Purification process of anhydrous-grade N, N-dimethylformamide

Abstract

The invention relates to the technical field of organic solvent purification, in particular to a purification process of anhydrous-grade N, N-dimethylformamide. The method comprises the following steps: firstly, carrying out reduced pressure distillation on an N, N-dimethylformamide wastewater raw material to remove liquid organic matter impurities with a lower boiling point and soluble solid components, then adsorbing and removing metal ions by adopting an ion exchange tower, and finally, carrying out osmotic dehydration by using a modified NaA molecular sieve membrane to realize purification of N, N-dimethylformamide. The modified NaA molecular sieve membrane is obtained through reaction of chitosan and chlorinated 1-hydroxyethyl-3-aminopropyl imidazole, a hydrophilic and solvent-resistant polymer molecular brush structure is formed on the surface of the molecular sieve membrane through modification, and the pervaporation dehydration efficiency of the NaA molecular sieve membrane is enhanced. The process is simple, the operation temperature is low, energy consumption is reduced, meanwhile, high-temperature decomposition of N, N-dimethylformamide is effectively avoided, the purity of N, N-dimethylformamide in the product is larger than or equal to 99.9 wt.%, the water content is smaller than or equal to 50 ppm, and the anhydrous grade standard is met.

Description

Technical Field

[0001] This invention relates to the field of organic solvent purification technology, specifically to a purification process for anhydrous N,N-dimethylformamide. Background Technology

[0002] N,N-Dimethylformamide (DMF) is a common polar aprotic organic solvent. It is colorless and transparent with a slight ammonia odor. Due to its low toxicity, good solubility, and strong physicochemical stability, it is widely used in organic synthesis, pharmaceuticals, electronics, and dyes. With the rise of the biopharmaceutical and electronic communication technologies, the demand for anhydrous N,N-dimethylformamide is increasing daily. Recycling N,N-dimethylformamide wastewater can not only avoid environmental pollution from waste liquid but also effectively reduce production costs.

[0003] Traditional processes for treating N,N-dimethylformamide wastewater primarily rely on distillation for separation and solvent recovery. However, this method suffers from low recovery rates and excessive energy consumption. Molecular sieve membrane pervaporation technology is a highly efficient and energy-saving separation method, mainly achieving mixture separation through pore sieving and adsorption. However, N,N-dimethylformamide, as a strongly polar solvent, tends to accumulate on the surface of the molecular sieve membrane or inside the pores during pervaporation, inhibiting water permeation and resulting in poor separation performance.

[0004] Chinese patent CN201711348483.4 discloses a method for modifying molecular sieve membranes for dehydration separation in highly polar solvents. This method uses a sol-gel method to modify the surface of a NaA molecular sieve membrane by coating it with metal oxides. While this effectively maintains the stability of the permeate flux of the molecular sieve membrane under long-term operation, it leads to a decrease in membrane selectivity. Patent CN202111560500.7 also reports a cation exchange method for modifying molecular sieve membranes; however, the membrane materials studied are mainly T-type and CHA-type molecular sieve membranes with high silicon-to-aluminum ratios. These membranes have lower hydrophilicity than NaA molecular sieve membranes, making it difficult to achieve satisfactory dehydration separation results at low water content. To address these technical problems, there is an urgent need to develop a novel purification process to obtain anhydrous N,N-dimethylformamide. Summary of the Invention

[0005] The purpose of this invention is to provide a purification process for anhydrous N,N-dimethylformamide to solve the problems mentioned in the background art.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a purification process for anhydrous N,N-dimethylformamide, comprising the following steps: Step 1: After preheating the N,N-dimethylformamide wastewater raw material to 40~50℃, it is fed into a vacuum distillation tower to remove low-boiling-point organic impurities and soluble solids, and the remaining material is collected and fed into the first storage tank. Step 2: Cool the material in the first storage tank to 25~30℃ and feed it into the ion exchange tower. After the ion exchange resin adsorbs and removes impurities, feed the liquid material into the second storage tank. Step 3: Heat the material in the second storage tank to 60~80℃ and feed it into the pervaporation membrane separation unit for dehydration treatment.

[0007] Furthermore, in step 1, the water content in the N,N-dimethylformamide wastewater raw material is 10~60 wt.%.

[0008] Furthermore, in step 1, the operating temperature of the vacuum distillation unit is 75~80℃.

[0009] Furthermore, in step 2, the ion exchange resin is a strongly acidic styrene-based cation exchange resin or a strongly basic styrene-based anion exchange resin.

[0010] Furthermore, in step 3, the absolute pressure on the permeate side of the pervaporation membrane separation unit is 100~1000 Pa.

[0011] Furthermore, in step 3, the pervaporation membrane is a modified NaA molecular sieve membrane, and the preparation method of the modified NaA molecular sieve membrane is as follows: S1: Disperse 1-(2-hydroxyethyl)imidazole and 3-chloropropylamine hydrochloride in acetonitrile, stir and reflux at 70-80℃ for 24h under nitrogen protection, cool to 0℃, filter and collect the lower solid layer, wash with anhydrous ether, dry, add sodium hydroxide solution with pH 8-8.5, rotary evaporate, add anhydrous ethanol to remove impurities, rotary evaporate and dry to obtain 1-hydroxyethyl-3-aminopropylimidazole chloride; S2: Dissolve chitosan in a 2-3% (w / w) acetic acid solution to obtain a 1-2% (w / w) chitosan acetic acid solution; add 1-hydroxyethyl-3-aminopropylimidazolium chloride and react at 80-90℃ for 2-3 hours to obtain a modified chitosan solution; seal both ends of a tube-type NaA molecular sieve membrane with polytetrafluoroethylene material, immerse it in the modified chitosan solution for 1-2 hours, and dry it at 40-50℃ to obtain a modified NaA molecular sieve membrane.

[0012] Furthermore, in S1, the molar ratio of 1-(2-hydroxyethyl)imidazole to 3-chloropropylamine hydrochloride is 1:1.

[0013] Furthermore, in S2, the mass ratio of 1-hydroxyethyl-3-aminopropylimidazolium chloride to chitosan is (0.3~0.5):1.

[0014] Compared with existing technologies, the beneficial effects achieved by this invention are as follows: This invention provides a purification process for anhydrous N,N-dimethylformamide. N,N-dimethylformamide is a high-boiling-point organic solvent with excellent performance. This invention first treats N,N-dimethylformamide wastewater using low-temperature vacuum distillation to separate the low-boiling-point impurity liquid organic components and soluble solid components. The remaining components are collected and sent to a first storage tank, cooled, and then fed into an ion exchange resin tower for further treatment. Ion exchange resin has an adsorption effect on formic acid, dimethylamine, salt impurities formed by formic acid and dimethylamine, and metal salt ions in N,N-dimethylformamide wastewater. These impurities are removed by ion exchange. After simple processing such as low-temperature vacuum distillation and ion exchange resin adsorption, damage to the molecular sieve membrane by metal ions and soluble solids in the raw material is effectively avoided, thereby ensuring the efficiency of subsequent pervaporation dehydration.

[0015] The NaA molecular sieve membrane has a silica-to-alumina ratio of 1, and this low ratio results in good hydrophilicity. Therefore, using a NaA molecular sieve membrane can remove water content from N,N-dimethylformamide wastewater to below 0.1 wt.%. To overcome the problems of decreased water flux and reduced separation selectivity that occur when dehydrating NaA molecular sieve membranes in strongly polar solvents, this invention modifies the surface of the NaA molecular sieve membrane with hydrophilic chitosan. Conventional organic modification methods for molecular sieve membrane surfaces, such as modification with siloxanes or sodium alginate, have been proven feasible. However, N,N-dimethylformamide is known as a "universal solvent" and can dissolve most organic matter. After long-term operation, the modified organic matter on the membrane surface will dissolve, meaning that the above modification methods are not applicable to wastewater systems containing N,N-dimethylformamide.

[0016] After comparative screening, researchers selected chitosan, a green and natural material with strong hydrophilicity and insoluble in N,N-dimethylformamide, as the modifying material. After modification, chitosan forms a hydrophilic film on the surface of the NaA molecular sieve membrane. This effectively maintains the membrane material's high selective permeability to water and, to a certain extent, inhibits the adsorption of N,N-dimethylacetamide on the surface or within the pores of the molecular sieve membrane during pervaporation. It also weakens the strong interaction between cations and highly polar solvents in the molecular sieve membrane, improving the stability and high selectivity of membrane flux during pervaporation. Since the pore size of a typical NaA molecular sieve membrane is 0.41 nm, much smaller than that of a chitosan membrane, the modification with chitosan has little impact on the membrane flux.

[0017] To prevent N,N-dimethylformamide molecules from diffusing into the membrane pores, this invention further reacts 1-hydroxyethyl-3-aminopropylimidazolium chloride with the amino groups on chitosan. The resulting modified chitosan forms a hydrophilic layer similar to a polymer molecular brush on the molecular sieve membrane surface. This effectively isolates N,N-dimethylformamide, which has a larger molecular volume (compared to water molecules), and the branched segments of the imidazole compound also act as a "probe," rapidly binding with water molecules in the component and improving the dehydration efficiency of pervaporation. Compared with traditional distillation purification processes, this invention features simpler front-end distillation and ion adsorption processes, and lower operating temperatures and lower energy consumption in the back-end pervaporation dehydration process, avoiding high-temperature decomposition of N,N-dimethylformamide. By modifying the NaA molecular sieve membrane, anhydrous N,N-dimethylformamide was successfully recovered, achieving green recycling of N,N-dimethylformamide waste liquid. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Materials and sources used in this invention: The N,N-dimethylformamide wastewater used in the embodiments and comparative examples of this invention is derived from untreated wastewater from pharmaceutical and chemical plants, containing 30-35% wt.% water, 50-60% N,N-dimethylformamide, and the remainder being other organic matter and impurities such as metal ions. The ion exchange resin is LH-007 strong acid styrene-based cation exchange resin from Jiangsu Linhai Resin Technology Co., Ltd.; the NaA molecular sieve membrane is a tubular membrane from Ningbo Xinyuan Membrane Industry Co., Ltd.

[0020] Example 1: A purification process for anhydrous N,N-dimethylformamide, comprising the following steps: Step 1: After preheating the N,N-dimethylformamide wastewater raw material to 40°C, it is fed into a vacuum distillation tower to remove low-boiling-point organic impurities and soluble solids. The remaining material is then collected and fed into the first storage tank. The vacuum distillation temperature is 75°C. Step 2: Cool the material in the first storage tank to 25°C and feed it into the ion exchange tower. After the ion exchange resin adsorbs and removes impurities, feed the liquid material into the second storage tank. Step 3: After heating the material in the second storage tank to 60°C, it is fed into the pervaporation membrane separation unit for dehydration treatment; in the pervaporation membrane separation unit, the absolute pressure on the permeate side is 100Pa; The pervaporation membrane is a modified NaA molecular sieve membrane, and its preparation method includes the following steps: S1: 1-(2-hydroxyethyl)imidazole and 3-chloropropylamine hydrochloride were dispersed in acetonitrile and stirred under nitrogen protection at 70°C for 24 h. After cooling to 0°C, the lower solid layer was collected by filtration, washed with anhydrous ether, dried, and then sodium hydroxide solution at pH=8 was added. After rotary evaporation, anhydrous ethanol was added to remove impurities. After rotary evaporation and drying, 1-hydroxyethyl-3-aminopropylimidazole chloride was obtained; the molar ratio of 1-(2-hydroxyethyl)imidazole to 3-chloropropylamine hydrochloride was 1:1. S2: Chitosan was dissolved in a 2% acetic acid solution to obtain a 1% chitosan acetic acid solution; 1-hydroxyethyl-3-aminopropylimidazolium chloride was added and reacted at 80℃ for 2 hours to obtain a modified chitosan solution; the two ends of a tube-type NaA molecular sieve membrane were sealed with polytetrafluoroethylene material, immersed in the modified chitosan solution for 1 hour, and dried at 40℃ to obtain a modified NaA molecular sieve membrane; wherein the mass ratio of 1-hydroxyethyl-3-aminopropylimidazolium chloride to chitosan was 0.3:1.

[0021] Example 2: A purification process for anhydrous N,N-dimethylformamide, comprising the following steps: Step 1: After preheating the N,N-dimethylformamide wastewater raw material to 45°C, it is fed into a vacuum distillation tower to remove low-boiling-point organic impurities and soluble solids. The remaining material is then collected and fed into the first storage tank. The vacuum distillation temperature is 78°C. Step 2: Cool the material in the first storage tank to 28°C and feed it into the ion exchange tower. After the ion exchange resin adsorbs and removes impurities, feed the liquid material into the second storage tank. Step 3: After heating the material in the second storage tank to 70°C, it is fed into the pervaporation membrane separation unit for dehydration treatment; in the pervaporation membrane separation unit, the absolute pressure on the permeate side is 500Pa; The pervaporation membrane is a modified NaA molecular sieve membrane, and its preparation method includes the following steps: S1: 1-(2-hydroxyethyl)imidazole and 3-chloropropylamine hydrochloride were dispersed in acetonitrile and stirred under nitrogen protection at 75°C for 24 h. After cooling to 0°C, the lower solid layer was collected by filtration, washed with anhydrous ether, dried, and then sodium hydroxide solution at pH 8.2 was added. After rotary evaporation, anhydrous ethanol was added to remove impurities. After rotary evaporation and drying, 1-hydroxyethyl-3-aminopropylimidazole chloride was obtained. The molar ratio of 1-(2-hydroxyethyl)imidazole to 3-chloropropylamine hydrochloride was 1:1. S2: Chitosan was dissolved in a 2.5% acetic acid solution to obtain a 1.5% chitosan acetic acid solution; 1-hydroxyethyl-3-aminopropylimidazolium chloride was added, and the mixture was reacted at 85°C for 2.5 h to obtain a modified chitosan solution; the ends of a tube-type NaA molecular sieve membrane were sealed with polytetrafluoroethylene material, then immersed in the modified chitosan solution for 1.5 h, and dried at 45°C to obtain a modified NaA molecular sieve membrane; wherein the mass ratio of 1-hydroxyethyl-3-aminopropylimidazolium chloride to chitosan was 0.4:1.

[0022] Example 3: A purification process for anhydrous N,N-dimethylformamide, comprising the following steps: Step 1: After preheating the N,N-dimethylformamide wastewater raw material to 50°C, it is fed into a vacuum distillation tower to remove low-boiling-point organic impurities and soluble solids. The remaining material is then collected and fed into the first storage tank. The vacuum distillation temperature is 80°C. Step 2: Cool the material in the first storage tank to 30°C and feed it into the ion exchange tower. After the ion exchange resin adsorbs and removes impurities, feed the liquid material into the second storage tank. Step 3: After heating the material in the second storage tank to 80°C, it is fed into the pervaporation membrane separation unit for dehydration treatment; in the pervaporation membrane separation unit, the absolute pressure on the permeate side is 1000Pa; The pervaporation membrane is a modified NaA molecular sieve membrane, and its preparation method includes the following steps: S1: 1-(2-hydroxyethyl)imidazole and 3-chloropropylamine hydrochloride were dispersed in acetonitrile and stirred under nitrogen protection at 80°C for 24 h. After cooling to 0°C, the lower solid layer was collected by filtration, washed with anhydrous ether, dried, and then added to a sodium hydroxide solution at pH 8.5. After rotary evaporation, anhydrous ethanol was added to remove impurities. After rotary evaporation and drying, 1-hydroxyethyl-3-aminopropylimidazole chloride was obtained; the molar ratio of 1-(2-hydroxyethyl)imidazole to 3-chloropropylamine hydrochloride was 1:1. S2: Chitosan was dissolved in a 3% acetic acid solution to obtain a 2% chitosan acetic acid solution; 1-hydroxyethyl-3-aminopropylimidazolium chloride was added, and the mixture was reacted at 90°C for 3 hours to obtain a modified chitosan solution; the ends of a tube-type NaA molecular sieve membrane were sealed with polytetrafluoroethylene material, then immersed in the modified chitosan solution for 2 hours and dried at 50°C to obtain a modified NaA molecular sieve membrane; wherein the mass ratio of 1-hydroxyethyl-3-aminopropylimidazolium chloride to chitosan was 0.5:1.

[0023] Comparative Example 1: The NaA molecular sieve membrane was not modified, and the other parameters were the same as in Example 1.

[0024] Step 1: After preheating the N,N-dimethylformamide wastewater raw material to 40°C, it is fed into a vacuum distillation tower to remove low-boiling-point organic impurities and soluble solids. The remaining material is then collected and fed into the first storage tank. The vacuum distillation temperature is 75°C. Step 2: Cool the material in the first storage tank to 25°C and feed it into the ion exchange tower. After the ion exchange resin adsorbs and removes impurities, feed the liquid material into the second storage tank. Step 3: After heating the material in the second storage tank to 60°C, it is fed into the pervaporation membrane separation unit for dehydration treatment; in the pervaporation membrane separation unit, the absolute pressure on the permeate side is 100Pa; The pervaporation membrane is a tubular NaA molecular sieve membrane.

[0025] Comparative Example 2: Silane coupling agent KH550 was used instead of chitosan, and the other parameters were the same as in Example 2.

[0026] Step 1: After preheating the N,N-dimethylformamide wastewater raw material to 45°C, it is fed into a vacuum distillation tower to remove low-boiling-point organic impurities and soluble solids. The remaining material is then collected and fed into the first storage tank. The vacuum distillation temperature is 78°C. Step 2: Cool the material in the first storage tank to 28°C and feed it into the ion exchange tower. After the ion exchange resin adsorbs and removes impurities, feed the liquid material into the second storage tank. Step 3: After heating the material in the second storage tank to 70°C, it is fed into the pervaporation membrane separation unit for dehydration treatment; in the pervaporation membrane separation unit, the absolute pressure on the permeate side is 500Pa; The pervaporation membrane is a modified NaA molecular sieve membrane, and its preparation method includes the following steps: S1: 1-(2-hydroxyethyl)imidazole and 3-chloropropylamine hydrochloride were dispersed in acetonitrile and stirred under nitrogen protection at 75°C for 24 h. After cooling to 0°C, the lower solid layer was collected by filtration, washed with anhydrous ether, dried, and then sodium hydroxide solution at pH 8.2 was added. After rotary evaporation, anhydrous ethanol was added to remove impurities. After rotary evaporation and drying, 1-hydroxyethyl-3-aminopropylimidazole chloride was obtained. The molar ratio of 1-(2-hydroxyethyl)imidazole to 3-chloropropylamine hydrochloride was 1:1. S2: Silane coupling agent KH550 was added to anhydrous ethanol to obtain a coupling agent solution with a mass concentration of 1.5%; 1-hydroxyethyl-3-aminopropylimidazolium chloride was added, and the mixture was reacted at 60℃ for 10 h to obtain an imidazolium-modified siloxane solution; the ends of a tube-type NaA molecular sieve membrane were sealed with polytetrafluoroethylene material, then immersed in the imidazolium-modified siloxane solution for 1.5 h, and dried at 45℃ to obtain a modified NaA molecular sieve membrane; wherein the mass ratio of 1-hydroxyethyl-3-aminopropylimidazolium chloride to chitosan was 0.4:1.

[0027] Comparative Example 3: A distillation process was used instead of a pervaporation process, and the remaining parameters were the same as in Example 3.

[0028] Step 1: After preheating the N,N-dimethylformamide wastewater raw material to 50°C, it is fed into a vacuum distillation tower to remove low-boiling-point organic impurities and soluble solids. The remaining material is then collected and fed into the first storage tank. The vacuum distillation temperature is 80°C. Step 2: Cool the material in the first storage tank to 30°C and feed it into the ion exchange tower. After the ion exchange resin adsorbs and removes impurities, feed the liquid material into the second storage tank. Step 3: The liquid material in the second storage tank is fed into the vacuum distillation column through a vacuum condenser pump to distill N,N-dimethylformamide, separate and collect N,N-dimethylformamide. The vacuum distillation is carried out at an operating temperature of 120°C, an operating pressure of 0.1 MPa and a reflux ratio of 0.39.

[0029] Experiment: The water content and organic matter content of the N,N-dimethylformamide materials purified in Examples 1-3 and Comparative Examples 1-3 were tested, and the results were as follows: Organic matter content detection: Refer to standard GB / T 9722-2023 and use gas chromatography for detection.

[0030] Water content detection: Weigh 10g of sample, use 10mL of methanol as solvent, and determine the water content according to GB / T 606.

[0031] The experimental results are shown in Table 1.

[0032] Conclusions: Data from Examples 1-3 show that, in this invention, the modified NaA molecular sieve membrane can reduce the water content of N,N-dimethylformamide to below 50 ppm after dehydration, achieving anhydrous reagent standards. Data from Example 1 and Comparative Example 1 indicate that modifying the NaA molecular sieve membrane using the method described in this invention and applying it for pervaporation effectively improves the membrane material's dehydration capacity under low water content conditions in highly polar solvents. Data from Example 2 and Comparative Example 2 show that using chitosan to modify the membrane yields better results; the increased organic impurity content in Comparative Example 2 indicates that the siloxane modifier dissolved in the solution. Data from Example 3 and Comparative Example 3 show that, compared to traditional vacuum distillation, the modified NaA molecular sieve membrane used in this invention achieves higher dehydration efficiency through pervaporation, and the significantly increased organic impurity content in Comparative Example 3 indicates that N,N-dimethylformamide decomposed upon heating during the distillation process.

[0033] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A purification process for anhydrous N,N-dimethylformamide, characterized in that: Includes the following steps: Step 1: Preheat the N,N-dimethylformamide wastewater raw material to 40~50℃, and feed it into a vacuum distillation tower to remove low-boiling-point organic impurities and soluble solids. Collect the remaining material and feed it into the first storage tank. Step 2: Cool the material in the first storage tank to 25~30℃ and feed it into the ion exchange tower. After the ion exchange resin adsorbs and removes impurities, feed the liquid material into the second storage tank. Step 3: Heat the material in the second storage tank to 60~80℃ and feed it into the pervaporation membrane separation unit for dehydration treatment to obtain anhydrous N,N-dimethylformamide.

2. The purification process according to claim 1, characterized in that: In step 1, the water content in the N,N-dimethylformamide wastewater raw material is 10~60 wt.%.

3. The purification process according to claim 1, characterized in that: In step 1, the operating temperature of the vacuum distillation unit is 75~80℃.

4. The purification process according to claim 1, characterized in that: In step 2, the ion exchange resin is either a strongly acidic styrene-based cation exchange resin or a strongly basic styrene-based anion exchange resin.

5. The purification process according to claim 1, characterized in that: In step 3, the absolute pressure on the permeate side of the pervaporation membrane separation unit is 100~1000 Pa.

6. The purification process according to claim 1, characterized in that: In step 3, the membrane material is a modified NaA molecular sieve membrane, and the preparation method of the modified NaA molecular sieve membrane is as follows: S1: 1-(2-hydroxyethyl)imidazolium and 3-chloropropylamine hydrochloride were dispersed in acetonitrile, stirred and refluxed at 70-80℃ for 24h under nitrogen protection, cooled to 0℃, filtered to collect the lower solid layer, washed with anhydrous ether, dried, added sodium hydroxide solution, rotary evaporated, added anhydrous ethanol to remove impurities, rotary evaporated and dried to obtain 1-hydroxyethyl-3-aminopropylimidazolium chloride; S2: Dissolve chitosan in acetic acid solution to obtain chitosan acetic acid solution; add 1-hydroxyethyl-3-aminopropylimidazolium chloride and react at 80~90℃ for 2~3h to obtain modified chitosan solution; seal both ends of the tube test NaA molecular sieve membrane with polytetrafluoroethylene material, immerse it in the modified chitosan solution for 1~2h, and dry it at 40~50℃ to obtain modified NaA molecular sieve membrane.

7. The purification process according to claim 6, characterized in that: In S1, the molar ratio of 1-(2-hydroxyethyl)imidazole to 3-chloropropylamine hydrochloride is 1:

1.

8. The purification process according to claim 6, characterized in that: In S1, the pH of the sodium hydroxide solution is 8~8.

5.

9. The purification process according to claim 6, characterized in that: In S1, the mass concentration of the acetic acid solution is 2-3%, and the mass concentration of the chitosan acetic acid solution is 1-2%.

10. The purification process according to claim 6, characterized in that: In S2, the mass ratio of 1-hydroxyethyl-3-aminopropylimidazolium chloride to chitosan is (0.3~0.5):1.

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