Method for depolymerizing polyethylene glycol terephthalate (PET) under catalyst-free condition

Abstract

The invention discloses a method for depolymerizing polyethylene glycol terephthalate (PET) under a catalyst-free condition. Under the condition of no catalyst, PET or a composite system thereof is treated in diester carbonate, chemical depolymerization of PET is realized, PET monomers of diester terephthalate and ethylene carbonate are obtained, and complete depolymerization of PET in a composite is realized. According to the method disclosed by the invention, through the reaction of the diester carbonate and the PET, the depolymerization of the PET under a mild condition can be realized, and the related technology research and development of chemical recovery of the PET monomer in a PET composite system or a mixed system are further carried out. The method provided by the invention has the advantages of high efficiency, greenness, mild reaction conditions and the like, can catalyze PET depolymerization to generate the monomer terephthalic acid diester, and has higher industrial application value.

Description

Technical Field

[0001] This invention belongs to the field of waste plastic recycling technology, specifically relating to a method for depolymerizing polyethylene terephthalate (PET) under catalyst-free conditions.

[0002] This application is a divisional application of application number 202411261062.8, filed on September 10, 2024, entitled "A method for preparing terephthalic acid diester by catalytic depolymerization of polyethylene terephthalate (PET)". Background Technology

[0003] Polyethylene terephthalate (PET) is a polymer formed by the condensation polymerization of terephthalic acid (ester) and ethylene glycol. It possesses excellent properties and has been widely used in various fields of production and daily life. With the rapid development of human society, the demand for PET has increased dramatically, leading to a corresponding increase in waste. my country's annual PET production exceeds 50 million tons, and the impact of waste PET products on our ecological environment is becoming increasingly serious. Therefore, it is urgent to solve this problem. Using chemical methods to convert it into useful substances is an important and effective way to solve this problem. Currently, methods such as hydrolysis, alcoholysis, ammonolysis, and hydrogenolysis can depolymerize PET into its monomers and chemicals, providing important means for its recycling. PET is often used in combination with other polymeric or inorganic materials, so its depolymerization process often involves high reaction temperatures and cross-contamination, making the chemical recycling of PET costly and technically difficult. Therefore, there is an urgent need for new methods that can depolymerize PET without damaging the composite components and achieve effective separation.

[0004] Ester exchange is a commonly used method in polyester depolymerization, and the exchange reagents used are mainly alcohols, acids, and esters. However, there are no reports on ester exchange using carbonates. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing the monomer terephthalate diester by depolymerizing polyethylene terephthalate (PET).

[0006] The method for preparing terephthalate monomer by depolymerization of PET provided by the present invention includes the following steps: treating PET or its composite system in diester carbonate without a catalyst or with a salt as a catalyst to achieve chemical depolymerization of PET, obtaining PET monomer terephthalate and ethylene carbonate, and achieving complete depolymerization of PET in the composite.

[0007] In the above method, the salt substance is any one of the following: an inorganic metal salt of a halide anion, an ionic liquid of a halide anion, or a composite system formed therefrom.

[0008] The inorganic metal salt of the halide anion is selected from at least one of the following: NaCl, KCl, ZnCl2, FeCl3, CuCl2, SnCl4, ZrCl4, CrCl3, NaBr, KBr, ZnBr2, FeBr3, CuBr2, SnBr4, ZrBr4, CrBr, NaI, KI, ZnI2, FeI3, CuI2, ZrI4, and CrI.

[0009] The ionic liquid containing the halogen anion has cations including, but not limited to, imidazole cations, pyridyl cations, tetraalkylammonium cations, tetraalkylphosphine cations, guanidine cations, and organic base cations.

[0010] The ionic liquid of the halide anion is selected from at least one of the following: 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), 1-butyl-3-methylimidazolium bromide ([BMIM]Br), 1-butyl-3-methylimidazolium iodide ([BMIM]I), 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl), 1-ethyl-3-methylimidazolium bromide ([EMIM]Br), and 1-ethyl-3-methylimidazolium iodide ([EMIM]I).

[0011] The carbonate diester may be at least one of dimethyl carbonate and diethyl carbonate.

[0012] The PET and its composite systems include: at least one of all samples, physical objects, and mixed or composite systems containing PET components.

[0013] The method involves placing a PET-containing sample (PET or its composite system) in a dicarbonate solution, heating the sample for a certain time without a catalyst or by adding a salt as a catalyst, cooling, and separating the samples to obtain terephthalic acid diester and ethylene carbonate.

[0014] The molar ratio of diester to PET structural unit can be 1:1 to 25:1, specifically 10:1 to 15:1, 10:1, 15:1, or 20:1. The molar content of the salt in the diester is 1% to 20%, specifically 1%; A salt is added as a catalyst, and the reaction temperature is 50~250℃; the reaction time is 0.5~36 h. Without a catalyst, the reaction is carried out at a temperature of 170-250°C for 15-36 hours.

[0015] The carbonate diester is dimethyl carbonate, and the resulting terephthalate diester is dimethyl terephthalate.

[0016] The carbonate diester is diethyl carbonate, and the resulting terephthalic acid diester is diethyl terephthalate.

[0017] The method further includes the following operations: filtering and separating unreacted solids; then cooling to precipitate terephthalic acid diester, filtering to obtain terephthalic acid diester; further, collecting excess diester carbonate and generated ethylene carbonate sequentially by conventional vacuum distillation to recover the catalyst.

[0018] The products obtained by the above methods are terephthalate and ethylene carbonate.

[0019] The recovered catalyst can be reused, and the resulting ethylene carbonate can be reacted with methanol or ethanol to produce the diester required for depolymerization of PET.

[0020] Regarding the separation of products after the reaction, it also includes adopting appropriate separation methods according to the different catalysts used.

[0021] This invention enables the depolymerization of PET under mild conditions through the reaction of PET diester and PET, and further promotes the research and development of related technologies for the chemical recovery of PET monomers in PET composite or mixed systems. The method provided by this invention has the advantages of high efficiency, environmental friendliness, and mild reaction conditions, and can catalyze the depolymerization of PET to generate its monomer, terephthalate diester, which has strong industrial application value. Attached Figure Description

[0022] Figure 1 The solid NMR spectrometer of the polyester-cotton blend (composition: 65% PET, 35% cotton) and its reaction residue in Example 5 of this invention. 13 C spectrum. Detailed Implementation

[0023] The present invention will now be described in further detail with reference to specific embodiments. The given embodiments are merely illustrative of the invention and not intended to limit its scope. The embodiments provided below can serve as a guide for further improvements by those skilled in the art and do not constitute a limitation on the invention in any way.

[0024] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Unless otherwise specified, the materials and reagents used in the following examples are commercially available.

[0025] Example 1, Ionic liquid N 4444 Cl-catalyzed depolymerization of PET to prepare dimethyl terephthalate 0.96 g of PET powder sample and 0.27 g of tetrabutylammonium chloride (N 44449 g of dimethyl terephthalate (C1) was added to a 25 mL round-bottom flask and stirred at 120 °C for 2 hours. Then, the mixture was cooled, and dimethyl terephthalate precipitated out. After filtration and drying, 0.93 g of dimethyl terephthalate was obtained (yield 95.9%). 1 H NMR (400 MHz, CDCl3, 25 ℃): / ppm = 8.10 (s, 4H), 3.95 (s, 6H); 13 C NMR (101 MHz, CDCl3, 25 ℃): / ppm = 166.57, 134.21, 129.84, 52.69). The filtrate was distilled under reduced pressure to give 0.43 g of ethylene carbonate (yield 97.7%). 1 H NMR (400MHz, CDCl3, 25℃): / ppm = 4.57 (s, 4H); 13 C NMR (101 MHz, CDCl3, 25 ℃): / ppm =155.12, 64.40.

[0026] Example 2: Preparation of dimethyl terephthalate from PET by ZnCl2-catalyzed depolymerization 0.96 g of PET powder sample, 0.13 g of zinc chloride (ZnCl2), and 9 g of dimethyl carbonate were added to a 25 mL round-bottom flask and stirred at 120 °C for 12 hours. After cooling, dimethyl terephthalate precipitated out, was filtered, and dried to obtain 0.92 g of dimethyl terephthalate (yield 94.8%). The filtrate was distilled under reduced pressure to obtain 0.42 g of ethylene carbonate (yield 95.5%).

[0027] Example 3, Ionic liquid N 4444 Cl Catalytic Recycling of PET Plastic Bottles to Prepare Dimethyl Terephthalate 0.96 g of PET plastic bottle fragments and 0.27 g of tetrabutylammonium chloride (N 4444 9 g of dimethyl carbonate was added to a 25 mL round-bottom flask and stirred at 120 °C for 4 hours. Then, the mixture was cooled, and dimethyl terephthalate precipitated out. The precipitate was filtered and dried to obtain 0.93 g of dimethyl terephthalate (yield 95.9%). The filtrate was distilled under reduced pressure to obtain 0.43 g of ethylene carbonate (yield 97.7%).

[0028] Example 4, Ionic liquid N 4444 Cl-catalyzed recycling of PET textiles to prepare methyl terephthalate 0.97 g of PET textile (composition: 100% PET) fragments and 0.28 g of tetrabutylammonium chloride (N 4444 8 g of dimethyl carbonate was added to a 25 mL round-bottom flask and stirred at 140 °C for 1 hour. Then, after cooling, dimethyl terephthalate precipitated out, was filtered, and dried to obtain 0.94 g of dimethyl terephthalate (yield 95.9%). The filtrate was distilled under reduced pressure to obtain 0.41 g of ethylene carbonate (yield 92.2%).

[0029] Example 5, Ionic liquid N 4444 Cl-catalyzed recovery of polyester / cotton textiles to prepare dimethyl terephthalate 1.48 g of PET textile fragments (composition: 65% polyester, 35% cotton fiber) and 0.38 g of tetrabutylammonium chloride (N 4444 10 g of dimethyl carbonate (C1) was added to a 25 mL round-bottom flask and stirred at 130 °C for 2 hours. The unreacted solid was then separated by filtration, washed, and dried to obtain 0.52 g of solid, which consisted of cellulose (C1). Figure 1 The filtrate was then cooled, and dimethyl terephthalate precipitated out. The precipitate was filtered and dried to obtain 0.93 g of dimethyl terephthalate (yield 96.0%). The filtrate was then distilled under reduced pressure to obtain 0.42 g of ethylene carbonate (95.5%).

[0030] Example 6, Ionic liquid N 4444 Cl-catalyzed recovery of polyester / cotton textiles to prepare dimethyl terephthalate 2.02 g of PET textile fragments (composition: 47.5% polyester, 47.5% cotton fiber, 5% spandex) and 0.27 g of tetrabutylammonium chloride (N 4444 9 g of dimethyl carbonate was added to a 25 mL round-bottom flask and stirred at 130 °C for 10 hours. The unreacted solid was then separated by filtration, washed, and dried to obtain 1.06 g of solid composed of cellulose and spandex. The filtrate was then cooled, and dimethyl terephthalate precipitated out. This precipitate was filtered and dried to obtain 0.94 g of dimethyl terephthalate (96.9% yield). The filtrate was then distilled under reduced pressure to obtain 0.43 g of ethylene carbonate (97.7% yield).

[0031] Example 7, Ionic liquid N 4444 I. Catalytic depolymerization of PET to prepare dimethyl terephthalate 0.97 g of PET powder sample and 0.37 g of tetrabutylammonium iodide (N 44441) 8 g of dimethyl carbonate was added to a 25 mL round-bottom flask and stirred at 110 °C for 2 hours. Then, the mixture was cooled, and dimethyl terephthalate precipitated out. After filtration and drying, 0.45 g of dimethyl terephthalate (yield 45.9%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.22 g of ethylene carbonate (yield 49.5%).

[0032] Example 8, Ionic liquid N 4444 Br-catalyzed depolymerization of PET to prepare dimethyl terephthalate 0.95 g of PET powder sample and 0.32 g of tetrabutylammonium bromide (N 4444 Br) and 9 g of dimethyl carbonate were added to a 25 mL round-bottom flask and stirred at 120 °C for 2 hours. Then, the mixture was cooled, and dimethyl terephthalate precipitated out. The precipitate was filtered and dried to obtain 0.69 g of dimethyl terephthalate (yield 71.9%). The filtrate was distilled under reduced pressure to obtain 0.30 g of ethylene carbonate (yield 68.9%).

[0033] Example 9, Ionic liquid P 4444 Cl-catalyzed depolymerization of PET to prepare dimethyl terephthalate 0.97 g of PET powder sample and 0.29 g of tetrabutylphosphine chloride (P) were mixed. 4444 8 g of dimethyl terephthalate was added to a 25 mL round-bottom flask and stirred at 120 °C for 2 hours. Then, the mixture was cooled, and dimethyl terephthalate precipitated out. The precipitate was filtered and dried to obtain 0.90 g of dimethyl terephthalate (yield 91.8%). The filtrate was distilled under reduced pressure to obtain 0.37 g of ethylene carbonate (yield 83.2%).

[0034] Example 10, Ionic liquid P 4444 Br-catalyzed depolymerization of PET to prepare dimethyl terephthalate 0.96 g of PET powder sample and 0.40 g of tetrabutylphosphine bromide (P 4444 Br) and 9 g of dimethyl carbonate were added to a 25 mL round-bottom flask and stirred at 120 °C for 2 hours. Then, the mixture was cooled, and dimethyl terephthalate precipitated out. The precipitate was filtered and dried to obtain 0.76 g of dimethyl terephthalate (yield 78.4%). The filtrate was distilled under reduced pressure to obtain 0.36 g of ethylene carbonate (yield 81.8%).

[0035] Example 11, Ionic liquid P 4444 I. Catalytic depolymerization of PET to prepare dimethyl terephthalate 0.97 g of PET powder sample and 0.36 g of tetrabutylphosphine iodide (P 44441) 7 g of dimethyl carbonate was added to a 25 mL round-bottom flask and stirred at 140 °C for 2 hours. Then, the mixture was cooled, and dimethyl terephthalate precipitated out. After filtration and drying, 0.47 g of dimethyl terephthalate (yield 48.0%) was obtained. The filtrate was distilled under reduced pressure to obtain 0.23 g of ethylene carbonate (yield 51.7%).

[0036] Example 12: Preparation of dimethyl terephthalate from PET by catalytic depolymerization of ionic liquid BMImCl 1.0 g of PET powder sample, 0.20 g of 1-butyl-3-methylimidazolium chloride (BMImCl), and 8 g of dimethyl carbonate were added to a 25 mL round-bottom flask and stirred at 110 °C for 6 hours. After cooling, dimethyl terephthalate precipitated out, was filtered, and dried to obtain 0.81 g of dimethyl terephthalate (yield 80.2%). The filtrate was distilled under reduced pressure to obtain 0.38 g of ethylene carbonate (yield 82.9%).

[0037] Example 13: Preparation of dimethyl terephthalate from PET by NaCl-catalyzed depolymerization 0.94 g of PET powder sample, 0.06 g of sodium chloride (NaCl), and 8 g of dimethyl carbonate were added to a 25 mL round-bottom flask and stirred at 180 °C for 24 hours. After cooling, dimethyl terephthalate precipitated out, was filtered, and dried to obtain 0.78 g of dimethyl terephthalate (yield 82.1%). The filtrate was distilled under reduced pressure to obtain 0.32 g of ethylene carbonate (yield 74.3%).

[0038] Example 14, Ionic Liquid N 4444 Cl-catalyzed depolymerization of PET to prepare dimethyl terephthalate 0.95 g of PET powder sample and 0.36 g of tetrabutylammonium chloride (N 4444 9 g of dimethyl terephthalate was added to a 25 mL round-bottom flask and stirred at 80 °C for 12 hours. Then, the mixture was cooled, and dimethyl terephthalate precipitated out. The precipitate was filtered and dried to obtain 0.30 g of dimethyl terephthalate (yield 42.2%). The filtrate was distilled under reduced pressure to obtain 0.14 g of ethylene carbonate (yield 38.5%).

[0039] Example 15, Ionic liquid N 4444 Cl-catalyzed depolymerization of PET to prepare dimethyl terephthalate 0.95 g of PET powder sample and 0.25 g of tetrabutylammonium chloride (N 44447 g of dimethyl carbonate was added to a 25 mL round-bottom flask and stirred at 80 °C for 24 hours. Then, the mixture was cooled, and dimethyl terephthalate precipitated out. The precipitate was filtered and dried to obtain 0.55 g of dimethyl terephthalate (yield 57.3%). The filtrate was distilled under reduced pressure to obtain 0.24 g of ethylene carbonate (yield 55.1%).

[0040] Example 16, Ionic Liquid N 4444 Cl-catalyzed depolymerization of PET to prepare diethyl terephthalate 0.94 g of PET powder sample and 0.26 g of tetrabutylammonium chloride (N 4444 10 g of diethyl carbonate was added to a 25 mL round-bottom flask and stirred at 130 °C for 4 hours. Then, the mixture was cooled, and diethyl terephthalate precipitated out. After filtration and drying, 0.96 g of diethyl terephthalate was obtained (yield 88.3%). 1 H NMR (400 MHz, CDCl3, 25 ℃): / ppm = 8.10 (s, 4H), 4.41 (q, 4H, J=7.3), 1.41 (t, 6H, J=7.0); 13 C NMR (101 MHz, CDCl3, 25 ℃): / ppm = 166.02, 134.41, 129.69, 61.60, 14.51. The filtrate was distilled under reduced pressure to give 0.33 g of ethylene carbonate (yield 76.6%).

[0041] Example 17, Ionic Liquid N 4444 Cl-catalyzed depolymerization of PET to prepare diethyl terephthalate 0.99 g of PET powder sample and 0.27 g of tetrabutylammonium chloride (N 4444 12 g of diethyl carbonate was added to a 25 mL round-bottom flask and stirred at 140 °C for 1 hour. Then, the mixture was cooled, and diethyl terephthalate precipitated out. The precipitate was filtered and dried to obtain 0.98 g of diethyl terephthalate (yield 85.6%). The filtrate was distilled under reduced pressure to obtain 0.40 g of ethylene carbonate (yield 88.2%).

[0042] Example 18, Ionic liquid P 4444 Cl-catalyzed depolymerization of PET to prepare diethyl terephthalate 0.97 g of PET powder sample and 0.46 g of tetrabutylphosphonium chloride (P) were added. 444412 g of diethyl carbonate was added to a 25 mL round-bottom flask and stirred at 160 °C for 1 hour. Then, the mixture was cooled, and diethyl terephthalate precipitated out. The precipitate was filtered and dried to obtain 1.05 g of diethyl terephthalate (yield 91.7%). The filtrate was distilled under reduced pressure to obtain 0.42 g of ethylene carbonate (yield 92.6%).

[0043] Example 19: ZnCl2-catalyzed recovery of dimethyl terephthalate from polyester-cotton textiles 1.48 g of PET textile fragments (composition: 65% polyester, 35% cotton fiber), 0.15 g of ZnCl2, and 8 g of dimethyl carbonate were added to a 25 mL round-bottom flask and stirred at 150 °C for 5 hours. The unreacted solid was then separated by filtration, washed, and dried to obtain 0.52 g of solid, which was composed of cellulose. The filtrate was then cooled, and dimethyl terephthalate precipitated out. This precipitate was filtered and dried to obtain 0.94 g of dimethyl terephthalate (yield 96.9%). The filtrate was then distilled under reduced pressure to obtain 0.43 g of ethylene carbonate (yield 97.7%).

[0044] Example 20, Ionic Liquid N 4444 Cl-catalyzed depolymerization and recycling of polyester / cotton textiles to prepare diethyl terephthalate 1.48 g of PET textile (composition: 65% polyester, 35% cotton fiber) fragments, 0.50 g of N 4444 Cl. 15 g of diethyl carbonate was added to a 25 mL round-bottom flask and stirred at 150 °C for 3 hours. The unreacted solid was then separated by filtration, washed, and dried to obtain 0.52 g of solid, which was composed of cellulose. Subsequently, the filtrate was cooled, and diethyl terephthalate precipitated out. This precipitate was filtered and dried to obtain 0.95 g of diethyl terephthalate (yield 97.9%). The filtrate was then distilled under reduced pressure to obtain 0.41 g of ethylene carbonate (yield 93.2%).

[0045] Example 21: Treatment of PET in dimethyl carbonate under catalyst-free conditions 0.96 g of PET powder sample and 9 g of dimethyl carbonate were added to a 25 mL pressure-resistant reactor and stirred at 180 °C for 24 hours until the solids completely disappeared. Then, the mixture was cooled, and dimethyl terephthalate precipitated out. After filtration and drying, 0.94 g of dimethyl terephthalate was obtained (yield 96.9%). The filtrate was distilled under reduced pressure to obtain 0.41 g of ethylene carbonate (yield 93.2%).

[0046] Comparative example: PET treatment in dimethyl carbonate under catalyst-free conditions 0.96 g of PET powder sample and 9 g of dimethyl carbonate were added to a 25 mL round-bottom flask and stirred at 120 °C for 10 hours. The mixture was then cooled, filtered, dried, and 0.96 g of solid sample was recovered. ¹H NMR spectroscopy revealed no product. This indicates that the reaction does not occur under these conditions.

[0047] The present invention has been described in detail above. Those skilled in the art will recognize that the invention can be practiced in a wide range of ways with equivalent parameters, concentrations, and conditions without departing from its spirit and scope, and without requiring unnecessary experiments. While specific embodiments have been provided, it should be understood that further modifications can be made to the invention. In summary, according to the principles of the invention, this application is intended to include any changes, uses, or improvements to the invention, including changes made using conventional techniques known in the art that depart from the scope disclosed herein.

Claims

1. A method for preparing the monomer diester terephthalate by depolymerization of polyethylene terephthalate (PET), comprising the following steps: treating PET or its composite system in diester carbonate under catalyst-free conditions to achieve chemical depolymerization of PET, obtaining PET monomer diester terephthalate and ethylene carbonate, and achieving complete depolymerization of PET in the composite.

2. The method according to claim 1, characterized in that, The carbonate diester is at least one of dimethyl carbonate (DMC) and diethyl carbonate (DEC).

3. The method according to claim 1, characterized in that, The PET and its composite systems include: at least one of all samples, physical objects, and mixed or composite systems containing PET components.

4. The method according to claim 1, characterized in that, The method involves placing PET or its composite system in dicarbonate, heating and reacting for a certain time, cooling, and separating to obtain terephthalic acid diester and ethylene carbonate.

5. The method according to claim 4, characterized in that, The molar ratio of diester to PET structural units is 1:1 to 25:

1.

6. The method according to claim 4, characterized in that, The reaction is carried out at a temperature of 170-250°C for 15-36 hours.

7. The method according to claim 4, characterized in that, The method further includes the following operations: filtering and separating unreacted solids; then cooling to precipitate terephthalic acid diester, filtering to obtain terephthalic acid diester; further, collecting excess diester carbonate and generated ethylene carbonate sequentially by conventional vacuum distillation to recover the catalyst.

Images