A method for purifying p-phenylenediamine by recrystallization

Abstract

The present application relates to the technical field of separation and purification, and particularly relates to a p-phenylenediamine recrystallization purification method, which comprises the following steps: S1, dissolving p-phenylenediamine crude product in a crystallization solvent by heating, wherein the crystallization solvent comprises NMP and ionic liquid choline acetate; S2, precipitating crystals under cooling and stirring; S3, performing solid-liquid separation, washing and drying at the temperature of the final crystallization temperature to obtain p-phenylenediamine crystals. The mass ratio of NMP to p-phenylenediamine crude product is (0.5-2):1; the addition amount of choline acetate is 1%-3% of the mass of p-phenylenediamine crude product. The p-phenylenediamine recrystallization purification method realizes molecular-level selective separation through the synergistic effect of ionic liquid and solvent, is simple to operate, and realizes efficient and green impurity removal of p-phenylenediamine in the form of high solubility and low energy consumption.

Description

Technical Field

[0001] This invention relates to a method for recrystallizing and purifying p-phenylenediamine, belonging to the field of separation and purification technology. Background Technology

[0002] p-Phenylenediamine (PPD), also known as Ulster D, with the chemical formula C6H8N2, is an aromatic diamine and an important chemical intermediate widely used in dyes, polymer materials, pharmaceuticals, and rubber additives. PPD is a key raw material for the synthesis of para-aramid fibers, which possess excellent properties such as high strength, high modulus, high temperature resistance, and chemical corrosion resistance, and are widely used in aerospace, defense, and automotive industries. The purity of p-phenylenediamine directly affects the quality and performance of para-aramid fibers; only high-purity p-phenylenediamine can synthesize high-performance para-aramid fibers. Furthermore, in the dye industry, p-phenylenediamine is used to synthesize high-grade dyes, and its purity affects the dye's color, fastness, and other properties. In the field of rubber additives, p-phenylenediamine is used as an antioxidant, and high purity ensures its protective effect on rubber products.

[0003] The preparation of p-phenylenediamine often results in the presence of mixed diamines such as m-phenylenediamine and o-phenylenediamine in the reaction system. Currently, there are several process routes for purifying p-phenylenediamine: 1) Sublimation crystallization: The industrial p-phenylenediamine purification method disclosed in patent application RU2448084C1 involves sublimation crystallization under an inert atmosphere, directly purifying the crude product containing tar residue to 99.5-99.9% p-phenylenediamine at 100-260℃; 2) Melt crystallization: The finned multi-stage method disclosed in patent application CN116059680A... The sweating crystallization device can achieve 99.99% ultrapure product. The patent application with publication number CN109232276A uses microwave zone melting to achieve directional enrichment of impurities; 3) Distillation purification: The patent application with publication number JP2011026224A uses reducing vacuum distillation, while the patent application with publication number CN221084691U improves separation efficiency through tower structure optimization; 4) Solution crystallization method: The aqueous crystallization process developed in the patent application with publication number CN101250113A can obtain 100% whiteness product, reference (Liu Yuguo). Research on a new synthetic process for p-phenylenediamine. Liaoning: Liaoning Normal University, 2001.) Purification is achieved by low-temperature recrystallization; 5) Combined process: The patent application with publication number CN101250113A couples distillation with melt crystallization; The literature (Ding Junwei. Purification of p-phenylenediamine by melt crystallization. Modern Chemical Industry, 2012, (3): 221-223.) 92% crude product is purified to 99.5% with a yield of over 70% by temperature-controlled sweating operation.

[0004] However, among the above-mentioned technical routes for purifying p-phenylenediamine, the traditional sublimation method and vacuum distillation method require high temperature or high vacuum conditions, resulting in high energy consumption and easy thermal decomposition of the product; the melt crystallization process is complex to operate, has a long production cycle, and has a low yield; the aqueous solution crystallization method requires a large amount of solvent and is accompanied by high evaporation energy consumption due to the low solubility of p-phenylenediamine in water; although the organic solvent extraction method can remove impurities, it uses toxic solvents such as benzene series, which poses significant environmental risks; and microwave zone melting equipment is expensive and only suitable for high-purity crude products.

[0005] Existing technologies are insufficient to meet the industrial requirements of high purity, high yield, and low cost. In particular, they have limited efficiency in separating ortho- and meta-isomers with similar molecular structures. There is an urgent need to develop new, efficient, and green purification processes. Summary of the Invention

[0006] This invention addresses the shortcomings of existing technologies by providing a method for recrystallizing and purifying p-phenylenediamine. This method achieves molecular-level selective separation through the synergistic effect of ionic liquids and solvents. It is simple to operate and achieves efficient and green impurity removal of p-phenylenediamine with high solubility and low energy consumption.

[0007] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: a method for recrystallization and purification of p-phenylenediamine, wherein the recrystallization and purification method is as follows:

[0008] S1. The crude p-phenylenediamine is dissolved in a crystallization solvent by heating, wherein the crystallization solvent includes NMP and the ionic liquid choline acetate;

[0009] S2. Under stirring conditions, cooling is performed to precipitate crystals;

[0010] S3. Solid-liquid separation and washing are carried out at the final crystallization temperature, and then dried to obtain p-phenylenediamine crystals.

[0011] Furthermore, the crude p-phenylenediamine contains p-phenylenediamine, m-phenylenediamine, and o-phenylenediamine, wherein the mass content of p-phenylenediamine is 90%-95%, the mass content of m-phenylenediamine is 3%-5%, and the mass content of o-phenylenediamine is 2%-5%.

[0012] Furthermore, the mass ratio of NMP to crude p-phenylenediamine is (0.5-2):1; the amount of choline acetate added is 1%-3% of the mass of crude p-phenylenediamine.

[0013] Furthermore, in step S1, the water content in the crystallization system is <200 ppm.

[0014] Furthermore, in step S1, the heating temperature is 60-90℃, and after dissolution, the mixture is kept at the heating temperature for 10-60 minutes.

[0015] Furthermore, in step S2, the stirring speed is 10-100 r / min.

[0016] Furthermore, in step S2, the temperature is cooled down to the final crystallization temperature of 45-50℃, and then kept at the final crystallization temperature for 1-2 hours before proceeding to step S3 for solid-liquid separation.

[0017] Furthermore, in step S2, a programmed cooling process is adopted during the cooling process: when the temperature is above 60°C, the cooling rate is 0.8-1.0°C / min; when the temperature drops from 60°C to 55°C, the cooling rate is 0.3-0.5°C / min; and when the temperature drops from 55°C to the final crystallization temperature, the cooling rate is 0.1-0.2°C / min.

[0018] Furthermore, in step S3, the solvent used for washing is NMP, and the drying temperature is 55-65℃.

[0019] Furthermore, deionized water is added to the mother liquor after solid-liquid separation to precipitate and recover choline acetate for recycling.

[0020] The beneficial effects of this invention are:

[0021] The p-phenylenediamine recrystallization purification method of the present invention uses N-methylpyrrolidone (NMP) as a solvent and adds choline acetate to recrystallize and purify the p-phenylenediamine. Molecular-level selective separation is achieved through the synergistic effect of ionic liquid (choline acetate) and solvent. The method is simple to operate and achieves efficient and green impurity removal of p-phenylenediamine in a form with high solubility and low energy consumption.

[0022] In the recrystallization and purification method for p-phenylenediamine described in this invention, acetic choline is introduced as a synergistic additive. The anion of acetic choline can form a bidentate hydrogen bond with the symmetrical amino group of p-phenylenediamine, while the cation of acetic choline inhibits the co-crystallization of isomers through charge shielding. This method breaks through the bottleneck of isomer separation with almost no increase in cost, and finally obtains high-purity p-phenylenediamine.

[0023] In the p-phenylenediamine recrystallization purification method of the present invention, the purity of the p-phenylenediamine product obtained by recrystallization in the first stage is not less than 99.9% (and can even reach 99.99%), and the purity of the p-phenylenediamine product obtained by recrystallization purification of the recovered choline acetate after reuse is also not less than 99%.

[0024] The p-phenylenediamine recrystallization purification method described in this invention is simpler to operate, has milder reaction conditions, significantly reduces energy consumption, and allows for multiple uses of the crystallization solvent. Choline acetate can be recycled at least five times (recovery rate >90%), resulting in highly efficient resource recycling. Compared to traditional solvents such as xylene, the use of low-toxicity, biodegradable NMP and the bio-derived ionic liquid choline acetate replaces traditional benzene-based toxic solvents, reducing the use and emission of toxic substances at the source, demonstrating significant environmental friendliness. Ionic liquid recovery is simple (simply add water for precipitation), significantly improving the greenness of the process and making it more suitable for industrial applications. Attached Figure Description

[0025] Figure 1 This is a process flow diagram of the p-phenylenediamine recrystallization and purification method described in this invention;

[0026] Figure 2 The above is a gas chromatogram of the purified p-phenylenediamine product in the examples;

[0027] Figure 3 The gas chromatogram of the purified p-phenylenediamine product in Comparative Example 1;

[0028] Figure 4 This is the gas chromatogram of the purified p-phenylenediamine product in Comparative Example 2;

[0029] Figure 5 This is the gas chromatogram of the p-phenylenediamine product after purification in Comparative Example 3;

[0030] Figure 6 The gas chromatogram of the purified p-phenylenediamine product in Comparative Example 4;

[0031] Figure 7 The image shows the gas chromatogram of the purified p-phenylenediamine product from Comparative Example 5. Detailed Implementation

[0032] The specific embodiments of the present invention will be described in detail below. The present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used is for describing particular embodiments only and is not intended to limit the invention.

[0034] like Figure 1 As shown, a method for recrystallizing and purifying p-phenylenediamine is described, wherein the recrystallization and purification method is as follows:

[0035] S1. The crude p-phenylenediamine is dissolved in a crystallization solvent by heating, wherein the crystallization solvent includes NMP and the ionic liquid choline acetate;

[0036] S2. Under stirring conditions, cooling is performed to precipitate crystals;

[0037] S3. Solid-liquid separation and washing are carried out at the final crystallization temperature, and then dried to obtain p-phenylenediamine crystals.

[0038] Specifically, the crude p-phenylenediamine used in the embodiments of the present invention comprises p-phenylenediamine, m-phenylenediamine and o-phenylenediamine, wherein the mass content of p-phenylenediamine is 90%-95%, the mass content of m-phenylenediamine is 3%-5%, and the mass content of o-phenylenediamine is 2%-5%.

[0039] Specifically, the ratio of NMP mass to crude p-phenylenediamine mass is (0.5-2):1; the amount of choline acetate added is 1%-3% of the crude p-phenylenediamine mass.

[0040] Specifically, in step S1, the water content in the crystallization system is <200 ppm.

[0041] Specifically, in step S1, the heating temperature is 60-90℃, and after dissolution, the mixture is kept at the heating temperature for 10-60 minutes.

[0042] More specifically, during the heating and dissolving process in step S1, the stirring speed is 100~500 r / min.

[0043] Specifically, in step S2, the stirring speed is 10-100 r / min.

[0044] Specifically, in step S2, the temperature is cooled down to the final crystallization temperature of 45-50℃, and then kept at the final crystallization temperature for 1-2 hours before proceeding to step S3 for solid-liquid separation.

[0045] Specifically, in step S2, a programmed cooling process is used during the cooling process: when the temperature is above 60℃, the cooling rate is 0.8-1.0℃ / min; when the temperature drops from 60℃ to 55℃, the cooling rate is 0.3-0.5℃ / min; and when the temperature drops from 55℃ to the final crystallization temperature, the cooling rate is 0.1-0.2℃ / min.

[0046] Specifically, in step S3, the solvent used for washing is NMP, and the drying temperature is 55-65℃.

[0047] Specifically, deionized water is added to the mother liquor after solid-liquid separation to precipitate and recover choline acetate for recycling.

[0048] The acetic choline can be recycled more than 5 times; preferably, the acetic choline is recycled 1-4 times to ensure that the purity of the p-phenylenediamine obtained by recrystallization is not less than 99.9%.

[0049] Example 1

[0050] A method for recrystallizing and purifying p-phenylenediamine, wherein the recrystallization and purification method comprises:

[0051] S1. Under dry nitrogen protection, add 100 g of crude p-phenylenediamine, 100 g of NMP (water content <200 ppm) and 1.5 g of choline acetate to the jacketed reactor. Control the stirring speed at 300 r / min, heat to 70 ℃ with continuous stirring to completely dissolve the crude p-phenylenediamine, and keep warm for 30 min.

[0052] S2. After the heat preservation is completed, adjust the stirring speed to 50 r / min and start the programmed cooling to cool the solution and precipitate crystals. The programmed cooling process is as follows:

[0053] The first cooling range: from 70 ℃ to 60 ℃, the cooling rate is 0.9 ℃ / min;

[0054] The second cooling range: the cooling rate from 60 ℃ to 55 ℃ is 0.3 ℃ / min;

[0055] The third cooling range: the cooling rate from 55 ℃ to 50 ℃ is 0.15 ℃ / min;

[0056] The crystallization temperature was maintained at 50℃ for 1.5 hours.

[0057] S3. Filter at 50°C, wash with NMP (10g), and dry at 60°C to obtain p-phenylenediamine product.

[0058] The yield of p-phenylenediamine was 90.2%, and the purity of p-phenylenediamine, as determined by gas chromatography, was 99.99% (e.g., Figure 2 (As shown), the color is white. Gas chromatographic data are shown in Table 1 below.

[0059] Table 1 Gas chromatographic data of Example 1

[0060]

[0061] Additive recovery: 50 mL of deionized water was added to the mother liquor after filtration, choline acetate crystals were precipitated, and the recovery rate was 96.3% after filtration and drying.

[0062] Example 2

[0063] A method for recrystallizing and purifying p-phenylenediamine, wherein the recrystallization and purification method comprises:

[0064] S1. Under dry nitrogen protection, add 100 g of crude p-phenylenediamine, 200 g of NMP (water content <200 ppm) and 3 g of choline acetate to the jacketed reactor. Control the stirring speed at 100 r / min, heat to 60 ℃ with continuous stirring to completely dissolve the crude p-phenylenediamine, and keep warm for 60 min.

[0065] S2. After the heat preservation is completed, adjust the stirring speed to 10 r / min and start the programmed cooling to cool the solution and precipitate crystals. The programmed cooling process is as follows:

[0066] The first cooling range: 60 ℃ to 55 ℃, with a cooling rate of 0.5 ℃ / min;

[0067] The second cooling range: the cooling rate from 55 ℃ to 45 ℃ is 0.1 ℃ / min;

[0068] The crystallization temperature was maintained at 45℃ for 2 hours.

[0069] S3. Filter at 45°C, wash with NMP (10g), and dry at 65°C to obtain p-phenylenediamine product.

[0070] The yield of p-phenylenediamine was 90.0%, and the purity of p-phenylenediamine was 99.99% according to gas chromatography. The color was white.

[0071] Additive recovery: 80 mL of deionized water was added to the mother liquor after filtration, choline acetate crystals were precipitated, and the recovery rate was 95.7% after filtration and drying.

[0072] Example 3

[0073] A method for recrystallizing and purifying p-phenylenediamine, wherein the recrystallization and purification method comprises:

[0074] S1. Under dry nitrogen protection, add 100 g of crude p-phenylenediamine, 50 g of NMP (water content <200 ppm) and 1 g of choline acetate to the jacketed reactor. Control the stirring speed at 500 r / min, heat to 90 ℃ with continuous stirring to completely dissolve the crude p-phenylenediamine, and keep warm for 10 min.

[0075] S2. After the heat preservation is completed, adjust the stirring speed to 100 r / min and start the programmed cooling to cool the solution and precipitate crystals. The programmed cooling process is as follows:

[0076] The first cooling range: the cooling rate from 90 ℃ to 60 ℃ is 1 ℃ / min;

[0077] The second cooling range: the cooling rate from 60 ℃ to 55 ℃ is 0.4 ℃ / min;

[0078] The third cooling range: the cooling rate from 55 ℃ to 50 ℃ is 0.2 ℃ / min;

[0079] The crystallization temperature was maintained at 50℃ for 1 hour.

[0080] S3. Filter at 50°C, wash with NMP (10g), and dry at 55°C to obtain p-phenylenediamine product.

[0081] The yield of p-phenylenediamine was 90.5%, and the purity of p-phenylenediamine was 99.97% as determined by gas chromatography. The color of p-phenylenediamine was white.

[0082] Additive recovery: 50 mL of deionized water was added to the mother liquor after filtration, choline acetate crystals were precipitated, and the recovery rate was 96.0% after filtration and drying.

[0083] Example 4

[0084] A method for recrystallizing and purifying p-phenylenediamine, wherein the recrystallization and purification method comprises:

[0085] S1. Under dry nitrogen protection, add 100 g of crude p-phenylenediamine, 150 g of NMP (water content <200 ppm) and 2 g of choline acetate to the jacketed reactor. Control the stirring speed at 200 r / min, heat to 80 ℃ with continuous stirring to completely dissolve the crude p-phenylenediamine, and keep warm for 30 min.

[0086] S2. After the heat preservation is completed, adjust the stirring speed to 60 r / min and start the programmed cooling to cool the solution and precipitate crystals. The programmed cooling process is as follows:

[0087] The first cooling range: from 80 ℃ to 60 ℃, the cooling rate is 0.8 ℃ / min;

[0088] The second cooling range: the cooling rate from 60 ℃ to 55 ℃ is 0.4 ℃ / min;

[0089] The third cooling range: the cooling rate from 55 ℃ to 50 ℃ is 0.1 ℃ / min;

[0090] The crystallization temperature was maintained at 50℃ for 1 h.

[0091] S3. Filter at 50°C, wash with NMP (10g), and dry at 60°C to obtain p-phenylenediamine product.

[0092] The yield of p-phenylenediamine was 90.0%, and the purity of p-phenylenediamine was 99.99% according to gas chromatography. The color was white.

[0093] Additive recovery: 50 mL of deionized water was added to the mother liquor after filtration, choline acetate crystals were precipitated, and the recovery rate was 95.9% after filtration and drying.

[0094] Comparative Example 1

[0095] The recrystallization and purification of p-phenylenediamine was carried out using the same method as in Example 1, except that the water content of the solvent in step S1 of Comparative Example 1 was 0.5%, which is higher than the water content limit specified in this invention. The specific process is as follows:

[0096] S1. Under dry nitrogen protection, add 100 g of crude p-phenylenediamine, 100 g of NMP (water content 0.5%) and 1.5 g of choline acetate to the jacketed reactor. Control the stirring speed at 300 r / min, heat to 70 ℃ while stirring continuously to completely dissolve the crude p-phenylenediamine, and keep warm for 30 min.

[0097] S2. After the heat preservation is completed, adjust the stirring speed to 50 r / min and start the programmed cooling to cool the solution and precipitate crystals. The programmed cooling process is as follows:

[0098] The first cooling range: from 70 ℃ to 60 ℃, the cooling rate is 0.9 ℃ / min;

[0099] The second cooling range: the cooling rate from 60 ℃ to 55 ℃ is 0.3 ℃ / min;

[0100] The third cooling range: the cooling rate from 55 ℃ to 50 ℃ is 0.15 ℃ / min;

[0101] The crystallization temperature was maintained at 50℃ for 1.5 hours.

[0102] S3. Filter at 50°C, wash with NMP (10g), and dry at 60°C to obtain p-phenylenediamine product.

[0103] The yield of p-phenylenediamine was 78.4%, and the purity of p-phenylenediamine, as determined by gas chromatography, was 99.181% (e.g., ...). Figure 3 (As shown in the image), the color is white. An oily byproduct appears in the crystallization system. Gas chromatographic data are shown in Table 2 below.

[0104] Table 2 Gas chromatographic data of Comparative Example 1

[0105]

[0106] Additive recovery: 50 mL of deionized water was added to the mother liquor after filtration, choline acetate crystals were precipitated, and the recovery rate was 85.1% after filtration and drying.

[0107] A comparison of the results from Comparative Example 1 and Example 1 shows that when the moisture content in the crystallization system exceeds the standard, the ionic liquid choline acetate undergoes a hydrolysis reaction upon heating, consuming some of the additives. This not only weakens the selective separation effect of choline acetate, leading to a decrease in purity, but the acetic acid and alkaline environment generated by the hydrolysis may also promote the oxidation of p-phenylenediamine or produce side reactions, forming an oily substance that coats the product, resulting in a decrease in yield and a deterioration in product color. Simultaneously, hydrolysis also damages the structure of choline acetate, reducing the recovery rate.

[0108] Comparative Example 2

[0109] p-Phenylenediamine was purified by recrystallization using the same method as in Example 1, except that choline acetate was not added in Comparative Example 2. The specific process is as follows:

[0110] S1. Under dry nitrogen protection, add 100 g of crude p-phenylenediamine and 100 g of NMP (water content <200 ppm) to the jacketed reactor. Control the stirring speed at 300 r / min, heat to 70 ℃ while stirring continuously to completely dissolve the crude p-phenylenediamine, and keep warm for 30 min.

[0111] S2. After the heat preservation is completed, adjust the stirring speed to 50 r / min and start the programmed cooling to cool the solution and precipitate crystals. The programmed cooling process is as follows:

[0112] The first cooling range: from 70 ℃ to 60 ℃, the cooling rate is 0.9 ℃ / min;

[0113] The second cooling range: the cooling rate from 60 ℃ to 55 ℃ is 0.3 ℃ / min;

[0114] The third cooling range: the cooling rate from 55 ℃ to 50 ℃ is 0.15 ℃ / min;

[0115] The crystallization temperature was maintained at 50℃ for 1.5 hours.

[0116] S3. Filter at 50°C, wash with NMP (10g), and dry at 60°C to obtain p-phenylenediamine product.

[0117] The yield of p-phenylenediamine was 80.4%, and the purity of p-phenylenediamine, as determined by gas chromatography, was 99.916% (e.g., ...). Figure 4 (As shown), the color is white. Gas chromatographic data are shown in Table 3 below.

[0118] Table 3 Gas chromatographic data of Comparative Example 2

[0119]

[0120] A comparison of the results from Comparative Example 2 and Example 1 shows that the yield of p-phenylenediamine significantly decreases if acetic choline is not added to the crystallization system. This is because, without acetic choline, the crystallization process relies solely on the solubility difference of NMP, resulting in limited selectivity for ortho- and meta-phenylenediamine isomers with extremely similar molecular structures. The lack of acetic choline's molecular recognition mechanism allows impurities to more easily infiltrate the crystal lattice, leading to a significant reduction in product yield, even though the purity remains acceptable.

[0121] Comparative Example 3

[0122] The recrystallization and purification of p-phenylenediamine was carried out using the same method as in Example 1, except that the programmed cooling in Comparative Example 3 differs from that specified in this invention. The specific process is as follows:

[0123] S1. Under dry nitrogen protection, add 100 g of crude p-phenylenediamine, 100 g of NMP (water content <200 ppm) and 1.5 g of choline acetate to the jacketed reactor. Control the stirring speed at 300 r / min, heat to 70 ℃ with continuous stirring to completely dissolve the crude p-phenylenediamine, and keep warm for 30 min.

[0124] S2. After the heat preservation is completed, adjust the stirring speed to 50 r / min and start the programmed cooling to cool the solution and precipitate crystals. The programmed cooling process is as follows:

[0125] The first cooling range: the cooling rate from 70 ℃ to 50 ℃ is 2 ℃ / min;

[0126] The second cooling range: the cooling rate from 50 ℃ to 40 ℃ is 1 ℃ / min;

[0127] The crystallization temperature was maintained at 450℃ for 1.5 hours.

[0128] S3. Filter at 50°C, wash with NMP (10g), and dry at 60°C to obtain p-phenylenediamine product.

[0129] The yield of p-phenylenediamine was 87.7%, and the purity of p-phenylenediamine, as determined by gas chromatography, was 98.724% (e.g., ...). Figure 5 (As shown), the color is white. Gas chromatographic data are shown in Table 4 below.

[0130] Table 4 Gas chromatographic data of Comparative Example 3

[0131]

[0132] Additive recovery: 50 mL of deionized water was added to the mother liquor after filtration, choline acetate crystals were precipitated, and the recovery rate was 95.7% after filtration and drying.

[0133] A comparison of the results from Comparative Example 3 and Example 1 shows that if the cooling rate is too fast, the yield and purity of p-phenylenediamine decrease significantly. This is because excessively rapid cooling leads to a sudden and excessively high supersaturation of the solution, triggering explosive nucleation and producing a large number of fine, irregular crystals. These crystals have a large specific surface area and easily encapsulate the mother liquor and impurities during growth, resulting in a decrease in product purity. Furthermore, the fine crystals make subsequent filtration and washing difficult, increasing product loss.

[0134] Comparative Example 4

[0135] The recrystallization and purification of p-phenylenediamine was carried out using the same method as in Example 1, except that the mass ratio of NMP to crude p-phenylenediamine in Comparative Example 4 was 3:1 (higher than the NMP dosage ratio specified in this invention). The specific process is as follows:

[0136] S1. Under dry nitrogen protection, add 100 g of crude p-phenylenediamine, 300 g of NMP (water content <200 ppm) and 1.5 g of choline acetate to the jacketed reactor. Control the stirring speed at 300 r / min, heat to 70 ℃ with continuous stirring to completely dissolve the crude p-phenylenediamine, and keep warm for 30 min.

[0137] S2. After the heat preservation is completed, adjust the stirring speed to 50 r / min and start the programmed cooling to cool the solution and precipitate crystals. The programmed cooling process is as follows:

[0138] The first cooling range: from 70 ℃ to 60 ℃, the cooling rate is 0.9 ℃ / min;

[0139] The second cooling range: the cooling rate from 60 ℃ to 55 ℃ is 0.3 ℃ / min;

[0140] The third cooling range: the cooling rate from 55 ℃ to 50 ℃ is 0.15 ℃ / min;

[0141] The crystallization temperature was maintained at 50℃ for 1.5 hours.

[0142] S3. Filter at 50°C, wash with NMP (10g), and dry at 60°C to obtain p-phenylenediamine product.

[0143] The yield of p-phenylenediamine was 88.2%, and the purity of p-phenylenediamine, as determined by gas chromatography, was 99.925% (e.g., ...). Figure 6 (As shown), the color is white. Gas chromatographic data are shown in Table 5 below.

[0144] Table 5 Gas chromatographic data of Comparative Example 4

[0145]

[0146] Additive recovery: 50 mL of deionized water was added to the mother liquor after filtration, choline acetate crystals were precipitated, and the recovery rate was 95.9% after filtration and drying.

[0147] A comparison of the results from Comparative Example 4 and Example 1 shows that adding too much NMP during crystallization significantly reduces the yield of p-phenylenediamine. This is because, according to the principle of solubility, the solubility of a solute in a solvent is constant at a given temperature. Excessive use of NMP means a decrease in the supersaturation of the solution when cooled to the same final crystallization temperature. Supersaturation is the driving force for crystal precipitation; insufficient driving force will result in a large amount of p-phenylenediamine remaining dissolved in the mother liquor and failing to precipitate, thus significantly reducing the product yield. This leads to raw material waste and increases the load on subsequent mother liquor treatment and the energy consumption for solvent recovery, which does not conform to the reduction principle of green chemistry.

[0148] Comparative Example 5

[0149] The recrystallization and purification of p-phenylenediamine was carried out using the same method as in Example 1, except that the amount of choline acetate added in Comparative Example 5 was 5% of the crude p-phenylenediamine mass (higher than the amount of choline acetate specified in this invention). The specific process is as follows:

[0150] S1. Under dry nitrogen protection, add 100 g of crude p-phenylenediamine, 100 g of NMP (water content <200 ppm) and 5 g of choline acetate to the jacketed reactor. Control the stirring speed at 300 r / min, heat to 70 ℃ with continuous stirring to completely dissolve the crude p-phenylenediamine, and keep warm for 30 min.

[0151] S2. After the heat preservation is completed, adjust the stirring speed to 50 r / min and start the programmed cooling to cool the solution and precipitate crystals. The programmed cooling process is as follows:

[0152] The first cooling range: from 70 ℃ to 60 ℃, the cooling rate is 0.9 ℃ / min;

[0153] The second cooling range: the cooling rate from 60 ℃ to 55 ℃ is 0.3 ℃ / min;

[0154] The third cooling range: the cooling rate from 55 ℃ to 50 ℃ is 0.15 ℃ / min;

[0155] The crystallization temperature was maintained at 50℃ for 1.5 hours.

[0156] S3. Filter at 50°C, wash with NMP (10g), and dry at 60°C to obtain p-phenylenediamine product.

[0157] The yield of p-phenylenediamine was 89.9%, and the purity of p-phenylenediamine, as determined by gas chromatography, was 99.943% (e.g., Figure 7The color shown is white. Gas chromatographic data are shown in Table 6 below.

[0158] Table 6 Gas chromatographic data of Comparative Example 5

[0159]

[0160] Additive recovery: 50 mL of deionized water was added to the mother liquor after filtration, choline acetate crystals were precipitated, and the recovery rate was 96.5% after filtration and drying.

[0161] A comparison of the results from Comparative Example 5 and Example 1 shows that if too much choline acetate is added during the crystallization process, the yield and purity of the p-phenylenediamine product will decrease slightly. This is because excessive addition may alter the overall physicochemical properties of the solution (such as viscosity and dielectric constant), potentially interfering with the crystallization behavior of the p-phenylenediamine molecules themselves, and even inhibiting crystal precipitation due to excessive complexation, leading to a stagnation or even a slight decrease in yield or purity. Furthermore, from an economic perspective, excessive use of ionic liquids also increases costs, even though they are recyclable, as it increases the recycling load per batch.

[0162] In addition, the following experiments were conducted to investigate the recovery and utilization of choline acetate:

[0163] The choline acetate recovered in Example 1 was recycled for the purification of a new batch of p-phenylenediamine (using the same purification method as in Example 1). The purified p-phenylenediamine product had a purity of 99.945% (result of the first recycling of choline acetate). Choline acetate was then recycled again, and the p-phenylenediamine product was purified by recrystallization using the same method as in Example 1. This recycling process was repeated 5 times, and the specific results are as follows:

[0164] Results of the first recovery and utilization of choline acetate: The purity of the p-phenylenediamine product was 99.945%, and the yield was 90.0%.

[0165] Results of the second recovery and utilization of choline acetate: The purity of the p-phenylenediamine product was 99.939%, and the yield was 89.8%.

[0166] Results of the third recovery and utilization of choline acetate: The purity of the p-phenylenediamine product was 99.932%, and the yield was 90.1%.

[0167] Results of the fourth recovery and utilization of choline acetate: The purity of the p-phenylenediamine product was 99.912%, and the yield was 89.7%.

[0168] Results of the fifth recovery and utilization of choline acetate: The purity of the p-phenylenediamine product was 99.889%, and the yield was 89.5%.

[0169] The experimental results above show that choline acetate used in the recrystallization process of p-phenylenediamine can be recycled multiple times, which can effectively reduce the cost in actual production.

[0170] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0171] For those skilled in the art, various modifications and improvements can be made without departing from the concept of the present invention, and these modifications and improvements are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A method for purifying p-phenylenediamine by recrystallization, characterized by, The recrystallization purification method is: S1, dissolving the crude p-phenylenediamine in a crystallization solvent by heating, the composition of the crystallization solvent being: NMP and ionic liquid choline acetate; S2, under stirring, cooling and temperature separation of crystals; S3, solid-liquid separation, washing, and then drying at the temperature of the final crystallization temperature to obtain p-phenylenediamine crystals; In step S1, the water content in the crystallization system is <200 ppm; The crude p-phenylenediamine contains p-phenylenediamine, m-phenylenediamine and o-phenylenediamine, wherein the mass content of p-phenylenediamine is 90%-95%, the mass content of m-phenylenediamine is 3%-5%, and the mass content of o-phenylenediamine is 2%-5%.

2. The method of claim 1, wherein the p-phenylenediamine is recrystallized and purified. The mass ratio of NMP to crude p-phenylenediamine is (0.5-2):1, and the addition amount of choline acetate is 1%-3% of the mass of the crude p-phenylenediamine.

3. The method of claim 1, wherein the recrystallization of the p-phenylenediamine is performed by adding water to the p-phenylenediamine and then heating the mixture to a temperature of 60°C to 80°C. In step S1, the heating temperature is 60-90℃, and after dissolving, the temperature is kept at the heating temperature for 10-60min.

4. The method of claim 1, wherein the recrystallization of the p-phenylenediamine is performed by adding water to the p-phenylenediamine. In step S2, the stirring speed is 10-100 r / min.

5. The method of claim 1, wherein the recrystallization of the p-phenylenediamine is performed at a temperature of about 50°C to about 60°C. In step S2, the cooling temperature is cooled to the final crystallization temperature of 45-50℃, and after keeping at the final crystallization temperature for 1-2h, the solid-liquid separation of step S3 is performed.

6. The method of claim 1, wherein the recrystallization of the p-phenylenediamine is performed at a temperature of about 50°C to about 60°C. In step S2, during the cooling process, the temperature is lowered in stages: when the temperature is higher than 60℃, the cooling speed is 0.8-1.0℃ / min; when the temperature is lowered from 60℃ to 55℃, the cooling speed is 0.3-0.5℃ / min; when the temperature is lowered from 55℃ to the final crystallization temperature, the cooling speed is 0.1-0.2℃ / min.

7. The method of claim 1, wherein the recrystallization is performed at a temperature of about 60°C to about 70°C. In step S3, the solvent used for washing is NMP, and the drying temperature is 55-65℃.

8. The method of claim 1, wherein the recrystallization of the p-phenylenediamine is performed at a temperature of about 50 °C to about 60 °C. In the mother liquor after solid-liquid separation, deionized water is added to precipitate and recover choline acetate for recycling.

Images