A PIM-1@ZIF-8 mixed matrix membrane based on in-situ polymerization method and a preparation method thereof

Abstract

The application belongs to the technical field of gas separation, and provides a PIM-1@ZIF-8 mixed matrix membrane based on an in-situ polymerization method, and the molar ratio of reverse-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene] propanedinitrile, TTSBI, a catalyst and ZIF-8 is 0.5-1.5:0.5-1.5:2-3:0.3-0.65. The application also provides a preparation method of the PIM-1@ZIF-8 mixed matrix membrane. The application forms a rich hydrogen bond network through a chemical cross-linking reaction between the -CN group of the monomer DCTB of PIM-1 and ZIF-8, thereby enhancing the interface interaction between ZIF-8 and PIM-1 and improving the interface compatibility thereof; the pore size screening effect of ZIF-8 and the characteristics of PIM-1, i.e., high specific surface area and interconnected micropores, are utilized, and the performance of the membrane is further improved.

Description

Technical Field

[0001] This invention relates to the field of gas separation technology, and in particular to a PIM-1@ZIF-8 mixed matrix membrane based on in-situ polymerization and its preparation method. Background Technology

[0002] Thermal power generation, as a pillar of electricity production, generates large amounts of carbon dioxide during combustion, posing a significant threat to the ecological environment. Traditional carbon dioxide waste gas treatment technologies from thermal power plants suffer from drawbacks such as complex operation, high cost, and low treatment efficiency. Under the strategic background of "carbon peaking and carbon neutrality," carbon capture and separation technologies in the thermal power industry have attracted widespread attention from the academic community. The core of this technology is how to achieve efficient carbon dioxide separation under low energy consumption conditions.

[0003] Membrane separation is a very popular gas separation technology in recent years. This method utilizes the difference in the permeation rates of different types of gases through a separation membrane to achieve gas separation, and has advantages such as simple equipment, convenient operation, and low energy consumption. Hybrid matrix membranes, as a new generation of high-efficiency gas separation membranes, combine the advantages of traditional organic and inorganic membranes, have lower costs, and are not limited by the "trade-off" effect, showing great potential for industrial applications.

[0004] Using hybrid matrix membranes for carbon dioxide treatment in thermal power plants, leveraging their selective permeability to CO2 / CH4 and CO2 / N2, achieves highly efficient CO2 capture while significantly reducing energy consumption. Therefore, obtaining a high-performance hybrid matrix membrane is a key technical challenge that needs to be addressed. Summary of the Invention

[0005] The purpose of this invention is to provide a PIM-1@ZIF-8 mixed matrix membrane and its preparation method based on in-situ polymerization to overcome the shortcomings of existing technologies, thereby overcoming the defects of poor interfacial compatibility, easy aggregation, and low membrane separation efficiency in existing traditional membrane preparation methods.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a PIM-1@ZIF-8 hybrid matrix membrane based on in-situ polymerization, comprising the following raw materials: trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile, TTSBI, ZIF-8, catalyst, and solvent;

[0008] The molar ratio of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile, TTSBI, and catalyst is 0.5–1.5:0.5–1.5:2–3;

[0009] The molar ratio of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile to ZIF-8 is 0.5–1.5:0.3–0.65.

[0010] Preferably, the molar volume ratio of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile to solvent is 0.5–1.5 mol: 20–35 L.

[0011] Preferably, the catalyst is potassium carbonate, and the solvent is N,N-dimethylformamide, tetrahydrofuran, chloroform, or dichloromethane.

[0012] The present invention also provides a method for preparing the PIM-1@ZIF-8 hybrid matrix membrane, comprising the following steps:

[0013] 1) A cross-linking reaction was carried out after mixing trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile, TTSBI, ZIF-8, catalyst and solvent to obtain a solution;

[0014] 2) The solution and methanol were mixed to obtain a flocculent precipitate. The flocculent precipitate was centrifuged to obtain a solid.

[0015] 3) The solid was sequentially ultrasonically washed with methanol, N,N-dimethylformamide and water, and then centrifuged to obtain a precipitate. The precipitate was then vacuum dried to obtain a powder.

[0016] 4) The powder and chloroform are mixed and then subjected to ultrasonic vibration and drying in sequence to obtain a preliminary film;

[0017] 5) The initial membrane was soaked in methanol and then vacuum dried to obtain the PIM-1@ZIF-8 mixed matrix membrane.

[0018] Preferably, the mixing in step 1) is carried out at room temperature for 10 to 30 minutes and at a speed of 400 to 600 rpm; the crosslinking reaction is carried out at a temperature of 70 to 80°C for 24 to 48 hours, and the mixing and crosslinking reactions are carried out in a nitrogen atmosphere.

[0019] Preferably, the solution in step 2) is cooled to room temperature before being mixed with methanol, and the centrifugation time is 10 to 30 minutes and the centrifugation speed is 3000 to 5000 rpm.

[0020] Preferably, in step 3), the ultrasonic washing time with methanol, N,N-dimethylformamide, and water is 10–30 min, the ultrasonic washing frequency is 30–50 kHz, the centrifugation time is 10–30 min, and the centrifugation speed is 3000–5000 rpm; the vacuum drying time is 12–24 h, the vacuum drying temperature is 80–100 °C, and the vacuum degree of vacuum drying is 0.08–0.1 MPa.

[0021] Preferably, in step 4), the mass-to-volume ratio of the powder to chloroform is 1-2 g: 100 mL; the mixing time is 3-5 h, and the mixing speed is 400-600 rpm; the ultrasonic vibration time is 1-2 h, and the ultrasonic vibration frequency is 30-50 Hz; and the drying time is ≥12 h.

[0022] Preferably, the soaking time in step 5) is 12-24 hours, the vacuum drying temperature is 80-140°C, the vacuum drying time is 12-24 hours, and the vacuum degree of vacuum drying is 0.08-0.1 MPa.

[0023] The beneficial effects of this invention include:

[0024] 1) This invention uses the inherently microporous polymer PIM-1 as a substrate and the metal-organic framework material ZIF-8 as a filler to prepare a hybrid matrix membrane for CO2 / CH4 separation and CO2 / N2 separation via in-situ polymerization. The hybrid matrix membrane of this invention can be applied to fields such as carbon dioxide treatment in thermal power plants. While reducing energy consumption in carbon capture, it increases the CO2 concentration during the carbon capture process, demonstrating significant economic and social value in commercial applications and in achieving sustainable development.

[0025] 2) The preparation method of this invention involves a chemical cross-linking reaction between the -CN group of the monomer DCTB of PIM-1 and ZIF-8, forming a rich hydrogen bond network. This enhances the interfacial interaction between ZIF-8 and PIM-1, improving their interfacial compatibility. Furthermore, the pore size sieving effect of ZIF-8 and the high specific surface area and interconnected micropores of PIM-1 further improve the membrane performance. This invention improves upon traditional synthesis steps, resulting in a significant enhancement in CO2 / N2 and CO2 / CH4 gas separation performance. Attached Figure Description

[0026] Figure 1 Here is an electron microscope image of the ZIF-8 material from Example 1;

[0027] Figure 2 The infrared spectrum of PIM-1 material in Example 1;

[0028] Figure 3The flowchart shows the conventional method for preparing the PIM-1@ZIF-8 hybrid matrix membrane of Comparative Example 1.

[0029] Figure 4 This is a flowchart of the in-situ polymerization preparation of the PIM-1@ZIF-8 hybrid matrix membrane in Example 1;

[0030] Figure 5 This is a photograph of the PIM-1@ZIF-8 hybrid matrix membrane from Example 1.

[0031] Figure 6 This is a cross-sectional scanning electron microscope image of the PIM-1@ZIF-8 hybrid matrix membrane of Example 1;

[0032] Figure 7 The curve showing the CO2 permeability of the PIM-1@ZIF-8 hybrid matrix membrane in Example 1 as a function of pressure. Detailed Implementation

[0033] This invention provides a PIM-1@ZIF-8 mixed matrix membrane based on in-situ polymerization, comprising the following raw materials: trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile, TTSBI (5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane), ZIF-8, catalyst, and solvent;

[0034] The molar ratio of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile, TTSBI, and catalyst is 0.5–1.5:0.5–1.5:2–3;

[0035] The molar ratio of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile to ZIF-8 is 0.5–1.5:0.3–0.65.

[0036] In this invention, the molar ratio of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile, TTSBI, and catalyst is preferably 0.7–1.3:0.7–1.3:2.2–2.8, more preferably 0.9–1.1:0.9–1.1:2.4–2.6, and even more preferably 1:1:2.5; the molar ratio of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile and ZIF-8 is preferably 0.7–1.3:0.43–0.6, more preferably 0.9–1.1:0.56–0.6, and even more preferably 1:0.5–0.55.

[0037] In this invention, the molar volume ratio of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile to solvent is preferably 0.5-1.5 mol: 20-35 L, more preferably 0.7-1.2 mol: 23-32 L, and even more preferably 0.9-1 mol: 25-30 L.

[0038] In this invention, the catalyst is preferably potassium carbonate, and the solvent is preferably N,N-dimethylformamide, tetrahydrofuran, chloroform, or dichloromethane, and more preferably N,N-dimethylformamide.

[0039] The present invention also provides a method for preparing the PIM-1@ZIF-8 hybrid matrix membrane, comprising the following steps:

[0040] 1) A cross-linking reaction was carried out after mixing trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile, TTSBI, ZIF-8, catalyst and solvent to obtain a solution;

[0041] 2) The solution and methanol were mixed to obtain a flocculent precipitate. The flocculent precipitate was centrifuged to obtain a solid.

[0042] 3) The solid was sequentially ultrasonically washed with methanol, N,N-dimethylformamide and water, and then centrifuged to obtain a precipitate. The precipitate was then vacuum dried to obtain a powder.

[0043] 4) The powder and chloroform are mixed and then subjected to ultrasonic vibration and drying in sequence to obtain a preliminary film;

[0044] 5) The initial membrane was soaked in methanol and then vacuum dried to obtain the PIM-1@ZIF-8 mixed matrix membrane.

[0045] In this invention, the mixing in step 1) is preferably carried out at room temperature, and the mixing time is preferably 10-30 min, more preferably 15-25 min, and more preferably 20 min; the mixing speed is preferably 400-600 rpm, more preferably 450-550 rpm, and more preferably 500 rpm; the temperature of the crosslinking reaction is preferably 70-80℃, more preferably 72-78℃, and more preferably 75-76℃; the crosslinking reaction time is preferably 24-48 h, more preferably 28-40 h, and more preferably 32-36 h; the mixing and crosslinking reaction are preferably carried out in a nitrogen atmosphere.

[0046] In this invention, the solution in step 2) is preferably cooled to room temperature before being mixed with methanol, the centrifugation time is preferably 10-30 min, more preferably 15-25 min, and even more preferably 20 min; the centrifugation speed is preferably 3000-5000 rpm, more preferably 3500-4500 rpm, and even more preferably 3800-4200 rpm.

[0047] In this invention, the ultrasonic washing time of methanol, N,N-dimethylformamide, and water in step 3) is preferably 10-30 min, more preferably 15-25 min, and more preferably 20 min; the ultrasonic washing frequency is preferably 30-50 kHz, more preferably 35-45 kHz, and more preferably 40 kHz; the centrifugation time is preferably 10-30 min, more preferably 15-25 min, and more preferably 20 min; the centrifugation speed is preferably 3000-5000 rpm, more preferably 3500-4500 rpm, and more preferably 3800-4200 rpm; the vacuum drying time is preferably 12-24 h, more preferably 15-20 h, and more preferably 17-18 h; the vacuum drying temperature is preferably 80-100℃, more preferably 85-95℃, and more preferably 90℃; the vacuum degree of vacuum drying is preferably 0.08-0.1 MPa, more preferably 0.085-0.095 MPa, and more preferably 0.09-0.093 MPa.

[0048] In this invention, the preferred mass-to-volume ratio of the powder and chloroform in step 4) is 1-2 g:100 mL, more preferably 1.2-1.8 g:100 mL, and even more preferably 1.4-1.6 g:100 mL; the preferred mixing time is 3-5 h, more preferably 3.5-4.5 h, and even more preferably 4 h; the preferred mixing speed is 400-600 rpm, more preferably 450-550 rpm, and even more preferably 500 rpm; the preferred ultrasonic oscillation time is 1-2 h, more preferably 1.5 h; the preferred ultrasonic oscillation frequency is 30-50 Hz, more preferably 35-45 Hz, and even more preferably 40 Hz; the preferred drying time is ≥12 h, more preferably 15-30 h, and even more preferably 20-24 h.

[0049] In this invention, the soaking time in step 5) is preferably 12-24 hours, more preferably 15-20 hours, and even more preferably 18 hours; the vacuum drying temperature is preferably 80-140°C, more preferably 100-120°C; the vacuum drying time is preferably 12-24 hours, more preferably 15-20 hours, and even more preferably 18 hours; the vacuum degree of vacuum drying is preferably 0.08-0.1 MPa, more preferably 0.085-0.095 MPa, and even more preferably 0.09-0.093 MPa.

[0050] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0051] Example 1

[0052] 0.005 mol of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile, 0.004 mol of TTSBI (5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane),

[0053] 0.0024 mol ZIF-8 and 0.01 mol potassium carbonate were added to a three-necked flask containing 100 mL of N,N-dimethylformamide. Under nitrogen purge, the mixture was stirred at 500 rpm at room temperature for 20 min. The temperature was then raised to 70 °C, and the mixture was stirred in a nitrogen-purged oil bath for 30 h to obtain a homogeneous solution. After cooling to room temperature, the homogeneous solution was poured into methanol, resulting in a flocculent precipitate. The precipitate was centrifuged at 4000 rpm for 20 min to obtain a yellow solid.

[0054] The yellow solid was sequentially ultrasonically washed with methanol, N,N-dimethylformamide and water for 20 min each time, at a frequency of 40 kHz. Then it was centrifuged at 4000 rpm for 20 min to obtain a precipitate. The precipitate was vacuum dried in a vacuum drying oven at 80℃ and 0.09 MPa for 12 h to obtain PIM-1@ZIF-8 powder.

[0055] 0.3 g of PIM-1@ZIF-8 powder was added to 20 mL of chloroform and stirred at 500 rpm for 3 h. The mixture was then ultrasonically vibrated at 40 Hz for 2 h. The ultrasonically vibrated membrane solution was cast onto a mold and dried at room temperature for 12 h to obtain a cured initial membrane. The initial membrane was then soaked in methanol for 20 h to allow it to fully expand. Finally, the initial membrane was vacuum dried in a vacuum drying oven at 120 °C and 0.093 MPa for 12 h to obtain a PIM-1@ZIF-8 mixed matrix membrane.

[0056] Example 2

[0057] 0.01 mol of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile, 0.009 mol of TTSBI, 0.0048 mol of ZIF-8, and 0.024 mol of potassium carbonate were added to a three-necked flask containing 280 mL of N,N-dimethylformamide. The mixture was stirred at 450 rpm for 25 min at room temperature under nitrogen protection. The temperature was then raised to 75 °C, and the mixture was stirred for 40 h in a constant-temperature oil bath with continuous nitrogen purging to obtain a homogeneous solution. After cooling the homogeneous solution to room temperature, it was poured into methanol, resulting in a flocculent precipitate. The precipitate was centrifuged at 3500 rpm for 18 min to obtain a yellow solid.

[0058] The yellow solid was sequentially ultrasonically washed with methanol, N,N-dimethylformamide and water for 15 min each and at a frequency of 45 kHz. Then it was centrifuged at 3500 rpm for 25 min to obtain a precipitate. The precipitate was vacuum dried in a vacuum drying oven at 90 ℃ and 0.085 MPa for 20 h to obtain PIM-1@ZIF-8 powder.

[0059] 0.24 g of PIM-1@ZIF-8 powder was added to 20 mL of chloroform and stirred at 450 rpm for 4 h. The mixture was then ultrasonically vibrated at 35 Hz for 1.5 h. The ultrasonically vibrated membrane solution was cast onto a mold and dried at room temperature for 18 h to obtain a cured initial membrane. The initial membrane was then soaked in methanol for 18 h to allow it to fully expand. Finally, the initial membrane was vacuum dried in a vacuum drying oven at 100 °C and 0.085 MPa for 18 h to obtain a PIM-1@ZIF-8 mixed matrix membrane.

[0060] Example 3

[0061] 0.013 mol of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile, 0.011 mol of TTSBI, 0.0058 mol of ZIF-8, and 0.028 mol of potassium carbonate were added to a three-necked flask containing 320 mL of N,N-dimethylformamide. The mixture was stirred at 550 rpm for 15 min at room temperature under nitrogen protection. The temperature was then raised to 78 °C, and the mixture was stirred for 26 h in a constant-temperature oil bath with continuous nitrogen purging to obtain a homogeneous solution. After cooling the homogeneous solution to room temperature, it was poured into methanol, resulting in a flocculent precipitate. The precipitate was centrifuged at 4500 rpm for 24 min to obtain a yellow solid.

[0062] The yellow solid was sequentially ultrasonically washed with methanol, N,N-dimethylformamide and water for 25 min each and at a frequency of 35 kHz. Then it was centrifuged at 4500 rpm for 15 min to obtain a precipitate. The precipitate was vacuum dried in a vacuum drying oven at 95 ℃ and a vacuum degree of 0.095 MPa for 16 h to obtain PIM-1@ZIF-8 powder.

[0063] 0.36 g of PIM-1@ZIF-8 powder was added to 20 mL of chloroform and stirred at 550 rpm for 4.5 h. The mixture was then ultrasonically vibrated at 45 Hz for 1 h. The ultrasonically vibrated membrane solution was cast onto a mold and dried at room temperature for 22 h to obtain a cured initial membrane. The initial membrane was then soaked in methanol for 14 h to allow it to fully expand. Finally, the initial membrane was vacuum dried in a vacuum drying oven at 90 °C and 0.095 MPa for 20 h to obtain a PIM-1@ZIF-8 mixed matrix membrane.

[0064] Comparative Example 1

[0065] Add 2.7 g of PIM-1 to 20 mL of chloroform and stir thoroughly at 25°C at a stirring speed of 600 rpm for 2 hours until fully dissolved to obtain a chloroform solution of PIM-1. Dissolve 0.5 g of ZIF-8 (20% by mass) in 5 mL of chloroform and stir thoroughly at 25°C at a stirring speed of 600 rpm for 1.5 hours until fully dissolved to obtain a chloroform solution of ZIF-8.

[0066] The chloroform solutions of PIM-1 and ZIF-8 were mixed and stirred at 600 rpm for 3 hours, followed by ultrasonic oscillation at a frequency of 40 kHz for 1 hour. The ultrasonically treated membrane solution was cast onto a mold and dried at room temperature for 30 hours to obtain a cured initial membrane. The initial membrane was then soaked in methanol for 15 hours to allow it to fully expand. Finally, the initial membrane was dried in a vacuum drying oven at 120°C and a vacuum degree of 0.093 MPa for 12 hours to obtain a mixed matrix membrane.

[0067] The preparation method of this invention incorporates the monomers TTSBI and DCTB of PIM-1 as reactants, simultaneously synthesizing PIM-1 during mixing with ZIF-8. Compared to the method in Comparative Example 1, which directly mixes PIM-1 and ZIF-8, in this method, the -CN groups of the PIM-1 monomer DCTB undergo a chemical cross-linking reaction with ZIF-8, forming a rich hydrogen bond network. This enhances the interfacial interaction between ZIF-8 and PIM-1, improving their interfacial compatibility. Furthermore, by utilizing the pore size sieving effect of ZIF-8 and the high specific surface area and interconnected micropores of PIM-1, the membrane performance is further improved. This invention improves upon traditional synthesis steps, resulting in a significant enhancement in CO2 / N2 and CO2 / CH4 gas separation performance. In the PIM-1@ZIF-8 hybrid matrix membrane of Example 1 of this invention, the ZIF-8 filling rate can reach 35wt%; under a pressure of 0.25MPa, the permeability of the PIM-1@ZIF-8 hybrid matrix membrane of Example 1 is 3473 barrer, and the CO2 / N2 selectivity is 2.5; the total cost of the PIM-1@ZIF-8 hybrid matrix membrane of this invention is 1.8 to 2.1 yuan / square centimeter.

[0068] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A PIM-1@ZIF-8 hybrid matrix membrane based on in-situ polymerization, characterized in that, The preparation materials include: trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile, TTSBI, ZIF-8, catalyst and solvent; The molar ratio of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile, TTSBI, and catalyst is 0.5~1.5:0.5~1.5:2~3; The molar ratio of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile to ZIF-8 is 0.5~1.5:0.3~0.65; In the preparation method, the monomers TTSBI and trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonium of PIM-1 are added as reaction raw materials, and PIM-1 is synthesized simultaneously during the mixing with ZIF-8.

2. The PIM-1@ZIF-8 hybrid matrix membrane according to claim 1, characterized in that, The molar volume ratio of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile to solvent is 0.5~1.5 mol: 20~35 L.

3. The PIM-1@ZIF-8 hybrid matrix membrane according to claim 1 or 2, characterized in that, The catalyst is potassium carbonate, and the solvent is N,N-dimethylformamide, tetrahydrofuran, chloroform, or dichloromethane.

4. The method for preparing the PIM-1@ZIF-8 hybrid matrix membrane according to any one of claims 1 to 3, characterized in that, It includes the following steps: 1) A cross-linking reaction was carried out after mixing trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile, TTSBI, ZIF-8, catalyst and solvent to obtain a solution; 2) The solution was mixed with methanol to obtain a flocculent precipitate, which was then centrifuged to obtain a solid. 3) The solid was sequentially ultrasonically washed with methanol, N,N-dimethylformamide and water, and then centrifuged to obtain a precipitate. The precipitate was then vacuum dried to obtain a powder. 4) The powder and chloroform are mixed and then subjected to ultrasonic vibration and drying in sequence to obtain a preliminary film; 5) The initial membrane was soaked in methanol and then vacuum dried to obtain the PIM-1@ZIF-8 mixed matrix membrane.

5. The preparation method according to claim 4, characterized in that, Step 1) The mixing is carried out at room temperature for 10-30 minutes and at a speed of 400-600 rpm; the crosslinking reaction is carried out at a temperature of 70-80°C for 24-48 hours and in a nitrogen atmosphere.

6. The preparation method according to claim 4 or 5, characterized in that, Step 2) The solution is cooled to room temperature and then mixed with methanol. The centrifugation time is 10-30 minutes and the centrifugation speed is 3000-5000 rpm.

7. The preparation method according to claim 6, characterized in that, Step 3) The ultrasonic washing time of methanol, N,N-dimethylformamide and water is 10~30 min, the ultrasonic washing frequency is 30~50 kHz, the centrifugation time is 10~30 min, the centrifugation speed is 3000~5000 rpm; the vacuum drying time is 12~24 h, the vacuum drying temperature is 80~100℃, and the vacuum degree of vacuum drying is 0.08~0.1 MPa.

8. The preparation method according to claim 7, characterized in that, Step 4) The mass-to-volume ratio of the powder to chloroform is 1~2g:100mL; the mixing time is 3~5h, and the mixing speed is 400~600rpm; the ultrasonic vibration time is 1~2h, and the ultrasonic vibration frequency is 30~50Hz; the drying time is ≥12h.

9. The preparation method according to claim 7 or 8, characterized in that, Step 5) The soaking time is 12~24h, the vacuum drying temperature is 80~140℃, the vacuum drying time is 12~24h, and the vacuum degree of vacuum drying is 0.08~0.1MPa.

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