A method for recovering waste polyester by methanolysis through acid-base synergistic catalysis

Abstract

The present invention discloses a method for recycling waste polyester by methanol decomposition with acid-base synergistic catalysis, comprising the following steps: adding polyester waste plastic, an acidic auxiliary agent and an alkaline catalyst to methanol, carrying out a depolymerization reaction to obtain a dicarboxylic acid ester, wherein the acidic auxiliary agent is a phenolic substance, and the alkaline catalyst is a strong base, acetate, phosphate, carbonate, hypochlorite, metaaluminate, alcohol salt or phenol salt. The acid-base synergistic catalytic waste polyester directional conversion method of the present invention successfully achieves the rapid and mild degradation of polyester waste plastic. This method not only improves the degradation efficiency, but also can obtain high-value-added chemicals, and has the advantages of low pollution, strong controllability, green and cheap catalysts, and low cost investment. In addition, the reaction system of the method of the present invention is close to neutral, has low corrosion to equipment, is safe and environmentally friendly, and has low energy consumption, providing a new idea for efficient recycling of waste polyester by methanol decomposition, and has extremely high industrial application value.

Description

Technical Field

[0001] The invention relates to the field of comprehensive utilization of waste resources, and in particular to a method for recovering waste polyester by methanolysis catalyzed by acid and base synergy. Background Art

[0002] Plastics, thanks to their lightweight and corrosion-resistant properties, have become widely used in modern society. According to statistics, global plastic production reached 450 million tons in 2020 and continues to increase annually. However, the widespread use of plastics also poses serious environmental challenges. A large number of plastic products are used as single-use items and subsequently discarded in the environment, accumulating in various ecosystems and causing so-called "white pollution." Furthermore, over 90% of plastics currently produced are derived from fossil fuels, which are limited and non-renewable. Due to insufficient plastic recycling and reuse, it is estimated that the economic losses caused by plastic waste exceed $80 billion annually. Therefore, developing appropriate methods for the resource utilization of plastics to achieve a win-win situation in terms of both environmental and economic benefits has become an urgent task.

[0003] Polyethylene terephthalate (PET) is one of the most widely used plastics, accounting for approximately 11% of the world's annual plastic production. Currently, the main recycling methods for waste PET plastic include mechanical recycling, energy recovery, and chemical recycling. The first two methods often result in resource waste, are inconsistent with sustainable development and the dual carbon goals, and limit PET recycling capacity. In contrast, chemical recycling can effectively reduce the consumption of petroleum raw materials while producing high-value-added materials. It is considered the most ideal recycling process and is the focus of current research.

[0004] Chemical recycling of PET primarily involves five degradation pathways: hydrolysis, glycolysis, alcoholysis, aminolysis, and ammonialysis. Methanolysis offers significant advantages in processing low-quality PET bottles. It not only handles low-quality raw materials, but also makes its product, ethylene terephthalate (DMT), easier to separate and purify. Consequently, methanolysis has become the subject of extensive research and large-scale application.

[0005] In the prior art, there are several patents on the methanolysis of PET. For example, patent document CN110938231A discloses a method for the methanolysis of waste PET catalyzed by a deep eutectic solvent. This method uses a self-synthesized urea-based deep eutectic solvent as a catalyst and methanol as a solvent to alcoholyze PET, obtaining a high yield of DMT. Patent document CN113735705A discloses a method for the methanolysis of waste PET catalyzed by a polyionic liquid. This method uses a polyionic liquid (such as PIL-Zn) to catalyze the methanolysis of waste PET. 2+ 、PIL-Co 2+ 、PIL-Mn 2+etc.) to catalyze methanolysis of waste PET polyester.

[0006] While the aforementioned alcoholysis catalytic systems can achieve complete degradation of PET, they still have some drawbacks: some catalysts are expensive and may cause environmental pollution; the reaction conditions are harsh and energy-intensive, typically requiring reaction temperatures above 170°C, making it difficult to obtain high yields of DMT under mild conditions; and the use of cosolvents in some methods may introduce additional environmental and safety risks. Therefore, there is an urgent need to develop a method that can efficiently depolymerize waste polyester under mild methanolysis conditions and obtain high-yield, high-value products. This approach would not only help reduce environmental pollution and production costs but would also significantly promote the upgrading and conversion of waste polyester, providing new technical support for the sustainable utilization of plastics. Summary of the Invention

[0007] To address the bottlenecks of traditional PET methanolysis, such as harsh reaction conditions, high catalyst costs, and environmental pollution, this paper proposes a method for recovering waste polyester by methanolysis using acid-base synergistic catalysis. This method rapidly degrades waste PET under mild conditions to produce the high-value chemical DMT. This method has a simple process flow and high economic benefits, enabling the high-value recovery of waste polyester plastics.

[0008] A method for recovering waste polyester by methanolysis through acid-base synergistic catalysis, comprising the following steps:

[0009] Polyester waste plastics, an acidic auxiliary agent and an alkaline catalyst are added to methanol to carry out a depolymerization reaction to obtain a dicarboxylic acid ester, wherein the acidic auxiliary agent is a phenolic substance and the alkaline catalyst is a strong base, acetate, phosphate, carbonate, hypochlorite, metaaluminate, alcoholate or phenate.

[0010] Acid-base synergistic catalysis is crucial for the efficient depolymerization of polyester waste plastics. Alkaline catalysts provide hydroxide ions to attack ester bonds, reducing their stability and promoting their breakage, forming alcohol anions that accelerate the alcoholysis reaction while also regulating the pH value to improve product selectivity. However, alkaline environments can trigger side reactions, leading to the introduction of acidic additives. Acidic additives can alter the reaction pathway, promote proton transfer, inhibit excessive base catalysis, regulate product distribution, and synergize with alkaline catalysts to improve catalytic efficiency.

[0011] The acidic auxiliary agent of the present invention is a phenolic substance, which ionizes H in a high-temperature methanol solution. +ions, fine-tuning the pH of the reaction system and maintaining acid-base balance, thereby optimizing catalytic efficiency. Simultaneously, the acidity of phenols promotes the protonation of ester bonds in polyester molecules, accelerating the alcoholysis reaction and improving efficiency. Furthermore, phenols can react with unstable intermediates or by-products, inhibiting side reactions and increasing the yield and purity of the target product. Furthermore, the mild acidity of phenolic additives reduces equipment corrosion and safety hazards, making the alcoholysis process more gentle and controllable.

[0012] The alkaline catalyst of the present invention is selected from strong bases, acetates, phosphates, carbonates, hypochlorites, metaaluminates, alcoholates or phenolates. These substances can effectively promote the breaking of lipid bonds, maintain the acid-base balance of the reaction system, regulate the reaction process, and make the alcoholysis reaction easier to proceed.

[0013] The present invention not only optimizes the reaction path and product distribution through the acid-base synergistic effect of phenolic substances and alkaline catalysts, but also improves the reaction rate and product quality, reduces energy consumption and the difficulty of subsequent processing, provides strong support for the recycling and utilization of polyester waste plastics, and realizes the precise control and efficient conversion of polyester molecular structure.

[0014] Preferably, the polyester waste plastic is made of polyethylene terephthalate, polybutylene terephthalate, polycarbonate, polyethylene succinate, or polyethylene adipate. Before being added to the reaction apparatus, the polyester waste plastic must be crushed into centimeter-sized flakes, granules, or powder to ensure efficient recycling of the waste polyester material and targeted upgrading to produce dicarboxylic acid ester monomers in high yield.

[0015] More preferably, the polyester waste plastic is made of polyethylene terephthalate (PET), including discarded PET bottles, discarded PET film, colored PET trays, colored PET twine, or non-woven PET fabric. The method of the present invention is applicable to a variety of PET samples, including difficult-to-process materials such as colored PET, with recovery rates reaching 72%-96%, demonstrating excellent efficiency and adaptability.

[0016] Preferably, the phenolic substances include at least one of guaiacol, methylguaiacol, ethylguaiacol, hydroquinone, pyrogallol, tannic acid, pomegranate polyphenols, 2,6-dimethylphenol, 2,3,5-trimethylphenol, p-bromophenol, p-cresol, phenol, 2-ethylphenol, 4-aminophenol, dichlorophenol, trichlorophenol, pentachlorophenol, p-methoxyphenol or bisphenol A. The present invention increases the diversity of products by introducing a variety of phenolic substances to meet specific needs. At the same time, the above-mentioned phenolic substances are mostly derived from natural products or bio-based chemicals. Compared with synthetic acidic additives, they have better environmental compatibility, comply with the principles of green chemistry, and reduce the impact on the environment.

[0017] Preferably, the molar ratio of the phenolic substance to the polyester waste plastic is (0.01-4):1. More preferably, the molar ratio of the phenolic substance to the polyester waste plastic is (0.05-1.5):1. A molar ratio of the phenolic substance to the polyester waste plastic that is too low will lead to incomplete reaction and low product yield; a molar ratio that is too high will cause waste of resources, increase costs, and increase the difficulty of product purification. Therefore, the present invention preferably limits the molar ratio of the phenolic substance to the polyester waste plastic to 0.05-1.5:1, which can ensure that the phenolic substance can fully exert its effect and promote the full depolymerization of the polyester waste plastic, while avoiding unnecessary cost increases and product purification problems, while taking into account both reaction efficiency and economy.

[0018] Preferably, the strong base comprises at least one of lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide, or calcium hydroxide. Such a strong base can effectively promote the cleavage of the ester bond and accelerate the alcoholysis reaction. It can also neutralize the acidic substances produced during the reaction, maintaining the acid-base balance of the reaction system and facilitating the continued progress of the reaction.

[0019] Preferably, the acetate includes at least one of lithium acetate, sodium acetate, potassium acetate, cesium acetate, calcium acetate, or zinc acetate; the phosphate includes at least one of lithium phosphate, sodium phosphate, potassium phosphate, disodium hydrogen phosphate, dipotassium hydrogen phosphate, or cesium phosphate; and the carbonate includes at least one of lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, cesium bicarbonate, lithium carbonate, sodium carbonate, potassium carbonate, cesium carbonate, or magnesium carbonate. Acetates, phosphates, and carbonates can ionize in solution to produce hydroxide ions, thus exhibiting alkalinity. When used as catalysts, these salts not only exhibit good catalytic activity, but also relatively mild reaction conditions and low corrosion to equipment. Furthermore, these salts are widely available and relatively inexpensive, helping to reduce production costs.

[0020] Preferably, the hypochlorite comprises at least one of sodium hypochlorite or potassium hypochlorite. Hypochlorite has a certain oxidizing property and can play a supporting catalytic role in the alcoholysis reaction. It can also inhibit the occurrence of side reactions to a certain extent, thereby improving the yield and purity of the target product.

[0021] Preferably, the metaaluminate comprises at least one of sodium metaaluminate or potassium metaaluminate. Metaaluminate can form stable aluminum hydroxy complexes in solution. These complexes catalyze the cleavage of ester bonds and can also interact with other components in the reaction system to regulate the reaction process.

[0022] Preferably, the alkoxide comprises at least one of lithium methoxide, lithium ethoxide, lithium isopropoxide, sodium methoxide, sodium ethoxide, sodium tert-butoxide, sodium propoxide, sodium isopropoxide, potassium methoxide, potassium ethoxide, potassium tert-butoxide, potassium propoxide, or potassium isopropoxide; and the phenoxide comprises at least one of sodium catechol, sodium hydroquinone, sodium resorcinol, sodium cresol, potassium phenoxide, or sodium phenoxide. The coordination between the metal ions in the alkoxides and phenoxides and the alcohol or phenol gives them unique catalytic properties. These alkoxides and phenoxides have high selectivity for the alcoholysis reaction of the present invention, effectively promoting the production of the target product. Furthermore, they are environmentally friendly, in line with the development trend of green chemistry.

[0023] Preferably, the molar ratio of the alkaline catalyst to the polyester waste plastic is (0.01-1):1. More preferably, the molar ratio of the alkaline catalyst to the polyester waste plastic is (0.05-0.3):1. Too low a catalyst dosage may result in incomplete alcoholysis, while excessive catalyst dosage increases costs and environmental risks. The present invention limits the molar ratio of the alkaline catalyst to the polyester waste plastic to the above range, effectively catalyzing the polyester alcoholysis reaction, ensuring product purity and selectivity, while simplifying the purification process and reducing costs.

[0024] Preferably, the amount of methanol added per mmol of polyester waste plastic is 0.1~6 mL. More preferably, the amount of methanol added per mmol of polyester waste plastic is 0.5~1.5 mL. The amount of methanol added is mainly based on considerations of reaction efficiency and product purity. Methanol is used as an alcoholysis agent, and its appropriate addition can effectively promote the decomposition of polyester, increase reaction rate and yield. If the amount of methanol added is insufficient, it may lead to incomplete alcoholysis reaction and affect product purity; while adding too much will not only cause waste, but may also trigger side reactions and increase the difficulty of product separation. In addition, an appropriate amount of methanol can also help control the reaction temperature, avoid local overheating, and ensure that the reaction proceeds smoothly. The present invention limits the amount of methanol added to the above range to achieve efficient and high-purity polyester alcoholysis.

[0025] Preferably, the reaction temperature of the depolymerization reaction is 60-200°C, and the reaction time is 0.5-18 hours. Further preferably, the reaction temperature of the depolymerization reaction is 90-150°C, and the reaction time is 1.5-6 hours. More preferably, the reaction temperature of the depolymerization reaction is 100-130°C, and the reaction time is 2-3 hours. The method of the present invention controls the reaction conditions within the above ranges, which can suppress the occurrence of side reactions and ensure high selectivity and yield of the target product.

[0026] Preferably, the polyester waste plastic is made of polyethylene terephthalate, the acidic additive is guaiacol, and the alkaline catalyst is potassium bicarbonate or cesium bicarbonate. The present invention utilizes guaiacol as the acidic additive and potassium bicarbonate as the alkaline catalyst. The combination of the two provides a good acid-base balance. The mild acidity of guaiacol and the alkalinity of potassium bicarbonate synergistically promote the protonation of ester bonds and accelerate the cleavage process, thereby improving the conversion rate and product selectivity of PET alcoholysis.

[0027] Preferably, the molar ratio of polyethylene terephthalate to guaiacol, potassium bicarbonate or cesium bicarbonate is 1:(0.05-1.5):(0.05-0.3), and the amount of methanol added per mmol of polyethylene terephthalate is 0.5-1.5 mL.

[0028] Preferably, the polyester waste plastic is made of polyethylene terephthalate, the acidic additive is methylguaiacol, and the alkaline catalyst is potassium methoxide or potassium phenoxide. Methylguaiacol's structural differences make it more stable than guaiacol under certain conditions. When combined with potassium methoxide or potassium phenoxide, it can effectively control side reactions and increase the yield of the alcoholysis product.

[0029] Preferably, the molar ratio of polyethylene terephthalate to methylguaiacol, potassium methoxide or potassium phenoxide is 1:(0.05-1.5):(0.05-0.3), and the amount of methanol added per mmol of polyethylene terephthalate is 0.5-1.5 mL.

[0030] Preferably, the polyester waste plastic is made of polyethylene terephthalate, the acidic additive is ethyl guaiacol, and the alkaline catalyst is sodium hypochlorite. The combination of ethyl guaiacol and sodium hypochlorite exhibits a more efficient catalytic effect when treating PET containing specific contaminants, and the oxidizing properties of sodium hypochlorite help remove certain difficult-to-treat impurities.

[0031] Preferably, the molar ratio of polyethylene terephthalate to ethyl guaiacol and sodium hypochlorite is 1:(0.05-1.5):(0.05-0.3), and the amount of methanol added is 0.5-1.5 mL per mmol of polyethylene terephthalate.

[0032] More preferably, the material of the polyester waste plastic is polyethylene terephthalate, the acidic auxiliary agent is guaiacol, the alkaline catalyst is potassium bicarbonate, the molar ratio of the polyester waste plastic to the acidic auxiliary agent and the alkaline catalyst is 1:1:0.1, and the amount of methanol added per mmol of polyester waste plastic is 1 mL. Under the reaction conditions, the conversion rate of the polyester waste plastic is close to 100%, and the yield of the product can reach up to 99%.

[0033] Furthermore, combinations of these guaiacols with specific alkaline catalysts (guaiacol + potassium bicarbonate or cesium bicarbonate, methylguaiacol + potassium methoxide or potassium phenoxide, and ethylguaiacol + sodium hypochlorite) demonstrate strong resistance to interference from dyes and impurities. The synergistic effect of the mild acidity of guaiacols and the alkalinity of specific catalysts effectively overcomes potential catalytic inhibition caused by dyes, significantly improving depolymerization efficiency and demonstrating significant advantages in processing colored plastics.

[0034] Compared with the prior art, the present invention has at least the following beneficial effects:

[0035] (1) This invention proposes an innovative acid-base synergistic catalytic method for the targeted conversion of waste polyester, which successfully achieves the rapid and gentle degradation of polyester waste plastics. This method not only improves the degradation efficiency but also produces high-value-added chemicals, opening up a new path for the resource utilization of polyester waste plastics.

[0036] (2) The acid-base synergistic catalytic system used in the present invention has significant advantages in terms of environmental friendliness and economy. Taking the preferred guaiacol / potassium bicarbonate catalytic system as an example, the weak alkalinity of potassium bicarbonate significantly reduces the corrosion resistance requirements of the equipment, which is conducive to industrial application; and guaiacol as an acid auxiliary agent is a bio-based solvent with the characteristics of being green and low-cost, which is fully in line with the concept of sustainable development. The design of this catalytic system fully reflects the innovation of the present invention in green chemistry and industrial practice;

[0037] (3) The acid-base synergistic catalytic directional conversion method of waste polyester of the present invention exhibits excellent catalytic performance, and can achieve a yield of ethylene glycol of more than 99% and a yield of dimethyl terephthalate of more than 97%. The method has a wide range of applicability and can efficiently process various types of waste PET raw materials, including but not limited to waste PET bottles, waste PET matrix films, PET colored ropes, and PET non-woven fabrics. All types of PET waste can be efficiently and directionally upgraded in a short period of time, significantly improving the resource utilization level and economic value of waste PET;

[0038] (4) The application prospects of the present invention are not limited to PET waste plastics, but can also be extended to other polyester materials, such as polyethylene succinate (PES) and polyethylene adipate (PEA). This high adaptability greatly broadens the range of materials for waste plastic treatment. While ensuring high depolymerization efficiency, this method can also achieve rapid reaction and quickly convert various polyester materials into corresponding high-value ester products. By simply adjusting the reaction conditions, this method can flexibly cope with different types of polyester waste, fully demonstrating its great potential and universality in industrial applications, and providing new technical support for the comprehensive utilization of polyester waste plastics;

[0039] (5) The method of the present invention is not only applicable to transparent plastics, but also exhibits good depolymerization effects on a variety of polyester waste materials, including colored bottle flakes. Even when dealing with highly crystalline materials such as PET woven tapes, the technology can achieve high yields. The method of the present invention provides a powerful tool for processing complex polyester waste materials, especially showing significant advantages in the treatment of colored plastics. DETAILED DESCRIPTION

[0040] In order to make the objects, features and advantages of the present invention more clearly understood, a detailed description is given below using specific embodiments. In the following description, many specific details are set forth to fully understand the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. The technical features in the various embodiments of the present invention can be combined accordingly without conflicting with each other.

[0041] In the following embodiments of the present invention, a method for recovering waste polyester by methanolysis through acid-base synergistic catalysis is provided, wherein the specific steps include:

[0042] S1. Place crushed polyester material, an acidic additive, an alkaline catalyst, and a methanol solvent in a reaction apparatus; the polyester material is made of polyethylene terephthalate (PET), polyethylene succinate, or polyethylene adipate. The molar ratio of the acidic additive to the polyester material is (0.01-4):1, the molar ratio of the alkaline catalyst to the polyester material is (0.01-1):1, and the amount of methanol added is 0.1-6 mL / mmol of polyester material.

[0043] S2. The above-mentioned reaction apparatus is placed at a depolymerization temperature of 60-200°C for 0.5-18 hours to conduct a methanol depolymerization reaction. This reaction process, based on an acid-base synergistic catalytic mechanism, achieves methanol depolymerization of polyester waste plastics under low temperature and low pressure conditions. After the reaction is completed, the temperature is cooled to room temperature to obtain high-value esters, achieving high-value recycling of polyester waste plastics.

[0044] The present invention can be used for the depolymerization and upgrading of waste PET materials. Other polyesters, such as polyethylene succinate and polyethylene adipate, are also suitable for the recycling strategy of the present invention. The products are the corresponding esters and ethylene glycol. For example, in the case of PET, the products are dimethyl terephthalate and ethylene glycol.

[0045] The application examples of the acid-base synergistic catalytic process of the present invention on common polyester waste plastics are as follows: Example 1

[0046] 3 mmol PET powder, 3 mmol guaiacol, 0.3 mmol potassium bicarbonate and 3 mL methanol were added to a pressure-resistant glass tube equipped with a stirrer, placed in an oil bath, opened for stirring and heating. The reaction system was warmed to 120 ° C and reacted at this temperature for 6 h. After the reaction was completed, it was cooled to room temperature, and the obtained mixed solution and the precipitated solid were products. Dichloromethane was added to dissolve, and the dissolved solution was measured by GC yield, with mesitylene as internal standard. The result showed that the ethylene glycol (EG) yield of the present embodiment was 99.0%, and the dimethyl terephthalate (DMT) yield was 97.2%.

[0047] Examples 2-5

[0048] The following examples investigate the reaction temperature. The difference from Example 1 is that the reaction temperature in the methanolysis reaction conditions is changed, corresponding to Examples 2-5 respectively.

[0049] In a pressure-resistant glass tube equipped with a stirrer, add 3 mmol of PET powder, 3 mmol of guaiacol, 0.3 mmol of potassium bicarbonate, and 3 mL of methanol. Place in an oil bath, stir, and heat. Heat the reaction system to 90-150°C and maintain this temperature for 6 hours. After completion, cool to room temperature. The resulting mixture and precipitated solid are the product. Dissolve the mixture in dichloromethane, and mesitylene is added as an internal standard. The yields of EG and DMT are determined by GC. The results are shown in Table 1.

[0050]

[0051] Examples 6-8

[0052] The following examples investigate the reaction time. The difference from Example 1 is that the reaction time in the methanolysis reaction conditions is changed, corresponding to Examples 6-8 respectively.

[0053] In a pressure-resistant glass tube equipped with a stirrer, add 3 mmol of PET powder, 3 mmol of guaiacol, 0.3 mmol of potassium bicarbonate, and 3 mL of methanol. Place in an oil bath, stir, and heat. Heat the reaction system to 120°C and allow to react for 1.5-6 hours. After the reaction is complete, cool to room temperature. The resulting mixture and precipitated solid are the product. Dissolve the mixture in dichloromethane, and mesitylene is added as an internal standard. The yields of EG and DMT are determined by GC. The results are shown in Table 2.

[0054]

[0055] Examples 9-12

[0056] The following examples investigate the dosage of the acidic auxiliary agent. The difference from Example 1 is that the dosage of guaiacol in the methanolysis reaction conditions is changed, corresponding to Examples 9-12 respectively.

[0057] In a pressure-resistant glass tube equipped with a stirrer, add 3 mmol of PET powder, 1.5–4.5 mmol of guaiacol, 0.3 mmol of potassium bicarbonate, and 3 mL of methanol. Place in an oil bath, stir, and heat. Heat the reaction system to 120°C and maintain this temperature for 2 h. After completion, cool to room temperature. The resulting mixture and precipitated solid are the product. Dissolve the mixture in dichloromethane, and mesitylene is added as an internal standard. The yields of EG and DMT are determined by GC. The results are shown in Table 3.

[0058]

[0059] Examples 13-16

[0060] The following examples investigate the dosage of the alkaline auxiliary agent. The difference from Example 1 is that the dosage of potassium bicarbonate in the methanolysis reaction conditions is changed, corresponding to Examples 13-16 respectively.

[0061] In a pressure-resistant glass tube equipped with a stirrer, add 3 mmol of PET powder, 3 mmol of guaiacol, 0.45–0.9 mmol of potassium bicarbonate, and 3 mL of methanol. Place the mixture in an oil bath, stir, and heat. Heat the reaction system to 120°C and maintain this temperature for 2 hours. After completion, cool the reaction to room temperature. The resulting mixture and precipitated solid are the product. Dissolve the mixture in dichloromethane, and mesitylene is added as an internal standard. The yields of EG and DMT are determined by GC. The results are shown in Table 4.

[0062]

[0063] Examples 17-20

[0064] The following examples investigate the amount of methanol used. The difference from Example 1 is that the amount of methanol used in the methanolysis reaction conditions is changed, corresponding to Examples 17-20, respectively.

[0065] In a pressure-resistant glass tube equipped with a stirrer, add 3 mmol of PET powder, 3 mmol of guaiacol, 0.3 mmol of potassium bicarbonate, and 0.5–4.5 mL of methanol. Place in an oil bath, stir, and heat. Heat the reaction system to 120°C and maintain this temperature for 2 hours. After completion, cool to room temperature. The resulting mixture and precipitated solid are the product. Dissolve the mixture in dichloromethane, and use mesitylene as an internal standard. The yields of EG and DMT are determined by GC. The results are shown in Table 5.

[0066]

[0067] Examples 21-24

[0068] The following examples investigate different acidic adjuvants. The difference from Example 1 is that guaiacol in the methanolysis reaction conditions is replaced with other acidic adjuvants, corresponding to Examples 21-24, respectively.

[0069] In a pressure-resistant glass tube equipped with a stirrer, add 3 mmol of PET powder, 3 mmol of an acidic additive, 0.3 mmol of potassium bicarbonate, and 3 mL of methanol. Place in an oil bath, stir, and heat. Heat the reaction system to 120°C and maintain this temperature for 2 hours. After completion, cool to room temperature. The resulting mixture and precipitated solid are the product. Dissolve the mixture in dichloromethane, and use mesitylene as an internal standard. The yields of EG and DMT are determined by GC. The results are shown in Table 6.

[0070]

[0071] Examples 25-32

[0072] The following examples investigate different alkaline catalysts. The difference from Example 1 is that potassium bicarbonate in the methanolysis reaction conditions is changed to other alkaline catalysts, corresponding to Examples 25-32 respectively.

[0073] In a pressure-resistant glass tube equipped with a stirrer, add 3 mmol of PET powder, 3 mmol of guaiacol, 0.3 mmol of a basic catalyst, and 3 mL of methanol. The mixture was placed in an oil bath, stirred, and heated. The reaction system was heated to 120°C and allowed to react for 2 hours. After completion, the reaction was cooled to room temperature. The resulting mixture and precipitated solid were the product. Dichloromethane was added for dissolution, and mesitylene was added as an internal standard. The yields of EG and DMT were determined by GC. The results are shown in Table 7.

[0074]

[0075] Examples 33-35

[0076] The following examples investigate the effects of different acidic additives and alkaline catalysts. The difference from Example 1 is that different combinations of acidic additives and alkaline catalysts are applied to the methanolysis reaction, corresponding to Examples 33-35, respectively.

[0077] In a pressure-resistant glass tube equipped with a stirrer, add 3 mmol of PET powder, 3 mmol of an acidic additive, 0.3 mmol of a basic catalyst, and 3 mL of methanol. Place in an oil bath, stir, and heat. Heat the reaction system to 120°C and maintain this temperature for 2 hours. After completion, cool to room temperature. The resulting mixture and precipitated solid are the product. Dissolve the mixture in dichloromethane, and mesitylene is added as an internal standard. The yields of EG and DMT are determined by GC. The results are shown in Table 8.

[0078]

[0079] Examples 36-40

[0080] The following examples investigate polyester types. The difference from Example 1 is that the PET in the methanolysis reaction conditions is changed to other polyester types: polybutylene terephthalate (PBT), polyethylene succinate (PES), polyethylene adipate (PEA), polycarbonate (PC), and polybutylene adipate / terephthalate (PBAT), corresponding to Examples 36-40, respectively.

[0081] In a pressure-resistant glass tube equipped with a stirrer, 3 mmol of polyester, 3 mmol of guaiacol, 0.3 mmol of potassium bicarbonate, and 3 mL of methanol were added. The mixture was placed in an oil bath, stirred, and heated. After the reaction was completed, the mixture was cooled to room temperature. The resulting mixture and precipitated solid were the product. Chloroform was added for dissolution, and mesitylene was added as an internal standard. The solution was then analyzed by nuclear magnetic resonance for the yield of the dicarboxylic acid ester product. The reaction results are shown in Table 9.

[0082]

[0083] Examples 41-45

[0084] The following examples investigate real plastics. The difference from Example 1 is that the PET in the methanolysis reaction conditions is changed to real PET plastic products, corresponding to Examples 41-45 respectively.

[0085] Discarded PET plastic products were crushed to centimeter size beforehand. 3 mmol of the crushed PET plastic, 3 mmol of guaiacol, 0.3 mmol of potassium bicarbonate, and 3 mL of methanol were added to a pressure-resistant glass tube equipped with a stirrer. The mixture was placed in an oil bath, stirred, and heated. The reaction system was heated to 120°C and allowed to react for 2 hours. After the reaction was complete, the temperature was cooled to room temperature. The resulting mixed solution and precipitated solid were the product. Dichloromethane was added for dissolution, and mesitylene was added as an internal standard. The yields of EG and DMT were determined by GC. The results are shown in Table 10.

[0086] Example 46

[0087] In an autoclave reactor equipped with an electromagnetic stirrer, thermocouple, and programmable temperature controller, 5.3 mmol of PET transparent plastic bottle flakes and colored plastic bottle flakes, 5.2 mmol of guaiacol, 0.52 mmol of potassium bicarbonate, and 5.2 L of methanol were added. After the autoclave reactor was placed, stirring and heating were started. The reaction system was heated to 120°C and reacted at this temperature for 4 hours. After the reaction was completed, the temperature was lowered to room temperature, and the resulting mixed solution and precipitated solid were the products. The DMT product was obtained by filtration, washing, and recrystallization, and the yield was calculated by weighing. The results showed that the DMT yield obtained from the transparent plastic bottle flakes in this example was 93%, and the DMT yield obtained from the colored plastic bottle flakes was 91%.

[0088] There are significant differences in the recycling process between transparent plastic and colored plastic bottle flakes, which are mainly determined by their raw material composition, added colorants, and possible impurities. Colored plastic bottle flakes are often more difficult to handle due to their complex composition. Specifically, colorants may interfere with the depolymerization reaction, reduce the activity of the catalyst, and even trigger undesirable side reactions, thereby affecting the depolymerization efficiency and product purity. At the same time, colored plastics often carry more impurities such as labels, adhesives, and oil stains during recycling, which require additional processing steps.

[0089] Experimental results demonstrate that the method is not only applicable to transparent plastics but also exhibits excellent depolymerization effectiveness on a variety of polyester waste materials, including colored bottle flakes. High yields are achieved even with highly crystalline materials, such as PET woven tape. Therefore, the method provides a powerful tool for processing complex polyester waste, particularly exhibiting significant advantages in processing colored plastic bottle flakes, effectively improving recycling efficiency and product quality.

[0090] The embodiment described above is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Persons skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent substitution or equivalent transformation falls within the scope of protection of the present invention.

Claims

1. A method for recovering waste polyester by methanolysis through acid-base synergistic catalysis, characterized in that: The following steps are involved: Adding polyester waste plastic, an acidic auxiliary agent and an alkaline catalyst into methanol to carry out a depolymerization reaction to obtain ethylene glycol and dicarboxylic acid ester, wherein the acidic auxiliary agent is a phenolic substance and the alkaline catalyst is a strong base, acetate, phosphate, carbonate, hypochlorite, metaaluminate, alkoxide or phenoxide; The molar ratio of the acidic auxiliary agent, the alkaline catalyst and the polyester waste plastic is (0.01-4): (0.01-1):1; the amount of methanol added per mmol of the polyester waste plastic is 0.1-6 mL; The reaction temperature of the depolymerization reaction is 60-200°C, and the reaction time is 0.5-18 h; The phenolic substance is at least one of guaiacol, methylguaiacol, ethylguaiacol, hydroquinone, pyrogallol, pomegranate polyphenol or phenol; The strong base is at least one of lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide or calcium hydroxide; and / or, the acetate is at least one of lithium acetate, sodium acetate, potassium acetate, cesium acetate, calcium acetate or zinc acetate; and / or, the phosphate is at least one of lithium phosphate, sodium phosphate, potassium phosphate, disodium hydrogen phosphate, dipotassium hydrogen phosphate or cesium phosphate; and / or, the carbonate is at least one of lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, cesium bicarbonate, lithium carbonate, sodium carbonate, potassium carbonate, cesium carbonate or magnesium carbonate; And / or, the hypochlorite is at least one of sodium hypochlorite or potassium hypochlorite; And / or, the metaaluminate is at least one of sodium metaaluminate or potassium metaaluminate; and / or, the alkoxide is at least one of lithium methoxide, lithium ethoxide, lithium isopropoxide, sodium methoxide, sodium ethoxide, sodium tert-butoxide, sodium propoxide, sodium isopropoxide, potassium methoxide, potassium ethoxide, potassium tert-butoxide, potassium propoxide or potassium isopropoxide; and / or, the phenate is at least one of sodium catechol, sodium hydroquinone, sodium resorcinol, sodium cresol, potassium phenate or sodium phenate; The material of the polyester waste plastic is polyethylene terephthalate, polybutylene terephthalate, polycarbonate, polyethylene succinate or polyethylene adipate; the dicarboxylic acid ester is dimethyl terephthalate, dimethyl carbonate, dimethyl succinate or dimethyl adipate.

2. The method according to claim 1, characterized in that The molar ratio of the acidic auxiliary agent, the alkaline catalyst and the polyester waste plastic is (0.05-1.5): (0.05-0.3):1, and the amount of methanol added per mmol of the polyester waste plastic is 0.3-1.5 mL.

3. The method according to claim 1 or 2, characterized in that The material of the polyester waste plastic is polyethylene terephthalate, the acidic auxiliary agent is guaiacol, and the alkaline catalyst is potassium bicarbonate or cesium bicarbonate.

4. The method according to claim 1 or 2, characterized in that The material of the polyester waste plastic is polyethylene terephthalate, the acidic auxiliary agent is methyl guaiacol, and the alkaline catalyst is potassium methoxide or potassium phenoxide.

5. The method according to claim 1 or 2, characterized in that The material of the polyester waste plastic is polyethylene terephthalate, the acidic auxiliary agent is ethyl guaiacol, and the alkaline catalyst is sodium hypochlorite.

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