Method for high-efficiency alcoholysis of monomer from waste polyester

Abstract

The invention discloses a method for high-efficiency alcoholysis of monomers from waste polyester, and belongs to the field of waste polyester recovery. The method comprises the following steps: firstly, by taking zinc acetate as a metal source and 2-methylimidazole (MI) and 2-ethylimidazole (EI) as organic ligands, coprecipitating at room temperature to form an MOFs precursor, and calcining at 350 DEG C in an air atmosphere to obtain a heterogeneous catalyst with nano ZnO anchored to a nitrogen-doped carbon skeleton, and applying the heterogeneous catalyst to ethylene glycol alcoholysis of waste polyester; the highest alcoholysis effect of the catalyst can reach that the PET conversion rate is 100%, and the BHET yield is 84.9%. The heterogeneous catalyst prepared by the invention has the advantages of high catalytic efficiency, simple preparation, green process and the like, is easy to filter and recover, solves the problems of difficult separation of metal salt, few active sites of metal oxide, easy sintering inactivation and the like, provides a new idea for chemical closed-loop recovery of waste polyester, and has a wide market application prospect.

Description

Technical Field

[0001] This invention relates to the field of waste polyester recycling, and specifically to a depolymerization method for catalytic alcoholysis of polyethylene terephthalate (PET). Background Technology

[0002] Polyethylene terephthalate (PET) has become the most widely used polyester plastic globally due to its excellent mechanical properties, transparency, and barrier properties, and is widely used in beverage bottles, textiles, and food packaging. However, its excellent chemical stability and durability also make it difficult to degrade in the natural environment, resulting in massive amounts of waste PET products and causing serious "white pollution" and resource waste problems. Therefore, developing efficient PET chemical recycling technologies to convert it into high-value monomers or raw materials is of great strategic significance for building a plastic circular economy and solving the environmental crisis. Among many chemical recycling pathways, ethylene glycol alcoholysis is recognized as one of the most industrially promising methods due to its relatively mild process, single product (mainly diethyl terephthalate, BHET), and ability to be repolymerized into high-quality PET. The core bottleneck of this technology lies in developing catalysts that possess high catalytic activity, high catalytic selectivity, easy separation, and controllable cost.

[0003] Existing catalysts can be broadly classified into homogeneous and heterogeneous catalysts. While homogeneous catalysts exhibit high catalytic activity, their inherent defects severely limit their industrial applications. After the catalytic reaction, the catalyst dissolves in the product system, making separation and recovery difficult. This necessitates complex purification steps (such as multiple water washes and recrystallization) for removal, and generates large amounts of organic wastewater containing metal salts, causing secondary pollution and the risk of metal residue in the product. Invention patent CN201810165381.7 discloses an alkali metal tungstate-potassium acetate composite catalyst. Although it exhibits high catalytic activity, the water-soluble potassium salt makes catalyst recovery impossible and wastewater treatment challenging. Furthermore, the complex synthesis route increases raw material costs and process control difficulty, sacrificing economic viability and scalability. To overcome the difficulty in separating and recovering homogeneous catalysts, researchers have begun to focus on the development of heterogeneous catalyst systems. However, these catalysts suffer from low specific surface area, limited and unevenly distributed active sites. The MoO2-C composite material disclosed in invention patent CN116891410A has a specific surface area of ​​only 8 m² / g for its MoO2 component, resulting in a PET conversion rate of less than 30% at 160℃ and low catalytic efficiency. Under high-temperature (typically >180℃) alcoholysis reaction conditions, metal oxide nanoparticles are prone to migration, aggregation, and sintering, leading to a rapid reduction in active sites and a sharp decline in catalytic activity and stability. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing a PET alcoholysis catalyst that is simple to prepare, low in cost, has high catalytic activity, and is easy to recover and separate, so as to overcome the problems of limited active sites, low specific surface area, and easy deactivation at high temperature in the heterogeneous catalysts of the prior art.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] This invention proposes a method for preparing a nitrogen-doped carbon-supported zinc oxide catalyst. The key lies in constructing a metal-organic framework (MOF) precursor through a mixed imidazole ligand strategy and calcining it in an air atmosphere to obtain the target catalyst.

[0007] 1. Promotes the formation of hierarchical porous structures: The different thermal decomposition behaviors of 2-methylimidazole (MI) and 2-ethylimidazole (EI) ligands facilitate the spontaneous generation of abundant micropores and mesopores in the carbon framework. The hierarchical porous structure provides efficient diffusion channels for PET macromolecular segments and the product BHET, solving the mass transfer limitation problem in non-porous or single-microporous materials.

[0008] 2. Optimized dispersion and anchoring of active sites: The mixed ligand environment of 2-methylimidazole (MI) and 2-ethylimidazole (EI) facilitates the dispersion and anchoring of active sites of zinc ions (Zn²⁺). + The coordination microenvironment in the precursor exhibits diversity. Calcination at 350℃ in air transforms these zinc species into highly dispersed nano-zinc oxide (ZnO) particles, firmly anchored within the nitrogen-doped carbon matrix. The nitrogen-doped carbon not only serves as a high specific surface area support, but its surface species, such as pyridine nitrogen and graphitic nitrogen, also engage in strong electronic interactions with ZnO, stabilizing the ZnO nanoparticles and effectively preventing aggregation and leaching during high-temperature alcoholysis, thus ensuring the catalyst's stability.

[0009] 3. Construction of Synergistic Catalytic Active Centers: In the final ZnO@NC catalyst, nano-ZnO acts as a Lewis acid center, activating the carbonyl carbon atom in the PET ester bond and enhancing its electrophilicity. Simultaneously, the basic nitrogen sites (such as pyridine nitrogen) in the nitrogen-doped carbon framework can activate the hydroxyl groups in the nucleophile ethylene glycol, enhancing its nucleophilic attack capability. This acid-base synergistic catalytic mechanism, combined with the mass transfer advantages brought by the catalyst's high specific surface area and hierarchical pores, achieves highly efficient and selective catalysis of the PET ethylene glycol alcoholysis reaction. This technology effectively overcomes the problems of limited active sites, low specific surface area, and easy deactivation at high temperatures in existing heterogeneous catalysts.

[0010] A method for preparing a nitrogen-doped carbon-supported ZnO catalyst specifically includes the following steps:

[0011] S1. A certain amount of zinc acetate is added to a certain amount of methanol solution and stirred thoroughly to prepare a zinc acetate methanol solution; then, a certain amount of imidazole ligand is added to a certain amount of methanol solution and stirred thoroughly to prepare an imidazole ligand methanol solution;

[0012] S2. The imidazole ligand methanol solution prepared in step S1 was mixed with the zinc acetate methanol solution and stirred at room temperature for 24 h. The mixture was then centrifuged to separate the solid material from the liquid. The resulting precipitate was washed three times with methanol to remove residual impurities. The washed precipitate was dried at 60 °C for 12 h to obtain the metal-organic framework (MOF) precursor.

[0013] S3. The metal-organic framework (MOF) precursor synthesized in step S2 is placed in a muffle furnace and calcined in air to obtain a nitrogen-doped carbon-supported ZnO catalyst.

[0014] In a preferred embodiment, in step S1, the molar ratio of zinc acetate to imidazole ligand is 0.125~0.25; and the molar ratio of 2-methylimidazole (MI) to 2-ethylimidazole (EI) is 0~3.

[0015] In a preferred embodiment, the centrifugation rate in step S2 is 6000~9000 rpm, and the centrifugation time is 2~4 min.

[0016] In a preferred embodiment, the calcination heating conditions in step S3 are to heat to 350°C at a rate of 2°C / min and hold at that temperature for 2 hours.

[0017] An application of a nitrogen-doped carbon-supported ZnO catalyst in the alcoholysis of waste polyester is characterized by placing a certain amount of waste polyester, ethylene glycol, and nitrogen-doped carbon-supported ZnO catalyst in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen gas. The temperature is slowly raised to a set temperature, and the reaction is continuously stirred until a set time is reached. After the reaction is completed, the solution is cooled to 150-160°C and filtered while hot. The solid is washed with deionized water, dried, and weighed to obtain the mass of undepolymerized PET, and the PET conversion rate is calculated. Then, deionized water is added to the filtrate, and the solution is filtered while hot. After the filtrate is cooled to room temperature, it is allowed to stand at a low temperature for 12-24 hours, filtered and dried to obtain BHET, and the BHET yield is calculated by weighing.

[0018] Preferably, the waste polyester has a mass of 2.0~10.0g, the ethylene glycol has a volume of 40~200ml, the catalyst has a mass of 0.025~0.1g, the alcoholysis temperature is 170~210℃, the alcoholysis time is 0.5~4h, and the low-temperature settling temperature is 3~8℃.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] 1. Thanks to the hierarchical channels, highly dispersed active sites, and acid-base synergistic centers constructed by the mixed ligand strategy, this catalyst exhibits higher catalytic activity and BHET selectivity for PET alcoholysis under high temperature conditions than heterogeneous metal oxide catalysts.

[0021] 2. The strong anchoring effect of nitrogen-doped carbon on nano-ZnO, as well as the good thermal and chemical stability of the material itself, enable the catalyst to maintain structural and activity stability under high temperature conditions, thus solving the problem of easy agglomeration and deactivation of heterogeneous catalysts. Detailed Implementation

[0022] The present invention will be further illustrated below with specific examples. These embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0023] Example 1:

[0024] This embodiment provides a method for efficient alcoholysis monomer preparation of waste polyester, and the catalyst preparation method specifically includes the following steps:

[0025] S1. Add 2g of zinc acetate to 48ml of methanol solution and stir thoroughly at room temperature for 10min to prepare zinc acetate methanol solution; then add 4.44g of 2-methylimidazole to 48ml of methanol solution and stir thoroughly to prepare 2-methylimidazole methanol solution;

[0026] S2. The 2-methylimidazolium methanol solution synthesized in step S1 was mixed with a zinc acetate methanol solution and stirred at room temperature for 24 h. The mixture was then centrifuged to separate the solid material from the liquid. The resulting precipitate was washed three times with 20 ml of methanol to remove residual impurities. The washed precipitate was dried at 60 °C for 12 h to obtain the metal-organic framework (MOF) precursor.

[0027] S3. The metal-organic framework (MOF) precursor synthesized in step S2 was placed in a muffle furnace and calcined at 350°C for 2 h in air at a heating rate of 2°C / min to obtain the ZnO@NC catalyst.

[0028] An application of a nitrogen-doped carbon-supported ZnO catalyst in the alcoholysis of waste polyester: 2.0 g PET, 40.0 ml EG, and 0.025 g ZnO@NC catalyst were placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen gas. The temperature was slowly raised to 200 °C with continuous stirring. The reaction time was set to 70 min. After the reaction, the solution was cooled to 160 °C and filtered while hot. The solid was washed with deionized water, dried, and weighed to obtain the mass of undepolymerized PET. The PET conversion rate was calculated to be 100%. Then, deionized water was added to the filtrate, and the solution was filtered while hot. After the filtrate was cooled to room temperature, it was allowed to stand at 3 °C for 24 h. The solution was then filtered and dried to obtain BHET. The BHET yield was calculated to be 71.1%.

[0029] Example 2:

[0030] This embodiment provides a method for efficient alcoholysis monomer preparation of waste polyester, and the catalyst preparation method specifically includes the following steps:

[0031] S1. Add 2g of zinc acetate to 48ml of methanol solution and stir thoroughly at room temperature for 10min to prepare zinc acetate methanol solution; then add 5.13g of 2-ethylimidazole to 48ml of methanol solution and stir thoroughly to prepare 2-ethylimidazole methanol solution;

[0032] S2. The 2-ethylimidazolium methanol solution synthesized in step S1 was mixed with a zinc acetate methanol solution and stirred at room temperature for 24 h. The mixture was then centrifuged to separate the solid material from the liquid. The resulting precipitate was washed three times with 20 ml of methanol to remove residual impurities. The washed precipitate was dried at 60 °C for 12 h to obtain the metal-organic framework (MOF) precursor.

[0033] S3. The metal-organic framework (MOF) precursor synthesized in step S2 was placed in a muffle furnace and calcined at 350°C for 2 h in air at a heating rate of 2°C / min to obtain the ZnO@NC catalyst.

[0034] An application of a nitrogen-doped carbon-supported ZnO catalyst in the alcoholysis of waste polyester: 2.0 g PET, 40.0 ml EG, and 0.025 g ZnO@NC catalyst were placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen gas. The temperature was slowly raised to 200 °C with continuous stirring. The reaction time was set to 70 min. After the reaction, the solution was cooled to 160 °C and filtered while hot. The solid was washed with deionized water, dried, and weighed to obtain the mass of undepolymerized PET. The PET conversion rate was calculated to be 100%. Then, deionized water was added to the filtrate, and the solution was filtered while hot. After the filtrate was cooled to room temperature, it was allowed to stand at 3 °C for 24 h. The solution was then filtered and dried to obtain BHET. The BHET yield was calculated to be 74.3%.

[0035] Example 3:

[0036] This embodiment provides a method for efficient alcoholysis monomer preparation of waste polyester, and the catalyst preparation method specifically includes the following steps:

[0037] S1. Add 2g of zinc acetate to 48ml of methanol solution and stir thoroughly. Stir at room temperature for 10min to prepare zinc acetate methanol solution; then add 2.83g of 2-methylimidazole and 1.11g of 2-ethylimidazole to 48ml of methanol solution and stir thoroughly to prepare mixed imidazole (MI:EI = 3:1) methanol solution.

[0038] S2. The mixed imidazole (MI:EI = 3:1) methanol solution synthesized in step S1 was mixed with a zinc acetate methanol solution and stirred at room temperature for 24 h. The mixture was then centrifuged to separate the solid material from the liquid. The resulting precipitate was washed three times with 20 ml of methanol to remove residual impurities. The washed precipitate was dried at 60 °C for 12 h to obtain the metal-organic framework (MOF) precursor.

[0039] S3. The metal-organic framework (MOF) precursor synthesized in step S2 was placed in a muffle furnace and calcined at 350°C for 2 h in air at a heating rate of 2°C / min to obtain the ZnO@NC catalyst.

[0040] An application of a nitrogen-doped carbon-supported ZnO catalyst in the alcoholysis of waste polyester: 2.0 g PET, 40.0 ml EG, and 0.025 g ZnO@NC catalyst were placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen gas. The temperature was slowly raised to 200 °C with continuous stirring. The reaction time was set to 70 min. After the reaction, the solution was cooled to 160 °C and filtered while hot. The solid was washed with deionized water, dried, and weighed to obtain the mass of undepolymerized PET. The PET conversion rate was calculated to be 100%. Then, deionized water was added to the filtrate, and the solution was filtered while hot. After the filtrate was cooled to room temperature, it was allowed to stand at 3 °C for 24 h. The solution was then filtered and dried to obtain BHET. The BHET yield was calculated to be 82.2%.

[0041] Example 4:

[0042] This embodiment provides a method for efficient alcoholysis monomer preparation of waste polyester, and the catalyst preparation method specifically includes the following steps:

[0043] S1. Add 2g of zinc acetate to 48ml of methanol solution and stir thoroughly. Stir at room temperature for 10min to prepare zinc acetate methanol solution; then add 2.22g of 2-methylimidazole and 2.62g of 2-ethylimidazole to 48ml of methanol solution and stir thoroughly to prepare mixed imidazole (MI:EI = 1:1) methanol solution.

[0044] S2. The mixed imidazole (MI:EI = 1:1) methanol solution synthesized in step S1 was mixed with a zinc acetate methanol solution and stirred at room temperature for 24 h. The mixture was then centrifuged to separate the solid material from the liquid. The resulting precipitate was washed three times with 20 ml of methanol to remove residual impurities. The washed precipitate was dried at 60 °C for 12 h to obtain the metal-organic framework (MOF) precursor.

[0045] S3. The metal-organic framework (MOF) precursor synthesized in step S2 was placed in a muffle furnace and calcined at 350°C for 2 h in air at a heating rate of 2°C / min to obtain the ZnO@NC catalyst.

[0046] An application of a nitrogen-doped carbon-supported ZnO catalyst in the alcoholysis of waste polyester: 2.0 g PET, 40.0 ml EG, and 0.025 g ZnO@NC catalyst were placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen gas. The temperature was slowly raised to 200 °C with continuous stirring. The reaction time was set to 70 min. After the reaction, the solution was cooled to 160 °C and filtered while hot. The solid was washed with deionized water, dried, and weighed to obtain the mass of undepolymerized PET. The PET conversion rate was calculated to be 100%. Then, deionized water was added to the filtrate, and the solution was filtered while hot. After the filtrate was cooled to room temperature, it was allowed to stand at 3 °C for 24 h. The solution was then filtered and dried to obtain BHET. The BHET yield was calculated to be 84.9%.

[0047] Example 5:

[0048] This embodiment provides a method for efficient alcoholysis of waste polyester monomers. The catalyst is prepared according to the method in Example 1, but the difference lies in its application in the alcoholysis of waste polyester: the alcoholysis temperature is raised to 180°C, the alcoholysis reaction time is 120 min, the calculated PET conversion rate is 79.6%, and the calculated BHET yield is 47.8%.

[0049] Example 6:

[0050] This embodiment provides a method for efficient alcoholysis of waste polyester monomers. The catalyst is prepared according to the method in Example 2, but the difference lies in its application in the alcoholysis of waste polyester: the alcoholysis temperature is raised to 180°C, the alcoholysis reaction time is 120 min, the calculated PET conversion rate is 91.6%, and the calculated BHET yield is 67.1%.

[0051] Example 7:

[0052] This embodiment provides a method for efficient alcoholysis of waste polyester monomers. The catalyst is prepared according to the method in Example 3, but the difference lies in its application in the alcoholysis of waste polyester: the alcoholysis temperature is raised to 180°C, the alcoholysis reaction time is 120 min, the calculated PET conversion rate is 96.3%, and the calculated BHET yield is 71.3%.

[0053] Example 8:

[0054] This embodiment provides a method for efficient alcoholysis of waste polyester monomers. The catalyst is prepared according to the method in Example 4, but the difference lies in its application in the alcoholysis of waste polyester: the alcoholysis temperature is raised to 180°C, the alcoholysis reaction time is 120 min, the calculated PET conversion rate is 97.9%, and the calculated BHET yield is 75.1%.

[0055] Comparative Example 1:

[0056] Application of commercial nano-ZnO catalyst in the alcoholysis of waste polyester: 2.0 g PET, 40.0 ml EG, and 0.025 g commercial nano-ZnO catalyst were placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature was slowly raised to 200 °C with continuous stirring. The reaction time was set to 70 min. After the reaction, the solution was cooled to 160 °C and filtered while hot. The solid was washed with deionized water, dried, and weighed to obtain the mass of undepolymerized PET. The PET conversion rate was calculated to be 93.7%. Then, deionized water was added to the filtrate, and the solution was filtered while hot. After the filtrate was cooled to room temperature, it was allowed to stand at 3 °C for 24 h. The solution was then filtered and dried to obtain BHET. The BHET yield was calculated to be 53.8%.

[0057] Comparative Example 2:

[0058] S1. Add 2g of zinc acetate to 48ml of methanol solution and stir thoroughly. Stir at room temperature for 10min to prepare a zinc acetate methanol solution. Then centrifuge to separate the solid material from the liquid. Wash the obtained precipitate three times with 20ml of methanol to remove residual impurities. Dry the washed precipitate at 60℃ for 12h to obtain the zinc acetate catalyst.

[0059] Application of zinc acetate catalyst in the alcoholysis of waste polyester: 2.0 g PET, 40.0 ml EG, and 0.025 g zinc acetate catalyst were placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature was slowly raised to 200 °C with continuous stirring. The reaction time was set to 70 min. After the reaction, the solution was cooled to 160 °C and filtered while hot. The solid was washed with deionized water, dried, and weighed to obtain the mass of undepolymerized PET. The PET conversion rate was calculated to be 92.3%. Then, deionized water was added to the filtrate, and the solution was filtered while hot. After the filtrate was cooled to room temperature, it was allowed to stand at 3 °C for 24 h. The solution was then filtered and dried to obtain BHET. The BHET yield was calculated to be 43.1%.

[0060] Comparative Example 3:

[0061] S1. Add 2g of zinc acetate to 48ml of methanol solution and stir thoroughly. Stir at room temperature for 10min to prepare zinc acetate methanol solution; then add 2.22g of 2-methylimidazole and 2.62g of 2-ethylimidazole to 48ml of methanol solution and stir thoroughly to prepare mixed imidazole (MI:EI = 1:1) methanol solution.

[0062] S2. The mixed imidazole (MI:EI = 1:1) methanol solution synthesized in step S1 was mixed with a zinc acetate methanol solution and stirred at room temperature for 24 h. The mixture was then centrifuged to separate the solid material from the liquid. The resulting precipitate was washed three times with 20 ml of methanol to remove residual impurities. The washed precipitate was dried at 60 °C for 12 h to obtain a metal-organic framework (MOF) catalyst.

[0063] Application of metal-organic framework (MOF) catalysts in the alcoholysis of waste polyester: 2.0 g PET, 40.0 ml EG, and 0.025 g MOF catalyst were placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature was slowly raised to 200 °C with continuous stirring. The reaction time was set to 70 min. After the reaction, the solution was cooled to 160 °C and filtered while hot. The solid was washed with deionized water, dried, and weighed to obtain the mass of undepolymerized PET. The PET conversion rate was calculated to be 96.7%. Then, deionized water was added to the filtrate, and the solution was filtered while hot. After the filtrate was cooled to room temperature, it was allowed to stand at 3 °C for 24 h. The solution was then filtered and dried to obtain BHET. The BHET yield was calculated to be 61.4%.

[0064] Comparative Example 4:

[0065] S1. Add 2g of zinc acetate to 48ml of methanol solution and stir thoroughly. Stir at room temperature for 10min to prepare zinc acetate methanol solution; then add 2.83g of 2-methylimidazole and 1.11g of 2-ethylimidazole to 48ml of methanol solution and stir thoroughly to prepare mixed imidazole (MI:EI = 3:1) methanol solution.

[0066] S2. The mixed imidazole (MI:EI = 3:1) methanol solution synthesized in step S1 was mixed with a zinc acetate methanol solution and stirred at room temperature for 24 h. The mixture was then centrifuged to separate the solid material from the liquid. The resulting precipitate was washed three times with 20 ml of methanol to remove residual impurities. The washed precipitate was dried at 60 °C for 12 h to obtain a metal-organic framework (MOF) catalyst.

[0067] Application of metal-organic framework (MOF) catalysts in the alcoholysis of waste polyester: 2.0 g PET, 40.0 ml EG, and 0.025 g MOF catalyst were placed in a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. The temperature was slowly raised to 200 °C with continuous stirring. The reaction time was set to 70 min. After the reaction, the solution was cooled to 160 °C and filtered while hot. The solid was washed with deionized water, dried, and weighed to obtain the mass of undepolymerized PET. The PET conversion rate was calculated to be 97.1%. Then, deionized water was added to the filtrate, and the solution was filtered while hot. After the filtrate was cooled to room temperature, it was allowed to stand at 3 °C for 24 h. The solution was then filtered and dried to obtain BHET. The BHET yield was calculated to be 64.3%.

[0068] The embodiments and comparative examples described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

[0069] Performance Evaluation:

[0070] The embodiments and comparative examples were tested according to the evaluation criteria proposed above:

[0071] (1) PET conversion rate (%):

[0072]

[0073] The initial mass of waste polyester (PET) fed into the reaction vessel.

[0074] After the depolymerization reaction is completed, the mass of unreacted waste polyester (PET) recovered through filtration is [data missing].

[0075] (2) BHET yield (%):

[0076]

[0077] W BHET After the reaction is complete, the actual mass of BHET is obtained by separation and drying.

[0078] M BHET BHET molar mass (g / mol)

[0079] W 聚酯 The initial mass of waste polyester (PET) fed into the reaction vessel.

[0080] M聚酯 Molar mass of a repeating PET unit (g / mol)

[0081]

[0082] Based on the test data presented in the table, the performance parameters of Examples 1-8 and Comparative Examples 1-4 were analyzed. The results showed that the nitrogen-doped carbon-supported ZnO catalyst derived from metal-organic frameworks (MOFs) has high catalytic activity and selectivity for catalyzing the depolymerization of waste polyester (PET) into BHET, and maintains high catalytic activity even under high temperature conditions.

Claims

1. A method for efficiently alcoholyzing waste polyester monomers, characterized in that, First, a heterogeneous catalyst is prepared, which includes the following raw materials: zinc acetate, methanol, 2-methylimidazole, and 2-ethylimidazole. The preparation method includes the following steps: S1. Add a certain amount of zinc acetate to a certain amount of methanol solution and stir thoroughly to prepare a zinc acetate methanol solution; then add a certain amount of imidazole ligand to a certain amount of methanol solution and stir thoroughly to prepare an imidazole ligand methanol solution. S2. The imidazole ligand methanol solution synthesized in step S1 is mixed with zinc acetate methanol solution and stirred at room temperature for 24 h. Then the mixture is centrifuged to separate the solid material from the liquid. The resulting precipitate is washed three times with methanol to remove residual impurities. The washed precipitate is dried at 60 °C for 12 h to obtain the metal-organic framework precursor. S3. The metal-organic framework precursor synthesized in step S2 is placed in a muffle furnace and calcined in air atmosphere to obtain a nitrogen-doped carbon-supported ZnO catalyst. S4. Place a certain amount of waste polyester, ethylene glycol, and catalyst into a three-necked flask connected to a thermometer, condenser, stirrer, and nitrogen. Slowly heat to the set temperature and stir continuously for the set time. After the reaction is complete, cool the solution to 150~160℃ and filter while hot. Wash the filtered solid with deionized water, dry and weigh to obtain the mass of undepolymerized PET, and calculate the PET conversion rate. Then add deionized water to the filtrate, filter while hot, cool the filtrate to room temperature, and let it stand at low temperature for 12~24h. Filter and dry to obtain BHET, weigh and calculate the BHET yield.

2. The method for efficient alcoholysis of waste polyester monomers according to claim 1, characterized in that, In step S1, the molar ratio of zinc acetate to imidazole ligand is 0.125~0.25; the imidazole ligand is any one or both of 2-methylimidazole and 2-ethylimidazole.

3. The method for efficient alcoholysis of waste polyester monomers according to claim 1, characterized in that, In step S2, the centrifugation rate is 6000~9000 rpm and the centrifugation time is 2~4 min.

4. The method for efficient alcoholysis of waste polyester monomers according to claim 1, characterized in that, In step S3, the calcination heating conditions are to raise the temperature to 350℃ at a rate of 2℃ / min and hold it at that temperature for 2 hours.

5. The method for efficient alcoholysis of waste polyester monomers according to claim 1, characterized in that, In step S4, the mass of waste polyester is 2.0~10.0g, the volume of ethylene glycol is 40~200ml, the mass of catalyst is 0.025~0.1g, the alcoholysis temperature is 170~210℃, the alcoholysis time is 0.5~4h, and the low-temperature settling temperature is 3~8℃.

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