A method for degrading pet waste plastics to monomers

Abstract

The application relates to a method for degrading PET waste plastics into monomers, which comprises the following steps: (1) sealing PET waste plastics and methanol in a reactor, introducing CO2 to pressurize the reactor to 1-4 MPa, and maintaining the reaction system in a subcritical state to carry out reaction; (2) after the reaction is completed, cooling to room temperature, carrying out solid-liquid separation to obtain a solid phase, and drying to obtain monomers DMT. By introducing CO2 into the methanol degradation PET system and maintaining the subcritical state, the PET degradation process can be effectively strengthened, the degradation performance and product selectivity are improved, and the reaction condition is more moderate.

Description

Technical Field

[0001] This invention belongs to the field of waste plastic recycling technology, specifically relating to a method for degrading PET waste plastic into monomers. Background Technology

[0002] In recent years, waste plastics have become a major environmental problem worldwide. Since the 1950s, a total of 8.3 billion tons of non-degradable plastics have been produced globally, of which only 11% has been recycled. Polyester (containing CO) plastics account for about 20% of the total plastics. Polyethylene terephthalate (PET), for example, is a thermoplastic polyester material formed by the condensation of terephthalic acid and ethylene glycol, widely used in the food and packaging industries. Most PET products are for single use, and waste PET accounts for 8% of the world's solid waste by weight and 12% by volume. Therefore, research on PET recycling and reuse has received widespread attention. One strategy, chemical recycling to monomers, involves separating and purifying the products after PET degradation, allowing them to be reused as raw materials for the production of polyesters (BHET, DMT, TPA) or other chemical products, thus achieving the recycling of PET resources. This strategy has significant advantages in energy conservation, environmental protection, and promoting a circular economy.

[0003] The main methods for recycling waste plastics include energy recovery, physical, chemical, and biological methods. Chemical recycling breaks down plastics into intermediates or monomers, which can be used to synthesize the same plastic with the properties of the original material (closed-loop recycling) or converted into another material with higher value (open-loop recycling). For PET, a polymer containing CO bonds, its depolymerization reaction typically exhibits a relatively low reaction barrier and near-neutral reaction free energy. Therefore, nucleophiles and corresponding catalysts are usually used to achieve PET depolymerization. Catalysts are generally metal salts, metal oxides, ionic liquids, deep eutectic solvents, etc., while nucleophiles are generally water, methanol, ethanol, ethylene glycol, amines, etc., to achieve PET depolymerization. However, issues such as metal residue, catalyst cost, and recycling limit its industrial application.

[0004] Non-catalytic chemical recycling—supercritical fluid technology for recycling waste plastics—has attracted attention from scholars both domestically and internationally due to its unique advantages. (Maja Colnik, Knez Z, Colnik M, Knez Z) M. Sub-and supercritical water for chemical recycling of polyethylene terephthalate waste[J]. Chemical Engineering Science, 2021, 233: 116389.) The optimal yield and purity of TPA were obtained under the conditions of 300℃, 95MPa, and 30min, which were 90±0.4% and 97.6±0.3%, respectively. Although supercritical water can achieve satisfactory degradation results, its reaction conditions are harsh, making industrial application difficult. However, using supercritical methanol to degrade PET into dimethyl terephthalate (DMT) and ethylene glycol (EG) monomers for recycling and regenerating chemical products provides a good approach to solving the challenges of PET waste plastics.

[0005] Sako et al. (Sako T, Okajima I, Sugeta T, et al. Recovery of Constituent Monomers from Polyethylene Terephthalate with Supercritical Methanol[J]. Polymer Journal, 2000, 32(02): 178-181.) added 0.5 g of PET to a 20 mL reactor, controlled the reaction temperature with a sand bath, and controlled the reaction pressure by adjusting the amount of methanol. Under the conditions of 330℃, 8.1 MPa, and 30 min of reaction, the optimal yield of DMT was 94%. Kim et al. (Kim B, Hwang G, Bae S, et al. Depolymerization of Polyethyleneterephthalate in Supercritical Methanol[J]. Journal of Applied Polymer Science, 2000, 81(9): 2102-2108.) conducted batch experiments using a 1.2 L reactor, with a reaction temperature of 270-310℃, a final reaction pressure of 7.0-10.1 MPa, and a reaction time of 0-70 min. Taking into account the effects of reaction time and temperature, the optimal reaction conditions are 300℃, 40 min, 8.26 MPa, and a methanol / PET mass ratio of 10, under which the final yield of DMT is 97.0%. Although the supercritical methanol alcoholysis method can achieve good monomer yields, methanol often needs to reach a supercritical state at temperatures above 300℃ and pressures above 8.0 MPa, resulting in high energy consumption.

[0006] While supercritical conditions can achieve satisfactory degradation results, their relatively high reaction temperatures pose challenges for industrial application. On one hand, high temperatures lead to side reactions that complicate the product composition, increasing the difficulty of subsequent separation and purification. On the other hand, high temperature and pressure place higher demands on equipment. Therefore, reducing reaction temperature and pressure while preserving reaction activity remains a challenge. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide a method for degrading PET waste plastics into monomers that is highly degradable, low-cost, and environmentally friendly. This invention introduces CO2 into the methanol-degradable PET system and maintains it in a subcritical state, which effectively enhances the PET degradation process, improves degradation performance and product selectivity, and provides milder reaction conditions.

[0008] The present invention provides a method for degrading PET waste plastic into monomers, comprising the following steps:

[0009] (1) Place waste PET plastic and methanol in a reactor and seal it. Then introduce CO2 to pressurize the reactor to 1-4 MPa and maintain the reaction system in a subcritical state.

[0010] (2) After the reaction is completed, the mixture is cooled to room temperature, and the solid phase is separated from the liquid phase and dried to obtain the monomer DMT.

[0011] In this invention, the PET waste plastic mentioned in step (1) comes from any one or more of waste PET bottle flakes, PET films, PET fibers, etc. Preferably, it is washed, dried, and cut before use, with a size of less than 10 mm.

[0012] In this invention, the mass ratio of PET waste plastic to methanol in step (1) is 1:5-15, preferably 1:6-10. After adding the PET waste plastic and methanol to the reactor and sealing it, it is preferable to first displace the air in the reactor, specifically by introducing at least one of nitrogen, inert gas, etc.

[0013] In this invention, step (1) involves introducing CO2 to pressurize the reactor to 1-4 MPa. Then, the reaction temperature is increased to 180-240°C at a heating rate of 10-15°C / min, and the reaction system is maintained in a subcritical state for 15-70 min.

[0014] In this invention, step (1) is carried out under stirring conditions, with a stirring rate of 500-1000 rpm.

[0015] Further, in step (1), the catalyst prepared by the present invention is added to the reaction system. The catalyst preparation method includes the following steps: (a) Zinc salt, ferrous salt and ferric salt are mixed to obtain mixture S1, which is transported together with precipitant to a microchannel reactor and micro-reaction is carried out under an inert atmosphere to obtain mixture S2; (b) Sodium citrate solution is added to mixture S2 and heated and stirred for a certain time; (c) The product obtained in step (2) is filtered, washed and dried and calcined to obtain the final catalyst.

[0016] In catalyst preparation step (a), the zinc salt is Zn 2+ The zinc salt is preferably at least one of zinc chloride, zinc acetate, zinc nitrate, etc. In mixture S1, the zinc salt is in the form of Zn... 2+ The concentration is 0.04-0.4 mol / L, preferably 0.05-0.4 mol / L.

[0017] In catalyst preparation step (a), the divalent iron salt is Fe 2+ The soluble salt, preferably at least one of ferrous chloride, ferrous sulfate, etc., is present in mixture S1. 2+ The concentration is 0.04-2.0 mol / L, preferably 0.2-0.4 mol / L.

[0018] In catalyst preparation step (a), the trivalent iron salt is Fe 3+ Soluble salts, preferably at least one of ferric chloride, ferric nitrate, and ferric sulfate. Ferric salts are predominantly Fe2+. 3+ Based on content, the molar concentration ratio with that of ferrous salt is 2:1.

[0019] In catalyst preparation step (a), the precipitant is at least one of sodium hydroxide solution, sodium carbonate solution, sodium bicarbonate solution, ammonium carbonate solution, etc., preferably sodium hydroxide solution; the concentration of the solution is 0.32-16 mol / L, preferably 1.7-4.0 mol / L.

[0020] In catalyst preparation step (a), mixture S1 and precipitant are fed into a microchannel reactor at a volume ratio of 1:1 for reaction. The reaction is carried out under an inert atmosphere at a pressure of 0.2-0.4 MPa. The inert atmosphere is an inert gas and / or nitrogen.

[0021] In catalyst preparation step (b), the concentration of sodium citrate solution is 0.3-2.0 mol / L, and the amount added is 1%-5% of the volume of mixture S2, preferably 2%-3%. The heating temperature is 60-80℃, the stirring speed is 200-600 rpm, and the reaction time is 6-8 h. The stirring method can be any one of magnetic stirring, mechanical stirring, etc.

[0022] In catalyst preparation step (c), conventional filtration methods such as vacuum filtration are used, and the product is washed until neutral. Drying temperature is 60-80℃, and time is 4-12 hours. Calcination temperature is 200-300℃, and time is 2-3 hours.

[0023] In this invention, step (1) involves adding a catalyst to the reaction system, with the amount being 0.05%-0.5% of the mass of methanol.

[0024] In this invention, the cooling in step (2) can be performed using an ice-water bath, preferably cooling to 10-30°C. Solid-liquid separation is performed using centrifugation, filtration, etc., preferably vacuum filtration. The drying temperature is 40-50°C, and the drying time is 1-2 hours. The liquid phase separated from the solid phase can be separated into ethylene glycol and methanol using equipment such as a rotary evaporator, and then recovered and reused respectively.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) This invention introduces CO2 into the methanol-degraded PET system and maintains it in a subcritical state, which can effectively enhance the PET degradation process and improve the selectivity of the monomer DMT while improving degradation performance. Under the conditions described in this invention, the PET degradation rate is higher than 98%, and the DMT monomer yield is higher than 90%.

[0027] (2) The CO2-enhanced subcritical methanol degradation PET system can achieve efficient depolymerization of PET under milder conditions, and CO2 can be directly removed after the reaction, reducing the difficulty and cost of separating the later products.

[0028] (3) Using the catalyst provided by this invention, the depolymerization temperature is reduced by more than 10°C compared with conventional catalysts while maintaining degradation capacity, and the reaction conditions are milder. Moreover, the degradation products and catalyst can be separated by applying an external magnetic field, further reducing the separation difficulty. Detailed Implementation

[0029] The technical solution and its effects of the present invention will be described in detail below with reference to specific embodiments. The embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures; however, the scope of protection of the present invention is not limited to the following embodiments.

[0030] Unless otherwise specified, the experimental methods used in the following examples are conventional methods in the art. Unless otherwise specified, the experimental materials used in the following examples were purchased from conventional biochemical reagent stores.

[0031] PET conversion rate = (initial PET addition amount - mass of PET remaining after degradation) / initial PET addition mass × 100%.

[0032] DMT yield = (mass of DMT obtained / molecular weight of DMT) / (mass of PET / molecular weight of PET) × 100%.

[0033] Example 1

[0034] Waste PET bottle flakes (3×3mm) and methanol were placed in a stainless steel reactor at a mass ratio of 1:6. After sealing, nitrogen was introduced to replace the air, and CO2 was introduced to pressurize the reactor to 2MPa. The reaction temperature was increased to 220℃ at a heating rate of 15℃ / min, and the reaction was carried out at a stirring rate of 500rpm for 50min. Heating was then stopped, and the mixture was cooled to room temperature. The material in the reactor was removed, and solid-liquid separation was performed using an SHZ-DⅢ circulating water vacuum pump. The solid was dried at 50℃ for 2h. The liquid phase separated from the solid phase was used to separate methanol using a rotary evaporator and collected. Liquid chromatography and DSC analysis showed that the PET degradation rate was 99.0%, and the DMT yield was 90.23%.

[0035] Example 2

[0036] Waste PET bottle flakes (3×3mm) and methanol were placed in a stainless steel reactor at a mass ratio of 1:10. After sealing, nitrogen was introduced to replace the air, and CO2 was introduced to pressurize the reactor to 3MPa. The reaction temperature was increased to 200℃ at a heating rate of 10℃ / min, and the reaction was carried out at a stirring rate of 500rpm for 70min. Heating was then stopped, and the reactor was cooled to room temperature. The material was removed from the reactor, and solid-liquid separation was performed using an SHZ-DⅢ circulating water vacuum pump. The solid was dried at 45℃ for 2h. The liquid phase separated from the solid phase was used to separate methanol using a rotary evaporator and collected. Liquid chromatography and DSC analysis showed that the PET degradation rate was 98.5%, and the DMT yield was 90.41%.

[0037] Example 3

[0038] Waste PET bottle flakes (3×3mm) and methanol were placed in a stainless steel reactor at a mass ratio of 1:15. After sealing, nitrogen was introduced to replace the air, and CO2 was introduced to pressurize the reactor to 4MPa. The reaction temperature was increased to 240℃ at a heating rate of 15℃ / min, and the reaction was carried out at a stirring rate of 500rpm for 60min. Heating was then stopped, and the mixture was cooled to room temperature. The material in the reactor was removed, and solid-liquid separation was performed using an SHZ-DⅢ circulating water vacuum pump. The solid was dried at 50℃ for 2h. The liquid phase separated from the solid phase was used to separate methanol using a rotary evaporator and collected. Liquid chromatography and DSC analysis showed that the PET degradation rate was 99.1%, and the DMT yield was 91.43%.

[0039] Example 4

[0040] Waste PET film and methanol (50 mL) were placed in a stainless steel reactor at a mass ratio of 1:10. After sealing, argon gas was introduced to replace the air, and CO2 was introduced to pressurize the reactor to 2 MPa. The reaction temperature was increased to 200°C at a heating rate of 15°C / min, and the reaction was carried out at a stirring rate of 500 rpm for 60 min. Heating was then stopped, and the mixture was cooled to room temperature. The material in the reactor was removed, and solid-liquid separation was performed using an SHZ-DⅢ circulating water vacuum pump. The solid was dried at 50°C for 2 h. The liquid phase separated from the solid phase was used to separate methanol using a rotary evaporator and collected. Liquid chromatography and DSC analysis showed that the PET degradation rate was 98.8%, and the DMT yield was 90.55%.

[0041] Example 5

[0042] Same as Example 1, except that the catalyst provided by this invention is used. The catalyst preparation process is as follows: (a) 2.176 g of zinc chloride, 22.24 g of ferrous sulfate heptahydrate and 43.248 g of ferric chloride hexahydrate are added to 200 mL of deionized water to obtain mixture S1(Zn 2+ =0.08mol / L, Fe 2+ =0.4mol / L, Fe 3+ =0.8mol / L). Take 26.88g of sodium hydroxide solid and add 200mL of water to prepare a precipitant of 3.36mol / L. Transfer the mixture S1 and the precipitant to a microchannel reactor and carry out a micro-reaction under a nitrogen atmosphere at a pressure of 0.4MPa to obtain mixture S2. (b) Take 1.032g of sodium citrate and add 10mL of water to prepare a sodium citrate solution of 0.4mol / L. Add the solution to mixture S2, heat to 80℃, and magnetically stir at 500 rpm for 6h. (c) After filtering and washing with deionized water until neutral, the product obtained in step (b) is dried at 60℃ for 5h and calcined at 250℃ for 2h to obtain the final catalyst. The catalyst dosage is 0.1% of the methanol mass, and the reaction temperature is reduced by 15℃. After the reaction is completed, liquid chromatography and DSC analysis show that the PET degradation rate is 99.5% and the DMT yield is 92.01%.

[0043] Comparative Example 1

[0044] Same as Example 1, except that CO2 was introduced to maintain a reactor pressure of 0.5 MPa. Post-reaction analysis showed a PET degradation rate of 75.62% and a DMT yield of 41.22%.

[0045] Comparative Example 2

[0046] Same as Example 1, except that the amount of CO2 introduced was adjusted to maintain a reactor pressure of 8.0 MPa. Post-reaction analysis showed a PET degradation rate of 99.1% and a DMT yield of 91.12%.

[0047] Comparative Example 3

[0048] Same as Example 5, except that CO2 was not introduced. Post-reaction analysis showed a PET degradation rate of 98.9% and a DMT yield of 85.3%.

Claims

1. A method for degrading PET waste plastics into monomers, characterized in that... The process includes the following steps: (1) Place waste PET plastic and methanol in a reactor and seal it. Then introduce CO2 to pressurize the reactor to 1-4 MPa and maintain the reaction system in a subcritical state. (2) After the reaction is completed, cool to room temperature, separate the solid and liquid phases to obtain the solid phase, and dry it to obtain the monomer DMT.

2. The method according to claim 1, characterized in that: The PET waste plastic mentioned in step (1) comes from any one or more of waste PET bottle flakes, PET films, and PET fibers. It is preferred that the plastic be washed, dried, and cut before use, and the size be less than 10 mm.

3. The method according to claim 1 or 2, characterized in that: The mass ratio of PET waste plastic to methanol in step (1) is 1:5-15, preferably 1:6-10.

4. The method according to claim 1, characterized in that: Step (1) After adding PET waste plastic and methanol to the reactor and sealing it, at least one of nitrogen or inert gas is introduced to replace the air in the reactor.

5. The method according to claim 1, characterized in that: Step (1) Introduce CO2 to pressurize the reactor to 1-4 MPa, and then raise the temperature to 180-240℃ at a rate of 10-15℃ / min, maintaining the reaction system in a subcritical state for 15-70 min.

6. The method according to claim 1 or 5, characterized in that: Step (1) is carried out under stirring conditions at a stirring rate of 500-1000 rpm.

7. The method according to claim 1, characterized in that: Step (1) Add a catalyst to the reaction system. The catalyst is prepared by: (a) mixing zinc salt, ferrous salt and ferric salt to obtain mixture S1, which is then transported to a microchannel reactor along with a precipitant and subjected to micro-reaction under an inert atmosphere to obtain mixture S2; (b) adding sodium citrate solution to mixture S2 and heating and stirring for a certain time; (c) after filtering and washing the product obtained in step (2), drying and calcining are performed to obtain the final catalyst.

8. The method according to claim 7, characterized in that: In catalyst preparation step (a), the zinc salt is Zn. 2+ The soluble salt, preferably at least one of zinc chloride, zinc acetate, and zinc nitrate; in mixture S1, the zinc salt is in the form of Zn 2+ The concentration is 0.04-0.4 mol / L, preferably 0.05-0.4 mol / L.

9. The method according to claim 7, characterized in that: In catalyst preparation step (a), the divalent iron salt is Fe. 2+ The soluble salt, preferably at least one of ferrous chloride and ferrous sulfate; in mixture S1, Fe... 2+ The concentration is 0.04-2.0 mol / L, preferably 0.2-0.4 mol / L.

10. The method according to claim 7, characterized in that: In catalyst preparation step (a), the trivalent iron salt is Fe 3+ Soluble salts, preferably at least one of ferric chloride, ferric nitrate, and ferric sulfate. Ferric salts are predominantly Fe2+. 3+ Based on content, the molar concentration ratio with that of ferrous salt is 2:

1.

11. The method according to claim 7, characterized in that: In catalyst preparation step (a), the precipitant is at least one of sodium hydroxide solution, sodium carbonate solution, sodium bicarbonate solution, and ammonium carbonate solution, preferably sodium hydroxide solution; the concentration of the solution is 0.32-16 mol / L, preferably 1.7-4.0 mol / L.

12. The method according to claim 7 or 11, characterized in that: In catalyst preparation step (a), mixture S1 and precipitant are transported to a microchannel reactor at a volume ratio of 1:1 for reaction; the reaction is carried out in an inert atmosphere at a pressure of 0.2-0.4 MPa.

13. The method according to claim 7, characterized in that: In catalyst preparation step (b), the concentration of sodium citrate solution is 0.3-2.0 mol / L, and the amount added is 1%-5% of the volume of mixture S2, preferably 2%-3%.

14. The method according to claim 7, characterized in that: In catalyst preparation step (b), the heating temperature is 60-80℃, the stirring speed is 200-600 rpm, and the reaction time is 6-8h.

15. The method according to claim 7, characterized in that: In catalyst preparation step (c), the drying temperature is 60-80℃ and the time is 4-12h; the calcination temperature is 200-300℃ and the time is 2-3h.

16. The method according to claim 7, characterized in that: Step (1) Add a catalyst to the reaction system, the amount of which is 0.05%-0.5% of the mass of methanol.

17. The method according to claim 1, characterized in that: The cooling in step (2) is performed using an ice-water bath, preferably cooling to 10-30℃; the solid-liquid separation is performed using centrifugation and filtration, preferably vacuum filtration; the drying temperature is 40-50℃, and the drying time is 1-2 hours.