Method for efficiently depolymerizing waste PET polyester at low temperature

Abstract

The invention discloses a method for efficiently depolymerizing waste PET polyester at low temperature, which comprises the following steps of: dissolving a cleaned and dried PET material with a good solvent, and distilling to recover the solvent after the PET material is completely dissolved, so as to obtain a PET sample; mixing the treated PET sample with a catalyst, ethylene glycol and a cosolvent, putting the mixture into a pressure-resistant reaction tube, stirring and reacting for a certain time at a specific temperature, and then ending the reaction; carrying out reduced pressure distillation on the reaction liquid to recover the cosolvent; stirring the residual solution in hot water for a certain time, performing hot filtration, washing and drying filter residues with hot water, refrigerating filtrate in a refrigerator, filtering, collecting solids, and drying to obtain the product. The method has the advantages of simplicity in operation, low reaction temperature, high reaction rate, no metal ions and the like, and can realize green and efficient recovery of waste polyester; the method can be applied to the fields of polyester fibers, bottle flakes, films and the like, and has relatively high industrial feasibility and application prospects.

Description

Technical Field

[0001] The present invention relates to a method for depolymerizing waste PET polyester, and more specifically, to a method for depolymerizing waste PET polyester at low temperature and high efficiency, belonging to the field of green recycling of waste polyester. Background Art

[0002] In recent years, plastic pollution has become a growing global concern. Polyethylene terephthalate (PET), one of the most produced plastics, boasts an annual global output exceeding 70 million tons. The massive production and use of PET has led to a rapid increase in the amount of PET waste generated. In China alone, 26 million tons of PET waste is needed for disposal each year. This waste not only causes irreversible damage to the environment but also wastes carbon resources.

[0003] Currently, the most common methods for PET waste are incineration, landfill and recycling. Incineration and landfill not only cause secondary pollution to the environment, but also waste land and carbon resources. Recycling is the most ideal method, which includes physical recycling and chemical recycling. Physical recycling can only be used for waste with less pollution, and the mechanical properties of the product after secondary processing will decrease. Chemical recycling includes alcoholysis, hydrogenolysis, pyrolysis, ammonialysis, enzymatic hydrolysis and hydrolysis. Among them, alcoholysis is considered to have the most promising industrial application prospects due to its green, energy-saving, high safety and high product added value.

[0004] Alcoholysis includes methanolysis and ethylene glycolysis. The methanol method has high equipment requirements, a low safety factor, and requires high temperature and high pressure. Compared to the methanol method, ethylene glycolysis offers advantages such as a high safety factor, simple operation, low equipment requirements, and no need for high pressure, making it more suitable for industrial applications. For example, companies such as DuPont, Goodyear, IO Niqa, and AIEs Co., Ltd. have established 10,000-ton-level ethylene glycol depolymerization production lines. However, current ethylene glycol depolymerization temperatures remain high, with industrial production temperatures exceeding 200°C. This not only increases energy consumption but also makes the product BHET prone to self-polymerization at high temperatures, leading to an increase in oligomer content. Furthermore, existing depolymerization systems struggle to simultaneously meet the requirements of low energy consumption, metal-free operation, high efficiency, and high yield. Most improve one performance aspect at the expense of other requirements.

[0005] For example, patent CN118786115A discloses a method for preparing and recovering bis(2-hydroxyethyl)terephthalate via multi-step depolymerization. Although the resulting product has a high purity, its depolymerization temperature is above 180°C, and the steps are complex. Low depolymerization temperatures often require longer reaction times or complex catalysts. For example, patent document CN114805776A discloses a polyester depolymerization or cyclic ester synthesis catalyst, preparation method, and application. Although it can slightly reduce the temperature, it introduces metal ions. The introduction of metal ions is often detrimental to subsequent repolymerization, which requires catalyst design. For example, patent document CN118684870A discloses a polyester recovery process. Although the designed MOF bimetallic catalyst can be used for both ethylene glycol depolymerization and repolymerization, the required temperature is high, the depolymerization time is long, and the catalyst preparation is complex, making it difficult to industrialize. Therefore, it is necessary to design an industrially feasible, green, efficient, and metal-free catalytic system to truly achieve green and efficient recycling of discarded PET bottles. Summary of the Invention

[0006] To address the above-mentioned problems in the prior art, the present invention aims to address the contamination of waste PET polyester and the high temperatures, long times, and susceptibility to metal ion introduction in existing depolymerization processes. A method for the metal-free, low-temperature catalytic depolymerization of waste PET polyester is disclosed. By using a metal-free organic base as a catalyst and introducing a cosolvent of similar polarity, the ratio of the cosolvent to ethylene glycol is adjusted to effectively lower the reaction temperature without introducing metal ions into the product, while also controlling the formation of oligomers. This method simultaneously meets the requirements of low energy consumption, metal-free operation, high efficiency, and high purity. Furthermore, the differences in boiling points between the cosolvent, ethylene glycol, catalyst, and product enable step-by-step distillation recovery.

[0007] In order to achieve the above object, the present invention is implemented through the following technical solutions:

[0008] The present invention provides a method for low-temperature and high-efficiency depolymerization of waste PET polyester, which comprises the following steps:

[0009] Step 1): dissolving the cleaned and dried PET material in a good solvent, and distilling and recovering the solvent after complete dissolution to obtain a PET sample;

[0010] Step 2): The treated PET sample is mixed with a catalyst, ethylene glycol, and a cosolvent in a pressure-resistant reaction tube, and the reaction is stirred at a specific temperature for a certain time before the reaction is terminated; the mass ratio of the catalyst to the PET sample is 1:100 to 1:5;

[0011] Step 3): Recovering the cosolvent by distillation under reduced pressure from the reaction solution;

[0012] Step 4): The remaining solution is stirred in hot water for a certain period of time, and after hot filtration, the filter residue is washed with hot water and dried. The filtrate is refrigerated and filtered, and the solid is collected and dried to obtain the product.

[0013] Preferably, the PET material is waste PET polyester, and the waste PET polyester is any one of plastic bottles, fibers, and films.

[0014] Preferably, the good solvent used is any one of hexafluoroisopropanol, trifluoroacetic acid, phenol, and tetrachloroethane.

[0015] Preferably, the catalyst in step 2) is an organic base, specifically any one of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 1,8-diazabicycloundec-7-ene (DBU), 4-dimethylaminopyridine (DMAP), 1,5-diazabicyclo[4.3.0]-5-nonene (DBN), N,N-dimethylacetamide (DMAC), 1,4-diazabicyclo[2.2.2]octane (DABCO), and tetraphenylphosphine phenolate.

[0016] Preferably, in step 2), the mass ratio of ethylene glycol to PET sample is 1:1 to 50:1; the mass ratio of cosolvent to PET sample is 1:10 to 10:1.

[0017] Preferably, the cosolvent used in step 2) is one or more of 1-methyl-2-pyrrolidone (NMP), anisole, acetonitrile (ACN), dichloromethane (DCM), dimethyl carbonate (DMC), chloroform, dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF).

[0018] Preferably, the specific temperature in step 2) is 60-150°C.

[0019] Preferably, the certain time in step 2) is 0.5 to 24 hours, and the stirring speed is 400 rpm.

[0020] Preferably, the distillation temperature in step 3) is 40-100°C.

[0021] Preferably, the stirring temperature in step 4) is 60-90° C., the certain time is 1 hour, and the stirring rate is 600 rpm.

[0022] Beneficial effects:

[0023] To address the problem of high reaction temperature in the industrial glycol decomposition process, the reaction temperature can be effectively lowered by introducing a co-solvent and regulating the proportion of the co-solvent; to address the problem of easy generation of oligomers in common ethylene glycol depolymerization processes, the generation of oligomers can be effectively reduced by introducing a co-solvent to lower the reaction temperature; to address the problem of easy introduction of metal ions in common industrial PET polyester depolymerization processes, the introduction of metal ions is avoided by adopting a metal-free organic base as a catalyst; to address the problem of high crystallinity and high density of textiles, their crystallinity and dense structure can be destroyed by solvent dissolution pretreatment; the co-solvent used in the present invention is low in toxicity and easy to recycle, and has extremely little harm to the environment; the solvent and catalyst used in the present invention can be recovered step by step through reduced pressure distillation; the process of the present invention can be used in the fields of bottle flakes, fibers, clothing, films, etc., and has a wide range of applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a diagram of the actual state of PET bottle flakes at different depolymerization times according to the present invention.

[0025] Figure 2 This is the appearance morphology of the product BHET of the present invention.

[0026] Figure 3 1 is a graph showing the change of PET conversion rate and BHET yield with depolymerization time in the present invention.

[0027] Figure 4 It is the DSC curve diagram of the product BHET of the present invention.

[0028] Figure 5 It is the 1H NMR spectrum of BHET, the product of the present invention.

[0029] Figure 6 UPLC chromatogram of commercial BHET and its depolymerization product. DETAILED DESCRIPTION

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. In addition to the following embodiments, the embodiments of the present application also include an embodiment formed by taking the left endpoint value of the entire range value, an embodiment formed by taking the middle value of the entire range value, an embodiment formed by taking the right endpoint value of the entire range value, and an embodiment formed by taking any numerical value in the entire range value.

[0031] The principle / inventiveness of this invention: Using a metal-free organic base as a catalyst, the process first dissolves the PET in a good solvent to disrupt its crystallinity and dense structure. An appropriate cosolvent is then introduced to increase the PET's swelling rate and reduce the activation energy required for the reaction, thereby increasing the depolymerization rate and lowering the reaction temperature. This process offers advantages such as simple operation, low reaction temperature, fast reaction rate, and the absence of metal ions, enabling green and efficient recycling of waste polyester. This process can be applied to polyester fibers, bottle flakes, films, and other fields, demonstrating high industrial feasibility and application prospects.

[0032] Example 1

[0033] Step 1) PET mineral water bottles are used as raw materials, washed and dried, dissolved in HFIP at room temperature and then rotary evaporated.

[0034] Step 2) Remove the solvent, dry, and weigh 1g of the bottle flakes and 80mg of DBU in a pressure-resistant reaction tube. Add 5.8mL of EG and 1.9mL of ACN. Add a stirrer and bubble with N2 for 10 minutes. Then, stir in a 100°C oil bath. React five groups of samples simultaneously, each for 1 to 5 hours, and then cool to room temperature.

[0035] Step 3) The reaction solution was poured into a 25 mL rotary evaporator and subjected to rotary evaporation at 60°C for 10 minutes to remove ACN.

[0036] Step 4) Pour the remaining reaction solution into a beaker, add 100 mL of water and heat to 80° C., and stir at 600 rpm for 1 hour.

[0037] Step 5) Hot filtration is performed using a suction filtration device to separate the solid and liquid. The solid is washed twice with hot water and then dried at 60°C. The liquid is cooled to room temperature and refrigerated for 24 hours at a temperature of 1-4°C. The solid is then collected by filtration and this step is repeated three times for crystallization.

[0038] Step 6) After purification and drying, white crystals are obtained. The reaction state of the sample is shown in the attached Figure 1 As shown in the figure, the PET bottle flakes gradually disappear as the reaction time increases, and the sample becomes a colorless transparent solution. After purification, it becomes a white crystalline powder, as shown in the attached figure. Figure 2 As the reaction time increases, the conversion rate and yield increase, as shown in the attached Figure 3 As shown, the conversion rate can reach 99.9% within 5 hours and the molar yield can reach 82.7%.

[0039] Example 2

[0040] Step 1) PET mineral water bottles were used as raw materials, chopped, washed, and dried, dissolved in HFIP at room temperature, and then the solvent was removed by rotary evaporation. After drying, 1 g of bottle pieces and 80 mg of DBU were weighed and placed in a pressure-resistant reaction tube. 4.35 mL of EG and 1.45 mL of ACN were added. After adding a stirrer, N2 was bubbled for 10 minutes, and then placed in an oil bath at 100°C and stirred for 5 hours. After completion, the reaction was cooled to room temperature.

[0041] Step 2) The reaction solution was poured into a 25 mL rotary evaporator and subjected to rotary evaporation at 60°C for 10 minutes to remove ACN.

[0042] Step 3) Pour the remaining reaction solution into a beaker, add 100 mL of water and heat to 80° C., and stir at 600 rpm for 1 hour.

[0043] Step 4) Hot filtration is performed using a suction filtration device to separate the solid and liquid. The solid is washed twice with hot water and then dried at 60°C. The liquid is cooled to room temperature and refrigerated for 24 hours at a temperature of 1-4°C. The solid is then collected by filtration and this step is repeated three times for crystallization.

[0044] Step 5) The purified sample was dried and weighed to obtain the conversion rate and BHET yield. The results showed that the bottle flakes could achieve nearly complete depolymerization after 5 hours. The conversion rate was found to be 99.5% and the BHET molar yield was 80.2%. This proves that under this catalytic system, PET bottle flakes can achieve low-temperature and efficient depolymerization. The melting point of BHET was tested by DSC, as shown in the attached figure. Figure 4 As shown in Figure 2, the melting point of BHET is 113°C. Figure 5 The 1H NMR spectrum shows that the product is highly pure and contains only a very small amount of dimer. This demonstrates that even at a PET to EG molar ratio of 1 / 15, waste PET bottle flakes can be rapidly depolymerized at 100°C.

[0045] Example 3

[0046] Step 1) PET industrial yarn is used as raw material, chopped, washed and dried, then dissolved with TFA at room temperature and evaporated to remove the solvent. After drying, 1 g of bottle flakes and 80 mg of TBD are weighed and placed in a pressure-resistant reaction tube, 5.0 mL of EG and 2.5 mL of ACN are added, and a stirring bar is added and N2 is bubbled for 10 minutes. Then, the mixture is placed in an oil bath at 90°C and stirred for 6 hours, and then cooled to room temperature.

[0047] Step 2) The reaction solution was poured into a 25 mL rotary evaporator and subjected to rotary evaporation at 60°C for 10 minutes to remove ACN.

[0048] Step 3) Pour the remaining reaction solution into a beaker, add 100 mL of water and heat to 80° C., and stir at 600 rpm for 1 hour.

[0049] Step 4) Hot filtration is performed using a suction filtration device to separate the solid and liquid. The solid is washed twice with hot water and then dried at 60°C. The liquid is cooled to room temperature and refrigerated for 24 hours at a temperature of 1-4°C. The solid is then collected by filtration and this step is repeated three times for crystallization.

[0050] The purified sample in step 5) was dried at 60°C and weighed to determine the conversion rate and BHET yield. The results showed that after 6 hours, the PET industrial yarn achieved a 93.6% conversion rate and a 79.2% BHET yield. This demonstrates that using TFA as a pretreatment agent and TBD as a catalyst, even with a 1 / 2 volume ratio of ACN to EG, PET bottle flakes can be efficiently depolymerized at 90°C.

[0051] Example 4

[0052] Step 1) A textile with a PET content of 80% was used as the raw material, chopped, washed, and dried, then dissolved in HFIP at room temperature and filtered to remove other polymers. The filtrate was rotary evaporated to remove the solvent. After drying, 1 g of the bottle pieces and 80 mg of DBU were weighed and placed in a pressure-resistant reaction tube. 8 mL of EG and 2 mL of DCM were added. After adding a stirrer, N2 was bubbled for 10 minutes, and then placed in an oil bath at 100°C and stirred for 10 hours. After completion, it was cooled to room temperature.

[0053] Step 2) The reaction solution was poured into a 25 mL rotary evaporator and evaporated at 30°C for 10 minutes to remove DCM.

[0054] Step 3) Pour the remaining reaction solution into a beaker, add 100 mL of water and heat to 80° C., and stir at 600 rpm for 1 hour.

[0055] Step 4) Hot filtration is performed using a suction filtration device to separate the solid and liquid. The solid is washed twice with hot water and then dried at 60°C. The liquid is cooled to room temperature and refrigerated for 24 hours at a temperature of 1-4°C. The solid is then collected by filtration and this step is repeated three times for crystallization.

[0056] Step 5) The conversion rate and BHET yield were obtained by weighing. The results showed that the sample could achieve a conversion rate of 99% and a BHET molar yield of 79.7% after 10 hours. It was proved that PET textiles can be efficiently depolymerized at 100°C using HFIP as a pre-treatment agent and DCM as a co-solvent under the catalytic action of DBU. Its purity was tested by ultra-high performance liquid chromatography (UPLC). Figure 6As shown, the product peak position is consistent with the commercial BHET peak position, and the BHET and dimer peaks appear at 1.3 minutes and 1.7 minutes, respectively. After integral calculation, it can be seen that the purity of commercial BHET is 87%, while the purity of the depolymerization product BHET is 95%, further confirming that BHET with higher purity can be obtained by repeated crystallization using water as solvent.

[0057] Example 5

[0058] Step 1) PET civilian silk is used as raw material, chopped, washed and dried, then dissolved with HFIP at room temperature and filtered to remove other polymers, the filtrate is rotary evaporated to remove the solvent, and after drying, 1g of bottle pieces and 80mg of DMAP are weighed and placed in a pressure-resistant reaction tube, 6mL of EG and 2.5mL of anisole are added, a stirring bar is added and N2 is bubbled for 10 minutes, and then placed in an oil bath at 120°C and stirred for 4 hours, and then cooled to room temperature.

[0059] Step 2) The reaction solution was poured into a 20 mL rotary evaporator and distilled under reduced pressure at 80° C. for 20 minutes to remove anisole.

[0060] Step 3) Pour the remaining reaction solution into a beaker, add 100 mL of water and heat to 80° C., and stir at 600 rpm for 1 hour.

[0061] Step 4) Hot filtration is performed using a suction filtration device to separate the solid and liquid. The solid is washed twice with hot water and then dried at 60°C. The liquid is cooled to room temperature and refrigerated for 24 hours at a temperature of 1-4°C. The solid is then collected by filtration and this step is repeated three times for crystallization.

[0062] Step 5) The conversion rate and BHET yield were determined gravimetrically, revealing that 97.8% conversion of PET consumer yarn was achieved after 4 hours. This demonstrates that pre-treating the sample with a good solvent and introducing a cosolvent can also effectively depolymerize PET consumer yarn at low temperatures.

[0063] Comparative Example 1

[0064] Step 1) PET mineral water bottles were used as raw materials, which were chopped, washed, and dried. 1 g of bottle flakes and 80 mg of DBU were weighed and placed in a pressure-resistant reaction tube. 5 mL of EG was added, and a stirring bar was added. N2 was bubbled for 10 minutes, and then the mixture was placed in an oil bath at 100°C and stirred for 8 hours. After the reaction, the mixture was cooled to room temperature.

[0065] Step 2) Pour the remaining reaction solution into a beaker, add 100 mL of water and heat to 80° C., and stir at 600 rpm for 1 hour.

[0066] Step 3) Perform hot filtration using a suction filtration device to separate the solid and liquid. The solid is washed twice with hot water and then dried at 60°C. The liquid is cooled to room temperature and refrigerated for 24 hours at a temperature of 1-4°C. The solid is then collected by filtration and this step is repeated three times for crystallization.

[0067] Step 4) The conversion rate was determined by weighing. The results are shown in Table 1. It was found that the conversion rate of the bottle flakes was only 11.9% after 8 hours. This result proves that PET is difficult to depolymerize without pretreatment or solvent.

[0068] Comparative Example 2

[0069] Step 1) PET mineral water bottles were used as raw materials, chopped, washed, and dried. 1 g of bottle flakes and 80 mg of DBU were weighed and placed in a pressure-resistant reaction tube. 5 mL of EG and 1 mL of ACN were added. A stirring bar was added and N2 was bubbled for 10 minutes. The mixture was then placed in an oil bath at 100°C and stirred for 8 hours. After the reaction was completed, it was cooled to room temperature.

[0070] Step 2) The reaction solution was placed at 60° C. and rotary evaporated for 10 minutes to remove ACN.

[0071] Step 3) Pour the remaining reaction solution into a beaker, add 100 mL of water and heat to 80° C., and stir at 600 rpm for 1 hour.

[0072] Step 4) Hot filtration is performed using a suction filtration device to separate the solid and liquid. The solid is washed twice with hot water and then dried at 60°C. The liquid is cooled to room temperature and refrigerated for 24 hours at a temperature of 1-4°C. The solid is then collected by filtration and this step is repeated three times for crystallization.

[0073] Step 5) The conversion rate was determined by weighing. The results are shown in Table 1. After 8 hours, the conversion rate of PET bottle flakes was only 34.6%. This indicates that acetonitrile can slightly promote the depolymerization of PET, but without pretreatment, PET still cannot depolymerize rapidly at low temperatures.

[0074] Comparative Example 3

[0075] Step 1) PET mineral water bottles were used as raw materials, chopped, washed, and dried, and completely dissolved with HFIP before removing the solvent. 1 g of the treated bottle pieces and 80 mg of DBU were weighed and placed in a pressure-resistant reaction tube. 5 mL of EG was added, and a stirring bar was added. N2 was bubbled for 10 minutes, and then the mixture was placed in an oil bath at 100°C and stirred for 8 hours. After the reaction, it was cooled to room temperature.

[0076] Step 2) Pour the remaining reaction solution into a beaker, add 100 mL of water and heat to 80° C., and stir at 600 rpm for 1 hour.

[0077] Step 3) Perform hot filtration using a suction filtration device to separate the solid and liquid. The solid is washed twice with hot water and then dried at 60°C. The liquid is cooled to room temperature and refrigerated for 24 hours at a temperature of 1-4°C. The solid is then collected by filtration and this step is repeated three times for crystallization.

[0078] Step 4) The conversion rate and BHET yield were obtained by weighing, as shown in Table 1. The results showed that the conversion rate of the bottle flakes was only 25.5% after 8 hours. This experiment demonstrated that pretreatment alone can slightly accelerate the depolymerization rate of PET, but cannot achieve efficient depolymerization.

[0079] Comparative Example 4

[0080] Step 1) PET mineral water bottles were used as raw materials, chopped, washed, and dried, and completely dissolved with HFIP before removing the solvent. 1 g of the treated bottle pieces and 80 mg of DBU were weighed and placed in a pressure-resistant reaction tube. 5 mL of ethylene glycol and 1 mL of n-hexane were added. A stirring bar was added and N2 was bubbled for 10 minutes. The mixture was then placed in an oil bath at 100°C and stirred for 8 hours. After completion, the mixture was cooled to room temperature.

[0081] Step 2) The reaction solution was rotary evaporated at 60° C. for 10 minutes to remove n-hexane.

[0082] Step 3) Pour the remaining reaction solution into a beaker, add 100 mL of water and heat to 80° C., and stir at 600 rpm for 1 hour.

[0083] Step 4) Hot filtration is performed using a suction filtration device to separate the solid and liquid. The solid is washed twice with hot water and then dried at 60°C. The liquid is cooled to room temperature and refrigerated for 24 hours at a temperature of 1-4°C. The solid is then collected by filtration and this step is repeated three times for crystallization.

[0084] Step 5) The conversion rate and BHET yield were obtained by weighing, as shown in Table 1. The results showed that the conversion rate of the bottle flakes was only 28.7% after 8 hours. This experiment proves that n-hexane cannot accelerate the depolymerization of PET.

[0085] Table 1 PET depolymerization behavior of comparative examples 1-4

[0086] Comparative Example raw material Pretreatment cosolvent Conversion rate (%) 1 PET bottle flakes 11.9 2 PET bottle flakes Acetonitrile 34.6 3 PET bottle flakes Hexafluoroisopropanol 25.5 4 PET bottle flakes Hexafluoroisopropanol n-hexane 28.7

[0087] Note: Catalyst 80 mg (DBU); Temperature: 100°C; Time: 8 hours.

[0088] Results show that solvent pretreatment and the introduction of a solvent removal method can achieve a 99% conversion rate and an 83% BHET yield from waste PET polyester at low temperatures of 60-150°C within a short period of time. This method can achieve efficient depolymerization of waste PET bottle flakes at low temperatures and has promising industrial application prospects, potentially alleviating the environmental impact of plastic pollution and enabling carbon resource recovery.

[0089] In summary, the present invention offers advantages such as simple operation, low reaction temperature, fast reaction rate, and the absence of metal ions, enabling green and efficient recycling of waste polyester. This process can be applied to polyester fibers, bottle flakes, films, and other fields, demonstrating high industrial feasibility and promising application prospects.

[0090] Finally, it should be noted that the present invention is not limited to the above embodiments and may be subject to many variations. All variations that can be directly derived or imagined by a person skilled in the art from the disclosure of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A method for depolymerizing waste PET polyester at low temperature and high efficiency, characterized in that The method comprises the following steps: Step 1): dissolving the cleaned and dried PET material in a good solvent, and distilling and recovering the solvent after complete dissolution to obtain a PET sample; Step 2): The treated PET sample is mixed with a catalyst, ethylene glycol, and a cosolvent in a pressure-resistant reaction tube, and the reaction is stirred at a specific temperature for a certain time before the reaction is terminated; the mass ratio of the catalyst to the PET sample is 1:100 to 1:5; Step 3): Recovering the cosolvent by distillation under reduced pressure from the reaction solution; Step 4): The remaining solution is stirred in hot water for a certain period of time, and after hot filtration, the filter residue is washed with hot water and dried. The filtrate is refrigerated and filtered, and the solid is collected and dried to obtain the product.

2. The method for depolymerizing waste PET polyester at low temperature and high efficiency according to claim 1, characterized in that: The PET material is waste PET polyester, which is any one of plastic bottles, fibers, and films.

3. The method for depolymerizing waste PET polyester at low temperature and high efficiency according to claim 1, characterized in that: The good solvent used is any one of hexafluoroisopropanol, trifluoroacetic acid, phenol and tetrachloroethane.

4. The method for depolymerizing waste PET polyester at low temperature and high efficiency according to claim 1, characterized in that: In step 2), the catalyst is an organic base, specifically any one of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 1,8-diazabicycloundec-7-ene (DBU), 4-dimethylaminopyridine (DMAP), 1,5-diazabicyclo[4.3.0]-5-nonene (DBN), N,N-dimethylacetamide (DMAC), 1,4-diazabicyclo[2.2.2]octane (DABCO), and tetraphenylphosphine phenolate.

5. The method for depolymerizing waste PET polyester at low temperature and high efficiency according to claim 1, characterized in that: In step 2), the mass ratio of ethylene glycol to the PET sample is 1:1 to 50:1; the mass ratio of the cosolvent to the PET sample is 1:10 to 10:

1.

6. The method for depolymerizing waste PET polyester at low temperature and high efficiency according to claim 1, characterized in that: The cosolvent used in step 2) is one or more of 1-methyl-2-pyrrolidone (NMP), anisole, acetonitrile (ACN), dichloromethane (DCM), dimethyl carbonate (DMC), chloroform, dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF).

7. The method for depolymerizing waste PET polyester at low temperature and high efficiency according to claim 1, characterized in that: The specific temperature in step 2) is 60-150°C.

8. The method for depolymerizing waste PET polyester at low temperature and high efficiency according to claim 1, characterized in that: The certain time mentioned in step 2) is 0.5 to 24 hours.

9. The method for depolymerizing waste PET polyester at low temperature and high efficiency according to claim 1, characterized in that: The distillation temperature in step 3) is 40-100°C.

10. The method for low-temperature and high-efficiency depolymerization of waste PET polyester according to claim 1, characterized in that: In step 4), the stirring temperature is 60-90° C., the certain time is 1 hour, and the stirring rate is 600 rpm.

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