Method for preparing high-purity 2, 6-naphthalic acid

Abstract

The invention relates to the technical field of organic synthesis, and provides a method for preparing high-purity 2, 6-naphthalic acid, which comprises the following steps: S1, uniformly mixing dimethyl 2, 6-naphthalic acid, water and a catalyst, hydrolyzing, and removing methanol to obtain a mixture; and S2, cooling the mixture, and separating to obtain the 2, 6-naphthalic acid. According to the technical scheme, the problems of low production efficiency, low yield and low purity of 2, 6-naphthalic acid in related technologies are solved.

Description

Technical Field

[0001] This invention relates to the field of organic synthesis technology, and more specifically, to a method for preparing high-purity 2,6-naphthalenedicarboxylic acid. Background Technology

[0002] 2,6-Naphthalenedicarboxylic acid is an important organic chemical intermediate and a key raw material for the synthesis of polyethylene naphthalate (PEN). Industrially, 2,6-naphthalenedicarboxylic acid is prepared via direct oxidation, which involves oxidizing 2,6-alkylnaphthalene. However, this direct oxidation process generates impurities such as 6-formyl-2-naphthoic acid, trimellitic acid, and various brominated compounds, resulting in relatively low purity of 2,6-naphthalenedicarboxylic acid, which is insufficient for the polymerization requirements of PEN. Therefore, currently, crude 2,6-naphthalenedicarboxylic acid is typically reacted to produce dimethyl 2,6-naphthalenedicarboxylic acid, purified, and then hydrolyzed back to 2,6-naphthalenedicarboxylic acid, achieving a purity sufficient for the polymerization of PEN.

[0003] One method involves hydrolysis, where dimethyl 2,6-naphthalenedicarboxylate is dissolved under alkaline conditions to form a salt, which is then acidified to obtain 2,6-naphthalenedicarboxylic acid. The main problem with this route is the extremely small particle size of the precipitated 2,6-naphthalenedicarboxylic acid during acidification, making filtration difficult. Furthermore, this route produces a significant amount of byproducts such as sodium sulfate or sodium carbonate. Alternatively, hydrolysis can be performed under acidic conditions, typically at high temperatures using organic acids such as sulfuric acid or hydrochloric acid as catalysts. However, this method suffers from poor catalytic performance, prolonged hydrolysis time, and extremely high requirements for reaction equipment. It also presents the technical challenge of handling large quantities of waste acid. Additionally, the presence of an acidic catalyst can lead to high levels of impurities such as 6-(methoxycarbonyl)-2-naphthic acid during hydrolysis, preventing complete conversion to 2,6-naphthalenedicarboxylic acid and resulting in insufficient product yield and purity.

[0004] Therefore, a method for preparing 2,6-naphthalenedicarboxylic acid is proposed, which can improve production efficiency and effectively increase the yield and purity of 2,6-naphthalenedicarboxylic acid. This is of great significance for ensuring the performance and quality of downstream products (PEN). Summary of the Invention

[0005] This invention proposes a method for preparing high-purity 2,6-naphthalenedicarboxylic acid, which solves the problems of low production efficiency, low yield and low purity in the preparation of 2,6-naphthalenedicarboxylic acid in related technologies.

[0006] The technical solution of the present invention is as follows: This invention proposes a method for preparing high-purity 2,6-naphthalenedicarboxylic acid, comprising the following steps: S1. After mixing dimethyl 2,6-naphthalenedicarboxylate, water I and catalyst evenly, hydrolyze the mixture to remove methanol and obtain a mixture. S2. Cool the mixture and separate it to obtain the 2,6-naphthalenedicarboxylic acid.

[0007] As a further technical solution, the temperature during methanol removal is 200~220℃.

[0008] As a further technical solution, the mass ratio of the dimethyl 2,6-naphthalenedicarboxylate, water and catalyst is 1:5~15:0.12~0.15.

[0009] As a further technical solution, the hydrolysis is divided into a first stage of hydrolysis and a second stage of hydrolysis; During the first stage of hydrolysis, the temperature is 200~220℃; The temperature during the second stage of hydrolysis is 240~260℃.

[0010] As a further technical solution, the hydrolysis time in the first stage is 0.5~1h; The second stage of hydrolysis takes 1 to 2 hours.

[0011] In this invention, dimethyl 2,6-naphthalenedicarboxylate, water, and catalyst are mixed uniformly and then subjected to staged hydrolysis. First, hydrolysis is performed at 200–220°C for 0.5–1 h, followed by hydrolysis at 240–260°C for 1–2 h. During the first stage of hydrolysis, the reaction is relatively slow, allowing dimethyl 2,6-naphthalenedicarboxylate to form 2,6-naphthalenedicarboxylic acid crystals at a slower rate. This slower hydrolysis rate is beneficial for forming larger crystal particles, facilitating subsequent filtration and downstream applications. During the second stage of hydrolysis, within this temperature range, the solubility of the intermediate 6-(methoxycarbonyl)-2-naphthic acid increases, accelerating hydrolysis and ultimately ensuring complete hydrolysis, resulting in improved product purity and yield.

[0012] As a further technical solution, during the first stage of hydrolysis, the temperature is increased to 200-220℃ at a heating rate of 5-10℃ / min.

[0013] As a further technical solution, the catalyst includes one or more of phthalic anhydride, trimellitic anhydride, and pyromellitic anhydride, preferably phthalic anhydride.

[0014] In the method for preparing high-purity 2,6-naphthalenedicarboxylic acid of the present invention, the catalyst is one or more anhydride catalysts selected from phthalic anhydride, trimellitic anhydride, and pyromellitic anhydride. Compared with traditional catalysts such as sulfuric acid or hydrochloric acid, the catalyst has relatively high reaction activity, and the 2,6-naphthalenedicarboxylic acid obtained by hydrolysis has a relatively large particle size, which can reduce the loss of 2,6-naphthalenedicarboxylic acid in the subsequent separation process.

[0015] As a further technical solution, the separation process also includes rinsing and drying.

[0016] As a further technical solution, during the rinsing process, methanol is first used to slurry and wash at 60°C, and the filter cake obtained by filtration is then rinsed with water II at room temperature. The amount of water II used is 0.5 to 3 times the amount of water I used; the amount of methanol added is 0.5 to 3 times the amount of water I used.

[0017] As a further technical solution, the particle size of the 2,6-naphthalenedicarboxylic acid is 100~150μm.

[0018] The working principle and beneficial effects of this invention are as follows: 1. In this invention, 2,6-naphthalenedicarboxylic acid is prepared by hydrolysis. During the hydrolysis process, the methanol generated in the reaction is discharged, which can prevent the newly generated methanol from reacting with 2,6-naphthalenedicarboxylic acid to regenerate dimethyl 2,6-naphthalenedicarboxylic acid and 6-(methoxycarbonyl)-2-naphthalic acid. This effectively reduces the presence of 6-(methoxycarbonyl)-2-naphthalic acid in the system, making the hydrolysis more thorough, facilitating the production of 2,6-naphthalenedicarboxylic acid, accelerating the hydrolysis process, improving production efficiency, and ultimately increasing the yield and purity of 2,6-naphthalenedicarboxylic acid.

[0019] 2. In the hydrolysis process of this invention, the methanol generated by the reaction is discharged, which reduces the contact between methanol and the catalyst, reduces the generation of dimethyl phthalate, a catalyst impurity, and greatly reduces the difficulty of catalyst recovery and treatment, which is conducive to the recycling and utilization of the catalyst. Detailed Implementation

[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0021] In the following examples and comparative examples, the purity of dimethyl 2,6-naphthalenedicarboxylate was 99.95%, and the CAS number was 840-65-3; The purity of phthalic anhydride is 99.95%, and the CAS number is 85-44-9.

[0022] Example 1 A method for preparing high-purity 2,6-naphthalenedicarboxylic acid includes the following steps: S1. Mix 1000g of dimethyl 2,6-naphthalenedicarboxylate, 10000g of water and 140g of phthalic anhydride evenly in a raw material tank, and then put them into a hydrolysis reactor. Heat the mixture to 215℃ at a heating rate of 8℃ / min, and then hydrolyze it at 215℃ for 6 hours. The methanol generated during the hydrolysis process is discharged through a distillation column above the hydrolysis reactor. The temperature at the top of the column is 205℃ when it is discharged. The water is returned to the hydrolysis reactor to obtain a mixture. S2. Filter the above mixture to obtain a solid. Wash the solid with 5000g of methanol at 60℃. Wash the filter cake obtained by filtration with 5000g of water at room temperature. After filtration and drying, 2,6-naphthalenedicarboxylic acid is obtained.

[0023] Example 2 A method for preparing high-purity 2,6-naphthalenedicarboxylic acid includes the following steps: S1. Mix 1000g of dimethyl 2,6-naphthalenedicarboxylate, 5000g of water and 120g of phthalic anhydride evenly in a raw material tank, and then put them into a hydrolysis reactor. Heat the mixture to 215℃ at a heating rate of 5℃ / min, and then hydrolyze it at 215℃ for 6 hours. The methanol generated during the hydrolysis process is discharged through a distillation column above the hydrolysis reactor. The temperature at the top of the column is 200℃ when it is discharged. The water is returned to the hydrolysis reactor to obtain a mixture. S2. Filter the mixture to obtain a solid. Wash the solid with 10,000 g of methanol at 60°C. Wash the filter cake obtained by filtration with 10,000 g of water at room temperature. After filtration and drying, 2,6-naphthalenedicarboxylic acid is obtained.

[0024] Example 3 A method for preparing high-purity 2,6-naphthalenedicarboxylic acid includes the following steps: S1. Mix 1000g of dimethyl 2,6-naphthalenedicarboxylate, 15000g of water and 150g of phthalic anhydride evenly in a raw material tank, and then feed the mixture through a preheater using a feed pump into a hydrolysis reactor. Heat the mixture to 215℃ at a heating rate of 10℃ / min, and hydrolyze it at 215℃ for 6 hours. The methanol generated during the hydrolysis process is discharged through a distillation column above the hydrolysis reactor. The temperature at the top of the column is 210℃ when the mixture is discharged. The water is returned to the hydrolysis reactor to obtain a mixture. S2. Filter the mixture to obtain a solid. Wash the solid with 15000g of methanol at 60℃. Filter the resulting filter cake and then wash it with 15000g of water at room temperature. Filter and dry to obtain 2,6-naphthalenedicarboxylic acid.

[0025] Example 4 A method for preparing high-purity 2,6-naphthalenedicarboxylic acid includes the following steps: S1. Mix 1000g of dimethyl 2,6-naphthalenedicarboxylate, 10000g of water and 140g of phthalic anhydride evenly in a raw material tank, and then put them into a hydrolysis reactor. Heat the mixture to 200℃ at a heating rate of 5℃ / min, and then hydrolyze it at 200℃ for 6 hours. The methanol generated during the hydrolysis process is discharged through a distillation column above the hydrolysis reactor. The temperature at the top of the column is 205℃ when it is discharged. The water is returned to the hydrolysis reactor to obtain a mixture. S2. Filter the mixture to obtain a solid. Wash the solid with 10,000 g of methanol at 60°C. Filter the resulting filter cake and then rinse it with 10,000 g of water at room temperature. After filtration and drying, 2,6-naphthalenedicarboxylic acid is obtained.

[0026] Example 5 A method for preparing high-purity 2,6-naphthalenedicarboxylic acid includes the following steps: S1. Mix 1000g of dimethyl 2,6-naphthalenedicarboxylate, 10000g of water and 140g of phthalic anhydride evenly in a raw material tank, and then put them into a hydrolysis reactor. Heat the mixture to 240℃ at a heating rate of 5℃ / min, and then hydrolyze it at 240℃ for 6 hours. The methanol generated during the hydrolysis process is discharged through a distillation column above the hydrolysis reactor. The temperature at the top of the column is 205℃ when it is discharged. The water is returned to the hydrolysis reactor to obtain a mixture. S2. Filter the mixture to obtain a solid. Wash the solid with 5000g of methanol at 60℃. Filter the resulting filter cake and then wash it with 5000g of water at room temperature. After filtration and drying, 2,6-naphthalenedicarboxylic acid is obtained.

[0027] Example 6 A method for preparing high-purity 2,6-naphthalenedicarboxylic acid includes the following steps: S1. Mix 1000g of dimethyl 2,6-naphthalenedicarboxylate, 10000g of water and 140g of phthalic anhydride evenly in a raw material tank, and then put them into a hydrolysis reactor. Heat the mixture to 200℃ at a heating rate of 8℃ / min, and then hydrolyze it at 200℃ for 1 hour. Then heat the mixture to 240℃ and continue hydrolyzing it at 240℃ for 2 hours. The methanol generated during the hydrolysis process is discharged through a distillation column above the hydrolysis reactor. The temperature at the top of the column is 205℃ when it is discharged. The water is refluxed back into the hydrolysis reactor to obtain a mixture. S2. Filter the above mixture to obtain a solid. Wash the solid with 5000g of methanol at 60℃. Wash the filter cake obtained by filtration with 5000g of water at room temperature. After filtration and drying, 2,6-naphthalenedicarboxylic acid is obtained.

[0028] Example 7 The only difference between this embodiment and Example 4 is that step S1 in the method for preparing high-purity 2,6-naphthalenedicarboxylic acid in this embodiment is different, specifically: S1. Mix 1000g of dimethyl 2,6-naphthalenedicarboxylate, 10000g of water and 140g of phthalic anhydride evenly in a raw material tank, and then put them into a hydrolysis reactor. Heat the mixture to 220℃ at a heating rate of 8℃ / min, and then hydrolyze it at 220℃ for 0.5h. Then heat the mixture to 260℃ and continue hydrolyzing it at 260℃ for 1h. The methanol generated during the hydrolysis process is discharged through a distillation column above the hydrolysis reactor. The temperature at the top of the column is 205℃ when it is discharged. The water is refluxed back into the hydrolysis reactor to obtain a mixture.

[0029] Comparative Example 1 A method for preparing high-purity 2,6-naphthalenedicarboxylic acid, characterized by comprising the following steps: 1000g of dimethyl 2,6-naphthalenedicarboxylate, 10000g of water, and 140g of phthalic anhydride were mixed evenly in a raw material tank and then transferred to a hydrolysis reactor. The mixture was heated to 215℃ at a heating rate of 8℃ / min and hydrolyzed at 215℃ for 6 hours to obtain a mixture. The mixture was filtered to obtain a solid. The solid was first washed with 5000g of methanol at 60℃, and the resulting filter cake was then washed with 5000g of water at room temperature. After filtration and drying, 2,6-naphthalenedicarboxylic acid was obtained.

[0030] Experimental Example The 2,6-naphthalenedicarboxylic acid prepared in Examples 1-7 and Comparative Example 1 was weighed, and the yield was calculated according to the formula: yield = actual mass of 2,6-naphthalenedicarboxylic acid / theoretical mass × 100%. The purity and impurity content of 2,6-naphthalenedicarboxylic acid were tested by liquid chromatography. In addition to 6-(methoxycarbonyl)-2-naphthalic acid and dimethyl 2,6-naphthalenedicarboxylic acid, some inert impurities were also present, but their content was negligible. The particle size of 2,6-naphthalenedicarboxylic acid prepared in Examples 1-7 and Comparative Example 1 was tested using a particle size analyzer. The test results are shown in Table 1.

[0031] Table 1 Test results of Examples 1-7 and Comparative Example 1

[0032] As can be seen from Table 1, compared with Comparative Example 1, the yield and purity of 2,6-naphthalenedicarboxylic acid in Examples 1-7 are improved. This indicates that the methanol generated during the hydrolysis process of the present invention is discharged, which can reduce the effect of methanol on the catalyst in the reaction system and reduce the generation of impurities such as 6-(methoxycarbonyl)-2-naphthalic acid. This makes the final hydrolysis more thorough, accelerates the hydrolysis process, improves production efficiency, and ultimately effectively improves the yield and purity of 2,6-naphthalenedicarboxylic acid.

[0033] In Examples 1-7, the purity and yield of 2,6-naphthalenedicarboxylic acid in Examples 5-7 were relatively higher. However, the particle size of 2,6-naphthalenedicarboxylic acid obtained in Example 5 was as low as 20 μm, making filtration difficult. In Examples 6-7, not only was the purity and yield of 2,6-naphthalenedicarboxylic acid higher, but the particle size also met the requirements. This indicates that the two-stage hydrolysis process can ensure that the 2,6-naphthalenedicarboxylic acid is within the range of 100-150 μm, and can also promote complete hydrolysis and reduce the impurity content in the product 2,6-naphthalenedicarboxylic acid.

[0034] Experiment Example 2 After filtering the mixture from Example 1 and Comparative Example 1, 2,6-naphthalenedicarboxylic acid solid and filtrate were obtained. The filtrate was then subjected to catalyst recovery treatment. The components and contents of the recovered substances are shown in Table 2. (The sum of the two is 99.9%, indicating trace impurities.) Table 2. Test results of recyclables from Example 1 and Comparative Example 1

[0035] Compared to Comparative Example 1, in Example 1, after recovering the filtrate, the phthalic acid content in the recovered product could reach over 97%, indicating that the methanol generated during hydrolysis was discharged, reducing the interaction between methanol and the catalyst and significantly improving the purity of the recovered catalyst, thus facilitating catalyst recycling. In contrast, in Comparative Example 1, the excessive generation of dimethyl phthalate increased the difficulty of catalyst post-processing, hindering catalyst recovery.

[0036] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing high-purity 2,6-naphthalenedicarboxylic acid, characterized in that, Includes the following steps: S1. After mixing dimethyl 2,6-naphthalenedicarboxylate, water I and catalyst evenly, hydrolyze the mixture to remove methanol and obtain a mixture. S2. Cool the mixture and separate it to obtain the 2,6-naphthalenedicarboxylic acid.

2. The method for preparing high-purity 2,6-naphthalenedicarboxylic acid according to claim 1, characterized in that, The temperature for methanol removal is 200~220℃. 3.The method for preparing high-purity 2,6-naphthalene dicarboxylic acid according to claim 1, characterized in that, The mass ratio of the dimethyl 2,6-naphthalenedicarboxylate, water, and catalyst is 1:5~15:0.12~0.

15. 4.The method for preparing high-purity 2,6-naphthalene dicarboxylic acid according to claim 1, characterized in that, The hydrolysis is divided into a first stage of hydrolysis and a second stage of hydrolysis. During the first stage of hydrolysis, the temperature is 200~220℃; The temperature during the second stage of hydrolysis is 240~260℃. 5.The method for preparing high-purity 2,6-naphthalene dicarboxylic acid according to claim 4, characterized in that, The first stage of hydrolysis takes 0.5 to 1 hour; The second stage of hydrolysis takes 1 to 2 hours.

6. The method for preparing high-purity 2,6-naphthalenedicarboxylic acid according to claim 4, characterized in that, During the first stage of hydrolysis, the temperature is increased to 200-220℃ at a rate of 5-10℃ / min.

7. The method for preparing high-purity 2,6-naphthalenedicarboxylic acid according to claim 1, characterized in that, The catalyst includes one or more of phthalic anhydride, trimellitic anhydride, and pyromellitic anhydride.

8. The method for preparing high-purity 2,6-naphthalenedicarboxylic acid according to claim 1, characterized in that, The separation process also includes rinsing and drying.

9. The method for preparing high-purity 2,6-naphthalenedicarboxylic acid according to claim 8, characterized in that, During the rinsing process, methanol is first used to slurry and wash the filter cake at 60°C, and the resulting filter cake is then rinsed with water II at room temperature. The amount of methanol added is 0.5 to 3 times the amount of water I used, and the amount of water II used is 0.5 to 3 times the amount of water I used.

10. The method for preparing high purity 2,6-naphthalene dicarboxylic acid according to any one of claims 1-9, characterized in that, The particle size of the 2,6-naphthalenedicarboxylic acid is 100~150 μm.

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