A method for preparing high-purity 4-chlorophthalic acid monosodium salt

Abstract

The application discloses a preparation method of 4-chlorophthalic acid monosodium salt, which comprises the following steps: step 1: mixing phthalic anhydride and a Lewis acid catalyst in an aqueous solution, then adding dropwise an alkaline sodium hypochlorite aqueous solution to perform a chlorination reaction, adjusting the pH value of the reaction solution after the reaction is completed, and then separating a crude product of the 4-chlorophthalic acid monosodium salt; and step 2: fully dissolving the crude product in an alkaline aqueous solution, removing part of impurities and colored substances through diatomite adsorption filtration, and precipitating the high-purity product 4-chlorophthalic acid monosodium salt by adjusting the filtrate to an acidic pH value. The Lewis acid is added as a catalyst for the chlorination reaction, the selectivity of the target product and the conversion rate of raw materials are improved, and the content of dichloro by-products is reduced; through the refining method of alkaline dissolution and acid precipitation combined with diatomite adsorption filtration, the 4-chlorophthalic acid monosodium salt product with low colority (<80) and high purity (>88%) is obtained.

Description

Technical Field

[0001] This invention relates to the field of organic synthesis and fine chemical intermediate preparation technology, and particularly to a method for preparing high-purity 4-chlorophthalic acid monosodium salt. Background Technology

[0002] Polyimide (PI) is one of the most high-performance specialty engineering plastics, possessing outstanding high-temperature resistance, radiation resistance, chemical corrosion resistance, and excellent mechanical and dielectric properties. It is widely used in key fields such as flexible displays, aerospace, new energy, microelectronics, and membrane separation. With the rapid development of next-generation information technology and high-end manufacturing, the market demand for high-performance polyimides continues to grow. Among these, 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) is the core monomer for preparing high-performance polyimides; its synthesis quality and cost directly determine the performance and industrial competitiveness of the final material. Monosodium 4-chlorophthalate is a key intermediate in the preparation of BPDA, and optimizing its preparation process is of great significance to promoting the development of the polyimide industry.

[0003] Currently, the mainstream industrial routes for preparing monosodium chlorophthalate are mainly divided into two categories: phthalic anhydride chlorination and phthalic anhydride sodium hypochlorite chlorination. One is the phthalic anhydride chlorination method. Chinese patent CN1824638A discloses an industrial production process for monosodium 4-chlorophthalate: using phthalic anhydride as raw material, chlorine gas is introduced for chlorination, followed by salting out, acid conditioning, filtration, and drying to obtain the product. The drawbacks of this process are difficulty in control, poor reaction selectivity, and a crude product purity of only 67%–76%. Furthermore, the use of chlorine gas imposes stringent requirements on process safety and tail gas treatment, limiting its industrial application.

[0004] The second method is the phthalic anhydride-sodium hypochlorite chlorination method. Chinese patent CN119118788A discloses a method for preparing monosodium 4-chlorophthalic acid: using phthalic anhydride as the starting material, a mixture of sodium hypochlorite aqueous solution and sodium hydroxide solution is added dropwise to the system. Through a chlorination reaction and pH adjustment, crude monosodium 4-chlorophthalic acid is generated. The crude product is then purified using pure water, but this patent does not disclose the specific quality of the crude and purified products. Additionally, Chinese patent CN110563678A discloses a method for preparing 4-chlorophthalic acid: reacting phthalic anhydride with alcohols to generate a monoalkyl ester, followed by a chlorination reaction, and then requiring hydrolysis protection to obtain the target product. This process is cumbersome and lengthy, increasing not only the reaction steps and material losses but also the costs of solvent recovery and waste treatment.

[0005] In summary, existing processes for preparing monosodium 4-chlorophthalate generally suffer from low product purity, insufficient yield, high cost, or poor environmental performance, making it difficult to balance the economic viability of industrial production with the quality requirements of downstream high-end applications. Therefore, developing a safe, cost-controllable, high-purity, and easily scalable method for preparing monosodium 4-chlorophthalate has significant industrial application value and market potential. Summary of the Invention

[0006] The present invention aims to optimize and improve the existing production process of 4-chlorophthalic acid monosodium salt, and to develop a method for preparing and purifying high-purity 4-chlorophthalic acid monosodium salt that is both economical and environmentally friendly.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution: A method for preparing high-purity monosodium 4-chlorophthalic acid salt, comprising the following steps: (a) Preparation of sodium hypochlorite reaction solution: Add pure water, sodium hypochlorite solution and sodium hydroxide to the reaction vessel, stir and mix to completely dissolve the sodium hydroxide and make the system uniformly mixed to obtain alkaline sodium hypochlorite reaction solution; (b) Chlorination reaction: Add refined mother liquor or pure water, phthalic anhydride and catalyst to the crude product reaction vessel; then add the alkaline sodium hypochlorite reaction solution obtained in step (a) dropwise; after the dropwise addition is completed, continue to keep the reaction at the temperature for 5-7 h to obtain a reaction solution containing monosodium 4-chlorophthalic acid salt; (c) Acidification precipitation and solid-liquid separation: After the heat preservation reaction in step (b) is completed, hydrochloric acid is added dropwise to the reaction solution to adjust the pH of the solution to 4.5-4.8, so that the product 4-chlorophthalic acid monosodium salt precipitates out; centrifugation is used to separate the crude product of 4-chlorophthalic acid monosodium salt. (d) Alkali melting treatment: Add pure water, crude 4-chlorophthalic acid monosodium salt obtained in step (c), and liquid alkali to the alkali melting kettle, stir for 4-5 h to fully dissolve and clear the material; then filter the obtained material through diatomaceous earth filter and press to the refining kettle to remove insoluble impurities and adsorbed colored substances. (e) Refining process: Hydrochloric acid was added dropwise to the refining vessel. After the pH value of the system reached 4.5-4.8, an antioxidant was added and stirring was continued for about 0.5 h. After stirring, the temperature was raised to 75 ℃ and kept at that temperature for 2 h. After the temperature was kept at that temperature, the temperature was lowered for 3-4 h. After the temperature was lowered to 30 ℃, centrifugation was started. After centrifugation, high-purity monosodium 4-chlorophthalic acid salt was obtained.

[0008] Preferably, in step (a), the molar ratio of sodium hypochlorite to phthalic anhydride is 1.01~1.15 : 1.0, and the molar ratio of sodium hydroxide to phthalic anhydride is 0.75~0.80 : 1.0.

[0009] Preferably, in step (b), the catalyst is selected from one or more of Lewis zinc salts, aluminum salts, and their hydrates.

[0010] Preferably, the catalyst is zinc chloride or aluminum chloride; the amount of the Lewis acid catalyst added is 0.1-10 wt% based on the mass of phthalic anhydride.

[0011] Preferably, the amount of the Lewis acid catalyst added is 0.5-2.0 wt% based on the mass of phthalic anhydride.

[0012] Preferably, in step (b), the system reaction temperature is 22-25°C. o C, the sodium hypochlorite solution is added dropwise over a period of 30-35 hours.

[0013] Preferably, in step (d), the concentration of the liquid alkali is 30%, and the amount used is 20-35 wt% of the crude product.

[0014] The method for preparing 4-chlorophthalic acid monosodium salt according to claim 1 is characterized in that, in step (e), the pH value is controlled at 4.6.

[0015] This invention has the following beneficial effects: By introducing a Lewis acid catalyst into the chlorination reaction stage, the chlorination reaction activity is enhanced through the complexation of Lewis acids zinc chloride and aluminum chloride with active hypochlorous acid, allowing the reaction to proceed at a lower temperature (22-25°C). o C) Improve the conversion rate of the chlorination reaction of phthalic anhydride. Higher reaction efficiency can be achieved under conditions of lower sodium hypochlorite concentration or lower available chlorine dosage, thereby reducing the probability of disubstituted side reactions, improving the conversion rate of raw materials and the selectivity for the formation of 4-chlorophthalic acid monosodium salt, and thus improving the quality of crude product.

[0016] Secondly, this invention employs a purification method combining alkali dissolution and acid precipitation with diatomaceous earth adsorption for decolorization and pressure filtration for impurity removal, to refine the crude product obtained from the chlorination reaction. Compared to the purification method in patent CN114478234, which extensively uses alcohols and acidic organic solvents for recrystallization or washing, this invention primarily uses water as the treatment medium, consumes less organic solvent, has milder process conditions, higher production safety, and lower wastewater treatment pressure, offering significant advantages in environmental friendliness. Simultaneously, the alkali dissolution process facilitates the complete dissolution of the target product, while the acid precipitation process promotes the redeposition of the target product and its separation from soluble inorganic salts and some byproduct impurities. Combined with diatomaceous earth filtration to adsorb pigment impurities, this significantly improves the product's color. Therefore, the preparation method provided by this invention combines high conversion rate, high selectivity, no organic solvent use, light product color, and environmental friendliness, making it suitable for the large-scale preparation of monosodium 4-chlorophthalate.

[0017] The reaction equation of this invention is as follows: . Phthalic anhydride undergoes a benzene ring chlorination reaction in the presence of sodium hypochlorite and a Lewis acid catalyst to produce monosodium 4-chlorophthalic acid. In the following examples, PA represents phthalic acid or a salt thereof, SCP represents monosodium 4-chlorophthalic acid, 3-CPA represents monosodium 4-chlorophthalic acid impurity, and 4,5-DCPA and 3,4-DCPA represent dichlorophthalic acid or its salt byproducts. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0019] A method for preparing and purifying a high-purity monosodium 4-chlorophthalic acid salt, comprising the following steps: (a) Add pure water, sodium hypochlorite solution and sodium hydroxide to the reaction vessel, stir and mix to completely dissolve the sodium hydroxide and make the system uniformly mixed to obtain an alkaline sodium hypochlorite reaction solution; (b) Add the refined mother liquor or pure water, phthalic anhydride and catalyst to the crude product reaction vessel. Then add the alkaline sodium hypochlorite reaction solution obtained in step (a) dropwise; after the addition is complete, continue to keep the reaction at the temperature for 5-7 h to obtain a reaction solution containing monosodium 4-chlorophthalic acid salt; (c) After the heat preservation reaction in step (b) is completed, hydrochloric acid is added dropwise to the reaction solution to adjust the pH of the solution to about 4.5-4.8, so that the product 4-chlorophthalic acid monosodium salt precipitates out, and the crude product 4-chlorophthalic acid monosodium salt is obtained by centrifugation.

[0020] (d) Add pure water, crude 4-chlorophthalic acid monosodium salt obtained in step (c), and liquid alkali to the alkali melting kettle, and stir for 4-5 h to fully dissolve and clear the material; then filter the obtained material through diatomaceous earth adsorption and pressure to the refining kettle to remove insoluble impurities and some colored substances. (e) Add hydrochloric acid dropwise to the refining vessel. Once the pH of the system reaches 4.6, add an antioxidant and continue stirring for about 0.5 h. After stirring, raise the temperature to 75 °C and hold for 2 h. After holding, begin cooling for 3-4 h. Once the temperature reaches 30 °C, begin centrifugation. After centrifugation and drying, obtain high-purity monosodium 4-chlorophthalic acid.

[0021] Example 1 (1) Preparation of sodium hypochlorite reaction solution: Add 2800 L of sodium hypochlorite solution and 150 kg of caustic soda flakes to the reaction vessel, stir and react for 4-5 h to completely dissolve the caustic soda flakes and mix the system evenly to obtain alkaline sodium hypochlorite reaction solution. Then, transfer the alkaline sodium hypochlorite reaction solution to a high-level tank for later use. (2) Chlorination reaction: Add 2100 L of pure water and 700 kg of phthalic anhydride to the crude product reactor, and control the system temperature to 22-25 °C; then add 7.5 kg of Lewis acid catalyst (ZnCl2), stir for 2 h, so that the Lewis acid catalyst is fully dispersed in the reaction system. Then add the alkaline sodium hypochlorite reaction solution obtained in step (1) dropwise at about 90 L / h, while maintaining the temperature at 22-25 °C, for 30-35 h; after the dropwise addition is completed, continue to keep the reaction at the temperature for 5-7 h to obtain a reaction solution containing monosodium 4-chlorophthalic acid. (3) Acidification precipitation and solid-liquid separation: After the heat preservation reaction in step (2) is completed, hydrochloric acid is added dropwise to the reaction solution to adjust the pH to about 4.6, so that the monosodium 4-chlorophthalic acid salt can be fully precipitated.

[0022] (4) Centrifugation of crude product: The centrifugation time was 4-5 h. After centrifugation, 1050 kg of crude wet product of monosodium 4-chlorophthalic acid was obtained. The crude product was analyzed by high performance liquid chromatography (HPLC). The PA content was 11.8%, the SCP content was 77.5%, the 4,5-DCPA content was 7.4%, the 3,4-DCPA content was 1.5%, the content of other impurities was 1.8%, and the color was 235.

[0023] (5) Alkali melting treatment: Add 3000 L of pure water, 1050 kg of crude 4-chlorophthalic acid monosodium salt obtained in step (4) and 300 L of 30% liquid alkali to the alkali melting kettle, stir for 4-5 h to fully dissolve and clear the material; then filter the obtained material through diatomaceous earth adsorption and pressure to the refining kettle for 1.5-2.5 h to remove insoluble impurities and some colored substances; (6) Refining: After pressure filtration, add approximately 500 L of hydrochloric acid dropwise to the refining vessel at a rate of approximately 100 L / h for 5-6 h. Monitor the addition process during the dropwise addition. Once the pH of the system reaches 4.6, add 4 kg of sodium sulfite, an antioxidant. Continue stirring for approximately 0.5 h after the addition is complete. After stirring, begin heating for 6-8 h, reaching 75 ℃ and holding for 4 h. After holding, begin cooling for 6-8 h, reaching 30 ℃, and then wait for centrifugation.

[0024] (7) Crystallization, centrifugation, and drying: After confirming the batch number of the product to be centrifuged and the centrifuge kettle number, begin centrifugation. After centrifugation, 100 oDrying at C to constant weight yielded 585 kg of a white, high-quality monosodium 4-chlorophthalate. High-performance liquid chromatography (HPLC) analysis revealed the following: PA content 2.3%, SCP content 88.2%, 4,5-DCPA content 8.8%, 3,4-DCPA content 0.23%, other impurities 0.47%, and color 72.

[0025] In this embodiment, zinc chloride is introduced as a Lewis acid catalyst during the chlorination reaction stage, which can improve the chlorination conversion rate of phthalic anhydride and its hydrolysis products, as well as the selectivity of the target product. Simultaneously, by combining alkali dissolution and acid precipitation, diatomaceous earth adsorption decolorization, and pressure filtration for impurity removal, a monosodium 4-chlorophthalic acid product with lighter color and stable purity can be obtained. This method mainly uses water, liquid alkali, and hydrochloric acid as treatment media, with low organic solvent usage, exhibiting significant advantages in environmental friendliness.

[0026] Example 2 1) Preparation of sodium hypochlorite reaction solution: Add 2800 L of sodium hypochlorite solution and 150 kg of caustic soda flakes to the reaction vessel, stir and react for 4-5 h to completely dissolve the caustic soda flakes and mix the system evenly to obtain alkaline sodium hypochlorite reaction solution. Then, transfer the alkaline sodium hypochlorite reaction solution to a high-level tank for later use. (2) Chlorination reaction: Add 2100 L of pure water and 700 kg of phthalic anhydride to the crude product reactor, and control the system temperature to 22-25 °C; then add 7.5 kg of Lewis acid catalyst (AlCl3), stir for 2 h to fully disperse the Lewis acid catalyst in the reaction system. Then add the alkaline sodium hypochlorite reaction solution obtained in step (1) dropwise at a rate of about 90 L / h while maintaining the temperature at 22-25 °C for 30-35 h; after the dropwise addition is completed, continue to keep the temperature for 5-7 h to obtain a reaction solution containing monosodium 4-chlorophthalic acid. (3) Acidification precipitation and solid-liquid separation: After the heat preservation reaction in step (2) is completed, hydrochloric acid is added dropwise to the reaction solution to adjust the pH to about 4.6, so that the monosodium 4-chlorophthalic acid salt can be fully precipitated.

[0027] (4) Centrifugation of crude product: The centrifugation time was 4-5 h. After centrifugation, 1030 kg of crude wet product of monosodium 4-chlorophthalic acid was obtained. The crude product was analyzed by high performance liquid chromatography (HPLC). The PA content was 10.2%, the SCP content was 77.6%, the 4,5-DCPA content was 8.6%, the 3,4-DCPA content was 1.7%, the content of other impurities was 1.9%, and the color was 266.

[0028] (5) Alkali melting treatment: Add 3000 L of pure water, 1068 kg of crude 4-chlorophthalic acid monosodium salt obtained in step (4) and 300 L of 30% liquid alkali to the alkali melting kettle, stir for 4-5 h to fully dissolve and clear the material; then filter the obtained material through diatomaceous earth adsorption and pressure to the refining kettle for 1.5-2.5 h to remove insoluble impurities and colored impurities; (6) Refining: After pressure filtration, add approximately 500 L of hydrochloric acid dropwise to the refining vessel at a rate of approximately 100 L / h for 5-6 h. Monitor the addition process during the dropwise addition. Once the pH of the system reaches 4.6, add 4 kg of sodium sulfite, an antioxidant. Continue stirring for approximately 0.5 h after the addition is complete. After stirring, begin heating for 6-8 h, reaching 75 ℃ and holding for 4 h. After holding, begin cooling for 6-8 h, reaching 30 ℃, and then wait for centrifugation.

[0029] (7) Crystallization, centrifugation, and drying: After confirming the batch number of the product to be centrifuged and the centrifuge kettle number, begin centrifugation. After centrifugation, 100 o Drying at C to constant weight yielded 576 kg of a white, high-quality monosodium 4-chlorophthalate. High-performance liquid chromatography (HPLC) analysis revealed a PA content of 2.2%, SCP content of 88.1%, 4,5-DCPA content of 8.9%, 3,4-DCPA content of 0.25%, other impurities of 0.55%, and a color of 76.

[0030] Example 3 (1) Preparation of sodium hypochlorite reaction solution: Add 2800 L of sodium hypochlorite solution and 150 kg of caustic soda flakes to the reaction vessel, stir and react for 4-5 h to completely dissolve the caustic soda flakes and mix the system evenly to obtain alkaline sodium hypochlorite reaction solution. Then, transfer the alkaline sodium hypochlorite reaction solution to a high-level tank for later use. (2) Chlorination reaction: Add 2100 L of pure water and 700 kg of phthalic anhydride to the crude product reactor, and control the system temperature to 22-25 °C; then add 7.5 kg of Lewis acid catalyst (AlCl3), stir for 2 h to fully disperse the Lewis acid catalyst in the reaction system. Then add the alkaline sodium hypochlorite reaction solution obtained in step (1) dropwise at a rate of about 90 L / h while maintaining the temperature at 22-25 °C for 30-35 h; after the dropwise addition is completed, continue to keep the temperature for 5-7 h to obtain a reaction solution containing monosodium 4-chlorophthalic acid. (3) Acidification precipitation and solid-liquid separation: After the heat preservation reaction in step (2) is completed, hydrochloric acid is added dropwise to the reaction solution to adjust the pH to about 4.8, so that the monosodium 4-chlorophthalic acid salt can be fully precipitated.

[0031] (4) Centrifugation of crude product: The centrifugation time was 4-5 h. After centrifugation, 1010 kg of crude wet product of monosodium 4-chlorophthalate was obtained. The crude product was analyzed by high performance liquid chromatography (HPLC). The PA content was 10.8%, the SCP content was 76.9%, the 4,5-DCPA content was 9.1%, the 3,4-DCPA content was 1.5%, the content of other impurities was 1.7%, and the color was 286.

[0032] (5) Alkali melting treatment: Add 3000 L of pure water, 1050 kg of crude 4-chlorophthalic acid monosodium salt obtained in step (4) and 300 L of 30% liquid alkali to the alkali melting kettle, stir for 4-5 h to fully dissolve and clear the material; then filter the obtained material through diatomaceous earth adsorption and pressure to the refining kettle for 1.5-2.5 h to remove insoluble impurities and colored impurities; (6) Refining: After pressure filtration, add approximately 500 L of hydrochloric acid dropwise to the refining vessel at a rate of approximately 100 L / h for 5-6 h. Monitor the dropwise addition process. Once the pH reaches 4.8, add 5 kg of sodium thiosulfate, an antioxidant. Continue stirring for approximately 0.5 h after the addition is complete. After stirring, begin heating for 6-8 h, reaching 75 °C and holding for 4 h. After holding, begin cooling for 6-8 h, reaching 30 °C, and then wait for centrifugation.

[0033] (7) Crystallization, centrifugation, and drying: After confirming the batch number of the product to be centrifuged and the centrifuge kettle number, begin centrifugation. After centrifugation, 100 o Drying at C to constant weight yielded 585 kg of a white, purified monosodium 4-chlorophthalate, with a purity of 56%. High-performance liquid chromatography (HPLC) analysis revealed a PA content of 1.9%, an SCP content of 88.6%, a 4,5-DCPA content of 8.8%, a 3,4-DCPA content of 0.22%, other impurities of 0.48%, and a color of 68.

[0034] Comparative Example 1 Comparative Example 1 provides a method for preparing 4-chlorophthalic acid monosodium salt without the addition of Lewis acid catalyst, and purification using only pure water. The difference between it and Example 1 is that, since no catalyst is added, the reaction temperature needs to be increased to 35-38℃ to promote the chlorination reaction. In step (2), zinc chloride is not added during the chlorination reaction, and the crude product is not purified by alkaline dissolution and acid precipitation. The remaining feed amount, sodium hypochlorite drop addition conditions, acidification conditions and post-treatment conditions are basically the same as those in Example 1.

[0035] (1) Preparation of sodium hypochlorite reaction solution: Add 2800 L of sodium hypochlorite solution and 150 kg of caustic soda flakes to the reaction vessel, stir and react for 4-5 h to completely dissolve the caustic soda flakes and mix the system evenly to obtain alkaline sodium hypochlorite reaction solution. Then, transfer the alkaline sodium hypochlorite reaction solution to a high-level tank for later use. (2) Chlorination reaction: Add 2100 L of pure water and 700 kg of phthalic anhydride to the crude product reaction vessel, and control the system temperature to 35-38 ℃. Then add the alkaline sodium hypochlorite reaction solution obtained in step (1) dropwise at a rate of about 90 L / h while maintaining the temperature at 35-38 ℃, for a dropwise time of 30-35 h; after the dropwise addition is completed, continue to keep the temperature for 5-7 h to obtain a reaction solution containing monosodium 4-chlorophthalic acid. (3) Acidification precipitation and solid-liquid separation: After the heat preservation reaction in step (2) is completed, hydrochloric acid is added dropwise to the reaction solution to adjust the pH to about 4.6, so that the monosodium 4-chlorophthalic acid salt can be fully precipitated.

[0036] (4) Centrifugation of crude product: The centrifugation time was 4-5 h. After centrifugation, 1050 kg of crude wet product of monosodium 4-chlorophthalate was obtained. The crude product was analyzed by HPLC and its PA content was 15.3%, the SCP content of the product was 71.7%, the 4,5-DCPA content was 10.3%, the 3,4-DCPA content was 1.5%, the content of other impurities was 1.2%, and the color was 565.

[0037] (5) Refining process: Add 3200 L of pure water, 1050 kg of crude 4-chlorophthalic acid monosodium salt obtained in step (4), and 4 kg of sodium sulfite antioxidant to the refining kettle. After the addition is complete, continue stirring for about 0.5 h. After stirring, start heating for 6-8 h, and keep the temperature at 75 ℃ for 4 h. After the temperature is kept at 30 ℃, start cooling for 6-8 h, and wait for centrifugation.

[0038] (6) Centrifugation and drying: After confirming the batch number and centrifuge vessel number of the product to be centrifuged, centrifugation was started. After centrifugation, 650 kg of high-purity 4-chlorophthalic acid monosodium salt product was obtained. The dry matter content of the product was 90.3%, and the total yield of the product was 56%. The PA content was 7.5%, the SCP content was 77.2%, the 4,5-DCPA content was 14.3%, the 3,4-DCPA content was 0.23%, the content of other impurities was 0.77%, and the color was 175.

[0039] Comparing Examples 1-3 with Comparative Example 1, it can be seen that under the same conditions, without the addition of Lewis acid catalysts ZnCl2 or AlCl3 during the chlorination reaction, the activation degree of the reaction substrate is lower, the chlorination conversion efficiency of phthalic anhydride and its hydrolysis products decreases, the selectivity of the target product decreases, the content of PA and reaction by-products in the crude product increases significantly, the SCP content in the obtained crude product is significantly lower than that in Examples 1-3, the color of the crude product is darker, and the subsequent purification burden increases.

[0040] Comparing Examples 1-3 with Comparative Example 1, it can be seen that the purification process is a single step. Comparative Example 1 did not use the alkali dissolution and acid precipitation purification method, but only used pure water to purify the crude product. Due to the limited solubility difference between the target product and some organic impurities, pigment impurities, and inorganic salt impurities during the pure water purification process, the impurity removal effect is weak. The resulting product has a relatively darker color, and the improvement in purity is limited. The purification yield and product appearance stability are both lower than those of Examples 1-3.

[0041] Finally, it should be noted that the above examples are merely some specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments and other modifications are possible. Any modifications, equivalent substitutions, or improvements made by those skilled in the art based on the disclosure of this invention should be considered within the scope of protection of this invention.

Claims

1. A method for preparing high-purity monosodium 4-chlorophthalic acid, characterized in that, Includes the following steps: (a) Preparation of sodium hypochlorite reaction solution: Add pure water, sodium hypochlorite solution and sodium hydroxide to the reaction vessel, stir and mix to completely dissolve the sodium hydroxide and make the system uniformly mixed to obtain alkaline sodium hypochlorite reaction solution; (b) Chlorination reaction: Add refined mother liquor or pure water, phthalic anhydride and catalyst to the crude product reaction vessel; then add the alkaline sodium hypochlorite reaction solution obtained in step (a) dropwise; after the dropwise addition is completed, continue to keep the reaction at the temperature for 5-7 h to obtain a reaction solution containing monosodium 4-chlorophthalic acid salt; (c) Acidification precipitation and solid-liquid separation: After the heat preservation reaction in step (b) is completed, hydrochloric acid is added dropwise to the reaction solution to adjust the pH of the solution to 4.5-4.8, so that the product 4-chlorophthalic acid monosodium salt precipitates out; centrifugation is used to separate the crude product of 4-chlorophthalic acid monosodium salt. (d) Alkali melting treatment: Add pure water, crude 4-chlorophthalic acid monosodium salt obtained in step (c), and liquid alkali to the alkali melting kettle, stir for 4-5 h to fully dissolve and clear the material; then filter the obtained material through diatomaceous earth filter and press to the refining kettle to remove insoluble impurities and adsorbed colored substances. (e) Refining process: Hydrochloric acid was added dropwise to the refining vessel. After the pH value of the system reached 4.5-4.8, an antioxidant was added and stirring was continued for about 0.5 h. After stirring, the temperature was raised to 75 ℃ and kept at that temperature for 2 h. After the temperature was kept at that temperature, the temperature was lowered for 3-4 h. After the temperature was lowered to 30 ℃, centrifugation was started. After centrifugation, high-purity monosodium 4-chlorophthalic acid salt was obtained.

2. The method for preparing monosodium 4-chlorophthalic acid according to claim 1, characterized in that, In step (a), the molar ratio of sodium hypochlorite to phthalic anhydride is 1.01~1.15 : 1.0, and the molar ratio of sodium hydroxide to phthalic anhydride is 0.75~0.80 : 1.

0.

3. The method for preparing monosodium 4-chlorophthalic acid according to claim 1, characterized in that, In step (b), the catalyst is selected from one or more of Lewis acid zinc salts, aluminum salts and their hydrates.

4. The method for preparing monosodium 4-chlorophthalic acid according to claim 3, characterized in that, The catalyst is zinc chloride or aluminum chloride; the amount of Lewis acid catalyst added is 0.1-10 wt% based on the mass of phthalic anhydride.

5. The method for preparing the monosodium 4-chlorophthalic acid salt according to claim 4, characterized in that, The amount of the Lewis acid catalyst added is 0.5-2.0 wt% based on the mass of phthalic anhydride.

6. The method for preparing the monosodium 4-chlorophthalic acid salt according to claim 1, characterized in that, In step (b), the reaction temperature of the system is 22-25°C. o C, the sodium hypochlorite solution is added dropwise over a period of 30-35 hours.

7. The method for preparing monosodium 4-chlorophthalic acid according to claim 1, characterized in that, In step (d), the concentration of the liquid alkali is 30%, and the amount used is 20-35 wt% of the crude product.

8. The method for preparing monosodium 4-chlorophthalic acid according to claim 1, characterized in that, In step (e), the centrally controlled pH value is 4.6.