Preparation method and application of fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane

Abstract

The application discloses a preparation method and application of fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane and belongs to the technical field of gas separation membranes. 4,4'-(hexafluoroisopropyl) diphthalic anhydride is used as a dianhydride monomer, 3,5'-diaminobenzoic acid and 4,4'-diamino diphenyl ether are used as common diamine monomers to copolymerize a polyamide acid solution; the polyamide acid solution is prepared into a casting solution, and after standing and defoaming, a polyamide acid membrane is prepared by using a coating method, and the polyamide acid membrane is imidized to prepare a polyimide membrane; the polyimide membrane is used as a polymer precursor membrane of the carbon molecular sieve membrane, is placed in a programmed temperature carbonization furnace with a protective atmosphere and is high-temperature baked to obtain the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane. The carbon molecular sieve membrane is not only easy to prepare but also excellent in anti-aging capacity; the carbon molecular sieve membrane is used for CO2 / N2 and CO2 / CH4 separation, and has a high CO2 permeation coefficient and CO2 / N2 and CO2 / CH4 separation factors.

Description

Technical Field

[0001] The invention relates to a preparation method and application of a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane, belonging to the technical field of gas separation membranes. Background Art

[0002] Natural gas contains significant amounts of harmful acidic gases, such as CO2, which can cause corrosion in process equipment and transportation pipelines. Therefore, removing CO2 from natural gas is essential to ensure that the natural gas product meets pipeline specifications. Currently, the main methods for CO2 capture include chemical adsorption, physical adsorption, cryogenic separation, and membrane separation. In recent years, membrane-based gas separation processes have emerged as a reliable alternative, offering significant advantages for separating sour natural gas, including a small footprint, simple operation, and low investment.

[0003] In addition to the increasingly popular commercial polymer membranes, carbon molecular sieve membranes are novel and attractive membranes with unique properties and excellent gas separation performance. The outstanding feature of carbon molecular sieve membranes is their good chemical and thermal stability. They can break the trade-off effect of traditional polymer membranes, where the improvement of gas permeability is at the expense of separation selectivity, and have achieved simultaneous improvement in permeability and selectivity. From a technical point of view, the porous structure of carbon molecular sieve membranes allows high permeability (high productivity), while the molecular sieving network provides effective differentiation of molecular size and shape (high selectivity). Carbon molecular sieve membranes are basically formed by pyrolysis of polymer precursors. A large number of studies have shown that the chemical structure of the polymer, the microstructure of the precursor and the pyrolysis process parameters are key factors in determining the transport characteristics of carbon molecular sieve membranes. In other words, these parameters play an important role in the sieving size and unique pore distribution and pore structure of carbon molecular sieve membranes. However, carbon molecular sieve membranes are brittle and have poor mechanical properties, and most basic research has only been conducted on flat membranes. There are few research results on large-scale carbon molecular sieve preparation technology required for long-term, continuous practical gas separation applications. How to overcome the brittleness of carbon molecular sieve membranes, prepare carbon molecular sieve membranes on a large scale, and effectively improve the long-term stability of carbon molecular sieve membranes in actual operating environments are huge challenges.

[0004] The fluorine-containing cross-linkable polyimide provided by the present invention has excellent gas separation performance and anti-aging ability after high-temperature calcination, and is expected to improve the service life and efficient gas separation performance of carbon molecular sieve membranes in industrial operating environments. Summary of the Invention

[0005] This invention provides a method for preparing and applying a fluorinated, cross-linked polyimide-based carbon molecular sieve membrane. Polyamic acid is first dehydrated and cyclized, followed by high-temperature calcination and carbonization to produce a carbon molecular sieve membrane. The membrane is then applied to CO₂ / N₂ and CO₂ / CH₄ separations. Results demonstrate that the membrane exhibits high CO₂ permeability and CO₂ / N₂ and CO₂ / CH₄ separation factors.

[0006] The present invention uses 4,4'-(hexafluoroisopropylene) diphthalic anhydride as the dianhydride monomer to prepare a fluorine-containing cross-linkable polyamic acid membrane. This membrane is then subjected to imidization and temperature-programmed carbonization to produce a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane. The mutual attraction of fluorine atoms between different segments causes the polymer chains to curl and entangle, limiting the dense packing of molecules. Furthermore, due to the strong attraction between fluorine atoms, the resulting molecular chain structure is strong and rigid. This helps increase the free volume of the carbon molecular sieve membrane, thereby improving permeability while maintaining good selectivity. Furthermore, 3,5-diaminobenzoic acid and 4,4'-diaminodiphenyl ether are used as common diamine monomers. The carboxyl group in the 3,5-diaminobenzoic acid monomer is an active substituent. When the carboxyl group is introduced into the polyimide molecular chain, intermolecular cross-linking occurs at high temperatures, reducing the local mobility of the polyimide segments and side groups, which helps improve gas selectivity.

[0007] The present invention provides a method for preparing a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane, comprising the steps of selecting 4,4'-(hexafluoroisopropylene) diphthalic anhydride as a dianhydride monomer, and copolymerizing 3,5'-diaminobenzoic acid and 4,4'-diaminodiphenyl ether as common diamine monomers to form a polyamic acid solution; preparing the polyamic acid solution into a casting solution, allowing it to stand for degassing, and then forming a polyamic acid membrane by a coating method; and imidizing the polyamic acid membrane to prepare a polyimide membrane; using the polyimide membrane as a polymer precursor membrane of the carbon molecular sieve membrane, and placing it in a programmed temperature carbonization furnace with a protective atmosphere for high-temperature calcination to obtain a polyimide-based carbon molecular sieve membrane containing a rigid paraphenylene structure. The structure of the carbon molecular sieve membrane is regulated by adjusting the ratio of 3,5'-diaminobenzoic acid and 4,4'-diaminodiphenyl ether, the carbonization temperature, the constant temperature time, and the cross-linking time to obtain a carbon molecular sieve membrane with better separation performance. Compared with other carbon molecular sieve membranes, fluorine-containing cross-linkable polyimide-based carbon molecular sieve membranes have better CO2 separation performance.

[0008] The present invention provides a method for preparing the above-mentioned fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane, comprising the following steps:

[0009] (1) Synthesis of fluorinated cross-linkable polyamic acid solution

[0010] The 3,5-diaminobenzoic acid and the 4,4'-diaminodiphenyl ether are dissolved in solvent A in a molar ratio of (1-6):(1.5-9), wherein the mass of the solvent A is 15-45 times the total mass of the 3,5-diaminobenzoic acid and the 4,4'-diaminodiphenyl ether. After the mixed solution is stirred on a mechanical stirrer at a temperature range of -5-5 °C and a rotation speed range of 40-1000 rpm for 0.5-12 h until complete dissolution, 4, 4'-(hexafluoroisopropylidene) diphthalic anhydride is added to the mixed solution, wherein the molar ratio of the 4, 4'-(hexafluoroisopropylidene) diphthalic anhydride to the total moles of the 3,5-diaminobenzoic acid and the 4,4'-diaminodiphenyl ether is 1:(1-5); and the stirring is continued on the mechanical stirrer at a temperature range of -5-5 °C and a rotation speed range of 40-1000 rpm for 8-48 h to obtain a polyamic acid solution.

[0011] wherein the type of the solvent A is any one of N,N-dimethylformamide, N,N-dimethylacetamide, tetrahydrofuran, chloroform, dimethyl sulfoxide and N-methyl pyrrolidone.

[0012] (2) Preparation of fluorine-containing cross-linkable polyimide film

[0013] The polyamic acid solution prepared in step one is weighed and added to solvent A to prepare solution B, ensuring that the polyamic acid accounts for 10-20% of the total mass of solution B. The solution is stirred on a magnetic stirrer at a temperature of 25-80 °C and a rotation speed of 100-800 rpm for 2-24 h to disperse uniformly, and is placed at -5-5 °C for 12-24 h to obtain a casting solution. The casting solution is uniformly coated onto a clean glass plate, a wet film thickness of 100-300 µm is controlled using a film doctor, a polyamic acid film is prepared, and the imidization is completed in a vacuum drying oven with programmed temperature increase. The imidized film is peeled off in a constant temperature water bath at 30-40 °C and is dried in a vacuum drying oven at 80-150 °C for 6-12 h to obtain a fluorine-containing cross-linkable polyimide film.

[0014] wherein the temperature increase program for imidization is: constant temperature at 40-80 °C for 1-2 h; constant temperature at 80-150 °C for 1-2 h; constant temperature at 150-200 °C for 1-2 h; and finally constant temperature at 200-250 °C for 1-2 h.

[0015] (3) Preparation of fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane

[0016] The fluorine-containing cross-linkable polyimide film prepared in step two is placed in a programmed temperature increase carbonization furnace with a protective atmosphere, and is carbonized by high temperature calcination. After cooling to room temperature in a protective atmosphere, the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane is taken out.

[0017] Wherein, the solvent A used in step 1 and step 2 is the same;

[0018] Among them, the protective atmosphere is one of nitrogen, helium or argon;

[0019] The high-temperature calcination carbonization process is as follows: the carbonization heating rate is increased from 0-20°C to 200-250°C at a heating rate of 5-10°C / min; the heating rate is increased from 200-250°C to 350-500°C at a heating rate of 0.01-5°C / min, and then the temperature is kept constant at this temperature for 0-120 min; the heating rate is increased from 350-500°C to 550-900°C at a heating rate of 0.01-5°C / min, and then the temperature is kept constant at this temperature for 0-360 min.

[0020] In the above method, the molecular weight of the polyamic acid polymer is 50,000-200,000.

[0021] The present invention provides the use of the above-mentioned fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane in separating CO2.

[0022] Fluorine-containing cross-linkable polyimide-based carbon molecular sieve membranes were used for CO2 / N2 and CO2 / CH4 separations. The gas permeability test was conducted using the constant volume pressure method. The effective test area of ​​the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membranes was 0.2~10cm 2 Before the test, both sides of the membrane are required to be vacuum treated and the pressure on both sides of the membrane is adjusted to less than 1000 Pa; the test temperature range is 5℃~95°C, the feed pressure is 0.1~0.5 MPa, and the pressure on the permeate side is monitored in real time using a pressure sensor. The test time is 2~12 h.

[0023] Beneficial effects of the present invention:

[0024] (1) During the high-temperature carbonization process of the present invention, two carboxyl groups on adjacent DABA units are dehydrated to form an anhydride intermediate; then, the intermediate removes the carboxyl groups to form a biphenyl network structure, which makes the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane have better thermal stability and more carbon residue.

[0025] (2) The preparation process of the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane of the present invention is simple and easy to operate; the prepared membrane has good chemical stability;

[0026] (3) The prepared fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane was applied to the separation of CO2. It was found that the CO2 permeability and selectivity of the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane were greatly improved compared with the fluorine-free polyimide-based carbon molecular sieve membrane, overcoming the trade-off effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The synthetic route diagram of the fluorine-containing cross-linkable polyimide in Example 1, Example 3 and Example 4 is shown. DETAILED DESCRIPTION

[0028] The present invention is further illustrated below by way of examples, but is not limited to the following examples.

[0029] Comparative Example 1: Preparation of a fluorine-free polyamic acid-based carbon molecular sieve pure membrane, the steps are as follows:

[0030] (1) Preparation of fluorine-free polyamic acid precursor film

[0031] Commercially available polyamic acid, prepared from pyromellitic dianhydride as the dianhydride monomer and diaminodiphenyl ether as the diamine monomer, was used as the polymer matrix. The polyamic acid was weighed and added to the N,N-dimethylacetamide solvent, ensuring that the polyamic acid accounted for 11% of the total solution by weight. The solution was stirred on a magnetic stirrer at 60°C and 500 rpm for 6 hours until uniformly dispersed. The solution was then allowed to stand at a constant temperature of 45°C for 12 hours to obtain a casting solution. The casting solution was evenly coated onto a clean glass plate, and a film scraper was used to control the wet film thickness to 150 μm. The solution was dried in a vacuum drying oven at 45°C for 12 hours, then at 60°C for another 12 hours. The dried film was slowly peeled off in a constant-temperature water bath at 35°C and dried in a vacuum drying oven at 60°C for 12 hours to obtain a polyamic acid precursor film.

[0032] (2) Preparation of fluorine-free polyamic acid-based carbon molecular sieve membrane

[0033] The polyamic acid precursor film prepared in step 1 is placed in a tubular furnace under a nitrogen atmosphere for high-temperature calcination and carbonization, and then cooled to room temperature under a nitrogen atmosphere and taken out to prepare a fluorine-free polyamic acid-based carbon molecular sieve membrane;

[0034] The calcination temperature program is as follows: the carbonization temperature is increased at a rate of 5°C / min to 400°C, and then kept at this temperature for 30 minutes; the temperature is increased from 400°C to 650°C at a rate of 3°C / min, and then kept at this temperature for 60 minutes.

[0035] The prepared carbon molecular sieve pure membrane was measured under dry conditions at 25 °C and a pressure difference of 0.1 MPa. The CO2 permeability coefficient was 183 Barrer, the CO2 / N2 selectivity was 27, and the CO2 / CH4 selectivity was 30.

[0036] Example 1: Preparation of a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane, the steps are as follows:

[0037] Step 1: Synthesis of fluorine-containing cross-linkable polyamic acid solution

[0038] 3,5-diaminobenzoic acid and 4,4-diaminodiphenyl ether are dissolved in N-methyl pyrrolidone in a molar ratio of 3:7 to prepare a mixed solution, wherein the mass of N-methyl pyrrolidone is 20 times the total mass of 3,5-diaminobenzoic acid and 4,4'-diaminodiphenyl ether; the mixed solution is stirred on a mechanical stirrer at 0°C and 200 rpm for 0.5 h until completely dissolved, and then 4,4'-(hexafluoroisopropylene) diphthalic anhydride is added to the mixed solution, wherein the ratio of the mole number of 4,4'-(hexafluoroisopropylene) diphthalic anhydride to the total mole number of 3,5-diaminobenzoic acid and 4,4-diaminodiphenyl ether is 1:1, and the mixture is stirred on a mechanical stirrer at 0°C and 200 rpm for 24 h to obtain a polyamic acid solution;

[0039] Step 2: Preparation of fluorine-containing cross-linkable polyimide film

[0040] The polyamic acid solution prepared in step 1 was weighed and added to N-methylpyrrolidone to prepare solution B, ensuring that the polyamic acid accounted for 20% of the total mass of solution B, and stirred on a magnetic stirrer at 30 ° C and a speed of 500 rpm for 2 h to disperse evenly, and then kept at a constant temperature of 0 ° C for 12 h to obtain a casting solution; the casting solution was evenly applied to a clean glass plate, and a scraper was used to control the wet film thickness to 150 μm to obtain a polyamic acid film, which was placed in a vacuum drying oven and heated at 80 ° C for 1 h; 150 ° C for 1 h; 200 ° C for 1 h; 250 ° C for 1 h. The imidization was performed, and the imidized film was peeled off in a constant temperature water bath at 35 ° C and dried in a vacuum drying oven at 80 ° C for 12 h to obtain a fluorine-containing cross-linkable polyimide film;

[0041] Step 3: Preparation of fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane

[0042] The fluorine-containing cross-linkable polyimide membrane obtained in step 2 is placed in a programmed temperature carbonization furnace in a nitrogen atmosphere, and the temperature is increased from 20°C to 250°C at a rate of 5°C / min, then increased from 250°C to 535°C at a rate of 3.5°C / min, and finally increased from 535°C to 550°C at a rate of 0.5°C / min, and carbonized at this temperature at a constant temperature for 120 min. After cooling to room temperature under a nitrogen atmosphere, the membrane is taken out to obtain a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane;

[0043] The prepared carbon molecular sieve membrane was tested under dry conditions at 25°C and a pressure difference of 0.1 MPa, and the CO2 permeability coefficient was

[0044] 3580 Barrer, CO2 / N2 selectivity is 9, CO2 / CH4 selectivity is 32.

[0045] Figure 1 The synthetic pathways for synthesizing the fluorinated cross-linkable polyimide in Examples 1, 3, and 4 are shown. The molar ratio of 4,4'-(hexafluoroisopropylene)diphthalic anhydride to the total molar ratio of 3,5-diaminobenzoic acid and 4,4'-diaminodiphenyl ether is 1:1; and the molar ratio of 3,5-diaminobenzoic acid to 4,4'-diaminodiphenyl ether is 3:7.

[0046] Example 2: Preparation of a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane, the steps are as follows:

[0047] Step 1: Synthesis of fluorine-containing cross-linkable polyamic acid solution

[0048] 3,5-diaminobenzoic acid and 4,4-diaminodiphenyl ether are dissolved in N-methylpyrrolidone in a molar ratio of 2:8 to prepare a mixed solution, wherein the mass of N-methylpyrrolidone is 21 times the total mass of 3,5-diaminobenzoic acid and 4,4'-diaminodiphenyl ether; the mixed solution is stirred on a mechanical stirrer at 0°C and 200 rpm for 0.5 h until completely dissolved, and then 4,4'-(hexafluoroisopropylene) diphthalic anhydride is added to the mixed solution, wherein the ratio of the mole number of 4,4'-(hexafluoroisopropylene) diphthalic anhydride to the total mole number of 3,5-diaminobenzoic acid and 4,4-diaminodiphenyl ether is 1:1, and the mixture is stirred on a mechanical stirrer at 0°C and 200 rpm for 24 h to obtain a polyamic acid solution;

[0049] Step 2: Preparation of fluorine-containing cross-linkable polyimide film

[0050] The polyamic acid solution prepared in step 1 was weighed and added to N-methylpyrrolidone to prepare solution B, ensuring that the polyamic acid accounted for 20% of the total mass of solution B, and stirred on a magnetic stirrer at 30 ° C and a speed of 500 rpm for 2 h to disperse evenly, and then kept at a constant temperature of 0 ° C for 12 h to obtain a casting solution; the casting solution was evenly applied to a clean glass plate, and a scraper was used to control the wet film thickness to 150 μm to obtain a polyamic acid film, which was placed in a vacuum drying oven and heated at 80 ° C for 1 h; 150 ° C for 1 h; 200 ° C for 1 h; 250 ° C for 1 h. The imidization was performed, and the imidized film was peeled off in a constant temperature water bath at 35 ° C and dried in a vacuum drying oven at 80 ° C for 12 h to obtain a fluorine-containing cross-linkable polyimide film;

[0051] Step 3: Preparation of fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane

[0052] The fluorine-containing cross-linkable polyimide membrane prepared in step 2 is placed in a programmed temperature carbonization furnace in a nitrogen atmosphere, and the temperature is increased from 20°C to 250°C at a rate of 5°C / min, then increased from 250°C to 535°C at a rate of 3.5°C / min, and finally increased from 535°C to 550°C at a rate of 0.5°C / min, and carbonized at this temperature at a constant temperature for 120 min. After cooling to room temperature under a nitrogen atmosphere, the membrane is taken out to obtain a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane;

[0053] The prepared carbon molecular sieve membrane was measured under dry conditions at 25°C and a pressure difference of 0.1 MPa. The CO2 permeability coefficient was 3150 Barrer, the CO2 / N2 selectivity was 11, and the CO2 / CH4 selectivity was 35.

[0054] Example 3: Preparation of a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane, the steps are as follows:

[0055] Step 1: Synthesis of fluorine-containing cross-linkable polyamic acid solution

[0056] 3,5-diaminobenzoic acid and 4,4-diaminodiphenyl ether were dissolved in N-methylpyrrolidone at a molar ratio of 3:7 to prepare a mixed solution, wherein the mass of N-methylpyrrolidone was 21 times the total mass of 3,5-diaminobenzoic acid and 4,4'-diaminodiphenyl ether. The mixed solution was stirred at 0°C and 100 rpm for 4 h until completely dissolved, and then 4,4'-(hexafluoroisopropylene) diphthalic anhydride was added to the mixed solution, wherein the ratio of the molar number of 4,4'-(hexafluoroisopropylene) diphthalic anhydride to the total molar number of 3,5-diaminobenzoic acid and 4,4'-diaminodiphenyl ether was 1:1. The mixture was stirred at 0°C and 150 rpm for 24 h to obtain a polyamic acid solution.

[0057] Step 2: Preparation of fluorine-containing cross-linkable polyimide film

[0058] The polyamic acid solution prepared in step 1 was weighed and added to tetrahydrofuran to prepare solution B, ensuring that the polyamic acid accounted for 20% of the total mass of solution B, and was stirred on a magnetic stirrer at 30 ° C and a speed of 500 rpm for 8 h to be evenly dispersed, and then kept at a constant temperature of 0 ° C for 12 h to obtain a casting solution. The casting solution was evenly coated on a clean glass plate, and a film scraper was used to control the wet film thickness to 150 μm to obtain a polyamic acid film, which was placed in a vacuum drying oven and imidized according to a heating program of constant temperature at 60 ° C for 2 h; constant temperature at 150 ° C for 1 h; constant temperature at 200 ° C for 1 h; constant temperature at 250 ° C for 1 h. The imidized film was peeled off in a constant temperature water bath at 35 ° C and dried in a vacuum drying oven at 80 ° C for 12 h to obtain a fluorine-containing cross-linkable polyimide film.

[0059] Step 3: Preparation of fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane

[0060] The fluorine-containing cross-linkable polyimide membrane obtained in step 2 is placed in a programmed temperature carbonization furnace in a helium atmosphere, and the temperature is increased to 250°C at a rate of 5°C / min, and then increased from 250°C to 500°C at a rate of 3°C / min, and kept constant at this temperature for 60 min; the temperature is increased from 500°C to 550°C at a rate of 1°C / min, and kept constant at this temperature for 120 min. After high-temperature calcination and carbonization, the membrane is cooled to room temperature under a helium atmosphere and taken out to obtain a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane;

[0061] The prepared carbon molecular sieve membrane was measured under dry conditions at 25°C and a pressure difference of 0.1 MPa. The CO2 permeability coefficient was 2060 Barrer, the CO2 / N2 selectivity was 14, and the CO2 / CH4 selectivity was 46.

[0062] Example 4: Preparation of a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane, the steps are as follows:

[0063] Step 1: Synthesis of fluorine-containing cross-linkable polyamic acid solution

[0064] 3,5-diaminobenzoic acid and 4,4-diaminodiphenyl ether were dissolved in N-methylpyrrolidone at a molar ratio of 3:7 to prepare a mixed solution, wherein the mass of N-methylpyrrolidone was 21 times the total mass of 3,5-diaminobenzoic acid and 4,4'-diaminodiphenyl ether. The mixture was stirred on a mechanical stirrer at 0°C and 200 rpm for 4 h until completely dissolved, and then 4,4'-(hexafluoroisopropylene) diphthalic anhydride was added to the mixed solution, wherein the ratio of the molar number of 4,4'-(hexafluoroisopropylene) diphthalic anhydride to the total molar number of 3,5-diaminobenzoic acid and 4,4'-diaminodiphenyl ether was 1:1. The mixture was further stirred on a mechanical stirrer at 0°C and 200 rpm for 12 h to obtain a polyamic acid solution.

[0065] Step 2: Preparation of fluorine-containing cross-linkable polyimide film

[0066] The polyamic acid solution prepared in step 1 was weighed and added to N-methylpyrrolidone to prepare solution B, ensuring that the polyamic acid accounted for 20% of the total mass of solution B, and stirred on a magnetic stirrer at 30 ° C and a speed of 500 rpm for 2 h to be evenly dispersed, and then kept at a constant temperature of 0 ° C for 12 h to obtain a casting solution; the casting solution was evenly applied to a clean glass plate, and a scraper was used to control the wet film thickness to 150 μm to obtain a polyamic acid film, which was placed in a vacuum drying oven and heated at 80 ° C for 1 h; 150 ° C for 1 h; 200 ° C for 2 h; 250 ° C for 1 h. The imidization was performed, and the imidized film was peeled off in a constant temperature water bath at 35 ° C and dried in a vacuum drying oven at 80 ° C for 12 h to obtain a fluorine-containing cross-linkable polyimide film;

[0067] Step 3: Preparation of fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane

[0068] The fluorine-containing cross-linkable polyimide membrane obtained in step 2 was placed in a programmed temperature carbonization furnace in a nitrogen atmosphere, and the temperature was increased to 250°C at a rate of 5°C / min, and then increased from 250°C to 350°C at a rate of 3°C / min, and kept constant at this temperature for 60 min; the temperature was increased from 350°C to 600°C at a rate of 1.5°C / min, and kept constant at this temperature for 360 min. After high-temperature calcination and carbonization, the membrane was cooled to room temperature under a nitrogen atmosphere and taken out to obtain a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane;

[0069] The prepared carbon molecular sieve membrane was measured under dry conditions at 25°C and a pressure difference of 0.1 MPa. The CO2 permeability coefficient was 2130 Barrer, the CO2 / N2 selectivity was 14, and the CO2 / CH4 selectivity was 42.

[0070] Example 5: Preparation of a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane, the steps are as follows:

[0071] Step 1: Synthesis of fluorine-containing cross-linkable polyamic acid solution

[0072] 3,5-diaminobenzoic acid and 4,4-diaminodiphenyl ether were dissolved in tetrahydrofuran at a molar ratio of 1:9, wherein the mass of tetrahydrofuran was 21 times the total mass of 3,5-diaminobenzoic acid and 4,4'-diaminodiphenyl ether; the mixed solution was stirred at 0°C and 200 rpm on a mechanical stirrer for 8 h until completely dissolved, and then 4,4'-(hexafluoroisopropylene) diphthalic anhydride was added to the mixed solution, wherein the ratio of the mole number of 4, 4'-(hexafluoroisopropylene) diphthalic anhydride to the total mole number of 3,5-diaminobenzoic acid and 4,4'-diaminodiphenyl ether was 1:1.2; and the mixture was stirred at 0°C and 200 rpm on a mechanical stirrer for 12 h to obtain a polyamic acid solution;

[0073] Step 2: Preparation of fluorine-containing cross-linkable polyimide film

[0074] Weighing polyamic acid and adding it to tetrahydrofuran to prepare solution B, ensuring that the polyamic acid accounts for 15% of the total mass of solution B, and stirring on a magnetic stirrer at 30°C and a speed of 300 rpm for 2 h to disperse evenly, and standing at a constant temperature of 0°C for 12 h to prepare a casting solution; the casting solution is evenly coated on a clean glass plate, and a film scraper is used to control the wet film thickness to 150 μm to prepare a polyamic acid film, which is placed in a vacuum drying oven according to a temperature increase program of constant temperature at 80°C for 1 h; constant temperature at 150°C for 1 h; constant temperature at 200°C for 1 h; and constant temperature at 250°C for 1 h for imidization, and the imidized film is peeled off in a constant temperature water bath at 35°C and dried in a vacuum drying oven at 80°C for 12 h to obtain a fluorine-containing cross-linkable polyimide film;

[0075] Step 3: Preparation of fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane

[0076] The fluorine-containing cross-linkable polyimide membrane obtained in step 2 was placed in a programmed temperature carbonization furnace in a nitrogen atmosphere, and the temperature was increased to 250°C at a rate of 5°C / min, and then increased from 250°C to 350°C at a rate of 2.5°C / min, and kept constant at this temperature for 60 min; the temperature was increased from 350°C to 700°C at a rate of 0.5°C / min, and kept constant at this temperature for 120 min. After high-temperature calcination and carbonization, the membrane was cooled to room temperature under a nitrogen atmosphere and taken out to obtain a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane;

[0077] The prepared carbon molecular sieve membrane was measured for CO2 permeation coefficient at 25°C under dry conditions with a pressure difference of 0.1 MPa, and the CO2 / N2 selectivity was 16 and the CO2 / CH4 selectivity was 60.

[0078] In summary, compared with Comparative Example 1, the CO2 permeability of the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membranes prepared in Examples 1-5 is significantly improved compared with the polyamide acid-based carbon molecular sieve membrane without fluorine in Comparative Example 1. The CO2 permeability of the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane prepared in Example 1 is greatly improved, because the 4, 4'-(hexafluoroisopropylidene) diphthalic anhydride contains a larger -CF3 group, which inhibits the accumulation of polymer chains, thereby increasing the free volume of the fluorine-containing cross-linkable polyimide membrane, and this characteristic is continued from the precursor membrane to the carbon molecular sieve membrane, so that the CO2 permeability is greatly improved; compared with Example 1, Examples 2-5 adjust the pore structure of the carbon molecular sieve membrane to fine-tune the separation performance of the carbon molecular sieve membrane. For example, Example 2 adjusts the pore structure of the carbon molecular sieve membrane by changing the ratio of the diamines to fine-tune the separation performance of the membrane. Compared with Comparative Example 1, the CO2 / N2 selectivity in Examples 1-5 is not improved but rather reduced, and the CO2 / CH4 selectivity is significantly improved, because the addition of fluorine groups increases the free volume of the carbon molecular sieve membrane, which changes the pore distribution of the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane, and the CO2 / CH4 selectivity is significantly improved while the CO2 permeability is improved. By adjusting the polymer structure of the prepared carbon molecular sieve membrane, it can be applied to the separation of different gas pairs (such as CO2 / N2, CO2 / CH4, N2 / CH4 and C3H6 / C3H8, etc.), and has good industrial application prospects.

[0079] Although the present application has been described above, the present application is not limited to the specific embodiments described above, which are merely illustrative and not restrictive, and many modifications can be made by those of ordinary skill in the art without departing from the spirit of the present application, and these are all within the scope of the present application.

Claims

1. Application of a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane in CO2 separation, characterized by: The method for preparing a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane comprises the following steps: using 4,4'-(hexafluoroisopropylene) diphthalic anhydride containing a large -CF3 group as a dianhydride monomer, cross-linkable 3,5-diaminobenzoic acid and 4,4'-diaminodiphenyl ether having a flexible ether bond as common diamine monomers, and copolymerizing them to form a polyamic acid solution; preparing a casting solution from the polyamic acid solution, standing it for degassing, and then scraping the wet film using a coating method to control the thickness of the wet film to prepare a polyamic acid membrane; and then performing an imidization process to prepare a polyimide membrane; and placing the obtained polyimide membrane in a programmed temperature carbonization furnace with a protective atmosphere for high-temperature calcination and carbonization, cooling it to room temperature in a protective atmosphere, and then taking it out to prepare a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane.

2. The use of the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane in separating CO2 according to claim 1, characterized in that: The method for preparing the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane comprises the following steps: Step 1: Synthesis of fluorine-containing cross-linkable polyamic acid solution 3,5-diaminobenzoic acid and 4,4'-diaminodiphenyl ether are dissolved in solvent A in a molar ratio of (1-6):(1.5-9), wherein the mass of solvent A is 15-45 times the total mass of 3,5-diaminobenzoic acid and 4,4'-diaminodiphenyl ether; the mixed solution is stirred on a mechanical stirrer at a temperature range of -5-5°C and a rotation speed range of 40-1000 rpm for 0.5-12 h until completely dissolved, and 4,4'-(hexafluoroisopropylene)diphthalic anhydride is added to the mixed solution, wherein the ratio of the mole number of 4,4'-(hexafluoroisopropylene)diphthalic anhydride to the total mole number of 3,5-diaminobenzoic acid and 4,4'-diaminodiphenyl ether is 1:(1-5); and the stirring is continued on the mechanical stirrer at a temperature range of -5-5°C and a rotation speed range of 40-1000 rpm for 8-48 h to obtain a polyamic acid solution; Wherein, solvent A is any one of N, N-dimethylformamide, N, N-dimethylacetamide, tetrahydrofuran, chloroform, dimethyl sulfoxide and N-methylpyrrolidone; Step 2: Preparation of fluorine-containing cross-linkable polyimide film The polyamic acid solution prepared in step 1 was weighed and added to solvent A to prepare solution B, ensuring that the polyamic acid accounted for 10-20% of the total mass of solution B, and the mixture was stirred on a magnetic stirrer at a temperature of 25-80 ° C and a speed of 100-800 rpm for 2-24 h to be evenly dispersed, and allowed to stand at a constant temperature of -5-5 ° C for 12-24 h to obtain a casting solution; the casting solution was evenly coated on a clean glass plate, and a scraper was used to control the wet film thickness to 100-300 μm to obtain a polyamic acid film, which was placed in a programmed temperature vacuum drying oven to complete imidization, and the imidized film was peeled off in a constant temperature water bath at 30-40 ° C, and dried in a vacuum drying oven at 80-150 ° C for 6-12 h to obtain a fluorine-containing cross-linkable polyimide film; Step 3: Preparation of fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane The fluorine-containing cross-linkable polyimide membrane prepared in step 2 is placed in a programmed temperature carbonization furnace with a protective atmosphere for high-temperature calcination and carbonization, and then cooled to room temperature under a protective atmosphere and taken out to obtain a fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane.

3. The use of the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane in separating CO2 according to claim 2, characterized in that: The molecular weight of the polyamic acid polymer synthesized in step 1 is 50,000-200,000.

4. Use of the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane in separating CO2 according to claim 2, characterized in that: The solvent A used in step 1 and step 2 is the same.

5. Use of the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane in separating CO2 according to claim 2, characterized in that: The temperature program for imidization in step 2 is: constant temperature at 40-80°C for 1-2 h; constant temperature at 80-150°C for 1-2 h; constant temperature at 150-200°C for 1-2 h; and finally constant temperature at 200-250°C for 1-2 h.

6. Use of the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane in separating CO2 according to claim 2, characterized in that: In step 3, the protective atmosphere used is one of nitrogen, helium or argon.

7. Use of the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane in separating CO2 according to claim 2, characterized in that: In step 3, the high-temperature calcination carbonization process is as follows: heating from 0-20°C to 200-250°C at a heating rate of 5-10°C / min; heating from 200-250°C to 350-500°C at a heating rate of 0.01-5°C / min, and maintaining the temperature at this temperature for 60-120min; heating from 350-500°C to 550-900°C at a heating rate of 0.01-5°C / min, and maintaining the temperature at this temperature for 120-360min.

8. Use of the fluorine-containing cross-linkable polyimide-based carbon molecular sieve membrane in CO2 separation according to claim 1, characterized in that: Fluorinated cross-linkable polyimide-based carbon molecular sieve membranes were used for CO2 / N2 and CO2 / CH4 separations. The gas permeation performance was tested using the constant volume pressure method. The effective test area of ​​the fluorinated cross-linkable polyimide-based carbon molecular sieve membranes was 0.2-10 cm 2 Before the test, both sides of the membrane are required to be vacuum treated and the pressure on both sides of the membrane is adjusted to less than 1000 Pa; the test temperature range is 5°C to 95°C, the feed pressure is 0.1~0.5 MPa, and the pressure on the permeate side is monitored in real time using a pressure sensor. The test time is 2~12h.

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