Preparation of directional depolymerization of waste pet and application in closed-loop preparation of degradable copolyester

Abstract

The application provides a preparation method of directional depolymerization of waste PET and application of the waste PET in closed-loop preparation of degradable copolyester, and comprises the following steps: a two-dimensional Zn / WO3 heterogeneous catalyst is prepared; waste PET raw material, 1,4-butanediol and the two-dimensional Zn / WO3 heterogeneous catalyst are added into a light-transmitting reactor; a depolymerization reaction is carried out under the conditions of external heating or external heating and light irradiation to obtain a depolymerization system, wherein the molar ratio of the 1,4-butanediol and the waste PET raw material is not higher than 3:1; the two-dimensional Zn / WO3 heterogeneous catalyst in the depolymerization system is recovered to obtain a depolymerization product; aliphatic dibasic acid is added into the depolymerization product to carry out esterification reaction, and then a polycondensation catalyst is added to carry out polycondensation reaction, so that aliphatic-aromatic degradable copolyester is prepared, and the effect of directional depolymerization of PET at a low diol ratio and direct polymerization utilization is achieved.

Description

Technical Field

[0001] This invention relates to the field of chemical recycling, specifically to the preparation of waste PET by directional depolymerization and its application in the closed-loop preparation of biodegradable copolyesters, and particularly to a method for the directional depolymerization of waste PET and the closed-loop preparation of biodegradable copolyesters based on a two-dimensional Zn / WO3 photothermal catalyst at a low glycol ratio. Background Technology

[0002] Polyethylene terephthalate (PET), as a general-purpose polyester material, is widely used in food packaging, textile fibers, electronic devices, engineering plastics, and many other fields due to its excellent mechanical strength, transparency, chemical resistance, and processability. It is one of the world's largest-produced synthetic polymer materials. With the continuous growth in PET market demand, its waste emissions are also increasing year by year. If large amounts of waste PET are disposed of through traditional landfill and incineration methods, it will not only cause a serious waste of petroleum-based resources, but the non-degradable nature of landfill will also damage soil structure. Incineration, on the other hand, easily releases toxic and harmful gases, placing a heavy burden on the ecological environment. Against this backdrop, achieving high-value closed-loop recycling of waste PET is currently a research hotspot and industrialization focus in the field of materials science and chemical engineering.

[0003] Among chemical recycling technologies for waste PET, the alcoholysis route has become one of the most promising technologies for industrial application due to its clear reaction path and the advantage that the depolymerization products can be directly used for repolymerization to prepare polyester materials. Specifically, the glycolysis process using 1,4-butanediol as a depolymerizing agent has become the preferred route for preparing aliphatic-aromatic biodegradable copolyesters because of the excellent compatibility of the product bis(2-hydroxybutyl) terephthalate (BHBT) and its oligomers with aliphatic polyesters. Existing research has confirmed that zinc salts can effectively catalyze the breaking of ester bonds in PET molecules, promoting the glycolysis reaction, and are one of the most widely used catalysts in PET alcoholysis systems. However, traditional PET glycolysis recycling technologies, even with the introduction of zinc salt catalysts, still have many technical bottlenecks, making it difficult to meet the industrial requirements for low-consumption, high-efficiency, and high-value recycling. The specific shortcomings are as follows: 1. High energy consumption and harsh process conditions: Traditional glycolysis reactions usually need to be carried out at temperatures of 190-220°C or even higher. Continuous high-temperature heating not only causes a large amount of energy consumption, but also easily leads to the volatilization of diol reagents in the reaction system and side reactions of products, affecting depolymerization efficiency and product purity.

[0004] 2. High glycol usage and high subsequent separation costs: Existing efficient depolymerization systems mostly rely on reaction conditions with a high glycol / PET molar ratio. The excess glycol drives the transesterification reaction to proceed in the positive equilibrium. However, the excess glycol needs to be separated and recovered through complex processes such as distillation and extraction, which greatly increases the complexity of the process and the overall production cost, and also reduces the overall economic efficiency of the process.

[0005] 3. Limited heat and mass transfer in low glycol ratio systems, resulting in a sharp drop in depolymerization efficiency: When the amount of glycol is reduced to an industrially acceptable low ratio, the viscosity of the reaction system will increase significantly, which will hinder the melting and dissolution process of PET, greatly reduce the contact efficiency between the catalyst and the reaction substrate, and cause uneven heat and mass transfer within the system. Ultimately, this will result in a slow PET chain breaking process and difficulty in achieving the depolymerization efficiency target, becoming the core problem restricting the industrialization of low glycol ratio processes.

[0006] 4. Homogeneous zinc salt catalysts are difficult to recover, affecting the performance of subsequent polymerization products: Conventional zinc salt catalysts are homogeneous systems, and are difficult to separate after mixing with depolymerization products. This not only wastes the catalyst, but the residual zinc ions can also cause side reactions in the subsequent polyester polycondensation process, resulting in problems such as yellowing of the polymerization products and decreased mechanical properties, thus limiting the direct reuse of depolymerization products.

[0007] 5. Depolymerization and repolymerization processes are disconnected, resulting in a lengthy process: Existing processes often focus on obtaining a single BHBT monomer as the core objective. After depolymerization, multiple purification and separation processes are required to remove impurities and unreacted raw materials before the pure monomer is used for polyester repolymerization. This forms an independent step-by-step process of "depolymerization-purification-repolymerization," which is lengthy, energy-intensive, and the purification process can cause product loss and reduce resource utilization.

[0008] 6. Uncontrollable depolymerization product structure, failing to meet the requirements of biodegradable copolyester preparation: For the preparation route of biodegradable aliphatic-aromatic copolyesters, complete monomerization of PET is not the only or optimal goal. More importantly, it is necessary to control the degree of depolymerization so that the product remains within the aromatic oligomer / ester window suitable for subsequent copolymerization. However, traditional processes cannot precisely control the chain length distribution of depolymerization products, making it difficult to meet the design requirements for aromatic chain length in copolyester synthesis, thus limiting the high-value conversion of waste PET into high-performance biodegradable polyesters.

[0009] To address the aforementioned issues, some improved solutions have emerged in existing technologies, including photothermal-assisted polyester degradation technology and short-process copolyester preparation technology. However, these technologies still have significant limitations and fail to fundamentally solve the technical challenge of efficiently and directionally depolymerizing PET and directly closing the ring to prepare biodegradable copolyesters under low glycol ratio conditions. In summary, existing PET glycolysis recovery technologies and related improvements cannot meet the industrialization requirements for efficiently and directionally depolymerizing PET and directly closing the ring to prepare high-performance biodegradable copolyesters under low glycol ratio conditions. Summary of the Invention

[0010] The purpose of this invention is to provide a method for the preparation of directional depolymerized waste PET and its application in the closed-loop preparation of biodegradable copolyesters. By using a two-dimensional supported zinc WO3 heterogeneous photothermal catalyst, a synergistic effect of local photothermal enhancement and Zn-WO3 interface activation is established in a low glycol ratio and high viscosity system, enabling PET to be directionally depolymerized into aromatic oligomers / esters suitable for subsequent direct polycondensation with adipic acid under relatively low external heating conditions.

[0011] To achieve the above objectives, this technical solution provides a method for preparing directional depolymerization waste PET, comprising the following steps: Two-dimensional WO3 nanosheet supports were prepared, and zinc active components were introduced onto the two-dimensional WO3 nanosheet supports to obtain a two-dimensional Zn / WO3 heterogeneous catalyst. Waste PET raw material, 1,4-butanediol and two-dimensional Zn / WO3 heterogeneous catalyst are added to a light-transmitting reactor and depolymerization reaction is carried out under external heating or external heating and light irradiation to obtain a depolymerization system, wherein the molar ratio of 1,4-butanediol to waste PET raw material is not higher than 3:1; The depolymerization product was obtained by recovering the two-dimensional Zn / WO3 heterogeneous catalyst in the depolymerization system.

[0012] This scheme utilizes a two-dimensional Zn / WO3 heterogeneous catalyst under low glycol ratio conditions to achieve directional depolymerization of waste PET, which reduces external heating requirements and post-processing burden, and enables the direct directional production of aromatic oligomers / esters suitable for subsequent biodegradable copolyester synthesis during the depolymerization stage.

[0013] While existing technologies also include photothermal-assisted polyester degradation schemes that involve adding polyester, alcohol, catalyst, and photothermal materials to a reactor to promote transesterification and recover monomers under light irradiation, traditional photothermal degradation schemes typically add photothermal materials and catalysts as two independent components, primarily aiming to promote the recovery of PET from monomers. This solution, however, abandons the simplistic approach of physically adding photothermal materials and catalysts in parallel. Through integrated catalyst structure design, synergistic coupling of photothermal and catalytic effects, and precise matching of process conditions, it constructs an integrated solution for efficient directional depolymerization of PET adapted to low glycol ratio, high viscosity systems. This fundamentally solves the problems of limited heat and mass transfer, low catalytic efficiency, and inability to directionally control products in such systems.

[0014] First, this approach does not mix independent photothermal materials and catalytic components. Instead, it first prepares highly active two-dimensional WO3 nanosheets and then atomically anchors the zinc active component onto the surface / lattice of the WO3 nanosheets to form a two-dimensional Zn / WO3 heterogeneous photothermal catalyst. This catalyst achieves structural integration between the photothermal substrate and the catalytic active center. The zinc active component and WO3 form a dedicated Zn-WO3 interface structure, rather than two physically mixed phases.

[0015] Preferably, two-dimensional WO3 nanosheet carriers are prepared using the spatial confinement method or the layered precursor method.

[0016] The preparation process of synthesizing two-dimensional Zn / WO3 heterogeneous catalysts using spatial confinement and saturated solution impregnation methods is as follows: Figure 1 As shown, the precursor Mg-Al-LDH possesses a unique multilayered structure, therefore using LDH as a template promotes the formation of the precursor WO4. 2- Enriched within the template space, LDH-WO4 was subsequently obtained through a structural reconstruction process. 2- Complex. LDH-WO4 2- LDO-WO3 composites are generated by calcination in air atmosphere, during which interlayer WO4 is produced. 2- WO3 was formed through in-situ decomposition and structural reconstruction. Because the interlayer structure of the LDO template only allows WO3 to grow in the same plane, the generated WO3 exists as a two-dimensional monolayer. The LDO template was then etched with acid to obtain monolayer WO3·H2O nanosheets. WO3·H2O was immersed in a saturated Zn(NO3)2 solution, dried, and calcined in a reducing atmosphere to obtain a monolayer Zn / WO3 sample. ICP-OES data showed that the sample loading reached saturation after 12 h of immersion in the saturated Zn(NO3)2 solution, with the Zn loading appearing to be approximately 2.55 wt%.

[0017] The two-dimensional WO3 nanosheets prepared by this method exhibit high defect rate, high monolayer ratio, and high specific surface area. The surface thickness of the WO3 nanosheets ranges from 0.6 to 1.4 nm, and the lateral dimensions are mostly in the range of 60 to 110 nm. The WO3 surface contains multiple planar defects, providing loading sites for active metals. The monolayer ratio is greater than 95%, and the specific surface area is 98.7 m² / Zn / WO3. 2 ·g -1 WO3101.9 m 2 ·g -1 It should be noted that the single-layer ultrathin nanosheets provide a high specific surface area and abundant loading sites for the active metal. The abundant surface defects provide loading sites for the active metal, reduce the band gap, broaden the light absorption range, and enhance the photothermal conversion effect.

[0018] Preferably, the thickness of the two-dimensional WO3 nanosheet carrier is 1~1.5 nm and the lateral dimension is 80~120 nm.

[0019] More preferably, the thickness of the two-dimensional WO3 nanosheet carrier is 1.2 nm and the lateral dimension is 100 nm.

[0020] Furthermore, this method introduces zinc active components into the surface or lattice of a two-dimensional WO3 nanosheet support through zinc salt impregnation and heat treatment reduction to obtain a two-dimensional Zn / WO3 heterogeneous catalyst.

[0021] Specifically, prepare 1 L of excess Zn(NO3)2 aqueous solution, add the above-mentioned WO3·H2O to the Zn(NO3)2 aqueous solution, stir at room temperature in the dark for 12 h, filter, dry at 60 °C for 12 h, and place the solid obtained by zinc salt impregnation and loading in an H2 / Ar atmosphere (5 vol% H2) and calcine at 300 °C for 2 h with a heating rate of 5 °C·min. -1 To achieve saturated impregnation loading of zinc metal components on WO3 nanosheets, a deep blue Zn / WO3 sample was obtained.

[0022] Preferably, Zn exists on the surface of the two-dimensional WO3 nanosheet carrier in a highly dispersed or atomically anchored form, and the two-dimensional WO3 nanosheet carrier forms a Zn-OW interface structure.

[0023] It is important to reiterate that the two-dimensional Zn / WO3 heterogeneous photothermal catalyst prepared by this method exhibits excellent visible light absorption and photothermal conversion capabilities. By introducing zinc active components and constructing the Zn-OW interface structure, the electronic structure of WO3 is regulated, broadening the light absorption range and enabling it to have high-efficiency absorption capacity in the visible light region. This allows for full utilization of low-cost light sources such as simulated sunlight (AM 1.5G), avoiding dependence on special light sources such as ultraviolet light. Furthermore, due to the synergistic effect of the Zn-OW interface structure and the two-dimensional WO3 substrate, its catalytic efficiency is far higher than that of pure WO3 photothermal materials or single homogeneous zinc salts. Moreover, the catalytic effect is targeted, precisely acting on the ester bond breaking process of PET.

[0024] In addition, the two-dimensional Zn / WO3 heterogeneous photothermal catalyst in this scheme is a heterogeneous catalytic system. Compared with traditional homogeneous zinc salt catalysts, its structure determines its advantage of easy solid-liquid separation after reaction. It can also be stably recycled after simple regeneration, which reduces the cost of catalyst use and avoids the contamination of products by catalyst residue.

[0025] In this scheme, after preparing a two-dimensional Zn / WO3 heterogeneous photothermal catalyst, waste PET raw material, 1,4-butanediol and the two-dimensional Zn / WO3 heterogeneous catalyst are added to a light-transmitting reactor, and a depolymerization reaction is carried out under the synergistic effect of external heating and light to obtain a depolymerization system.

[0026] Preferably, the molar ratio of 1,4-butanediol to waste PET raw material is 2:1. Unlike traditional processes, this method performs directional depolymerization of PET under low glycol ratio conditions, which significantly reduces the amount of 1,4-butanediol used. This reduces the complexity, energy consumption, and reagent consumption of subsequent glycol separation and recovery processes from the source, resulting in a significant reduction in raw material costs and post-processing costs of the depolymerization process.

[0027] It should be noted that under low glycol ratio conditions, the PET / glycol system has high viscosity and shallow light penetration depth. If conventional external heating is used alone, the internal temperature rise of the system is slow, and the melting and dissolution of PET are limited, resulting in a decrease in transesterification chain scission efficiency. The two-dimensional Zn / WO3 heterogeneous catalyst introduced in this system plays a dual role: Firstly, the two-dimensional WO3 nanosheets have excellent visible light absorption and non-radiative relaxation capabilities, and can rapidly convert light energy into heat energy under illumination, forming a distributed local heat source, thus enhancing the heating, melting, and interfacial heat transfer processes of the high-viscosity solid-liquid mixture. Secondly, the Zn-WO3 interface formed by the Zn active center and WO3 can synergistically polarize the PET ester groups and glycol hydroxyl groups, promoting the nucleophilic attack of the glycol on the ester carbonyl group, thereby lowering the reaction energy barrier of key steps in glycolysis. Therefore, this invention is not a simple superposition of "heat generated by light" and "zinc catalytic chain breaking", but rather a two-dimensional Zn / WO3 heterogeneous catalyst that simultaneously completes local photothermal enhancement and interfacial catalytic activation in a low glycol ratio and high viscosity environment, thereby achieving low energy consumption and directional depolymerization that are difficult to achieve in traditional processes.

[0028] Preferably, the amount of the two-dimensional Zn / WO3 heterogeneous catalyst is 1 to 5 wt% of the mass of PET.

[0029] More preferably, the amount of the two-dimensional Zn / WO3 heterogeneous catalyst is 3 wt% of the mass of PET.

[0030] Our research found that if the proportion of the two-dimensional Zn / WO3 heterogeneous catalyst to PET mass is less than 1 wt%, the photothermal and catalytic effects are weak. If it is higher than 5 wt%, the photothermal and catalytic effects will be improved, but the viscosity will increase, which will greatly increase the difficulty of catalyst separation after the reaction.

[0031] It should be noted that the depolymerization reaction can be carried out under the combined effect of external heating and light, or under external heating only, or under light only.

[0032] Preferably, the external heating temperature is 160–220°C, and the illumination is simulated sunlight.

[0033] More preferably, the external heating temperature is 180~220°C.

[0034] More preferably, the illumination is under AM 1.5G conditions with a light intensity of 0.1 W / cm². 2 .

[0035] Preferably, the reaction time for the depolymerization reaction is 0.5 to 3 hours.

[0036] More preferably, the depolymerization reaction time is 2 hours.

[0037] Preferably, the depolymerization products include BHBT and aromatic low polyesters with different degrees of polymerization.

[0038] In addition, this scheme can control the degree of depolymerization of the depolymerization products by controlling the time, temperature, light conditions and catalyst dosage of the depolymerization reaction. For example, by controlling the reaction time of the depolymerization reaction to 0.5 h, 1 h, 2 h or 3 h, depolymerization products with different chain length distributions can be obtained.

[0039] Preferably, under the conditions of simulated sunlight-assisted 180°C oil bath, a molar ratio of 1,4-butanediol and waste PET raw material of about 2:1, and a dosage of about 3 wt% of two-dimensional Zn / WO3 heterogeneous catalyst, the reaction can achieve almost complete depolymerization of PET in 2 h, and obtain depolymerization products mainly composed of bis(2-hydroxybutyl) terephthalate (BHBT) and aromatic low polyester.

[0040] More preferably, under pure heat conditions without light, a similar depolymerization effect can be achieved by using a 220°C oil bath with the other conditions remaining the same; this shows that under photothermal coupling conditions, the external heating demand can be reduced while maintaining the depolymerization effect.

[0041] This scheme allows for the recovery of the two-dimensional Zn / WO3 heterogeneous catalyst from the depolymerization system after obtaining the depolymerization system, thus yielding the depolymerization product. Furthermore, the recovered two-dimensional Zn / WO3 heterogeneous catalyst can be regenerated and fully utilized.

[0042] Preferably, the two-dimensional Zn / WO3 heterogeneous catalyst in the depolymerization system is recovered through solid-liquid separation.

[0043] Preferably, the solid-liquid separation method includes hot filtration. When hot filtration is used to recover the two-dimensional Zn / WO3 heterogeneous catalyst, the filter cake in the reaction system is collected by hot vacuum filtration. NMP is added to the filter cake for thorough dispersion, followed by vacuum filtration to remove insoluble impurities from the system. The obtained solid is repeatedly washed with deionized water until residual organic solvents are removed, and then dried in a 60°C drying oven for 12 h. The dried solid is then placed under a nitrogen atmosphere and dried at 5°C·min. -1 The temperature was programmed to rise to 300℃ and calcined at this temperature for 2 h to obtain a regenerated Zn / WO3 catalyst sample. The recovery rate of the Zn / WO3 catalyst obtained by this regeneration method is 80%~95%, which can be directly used in subsequent PET depolymerization cycle experiments. Zn / WO3 catalyst was added during the experiment to ensure that the amount of catalyst added in the system remained consistent in each cycle.

[0044] Furthermore, the recovered two-dimensional Zn / WO3 heterogeneous catalyst is reused after being regenerated by washing, drying, and calcination.

[0045] Furthermore, after removing organic matter from the two-dimensional Zn / WO3 heterogeneous catalyst with deionized water, it is adsorbed with deionized water, then dried and calcined at high temperature under an inert atmosphere.

[0046] Preferably, the product is dried at 60°C and then calcined at 300°C for 2 hours under an inert atmosphere.

[0047] Preferably, the recovery rate of the two-dimensional Zn / WO3 heterogeneous catalyst is 80%–95%. Preferably, after five consecutive cycles of the two-dimensional Zn / WO3 heterogeneous catalyst, the PET depolymerization rate remains at 100%, and the BHBT yield remains above 70%.

[0048] As mentioned above, the depolymerization products obtained by the above-mentioned method for preparing waste PET through directional depolymerization include BHBT and aromatic low polyesters with different degrees of polymerization. Furthermore, the depolymerization products can be directly used as polymerization intermediates to add aliphatic diacids for esterification and polycondensation reactions to obtain aliphatic-aromatic biodegradable copolyesters.

[0049] Secondly, this solution provides an application method for the closed-loop preparation of biodegradable copolyesters, based on the method for preparing waste PET through directional depolymerization mentioned in the first aspect, including the following steps: Aliphatic diacids and polycondensation catalysts were added to the depolymerization products to carry out esterification and polycondensation reactions, thereby obtaining aliphatic-aromatic biodegradable copolyesters.

[0050] Preferably, the aliphatic dicarboxylic acid is adipic acid.

[0051] Preferably, the molar ratio of adipic acid to the repeating aromatic acid units of waste PET is (0.8~1.2):1. In this case, the aromatic acid units in the aliphatic-aromatic biodegradable copolyester account for 40~60% of the total acid molar amount.

[0052] More preferably, the molar ratio of adipic acid to the repeating aromatic acid units of waste PET is 1:1, in which case the aromatic acid units in the aliphatic-aromatic biodegradable copolyester account for 50% of the total acid molar amount.

[0053] Preferably, the polycondensation catalyst is tetrabutyl titanate, and its addition amount is 0.05 to 0.5 mol% of the molar amount of the aliphatic dicarboxylic acid, more preferably 0.15 mol%.

[0054] Preferably, the esterification reaction is carried out in a nitrogen atmosphere at around 220-230°C until the amount of water collected reaches more than 90% of the theoretical value. Then, the temperature is raised to 235-245°C and the system is pumped to below about 100 Pa for polycondensation for 3-4 hours to obtain an aliphatic-aromatic biodegradable copolyester.

[0055] Preferably, the aliphatic-aromatic biodegradable copolyester is an aliphatic-aromatic copolyester containing butylene terephthalate units and butylene adipate units, denoted as PBEAT.

[0056] Preferably, by controlling the degree of depolymerization of the depolymerization product, an aliphatic-aromatic biodegradable copolyester with an aromatic sequence length of about 2.31 to 2.58 can be obtained.

[0057] After obtaining the aliphatic-aromatic biodegradable copolyester, this method allows for melt granulation, extrusion, and blown film processing to prepare biodegradable films.

[0058] Preferably, the thickness of the resulting biodegradable film is 15–20 μm.

[0059] Preferably, the resulting biodegradable film has high ductility, with an elongation at break greater than 1000%, and can reach or exceed the modulus and strength levels of commercial PBAT.

[0060] This solution addresses the core shortcomings of existing PET chemical recycling technologies, such as high glycol consumption, high depolymerization energy consumption, difficult catalyst recovery, uncontrollable product structure, and disconnect between depolymerization and repolymerization processes. Through catalyst structure innovation, photothermal-catalytic synergistic coupling, and integrated process design, it achieves efficient, targeted depolymerization and high-value closed-loop recycling of waste PET under low glycol ratio conditions. Compared to existing technologies, its core features and beneficial effects are as follows: 1. The integrated dual-function catalyst design achieves deep synergy between photothermal enhancement and catalytic activation, addressing the shortcomings of existing technologies where photothermal materials and catalytic components are physically superimposed and lack synergy. The two-dimensional Zn / WO3 heterogeneous catalyst prepared by this method simultaneously integrates excellent visible light photothermal conversion performance and targeted PET ester bond breaking catalytic activity. Molecular-level synergy between the two-dimensional WO3 substrate and the zinc active center is achieved through the construction of a Zn-WO3 surface structure. At the same time, as a heterogeneous system, it can be rapidly recovered through conventional methods such as thermal filtration and vacuum filtration, and can be continuously recycled after simple regeneration.

[0061] 2. This method overcomes the bottleneck of depolymerization efficiency at low glycol ratios, addressing the core shortcomings of existing technologies that rely on high glycol ratios, have high separation costs, and experience a sharp drop in depolymerization efficiency at low glycol ratios. This solution can achieve highly efficient and complete depolymerization of PET under near-stoichiometric conditions where the molar ratio of 1,4-butanediol to PET repeating units is as low as 3:1. This reduces the amount of glycol reagent used at the source, significantly lowering the complexity and overall cost of subsequent glycol separation and recovery processes.

[0062] 3. The depolymerization process is precise and controllable, solving the problems of existing technologies that only pursue monomer recovery, resulting in uncontrollable product structure and disconnection from subsequent copolymerization processes. This solution abandons the traditional single depolymerization target of complete monomerization. By adjusting parameters such as reaction time, temperature, and light, the degree of PET depolymerization can be precisely controlled, ensuring that the product remains stably within the aromatic oligomer / ester window suitable for copolymerization with adipic acid. The chain length distribution of the depolymerization product can be flexibly adjusted, providing a precise raw material basis for subsequent copolyester aromatic sequence length design and optimization of the balance between degradation performance and mechanical properties. Attached Figure Description

[0063] Figure 1 This is a schematic diagram of the route for preparing the two-dimensional Zn / WO3 heterogeneous catalyst in Example 1.

[0064] Figure 2 This is a schematic diagram of the closed-loop preparation route for PBEAT.

[0065] Figure 3 This is a schematic diagram illustrating the changes in molecular weight of PET depolymerization products and subsequent copolyester chain segment regulation under different depolymerization times.

[0066] Figure 4 This is a graph showing the test results of Example 4.

[0067] Figure 5 This is a graph showing the test results of Example 7.

[0068] Figure 6 This is a graph showing the degradation results at different temperatures under oil bath heating only.

[0069] Figure 7 This is a graph showing the degradation results under different temperatures combined with oil bath heating and light irradiation. Detailed Implementation

[0070] The chemical reagents involved in this solution are all conventional raw materials disclosed in the prior art. Their purity and specifications can be adjusted according to actual experimental / production needs. Any selection of raw materials that can achieve the technical effect of this solution falls within the protection scope of this invention. Unless otherwise specified, the experimental conditions in the following examples are all carried out according to conventional conditions in the prior art or according to the conditions recommended by the reagent / equipment manufacturer; unless otherwise specified, the raw materials used are all commercially available conventional products.

[0071] Example 1: Preparation of two-dimensional Zn / WO3 heterogeneous catalyst The specific preparation process is as follows: Figure 1 As shown: First, a magnesium-aluminum layered double hydroxide precursor is prepared, which is then calcined to obtain a layered bimetallic oxide; then, a tungsten source is introduced to form LDO-WO4. 2- The composite was calcined and acid-washed to obtain two-dimensional WO3·H2O nanosheets.

[0072] The WO3·H2O was then impregnated in an aqueous solution of Zn(NO3)2, stirred at room temperature in the dark for 12 h, filtered, dried, and then calcined at 300 °C for 2 h under an H2 / Ar atmosphere to obtain a deep blue two-dimensional Zn / WO3 heterogeneous catalyst.

[0073] In the obtained two-dimensional Zn / WO3 heterogeneous catalyst, Zn forms a Zn-WO3 interface with WO3 in a highly dispersed form, exhibiting excellent photothermal conversion performance and PET depolymerization activity.

[0074] Example 2: Photothermal Directed Depolymerization of PET under Low Glycol Ratio Conditions Weigh 38.4 g of PET powder, 36 g of 1,4-butanediol and 1.152 g of the two-dimensional Zn / WO3 heterogeneous catalyst prepared in Example 1, add them to a light-transmitting reactor, ultrasonically disperse for 10 min, and react for 2 h under simulated sunlight-assisted 180℃ oil bath conditions. After the reaction is completed, the catalyst is separated by hot filtration and the depolymerization liquid is collected.

[0075] Visually, the depolymerization solution becomes clear (blue and transparent), and no residual PET is observed after separation, indicating complete depolymerization of PET. Furthermore, the weights of the separated degraded BHBT and oligomers can be determined by weighing. The molecular weight composition of the depolymerization products was determined by GPC, allowing for a rough estimation of the molecular chain length. It is evident that under these conditions, PET can achieve near-complete depolymerization, yielding a depolymerization product primarily composed of BHBT and aromatic oligomers.

[0076] Example 3: Depolymerization under no-light control Using the same amounts of PET, butanediol, and catalyst as in Example 2, but without simulating sunlight, the oil bath temperature was increased to 220°C, and the reaction was carried out for 2 hours. After separating the products, based on their respective weights (BHBT and oligomers), it was found that although a similar depolymerization effect could be achieved, the external heating temperature was higher, indicating that photothermal coupling has a significant effect on reducing the external heating requirement.

[0077] Example 4: Catalyst synergistic effect control Using WO3 and WO respectively 3-x The two-dimensional Zn / WO3 heterogeneous catalyst was replaced by ZnCl2, Zn(OAc)2, and Zn(NO3)2 with the same metal content as in Zn / WO3. The PET depolymerization was carried out at 220°C without light and for 2 h using the same amount of PET, butanediol, and catalyst as in Example 2.

[0078] like Figure 4 The test results shown are as follows: Figure 4Yellow in the figure represents the PET degradation rate on the left axis, and blue represents the PET degradation rate on the left axis. The control group without any catalyst and the original support (WO) are also shown. 3-x The degradation rate of PET is relatively low (<30%), i.e., WO3 and WO 3-x The catalyst has a limited effect on promoting the depolymerization of PET, and homogeneous zinc salts are also difficult to achieve the same depolymerization efficiency as Zn / WO3 under low glycol ratio conditions. This indicates that the activity of the catalyst in this invention comes from the synergy between the two-dimensional WO3 support and the zinc active sites, rather than a simple physical superposition of "zinc + photothermal material".

[0079] Example 5: One-pot closed-loop preparation of biodegradable copolyester The closed-loop preparation route for PBEAT is as follows: Figure 2 As shown, the depolymerization product obtained in Example 2 was transferred to a polymerization reactor after the catalyst was separated, and adipic acid with an amount approximately equal to that of the aromatic acid units from PET was added, along with 0.15 mol% of tetrabutyl titanate as a catalyst.

[0080] Esterification was carried out in a nitrogen atmosphere at 220°C. When the water output reached more than 90% of the theoretical value, the temperature was raised to 240°C and the vacuum was drawn to about 100 Pa. Polycondensation was continued for 3-4 hours to obtain the copolyester PBEAT.

[0081] Example 6: Controlling the copolyester chain structure by the degree of depolymerization Keeping other conditions unchanged in Example 2, only the PET depolymerization time was adjusted to 0.5 h, 1 h, 2 h and 3 h respectively to obtain depolymerization products with different molecular weights and chain length distributions. Then, esterification and polycondensation were carried out according to Example 5 to obtain PBEAT copolyesters with different aromatic sequence lengths.

[0082] The following is a graph showing the changes in molecular weight of PET depolymerization products and the subsequent copolyester segment regulation results under different depolymerization times: Figure 3 As shown, Figure 3 As shown, by controlling the degree of depolymerization in the front stage, the length of aromatic segments in the copolyester and its degradation-mechanical property balance can be adjusted.

[0083] Example 7: Catalyst recycling: The Zn / WO3 catalyst recovered in Example 2 was sequentially washed, dried, and calcined at 300°C under an inert atmosphere for regeneration, and then reused for PET depolymerization. The results after five consecutive cycles are as follows: Figure 5 As shown, Figure 5The yellow and blue axes represent the PET degradation rate on the left and right sides, respectively. It can be seen that the Zn / WO3 catalyst maintained a 100% PET depolymerization rate throughout the five cycles, while the BHBT yield remained at a high level exceeding 70%. This indicates that the Zn / WO3 catalyst retains good catalytic activity and stability even after multiple uses, effectively maintaining depolymerization efficiency and ensuring high BHBT production.

[0084] Example 8: Exploring the effect of external heating temperature: Weigh 38.4 g of PET powder, 36 g of 1,4-butanediol and 1.152 g of the two-dimensional Zn / WO3 heterogeneous catalyst prepared in Example 1, add them to a light-transmitting reactor, ultrasonically disperse for 10 min, and react for 2 h under external heating only without light synergy. After the reaction is completed, the catalyst is separated by hot filtration and the depolymerization liquid is collected.

[0085] The specific experiments were conducted at temperature gradients of 10℃, ranging from 160℃ to 220℃, and included pure oil bath heating and simulated sunlight irradiation conditions (AM 1.5G, 0.1 W·cm⁻¹). -2 A comparative experiment was conducted with auxiliary oil bath heating, and the results of the degradation at different temperatures under oil bath heating alone are shown in the figure below. Figure 6 The results of oil bath heating combined with light irradiation (degradation at different temperatures) are shown in the figure below. Figure 7 As shown, yellow represents the PET degradation rate on the left axis, and blue represents the PET degradation rate on the left axis.

[0086] The results showed that, with the assistance of simulated sunlight, the Zn / WO3 catalyst could achieve complete depolymerization of PET at a significantly reduced oil bath temperature of 180℃, yielding a high BHBT yield (75.1%). In contrast, without sunlight, simple oil bath heating required 220℃ to achieve the same depolymerization effect. With the assistance of simulated sunlight, the reaction temperature was reduced by 40℃, decreasing the need for external heating and demonstrating the energy-saving potential of the Zn / WO3 catalyst.

[0087] Under simulated sunlight, the BHBT yield increases slowly with increasing temperature, eventually stabilizing at around 80%. This may be due to the low glycol ratio, which brings the reaction close to equilibrium at 80%. However, considering the need to reduce raw material waste in PET recycling, a reaction system with a glycol ratio of 2:1 is chosen. This system satisfies the requirements of efficient depolymerization of PET with low external heat supply, achieves a high BHBT yield, and reduces the waste of glycolysis agents. This meets the economic and environmental requirements of PET recycling and reduces raw material waste at the source for subsequent one-pot copolyester production.

[0088] This indicates that the Zn / WO3 catalyst not only has excellent catalytic performance, but also reduces the temperature and energy consumption required in traditional thermocatalysis processes due to its good photothermal conversion performance. This energy-saving effect is consistent with the photothermal catalysis process reported in the literature. Related studies have shown that photothermal catalysts can effectively reduce the temperature required for the reaction by utilizing renewable energy sources such as sunlight, and can achieve efficient catalysis with lower energy input.

[0089] Those skilled in the art should understand that the technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments have been described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0090] The above embodiments are merely illustrative of several implementation methods of this application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A method for preparing waste PET through directional depolymerization, characterized in that, Includes the following steps: Two-dimensional WO3 nanosheet supports were prepared, and zinc active components were introduced onto the two-dimensional WO3 nanosheet supports to obtain a two-dimensional Zn / WO3 heterogeneous catalyst. Waste PET raw material, 1,4-butanediol and two-dimensional Zn / WO3 heterogeneous catalyst are added to a light-transmitting reactor and depolymerization reaction is carried out under external heating or external heating and light irradiation to obtain a depolymerization system, wherein the molar ratio of 1,4-butanediol to waste PET raw material is not higher than 3:1; The depolymerization product was obtained by recovering the two-dimensional Zn / WO3 heterogeneous catalyst in the depolymerization system.

2. The method for preparing waste PET by directional depolymerization according to claim 1, characterized in that, The surface thickness of the two-dimensional WO3 nanosheet carrier ranges from 0.6 to 1.4 nm, the lateral dimension ranges from 60 to 110 nm, and the monolayer ratio is greater than 95%.

3. The method for preparing waste PET by directional depolymerization according to claim 1, characterized in that, Two-dimensional Zn / WO3 heterogeneous catalysts are obtained by introducing zinc active components into the surface or lattice of two-dimensional WO3 nanosheet supports through zinc salt impregnation and heat treatment reduction. Zn exists on the surface of the two-dimensional WO3 nanosheet supports in a highly dispersed or atomically anchored form, and the two-dimensional WO3 nanosheet supports form a Zn-WO3 interface structure.

4. The method for preparing directional depolymerization waste PET according to claim 1, characterized in that, The amount of two-dimensional Zn / WO3 heterogeneous catalyst used is 1-5 wt% of PET mass, the reaction time of the depolymerization reaction is 0.5-3 h, the external heating temperature is 160-220℃, and the light is simulated sunlight.

5. The method for preparing directional depolymerization waste PET according to claim 1, characterized in that, The depolymerization products include BHBT and aromatic low polyesters with different degrees of polymerization.

6. The method for preparing waste PET by directional depolymerization according to claim 1, characterized in that, The degree of depolymerization of the depolymerization products can be controlled by adjusting the time, temperature, light conditions, and catalyst dosage of the depolymerization reaction.

7. The method for preparing directional depolymerization waste PET according to claim 1, characterized in that, The two-dimensional Zn / WO3 heterogeneous catalyst in the depolymerization system is recovered by solid-liquid separation. The recovered two-dimensional Zn / WO3 heterogeneous catalyst is then regenerated by washing, drying and calcination and reused.

8. A method for the closed-loop preparation of biodegradable copolyesters, characterized in that, Includes the following steps: Obtain the depolymerization product as described in any one of claims 1 to 7; Aliphatic diacids and polycondensation catalysts were added to the depolymerization products to carry out esterification and polycondensation reactions, thereby obtaining aliphatic-aromatic biodegradable copolyesters.

9. The method for preparing biodegradable copolyesters using a closed-loop circuit according to claim 8, characterized in that, The aliphatic dicarboxylic acid is adipic acid, and the molar ratio of adipic acid to the repeating aromatic acid unit of waste PET is (0.8~1.2):1; the polycondensation catalyst is tetrabutyl titanate, and its addition amount is 0.05~0.5 mol of the molar amount of aliphatic dicarboxylic acid.

10. The method for preparing biodegradable copolyesters using a closed-loop system according to claim 8, characterized in that, Aliphatic-aromatic biodegradable copolyesters are melt-granulated, extruded, and blown into films to prepare biodegradable films.

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