Application of NaOH-KOH molten salt in depolymerized PET plastic

Abstract

This invention discloses the application of NaOH-KOH molten salt in the depolymerization of PET plastics, belonging to the field of polyester plastic depolymerization technology. This invention uses NaOH-KOH molten salt as the substrate for PET depolymerization and employs a xenon lamp to simulate sunlight as the heat source for the reactor. Utilizing the heat storage and strong alkalinity of NaOH-KOH molten salt, efficient and green depolymerization of PET plastics is achieved, yielding EG and high-purity TPA. Compared to traditional alkaline hydrolysis technology for PET plastics, the method of this invention has milder reaction conditions and advantages such as simple subsequent processing steps, low cost, and environmental friendliness, demonstrating broad commercial application prospects.

Description

Technical Field

[0001] This invention belongs to the field of polyester depolymerization technology, and particularly relates to the application of NaOH-KOH molten salt in depolymerized PET plastic. Background Technology

[0002] The industrial production of plastics can be traced back to the early 20th century. Due to their low cost and excellent properties such as wear resistance, chemical resistance, and heat resistance, they have been widely used. Existing plastics can be mainly classified into two categories based on their manufacturing process: addition polymerization plastics and condensation polymerization plastics. Addition polymerization plastics are high-molecular-weight polymers formed by the addition polymerization reaction of small-molecule olefins or their substituted derivatives under heating and catalysis. These mainly include polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC). Condensation polymerization plastics are high-molecular-weight condensation polymers formed by multiple condensation reactions between multifunctional monomers, accompanied by the formation of small-molecule byproducts such as water, alcohols, ammonia, or hydrogen chloride. These mainly include polyamide (PA), polyethylene terephthalate (PET), polycarbonate (PC), and polyurethane (PU). Among these, PET plastic, due to the rigidity of its macromolecular chains, exhibits excellent mechanical properties and chemical stability, and is widely used in food packaging, agricultural films, and industrial components. However, due to a lack of effective recycling technologies, only about 20% of waste plastics worldwide are recycled. Waste plastics contain abundant carbon and hydrogen resources; therefore, realizing the resource utilization of waste plastics is of great significance for promoting high-value recycling of waste plastics and fostering green and sustainable economic development.

[0003] Currently, the main methods for treating waste plastics are traditional recycling methods such as incineration, landfill, and mechanical recycling. Incineration not only consumes a large amount of energy but also easily causes secondary pollution such as the greenhouse effect. Furthermore, mechanical recycling of waste plastics is limited by the quality of plastic recycling, producing only low-value-added chemicals such as recycled plastics. The large accumulation of waste plastics not only leads to soil fertility degradation and damage to marine ecosystems, but more seriously, some non-degradable microplastics pose a serious threat to the health of the entire ecosystem. To overcome the problems of traditional waste plastic disposal methods, in recent years, research has focused on chemical recycling technologies that can realize the resource recovery and energy recovery of waste plastics.

[0004] Existing chemical recycling technologies for waste plastics mainly include pyrolysis, hydrolysis, and hydrocracking, with the specific technology chosen depending on the type of waste plastic. For polymeric plastics, common treatment methods include pyrolysis and hydrocracking. Under conditions of high temperature, high pressure, and catalysts, the polymer macromolecular chains break down, generating oils, gases, or monomers. These methods are suitable not only for single-component plastics such as PP, PE, PS, and PVC, but also for mixed plastics composed of them. However, they have high energy consumption and relatively complex processes, posing a significant challenge to their industrial application. For condensation plastics, the common treatment method is depolymerization, which involves degrading the plastic polymer chains into high-purity oligomers or corresponding monomers in the presence of depolymerizing agents such as water, alcohol, or ammonia, thereby achieving closed-loop recycling of waste plastics.

[0005] Currently, chemical recycling methods for waste PET plastics can be categorized based on the type of depolymerizing agent used in the depolymerization reaction, including hydrolysis, alcoholysis, ammonolysis, and amine hydrolysis. The depolymerizing agent undergoes a nucleophilic reaction with the carbonyl carbon atoms in the PET macromolecular chain, subsequently breaking the CO bond to generate terephthalic acid (TPA), terephthalamide, diamine terephthalate, and ethylene glycol (EG). Hydrolysis can be further classified into acidic, neutral, and alkaline hydrolysis based on the pH value of the depolymerization system. All of these methods require high temperatures and pressures to achieve high TPA and EG yields, inevitably leading to energy waste and resource loss. Alkaline hydrolysis typically requires high temperatures (210-250℃) and pressures (1.4-2.0 MPa) in a 4-20 wt% NaOH or KOH aqueous solution for 3-5 hours, followed by acidification, filtration, and other post-treatment steps to obtain EG and high-purity TPA. However, alkaline hydrolysis involves harsh reaction conditions, high consumption of depolymerizing agents, and the generation of large amounts of alkaline waste liquid, leading to severe equipment corrosion and significant treatment costs. Therefore, although PET plastic hydrolysis technology has been researched for a long time and is relatively mature, its low economic viability hinders its industrial application. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention proposes the application of NaOH-KOH molten salt in the depolymerization of PET plastics. This invention provides a highly efficient, low-cost, and low-energy-consumption method for depolymerizing PET plastics. By utilizing NaOH-KOH molten salt, efficient and green depolymerization of PET plastics is achieved under relatively mild conditions.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] This invention provides an application of NaOH-KOH molten salt in depolymerized PET plastic, comprising the following steps:

[0009] NaOH-KOH molten salt and PET plastic are mixed and reacted at 170-190℃ and 0.1 MPa.

[0010] The mass ratio of NaOH-KOH molten salt to PET plastic is 1:2, and the PET plastic is added in batches.

[0011] The molar ratio of NaOH (sodium hydroxide) to KOH (potassium hydroxide) in the NaOH-KOH molten salt is 1.08.

[0012] Although traditional alkaline hydrolysis technology for PET plastics has been extensively studied, it typically requires harsh conditions (210-250℃, 1.4-2.0 MPa), which severely hinders its industrial production. In contrast, the KOH-NaOH molten salt used in this invention can achieve efficient depolymerization of PET plastics under mild conditions of 170℃ and 0.1 MPa. Furthermore, the reaction system of this invention produces a neutral aqueous solution after treatment, and no alkaline waste liquid is generated during the entire post-treatment process. This not only avoids equipment corrosion problems but also effectively reduces subsequent treatment costs, significantly improving the economic efficiency and environmental friendliness of the entire process, demonstrating broad commercial prospects.

[0013] Furthermore, the preparation method of the NaOH-KOH molten salt includes the following steps: mixing NaOH and KOH at a molar ratio of 1.08, grinding to form a mixed salt, subjecting the mixed salt to constant temperature treatment at 170°C, stopping heating after the mixed salt has completely melted into a colorless and transparent solution, cooling to room temperature, and thoroughly grinding the resulting solid to obtain the NaOH-KOH molten salt.

[0014] Furthermore, in the preparation method of NaOH-KOH molten salt, the isothermal treatment at 170℃ is carried out for 30 minutes.

[0015] Furthermore, the mass ratio of the NaOH-KOH molten salt to PET plastic is 1:2, and the PET plastic is added in five batches.

[0016] Furthermore, the reaction time of the NaOH-KOH molten salt and PET plastic at 170-190℃ and 0.1 MPa is 2-6 h.

[0017] Preferably, the reaction temperature between the NaOH-KOH molten salt and PET plastic is 170°C, and the reaction time is 4 hours.

[0018] This invention uses NaOH-KOH molten salt as the substrate for PET depolymerization and employs a xenon lamp to simulate sunlight as the heat source for the reactor. Leveraging the heat storage and strong alkalinity of NaOH-KOH molten salt, efficient and environmentally friendly depolymerization of PET plastics is achieved, yielding EG and high-purity TPA. Compared to traditional alkaline hydrolysis techniques for PET plastics, the method of this invention offers milder reaction conditions and advantages such as simple subsequent processing steps, low cost, and environmental friendliness, demonstrating broad commercial application prospects.

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

[0020] This invention, based on the heat storage and strong alkalinity of NaOH-KOH molten salt, provides a highly efficient, low-cost, and low-energy-consumption method for depolymerizing polyester-type PET plastics. The NaOH-KOH molten salt designed in this invention can achieve highly efficient alkaline depolymerization of PET plastics under mild conditions, ultimately yielding TPA and EG products, with a PET conversion rate as high as 89.4% and a TPA yield as high as 88.6%. Furthermore, compared to the traditional alkaline hydrolysis process of PET plastics, this invention significantly improves the required stringent reaction conditions and offers advantages such as simple subsequent processing steps, low cost, and environmental friendliness. Simultaneously, NaOH-KOH molten salt is easy to mass-produce and has broad commercial application prospects. Attached Figure Description

[0021] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0022] Figure 1 This is a flowchart illustrating the depolymerization process of PET plastic using NaOH-KOH molten salt according to the present invention.

[0023] Figure 2 A comparison diagram of the technical routes for traditional alkaline hydrolysis of waste PET plastics and depolymerization of PET plastics using NaOH-KOH molten salt of the present invention;

[0024] Figure 3 The graph shows the effect of different feeding methods on the performance of PET plastic (9.6 g) depolymerized by NaOH-KOH molten salt in Example 1 and Comparative Example 2 of this invention.

[0025] Figure 4 The graph shows the performance of PET plastic (1.0 g) depolymerized by NaOH-KOH molten salt at 170°C in Comparative Example 3 of this invention.

[0026] Figure 5 The graphs show the performance of NaOH-KOH molten salt depolymerizing PET plastic (9.6g) at different temperatures in Comparative Examples 2 and 4-5 of this invention.

[0027] Figure 6 The graphs show the performance of PET plastic (9.6g) depolymerized by NaOH-KOH molten salt at different times in Comparative Examples 2 and 6-7 of this invention.

[0028] Figure 7 Infrared spectra of the PET plastic used in Examples 1-5 and Comparative Examples 1-9 of this invention;

[0029] Figure 8 X-ray diffraction patterns of the PET plastic used in Examples 1-5 and Comparative Examples 1-9 of this invention;

[0030] Figure 9 The 1H NMR spectrum of TPA, the PET plastic product depolymerized by NaOH-KOH molten salt in Example 1 of this invention;

[0031] Figure 10 The image shows the carbon nuclear magnetic resonance spectrum of TPA, the PET plastic product depolymerized by NaOH-KOH molten salt in Example 1 of this invention.

[0032] Figure 11 The infrared spectrum of TPA, the PET plastic product depolymerized by NaOH-KOH molten salt in Example 1 of this invention;

[0033] Figure 12 This is an X-ray diffraction pattern of TPA, the PET plastic product depolymerized by NaOH-KOH molten salt in Example 1 of the present invention.

[0034] Figure 13 The above is the 1H NMR spectrum of EG, the PET plastic product depolymerized by NaOH-KOH molten salt in Example 1 of this invention.

[0035] Figure 14 The image shows the carbon nuclear magnetic resonance spectrum of EG, the PET plastic product depolymerized by NaOH-KOH molten salt in Example 1 of this invention.

[0036] Figure 15 The diagram shows the alkaline hydrolysis performance of PET plastic (1.0 g) in Comparative Example 8 of this invention.

[0037] Figure 16 The diagram shows the alkaline hydrolysis performance of PET plastic (9.6 g) in Comparative Example 9 of this invention.

[0038] Figure 17 This is a graph showing the performance of PET plastic in real-world applications during the depolymerization of NaOH-KOH molten salt in Test Example 1 of this invention. Detailed Implementation

[0039] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0040] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0041] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0042] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0043] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0044] An embodiment of the present invention provides an application of NaOH-KOH molten salt in depolymerized PET plastic, comprising the following steps:

[0045] NaOH-KOH molten salt and PET plastic are mixed and reacted at 170-190℃ and 0.1 MPa for 2-6 h.

[0046] The mass ratio of NaOH-KOH molten salt to PET plastic is 1:2, and the PET plastic is added in batches.

[0047] The molar ratio of NaOH to KOH in the NaOH-KOH molten salt is 1.08.

[0048] In an embodiment of the present invention, the preparation method of NaOH-KOH molten salt includes the following steps: mixing NaOH and KOH at a molar ratio of 1.08, grinding to form a mixed salt, isothermal treatment of the mixed salt at 170°C, stopping heating after the mixed salt has completely melted into a colorless and transparent solution, cooling to room temperature, and thoroughly grinding the resulting solid to obtain NaOH-KOH molten salt. To achieve a lower reaction temperature and higher reaction efficiency, the present invention selects a suitable NaOH-KOH molten salt composition ratio near the eutectic point (approximately 170°C), determining that a NaOH-KOH molten salt system with a NaOH to KOH molar ratio of 1.08 is used. Based on the NaOH-KOH phase diagram, the present invention prepares a NaOH-KOH molten salt with a specific molar ratio. This molten salt has characteristics such as low melting point, strong alkalinity, and good fluidity, allowing PET plastic to fully contact the molten salt medium, thereby effectively reducing the reaction energy barrier, accelerating reaction kinetics, and increasing the reaction rate.

[0049] In an embodiment of the present invention, in the preparation method of NaOH-KOH molten salt, the isothermal treatment at 170°C is carried out for 30 min.

[0050] In an embodiment of the present invention, when the mass ratio of NaOH-KOH molten salt to PET plastic is 1:2, the application of NaOH-KOH molten salt in depolymerizing PET plastic includes the following steps:

[0051] (1) First, add 4.8 g of NaOH-KOH molten salt to the reactor at once, and then add 9.6 g of PET plastic to the reactor in batches. The specific process includes: adding 4.8 g of NaOH-KOH molten salt and 2.0 g of PET plastic during the first reaction, and adding 2.0 g of PET plastic after the reaction is completed. Repeat the above steps until all the PET plastic is added.

[0052] (2) Use a xenon lamp to simulate sunlight to provide heat source for the reactor. After each PET plastic is added, the reaction conditions are controlled to be 2-6 h, 170-190℃ and 0.1 MPa. After the reaction is completed, add enough deionized water to the reactor to fully dissolve the PET plastic. Separate the depolymerization products (disodium terephthalate and ethylene glycol) and unreacted PET plastic by filtration.

[0053] (3) Add a small amount of dilute hydrochloric acid to the depolymerization product of step (2) to acidify it, and a large amount of milky white terephthalic acid suspension can be obtained. Further, the depolymerization product terephthalic acid and the acidified ethylene glycol aqueous solution are separated by vacuum filtration.

[0054] (4) The acidified ethylene glycol aqueous solution is first pre-concentrated to obtain an ethylene glycol aqueous solution with reduced water content, and then vacuum distilled to obtain a higher purity ethylene glycol product.

[0055] In an embodiment of the present invention, PET plastic is added in five batches.

[0056] In embodiments of the present invention, the preferred reaction temperature is 170°C and the preferred reaction time is 4 h.

[0057] In the chemical recycling of waste plastics, the rational control of reaction conditions (including reaction time, temperature, and pressure) is crucial to achieving high recycling rates and reducing reaction energy consumption. Molten salt refers to the melt formed by melting inorganic salts at high temperatures. Common molten salts can be classified into nitrates, carbonates, sulfates, and chlorides based on the type of anion. In the molten state, molten salts possess advantages such as high specific heat capacity, high thermal stability, and good fluidity. They can effectively store heat converted from solar energy and increase the fluidity of reactants within the molten salt, thereby improving diffusion rates and accelerating chemical reaction kinetics. Based on their excellent heat transfer and energy storage characteristics, molten salts have been widely used in thermochemical reactions, solar thermal power generation, industrial waste heat recovery, and thermal power plant retrofitting. Compared to other thermochemical reaction processes, molten salt thermochemical reactions have several significant advantages. First, this process is suitable for the thermochemical decomposition of various materials, including carbon-containing gases, waste polymers, and organic compounds such as biomass. Second, the large heat capacity of the molten medium can effectively lower the thermochemical reaction temperature. Furthermore, the molten medium can also act as a solvent or reactant, participating in the chemical reaction and thus improving the efficiency of the chemical reaction. In thermochemical reactions involving molten salts, the melting point of the molten salt is a crucial physical property parameter, determining the operating temperature and fluidity of the reaction medium, thus directly impacting the reaction pathway and energy consumption. The melting point of molten salts is influenced by various factors, including the proportions of constituent substances, impurity content, and usage conditions. Among these, the proportions of constituent substances are the key factor determining the melting point. The melting points of common molten salts typically range from 150 to 800°C, with most exceeding 200°C. Therefore, selecting a suitable molten salt composition system is of great significance for effectively controlling reaction conditions. To achieve high-value conversion of waste plastics and significantly improve process energy efficiency, this invention utilizes a molten salt system (NaOH-KOH molten salt) with high specific heat capacity and high thermal stability in the chemical recycling process of waste plastics to reduce reaction temperature, increase the fluidity of the reaction medium, accelerate reaction kinetics, and improve reaction efficiency. The selection of molten salt systems in existing chemical recycling processes for waste plastics still faces bottlenecks. Some molten salt systems have high melting points, leading to excessive energy consumption and low reaction efficiency, thus limiting their further development and application in the field of waste plastic recycling. Therefore, this invention rationally selects and develops a suitable low-melting-point, high-stability molten salt system (NaOH-KOH molten salt), which is of great significance for realizing the green resource recycling of waste plastics. Moreover, the further development and application of molten salt systems in the chemical recycling of waste plastics can not only achieve large-scale waste plastic disposal but also effectively alleviate the fossil energy crisis, improve the carbon resource recycling rate, and demonstrate broad application prospects.

[0058] In this embodiment of the invention, the experimental conditions for alkaline depolymerization of polyethylene terephthalate using NaOH-KOH molten salt are as follows: the mass ratio of NaOH-KOH molten salt to PET is 1:2, the reaction temperature is 170-190℃ (preferably 170℃), the reaction time is 2-6 h (preferably 4 h), and a xenon lamp is used as the heat source for the preparation of NaOH-KOH molten salt and the alkaline depolymerization experiment of polyethylene terephthalate (PET).

[0059] All raw materials used in the embodiments of the present invention were obtained through commercial purchase.

[0060] The technical solution of the present invention will be further illustrated by the following embodiments.

[0061] Example 1

[0062] The application of a NaOH-KOH molten salt in the depolymerization of PET plastic is described in the following steps:

[0063] Step 1: Weigh 2.079 g (0.052 mol) NaOH and 2.693 g (0.048 mol) KOH, mix them, and grind them thoroughly in a mortar to form a mixed salt (i.e., the molar percentage of NaOH in the mixed salt is 52 mol%, and the molar percentage of KOH in the mixed salt is 48 mol%). Then, transfer the ground mixed salt into a reactor, heat it to 170℃ and maintain the temperature for 30 min. After the mixed salt has completely melted into a colorless and transparent solution, stop heating. Cool the reactor to room temperature, take out the mixed salt solid, and grind it thoroughly to obtain NaOH-KOH molten salt.

[0064] Step two: First, add the prepared NaOH-KOH molten salt (4.8 g) to the reactor all at once. Then, add PET plastic (9.6 g) to the reactor intermittently in batches. The specific process includes: for the first reaction, add 4.8 g of NaOH-KOH molten salt and 2.0 g of PET to the reactor. After the reaction is completed, add another 2.0 g of PET. Repeat the above steps until all PET is added. After each PET addition, set the reaction conditions to 4 h, 170℃, and 0.1 MPa to conduct an alkaline depolymerization experiment of NaOH-KOH molten salt on PET plastic. After the depolymerization reaction is completed, add sufficient deionized water to the reactor for complete dissolution. Then, separate the filtrate (containing the depolymerization products disodium terephthalate and ethylene glycol) and the filter residue (unreacted PET plastic) by vacuum filtration. Add 10 mL of 0.50... Acidification with mol / L HCl yields a milky white terephthalic acid suspension. Further separation of the filter residue (terephthalic acid) and filtrate (ethylene glycol aqueous solution) via vacuum filtration is then performed. The ethylene glycol aqueous solution is pre-concentrated to obtain an ethylene glycol aqueous solution with lower water content, followed by vacuum distillation to obtain the ethylene glycol product. Finally, infrared and nuclear magnetic resonance spectroscopy are used to analyze the depolymerization products (terephthalic acid and ethylene glycol), and the performance of the PET plastic depolymerized by NaOH-KOH molten salt is evaluated.

[0065] Example 2

[0066] The application of a NaOH-KOH molten salt in the depolymerization of PET plastic is described in the following steps:

[0067] Step 1 is the same as in Example 1;

[0068] Step two is the same as in Example 1, except that the reaction temperature is 180°C.

[0069] Example 3

[0070] The application of a NaOH-KOH molten salt in the depolymerization of PET plastic is described in the following steps:

[0071] Step 1 is the same as in Example 1;

[0072] Step two is the same as in Example 1, except that the reaction temperature is 190°C.

[0073] The reaction temperatures in Examples 1-3 were 170-190℃, all of which are not lower than the melting point of NaOH-KOH molten salt (170℃). This allows the NaOH-KOH molten salt to melt and fully contact the PET plastic, thereby increasing the reaction rate. Examples 1-3 achieved similar PET conversion rates and TPA yields. To minimize energy consumption during the reaction process, a reaction temperature of 170℃ is preferred for subsequent reactions.

[0074] Example 4

[0075] The application of a NaOH-KOH molten salt in the depolymerization of PET plastic is described in the following steps:

[0076] Step 1 is the same as in Example 1;

[0077] Step two is the same as in Example 1, except that the reaction time is 2 h and the amount of HCl consumed during acidification is 30 mL.

[0078] Example 5

[0079] The application of a NaOH-KOH molten salt in the depolymerization of PET plastic is described in the following steps:

[0080] Step 1 is the same as in Example 1;

[0081] Step two is the same as in Example 1, except that the reaction temperature is 6 h.

[0082] In Examples 1 and 4-5, the reaction temperature was the melting point of the NaOH-KOH molten salt (170°C), which allowed the NaOH-KOH molten salt to melt and fully contact the PET plastic. However, in Example 4, the reaction time was too short, which was not conducive to the full reaction between the NaOH-KOH molten salt and the PET plastic, resulting in incomplete depolymerization of the PET plastic. To obtain the best PET depolymerization effect and the lowest reaction energy consumption, the preferred reaction time in subsequent reactions is 4 hours.

[0083] To illustrate the effect of reaction temperature on the depolymerization ability of PET plastic when the melting point of the NaOH-KOH molten salt is below 170°C, Comparative Example 1 was set up in this invention.

[0084] Comparative Example 1

[0085] The application of a NaOH-KOH molten salt in the depolymerization of PET plastic is described in the following steps:

[0086] Step 1 is the same as in Example 1;

[0087] Step two is the same as in Example 1, except that the reaction temperature is 150°C and the amount of HCl consumed during acidification is 60 mL.

[0088] Compared with Comparative Example 1, the reaction temperature in Example 1 was lower than the melting point of NaOH-KOH molten salt (170°C). As a result, the NaOH-KOH molten salt could not be completely melted and could not fully contact the PET plastic, thus the reaction efficiency was low.

[0089] To illustrate the effect of different reactant feeding methods on PET plastic conversion rate and TPA yield, Comparative Example 2 was set up in this invention. Compared with the batch feeding of PET plastic in Example 1, Comparative Example 2 used a one-time feeding method for PET plastic.

[0090] Comparative Example 2

[0091] The application of a NaOH-KOH molten salt in the depolymerization of PET plastic is described in the following steps:

[0092] Step 1 is the same as in Example 1;

[0093] Step 2: The prepared NaOH-KOH molten salt (4.8 g) and PET plastic (9.6 g) were added to the reactor at one time. The reaction conditions were set to 20 h, 170 °C, and 0.1 MPa to conduct an alkaline depolymerization experiment on PET plastic using NaOH-KOH molten salt. After the depolymerization reaction was completed, sufficient deionized water was added to the reactor for complete dissolution. The filtrate (containing the depolymerization products disodium terephthalate and ethylene glycol) and the filter residue (unreacted PET plastic) were separated by vacuum filtration. 70 mL of 0.50 mol / L HCl was added to the filtrate for acidification to obtain a milky white terephthalic acid suspension. The filter residue (terephthalic acid) and the filtrate (ethylene glycol aqueous solution) were further separated by vacuum filtration. The ethylene glycol aqueous solution was pre-concentrated to obtain an ethylene glycol aqueous solution with low water content, and then subjected to vacuum distillation to obtain the ethylene glycol product. Finally, the performance of the depolymerized PET plastic using NaOH-KOH molten salt was evaluated.

[0094] Compared with Comparative Example 2, Example 1 shows that when the mass of PET is greater than the mass of NaOH-KOH molten salt, adding PET plastic in batches can achieve a higher PET conversion rate and TPA yield. Furthermore, the subsequent reaction solution is neutral, simple to process, and has low corrosiveness to equipment.

[0095] To illustrate the effect of NaOH-KOH molten salt on the depolymerization ability of PET plastic when the mass of PET (1.0 g) is less than the mass of NaOH-KOH molten salt (4.8 g), Comparative Example 3 was set up in this invention.

[0096] Comparative Example 3

[0097] The application of a NaOH-KOH molten salt in the depolymerization of PET plastic is described in the following steps:

[0098] Step 1 is the same as in Example 1;

[0099] Step 2: The prepared NaOH-KOH molten salt (4.8 g) and PET plastic (1.0 g) were added to the reactor at one time. The reaction time was set to 4 h and the reaction temperature to 170 °C. The alkaline depolymerization experiment of NaOH-KOH molten salt on PET plastic was carried out at 0.1 MPa. After the depolymerization reaction was completed, sufficient deionized water was added to the reactor to fully dissolve the PET plastic. The filtrate (containing the depolymerization products disodium terephthalate and ethylene glycol) and the filter residue (unreacted PET plastic) were separated by vacuum filtration. 80 mL of 0.50 mol / L HCl was added to the filtrate for acidification to obtain a milky white terephthalic acid suspension. The filter residue (terephthalic acid) and the filtrate (ethylene glycol aqueous solution) were further separated by vacuum filtration. The ethylene glycol aqueous solution was pre-concentrated to obtain an ethylene glycol aqueous solution with low water content, and then subjected to vacuum distillation to obtain the ethylene glycol product. Finally, the performance of NaOH-KOH molten salt in depolymerizing PET plastic at different temperatures was evaluated.

[0100] To illustrate the effect of reaction temperature on the depolymerization ability of NaOH-KOH molten salt on PET plastic during a one-time feeding reaction when the mass of PET is greater than the mass of NaOH-KOH molten salt, comparative examples 4-5 were set up in this invention.

[0101] Comparative Example 4

[0102] The application of a NaOH-KOH molten salt in the depolymerization of PET plastic is described in the following steps:

[0103] Step 1 is the same as in Example 1;

[0104] Step two is the same as Comparative Example 2, except that the reaction temperature is 150℃.

[0105] Comparative Example 5

[0106] The application of a NaOH-KOH molten salt in the depolymerization of PET plastic is described in the following steps:

[0107] Step 1 is the same as in Example 1;

[0108] Step two is the same as Comparative Example 2, except that the reaction temperature is 190℃.

[0109] To illustrate the effect of reaction time on the depolymerization ability of NaOH-KOH molten salt on PET plastic during a one-time feeding reaction when the mass of PET is greater than the mass of NaOH-KOH molten salt, comparative examples 6-7 were set up in this invention.

[0110] Comparative Example 6

[0111] The application of a NaOH-KOH molten salt in the depolymerization of PET plastic is described in the following steps:

[0112] Step 1 is the same as in Example 1;

[0113] Step two is the same as Comparative Example 2, except that the reaction time is 6 hours.

[0114] Comparative Example 7

[0115] The application of a NaOH-KOH molten salt in the depolymerization of PET plastic is described in the following steps:

[0116] Step 1 is the same as in Example 1;

[0117] Step two is the same as in Example 2, except that the reaction time is 8 hours.

[0118] To compare with traditional PET alkaline hydrolysis technology, comparative examples 8-9 were provided in this invention. Since the chemical reactions follow the same process in alkaline hydrolysis involving sodium hydroxide or potassium hydroxide, an 8 wt% NaOH solution was selected as the depolymerizing agent for PET alkaline hydrolysis in the comparative examples.

[0119] Comparative Example 8

[0120] Step 1: Prepare 60 mL of 8 wt% NaOH solution and add it to the reactor along with 1.0 g of PET plastic at once. Set the reaction time to 4 h and the reaction temperature to 170 °C. Conduct the NaOH alkaline hydrolysis of PET experiment at 1.6 MPa.

[0121] Step 2: After the reaction is complete, the unreacted PET plastic is first collected by filtration, and then the filtrate is acidified with 80 mL of 0.50 mol / L HCl. The filter residue (terephthalic acid) is collected by vacuum filtration, and the performance of NaOH alkaline hydrolysis of PET is evaluated.

[0122] Comparative Example 9

[0123] Step 1: Prepare 60 mL of 8 wt% NaOH solution and add it to the reactor along with 9.6 g of PET plastic at once. Set the reaction time to 4 h and the reaction temperature to 170 °C. Conduct the NaOH alkaline hydrolysis of PET experiment at 1.6 MPa.

[0124] Step two is the same as Comparative Example 8, except that the amount of HCl used is 60 mL.

[0125] Compared to the traditional alkaline hydrolysis method for PET in Comparative Examples 8 and 9, the present invention's use of NaOH-KOH molten salt to depolymerize PET plastic has significant advantages: Firstly, the present invention significantly improves the harsh reaction conditions during the alkaline hydrolysis of PET plastic, requiring only 170℃ and 0.1 MPa to complete the reaction; secondly, the aqueous solution is neutral in the subsequent treatment process, requiring only a small amount of acid to complete the TPA acidification process, effectively reducing corrosion problems on the reaction equipment. The present invention's use of NaOH-KOH molten salt to depolymerize PET plastic has the characteristics of reduced process costs and simple operation, demonstrating broad commercial prospects.

[0126] To verify the scalability of the NaOH-KOH molten salt of this invention for practical applications in PET plastics, the following test examples were set up:

[0127] Test Example 1

[0128] The specific steps for the depolymerization of NaOH-KOH molten salt in PET plastics in real-world applications are as follows:

[0129] Step 1: Wash, dry, and crush commercially available PET plastics (plastic bottles, plastic cups, or packaging boxes) to obtain PET plastic granules.

[0130] Step 2: First, add 4.8 g of the NaOH-KOH molten salt prepared in Example 1 to the reactor all at once. Then, add 9.6 g of PET plastic particles from Step 1 to the reactor intermittently in batches. The specific process includes: adding 4.8 g of NaOH-KOH molten salt and 2.0 g of PET plastic particles to the reactor for the first reaction. After the reaction is completed, add another 2.0 g of PET plastic particles and repeat the above steps until all the PET plastic particles are added. After each addition of PET plastic particles, set the reaction conditions to 4 h, 170 °C, and 0.1 MPa to conduct an alkaline depolymerization experiment of NaOH-KOH molten salt on PET plastic. After the depolymerization reaction is completed, add sufficient deionized water to the reactor to fully dissolve the particles. Then, separate the filtrate (containing the depolymerization products disodium terephthalate and ethylene glycol) and the filter residue (unreacted PET plastic particles) by vacuum filtration. Add 20-50 mL of the filtrate to the filter residue. Acidification with 0.50 mol / L HCl (HCl consumption of 20, 50, and 40 mL for plastic bottles, plastic cups, and packaging boxes, respectively) yields a milky white terephthalic acid suspension. Further separation of the filter residue (terephthalic acid) and filtrate (ethylene glycol aqueous solution) is achieved by vacuum filtration. The ethylene glycol aqueous solution is pre-concentrated to obtain an ethylene glycol aqueous solution with lower water content, followed by vacuum distillation to obtain the ethylene glycol product. Finally, infrared and nuclear magnetic resonance spectroscopy are used to analyze the depolymerization products (terephthalic acid and ethylene glycol), and the performance of the PET plastic depolymerized by NaOH-KOH molten salt is evaluated.

[0131] Figure 1 This is a flowchart illustrating the depolymerization process of PET plastic using NaOH-KOH molten salt according to the present invention.

[0132] Figure 2 The diagram compares the traditional alkaline hydrolysis of waste PET plastics with the NaOH-KOH molten salt depolymerization of PET plastics of this invention. It can be seen that compared with the traditional alkaline hydrolysis of PET, the NaOH-KOH molten salt depolymerization of PET plastics has advantages such as less corrosion to equipment and lower subsequent processing costs, showing broad prospects for commercial application.

[0133] Figure 3 The graph shows the effect of different feeding methods on the performance of PET depolymerization by NaOH-KOH molten salt in Example 1 and Comparative Example 2 of this invention. It can be seen that when the mass of PET plastic is greater than the mass of NaOH-KOH molten salt, the batch feeding method of PET plastic in Example 1 can obtain a higher PET conversion rate and product yield. At the same time, the subsequent acidification only requires a small amount of dilute hydrochloric acid, which is more economical.

[0134] Figure 4 The comparison of NaOH-KOH molten salt in Comparative Example 3 of this invention shows the PET conversion rate and TPA yield of PET plastic (1.0 g) under alkaline depolymerization at 170°C. It can be seen that when the mass of PET plastic is less than the mass of NaOH-KOH molten salt, the one-time addition of PET plastic in Comparative Example 3 can achieve a high PET conversion rate and a high product yield, but the subsequent acidification requires a large amount of dilute hydrochloric acid, which is less economical.

[0135] Figure 5 The graphs show the performance of NaOH-KOH molten salt in Comparative Examples 2 and 4-5 of this invention at different temperatures in depolymerizing PET plastic (9.6g). It can be seen that when the mass of PET plastic is greater than the mass of NaOH-KOH molten salt, the PET conversion rate and TPA yield of the one-time addition of PET plastic in Comparative Examples 2 and 4-5 are low, and the reaction efficiency does not increase significantly with increasing temperature.

[0136] Figure 6 The graphs show the performance of PET plastic (9.6g) depolymerized by NaOH-KOH molten salt at different times in Comparative Examples 2 and 6-7 of this invention. It can be seen that when the mass of PET plastic is greater than the mass of NaOH-KOH molten salt, the PET conversion rate and TPA yield of the one-time PET plastic input in Comparative Examples 2 and 6-7 are low, and the reaction efficiency does not show a significant increase with the extension of time.

[0137] Figure 7 and Figure 8The infrared spectrum and X-ray diffraction pattern of the PET used in Examples 1-5 and Comparative Examples 1-9 of this invention are shown.

[0138] Figure 9 The 1H NMR spectrum of TPA, the PET plastic product depolymerized by NaOH-KOH molten salt in Example 1 of this invention; Figure 10 The image shows the carbon nuclear magnetic resonance spectrum of TPA, the PET plastic product depolymerized by NaOH-KOH molten salt in Example 1 of this invention. Figure 11 The infrared spectrum of TPA, the PET plastic product depolymerized by NaOH-KOH molten salt in Example 1 of this invention; Figure 12 This is an X-ray diffraction pattern of TPA, the PET plastic product depolymerized by NaOH-KOH molten salt in Example 1 of this invention. Figure 9-12 It can be seen that one of the products of depolymerization of PET plastic by NaOH-KOH molten salt is high-purity TPA.

[0139] Figure 13 The above is the 1H NMR spectrum of EG, the PET plastic product depolymerized by NaOH-KOH molten salt in Example 1 of this invention. Figure 14 This is the carbon NMR spectrum of EG, the PET plastic product depolymerized by NaOH-KOH molten salt in Example 1 of this invention; Figure 13 , 14 It can be seen that another product of the depolymerization of PET plastic by NaOH-KOH molten salt is EG.

[0140] Figure 15 The graph shows the alkaline hydrolysis performance of PET (1.0 g) in Comparative Example 8 of this invention. Figure 16 This is a graph showing the alkaline hydrolysis performance of PET (9.6 g) in Comparative Example 9 of this invention. (The graph is derived from...) Figure 15 , 16 It can be seen that although the traditional alkaline hydrolysis process of PET has a high conversion rate and yield, the entire reaction process is subject to harsh conditions and has low economic efficiency.

[0141] Figure 17 This is a performance graph of PET plastic in real-world applications, obtained from the depolymerization of NaOH-KOH molten salt in Test Example 1 of this invention. Figure 17 It can be seen that NaOH-KOH molten salt can achieve alkaline depolymerization of commercial PET plastics, demonstrating the excellent scalability of NaOH-KOH molten salt alkaline depolymerization of PET plastics.

[0142] The alkaline NaOH-KOH molten salt of this invention achieves efficient and green depolymerization of PET plastic, ultimately obtaining a PET conversion rate of 89.4% and a TPA yield of 88.6%, while also producing EG. Compared with the traditional alkaline hydrolysis process of PET plastic, this invention utilizes the heat storage and strong alkalinity of NaOH-KOH molten salt for alkaline depolymerization of PET plastic, significantly improving the harsh reaction conditions in the alkaline hydrolysis process of PET. It has advantages such as simple subsequent processing steps, low cost, and environmental friendliness, showing broad commercial application prospects.

[0143] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. Use of NaOH-KOH molten salt in depolymerization of PET plastic, characterized in that, Includes the following steps: NaOH-KOH molten salt and PET plastic are mixed and reacted at 170-190℃ and 0.1 MPa. The mass ratio of the NaOH-KOH molten salt to the PET plastic is 1:2, and it is added to the PET plastic in batches. The molar ratio of NaOH to KOH in the NaOH-KOH molten salt is 1.

08.

2. Use according to claim 1, characterized in that, The preparation method of the NaOH-KOH molten salt includes the following steps: mixing NaOH and KOH in a molar ratio of 1.08, grinding to form a mixed salt, subjecting the mixed salt to constant temperature treatment at 170°C, stopping heating after the mixed salt has completely melted into a colorless and transparent solution, cooling to room temperature, and grinding the resulting solid thoroughly to obtain the NaOH-KOH molten salt.

3. The application according to claim 2, characterized in that, The isothermal treatment at 170℃ lasted for 30 minutes.

4. The application according to claim 1, characterized in that, The reaction time is 2-6 h at 170-190℃ and 0.1 MPa.

5. The application according to claim 4, characterized in that, The reaction temperature is 170℃.

6. The application according to claim 4, characterized in that, The reaction time was 4 hours.

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