Recrystallization purification process of 2, 4-dichlorobenzonitrile

Abstract

The invention relates to the technical field of compound purification, in particular to a recrystallization purification process of 2, 4-dichlorobenzonitrile. The invention solves the problem that the purity and yield of 2, 4-dichlorobenzonitrile are difficult to consider at the same time in the prior art. The method comprises the following steps: carrying out rectification and column chromatography pretreatment on a crude product; dissolving in a mixed solvent of isopropanol and n-heptane, and carrying out decoloration and adsorption treatment by using activated carbon; then adding an ethylenediamine disuccinic acid and hydroxypropyl-beta-cyclodextrin composite complexing agent into the solution; crystal growth is promoted through staged cooling and ultrasonic-assisted crystallization; according to the method, 2, 4-dichlorobenzonitrile is taken as a raw material, pure 2, 4-dichlorobenzonitrile is obtained through centrifugal separation, washing and drying, the purity and yield of 2, 4-dichlorobenzonitrile are improved through cooperation of the components and introduction optimization of the process, and various application conditions of 2, 4-dichlorobenzonitrile are met.

Description

Technical Field

[0001] The present invention relates to the technical field of compound purification, and specifically to a recrystallization purification process for 2,4-dichlorobenzonitrile. Background Art

[0002] As an important organic synthesis intermediate, 2,4-dichlorobenzonitrile is widely used in the fields of pesticides, pharmaceuticals, dyes, etc. Its purity directly affects the quality and performance of subsequent products. However, there are many problems in the existing recrystallization purification process for 2,4-dichlorobenzonitrile, mainly focusing on two aspects: low product purity and low purification efficiency.

[0003] The traditional recrystallization process for 2,4-dichlorobenzonitrile often uses a single solvent, and the discrimination of the dissolution characteristics of 2,4-dichlorobenzonitrile and impurities is limited. During the recrystallization process, it is difficult to effectively separate the impurities. In addition, the solubility curves of some impurities and 2,4-dichlorobenzonitrile in the selected solvent are similar, so that during the crystallization process, the impurities are likely to precipitate together with the product, reducing the product purity.

[0004] In addition, in the existing crystallization operation, the control of crystallization conditions is relatively rough, and parameters such as the heating rate, cooling rate, and stirring speed during the crystallization process lack precise regulation. This not only affects the growth quality of crystals, but also has an adverse impact on product purity and purification efficiency. In actual production, due to the lack of effective process control means, it often takes a lot of time for crystallization operations, and the product purity still cannot be effectively guaranteed.

[0005] In summary, the crude product of 2,4-dichlorobenzonitrile synthesis often contains various impurities, including unreacted raw materials and by-products generated during the reaction process. At the same time, the chemical properties of some impurities are similar to those of 2,4-dichlorobenzonitrile. It is difficult to effectively reduce the content of such impurities by the existing recrystallization methods. Therefore, there is a problem that it is difficult to balance the purity and yield of 2,4-dichlorobenzonitrile products, which limits the application range of 2,4-dichlorobenzonitrile.

[0006] Therefore, a recrystallization purification process for 2,4-dichlorobenzonitrile is proposed. Summary of the Invention

[0007] The object of the present invention is to provide a recrystallization purification process for 2,4-dichlorobenzonitrile. The present invention pre-treats the crude product by distillation and column chromatography; then dissolves it in a mixed solvent of isopropanol and n-heptane, and performs decolorization and adsorption treatment with activated carbon; then adds a composite complexing agent of ethylenediamine disuccinic acid and hydroxypropyl-β-cyclodextrin to the solution; promotes crystal growth by stepwise cooling and ultrasonic-assisted crystallization; and then obtains pure 2,4-dichlorobenzonitrile through centrifugal separation, washing and drying. Through the synergy between components and the introduction and optimization of the process, the purity and yield of 2,4-dichlorobenzonitrile are improved to meet various application conditions of 2,4-dichlorobenzonitrile.

[0008] To achieve the above object, the present invention provides the following technical solutions: The present invention provides a recrystallization purification process for 2,4-dichlorobenzonitrile, and the purification process includes the following steps: S1: Subject the 2,4-dichlorobenzonitrile crude product to distillation and column chromatography to obtain a pre-treated component; S2: Add the pre-treated component to a mixed solvent and stir to dissolve it; obtain a filtered component through treatment with activated carbon; S3: Dissolve the composite complexing agent in the mixed solvent to obtain a complexing agent solution; add the complexing agent solution to the filtered component and stir to mix; then perform stepwise cooling treatment and ultrasonic-assisted crystallization to obtain a crystallization system; S4: Subject the crystallization system to centrifugal separation, washing and drying to obtain pure 2,4-dichlorobenzonitrile; Among them, the composite complexing agent is prepared by spray drying a mixture of ethylenediamine disuccinic acid and hydroxypropyl-β-cyclodextrin with a mass ratio of 2-5:1.

[0009] Preferably, the pre-treatment component in S1 includes the following process: Subject the 2,4-dichlorobenzonitrile crude product to a distillation column, adjust the temperature of the distillation column to 120-150 °C, the pressure to 8-10 kPa, keep the number of theoretical plates at 18, and keep the reflux ratio at 8:1 to obtain a distilled component; subject the distilled component to atmospheric column chromatography, use silica gel as the stationary phase, and perform gradient elution according to the volume ratio of n-heptane to ethyl acetate of 10:1-5:1, combine the 2,4-dichlorobenzonitrile distillation segments, and obtain the pre-treated component through vacuum concentration.

[0010] Preferably, the filtered component in S2 includes the following process: Add isopropanol to a beaker, slowly add n-heptane, and stir to mix to obtain a mixed solvent; add the pre-treated component to a crystallizer, add the mixed solvent, raise the temperature of the crystallizer to 70-80 °C, and stir at 150-200 rpm to dissolve to obtain a clear solution; add activated carbon to the clear solution, stir and treat it, and perform vacuum filtration using a Buchner funnel under hot conditions, and assist filtration with diatomaceous earth to obtain a filtered component; Preferably, the activated carbon of the present invention is selected from YP-300, and the pore diameter of the filter paper for vacuum filtration is 1-2 μm Among them, the volume-mass ratio of the pretreatment component to the mixed solvent is 4-8:1.

[0011] Preferably, the activated carbon accounts for 0.8-1.5% of the mass of the pretreatment component; the rotation speed of the stirring treatment is 150-250 rpm; the stirring treatment time is 30-60 min.

[0012] Preferably, the crystallization system in S3 includes the following processes: Add a complexing agent solution to the filtered component, and stir at 150-200 rpm for 20-30 min under insulation conditions to obtain a mixed system; add the mixed system to a crystallizer, place it in an ultrasonic device, and obtain a crystallization system through staged cooling treatment.

[0013] Preferably, the complexing agent solution includes the following processes: Dissolve ethylenediamine disuccinic acid and hydroxypropyl-β-cyclodextrin in deionized water, mix evenly, and perform spray drying, where the inlet temperature of the spray drying is 120 °C and the outlet temperature is 60 °C to obtain a composite complexing agent; add a mixed solvent to the composite complexing agent, and the mass-volume ratio of the composite complexing agent to the mixed solvent is 1:2, and stir at 200 rpm for 30 min to dissolve evenly to obtain a complexing agent solution; Among them, the mixed solvent is obtained by mixing isopropyl alcohol and n-heptane in a volume ratio of 3:1-6:1.

[0014] Preferably, the staged cooling treatment includes the following processes: Add the mixed system to a crystallizer, place it in an ultrasonic device, in the first stage, lower the system temperature to 40-45 °C at a cooling rate of 1-1.5 °C / min, and perform heat preservation treatment for 1-3 h to obtain a dispersion system. During this period, use ultrasonic-assisted crystallization, maintain a frequency of 30 kHz and a power of 120 W for ultrasonic dispersion, adopt a pulsed working mode, work for 5 seconds and then rest for 10 seconds, and control the total working time within 20 min; stop the ultrasonic treatment of the dispersion system, and in the second stage, lower the system temperature to 5-10 °C at a cooling rate of 0.5-1 °C / min, and keep it standing for 4-6 h to obtain a crystallization system.

[0015] Compared with the prior art, the beneficial effects of the present invention are: 1. Before the recrystallization process, the present invention pre-treats the crude 2,4-dichlorobenzonitrile by distillation and column chromatography, and utilizes the synergistic effect of the different impurity removal principles of the two to reduce the introduction of impurities before recrystallization; then, through the synergistic effect of the mixed solvent of isopropyl alcohol and n-heptane, the solubility of impurities in the mother liquor is increased, the precipitation of impurities during the crystallization process is reduced, and the yield and purity of 2,4-dichlorobenzonitrile are further improved.

[0016] 2. By introducing a composite complexing agent of ethylenediamine disuccinic acid and hydroxypropyl-β-cyclodextrin, the present invention utilizes the complexation of ethylenediamine disuccinic acid with metal ion impurities and the inclusion effect of hydroxypropyl-β-cyclodextrin to remove small molecule impurities, and removes impurity molecules in the product through different action mechanisms, synergistically improving the product purity and yield. At the same time, the use of spray drying can adjust the solubility of the complexing agent, avoid introducing complexing agent molecules during the crystallization process, further improve the product purity, and obtain 2,4-dichlorobenzonitrile with high purity and high yield.

[0017] 3. Through staged cooling crystallization treatment, the present invention uses rapid cooling to promote the rapid formation of crystal nuclei. During the slow cooling process, the crystal nuclei slowly grow into complete crystals, promoting the orderly growth of crystal nuclei; introducing ultrasonic-assisted crystallization process, and using the synergistic effect of staged cooling and ultrasonic assistance throughout the crystallization process to ensure that the formation of crystal nuclei and the growth of crystals are carried out under the best conditions, reducing the entrapment of impurities, and improving the purity and yield of 2,4-dichlorobenzene.

[0018] 4. Through the pretreatment of rectification and column chromatography, impurity removal by composite complexing agent, and staged cooling combined with ultrasonic-assisted crystallization process, the present invention significantly improves the purity and yield of 2,4-dichlorobenzonitrile, effectively removes metal catalyst residues and various organic by-products introduced during the synthesis process by ammoxidation, uses solvents with low toxicity and high environmental compatibility, reduces the difficulty and cost of solvent recovery, improves resource utilization rate, and reduces waste generation. Brief Description of the Drawings

[0019] Figure 1 It is the process flow chart of the recrystallization purification of 2,4-dichlorobenzonitrile of the present invention. Detailed Embodiments

[0020] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0021] For those not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0022] Please refer to Figure 1 , the present invention provides a recrystallization purification process for 2,4-dichlorobenzonitrile, and the technical solution is as follows: Example 1 The crude product of 2,4-dichlorobenzonitrile synthesized by the ammonia oxidation method is passed through a rectification column. The temperature of the rectification column is adjusted to 140 °C, the pressure is 10 kPa, the number of theoretical plates is maintained at 18, and the reflux ratio is maintained at 8:1. Low-boiling and high-boiling impurities are removed to obtain a rectified component; the rectified component is subjected to atmospheric column chromatography with silica gel of 300-400 mesh as the stationary phase, and n-heptane and ethyl acetate are used as eluents. Gradient elution is carried out according to the volume ratio of n-heptane to ethyl acetate of 10:1 - 5:1. The effluent is detected by thin-layer chromatography, and the 2,4-dichlorobenzonitrile fraction is combined and a pretreated component is obtained by rotary evaporation; Isopropanol is added to a beaker, and n-heptane is slowly added while maintaining the volume ratio of isopropanol to n-heptane at 5:1. Stir and mix to obtain a mixed solvent; the pretreated component is added to a crystallizer, the mixed solvent is added, the temperature of the crystallizer is raised to 75 °C, and the mixed solvent is slowly supplemented while maintaining the volume-mass ratio of the pretreated component to the mixed solvent at 6:1. Stir at 200 rpm to obtain a clear solution; activated carbon is added to the clear solution, and the activated carbon accounts for 1.5% of the mass of the pretreated component. Stir at 200 rpm for 45 min under heat preservation. Under hot conditions, vacuum filtration is carried out using a Buchner funnel, and diatomaceous earth is used to assist filtration to obtain a filtered component; Ethylenediamine disuccinic acid and hydroxypropyl-β-cyclodextrin are dissolved in deionized water according to a mass ratio of 3:1 and uniformly mixed and spray-dried. The inlet temperature of the spray dryer is 120 °C, and the outlet temperature is 60 °C to obtain a composite complexing agent. The composite complexing agent is added to the mixed solvent, and the mass-volume ratio of the composite complexing agent to the mixed solvent is 1:2. Stir at 200 rpm for 30 min to dissolve uniformly to obtain a complexing agent solution; the complexing agent solution is added to the filtered component, and under heat preservation conditions, stir at 180 rpm for 30 min to obtain a mixed system; the mixed system is added to a crystallizer and placed in an ultrasonic device, and the mixed system is subjected to staged cooling treatment. In the first stage, the system temperature is reduced to 40 °C at a cooling rate of 1.5 °C / min, and heat preservation treatment is carried out for 2 h to obtain a dispersion system. During this period, ultrasonic-assisted crystallization is used, maintaining a frequency of 30 kHz and a power of 120 W for ultrasonic dispersion, adopting a pulsed working mode, working for 5 s and then resting for 10 s, and the total working time is controlled within 20 min; the ultrasonic treatment of the dispersion system is stopped, and in the second stage, the system temperature is reduced to 10 °C at a cooling rate of 0.8 °C / min, and heat preservation and static settlement are carried out for 5 h to obtain a crystallization system; The crystallization system is centrifuged, and the centrifuge speed is controlled at 5000 rpm. Centrifugation is carried out for 20 min to obtain a crystalline product; the crystalline product is washed twice with deionized water at 40 °C, cooled and then washed three times with the same proportion of mixed solvent at 5 °C, and placed in a vacuum drying oven. The temperature is raised to 50 °C, and the pressure is maintained at 15 kPa. Vacuum drying is carried out for 8 h to obtain pure 2,4-dichlorobenzonitrile.

[0023] Examples 2 - 6 Refer to the preparation method and parameter conditions of Example 1, and the differences are shown in Table 1.

[0024] Table 1 Parameter Changes of Examples 1-6

[0025] Comparative Example 1 Referring to Example 1, the difference is that the crude 2,4-dichlorobenzonitrile is not subjected to rectification and column chromatography pretreatment.

[0026] Comparative Example 2 Referring to Example 1, the difference is that the crude 2,4-dichlorobenzonitrile is only subjected to rectification treatment.

[0027] Comparative Example 3 Referring to Example 1, the difference is that the crude 2,4-dichlorobenzonitrile is only subjected to column chromatography pretreatment.

[0028] Comparative Example 4 Referring to Example 1, the difference is that an equal amount of isopropanol single solvent is used as the recrystallization solvent.

[0029] Comparative Example 5 Referring to Example 1, the difference is that an equal amount of n-heptane single solvent is used as the recrystallization solvent.

[0030] Comparative Example 6 Referring to Example 1, the difference is that a mixed solvent with a volume ratio of ethanol to water of 3:1 is used, and the others are the same.

[0031] Experimental Example 1 Purity and Yield Test The purity of the pure 2,4-dichlorobenzonitrile prepared in Examples 1-6 and Comparative Examples 1-6 was determined by high performance liquid chromatography, and the yield was calculated by the ratio of the pure 2,4-dichlorobenzonitrile to the crude 2,4-dichlorobenzonitrile. The test results are shown in Table 2.

[0032] Table 2 Yield and Purity Tests of Examples 1-6 and Comparative Examples 1-6

[0033] It can be seen from the results in Table 2 that in Comparative Examples 1-3, for the crude 2,4-dichlorobenzonitrile synthesized by the ammoxidation method, without the pretreatment process of distillation or column chromatography, the obtained 2,4-dichlorobenzonitrile has a significant decrease in both purity and yield compared with Examples 1-6. Since distillation can effectively remove impurities with large boiling point differences, and column chromatography can further separate impurities based on the polarity differences of substances, avoiding the presence of impurities; during the subsequent crystallization process, the presence of impurities will interfere with the normal growth of crystals, making the crystal structure disordered, enclosing more impurities, seriously reducing the purity. At the same time, the presence of impurities also affects the kinetic and thermodynamic processes of crystallization, hindering the crystallization of the target product and resulting in a decrease in yield; in addition, distillation and column chromatography are important preliminary steps for removing impurities, creating good conditions for subsequent crystallization. Distillation and column chromatography remove impurities from different principles and, through mutual cooperation, comprehensively remove impurities, effectively improving the yield and purity of 2,4-dichlorobenzonitrile; from the results of Comparative Examples 4-6, a single solvent cannot effectively control the precipitation rate and crystal growth of the target product during the crystallization process, resulting in impurities being enclosed in the crystals and reducing the purity. In addition, it does not have a good solubility difference for impurities like a mixed solvent, and cannot fully remove impurities, further affecting the product purity; isopropanol is a polar organic solvent with a certain solubility for 2,4-dichlorobenzonitrile, which can make the pretreatment components dissolve well during the heating stage to form a homogeneous solution. n-heptane is a non-polar solvent, which can adjust the polarity of the mixed solvent to make the solubility of the mixed solvent for 2,4-dichlorobenzonitrile meet the crystallization requirements; during the dissolution stage, the mixed solvent can fully dissolve the pretreatment components, avoiding partial product loss due to insufficient solubility; during the crystallization stage, as the temperature decreases, the change in the polarity of the mixed solvent promotes the orderly crystallization of 2,4-dichlorobenzonitrile, reducing impurity entrapment and improving the product purity; the solubility of the mixed solvent of ethanol and water for impurities is different from that of the original mixed solvent. Due to the high solubility of ethanol, impurities with poor solubility in the isopropanol-n-heptane mixed solvent, such as polar organic compounds in the impurities, may have better solubility in the ethanol-water mixed solvent. This makes it easier for impurities to dissolve in the solvent with 2,4-dichlorobenzonitrile during the crystallization process and cannot be effectively separated by crystallization, further increasing the product purity.

[0034] Example 7 is the same as Example 1; Examples 8-11 refer to the preparation method and parameter conditions of Example 7, with the differences shown in Table 3.

[0035] Table 3 Parameter changes in Examples 7-11

[0036] Comparative Example 7 refers to Example 7, with the difference that activated carbon treatment is not used.

[0037] Comparative Example 8 Refer to Example 7, the difference is that no complexing agent is used for treatment.

[0038] Comparative Example 9 Refer to Example 7, the difference is that no activated carbon and complexing agent are used for treatment.

[0039] Comparative Example 10 Refer to Example 7, the difference is that ethylenediamine disuccinic acid is used as the complexing agent with the same dosage.

[0040] Comparative Example 11 Refer to Example 7, the difference is that hydroxypropyl-β-cyclodextrin is used as the complexing agent with the same dosage.

[0041] Comparative Example 12 Refer to Example 7, the difference is that hydroxypropyl-β-cyclodextrin is replaced with an equal amount of β-cyclodextrin.

[0042] Comparative Example 13 Refer to Example 7, the difference is that the complexing agent is not spray-dried.

[0043] Experimental Example 2 Purity and Yield Tests The purity of the pure 2,4-dichlorobenzonitrile prepared in Examples 7 - 11 and Comparative Examples 7 - 13 was determined by high performance liquid chromatography, and the yield was calculated from the ratio of the pure 2,4-dichlorobenzonitrile and the crude 2,4-dichlorobenzonitrile. The test results are shown in Table 4.

[0044] Table 4 Test Results of Examples 7 - 11 and Comparative Examples 7 - 13

[0045] From the results in Table 4 and Comparative Examples 7 - 9, it can be seen that activated carbon can effectively adsorb impurities such as pigments and colloids in the solution. Without using activated carbon for treatment, impurities in the solution cannot be effectively adsorbed. During the crystallization process, impurities will adhere to the crystal surface or be wrapped inside the crystal, reducing the product purity. However, the main function of activated carbon is to adsorb impurities, and it has little impact on the crystallization process of the target product itself. Therefore, the yield decreases slightly. Chelating agents can complex or inclusion - complex with specific impurities to remove the influence of these impurities on the crystallization behavior. Combining the results of Comparative Examples 10 - 11, ethylenediamine disuccinic acid mainly complexes with metal catalyst ionic impurities and has limited ability to remove some small - molecule impurities. During the crystallization process, the unremoved small - molecule impurities will mix into the crystal, reducing the product purity. Hydroxypropyl - β - cyclodextrin mainly removes small - molecule impurities through inclusion - complexation and has poor removal effect on impurities such as metal ions. Without using a chelating agent for treatment, these impurities cannot be effectively removed and will exist in large quantities in the solution. During the crystallization process, the impurities will interfere with the normal growth of the crystal, causing defects in the crystal structure and enclosing more impurities, resulting in a significant decrease in purity. In addition, the presence of impurities also affects the thermodynamic and kinetic processes of crystallization, hindering the crystallization and precipitation of the target product, thereby reducing the yield. From the perspective of process synergy, the treatment with chelating agents cooperates with other process steps to jointly improve the product purity and yield, and utilize their synergistic effect to reduce the influence of residual metal catalysts and organic small - molecule impurities on the crystallization behavior. In Comparative Example 12, the structure of β - cyclodextrin is different from that of hydroxypropyl - β - cyclodextrin, and there are differences in its inclusion - complexation ability, solubility, and selectivity for impurities. Due to the introduction of hydroxypropyl groups, the solubility and inclusion - complexation performance of hydroxypropyl - β - cyclodextrin are improved, which can more effectively remove impurities and further enhance the purity and yield of the product. From the results of Comparative Example 13, after spray - drying, the chelating agent forms fine dry particles with an increased specific surface area, which can be quickly dispersed in the mixed solvent, easily wetted and dissolved, making the chelating agent more evenly distributed in the mixed solvent, providing good conditions for subsequent impurity removal. During crystallization, 2,4 - dichlorobenzonitrile crystals preferentially adsorb and bind to target molecules, and the fine chelating agent particles are not easily involved in crystal growth and are more easily separated out in the mother liquor, thus reducing the residue in the product and improving the product purity.

[0046] Example 12 is the same as Example 1; Examples 13 - 16 refer to the preparation method and parameter conditions of Example 12, with the differences shown in Table 5.

[0047] Table 5 Parameter Changes in Examples 12 - 16

[0048] Comparative Example 14 refers to Example 12, with the difference that the step - by - step cooling is not used, and the cooling rate in the first stage is maintained, and the heat - preservation time is 7 h.

[0049] Comparative Example 15 Refer to Example 12, the difference is that staged cooling is not used, the cooling rate in the second stage is maintained, and the heat preservation time is 7 h.

[0050] Comparative Example 16 Refer to Example 12, the difference is that ultrasonic-assisted crystallization is not used.

[0051] Comparative Example 17 Refer to Example 12, the difference is that ultrasonic assistance is applied to the second-stage cooling process.

[0052] Comparative Example 18 Refer to Example 12, the difference is that ultrasonic assistance is applied to the cooling process of each stage, and the parameter conditions are the same.

[0053] Comparative Example 19 Refer to Example 12, the difference is that a continuous ultrasonic mode is adopted, and the ultrasonic time is 20 min.

[0054] Comparative Example 20 Refer to Example 12, the difference is that the ultrasonic power is 300 W.

[0055] Experimental Example 3 Purity and Yield Tests The purity of the pure 2,4-dichlorobenzonitrile prepared in Examples 12-16 and Comparative Examples 14-20 was determined by high performance liquid chromatography, and the yield was calculated by the ratio of the pure 2,4-dichlorobenzonitrile and the crude 2,4-dichlorobenzonitrile. The test results are shown in Table 6.

[0056] Table 6 Test Results of Examples 12-16 and Comparative Examples 14-20

[0057] The results in Table 6 show that in Comparative Examples 14-20, the change in the crystallization process has a significant effect on the purity and yield of 2,4-dichlorobenzonitrile obtained by recrystallization; the results in Comparative Examples 14-15 show that without adopting staged cooling, a fixed cooling rate is used. Too fast a cooling rate will cause excessive and too fast crystal nuclei to form. A large number of crystal nuclei grow at the same time, which will compete with each other for solutes in the solution, resulting in incomplete crystal growth, easy inclusion of impurities, and reduced purity. In addition, excessive crystal nuclei formation consumes too much solute, which limits subsequent crystal growth and affects the yield; too slow a cooling rate will cause the early crystals to grow too slowly. Insufficient nucleus formation gives impurities in the solution more opportunities to adhere to the crystal surface or enter the crystal interior, reducing purity. At the same time, the slow cooling process causes the solute to be in an oversaturated state in the solution for a long time, which is prone to solute aggregation or precipitation, rather than orderly crystallization, affecting the product yield. As can be seen from the structure of Comparative Example 16, the mechanical vibration generated by ultrasound can promote the diffusion of solutes in the solution, making the solutes more evenly distributed in the solution, which is conducive to the formation and growth of crystal nuclei. Combined with Comparative Example 17, ultrasound assistance is applied to the second stage cooling process. The second stage is mainly the crystal growth stage. At this time, the mechanical vibration of ultrasound will interfere with the normal growth of the crystal, destroy the lattice structure of the crystal, cause defects in the crystal, wrap more impurities, and reduce purity. In addition, the energy input of ultrasound will change the kinetic process of crystal growth, resulting in unstable crystal growth rate, thereby reducing the yield. In Comparative Example 18, ultrasound helps promote crystal nucleus formation in the crystal nucleus formation stage, but the crystal needs a relatively stable environment to form a regular lattice structure during the growth process. Continuous ultrasound will destroy this stability, resulting in an increase in crystal defects. In addition, excessive ultrasonic energy input will make the solute in the solution too active. The results of comparative example 19 show that continuous ultrasonic energy input will make the energy distribution in the solution uneven, produce strong mechanical vibration, and in the crystal nucleus formation stage, it will lead to excessive and too fast crystal nucleus formation, which is not conducive to the orderly growth of the crystal, further affecting the purity and yield; in comparative example 20, similarly, too high ultrasonic power will produce stronger mechanical vibration and cavitation effect, resulting in competition during the crystal growth process, affecting the integrity and purity of the crystal, thereby improving the purity and yield of 2,4-dichlorobenzene.

[0058] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A recrystallization purification process for 2,4-dichlorobenzonitrile, characterized in that: The purification process includes the following steps: S1: The crude 2,4-dichlorobenzonitrile is subjected to distillation and column chromatography to obtain a pretreated component; S2: The pretreated component is added to a mixed solvent and stirred to dissolve; it is treated with activated carbon to obtain a filtered component; S3: A complexing agent solution is obtained by dissolving a composite complexing agent in the mixed solvent; the complexing agent solution is added to the filtered component and stirred and mixed; then, through staged cooling treatment and ultrasonic-assisted crystallization, a crystallization system is obtained; S4: The crystallization system is separated by centrifugation, washed and dried to obtain pure 2,4-dichlorobenzonitrile; Among them, the mixed solvent is obtained by mixing isopropyl alcohol and n-heptane at a volume ratio of 3-6:1; The composite complexing agent is prepared by spray drying after mixing ethylenediamine disuccinic acid and hydroxypropyl-β-cyclodextrin at a mass ratio of 2-5:1; 2. The recrystallization purification process of 2,4-dichlorobenzonitrile according to claim 1, wherein: The preparation of the pretreated component in S1 includes the following process: The crude 2,4-dichlorobenzonitrile is passed through a distillation column, the temperature of the distillation column is adjusted to 120-150 °C, the pressure is 8-10 kPa, the number of theoretical plates is maintained at 18, and the reflux ratio is maintained at 8:1 to obtain a distilled component; the distilled component is subjected to atmospheric column chromatography with silica gel as the stationary phase, and gradient elution is carried out according to the volume ratio of n-heptane to ethyl acetate of 10:1-5:1, and the 2,4-dichlorobenzonitrile fraction is combined and concentrated under reduced pressure to obtain the pretreated component.

3. The recrystallization purification process of 2,4-dichlorobenzonitrile according to claim 1, characterized in that: The preparation of the filtered component in S2 includes the following process: The isopropyl alcohol is added to a beaker, and the n-heptane is slowly added and stirred and mixed to obtain a mixed solvent; the pretreated component is added to a crystallizer, the mixed solvent is added, and the temperature of the crystallizer is raised to 70-80 °C and stirred at 150-200 rpm to dissolve to obtain a clear solution; activated carbon is added to the clear solution, stirred, and filtered under reduced pressure using a Buchner funnel while hot, and diatomaceous earth is used for auxiliary filtration to obtain the filtered component; Among them, the volume-mass ratio of the pretreated component to the mixed solvent is 4-8:

1.

4. The recrystallization purification process of 2,4-dichlorobenzonitrile according to claim 3, characterized in that: The prepared activated carbon accounts for 0.8%-1.5% of the mass of the pretreated component; the rotation speed of the stirring treatment is 150-250 rpm; the time of the stirring treatment is 30-60 min.

5. The recrystallization purification process of 2,4-dichlorobenzonitrile according to claim 1, characterized in that: The preparation of the crystallization system in S3 includes the following process: The complexing agent solution is added to the filtered component, and under the condition of heat preservation and stirring at 150-200 rpm for 20-30 min, a mixed system is obtained; the mixed system is added to a crystallizer and placed in an ultrasonic device, and through staged cooling treatment, the crystallization system is obtained.

6. The recrystallization purification process of 2,4-dichlorobenzonitrile according to claim 5, characterized in that: The preparation of the complexing agent solution includes the following process: Ethylenediamine disuccinic acid and hydroxypropyl-β-cyclodextrin are dissolved in deionized water, uniformly mixed, and spray dried, where the inlet temperature of the spray drying is 120 °C and the outlet temperature is 60 °C to obtain a composite complexing agent; the composite complexing agent is added to the mixed solvent, and the mass-volume ratio of the composite complexing agent to the mixed solvent is 1:2, and stirred at 200 rpm for 30 min to dissolve uniformly to obtain the complexing agent solution.

7. A recrystallization purification process for 2,4-dichlorobenzonitrile according to claim 5, characterized in that: The preparation of the staged cooling treatment includes the following process: Add the said mixed system into a crystallizer and place it in an ultrasonic device. In the first stage, lower the temperature of the system to 40 - 45 °C at a cooling rate of 1 - 1.5 °C / min, and keep it warm for 1 - 3 h to obtain a dispersion system. During this period, use ultrasonic-assisted crystallization, maintain a frequency of 30 kHz and a power of 120 W for ultrasonic dispersion, adopt a pulsed working mode, work for 5 seconds and then rest for 10 seconds, and control the total working time within 20 min. Stop the ultrasonic treatment of the said dispersion system. In the second stage, lower the temperature of the system to 5 - 10 °C at a cooling rate of 0.5 - 1 °C / min, and keep it warm and static for 4 - 6 h to obtain a crystallization system.

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