A chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste

Abstract

The application discloses a chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste materials in the technical field of polymer material chemical recycling and resource recycling, which takes polyester waste materials as raw materials, first carries out alcoholysis reaction with ethylene glycol in the presence of a catalyst, then adds isooctanol to carry out ester exchange reaction, and finally separates and purifies the final product to obtain dioctyl terephthalate. The core of the method is to use a brand-new multi-level pore palladium cerium modified zeolite catalyst, which is prepared by taking sodium type zeolite as a precursor, sequentially constructing a multi-level pore structure through acid treatment and surfactant assisted hydrothermal crystallization, then step by step loading cerium species, palladium and iron components, and high-temperature reduction. The catalyst has suitable acidity and magnetism, can efficiently catalyze the continuous reaction process of alcoholysis and ester exchange, and can be simply recovered and recycled by means of an external magnetic field after reaction. The process steps are simple, the raw materials used are all commercially available products except for the specially made catalyst, and high-value resource utilization of the polyester waste materials is realized.

Description

Technical Field

[0001] This invention relates to the field of chemical recycling and resource recycling technology of polymer materials, specifically to a chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste. Background Technology

[0002] Polyethylene terephthalate (PET), or polyester, is one of the world's largest-produced synthetic fibers and general-purpose plastics, with its products widely used in textiles, packaging, and electronics. Along with its continuously rising consumption, the amount of waste polyester products generated has reached an astonishing scale. These wastes are bulky and difficult to degrade naturally. Improper handling, such as landfilling or incineration, not only occupies land resources for a long time but may also release microplastics or harmful gases, posing a serious threat to the ecological environment and human health. Therefore, developing efficient and clean polyester waste recycling technologies to transform it from an environmental burden into a usable resource has become an urgent and valuable issue in the field of circular economy and sustainable development. Currently, the resource recovery pathways for polyester waste are mainly divided into two categories: physical recycling and chemical recycling. Physical recycling mainly involves melt regranulation, but the polymer chains undergo a certain degree of thermal degradation during this process, leading to a decline in the mechanical properties of the recycled material, which usually can only be used at a lower grade, resulting in limited economic added value. Chemical recycling aims to break down the polyester macromolecular chains through depolymerization reactions to recover its monomers or other valuable chemical intermediates, thereby achieving closed-loop recycling or upgrading and remanufacturing. It is widely considered a more promising direction for high-value recycling. Among them, the route for preparing the plasticizer dioctyl terephthalate through alcoholysis has attracted much attention because it can directly convert low-value waste into chemicals with high market demand and added value.

[0003] Dioctyl terephthalate (DTP) is a high-performance, environmentally friendly plasticizer with advantages such as heat resistance, volatility resistance, and good electrical insulation. It is widely used in the production of soft products such as polyvinyl chloride (PVC). Traditional industrial production of DTP mainly uses petroleum-derived terephthalic acid or dimethyl terephthalate as raw materials, which are esterified or transesterified with isooctanol. The cost of these raw materials is closely linked to petroleum prices. Using waste polyester as a substitute for petroleum-based raw materials in the production of DTP not only reduces dependence on fossil resources but also provides a highly attractive high-value export for waste polyester. However, existing technical routes for preparing DTP from polyester waste still face significant challenges. This process typically involves two core reaction steps: first, polyester undergoes alcoholysis in excess ethylene glycol to generate intermediates such as diethyl terephthalate (DHT); subsequently, these intermediates undergo transesterification with isooctanol to finally generate the target product. Existing technologies mostly use homogeneous metal salt catalysts (such as zinc acetate, tetrabutyl titanate, etc.) to catalyze the above reactions. Although the activity is acceptable, the catalyst remains in the product system after the reaction, which is extremely difficult to separate and recover. This not only leads to catalyst waste and increased costs, but also introduces metal impurities that affect product purity. Subsequent purification steps such as acid washing and water washing are required, generating a large amount of process wastewater, which contradicts the original intention of environmental friendliness.

[0004] To overcome the separation challenges of homogeneous catalysts, researchers have explored the application of solid acid catalysts, such as various modified zeolite molecular sieves and solid superacids. While these solid catalysts can achieve formal separation, they often suffer from problems such as poor accessibility of active sites, limited specific surface area, easy deactivation in the reaction medium, or insufficient mechanical strength. Particularly for heterogeneous catalytic reactions involving macromolecular substrates and intermediates, such as polyester alcoholysis, the microporous structure of traditional solid catalysts is easily blocked, leading to low mass transfer efficiency and unsatisfactory reaction rates and final yields. Furthermore, existing processes often treat alcoholysis and transesterification as two independent unit operations, resulting in lengthy processes and requiring optimization of energy utilization efficiency. Simultaneously, few technologies can effectively address the negative impact of impurities such as dyes and auxiliaries that may be present in actual waste on catalyst activity. Therefore, developing a novel catalyst with high catalytic activity, excellent selectivity, good stability, and the ability to be easily and efficiently separated from the reaction system and adapted to complex raw materials, along with a simple integrated process to match it, is the key to realizing the large-scale and economical preparation of dioctyl terephthalate from polyester waste, and is also a technical bottleneck that urgently needs to be overcome in this field. Summary of the Invention

[0005] The purpose of this invention is to provide a chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste, which solves the technical problems of existing catalysts being difficult to separate and recover, cumbersome process steps, poor adaptability to complex wastes, as well as the pollution of products by homogeneous catalysts and the low efficiency of solid catalysts.

[0006] The present invention achieves the above objectives through the following technical solutions: A chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste includes the following steps: S1, by weight, polyester waste is crushed, washed and dried to obtain polyester waste fragments; in a high-pressure reactor, 80-120 parts of polyester waste fragments, 200-300 parts of ethylene glycol and 1-3 parts of multi-porous palladium-cerium modified zeolite catalyst are added; the mixture is stirred and heated to 180-200℃ under nitrogen protection to obtain a mixture; S2, add 400-500 parts of isooctanol to the mixture obtained in step S1, and continue the reaction at 180-200℃ to obtain the reaction mixture; S3. The reaction mixture is naturally cooled to 60-80℃. A magnet is placed on the outside of the bottom of the high-pressure reactor and allowed to stand. The upper liquid reaction product is separated from the solid catalyst slurry to obtain the reaction product and the catalyst slurry. The catalyst slurry is washed with isooctanol to obtain the washing liquid and the washed catalyst. The washing liquid is incorporated into the reaction product to obtain the liquid product. The washed catalyst is dried and heat-treated under an argon atmosphere for recycling. S4. Transfer the liquid product obtained in step S3 to a distillation apparatus and distill at 100-120°C. Distill under reduced pressure to recover isooctanol and crude product, and then distill the crude product.

[0007] In this invention, a complete chemical depolymerization and conversion mechanism from polyester waste to dioctyl terephthalate is efficiently and sequentially achieved based on a hierarchical palladium-cerium modified zeolite catalyst. The entire reaction process can be clearly divided into two consecutive stages: alcoholysis and transesterification, both sequentially catalyzed by the same catalyst. In the alcoholysis reaction, pretreated polyester fragments, ethylene glycol, and the catalyst are heated together under nitrogen protection. The catalyst's abundant hierarchical channels first adsorb and accommodate the polyester macromolecular chains and ethylene glycol. The acid-base synergistic active sites provided by the zeolite framework and cerium species on its surface and within the channels effectively polarize and attack the ester bonds in the polyester polymer chains. Brønsted acid sites are responsible for protonating the carbonyl oxygen on the ester bonds, increasing the electrophilicity of the carbon atoms, while Lewis acid sites activate the oxygen atoms at the other end through complexation. Under the nucleophilic attack of the ethylene glycol molecules, the ester bonds of the polyester break, generating diethyl terephthalate as the main intermediate product, and possibly a small amount of oligomers. The mesoporous structure of the catalyst in this stage ensures that even large polyester molecular segments can smoothly enter the reaction region, while the micropores provide precise size selectivity. Once the alcoholysis reaction is essentially complete, the system does not require cooling or replacement; isooctanol is directly added to initiate the transesterification reaction. At this point, the main components of the reaction system become the diethyl terephthalate intermediate generated in the first stage and excess isooctanol. The acidic sites of the catalyst continue to play a crucial role, catalyzing the breaking of the ester bond linked to the hydroxyethyl group in the diethyl terephthalate molecule. Simultaneously, highly dispersed palladium nanoparticles may promote the forward transesterification reaction through the adsorption and activation of reaction intermediates or trace impurities, and may also act as a byproduct catalytic agent, removing unsaturated bonds generated by waste impurities or trace side reactions, thus improving the color and stability of the final product. Under the synergistic catalysis of the acidic and metal sites, isooctanol gradually replaces the ethylene glycol units in diethyl terephthalate, ultimately generating the target product, dioctyl terephthalate, and releasing ethylene glycol. The released ethylene glycol can participate in the cycle, continuing to attack incompletely depolymerized ester bonds. The entire conversion process is completed in series in the same reactor at a relatively mild temperature. The dual-functional design of the catalyst allows it to perfectly meet the catalytic requirements of both reaction stages. After the reaction, the catalyst's unique magnetic properties allow for extremely simple and complete separation from the liquid-phase product using an external magnetic field. The separated catalyst can be recycled after simple regeneration, while the liquid-phase product is purified by distillation to separate excess alcohols, yielding high-purity dioctyl terephthalate. This mechanism is clear and efficient, fully demonstrating the rationality and innovation of the catalyst and process design in this invention.

[0008] According to a preferred embodiment of the present invention, in step S1, the reaction time at 180-200°C is 0.8-1.2 h.

[0009] According to a preferred embodiment of the present invention, in step S2, the reaction continues at 180-200°C for 1-1.5 hours.

[0010] According to a preferred embodiment of the present invention, in step S3, the settling time is 10-20 minutes.

[0011] According to a preferred embodiment of the present invention, in step S4, the pressure of distillation under reduced pressure is -0.094 to -0.096 MPa.

[0012] According to a preferred embodiment of the present invention, the preparation steps of the hierarchical porous palladium-cerium modified zeolite catalyst include: A1, by weight, 100-120 parts of sodium-type ZSM-5 zeolite were calcined at 545-555℃ to obtain calcined zeolite; the calcined zeolite was dispersed in hydrochloric acid solution and stirred at 78-82℃ to obtain HZSM-5; HZSM-5 was mixed with 5-20 parts of hexadecyltrimethylammonium bromide and hydrothermally crystallized at 118-122℃; after the reaction was completed, the solid was collected by centrifugation, washed with deionized water, and calcined at 545-555℃ to obtain HZSM-5-H support; A2, HZSM-5-H support is impregnated in an aqueous solution containing 3.5-10 parts of cerium nitrate, and the impregnation is followed by rotary evaporation at 78-82℃ to obtain the impregnated sample; the impregnated sample is then calcined in air at 395-405℃ to obtain a mixture; the mixture is then placed in a tube furnace and reduced at 495-505℃ under a hydrogen / argon mixed atmosphere to obtain Ce / HZSM-5-H powder; A3. Disperse Ce / HZSM-5-H powder in deionized water, add an aqueous solution containing 0.8-2.5 parts of chloropalladic acid and an aqueous solution containing 4-15 parts of ferric nitrate, add 10-30 parts of urea aqueous solution dropwise under an ice-water bath, and sonicate to obtain a reaction mixture; A4. Centrifuge the reaction mixture to collect the solid, wash the solid with deionized water to obtain the washed solid, dry the washed solid, and react it in a hydrogen / argon mixed atmosphere at 295-305℃.

[0013] In this invention, the hierarchical palladium-cerium modified zeolite catalyst is designed and functionally constructed based on the ingenious concept of integrating composite catalysis with convenient separation. This catalyst uses a zeolite molecular sieve with a specific silicon-to-aluminum ratio as a matrix. First, it is converted to the hydrogen form through mild acid treatment to provide initial Brønsted acid active sites. Then, with the guiding effect of a long-chain organic template agent under hydrothermal conditions, mesoporous pore-forming engineering is performed on the zeolite crystals. Without destroying the original regular microporous framework, an interconnected mesoporous network is constructed within the particles, forming a unique microporous-mesoporous hierarchical pore system. This structure greatly increases the overall specific surface area and pore volume of the catalyst, providing ample space for the subsequent loading of active components. More importantly, its open mesoporous channels act like a highway network, significantly alleviating the bottleneck problem of hindered diffusion and mass transfer of large molecular fragments and intermediates generated from the depolymerization of waste polyester within traditional microporous catalysts, allowing reactants to quickly contact the active sites hidden deep within the pores. Subsequently, cerium and palladium, two key metal components, are introduced through a stepwise loading strategy. After impregnation with cerium species as precursors and followed by high-temperature calcination and reduction in a specific atmosphere, the cerium species are highly dispersed on the surface and within the pores of the support, primarily in the form of cerium oxide clusters and partially reduced cerium ions. These cerium species not only serve as mild Lewis acid sites themselves but also exhibit synergistic modulation effects with the Brønsted acid sites of the zeolite itself, collectively forming a gradient acid catalytic center adapted to ester bond breaking and recombination reactions. The palladium component is supported by iron species, which serve as the magnetic source, through co-precipitation and is activated in a reducing atmosphere to form highly dispersed palladium nanoparticles. These nanoparticles exhibit significant metal-support interactions with the adjacent cerium oxide clusters. These interactions not only stabilize the palladium nanoparticles and prevent sintering and agglomeration but may also optimize the electronic state of palladium, endowing it with better catalytic activity. Finally, the embedded magnetic oxide particles give the entire catalyst complex superparamagnetism, laying the physical foundation for the revolutionary operation of high-speed magnetic separation and recovery after the reaction. The entire catalyst preparation process is interconnected, and the final product is a highly efficient, stable, and recyclable multifunctional catalytic platform that integrates three major functions: shape-selective acid catalysis, metal-promoted conversion, and magnetic response separation.

[0014] According to a preferred embodiment of the present invention, in step A1, the hydrothermal crystallization time at 118-122°C is 24-30 hours.

[0015] According to a preferred embodiment of the present invention, in step A2, the reduction treatment at 495-505°C is carried out for 2-4 hours.

[0016] According to a preferred embodiment of the present invention, in step A3, the ultrasonic treatment time is 2-4 hours.

[0017] According to a preferred embodiment of the present invention, in step A4, the reaction time at 295-305°C is 3-5 hours.

[0018] The beneficial effects of this invention are as follows: The technical solution provided by this invention brings significant and multifaceted positive effects, primarily reflected in the revolutionary improvement in reaction efficiency brought about by the superior catalytic performance and structural innovation of its core catalyst. This hierarchical palladium-cerium modified zeolite catalyst is not a simple blend of traditional materials, but rather, through a sophisticated step-by-step preparation process, it constructs a highly efficient catalytic platform integrating multiple advantages. The hierarchical pore structure formed internally, interwoven with micropores and mesopores, acts like a highly efficient highway network for the massive polyester molecules and their reaction intermediates, greatly promoting the diffusion and mass transfer of reactants and products within the catalyst particles. This effectively solves the problem of "idle" internal active sites and low reaction rates caused by the narrow pores of previous solid catalysts. More importantly, by introducing cerium species and palladium metal and subjecting them to reduction treatment under specific conditions, a bifunctional catalytic system was precisely constructed on the surface and within the pores of the support, where acidic sites and metal active centers work synergistically. The suitable acidic environment efficiently cleaves the ester bonds in the polyester molecular chain, while the highly dispersed metal component further promotes the transformation and stabilization of intermediates, thus ensuring that the tandem reaction from alcoholysis to transesterification proceeds smoothly with high selectivity and high conversion rate. Furthermore, the magnetic component introduced into the catalyst endows it with unique magnetic response characteristics; this ingenious design lays a solid foundation for subsequent separation and recovery.

[0019] Secondly, the outstanding technical advantages of this invention are the high degree of integration and ease of operation of the entire chemical depolymerization and product purification process, which greatly simplifies the production process and significantly reduces energy and material consumption. Unlike traditional processes that treat alcoholysis and transesterification as two isolated unit operations that may require different reaction conditions and catalytic systems, this invention creatively integrates the two reactions into the same reaction device and uses the same multifunctional catalyst for sequential catalysis. This allows for continuous completion under similar temperature conditions, eliminating the need for intermediate product separation, transfer, and refeeding. This not only shortens the total reaction time but also significantly reduces equipment investment and operational complexity. After the reaction, thanks to the magnetic characteristics of the catalyst, rapid, thorough, and low-loss separation of the catalyst particles and the liquid reaction mixture can be achieved simply by applying an external magnetic field, completely avoiding the troublesome problems of filter membrane clogging and catalyst powder loss in traditional filtration processes. The subsequent product purification process is also simple and efficient. Through the organic combination of atmospheric and vacuum distillation, excess ethylene glycol and isooctanol can be recovered sequentially. These recovered raw materials can be directly reused in the production process, realizing the internal circulation of materials. The crude dioctyl terephthalate obtained in the end has high purity and only requires simple purification to reach the standard of high-quality products. The entire process route is smooth and produces almost no difficult-to-treat process wastewater, reflecting the principles of green chemistry.

[0020] Finally, this invention achieves a perfect balance between economic and environmental benefits, opening up a practical new path for the high-value utilization of polyester waste. From an economic perspective, this method directly transforms inexpensive polyester waste into high-value-added plasticizer products with strong market demand, greatly enhancing the economic driving force of the recycling process. Although the developed catalyst has a rigorous preparation process, its raw materials are all conventional chemicals, and thanks to its excellent magnetic separation performance and experimentally verified good stability, it can be reused multiple times, effectively reducing the cost per use of the catalyst. The integrated process reduces energy consumption and labor costs, and the efficient recovery of solvents further reduces raw material consumption. From an environmental and social perspective, this technology provides an advanced chemical recycling solution for massive amounts of polyester waste, helping to alleviate the environmental pollution pressure caused by waste plastics, promoting the recycling of carbon resources, and conforming to the strategic direction of circular economy and sustainable development. At the same time, the products produced by this process have excellent performance and can replace some traditional petroleum-based plasticizers, indirectly reducing dependence on fossil resources. In summary, this invention has achieved significant progress in multiple aspects, including technology, economy, and environment, through synergistic innovation of catalysts and processes, and possesses outstanding practical value and broad prospects for industrial application. Detailed Implementation

[0021] The following detailed embodiments are only used to further illustrate this application and should not be construed as limiting the scope of protection of this application. Those skilled in the art can make some non-essential improvements and adjustments to this application based on the above application content.

[0022] Example 1 Preparation of hierarchical porous palladium-cerium modified zeolite catalysts: A1. Preparation of a hierarchical porous zeolite support. 110.0 g of sodium-form ZSM-5 zeolite was weighed and placed in a muffle furnace, calcined at 550 °C for 4.0 h, and then allowed to cool naturally to obtain calcined zeolite. All the calcined zeolite was transferred to a 2.0 L flask, and 800 mL of 1.0 mol / L hydrochloric acid solution was added. The flask was placed in an 80 °C oil bath and mechanically stirred at 300 rpm for 6.0 h for ion exchange. After the reaction, the mixture was transferred to a centrifuge tube and centrifuged at 8000 rpm for 10 min, discarding the supernatant. 800 mL of deionized water was added to the precipitate, stirred to disperse, and centrifuged again. This washing process was repeated until the pH of the washing solution was approximately 6-7. The washed solid was transferred to an oven and dried at 120 °C for 12 h to obtain hydrogen-form zeolite (HZSM-5). 100.0 g of the hydrogen-form zeolite and 12.0 g of cetyltrimethylammonium bromide (CTAB) were weighed and added together to a 500 mL polytetrafluoroethylene liner containing 300 mL of deionized water. The mixture was magnetically stirred for 2.0 h at room temperature to ensure thorough mixing and form a suspension. The liner was then placed in a stainless steel hydrothermal reactor and sealed. The reactor was then placed in an oven and statically crystallized at 120 °C for 24.0 h. After crystallization, the mixture was allowed to cool naturally to room temperature. The reactor was then opened, and the contents were transferred to a centrifuge cup. The solid was collected by centrifugation at 8000 rpm for 10 min. The solid was washed three times with 300 mL of deionized water, and centrifuged after each wash. The washed solid was placed in a ceramic boat and placed in a muffle furnace. The temperature was programmed to rise from room temperature to 550 °C at a rate of 2 °C / min, and calcined at this temperature for 6.0 h to completely remove the template agent. After furnace cooling, a white hierarchical porous zeolite support was obtained, designated as HZSM-5-H support.

[0023] A2, Cerium-supported species. Accurately weigh 100.0 g of the above HZSM-5-H support and place it in a 500 mL round-bottom flask. Dissolve 6.5 g of cerium nitrate hexahydrate in 50 mL of deionized water to prepare an impregnation solution. Slowly add this impregnation solution dropwise to the flask containing the support, continuously shaking the flask manually to ensure the support is evenly wetted. After the addition is complete, seal the flask with sealing film and allow it to stand at room temperature for 12.0 h. Subsequently, connect the flask to a rotary evaporator and evaporate it completely in an 80 °C water bath to obtain a light yellow impregnated sample. Transfer the sample to a muffle furnace and calcine it at 400 °C at a rate of 2 °C / min under air atmosphere for 4.0 h. After cooling, weigh 50.0 g of the calcined sample and place it in a quartz boat, then place it in a tube reduction furnace. A mixture of 5 vol% hydrogen and 95 vol% argon was introduced at a flow rate of 50 mL / min and purged for 30 min to replace the air. Then, the furnace temperature was increased to 500 °C at a rate of 5 °C / min and maintained at this temperature for 2.0 h. After reduction, the mixture was cooled to room temperature under the protection of a hydrogen-argon mixture to obtain a pale yellow cerium-modified zeolite powder, denoted as Ce / HZSM-5-H powder.

[0024] A3, co-loaded palladium and magnetic iron components. Weigh 100.0 g of Ce / HZSM-5-H powder obtained in step A2, place it in a 1.0 L beaker, add 200 mL of deionized water, and stir magnetically to form a homogeneous slurry. Place the beaker in an ice-water bath and then in an ultrasonic cleaner tank (keeping the water bath level higher than the liquid level in the beaker). Turn on the ultrasonic cleaner and set the power to 300 W. Add 20 mL of an aqueous solution containing 1.50 g of palladium chloroacetic acid and 30 mL of an aqueous solution containing 8.00 g of ferric nitrate nonahydrate to the beaker sequentially, and ultrasonically stir for 10 min to mix thoroughly. Then, using a constant pressure dropping funnel, slowly add 20 mL of a 2.0 mol / L urea aqueous solution, controlling the addition time to within 30 min. After the addition is complete, continue to maintain the ice-water bath and ultrasonic conditions, and react for 2.0 h.

[0025] After reaction A4, the mixture in the beaker was transferred to a centrifuge tube and centrifuged at 8000 rpm for 10 min, discarding the supernatant. The precipitate was redispersed with 200 mL of deionized water and washed by centrifugation; this process was repeated three times. The washed solid was transferred to an oven and dried at 120 °C for 12 h. The dried solid was ground and placed in a quartz boat, which was then placed in a tube furnace. A 5 vol% H2 / Ar mixed gas (50 mL / min) was introduced to purge for 30 min, and then the temperature was increased to 300 °C at 5 °C / min, and reduced at this temperature for 3.0 h. After reduction, the temperature was lowered to room temperature under a protective gas to obtain the final hierarchical porous palladium-cerium modified zeolite catalyst, denoted as Pd-Ce / HZSM-5-H.

[0026] Chemical depolymerization of dioctyl terephthalate prepared from polyester waste: S1, Raw material pretreatment and alcoholysis reaction. Waste polyester textiles are collected and cut into pieces with an area less than 1 cm² using a shredder. 2 The polyester fragments were weighed and soaked in 200 mL of anhydrous ethanol for 30 min with stirring to remove surface stains. After filtration, the fragments were dried in a vacuum oven at 80 °C for 6 h to obtain clean and dry polyester fragments. The 100 g polyester fragments, 250 g ethylene glycol (EG), and 2.50 g of self-made Pd-Ce / HZSM-5-H catalyst were added sequentially to a 500 mL high-pressure reactor equipped with a magnetic stirrer, thermocouple, pressure gauge, reflux condenser, and nitrogen inlet / outlet. The reactor was sealed, and the nitrogen valve was opened to purge nitrogen into the reactor until the pressure reached 0.5 MPa. Then, the vent valve was opened to release the pressure to atmospheric pressure. This process was repeated three times to completely replace the air in the reactor. After replacement, under the protection of a continuous weak nitrogen flow (approximately 10 mL / min), the stirrer was started and the speed was set to 300 rpm. Heating was initiated, and the reaction mixture was heated to 190°C at a rate of approximately 5°C / min. At this point, the autogenous pressure inside the reactor was approximately 0.4 MPa. The reaction was maintained at 190°C ± 2°C with stirring for 1.0 h. During this period, the solid polyester fragments were observed to gradually dissolve, and the system became homogeneous.

[0027] S2, transesterification reaction. After the first step of alcoholysis has proceeded for 1.0 h, stirring is stopped. 450.0 g of isooctanol is injected into the reactor in a single batch using a high-pressure metering pump through the feed tube provided on the reactor. Stirring is restarted (300 rpm), and the reaction temperature is maintained at 190℃±2℃ for another 1.2 h.

[0028] S3, Product Separation and Catalyst Recovery. After a total reaction time of 2.2 hours, the heating was turned off, and the reactor was removed from the heating jacket to allow it to cool naturally to 70°C. At this point, the pressure inside the reactor dropped to atmospheric pressure. The reactor lid was opened, and the entire reaction mixture was poured into a 1.0L glass beaker. A neodymium iron boron circular magnet with a diameter of approximately 5cm and a surface magnetic induction intensity greater than 0.3T was placed firmly against the bottom of the beaker. After standing for 15 minutes, it was clearly observed that the grayish-black catalyst powder was completely adsorbed to the bottom of the beaker, and the upper layer was a brownish-yellow transparent liquid. The clear liquid was carefully poured into another 1.0L round-bottom flask. 20.0mL of isooctanol was added to the beaker containing the solid catalyst residue, and after gently stirring and washing with a glass rod, the residue was separated again by adsorption with a magnet. The washing liquid was poured out and added to the aforementioned round-bottom flask, and the liquid product was obtained by combining the two. The catalyst solid adsorbed at the bottom of the cup was transferred to a ceramic boat with deionized water, dried at 120°C, placed in a tube furnace, and heat-treated at 300°C for 1.0 h under an argon atmosphere. After cooling, it was recovered for later use.

[0029] S4, Product Purification. Connect the round-bottom flask containing the liquid product to a rotary evaporator. First, distill at atmospheric pressure and a water bath temperature of 110°C, recovering the fraction mainly as excess ethylene glycol, until the distillation temperature rises significantly. Then, connect the system to a vacuum pump, reduce the pressure to -0.095 MPa, and raise the water bath temperature to 150°C to distill and recover isooctanol. Stop distillation when the distillation rate becomes extremely slow. The remaining crude product in the flask is mainly dioctyl terephthalate (DOTP). Transfer the crude product to a short-path distillation apparatus and perform high-vacuum distillation under a vacuum of less than 100 Pa, collecting the main fraction to obtain a colorless, transparent liquid product with suitable viscosity. The recovered catalyst is regenerated as described above and used in the next reaction cycle under the same conditions.

[0030] Example 2 The specific implementation method is the same as in Example 1, except that the preparation of the hierarchical porous palladium-cerium modified zeolite catalyst is as follows: A1. Preparation of a hierarchical porous zeolite support. 115.0 g of sodium-form ZSM-5 zeolite was weighed and placed in a muffle furnace, calcined at 548 °C for 4.5 h, and then allowed to cool naturally. The calcined zeolite was added to 850 mL of a 1.2 mol / L hydrochloric acid solution and mechanically stirred in an oil bath at 82 °C for 6.5 h. After centrifugation, the solid was washed with deionized water until neutral and dried at 118 °C for 14 h to obtain hydrogen-form zeolite (HZSM-5). 100.0 g of this hydrogen-form zeolite and 18.0 g of cetyltrimethylammonium bromide (CTAB) were weighed and added together to a 500 mL polytetrafluoroethylene liner containing 350 mL of deionized water, and stirred at room temperature for 2.5 h. The liner was then placed in a reaction vessel and statically crystallized in an oven at 122 °C for 28.0 h. After crystallization, the solid was collected by centrifugation, washed three times with water, and then placed in a muffle furnace. The temperature was increased to 552℃ at 2℃ / min and calcined for 5.5h. The solid was then cooled in the furnace to obtain the carrier HZSM-5-H.

[0031] A2, Cerium-supported species. 100.0 g of HZSM-5-H support was weighed and placed in a round-bottom flask. 8.00 g of cerium nitrate hexahydrate was dissolved in 55 mL of deionized water to prepare an impregnation solution, which was then uniformly added to the support. The solution was allowed to stand at room temperature for 14 h. The solution was then rotary evaporated to dryness at 78 °C. The solid was transferred to a muffle furnace and calcined at 398 °C for 3.5 h under air atmosphere at a rate of 2 °C / min. 50.0 g of the calcined sample was placed in a tube furnace, and a 5 vol% H2 / Ar mixed gas was introduced (55 mL / min). The temperature was increased to 498 °C at a rate of 5 °C / min, and the solution was reduced for 3.0 h. The temperature was then lowered under protective conditions to obtain Ce / HZSM-5-H powder.

[0032] A3, co-loaded with palladium and magnetic iron components. Weigh 100.0 g of Ce / HZSM-5-H powder into a 1 L beaker, add 220 mL of deionized water and stir to form a slurry. Place the beaker in an ice-water bath and sonicate (300 W). Add 25 mL of aqueous solution containing 2.00 g of palladium chloroacetic acid and 40 mL of aqueous solution containing 12.00 g of ferric nitrate nonahydrate sequentially, and sonicate for 10 min. Slowly add 25 mL of 1.8 mol / L urea aqueous solution over 35 min, and then continue sonication for 2.5 h.

[0033] After the reaction, A4 was centrifuged, and the solid was washed three times with 220 mL of deionized water and dried at 118 °C for 14 h. The dried solid was then ground and placed in a tube furnace. Under a gas flow of 5 vol% H2 / Ar (55 mL / min), the temperature was increased to 298 °C at a rate of 5 °C / min, and the reduction was carried out for 3.5 h. After the reduction was completed, the temperature was lowered under protective conditions to obtain a gray-black Pd-Ce / HZSM-5-H catalyst powder.

[0034] Chemical depolymerization of dioctyl terephthalate prepared from polyester waste: Weigh 120.0 g of pretreated polyester flakes, 300.0 g of ethylene glycol, and 3.00 g of the above catalyst, and add them to a 500 mL high-pressure reactor. After purging the air with nitrogen three times, the temperature was increased to 185 °C at approximately 5 °C / min under a continuous weak nitrogen flow and stirring at 300 rpm for 1.1 h of alcoholysis reaction. Subsequently, 500.0 g of isooctanol was injected through the feed tube, and the temperature was maintained at 185 °C and stirring at 300 rpm for another 1.5 h. After a total reaction time of 2.6 h, heating was stopped, and the mixture was allowed to cool naturally to 65 °C. The reaction mixture was poured into a beaker, and the catalyst was separated by adsorption using an external magnet for 20 min. The supernatant was poured off, and the solid catalyst was washed with 25.0 mL of isooctanol. The liquid products were combined. The catalyst was dried and then heat-treated at 300 °C under argon for 1 h before recovery. The liquid product was first distilled at atmospheric pressure and 108°C to recover ethylene glycol, and then isooctyl alcohol was recovered under reduced pressure at -0.094 MPa and 150°C. The remaining crude product was purified by high-vacuum distillation to obtain the DOTP product.

[0035] Example 3 The specific implementation method is the same as in Example 1, except that the preparation of the hierarchical porous palladium-cerium modified zeolite catalyst is as follows: A1. Preparation of a hierarchical porous zeolite support. 105.0 g of sodium-type ZSM-5 zeolite was weighed and calcined in a muffle furnace at 552 °C for 3.5 h. After cooling, 750 mL of 0.8 mol / L hydrochloric acid solution was added, and the mixture was stirred at 79 °C for 5.5 h. The mixture was centrifuged, washed with water until neutral, and dried at 122 °C for 10 h to obtain HZSM-5. 100.0 g of this HZSM-5 and 8.00 g of CTAB were weighed and added to a polytetrafluoroethylene liner containing 280 mL of deionized water. The mixture was stirred at room temperature for 1.5 h. The liner was placed in a reaction vessel and crystallized in an oven at 119 °C for 26.0 h. The solid was collected by centrifugation, washed three times with water, and then calcined in a muffle furnace at a temperature increased to 548 °C at 2 °C / min for 6.5 h to obtain the support HZSM-5-H.

[0036] A2, Cerium-supported species. Weigh 100.0 g of HZSM-5-H support. Dissolve 4.50 g of cerium nitrate hexahydrate in 45 mL of deionized water to obtain an impregnation solution, add it evenly to the support, and let it stand at room temperature for 13 h. Evaporate to dryness at 81 °C. Calcinate the solid in a muffle furnace under air atmosphere at 2 °C / min to 402 °C for 4.5 h. Take 50.0 g of the calcined sample and place it in a tube furnace, introduce 5 vol% H2 / Ar (48 mL / min), heat to 502 °C at 5 °C / min, reduce for 3.0 h, and then cool under protection to obtain Ce / HZSM-5-H powder.

[0037] A3, co-loaded with palladium and magnetic iron components. Weigh 100.0g of Ce / HZSM-5-H powder, add 180mL of deionized water and stir to form a slurry, then sonicate in an ice-water bath (300W). Add 15mL of aqueous solution containing 0.90g of palladium chloroacetic acid and 25mL of aqueous solution containing 5.00g of ferric nitrate nonahydrate sequentially, and sonicate for 10min. Slowly add 15mL of 2.2mol / L urea aqueous solution (completed over 30min), and continue sonication for 1.8h.

[0038] A4 was centrifuged, the solid was washed three times with 180 mL of water, and dried at 122 °C for 10 h. The dried solid was then placed in a tube furnace and reduced to 302 °C at 5 °C / min under 5 vol% H2 / Ar (48 mL / min) conditions for 4.0 h. The solid was then cooled under protective conditions to obtain a gray-black Pd-Ce / HZSM-5-H catalyst.

[0039] Chemical depolymerization of dioctyl terephthalate prepared from polyester waste: Weigh 85.0 g of pretreated polyester flakes, 220.0 g of ethylene glycol, and 1.50 g of the above catalyst, and add them to a 500 mL high-pressure reactor. After three nitrogen purgings, the mixture was heated to 195 °C for alcoholysis under continuous nitrogen protection and stirring at 300 rpm for 0.9 h. Then, 420.0 g of isooctanol was added, and the reaction was continued for another 1.0 h while maintaining 195 °C and stirring at 300 rpm. Heating was stopped after a total reaction time of 1.9 h, and the mixture was cooled to 75 °C. The mixture was transferred and allowed to stand with a magnet for 12 min to separate the catalyst. The liquid was decanted, and the catalyst was washed with 15.0 mL of isooctanol. The liquids were combined. The catalyst was dried and heat-treated at 300 °C under argon for 1 h for recovery. Ethylene glycol was recovered from the liquid product at atmospheric pressure and 105 °C, and isooctanol was recovered at -0.096 MPa and 150 °C. The crude product was distilled under high vacuum to obtain DOTP.

[0040] Comparative Example 1 The specific implementation method is the same as in Example 1, except that unmodified hydrogen-form zeolite HZSM-5 is used as the catalyst in this comparative example. Preparation: 110.0 g of sodium-form ZSM-5 zeolite was weighed and calcined at 550 °C for 4.0 h. The calcined zeolite was dispersed in 800 mL of 1.0 mol / L hydrochloric acid and stirred at 80 °C for 6.0 h. After centrifugation, it was washed with water until neutral and dried at 120 °C for 12 h to obtain the HZSM-5 catalyst, without mesoporous construction or metal loading.

[0041] Depolymerization experiment: Except for replacing the catalyst with 2.50g of the above-mentioned HZSM-5, all other conditions (raw material mass, steps, temperature, time) were exactly the same as in Example 1. After the reaction, the catalyst was non-magnetic and needed to be separated from the viscous liquid by complex filtration.

[0042] Comparative Example 2 The specific implementation method is the same as in Example 1, except that this comparative example uses a Pd-Ce / HZSM-5 catalyst without magnetic iron components. Its preparation: Steps A1 and A2 are exactly the same as in Example 1. In step A3, only 20 mL of an aqueous solution containing 1.50 g of palladium chloroacetic acid is added to the Ce / HZSM-5-H slurry; ferric nitrate solution is not added, and the amount of urea solution is reduced to 10 mL. The reduction conditions in step A4 are the same as in Example 1, yielding a powdered non-magnetic catalyst.

[0043] Depolymerization experiment: Except for using 2.50g of the above-mentioned non-magnetic catalyst, the other conditions were the same as in Example 1. After the reaction, the catalyst could not be magnetically separated and had to be removed from the reaction solution by centrifugation and filtration. The separation process took more than 60 minutes and resulted in losses.

[0044] Comparative Example 3 The specific implementation method is the same as in Example 1, except that this comparative example uses a one-step mixing method for all raw materials. Depolymerization experiment: 100.0g of polyester fragments, 250.0g of ethylene glycol, 450.0g of isooctanol, and 2.50g of the hierarchical porous palladium-cerium modified zeolite catalyst prepared in Example 1 were added to a high-pressure reactor in one step. After nitrogen purging, the reaction was carried out continuously at 190℃ and 300rpm for 2.2h with stirring. After cooling to 70℃, the catalyst was magnetically separated, and subsequent purification steps were the same as in Example 1.

[0045] Performance testing The dioctyl terephthalate prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to performance testing according to the following method, which included the following steps: Polyester conversion rate, dioctyl terephthalate yield, and product purity were determined using a gas chromatograph equipped with a flame ionization detector and a polar capillary column. The specific steps were as follows: 50.0 mg of the liquid product sample was accurately weighed into a 2.0 mL chromatographic sample vial, dissolved in 1.00 mL of chromatographically pure N,N-dimethylformamide solvent, and shaken well. Quantification was performed using the internal standard method, with 10.0 mg of dimethyl terephthalate accurately added as an internal standard in parallel experiments. The chromatographic conditions were set as follows: carrier gas was high-purity nitrogen, flow rate was 1.0 mL / min, injection port temperature was 280 °C, detector temperature was 300 °C, column temperature program was: initial temperature 150 °C held for 2.0 min, increased to 280 °C at a rate of 15 °C / min and held for 10.0 min, injection volume was 1.0 μL, and split ratio was 50:1. Polyester conversion rate was calculated using the mass difference before and after the reaction: Conversion rate = (Mass of polyester fragments before reaction - Mass of recovered dry solid insoluble matter after reaction) / Mass of polyester fragments before reaction × 100%. Dioctyl terephthalate yield was calculated using the internal standard method as the ratio of product mass to theoretical yield: Yield = (Actual mass of dioctyl terephthalate determined by gas chromatography internal standard method / Theoretical mass based on the conversion of all polyester to dioctyl terephthalate) × 100%. Product purity was directly obtained from the gas chromatogram using the area normalization method, calculated as: Purity = (Peak area of ​​dioctyl terephthalate / Sum of peak areas of all detected components) × 100%. All tests were performed three times under the same chromatographic conditions, and the average value was taken.

[0046] Catalyst separation efficiency is assessed by measuring the total time required from the start of cooling the reaction mixture to a specified temperature until decantation is complete and a fully clear supernatant is obtained, using a stopwatch, and the results are recorded in minutes (min).

[0047] Catalyst cycle stability was assessed by regenerating the catalyst after each use using a standard process and then reusing it under the same conditions. The yield of dioctyl terephthalate was recorded after each cycle, with this report highlighting the yield data after the 5th cycle.

[0048] The acid value of the product was determined by acid-base titration: 5.00 g of sample was accurately weighed and dissolved in 50 mL of 95% ethanol solution that had been neutralized to a slightly pink color with phenolphthalein as an indicator. After complete dissolution, 2-3 drops of 1% phenolphthalein indicator were added, and titration was performed with 0.050 mol / L potassium hydroxide standard ethanol solution until the solution turned slightly pink and remained so for 30 seconds. The volume of potassium hydroxide standard solution consumed was recorded. The acid value was calculated using the formula: Acid value = (V × C × 56.1) / m, where V is the volume of potassium hydroxide standard solution consumed in the titration (unit: mL), C is its concentration (unit: mol / L), m is the sample mass (unit: g), and 56.1 is the molar mass of potassium hydroxide (unit: g / mol). The result is expressed as mgKOH / g.

[0049] The colorimetry of the product was determined using the platinum-cobalt colorimetric method: the sample to be tested was injected into a 50 mL Nessler tube to the graduation mark. Under a standard light source box or fluorescent lamp, with a white background, the color depth of the sample was visually compared with a series of standard platinum-cobalt color solutions in the vertical direction. The standard color solution that is closest to the color of the sample was selected, and its number is the colorimetry of the sample. The result is expressed as the platinum-cobalt color number (Pt-Co).

[0050] Test results: Table 1: Test results of each embodiment and comparative example ; As can be seen from Table 1, the technical solutions represented by Embodiments 1-3 of the present invention effectively and comprehensively solve a series of core problems raised in the background art.

[0051] First, addressing the challenge of separating and recovering catalysts, the multi-porous palladium-cerium modified zeolite catalyst used in the examples achieved revolutionary and simple separation due to its built-in magnetic components. Test data showed that its separation time was only 12 to 20 minutes. In contrast, although the catalytic activity of Comparative Example 2, which lacked magnetic design, was acceptable, the separation time exceeded 60 minutes and the process was cumbersome. This directly confirms the decisive role of magnetic functional design in solving the separation and recovery bottleneck.

[0052] Secondly, in overcoming the low efficiency of solid catalysts, the catalysts in the examples showed significant advantages, with conversion rates of polyester exceeding 99% and dioctyl terephthalate yields exceeding 91%. In contrast, Comparative Example 1 (using only ordinary zeolite) without hierarchical pore construction and metal modification had a conversion rate of only 76% and a yield as low as 61%, and the product had a deep color and high acid value. This clearly demonstrates that improving mass transfer by constructing hierarchical channels and introducing palladium-cerium bimetallic active centers for synergistic catalysis is the key to significantly improving the intrinsic activity and selectivity of solid catalysts.

[0053] Third, regarding the issue of cumbersome process steps, the two-step sequential feeding method (first alcoholysis then transesterification) used in the example is completed in the same reactor. Compared with the simplified operation of Comparative Example 3, which mixes all raw materials in one step, this method not only maintains a high conversion rate but also increases the yield of dioctyl terephthalate from 84.7% to over 92% and the purity from 97.5% to over 99%. This demonstrates that the process design simplifies the process while significantly improving the generation efficiency and purity of the target product by optimizing the reaction path, thus avoiding the problem of complex by-products caused by competing reactions.

[0054] Fourth, and finally, considering the high-quality performance of the products in the comprehensive examples (acid value below 0.06 mg KOH / g and color below 20 Pt-Co), and the product quality degradation caused by low catalytic efficiency in Comparative Example 1, it is evident that the present invention completely avoids the problem of product contamination by the residue of traditional homogeneous catalysts. At the same time, the excellent cycle stability of its catalyst (yield retention rate exceeding 96% after 5 uses) further enhances its adaptability to complex waste systems and the economy of the process.

[0055] Fifth, the test data fully demonstrates that this invention, through the integrated design of the catalyst's structure and function (multi-level pores, dual active centers, and magnetism) combined with a matching optimized process, has synergistically overcome multiple technical obstacles such as catalytic efficiency, separation and recovery, product purity, and process simplicity, thereby achieving improved efficiency and greening of the high-value conversion process of polyester waste.

[0056] The embodiments described above are merely examples 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 present invention. 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 modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste, characterized in that, Includes the following steps: S1, by weight, polyester waste is crushed, washed and dried to obtain polyester waste fragments; in a high-pressure reactor, 80-120 parts of polyester waste fragments, 200-300 parts of ethylene glycol and 1-3 parts of multi-porous palladium-cerium modified zeolite catalyst are added; the mixture is stirred and heated to 180-200℃ under nitrogen protection to obtain a mixture; S2, add 400-500 parts of isooctanol to the mixture obtained in step S1, and continue the reaction at 180-200℃ to obtain the reaction mixture; S3, let the reaction mixture cool naturally to 60-80℃, place the magnet on the outside of the bottom of the high-pressure reactor, let it stand, and separate the upper liquid reaction product from the solid catalyst slurry to obtain the reaction product and the catalyst slurry; The catalyst slurry was washed with isooctanol to obtain a washing liquid and the washed catalyst. The washing liquid was then incorporated into the reaction product to obtain a liquid product. The washed catalyst is dried and heat-treated under an argon atmosphere for recycling. S4. Transfer the liquid product obtained in step S3 to a distillation apparatus and distill at 100-120°C. Distill under reduced pressure to recover isooctanol and crude product, and then distill the crude product.

2. The chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste according to claim 1, characterized in that, In step S1, the reaction time is 0.8-1.2 h after heating to 180-200℃.

3. The chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste according to claim 1, characterized in that, In step S2, the reaction continues at 180-200℃ for 1-1.5 hours.

4. The chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste according to claim 1, characterized in that, In step S3, the settling time is 10-20 minutes.

5. The chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste according to claim 1, characterized in that, In step S4, the pressure for distillation under reduced pressure is -0.094 to -0.096 MPa.

6. The chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste according to any one of claims 1-5, characterized in that, The preparation steps of the hierarchical porous palladium-cerium modified zeolite catalyst include: A1, by weight, 100-120 parts of sodium-type ZSM-5 zeolite were calcined at 545-555℃ to obtain calcined zeolite; the calcined zeolite was dispersed in hydrochloric acid solution and stirred at 78-82℃ to obtain HZSM-5; HZSM-5 was mixed with 5-20 parts of hexadecyltrimethylammonium bromide and hydrothermally crystallized at 118-122℃; after the reaction was completed, the solid was collected by centrifugation, washed with deionized water, and calcined at 545-555℃ to obtain HZSM-5-H support; A2, HZSM-5-H support is impregnated in an aqueous solution containing 3.5-10 parts of cerium nitrate, and the impregnation is followed by rotary evaporation at 78-82℃ to obtain the impregnated sample; the impregnated sample is then calcined in air at 395-405℃ to obtain a mixture; the mixture is then placed in a tube furnace and reduced at 495-505℃ under a hydrogen / argon mixed atmosphere to obtain Ce / HZSM-5-H powder; A3. Disperse Ce / HZSM-5-H powder in deionized water, add an aqueous solution containing 0.8-2.5 parts of chloropalladic acid and an aqueous solution containing 4-15 parts of ferric nitrate, add 10-30 parts of urea aqueous solution dropwise under an ice-water bath, and sonicate to obtain a reaction mixture; A4. Centrifuge the reaction mixture to collect the solid, wash the solid with deionized water to obtain the washed solid, dry the washed solid, and react it in a hydrogen / argon mixed atmosphere at 295-305℃.

7. The chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste according to claim 6, characterized in that, In step A1, the hydrothermal crystallization time at 118-122℃ is 24-30 hours.

8. The chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste according to claim 6, characterized in that, In step A2, the reduction treatment at 495-505℃ takes 2-4 hours.

9. The chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste according to claim 6, characterized in that, In step A3, the ultrasonic treatment time is 2-4 hours.

10. The chemical depolymerization method for preparing dioctyl terephthalate based on polyester waste according to claim 6, characterized in that, In step A4, the reaction time at 295-305℃ is 3-5 hours.