A method for purifying trans-chda by efficient isomerization coupled with crystallization

Abstract

The application discloses a kind of efficient isomerization coupling crystallization trans CHDA purification method, it is related to fine chemical purification technical field, the present application will CHDA crude product and N, N-dimethylformamide are mixed, sodium methoxide is added as isomerization catalyst, isomerization reaction is carried out under reflux condition, realize the efficient conversion of cis CHDA to trans CHDA;Subsequently combined gradient cooling crystallization process, with segmented heat preservation and low temperature DMF washing operation, for trans CHDA crystal nucleus uniform formation and crystal fully grown provide suitable conditions, effectively avoid the problem of cis isomer co-crystallization;After subsequent refining treatment, high-purity trans CHDA product is prepared.The present application realizes the directional preparation and separation of trans CHDA by the cooperation of isomerization and crystallization, the product purity stability is ≥99.0%, total yield is ≥88.0%, with purification efficiency and product quality, process stability is good, suitable for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of fine chemical purification technology, and more specifically to a highly efficient method for purifying trans-CHDA through isomerization coupling crystallization. Background Technology

[0002] Trans-cyclohexanedicarboxylic acid (trans-CHDA) is a key intermediate in the preparation of high-performance polyesters, polyurethanes, epoxy resins, and other high-end engineering materials. Its purity directly determines the core performance indicators of downstream products, such as mechanical strength, thermal stability, and chemical corrosion resistance. Currently, there is an urgent market demand for high-purity trans-CHDA with a purity of ≥99%, and the requirements for product quality stability are becoming increasingly stringent. In industrial production, the N,N-dimethylformamide solvent method is the mainstream process for purifying trans-CHDA. However, existing processes still have significant technical shortcomings, making it difficult to meet the demands for large-scale and stable production of high-quality trans-CHDA, thus limiting its application in the field of high-end engineering materials.

[0003] Specifically, the core technical defects of the existing N,N-dimethylformamide solvent method for purifying trans CHDA are concentrated in four aspects: First, the isomerization efficiency is low. The cis-CHDA contained in the crude CHDA product is difficult to fully convert into the trans isomer, directly affecting the purity of the trans-isomer in subsequent crystallization products. For example, the production method of trans-1,4-cyclohexanedicarboxylic acid disclosed in Chinese patent document "CN1310868C" uses a simple thermal isomerization method to convert the cis-isomer, and its cis-to-trans conversion rate can only reach about 60%, resulting in poor isomerization effect. Second, the crystallization yield stability is poor. Traditional processes often use a single-rate cooling crystallization mode. For example, the isothermal crystallization process disclosed in Japanese patent document "JP2021528389A" is prone to co-crystallization of cis-CHDA and trans-CHDA if the cooling rate is too fast, which will not only reduce the purity of the product but also cause... The following issues arise: First, the yield fluctuations between batches exceed 5%, affecting production stability. Second, product purity is difficult to meet standards. For example, the production method of trans-1,4-cyclohexanedicarboxylic acid disclosed in Chinese patent document "CN1310868C" cannot stably prepare trans-CHDA products with a purity ≥99%. Multiple recrystallization operations are required to compensate for insufficient purity, which leads to a significant loss in product yield and increases production costs. Third, the solvent recovery efficiency of N,N-dimethylformamide is low. In existing processes, the solvent recovery rate of N,N-dimethylformamide is generally below 85%. Even with a novel N,N-dimethylformamide recovery device and its usage method disclosed in Chinese patent document "CN114057340A", when applied to the CHDA purification system, the N,N-dimethylformamide recovery rate is still difficult to exceed 90%, resulting in large solvent losses and further increasing the total production cost.

[0004] In summary, existing trans-CHDA purification processes are inadequate and cannot meet market demand for high-purity trans-CHDA, nor can they meet the economic requirements of industrial-scale production. Therefore, it is necessary to propose an efficient isomerization-coupled crystallization method for trans-CHDA purification to solve these problems. Summary of the Invention

[0005] The purpose of this invention is to address the problems mentioned in the background section by providing an efficient method for purifying trans-CHDA through isomerization coupling crystallization. This method achieves full conversion of cis-CHDA, directional crystallization of trans-CHDA, and efficient recovery of N,N-dimethylformamide solvent, ultimately yielding a high-purity trans-CHDA product.

[0006] To achieve the above objectives, the present invention specifically adopts the following technical solution: A highly efficient method for purifying trans-CHDA via isomerization coupling crystallization includes the following steps: S1. Mix crude CHDA and N,N-dimethylformamide at a mass ratio of 1:1 to 1.5, add 1 to 5 g of isomerization catalyst, and carry out the isomerization reaction under reflux conditions. The reaction temperature for the isomerization reaction is 150~170℃; S2. The product after the isomerization reaction is subjected to first crystallization and second crystallization in sequence. The crystals obtained from the second crystallization are collected by vacuum filtration and washed with N,N-dimethylformamide at 0~10℃. S3. Purify the washed crystals to obtain high-purity trans-CHDA product; recover N,N-dimethylformamide from S2.

[0007] Preferably, the trans content of the crude CHDA product is ≥85%, and the reaction time of the isomerization reaction is 12h.

[0008] Preferably, the mass ratio of the crude CHDA to N,N-dimethylformamide is 1:1.1, and the isomerization catalyst is preferably sodium methoxide, with a preferred mass of 1g.

[0009] Preferably, the specific methods for the first and second crystallizations include: After cooling to 70~90℃ at a rate of 5℃ / h and holding at that temperature for 1~5h, cool down to 5~15℃ at a rate of 1℃ / h and hold at that temperature for 5~10h.

[0010] Preferably, the temperature for the first crystallization is 80°C and the time is 2 hours, and the temperature for the second crystallization is 8°C and the time is 8 hours.

[0011] Preferably, the temperature of N,N-dimethylformamide in S2 is 5°C, the mass of N,N-dimethylformamide is 0.5 times the mass of the crystal, and the crystal is washed twice with N,N-dimethylformamide.

[0012] Preferably, the specific method for purifying the washed crystals to obtain high-purity trans-CHDA products includes: The washed crystals and deionized water were mixed at a mass ratio of 1:3~10 and placed in a reaction vessel. The mixture was refluxed at 80~120℃ for 1~5 hours and then cooled to 20℃ for crystallization for 10~15 hours. The crystals were collected by centrifugation and washed with deionized water until the pH of the washing solution was 7. The washed crystals were dried in a vacuum drying oven at 80°C for 12 hours to obtain a high-purity trans-CHDA product. Among them, the purity of high-purity trans-CHDA products is ≥99% and the water content is <0.5%.

[0013] Preferably, the mass ratio of the washed crystals to deionized water is 1:5, the reflux hydrolysis temperature is preferably 100°C, the time is preferably 2 hours, and the crystallization time at 20°C is preferably 12 hours.

[0014] Preferably, the specific method for recovering N,N-dimethylformamide from S2 includes: Collect the mother liquor obtained from vacuum filtration in S2 and the washing liquid obtained from washing the crystals, and mix the mother liquor and washing liquid to obtain a mixture. The mixture was subjected to vacuum distillation to recover N,N-dimethylformamide; The vacuum degree of the reduced pressure distillation is -0.095 MPa and the distillation temperature is 150℃.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. By using sodium methoxide as an isomerization catalyst, the efficient conversion of cis-CHDA to trans-CHDA is achieved; simultaneously, gradient cooling crystallization effectively avoids co-crystallization of the cis isomers, resulting in a final product purity consistently reaching ≥99.0%. 2. By using gradient cooling and segmented heat preservation crystal growth process, the trans CHDA crystals grow fully and uniformly, effectively avoiding co-crystallization of cis CHDA, and the final product yield is ≥88.0%. Detailed Implementation

[0016] 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 skilled in the art without creative effort are within the scope of protection of the present invention. The materials and instruments used in the following embodiments are all commercially available.

[0017] A highly efficient method for purifying trans-CHDA via isomerization coupling crystallization includes steps S1 to S3.

[0018] S1. Mix crude CHDA and N,N-dimethylformamide at a mass ratio of 1:1 to 1.5, add 1 to 5 g of isomerization catalyst, and carry out the isomerization reaction under reflux conditions. The isomerization reaction is carried out at a temperature of 150-170℃ for 12 hours, with the crude CHDA containing ≥85% trans-formamide. The preferred mass ratio of crude CHDA to N,N-dimethylformamide is 1:1.1, and the preferred isomerization catalyst is sodium methoxide, with a preferred mass of 1 g. Furthermore, the isomerization reaction in this step is carried out with stirring in a reactor equipped with a reflux device.

[0019] This invention preferentially uses sodium methoxide as an isomerization catalyst to achieve efficient conversion of cis-CHDA to trans-CHDA. Specifically, sodium methoxide dissociates into a methoxy anion in a homogeneous N,N-dimethylformamide system. Sodium methoxide, acting as a strong base, selectively removes the active proton from the carbon atom bonded to the carboxyl group on the cyclohexane ring of CHDA, forming a carbanion-active intermediate. This breaks the rigid structure of the cyclohexane ring, giving it flexible rotational capability. Because the two carboxyl groups in trans-CHDA are on opposite sides of the cyclohexane ring, the steric hindrance is minimal, resulting in the most thermodynamically stable configuration. The flexible cyclohexane ring spontaneously undergoes conformational rearrangement, and the carbanion intermediate recombines protons to generate trans-CHDA. Furthermore, sodium methoxide forms a catalytic cycle in the reaction, is not consumed, and continuously promotes the isomerization of cis-CHDA until the system reaches thermodynamic equilibrium, achieving a complete conversion from cis to trans.

[0020] S2. The product after the isomerization reaction is subjected to a first crystallization and a second crystallization in sequence. The crystals obtained from the second crystallization are collected by vacuum filtration and washed with N,N-dimethylformamide at 0~10℃.

[0021] The specific methods for the first and second crystallization include: After cooling to 70~90℃ at a rate of 5℃ / h and holding at that temperature for 1~5h, cool down to 5~15℃ at a rate of 1℃ / h and hold at that temperature for 5~10h. The preferred crystallization temperature for the first crystallization is 80℃ and the preferred crystallization time is 2 hours. The preferred crystallization temperature for the second crystallization is 8℃ and the preferred crystallization time is 8 hours. The preferred crystallization temperature for N,N-dimethylformamide is 5℃. The mass of N,N-dimethylformamide is 0.5 times the mass of the crystal. The crystal is washed twice with N,N-dimethylformamide.

[0022] This invention employs a gradient cooling crystallization method that combines primary and secondary crystallization, enabling precise control of the crystallization process and resulting in an overall process yield of ≥88% and batch-to-batch yield fluctuations of <2%. Specifically, the first crystallization is carried out by slowly cooling the temperature to 70~90℃ at a rate of 5℃ / h. The crystals are then kept at this temperature for 1~5 hours to avoid the sudden increase in supercooling of the system and the generation of impurities. The supersaturation of trans-CHDA is precisely controlled to a suitable range for crystal nucleus formation. At the same time, kinetic energy is provided for the orderly arrangement of trans-CHDA molecules, promoting their directional aggregation to form stable pure trans-CHDA crystal nuclei, thus providing conditions for the formation of trans-CHDA crystal nuclei. The second crystallization involves slowly cooling the system to 5-15°C at a rate of 1°C / h, and then maintaining this temperature for 5-10 hours. This cooling rate can uniformly increase the supersaturation of the system, providing sufficient time for the directional growth of trans-CHDA crystals and enabling them to grow fully. At the same time, the low temperature environment of 5-15°C significantly reduces the solubility of trans-CHDA to promote its continuous precipitation and growth, while also making it difficult for the cis isomer to reach supersaturation, effectively preventing its co-crystallization.

[0023] This invention thoroughly removes adsorbed cis-CHDA and residual impurities from the crystal surface by washing the crystal with N,N-dimethylformamide at low temperature.

[0024] S3. Purify the washed crystals to obtain high-purity trans-CHDA product; recover N,N-dimethylformamide from S2.

[0025] Specifically, the washed crystals and deionized water are mixed at a mass ratio of 1:3~10 and placed in a reaction vessel. The mixture is refluxed at 80~120℃ for 1~5 hours and then cooled to 20℃ for 10~15 hours to crystallize. The crystals are collected by centrifugation and washed with deionized water until the pH of the washing solution is 7. The washed crystals were dried in a vacuum drying oven at 80°C for 12 hours to obtain a high-purity trans-CHDA product. Among them, the purity of high-purity trans-CHDA products is ≥99% and the water content is <0.5%.

[0026] Furthermore, the preferred mass ratio of washed crystals to deionized water is 1:5, the preferred temperature for reflux hydrolysis is 100°C, the preferred time is 2 hours, and the preferred time for crystallization under cooling to 20°C is 12 hours.

[0027] This invention involves mixing washed crystals and deionized water at a mass ratio of 1:3~10 and placing the mixture in a reaction vessel. The mixture is then refluxed at 80~120℃ for 1~5 hours to fully dissolve residual N,N-dimethylformamide in the deionized water. Simultaneously, this temperature range avoids significant dissolution and loss of the target product, namely high-purity trans-CHDA. The mixture is then cooled to 20℃ and crystallized for 10~15 hours to allow the trace amounts of dissolved trans-CHDA in the system to reach supersaturation, spontaneously precipitating into crystals and achieving the recovery of the target product. This process also avoids reduced product yield due to insufficient crystallization time.

[0028] Specific methods for recovering N,N-dimethylformamide from S2 include: Collect the mother liquor obtained from vacuum filtration in S2 and the washing liquid obtained from washing the crystals, and mix the mother liquor and washing liquid to obtain a mixture. The mixture was subjected to vacuum distillation to recover N,N-dimethylformamide; The vacuum degree of the reduced pressure distillation is -0.095 MPa and the distillation temperature is 150℃.

[0029] The N,N-dimethylformamide recovered by this invention has a recovery rate of ≥92%. Furthermore, the recovered N,N-dimethylformamide, after dehydration using molecular sieves, can be recycled for use in S1. This invention generates only a small amount of wastewater containing N,N-dimethylformamide during the purification process, and the high recovery rate of N,N-dimethylformamide reduces solvent emissions.

[0030] The present invention will be further illustrated by the following examples, but these examples do not limit the scope of the invention.

[0031] Example 1 1. Isomerization reaction: Weigh 100g of crude CHDA (trans content of crude CHDA ≥ 85%), add 110g of N,N-dimethylformamide, mix well and place in a reactor equipped with a reflux device, add 1g of sodium methoxide, stir at 600rpm for 12h, and the temperature inside the reactor is 160℃. 2. Gradient cooling crystallization: After the isomerization reaction is completed, the temperature is lowered to 80℃ at a rate of 5℃ / h and kept at this temperature for 2 hours. Then, the temperature is lowered to 8℃ at a rate of 1℃ / h and kept at this temperature for 8 hours. The crystals are collected by vacuum filtration and washed twice with N,N-dimethylformamide at 5℃. The amount of N,N-dimethylformamide used each time is 0.5 times the mass of the crystals. Specifically, 45g of N,N-dimethylformamide is used in the first wash and 43g of N,N-dimethylformamide is used in the second wash. 3. Hydrolysis purification: Mix the washed crystals with 425g of deionized water, with a mass ratio of crystals to deionized water of 1:5; heat to 100℃ and reflux for 2h; cool the hydrolysate to 20℃ and crystallize for 12h; centrifuge to separate and collect the crystals, then wash with deionized water until the pH of the deionized water is 7; place the crystals in a vacuum drying oven at 80℃ and dry for 12h to obtain 82.5g of trans-CHDA product; 4. Recovery of N,N-dimethylformamide: The mother liquor obtained from vacuum filtration and the washing liquid obtained from washing the crystals were collected and mixed to obtain 158g of a mixture; the mixture was subjected to vacuum distillation to recover 98.5g of N,N-dimethylformamide; the vacuum degree of vacuum distillation was -0.095MPa and the distillation temperature was 150℃; the recovery rate of N,N-dimethylformamide was 92.8%; Test results: The purity of the trans-CHDA product was 99.3%, the water content was 0.35%, and the total yield was 88.6%.

[0032] Example 2 The main difference between this embodiment and Embodiment 1 is that: Add 100g of N,N-dimethylformamide and 5g of sodium methoxide. The temperature inside the reactor is 150℃. After the isomerization reaction was completed, the temperature was first lowered to 70℃ and kept at that temperature for 1 hour to grow crystals. Then the temperature was lowered to 5℃ and kept at that temperature for 5 hours to grow crystals. The crystals were collected by vacuum filtration and washed twice with N,N-dimethylformamide at 0℃. The washed crystals and deionized water were mixed at a mass ratio of 1:3; the mixture was heated to 80°C and refluxed for 1 hour; the hydrolysate was then cooled to 20°C and crystallized for 10 hours. Test results: The purity of the trans-CHDA product was 99.0%, the water content was 0.42%, and the total yield was 86.2%.

[0033] Example 3 The main difference between this embodiment and Embodiment 1 is that: Add 150g of N,N-dimethylformamide, and the temperature inside the reactor is 170℃; After the isomerization reaction was completed, the temperature was first lowered to 90℃ and kept at that temperature for 5 hours to grow crystals. Then the temperature was lowered to 15℃ and kept at that temperature for 10 hours to grow crystals. The crystals were collected by vacuum filtration and washed twice with N,N-dimethylformamide at 10℃. The washed crystals and deionized water were mixed at a mass ratio of 1:10; the mixture was heated to 120°C and refluxed for 5 hours; the hydrolysate was then cooled to 20°C and crystallized for 15 hours. Test results: The purity of the trans-CHDA product was 99.2%, the water content was 0.38%, and the total yield was 85.5%.

[0034] Comparative Example Verification of the effect of different cooling rates on product purity Compared with the process parameters of Example 1, only the cooling rate of the gradient cooling crystallization step was changed, while the remaining steps and process parameters remained the same as in Example 1; specifically, the cooling rate of the first stage was changed to 8℃ / h, and the cooling rate of the second stage was changed to 3℃ / h. Testing revealed that the purity of the final trans-CHDA product was 97.8%, and the yield was 82.3%, with a batch-to-batch yield fluctuation of 6.1%.

[0035] The following is a comparative analysis of Examples 1, 2, and 3 with Comparative Example 1: The detection results of Examples 1, 2, and 3 and Comparative Example 1 are shown in Table 1. Table 1 Test Results The trans-CHDA products obtained in Examples 1 to 3 all had a purity of 99.0% or higher, a water content of less than 0.45%, and a total yield of 88.0% or higher, indicating that the process parameters defined in this invention can stably produce high-purity trans-CHDA products. Among them, Example 1 is the optimal parameter combination, with the highest product purity and total yield, and the best overall effect. Compared with Examples 1 to 3, the comparative example had an excessively fast cooling rate in the first stage, which caused co-crystallization of cis-CHDA and trans-CHDA during the crystallization process. This not only significantly reduced the trans purity of the product, but also caused a decrease in yield and a deterioration in yield stability between batches. This fully verifies the criticality and rationality of the selection of cooling rate parameters in the gradient cooling crystallization process of the present invention.

Claims

1. A highly efficient method for purifying trans-CHDA via isomerization coupling crystallization, characterized in that, Includes the following steps: S1. Mix crude CHDA and N,N-dimethylformamide at a mass ratio of 1:1 to 1.5, add 1 to 5 g of isomerization catalyst, and carry out the isomerization reaction under reflux conditions. The reaction temperature for the isomerization reaction is 150~170℃; S2. The product after the isomerization reaction is subjected to first crystallization and second crystallization in sequence. The crystals obtained from the second crystallization are collected by vacuum filtration and washed with N,N-dimethylformamide at 0~10℃. S3. Purify the washed crystals to obtain high-purity trans-CHDA product; recover N,N-dimethylformamide from S2.

2. The method for purifying trans-CHDA by efficient isomerization coupling crystallization according to claim 1, characterized in that, The crude CHDA product has a trans content of ≥85%, and the isomerization reaction takes 12 hours.

3. The method for purifying trans-CHDA by efficient isomerization coupling crystallization according to claim 1, characterized in that, The preferred mass ratio of the crude CHDA to N,N-dimethylformamide is 1:1.1, and the preferred isomerization catalyst is sodium methoxide, with a preferred mass of 1g.

4. The method for purifying trans-CHDA by efficient isomerization coupling crystallization according to claim 1, characterized in that, The specific methods for the first and second crystallizations include: After cooling to 70~90℃ at a rate of 5℃ / h and holding at that temperature for 1~5h, cool down to 5~15℃ at a rate of 1℃ / h and hold at that temperature for 5~10h.

5. The method for purifying trans-CHDA by efficient isomerization coupling crystallization according to claim 4, characterized in that, The preferred temperature for the first crystallization is 80℃ and the preferred time is 2 hours, while the preferred temperature for the second crystallization is 8℃ and the preferred time is 8 hours.

6. The method for purifying trans-CHDA by efficient isomerization coupling crystallization according to claim 1, characterized in that, The preferred temperature for N,N-dimethylformamide in S2 is 5°C, the mass of N,N-dimethylformamide is 0.5 times the mass of the crystal, and the crystal is washed twice with N,N-dimethylformamide.

7. The method for purifying trans-CHDA by efficient isomerization coupling crystallization according to claim 1, characterized in that, The specific methods for purifying the washed crystals to obtain high-purity trans-CHDA products include: The washed crystals and deionized water were mixed at a mass ratio of 1:3~10 and placed in a reaction vessel. The mixture was refluxed at 80~120℃ for 1~5 hours and then cooled to 20℃ for crystallization for 10~15 hours. The crystals were collected by centrifugation and washed with deionized water until the pH of the washing solution was 7. The washed crystals were dried in a vacuum drying oven at 80°C for 12 hours to obtain a high-purity trans-CHDA product. Among them, the purity of high-purity trans-CHDA products is ≥99% and the water content is <0.5%.

8. The method for purifying trans-CHDA by efficient isomerization coupling crystallization according to claim 7, characterized in that, The preferred mass ratio of the washed crystals to deionized water is 1:

5. The preferred temperature for reflux hydrolysis is 100°C, and the preferred time is 2 hours. The preferred time for crystallization under cooling to 20°C is 12 hours.

9. The method for purifying trans-CHDA by efficient isomerization coupling crystallization according to claim 1, characterized in that, Specific methods for recovering N,N-dimethylformamide from S2 include: Collect the mother liquor obtained from vacuum filtration in S2 and the washing liquid obtained from washing the crystals, and mix the mother liquor and washing liquid to obtain a mixture. The mixture was subjected to vacuum distillation to recover N,N-dimethylformamide; The vacuum degree of the reduced pressure distillation is -0.095 MPa and the distillation temperature is 150℃.

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