Method for synthesizing dimethyl terephthalate

Abstract

The present invention provides a method for synthesizing dimethyl terephthalate, comprising: adding a sulfonic acid catalyst into methanol and terephthalic acid, performing a primary esterification reaction, then adding methanol, and performing a secondary esterification reaction; and crystallizing and distilling a secondary esterification product to obtain dimethyl terephthalate, wherein the catalyst comprises hexylbenzenesulfonic acid, dodecyl sulfonic acid, pentadecyl sulfonic acid or dodecylbenzene sulfonic acid. Compared with a concentrated sulfuric acid catalyst, the catalyst used in the present invention greatly weakens the corrosion of the catalyst to a device, and has good repeated use performance.

Description

A method for synthesizing dimethyl terephthalate Technical Field

[0001] This invention relates to the field of dimethyl terephthalate preparation technology, and specifically to a method for synthesizing dimethyl terephthalate. Background Technology

[0002] Dimethyl terephthalate (DMT) is a white, needle-like crystalline solid at room temperature. It readily sublimates, is insoluble in water, but soluble in organic solvents such as methanol, ether, chloroform, and ethyl acetate. DMT is primarily used in the synthesis of polyester resins, polyester films, fibers, and engineering plastics. For example, DMT can undergo transesterification with ethylene glycol, followed by polycondensation to produce polyethylene terephthalate (PET), a crucial engineering plastic with excellent toughness, abrasion resistance, and mechanical properties. DMT can also be used to synthesize polybutylene adipate / terephthalate (PBAT) via transesterification, exhibiting good heat resistance, plasticity, and biodegradability. Furthermore, PTT, a polymer formed by the polymerization of DMT and 1,3-propanediol (1,3-PDO), is a globally popular polymer material with excellent processing properties, high strength, and good resilience. It has broad applications in synthetic fibers and engineering plastics, and can be used to manufacture carpets, nonwoven fabrics, and staple fibers.

[0003] Currently, the production of dimethyl terephthalate from terephthalic acid mostly employs a concentrated sulfuric acid catalytic process. CN104072374A discloses a method using concentrated sulfuric acid as a catalyst, in which terephthalic acid is reacted with excess methanol at 170-180℃ for 10-18 hours. After filtration, alcohol removal, water washing, impurity removal, and stripping, a purity of 99.8-99.9% can be obtained. This method uses excess alcohol as a dehydrating agent, allowing the esterification reaction to proceed gradually to completion, simplifying the process and producing high-quality products. However, this method uses concentrated sulfuric acid as a catalyst, which causes severe corrosion to equipment, and the production process generates a large amount of waste acid and washing wastewater, which are difficult to treat and cause significant environmental pollution. Therefore, this process is relatively outdated and unsuitable for long-term operation.

[0004] CN115353453A discloses a method for producing dimethyl terephthalate from terephthalic acid di-long-chain esters and methanol via transesterification under the action of a catalyst. This method uses C6-C10 long-chain terephthalic acid esters and methanol as raw materials, and organotitanium compounds as catalysts. The reaction is carried out at a temperature of 140-270℃, a pressure of 1.0-4.0 MPa, a feed molar ratio of terephthalic acid di-long-chain esters to methanol of 1:(4-30), and a mass ratio of terephthalic acid di-long-chain esters to titanate catalyst of 1:(0.001-0.2), for 10-120 min, achieving a yield of 86-94%. This method avoids direct esterification, and the raw materials used are all in the liquid phase at room temperature, avoiding solid feed and the resulting equipment blockage and subsequent cleaning problems. The reaction process is easier to control, production efficiency is higher, and equipment investment and energy consumption can be effectively reduced. However, this method uses C6-C10 long-chain terephthalic acid esters as raw materials, which are rare and expensive. The process is complex and introduces impurities such as long-chain alcohol esters, making it unsuitable for large-scale industrial production.

[0005] CN1048542A discloses a method for producing dimethyl terephthalate (DTB) from p-xylene via oxidation, extraction, esterification, distillation, rectification, and cooling / slab formation. The method first catalyzes p-xylene to terephthalic acid and its mixture in an oxidation tower using cobalt acetate and manganese acetate. The oxidized terephthalic acid and its mixture are then sent to an extraction tower, where they are contacted countercurrently with an extractant. Under conditions of 0.1-0.3 MPa and 94-105°C, meta-, ortho-, and non-para-terminal derivatives of the terephthalic acid and its mixture are removed. The mixture is then sent to an esterification tower for esterification with methanol. Finally, the resulting DTB and its monoester mixture are distilled and rectified to further remove impurities, ultimately yielding DTB with a purity of 99.9%. This method provides stable production and high-quality products, but the process is lengthy, requiring three crystallization and re-distillation cycles to achieve 99.9% purity. It also involves multiple pieces of equipment and has high operating costs.

[0006] CN107151208A discloses a method for synthesizing dimethyl terephthalate crystals. The method involves weighing 0.4085 g (0.002 mol) of 7,7,8,8-tetracyano-terebenzoquinone dimethane and 0.1225 g (0.0005 mol) of manganese acetate tetrahydrate into a round-bottom flask. Using anhydrous methanol (approximately 40 ml) as the solvent, the mixture is heated under reflux for 48 hours. After hot filtration, the crystals are obtained by natural evaporation. This method is simple to operate, but the yield of dimethyl terephthalate is relatively low.

[0007] CN1320593A discloses a method for catalyzing the reaction of methanol and terephthalic acid to produce dimethyl terephthalate using sulfate as a catalyst. First, terephthalic acid, methanol, and sulfate are added to a reaction vessel and reacted at 90℃-130℃ for 2-4 hours. Excess methanol and water are evaporated under reduced pressure. Then, the same methanol and sulfate as in the first reaction are added for a second reaction. After the second reaction, the mixture is unloaded, dried, and then subjected to reduced pressure distillation to obtain dimethyl terephthalate. The catalyst is selected from at least one of ferric sulfate, titanium sulfate, manganese sulfate, barium sulfate, and cobalt sulfate that have been calcined at high temperature. Compared to using concentrated sulfuric acid as a catalyst, this method reduces equipment corrosion. However, since monomethyl terephthalate and terephthalic acid in DMT synthesis have low solubility in DMT, secondary esterification is required to improve the conversion rate. Secondary esterification is usually carried out in a tower reactor. When using sulfate as a catalyst, such as ferric sulfate, the solubility in methanol is low. As the reactants enter the tower reactor for secondary esterification, there is a problem of clogging the trays and bottom of the tower. Therefore, it is not suitable for continuous industrial production.

[0008] Therefore, given the current methods for synthesizing dimethyl terephthalate (DTPA) suffer from severe equipment corrosion, cumbersome operations, and low yields, it is urgent to develop a method for preparing DTPA that is simple in steps, less corrosive, has a high yield, high thermal and chemical stability, is environmentally friendly, and meets the requirements of green chemistry. Summary of the Invention

[0009] To address the problems existing in the current dimethyl terephthalate production process, the present invention aims to provide a method for synthesizing dimethyl terephthalate, which uses methanol and terephthalic acid as raw materials, adds a sulfonic acid catalyst, and obtains high-purity dimethyl terephthalate through esterification and crystallization.

[0010] To achieve the above objectives, the present invention provides a method for synthesizing dimethyl terephthalate, comprising the following steps:

[0011] S1: Add a sulfonic acid catalyst to methanol and terephthalic acid to carry out a primary esterification reaction to obtain a primary esterification product;

[0012] S2: Add methanol to the primary esterification product to perform secondary esterification and obtain the secondary esterification product.

[0013] S3: Add methanol to the secondary esterification product and crystallize to obtain a crystalline product;

[0014] S4: Distill the crystalline product to obtain dimethyl terephthalate;

[0015] The catalyst includes one or more of hexylbenzenesulfonic acid, dodecyl sulfonic acid, pentadecyl sulfonic acid, and dodecylbenzenesulfonic acid.

[0016] The process of this invention uses readily available general chemicals as raw materials and catalysts. The catalysts used have high catalytic efficiency, high thermal stability and chemical stability. In particular, after the addition of additives, the corrosiveness to equipment is significantly reduced compared with traditional concentrated sulfuric acid and p-toluenesulfonic acid catalysts. Furthermore, the process of this invention is simple to produce, produces high-purity products, and the overall process meets the requirements of green chemistry and economy.

[0017] According to a specific embodiment of the present invention, preferably, the mass of the catalyst is 0.5-3% of the mass of terephthalic acid, more preferably 1.5-2.5%.

[0018] According to a specific embodiment of the present invention, preferably, in S1, an additive is added along with the catalyst. The additive includes one or more of N-methylpyrrolidone, benzotriazole, dihexylamine nitrite, and 2-mercaptobenzothiazole, more preferably N-methylpyrrolidone.

[0019] According to a specific embodiment of the present invention, preferably, the amount of additive added is 500-1000 ppm, more preferably 500-800 ppm, based on the mass of the catalyst.

[0020] According to a specific embodiment of the present invention, preferably, in S1, the molar ratio of methanol to terephthalic acid is 10-15:1, more preferably 12-14:1.

[0021] According to a specific embodiment of the present invention, preferably, in S2, the amount of methanol added is 5-12 times the molar amount of terephthalic acid, more preferably 5-10 times.

[0022] According to a specific embodiment of the present invention, preferably, in S1 and / or S2, the temperature of the primary esterification reaction and / or the secondary esterification reaction is 140-180°C, more preferably 150°C-170°C, and the time is 1-2 hours.

[0023] According to a specific embodiment of the present invention, preferably, in S1, the temperature of the first esterification reaction is 140-180°C, more preferably 150°C-170°C, and the time is 1-2 hours.

[0024] According to a specific embodiment of the present invention, preferably, in S2, the temperature of the secondary esterification reaction is 140-180℃, more preferably 150℃-170℃, and the time is 1-2h.

[0025] According to a specific embodiment of the present invention, preferably, S1 and / or S2 further include: after the completion of the primary esterification reaction and / or the secondary esterification reaction, removing the remaining methanol and the generated water in the reaction system by distillation, thereby obtaining the primary esterification product and / or the secondary esterification product.

[0026] According to a specific embodiment of the present invention, preferably, in S3, the mass ratio of methanol to the secondary esterification product is 1-4:1, more preferably 1.5-3:1.

[0027] According to a specific embodiment of the present invention, preferably, in S3, when dimethyl terephthalate is purified by crystallization, the crystallization start temperature is 80-120°C, more preferably 90-110°C.

[0028] According to a specific embodiment of the present invention, preferably, in S4, the distillation temperature is 180-210°C, more preferably 190°C.

[0029] According to a specific embodiment of the present invention, preferably, the distillation is vacuum distillation.

[0030] The technical solution provided by this invention has the following beneficial effects:

[0031] (1) Compared with traditional concentrated sulfuric acid catalysts, the sulfonic acid catalysts used in this invention can greatly reduce the corrosion of equipment (especially after the addition of additives), and have good reusability, which is more in line with the requirements of green chemistry.

[0032] (2) The catalyst used in this invention has low cost, high catalytic efficiency, good thermal and chemical stability, low loss, does not clog equipment, and is not easy to contaminate products.

[0033] (3) The method for synthesizing dimethyl terephthalate of the present invention has a simple production process, high product purity (up to 99.9%), and high raw material utilization. Detailed Implementation

[0034] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0035] Example 1

[0036] This embodiment provides a method for synthesizing dimethyl terephthalate, comprising the following steps:

[0037] (1) The molar ratio of methanol to terephthalic acid was selected as 12:1. 33.23 g (0.2 mol) of terephthalic acid, 77.90 g (2.4 mol) of methanol and 0.50 g of hexylbenzenesulfonic acid (the mass of the catalyst was 1.5% of the mass of terephthalic acid) were added to the reactor and reacted at 160 °C for 2 h to carry out the esterification reaction. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after the first esterification. The yield of dimethyl terephthalate was 88.43%.

[0038] (2) The obtained primary esterification product was placed in a reaction vessel, and 51.26 g (1.6 mol) of methanol was added. The reaction was carried out at 150 °C for 1 h to perform secondary esterification. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after secondary esterification. The yield of dimethyl terephthalate was 97.90%.

[0039] (3) Take 30g of the product after secondary esterification and place it in a reaction vessel. Add 60g of methanol and stir at 100℃ for 15min. Then cool down to crystallize at 30℃ to obtain the crystallized crude dimethyl terephthalate with a yield of 93.79%.

[0040] (4) Take 10g of the crystallized crude dimethyl terephthalate and place it in a flask. Distill it under reduced pressure at 190℃ to obtain high-purity dimethyl terephthalate after distillation. The yield is 99.14% and the purity is 99.91%. The acid value of the product is 0.20 according to GB / T 264-83 "Determination of Acid Value of Petroleum Products".

[0041] Example 2

[0042] This embodiment provides a method for synthesizing dimethyl terephthalate, comprising the following steps:

[0043] (1) The molar ratio of methanol to terephthalic acid was selected as 14:1. 33.23 g (0.2 mol) of terephthalic acid, 89.71 g (2.8 mol) of methanol and 0.50 g of dodecylbenzenesulfonic acid (the mass of the catalyst was 1.5% of the mass of terephthalic acid) were added to the reactor and reacted at 160 °C for 2 h to carry out the esterification reaction. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after the first esterification. The yield of dimethyl terephthalate was 90.15%.

[0044] (2) The obtained primary esterification product was placed in a reaction vessel, and 51.26 g (1.6 mol) of methanol was added. The reaction was carried out at 150 °C for 1 h to perform secondary esterification. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after secondary esterification. The yield of dimethyl terephthalate was 98.25%.

[0045] (3) Take 30g of the product after secondary esterification and place it in a reaction vessel. Add 60g of methanol and stir at 90℃ for 15min. Then cool down to crystallize at 30℃ to obtain the crystallized crude dimethyl terephthalate with a yield of 93.86%.

[0046] (4) Take 10g of the crystallized crude dimethyl terephthalate and place it in a flask. Distill it under reduced pressure at 190℃ to obtain high-purity dimethyl terephthalate after distillation. The yield is 99.16% and the purity is 99.92%. The acid value of the product is 0.20 according to GB / T 264-83 "Determination of Acid Value of Petroleum Products".

[0047] Example 3

[0048] This embodiment provides a method for synthesizing dimethyl terephthalate, comprising the following steps:

[0049] (1) The molar ratio of methanol to terephthalic acid was selected as 12:1. 33.23 g (0.2 mol) of terephthalic acid, 77.90 g (2.4 mol) of methanol and 0.83 g of dodecyl sulfonic acid (the mass of the catalyst was 2.5% of the mass of terephthalic acid) were added to the reaction vessel and reacted at 170 °C for 2 h to carry out the esterification reaction. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after the first esterification. The yield of dimethyl terephthalate was 88.22%.

[0050] (2) The obtained primary esterification product was placed in a reaction vessel, and 51.26 g (1.6 mol) of methanol was added. The reaction was carried out at 150 °C for 1 h to perform secondary esterification. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after secondary esterification. The yield of dimethyl terephthalate was 97.75%.

[0051] (3) Take 30g of the product after secondary esterification and place it in a reaction vessel. Add 60g of methanol and stir at 100℃ for 15min. Then cool down to crystallize at 30℃ to obtain the crystallized crude dimethyl terephthalate with a yield of 93.71%.

[0052] (4) Take 10g of the crystallized crude dimethyl terephthalate and place it in a flask. Distill it under reduced pressure at 190℃ to obtain high-purity dimethyl terephthalate after distillation. The yield is 99.26% and the purity is 99.91%. The acid value of the product is 0.21 according to GB / T 264-83 "Determination of Acid Value of Petroleum Products".

[0053] Example 4

[0054] This embodiment provides a method for synthesizing dimethyl terephthalate, comprising the following steps:

[0055] (1) The molar ratio of methanol to terephthalic acid was selected as 14:1. 33.23 g (0.2 mol) of terephthalic acid, 89.71 g (2.8 mol) of methanol, 0.30 g of pentadecyl sulfonic acid and 0.20 g of dodecylbenzene sulfonic acid (the mass of the catalyst was 1.5% of the mass of terephthalic acid) were added to the reactor and reacted at 170 °C for 2 h to carry out the esterification reaction. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after the first esterification. The yield of dimethyl terephthalate was 89.52%.

[0056] (2) The obtained primary esterification product was placed in a reaction vessel, and 51.26 g (1.6 mol) of methanol was added. The reaction was carried out at 150 °C for 1 h to perform secondary esterification. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after secondary esterification. The yield of dimethyl terephthalate was 97.72%.

[0057] (3) Take 30g of the product after secondary esterification and place it in a reaction vessel. Add 60g of methanol and stir at 110℃ for 15min. Then cool down to crystallize at 30℃ to obtain the crystallized crude dimethyl terephthalate with a yield of 93.65%.

[0058] (4) Take 10g of the crystallized crude dimethyl terephthalate and place it in a flask. Distill it under reduced pressure at 190℃ to obtain high-purity dimethyl terephthalate after distillation. The yield is 99.08% and the purity is 99.90%. The acid value of the product is 0.20 according to GB / T 264-83 "Determination of Acid Value of Petroleum Products".

[0059] As shown in Examples 1-4, increasing the amount of methanol helps the reaction proceed in the forward direction, improves the esterification effect, and ultimately achieves a purity of over 99.9% for dimethyl terephthalate, with the acid value meeting the requirements. This demonstrates that the process method of this invention for preparing DMT is indeed feasible. Furthermore, the sulfonic acid catalyst used in this invention has high catalytic efficiency and yields a high product yield.

[0060] Comparative Example 1

[0061] A method for synthesizing dimethyl terephthalate, comprising:

[0062] (1) The molar ratio of methanol to terephthalic acid was selected as 12:1. 33.23 g (0.2 mol) of terephthalic acid, 77.90 g (2.4 mol) of methanol and 0.50 g of p-toluenesulfonic acid (the mass of the catalyst was 1.5% of the mass of terephthalic acid) were added to the reactor and reacted at 160 °C for 2 h to carry out the esterification reaction. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after the first esterification. The yield of dimethyl terephthalate was 86.98%.

[0063] (2) The obtained primary esterification product was placed in a reaction vessel, and 51.26 g (1.6 mol) of methanol was added. The reaction was carried out at 150 °C for 1 h to perform secondary esterification. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after secondary esterification. The yield of dimethyl terephthalate was 96.74%.

[0064] (3) Take 30g of the product after secondary esterification and place it in a reaction vessel. Add 60g of methanol and stir at 100℃ for 15min. Then cool down to crystallize at 30℃ to obtain crude dimethyl terephthalate after primary crystallization, with a yield of 93.11%.

[0065] (4) Take 10g of crude dimethyl terephthalate after one crystallization and place it in a flask. Distill it under reduced pressure at 190℃ to obtain high-purity dimethyl terephthalate after distillation. The yield is 99.18% and the purity is 99.84%. The acid value of the product is measured to be 3.16 according to GB / T264-83 "Determination of Acid Value of Petroleum Products".

[0066] Comparative Example 2

[0067] A method for synthesizing dimethyl terephthalate, comprising:

[0068] (1) The molar ratio of methanol to terephthalic acid was selected as 12:1. 33.23 g (0.2 mol) of terephthalic acid, 77.90 g (2.4 mol) of methanol and 0.50 g of concentrated sulfuric acid (the mass of the catalyst was 1.5% of the mass of terephthalic acid) were added to the reactor and reacted at 150 °C for 2 h to carry out the esterification reaction. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after the first esterification. The yield of dimethyl terephthalate was 65.44%.

[0069] (2) The obtained primary esterification product was placed in a reaction vessel, and 51.26 g (1.6 mol) of methanol was added. The reaction was carried out at 150 °C for 1 h to perform secondary esterification. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after secondary esterification. The yield of dimethyl terephthalate was 79.94%.

[0070] As can be seen from Example 1 and Comparative Examples 1 and 2, benzenesulfonic acid and p-toluenesulfonic acid catalysts have higher esterification efficiency than concentrated sulfuric acid catalysts. However, since p-toluenesulfonic acid has a lower boiling point, which is close to that of DMT products, distillation will cause catalyst loss, resulting in excessively high acid value of the product and affecting downstream applications.

[0071] Example 5

[0072] This embodiment provides a method for synthesizing dimethyl terephthalate, comprising the following steps:

[0073] (1) The molar ratio of methanol to terephthalic acid was selected as 12:1. 33.23 g (0.2 mol) of terephthalic acid, 77.90 g (2.4 mol) of methanol, 0.50 g of hexylbenzenesulfonic acid (catalyst, whose mass is 1.5% of the mass of terephthalic acid) and 500 ppm (based on the mass of the catalyst) of N-methylpyrrolidone additive were added to the reactor and reacted at 150 °C for 2 h to carry out the esterification reaction. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after the first esterification. The yield of dimethyl terephthalate was 88.47%.

[0074] (2) The obtained primary esterification product was placed in a reaction vessel, and 51.26 g (1.6 mol) of methanol was added. The reaction was carried out at 150 °C for 1 h to perform secondary esterification. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after secondary esterification. The yield of dimethyl terephthalate was 97.83%.

[0075] (3) Take 30g of the product after secondary esterification and place it in a reaction vessel. Add 60g of methanol and stir at 100℃ for 15min. Then cool down to crystallize at 30℃ to obtain crude dimethyl terephthalate after primary crystallization, with a yield of 93.49%.

[0076] (4) Take 10g of the crystallized crude dimethyl terephthalate and place it in a flask. Distill at 190℃ for 2h to obtain high-purity dimethyl terephthalate after distillation. The yield is 99.13% and the purity is 99.90%. The acid value of the product is 0.20 according to GB / T 264-83 "Determination of Acid Value of Petroleum Products".

[0077] Example 6

[0078] This embodiment provides a method for synthesizing dimethyl terephthalate, comprising the following steps:

[0079] (1) 33.23 g (0.2 mol) terephthalic acid, 77.90 g (2.4 mol) methanol, 0.50 g dodecylbenzenesulfonic acid (the mass of the catalyst is 1.5% of the mass of terephthalic acid) and 800 ppm (based on the mass of the catalyst) of N-methylpyrrolidone additive were added to the reactor and reacted at 150 °C for 2 h to undergo esterification. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after one esterification. The yield of dimethyl terephthalate was 88.44%.

[0080] (2) The obtained primary esterification product was placed in a reaction vessel, and 51.26 g (1.6 mol) of methanol was added. The reaction was carried out at 150 °C for 1 h to perform secondary esterification. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after secondary esterification. The yield of dimethyl terephthalate was 97.80%.

[0081] (3) Take 30g of the product after secondary esterification and place it in a reaction vessel. Add 60g of methanol and stir at 100℃ for 15min. Then cool down to crystallize at 30℃ to obtain crude dimethyl terephthalate after primary crystallization, with a yield of 93.55%.

[0082] (4) Take 10g of the crystallized crude dimethyl terephthalate and place it in a flask. Distill at 190℃ for 2h to obtain high-purity dimethyl terephthalate after distillation. The yield is 99.11% and the purity is 99.91%. The acid value of the product is 0.20 according to GB / T 264-83 "Determination of Acid Value of Petroleum Products".

[0083] Experimental Example 1

[0084] Thermal stability of benzenesulfonic acid catalysts:

[0085] (1) The molar ratio of methanol to terephthalic acid was selected as 12:1. 33.23 g (0.2 mol) of terephthalic acid, 77.90 g (2.4 mol) of methanol and 0.50 g of benzenesulfonic acid (the mass of the catalyst was 1.5% of the mass of terephthalic acid) were added to the reactor and reacted at 150 °C for 2 h to carry out the esterification reaction. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after the first esterification. The yield of dimethyl terephthalate was 88.48%.

[0086] (2) The obtained primary esterification product was placed in a reaction vessel, and 51.26 g (1.6 mol) of methanol was added. The reaction was carried out at 150 °C for 1 h to perform secondary esterification. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after secondary esterification. The yield of dimethyl terephthalate was 97.86%.

[0087] (3) The product after secondary esterification was placed in a flask and distilled under reduced pressure at 190°C for 2 hours to recover the dodecylbenzenesulfonic acid catalyst after reduced pressure distillation.

[0088] (4) The recovered benzenesulfonic acid catalyst, 33.23 g (0.2 mol) terephthalic acid and 77.90 g (2.4 mol) methanol were added to the reactor and reacted at 150 °C for 2 h to undergo esterification. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after one esterification. The yield of dimethyl terephthalate was 88.53%.

[0089] (5) The obtained primary esterification product was placed in a reaction vessel, 51.26 g (1.6 mol) of methanol was added, and the reaction was carried out at 150 °C for 1 h to perform secondary esterification. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after secondary esterification. The yield of dimethyl terephthalate was 97.84%.

[0090] As shown in Experiment 1, the benzenesulfonic acid catalyst after vacuum distillation still has good thermal stability and can maintain catalytic efficiency at higher temperatures, indicating that the benzenesulfonic acid separated at high temperature in the distillation column still has good catalytic activity.

[0091] Experiment Example 2

[0092] Corrosiveness of benzenesulfonic acid after the addition of additives:

[0093] (1) The experiment was conducted in accordance with GB / T4334.6-2015 "Test Method for Stainless Steel Sulfuric Acid Corrosion". Three parallel 316L stainless steel samples were taken and polished with sandpaper while avoiding heating the samples. The dimensions of the samples were measured with vernier calipers and the total surface area of ​​the samples was calculated. After ultrasonic cleaning in clean water, the samples were dried in an oven for 30 minutes and cooled to room temperature. The samples were then weighed on an analytical balance with an accuracy of 0.0001 g.

[0094] (2) Place the sample in a glass vertical reflux condenser with a conical ground joint and sufficient cooling effect, add dodecylbenzenesulfonic acid to immerse the stainless steel sample, add 500 ppm (based on the mass of the catalyst) of N-methylpyrrolidone additive, heat to 150°C and keep for 12 h.

[0095] (3) After the predetermined corrosion time is reached, the stainless steel corrosion test sample is taken out, and it is washed with a soft brush in running water to remove the corrosion products on the sample surface. After drying in an oven, the weight of the corroded sample is measured.

[0096] (4) Calculate the rate of weight loss of 316L stainless steel after the addition of dodecylbenzenesulfonic acid as an additive.

[0097] Experimental Example 3

[0098] Corrosiveness of dodecyl sulfonic acid after the addition of additives:

[0099] (1) The experiment was conducted in accordance with GB / T4334.6-2015 "Test Method for Corrosion of Stainless Steel". Three parallel 316L stainless steel samples were taken and polished with sandpaper while avoiding heating the samples. The dimensions of the samples were measured with vernier calipers and the total surface area of ​​the samples was calculated. After ultrasonic cleaning in clean water, the samples were dried in an oven for 30 minutes and cooled to room temperature. The samples were weighed on an analytical balance with an accuracy of 0.0001 g.

[0100] (2) Place the sample in a glass vertical reflux condenser with a conical ground joint and sufficient cooling effect, add dodecyl sulfonic acid to immerse the stainless steel sample, add 800 ppm (based on the mass of the catalyst) of N-methylpyrrolidone additive, heat to 150°C and maintain for 12 h.

[0101] (3) After the predetermined corrosion time is reached, the stainless steel corrosion test sample is taken out, and it is washed with a soft brush in running water to remove the corrosion products on the sample surface. After drying in an oven, the weight of the corroded sample is measured.

[0102] (4) Calculate the rate of weight loss of 316L stainless steel after the addition of dodecyl sulfonic acid as an additive.

[0103] Comparative Experiment Example 1

[0104] Corrosiveness of dodecylbenzenesulfonic acid catalyst:

[0105] (1) The experiment was conducted in accordance with GB / T4334.6-2015 "Test Method for Corrosion of Stainless Steel". Three parallel 316L stainless steel samples were taken and polished with sandpaper while avoiding heating the samples. The dimensions of the samples were measured with vernier calipers and the total surface area of ​​the samples was calculated. After ultrasonic cleaning in clean water, the samples were dried in an oven for 30 minutes and cooled to room temperature. The samples were weighed on an analytical balance with an accuracy of 0.0001 g.

[0106] (2) Place the sample in a glass vertical reflux condenser with a conical ground joint and sufficient cooling effect, add dodecylbenzenesulfonic acid to immerse the stainless steel sample, heat to 150°C and maintain for 12 hours.

[0107] (3) After the predetermined corrosion time is reached, the stainless steel corrosion test sample is taken out, and it is washed with a soft brush in running water to remove the corrosion products on the sample surface. After drying in an oven, the weight of the corroded sample is measured.

[0108] (4) Calculate the weight loss rate of 316L stainless steel corrosion caused by dodecylbenzenesulfonic acid catalyst.

[0109] Comparative Example 2

[0110] Corrosiveness of p-toluenesulfonic acid catalysts:

[0111] (1) The experiment was conducted in accordance with GB / T4334.6-2015 "Test Method for Corrosion of Stainless Steel". Three parallel 316L stainless steel samples were taken and polished with sandpaper while avoiding heating the samples. The dimensions of the samples were measured with vernier calipers and the total surface area of ​​the samples was calculated. After ultrasonic cleaning in clean water, the samples were dried in an oven for 30 minutes and cooled to room temperature. The samples were weighed on an analytical balance with an accuracy of 0.0001 g.

[0112] (2) Place the sample in a glass vertical reflux condenser with a conical ground joint and sufficient cooling effect, add p-toluenesulfonic acid to immerse the stainless steel sample, heat to 150°C and keep for 12 hours.

[0113] (3) After the predetermined corrosion time is reached, the stainless steel corrosion test sample is taken out, and it is washed with a soft brush in running water to remove the corrosion products on the sample surface. After drying in an oven, the weight of the corroded sample is measured.

[0114] (4) Calculate the weight loss rate of 316L stainless steel corrosion caused by p-toluenesulfonic acid catalyst.

[0115] Comparative Experiment Example 3

[0116] The corrosiveness of sulfuric acid:

[0117] (1) The experiment was conducted in accordance with GB / T4334.6-2015 "Test Method for Corrosion of Stainless Steel". Three parallel 316L stainless steel samples were taken and polished with sandpaper while avoiding heating the samples. The dimensions of the samples were measured with vernier calipers and the total surface area of ​​the samples was calculated. After ultrasonic cleaning in clean water, the samples were dried in an oven for 30 minutes and cooled to room temperature. The samples were weighed on an analytical balance with an accuracy of 0.0001 g.

[0118] (2) Place the sample in a glass vertical reflux condenser with a conical ground joint and sufficient cooling effect, add DMT containing 5% sulfuric acid to immerse the stainless steel sample, heat to 150°C and keep for 12 hours.

[0119] (3) After the predetermined corrosion time is reached, the stainless steel corrosion test sample is taken out, and it is washed with a soft brush in running water to remove the corrosion products on the sample surface. After drying in an oven, the weight of the corroded sample is measured.

[0120] (4) Calculate the weight loss rate of 316L stainless steel corroded by DMT containing 5% sulfuric acid.

[0121] Table 1 Weight loss rate in stainless steel corrosion experiments

[0122] As shown in the comparative experimental examples 1-3 above, the corrosion rate when using the sulfonic acid catalyst of the present invention is significantly lower than the corrosion rate of stainless steel when using p-toluenesulfonic acid and DMT containing 5% sulfuric acid. Therefore, compared with p-toluenesulfonic acid and concentrated sulfuric acid catalysts, the use of benzenesulfonic acid catalyst can reduce the corrosion of equipment while improving the esterification efficiency.

[0123] As shown in Experimental Examples 1-2 and Comparative Experimental Example 1 above, the addition of additives significantly reduced the corrosion rate of stainless steel by the sulfonic acid catalyst of this invention, and the increased amount of additives further reduced the corrosiveness to the steel. Therefore, the addition of additives further reduces equipment corrosion and environmental pollution, meeting the requirements of green chemistry.

[0124] The difference between Example 1 and Examples 5-6 is that an additive was added to the reaction system. The results show that the addition of the additive can reduce the corrosion of the equipment without affecting the esterification efficiency and the acid value of the product.

[0125] Experiment Example 4

[0126] Thermal stability of benzenesulfonic acid catalyst after adding additives:

[0127] (1) The molar ratio of methanol to terephthalic acid was selected as 12:1. 33.23 g (0.2 mol) of terephthalic acid, 77.90 g (2.4 mol) of methanol, 0.50 g of benzenesulfonic acid (the mass of the catalyst was 1.5% of the mass of terephthalic acid) and 500 ppm (based on the mass of the catalyst) of N-methylpyrrolidone additive were added to the reactor and reacted at 150 °C for 2 h to carry out the esterification reaction. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after the first esterification. The yield of dimethyl terephthalate was 88.41%.

[0128] (2) The obtained primary esterification product was placed in a reaction vessel, and 51.26 g (1.6 mol) of methanol was added. The reaction was carried out at 150 °C for 1 h to perform secondary esterification. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after secondary esterification. The yield of dimethyl terephthalate was 97.89%.

[0129] (3) The product after secondary esterification was placed in a flask and distilled at 190°C for 2 hours. The bottom liquid (dodecylbenzenesulfonic acid catalyst) after DMT was recovered by vacuum distillation was collected.

[0130] (4) The recovered benzenesulfonic acid catalyst, 33.23 g (0.2 mol) terephthalic acid and 77.90 g (2.4 mol) methanol were added to the reactor and reacted at 150 °C for 2 h to carry out the esterification reaction. After the reaction was completed, the methanol and water were removed by rotary evaporation to obtain the product after the first esterification.

[0131] (5) The obtained primary esterification product was placed in a reaction vessel, 51.26 g (1.6 mol) of methanol was added, and the reaction was carried out at 150 °C for 1 h to perform secondary esterification. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after secondary esterification. The yield of dimethyl terephthalate was 97.86%.

[0132] As shown in Experiment 4, the benzenesulfonic acid catalyst with additives still has good thermal stability after vacuum distillation, indicating that the benzenesulfonic acid with additives separated at high temperature in the distillation column still has good catalytic activity.

[0133] This invention enables the production of dimethyl terephthalate with a purity of over 99.9% by using methanol and terephthalic acid as raw materials, high-boiling-point sulfonic acid as catalyst, and adding additives, through a process of primary esterification, secondary esterification, crystallization, and distillation. The process is simple and, compared with the traditional concentrated sulfuric acid catalytic process for esterification, it improves esterification efficiency while ensuring catalyst stability and reducing equipment corrosion, thus meeting the requirements of green chemistry.

[0134] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any modifications or equivalent changes made based on the technical essence of the present invention shall still fall within the scope of protection claimed by the present invention.

Claims

1. A method for synthesizing dimethyl terephthalate, comprising the following steps: S1: Add a sulfonic acid catalyst to methanol and terephthalic acid to carry out a primary esterification reaction to obtain a primary esterification product; S2: Add methanol to the primary esterification product to perform secondary esterification and obtain the secondary esterification product. S3: Add methanol to the secondary esterification product and crystallize to obtain a crystalline product; S4: Distill the crystalline product to obtain dimethyl terephthalate; in, The catalyst comprises one or more of hexylbenzenesulfonic acid, dodecyl sulfonic acid, pentadecyl sulfonic acid, and dodecylbenzenesulfonic acid.

2. The method for synthesizing dimethyl terephthalate according to claim 1, wherein, The mass of the catalyst is 0.5-3% of the mass of terephthalic acid.

3. The method for synthesizing dimethyl terephthalate according to claim 2, wherein, The mass of the catalyst is 1.5-2.5% of the mass of terephthalic acid.

4. The method for synthesizing dimethyl terephthalate according to claim 1, wherein, In S1, an additive is added along with the catalyst. The additive includes one or more of N-methylpyrrolidone, benzotriazole, dihexylamine nitrite, and 2-mercaptobenzothiazole.

5. The method for synthesizing dimethyl terephthalate according to claim 4, wherein, The amount of additive added is 500-1000 ppm, based on the mass of the catalyst.

6. The method for synthesizing dimethyl terephthalate according to claim 1, wherein, In S1, the molar ratio of methanol to terephthalic acid is 10-15:

1.

7. The method for synthesizing dimethyl terephthalate according to claim 6, wherein, In S1, the molar ratio of methanol to terephthalic acid is 12-14:

1.

8. The method for synthesizing dimethyl terephthalate according to claim 1, wherein, In S1, the temperature of the first esterification reaction is 140-180℃ and the time is 1-2h.

9. The method for synthesizing dimethyl terephthalate according to claim 1, wherein, In S2, the amount of methanol added is 5-12 times the molar amount of terephthalic acid.

10. The method for synthesizing dimethyl terephthalate according to claim 1, wherein, In S2, the secondary esterification reaction is carried out at a temperature of 140-180℃ for 1-2 hours.

11. The method for synthesizing dimethyl terephthalate according to claim 1, wherein, S1 and / or S2 further include: after the completion of the primary esterification reaction and / or the secondary esterification reaction, distilling to remove the remaining methanol and the generated water in the reaction system, thereby obtaining the primary esterification product and / or the secondary esterification product.

12. The method for synthesizing dimethyl terephthalate according to claim 1, wherein, In S3, the mass ratio of methanol to secondary esterification product is 1-4:

1.

13. The method for synthesizing dimethyl terephthalate according to claim 12, wherein, In S3, the mass ratio of methanol to secondary esterification product is 1.5-3:

1.

14. The method for synthesizing dimethyl terephthalate according to claim 1, wherein, In S3, when dimethyl terephthalate is purified by crystallization, the crystallization initiation temperature is 80-120℃.

15. The method for synthesizing dimethyl terephthalate according to claim 1, wherein, In S4, the distillation temperature is 180-210℃.

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