through commercial channels.

[0052] Embodiment 1

[0053] Place a 1000 ml dry four-necked flask equipped with mechanical stirring, a thermometer, a nitrogen inlet, and a water separator in a 20°C constant temperature water bath. Under nitrogen protection, add 500 ml of N-methylpyrrolidone (NMP), and add 0.1 mol of 4,4'-ODA under mechanical stirring. After stirring until homogeneous, add 0.1 mol of mPDA. After complete dissolution, add 0.2 mol of 6FDA in three portions. React at 20°C for 4 - 24 h to obtain a polyamic acid solution. Add 1 mol of acetic anhydride and 1 mol of triethylamine for chemical imidization. After reacting at room temperature for 24 h, end the reaction. Let the polymerization product precipitate in anhydrous methanol, then wash it three times with anhydrous methanol, air-dry it at room temperature for 1 h, and further dry it in a vacuum oven at 250°C for 24 h to remove residual moisture and solvents. Wait for the oven to cool naturally to room temperature and then take it out. Test the gas permeability of the polyimide by the method specified in GB / T 40260-2021. Gas permeability at 25°C and 0.2 MPa: PH2 = 16.705 Barrer, PO2 = 1.330 Barrer, PHe = 21.877 Barrer, PCO2 = 6.843 Barrer; Ideal separation factor αH2 / N2 = 82.24, αO2 / N2 = 6.55, αHe / CH4 = 158.59,

[0054] αCO2 / CH4 = 49.59.

[0055] Figure 1 is the infrared spectrum of 6FDA-mPDA / ODA polyimide; Figure 2 is the gel permeation chromatography analysis result of 6FDA-mPDA / ODA polyimide, from Figure 1 It can be seen that the formation of imide can be confirmed by 1787 cm-1 (asymmetric stretching peak of C=O), 1728 cm-1 (symmetric stretching peak of C=O), 1358 cm-1 (C-N stretching peak), and 1064 cm-1 (bending peak of imide ring). At the same time, since no obvious absorption peak appears near 3300 cm-1 (stretching peaks of N-H and -OH), it can be confirmed that the imidization is relatively complete.

[0056] From Figure 2It can be seen that the molecular weight distribution of 6FDA-mPDA / ODA polyimide is 2.05, which is a better raw material for preparing hollow fiber membranes.

[0057] Example 2

[0058] Add 240 g of 6FDA-mPDA / ODA polyimide, 60 g of salicylic acid and 500 g of solvent NMP into a stirring kettle, and continuously stir at 100 °C for 24 h. After keeping the casting solution at a constant temperature and standing for defoaming for 3 days, use dry nitrogen as the spinning driving force. The core liquid (the core liquid is a mixed solution of NMP and deionized water with a mass ratio of 60:40) is supplied by a liquid-phase pump. The membrane solution forms a nascent membrane (air gap 10 cm, extrusion temperature 70 °C) after passing through the spinning nozzle by the dry-wet method, and enters the coagulation bath (water, temperature 25 °C), and then gels and cures into a membrane; the hollow membrane is washed in flowing water for 24 h and then post-treated by the solvent replacement method. Transfer the hollow fiber into an ethanol tank and circulate for 45 min, then replace the ethanol, for a total of three times; then replace it with n-hexane three times and dry at room temperature for 12 h. In order to fill the tiny defects on the surface of the hollow fiber membrane and improve its gas separation effect, immerse the hollow fiber membrane module in a 2 wt% polydimethylsiloxane (PDMS) / petroleum ether solution for coating operation for 30 min, and then put the module into a forced-air oven at 70 °C for crosslinking for 3 h, and then test its gas permeability at 25 °C and 0.5 MPa. The results are shown in Table 1. By comparing with the results of Example 1, the functional layer thickness of the hollow fiber membrane can be calculated to be about 0.094 μm. The tensile strength and elongation at break of the hollow fiber membrane were tested: 7.45 MPa, 37.23%.

[0059] Figure 3 It is the electron microscope photograph of the prepared membrane. The cross-sectional structure of the hollow fiber membrane is a sponge-like asymmetric structure, without finger holes and large cavities. Gas separation membranes are usually applied at higher pressures. Finger holes and large cavity structures are prone to collapse during high-pressure or long-term operation, while the sponge-like structure has better resistance to compaction.

[0060] Table 1

[0061]

[0062] Note: 1 GPU = 1×10 -6 cm 3 (STP)·cm -2 ·s -1 cmHg -1

[0063] Example 3

[0064] Add 240 g of 6FDA-mPDA / ODA polyimide, 120 g of salicylic acid and 440 g of solvent NMP into a stirring kettle, and continuously stir at 100 °C for 24 h. After keeping the casting solution at a constant temperature and standing for defoaming for 3 days, use dry nitrogen as the spinning driving force. The core liquid (the core liquid is a mixed solution of NMP and deionized water with a mass ratio of 60:40) is supplied by a liquid-phase pump. The dry-wet method is used to form a nascent membrane after the membrane solution passes through the spinning nozzle (air gap 10 cm, extrusion temperature 70 °C). After entering the coagulation bath (water, temperature 25 °C), it gels and solidifies into a membrane; the hollow membrane is washed in flowing water for 24 h and then post-treated by the solvent replacement method. Transfer the hollow fiber into an ethanol tank and circulate for 45 min, then replace the ethanol, for a total of three times; then replace it with n-hexane three times and dry at room temperature for 12 h. In order to fill the tiny defects on the surface of the hollow fiber membrane and improve its gas separation effect, immerse the hollow fiber membrane module in a 2 wt% polydimethylsiloxane (PDMS) / petroleum ether solution for coating operation for 30 min, and then put the module into a forced-air oven at 70 °C for cross-linking for 3 h, and then test its gas permeability at 25 °C and 0.5 MPa. The results are shown in Table 2. By comparing with the results of Example 1, the functional layer thickness of the hollow fiber membrane can be calculated to be about 0.081 μm. The tensile strength and elongation at break of the hollow fiber membrane were tested: 6.46 MPa, 28.44%.

[0065] Table 2

[0066]

[0067] Note: 1 GPU = 1×10 -6 cm 3 (STP)·cm -2 ·s -1 cmHg -1

[0068] The above are only several embodiments of the present application, and do not impose any form of limitation on the present application. Although the present application is disclosed above with preferred embodiments, it is not intended to limit the present application. Any person skilled in the art, without departing from the scope of the technical solution of the present application, makes some changes or modifications using the technical content disclosed above, which are equivalent to equivalent implementation cases and all belong to the scope of the technical solution.

Claims

1. A polyimide hollow fiber gas separation membrane, characterized in that, The polyimide hollow fiber gas separation membrane is prepared by a phase inversion method from a mixture containing polyimide and an additive; The polyimide is prepared by solution polycondensation using 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride as the dianhydride monomer, 1,3-phenylenediamine and 4,4'-diaminodiphenyl ether as the diamine monomers; The polyimide has a weight average molecular weight of ≥60,000.

2. The polyimide hollow fiber gas separation membrane according to claim 1, characterized in that, The polyimide hollow fiber gas separation membrane has an asymmetric structure, with an ultra-thin dense layer on the skin and a sponge-like pore structure in the support layer.

3. The polyimide hollow fiber gas separation membrane according to claim 1, characterized in that, The additive is salicylic acid.

4. A method for preparing the polyimide hollow fiber gas separation membrane according to any one of claims 1 to 3, characterized in that, The preparation method includes: Step S1, mixing polyimide, additive and solvent, and degassing to obtain a casting solution; Step S2, feeding the casting solution into a spinneret through a metering pump, and feeding the core liquid into the spinneret through a liquid phase pump. The casting solution is extruded in the spinneret and enters a gel bath. The outer part of the membrane filament is formed by the gel bath, and the inner part is formed by the gelation of the core liquid to form a hollow cavity inside the hollow fiber; Step S3, passing the hollow fiber obtained in Step S2 through a washing tank, and then collecting the hollow fiber membrane filaments on a reel and washing them; Step S4, performing solvent exchange on the washed hollow fiber membrane, drying it, and then coating it with silicone rubber to obtain the polyimide hollow fiber gas separation membrane.

5. The preparation method according to claim 4, characterized in that, In the casting solution, the mass content of each component is: Polyimide 20 - 40% Additive 5 - 35% Solvent 40 - 65%.

6. The preparation method according to claim 4, characterized in that, In Step S1, the solvent is selected from one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and N-methylpiperidine.

7. The preparation method according to claim 4, characterized in that, The preparation temperature of the casting solution is from room temperature to 100°C.

8. The preparation method according to claim 4, characterized in that, The core liquid is selected from an aqueous solution of Compound A; Compound A is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, methanol, and ethanol; Preferably, in the core liquid, the mass concentration of Compound A is 5 - 100%; Preferably, the gel bath is a water bath with a temperature of 0 - 50°C.

9. The preparation method according to claim 4, characterized in that, The extrusion temperature of the casting solution in the spinneret is from room temperature to 100°C.

10. The preparation method according to claim 4, characterized in that, In Step S2, the casting solution is extruded in the spinneret and enters the gel bath through an air gap; the distance of the air gap is 0 - 20 cm.

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