Composite catalyst for pet alcoholysis and use thereof

Abstract

The present application belongs to the field of polymer material recycling, and particularly relates to a composite catalyst for PET alcoholysis and use thereof. The present application prepares a new composite catalyst by skillfully regulating the ratio of polyethylene glycol and titanium sulfate, which is suitable for the process of degrading PET with mono-alcohol and poly-alcohol. By fine regulation of the reaction conditions, high conversion rate of PET is achieved, and the highest conversion rate can reach 100%. When using ethanol as the alcoholysis agent, the yield of the main product DET can reach 95%. When using methanol as the alcoholysis agent, the yield of the main product DMT can reach 93%. When using ethylene glycol as the alcoholysis agent, the yield of the main product BHET can reach 92% at the highest, and the content of the byproduct diethylene glycol is 0. This shows that the composite catalyst of the present application can completely inhibit the occurrence of side reactions. The results show that the composite catalyst of the present application can achieve efficient and clean catalytic reaction, and avoid unnecessary side reactions.

Description

Technical Field

[0001] This invention belongs to the field of polymer material recycling, and in particular relates to a composite catalyst for PET alcoholysis and its applications. Background Technology

[0002] Polyethylene terephthalate (PET) is a high-performance polyester material, renowned for its high transparency and excellent impact resistance. PET is easy to process and mold, making it particularly suitable for producing thick-walled, transparent plastic products. This material is not only suitable for packaging auxiliary materials and product components, but can also be used to manufacture a variety of plastic products on its own. PET's applications extend beyond traditional manufacturing, encompassing the production of food, medical, cosmetic, and electronic products. Despite its wide range of uses, the recycling and reuse of PET waste remains a challenge. Industrial recycling processes require high levels of waste cleanliness and have limited recyclability. In China, PET recycling primarily employs thermophysical methods, with the recycled PET mainly used to manufacture polyester fibers and PET cable ties. However, physically recycled PET is prone to aging and performance degradation, limiting its use to lower-end applications. To improve the efficiency and quality of PET recycling, chemical recycling methods, such as methanol hydrolysis, hydrolysis, and ethylene glycol hydrolysis, are being actively researched. These methods can degrade PET into monomers, offering advantages in terms of environmental friendliness and sustainability.

[0003] To accelerate the reaction rate, a certain amount of catalyst is usually added during the alcoholysis of PET. Currently, zinc acetate has the best catalytic performance (reaction temperature 170-190℃, reaction time 3-5h, yield 85-90%). However, the addition of catalysts brings new problems. For example, most alcoholysis catalysts have a catalytic polymerization effect on alcoholysis agents (methanol, ethanol, ethylene glycol). At high temperatures, alcohols are prone to dehydration to form ethers, diethylene glycol, and other substances, which is very detrimental to the subsequent separation and purification of alcoholysis products. This indirectly leads to the monomers failing to meet standards in subsequent esterification / transesterification synthesis. Furthermore, these alcoholysis catalysts suffer from difficulties in separation, unreliable catalyst recycling efficiency, and the impact of catalyst residue on monomer quality.

[0004] Patent application CN114849714A discloses a method for preparing a solid base catalyst for the alcoholysis of PET: magnesium chloride, β-alanine, and nickel chloride are dissolved in deionized water to obtain a mixed aqueous solution; sodium hydroxide and sodium bicarbonate are dissolved in deionized water to obtain an alkaline solution; the alkaline solution is added to the mixed aqueous solution, and the mixture is stirred to obtain a suspension. The suspension is stirred to obtain a reaction solution containing magnesium-nickel bimetallic hydroxide. After solid-liquid separation, the solid fraction is washed and vacuum dried to obtain a solid base catalyst precursor. The solid base catalyst precursor is then calcined to obtain the final catalyst. However, this catalyst suffers from problems such as high metal content, a high risk of metal ion leakage, and low product purity.

[0005] Patent CN115055175A discloses a method for preparing defective zinc oxide nanosheet catalysts for the alcoholysis of PET: zinc chloride and a surfactant are added to a first mixed solvent, stirred, and reacted to obtain a mixed reaction solution containing Zn-CTAB. An aqueous ethanolamine solution and a second mixed solvent are then added to the Zn-CTAB mixed reaction solution. After stirring, solid-liquid separation, and other steps, a defective ZnO nanosheet precursor is obtained, which is then calcined to obtain the catalyst. The defective zinc oxide nanosheets prepared by this invention can be used as a catalyst for the alcoholysis of waste PET. However, the preparation process of the above catalyst is relatively complex, energy-intensive, and has a high metal content, and the metal overflow affects the purity of the alcoholysis products.

[0006] Chinese invention patent application CN116981725A, entitled "Integrated Chemical and Biological Degradation Method for Recycling Polyethylene Terephthalate (PET)," describes a method that uses glycolysis to depolymerize PET into oligomers, with betaine acting as a catalyst. However, the catalyst generates some byproducts, the reaction conditions are harsh, and the conversion rate needs improvement.

[0007] In summary, although existing catalysts have demonstrated certain activity and selectivity in the alcoholysis of PET, they still face several challenges. These challenges include the harsh conditions under which most reactions occur at high temperatures and pressures; severe side reactions due to the catalytic polymerization effect of the alcoholysis agent; a significant risk of metal ion spillage, affecting product purity; and the complexity and high energy consumption of the catalyst preparation process. These problems limit further improvements in catalyst performance and the expansion of its application range. Therefore, providing a simple preparation method for a PET alcoholysis catalyst that exhibits high selectivity, low pollution, and operates under mild reaction conditions has become a critical issue that urgently needs to be addressed in current research. The advent of such a catalyst is expected to overcome the limitations of existing technologies and provide a more efficient and sustainable solution for the green chemical conversion of PET. Summary of the Invention

[0008] To address the problems existing in the prior art, this invention proposes a composite catalyst for PET alcoholysis and its applications. The composite catalyst of this invention is composed of PEG and titanium sulfate, with a mass ratio of PEG to titanium sulfate of 1:(0.5-5). This composite catalyst achieves a 100% PET alcoholysis conversion rate. When using ethanol as the alcoholysis agent, the yield of the main product DET can reach 95%; when using methanol as the alcoholysis agent, the yield of the main product DMT can reach 93%; and when using ethylene glycol as the alcoholysis agent, the yield of the main product BHET can reach up to 92%, while the content of the byproduct diethylene glycol is 0.

[0009] Furthermore, the molecular weight of the PEG is 300-600.

[0010] The present invention also provides an application of the aforementioned composite catalyst in catalyzing the alcoholysis of PET, specifically including the following steps:

[0011] PET and the alcoholysis agent were placed in a mechanically stirred high-pressure reactor. The composite catalyst was added, and N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor 3-4 times. The reactor was then sealed and mechanical stirring was started. The temperature was raised to 130-180℃, the reaction pressure was controlled at 0.2-1.6 MPa, the heating rate was 2-10℃ / min, and the reaction time was 1-5 h. After the reaction was completed, the temperature was lowered to room temperature. The liquid after the reaction was taken for high-performance liquid chromatography analysis to quantitatively analyze the content of the main product and by-reaction products in the liquid. The filter residue after the reaction was incompletely degraded PET. The filter residue was weighed, and the PET conversion rate was calculated to be up to 100%, the main product yield to be up to 95%, and the diethylene glycol content generated by the by-reaction was 0-0.2%.

[0012] Furthermore, the mass ratio of PET to alcoholysis agent is 1:(4-7).

[0013] Furthermore, the alcoholysis agent is any one of ethanol, methanol, or ethylene glycol.

[0014] Furthermore, based on the mass of the PET, the amount of composite catalyst used is 0.5-3%wt.

[0015] Furthermore, the mechanical stirring speed is 200-800 rpm.

[0016] Furthermore, the mass ratio of PEG to titanium sulfate in the composite catalyst is 1:(0.5-5).

[0017] More preferably, the mass ratio of PEG to titanium sulfate in the composite catalyst is 1:(1-3).

[0018] Furthermore, when the alcoholysis agent is ethanol, the main product is diethyl terephthalate (DET).

[0019] Furthermore, when the alcoholysis agent is methanol, the main product is dimethyl terephthalate (DMT).

[0020] Furthermore, when the alcoholysis agent is ethylene glycol, the main product is bis(2-hydroxyethyl) terephthalate (abbreviated as BHET).

[0021] The beneficial effects of this invention are as follows:

[0022] 1. This invention prepares a novel composite catalyst by combining existing catalysts polyethylene glycol and titanium sulfate and cleverly controlling their ratio. It was found that the composite catalyst unexpectedly exhibits excellent catalytic performance in the catalytic alcoholysis of PET. It not only effectively prevents the formation of the byproduct diethylene glycol and improves the selectivity of the reaction, but also significantly increases the yield of the main product, providing high-quality monomers for possible subsequent esterification / transesterification synthesis. At the same time, it significantly reduces the alcoholysis temperature in terms of reaction conditions, making industrialization feasible and possessing important industrial application value.

[0023] 2. By precisely controlling the ratio of polyethylene glycol and titanium sulfate, this invention unexpectedly achieves a high conversion rate of PET, up to 100%. When ethanol is used as the alcoholysis agent, the yield of the main product DET can reach 95%; when methanol is used as the alcoholysis agent, the yield of the main product DMT can reach 93%; and when ethylene glycol is used as the alcoholysis agent, the yield of the main product BHET can reach up to 92%. At the same time, the content of the byproduct diethylene glycol is 0. This indicates that the composite catalyst of this invention can completely suppress the occurrence of side reactions. This result shows that the composite catalyst of this invention can achieve a highly efficient and clean catalytic reaction, avoiding unnecessary side reactions.

[0024] 3. The reaction conditions for alcoholysis of PET using the composite catalyst of the present invention have been significantly optimized, with both reaction temperature and catalyst dosage reduced. These improvements not only reduce energy consumption and production costs, but also increase production efficiency and create significant economic benefits.

[0025] 4. The composite catalyst of the present invention has a simple preparation process. Due to its low metal content, the risk of metal ion spillover is reduced, thereby effectively improving the purity of the alcoholysis products. This not only improves the quality of the final product, but also enhances its competitiveness in the market and meets the market demand for high-quality products. Attached Figure Description

[0026] Figure 1 High-performance liquid chromatogram of DET standard sample

[0027] Figure 2 The high-performance liquid chromatogram of the ethanol-hydrolyzed PET product in Example 4 is shown below.

[0028] Figure 3 High-performance liquid chromatogram of DMT standard sample

[0029] Figure 4 The high-performance liquid chromatogram of the methanol alcoholysis PET product of Example 11 is shown below.

[0030] Figure 5 High-performance liquid chromatogram of BHET standard sample

[0031] Figure 6 The high-performance liquid chromatogram of the ethylene glycol alcoholysis PET product from Example 12 is shown below.

[0032] In a high-performance liquid chromatogram, the horizontal axis represents time (min) and the vertical axis represents peak height (mAu). Detailed Implementation

[0033] To better understand the technical solution of the present invention, the following detailed embodiments are provided to further illustrate the above-mentioned content of the present invention. However, this should not be construed as limiting the scope of the subject matter of the present invention to the following examples. All technologies implemented based on the above-mentioned content of the present invention fall within the scope of the present invention.

[0034] Example 1

[0035] 50g of PET and 300g of ethanol were placed in a 1L mechanically stirred high-pressure reactor. 0.33g of PEG300 and 0.17g of titanium sulfate were added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400 rpm. The temperature was raised to 140°C, the reaction pressure was controlled at 0.9 MPa, the heating rate was 4°C / min, and the reaction time was 2 hours. After the reaction was completed, the mixture was cooled to room temperature. The resulting liquid was analyzed by high-performance liquid chromatography (HPLC) to quantitatively analyze the DET and diethylene glycol contents. The filter residue was weighed, and the PET conversion rate was calculated to be 93%, the main product DET yield was 76%, and the diethylene glycol content generated by the side reaction was 0%.

[0036] Example 2

[0037] 50g of PET and 300g of ethanol were placed in a 1L mechanically stirred high-pressure reactor. 0.25g of PEG300 and 0.25g of titanium sulfate were added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400 rpm. The temperature was raised to 140°C, the reaction pressure was controlled at 0.9 MPa, the heating rate was 4°C / min, and the reaction time was 2 hours. After the reaction was completed, the mixture was cooled to room temperature. The resulting liquid was analyzed by high-performance liquid chromatography (HPLC) to quantitatively analyze the DET and diethylene glycol contents. The filter residue was weighed, and the PET conversion rate was calculated to be 95%, the main product DET yield was 80%, and the diethylene glycol content generated by the side reaction was 0%.

[0038] Example 3

[0039] 50g of PET and 300g of ethanol were placed in a 1L mechanically stirred high-pressure reactor. 0.17g of PEG300 and 0.33g of titanium sulfate were added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400 rpm. The temperature was raised to 140°C, the reaction pressure was controlled at 0.9 MPa, the heating rate was 4°C / min, and the reaction time was 2 hours. After the reaction was completed, the mixture was cooled to room temperature. The resulting liquid was analyzed by high-performance liquid chromatography (HPLC) to quantitatively analyze the DET and diethylene glycol contents. The filter residue was weighed, and the PET conversion rate was calculated to be 100%, the main product DET yield was 85%, and the diethylene glycol content generated by the side reaction was 0%.

[0040] Example 4

[0041] 50g of PET and 300g of ethanol were placed in a 1L mechanically stirred high-pressure reactor. 0.125g of PEG300 and 0.375g of titanium sulfate were added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400 rpm. The temperature was raised to 140°C, the reaction pressure was controlled at 0.9 MPa, the heating rate was 4°C / min, and the reaction time was 2 hours. After the reaction was completed, the mixture was cooled to room temperature. The resulting liquid was analyzed by high-performance liquid chromatography (HPLC) to quantitatively analyze the DET and diethylene glycol contents. The filter residue was weighed, and the PET conversion rate was calculated to be 100%, the main product DET yield was 86%, and the diethylene glycol content generated by the side reaction was 0.2%.

[0042] Example 5

[0043] 50g of PET and 300g of ethanol were placed in a 1L mechanically stirred high-pressure reactor. 0.33g of PEG600 and 0.17g of titanium sulfate were added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400 rpm. The temperature was raised to 140°C, the reaction pressure was controlled at 0.9 MPa, the heating rate was 4°C / min, and the reaction time was 2 hours. After the reaction was completed, the mixture was cooled to room temperature. The resulting liquid was analyzed by high-performance liquid chromatography (HPLC) to quantitatively analyze the DET and diethylene glycol contents. The filter residue was weighed, and the PET conversion rate was calculated to be 92%, the yield of the main product DET was 74%, and the content of diethylene glycol generated by the side reaction was 0.

[0044] Example 6

[0045] 50g of PET and 300g of ethanol were placed in a 1L mechanically stirred high-pressure reactor. 0.25g of PEG600 and 0.25g of titanium sulfate were added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400 rpm. The temperature was raised to 140°C, the reaction pressure was controlled at 0.9 MPa, the heating rate was 4°C / min, and the reaction time was 2 hours. After the reaction was completed, the mixture was cooled to room temperature. The resulting liquid was analyzed by high-performance liquid chromatography (HPLC) to quantitatively analyze the DET and diethylene glycol contents. The filter residue was weighed, and the PET conversion rate was calculated to be 92%, the main product DET yield was 73%, and the diethylene glycol content generated by the side reaction was 0%.

[0046] Example 7

[0047] 50g of PET and 300g of ethanol were placed in a 1L mechanically stirred high-pressure reactor. 0.17g of PEG600 and 0.33g of titanium sulfate were added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400 rpm. The temperature was raised to 140°C, the reaction pressure was controlled at 0.9 MPa, the heating rate was 4°C / min, and the reaction time was 2 hours. After the reaction was completed, the mixture was cooled to room temperature. The resulting liquid was analyzed by high-performance liquid chromatography (HPLC) to quantitatively analyze the DET and diethylene glycol contents. The filter residue was weighed, and the PET conversion rate was calculated to be 98%, the main product DET yield was 78%, and the diethylene glycol content generated by the side reaction was 0%.

[0048] Example 8

[0049] 50g of PET and 300g of ethanol were placed in a 1L mechanically stirred high-pressure reactor. 0.125g of PEG600 and 0.375g of titanium sulfate were added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400 rpm. The temperature was raised to 140°C, the reaction pressure was controlled at 0.9 MPa, the heating rate was 4°C / min, and the reaction time was 2 hours. After the reaction was completed, the mixture was cooled to room temperature. The resulting liquid was analyzed by high-performance liquid chromatography (HPLC) to quantitatively analyze the DET and diethylene glycol contents. The filter residue was weighed, and the PET conversion rate was calculated to be 100%, the main product DET yield was 83%, and the diethylene glycol content generated by the side reaction was 0.1%.

[0050] Example 9

[0051] 50g of PET and 300g of ethanol were placed in a 1L mechanically stirred high-pressure reactor. 0.33g of PEG300 and 0.67g of titanium sulfate were added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400 rpm. The temperature was raised to 160°C, the reaction pressure was controlled at 1.2 MPa, the heating rate was 4°C / min, and the reaction time was 2 hours. After the reaction was completed, the mixture was cooled to room temperature. The resulting liquid was analyzed by high-performance liquid chromatography (HPLC) to quantitatively analyze the DET and diethylene glycol contents. The filter residue was weighed, and the PET conversion rate was calculated to be 97%, the main product DET yield was 95%, and the diethylene glycol content generated by the side reaction was 0%.

[0052] Example 10

[0053] 50g of PET and 300g of ethanol were placed in a 1L mechanically stirred high-pressure reactor. 0.33g of PEG600 and 0.67g of titanium sulfate were added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400 rpm. The temperature was raised to 160°C, the reaction pressure was controlled at 1.2 MPa, the heating rate was 4°C / min, and the reaction time was 2 hours. After the reaction was completed, the mixture was cooled to room temperature. The resulting liquid was analyzed by high-performance liquid chromatography (HPLC) to quantitatively analyze the DET and diethylene glycol contents. The filter residue was weighed, and the PET conversion rate was calculated to be 95%, the main product DET yield was 93%, and the diethylene glycol content generated by the side reaction was 0%.

[0054] Example 11

[0055] 50g of PET and 250g of methanol were placed in a 1L mechanically stirred high-pressure reactor. 0.33g of PEG300 and 0.67g of titanium sulfate were added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400 rpm. The temperature was raised to 160°C, the reaction pressure was controlled at 1.3 MPa, the heating rate was 4°C / min, and the reaction time was 2 hours. After the reaction was completed, the mixture was cooled to room temperature. The resulting liquid was analyzed by high-performance liquid chromatography (HPLC) to quantitatively analyze the DMT and diethylene glycol contents. The filter residue was weighed, and the PET conversion rate was calculated to be 100%, the main product DMT yield was 93%, and the diethylene glycol content generated by the side reaction was 0%.

[0056] Example 12

[0057] 50g of PET and 300g of ethylene glycol were placed in a 1L mechanically stirred high-pressure reactor. 0.2g of PEG300 and 0.8g of titanium sulfate were added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400 rpm. The temperature was raised to 210°C, the reaction pressure was controlled at 0.2 MPa, the heating rate was 4°C / min, and the reaction time was 3 hours. After the reaction was completed, the mixture was cooled to room temperature. The resulting liquid was analyzed by high-performance liquid chromatography (HPLC) to quantitatively analyze the BHET and diethylene glycol contents. The filter residue was weighed, and the PET conversion rate was calculated to be 98%, the main product BHET yield was 92%, and the diethylene glycol content generated by the side reaction was 0%.

[0058] Comparative Example 1

[0059] 50g of PET and 300g of ethanol were placed in a 1L mechanically stirred high-pressure reactor. 0.5g of titanium sulfate was added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400 rpm. The temperature was raised to 140°C, the reaction pressure was controlled at 0.9 MPa, the heating rate was 4°C / min, and the reaction time was 2 hours. After the reaction was completed, the mixture was cooled to room temperature. The resulting liquid was analyzed by high-performance liquid chromatography (HPLC) to quantitatively analyze the DET and diethylene glycol contents. The filter residue was weighed, and the PET conversion rate was calculated to be 95%, the main product DET yield was 78%, and the diethylene glycol content generated by the side reaction was 1.2%.

[0060] Comparative Example 2

[0061] 50g of PET and 300g of ethanol were placed in a 1L mechanically stirred high-pressure reactor. 0.5g of PEG300 was added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400rpm. The temperature was raised to 140℃, the reaction pressure was controlled at 0.9MPa, the heating rate was 4℃ / min, and the reaction time was 2h. After the reaction was completed, the mixture was cooled to room temperature. The liquid after the reaction was taken for high-performance liquid chromatography analysis to quantitatively analyze the DET and diethylene glycol contents in the liquid. The filter residue was weighed, and the PET conversion rate was calculated to be 25%, the yield of the main product DET was 16%, and the content of diethylene glycol generated by the side reaction was 0.

[0062] Comparative Example 3

[0063] 50g of PET and 300g of ethanol were placed in a 1L mechanically stirred high-pressure reactor. 0.5g of PEG600 was added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400rpm. The temperature was raised to 140℃, the reaction pressure was controlled at 0.9MPa, the heating rate was 4℃ / min, and the reaction time was 2h. After the reaction was completed, the mixture was cooled to room temperature. The liquid after the reaction was taken for high-performance liquid chromatography analysis to quantitatively analyze the DET and diethylene glycol contents in the liquid. The filter residue was weighed, and the PET conversion rate was calculated to be 23%, the yield of the main product DET was 14%, and the content of diethylene glycol generated by the side reaction was 0.

[0064] Comparative Example 4

[0065] 50g of PET and 250g of ethanol were placed in a 1L mechanically stirred high-pressure reactor. 1g of zinc acetate was added. N2 was introduced into the high-pressure reactor as a protective gas to replace the air in the reactor three times. The reactor was then sealed and mechanical stirring was started at 400 rpm. The temperature was raised to 140°C, the reaction pressure was controlled at 0.9 MPa, the heating rate was 4°C / min, and the reaction time was 3 hours. After the reaction was completed, the mixture was cooled to room temperature. The resulting liquid was analyzed by high-performance liquid chromatography (HPLC) to quantitatively analyze the DET and diethylene glycol contents. The filter residue was weighed, and the PET conversion rate was calculated to be 95%, the main product DET yield was 75%, and the diethylene glycol content generated by the side reaction was 2%.

[0066] Table 1. Comparison of catalyst composition, dosage, PET conversion rate, and yield of main product and by-product.

[0067]

[0068] Because ethanol, methanol, and ethylene glycol readily undergo polymerization and dehydration reactions under high temperature and pressure to generate a small amount of diethylene glycol, using titanium sulfate or zinc acetate alone as catalysts for the PET alcoholysis reaction will produce diethylene glycol, as shown in Comparative Examples 1 and 4. The byproduct diethylene glycol is difficult to separate during subsequent product purification, affecting product purity. Polyethylene glycol 300 / 600 can effectively inhibit the dehydration polymerization of the alcoholysis agents (ethanol, methanol, and ethylene glycol) during PET catalytic alcoholysis, preventing the formation of the byproduct diethylene glycol. However, using polyethylene glycol 300 / 600 alone as a catalyst has a weak catalytic effect, specifically manifested in lower PET conversion and DET yield, as shown in Comparative Examples 2 and 3. Therefore, this invention cleverly combines two existing catalysts, polyethylene glycol and titanium sulfate, to prepare a composite catalyst, achieving the effect of maximizing strengths and minimizing weaknesses, ensuring high catalytic activity while preventing side reactions. Furthermore, as shown in Table 1, when ethanol and methanol are used as alcoholysis agents, the conversion rate of PET, the yield of the main product, and the inhibition rate of side reactions are optimal when the mass ratio of polyethylene glycol to titanium sulfate is 1:2, as in Examples 3, 7, and 9-11. When ethylene glycol is used as an alcoholysis agent, the mass ratio of polyethylene glycol to titanium sulfate is 1:4, and no side reactions occur while maintaining a high conversion rate of PET and a high yield of the main product, as in Example 12.

[0069] Table 2. Purity specifications and manufacturers of the composite catalysts purchased.

[0070] Medicines and reagents Purity Specifications factory Polyethylene glycol 300 Synthesis Shanghai Aladdin Biochemical Technology Co., Ltd. Polyethylene glycol 600 Synthesis Shanghai Aladdin Biochemical Technology Co., Ltd. Anhydrous titanium sulfate ≥96% Shanghai McLean Biochemical Technology Co., Ltd.

[0071] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A composite catalyst for PET alcoholysis, characterized in that, The composite catalyst is composed of polyethylene glycol and titanium sulfate, with a mass ratio of polyethylene glycol to titanium sulfate of 1:(0.5-5). By adjusting the ratio of polyethylene glycol to titanium sulfate, the composite catalyst achieves a PET alcoholysis conversion rate of up to 100%. When ethanol is used as the alcoholysis agent, the yield of the main product DET can reach up to 95%. When methanol is used as the alcoholysis agent, the yield of the main product DMT can reach up to 93%. When ethylene glycol is used as the alcoholysis agent, the yield of the main product BHET can reach up to 92%, while the content of the byproduct diethylene glycol is 0.

2. The composite catalyst according to claim 1, characterized in that, The polyethylene glycol has a molecular weight of 300-600.

3. A method for catalytic alcoholysis of PET, characterized in that, The composite catalyst according to any one of claims 1 or 2 is used, specifically comprising the following steps: PET and an alcoholysis agent are placed in a mechanically stirred high-pressure reactor, the composite catalyst is added, N2 is introduced into the high-pressure reactor as a protective gas, the air in the high-pressure reactor is replaced 3-4 times, the high-pressure reactor is sealed and mechanical stirring is turned on, the temperature is raised to the reaction temperature, the reaction pressure is controlled, the reaction time is 1-5 hours, the reaction is cooled to room temperature after completion, the liquid after reaction is taken for high-performance liquid chromatography analysis, the content of the main product and the content of the by-reaction product in the liquid are quantitatively analyzed, the filter residue after reaction is weighed, and the PET conversion rate can be calculated to be up to 100%, the main product yield can be up to 95%, and the by-reaction product is diethylene glycol, the content of which is 0-0.2%.

4. The method for catalytic alcoholysis of PET according to claim 3, characterized in that, The mass ratio of PET to alcoholysis agent is 1:(4-7).

5. The method for catalytic alcoholysis of PET according to claim 3, characterized in that, The alcoholysis agent is any one of ethanol, methanol, or ethylene glycol.

6. The method for catalytic alcoholysis of PET according to claim 4, characterized in that, The alcoholysis agent is any one of ethanol, methanol, or ethylene glycol.

7. The method for catalytic alcoholysis of PET according to any one of claims 3-6, characterized in that, The amount of composite catalyst used is 0.5-3 wt% based on the mass of the PET.

8. The method for catalytic alcoholysis of PET according to any one of claims 3-6, characterized in that, The mechanical stirring speed is 200-800 rpm.

9. The method for catalytic alcoholysis of PET according to claim 7, characterized in that, The mechanical stirring speed is 200-800 rpm.

10. The method for catalytic alcoholysis of PET according to any one of claims 3-6, characterized in that, The reaction temperature is 130-180℃.

11. The method for catalytic alcoholysis of PET according to claim 7, characterized in that, The reaction temperature is 130-180℃.

12. The method for catalytic alcoholysis of PET according to claim 8, characterized in that, The reaction temperature is 130-180℃.

13. The method for catalytic alcoholysis of PET according to claim 9, characterized in that, The reaction temperature is 130-180℃.

14. The method for catalytic alcoholysis of PET according to any one of claims 3-6, characterized in that, The reaction pressure is 0.2-1.6 MPa.

15. The method for catalytic alcoholysis of PET according to claim 7, characterized in that, The reaction pressure is 0.2-1.6 MPa.

16. The method for catalytic alcoholysis of PET according to claim 8, characterized in that, The reaction pressure is 0.2-1.6 MPa.

17. The method for catalytic alcoholysis of PET according to claim 9, characterized in that, The reaction pressure is 0.2-1.6 MPa.

18. The method for catalytic alcoholysis of PET according to claim 10, characterized in that, The reaction pressure is 0.2-1.6 MPa.

19. The method for catalytic alcoholysis of PET according to claim 11, characterized in that, The reaction pressure is 0.2-1.6 MPa.

20. The method for catalytic alcoholysis of PET according to claim 12, characterized in that, The reaction pressure is 0.2-1.6 MPa.

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