A method for polyethylene terephthalate glycolysis with staged control of ethylene glycol dosage

Abstract

This invention discloses a method for the ethylene glycol hydrolysis of polyethylene terephthalate (PET) with staged control of ethylene glycol dosage. The method includes: in a first stage, adding PET and ethylene glycol to the reaction system at a molar ratio of 1:1 to 1:3, and reacting at 180–196 °C for 0.5–2 h under the action of a solid catalyst; in a second stage, adding ethylene glycol to the reaction system to achieve a molar ratio of ethylene glycol to PET of 1:5 to 1:8, and continuing the reaction for another 2–4 h. By adding ethylene glycol in stages, the conversion degree of oligomers during the reaction is improved, thereby significantly increasing the monomer yield under the same ethylene glycol dosage conditions, reducing oligomer residue, and lowering subsequent separation costs.

Description

Technical Field

[0001] This invention relates to the field of chemical recycling technology of polyethylene terephthalate, specifically to a method for the ethylene glycol hydrolysis of polyethylene terephthalate by controlling the amount of ethylene glycol used in stages. Background Technology

[0002] With the widespread use of plastic products, the resource utilization of waste plastics has become an important research direction. Polyethylene terephthalate (PET), as an important polyester material, is of great significance in its chemical recycling. Glycol hydrolysis, as an important method for the chemical depolymerization of PET, can achieve the conversion of PET under relatively mild conditions (such as low pressure and low temperature) to generate monomer products such as polyethylene terephthalate (BHET), showing the potential to achieve closed-loop recycling of materials.

[0003] In existing technologies, the ethylene glycololysis of PET typically involves a one-time addition of ethylene glycol. During this process, monomers and oligomers are usually generated simultaneously in the reaction system. The presence of oligomers reduces monomer yield and increases the difficulty of subsequent separation and purification. Furthermore, existing methods often improve monomer selectivity by increasing the amount of ethylene glycol used; however, excessive ethylene glycol leads to increased solvent consumption, resulting in resource waste and increased separation costs. Therefore, how to control the amount of ethylene glycol used while suppressing oligomer formation and increasing monomer yield has become a pressing technical problem to be solved in this field. Summary of the Invention

[0004] The purpose of this invention is to provide a method for the ethylene glycol hydrolysis of polyethylene terephthalate by controlling the amount of ethylene glycol used in stages, thereby increasing the monomer yield while controlling the amount of ethylene glycol used.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for the staged control of ethylene glycol dosage in the hydrolysis of polyethylene terephthalate, characterized by comprising the following steps:

[0007] S1. Polyethylene terephthalate and ethylene glycol are added to a reaction vessel in a molar ratio of 1:1 to 1:3. The temperature is raised to 180 to 196 °C in the presence of a solid catalyst, and the reaction is carried out for 0.5 to 2 h.

[0008] S2. Add ethylene glycol to the reaction vessel to make the molar ratio of polyethylene terephthalate to ethylene glycol in the system 1:5 to 1:8, and continue the reaction for 2 to 4 hours.

[0009] As a preferred embodiment, in step S1, the solid catalyst is a solid catalyst with a mesoporous structure, and the solid catalyst is selected from one of MCM-41 and SBA-15 supported on metal oxides, and the metal oxide is one of zinc oxide (ZnO), ferric oxide (Fe2O3), chromium oxide (Cr2O3), and tin oxide (SnO2).

[0010] As a preferred embodiment, in step S2, the added ethylene glycol is preheated to the reaction temperature.

[0011] As a preferred embodiment, after the reaction in step S2 is completed, the reaction system is separated and purified to obtain the monomer product.

[0012] As a preferred embodiment, the separation and purification includes the following steps:

[0013] S31. Filter the reaction solution while it is hot to separate the solid catalyst;

[0014] S32. Add water to the filtrate and filter to separate the oligomers;

[0015] S33. The obtained filtrate is recrystallized at low temperature and filtered to obtain the monomer product.

[0016] As a preferred embodiment, in step S31, the separated solid catalyst can be reused after washing and drying.

[0017] The technical principles and beneficial effects of this invention are as follows:

[0018] This invention achieves stepwise control of the ethylene glycololysis reaction of polyethylene terephthalate (PET) by adjusting the amount of ethylene glycol used in stages. In the first stage, the amount of ethylene glycol is relatively low, and PET undergoes a chain scission reaction to generate oligomers. In the second stage, the concentration of ethylene glycol in the system is increased by adding more ethylene glycol, which is beneficial for the further conversion of oligomers, thereby promoting monomer formation. Simultaneously, the presence of a solid catalyst further facilitates the conversion of oligomers and reduces side reactions, thus increasing the monomer yield.

[0019] Compared with the existing technology of adding ethylene glycol all at once, the present invention improves the monomer yield by controlling the addition of ethylene glycol in stages, so as to make the conversion of intermediate products more complete during the reaction process, reduce the residue of oligomers in the system, and thus improve the monomer yield.

[0020] Furthermore, in this invention, by controlling the amount of ethylene glycol used during the reaction process, the monomer yield can be increased without significantly increasing the total amount of ethylene glycol used, thereby reducing the amount of solvent used and facilitating the separation and purification of subsequent products. Attached Figure Description

[0021] To more clearly illustrate the specific embodiments of the present invention, the accompanying drawings used in the embodiments of the present invention are briefly described below:

[0022] Figure 1 This is a process flow diagram of the present invention;

[0023] Figure 2 This is the DSC spectrum of the substances in the reaction system before the addition of ethylene glycol in Example 1 of the present invention;

[0024] Figure 3 The liquid NMR spectrum of the monomer product obtained in Example 2 of this invention; Detailed Implementation

[0025] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the invention will now be further described in conjunction with the accompanying drawings and embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of this invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

[0026] Figure 1 A process flow diagram of the present invention is shown.

[0027] Example 1

[0028] See Figure 1 Polyethylene terephthalate (PET) and ethylene glycol were added to a reactor at a molar ratio of 1:2, and SBA-15 solid catalyst with a zinc oxide loading of 1% was added. The amount of catalyst added was 2% of the mass of PET, and the reaction was carried out at 196 °C for 0.5 h.

[0029] After the first stage of reaction is completed, ethylene glycol is added to the reaction system to make the molar ratio of PET to ethylene glycol in the system reach 1:8, and the reaction continues for 2 hours.

[0030] After the ethylene glycol hydrolysis reaction was completed, the reaction system was separated and purified to obtain the monomer product BHET;

[0031] The monomer yield was determined to be 88.4%.

[0032] Figure 2 The DSC spectrum of the substances in the reaction system before the addition of ethylene glycol in Example 1 is shown. The presence of characteristic peaks attributable to oligomers in the spectrum indicates that oligomer intermediates were generated in the reaction system.

[0033] Example 2

[0034] See Figure 1Polyethylene terephthalate (PET) and ethylene glycol were added to a reactor at a molar ratio of 1:1, and a solid catalyst of MCM-41 with a loading of 5% ferric oxide was added. The amount of catalyst added was 1% of the mass of PET, and the reaction was carried out at 180 °C for 2 h.

[0035] After the first stage of reaction is completed, ethylene glycol is added to the reaction system to make the molar ratio of PET to ethylene glycol in the system reach 1:6, and the reaction continues for 4 hours.

[0036] After the ethylene glycol hydrolysis reaction was completed, the reaction system was separated and purified to obtain the monomer product BHET;

[0037] The monomer yield was determined to be 81.6%.

[0038] Figure 3 The liquid NMR spectrum of the monomer product obtained in Example 2 is shown. The spectrum shows characteristic peaks belonging to polyethylene terephthalate (BHET), indicating that the obtained product is the target monomer.

[0039] Comparative Example 1

[0040] Polyethylene terephthalate and ethylene glycol were added to a reactor in a molar ratio of 1:8, and SBA-15 solid catalyst with a zinc oxide loading of 1% was added. The amount of catalyst added was 2% of the mass of PET. The reaction was carried out at 196 °C for 2.5 h.

[0041] After the ethylene glycol hydrolysis reaction was completed, the reaction system was separated and purified to obtain the monomer product BHET;

[0042] The monomer yield was determined to be 63.4%, which is significantly lower than the monomer yield in Example 1.

[0043] The above comparison results show that, under the same reaction time and catalyst conditions, the present invention effectively promotes the further conversion of oligomer intermediates by adding ethylene glycol in stages, and significantly improves the monomer yield.

[0044] The present invention provides a method for the depolymerization of polyethylene terephthalate by controlling the amount of ethylene glycol in stages. This method differs significantly from traditional methods in terms of process flow and depolymerization effect, demonstrating excellent technical performance.

[0045] Please note that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments have been described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification. The above embodiments only illustrate several implementation methods of this application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent.

Claims

1. A method for the staged control of ethylene glycol dosage in the hydrolysis of polyethylene terephthalate, characterized in that, Includes the following steps: S1. Polyethylene terephthalate and ethylene glycol are added to a reaction vessel in a molar ratio of 1:1 to 1:

3. The temperature is raised to 180 to 196 °C in the presence of a solid catalyst, and the reaction is carried out for 0.5 to 2 h. S2. Add ethylene glycol to the reaction vessel to make the molar ratio of polyethylene terephthalate to ethylene glycol in the system 1:5 to 1:8, and continue the reaction for 2 to 4 hours.

2. The method for staged regulation of ethylene glycol dosage in the hydrolysis of polyethylene terephthalate according to claim 1, characterized in that, In step S1, the solid catalyst is a solid catalyst with a mesoporous structure. Further, the solid catalyst is selected from one of MCM-41 and SBA-15 supported on metal oxides, and the metal oxide is one of zinc oxide (ZnO), ferric oxide (Fe2O3), chromium oxide (Cr2O3), and tin oxide (SnO2).

3. The method for staged regulation of ethylene glycol dosage in the hydrolysis of polyethylene terephthalate according to claim 1, characterized in that, In step S2, the added ethylene glycol is preheated to the reaction temperature.

4. The method for staged regulation of ethylene glycol dosage in the hydrolysis of polyethylene terephthalate according to claim 1, characterized in that, After the reaction in step S2 is completed, the reaction system is separated and purified to obtain the monomer product.

5. The separation and purification process according to claim 4, characterized in that, Includes the following steps: S31. Filter the reaction solution while it is hot to separate the solid catalyst; S32. Add water to the filtrate and filter to separate the oligomers; S33. The obtained filtrate is recrystallized at low temperature and filtered to obtain the monomer product.

6. The separation and purification step according to claim 5, characterized in that, In step S31, the separated solid catalyst can be reused after washing and drying.

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