Catalyst for alcoholysis of waste pet and its preparation method and application

Abstract

This invention provides a catalyst for the alcoholysis of waste PET, its preparation method, and its application, relating to the field of polymer material recycling technology. The catalyst is prepared in situ from a raw material comprising a zinc source, urea, and at least one auxiliary metal salt. The zinc source is a soluble zinc salt, and the metal element in the auxiliary metal salt is selected from one or more of Mg, Mn, Al, Co, and Na. This catalyst, through the synergistic effect of the urea ligand and the Zn-based active center, effectively promotes the complete cleavage of the PET molecular chain via coordination activation and acid-base synergistic mechanisms. Simultaneously, the introduction of a specific auxiliary metal optimizes the distribution of Lewis acidic sites, improving the selectivity of the target product, diethylene phthalate (DPET). The catalyst system of this application is homogeneous and transparent, exhibits good mass transfer performance, requires no neutralization treatment after the reaction, does not produce saline wastewater, leaves no toxic residues, and is environmentally friendly, making it suitable for the green and efficient chemical recycling of waste PET.

Description

Technical Field

[0001] This invention relates to the field of polymer material recycling technology, and in particular to a catalyst for the alcoholysis of waste PET, its preparation method, and its application. Background Technology

[0002] With the widespread use of polyethylene terephthalate (PET) plastic products in packaging containers (such as beverage bottles and food packaging boxes), textile fibers (such as polyester fibers), and functional films, the global generation of waste PET is increasing year by year. Due to the stable molecular chain structure of PET and its long natural degradation cycle of hundreds of years, large amounts of waste PET have formed serious "white pollution" in terrestrial and marine environments, posing a dual threat to ecosystems and the human living environment. Therefore, there is an urgent need to develop efficient and sustainable recycling technologies.

[0003] Currently, waste PET recycling technologies are mainly divided into two categories: physical recycling and chemical recycling. Physical recycling achieves the recycling of PET through melt regeneration, but it suffers from the problem of gradual performance degradation with each iteration, making it only suitable for low-value-added applications. Chemical recycling, on the other hand, degrades waste PET into smaller molecule compounds by breaking chemical bonds, effectively avoiding performance degradation. In particular, chemical alcoholysis recycling technology can, under specific conditions, directionally convert waste PET into high-purity chemical raw materials such as dimethyl terephthalate (DMT) or diethylene terephthalate (BHET). These raw materials can be directly used to resynthesize new PET products, truly achieving a closed-loop cycle "from plastic to plastic."

[0004] However, in the alcoholysis reaction of waste PET, the ester bonds in the PET molecular chain need to be efficiently broken under the action of a catalyst. The catalytic activity, product selectivity, stability, and recyclability of the catalyst directly determine the rate, energy consumption, product purity, and process economy of the alcoholysis reaction. Therefore, developing a highly efficient catalyst with high activity, high selectivity, low corrosivity, and good recyclability is key to overcoming the industrialization bottleneck of waste PET alcoholysis recycling technology and promoting its large-scale application.

[0005] Currently, the catalysts used for the alcoholysis of waste PET mainly include four types: alkali metals, organometallic compounds, single metal oxides, and supported single metals. However, all of them have insurmountable technical defects: (1) Alkali metal catalysts (such as NaOH, CH3ONa, etc.): Although they have certain catalytic activity, they are strongly alkaline and will cause serious corrosion to commonly used reaction equipment such as stainless steel, shorten the equipment life and increase maintenance costs. At the same time, they may cause metal ions to dissolve and pollute the products. After the reaction, the system is strongly alkaline and needs to be neutralized by adding acid, generating a large amount of salt wastewater, increasing the wastewater treatment cost, and does not meet the environmental protection requirements of green chemical industry.

[0006] (2) Organometallic catalysts (such as tetrabutyl titanate, stannous octoate, etc.): As homogeneous catalysts, they have high catalytic activity and mild reaction conditions, but they have obvious toxicity problems. These metal compounds are bioaccumulative and easily remain in target products such as DMT and BHET. They need to be removed through complex purification processes such as multiple distillations and recrystallization, which significantly increases production costs. If not handled properly, they may also cause secondary environmental pollution through the discharge of waste residue and wastewater, which cannot meet the safety requirements of food-grade PET raw materials.

[0007] (3) Single metal oxide catalysts (such as ZnO, MgO, etc.): These are heterogeneous systems, environmentally friendly and without toxic residues, but the catalytic performance of a single component has shortcomings. Due to the fixed electronic structure and adsorption capacity of a single metal active site, it is difficult to simultaneously achieve efficient activation of PET ester bonds and directional adsorption of alcohol solvents (such as methanol and ethylene glycol), resulting in difficulty in achieving both catalytic activity and product selectivity. To achieve ideal alcoholysis efficiency, it is usually necessary to continue the reaction at a high temperature of 180–220°C for 4–6 hours, which consumes a lot of energy and may trigger side reactions such as PET carbonization and alcohol dehydration, reducing product purity and hindering energy-saving industrial production.

[0008] (4) Supported single-metal catalysts (such as Zn / HAAP, Ti / SiO2, etc.): These catalysts improve dispersibility by loading single-metal active components onto the surface of mesoporous supports, but the fundamental problems remain unresolved. On the one hand, the types and numbers of single-metal active sites are limited, making it difficult to form a synergistic catalytic effect, resulting in limited improvement in catalytic efficiency. On the other hand, most supports and metal components are only bound by weak interactions, making it easy for metal particles to aggregate or detach during the reaction, leading to rapid decay of catalytic activity. Data shows that the activity of such catalysts typically decreases by more than 30% after 3–4 uses, requiring frequent replacement and significantly increasing operating costs.

[0009] In summary, none of the four existing catalyst types can simultaneously meet the comprehensive requirements of "high catalytic activity, high product selectivity, low energy consumption, environmental friendliness, and recyclability" for the recovery of waste PET through alcoholysis. These technical pain points have become key bottlenecks restricting the industrialization of waste PET alcoholysis technology. Therefore, it is urgent to develop a new type of high-efficiency catalyst to overcome the shortcomings of existing technologies and promote the development of waste PET towards high-value, closed-loop recycling.

[0010] In view of this, the present invention is hereby proposed. Summary of the Invention

[0011] The first objective of this invention is to provide a catalyst for the alcoholysis of waste PET. The catalyst achieves efficient and complete depolymerization of waste PET through in-situ reaction of a zinc source, urea and a specific auxiliary metal salt, and improves the selectivity of the target product BHET. It also has the advantages of being environmentally friendly, non-toxic and requiring no neutralization treatment.

[0012] A second objective of this invention is to provide a method for preparing the above-mentioned catalyst for the alcoholysis of waste PET.

[0013] A third objective of the present invention is to provide an application of the above-mentioned catalyst for the alcoholysis of waste PET.

[0014] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: This invention provides a catalyst for the alcoholysis of waste PET, wherein the catalyst is prepared by in-situ reaction from a raw material comprising a zinc source, urea, and at least one auxiliary metal salt, wherein: The zinc source is a soluble zinc salt; The metal element in the auxiliary metal salt is selected from one or more of Mg, Mn, Al, Co, and Na.

[0015] Preferably, the metal element in the auxiliary metal salt includes Mg.

[0016] Furthermore, the soluble zinc salt is zinc acetate hydrate.

[0017] Furthermore, the auxiliary metal salt is one or a mixture of Mg(Ac)2•4H2O (magnesium acetate tetrahydrate), Mn(Ac)2•4H2O (manganese(II) acetate tetrahydrate), NaAc (sodium acetate), Al(Ac)3•4H2O (aluminum acetate tetrahydrate), and Co(Ac)2•4H2O (cobalt(II) acetate tetrahydrate).

[0018] Furthermore, the mass ratio of the auxiliary metal salt to the zinc source is 3~5:15~25.

[0019] Furthermore, the mass ratio of urea to zinc source is 25~30:15~25.

[0020] The present invention provides a method for preparing the above-mentioned catalyst for the alcoholysis of waste PET, the preparation method comprising the following steps: The zinc source, urea, and auxiliary metal salt are mixed, heated and stirred under an inert atmosphere to allow the components to react in situ and generate a homogeneous and transparent catalytic system. After cooling, the catalyst is obtained.

[0021] Furthermore, the reaction temperature is 90~110°C, and the reaction time is 2~3 hours; Preferably, the stirring speed is 300~450 r / min.

[0022] The application of the catalyst for alcoholysis of waste PET provided by the present invention in the chemical recycling of waste PET is to catalyze the alcoholysis reaction between PET and ethylene glycol, thereby depolymerizing waste PET into diethylene terephthalate.

[0023] Furthermore, the amount of catalyst used in the alcoholysis reaction is 0.5 to 0.8 wt% of the waste PET mass.

[0024] Furthermore, the alcoholysis reaction is carried out at a temperature of 160~170°C.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a catalyst for the alcoholysis of waste PET. The catalyst is prepared in situ from a raw material comprising a zinc source, urea, and at least one auxiliary metal salt. The zinc source is a soluble zinc salt, and the metal element in the auxiliary metal salt is selected from one or more of Mg, Mn, Al, Co, and Na. The catalyst, through the synergistic effect of the urea ligand and the Zn-based active center, can effectively promote the breaking of ester bonds in the PET molecular chain via coordination activation and acid-base synergy mechanisms, achieving efficient depolymerization. Different catalyst compositions have a significant impact on product distribution; the introduction of specific auxiliary metals helps optimize the distribution of Lewis acidic sites and improves the selectivity of the target product BHET. Furthermore, the catalyst of this application uses urea as a ligand, posing no risk of toxic residues. The reaction process requires no neutralization treatment and does not generate saline wastewater, making it environmentally friendly and meeting the requirements of green chemistry.

[0026] The present invention provides a method for preparing a catalyst for the alcoholysis of waste PET. The method includes: mixing a zinc source, urea, and an auxiliary metal salt; heating and stirring under an inert atmosphere to allow the components to react in situ, generating a homogeneous and transparent catalytic system; and cooling to obtain the catalyst. This preparation method has the advantages of simple processing and ease of industrial production.

[0027] The catalyst for alcoholysis of waste PET provided by this invention can be widely used to catalyze the alcoholysis reaction between PET and ethylene glycol, thereby depolymerizing waste PET into diethylene terephthalate. Attached Figure Description

[0028] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0029] Figure 1 The images show observations of PET depolymerization experiments in the various embodiments and comparative examples provided in Experimental Example 1 of this invention.

[0030] Figure 2 The diagram shows the depolymerization solution treatment and product observation of each embodiment and comparative example provided in Experimental Example 1 of this invention.

[0031] Figure 3 Liquid chromatograms of the depolymerization products of the various embodiments and comparative examples provided in Experimental Example 1 of the present invention. Detailed Implementation

[0032] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] According to one aspect of the invention, a catalyst for the alcoholysis of waste PET is prepared by in-situ reaction of a raw material comprising a zinc source, urea, and at least one auxiliary metal salt, wherein: The zinc source is a soluble zinc salt; the metal element in the auxiliary metal salt is selected from one or more of Mg, Mn, Al, Co, and Na.

[0034] This invention provides a catalyst for the alcoholysis of waste PET. The catalyst is prepared in situ from a raw material comprising a zinc source, urea, and at least one auxiliary metal salt. The zinc source is a soluble zinc salt, and the metal element in the auxiliary metal salt is selected from one or more of Mg, Mn, Al, Co, and Na. The catalyst, through the synergistic effect of the urea ligand and the Zn-based active center, can effectively promote the breaking of ester bonds in the PET molecular chain via coordination activation and acid-base synergy mechanisms, achieving efficient depolymerization. Different catalyst compositions have a significant impact on product distribution; the introduction of specific auxiliary metals helps optimize the distribution of Lewis acidic sites and improves the selectivity of the target product BHET.

[0035] Furthermore, the catalyst in this application uses urea as a ligand, which poses no risk of toxic residues. The reaction process does not require neutralization treatment and does not generate saline wastewater, making it environmentally friendly and in line with green chemical requirements.

[0036] In a preferred embodiment of the present invention, the soluble zinc salt is zinc acetate hydrate.

[0037] In a preferred embodiment of the present invention, the auxiliary metal salt is one or a mixture of Mg(Ac)2•4H2O, Mn(Ac)2•4H2O, NaAc, Al(Ac)3•4H2O, and Co(Ac)2•4H2O.

[0038] In a preferred embodiment of the present invention, the mass ratio of the auxiliary metal salt to the zinc source is 3~5:15~25.

[0039] In a preferred embodiment of the present invention, the mass ratio of urea to zinc source is 25~30:15~25.

[0040] According to one aspect of the present invention, a method for preparing the above-described catalyst for the alcoholysis of waste PET, the method comprising the following steps: The zinc source, urea, and auxiliary metal salt are mixed, heated and stirred under an inert atmosphere to allow the components to react in situ and generate a homogeneous and transparent catalytic system. After cooling, the catalyst is obtained.

[0041] The present invention provides a method for preparing a catalyst for the alcoholysis of waste PET. The method includes: mixing a zinc source, urea, and an auxiliary metal salt; heating and stirring under an inert atmosphere to allow the components to react in situ, generating a homogeneous and transparent catalytic system; and cooling to obtain the catalyst. This preparation method has the advantages of simple processing and ease of industrial production.

[0042] In a preferred embodiment of the present invention, the reaction temperature is 90~110°C and the reaction time is 2~3 hours; As an optional implementation, the reaction temperature is 90~110°C, for example, it can be 90°C, 95°C, 100°C, 105°C or 110°C, or any value between 90~110°C; the reaction time is 2~3 hours, for example, it can be 2 hours, 2.5 hours or 3 hours, or any value between 2~3 hours.

[0043] In a preferred embodiment of the present invention, the stirring speed is 300~450 r / min.

[0044] As an optional implementation, the stirring speed is 300~450 r / min, for example, it can be 300 r / min, 350 r / min, 400 r / min, 450 r / min, or any value between 300~450 r / min.

[0045] According to one aspect of the present invention, the above-mentioned catalyst for the alcoholysis of waste PET is applied in the chemical recycling of waste PET, wherein the application is to catalyze the alcoholysis reaction between PET and ethylene glycol, thereby depolymerizing waste PET into diethylene terephthalate.

[0046] The catalyst for alcoholysis of waste PET provided by this invention can be widely used to catalyze the alcoholysis reaction between PET and ethylene glycol, thereby depolymerizing waste PET into diethylene terephthalate.

[0047] In a preferred embodiment of the present invention, the amount of catalyst used in the alcoholysis reaction is 0.5 to 0.8 wt% of the mass of waste PET.

[0048] As an optional implementation, the amount of catalyst used in the alcoholysis reaction is 0.5 to 0.8 wt% of the mass of waste PET, for example, it can be 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, or any value between 0.5 and 0.8 wt%.

[0049] In a preferred embodiment of the present invention, the alcoholysis reaction is carried out at a temperature of 160-170°C.

[0050] As an optional implementation, the alcoholysis reaction is carried out at a temperature of 160~170°C, for example, 160°C, 162°C, 165°C, 168°C, 170°C, or any value between 160~170°C.

[0051] The technical solution of the present invention will be further described below with reference to the embodiments.

[0052] Example 1 A catalyst for the alcoholysis of waste PET, the catalyst preparation method comprising the following steps: 1. Catalyst preparation: Weigh 18.0 g of zinc acetate dihydrate (Zn(Ac)₂•2H₂O) and 25.0 g of urea, place them in a three-necked round-bottom flask, add a magnetic stir bar, and place the flask in an oil bath. Turn on the magnetic stirrer, controlling the speed at 300-450 r / min, purge with nitrogen three times, reflux, and then turn on the heater, controlling the temperature at 90-110 ℃. With continuous heating and stirring, the solid gradually dissolves and reacts. Continue stirring for 2-3 h until a homogeneous and transparent liquid is formed.

[0053] Add 4.5 g Mg(Ac)2•4H2O and stir continuously for 1-2 h to form a homogeneous solution. Stop the oil bath heating and allow the flask to cool naturally to room temperature in the oil bath. Transfer it to a clean, dry reagent bottle, seal and store for later use, to obtain the Zn-based catalyst ZnMgUA from the alcoholysis of waste PET.

[0054] 2. The process of PET alcoholysis: Weigh 100 g of waste PET and 300 ml of MEG (ethylene glycol) into a 500 ml three-necked flask. After purging with N2 three times, mechanically stir and heat at 200 rpm. When the temperature inside the flask reaches 160 °C, add 0.5–0.8 g of catalyst ZnMgUA and maintain the temperature at 160–165 °C with continuous stirring to depolymerize until the PET is completely depolymerized and a homogeneous, transparent solution appears in the flask. Stop the reaction and cool to room temperature.

[0055] 3. Separation and purification of the product: The mixed solution, cooled to room temperature, was poured into 2 L of deionized water, and a white precipitate was immediately formed. After standing for 2 h, the precipitate was filtered, washed three times with deionized water, and the resulting white product was placed in an oven and dried at 80 ℃ for 12 h to obtain the depolymerization product, which was named BHET-ZnMgUA.

[0056] Example 2 A catalyst for the alcoholysis of waste PET, the catalyst preparation method comprising the following steps: 1. Catalyst preparation: Weigh out 18 g of Zn(Ac)₂•2H₂O and 25 g of Urea, respectively. Then, place the weighed Zn(Ac)₂•2H₂O and Urea into a three-necked round-bottom flask, add a magnetic stir bar, and place the flask in an oil bath. Turn on the magnetic stirrer, controlling the speed at 300~450 r / min, purge with nitrogen three times, reflux, and then turn on the heater, controlling the temperature at 90~110 ℃. With continuous heating and stirring, the solid gradually dissolves and reacts. Continue stirring for 2-3 h until a homogeneous and transparent liquid is formed.

[0057] Add 2.2 g Mg(Ac)₂•4H₂O and 1.0 g NaAc, and continue stirring for 1-2 h to form a homogeneous solution. Stop the oil bath heating and allow the flask to cool naturally to room temperature in the oil bath. Transfer it to a clean, dry reagent bottle, seal and store for later use, to obtain the Zn-based catalyst ZnMgNaUA from the alcoholysis of waste PET.

[0058] 2. The process of PET alcoholysis: Weigh 100 g of waste PET and 300 ml of MEG into a 500 ml three-necked flask. After purging with N2 three times, mechanically stir and heat at 200 rpm. When the temperature inside the flask reaches 160 °C, add 0.5–0.8 g of catalyst ZnMgNaUA. Maintain the temperature at 160–165 °C and continue stirring to depolymerize until the PET is completely depolymerized and a homogeneous, transparent solution appears in the flask. Stop the reaction and cool to room temperature.

[0059] 3. Separation and purification of the product: The obtained mixed solution was poured into 2 L of deionized water, and a white precipitate was immediately formed. After standing for 2 h, the solution was filtered and washed 3 times with deionized water. The obtained white product was placed in an oven and dried at 80 ℃ for 12 h to obtain the depolymerization product, which was named BHET-ZnMgNaUA.

[0060] Example 3 A catalyst for the alcoholysis of waste PET, the catalyst preparation method comprising the following steps: 1. Catalyst preparation: Weigh out 18 g of Zn(Ac)₂•2H₂O and 25 g of Urea, respectively. Then, place the weighed Zn(Ac)₂•2H₂O and Urea into a three-necked round-bottom flask, add a magnetic stir bar, and place the flask in an oil bath. Turn on the magnetic stirrer, controlling the speed at 300~450 r / min, purge with nitrogen three times, reflux, and then turn on the heater, controlling the temperature at 90~110 ℃. With continuous heating and stirring, the solid gradually dissolves and reacts. Continue stirring for 2-3 h until a homogeneous and transparent liquid is formed.

[0061] Add 2.2 g Mg(Ac)₂•4H₂O and 2.5 g Mn(Ac)₂•4H₂O, and stir continuously for 1-2 h to form a homogeneous solution. Stop the oil bath heating and allow the flask to cool naturally to room temperature in the oil bath. Transfer it to a clean, dry reagent bottle, seal and store for later use, to obtain the Zn-based catalyst ZnMgMnUA from the alcoholysis of waste PET.

[0062] 2. The process of PET alcoholysis: Weigh 100 g of waste PET and 300 ml of MEG into a 500 ml three-necked flask. After purging with N2 three times, mechanically stir and heat at 200 rpm. When the temperature inside the flask reaches 160 °C, add 0.5–0.8 g of catalyst ZnMgMnUA and maintain the temperature at 160–165 °C with continuous stirring to depolymerize until the PET is completely depolymerized and a homogeneous, transparent solution appears in the flask. Stop the reaction and cool to room temperature.

[0063] 3. Separation and purification of the product: The obtained mixed solution was poured into 2 L of deionized water, and a white precipitate was immediately formed. After standing for 2 h, the solution was filtered and washed 3 times with deionized water. The obtained white product was placed in an oven and dried at 80 ℃ for 12 h to obtain the depolymerization product, which was named BHET-ZnMgMnUA.

[0064] Example 4 A catalyst for the alcoholysis of waste PET, the catalyst preparation method comprising the following steps: 1. Catalyst preparation: Weigh out 18 g of Zn(Ac)₂•2H₂O and 25 g of Urea, respectively. Then, place the weighed Zn(Ac)₂•2H₂O and Urea into a three-necked round-bottom flask, add a magnetic stir bar, and place the flask in an oil bath. Turn on the magnetic stirrer, controlling the speed at 300~450 r / min, purge with nitrogen three times, reflux, and then turn on the heater, controlling the temperature at 90~110 ℃. With continuous heating and stirring, the solid gradually dissolves and reacts. Continue stirring for 2-3 h until a homogeneous and transparent liquid is formed.

[0065] Add 2.2 g Mg(Ac)₂•4H₂O and 2.0 g Al(Ac)₃•4H₂O, and stir continuously for 1-2 h to form a homogeneous solution. Stop the oil bath heating and allow the flask to cool naturally to room temperature in the oil bath. Transfer it to a clean, dry reagent bottle, seal and store for later use, to obtain the Zn-based catalyst ZnMgAlUA from the alcoholysis of waste PET.

[0066] 2. The process of PET alcoholysis: Weigh 100 g of waste PET and 300 ml of MEG into a 500 ml three-necked flask. After purging with N2 three times, mechanically stir and heat at 200 rpm. When the temperature inside the flask reaches 160 °C, add 0.5–0.8 g of catalyst ZnMgAlUA. Maintain the temperature at 160–165 °C and continue stirring to depolymerize until the PET is completely depolymerized and a homogeneous, transparent solution appears in the flask. Stop the reaction and cool to room temperature.

[0067] 3. Separation and purification of the product: The obtained mixed solution was poured into 2 L of deionized water, and a white precipitate was immediately formed. After standing for 2 h, the solution was filtered and washed 3 times with deionized water. The obtained white product was placed in an oven and dried at 80 ℃ for 12 h to obtain the depolymerization product, which was named BHET-ZnMgAlUA.

[0068] Example 5 A catalyst for the alcoholysis of waste PET, the catalyst preparation method comprising the following steps: 1. Catalyst preparation: Weigh out 18 g of Zn(Ac)₂•2H₂O and 25 g of Urea, respectively. Then, place the weighed Zn(Ac)₂•2H₂O and Urea into a three-necked round-bottom flask, add a magnetic stir bar, and place the flask in an oil bath. Turn on the magnetic stirrer, controlling the speed at 300~450 r / min, purge with nitrogen three times, reflux, and then turn on the heater, controlling the temperature at 90~110 ℃. With continuous heating and stirring, the solid gradually dissolves and reacts. Continue stirring for 2-3 h until a homogeneous and transparent liquid is formed.

[0069] Add 2.2 g Mg(Ac)₂•4H₂O and 2.5 g Co(Ac)₂•4H₂O, and stir continuously for 1-2 h to form a homogeneous solution. Stop the oil bath heating and allow the flask to cool naturally to room temperature in the oil bath. Transfer it to a clean, dry reagent bottle, seal and store for later use, to obtain the Zn-based catalyst ZnMgCoUA from the alcoholysis of waste PET.

[0070] 2. The process of PET alcoholysis: Weigh 100 g of waste PET and 300 ml of MEG into a 500 ml three-necked flask. After purging with N2 three times, mechanically stir and heat at 200 rpm. When the temperature inside the flask reaches 160 °C, add 0.5–0.8 g of catalyst ZnMgCoUA and maintain the temperature at 160–165 °C with continuous stirring to depolymerize until the PET is completely depolymerized and a homogeneous, transparent solution appears in the flask. Stop the reaction and cool to room temperature.

[0071] 3. Separation and purification of the product: The obtained mixed solution was poured into 2 L of deionized water, and a white precipitate was immediately formed. After standing for 2 h, the solution was filtered and washed 3 times with deionized water. The obtained white product was placed in an oven and dried at 80 ℃ for 12 h to obtain the depolymerization product, which was named BHET-ZnMgCoUA.

[0072] Example 6 A catalyst for the alcoholysis of waste PET, the catalyst preparation method comprising the following steps: 1. Catalyst preparation: Weigh out 18 g of Zn(Ac)₂•2H₂O and 25 g of Urea, respectively. Then, place the weighed Zn(Ac)₂•2H₂O and Urea into a three-necked round-bottom flask, add a magnetic stir bar, and place the flask in an oil bath. Turn on the magnetic stirrer, controlling the speed at 300~450 r / min, purge with nitrogen three times, reflux, and then turn on the heater, controlling the temperature at 90~110 ℃. With continuous heating and stirring, the solid gradually dissolves and reacts. Continue stirring for 2-3 h until a homogeneous and transparent liquid is formed.

[0073] Add 5 g of Mn(Ac)2•4H2O and stir continuously for 1-2 h to form a homogeneous solution. Stop the oil bath heating and allow the flask to cool naturally to room temperature in the oil bath. Transfer it to a clean, dry reagent bottle, seal and store for later use, to obtain the Zn-based catalyst ZnMnUA from the alcoholysis of waste PET.

[0074] 2. The process of PET alcoholysis: Weigh 100 g of waste PET and 300 ml of MEG into a 500 ml three-necked flask. After purging with N2 three times, mechanically stir and heat at 200 rpm. When the temperature inside the flask reaches 160 °C, add 0.5–0.8 g of catalyst ZnMnUA and maintain the temperature at 160–165 °C with continuous stirring to depolymerize until R-PET is completely depolymerized and a homogeneous, transparent solution appears in the flask. Stop the reaction and cool to room temperature.

[0075] 3. Separation and purification of the product: The obtained mixed solution was poured into 2 L of deionized water, and a white precipitate was immediately formed. After standing for 2 h, the solution was filtered and washed 3 times with deionized water. The obtained white product was placed in an oven and dried at 80 ℃ for 12 h to obtain the depolymerization product, which was named BHET-ZnMnUA.

[0076] Example 7 A catalyst for the alcoholysis of waste PET, the catalyst preparation method comprising the following steps: 1. Catalyst preparation: Weigh out 18 g of Zn(Ac)₂•2H₂O and 25 g of Urea, respectively. Then, place the weighed Zn(Ac)₂•2H₂O and Urea into a three-necked round-bottom flask, add a magnetic stir bar, and place the flask in an oil bath. Turn on the magnetic stirrer, controlling the speed at 300~450 r / min, purge with nitrogen three times, reflux, and then turn on the heater, controlling the temperature at 90~110 ℃. With continuous heating and stirring, the solid gradually dissolves and reacts. Continue stirring for 2-3 h until a homogeneous and transparent liquid is formed.

[0077] Add 2.5 g Mn(Ac)₂•4H₂O and 2.5 g Co(Ac)₂•4H₂O, and stir continuously for 1-2 h to form a homogeneous solution. Stop the oil bath heating and allow the flask to cool naturally to room temperature in the oil bath. Transfer it to a clean, dry reagent bottle, seal and store for later use, to obtain the Zn-based catalyst ZnMnCoUA from the alcoholysis of waste PET.

[0078] 2. The process of PET alcoholysis: Weigh 100 g of waste PET and 300 ml of MEG into a 500 ml three-necked flask. After purging with N2 three times, mechanically stir and heat at 200 rpm. When the temperature inside the flask reaches 160 °C, add 0.5–0.8 g of catalyst ZnMnCoUA and maintain the temperature at 160–165 °C with continuous stirring to depolymerize until R-PET is completely depolymerized and a homogeneous, transparent solution appears in the flask. Stop the reaction and cool to room temperature.

[0079] 3. Separation and purification of the product: The obtained mixed solution was poured into 2 L of deionized water, and a white precipitate was immediately formed. After standing for 2 h, the mixture was filtered and washed 3 times with deionized water. The obtained white product was placed in an oven and dried at 80 ℃ for 12 h to obtain the depolymerization product, which was named BHET-ZnMnCoUA.

[0080] Comparative Example 1 A catalyst for the alcoholysis of waste PET, the catalyst preparation method comprising the following steps: 1. Catalyst preparation: Weigh out 22 g of Zn(Ac)₂•2H₂O and 25 g of Urea, respectively. Then, place the weighed Zn(Ac)₂•2H₂O and Urea into a three-necked round-bottom flask, add a magnetic stir bar, and place the flask in an oil bath. Turn on the magnetic stirrer, controlling the speed at 300~450 r / min, purge with nitrogen three times, reflux, and then turn on the heater, controlling the temperature at 90~110℃. With continuous heating and stirring, the solid gradually dissolves and reacts. Continue stirring for 2-3 hours until a homogeneous and transparent liquid is formed.

[0081] Stop heating in the oil bath and allow the flask to cool naturally to room temperature in the oil bath. Transfer it to a clean, dry reagent bottle, seal and store for later use, to obtain ZnUA, a Zn-based catalyst from the alcoholysis of waste PET.

[0082] 2. The process of PET alcoholysis: Weigh 100 g of waste PET and 300 ml of MEG into a 500 ml three-necked flask. After purging with N2 three times, mechanically stir and heat at 200 rpm. When the temperature inside the flask reaches 160 °C, add 0.5–0.8 g of catalyst ZnUA and maintain the temperature at 160–165 °C with continuous stirring to depolymerize until the PET is completely depolymerized and a homogeneous, transparent solution appears in the flask. Stop the reaction and cool to room temperature.

[0083] 3. Separation and purification of the product: The obtained mixed solution was poured into 2 L of deionized water, and a white precipitate was immediately formed. After standing for 2 h, the solution was filtered and washed 3 times with deionized water. The obtained white product was placed in an oven and dried at 80 ℃ for 12 h to obtain the depolymerization product, which was named BHET-ZnUA.

[0084] Experimental Example 1 To evaluate the catalytic performance of different catalysts on the alcoholysis reaction of waste PET, the catalysts prepared in Examples 1-7 and Comparative Example 1 were used to conduct alcoholysis experiments under the same reaction conditions. The PET depolymerization rate and product distribution were determined by product analysis, as detailed below: Figure 1 The images show observations of PET depolymerization experiments in the various embodiments and comparative examples provided for this experimental case.

[0085] like Figure 1 As shown, after using eight catalysts—ZnUA, ZnMgUA, ZnMgNaUA, ZnMgMnUA, ZnMgAlUA, ZnMgCoUA, ZnMnUA, and ZnMnCoUA—for alcoholysis, the reaction systems ultimately formed homogeneous and transparent solutions without any unreacted PET particles or other suspended matter.

[0086] The formula for calculating the depolymerization rate of PET is as follows: (Equation 1) In the formula, —PET depolymerization rate —The mass of PET added before the reaction, —The mass of PET remaining after the reaction is complete.

[0087] The experimental results show that no solid PET residue was left after the reaction. This indicates that the catalysts prepared in Examples 1-7 and Comparative Example 1 all have a 100% depolymerization rate for PET and have good depolymerization effects.

[0088] Figure 2 The depolymerization solution treatment and product observation diagrams for each embodiment and comparative example provided in this experimental example.

[0089] Depending on the catalyst used, the depolymerization products were named BHET-ZnUA, BHET-ZnMgUA, BHET-ZnMgNaUA, BHET-ZnMgMnUA, BHET-ZnMgAlUA, BHET-ZnMgCoUA, BHET-ZnMnUA, and BHET-ZnMnCoUA, respectively. Specific component analysis was performed using liquid chromatography, and the results are as follows: Figure 3 As shown.

[0090] Figure 3 Liquid chromatograms of the depolymerization products of the various embodiments and comparative examples provided for this experimental example.

[0091] Depend on Figure 3 It can be seen that the characteristic peak at 13.5 min is the characteristic peak of the mobile phase CH3OH. From the characteristic peaks of the standard samples BHET and PTA, the characteristic peaks at 1.7 min and 5.7 min are attributed to PTA and BHET, respectively. BHET-ZnUA, BHET-ZnMgUA, BHET-ZnMgNaUA, BHET-ZnMgMnUA, BHET-ZnMgAlUA, BHET-ZnMgCoUA, and BHET-ZnMnCoUA contain only two small molecules: BHET and its dimer (BHET-Dimer, 13.5 min). However, BHET-ZnMnUA also contains the PTA monomer, indicating that ZnMnUA has a stronger depolymerization ability and can depolymerize into smaller structural units.

[0092] according to Figure 3 The peak areas of polymers PTA, BHET, and BHET-Dimer can be calculated using liquid chromatography software, and the results are shown in Table 1.

[0093] Table 1. Peak areas of PTA, BHET, and BHET-Dimer in the liquid phase spectra of depolymerization products from different catalysts.

[0094]

[0095] BHET's yield calculation formula is as follows: (Equation 2); In the formula, —BHET's yield, —Peak area of ​​PTA in the liquid phase spectrum. —Peak area of ​​BHET in the liquid phase spectrum. —Peak area of ​​BHET-Dimer in liquid phase spectrum.

[0096] The yield calculation methods for PTA and BHET-Dimer are similar to those for BHET, and the calculation results are shown in Table 2.

[0097] Table 2: Yields of PTA, BHET and BHET-Dimer depolymerization products based on different catalysts and depolymerization rate of PET.

[0098]

[0099] As shown in Table 2, the depolymerization rate of PET reached 100% under all catalyst systems, indicating that the catalytic system based on zinc source and urea has excellent catalytic activity and can achieve complete cleavage of PET molecular chains. This demonstrates the unique promoting effect of Zn-based catalysts on PET depolymerization. The urea ligand can synergistically enhance the nucleophilic / electrophilic activation ability of ester bonds with the metal center, which is different from the limited depolymerization performance of traditional catalysts.

[0100] Meanwhile, in the depolymerization reaction of waste PET ethylene glycol, the BHET yield was significantly affected by the catalyst composition. Among them, the ZnMgUA catalytic system exhibited the highest BHET yield, reaching 90.67%, significantly superior to other systems, indicating that Mg²⁺… + The introduction of [a specific metal] effectively optimizes the distribution of Lewis acidic sites in the catalyst, promotes the nucleophilic substitution reaction of PET terminal hydroxyl groups on ester bonds, and simultaneously suppresses secondary side reactions of BHET. In contrast, other auxiliary metal combinations have varying effects on selectivity; some systems show a higher proportion of byproducts such as BHET-Dimer or PTA, reflecting the crucial role of catalyst composition in product selectivity. These results demonstrate that by controlling the type of auxiliary metal, the selectivity of BHET can be significantly improved while maintaining complete depolymerization capability, validating the effectiveness of the multi-metal synergistic strategy.

[0101] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A catalyst for the alcoholysis of waste PET, characterized in that, The catalyst is prepared by in-situ reaction of a raw material comprising a zinc source, urea, and at least one auxiliary metal salt, wherein: The zinc source is a soluble zinc salt; The metal element in the auxiliary metal salt is selected from one or more of Mg, Mn, Al, Co, and Na.

2. The catalyst for alcoholysis of waste PET according to claim 1, characterized in that, The soluble zinc salt is zinc acetate hydrate.

3. The catalyst for alcoholysis of waste PET according to claim 1, characterized in that, The auxiliary metal salt is one or a mixture of Mg(Ac)2•4H2O, Mn(Ac)2•4H2O, NaAc, Al(Ac)3•4H2O, and Co(Ac)2•4H2O.

4. The catalyst for alcoholysis of waste PET according to claim 1, characterized in that, The mass ratio of the auxiliary metal salt to the zinc source is 3~5:15~25.

5. The catalyst for alcoholysis of waste PET according to claim 1, characterized in that, The mass ratio of urea to zinc source is 25~30:15~25.

6. A method for preparing a catalyst for the alcoholysis of waste PET according to any one of claims 1 to 5, characterized in that, The preparation method includes the following steps: The zinc source, urea, and auxiliary metal salt are mixed, heated and stirred under an inert atmosphere to allow the components to react in situ and generate a homogeneous and transparent catalytic system. After cooling, the catalyst is obtained.

7. The method for preparing the catalyst for the alcoholysis of waste PET according to claim 6, characterized in that, The reaction temperature is 90~110°C, and the reaction time is 2~3 hours; Preferably, the stirring speed is 300~450 r / min.

8. The application of the catalyst for alcoholysis of waste PET according to any one of claims 1 to 5 in the chemical recycling of waste PET, characterized in that, The application is to catalyze the alcoholysis reaction between PET and ethylene glycol, converting waste PET into diethylene terephthalate.

9. The application according to claim 8, characterized in that, The amount of catalyst used in the alcoholysis reaction is 0.5~0.8 wt% of the waste PET mass.

10. The application according to claim 8, characterized in that, The alcoholysis reaction was carried out at a temperature of 160-170°C.

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