Method for alcoholysis of polyethylene glycol terephthalate

Abstract

The invention provides a method for alcoholysis of polyethylene glycol terephthalate, and relates to the technical field of PET degradation. The PET alcoholysis method provided by the invention comprises the following steps: mixing PET, ethylene glycol, a cosolvent and a composite catalyst, and carrying out alcoholysis reaction to obtain bis (hydroxyethyl) terephthalate (BHET). According to the invention, dimethyl sulfoxide is taken as a cosolvent, zinc acetate and 1, 8-diazabicyclo [5.4. 0] undec-7-ene are taken as a composite catalyst, catalysis and solvent effect driving are combined, and ethylene glycol alcoholysis of PET is carried out under the synergistic effect of the composite catalyst and the cosolvent. According to the method, effective decomposition of PET can be achieved at the low temperature (140-155 DEG C) and within the short reaction time (60-100 min), and the yield of BHET is high (66% or above and can reach 80% or above). The method provided by the invention can realize high-efficiency alcoholysis of PET with ethylene glycol.

Description

Technical Field

[0001] The present invention relates to the technical field of PET degradation, in particular to a method for alcoholysis of polyethylene terephthalate. Background Art

[0002] Polyethylene terephthalate (PET) is widely used due to its various favorable properties. However, PET's natural degradation time is very long, making it difficult to remove through environmental purification. Furthermore, the use of PET products is often short-term, and the accumulation of large amounts of waste plastic can cause serious pollution and energy waste. Therefore, the recycling of waste PET is of great significance.

[0003] Chemical methods for recycling PET typically use ethylene glycol (EG) as a substrate for PET degradation (i.e., PET alcoholysis). The product is bis(hydroxyethyl) terephthalate (BHET), which can be used to regenerate PET. Currently, the temperature for PET alcoholysis is typically above 180°C, even exceeding 190°C, which is close to the boiling point of ethylene glycol (197°C, 1 bar). This high-temperature reaction results in high process energy consumption. Some researchers have proposed alcoholyzing PET to BHET under the catalysis of lysine choline. This solution can reduce the alcoholysis reaction temperature to 150°C, but the reaction time is too long (360 minutes), and the BHET yield is low, at only 51%. Summary of the Invention

[0004] In view of this, the present invention aims to provide a method for the alcoholysis of polyethylene terephthalate. The method can effectively decompose PET at a relatively low temperature (140-155°C) and a relatively short reaction time (60-100 minutes), and has a high yield of BHET (above 66%, and can reach over 80%).

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] The present invention provides a method for alcoholysis of polyethylene terephthalate, comprising the following steps:

[0007] Polyethylene terephthalate, ethylene glycol, a cosolvent and a composite catalyst are mixed and subjected to an alcoholysis reaction to obtain bis(hydroxyethyl) terephthalate; the cosolvent is dimethyl sulfoxide; the composite catalyst comprises zinc acetate and 1,8-diazabicyclo[5.4.0]undec-7-ene, and the molar ratio of the zinc acetate to the 1,8-diazabicyclo[5.4.0]undec-7-ene is 1:2; the alcoholysis reaction temperature is 140-155° C., and the reaction time is 60-100 minutes.

[0008] Preferably, the polyethylene terephthalate comprises polyethylene terephthalate waste.

[0009] Preferably, the molar ratio of the polyethylene terephthalate to ethylene glycol is 1:7-10; the molar amount of the polyethylene terephthalate is calculated based on the molar amount of repeating units in the polyethylene terephthalate.

[0010] Preferably, the molar ratio of the cosolvent to polyethylene terephthalate is 13 to 15:1, and the molar amount of the polyethylene terephthalate is calculated based on the molar amount of repeating units in the polyethylene terephthalate.

[0011] Preferably, the mass of the composite catalyst is 2-3% of the mass of polyethylene terephthalate.

[0012] Preferably, the alcoholysis reaction temperature is 150° C. and the time is 90 minutes.

[0013] Preferably, the alcoholysis reaction is carried out under normal pressure.

[0014] Preferably, after the alcoholysis reaction, the obtained reaction solution is subjected to post-treatment, and the post-treatment includes:

[0015] Performing a first hot filtration on the reaction solution to collect a first filtrate;

[0016] The first filtrate is mixed with water and then subjected to a second hot filtration to collect the second filtrate; the temperature of the mixed solution during the second hot filtration is 70 to 85°C;

[0017] The second filtrate is cooled to crystallize, and the obtained crystals are dried to obtain bis(hydroxyethyl) terephthalate.

[0018] Preferably, the first filtrate is mixed with water for 0.5 to 1.5 hours, and the mixing is performed under stirring.

[0019] Preferably, the final temperature of the cooling is 0-4°C.

[0020] The present invention provides a method for alcoholysis of polyethylene terephthalate, comprising the following steps: mixing polyethylene terephthalate, ethylene glycol, a cosolvent, and a composite catalyst for alcoholysis reaction to obtain bis(hydroxyethyl) terephthalate; the cosolvent is dimethyl sulfoxide; the composite catalyst comprises zinc acetate and 1,8-diazabicyclo[5.4.0]undec-7-ene, the molar ratio of zinc acetate to 1,8-diazabicyclo[5.4.0]undec-7-ene being 1:2; and the alcoholysis reaction is carried out at a temperature of 140-155°C and for a time of 60-100 minutes. Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention uses dimethyl sulfoxide as a cosolvent, zinc acetate and 1,8-diazabicyclo[5.4.0]undec-7-ene as a composite catalyst, combines catalysis with solvent effect driving, and performs ethylene glycol alcoholysis of polyethylene terephthalate (PET) under the synergistic action of the composite catalyst and the cosolvent. The method can achieve effective decomposition of PET at a relatively low temperature (140-155° C.) and a relatively short reaction time (60-100 minutes), and has a high yield of BHET (above 66%, which can be above 80%). The present invention can achieve efficient ethylene glycol alcoholysis of polyethylene terephthalate.

[0022] The results of the embodiment show that when dimethyl sulfoxide is used as a cosolvent, zinc acetate and 1,8-diazabicyclo[5.4.0]undec-7-ene are used as a composite catalyst, and the reaction temperature is 150°C for 90 minutes, the conversion rate of PET is 98.60% and the yield of BHET is 80.11%. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The figure is a bar graph showing the conversion rate of PET and the yield of the product BHET in Example 1 and Comparative Examples 1-2;

[0024] Figure 2 is the PET conversion rate and BHET yield of PET alcoholysis under different catalysts in Comparative Example 3;

[0025] Figure 3 The PET conversion rate and BHET yield of PET alcoholysis under different ratios of composite catalysts in Comparative Example 4;

[0026] Figure 4 Non-renewable energy consumption (NREU) of virgin PET and PET waste recovered through PET alcoholysis + PET resynthesis;

[0027] Figure 5 It is the global warming potential (GWP) of virgin PET and PET waste recycled through PET alcoholysis + PET resynthesis. DETAILED DESCRIPTION

[0028] The present invention provides a method for alcoholysis of polyethylene terephthalate, comprising the following steps:

[0029] Polyethylene terephthalate (PET), ethylene glycol (EG), a cosolvent and a composite catalyst are mixed and subjected to an alcoholysis reaction to obtain bis(hydroxyethyl) terephthalate (BHET); the cosolvent is dimethyl sulfoxide (DMSO); the composite catalyst comprises zinc acetate (Zn(OAc)2) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU); the molar ratio of zinc acetate to 1,8-diazabicyclo[5.4.0]undec-7-ene is 1:2; the alcoholysis reaction temperature is 140-155°C, and the reaction time is 60-100 minutes.

[0030] In the present invention, unless otherwise specified, all raw materials involved are commercially available products well known in the art.

[0031] In the present invention, the polyethylene terephthalate preferably comprises polyethylene terephthalate waste. In the present invention, the polyethylene terephthalate is preferably in the form of powder; and the number average molecular weight of the polyethylene terephthalate is preferably 20,000 to 30,000.

[0032] In the present invention, the molar ratio of polyethylene terephthalate to ethylene glycol is preferably 1:7 to 10, and can be 1:7, 1:8, 1:9 or 1:10, preferably 1:8; the molar amount of the polyethylene terephthalate is based on the repeating units (i.e., C 10 The molar amount of ethylene glycol (H8O4) is calculated based on the amount of cosolvent (cosolvent: ethylene glycol). Because the alcoholysis of polyethylene terephthalate (PET) is a two-phase solid-liquid transesterification reaction, an appropriate amount of ethylene glycol must be used. Too little ethylene glycol cannot penetrate the surface of PET. However, if too much ethylene glycol is used, the ethylene glycol itself will undergo dimerization, which will increase ethylene glycol adsorption, and the dimerization of ethylene glycol will also affect the recombination reaction of BHET. The present invention controls the molar ratio of PET to ethylene glycol within the above range, which not only reduces the amount of ethylene glycol used and improves the economic efficiency of the process, but also reduces the production of single crystal ethylene glycol as a byproduct. In the present invention, the molar ratio of the cosolvent to PET is preferably 13 to 15:1, and can be 13:1, 14:1, or 15:1, more preferably 14:1. The molar amount of PET is calculated based on the molar amount of repeating units in PET. In the present invention, adding too much or too little co-solvent will result in a decrease in BHET yield.

[0033] In the present invention, the composite catalyst is an organic base composite catalyst, and the mass of the composite catalyst (denoted as Zn(OAc)2 / DBU) is preferably 2-3% of the mass of polyethylene terephthalate, and can be 2%, 2.5% or 3%.

[0034] In the present invention, polyethylene terephthalate, ethylene glycol, a cosolvent and a composite catalyst are preferably added into a reaction vessel equipped with a condenser, heated to a specified temperature, and the reaction device is placed in an oil bath to carry out an alcoholysis reaction.

[0035] In the present invention, the alcoholysis reaction (also referred to as glycolysis reaction) is carried out at a temperature of 140 to 155° C., preferably 145 or 150° C., and preferably 150° C., for a time of 60 to 100 min, preferably 60, 70, 80, or 90 min, and preferably 90 min. In the present invention, the alcoholysis reaction is preferably carried out under normal pressure.

[0036] After the alcoholysis reaction, the present invention preferably further comprises post-processing the obtained reaction solution, and the post-processing preferably comprises:

[0037] Performing a first hot filtration on the reaction solution to collect a first filtrate;

[0038] The first filtrate is mixed with water and then subjected to a second hot filtration to collect the second filtrate; the temperature of the mixed solution during the second hot filtration is 70 to 85°C;

[0039] The second filtrate is cooled to crystallize, and the obtained crystals are dried to obtain bis(hydroxyethyl) terephthalate.

[0040] In the present invention, the first hot filtration specifically involves filtering the reaction liquid obtained from the alcoholysis reaction while it is hot; the purpose of the first hot filtration is to remove unreacted PET. In the present invention, the water is preferably deionized water. A large amount of water is preferably added to the first filtrate, with the volume ratio of water to the first filtrate preferably being 10-12:1. In the present invention, the mixing time of the first filtrate and water is preferably 0.5-1.5 hours, and can be 1 hour, and the mixing is preferably performed under stirring. The temperature of the mixed liquid during the second hot filtration is preferably 70-85°C, and can be 80°C. In the present invention, the purpose of the second hot filtration is to separate water-insoluble oligomers. In the present invention, the final cooling temperature is preferably 0-4°C, and the second filtrate can be cooled in a refrigerator. The crystallization time is not particularly critical; sufficient crystallization is sufficient, and can be 24 hours. The drying method and conditions are not particularly critical; drying in a vacuum oven at 60-70°C overnight is acceptable.

[0041] The present invention combines catalysis with solvent-driven reactions, using a composite catalyst and a cosolvent to perform glycolysis of PET. This method effectively decomposes PET at relatively low temperatures (140-155°C) and short reaction times (60-100 minutes), while producing a high yield of BHET (over 66%, potentially exceeding 80%). This new alcoholysis reaction system efficiently depolymerizes PET waste into BHET, enabling PET recycling. This method improves the energy efficiency and cost-effectiveness of PET degradation, offering a more energy-efficient and efficient degradation method.

[0042] After obtaining bis(hydroxyethyl) terephthalate (i.e., a degradation product of PET), the obtained bis(hydroxyethyl) terephthalate (BHET) can be used for the resynthesis of PET, thereby recycling PET waste into new PET. In the present invention, the resynthesis (or repolymerization) is carried out in three stages, specifically including the following steps:

[0043] Mixing the bis(hydroxyethyl) terephthalate and antimony trioxide under heating to obtain a mixture;

[0044] Pre-condensing the mixture to obtain low molecular weight PET;

[0045] The low molecular weight PET is subjected to polycondensation to obtain a solid product PET (i.e. recycled PET, denoted as rPET)

[0046] In the present invention, the entire resynthesis process is carried out in a nitrogen environment, and the reaction vessel used for the resynthesis must be equipped with a mechanical stirrer, a cooler, and an ethylene glycol receiving device. In the present invention, antimony trioxide is used as a catalyst, and the mass of the antimony trioxide is preferably 0.1% of the mass of BHET. The heating temperature is preferably 220°C, and the mixing time is preferably 20 minutes. The mixing is preferably carried out under stirring conditions. During the mixing process, BHET melts, changing from a solid state to a liquid state, and the liquid BHET and antimony trioxide are thoroughly mixed.

[0047] In the present invention, the pre-condensation temperature is preferably 260° C., preferably for 2 hours, and the pre-condensation is preferably carried out under a vacuum condition of -0.05 MPa to -0.06 MPa (vacuum gauge reading); performing the pre-condensation first can minimize the loss of BHET under vacuum conditions compared to the one-step polycondensation.

[0048] In the present invention, the polycondensation temperature is preferably 270° C., and the time is preferably 4 hours; and the polycondensation is preferably carried out under a vacuum condition of -0.1 MPa (vacuum gauge reading).

[0049] Compared with traditional processes, the PET alcoholysis + PET resynthesis process provided by the present invention can significantly reduce energy consumption; in terms of carbon footprint, compared with virgin PET, the greenhouse gas emissions of the PET alcoholysis + PET resynthesis provided by the present invention are significantly reduced.

[0050] In order to further illustrate the present invention, the method for alcoholysis of polyethylene terephthalate provided by the present invention is described in detail below with reference to examples, but they should not be construed as limiting the scope of protection of the present invention.

[0051] Example 1

[0052] PET glycol alcoholysis:

[0053] 1g (1eq) of PET powder (50μm particle size, 30,000 number-average molecular weight), 2.35mL of ethylene glycol (EG, 8eq), 5.2mL of cosolvent (DMSO, 14eq), and 3% (mass percentage relative to the mass of the PET powder) of a composite catalyst (Zn(OAc)2) and DBU (molar ratio 1:2) were added to a round-bottom flask equipped with a condenser. The mixture was heated to 150°C and placed in an oil bath for alcoholysis. After 90 minutes of reaction at 150°C, the PET was essentially completely degraded. After completion of the reaction, the hot reaction solution was filtered; 80mL of 80°C hot water was added to the filtrate, and stirring was continued for 1 hour. A second hot filtration (80°C mixture) was then performed. The filtrate was collected and placed in a 0°C refrigerator for 24 hours for crystallization. The resulting crystals were then filtered and dried in a vacuum oven at 60°C overnight to yield white, needle-shaped BHET product.

[0054] The calculation formulas for the conversion rate of PET and the yield of product BHET are as follows:

[0055]

[0056] Among them, C PET is the conversion rate of PET, Y BHET is the yield of product BHET, W PET.0 is the initial weight of PET, W PET is the weight of unreacted PET, W BHET is the weight of BHET, M BHET is the molecular weight of BHET (254 g / mol), M PET is the molecular weight of the PET repeating unit (192 g / mol).

[0057] Example 2

[0058] The molar ratio of PET to EG was varied by changing the amount of ethylene glycol (EG). The remaining conditions were the same as in Example 1, with 1 g (1 eq) of PET powder. The conversion rates and BHET yields of PET glycol hydrolysis at different PET to EG molar ratios are shown in Table 1. The molar amount of PET was calculated based on the molar amount of the repeating unit in PET.

[0059] Table 1 Conversion rate and BHET yield of PET glycol hydrolysis at different PET to EG molar ratios

[0060] PET:EG (molar ratio) PET conversion rate% BHET yield % 1:6 96.39 66.31 1:7 93.54 67.65 1:8 98.60 80.11 1:9 95.57 72.16 1:10 94.95 67.49

[0061] Example 3

[0062] The molar ratio of the cosolvent to PET was varied by changing the amount of cosolvent (DMSO). The remaining conditions were the same as in Example 1, with 1 g (1 eq) of PET powder. The conversion rates and BHET yields of PET glycol hydrolysis at different cosolvent to PET molar ratios are shown in Table 2. The molar amount of PET was calculated based on the molar amount of the repeating units in PET.

[0063] Table 2 Conversion rate and BHET yield of PET glycol hydrolysis under different cosolvent to PET molar ratios

[0064] PET:DMSO (molar ratio) PET conversion rate% BHET yield % 1:12 95.94 62.32 1:13 96.14 66.44 1:14 98.60 80.11 1:15 97.90 66.15 1:16 96.83 64.87

[0065] The addition of dimethyl sulfoxide significantly increased the reaction activity, raising the final BHET yield (at a PET:DMSO ratio of 1:14) to 80.11%. Furthermore, if the amount of cosolvent remained below the initial level for complete PET decomposition (at a PET:DMSO ratio of 1:14), the catalytic activity of the composite catalytic reaction system decreased. With further increases in the DMSO content, the BHET yield gradually decreased.

[0066] Example 4

[0067] The reaction temperature was changed, and the rest was the same as in Example 1. The conversion rate and BHET yield of PET glycol alcoholysis at different reaction temperatures are shown in Table 3.

[0068] Table 3 Conversion rate and BHET yield of PET glycol alcoholysis at different reaction temperatures

[0069] Reaction temperature ℃ PET conversion rate% BHET yield % 130 93.52 65.14 140 93.65 69.32 150 98.60 80.11 160 99.14 69.55 170 95.99 67.81

[0070] Example 5

[0071] The reaction time was changed, and the rest was the same as in Example 1. The conversion rate and BHET yield of PET glycol alcoholysis under different reaction times are shown in Table 4.

[0072] Table 4 Conversion rate and BHET yield of PET glycol alcoholysis at different reaction times

[0073] Time PET conversion rate% BHET yield % 0.5 94.74 67.01 1 94.63 73.68 1.5 98.60 80.11 2 96.63 67.95 2.5 96.31 64.12

[0074] Example 6

[0075] The conversion rate and BHET yield of PET glycol alcoholysis were shown in Table 5 when the catalyst dosage was changed (mass percentage relative to PET powder).

[0076] Table 5 Conversion rate and BHET yield of PET glycol alcoholysis under different catalyst dosages

[0077]

[0078]

[0079] Comparative Example 1

[0080] PET glycol alcoholysis:

[0081] 1g (1eq) of PET powder (50μm particle size, number-average molecular weight 30,000), 5mL of ethylene glycol (EG), and 3% (mass percentage relative to the mass of the PET powder) of a composite catalyst (Zn(OAc)2:DBU in a 1:2 molar ratio) were added to a round-bottom flask equipped with a condenser. The mixture was heated to 150°C and placed in an oil bath for alcoholysis. The reaction was carried out at 150°C for 90 minutes. After completion, the hot reaction solution was filtered. 80mL of hot water was added to the filtrate, and stirring was continued at 80°C for 1 hour. A second hot filtration was then performed. The filtrate was collected and placed in a refrigerator at 0°C for 24 hours to crystallize. The resulting crystals were then filtered and dried in a vacuum oven at 60°C overnight to yield white, needle-shaped BHET product.

[0082] Comparative Example 2

[0083] Comparison of co-solvent effects:

[0084] There seems to be a lack of in-depth research on the mechanisms involved in PET dissolution, which limits the choice of co-solvents. In order to make the solvent-assisted depolymerization process more efficient, the Hansen Solubility Parameter (HSP) theory was used to study the selectivity of solvents in PET by selecting a series of solvents that are essentially chemically inert to the environment during the depolymerization process.

[0085] For amorphous PET, Ra (Hansen Distance) < 9.0 MPa 1 / 2It is considered to be a good potential solvent. According to the prediction, polymer-solvent pairing systems with similar HSP can be miscible. However, for the PET (polyethylene terephthalate) solvent system, this prediction method has certain limitations. This is because the prediction method is mainly applicable to amorphous and non-polar polymers, and the prediction for semi-crystalline and polar materials may not be accurate enough. If the Ra value of the solvent is less than 11.0 MPa 1 / 2 , it is considered possible to react with PET. According to HSP, the molecular solvents with the highest affinity for PET are cyclic ketones and ethers, followed by strong dipolar, non-protonated solvents. However, there is no obvious interaction between PET and non-polar solvents (such as cyclohexane) and polar protonated solvents (such as alcohols, ethylene glycol and water). Solvents with boiling points above 150 ° C were selected for this experiment, including ketones (such as γ-valerolactone, Ra = 5.06 MPa 1 / 2 ), linear ethers and cyclic ethers (such as anisole, Ra = 7.58 MPa 1 / 2 ; Dimethyl isosorbide (DMI), Ra = 5.38 MPa 1 / 2 ) and aprotic dipolar solvents (such as dimethyl sulfoxide (DMSO), Ra = 8.69 MPa 1 / 2 Although the Ra values of halogenated solvents met the threshold requirements, they were not included in the initial screening due to their low boiling points and environmental, health, and safety concerns. Table 6 shows the results of the theoretical solvent screening tool.

[0086] Table 6 Results of the Solvent Theoretical Screening Tool

[0087]

[0088] The experimental method for comparing the co-solvent effect is as follows:

[0089] The amount of EG was fixed at 8 eq, the amount of cosolvent was fixed at 14 eq, the amount of composite catalyst was fixed at 3 wt %, and the DMSO in Example 1 was replaced by anisole, dimethyl isosorbide (DMI), and γ-valerolactone, respectively. The rest was the same as in Example 1.

[0090] Figure 1 The figure is a bar graph showing the conversion rate of PET and the yield of BHET in Example 1 and Comparative Examples 1 to 2. Figure 1 EG represents Comparative Example 1, DMSO represents Example 1, and the rest represent the experiments under different cosolvents in Comparative Example 2. Figure 1 The corresponding data are listed in Table 7.

[0091] Table 7 Conversion rate of PET and yield of product BHET in Example 1 and Comparative Examples 1-2

[0092]

[0093] Depend on Figure 1 As can be seen from Table 7, at 150°C, when only EG was present in the liquid phase of the system, the reaction yield did not exceed 50%. However, when DMSO was added, PET was essentially completely depolymerized within 90 minutes. This is likely due to the polymer swelling in the cosolvent facilitating the migration of the nucleophile (ethylene glycol) and catalyst into the polymer matrix, thereby promoting their diffusion and enabling better access to the ester groups (active sites).

[0094] Comparative Example 3

[0095] Different catalysts, including 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), DBU, Zn(OAc)2 / TBD (i.e., Zn(OAc)2+TBD), and Zn(OAc)2 / DBU (i.e., Zn(OAc)2+DBU), were used to decompose PET ethylene glycol at 150°C for 90 min. The reaction conditions were PET (1 eq), EG (8 eq), cosolvent (DMSO, 14 eq), and catalyst (n(Zn(OAc)2) / n(organic base) = 1:1) at 3 wt%.

[0096] Figure 2 The PET conversion rate and BHET yield of PET alcoholysis under different catalysts in Comparative Example 3 are listed in Table 8. It can be seen that the PET alcoholysis using Zn(OAc)2 / DBU as a catalyst has the highest PET conversion rate and the highest BHET yield.

[0097] Table 8 PET conversion and BHET yield of PET alcoholysis under different catalysts

[0098] catalyst Conversion Rate % BHET yield % TBD 96.07 65.06 DBU 95.55 67.9 <![CDATA[Zn(OAc)2+TBD]]> 97.13 71.5 <![CDATA[Zn(OAc)2+DBU]]> 98.24 72.09

[0099] Comparative Example 4

[0100] Zn(OAc)2 / TBD and Zn(OAc)2 / DBU were used as catalysts respectively, and the molar ratios of Zn(OAc)2 to TBD and Zn(OAc)2 to DBU were changed (2:1, 1:1, 1:2, 1:3). PET ethylene glycol was decomposed at 150°C for 90 min. The reaction conditions were PET (1eq), EG (8eq), co-solvent (DMSO, 14eq), and catalyst 3wt%.

[0101] Figure 3The PET conversion rate and BHET yield of PET alcoholysis under different ratios of composite catalysts in Comparative Example 4 are shown in Table 9. The specific data are as follows. It can be seen that the two organic bases, TBD and DBU, exhibit different acid-base properties and catalytic effects in the catalytic process. TBD, as an organic base that is more alkaline than DBU, has a lower catalytic effect on the reaction than DBU. This may be attributed to the structural characteristics of DBU, which enables it to form a more effective active complex with zinc acetate during the catalytic process, thereby improving the catalytic efficiency. In addition, the moderate alkalinity of DBU can better support the alcoholysis reaction at low temperatures without over-catalyzing the formation of side reactions. The addition of Zn(OAc)2 makes the composite catalyst more effective, and this result is explained by the Lewis acid of zinc, which interacts closely with carbon atoms and enhances the electropositivity of carbon. Zinc acetate, as a Lewis acid, through coordination, Zn 2+ Combined with the carbonyl oxygen atom in the PET chain, it enhances the electronegativity of the carbonyl group and makes the carbonyl carbon more susceptible to nucleophilic attack. In addition, in the alcoholysis reaction, Zn 2+ The coordination with the carbonyl oxygen stabilizes the tetrahedral transition state and accelerates the reaction process. DBU, as a strong organic base, deprotonates the hydroxyl group in ethylene glycol to produce a more nucleophilic alkoxy anion (RO - ), thereby accelerating the nucleophilic attack, forming a new ester bond and releasing the monomer. Figure 3 As shown, the reaction was carried out at 150 °C for 90 min, and the maximum BHET yield could be obtained when the molar ratio of Zn(OAc)2 to DBU in the composite catalyst Zn(OAc)2 / DBU was 1:2.

[0102] Table 9 PET conversion and BHET yield of PET alcoholysis under different ratios of composite catalysts

[0103]

[0104] The present invention has developed a novel catalytic alcoholysis reaction system that efficiently depolymerizes PET waste into BHET, thereby enabling PET recycling. By using DMSO as a cosolvent and using a Zn(OAc)2 / DBU composite catalyst, PET can be effectively decomposed at a relatively low temperature (150°C) and a short reaction time (90 minutes), achieving a PET yield of 98.60% and a BHET yield of 80.11%. Table 10 below compares the reaction conditions and catalytic activity of ethylene glycol alcoholysis of PET using different catalysts in related art. The present invention demonstrates a more efficient ethylene glycol alcoholysis of PET.

[0105] Table 10 Comparison of reaction conditions and catalytic activity of ethylene glycol hydrolysis of PET in related technologies

[0106]

[0107] Process sustainability assessment: The closed-loop recycling process was evaluated using the life cycle assessment (LCA) method, mainly selecting global warming potential (GWP) and non-renewable energy consumption (NREU) as key environmental indicators, and compared with the current industrial recycling process. Figure 4 and Figure 5 The non-renewable energy consumption (NREU) and global warming potential (GWP) of virgin PET (petroleum-based PET) and post-consumer PET waste recycled by PET alcoholysis + PET resynthesis, of which the recycling method of PET alcoholysis + PET resynthesis ( Figure 4 and Figure 5 In case PET), the method is as follows:

[0108] Mechanical shredding: PET bottle flakes are ultrasonically cleaned with ethylene glycol, dried, and then shredded with liquid nitrogen;

[0109] Ethylene glycol hydrolysis: The PET powder obtained by mechanical pulverization was subjected to ethylene glycol hydrolysis. The hydrolysis method and process conditions were the same as those in Example 1.

[0110] Repolymerization: The BHET obtained from the glycolysis is repolymerized using antimony trioxide as a polymerization catalyst, with the mass of the catalyst being 0.1% of the mass of the BHET. The first stage of the reaction involves melting the BHET with stirring at 220°C for 20 minutes, converting the solid BHET into a liquid, where the liquid BHET and the catalyst are thoroughly mixed. The second stage is a pre-condensation process, in which the BHET is pre-condensed at 260°C under a vacuum of -0.05 MPa for 2 hours to form low-molecular-weight PET. The third step is a polycondensation reaction: the low-molecular-weight PET is maintained at 270°C under a vacuum of -0.1 MPa for an additional 4 hours to obtain the regenerated PET product.

[0111] Figure 4 and Figure 5Data analysis results show that the traditional petroleum-based virgin PET production process consumes a lot of energy, up to 90MJ / kg. The decomposition process proposed in the present invention (NREU=31.92MJ / kg) significantly reduces energy consumption. Compared with petroleum-based production of PET, energy consumption is reduced by 64.54%. In terms of carbon footprint, the emissions from the production of virgin PET are 4.99 kg CO2 equivalent / kg. Studies have shown that the depolymerization and repolymerization stages of PET are the main sources of greenhouse gas emissions, which is mainly attributed to the consumption of electricity and fuel. In contrast, the emissions of BHET produced using the waste PET recycling route are 3.13 kg CO2 equivalent / kg, which is a 37.32% reduction in greenhouse gas emissions compared to virgin PET, indicating that PET recycling has great potential in achieving carbon neutrality and low carbon.

[0112] As demonstrated in the above examples, the present invention enables alcoholysis of PET at relatively low temperatures, using a low-cost co-solvent. Furthermore, LCA analysis demonstrates that the present method can reduce the carbon footprint and improve the energy efficiency of PET waste recycling. This invention provides a novel approach to addressing the high energy consumption of PET recycling technology.

[0113] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation thereto. It should be noted that those skilled in the art may make various improvements and modifications without departing from the principles of the present invention, and such improvements and modifications shall also be considered within the scope of protection of the present invention.

Claims

1. A method for alcoholysis of polyethylene terephthalate, characterized in that: The following steps are involved: Polyethylene terephthalate, ethylene glycol, a cosolvent and a composite catalyst are mixed and subjected to an alcoholysis reaction to obtain bis(hydroxyethyl) terephthalate; the cosolvent is dimethyl sulfoxide; the composite catalyst comprises zinc acetate and 1,8-diazabicyclo[5.4.0]undec-7-ene, and the molar ratio of the zinc acetate to the 1,8-diazabicyclo[5.4.0]undec-7-ene is 1:2; the alcoholysis reaction temperature is 140-155° C., and the reaction time is 60-100 minutes.

2. The method according to claim 1, characterized in that The polyethylene terephthalate includes polyethylene terephthalate waste.

3. The method according to claim 1, characterized in that The molar ratio of the polyethylene terephthalate to ethylene glycol is 1:7-10, and the molar amount of the polyethylene terephthalate is calculated based on the molar amount of the repeating units in the polyethylene terephthalate.

4. The method according to claim 1 or 3, characterized in that The molar ratio of the cosolvent to polyethylene terephthalate is 13 to 15:1, and the molar amount of the polyethylene terephthalate is calculated based on the molar amount of the repeating units in the polyethylene terephthalate.

5. The method according to claim 1, wherein The mass of the composite catalyst is 2-3% of the mass of polyethylene terephthalate.

6. The method according to claim 1, characterized in that The temperature of the alcoholysis reaction is 150° C. and the time is 90 minutes.

7. The method according to claim 1 or 6, characterized in that The alcoholysis reaction is carried out under normal pressure.

8. The method according to claim 1, characterized in that After the alcoholysis reaction, the obtained reaction solution is subjected to post-treatment, and the post-treatment includes: Performing a first hot filtration on the reaction solution to collect a first filtrate; The first filtrate is mixed with water and then subjected to a second hot filtration to collect the second filtrate; the temperature of the mixed solution during the second hot filtration is 70 to 85°C; The second filtrate is cooled to crystallize, and the obtained crystals are dried to obtain bis(hydroxyethyl) terephthalate.

9. The method according to claim 8, characterized in that The first filtrate is mixed with water for 0.5 to 1.5 hours, and the mixing is performed under stirring.

10. The method according to claim 8, characterized in that The final temperature of the cooling is 0-4°C.

Images