Process for efficiently degrading pet under low temperature condition by alcoholysis method

Abstract

This invention relates to the field of polymer material degradation and recycling technology, and provides a process for the efficient degradation of PET using alcoholysis under low-temperature conditions, comprising the following steps: S1, raw material preparation and degradation reaction: waste PET fragments and ethylene glycol are added to a reaction vessel, stirred at 120-150℃, a composite catalyst is added, and the degradation temperature and pressure are controlled. The reaction is carried out for 2-6 hours to obtain a reaction solution; S2, separation: the reaction solution is filtered to separate solid and liquid components. The solid component enters the recycling system, and the liquid component enters the recrystallization system; S3, recrystallization: water is added to the recrystallization system, the mixture is cooled and crystallized, filtered, and the resulting solid enters the degradation system. The resulting liquid undergoes secondary crystallization. After crystallization, the solid obtained is filtered to obtain the product BHET, which is dried and collected. The filtered liquid enters the purification system; S4, purification; S5, crystallization. The preparation method of this invention provides a process for the alcoholysis degradation of PET, which has the advantages of low PET degradation temperature and high BHET yield.

Description

Technical Field

[0001] This invention relates to the field of polymer material degradation and recycling technology, and in particular to a process for efficiently degrading PET using alcoholysis under low-temperature conditions. Background Technology

[0002] Polyethylene terephthalate (PET), as an important thermoplastic polyester material, is widely used in food packaging, textiles, construction, and automotive industries. However, with the widespread use of PET, the disposal of waste PET has become a prominent issue. Traditional physical recycling methods, while simple in process, suffer from drawbacks such as high energy consumption, reduced product quality, and difficulty in achieving multiple recycling cycles. Chemical recycling methods, while capable of converting waste PET into useful monomers or intermediate raw materials, typically require the use of organic solvents, raising concerns about safety and environmental friendliness.

[0003] Currently, among chemical recycling methods, alcoholysis has received widespread attention due to its mild reaction conditions and good product selectivity. However, existing alcoholysis processes still have some problems, such as large catalyst usage, high energy consumption, and long operating cycles, which make it difficult to achieve efficient continuous operation. In addition, the degradation reaction temperature of PET is still relatively high, often requiring temperatures above 195°C, resulting in high energy consumption in the PET degradation process.

[0004] Patent CN 121085781 A discloses a reaction process for depolymerizing PET to recover BHET. The process includes: mixing raw materials containing waste PET particles and ethylene glycol with a potassium salt catalyst, placing them in a sealed container, and carrying out a depolymerization reaction under an inactive atmosphere to obtain a depolymerization liquid. Then, the depolymerization liquid is mixed with methanol and BHET seed crystals and subjected to an ester exchange reaction to obtain a reaction liquid. The reaction liquid is then cooled to 25°C at a rate of 10°C / min, crystallized for 3-6 hours, acid-washed, and dried to obtain BHET crystals. This process yields a high BHET yield, but the depolymerization reaction temperature still reaches 225-235°C, which does not meet the requirements for low-temperature degradation.

[0005] Therefore, there is an urgent need in the market for a process that can efficiently degrade PET using alcoholysis under low-temperature conditions. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention discloses a process for the efficient degradation of PET using alcoholysis under low-temperature conditions, comprising five steps: raw material preparation and degradation reaction, separation, recrystallization, purification, and crystallization. A composite catalyst is designed and added during the degradation reaction stage, which effectively reduces the degradation temperature of PET and increases the yield of the product BHET.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: This invention provides a process for the efficient degradation of PET using alcoholysis under low-temperature conditions, comprising the following steps: S1. Raw material preparation and degradation reaction: Add waste PET fragments and ethylene glycol to the reactor, stir at 120-150℃, add composite catalyst, control degradation temperature and pressure, react for 2-6 hours, and obtain reaction solution for later use. S2. Separation: The reaction liquid from step S1 is filtered and separated into solid phase and liquid phase components. The solid phase component enters the recycling system, and the liquid phase component enters the recrystallization system. S3. Recrystallization: Add water to the recrystallization system, cool and crystallize, filter, the resulting solid enters the degradation system, the resulting liquid undergoes secondary crystallization, after crystallization, filter to obtain the solid as product BHET, dry and collect, the filtered liquid enters the purification system. S4. Purification: The liquid in step S4 is purified by distillation to obtain water with a purity of ≥99.9%. The water is then introduced into a recrystallization system to determine the purity of ethylene glycol. If the purity is ≥99.5%, the process returns to step S1. If the purity is <99.5%, the process proceeds to step S5. S5. Crystallization: Ethylene glycol is crystallized at -15~-25℃ for 4-6 hours to obtain ethylene glycol with a purity ≥99.5%.

[0008] In some embodiments of the present invention, in step S1, the mass ratio of the waste PET fragments, ethylene glycol and composite catalyst is 1:(5.5-6.5):(0.08-0.15).

[0009] In some embodiments of the present invention, in step S1, the composite catalyst is a mixture of an iron-based catalyst and a cobalt-based catalyst.

[0010] In some embodiments of the present invention, in step S1, the mass ratio of the iron-based catalyst to the cobalt-based catalyst in the composite catalyst is (2-4):1.

[0011] In some embodiments of the present invention, step S4, the method for preparing the cobalt-based catalyst includes the following steps: (1) After ball milling sodium chloride, add γ-glycidoxypropyltrimethoxysilane to anhydrous ethanol, sonicate, add ball-milled sodium chloride, stir at 75-85℃ for 1.5-2.5h, cool to 25-35℃, add N-aminoethyl-γ-aminopropyltrimethoxysilane, stir, and vacuum dry to obtain modified template agent for later use; (2) Add 2-methylimidazole to anhydrous methanol, stir, and obtain a liquid for later use. Add cobalt nitrate hexahydrate and the modified template agent from step (1) to anhydrous methanol, stir, add to the liquid, stir, age at 35-45℃, freeze dry, keep warm at 750-820℃ in a nitrogen-hydrogen mixed atmosphere for 1-2 hours, wash, and dry to obtain the cobalt-based catalyst.

[0012] In some embodiments of the present invention, in step (1), the molar ratio of sodium chloride and N-aminoethyl-γ-aminopropyltrimethoxysilane, γ-glycidyl etheroxypropyltrimethoxysilane is 1:(0.008-0.012):(0.025-0.035).

[0013] Preferably, in step (1), the molar ratio of sodium chloride to N-aminoethyl-γ-aminopropyltrimethoxysilane and γ-glycidyl etheroxypropyltrimethoxysilane is 1:0.01:0.03.

[0014] In some embodiments of the present invention, the molar ratio of sodium chloride, 2-methylimidazole and cobalt nitrate hexahydrate is 1:(5.5-6.5):(1-2).

[0015] Preferably, the molar ratio of sodium chloride, 2-methylimidazole and cobalt nitrate hexahydrate is 1:6:1.5.

[0016] This application adds ionic liquid functionalized magnetic nanoparticles as an iron-based catalyst, which has the effect of synergistic effect of multiple active sites and efficient separation, and can meet the requirements of PET alcoholysis catalyst. However, using only this iron-based catalyst to catalyze the degradation of PET still has the problem of high degradation temperature.

[0017] To further reduce the temperature of PET degradation via alcoholysis, the applicant designed and synthesized a cobalt-based catalyst, using Co provided by cobalt nitrate hexahydrate. 2+ A ZIF precursor was formed with 2-methylimidazole. A modified template agent was obtained by modifying sodium chloride with a specific ratio of composite silane coupling agent. This was then combined with a high-temperature carbonization process to prepare a cobalt-based catalyst. The ball-milled sodium chloride has a macroporous channel structure that can fully load the cobalt active component and disperse it well. The introduction of silane coupling agents N-aminoethyl-γ-aminopropyltrimethoxysilane and γ-glycidoxypropyltrimethoxysilane can fill coordination defects through the diamino group and construct a silane cross-linked network structure, which synergistically enhances the structural stability. Furthermore, the combination of the ZIF precursor and the modified template agent can improve the dispersion of cobalt active sites, thereby enabling the cobalt-based catalyst to have both good catalytic activity and stability.

[0018] Furthermore, the applicant controls the ratio of iron-based catalyst and cobalt-based catalyst to obtain a composite catalyst in a ratio of (2-4):1. This can increase the density of active sites of the iron-cobalt bimetallic catalyst and make it have the most suitable spatial distribution, effectively reducing the activation energy of PET degradation, thereby improving the catalytic effect of the composite catalyst and effectively reducing the temperature of the PET degradation stage.

[0019] In some embodiments of the present invention, in step S1, the degradation temperature and pressure are 150-165°C and 0-0.2 MPa, respectively.

[0020] In some embodiments of the present invention, the mass ratio of water in step S3 to waste PET fragments in step S1 is (23-25):1.

[0021] In some embodiments of the present invention, the yield of the product BHET obtained in step S3 is not less than 98.5%.

[0022] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention discloses a process for efficient degradation of PET by alcoholysis under low temperature conditions, including five steps: raw material preparation and degradation reaction, separation, recrystallization, purification and crystallization. A composite catalyst is designed and added in the degradation reaction stage. Through the synergistic effect between the components, the temperature of PET degradation is effectively reduced and the yield of product BHET is increased.

[0023] (2) This invention designs and synthesizes a cobalt-based catalyst, using Co provided by cobalt nitrate hexahydrate. 2+ A ZIF precursor was formed with 2-methylimidazole, and a modified template agent was obtained by modifying sodium chloride with N-aminoethyl-γ-aminopropyltrimethoxysilane and γ-glycidoxypropyltrimethoxysilane. Then, a cobalt-based catalyst was prepared by high-temperature carbonization process, which gave the cobalt-based catalyst both good catalytic activity and stability. By controlling the ratio of iron-based catalyst and cobalt-based catalyst to be combined in a ratio of (2-4):1, a composite catalyst was obtained. This can improve the density of iron-cobalt bimetallic active sites and make them have the most suitable spatial distribution, effectively reducing the activation energy of PET degradation, thereby improving the catalytic effect of the composite catalyst and effectively reducing the temperature of PET degradation stage. Detailed Implementation

[0024] The present invention will be described below with reference to specific embodiments. It should be noted that the following embodiments are examples of the present invention and are used only to illustrate the invention, not to limit it. Other combinations and various modifications within the scope of the present invention can be made without departing from its spirit or scope.

[0025] In the following examples and comparative examples, except for the iron-based catalyst and the cobalt-based catalyst, all other compound monomers and related reagents used can be purchased from the market.

[0026] Preparation Example 1 The synthesis method of iron-based catalysts includes the following steps: 1) Mix 5g of Fe3O4 nanoparticles, 100mL of ethanol and 15mL of 25wt% ammonia water, stir at 30℃ for 15min, add 4mL of tetraethyl silicate, and react at 360rpm for 6h under nitrogen atmosphere and at 30℃. After the reaction is completed, the solid is recovered with a magnet, washed with anhydrous ethanol and dried at 45℃ for 12h to obtain magnetic nanoparticle precursor for later use. 2) Mix 3g of the magnetic nanoparticle precursor from step 1), 0.6g of 3-chloropropyltrimethoxysilane and 15g of toluene, and react at 50°C for 6h under a nitrogen atmosphere. After the reaction is complete, cool naturally to room temperature, recover the solid with a magnet, wash twice with toluene and twice with anhydrous ethanol, and dry at 60°C for 4h to obtain the magnetic nanoparticle intermediate for later use. 3) Mix 3g of magnetic nanoparticle intermediate, 5g of N-methylimidazole, 0.1g of potassium iodide and 15g of toluene from step 2), reflux at 110℃ for 12h, filter to obtain solid, wash with toluene 3 times and ethanol 3 times, and then dry at 80℃ to constant weight to obtain iron-based catalyst.

[0027] Preparation Example 2 The synthesis method of cobalt-based catalyst A includes the following steps: (1) 0.1 mol sodium chloride was ball-milled at 500 rpm for 3 h and set aside. 0.03 mol γ-glycidoxypropyltrimethoxysilane was added to 100 ml anhydrous ethanol and sonicated for 10 min. The ball-milled sodium chloride was added and stirred at 80 °C for 2 h. The temperature was lowered to 30 °C and 0.01 mol N-aminoethyl-γ-aminopropyltrimethoxysilane was added and stirred for 1 h. The mixture was then vacuum dried at 55 °C for 2 h to obtain the modified template agent for use. (2) Add 0.6 mol 2-methylimidazole to 200 ml anhydrous methanol and stir for 30 min to obtain a liquid for later use. Add 0.15 mol cobalt nitrate hexahydrate and the modified template agent from step (1) to 300 ml anhydrous methanol and stir for 1 h. Add the liquid and stir for 30 min. Aging at 40 °C for 24 h, freeze drying for 12 h, heating to 800 °C at 5 °C / min, and holding at a nitrogen-hydrogen mixed atmosphere (hydrogen volume fraction of 5%) for 2 h. Wash (sequentially wash with 0.5 mol / L sulfuric acid aqueous solution, 0.1 mol / L sodium hydroxide aqueous solution and deionized water at 60 °C until neutral), and dry at 60 °C for 12 h to obtain cobalt-based catalyst A.

[0028] Preparation Example 2 Cobalt-based catalyst B is implemented in the same way as cobalt-based catalyst A, except that the molar amount of γ-glycidoxypropyltrimethoxysilane in step (1) is replaced with 0.02 mol.

[0029] Preparation Example 3 Cobalt-based catalyst C is implemented in the same way as cobalt-based catalyst A, except that the preparation method of step (1) is replaced by: 0.1 mol sodium chloride is ball-milled at 500 rpm for 3 h and then set aside; 0.04 mol γ-glycidyl etheroxypropyltrimethoxysilane is added to 100 ml of anhydrous ethanol, sonicated for 10 min, and then the ball-milled sodium chloride is added. The mixture is stirred at 30 °C for 1 h and then vacuum dried at 55 °C for 2 h to obtain the modified template agent for use.

[0030] Preparation Example 4 Cobalt-based catalyst D, the specific implementation method is the same as that of cobalt-based catalyst A, the difference is that the preparation method of step (1) is replaced by: 0.1 mol sodium chloride is ball-milled at 500 rpm for 3 h and then set aside, 0.04 mol γ-glycidyl etheroxypropyltrimethoxysilane is added to 100 ml of anhydrous ethanol, sonicated for 10 min, the ball-milled sodium chloride is added, stirred at 80 °C for 2 h, and vacuum dried at 55 °C for 2 h to obtain the modified template agent for use.

[0031] Preparation Example 5 Cobalt-based catalyst E is implemented in the same way as cobalt-based catalyst A, except that the molar amount of 2-methylimidazole in step (2) is replaced with 0.5 mol.

[0032] Preparation Example 6 The cobalt-based catalyst F is implemented in the same way as the cobalt-based catalyst A, except that the molar amount of cobalt nitrate hexahydrate in step (2) is replaced with 0.07 mol.

[0033] Example 1 A process for efficiently degrading PET using alcoholysis under low-temperature conditions includes the following steps: S1. Raw material preparation and degradation reaction: Add 10g of waste PET fragments (passed through a 40-mesh sieve) and 60g of ethylene glycol to the reactor, stir evenly at 150℃, add 1g of composite catalyst, control the degradation temperature and pressure (160℃, 0.1MPa), react for 4h, and obtain the reaction solution for later use. The composite catalyst is a mixture of iron-based catalyst and cobalt-based catalyst A, with a mass ratio of 3:1. S2. Separation: The reaction liquid from step S1 is filtered and separated into solid phase and liquid phase components. The solid phase component enters the recycling system, and the liquid phase component enters the recrystallization system. S3. Recrystallization: Add 240g of water to the recrystallization system and cool to crystallize (crystallization temperature is 80℃, crystallization time is 3h). Filter and the obtained solid enters the degradation system. The obtained liquid undergoes secondary crystallization (crystallization temperature is -5℃, crystallization time is 12h). After crystallization, filter to obtain the solid, which is the product BHET. Dry at 60℃ and collect. The filtered liquid enters the purification system. S4. Purification: The liquid in step S4 is purified by distillation to obtain water with a purity of ≥99.9%. The water is then introduced into a recrystallization system to determine the purity of ethylene glycol. If the purity is ≥99.5%, the process returns to step S1. If the purity is <99.5%, the process proceeds to step S5. S5. Crystallization: Ethylene glycol is crystallized at -20℃ for 5 hours to obtain ethylene glycol with a purity ≥99.5%.

[0034] Example 2 A process for efficiently degrading PET using alcoholysis under low-temperature conditions includes the following steps: S1. Raw material preparation and degradation reaction: Add 10g of waste PET fragments (passed through a 40-mesh sieve) and 55g of ethylene glycol to the reactor, stir evenly at 150℃, add 0.8g of composite catalyst, control the degradation temperature and pressure (150℃, 0MPa), react for 6h, and obtain the reaction solution for later use. The composite catalyst is a mixture of iron-based catalyst and cobalt-based catalyst A in a mass ratio of 2:1. S2. Separation: The reaction liquid from step S1 is filtered and separated into solid phase and liquid phase components. The solid phase component enters the recycling system, and the liquid phase component enters the recrystallization system. S3. Recrystallization: Add 230g of water to the recrystallization system and cool to crystallize (crystallization temperature is 80℃, crystallization time is 3h). Filter and the obtained solid enters the degradation system. The obtained liquid undergoes secondary crystallization (crystallization temperature is -5℃, crystallization time is 12h). After crystallization, filter to obtain the solid, which is the product BHET. Dry at 60℃ and collect. The filtered liquid enters the purification system. S4. Purification: The liquid in step S4 is purified by distillation to obtain water with a purity of ≥99.9%. The water is then introduced into a recrystallization system to determine the purity of ethylene glycol. If the purity is ≥99.5%, the process returns to step S1. If the purity is <99.5%, the process proceeds to step S5. S5. Crystallization: Ethylene glycol is crystallized at -20℃ for 5 hours to obtain ethylene glycol with a purity ≥99.5%.

[0035] Example 3 A process for efficiently degrading PET using alcoholysis under low-temperature conditions includes the following steps: S1. Raw material preparation and degradation reaction: Add 10g of waste PET fragments (passed through a 40-mesh sieve) and 65g of ethylene glycol to the reactor, stir evenly at 150℃, add 1.5g of composite catalyst, control the degradation temperature and pressure (165℃, 0.2MPa), react for 2h, and obtain the reaction solution for later use. The composite catalyst is a mixture of iron-based catalyst and cobalt-based catalyst A, with a mass ratio of 4:1. S2. Separation: The reaction liquid from step S1 is filtered and separated into solid phase and liquid phase components. The solid phase component enters the recycling system, and the liquid phase component enters the recrystallization system. S3. Recrystallization: Add 240g of water to the recrystallization system and cool to crystallize (crystallization temperature is 80℃, crystallization time is 3h). Filter and the obtained solid enters the degradation system. The obtained liquid undergoes secondary crystallization (crystallization temperature is -5℃, crystallization time is 12h). After crystallization, filter to obtain the solid, which is the product BHET. Dry at 60℃ and collect. The filtered liquid enters the purification system. S4. Purification: The liquid in step S4 is purified by distillation to obtain water with a purity of ≥99.9%. The water is then introduced into a recrystallization system to determine the purity of ethylene glycol. If the purity is ≥99.5%, the process returns to step S1. If the purity is <99.5%, the process proceeds to step S5. S5. Crystallization: Ethylene glycol is crystallized at -20℃ for 5 hours to obtain ethylene glycol with a purity ≥99.5%.

[0036] Example 4 This embodiment provides a process for the efficient degradation of PET using alcoholysis under low-temperature conditions. The specific implementation method is the same as in Embodiment 1, except that in step S1, the amount of composite catalyst added is 0.5g.

[0037] Example 5 This embodiment provides a process for the efficient degradation of PET using alcoholysis under low-temperature conditions. The specific implementation method is the same as that in Embodiment 1, except that in step S1, the mass ratio of iron-based catalyst and cobalt-based catalyst A in the composite catalyst is 1:1.

[0038] Example 6 This embodiment provides a process for the efficient degradation of PET using alcoholysis under low-temperature conditions. The specific implementation method is the same as in Embodiment 1, except that in step S1, the composite catalyst is replaced by an iron-based catalyst in equal amounts.

[0039] Example 7 This embodiment provides a process for the efficient degradation of PET using alcoholysis under low-temperature conditions. The specific implementation method is the same as that in Embodiment 1, except that in step S1, the composite catalyst is replaced by a cobalt-based catalyst in equal amounts.

[0040] Example 8 This embodiment provides a process for the efficient degradation of PET using alcoholysis under low-temperature conditions. The specific implementation method is the same as in Embodiment 1, except that cobalt-based catalyst A is replaced by cobalt-based catalyst B in an equal amount.

[0041] Example 9 This embodiment provides a process for the efficient degradation of PET using alcoholysis under low-temperature conditions. The specific implementation method is the same as in Embodiment 1, except that cobalt-based catalyst A is replaced by cobalt-based catalyst C in an equal amount.

[0042] Example 10 This embodiment provides a process for the efficient degradation of PET using alcoholysis under low-temperature conditions. The specific implementation method is the same as in Embodiment 1, except that cobalt-based catalyst A is replaced by cobalt-based catalyst D in an equal amount.

[0043] Example 11 This embodiment provides a process for the efficient degradation of PET using alcoholysis under low-temperature conditions. The specific implementation method is the same as in Embodiment 1, except that cobalt-based catalyst A is replaced by cobalt-based catalyst E in an equal amount.

[0044] Example 12 This embodiment provides a process for the efficient degradation of PET using alcoholysis under low-temperature conditions. The specific implementation method is the same as in Embodiment 1, except that cobalt-based catalyst A is replaced by cobalt-based catalyst F in an equal amount.

[0045] Example 13 This embodiment provides a process for the efficient degradation of PET using alcoholysis under low-temperature conditions. The specific implementation method is the same as in Embodiment 1, except that the cobalt-based catalyst A is replaced by an equal amount of Co3O4.

[0046] Performance testing The yield of BHET, a product obtained by the low-temperature process of efficiently degrading PET using alcoholysis as described in Examples 1-13 above, was tested, and the test results are shown in Table 1.

[0047] The yield of BHET was determined by HPLC.

[0048] Table 1

[0049] As shown in Table 1, the BHET product obtained by the alcoholysis degradation of PET in Examples 1-3 of this invention still exhibits high yield under low-temperature degradation conditions. Specifically, Example 4 altered the amount of composite catalyst added, resulting in insufficient degradation of PET and a certain degree of decrease in the BHET yield. Examples 5-7 changed the ratio of iron-based catalyst and cobalt-based catalyst A in the composite catalyst, or replaced the composite catalyst with a single catalyst, leading to a decrease or disappearance of the synergistic catalytic effect between the two catalysts, thus resulting in a poorer BHET yield. Examples 8-12 changed the proportion of key modifying substances in the synthesis of the cobalt-based catalyst, leading to a decrease in the catalytic activity and stability of the cobalt-based catalyst, thus resulting in a lower BHET yield. Example 13 used an equal amount of Co3O4 to replace cobalt-based catalyst A, and the test showed a poor BHET yield.

[0050] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it. They should not be used to limit the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A process for efficiently degrading PET using alcoholysis under low-temperature conditions, characterized in that, Includes the following steps: S1. Raw material preparation and degradation reaction: Add waste PET fragments and ethylene glycol to the reactor, stir at 120-150℃, add composite catalyst, control degradation temperature and pressure, react for 2-6 hours, and obtain reaction solution for later use. S2. Separation: The reaction liquid from step S1 is filtered and separated into solid phase and liquid phase components. The solid phase component enters the recycling system, and the liquid phase component enters the recrystallization system. S3. Recrystallization: Add water to the recrystallization system, cool and crystallize, filter, the resulting solid enters the degradation system, the resulting liquid undergoes secondary crystallization, after crystallization, filter to obtain the solid as product BHET, dry and collect, the filtered liquid enters the purification system. S4. Purification: The liquid in step S4 is purified by distillation to obtain water with a purity of ≥99.9%. The water is then introduced into a recrystallization system to determine the purity of ethylene glycol. If the purity is ≥99.5%, the process returns to step S1. If the purity is <99.5%, the process proceeds to step S5. S5. Crystallization: Ethylene glycol is crystallized at -15~-25℃ for 4-6 hours to obtain ethylene glycol with a purity ≥99.5%.

2. The process for efficient degradation of PET using alcoholysis under low-temperature conditions according to claim 1, characterized in that, In step S1, the mass ratio of the waste PET fragments, ethylene glycol, and composite catalyst is 1:(5.5-6.5):(0.08-0.15).

3. The process for efficient degradation of PET using alcoholysis under low-temperature conditions according to claim 1, characterized in that, In step S1, the composite catalyst is a mixture of iron-based catalyst and cobalt-based catalyst.

4. The process for efficient degradation of PET using alcoholysis under low-temperature conditions according to claim 3, characterized in that, In step S1, the mass ratio of iron-based catalyst to cobalt-based catalyst in the composite catalyst is (2-4):

1.

5. The process for efficient degradation of PET using alcoholysis under low-temperature conditions according to claim 3, characterized in that, The preparation method of the cobalt-based catalyst includes the following steps: (1) After ball milling sodium chloride, add γ-glycidoxypropyltrimethoxysilane to anhydrous ethanol, sonicate, add ball-milled sodium chloride, stir at 75-85℃ for 1.5-2.5h, cool to 25-35℃, add N-aminoethyl-γ-aminopropyltrimethoxysilane, stir, and vacuum dry to obtain modified template agent for later use; (2) Add 2-methylimidazole to anhydrous methanol, stir, and obtain a liquid for later use. Add cobalt nitrate hexahydrate and the modified template agent from step (1) to anhydrous methanol, stir, add to the liquid, stir, age at 35-45℃, freeze dry, keep warm at 750-820℃ in a nitrogen-hydrogen mixed atmosphere for 1-2 hours, wash, and dry to obtain the cobalt-based catalyst.

6. The process for efficient degradation of PET using alcoholysis under low-temperature conditions according to claim 5, characterized in that, In step (1), the molar ratio of sodium chloride to N-aminoethyl-γ-aminopropyltrimethoxysilane and γ-glycidyl etheroxypropyltrimethoxysilane is 1:(0.008-0.012):(0.025-0.035).

7. The process for efficient degradation of PET using alcoholysis under low-temperature conditions according to claim 5, characterized in that, The molar ratio of sodium chloride, 2-methylimidazole and cobalt nitrate hexahydrate is 1:(5.5-6.5):(1-2).

8. The process for efficient degradation of PET using alcoholysis under low-temperature conditions according to claim 1, characterized in that, In step S1, the degradation temperature and pressure are 150-165℃ and 0-0.2MPa, respectively.

9. The process for efficient degradation of PET using alcoholysis under low-temperature conditions according to claim 1, characterized in that, In step S3, the mass ratio of water to waste PET fragments in step S1 is (23-25):

1.

10. The process for efficient degradation of PET using alcoholysis under low-temperature conditions according to claim 1, characterized in that, The yield of the product BHET obtained in step S3 is not less than 98.5%.

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