A method for synthesizing dimethyl terephthalate

Abstract

The application provides a synthesis method of dimethyl terephthalate, which comprises the following steps: adding a sulfonic acid catalyst into methanol and terephthalic acid to perform a first esterification reaction to obtain a first esterification product; adding methanol into the first esterification product to perform a second esterification to obtain a second esterification product; adding methanol into the second esterification product to perform crystallization to obtain a crystallization product; and performing distillation on the crystallization product to obtain dimethyl terephthalate; wherein the catalyst comprises one or more than two combinations of hexyl benzene sulfonic acid, dodecyl sulfonic acid, pentadecyl sulfonic acid and dodecyl benzene sulfonic acid. Compared with a traditional concentrated sulfuric acid catalyst, the catalyst used in the application itself, especially after adding an additive, greatly reduces the corrosion of the catalyst to equipment, has good reusability and is more in line with the requirements of green chemistry.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of dimethyl terephthalate preparation, and particularly relates to a synthesis method of dimethyl terephthalate. BACKGROUND

[0002] Dimethyl terephthalate (DMT for short) is a white needle-like crystalline body in normal state, is easy to sublimate, is insoluble in water, and is soluble in organic solvents such as methanol, diethyl ether, chloroform, ethyl acetate and the like. DMT is mainly used for the synthesis of polyester resin, polyester film, fiber and engineering plastic materials and the like. For example, DMT can be subjected to an ester exchange reaction with ethylene glycol, and then is subjected to polycondensation to generate polyethylene terephthalate (PET), which is the most important engineering plastic, and has excellent toughness, wear resistance and mechanical properties; DMT can also be synthesized into polybutylene adipate / terephthalate (PBAT) through an ester exchange method, and has good heat resistance, plasticity and biodegradability. In addition, PTT material synthesized by DMT and 1,3-propanediol (1,3-PDO) is also a global popular polymer material, has good processing performance, and has high strength and good resilience, and has a wide development in the fields of synthetic fibers and engineering plastics, and can be used to manufacture carpets, non-woven fabrics and short fibers and the like.

[0003] At present, when dimethyl terephthalate is produced by using terephthalic acid, a concentrated sulfuric acid catalytic process is mostly adopted. CN104072374A discloses a method using concentrated sulfuric acid as a catalyst, and terephthalic acid is reacted with excessive methanol at 170-180℃ for 10-18 hours, and then pure product with a purity of 99.8-99.9% can be obtained through filtration, dealcoholization, water washing, impurity removal and stripping. The method uses excessive alcohol as a water-carrying agent, and makes the esterification reaction gradually proceed to the end, simplifies the process, and has good product quality, but the method uses concentrated sulfuric acid as a catalyst, and the equipment is seriously corroded, and a large amount of waste acid and a large amount of washing wastewater are generated in the production process, which has great difficulty in treatment, and causes great environmental pollution, so the process is relatively backward, and is not suitable for long-term operation.

[0004] CN115353453A discloses a method for preparing dimethyl terephthalate by transesterification of long-chain terephthalate ester and methanol in the presence of a catalyst. The method uses long-chain terephthalate ester C6-C10 and methanol as raw materials, and organic titanium as catalyst. The reaction is carried out at a temperature of 140-270°C and a pressure of 1.0-4.0MPA, with a molar ratio of long-chain terephthalate ester to methanol of 1:(4-30) and a mass ratio of long-chain terephthalate ester to titanium ester catalyst of 1:(0.001-0.2), for 10-120 minutes. The yield can reach 86-94%. This method avoids direct esterification, and the raw materials are all in liquid phase at room temperature, which can avoid equipment blockage and subsequent cleaning problems, making the reaction process easier to control, the production efficiency higher, and the equipment investment and energy consumption lower. However, this method uses long-chain terephthalate ester C6-C10 as raw material, which is difficult to obtain and expensive, and the process is complex, and introduces long-chain alcohol ester impurities, which is not suitable for large-scale industrial production.

[0005] CN1048542A discloses a method for producing dimethyl terephthalate by oxidizing, extracting, esterifying, distilling, rectifying, and cooling crystallization of p-xylene. First, p-xylene is catalyzed by cobalt acetate and manganese acetate in an oxidation tower to produce terephthalic acid and its mixture. Then, the oxidized terephthalic acid and its mixture are sent to an extraction tower to remove meta-, ortho-, and non-para derivatives under the conditions of a pressure of 0.1-0.3mpa and a temperature of 94°C-105°C. After that, the mixture is esterified with methanol in an esterification tower. Finally, the generated dimethyl terephthalate and its monoester mixture are distilled and rectified to further remove impurities, and finally, dimethyl terephthalate with a purity of 99.9% is obtained. This method produces stable products with good quality, but the process is long and requires three times of crystallization and distillation to obtain 99.9% pure product, which requires more equipment and higher operating costs.

[0006] CN107151208A discloses a method for synthesizing dimethyl terephthalate crystals. 7,7,8,8-tetracyanoquinodimethane and manganese acetate tetrahydrate are weighed into a round-bottom flask, and anhydrous methanol is used as solvent. After heating and refluxing for 48 hours, the mixture is filtered and naturally volatilized to obtain crystals. This method is simple to operate, but the yield of dimethyl terephthalate is low.

[0007] CN1320593A discloses a method for preparing dimethyl terephthalate by using sulfate as catalyst to catalyze the reaction of methanol and terephthalic acid. The terephthalic acid, methanol and sulfate are put into a reaction kettle, and reacted at 90-130 DEG C for 2-4 hours. The excess methanol and water are distilled out under reduced pressure. The same amount of methanol and sulfate as the first time are added to carry out the second reaction. After the second reaction, the product is unloaded, dried and baked, and then distilled under reduced pressure to obtain dimethyl terephthalate. The catalyst is at least one selected from high-temperature calcined iron sulfate, titanium sulfate, manganese sulfate, barium sulfate and cobalt sulfate. Compared with using concentrated sulfuric acid as catalyst, the method reduces the corrosion of equipment. However, since the solubility of monomethyl terephthalate and terephthalic acid in DMT is small, the second esterification is needed to improve the conversion rate. The second esterification is usually carried out in a tower reactor. When the reactants enter the tower reactor for the second esterification, the tower tray and tower kettle are blocked due to the low solubility of the sulfate catalyst such as iron sulfate in methanol. Therefore, the method is not suitable for continuous industrial production.

[0008] Therefore, in view of the problems of serious corrosion of equipment, complicated operation and low yield in the method for synthesizing dimethyl terephthalate, it is urgent to develop a method for preparing dimethyl terephthalate with simple steps, small corrosion, high yield, high thermal stability and chemical stability, environmental friendliness and green chemistry requirements. SUMMARY

[0009] In view of the problems in the existing dimethyl terephthalate production process, the purpose of the present application is to provide a method for synthesizing dimethyl terephthalate. Methanol and terephthalic acid are used as raw materials, and a sulfonic acid catalyst is added. After esterification and crystallization, high-purity dimethyl terephthalate is obtained.

[0010] To achieve the above purpose, the present application provides a method for synthesizing dimethyl terephthalate, which comprises the following steps:

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

[0012] S2: adding methanol to the first esterification product to carry out a second esterification to obtain a second esterification product;

[0013] S3: adding methanol to the second esterification product to carry out crystallization to obtain a crystallization product;

[0014] S4: distilling the crystallization product to obtain dimethyl terephthalate;

[0015] The catalyst comprises one or a combination of more than two of hexyl benzene sulfonic acid, dodecyl sulfonic acid, pentadecyl sulfonic acid and dodecyl benzene sulfonic acid.

[0016] The process flow of the present application uses market-available general chemicals as raw materials and a catalyst, the catalyst has high catalytic efficiency, high thermal stability and chemical stability, especially after adding an additive, the corrosion of the catalyst to the equipment is obviously reduced compared with traditional concentrated sulfuric acid and p-toluene sulfonic acid catalysts; and the process flow of the present application has simple production process, high product purity, and the overall process meets the requirements of green chemistry and economy.

[0017] According to the specific embodiment of the present application, 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 the specific embodiment of the present application, preferably, in S1, the catalyst is added together with an additive, the additive comprises one or a combination of more than two of N-methyl pyrrolidone, benzotriazole, dihexylamine nitrite and 2-mercaptobenzothiazole, more preferably N-methyl pyrrolidone.

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

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

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

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

[0023] According to the specific embodiment of the present application, preferably, S1 and / or S2 further comprises: after the primary esterification reaction and / or the secondary esterification reaction is completed, distilling to remove the remaining methanol and generated water in the reaction system, thereby obtaining the primary esterification product and / or the secondary esterification product.

[0024] According to the specific embodiment of the present application, preferably, in S3, the mass ratio of methanol to the secondary esterification product is 1-4:1, more preferably 1.5-3:1.

[0025] According to the specific embodiment of the present application, preferably, in S3, when crystallization purification of dimethyl terephthalate is adopted, the crystallization starting temperature is 80-120℃, more preferably 90-110℃.

[0026] According to the specific embodiment of the present application, preferably, in S4, the distillation temperature is 180-210℃, more preferably 190℃.

[0027] According to the specific embodiment of the present application, preferably, the distillation is reduced pressure distillation.

[0028] The technical solution provided by the present application has the following beneficial effects:

[0029] (1) Compared with the traditional concentrated sulfuric acid catalyst, the sulfonic acid catalyst used in the present application can greatly weaken the corrosion of the equipment (especially after adding the additive), has good repeated use performance, and is more in line with the requirements of green chemistry.

[0030] (2) The catalyst used in the present application has low cost, high catalytic efficiency, good thermal stability and chemical stability, small loss, does not block the equipment, and is not easy to contaminate the product.

[0031] (3) The dimethyl terephthalate synthesis method of the present application has a simple production process, high product purity (up to more than 99.9%), and high raw material utilization rate. DETAILED DESCRIPTION

[0032] In order to have a clearer understanding of the technical features, objectives and beneficial effects of the present application, the technical solution of the present application will be described in detail below, but it cannot be understood as limiting the scope of the present application.

[0033] Example 1

[0034] The present embodiment provides a dimethyl terephthalate synthesis method, which comprises the following steps:

[0035] (1) The molar ratio of methanol to terephthalic acid is selected as 12:1, 33.23g (0.2mol) terephthalic acid, 77.90g (2.4mol) methanol and 0.50g hexylbenzene sulfonic acid (the mass of the catalyst is 1.5% of the mass of terephthalic acid) are added to the reaction kettle, and the esterification reaction occurs at a temperature of 160℃ for 2h. After the reaction is completed, the methanol and water are removed by rotary evaporation to obtain the product after the first esterification, and the yield of dimethyl terephthalate is 88.43%.

[0036] (2) The primary esterification product obtained is placed in a reaction kettle, 51.26 g (1.6 mol) of methanol is added, and secondary esterification is carried out at 150°C for 1 h. After the reaction is completed, methanol and water are removed by rotary evaporation to obtain the secondary esterification product. The dimethyl terephthalate yield is 97.90%.

[0037] (3) 30 g of the secondary esterification product is placed in a reaction kettle, 60 g of methanol is added, and after stirring at 100°C for 15 min, crystallization is carried out at 30°C to obtain the crystallized crude dimethyl terephthalate. The yield is 93.79%.

[0038] (4) 10 g of the crystallized crude dimethyl terephthalate is placed in a flask, and vacuum distillation is carried out at 190°C to obtain the high-purity dimethyl terephthalate after distillation. The yield is 99.14%, and the purity is 99.91%. The product acid value is 0.20, which is measured according to GB / T264-83 "Determination of Acid Value of Petroleum Products".

[0039] Example 2

[0040] The present embodiment provides a synthesis method of dimethyl terephthalate, which comprises the following steps:

[0041] (1) The molar ratio of methanol to terephthalic acid is selected to be 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 is 1.5% of the mass of terephthalic acid) are added to a reaction kettle, and esterification is carried out at a temperature of 160°C for 2 h. After the reaction is completed, methanol and water are removed by rotary evaporation to obtain the primary esterification product. The dimethyl terephthalate yield is 90.15%.

[0042] (2) The primary esterification product obtained is placed in a reaction kettle, 51.26 g (1.6 mol) of methanol is added, and secondary esterification is carried out at 150°C for 1 h. After the reaction is completed, methanol and water are removed by rotary evaporation to obtain the secondary esterification product. The dimethyl terephthalate yield is 98.25%.

[0043] (3) 30 g of the secondary esterification product is placed in a reaction kettle, 60 g of methanol is added, and after stirring at 90°C for 15 min, crystallization is carried out at 30°C to obtain the crystallized crude dimethyl terephthalate. The yield is 93.86%.

[0044] (4) 10 g of the crystallized crude dimethyl terephthalate is placed in a flask, and vacuum distillation is carried out at 190°C to obtain the high-purity dimethyl terephthalate after distillation. The yield is 99.16%, and the purity is 99.92%. The product acid value is 0.20, which is measured according to GB / T264-83 "Determination of Acid Value of Petroleum Products".

[0045] Example 3

[0046] The present example provides a method for synthesizing dimethyl terephthalate, comprising the following steps:

[0047] (1) The molar ratio of methanol to terephthalic acid is selected to be 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 is 2.5% of the mass of terephthalic acid) are added to a reaction kettle, and esterification reaction occurs at a temperature of 170°C for 2 h. After the reaction is completed, methanol and water are removed by rotary evaporation to obtain a product after the first esterification, and the yield of dimethyl terephthalate is 88.22%.

[0048] (2) The product after the first esterification is placed in a reaction kettle, 51.26 g (1.6 mol) of methanol is added, and secondary esterification is carried out at 150°C for 1 h. After the reaction is completed, methanol and water are removed by rotary evaporation to obtain a product after the second esterification, and the yield of dimethyl terephthalate is 97.75%.

[0049] (3) 30 g of the product after the second esterification is placed in a reaction kettle, 60 g of methanol is added, and after stirring at 100°C for 15 min, the temperature is lowered to 30°C to obtain crystallized crude dimethyl terephthalate, and the yield is 93.71%.

[0050] (4) 10 g of the crystallized crude dimethyl terephthalate is placed in a flask, and distilled under reduced pressure at 190°C to obtain distilled high-purity dimethyl terephthalate, and the yield is 99.26% and the purity is 99.91%. The acid value of the product is 0.21 according to GB / T264-83 "Determination of Acid Value of Petroleum Products".

[0051] Example 4

[0052] The present example provides a method for synthesizing dimethyl terephthalate, comprising the following steps:

[0053] (1) The molar ratio of methanol to terephthalic acid is selected to be 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 dodecyl benzene sulfonic acid (the mass of the catalyst is 1.5% of the mass of terephthalic acid) are added to a reaction kettle, and esterification reaction occurs at a temperature of 170°C for 2 h. After the reaction is completed, methanol and water are removed by rotary evaporation to obtain a product after the first esterification, and the yield of dimethyl terephthalate is 89.52%.

[0054] (2) The primary esterification product obtained is placed in a reaction kettle, 51.26 g (1.6 mol) of methanol is added, and secondary esterification is carried out at 150 °C for 1 h. After the reaction is completed, methanol and water are removed by rotary evaporation to obtain the product after secondary esterification. The dimethyl terephthalate yield is 97.72%.

[0055] (3) 30 g of the product after secondary esterification is taken and placed in a reaction kettle, 60 g of methanol is added, and after stirring at 110 °C for 15 min, crystallization is carried out at 30 °C to obtain the crystallized crude dimethyl terephthalate. The yield is 93.65%.

[0056] (4) 10 g of the crystallized crude dimethyl terephthalate is taken and placed in a flask, and distillation is carried out at 190 °C under reduced pressure to obtain the distilled high-purity dimethyl terephthalate. The yield is 99.08%, and the purity is 99.90%. The product acid value is 0.20, which is measured according to GB / T264-83 “Determination of Acid Value of Petroleum Products”.

[0057] It can be known from Examples 1-4 that increasing the amount of methanol helps the reaction to proceed in the forward direction, improves the esterification effect, and the purity of the final dimethyl terephthalate can all reach more than 99.9%, and the acid value meets the requirements, thus indicating that it is feasible to use the process method of the application to prepare DMT. In addition, the sulfonic acid catalyst used in the application has high catalytic efficiency, and the product yield obtained is high.

[0058] Comparative Example 1

[0059] A dimethyl terephthalate synthesis method comprises the following steps:

[0060] (1) The molar ratio of methanol to terephthalic acid is selected to be 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 is 1.5% of the mass of terephthalic acid) are added to a reaction kettle, and esterification is carried out at a temperature of 160 °C for 2 h. After the reaction is completed, methanol and water are removed by rotary evaporation to obtain the product after primary esterification. The dimethyl terephthalate yield is 86.98%.

[0061] (2) The primary esterification product obtained is placed in a reaction kettle, 51.26 g (1.6 mol) of methanol is added, and secondary esterification is carried out at 150 °C for 1 h. After the reaction is completed, methanol and water are removed by rotary evaporation to obtain the product after secondary esterification. The dimethyl terephthalate yield is 96.74%.

[0062] (3) 30 g of the product after secondary esterification is taken and placed in a reaction kettle, 60 g of methanol is added, and after stirring at 100 °C for 15 min, crystallization is carried out at 30 °C to obtain the product after primary crystallization. The yield is 93.11%.

[0063] (4) Take 10 g of the crude dimethyl terephthalate after the first crystallization and place it in a flask, distill under reduced pressure at 190°C to obtain high-purity dimethyl terephthalate after distillation, with a yield of 99.18% and a purity of 99.84%. The product acid value is 3.16 according to GB / T 264-83 "Determination of Acid Value of Petroleum Products".

[0064] Comparative Example 2

[0065] A method for synthesizing dimethyl terephthalate, comprising:

[0066] (1) The molar ratio of methanol to terephthalic acid is selected to be 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 is 1.5% of the mass of terephthalic acid) are added to a reaction kettle, and esterification occurs at a temperature of 150°C for 2 h. After the reaction is completed, the methanol and water are removed by rotary evaporation to obtain the product after the first esterification, with a yield of dimethyl terephthalate of 65.44%.

[0067] (2) The product after the first esterification is placed in a reaction kettle, 51.26 g (1.6 mol) of methanol is added, and secondary esterification is carried out at 150°C for 1 h. After the reaction is completed, the methanol and water are removed by rotary evaporation to obtain the product after the second esterification, with a yield of dimethyl terephthalate of 79.94%.

[0068] As can be seen from Example 1, Comparative Examples 1 and 2, compared with the concentrated sulfuric acid catalyst, the benzene sulfonic acid and p-toluene sulfonic acid catalysts have high esterification efficiency, but because the boiling point of p-toluene sulfonic acid is relatively low and is close to the boiling point of DMT product, the catalyst is lost during distillation, resulting in a product with a high acid value and affecting the downstream application of the product.

[0069] Example 5

[0070] The present embodiment provides a method for synthesizing dimethyl terephthalate, comprising the following steps:

[0071] (1) The molar ratio of methanol to terephthalic acid is selected to be 12:1, 33.23 g (0.2 mol) of terephthalic acid, 77.90 g (2.4 mol) of methanol, 0.50 g of hexyl benzene sulfonic acid (the catalyst, with a mass of 1.5% of the mass of terephthalic acid), and 500 ppm (based on the mass of the catalyst) of the additive N-methyl pyrrolidone are added to a reaction kettle, and esterification occurs at a temperature of 150°C for 2 h. After the reaction is completed, the methanol and water are removed by rotary evaporation to obtain the product after the first esterification, with a yield of dimethyl terephthalate of 88.47%.

[0072] (2) The primary esterification product was placed in a reaction kettle, 51.26 g (1.6 mol) of methanol was added, and secondary esterification was carried out at 150°C for 1 h. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the secondary esterification product. The yield of dimethyl terephthalate was 97.83%.

[0073] (3) 30 g of the secondary esterification product was placed in a reaction kettle, 60 g of methanol was added, and after stirring at 100°C for 15 min, the temperature was lowered to 30°C to obtain the primary crystallized crude dimethyl terephthalate. The yield was 93.49%.

[0074] (4) 10 g of the crystallized crude dimethyl terephthalate was placed in a flask, and distilled at 190°C for 2 h to obtain the distilled high-purity dimethyl terephthalate. The yield was 99.13%, and the purity was 99.90%. The product acid value was 0.20, which was measured according to GB / T264-83 "Determination of Acid Value of Petroleum Products".

[0075] Example 6

[0076] The present embodiment provides a method for synthesizing dimethyl terephthalate, which comprises the following steps:

[0077] (1) 33.23 g (0.2 mol) of terephthalic acid, 77.90 g (2.4 mol) of methanol, 0.50 g of dodecylbenzenesulfonic acid (the mass of the catalyst was 1.5% of the mass of terephthalic acid), and 800 ppm (based on the mass of the catalyst) of the additive N-methylpyrrolidone were added to a reaction kettle, and esterification was carried out at a temperature of 150°C for 2 h. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the primary esterification product. The yield of dimethyl terephthalate was 88.44%.

[0078] (2) The primary esterification product was placed in a reaction kettle, 51.26 g (1.6 mol) of methanol was added, and secondary esterification was carried out at 150°C for 1 h. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the secondary esterification product. The yield of dimethyl terephthalate was 97.80%.

[0079] (3) 30 g of the secondary esterification product was placed in a reaction kettle, 60 g of methanol was added, and after stirring at 100°C for 15 min, the temperature was lowered to 30°C to obtain the primary crystallized crude dimethyl terephthalate. The yield was 93.55%.

[0080] (4) 10 g of the crystallized crude dimethyl terephthalate was placed in a flask, and distilled at 190°C for 2 h to obtain the distilled high-purity dimethyl terephthalate. The yield was 99.11%, and the purity was 99.91%. The product acid value was 0.20, which was measured according to GB / T264-83 "Determination of Acid Value of Petroleum Products".

[0081] Experimental Example 1

[0082] Thermal stability of benzene sulfonic acid catalyst:

[0083] (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 benzene sulfonic acid (the mass of catalyst was 1.5% of the mass of terephthalic acid) were added into a reaction kettle, and esterification reaction occurred at a temperature of 150°C for 2 h. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after the first esterification, and the yield of dimethyl terephthalate was 88.48%.

[0084] (2) The product after the first esterification was placed in a reaction kettle, 51.26 g (1.6 mol) of methanol was added, and secondary esterification was carried out at 150°C for 1 h. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after the second esterification, and the yield of dimethyl terephthalate was 97.86%.

[0085] (3) The product after the second esterification was placed in a flask, and the dodecyl benzene sulfonic acid catalyst after vacuum distillation was recovered after vacuum distillation at 190°C for 2 h.

[0086] (4) The recovered benzene sulfonic acid catalyst, 33.23 g (0.2 mol) of terephthalic acid and 77.90 g (2.4 mol) of methanol were added into a reaction kettle, and esterification reaction occurred at a temperature of 150°C for 2 h. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after the first esterification, and the yield of dimethyl terephthalate was 88.53%.

[0087] (5) The product after the first esterification was placed in a reaction kettle, 51.26 g (1.6 mol) of methanol was added, and secondary esterification was carried out at 150°C for 1 h. After the reaction was completed, methanol and water were removed by rotary evaporation to obtain the product after the second esterification, and the yield of dimethyl terephthalate was 97.84%.

[0088] As can be seen from Experimental Example 1, the benzene sulfonic acid catalyst after vacuum distillation still has good thermal stability and can maintain catalytic efficiency at a higher temperature, indicating that the benzene sulfonic acid separated by the kettle of the rectifying tower at a high temperature still has very good catalytic activity.

[0089] Experimental Example 2

[0090] Corrosiveness of benzene sulfonic acid after adding an additive:

[0091] (1) The experiment was carried out according to GB / T4334.6-2015 "Stainless Steel Sulfuric Acid Corrosion Test Method". Three parallel 316L stainless steel samples were taken, the sample was ground with sandpaper without heating the sample, the size of the sample was measured with a vernier caliper, the total surface area of the sample was calculated, then the sample was placed in clean water for ultrasonic cleaning, dried in an oven for 30 min, cooled to room temperature, and weighed on an analytical balance with an accuracy of 0.0001 g.

[0092] (2) The sample was placed in a flask with a conical ground and a glass vertical reflux condenser with sufficient cooling effect, dodecylbenzenesulfonic acid was added to immerse the stainless steel sample, and 500 ppm (based on the mass of the catalyst) of the additive N-methyl pyrrolidone was added, heated to 150°C and kept for 12 h.

[0093] (3) After reaching the predetermined corrosion time, the stainless steel corrosion test sample was taken out, washed with a soft brush in running water to remove the corrosion products on the surface of the sample, and weighed after drying in an oven.

[0094] (4) The weight loss rate of dodecylbenzenesulfonic acid after adding the additive to corrode 316L stainless steel was calculated.

[0095] Experimental Example 3

[0096] Corrosion of dodecylsulfonic acid after adding an additive:

[0097] (1) The experiment was carried out according to GB / T4334.6-2015 "Stainless Steel Corrosion Test Method". Three parallel 316L stainless steel samples were taken, the sample was ground with sandpaper without heating the sample, the size of the sample was measured with a vernier caliper, the total surface area of the sample was calculated, then the sample was placed in clean water for ultrasonic cleaning, dried in an oven for 30 min, cooled to room temperature, and weighed on an analytical balance with an accuracy of 0.0001 g.

[0098] (2) The sample was placed in a flask with a conical ground and a glass vertical reflux condenser with sufficient cooling effect, dodecylsulfonic acid was added to immerse the stainless steel sample, and 800 ppm (based on the mass of the catalyst) of the additive N-methyl pyrrolidone was added, heated to 150°C and kept for 12 h.

[0099] (3) After reaching the predetermined corrosion time, the stainless steel corrosion test sample was taken out, washed with a soft brush in running water to remove the corrosion products on the surface of the sample, and weighed after drying in an oven.

[0100] (4) The weight loss rate of dodecylsulfonic acid after adding the additive to corrode 316L stainless steel was calculated.

[0101] Comparative Experimental Example 1

[0102] Corrosivity of dodecylbenzenesulfonic acid catalyst:

[0103] (1) The experiment was carried out according to GB / T4334.6-2015 "Stainless Steel Corrosion Test Method". Three parallel 316L stainless steel samples were taken, the samples were ground with sandpaper without heating the samples, the size of the samples was measured with a vernier caliper, the total surface area of the samples was calculated, then the samples were placed in clean water for ultrasonic cleaning, dried in an oven for 30 min, cooled to room temperature, and weighed on an analytical balance with an accuracy of 0.0001 g.

[0104] (2) The samples were placed in a flask with a conical ground and a glass vertical reflux condenser with sufficient cooling effect, dodecylbenzenesulfonic acid was added to immerse the stainless steel samples, and the temperature was raised to 150°C and maintained for 12 h.

[0105] (3) After reaching the predetermined corrosion time, the stainless steel corrosion test sample was taken out, washed with a soft brush in running water to remove the corrosion products on the surface of the sample, and weighed after drying in an oven.

[0106] (4) The weight loss rate of dodecylbenzenesulfonic acid catalyst on 316L stainless steel corrosion was calculated.

[0107] Comparative Experiment Example 2

[0108] Corrosivity of p-toluenesulfonic acid catalyst:

[0109] (1) The experiment was carried out according to GB / T4334.6-2015 "Stainless Steel Corrosion Test Method". Three parallel 316L stainless steel samples were taken, the samples were ground with sandpaper without heating the samples, the size of the samples was measured with a vernier caliper, the total surface area of the samples was calculated, then the samples were placed in clean water for ultrasonic cleaning, dried in an oven for 30 min, cooled to room temperature, and weighed on an analytical balance with an accuracy of 0.0001 g.

[0110] (2) The samples were placed in a flask with a conical ground and a glass vertical reflux condenser with sufficient cooling effect, p-toluenesulfonic acid was added to immerse the stainless steel samples, and the temperature was raised to 150°C and maintained for 12 h.

[0111] (3) After reaching the predetermined corrosion time, the stainless steel corrosion test sample was taken out, washed with a soft brush in running water to remove the corrosion products on the surface of the sample, and weighed after drying in an oven.

[0112] (4) The weight loss rate of p-toluenesulfonic acid catalyst on 316L stainless steel corrosion was calculated.

[0113] Comparative Experiment Example 3

[0114] Corrosion of sulfuric acid:

[0115] (1) The experiment was carried out according to GB / T 4334.6-2015 "Stainless Steel Corrosion Test Method". Three parallel 316L stainless steel samples were taken, and the samples were ground using sandpaper under the condition of avoiding heating of the samples. The size of the sample was measured using a vernier caliper, the total surface area of the sample was calculated, and then the sample was placed in clean water for ultrasonic cleaning, dried in an oven for 30 min, cooled to room temperature, and weighed on an analytical balance with an accuracy of 0.0001 g.

[0116] (2) The sample was placed in a flask with a conical ground and a glass vertical reflux condenser with sufficient cooling effect, and DMT containing 5% sulfuric acid was added to immerse the stainless steel sample. The temperature was raised to 150°C and maintained for 12 h.

[0117] (3) After reaching the predetermined corrosion time, the stainless steel corrosion test sample was taken out, washed with a soft brush in running water to remove the corrosion products on the surface of the sample, and weighed after drying in an oven.

[0118] (4) The weight loss rate of 316L stainless steel corrosion by DMT containing 5% sulfuric acid was calculated.

[0119] Table 1 Weight loss rate of stainless steel corrosion test

[0120]

[0121]

[0122] As shown in the above table comparative experiment examples 1-3, the corrosion rate using the sulfonic acid catalyst of the present application is significantly lower than the corrosion rate of stainless steel using p-toluene sulfonic acid and DMT containing 5% sulfuric acid. Therefore, compared with p-toluene sulfonic acid and concentrated sulfuric acid catalysts, the use of benzene sulfonic acid catalyst can improve the esterification efficiency while reducing the corrosion of the equipment.

[0123] As shown in the above table experimental examples 1-2 and comparative experiment example 1, after adding the additive, the corrosion rate of the sulfonic acid catalyst of the present application to stainless steel is significantly reduced, and the amount of additive increases, the corrosion of the steel material decreases. Therefore, after adding the additive, the corrosion of the equipment is further reduced, the pollution to the environment is reduced, and the requirements of green chemistry are met.

[0124] The difference between example 1 and examples 5-6 is that an additive is added to the reaction system. As can be seen from the results, after adding the additive, the corrosion of the equipment can be reduced without affecting the esterification efficiency and the acid value of the product.

[0125] Experimental example 4

[0126] Thermal stability of the catalyst of benzene sulfonic acid after adding the additive:

[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 benzene sulfonic 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 the additive N-methyl pyrrolidone were added to a reaction kettle, and esterification was carried out at a temperature of 150 DEG C for 2 h, after which the methanol and water were removed by rotary evaporation to obtain the product after the first esterification, and the yield of dimethyl terephthalate was 88.41%.

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

[0129] (3) The product after the secondary esterification was placed in a flask, and after distillation at 190 DEG C for 2 h, the bottom liquid (dodecyl benzene sulfonic acid catalyst) after the recovery of DMT by reduced pressure distillation was collected.

[0130] (4) The recovered benzene sulfonic acid catalyst, 33.23 g (0.2 mol) of terephthalic acid and 77.90 g (2.4 mol) of methanol were added to a reaction kettle, and esterification was carried out at a temperature of 150 DEG C for 2 h, after which the methanol and water were removed by rotary evaporation to obtain the product after the first esterification.

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

[0132] As can be seen from the experimental example 4, the benzene sulfonic acid catalyst after the addition of the additive still has good thermal stability after reduced pressure distillation, which indicates that the benzene sulfonic acid after the high-temperature separation of the kettle of the rectifying tower still has very good catalytic activity after the addition of the additive.

[0133] The present application realizes that dimethyl terephthalate with a purity of more than 99.9% is obtained by using methanol and terephthalic acid as raw materials, high-boiling point sulfonic acid as a catalyst and adding an additive after the processes of first esterification, secondary esterification, crystallization and distillation, and the process flow is simple, the esterification efficiency is improved under the conditions of ensuring the stability of the catalyst and reducing the corrosion of the equipment compared with the traditional concentrated sulfuric acid catalytic process, and the requirements of green chemistry are met.

[0134] The above merely describes the preferred embodiments of the present application, and is not intended to limit the present application in any other form, and any modification or equivalent variation made according to the technical essence of the present application still falls within the scope of the present application.

Claims

1. A method for synthesizing dimethyl terephthalate, comprising the following steps: S1: adding a sulfonic acid catalyst and an additive to methanol and terephthalic acid to perform a first esterification reaction, to obtain a first esterification product, the additive being N-methyl pyrrolidone, and the additive being added in an amount of 500-1000 ppm based on the mass of the catalyst; S2: adding methanol to the first esterification product to perform a second esterification, to obtain a second esterification product; S3: adding methanol to the second esterification product to perform crystallization, to obtain a crystallization product; S4: distilling the crystallization product to obtain dimethyl terephthalate; wherein the catalyst being one or a combination of more than two of hexyl benzene sulfonic acid, dodecyl sulfonic acid, pentadecyl sulfonic acid, and dodecyl benzene sulfonic acid.

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

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

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

1.

5. The method of synthesizing dimethyl terephthalate according to claim 4, wherein, In S1, the molar ratio of methanol to terephthalic acid is 12-14:

1.

6. The method of 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.

7. The method of 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.

8. The method of synthesizing dimethyl terephthalate according to claim 1, wherein, In S2, the temperature of the second esterification reaction is 140-180℃, and the time is 1-2h.

9. The method of synthesizing dimethyl terephthalate according to claim 1, wherein, S1 and / or S2 further comprise: after the first esterification reaction and / or the second esterification reaction, distilling to remove the remaining methanol and the generated water in the reaction system, to obtain the first esterification product and / or the second esterification product.

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

1.

11. The method of synthesizing dimethyl terephthalate according to claim 10, wherein, In S3, the mass ratio of methanol to the second esterification product is 1.5-3:

1.

12. The method of synthesizing dimethyl terephthalate according to claim 1, wherein, In S3, when crystallization is used to purify dimethyl terephthalate, the starting temperature of crystallization is 80-120℃.

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

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