A method for preparing dimethyl terephthalate

Abstract

This invention discloses a method for preparing dimethyl terephthalate, comprising the following steps: using terephthalic acid and methanol as raw materials, carrying out an esterification reaction, primary separation, and multi-stage purification in the presence of an imidazole-type ionic liquid catalyst and a co-solvent to obtain dimethyl terephthalate. The multi-stage purification includes: vacuum distillation, recrystallization, and drying. This invention provides a method for preparing dimethyl terephthalate that, by using an imidazole-type ionic liquid as a catalyst and optimizing reaction process parameters, features a simple process, high catalytic efficiency, high selectivity, high catalyst cycle stability, and high product purity.

Description

Technical Field

[0001] This invention relates to the field of organic chemical technology, and in particular to a method for preparing dimethyl terephthalate. Background Technology

[0002] Dimethyl terephthalate (DMT) is a key monomer in polyester synthesis and is widely used in high-performance fields such as high-end polyester fibers, optical-grade polyester films, and specialty engineering plastics. These applications place extremely stringent purity requirements on DMT, typically requiring a purity of 99.9% or higher. This is because excessively high impurity content directly affects the stability of the polymerization reaction, thereby deteriorating the mechanical and optical properties of the product.

[0003] Currently, the mainstream industrial process for producing dimethyl terephthalate (DMT) involves the esterification of terephthalic acid (PTA) with methanol, with concentrated sulfuric acid commonly used as a catalyst in traditional processes. However, this process suffers from poor selectivity due to the use of concentrated sulfuric acid, easily triggering side reactions such as methanol dehydration and etherification, PTA carbonization, and ester isomerization, generating impurities like methyl ethers, methyl p-methylbenzoate, and carbon black. These impurities have similar boiling points and polarities to DMT, making subsequent separation and purification extremely difficult, hindering the achievement of a purity of over 99.9%. Furthermore, concentrated sulfuric acid is highly corrosive, requiring specialized corrosion-resistant equipment, and the resulting acidic wastewater easily carries organic impurities, polluting the environment and potentially introducing secondary pollution during product purification.

[0004] Therefore, developing a novel green process that combines high selectivity, low corrosivity, easy separation, and stable preparation of DMT with a purity of ≥99.9% is a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0005] In view of the problems of poor catalyst selectivity, many by-products, difficulty in separation and purification, and insufficient DMT purity in the existing DMT production process, the present invention provides a method for preparing dimethyl terephthalate. By using imidazole ionic liquid as catalyst and optimizing reaction process parameters, the method has the characteristics of simple process, high catalytic efficiency, high selectivity, high catalyst cycle stability and high product purity.

[0006] To achieve the above objectives, the embodiments of the present invention adopt the following technical solutions:

[0007] A method for preparing dimethyl terephthalate, comprising the steps of:

[0008] Dimethyl terephthalate is obtained by esterification of terephthalic acid and methanol in the presence of a catalyst and a co-solvent. The catalyst is an imidazole ionic liquid.

[0009] According to one aspect of the present invention, the imidazole-type ionic liquid is one or a combination of 1-ethyl-3-methylimidazolium acetate or 1-butyl-3-methylimidazolium acetate. Preferably, the ionic liquid is 1-ethyl-3-methylimidazolium acetate, which not only has higher catalytic activity but also better separation performance between the product and the solvent, facilitating subsequent separation and purification.

[0010] According to one aspect of the present invention, the esterification reaction is carried out at a temperature of 120–160°C, a pressure of 1.3–1.8 MPa, and a time of 4–8 hours.

[0011] According to one aspect of the present invention, the molar ratio of terephthalic acid to methanol is 1:4 to 8, the molar ratio of ionic liquid to terephthalic acid is 10 to 25:100, and the volume molar ratio of cosolvent to terephthalic acid is 3 to 6 mL:1 mmol.

[0012] According to one aspect of the present invention, the terephthalic acid has a purity of ≥99.8%, a particle size of 100-200 mesh, and a moisture content of ≤0.5%.

[0013] According to one aspect of the invention, the methanol purity is ≥99.5%.

[0014] According to one aspect of the present invention, the co-solvent is one or more combinations of N-methylpyrrolidone, dimethyl sulfoxide, or N,N-dimethylformamide. Preferably, the co-solvent is N-methylpyrrolidone, which has good solubility in the raw material and good compatibility with ionic liquids, facilitating subsequent separation and recovery.

[0015] According to one aspect of the present invention, the esterification reaction further includes primary separation and multi-stage purification, wherein the primary separation includes: cooling the reaction system, allowing it to stand and separate into layers, and separating the mixture and crude dimethyl terephthalate.

[0016] According to one aspect of the present invention, the multi-stage purification includes: subjecting crude dimethyl terephthalate to vacuum distillation and recrystallization.

[0017] According to one aspect of the present invention, the vacuum distillation comprises: distilling crude dimethyl terephthalate under vacuum conditions of 0.09–0.095 MPa and 180–200°C to obtain a semi-finished product of dimethyl terephthalate; the recrystallization comprises: mixing the semi-finished product of dimethyl terephthalate with methanol and crystallizing at 0–5°C for 2–4 hours.

[0018] According to one aspect of the invention, the separation of the mixture further includes recovering the ionic liquid and the co-solvent from the mixture.

[0019] Advantages of implementing this invention:

[0020] 1. Green and environmentally friendly with low corrosivity: Ionic liquids are used instead of concentrated sulfuric acid as catalysts. Ionic liquids are non-volatile and non-corrosive to equipment, eliminating the need for corrosion-resistant equipment and reducing equipment investment and maintenance costs. No acidic wastewater is generated after the reaction, and ionic liquids can be recycled. The catalyst utilization rate is high, which can significantly reduce environmental pollution and meet the requirements of green chemical development.

[0021] 2. High catalytic efficiency and few side reactions: The selected imidazole ionic liquid has excellent catalytic activity, which can significantly accelerate the esterification reaction of terephthalic acid and methanol. Under optimal conditions, the conversion rate of terephthalic acid can reach more than 99%. At the same time, the ionic liquid has high catalytic selectivity, which can effectively suppress side reactions such as methanol dehydration and raw material carbonization. The DMT selectivity is ≥98.5%, and the purity of the finished product can reach more than 99.8%, without the need for complex purification processes.

[0022] 3. Simple process and controllable cost: The reaction conditions are mild and do not require extreme environments such as high temperature and high pressure, making it easy to implement industrially; the ionic liquid catalyst can be recycled after simple separation, and its catalytic activity does not decrease significantly after 5 cycles; the solvent can be recovered and reused, greatly reducing the cost of raw materials and catalysts; unreacted methanol can be recovered and recycled in real time to participate in the reaction, improving the utilization rate of raw materials.

[0023] 4. Wide range of applications: This method is not only applicable to high-purity terephthalic acid raw materials, but also to crude terephthalic acid obtained from the depolymerization of PET waste plastics, providing a new path for the resource utilization of PET waste, expanding the source of raw materials, and improving the practicality and economy of the process. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] A method for preparing dimethyl terephthalate, comprising esterification reaction of terephthalic acid and methanol as raw materials in the presence of a catalyst and a cosolvent to obtain dimethyl terephthalate, wherein the catalyst is an imidazole ionic liquid.

[0026] In practical applications, the imidazole-type ionic liquid is one or a combination of 1-ethyl-3-methylimidazolium acetate or 1-butyl-3-methylimidazolium acetate. Preferably, the ionic liquid is 1-ethyl-3-methylimidazolium acetate, which not only has higher catalytic activity but also better separation performance between the product and the co-solvent, facilitating subsequent separation and purification.

[0027] In practical applications, the esterification reaction is carried out at a temperature of 120–160°C, a pressure of 1.3–1.8 MPa, and a time of 4–8 hours.

[0028] In practical applications, the molar ratio of terephthalic acid to methanol is 1:4 to 8, the molar ratio of ionic liquid to terephthalic acid is 10 to 25:100, and the volume molar ratio of cosolvent to terephthalic acid is 3 to 6 mL:1 mmol.

[0029] In practical applications, the terephthalic acid has a purity ≥99.8%, a particle size of 100-200 mesh, and a moisture content ≤0.5%, thus eliminating the influence of moisture on the esterification reaction and product purity. The methanol has a purity ≥99.5%.

[0030] In practical applications, the co-solvent is one or more combinations of N-methylpyrrolidone, dimethyl sulfoxide, or N,N-dimethylformamide. Preferably, the co-solvent is N-methylpyrrolidone, which has good solubility for the raw materials and good compatibility with ionic liquids, facilitating subsequent separation and recovery.

[0031] In practical applications, the esterification reaction is followed by primary separation and multi-stage purification. The primary separation includes: cooling the reaction system to room temperature and allowing it to stand; using the density difference between the ionic liquid and the product to achieve preliminary separation; the lower layer is a mixture of ionic liquid and co-solvent, which is transferred to a recovery system for ionic liquid recovery; the upper layer is crude dimethyl terephthalate containing a small amount of methanol and trace by-products, which is sent to multi-stage purification treatment.

[0032] In practical applications, the multi-stage purification process includes: vacuum distillation and recrystallization of crude dimethyl terephthalate. The vacuum distillation involves distilling the crude dimethyl terephthalate under reduced pressure at a vacuum of 0.09–0.095 MPa and a temperature of 180–200°C to obtain a semi-finished dimethyl terephthalate product. The recrystallization involves mixing the semi-finished dimethyl terephthalate product with methanol and crystallizing at 0–5°C for 2–4 hours. Recrystallization can be repeated 1–2 times depending on the required purity to ensure that the purity of the finished dimethyl terephthalate product is ≥99.9%.

[0033] In practical applications, the separation of the mixture further includes recovery, which includes: sending the mixture into a recovery system and distilling and recovering the co-solvent under a vacuum of 0.09 MPa and a temperature of 120-130°C. The recovered co-solvent can be returned to the reaction vessel for recycling; the remaining ionic liquid catalyst is directly returned to the reaction vessel for reuse.

[0034] The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the invention. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0035] Example 1

[0036] A method for preparing dimethyl terephthalate includes the following steps:

[0037] S1. Raw material pretreatment: Terephthalic acid is pulverized into 150-mesh powder and vacuum dried at 80°C for 2 hours until the moisture content drops to 0.3%; methanol is purified by double-tower distillation to a purity of 99.6% to remove trace amounts of ethanol, acetaldehyde and other impurities from methanol and prevent them from participating in the reaction to form ester byproducts.

[0038] S2. Catalytic esterification reaction: Add 0.1 mol terephthalic acid, 0.6 mol methanol, 0.02 mol 1-ethyl-3-methylimidazolium acetate ionic liquid catalyst, and 40 mL N-methylpyrrolidone solvent to a 500 mL high-pressure reactor equipped with a distillation reflux device. After sealing, stir evenly, raise the reactor temperature to 140℃, adjust the reaction pressure to 1.5 MPa, and stir the reaction at constant temperature and pressure for 6 hours. During the reaction, unreacted methanol is refluxed in real time through the distillation reflux device for recycling. The reflux ratio is controlled at 2.5:1 to ensure that methanol is in excess and participates in the reaction stably, and to inhibit the formation of by-products.

[0039] S3. Primary separation: After the reaction is completed, the high-pressure reactor is cooled to room temperature and allowed to stand to separate into layers. The lower layer is a mixture of ionic liquid and co-solvent, and the upper layer is crude dimethyl terephthalate.

[0040] S4. Multi-stage purification: The crude dimethyl terephthalate was distilled under reduced pressure at 0.092 MPa and 200°C to remove methanol and a small amount of impurities. Then, it was recrystallized at 0°C for 3 hours using methanol as solvent. After filtration and drying, DMT product was obtained with a purity of 99.85% and a terephthalic acid conversion rate of 99.2%.

[0041] S5. Catalyst recovery and reuse: The mixture is transferred to the recovery system, and the co-solvent is recovered by vacuum distillation under the conditions of 0.09 MPa and 120℃. 1-Ethyl-3-methylimidazolium acetate is directly returned to the reactor for reuse.

[0042] Example 2

[0043] A method for preparing dimethyl terephthalate includes the following steps:

[0044] S1. Raw material pretreatment: Terephthalic acid is pulverized into 200-mesh powder and vacuum dried at 85°C for 1.5 hours until the moisture content drops to 0.2%; methanol is purified to 99.7% purity by double-tower distillation to remove trace amounts of ethanol, acetaldehyde and other impurities from methanol and prevent them from participating in the reaction to generate ester byproducts.

[0045] S2. Catalytic esterification reaction: Add 0.1 mol terephthalic acid, 0.8 mol methanol, 0.025 mol 1-butyl-3-methylimidazolium acetate ionic liquid catalyst, and 50 mL dimethyl sulfoxide solvent to a 500 mL high-pressure reactor equipped with a distillation reflux device. After sealing, stir evenly, heat the reactor to 150 °C, adjust the reaction pressure to 1.6 MPa, and stir the reaction at constant temperature and pressure for 7 hours. During the reaction, unreacted methanol is refluxed in real time through the distillation reflux device, and the reflux ratio is controlled at 2.5:1 to ensure that methanol is in excess and participates in the reaction stably, and to inhibit the formation of by-products.

[0046] S3. Primary separation: After the reaction is completed, cool to room temperature and allow to stand to separate into layers. The lower layer is a mixture of ionic liquid and co-solvent, and the upper layer is crude dimethyl terephthalate.

[0047] S4. Multi-stage purification: The crude dimethyl terephthalate was distilled under reduced pressure at 0.095 MPa and 190°C to remove methanol and a small amount of impurities. Then, it was recrystallized at 55°C for 2.5 hours using methanol as solvent. After filtration and drying, DMT product was obtained with a purity of 99.82% and a terephthalic acid conversion rate of 98.8%.

[0048] S5. Catalyst recovery and reuse: The mixture is transferred to the recovery system, and the co-solvent is recovered by vacuum distillation under the conditions of 0.09 MPa and 120℃. 1-Ethyl-3-methylimidazolium acetate is directly returned to the reactor for reuse.

[0049] Comparative Example 1

[0050] The only difference between Comparative Example 1 and Example 1 is that Comparative Example 1 uses concentrated sulfuric acid as a catalyst; otherwise, they are the same as Example 1.

[0051] Testing revealed that the final dimethyl terephthalate obtained in Comparative Example 1 had a purity of 99.14% and an impurity content of 0.86%. It is evident that the amount of byproducts generated in Comparative Example 1 was 5.7 times that of Example 1, and it also produced a large amount of acidic wastewater and significant equipment corrosion.

[0052] Comparative Example 2

[0053] The only difference between Comparative Example 2 and Example 1 is that in Comparative Example 2, the unreacted methanol is not recovered in real time through a distillation reflux device during the catalytic esterification reaction. All other aspects are the same as in Example 1.

[0054] The purity of the dimethyl terephthalate obtained in Comparative Example 2 was found to be 99.63%, with an impurity content of 0.37%.

[0055] Comparative Example 3

[0056] The only difference between Comparative Example 3 and Example 1 is that no pretreatment of raw materials is performed during the catalytic esterification reaction in Comparative Example 3; otherwise, they are the same as in Example 1.

[0057] The purity of the dimethyl terephthalate obtained in Comparative Example 3 was 99.51%, and the impurity content was 0.49%.

[0058] Catalyst cycle performance test

[0059] The 1-ethyl-3-methylimidazolium acetate ionic liquid catalyst recovered in Example 1 was reused in the preparation of dimethyl terephthalate to test the recycling performance of the 1-ethyl-3-methylimidazolium acetate ionic liquid recovered in Example 1.

[0060] Specifically, the 1-ethyl-3-methylimidazolium acetate ionic liquid catalyst recovered in Example 1 was reused in the preparation of dimethyl terephthalate. The preparation method of dimethyl terephthalate is as in Example 1, and the results are shown in Table 1 below.

[0061] Table 1

[0062]

[0063] As shown in Table 1 above, after the ionic liquid catalyst is recycled 5 times, the conversion rate of terephthalic acid remains above 98.8%, and the selectivity of DMT and the purity of the finished product do not decrease significantly, demonstrating excellent cycle stability.

[0064] Advantages of implementing this invention:

[0065] 1. Green and environmentally friendly with low corrosivity: Ionic liquids are used instead of concentrated sulfuric acid as catalysts. Ionic liquids are non-volatile and non-corrosive to equipment, eliminating the need for corrosion-resistant equipment and reducing equipment investment and maintenance costs. No acidic wastewater is generated after the reaction, and the ionic liquids can be recycled. The catalyst utilization rate is high, significantly reducing environmental pollution and meeting the requirements of green chemical development.

[0066] 2. High catalytic efficiency and few side reactions: The selected imidazole ionic liquid has excellent catalytic activity, which can significantly accelerate the esterification reaction of terephthalic acid and methanol. Under optimal conditions, the conversion rate of terephthalic acid can reach more than 99%. At the same time, the ionic liquid has high catalytic selectivity, which can effectively suppress side reactions such as methanol dehydration and raw material carbonization. The DMT selectivity is ≥98.5%, and the purity of the finished product can reach more than 99.8%, without the need for complex purification processes.

[0067] 3. Simple process and controllable cost: The reaction conditions are mild and do not require extreme environments such as high temperature and high pressure, making it easy to implement industrially; the ionic liquid catalyst can be recycled after simple separation, and its catalytic activity does not decrease significantly after 5 cycles; the solvent can be recovered and reused, greatly reducing the cost of raw materials and catalysts; unreacted methanol can be recovered and recycled in real time to participate in the reaction, improving the utilization rate of raw materials.

[0068] 4. Wide range of applications: This method is not only applicable to high-purity terephthalic acid raw materials, but also to crude terephthalic acid obtained from the depolymerization of PET waste plastics, providing a new path for the resource utilization of PET waste, expanding the source of raw materials, and improving the practicality and economy of the process.

[0069] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing dimethyl terephthalate, characterized in that, Includes the following steps: Dimethyl terephthalate is obtained by esterification reaction of terephthalic acid and methanol in the presence of a catalyst and a cosolvent. The ionic liquid is an imidazole type ionic liquid, which is one or a combination of 1-ethyl-3-methylimidazolium acetate or 1-butyl-3-methylimidazolium acetate.

2. The method for preparing dimethyl terephthalate according to claim 1, characterized in that, The esterification reaction is carried out at a temperature of 120–160°C, a pressure of 1.3–1.8 MPa, and a time of 4–8 hours.

3. The method for preparing dimethyl terephthalate according to claim 1, characterized in that, The molar ratio of terephthalic acid to methanol is 1:4 to 8, and the molar ratio of ionic liquid to terephthalic acid is 10 to 25:

100.

4. The method for preparing dimethyl terephthalate according to claim 1, characterized in that, The volume molar ratio of the cosolvent to terephthalic acid is 3–6 mL:1 mmol.

5. The method for preparing dimethyl terephthalate according to claim 1, characterized in that, The terephthalic acid has a purity of ≥99.8%, a particle size of 100-200 mesh, and a moisture content of ≤0.5%.

6. The method for preparing dimethyl terephthalate according to claim 1, characterized in that, The co-solvent is one or more combinations of N-methylpyrrolidone, dimethyl sulfoxide, or N,N-dimethylformamide.

7. The method for preparing dimethyl terephthalate according to claim 1, characterized in that, The esterification reaction is followed by primary separation and multi-stage purification. The primary separation includes: cooling the reaction system, separating the layers, and obtaining a mixture and crude dimethyl terephthalate.

8. The method for preparing dimethyl terephthalate according to claim 7, characterized in that, The multi-stage purification process includes: vacuum distillation and recrystallization of crude dimethyl terephthalate.

9. The method for preparing dimethyl terephthalate according to claim 8, characterized in that, The vacuum distillation includes: distilling crude dimethyl terephthalate under vacuum conditions of 0.09–0.095 MPa and 180–200°C to obtain a semi-finished product of dimethyl terephthalate; the recrystallization includes: mixing the semi-finished product of dimethyl terephthalate with methanol and crystallizing at 0–5°C for 2–4 hours.

10. The method for preparing dimethyl terephthalate according to claim 1, characterized in that, It also includes the recovery of ionic liquids and cosolvents from the mixture.

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