Method for efficiently producing terephthalate by depolymerizing waste polyester with carbonate ester

Abstract

The present invention belongs to the technical field of polyester recycling, and relates to a method for efficiently producing terephthalate by depolymerizing a waste polyester with a carbonate ester. The method comprises subjecting a waste polyester, a carbonate ester and a catalyst to a depolymerization reaction under the protection of nitrogen or an inert gas and under solvent-free conditions, wherein the catalyst is a mixture of A and B at a mass ratio of 10-20:1, A is one or more of zinc acetate, sodium acetate, potassium chloride, lithium chloride, potassium carbonate, sodium carbonate, antimony glycolate, antimony trioxide, antimony acetate, titanium glycolate and titanium dioxide, and B is sodium ethoxide. The present invention solves the problems of the limited conversion of monomers and the long production process during the chemical recovery of waste polyesters in the prior art, significantly improves the catalytic efficiency and the depolymerization speed by means of the compound catalyst, simplifies the depolymerization process, reduces the energy consumption, and is beneficial to industrial application.

Description

A method for efficiently producing terephthalate from waste polyester through carbonate depolymerization Technical Field

[0001] This invention belongs to the field of polyester recycling technology and relates to a method for efficiently producing terephthalate from waste polyester through carbonate depolymerization. Background Technology

[0002] Polyethylene terephthalate (PET), a high-performance thermoplastic polymer, is widely used in various aspects of life. However, post-consumer polyester is difficult to process directly due to its non-degradability. Chemical recycling of polyester, due to its simplicity and efficiency, has become one of the most direct methods for recycling large quantities of waste polyester. Chemical recycling refers to the process in which polyester undergoes transesterification with depolymerized molecules under the action of a catalyst, gradually breaking the chain to obtain monomer products. Depending on the depolymerized molecules, chemical recycling can be divided into methanol depolymerization, ethylene glycol depolymerization, hydrolysis, and ammonolysis, with the corresponding end products being monomers such as dimethyl terephthalate (DMT), diethyl terephthalate (BHET), and terephthalic acid (TPA). In particular, ethylene glycol depolymerization of polyester, due to its simple and mild reaction conditions and relatively easy product separation, has become the most commonly used method for polyester chemical recycling.

[0003] However, the depolymerization of polyesters with ethylene glycol suffers from chemical equilibrium issues, with the main products being small-molecule BHET and partially soluble PET oligomers. To promote the depolymerization reaction in the forward direction, high doses of ethylene glycol are typically required, which limits monomer conversion and yield. For example, patent application CN102875382A discloses a method for the alcohol depolymerization of ethylene glycol terephthalate catalyzed by metal acetate ionic liquids, but with a polyester segment content to ethylene glycol molar ratio of 1:20, the monomer product yield is only 88.5%.

[0004] Furthermore, to ensure the stable preparation of subsequent polymers, the product generated from the depolymerization of ethylene glycol typically requires secondary transesterification, reacting with methanol to generate dimethyl terephthalate (DMT) to ensure batch stability. For example, patent application CN118666683A discloses a solidification extraction process for chemically recycled dimethyl terephthalate; however, this method involves a lengthy overall depolymerization process for obtaining the monomer DMT and consumes a large amount of energy.

[0005] Depolymerization via carbonates is simple and efficient. For example, patent application CN117986114A discloses a method for alkylating polyester materials involving carbonates or carboxylic esters, using carbonates to catalyze the depolymerization of polyesters under solvent conditions via ionic liquids. However, even under solvent-enhanced conditions, waste polyesters still require a long reaction time (4-8 hours) for degradation, resulting in insufficient catalytic efficiency. The strategy of enhancing depolymerization through solvent effects leads to problems such as complex product separation operations, high equipment requirements, and significant challenges in industrialization. Summary of the Invention

[0006] The purpose of this invention is to solve the problems existing in the prior art and to provide a method for the efficient production of terephthalate from carbonate depolymerization waste polyester.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for efficiently producing terephthalic acid esters from waste polyester by depolymerization of carbonate involves carrying out a depolymerization reaction of waste polyester, carbonate, and catalyst under nitrogen or inert gas protection and solvent-free conditions. The catalyst is a mixture of A and B in a mass ratio of 10-20:1. A is one or more of zinc acetate, sodium acetate, potassium chloride, lithium chloride, potassium carbonate, sodium carbonate, antimony glycolate, antimony trioxide, antimony acetate, titanium glycolate, and titanium dioxide. B is sodium ethoxide.

[0009] As a preferred technical solution:

[0010] In the method for efficiently producing terephthalate from waste polyester via carbonate depolymerization as described above, the amount of catalyst added is 0.1 to 5 wt% of the mass of the waste polyester.

[0011] The method described above for the efficient production of terephthalate from waste polyester by depolymerization of carbonate has a molar ratio of carbonate to waste polyester of 3 to 30:1.

[0012] The above-described method for efficiently producing terephthalic acid esters from waste polyester by depolymerization of carbonate, wherein the carbonate is one or more of dimethyl carbonate, diethyl carbonate, and dibutyl carbonate.

[0013] The above describes a method for efficiently producing terephthalic acid esters from carbonate depolymerization waste polyester. The waste polyester is any form of product or waste product made from X or Y, where X is polyethylene terephthalate and Y is a copolyester obtained by copolymerizing polyethylene terephthalate with diacids and / or diols. Specifically, it can be solution-dyed fibers containing pigments / dyes, waste polyester fibers dyed with disperse dyes / cationic dyes, and their products.

[0014] The method for efficiently producing terephthalic acid esters from waste polyester by depolymerization of carbonate, as described above, involves a depolymerization reaction temperature of 160–200°C and a depolymerization reaction time of 30–200 min.

[0015] The method described above for the efficient production of terephthalic acid esters from waste polyester via carbonate depolymerization involves the following steps: After the depolymerization reaction, the depolymerization product is first subjected to vacuum devolatilization at 140–190°C. The removed volatile components are collected to obtain a mixture of terephthalic acid esters and carbonates. Then, the mixture of terephthalic acid esters and carbonates is subjected to vacuum devolatilization at 70–100°C. The removed volatile components are collected to obtain carbonates. The remaining components are collected to obtain terephthalic acid esters (the main product).

[0016] The method described above for the efficient production of terephthalic acid esters from waste polyester by depolymerization of carbonate involves performing vacuum devolatilization on the depolymerization reaction product at 140–160°C, collecting the residual components, and then washing, separating, and filtering the residual components with water to obtain ethylene carbonate.

[0017] This invention can also achieve efficient separation of products. Through gas-liquid separation, catalysts, ethylene carbonate, carbonates, and terephthalates can be separated efficiently and with high quality.

[0018] The method described above for the efficient production of terephthalic acid esters from waste polyester via carbonate depolymerization has the following characteristics: the solid-phase conversion rate of the waste polyester is 80-100%, the conversion rate of terephthalic acid ester is 80-100%, the purity of the terephthalic acid ester is 99%-99.9%, and the oligomer content in the terephthalic acid ester is 0-0.5 wt%.

[0019] Invention principle:

[0020] The reaction system of this invention adopts a near-molten system, which avoids the use of other solvents, reduces the difficulty of product separation and the requirements for equipment;

[0021] The catalyst of this invention is a compound of A (conventional metal salt) and B (sodium ethoxide). If the catalyst is only A, the catalytic efficiency is low due to the insufficient nucleophilicity of conventional metal salt. If the catalyst is only B, the depolymerization reaction is difficult to control due to the strong hydrolysis side reaction of sodium ethoxide, resulting in inconsistent depolymerization between batches. By compounding A and B, B (as a co-catalyst) can make up for the deficiency of insufficient nucleophilicity of A (the main catalyst), resulting in higher catalytic efficiency. At the same time, A has excellent stability, which can ensure the excellent high-temperature stability of the catalyst, thus enabling the depolymerization reaction to be carried out in a near-molten system, significantly improving the depolymerization efficiency.

[0022] In a near-molten system, a strong base catalytic system composed of metal salt and sodium ethoxide as a co-catalyst can enhance the nucleophilic reaction of carbonates. Carbonates attack the ester groups in the polyester molecular chain through nucleophilic reaction, causing the polyester chain to break. At the same time, carbonates spontaneously form cyclization with the generated ethylene glycol to generate ethylene carbonate, thereby driving the depolymerization reaction equilibrium to the right. Finally, efficient depolymerization is achieved to obtain terephthalate (taking waste polyester as polyethylene terephthalate as an example, the depolymerization process is shown in Figure 1), realizing rapid depolymerization of waste polyester and high monomer conversion rate. Beneficial effects:

[0023] (1) The present invention uses a catalyst composed of A (conventional metal salt) and B (sodium ethoxide). B, as a co-catalyst, can make up for the deficiency of A's insufficient nucleophilicity, thereby significantly improving the catalytic efficiency of the catalyst.

[0024] (2) Under nitrogen or inert gas protection and solvent-free conditions, waste polyester, carbonate and catalyst undergo depolymerization reaction. Due to the efficient catalytic effect of the catalyst, the depolymerization reaction proceeds rapidly, significantly improving the depolymerization efficiency.

[0025] (3) The present invention does not require secondary transesterification of the depolymerization product, and directly obtains terephthalate monomer, which simplifies the overall depolymerization process and reduces energy consumption.

[0026] (4) The method of the present invention carries out the depolymerization reaction in a near-molten system, which has relatively low requirements for equipment and does not require complex product separation operations, which is conducive to industrial application. Attached Figure Description

[0027] Figure 1 illustrates the mechanism of carbonate depolymerization in waste polyester.

[0028] Figure 2 shows the DSC melt curve of terephthalate in Example 1. Detailed Implementation

[0029] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0030] The following are the calculation formulas for the relevant performance indicators in each embodiment and comparative example:

[0031] In the formula, m0 is the added mass of waste polyester (g), and m1 is the mass of the waste polyester (i.e., residual solids) that has not been depolymerized after the depolymerization reaction (g).

[0032] In the formula, m2 is the mass (g) of the collected terephthalate, M1 is the molar mass (g / mol) of the collected terephthalate, m0 is the added mass (g) of waste polyester, and M2 is the molar mass of the repeating unit of the waste polyester (for example, if the waste polyester is polyethylene terephthalate, M2 = 192 g / mol. Note that when the waste polyester contains two repeating units, the molar mass here is the average of the two).

[0033] The following are the test methods for the relevant performance indicators in each embodiment:

[0034] Purity and oligomer content of terephthalic acid esters: The purity and oligomer content of the collected terephthalic acid esters were qualitatively and quantitatively analyzed using a Shimadzu LC-16 high-performance liquid chromatograph (HPLC) equipped with a WondaSil C18-WR (200 mm, 5 μm packed particle size) column and a UV detector. The detection wavelength was 254 nm, the detection temperature was 40 °C, and a binary gradient test method was used. The mobile phases were methanol and water (3:1, V / V), and the flow rate was 0.8 mL / min. The purity was fitted using the external standard method.

[0035] Melting point: Weigh 5-10 mg of terephthalate into an aluminum crucible, and then determine the melting point (T) using a TA Q-20 differential scanning calorimeter (DSC). m The specific process is as follows: the temperature is increased from 30℃ to 250℃ at a rate of 10℃ / min, and after holding at that temperature for 5 minutes, the melting point of the product is obtained by analysis using Universal Analysis software.

[0036] Example 1

[0037] A method for efficiently producing terephthalate from waste polyester via carbonate depolymerization, comprising the following specific steps:

[0038] (1) Preparation of raw materials;

[0039] Waste polyester: waste fibers made from polyethylene terephthalate;

[0040] Dimethyl carbonate;

[0041] Catalyst: A mixture of zinc acetate and sodium ethoxide in a mass ratio of 10:1;

[0042] (2) Waste polyester, dimethyl carbonate and catalyst were depolymerized at 160°C for 200 min under nitrogen or inert gas protection and solvent-free conditions to obtain depolymerization products. The depolymerization products were then subjected to vacuum devolatilization at 140°C. The removed volatile components were collected to obtain a mixture of terephthalate and dimethyl carbonate. The residual components were collected and separated by water washing and filtration to obtain ethylene carbonate. The mixture of terephthalate and dimethyl carbonate was then subjected to vacuum devolatilization at 70°C. The removed volatile components were collected to obtain dimethyl carbonate. The residual components were collected to obtain terephthalate.

[0043] The catalyst was added at a rate of 0.1 wt% of the mass of the waste polyester, and the molar ratio of dimethyl carbonate to waste polyester was 30:1.

[0044] Tests showed that the solid-phase conversion rate of waste polyester was 100%, the conversion rate of terephthalate was 100%, the purity of terephthalate was 99.9%, the melting point of terephthalate was 140℃ (DSC melting curve is shown in Figure 2), and the oligomer content in terephthalate was 0.01wt%.

[0045] Comparative Example 1

[0046] A method for producing terephthalate from waste polyester by depolymerization of carbonate is basically the same as in Example 1, except that the catalyst is replaced with an equal mass of sodium ethoxide.

[0047] Tests showed that the solid-phase conversion rate of waste polyester was 80%, and the conversion rate of terephthalate was 75%.

[0048] Compared with Example 1, the solid-phase conversion rate and terephthalate conversion rate of waste polyester in Comparative Example 1 decreased significantly. This is because Comparative Example 1 used sodium ethoxide as a catalyst alone. Sodium ethoxide has a strong hydrolysis side reaction, which makes it difficult to control the depolymerization reaction and leads to inconsistent depolymerization between batches. This makes it impossible for the entire depolymerization reaction to proceed stably and efficiently, resulting in a significant decrease in the solid-phase conversion rate and terephthalate conversion rate of waste polyester.

[0049] Comparative Example 2

[0050] A method for producing terephthalate from waste polyester by depolymerization of carbonate is basically the same as in Example 1, except that the catalyst is replaced with an equal mass of zinc acetate.

[0051] Tests showed that the solid-phase conversion rate of waste polyester was 75%, and the conversion rate of terephthalate was 70%.

[0052] Compared with Example 1, the solid-phase conversion rate and terephthalate conversion rate of waste polyester in Comparative Example 2 decreased significantly. This is because Zinc acetate alone was used as a catalyst in Comparative Example 2, and its nucleophilic ability was insufficient. This made it difficult to effectively promote the carbonate to attack the ester groups in the polyester molecular chain to break the polyester chain during the depolymerization reaction. Consequently, the entire depolymerization reaction could not proceed efficiently, resulting in a significant decrease in the solid-phase conversion rate and terephthalate conversion rate of waste polyester.

[0053] Example 2

[0054] A method for efficiently producing terephthalate from waste polyester via carbonate depolymerization, comprising the following specific steps:

[0055] (1) Preparation of raw materials;

[0056] Waste polyester: Waste colored fibers made from polyethylene terephthalate;

[0057] Diethyl carbonate;

[0058] Catalyst: A mixture of potassium carbonate and sodium ethoxide in a mass ratio of 20:1;

[0059] (2) Waste polyester, diethyl carbonate and catalyst were depolymerized at 170°C for 150 min under nitrogen or inert gas protection and solvent-free conditions to obtain depolymerization reaction products. The depolymerization reaction products were first subjected to vacuum devolatilization at 150°C, and the removed volatile components were collected to obtain a mixture of terephthalate and diethyl carbonate. The residual components were collected and separated by water washing and filtration to obtain ethylene carbonate. The mixture of terephthalate and diethyl carbonate was then subjected to vacuum devolatilization at 80°C, and the removed volatile components were collected to obtain diethyl carbonate. The residual components were collected to obtain terephthalate.

[0060] The catalyst is added at 1 wt% of the mass of waste polyester, and the molar ratio of diethyl carbonate to waste polyester is 20:1.

[0061] Tests showed that the solid-phase conversion rate of waste polyester was 100%, the conversion rate of terephthalate was 99%, the purity of terephthalate was 99.9%, and the oligomer content in terephthalate was 0.03 wt%.

[0062] Example 3

[0063] A method for efficiently producing terephthalate from waste polyester via carbonate depolymerization, comprising the following specific steps:

[0064] (1) Preparation of raw materials;

[0065] Waste polyester: Waste fibers made from propylene glycol-modified polyethylene terephthalate;

[0066] The structural formula of propylene glycol-modified polyethylene terephthalate is as follows:

[0067] In the formula, x and y are integers from 10 to 100;

[0068] Dibutyl carbonate;

[0069] Catalyst: A mixture of lithium chloride and sodium ethoxide in a mass ratio of 15:1;

[0070] (2) Waste polyester, dibutyl carbonate and catalyst were depolymerized at 180°C for 100 min under nitrogen or inert gas protection and solvent-free conditions to obtain depolymerization products. The depolymerization products were then subjected to vacuum devolatilization at 190°C. The volatile components removed were collected to obtain a mixture of terephthalate and dibutyl carbonate. The residual components were collected and separated by water washing and filtration to obtain ethylene carbonate. The mixture of terephthalate and dibutyl carbonate was then subjected to vacuum devolatilization at 80°C. The volatile components removed were collected to obtain dibutyl carbonate. The residual components were collected to obtain terephthalate.

[0071] The catalyst is added at 2 wt% of the mass of waste polyester, and the molar ratio of dibutyl carbonate to waste polyester is 15:1.

[0072] Tests showed that the solid-phase conversion rate of waste polyester was 95%, the conversion rate of terephthalate was 92%, the purity of terephthalate was 99.3%, and the oligomer content in terephthalate was 0.1 wt%.

[0073] Example 4

[0074] A method for efficiently producing terephthalate from waste polyester via carbonate depolymerization, comprising the following specific steps:

[0075] (1) Preparation of raw materials;

[0076] Waste polyester: waste fibers made from polyethylene terephthalate-1,4-cyclohexanediol ester;

[0077] The structural formula of polyethylene terephthalate-1,4-cyclohexanediethanol ester is as follows:

[0078] In the formula, x and y are integers from 10 to 100;

[0079] Dimethyl carbonate;

[0080] Catalyst: A mixture of antimony glycolate and sodium ethoxide in a mass ratio of 14:1;

[0081] (2) Waste polyester, dimethyl carbonate and catalyst were depolymerized at 190°C for 30 min under nitrogen or inert gas protection and solvent-free conditions to obtain depolymerization products. The depolymerization products were then subjected to vacuum devolatilization at 160°C. The removed volatile components were collected to obtain a mixture of terephthalate and dimethyl carbonate. The residual components were collected and separated by water washing and filtration to obtain ethylene carbonate. The mixture of terephthalate and dimethyl carbonate was then subjected to vacuum devolatilization at 90°C. The removed volatile components were collected to obtain dimethyl carbonate. The residual components were collected to obtain terephthalate.

[0082] The catalyst is added at 5 wt% of the mass of waste polyester, and the molar ratio of dimethyl carbonate to waste polyester is 10:1.

[0083] Tests showed that the solid-phase conversion rate of waste polyester was 90%, the conversion rate of terephthalate was 85%, the purity of terephthalate was 99.2%, and the oligomer content in terephthalate was 0.2 wt%.

[0084] Example 5

[0085] A method for efficiently producing terephthalate from waste polyester via carbonate depolymerization, comprising the following specific steps:

[0086] (1) Preparation of raw materials;

[0087] Waste polyester: Waste bottle flakes made from polyethylene terephthalate;

[0088] Dibutyl carbonate;

[0089] Catalyst: A mixture of titanium glycolate and sodium ethoxide in a mass ratio of 18:1;

[0090] (2) Waste polyester, dibutyl carbonate and catalyst were depolymerized at 200°C for 50 min under nitrogen or inert gas protection and solvent-free conditions to obtain depolymerization reaction products. The depolymerization reaction products were first subjected to vacuum devolatilization at 160°C. The removed volatile components were collected to obtain a mixture of terephthalate and dibutyl carbonate. The residual components were collected and separated by water washing and filtration to obtain ethylene carbonate. The mixture of terephthalate and dibutyl carbonate was then subjected to vacuum devolatilization at 100°C. The removed volatile components were collected to obtain dibutyl carbonate. The residual components were collected to obtain terephthalate.

[0091] The catalyst is added at 4 wt% of the mass of waste polyester, and the molar ratio of dibutyl carbonate to waste polyester is 3:1.

[0092] Tests showed that the solid-phase conversion rate of waste polyester was 80%, the conversion rate of terephthalate was 80%, the purity of terephthalate was 99%, and the oligomer content in terephthalate was 0.5 wt%.

Claims

1. A method for efficient production of terephthalate from depolymerization of waste polyester carbonate, characterized by, Waste polyester, carbonate, and catalyst are subjected to depolymerization under nitrogen or inert gas protection and solvent-free conditions. The catalyst is a mixture of A and B in a mass ratio of 10 to 20:

1. A is one or more of zinc acetate, sodium acetate, potassium chloride, lithium chloride, potassium carbonate, sodium carbonate, antimony glycolate, antimony acetate, and titanium glycolate, and B is sodium ethoxide.

2. The method for efficiently producing terephthalate from depolymerization of waste polyester carbonate according to claim 1, characterized by, The amount of catalyst added is 0.1 to 5 wt% of the mass of waste polyester.

3. The method for efficiently producing terephthalate from depolymerization of waste polyester carbonate according to claim 1, characterized in that, The molar ratio of carbonate to waste polyester is 3 to 30:

1.

4. The method for efficiently producing terephthalate from depolymerization of waste polyester carbonate according to claim 3, characterized by, The carbonate is one or more of dimethyl carbonate, diethyl carbonate, and dibutyl carbonate.

5. The method for efficiently producing terephthalate from depolymerization of waste polyester carbonate according to claim 1, characterized in that, Waste polyester is any form of product or waste product made from X or Y, where X is polyethylene terephthalate and Y is a copolyester obtained by copolymerizing polyethylene terephthalate with diacids and / or diols.

6. The method for efficiently producing terephthalate from depolymerization of waste polyester carbonate according to claim 1, characterized by, The depolymerization reaction temperature is 160–200℃, and the depolymerization reaction time is 30–200 min.

7. The method for efficiently producing terephthalate from depolymerization of waste polyester carbonate according to claim 1, characterized by, After the depolymerization reaction, the depolymerization product is first subjected to vacuum devolatilization at 140–190°C. The removed volatile components are collected to obtain a mixture of terephthalate and carbonate. Then, the mixture of terephthalate and carbonate is subjected to vacuum devolatilization at 70–100°C. The removed volatile components are collected to obtain carbonate. The residual components are collected to obtain terephthalate.

8. The method according to claim 7, wherein the carbonic acid depolymerization of waste polyester is carried out at a temperature of 100- 200°C, preferably 120- 180°C, and a pressure of 0.1- 10 MPa, preferably 0.5- 5 MPa. After the depolymerization reaction product was subjected to vacuum devolatilization at 140–160 °C, the residual components were collected and then washed with water and filtered to obtain ethylene carbonate.

9. The method for efficiently producing terephthalate from depolymerization of waste polyester carbonate according to claim 1, characterized by, The solid-phase conversion rate of waste polyester is 80-100%, the conversion rate of terephthalate is 80-100%, the purity of terephthalate is 99%-99.9%, and the oligomer content in terephthalate is 0-0.5 wt%.

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