An upgrading recovery method for catalyzing alcoholysis of PET based on in-situ generation of rich-alkali type proton salt

Abstract

The application discloses an upgrading and recycling method for catalytic alcoholysis of PET based on in-situ generation of alkali-rich proton salt, and relates to the fields of polymer degradation and green catalysis technology. The method uses organic alkali and amino acid as catalytic components, constructs an alkali-rich proton salt composite catalytic system in-situ in an ethylene glycol or methanol alcoholysis medium, and does not need to synthesize and purify the catalyst in advance. The pretreated PET fragments are added into the system, and catalytic alcoholysis is carried out under mild conditions to efficiently prepare BHET or DMT. The alkali-rich proton salt composite catalytic system is constructed in-situ, the catalyst does not need to be synthesized and purified in advance, the process steps are simplified, the energy consumption and cost are reduced, the efficient alcoholysis of PET under mild conditions is realized, the high-purity BHET or DMT product is obtained, the catalyst can be recycled, and the problems of complicated preparation of the existing PET alcoholysis catalyst, high reaction energy consumption and difficult product purification are solved, which meets the needs of green environmental protection and industrial production.

Description

Technical Field

[0001] This invention relates to the fields of polymer degradation and green catalysis, and in particular to an upgraded recycling method for PET based on in-situ generation of base-rich proton salts for catalytic alcoholysis. Background Technology

[0002] With the continued rise in global plastic consumption, the environmental accumulation of waste polyethylene terephthalate (PET) has become an urgent ecological problem that needs to be addressed. Developing efficient and green PET chemical recycling technologies to depolymerize it into polymer monomers is a key path to achieving a closed-loop carbon resource cycle.

[0003] Among various upgrading and recycling strategies, using ethylene glycol as a medium to degrade PET into dihydroxyethyl terephthalate (BHET) can achieve a closed-loop regeneration path of "PET→BHET→PET", and is considered one of the recycling methods with the greatest industrial potential. However, the efficiency of alcoholysis, the purity of the product, and the energy consumption level are all highly dependent on the type and performance of the catalyst used. Although traditional metal acetates are effective, they pose environmental risks such as difficulty in product purification and metal residues. In recent years, organic catalysts have become a research hotspot due to their advantages such as being green and free of metal pollution. The Hedrick team at IBM first demonstrated the excellent activity of the bicyclic guanidine superbase 1,5,7-triazacyclo[4.4.0]dodecyl-5-ene (TBD) in the alcoholysis of PET with ethylene glycol (J. Polym. Sci. Pol. Chem. 49 (5) (2011) 1273–1281), achieving a yield of 78% after 3.5 h of reaction at 190 °C. To further improve catalyst activity, the academic community has developed strategies using "proton-type ionic salts" or "ionic liquids." For example, Jehanno et al. studied the alcoholysis of PET ethylene glycol using an ionic liquid synthesized from TBD and methanesulfonic acid (MSA), achieving a BHET yield of 91%, but with a high catalyst loading exceeding 50% (J. Name., 2013, 00, 1-3). In recent years, Tomonori Saito's team attempted to combine TBD with trifluoroacetic acid, achieving excellent conversion and yield at a lower temperature, but trifluoroacetic acid is highly toxic and expensive (Mater. Horiz., 2023, 10, 3360–3368). This team also systematically studied proton salt catalysts formed from TBD and benzoic acid derivatives, but these still require a high-temperature reaction of 190 °C for 2 h (J. Mater. Chem. A, 2025, 13, 32111–32121). In addition, Professor Zhang Fan's research group at Sichuan University reported the use of TBD and acetic acid in an equimolar ratio to form a proton ion salt for methanol hydrolysis of PET (Nature Communications (2025) 16: 2482). However, this ion salt catalyst needs to be synthesized, purified and dried in advance, and the operation steps are relatively complicated. The step of removing solvent water by evaporation has high energy consumption.

[0004] Therefore, developing a novel organic catalytic system that does not require pre-synthesis, can be generated in situ, and can maintain high activity under mild conditions is of great significance for promoting the industrialization of PET upgrading and recycling. Summary of the Invention

[0005] This invention addresses the problems of cumbersome catalyst preparation and high energy consumption in alcoholysis reactions in existing technologies by providing an upgraded recovery method for PET alcoholysis based on in-situ generation of a base-rich proton salt. This method constructs a base-rich proton salt composite catalytic system in situ, eliminating the need for pre-synthesization and purification of catalysts, simplifying process steps, reducing energy consumption and costs, and achieving efficient alcoholysis of PET under mild conditions to obtain high-purity BHET or DMT products. Furthermore, the catalyst is recyclable, meeting the requirements of green environmental protection and industrial production.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] Firstly, this invention provides an upgraded recycling method for PET based on the in-situ generation of a base-rich proton salt and its catalytic alcoholysis in ethylene glycol, comprising the following steps: PET waste is cut and pre-treated to obtain PET fragments; Dissolve an organic base in an appropriate amount of ethylene glycol, ultrasonically disperse until homogeneous, add amino acids, continue ultrasonic dispersion until homogeneous, add PET fragments, and catalytically hydrolyze the PET fragments at 170-180℃ for 120-240 min under an inert gas atmosphere to obtain the alcoholysis product. The alcoholysis product was cooled to room temperature and then crystallized at 0–4 °C. The crystal precipitate was filtered to obtain crude diethyl terephthalate (BHET). Crude BHET was dissolved in an appropriate amount of water at 70–80 °C, filtered, and the filtrate was placed at 0–4 °C for crystallization. The crystal precipitate was then filtered to obtain dihydroxyethyl terephthalate (BHET).

[0008] Furthermore, the mass ratio of the ethylene glycol to the PET fragments is (5-5.5):1.

[0009] Secondly, this invention provides an upgraded recovery method for PET based on in-situ generation of a base-rich proton salt via catalytic alcoholysis in methanol, comprising the following steps: PET waste is cut and pre-treated to obtain PET fragments; Dissolve an organic base in an appropriate amount of methanol, ultrasonically disperse until homogeneous, add amino acids, continue ultrasonic dispersion until homogeneous, add PET fragments, and catalytically alcoholyze the PET fragments at 145-150℃ for 120-180 min to obtain the alcoholysis product. The alcoholysis product was cooled to room temperature, filtered, washed with methanol, and dried under vacuum to obtain dimethyl terephthalate (DMT).

[0010] Furthermore, the mass ratio of methanol to PET fragments is (3-5):1.

[0011] In both of the above technical solutions, the organic base is at least one of 1,5,7-triazabicyclo[4.4.0]dodec-5-ene (TBD), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU); and the amino acid is L-valine (Val) or proline (Pro). The molar ratio of the organic base to the amino acid is 1:(0.25~0.5). The amount of the organic strong alkali used is 6% to 10% of the mass of the PET fragments; Further, the organic base is 1,5,7-triazacyclo[4.4.0]dodecyl-5-ene (TBD), and the amino acid is L-valine (Val).

[0012] The PET waste mentioned above refers to waste PET bottles, PET fibers, or PET textiles.

[0013] The beneficial effects of this invention are: This invention utilizes the proton transfer equilibrium between organic bases and natural amino acids to construct an in-situ base-rich organic base / proton salt composite catalytic system in an alcoholysis medium. This eliminates the need for pre-synthesizing, purifying, and drying the catalyst, thus completely simplifying the process steps and avoiding energy-intensive and cumbersome steps such as solvent evaporation in the preparation of traditional ionic salt catalysts. This significantly reduces process costs and lowers the barriers to industrialization.

[0014] This invention creatively constructs a synergistic catalytic system of "free organic base + in-situ proton salt". Excess free organic base (such as TBD) can efficiently activate the hydroxyl groups of alcohols (ethylene glycol or methanol), enhancing their nucleophilic attack ability. The in-situ generated proton salt (such as [TBDH][Val]) can stabilize the alcoholysis reaction intermediate through a hydrogen bonding network, lowering the activation energy. The synergistic catalytic effect of both is significantly better than using an organic base or a proton salt alone. In the ethylene glycol alcoholysis system, complete conversion of PET can be achieved at 170℃ for 3.5 h, with a BHET yield as high as 94.2%. In the methanol alcoholysis system, high-purity DMT can be obtained by reacting at 145–150℃ for 120–180 min. Compared with existing technologies, the reaction temperature is reduced by 10–20℃, significantly improving reaction efficiency and greatly reducing energy consumption.

[0015] The amino acids used in this invention are natural amino acids (L-valine or proline), which are inexpensive, readily available, non-toxic, and harmless, and will not cause pollution to the environment. Although the organic bases (TBD, DBN, DBU) are chemical reagents, they are used in small quantities and can be recycled. The overall catalytic system conforms to the development concept of green catalysis and environmentally friendly recycling, and solves the problems of high catalyst toxicity and high cost in the existing technology.

[0016] This invention obtains high-purity BHET or DMT products through simple crystallization, filtration, and washing steps. The purity of the products meets the requirements for repolymerization to prepare polymer materials such as PET, truly realizing a closed-loop recycling process of "PET → monomer → PET". This provides an efficient and feasible technical solution for the high-value resource utilization of PET waste, and has significant industrial application value and ecological significance. The base-rich proton salt catalytic system prepared by this invention has excellent recyclability. After three cycles, it still maintains excellent catalytic activity, and the PET conversion rate and product yield do not decrease significantly. This further reduces the catalyst cost for industrial production and improves the economics and feasibility of the technology. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of the organic base and amino acid used in this invention; Figure 2 This is a schematic diagram of the ethylene glycol alcoholysis of PET according to the present invention; Figure 3 This is a schematic diagram of the methanol alcoholysis of PET according to the present invention; Figure 4 Figure 1 shows experimental data of PET ethylene glycol hydrolysis cycle in the TBD / Val catalytic system. Figure 5 The bis(hydroxyethyl) terephthalate (BHET) obtained in Example 1 1 H-NMR spectrum; Figure 6 The dimethyl terephthalate (DMT) obtained in Example 32 1 H-NMR spectrum. Detailed Implementation

[0018] To further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the scope of the claims of the present invention.

[0019] The PET waste used in this invention is discarded PET mineral water bottles after single consumption, with the bottle body cut into 1 cm pieces. The 1 cm thin slices were first washed with water and then with anhydrous ethanol, and the washing was repeated 3 times. Then they were dried in a vacuum at 80 °C for 8 h for later use.

[0020] The reagents used in this invention include ethylene glycol (EG), methanol (MeOH), 1,5,7-triazacyclo[4.4.0]dodec-5-ene (TBD), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), L-valine (Val), proline (Pro), arginine (Arg), glycine (Gly), para-aminobenzoic acid (PABA), and L-phenylalanine (L-Phe), all of which are analytical grade and purchased from Shanghai Anaiji Chemical Co., Ltd. It should be noted that reagents or instruments used in this invention without a specified manufacturer are all commercially available conventional products.

[0021] 1. Effects of different proton ion salt systems on the alcoholysis of PET in ethylene glycol Examples 1-9 Example 1 Weigh 70.0 mg (0.50 mmol) TBD and dissolve it in 4.45 mL of ethylene glycol. Sonicate for 5 minutes until completely dissolved. Then add 29.3 mg (0.25 mmol) valine (TBD:Val molar ratio = 2:1) and continue sonicating for 5 minutes to obtain an organic base / proton salt composite catalyst solution. Then, 0.96 g of PET sheet was added to the above organic base / proton salt composite catalyst solution, and the reaction system was subjected to three "vacuum-nitrogen purging" operations. Subsequently, under nitrogen protection, the reaction device was placed in a 170 ℃ oil bath, the rotation speed was maintained at 500 r / min, and the reaction was stirred for 3.5 hours to obtain the alcoholysis product. The above reactions take place in a Shrek tube; After the alcoholysis product was cooled to room temperature, it was cooled at 0–4 °C for 24 h. The precipitated crude product was filtered, and the recovered filtrate was used in subsequent recycling experiments. The crude product was dissolved in 200 mL of 80 °C hot water, filtered to remove a small amount of insoluble substances, and the filtrate was cooled and crystallized again at 0–4 °C for 24 h. After filtration, the resulting crystals were vacuum dried at 80 °C for 24 h to obtain a white needle-like BHET product.

[0022] Meanwhile, following the steps in Example 1, after the alcoholysis reaction was completed, 10 μL of the reaction solution was extracted, dissolved in acetonitrile, mixed thoroughly, diluted 5000 times, and analyzed by HPLC to obtain the yield and selectivity of BHET.

[0023] The formula for calculating the conversion rate of PET is as follows: , in, C PET This indicates the conversion rate of PET by alcoholysis. W0 represents the PET sheet weighed before the reaction. W 1 represents the mass of undegraded PET obtained after filtration and drying of the alcoholysis product.

[0024] The formula for calculating the BHET selectivity of the product is as follows: , in, S BHET This indicates the selectivity of the product BHET. MW BHET , MW PET The relative molecular mass of the main product BHET is 245 g / mol, and the molecular weight of the repeating unit of PET is 192 g / mol.

[0025] The formula for calculating the yield of the product BHET is as follows: , in, Y BHET This represents the overall yield of BHET. S BHET This indicates the selectivity of the product BHET. C PET This indicates the conversion rate of PET by alcoholysis.

[0026] Examples 1-9 only changed the type of composite catalyst solution; the rest of the process was the same as in Example 1. The effects of different proton ion salt systems on the PET ethylene glycol alcoholysis conversion and BHET yield are shown in Table 1.

[0027] Table 1. Effects of different proton ion salt systems on the alcoholysis of PET in ethylene glycol.

[0028] 2. Effect of different molar ratios of TBD and valine on the alcoholysis of PET in ethylene glycol Examples 10-15 Examples 10-15 investigated the effects of different acid-base ratios (1:0, 1:0.25, 1:1, 0.5:1, 0.25:1, 0:1) on the alcoholysis of PET ethylene glycol by changing the ratio of TBD to valine (Val). The remaining process was the same as in Example 1. The effects of different molar ratios of TBD and valine on the alcoholysis conversion rate and BHET yield of PET ethylene glycol are shown in Table 2.

[0029] Table 2. Effects of different molar ratios of TBD and valine on the alcoholysis of PET in ethylene glycol.

[0030] 3. Effect of different EG dosages on the alcoholysis of PET Examples 16-19 Examples 16-19 investigated the effect of EG dosage on the alcoholysis of PET by varying the amount of ethylene glycol added; the remaining process procedures were the same as in Example 1. The effects of different EG dosages on the alcoholysis conversion rate and BHET yield of PET are shown in Table 3.

[0031] Table 3 Effect of different EG dosages on the alcoholysis of PET in ethylene glycol

[0032] 4. Effect of different reaction temperatures on the alcoholysis of PET in ethylene glycol Examples 20-23 Examples 20-23 investigated the effect of different reaction temperatures on the alcoholysis of PET with ethylene glycol by varying the reaction temperature; the remaining process procedures were the same as in Example 1. The effects of different temperatures on the alcoholysis conversion rate and BHET yield of PET with ethylene glycol are shown in Table 4.

[0033] Table 4. Effect of different reaction temperatures on the alcoholysis of PET in ethylene glycol

[0034] 5. Effect of different reaction times on the alcoholysis of PET in ethylene glycol Examples 24-28 Examples 24-28 investigated the effect of different reaction times on the alcoholysis of PET with ethylene glycol by varying the reaction time; the remaining process procedures were the same as in Example 1. The effects of different reaction times on the PET alcoholysis conversion rate and BHET yield are shown in Table 5.

[0035] Table 5. Effect of different reaction times on the alcoholysis of PET in ethylene glycol.

[0036] 6. Effect of different TBD dosages on the alcoholysis of PET Examples 29-31 Examples 29-31 differed from Example 1 by varying the amount of TBD (TBD:Val molar ratio = 2:1). The effects of different TBD catalyst dosages on ethylene glycol alcoholysis conversion and BHET yield are shown in Table 6.

[0037] Table 6 Effect of different catalyst dosages on ethylene glycol alcoholysis

[0038] 7. Cyclic Experiment of TBD / Val Catalytic System The filtrate recovered in Example 1 was dried under vacuum at 80 °C for 24 h; Add 0.96 g of PET film to the above filtrate, and perform the "vacuum-nitrogen purging" operation three times on the reaction system. Then, under nitrogen protection, place the reaction device in an oil bath at 170 °C, maintain the rotation speed at 500 r / min, and stir the reaction for 3.5 hours to complete the second cycle experiment. After the reaction is completed, calculate the yield and selectivity of BHET in the second cycle according to the operation steps of Example 1. Following the above operating steps, continue to complete the third cycle experiment. After the reaction is completed, calculate the yield and selectivity of BHET in the third cycle.

[0039] The test data of the TBD / Val catalytic system cycling experiment are as follows: Figure 4 As shown.

[0040] Example 32 Weigh 70.0 mg (0.50 mmol) TBD and dissolve it in 4 mL of methanol solvent. Sonicate for 5 minutes until completely dissolved. Then add 29.3 mg (0.25 mmol) valine (TBD:Val molar ratio = 2:1) and continue sonicating for 5 minutes to obtain an organic base / proton salt composite catalyst solution. Then, 0.96 g of PET sheet was added to the organic base / proton salt composite catalyst solution, and the reaction system was placed in an oil bath for alcoholysis reaction at 140 °C for 2.5 h with the rotation speed maintained at 500 r / min to obtain the alcoholysis product; The above reactions were carried out in a 10 mL pressure-resistant tube; After the alcoholysis product was cooled to room temperature, it was filtered, and the recovered filtrate was used in subsequent recycling experiments. The resulting precipitate was washed three times with methanol and then dried under vacuum at 80 °C for 24 h to obtain white crystals of dimethyl terephthalate (DMT) with a yield of 96.4%.

[0041] Meanwhile, following the steps of Example 32, after the alcoholysis reaction was completed, 10 μL of the reaction solution was extracted while hot and dissolved in methanol-water with a volume ratio of 1:1. The solution was mixed evenly and diluted 5000 times. HPLC analysis was performed to obtain the yield and selectivity data of dimethyl terephthalate (DMT).

[0042] As can be seen from the above embodiments, the present invention can achieve efficient alcoholysis of PET under relatively mild conditions. By generating the catalyst in situ, energy consumption and process complexity are reduced, paving a practical path for industrialization.

[0043] Based on the disclosure in the foregoing specification, those skilled in the art can make appropriate changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and some modifications and changes to the present invention should also fall within the protection scope of the claims of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. An upgraded recycling method for PET based on in-situ generation of base-rich proton salts for catalytic alcoholysis, characterized in that: Includes the following steps: PET waste is cut and pre-treated to obtain PET fragments; An organic base was dissolved in an appropriate amount of ethylene glycol and ultrasonically dispersed until homogeneous. Amino acids were added and ultrasonically dispersed again until homogeneous. PET fragments were added and the PET fragments were catalytically hydrolyzed under an inert gas atmosphere to obtain the hydrolysis product. The molar ratio of the organic base to the amino acid is 1:(0.25-0.5), and the amount of the strong organic base used is 6%-10% of the mass of the PET fragments.

2. The upgraded recycling method for PET based on in-situ generation of base-rich proton salt catalytic alcoholysis according to claim 1, characterized in that: The mass ratio of ethylene glycol to PET fragments is (5-5.5):

1.

3. The upgraded recycling method for PET based on in-situ generation of base-rich proton salt catalytic alcoholysis according to claim 1, characterized in that: The alcoholysis temperature is 170–180°C, and the alcoholysis time is 120–240 min.

4. The upgraded recycling method for PET based on in-situ generation of base-rich proton salt catalytic alcoholysis according to claim 3, characterized in that: It also includes the following steps: The alcoholysis product was cooled to room temperature and then crystallized at 0–4 °C. The crystal precipitate was filtered to obtain crude diethyl terephthalate. Crude diethyl terephthalate was dissolved in an appropriate amount of water at 70–80 °C, filtered, and the filtrate was placed at 0–4 °C for crystallization. The crystal precipitate was then filtered to obtain diethyl terephthalate.

5. An upgraded recycling method for PET based on in-situ generation of base-rich proton salts for catalytic alcoholysis, characterized in that: Includes the following steps: PET waste is cut and pre-treated to obtain PET fragments; Dissolve an organic base in an appropriate amount of methanol, ultrasonically disperse until homogeneous, add amino acids, continue ultrasonic dispersion until homogeneous, add PET fragments, and stir at 145–150°C for 120–180 min to catalyze the alcoholysis of PET fragments to obtain methanolysis products; The molar ratio of the organic base to the amino acid is 1:(0.25-0.5), and the amount of the strong organic base used is 6%-10% of the mass of the PET fragments.

6. The upgraded recycling method for PET based on in-situ generation of base-rich proton salt catalytic alcoholysis according to claim 5, characterized in that: The mass ratio of methanol to PET fragments is (3-5):

1.

7. The upgraded recycling method for PET based on in-situ generation of base-rich proton salt catalytic alcoholysis according to claim 5, characterized in that: It also includes the following steps: The methanol hydrolysis product was cooled to room temperature, filtered, washed with methanol, and dried under vacuum to obtain dimethyl terephthalate.

8. The upgraded recycling method for PET based on in-situ generation of base-rich proton salt catalytic alcoholysis according to any one of claims 1 to 7, characterized in that: The organic base is at least one of 1,5,7-triazabicyclo[4.4.0]dodec-5-ene, 1,5-diazabicyclo[4.3.0]non-5-ene, and 1,8-diazabicyclo[5.4.0]undec-7-ene; the amino acid is L-valine or proline.

9. The upgraded recycling method for PET based on in-situ generation of base-rich proton salt catalytic alcoholysis according to claim 8, characterized in that: The organic base is 1,5,7-triazacyclo[4.4.0]dodecyl-5-ene, and the amino acid is L-valine.

10. The upgraded recovery method for PET based on in-situ generation of base-rich proton salt catalytic alcoholysis according to any one of claims 1 to 7, characterized in that: The PET waste is discarded PET bottles.

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