Recovery process of waste polyethylene glycol terephthalate

Abstract

The invention discloses a recovery process of waste PET (polyethylene glycol terephthalate), which comprises the following steps: dissolving waste PET by using a good solvent, dropwise adding into cold methanol, precipitating, filtering, separating, drying, and hydrolyzing the waste PET by using a low-concentration sodium hydroxide aqueous solution, so that the PET can be effectively hydrolyzed at a relatively low temperature (25-40 DEG C), the reaction conditions are mild, the reaction time is short, the yield is high, and the recovery process is suitable for industrial production. The yield of the product TPA is up to 99%, the product TPA is easy to separate, and the solvent can be recycled.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of green / chemical recycling technology and polymer plastic degradation, and particularly relates to a mild hydrolysis recycling process for waste polyethylene terephthalate. BACKGROUND

[0002] Since the invention of celluloid as the first synthetic plastic in 1869, plastics have become one of the most rapidly growing materials in human history, with an estimated 830 million tons of virgin plastics produced, of which only about 9% have been recycled. Non-degradable plastics accumulate in the environment, and the amount of plastics present in some ecosystems is even comparable to the amount of natural organic carbon. At present, the global annual production of plastics is about 400 million tons, accounting for 9% of global oil consumption and 4.5% of greenhouse gas emissions. The continuous consumption of non-renewable resources and the lack of plastic waste management have led to huge economic waste and environmental pressure.

[0003] Polyethylene terephthalate (PET) is the most widely used thermoplastic polyester material in the world, with excellent mechanical strength, thermal stability, gas barrier properties, and processability. It is widely used in packaging, textiles, and engineering plastics. According to industry statistics, global PET production exceeded 80 million tons in 2022, with an average annual growth rate of 4.5%. However, the traditional economic model has led to about 60% of PET products being directly disposed of in landfills or the natural environment after use, with a natural degradation period of 300-450 years, causing serious microplastic pollution and ecological toxicity problems, and resulting in the loss of more than $100 billion in resource value annually. Traditional PET recycling methods such as incineration and landfill pose a threat to the environment and health.

[0004] The current PET recycling technology system mainly includes three levels: (1) Primary mechanical recycling reduces material performance by melt reprocessing; (2) Biological recycling relies on the selective depolymerization of PET hydrolase under mild conditions, but the enzymatic efficiency of high crystallinity (>40%) bottle-grade PET is less than 0.5 mg / cm² / h; (3) Chemical recycling realizes molecular-level recycling through depolymerization-polymerization, mainly including hydrolysis, ammonolysis / aminolysis, pyrolysis, methanolysis, and ethylene glycolysis, and is considered to be the most sustainable development potential technology path. Among them, the hydrolysis method has become the focus of research in the field of chemical recycling due to its technical advantages of using water as a reaction medium and directly regenerating terephthalic acid (TPA) monomers.

[0005] Current research shows that aqueous sodium hydroxide solution can achieve PET degradation and recovery under certain temperature conditions, and the product is easy to separate, which is conducive to industrial application. However, the hydrolysis method generally requires high temperature or external pressure, which is not conducive to reducing the cost of recycling and is not economically feasible. SUMMARY

[0006] The application provides a recycling process for waste polyethylene terephthalate, which greatly reduces the temperature required for hydrolysis by destroying the compact structure of PET through solvent treatment, and regulates the crystalline structure of waste PET through dissolution / reprecipitation, and is applied to the hydrolysis and recovery of waste PET at room temperature and normal pressure.

[0007] To solve the above technical problems, the application discloses a recycling process for waste polyethylene terephthalate, characterized in that it comprises the following steps: (1) dissolving waste PET in a good solvent to obtain a mixed solution; (2) slowly dropping the mixed solution obtained in step (1) into sufficient cold methanol (commercially available, refrigerator cold storage temperature 0-5 degrees Celsius), and obtaining a suspension after fully stirring and standing; (3) performing solid-liquid separation on the suspension obtained in step (2), and obtaining D-PET by washing and vacuum drying the obtained solid, wherein D-PET represents loose PET after treatment; (4) mixing the D-PET obtained in step (3) with an aqueous sodium hydroxide solution and reacting until the solution is clear; (5) adding hydrochloric acid to the clear solution obtained in step (4) under stirring to obtain a white emulsion; (6) performing solid-liquid separation on the emulsion obtained in step (5), and obtaining terephthalic acid (TPA) by washing and vacuum drying the obtained solid part several times, specifically using ethanol for washing.

[0008] Specifically, in step (1), the good solvent is any one of trifluoroacetic acid (TFA), hexafluoroisopropanol (HFIP), or a mixed solvent of tetrachloroethane / phenol, wherein when the good solvent is the mixed solvent of tetrachloroethane / phenol, the two are mixed in a mass ratio of 1:1. Preferably, the good solvent is hexafluoroisopropanol.

[0009] In step (1), magnetic stirring is used to make PET completely dissolve in the good solvent, and the mass concentration of PET in the mixed solution is not more than 30%.

[0010] Preferably, in step (1), the good solvent is hexafluoroisopropanol, and the mass concentration of PET in the mixed solution is 10%-15%.

[0011] In step (2), the amount of cold methanol used is 3 to 5 times the volume of the mixed solution, and the mixed solution is added to the cold methanol at a rate of 0.2 mL / min to 1.0 mL / min.

[0012] Specifically, in step (2), after the mixed solution is added to cold methanol, it is stirred thoroughly for 30 to 60 minutes and then left to stand for more than 3 hours.

[0013] Preferably, in step (3), solid-liquid separation is performed by centrifugation; the solid part is washed with anhydrous ethanol; the vacuum drying temperature is 70~90℃ and the drying time is 24~48h; the liquid part is separated and recovered by vacuum distillation.

[0014] Preferably, in step (4), the concentration of the sodium hydroxide aqueous solution is 1~2 mol / L, and it is added according to a stoichiometric ratio of 1:1.1~1:1.2. The reaction temperature is 15-25℃, and the reaction time is 3-5h. Increasing the temperature can shorten the reaction time.

[0015] In step (5), the concentration of hydrochloric acid used is 2~3 mol / L, and the pH of the solution is adjusted to 2~3.

[0016] Preferably, in step (6), solid-liquid separation is performed by centrifugation; the solid fraction is washed with anhydrous ethanol; and the vacuum drying temperature is 70~90℃, and the drying time is 24~48h. Beneficial effects: This invention uses a suitable solvent to dissolve and reprecipitate waste PET, thereby destroying the crystalline structure of PET and greatly reducing the temperature required for hydrolysis. It can effectively hydrolyze PET, with mild reaction conditions, high product yield, and TPA yield as high as 99%, showing potential application prospects. Attached Figure Description

[0017] Figure 1 XRD patterns of waste PET treated with PET dissolution / reprecipitation at different mass concentrations; Figure 2 DSC diagrams of waste PET treated with PET dissolution / reprecipitation at different mass concentrations; Figure 3 Hydrolysis kinetics and thermodynamic fitting of waste PET for dissolution / reprecipitation treatment of PET with different mass concentrations; Figure 4 The image shows the 1H NMR spectrum of the product TPA. Figure 5 A comparison chart showing the degradation capacity of virgin PET and waste PET treated by dissolution / reprecipitation. Figure 6 The relationship between crystallinity and degradation thermokinetic parameters of waste PET treated with different mass concentrations of dissolution / reprecipitation. Detailed Implementation

[0018] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, and the advantages of the present invention in the above and / or other aspects will become clearer.

[0019] In the following examples, discarded polyethylene terephthalate (PET) that had undergone dissolution / reprecipitation treatment was mixed with an aqueous sodium hydroxide solution and magnetically stirred. After the reaction was complete, insoluble matter was removed by filtration. Disodium terephthalate was retained in the filtrate, and the product was qualitatively and quantitatively analyzed using a UV-Vis spectrophotometer. Hydrochloric acid was added to the reaction system to adjust the pH to approximately 3, the insoluble matter was separated by centrifugation, repeatedly washed with anhydrous ethanol, and vacuum dried to obtain terephthalic acid (TPA).

[0020] The conversion rate of PET and the yield of TPA are calculated according to formulas (1) and (2), respectively. (1) (2) m PET This represents the initial weight of PET. m NA This represents the weight of undegraded PET. M PET Represents the relative molecular mass of one cycle unit of PET. M TPA Represents the relative molecular mass of TPA. m TPA This represents the quality of the obtained TPA.

[0021] Example 1

[0022] First, waste polyethylene terephthalate (PET) was dissolved in a good solvent (a mixture of trifluoroacetic acid (TFA), hexafluoroisopropanol (HFIP), and tetrachloroethane / phenol (T-phOH)) to maintain a PET mass concentration of 10%. Then, the mixed solution was slowly added to three times its volume of cold methanol at a constant rate of 1 mL / min, and stirred at room temperature for 30 min. After stirring, the mixture was allowed to stand for 3 h, centrifuged to separate the precipitate, and the dissolved and reprecipitated polyethylene terephthalate was obtained. This was washed with anhydrous ethanol and dried in a vacuum drying oven at 80 °C for 24 h to obtain D-PET 10% (representing D-PET obtained after treatment at a PET mass concentration of 10%). 1.92 g of each of the D-PET obtained after treatment with different solvents was weighed and mixed with 11 mL of 1 mol / L NaOH at room temperature (20–25 °C) for 7 days. Samples were taken at 12-h intervals to analyze the TPA yield.

[0023] 1.92 g of untreated PET raw material was weighed and mixed with 11 mL of 1 mol / L NaOH. The mixture was reacted at room temperature (20-25℃) for 7 days, and samples were taken at 12-hour intervals to analyze the TPA yield. Figure 5 It is evident that untreated PET showed almost no mass loss, and no TPA was detected after the reaction. In contrast, treated PET achieved a maximum mass loss rate of 92% and a TPA yield of 85% after reaction at room temperature. This indicates that dissolution / reprecipitation treatment significantly enhances the reactivity of PET, enabling it to degrade to monomers under mild conditions.

[0024] Example 2

[0025] First, waste polyethylene terephthalate (PET) was dissolved in a good solvent (a mixture of trifluoroacetic acid (TFA), hexafluoroisopropanol (HFIP), and tetrachloroethane / phenol (T-phOH)) to maintain a PET mass concentration of 10%. Then, the mixed solution was slowly added to three volumes of cold methanol at a constant rate of 1 mL / min, and stirred at room temperature for 30 min. After stirring, the mixture was allowed to stand for 3 h, and the precipitated solid was separated by centrifugation, washed with anhydrous ethanol, and dried in a vacuum drying oven at 80 °C for 24 h to obtain PET treated with different solvents, labeled as TFA-PET, HFIP-PET, and T-phOH-PET, respectively. 1.92 g of each of the obtained TFA-PET, HFIP-PET, and T-phOH-PET were weighed and mixed with 11 mL of 1 mol / L NaOH at room temperature for reaction. TFA-PET and HFIP-PET both achieved TPA yields of over 83% at room temperature, while T-phOH-PET had a relatively slower reaction rate, requiring 12 days to achieve a 64% TPA yield.

[0026] Experimental results show that PET treated with different solvents can undergo hydrolysis with sodium hydroxide aqueous solution at room temperature. However, TFA is relatively more toxic, and the tetrachloroethane / phenol mixed solvent requires 80°C to dissolve PET and has a slow hydrolysis rate. Therefore, HFIP is the preferred solvent for treating waste PET.

[0027] Example 3

[0028] In this embodiment, HFIP was used as a solvent to dissolve waste PET with mass concentrations of 10%, 15%, 20%, and 25%. All PET precipitates were formed by precipitating the PET solution with 10 times its volume of cold methanol. After washing three times with anhydrous ethanol, the precipitates were dried in a vacuum drying oven at 80°C for 24 hours.

[0029] XRD analysis was performed on the dissolved / reprecipitated PET samples, and the results were compared with those of untreated PET samples. Figure 1As shown, untreated PET is a typical semi-crystalline polyester, and its peak positions match those on the PDF card. After HFIP dissolution and cold methanol precipitation, the XRD peaks of PET became blurred and broadened, indicating that the crystalline regions of PET were significantly disrupted.

[0030] The dissolved / reprecipitated PET samples were subjected to DSC analysis and compared with untreated PET samples. Figure 2 As shown, the crystallization enthalpy of untreated PET is 59.21 J / g. However, after dissolution with HFIP and precipitation with cold methanol, the DSC of PET shows a significant broadening of the melting range, and the melting enthalpy decreases to varying degrees. This also indicates that the crystallization of PET is disrupted. Furthermore, the quantification of the co-melting enthalpy suggests that the lower the mass concentration of the PET solution, the lower the crystallization enthalpy of its redeposition products and the more thorough the crystallization disruption.

[0031] Example 4

[0032] In this embodiment, HFIP was used as a solvent to dissolve waste PET with mass concentrations of 10%, 15%, 20%, and 25%. All PET were precipitated with excess cold methanol, washed three times with anhydrous ethanol, and then dried in a vacuum drying oven at 80°C for 24 hours.

[0033] 1.92 g of treated PET was weighed and added to 11 mL of 1 mol / L NaOH for mixing and reaction. The reaction temperature was increased to 85 °C, and samples were taken at 5-minute intervals to calculate the TPA yield until the reaction reached equilibrium. The reaction time and TPA yield were fitted using a nucleation kinetic model to obtain the hydrolysis kinetics curve of the dissolved / reprecipitated waste PET. Figure 3 As shown, the higher the mass concentration of the PET solution, the slower the hydrolysis rate of its redeposition products. This indicates that crystallization hinders the hydrolysis process of PET.

[0034] 1.92 g of treated PET was weighed and added to 11 mL of 1 mol / L NaOH for mixing and reaction. The reaction temperature was increased to 70℃, 75℃, 80℃, 85℃, and 90℃, with samples taken at 5-minute intervals to calculate the TPA yield until the reaction reached equilibrium. The hydrolysis kinetic constants of waste PET at different temperatures were calculated, and the hydrolysis thermodynamic curves of waste PET were obtained through linear fitting. Figure 3 As shown, the higher the mass concentration of the PET solution, the higher the activation energy of the hydrolysis of its redeposition products. The highest activation energy is 82.74 kJ / mol for 25% PET, and the lowest is 63.08 kJ / mol for 10% PET. This indicates that the reaction of treated waste PET in sodium hydroxide aqueous solution has an activation energy similar to that of transesterification (40-100 kJ / mol), and the reaction can be carried out at room temperature and pressure.

[0035] likeFigure 6 As shown, XRD peak fitting calculations revealed different crystallinities in PET at different mass concentrations. Specifically, the hydrolysis activation energy of PET was positively correlated with its crystallinity, while the hydrolysis kinetic coefficient was negatively correlated. This indicates a significant difference in the degradation capacity of D-PET with different crystallinities. Its alkaline hydrolysis capacity can be further controlled by adjusting the crystallinity of D-PET.

[0036] Example 5

[0037] In this embodiment, HFIP was used as a solvent to dissolve waste PET, with a PET mass concentration of 25%. The HFIP-PET-25% solution was then subjected to alkaline hydrolysis for 180 minutes.

[0038] A 25% (w / w) PET solution was prepared using HFIP as a solvent. The precipitate was precipitated by adding 10 times the volume of cold methanol to the PET solution. The precipitate was filtered to obtain insoluble matter, which was washed three times with anhydrous ethanol and then dried in a vacuum drying oven at 80°C for 24 hours to prepare HFIP-PET-25%. The filtrate was placed in a rotary evaporator and HFIP was separated and recovered under conditions of 55°C water bath and 0.1 mPa.

[0039] 1.92 g of HFIP-PET-25% and 11 mL of 1 mol / L NaOH were weighed and placed into a 25 mL reaction vessel. The reaction vessel was then placed on a platform equipped with a magnetic stirrer and the hydrolysis reaction was continued at room temperature (25°C) for 180 min. After the reaction was completed, the mixture was filtered. The filter paper contained unreacted PET and a small amount of oligomers, and the filtrate contained the product disodium terephthalate. The unreacted PET and the small amount of oligomers were washed three times with anhydrous ethanol and dried in a vacuum drying oven at 80°C for 24 hours. The conversion rate of PET was calculated to be 92.3%. The pH of the filtrate was adjusted to 3 with hydrochloric acid, and the white insoluble substance obtained by centrifugation was the product TPA. This substance was washed three times with anhydrous ethanol and dried in a vacuum drying oven at 80°C for 24 hours. Under these conditions, the TPA yield was 74.98%. The dried TPA was dissolved in deuterated DMSO and analyzed by 1H NMR. The 1H NMR spectrum confirmed that the product was indeed terephthalic acid (TPA) with a purity higher than 99%.

[0040] Example 6

[0041] In this embodiment, HFIP was used as a solvent to dissolve waste PET, with a PET mass concentration of 20%. HFIP-PET-20% was then subjected to alkaline hydrolysis for 180 min.

[0042] A 20% (w / w) PET solution was prepared using HFIP as a solvent. The precipitate was precipitated by adding 10 times the volume of cold methanol to the PET solution. The precipitate was filtered to obtain insoluble matter, which was washed three times with anhydrous ethanol and then dried in a vacuum drying oven at 80°C for 24 hours to prepare HFIP-PET-20%. The filtrate was placed in a rotary evaporator and HFIP was separated and recovered under conditions of 55°C water bath and 0.1 mPa.

[0043] 1.92 g of HFIP-PET-20% and 11 mL of 1 mol / L NaOH were weighed and placed into a 25 mL reaction vessel. The reaction vessel was then placed on a platform equipped with a magnetic stirrer and the hydrolysis reaction was continued at room temperature (25°C) for 180 min. After the reaction was completed, the mixture was filtered. The filter paper contained unreacted PET and a small amount of oligomers, and the filtrate contained the product disodium terephthalate. The unreacted PET and the small amount of oligomers were washed three times with anhydrous ethanol and dried in a vacuum drying oven at 80°C for 24 hours. The conversion rate of PET was calculated to be 97.4%. The pH of the filtrate was adjusted to 3 with hydrochloric acid, and the white insoluble substance obtained by centrifugation was the product TPA. This substance was washed three times with anhydrous ethanol and dried in a vacuum drying oven at 80°C for 24 hours. Under these conditions, the TPA yield was 88.86%. The dried TPA was dissolved in deuterated DMSO and analyzed by 1H NMR. The 1H NMR spectrum confirmed that the product was indeed terephthalic acid (TPA) with a purity higher than 99%.

[0044] Example 7

[0045] In this embodiment, HFIP was used as a solvent to dissolve waste PET, with a PET mass concentration of 15%. The HFIP-PET-15% solution was then subjected to alkaline hydrolysis for 180 minutes.

[0046] A 15% (w / w) PET solution was prepared using HFIP as a solvent. The precipitate was precipitated by adding 10 times the volume of cold methanol to the PET solution. The precipitate was filtered to obtain insoluble matter, which was washed three times with anhydrous ethanol and then dried in a vacuum drying oven at 80°C for 24 hours to prepare HFIP-PET-15%. The filtrate was placed in a rotary evaporator and HFIP was separated and recovered under conditions of 55°C water bath and 0.1 mPa.

[0047] 1.92 g of HFIP-PET-15% and 11 mL of 1 mol / L NaOH were weighed and placed into a 25 mL reaction vessel. The reaction vessel was then placed on a platform equipped with a magnetic stirrer and reacted at room temperature (25°C) for 180 min. After the reaction, the mixture was filtered. The filter paper contained unreacted PET and a small amount of oligomers, and the filtrate contained the product disodium terephthalate. The unreacted PET and the small amount of oligomers were washed three times with anhydrous ethanol and dried in a vacuum drying oven at 80°C for 24 hours. The conversion rate of PET was calculated to be 99.3%. The pH of the filtrate was adjusted to 3 with hydrochloric acid, and the white insoluble residue obtained by centrifugation was the product TPA. This residue was washed three times with anhydrous ethanol and dried in a vacuum drying oven at 80°C for 24 hours. Under these conditions, the TPA yield was 92.03%. The dried TPA was dissolved in deuterated DMSO and analyzed by 1H NMR. The 1H NMR spectrum confirmed that the product was indeed terephthalic acid (TPA) with a purity higher than 99%.

[0048] Example 8

[0049] In this embodiment, HFIP was used as a solvent to dissolve waste PET, with a PET mass concentration of 10%. The HFIP-PET-10% solution was then subjected to alkaline hydrolysis for 180 minutes.

[0050] A 10% (w / w) PET solution was prepared using HFIP as a solvent. The precipitate was precipitated by adding 10 times the volume of cold methanol to the PET solution. The precipitate was filtered to obtain insoluble matter, which was washed three times with anhydrous ethanol and then dried in a vacuum drying oven at 80°C for 24 hours to prepare HFIP-PET-10%. The filtrate was placed in a rotary evaporator and HFIP was separated and recovered under conditions of 55°C water bath and 0.1 mPa.

[0051] 1.92 g of HFIP-PET-10% and 11 mL of 1 mol / L NaOH were weighed and placed into a 25 mL reaction vessel. The reaction vessel was then placed on a platform equipped with a magnetic stirrer and the hydrolysis reaction was continued at room temperature (25°C) for 180 min. After the reaction was completed, the mixture was filtered. The filter paper contained unreacted PET and a small amount of oligomers, and the filtrate contained the product disodium terephthalate. The unreacted PET and the small amount of oligomers were washed three times with anhydrous ethanol and dried in a vacuum drying oven at 80°C for 24 hours. The conversion rate of PET was calculated to be 99.4%. The pH of the filtrate was adjusted to 3 with hydrochloric acid, and the white insoluble substance obtained by centrifugation was the product TPA. This substance was washed three times with anhydrous ethanol and dried in a vacuum drying oven at 80°C for 24 hours. Under these conditions, the TPA yield was 96.67%. The dried TPA was dissolved in deuterated DMSO and analyzed by 1H NMR. The 1H NMR spectrum confirmed that the product was indeed terephthalic acid (TPA) with a purity higher than 99%.

[0052] This invention provides a method and approach for the mild chemical recycling of waste PET through dissolution / reprecipitation. Many methods and approaches exist for implementing this technical solution; the above description is merely a preferred embodiment. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications should also be considered within the scope of protection of this invention. All components not explicitly stated in this embodiment can be implemented using existing technologies.

Claims

1. A recycling process for waste polyethylene terephthalate, characterized in that, Includes the following steps: (1) Dissolve waste PET in a good solvent to obtain a mixed solution; (2) Add the mixed solution obtained in step (1) to a sufficient amount of cold methanol, stir thoroughly and let stand to obtain a suspension; (3) The suspension obtained in step (2) is subjected to solid-liquid separation. The obtained solid is washed and vacuum dried to obtain the treated PET, which is denoted as D-PET. (4) Mix the D-PET obtained in step (3) with an aqueous solution of sodium hydroxide and react until the solution is clear; (5) Acidify the clear solution obtained in step (4) with hydrochloric acid under stirring to obtain a white emulsion; (6) The emulsion obtained in step (5) is subjected to solid-liquid separation. The solid part is washed and vacuum dried multiple times to obtain terephthalic acid.

2. The recycling process according to claim 1, characterized in that, In step (1), the good solvent is any one of trifluoroacetic acid, hexafluoroisopropanol, tetrachloroethane / phenol mixed solvent, etc.

3. The recycling process according to claim 1, characterized in that, In step (1), magnetic stirring is used to completely dissolve PET in a good solvent, and the mass concentration of PET in the mixed solution does not exceed 30%.

4. The recycling process according to claim 1, characterized in that, In step (2), the amount of cold methanol used is 9 to 10 times the volume of the mixed solution, and the mixed solution is added to the cold methanol at a rate of 0.2 mL / min to 1.0 mL / min.

5. The recycling process according to claim 1, characterized in that, In step (2), after the mixed solution is added to cold methanol, it is stirred thoroughly for 30-60 minutes and then left to stand for more than 3 hours.

6. The recycling process according to claim 1, characterized in that, In step (3), solid-liquid separation is carried out by centrifugation; the solid part is washed multiple times with anhydrous ethanol; the vacuum drying temperature is 70~90℃ and the drying time is 24~48h; the liquid part is separated and recovered by vacuum distillation.

7. The recycling process according to claim 1, characterized in that, In step (4), the concentration of the sodium hydroxide aqueous solution is 1~2 mol / L, and it is added according to a stoichiometric ratio of 1:1.1~1:1.

2. The reaction temperature is 15-25℃ and the reaction time is 3-5h.

8. The recycling process according to claim 1, characterized in that, In step (5), the concentration of hydrochloric acid used is 2~3 mol / L, and the pH of the solution is adjusted to 2~3.

9. The method for dissolving / reprecipitating and gently chemically recovering PET according to claim 1, characterized in that, In step (6), solid-liquid separation is performed by centrifugation; the solid part is washed with anhydrous ethanol; the vacuum drying temperature is 70~90℃ and the drying time is 24~48h.

Images