A method for catalytic, controlled degradation of polyethylene terephthalate melt

Abstract

The present application relates to a kind of catalytic polyethylene terephthalate melt controllable degradation method, belong to the upgrading chemical recycling technical field of waste polyethylene terephthalate.The fragments of polyethylene terephthalate are mixed with catalyst sufficiently, the catalyst is alkaline earth metal hydroxide or alkaline earth metal oxide;The mixture is heated, so that the catalyst degrades polyethylene terephthalate and obtains intermediate product terephthalic acid vinyl ester, after adding water, the terephthalic acid vinyl ester hydrolysis reaction generates terephthalic acid and ethylene glycol, realizes the degradation of polyethylene terephthalate.The present application does not use organic solvent, cost is low, degradation time is short, reaction temperature is low, yield is high, catalyst can be recycled and used again, and the whole reaction is carried out under normal pressure, and nitrogen gas and the like are not needed Protection.

Description

Technical Field

[0001] This invention belongs to the field of upgraded chemical recycling technology of waste polyethylene terephthalate (PET), and more specifically, it relates to a method for the controlled degradation of PET melt, particularly a method for the low-temperature controlled degradation of waste PET melt into terephthalic acid and ethylene glycol. Background Technology

[0002] PET is typically produced by the polycondensation reaction of terephthalic acid and ethylene glycol. PET possesses excellent creep resistance, fatigue resistance, abrasion resistance, dimensional stability, and mechanical properties. It also boasts advantages such as cleanliness, transparency, non-toxicity, odorlessness, lightweight, and good processability, making it widely used in beverage bottles, textiles, fibers, food packaging materials, and films. In the thermoplastics sector, PET is currently the world's most in-demand and fastest-growing polyester product. However, PET is chemically inert and highly resistant to atmospheric and microbial agents, meaning it will not naturally decompose for decades. The resulting waste accumulation not only represents a huge waste of resources but also places immense pressure on society and the environment. Therefore, the disposal of waste PET has become an urgent problem to be solved. Traditional methods for waste PET disposal mainly include incineration and landfill. Incineration can recover heat energy, but its energy utilization rate is low, and it produces large amounts of toxic and harmful gases, causing secondary pollution. Landfill is the simplest and most convenient method, but it requires enormous space, occupies a large amount of land resources, and causes serious soil pollution.

[0003] Recycling and reusing waste PET not only solves environmental pollution problems but also achieves better resource utilization. There are three main pathways for recycling waste PET. First is physical recycling, which involves melting and reshaping or regranulating PET to produce low-value-added products. Second is biodegradation, where microorganisms adhere to the surface of PET materials and secrete enzymes that break down PET into fragments through hydrolysis and oxidation reactions, ultimately forming small molecules. Third is chemical recycling, where PET is depolymerized through heat or chemical reagents to form low-molecular-weight products or monomers, achieving efficient resource recycling. This is the most common degradation method in industry today, mainly including hydrolysis, alcoholysis, ammonolysis, and catalytic cracking.

[0004] PET can be hydrolyzed into terephthalic acid and ethylene glycol under different acidic or alkaline media. Based on the electron-donating properties of the reaction medium, hydrolysis methods are classified into acidic, alkaline, and neutral hydrolysis methods. Acidic hydrolysis uses inorganic acids as catalysts, most commonly concentrated sulfuric acid. Although the hydrolysis effect is significant, the degradation process severely corrodes equipment, the catalyst is difficult to recycle, and a large amount of wastewater is generated, causing serious environmental pollution. Alkaline hydrolysis has similar problems. An earlier method for depolymerizing PET was methanol hydrolysis, which yielded reaction products with relatively few impurities. Given the high purification costs of products obtained from methanol hydrolysis, methanol has gradually been replaced by diols. Currently, the main diols used for PET hydrolysis include ethylene glycol, propylene glycol, and triethylene glycol. Ethylene glycol hydrolysis of PET is relatively simple to purify, leading to its widespread use. The principle is that PET reacts with ethylene glycol to obtain diethyl terephthalate and PET oligomers. However, many PET oligomers exist in the reaction solution, making purification relatively difficult. Amine hydrolysis generally involves using primary amines such as ethylenediamine, ethanolamine, or ammonia as degrading agents in a primary amine solution to promote the depolymerization of PET into diamines of terephthalic acid and ethylene glycol. The disadvantages are that the raw materials are expensive, a large amount of organic solvents are required, the product purification process is cumbersome, and the cost is high.

[0005] Catalytic degradation is a method that uses a catalyst to break ester bonds in PET to produce small-molecule degradation products. Based on the reaction temperature, it can be divided into high-temperature catalytic degradation and low-temperature catalytic degradation. High-temperature catalytic degradation is generally carried out at 600–900℃, with the advantage of fast reaction rate, but disadvantages of high reaction temperature, high energy consumption, and complex degradation products that are difficult to purify. Low-temperature catalytic degradation is an emerging method, with reaction temperatures generally between 230–350℃, offering advantages such as low reaction temperature and low energy consumption. Kratish et al. used MoO2 / C as a catalyst to catalyze the degradation of waste PET to prepare terephthalic acid at 260℃ and in a hydrogen atmosphere of 1 bar. The disadvantages of this method are the need for hydrogen protection, the expensive catalyst, and the difficulty in large-scale preparation (Polyethylene terephthalate deconstruction catalyzed by a carbon-supported single-site molybdenum-dioxo complex, Angew. Chem. Int. Ed. 2020, 59, 19857). In patent ZL202110360655.X, researchers proposed using zinc oxide or compounds that can be decomposed by heating to produce zinc oxide as catalysts to catalyze the preparation of terephthalic acid and ethylene glycol from waste PET. This technology achieves low-temperature controllable degradation of waste PET. The invention uses water vapor as a reactant. After the low-temperature degradation reaction between PET and the catalyst is completed, an intermediate is obtained, and a small amount of water is added to it to undergo an addition reaction. However, the catalyst is expensive, hydrochloric acid solution is required to obtain the terephthalic acid product, the filtrate needs to be treated with ammonia solution to recover the catalyst, and the liquid product in the condensation device needs to be dried with magnesium sulfate to obtain ethylene glycol, thus limiting its industrial application.

[0006] Therefore, there is an urgent need for an economical and environmentally friendly method for the chemical recycling of waste PET that does not use organic solvents, has low cost and energy consumption, short degradation time, high yield, fast reaction speed, simple post-processing, and allows for the separation, recycling, and reuse of catalysts. The entire process should be carried out under normal pressure and without the need for protective gases such as hydrogen or nitrogen. This method would utilize efficient catalytic depolymerization technology to transform waste PET and other polyesters into valuable chemicals, thereby achieving true green and biodegradable material recycling. This method has extremely important research value and application prospects. Summary of the Invention

[0007] To address the shortcomings or improvement needs of existing technologies, the purpose of this invention is to provide a method for the low-temperature, controllable degradation of waste PET melt to prepare terephthalic acid and ethylene glycol. This invention uses an alkaline earth metal hydroxide or alkaline earth metal oxide as the catalyst. The catalyst activates the C=O bonds in the PET polymer chain, generating an intermediate, ethylene terephthalate, under the combined action of the catalyst and heating. This intermediate then undergoes hydrolysis to produce terephthalic acid and ethylene glycol. This invention does not use organic solvents. Compared to existing catalytic degradation methods that often require high temperature and high pressure, this invention achieves highly efficient catalytic depolymerization of waste PET under low temperature and normal pressure conditions, without the formation of oligomer byproducts such as dimers or polymers, yielding high-purity and high-yield terephthalic acid and ethylene glycol. This solves the technical problems of existing PET degradation methods, such as the need for organic solvents, high cost and energy consumption, long degradation time, low yield, slow reaction rate, complex post-processing, and the inability to separate and recycle the catalyst for reuse.

[0008] According to a first aspect of the present invention, a method for catalytic degradation of polyethylene terephthalate is provided, comprising the following steps:

[0009] (1) The polyethylene terephthalate fragments are thoroughly mixed with the catalyst, wherein the catalyst is an alkaline earth metal hydroxide or an alkaline earth metal oxide.

[0010] (2) The mixture obtained in step (1) is heated to degrade polyethylene terephthalate by the catalyst to obtain the intermediate product ethylene terephthalate; after adding water, the ethylene terephthalate undergoes a hydrolysis reaction to generate terephthalic acid and ethylene glycol, thereby realizing the degradation of polyethylene terephthalate.

[0011] According to another aspect of the present invention, a method for catalytic degradation of polyethylene terephthalate is provided, comprising the following steps:

[0012] (1) The polyethylene terephthalate fragments are thoroughly mixed with the catalyst, wherein the catalyst is an alkaline earth metal hydroxide or an alkaline earth metal oxide.

[0013] (2) The mixture obtained in step (1) is heated to degrade polyethylene terephthalate by the catalyst to obtain the intermediate product ethylene terephthalate; then it is soaked in alkaline solution to react the intermediate product ethylene terephthalate to obtain terephthalate and ethylene glycol. The white solid after filtration is the recovered catalyst. Then an acid solution is added to the filtrate to react the terephthalate to obtain a white precipitate of terephthalic acid. After solid-liquid separation, the terephthalic acid is recovered. The filtrate is distilled to obtain ethylene glycol, thus realizing the recovery of ethylene glycol.

[0014] Preferably, the heating temperature in step (2) is 200℃~350℃ and the heating time is 5min~180min.

[0015] Preferably, the alkaline earth metal hydroxide is magnesium hydroxide, calcium hydroxide, or barium hydroxide.

[0016] Preferably, the alkaline earth metal oxide is magnesium oxide, calcium oxide, or barium oxide.

[0017] Preferably, the polyethylene terephthalate is waste polyethylene terephthalate.

[0018] Preferably, the waste polyethylene terephthalate is at least one of waste polyethylene terephthalate bottles, waste polyethylene terephthalate packaging sheets, waste polyethylene terephthalate films, waste polyethylene terephthalate fibers, and waste polyethylene terephthalate textiles.

[0019] Preferably, the mass ratio of polyethylene terephthalate to catalyst is (1-100):10.

[0020] Preferably, the heating rate is 1℃ / min to 100℃ / min.

[0021] In summary, compared with the prior art, the above-described technical solutions conceived by this invention mainly possess the following technical advantages:

[0022] (1) This invention not only simplifies the operation process of degrading and recycling waste PET products, but also makes the degradation process green and environmentally friendly, without the use of organic solvents in the reaction. Compared with existing catalytic degradation methods that often require high temperature and high pressure, this invention can achieve efficient catalytic depolymerization of waste PET under low temperature and normal pressure conditions, without the generation of oligomer byproducts such as dimers or polymers, and obtains high purity and high yield of terephthalic acid and ethylene glycol.

[0023] (2) The alkaline earth metal catalyst used in this invention activates the C=O bonds in the PET polymer chain (for example, magnesium ions in magnesium oxide can activate the O in the C=O bond). Under the combined action of the catalyst and heating, an intermediate, ethylene terephthalate, is generated, which then undergoes a hydrolysis reaction to generate terephthalic acid and ethylene glycol. This technology is green and environmentally friendly, does not use organic solvents, has low cost, short degradation time, low reaction temperature, high yield, and the catalyst can be recycled and reused. Moreover, the entire reaction is carried out under normal pressure and does not require the protection of gases such as nitrogen.

[0024] (3) This invention uses waste PET as raw material to prepare terephthalic acid and ethylene glycol with high yield and high purity through catalytic degradation of PET. It not only provides a new strategy for the resource recycling of waste PET, but also provides a new method for the preparation of terephthalic acid and ethylene glycol. It has important practical significance and broad application prospects.

[0025] (4) In addition to the low cost and easy availability of waste PET, the catalyst used in this invention is inexpensive, widely available, and can be easily recovered through simple separation, enabling its recycling. For example, when magnesium hydroxide is used as a catalyst, the crystal structure and catalytic efficiency remain unchanged after 10 uses. When calcium hydroxide is used as a catalyst, the terephthalic acid yield is 98%. Calcium hydroxide, also known as quicklime, is extremely inexpensive, costing only 300-800 yuan / ton, and has the potential for industrial application. Overall, this invention combines the above advantages to turn waste PET into a valuable resource, fundamentally achieving a permanent closed-loop recycling of PET, which will generate significant social and economic benefits and has a very clear industrialization prospect.

[0026] (5) This invention uses alkaline earth metal hydroxide or alkaline earth metal oxide catalysts to degrade polyethylene terephthalate to obtain the intermediate product terephthalic acid monoethylene ester. After adding water and stirring for a period of time, the terephthalic acid monoethylene ester undergoes a hydrolysis reaction to generate terephthalic acid and ethylene glycol, thereby achieving the degradation of polyethylene terephthalate. The catalyst can be recovered by subsequent alkaline solution treatment. The filtrate is treated with hydrochloric acid solution to obtain terephthalic acid powder. The product after hydrolysis of the intermediate can be directly obtained by distillation. This invention has the advantages of simple operation, feasible method and efficient recovery of degradation products, while achieving the separation of terephthalic acid and ethylene glycol.

[0027] (6) This method solves the problem of recycling and reusing waste PET in urban and industrial settings, providing a new green pathway for the recycling and reuse of large quantities of waste PET. It also provides a new and convenient method for preparing important chemical raw materials such as terephthalic acid and ethylene glycol, which is of great significance for environmental protection, resource recovery, and sustainable development. This invention fully meets the needs of waste PET recycling, enables the ecological development of the industry, and provides strong support for the current operation of the waste PET recycling industry.

[0028] (7) This invention provides a new way for the large-scale upgrading and reuse of waste PET in cities and industries, and also provides a new method for preparing high-value-added industrial raw materials terephthalic acid and ethylene glycol. It has important practical significance for environmental protection, resource recycling and sustainable development, and has the prospect of industrial application, helping to achieve the goal of "carbon neutrality". Attached Figure Description

[0029] Figure 1 (a) Carbon NMR spectrum and (b) Hydrogen NMR spectrum of terephthalic acid prepared by degradation of waste PET at 255 °C using magnesium hydroxide as a catalyst.

[0030] Figure 2 The X-ray diffraction pattern is shown for terephthalic acid prepared by degradation of waste PET at 270°C using magnesium hydroxide as a catalyst.

[0031] Figure 3 (a) Carbon NMR spectrum and (b) Hydrogen NMR spectrum of terephthalic acid prepared by degradation of waste PET at 220 °C using magnesium oxide as a catalyst.

[0032] Figure 4 The X-ray diffraction pattern is shown for terephthalic acid prepared by degradation of waste PET at 260°C using magnesium oxide as a catalyst.

[0033] Figure 5 (a) Low-magnification and (b) high-magnification scanning electron microscope images of terephthalic acid prepared by degradation of waste PET at 240°C with calcium oxide as a catalyst.

[0034] Figure 6 The infrared spectrum of terephthalic acid prepared by degradation of waste PET at 200℃ using calcium oxide as a catalyst.

[0035] Figure 7 This is the solid-state carbon NMR spectrum of terephthalic acid prepared by degradation of waste PET at 290℃ using calcium hydroxide as a catalyst.

[0036] Figure 8 The infrared spectrum of terephthalic acid prepared by degradation of waste PET at 280℃ using calcium hydroxide as a catalyst.

[0037] Figure 9 The X-ray diffraction pattern is shown for terephthalic acid prepared by degradation of waste PET at 210℃ using barium hydroxide as a catalyst.

[0038] Figure 10 The X-ray diffraction pattern of (a) recovered magnesium hydroxide and (b) yield change of terephthalic acid are shown for waste PET after degradation at 230°C with magnesium hydroxide as a catalyst. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0040] This invention discloses a method for the low-temperature controlled degradation of waste polyethylene terephthalate (PET) melt to prepare terephthalic acid and ethylene glycol. Using waste PET as raw material and an alkaline earth metal hydroxide or alkaline earth metal oxide as catalyst, terephthalic acid monoethylene ester is prepared, followed by hydrolysis to obtain terephthalic acid and ethylene glycol in high yield and high purity. The specific process steps are as follows:

[0041] (1) Clean and cut the waste PET to obtain waste PET fragments with a size of 1mm to 6mm;

[0042] (2) Mix the waste PET fragments obtained in step (1) with the catalyst evenly to obtain a mixture of waste PET and catalyst;

[0043] (3) Place the mixture obtained in step (2) into a heating table and heat it at 200℃~350℃ for 5min~180min; after cooling to room temperature, the solid product is a mixture of catalyst and ethylene terephthalate. After adding water, ethylene terephthalate undergoes a hydrolysis reaction to produce terephthalic acid and ethylene glycol. After separation, the catalyst, as well as high-purity terephthalic acid and ethylene glycol, are obtained. The catalyst can be reused.

[0044] In some embodiments, the waste PET is selected from at least one of PET bottles, PET packaging sheets, PET films, PET fibers, and PET textiles.

[0045] In some embodiments, the catalyst is magnesium oxide, calcium oxide, or barium oxide, and the corresponding hydroxides include magnesium hydroxide, calcium hydroxide, and barium hydroxide.

[0046] In some embodiments, a certain amount of waste PET fragments and catalyst are weighed and added to a ball mill, and stirred and mixed at a speed of 50 r / min to 600 r / min for 5 min to 90 min to obtain a mixture of waste PET and catalyst.

[0047] In some embodiments, the mass ratio of the waste PET to the catalyst is 10:1 to 1:10.

[0048] In some embodiments, the heating rate is 1°C / min to 100°C / min, and the holding time after reaching the reaction temperature is 5 min to 180 min.

[0049] Taking magnesium hydroxide as an example, the reaction of this invention is as follows:

[0050]

[0051] According to the purpose of this invention, a method is provided for the low-temperature controllable degradation of waste PET to prepare terephthalic acid and ethylene glycol using alkaline earth metal hydroxides, alkaline earth metal oxides, or alkaline earth metal compounds that generate corresponding alkaline earth metal oxides during heating as catalysts. Specifically, the waste PET material is cleaned and cut to obtain waste PET fragments with a size of 1mm to 6mm; then, it is mixed evenly with the catalyst at a certain mass ratio and placed in a heating platform. After heating to the reaction temperature, it is held at that temperature for a period of time. After the reaction is completed, the reaction product is post-processed to recover the catalyst, and high-purity, high-yield terephthalic acid and ethylene glycol are obtained simultaneously.

[0052] The following are specific embodiments.

[0053] Example 1

[0054] (1) After cleaning and drying the waste PET bottles, place them in a crusher to crush them to obtain waste PET fragments.

[0055] (2) Weigh 5g of waste PET fragments and 3.5g of calcium hydroxide, put them into a ball mill, and stir and mix them for 10 minutes at a speed of 60r / min to obtain a uniform mixture of the two.

[0056] (3) Transfer the mixture obtained in step (2) to a tubular reactor, place the tubular reactor in a heating platform, set the heating rate to 5℃ / min, raise the temperature to 260℃, add an appropriate amount of water to the tubular reactor, and keep it at this temperature for 30min.

[0057] (4) After the heating platform cools naturally, a solid product and a small amount of liquid product are obtained from the tubular reactor. Ethylene glycol is obtained by distillation of this mixture. The solid product is then soaked in 0.1 mol / L NaOH solution for 40 min and separated by filtration. The upper white solid is washed and dried to obtain recovered calcium hydroxide. A white precipitate is obtained by adding 1 mol / L hydrochloric acid solution to the filtrate. Terephthalic acid is obtained by filtration, washing, and drying of the filtrate, with a yield of 99 wt%. Ethylene glycol is obtained by distillation of the filtrate, with a yield of 98 wt%.

[0058] Example 2

[0059] (1) After cleaning and drying the waste PET bottles, place them in a crusher to crush them to obtain waste PET fragments.

[0060] (2) Weigh 4g of waste PET fragments and 4g of magnesium hydroxide, put them into a ball mill, and stir and mix them for 5 minutes at a speed of 100r / min to obtain a uniform mixture of the two.

[0061] (3) Transfer the mixture obtained in step (2) to a tubular reactor, place the tubular reactor in a heating platform, set the heating rate to 5℃ / min, raise the temperature to the reaction temperature of 255℃, and keep it at this temperature for 10min.

[0062] (4) After the heating platform cools naturally, the solid product from the tubular reactor is obtained. The solid product from the tubular reactor is soaked in 0.1 mol / L NaOH solution for 1 hour and then filtered. The upper white solid is washed and dried to obtain recovered magnesium hydroxide. A white precipitate is obtained by adding 1 mol / L hydrochloric acid solution to the filtrate. The filtrate is filtered, washed, and dried to obtain terephthalic acid with a yield of 99 wt%. Ethylene glycol is obtained by distillation of the filtrate with a yield of 99 wt%.

[0063] Figure 1 (a) and (b) in the image show the carbon NMR spectrum and hydrogen NMR spectrum of the prepared terephthalic acid, respectively. This indicates that the purity of the terephthalic acid is very high, close to 100%.

[0064] Example 3

[0065] The reaction temperature in Example 2 was changed to 270°C and maintained at this temperature for 30 min, while keeping other steps unchanged. This yielded a colorless liquid ethylene glycol and a white powder terephthalic acid, with yields of 98 wt% and 98 wt%, respectively. The X-ray diffraction patterns of the prepared terephthalic acid and commercially available terephthalic acid (purchased from Sinopharm Chemical Reagent Co., Ltd., purity >99 wt%) are shown below. Figure 2 As shown, this indicates that the crystal form of the prepared terephthalic acid is almost identical to that of commercially available terephthalic acid, thus confirming that the product is terephthalic acid.

[0066] Example 4

[0067] (1) After cleaning and drying the waste PET bottles, place them in a crusher to crush them to obtain waste PET fragments.

[0068] (2) Weigh 6g of waste PET fragments and 5g of magnesium oxide, put them into a ball mill, and stir and mix them for 10 minutes at a speed of 150r / min to obtain a uniform mixture of the two.

[0069] (3) Transfer the mixture obtained in step (2) to a tubular reactor, place the tubular reactor in a heating platform, set the heating rate to 10℃ / min, raise the temperature to the reaction temperature of 220℃, and keep it at this temperature for 20min.

[0070] (4) After the heating platform cools naturally, the solid product from the tubular reactor is obtained. The solid product from the tubular reactor is soaked in 0.1 mol / L NaOH solution for 1 hour and then filtered. The upper white solid is washed and dried to obtain recovered magnesium oxide. A white precipitate is obtained by adding 1 mol / L hydrochloric acid solution to the filtrate. The filtrate is filtered, washed, and dried to obtain terephthalic acid with a yield of 99 wt%. The filtrate is then distilled to obtain ethylene glycol with a yield of 99 wt%.

[0071] Figure 3 (a) and (b) in the image show the carbon NMR spectrum and hydrogen NMR spectrum of the prepared terephthalic acid, respectively. This indicates that the purity of the terephthalic acid is very high, close to 100%.

[0072] Example 5

[0073] (1) After cleaning and drying the waste PET bottles, place them in a crusher to obtain waste PET fragments with a size of 0.5mm to 6mm.

[0074] (2) Weigh 4g of waste PET fragments and 6.5g of magnesium oxide, put them into a ball mill, and stir and mix them at 120r / min for 15min to obtain a uniform mixture of the two.

[0075] (3) Transfer the mixture obtained in step (2) into a crucible, then place the crucible into a tube furnace, set the tube furnace to heat at a heating rate of 10℃ / min, raise it to the reaction temperature of 260℃, and keep it at this temperature for 30min.

[0076] (4) After the heating platform cools naturally, the solid product from the tubular reactor is obtained. The solid product from the tubular reactor is soaked in 0.5 mol / L NaOH solution for 1 hour and then filtered. The upper white solid is washed and dried to obtain recovered magnesium oxide. A white precipitate is obtained by adding 1 mol / L hydrochloric acid solution to the filtrate. The filtrate is filtered, washed, and dried to obtain terephthalic acid with a yield of 98 wt%. The filtrate is then distilled to obtain ethylene glycol with a yield of 98 wt%.

[0077] The X-ray diffraction patterns of the prepared terephthalic acid and commercially available terephthalic acid (purchased from Sinopharm Chemical Reagent Co., Ltd., purity >99wt%) are shown below. Figure 4As shown in the figure. This indicates that the crystal form of the prepared terephthalic acid is almost identical to that of commercially available terephthalic acid, thus confirming that the product is terephthalic acid.

[0078] Example 6

[0079] The reaction temperature in Example 5 was changed to 240℃ and maintained at this temperature for 60 min, while keeping other steps unchanged. This yielded a colorless liquid ethylene glycol and a white powder terephthalic acid, with yields of 96 wt% and 96 wt%, respectively. A scanning electron microscope image of the prepared terephthalic acid is shown below. Figure 5 As shown, this indicates that terephthalic acid prepared by using calcium oxide as a catalyst is a crystal, exhibiting rod-shaped and granular cluster structures.

[0080] Example 7

[0081] The reaction temperature in Example 5 was changed to 200°C and kept at that temperature for 90 min, while other steps remained unchanged. After post-treatment, colorless ethylene glycol liquid and white terephthalic acid powder were obtained, with yields of 95 wt% and 94 wt%, respectively.

[0082] The infrared spectrum of the prepared terephthalic acid is as follows: Figure 6 As shown, the results confirm that the product is terephthalic acid.

[0083] Example 8

[0084] (1) After cleaning and drying the waste PET bottles, place them in a crusher to obtain waste PET fragments with a size of 0.5mm to 6mm.

[0085] (2) Weigh 7g of waste PET fragments and 7g of calcium hydroxide, put them into a ball mill, and stir and mix them at 80r / min for 30min to obtain a uniform mixture of the two.

[0086] (3) Transfer the mixture obtained in step (2) into a crucible, place the crucible into a tube furnace, set the tube furnace to heat at a heating rate of 10℃ / min, raise it to the reaction temperature of 290℃, and keep it at this temperature for 10min.

[0087] (4) After the heating platform cools naturally, the solid product from the tubular reactor is obtained. The solid product from the tubular reactor is soaked in a 1 mol / L NaOH solution for 1 hour and then filtered. The upper white solid is washed and dried to obtain recovered calcium hydroxide. A white precipitate is obtained by adding a 1 mol / L hydrochloric acid solution to the filtrate. The filtrate is filtered, washed, and dried to obtain terephthalic acid with a yield of 96 wt%. Ethylene glycol is obtained by distillation of the filtrate with a yield of 96 wt%.

[0088] The solid-state carbon NMR spectrum of the prepared terephthalic acid is shown below. Figure 7 As shown, this result confirms that the product is terephthalic acid.

[0089] Example 9

[0090] The reaction temperature in Example 8 was changed to 280°C and kept at that temperature for 40 min, while other steps remained unchanged, to obtain colorless ethylene glycol liquid and white terephthalic acid powder, both with a yield of 95 wt%.

[0091] Infrared spectrum of the prepared terephthalic acid Figure 8 As shown, this result confirms that the product is terephthalic acid.

[0092] Example 10

[0093] (1) After cleaning and drying the waste PET bottles, place them in a crusher to obtain waste PET fragments with a size of 0.5mm to 6mm.

[0094] (2) Weigh 2g of waste PET fragments and 2g of barium hydroxide, put them into a ball mill, and stir and mix them for 20 minutes at a speed of 40r / min to obtain a uniform mixture of the two.

[0095] (3) Transfer the mixture obtained in step (2) into a crucible, place the crucible into a tube furnace, set the tube furnace to heat at a heating rate of 10℃ / min, raise it to the reaction temperature of 210℃, and keep it at this temperature for 40min.

[0096] (4) After the heating platform cools naturally, the solid product from the tubular reactor is obtained. The solid product from the tubular reactor is soaked in 1 mol / L NaOH solution for 1 hour and then filtered. The upper white solid is washed and dried to obtain recovered barium hydroxide. A white precipitate is obtained by adding 1 mol / L hydrochloric acid solution to the filtrate. The filtrate is filtered, washed, and dried to obtain terephthalic acid with a yield of 98 wt%. Ethylene glycol is obtained by distillation of the filtrate with a yield of 97 wt%.

[0097] The X-ray diffraction pattern of the prepared terephthalic acid product is shown below. Figure 9 As shown, this indicates that the crystal form of the prepared terephthalic acid is almost identical to that of commercially available terephthalic acid (purchased from Sinopharm Chemical Reagent Co., Ltd., with a purity of >99wt%), thus confirming that the product is terephthalic acid.

[0098] Example 11

[0099] The reaction temperature in Example 9 was changed to 230°C, the catalyst was changed to magnesium hydroxide, and the temperature was maintained for 120 min. Other steps remained unchanged, and colorless ethylene glycol liquid and white terephthalic acid powder were obtained with yields of 99 wt% and 98 wt%, respectively.

[0100] The recovered magnesium hydroxide was reused 10 times, with the yield of terephthalic acid remaining above 99 wt%. The X-ray diffraction pattern of the reused Mg(OH)₂ is shown below. Figure 10 As shown, the recovered Mg(OH)₂ has the same crystal structure as the Mg(OH)₂ before the reaction. This indicates that Mg(OH)₂ is easy to recover, and the crystal structure of the Mg(OH)₂ catalyst before and after the reaction remains unchanged, allowing it to be reused directly. This achieves the goal of recycling the catalyst to degrade PET.

[0101] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method of catalyzing the degradation of polyethylene terephthalate, characterized by, The method comprises the following steps: (1) mixing the polyethylene terephthalate pieces with a catalyst, wherein the catalyst is an alkaline earth metal hydroxide or an alkaline earth metal oxide; (2) heating the mixture obtained in step (1) at a temperature of 200-350 DEG C, so that the catalyst activates the C=O bond in the polyethylene terephthalate polymer chain, and under the combined action of the catalyst and the heating, an intermediate, polyethylene terephthalate vinyl ester, is generated; after adding water, the polyethylene terephthalate vinyl ester undergoes a hydrolysis reaction to generate terephthalic acid and ethylene glycol, so that the polyethylene terephthalate is degraded.

2. A method of catalyzing the degradation of polyethylene terephthalate, characterized by, The method comprises the following steps: (1) mixing the polyethylene terephthalate pieces with a catalyst, wherein the catalyst is an alkaline earth metal hydroxide or an alkaline earth metal oxide; (2) heating the mixture obtained in step (1) at a temperature of 200-350 DEG C, so that the catalyst activates the C=O bond in the polyethylene terephthalate polymer chain, and under the combined action of the catalyst and the heating, an intermediate, polyethylene terephthalate vinyl ester, is generated; then, the polyethylene terephthalate vinyl ester is immersed in a lye, so that terephthalate and ethylene glycol are generated; after filtration, the white solid is the recovered catalyst; then, an acid solution is added to the filtrate, so that the terephthalate reacts to generate terephthalic acid white precipitate; after solid-liquid separation, the terephthalic acid is recovered; and after distillation of the filtrate, the ethylene glycol is recovered.

3. The method of catalytic polyethylene terephthalate degradation according to claim 1 or 2, characterized in that, The heating in step (2) is performed for 5-180 minutes.

4. The method of catalytic polyethylene terephthalate degradation according to claim 1 or 2, wherein, The alkaline earth metal hydroxide is magnesium hydroxide, calcium hydroxide or barium hydroxide.

5. The method of catalytic polyethylene terephthalate degradation according to claim 1 or 2, wherein, The alkaline earth metal oxide is magnesium oxide, calcium oxide or barium oxide.

6. The method of catalytic polyethylene terephthalate degradation according to claim 1 or 2, wherein, The polyethylene terephthalate is waste polyethylene terephthalate.

7. The method of catalytic polyethylene terephthalate degradation according to claim 6, wherein, The waste polyethylene terephthalate is at least one of waste polyethylene terephthalate bottles, waste polyethylene terephthalate packaging sheets, waste polyethylene terephthalate films, waste polyethylene terephthalate fibers and waste polyethylene terephthalate textiles.

8. The method of catalytic polyethylene terephthalate degradation according to claim 1 or 2, wherein, The mass ratio of the polyethylene terephthalate to the catalyst is (1-100):

10.

9. The method of catalytic polyethylene terephthalate degradation according to claim 1 or 2, wherein, The heating rate is 1-100 DEG C / min.

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