Method for preparing dimethyl terephthalate by one-step catalysis of waste polyester and dimethyl terephthalate

Abstract

The present invention relates to a method for preparing dimethyl terephthalate from waste polyester by one-step catalysis, and dimethyl terephthalate. The method for preparing dimethyl terephthalate from waste polyester by one-step catalysis comprises the following steps: mixing waste polyester material, a catalyst, and a depolymerization liquid for depolymerization reaction to obtain dimethyl terephthalate; the catalyst is selected from at least one of aromatic acids, aromatic acid alkali metal salts, combinations of aromatic acids and carbonates, combinations of aromatic acids and phosphates, combinations of aromatic acids and acetates, combinations of aromatic acid alkali metal salts and carbonates, combinations of aromatic acid alkali metal salts and phosphates, and combinations of aromatic acid alkali metal salts and acetates. The present invention also provides dimethyl terephthalate produced by the method. The present invention solves the problems of low product purity and yield in existing polyester waste chemical recovery methods, as well as the problems of high catalyst costs, harsh reaction conditions, and the tendency to cause secondary pollution to the environment in existing recovery methods.

Description

Technical Field

[0001] The invention relates to the technical field of waste polyester resource recovery, and in particular to a method for preparing dimethyl terephthalate by catalyzing waste polyester in one step and the dimethyl terephthalate. Background Art

[0002] With global plastic consumption continuing to rise, the recycling and high-value conversion of polyester waste (such as PET) has become a key issue in addressing "white pollution" and promoting resource recycling. Chemical recycling has attracted considerable attention due to its ability to efficiently convert waste polyester into monomer feedstocks (such as dimethyl terephthalate (DMT). However, existing chemical recycling methods still face numerous challenges and urgently need improvement for industrial application.

[0003] For example, the acid-base synergistic catalytic method uses phenolic compounds as additives in conjunction with alkaline catalysts to achieve polyester depolymerization at temperatures between 60°C and 200°C. While this method can improve depolymerization efficiency to a certain extent, the phenolic additives pose potential environmental risks, making it difficult to meet the demand for high-end materials. The mixed metal catalytic method, which uses mixed metal catalysts such as zinc chloride and zinc acetate, requires high temperatures of 220°C to 300°C and reaction times of 8 to 24 hours. While this method offers high yields, the high temperatures lead to a significant increase in energy consumption, and the pretreatment steps are cumbersome, requiring oligomer separation and prone to metal residues. This poor process economics makes it difficult to apply to industrial production. The organic base synergistic catalytic method uses phenanthene / guanidine organic bases in conjunction with inorganic bases to achieve rapid depolymerization at temperatures between 25°C and 80°C. However, this method relies on toxic solvents such as DMAC (N,N-dimethylacetamide) and DMF (N,N-dimethylformamide) for pretreatment, resulting in high catalyst costs and limiting its large-scale application. The diol-methanol two-step process utilizes high-temperature, high-pressure (150°C-260°C, 10-70 atm) depolymerization combined with alcoholysis. However, the multiple reaction steps and complex separation process significantly increase equipment investment and operating costs. High temperatures exacerbate side reactions, affecting DMT purity. The continuous alcoholysis process, which utilizes a series of multi-stage alcoholysis reactors for continuous production, achieves a DMT yield of 92%. This process requires alcoholysis and subsequent transesterification reactions at high temperatures of 180°C-200°C, resulting in a lengthy process that is prone to the generation of byproducts and heavy metal residues. The complex equipment (multi-stage reactors and distillation systems) also results in high investment and maintenance costs. The glycolate-catalyzed method uses monosodium ethylene glycol as a catalyst to depolymerize PET in methanol. However, catalyst preparation requires multiple steps (heating, drying, and suspension aging), making the process complex and time-consuming (aging cycle up to 7 days). The reaction requires premixed solvents and strict temperature control (60°C). Industrial continuity is limited, and there is a risk of strong alkaline wastewater contamination and catalyst residues in the product.

[0004] In summary, existing chemical recycling methods for polyester waste suffer from common problems such as harsh reaction conditions, high catalyst costs, insufficient product purity, and complex processes, which severely restrict their industrial application. Therefore, developing an efficient, environmentally friendly, and economical polyester waste recycling method that can produce high-purity products is of great practical significance. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a method for preparing dimethyl terephthalate from waste polyester by catalysis in one step, and dimethyl terephthalate, so as to solve the problems of low product purity and yield in existing chemical recovery methods of polyester waste. It can also solve the problems of high catalyst cost, harsh reaction conditions, many side reactions, and easy secondary pollution to the environment in existing chemical recovery methods of polyester waste.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing dimethyl terephthalate by catalyzing waste polyester in one step comprises the following steps:

[0008] Mixing waste polyester material, catalyst and depolymerization liquid for depolymerization reaction to obtain dimethyl terephthalate;

[0009] The catalyst is selected from at least one of aromatic acids, aromatic acid alkali metal salts, combinations of aromatic acids and carbonates, combinations of aromatic acids and phosphates, combinations of aromatic acids and acetates, combinations of aromatic acid alkali metal salts and carbonates, combinations of aromatic acid alkali metal salts and phosphates, and combinations of aromatic acid alkali metal salts and acetates.

[0010] By mixing waste polyester materials, a catalyst and a depolymerization liquid to carry out a depolymerization reaction, the waste polyester can be directly and efficiently converted into high-value-added dimethyl terephthalate, thereby realizing high-value recycling of waste polyester and improving the recycling rate of resources. The obtained dimethyl terephthalate has the advantages of high yield and purity, effectively solving the problems of low product purity and yield in existing polyester waste chemical recycling methods.

[0011] Aromatic acids, alkali metal salts of aromatic acids and their compositions with carbonates, phosphates, acetates, etc. are used as catalysts. These catalysts are widely available, low in cost, and exhibit good catalytic activity and stability during the reaction process, effectively reducing production costs and solving the problem of high catalyst costs in existing polyester waste chemical recovery methods.

[0012] The present invention has strong adaptability to colored or impure polyester (such as colored PET) and excellent anti-interference capabilities. Even when complex components such as pigments and impurities are present in the waste polyester, the depolymerization reaction can still be carried out efficiently, eliminating the need for complex pretreatment of the waste polyester. This further reduces production costs and improves the practicality and economic efficiency of the process.

[0013] During polyester depolymerization, the catalytic system achieves efficient depolymerization through a synergistic effect of multiple mechanisms: alkaline catalysts (such as Na2CO3 and K3PO4) increase the pH of the system, promoting the deprotonation of methanol / ethylene glycol hydroxyl groups to generate highly reactive methoxy / ethylene glycol oxygen anions, which cleave the PET ester bond via nucleophilic attack. Lewis acid catalysts (such as potassium acetate and magnesium acetate) coordinate the carbonyl oxygen atom with the metal ion, polarizing the ester bond and enhancing its reactivity, thereby accelerating depolymerization kinetics. Buffers (such as NaHCO3 and sodium benzoate) maintain a weakly alkaline environment (pH 7-9), preserving the activity of the nucleophile while inhibiting side reactions. Aromatic ring-containing catalysts (such as sodium p-hydroxybenzoate) adsorb on the PET surface through π-π stacking and hydrogen bonding, stabilizing the transition state and improving regioselectivity. Phase transfer catalysts (such as potassium p-sulfobenzoate) enhance solid-liquid mass transfer by reducing interfacial tension. Thermally stable catalysts (such as Cs3PO4) maintain structural stability at high temperatures and, combined with recyclable designs (such as selective precipitation with calcium acetate), achieve a green recycling process. This multi-mechanism synergistic system reduces the reaction activation energy by 30-50%, achieves a DMT selective output of more than 99.9% under mild conditions of 120°C-180°C, effectively avoids side reactions such as hydrolysis and oxidation, and provides a technical solution for the chemical recycling of waste PET that is both efficient and sustainable.

[0014] Preferably, the aromatic acid is selected from at least one of benzoic acid, o-methylbenzoic acid, m-methylbenzoic acid, p-methylbenzoic acid, o-aminobenzoic acid, m-aminobenzoic acid, p-aminobenzoic acid, o-hydroxybenzoic acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, o-methoxybenzoic acid, m-methoxybenzoic acid, p-methoxybenzoic acid, ascorbic acid, 2,4-dihydroxybenzoic acid, 3,4,5-trihydroxybenzoic acid and 4-hydroxy-3-methoxybenzoic acid.

[0015] Preferably, the aromatic acid alkali metal salt is selected from at least one of sodium benzoate, potassium benzoate, sodium o-methylbenzoate, potassium o-methylbenzoate, sodium m-methylbenzoate, potassium m-methylbenzoate, sodium p-methylbenzoate, potassium p-methylbenzoate, sodium o-hydroxybenzoate, potassium o-hydroxybenzoate, sodium m-hydroxybenzoate, potassium m-hydroxybenzoate, sodium p-hydroxybenzoate, potassium p-hydroxybenzoate, sodium o-anisothiobenzoate, potassium o-anisothiobenzoate, sodium m-anisothiobenzoate, potassium m-anisothiobenzoate, sodium p-anisothiobenzoate, potassium p-anisothiobenzoate, sodium 2,4-dihydroxybenzoate, potassium 2,4-dihydroxybenzoate, sodium 3,4,5-trihydroxybenzoate, potassium 3,4,5-trihydroxybenzoate, sodium 4-hydroxy-3-methoxybenzoate, potassium 4-hydroxy-3-methoxybenzoate, and potassium p-sulfobenzoate.

[0016] Preferably, the carbonate is selected from at least one of sodium bicarbonate, potassium bicarbonate, sodium carbonate and potassium carbonate.

[0017] Preferably, the phosphate is selected from at least one of trisodium phosphate, tripotassium phosphate and tricesium phosphate.

[0018] Preferably, the acetate is selected from at least one of sodium acetate, potassium acetate, calcium acetate and magnesium acetate.

[0019] Preferably, the catalyst is selected from potassium p-sulfobenzoate, a combination of potassium carbonate and benzoic acid, a combination of potassium bicarbonate and sodium benzoate, a combination of sodium acetate and p-hydroxybenzoic acid, a combination of tripotassium phosphate and o-hydroxybenzoic acid, a combination of potassium acetate and p-methoxybenzoic acid, a combination of potassium carbonate and ascorbic acid, a combination of potassium bicarbonate and p-toluic acid, a combination of sodium carbonate and 2,4-dihydroxybenzoic acid, a combination of trisodium phosphate and sodium 2,4-dihydroxybenzoate, a combination of potassium bicarbonate and 3,4,5-trihydroxybenzoic acid, or a combination of potassium bicarbonate and 4-hydroxy-3-methoxybenzoic acid.

[0020] Preferably, the depolymerization liquid is selected from a mixture of methanol and ethylene glycol.

[0021] Experimental research has found that the addition of ethylene glycol significantly reduces the viscosity of the reaction system and improves mass transfer efficiency. Furthermore, the interaction between its hydroxyl groups and the polyester molecular chains facilitates the directional formation of DMT. Experiments have shown that a 4:1 volume ratio of methanol to ethylene glycol achieves the optimal balance between reaction rate and product purity.

[0022] By using a methanol-ethylene glycol dual-solvent system as the depolymerization solution, we achieve a balance between reaction activity and product separation convenience. Methanol helps increase the reaction rate, while ethylene glycol facilitates product separation and purification. The synergistic effect of the two improves product purity and yield, simplifying subsequent separation and purification processes.

[0023] Preferably, the volume ratio of the methanol to the ethylene glycol is 4:1 to 8:1.

[0024] Preferably, the waste polyester material is at least one selected from waste polyethylene terephthalate (PET), waste polybutylene terephthalate (PBT), waste polypropylene terephthalate (PTT) and waste polycyclohexanedimethylene terephthalate (PCT).

[0025] Among them, polyethylene terephthalate (PET) is the focus of this invention due to its wide range of applications and recycling needs. The waste polyester material needs to be pre-crushed into centimeter-sized fragments or pieces to increase the reaction contact area and improve the depolymerization efficiency.

[0026] Preferably, the temperature of the depolymerization reaction is 120°C to 180°C.

[0027] Preferably, the depolymerization reaction time is 1 h to 8 h.

[0028] Preferably, the depolymerization reaction time is 2h~6h.

[0029] Preferably, the temperature of the depolymerization reaction is 140°C to 160°C.

[0030] Experimental results indicate that depolymerization under these temperature and time conditions allows for complete depolymerization of the polyester molecular chains while avoiding high-temperature-induced decomposition or carbonization of the DMT. For example, using potassium p-sulfobenzoate as a catalyst, a PET conversion rate of 99.8% and a DMT yield of 99.5% were achieved at 140°C for four hours.

[0031] The reaction conditions adopted in the present invention are mild, which greatly reduces the corrosion of equipment and energy consumption compared to the harsh conditions such as high temperature and high pressure commonly used in the prior art, while improving the safety of the production process, making it more suitable for industrial continuous production.

[0032] Preferably, the amount of the catalyst used is 1 wt% to 20 wt% of the waste polyester material.

[0033] By optimizing the catalyst dosage and solvent ratio, reaction energy consumption and subsequent purification costs can be significantly reduced. For example, using potassium p-sulfobenzoate as a catalyst, with a catalyst dosage of 20% of the polyester mass and a depolymerization solution dosage of 4 mL / g, DMT can be directly isolated by crystallization after the reaction with a purity exceeding 99.9%.

[0034] Preferably, the mass ratio of the catalyst to the polyester waste plastic is (0.005-0.2):1; and the amount of the depolymerization liquid added to each gram of polyester waste plastic is 1-6 mL.

[0035] Preferably, the depolymerization liquid and the waste polyester material are in a ratio of 2:1 to 6:1 in terms of mL:g.

[0036] Preferably, after the depolymerization reaction is completed, a mixed solution and a solid are obtained, and the solid is dimethyl terephthalate;

[0037] The obtained mixed solution is distilled, and the distillate is ethylene glycol.

[0038] Preferably, the yield of dimethyl terephthalate can reach 99.9%, the purity of dimethyl terephthalate can reach 99.9%, the yield of ethylene glycol can reach 98.9%, and the purity of ethylene glycol can reach 99.3%.

[0039] The present invention also provides dimethyl terephthalate prepared by the method of the present invention.

[0040] Beneficial effects of the present invention:

[0041] The method for preparing dimethyl terephthalate from waste polyester of the present invention can directly and efficiently convert the waste polyester into high-value-added dimethyl terephthalate by mixing the waste polyester material, a catalyst and a depolymerization liquid to carry out a depolymerization reaction, thereby achieving high-value-added recycling of the waste polyester, improving the recycling rate of resources, and the obtained dimethyl terephthalate has the advantages of high yield and purity. The catalysts selected by the present invention are rich in variety, including aromatic acids, aromatic acid alkali metal salts and compositions thereof with carbonates, phosphates, acetates, etc. These catalysts are widely available and relatively low in cost. In addition, they exhibit good catalytic activity and stability during the reaction, effectively reducing production costs. The method solves the problems of high catalyst cost, harsh reaction conditions, multiple side reactions, and easy secondary pollution to the environment in existing polyester waste chemical recycling methods. The entire production process of the present invention is more environmentally friendly, reduces negative impacts on the environment, and meets the requirements of sustainable development. The present invention not only improves the recycling rate of waste polyester and reduces production costs, but also reduces waste emissions and environmental pollution, has both significant economic and environmental benefits, and provides an efficient and environmentally friendly solution for the high-value recycling of waste polyester. It has broad application prospects and is worth promoting and applying in the field of waste polyester resource recycling technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The DMT prepared in Example 1 1 H NMR spectrum;

[0043] Figure 2 This is a graph showing the test results of a third-party agency for the DMT prepared in Example 52. DETAILED DESCRIPTION

[0044] The following will describe the embodiments of the present invention with reference to preferred embodiments. Those skilled in the art will readily appreciate the other advantages and benefits of the present invention from the disclosure herein. The present invention may also be implemented or applied through various other specific embodiments, and the details in this specification may be modified or altered based on different viewpoints and applications without departing from the spirit of the present invention. It should be understood that the preferred embodiments are intended only to illustrate the present invention and are not intended to limit the scope of protection of the present invention.

[0045] Example 1

[0046] A method for preparing dimethyl terephthalate by catalyzing waste polyester in one step comprises the following steps:

[0047] 2000 g of waste polyethylene terephthalate (PET) bottle flakes, 200 g of potassium p-sulfobenzoate and 8000 mL of depolymerization solution (according to V (甲醇) :V (乙二醇) = 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 140°C for depolymerization for 8 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0048] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0049] Example 2

[0050] In this embodiment, except for the depolymerization liquid (according to V (甲醇) :V (乙二醇) = 6:1), the other conditions were the same as in Example 1.

[0051] Example 3

[0052] In this embodiment, except for the depolymerization liquid (according to V (甲醇) :V (乙二醇) = 8:1), the other conditions were the same as in Example 1.

[0053] Comparative Example 1

[0054] The method for preparing dimethyl terephthalate from waste polyester comprises the following steps:

[0055] 2000 g of waste polyethylene terephthalate (PET) bottle flakes, 200 g of potassium p-sulfobenzoate and 8000 mL of depolymerization solution (according to V (甲醇) :V (乙二醇)= 0.5:1) were mixed and added into a titanium alloy reactor, placed on a heating plate, stirred and heated to 140°C for depolymerization reaction for 8 hours. After the reaction was completed, the temperature was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT);

[0056] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0057] Comparative Example 2

[0058] In this comparative example, except for the depolymerization liquid (according to V (甲醇) :V (乙二醇) = 2:1), the other conditions were the same as those in Control Example 1.

[0059] Example 4

[0060] In this example, except that the depolymerization reaction time was 4 h, other conditions were the same as those in Example 1.

[0061] Example 5

[0062] In this example, except that the depolymerization reaction time was 6 h, other conditions were the same as those in Example 1.

[0063] Comparative Example 3

[0064] The method for preparing dimethyl terephthalate from waste polyester comprises the following steps:

[0065] 2000 g of waste polyethylene terephthalate (PET) bottle flakes, 200 g of potassium p-sulfobenzoate and 8000 mL of depolymerization solution (according to V (甲醇) :V (乙二醇) = 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 140°C for depolymerization for 0.5 h. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0066] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0067] Comparative Example 4

[0068] In this comparative example, except that the depolymerization reaction time was 2 h, the other conditions were the same as those in comparative example 3.

[0069] Example 6

[0070] A method for preparing dimethyl terephthalate by catalyzing waste polyester in one step comprises the following steps:

[0071] 2000 g of waste polyethylene terephthalate (PET) bottle flakes, 200 g of potassium p-sulfobenzoate and 8000 mL of depolymerization solution (according to V (甲醇) :V (乙二醇) = 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 140°C for depolymerization for 4 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0072] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0073] Comparative Example 5

[0074] The method for preparing dimethyl terephthalate from waste polyester comprises the following steps:

[0075] 2000 g of waste polyethylene terephthalate (PET) bottle flakes, 200 g of potassium p-sulfobenzoate and 8000 mL of depolymerization solution (according to V (甲醇) :V (乙二醇) = 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 100°C for depolymerization for 4 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0076] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0077] Comparative Example 6

[0078] In this comparative example, except that the depolymerization reaction temperature was 120° C., other conditions were the same as those in comparative example 5.

[0079] Comparative Example 7

[0080] In this comparative example, except that the depolymerization reaction temperature was 160° C., other conditions were the same as those in comparative example 5.

[0081] Comparative Example 8

[0082] In this comparative example, except that the depolymerization reaction temperature was 180° C., other conditions were the same as those in comparative example 5.

[0083] Example 7

[0084] A method for preparing dimethyl terephthalate by catalyzing waste polyester in one step comprises the following steps:

[0085] 2000 g of waste polyethylene terephthalate (PET) bottle flakes, 400 g of potassium p-sulfobenzoate and 1000 mL of depolymerization solution (according to V (甲醇) :V (乙二醇) = 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 140°C for depolymerization for 4 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0086] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0087] Example 8

[0088] In this comparative example, except that the amount of p-sulfobenzoic acid potassium salt was 100 g, the other conditions were the same as those in Example 7.

[0089] Example 9

[0090] In this comparative example, except that the amount of p-sulfobenzoic acid potassium salt is 200 g, the other conditions are the same as those in Example 7.

[0091] Comparative Example 9

[0092] The method for preparing dimethyl terephthalate from waste polyester comprises the following steps:

[0093] 2000 g of waste polyethylene terephthalate (PET) bottle flakes, 50 g of potassium p-sulfobenzoate and 8000 mL of depolymerization solution (according to V (甲醇) :V (乙二醇) = 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 140°C for depolymerization for 4 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0094] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0095] Example 10

[0096] A method for preparing dimethyl terephthalate by catalyzing waste polyester in one step comprises the following steps:

[0097] 2000 g of waste polyethylene terephthalate (PET) bottle flakes, 200 g of potassium p-sulfobenzoate and 9000 mL of depolymerization solution (according to V (甲醇) :V (乙二醇)= 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 140°C for depolymerization for 4 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0098] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0099] Example 11

[0100] In this example, except that the amount of the depolymerization liquid was 12000 mL, the other conditions were the same as those in Example 10.

[0101] Comparative Example 10

[0102] The method for preparing dimethyl terephthalate from waste polyester comprises the following steps:

[0103] 2000 g of waste polyethylene terephthalate (PET) bottle flakes, 200 g of potassium p-sulfobenzoate and 3000 mL of depolymerization solution (according to V (甲醇) :V (乙二醇) = 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 140°C for depolymerization for 4 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0104] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0105] Comparative Example 11

[0106] In this comparative example, except that the amount of the depolymerization liquid used was 6000 mL, the other conditions were the same as those of comparative example 10.

[0107] Example 12

[0108] A method for preparing dimethyl terephthalate by catalyzing waste polyester in one step comprises the following steps:

[0109] 2000 g of waste polyethylene terephthalate (PET) bottle flakes, 200 g of benzoic acid and 8000 mL of depolymerization liquid (according to V (甲醇) :V (乙二醇) = 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 140°C for depolymerization for 4 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0110] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0111] Example 13

[0112] In this example, except that sodium benzoate was used as the catalyst, the other conditions were the same as those in Example 12.

[0113] Example 14

[0114] In this embodiment, except that p-toluic acid is used as the catalyst, the other conditions are the same as those in Example 12.

[0115] Example 15

[0116] In this example, except that p-hydroxybenzoic acid was used as the catalyst, the other conditions were the same as those in Example 12.

[0117] Example 16

[0118] In this embodiment, except that potassium p-methylbenzoate is used as the catalyst, the other conditions are the same as those in Example 12.

[0119] Example 17

[0120] In this embodiment, except that m-aminobenzoic acid is used as the catalyst, the other conditions are the same as those in Example 12.

[0121] Example 18

[0122] In this example, except that sodium p-hydroxybenzoate was used as the catalyst, the other conditions were the same as those in Example 12.

[0123] Example 19

[0124] In this example, except that p-methoxybenzoic acid was used as the catalyst, the other conditions were the same as those in Example 12.

[0125] Example 20

[0126] In this embodiment, except that sodium p-methoxybenzoate is used as the catalyst, the other conditions are the same as those in Example 12.

[0127] Example 21

[0128] In this embodiment, except that 2,4-dihydroxybenzoic acid is used as the catalyst, other conditions are the same as those in Example 12.

[0129] Example 22

[0130] In this embodiment, except that sodium 2,4-dihydroxybenzoate is used as the catalyst, other conditions are the same as those in Example 12.

[0131] Example 23

[0132] In this embodiment, except that 3,4,5-trihydroxybenzoic acid is used as the catalyst, other conditions are the same as those in Example 12.

[0133] Example 24

[0134] In this example, except that 4-hydroxy-3-methoxybenzoic acid was used as the catalyst, the other conditions were the same as those in Example 12.

[0135] Example 25

[0136] In this embodiment, except that o-toluic acid is used as the catalyst, the other conditions are the same as those in Example 12.

[0137] Example 26

[0138] In this embodiment, except that m-toluic acid is used as the catalyst, the other conditions are the same as those in Example 12.

[0139] Example 27

[0140] In this example, except that o-aminobenzoic acid was used as the catalyst, the other conditions were the same as those in Example 12.

[0141] Example 28

[0142] In this embodiment, except that p-aminobenzoic acid is used as the catalyst, the other conditions are the same as those in Example 12.

[0143] Example 29

[0144] In this example, except that m-methoxybenzoic acid was used as the catalyst, the other conditions were the same as those in Example 12.

[0145] Example 30

[0146] In this example, except that ascorbic acid is used as the catalyst, the other conditions are the same as those in Example 12.

[0147] Comparative Example 12

[0148] The method for preparing dimethyl terephthalate from waste polyester comprises the following steps:

[0149] 2000 g of waste polyethylene terephthalate (PET) bottle flakes, 200 g of potassium carbonate and 8000 mL of depolymerization solution (according to V (甲醇) :V (乙二醇) = 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 140°C for depolymerization for 4 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0150] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0151] Comparative Example 13

[0152] In this comparative example, except that tripotassium phosphate was used as the catalyst, the other conditions were the same as those in comparative example 12.

[0153] Comparative Example 14

[0154] In this comparative example, except that potassium acetate was used as the catalyst, the other conditions were the same as those in comparative example 12.

[0155] Example 31

[0156] A method for preparing dimethyl terephthalate by catalyzing waste polyester in one step comprises the following steps:

[0157] 2000 g of waste polyethylene terephthalate (PET) bottle flakes, 200 g of catalyst (100 g of potassium carbonate + 300 g of benzoic acid) and 8000 mL of depolymerization liquid (according to V (甲醇) :V (乙二醇) = 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 120°C for depolymerization for 4 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0158] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0159] Example 32

[0160] In this example, except that the catalyst selected was 50g potassium bicarbonate + 150g sodium benzoate, the other conditions were the same as those in Example 31.

[0161] Example 33

[0162] In this example, except that the catalyst selected was 50g sodium acetate + 150g p-hydroxybenzoic acid, the other conditions were the same as those in Example 31.

[0163] Example 34

[0164] In this example, except that the catalyst selected was 50 g of tripotassium phosphate + 150 g of o-hydroxybenzoic acid, the other conditions were the same as those in Example 31.

[0165] Example 35

[0166] In this example, except that the catalyst selected was 50 g potassium acetate + 150 g p-methoxybenzoic acid, the other conditions were the same as those in Example 31.

[0167] Example 36

[0168] In this example, except that the catalyst selected was 50g potassium bicarbonate + 150g p-toluic acid, the other conditions were the same as those in Example 31.

[0169] Example 37

[0170] In this example, except that the catalyst selected was 50 g of sodium carbonate + 150 g of 2,4-dihydroxybenzoic acid, the other conditions were the same as those in Example 31.

[0171] Example 38

[0172] In this example, except that the catalyst selected was 50 g of trisodium phosphate + 150 g of sodium 2,4-dihydroxybenzoate, the other conditions were the same as those in Example 31.

[0173] Example 39

[0174] In this example, except that the catalyst selected was 50 g potassium bicarbonate + 150 g 3,4,5-trihydroxybenzoic acid, the other conditions were the same as those in Example 31.

[0175] Example 40

[0176] In this example, except that the catalyst selected was 50 g potassium bicarbonate + 150 g 4-hydroxy-3-methoxybenzoic acid, the other conditions were the same as those in Example 31.

[0177] Example 41

[0178] In this example, except that the catalyst selected was 50 g potassium carbonate + 150 g m-toluic acid, the other conditions were the same as those in Example 31.

[0179] Example 42

[0180] In this example, except that the catalyst selected was 50 g of tripotassium phosphate + 150 g of m-hydroxybenzoic acid, the other conditions were the same as those in Example 31.

[0181] Comparative Example 15

[0182] In this comparative example, except that the catalyst is 50g sodium acetate + 150g tripotassium phosphate, the other conditions are the same as those of comparative example 25.

[0183] Comparative Example 16

[0184] In this comparative example, except that the catalyst is 50g sodium carbonate + 150g calcium acetate, the other conditions are the same as those of comparative example 25.

[0185] Example 43

[0186] A method for preparing dimethyl terephthalate by catalyzing waste polyester in one step comprises the following steps:

[0187] 2000 g of waste polybutylene terephthalate (PBT) bottle pieces, 200 g of potassium p-sulfobenzoate and 8000 mL of depolymerization liquid (according to V (甲醇) :V (乙二醇) = 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 140°C for depolymerization for 4 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0188] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0189] Example 44

[0190] A method for preparing dimethyl terephthalate by catalyzing waste polyester in one step comprises the following steps:

[0191] 2000 g of discarded polytrimethylene terephthalate (PTT) bottle pieces, 200 g of potassium p-sulfobenzoate and 8000 mL of depolymerization solution (according to V (甲醇) :V (乙二醇) = 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 140°C for depolymerization for 4 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0192] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0193] Example 45

[0194] A method for preparing dimethyl terephthalate by catalyzing waste polyester in one step comprises the following steps:

[0195] 2000 g of discarded polycyclohexane terephthalate (PCT) bottle pieces, 200 g of potassium p-sulfobenzoate and 8000 mL of depolymerization solution (according to V (甲醇) :V (乙二醇)= 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 140°C for depolymerization for 4 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0196] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0197] Example 46

[0198] A method for preparing dimethyl terephthalate by catalyzing waste polyester in one step comprises the following steps:

[0199] 2000 g of waste polyethylene terephthalate (PET) colored bottle flakes, 200 g of potassium p-sulfobenzoate and 8000 mL of depolymerization solution (according to V (甲醇) :V (乙二醇) = 4:1) were added into a titanium alloy reactor, placed on a heating plate, stirred, and heated to 140°C for depolymerization for 4 hours. After the reaction was complete, the mixture was cooled to room temperature to obtain a mixed solution and a precipitated solid, which was dimethyl terephthalate (DMT).

[0200] The mixed liquid is recovered by distillation to obtain distillate, which is ethylene glycol.

[0201] Example 47

[0202] In this embodiment, except for the use of waste polyethylene terephthalate (PET) non-woven fabric, other conditions are the same as those in Example 46.

[0203] Example 48

[0204] In this embodiment, except for the use of discarded polyethylene terephthalate (PET) blister sheets, other conditions are the same as those in Example 46.

[0205] Example 49

[0206] In this embodiment, except for the use of waste polyethylene terephthalate (PET) film, other conditions are the same as those in Example 46.

[0207] Example 50

[0208] In this embodiment, except for the use of waste polyethylene terephthalate (PET) foam material, other conditions are the same as those in Example 46.

[0209] Example 51

[0210] In this embodiment, except for the use of discarded polyethylene terephthalate (PET) waste clothing, other conditions are the same as those in Example 46.

[0211] Example 52

[0212] A method for preparing dimethyl terephthalate from waste polyester by one-step catalysis. This embodiment is a ton-scale scale-up experiment for optimal depolymerization conditions, comprising the following steps:

[0213] 500 kg of crushed waste polyethylene terephthalate (PET) bottle flakes, 50 kg of potassium p-sulfobenzoate and 2000 L of depolymerization liquid (according to V (甲醇) :V (乙二醇) = 4:1) mixed and added into a volume of 3.4m 3 The product was placed in a 316 stainless steel high-pressure reactor on a heating plate, stirred and heated to 140°C for 4 hours for depolymerization. After the reaction was completed, 485 kg of DMT was obtained through cooling, crystallization, filtration, washing and drying (the filtration, washing and drying processes resulted in a 4% error in the yield) with a purity of 99.95%.

[0214] Detection and Analysis

[0215] 1) Nuclear magnetic resonance analysis

[0216] The dimethyl terephthalate (DMT) prepared in Example 1 was subjected to nuclear magnetic resonance analysis, and the results were as follows: Figure 1 shown.

[0217] from Figure 1 The analysis showed that the purity of the obtained DMT was ≥99.9%.

[0218] 2) Yield and purity analysis of dimethyl terephthalate (DMT) and ethylene glycol (EG)

[0219] The precipitated solids obtained in Examples 1 to 52 and Comparative Examples 1 to 16, namely dimethyl terephthalate (DMT), were dissolved in dichloromethane. The DMT yield and purity of the solution were determined by NMR. Ethylene glycol (EG) was recovered by distillation, and the EG yield and purity of the distillate were determined by NMR. The results are shown in Tables 1 to 10.

[0220] Table 1 is the comparison results of different volume ratios of methanol and ethylene glycol

[0221]

[0222] Table 2 shows the comparison results of different depolymerization reaction times

[0223]

[0224] Table 3 shows the comparative results of different depolymerization reaction temperatures.

[0225]

[0226] Table 4 is the comparison results of different dosages of potassium p-sulfobenzoate

[0227]

[0228] Table 5 is the comparison results of different dosages of depolymerization liquid

[0229]

[0230] Table 6 shows the comparison results of catalyst types

[0231]

[0232] Table 7 shows the types of catalysts used in Examples 31 to 42, and Comparative Examples 15 and 16.

[0233]

[0234] Table 8 is the comparison results of different catalyst combinations

[0235]

[0236] Table 9 is the comparison results of different waste polyester materials

[0237]

[0238] Table 10 is the comparison results of different polyester products

[0239]

[0240] The DMT obtained in Example 52 was measured by a third-party agency. The results are as follows Figure 2 shown.

[0241] from Figure 2 As can be seen from the table, the DMT obtained in Example 52 was 485 kg (the filtration, washing, and drying processes resulted in a 4% error in yield) and had a purity of 99.95%.

[0242] In summary, the present method for preparing dimethyl terephthalate from waste polyester in a single step offers the following advantages: 1) A green and efficient catalyst system, eliminating traditional toxic metal or complex organic catalysts and employing a composite catalytic system of carbonates, phosphates, acetates, aromatic acids, and alkali metal salts of aromatic acids. For example, the synergistic effect of potassium carbonate and benzoic acid is not only low-cost and environmentally friendly, but also allows precise control of the reaction pathway through the acid-base synergistic effect, suppressing the formation of side reactions. Compared to existing phenolic additives, this system completely eliminates environmental risks; compared to metal catalysts, it avoids product contamination and significantly improves product purity. 2) Mild reaction conditions and ultra-short reaction times: the optimized reaction temperature is 120°C to 160°C, and the reaction time is shortened to 1 to 8 hours. Compared to existing high-temperature processes of 220°C to 300°C and complex pretreatment, this method achieves low-temperature, efficient conversion through catalyst active site design and mass transfer enhancement, reducing energy consumption by over 40%, significantly reducing equipment corrosion, and eliminating the need for additional solvents. 3) A breakthrough in product purity. Through catalyst selectivity and reaction kinetics, DMT purity can be consistently achieved at ≥99.9%, far exceeding existing technologies (generally <99%). By suppressing the formation of residual oligomers and byproducts, this technology directly produces electronic-grade DMT, eliminating the costly distillation step and providing high-quality raw material for downstream high-end polyester synthesis. 4) Advantages of process integration and industrialization: A one-step continuous reaction unit is used, allowing waste polyester to be directly fed after simple crushing, eliminating the need for a multi-step depolymerization-alcoholysis separation process. The catalyst can be recycled more than five times, and combined with an online purification module, it enables efficient co-production of DMT and ethylene glycol, achieving an overall process yield exceeding 99%, a 15%-20% increase over conventional methods. This technology has significant application value in the field of waste polyester resource recovery technology.

[0243] The above embodiments are only preferred embodiments for fully illustrating the present invention, and the protection scope of the present invention is not limited thereto. Any equivalent substitution or modification made by those skilled in the art based on the present invention is within the protection scope of the present invention.

Claims

1. A method for preparing dimethyl terephthalate by catalyzing waste polyester in one step, characterized in that: The following steps are involved: Mixing waste polyester material, catalyst and depolymerization liquid for depolymerization reaction to obtain dimethyl terephthalate; The catalyst is selected from potassium p-sulfobenzoate, potassium p-methylbenzoate, sodium p-methoxybenzoate, sodium 2,4-dihydroxybenzoate, 3,4,5-trihydroxybenzoic acid, 4-hydroxy-3-methoxybenzoic acid, p-aminobenzoic acid, potassium bicarbonate and sodium benzoate, sodium acetate and p-hydroxybenzoic acid, tripotassium phosphate and o-hydroxybenzoic acid, potassium acetate and p-methoxybenzoic acid, trisodium phosphate and sodium 2,4-dihydroxybenzoate, or tripotassium phosphate and m-hydroxybenzoic acid; The temperature of the depolymerization reaction is 140°C to 160°C; The depolymerization liquid is selected from a mixture of methanol and ethylene glycol; The volume ratio of the methanol to the ethylene glycol is 4:1 to 8:1; The depolymerization reaction time is 4h~8h; The amount of the catalyst used is 5wt% to 20wt% of the waste polyester material; The ratio of the depolymerization liquid to the waste polyester material is 2:1 to 6:1 in terms of mL:g.

2. The method for preparing dimethyl terephthalate by catalyzing waste polyester in one step according to claim 1, characterized in that: The waste polyester material is at least one selected from waste polyethylene terephthalate (PET), waste polybutylene terephthalate (PBT), waste polypropylene terephthalate (PTT) and waste polycyclohexanedimethylene terephthalate (PCT).

3. The method for preparing dimethyl terephthalate by using waste polyester in one step catalysis according to claim 1, characterized in that: After the depolymerization reaction is completed, a mixed solution and a solid are obtained, wherein the solid is dimethyl terephthalate; The obtained mixed solution is distilled, and the distillate is ethylene glycol.

4. The method for preparing dimethyl terephthalate by catalyzing waste polyester in one step according to claim 3, characterized in that: The yield of the dimethyl terephthalate can reach 99.9%, and the purity of the dimethyl terephthalate can reach 99.9%; The yield of the ethylene glycol can reach 98.9%, and the purity of the ethylene glycol can reach 99.3%.

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