Method for purifying 2-phenylimidazoline
By combining washing and recrystallization, impurities in 2-phenylimidazoline are effectively removed, solving the problem of poor purification effect in the prior art, achieving the production of high-purity and high-yield products, and reducing operating costs and wastewater emissions.
Patent Information
- Application Number
- CN202510799994.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-06-16
- Publication Date
- 2025-09-09
AI Technical Summary
Existing technologies make it difficult to effectively remove impurities from 2-phenylimidazoline, especially benzoylethylenediamine, resulting in unqualified product purity and color quality. Common purification methods such as activated carbon adsorption and sublimation crystallization have problems such as high operational difficulty, high cost, or the introduction of impurities.
A method combining washing and recrystallization is adopted. By selecting suitable solvents such as ethanol, methanol, water, etc., combined with appropriate temperature and stirring rate, multiple cooling crystallizations are carried out to remove impurities and recover the mother liquor to optimize product quality.
The purity and yield of 2-phenylimidazoline were improved, the product color increased from L value 83 to 96, the melting range increased from 88-95°C to 97-101°C, and the yield increased from 85% to 98%, thereby reducing wastewater treatment costs and improving economic benefits.
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Abstract
Description
Technical Field
[0001] The present invention relates to a method for purifying 2-phenylimidazoline. Background Art
[0002] Phenylimidazoline, also known as 2-phenyl-2-imidazoline, molecular formula C9H 10 N2 can be used as a curing agent for epoxy resin powder coatings, as a pharmaceutical raw material, and in organic synthesis. Its chemical structure contains tertiary nitrogen atoms and active hydrogen atoms, which can polymerize epoxy groups at relatively low temperatures. The cured product exhibits excellent mechanical and physical properties and chemical stability. It appears as white or pale yellow crystals with a melting point of 101°C, comparable to common solid epoxy resins (such as Chinese E-12, US DOW663U, and Japanese YD-013). It also exhibits good compatibility and processability. Because it meets the requirements of epoxy resin-based powder coatings, it has been developed as a low-temperature, fast curing agent and curing accelerator for epoxy powder coatings.
[0003] The industrial synthesis method of 2-phenylimidazoline mainly adopts the synthesis method of benzonitrile and ethylenediamine as substrate at present.The step of the method is to take benzonitrile and ethylenediamine as raw materials, and thioacetamide as catalyst, stirs under vacuum state, generates 2-phenylimidazoline, and releases gaseous by-product ammonia simultaneously.The method has the advantages such as raw materials are easy to get, process maturity, low cost, and is therefore the synthetic method of current mainstream.But, in the 2-phenylimidazoline synthesized by the method, often contain impurities such as benzonitrile, ethylenediamine, benzoylethylenediamine, dimethylimidazoline and thiobenzoylethylenediamine, the source of these impurities is mainly the intermediate product of reaction process by-product and the product after 2-phenylimidazoline thermal decomposition.The existence of impurity can have a negative impact on 2-phenylimidazoline quality, especially on product color appearance quality, market acceptance, so that the 2-phenylimidazoline that should originally be white crystal presents yellow, and purity analysis is unqualified. Therefore, in order to improve the quality of this part of defective products and increase their economic value, it is necessary to purify 2-phenylimidazoline to remove impurities therein.
[0004] Currently, common purification methods include activated carbon adsorption, recrystallization, and sublimation crystallization. While activated carbon adsorption effectively adsorbs colored impurities, practice has shown that when heated, a small amount of the activated carbon dissolves in the solvent. During cooling, the dissolved activated carbon also precipitates, generating new impurities that can affect product quality. Sublimation crystallization requires precise control of temperature and condensation conditions during the sublimation process, requiring high-quality equipment and increasing operational complexity and cost. The sublimation process can also result in incomplete sublimation, low yields, and sometimes decomposition. Recrystallization can significantly improve the purity of the target compound and is particularly suitable for purifying organic compounds containing small amounts of soluble or insoluble impurities. By selecting an appropriate solvent and exploiting solubility changes caused by temperature fluctuations, this method is simple to operate, cost-effective, and effectively removes impurities, yielding a high-purity target product.
[0005] There are few studies on the purification of defective 2-phenylimidazoline products. In the paper "Discussion on the Synthesis Mechanism of 2-Phenylimidazoline", Bao Xinhao et al. from Ningbo Weikai Chemical Co., Ltd. used benzonitrile and ethylenediamine as raw materials to explore the synthesis mechanism of 2-phenylimidazoline. The paper pointed out that the substances that may affect the purity of 2-phenylimidazoline in this synthesis route are benzoylethylenediamine, thiobenzoylethylenediamine, dimethylimidazoline and raw material residues.
[0006] Under current process conditions, impurities are difficult to effectively remove during the reaction, and the resulting material easily absorbs water and decomposes. These unfavorable conditions lead to the production of a large amount of defective 2-phenylimidazoline. To dispose of these defective products, manufacturers generally choose to sell them at a low price or return them to the reaction separation vessel for refining. While selling at a low price reduces the product's original value, resulting in reduced economic benefits, refining requires high energy consumption, and neither solution solves the problem of defective product disposal.
[0007] Currently, the most common purification methods include activated carbon adsorption, recrystallization, and sublimation crystallization. A literature review revealed that there was little literature on the purification of 2-phenylimidazoline. Therefore, we attempted to use other common purification methods for organic compounds to conduct an experimental exploration of 2-phenylimidazoline purification.
[0008] Experiments have shown that although the activated carbon adsorption method can effectively adsorb colored impurities, it cannot remove the soluble colorless impurities, so that the purity of the purified 2-phenylimidazoline still does not meet the industry standard. At the same time, under heating conditions, the activated carbon will also be affected by the solvent, and a small amount of activated carbon will dissolve in the solvent. In the subsequent cooling and crystallization process, the dissolved activated carbon will also precipitate, thereby generating new impurities, causing the product color to turn gray, affecting product quality.
[0009] Experiments have shown that the sublimation crystallization method has the following problems: the difficulty in accurately controlling the temperature during the sublimation process, the stringent condensation conditions, and the high equipment requirements, which increase the difficulty and cost of industrial operation. At the same time, the sublimation process is also prone to incomplete sublimation and low yield. 2-phenylimidazoline also decomposes at high temperatures to produce impurities such as hydrogen sulfide, ammonia, and 2-methylimidazoline, which reduces the purification yield and also introduces new impurities. Summary of the Invention
[0010] The technical problem to be solved by the present invention is to provide a purification method for defective 2-phenylimidazoline, thereby reducing impurities, especially residual benzoylethylenediamine.
[0011] Therefore, the present invention proposes a purification process for 2-phenylimidazoline. The process can efficiently remove impurities such as benzoylethylenediamine in 2-phenylimidazoline by combining washing and recrystallization, thereby optimizing product quality. The process is simple to operate, has mild purification conditions, uses readily available detergents, can recycle mother liquor, and discharges a small amount of waste liquid.
[0012] The object of the present invention is to provide a method for purifying 2-phenylimidazoline, which can effectively remove impurities such as benzoylethylenediamine, and inhibit the decomposition and deterioration of 2-phenylimidazoline during the purification process, thereby improving the recovery rate. The method comprises the following steps: (1) adding crude 2-phenylimidazoline to a solvent and stirring at a temperature of 20-50°C, preferably 21-45°C, more preferably 22-40°C, 23-38°C, and further preferably 25-35°C, such as 27, 28, 29, 30, 31, 32, or 33°C, to fully dissolve impurities in the crude product. After stirring, cooling the suspension containing the solvent for crystallization, and then filtering to collect the filtrate; (2) adding deionized water to the filtrate obtained in step (1), stirring at room temperature, cooling and crystallizing, and collecting the filtrate by filtration; (3) Drying the filtered product in step (2) to obtain the final purified product.
[0013] The method of the present invention may further comprise: (4) Collect the mother liquor of step (1) and / or step (2) as a solvent for the purification of the next batch of crude 2-phenylimidazoline. Preferably, the mass ratio of crude 2-phenylimidazoline to mother liquor is 1:(1-8), more preferably 1:(3-5), stirring is performed at a temperature of 20-50°C, preferably 25-35°C, the stirring rate is preferably 150-400 rpm, more preferably 200-300 rpm, the stirring time is preferably 1-8h, more preferably 3-5h, after stirring, the suspension containing the solvent is cooled and crystallized, the cooling temperature is preferably 0-15°C, more preferably 5-10°C, the cooling time is preferably 5-40min, more preferably 10-30min, and the filtrate is collected by filtration; according to step (2), deionized water is added to the filtrate and stirred at room temperature, then cooled and crystallized and filtered, and then dried according to step (3) to obtain a pure product.
[0014] In the present application, the mother liquor of step (1) and / or step (2) refers to the filtrate obtained by filtration in step (1) and / or the filtrate obtained by filtration in step (2).
[0015] Furthermore, in step (1), the solvent is preferably one or more of ethanol, methanol, acetone, and water (preferably deionized water), and water is further preferably used as the solvent.
[0016] Furthermore, in step (1), the mass ratio of the crude 2-phenylimidazoline to the solvent is 1:(1-8), and more preferably 1:(3-5).
[0017] Furthermore, in step (1), the stirring rate is 150-400 rpm, more preferably 200-300 rpm, and the stirring time is preferably 1-8 h, more preferably 3-5 h.
[0018] Furthermore, in the cooling crystallization in step (1), the cooling temperature is 0-15°C, more preferably 5-10°C, and the cooling time is preferably 5-40 min, more preferably 10-30 min.
[0019] Furthermore, in step (2), the mass ratio of deionized water to 2-phenylimidazoline (or the filtrate) is 1:(1-6), and more preferably 1:(2-4).
[0020] Furthermore, in step (2), the stirring time at room temperature is 5-30 min, more preferably 10-20 min.
[0021] Furthermore, in step (2), the cooling temperature of the cooling crystallization is 0-15°C, more preferably 5-10°C, and the cooling time is preferably 1-20 min, more preferably 10-15 min.
[0022] Furthermore, in step (3), the filtrate in step (2) is placed in an oven for drying, the oven temperature is 50-120°C, more preferably 70-90°C, and the drying time is preferably 10-60 min, more preferably 20-40 min.
[0023] It is further preferred that a pretreatment be performed before washing and purification in step (1) so that the benzoylethylenediamine impurity is converted into 2-phenylimidazoline through a dehydration reaction. The pretreatment comprises heating the crude 2-phenylimidazoline at 100-160°C, further optimizing the heating temperature to 120-140°C, and heating for 2-5 hours, more preferably 3-4 hours. The pretreatment process is ensured to be carried out in a nitrogen environment. After the heating is completed, a dry powdered crude 2-phenylimidazoline is obtained and purified according to steps (1)-(3) or steps (1)-(4).
[0024] In the present application, the crude 2-phenylimidazoline can be obtained by using benzonitrile and ethylenediamine as raw materials, thioacetamide as a catalyst, stirring under a vacuum state to generate 2-phenylimidazoline, and releasing gaseous by-product ammonia. According to the spectrocolorimeter, the L value in the colorimetric analysis of the crude product is, for example, 83±2, further 83±1, and the melting range determined according to GB / T617-2006 is 88-95°C.
[0025] In the present application, filtration can be performed using a vacuum filter, for example, a stainless steel vacuum filter manufactured by Zhengzhou Yuhui, model ZF-5L.
[0026] Advantages of the present invention: The present invention proposes a purification process for 2-phenylimidazoline, which can efficiently remove impurities such as benzonitrile, ethylenediamine, benzoylethylenediamine, dimethylimidazoline and thiobenzoylethylenediamine in 2-phenylimidazoline by combining washing and recrystallization. To this end, the present invention optimizes the purification time, purification solvent ratio, purification temperature, cooling crystallization and mother liquor recycling methods, thereby improving the yield of 2-phenylimidazoline from 85% (the yield of comparative example 1) to 98%, and optimizing the product purity and appearance quality. The L value in the colorimetric analysis of 2-phenylimidazoline is increased from 83 of the original crude product to 96 (the L value refers to the color of black and white colors, and the color of the product refers to the color of the product). The larger the value, the whiter and purer the product). The melting range was also increased from 88-95°C to 97-101°C (the melting point range of pure 2-phenylimidazoline is 94-104°C, and the melting range of the premium grade in the product quality index is 96-101°C. The narrower the melting point range and the range within the pure grade melting range, the purer the product). After purification, the indicators reached or even exceeded those of the premium grade. By recycling the mother liquor, on the one hand, the wastewater discharge was effectively reduced, thereby lowering the treatment cost of the purified wastewater. On the other hand, the effective ingredients in the mother liquor were further reused, reducing material loss. The technical solution was rationally designed and simple to operate, facilitating the subsequent processing of 2-phenylimidazoline and conducive to industrialization. DETAILED DESCRIPTION
[0027] The embodiments of the present invention are described in more detail below through examples.
[0028] The crude 2-phenylimidazoline used in the following examples was produced by Hunan Zhongchuang Chemical Industry Co., Ltd., and its colorimetric analysis results showed an L value of 83.77, an a value of 1.51, a b value of 14.65, and a melting range of 88-96.4° C. Analysis showed that the impurities in the 2-phenylimidazoline were mainly benzoylethylenediamine, thiobenzoylethylenediamine, and dimethylimidazoline.
[0029] The colorimetric analysis was performed using the Chubby Series spectrocolorimeter from Shenzhen Sanenshi Technology Co., Ltd.
[0030] Melting range analysis is carried out in accordance with GB / T617-2006. Example 1
[0031] (1) Add 400 g of water to a 1 L flask, then add 100 g of crude 2-phenylimidazoline, control the temperature to 25 ° C, stir for 3 h, and rotate at 250 rpm. After stirring, cool and crystallize in an ice water bath at 10 ° C for 20 min, and filter.
[0032] (2) Take the filter cake and add it to a 1L flask. Add 400g of deionized water and stir at room temperature for 10min at a speed of 150rpm. After stirring, cool and crystallize in an ice water bath at 10℃ for 10min. Pour out the slurry and filter thoroughly.
[0033] (3) The filter cake was crushed and placed in an oven for drying at 70°C for 30 min to obtain purified 2-phenylimidazoline. Example 2
[0034] (1) Add 400 g of water to a 1 L flask, then add 100 g of crude 2-phenylimidazoline, control the temperature to 30 ° C, stir for 5 h, rotate at 300 rpm, cool in an ice water bath at 9 ° C for 18 min to crystallize, and filter.
[0035] (2) Take the filter cake and add it to a 1L flask. Add 300g of deionized water and stir at room temperature for 15min at a speed of 300rpm. After stirring, cool and crystallize in an ice water bath at 9℃ for 13min. Release the slurry and filter.
[0036] (3) The filter cake was crushed and placed in an oven for drying at 75°C for 40 min to obtain purified 2-phenylimidazoline. Example 3
[0037] (1) The crude 2-phenylimidazoline was pretreated by heating at 130°C for 3.5 hours. The pretreatment process was carried out in a nitrogen environment. After the heating was completed, a dry powdered crude 2-phenylimidazoline was obtained.
[0038] (2) Add 400 g of the mother liquor collected in Example 1 to a 1 L flask, then add 100 g of the pretreated crude 2-phenylimidazoline, control the temperature to 35 ° C, stir for 4 h, rotate at 200 rpm, cool in an ice-water bath at 8 ° C for 20 min to crystallize, and filter.
[0039] (3) Take the filter cake and add it to a 1L flask. Add 400g of deionized water and stir at room temperature for 20min at a speed of 200rpm. After stirring, cool and crystallize in an ice water bath at 8℃ for 15min. Pour out the slurry and filter.
[0040] (4) The filter cake was crushed and placed in an oven for drying at 80°C for 40 min to obtain purified 2-phenylimidazoline. Example 4
[0041] (4) The mother liquors of step (1) and step (2) of Example 1 were collected and used as solvents for the purification of the next batch of crude 2-phenylimidazoline, wherein the mass ratio of crude 2-phenylimidazoline to mother liquor was about 1:4, and the mixture was stirred at 30°C, at a stirring rate of 300 rpm, and for 5 h. After the stirring was completed, the suspension containing the solvent was cooled and crystallized at a cooling temperature of 9°C and a cooling time of 15 min, and the filtrate was collected by filtration; deionized water was added to the filtrate according to step (2) and stirred at room temperature, and then cooled and crystallized and filtered. After filtration, the mixture was dried according to step (3) to obtain a pure product. Example 5
[0042] (4) The mother liquors of step (1) and step (2) of Example 2 were collected and used as solvents for the purification of the next batch of crude 2-phenylimidazoline, wherein the mass ratio of crude 2-phenylimidazoline to mother liquor was about 1:4, and the mixture was stirred at 25°C, at a stirring rate of 150 rpm, and for 3 h. After the stirring was completed, the suspension containing the solvent was cooled and crystallized at a cooling temperature of 10°C and a cooling time of 20 min, and the filtrate was collected by filtration; deionized water was added to the filtrate according to step (2) and stirred at room temperature, and then cooled and crystallized and filtered. After filtration, the mixture was dried according to step (3) to obtain a pure product. Example 6
[0043] (1) Add 400 g of ethanol (anhydrous ethanol) to a 1 L flask, then add 100 g of crude 2-phenylimidazoline, control the temperature to 25 ° C, stir for 3 h, and rotate at 250 rpm. After stirring, cool and crystallize in an ice water bath at 10 ° C for 20 min, and filter.
[0044] (2) Take the filter cake and add it to a 1L flask. Add 400g of deionized water and stir at room temperature for 10min at a speed of 150rpm. After stirring, cool and crystallize in an ice water bath at 10℃ for 10min. Pour out the slurry and filter thoroughly.
[0045] (3) The filter cake was crushed and placed in an oven for drying at 70°C for 30 min to obtain purified 2-phenylimidazoline. Example 7
[0046] (1) Add 400 g of acetone (anhydrous acetone) to a 1 L flask, then add 100 g of crude 2-phenylimidazoline, control the temperature to 25 ° C, stir for 3 h, and rotate at 250 rpm. After stirring, cool and crystallize in an ice water bath at 10 ° C for 20 min, and filter.
[0047] (2) Take the filter cake and add it to a 1L flask. Add 400g of deionized water and stir at room temperature for 10min at a speed of 150rpm. After stirring, cool and crystallize in an ice water bath at 10℃ for 10min. Pour out the slurry and filter thoroughly.
[0048] (3) The filter cake was crushed and placed in an oven for drying at 70°C for 30 min to obtain purified 2-phenylimidazoline. Comparative Example 1
[0049] Add 400g of 1,2-dichloroethane to a 1L flask, followed by 100g of crude 2-phenylimidazoline. Heat to 100°C and stir at 250 rpm for 3 hours. After stirring, cool at room temperature for 30 minutes and filter. Add the filter cake to a 1L flask and add 400g of 1,2-dichloroethane. Stir at room temperature for 3 minutes at 250 rpm. Let stand at room temperature for 20 minutes, then pour out the slurry and filter. Crush the filter cake and place it in an oven. Dry at 100°C for 1 hour to obtain purified 2-phenylimidazoline.
[0050] The present invention conducted performance tests on the purified 2-phenylimidazoline prepared in the above Examples 1-3 and Comparative Example 1, and the test method is as follows.
[0051] 1. Chromaticity Analysis The dried sample was placed in a grinder for grinding. The ground 2-phenylimidazoline was added to a cuvette with a spoon, and then a colorimeter (a spectrophotometer of the Fat Girl series produced by Shenzhen Sanenshi Technology Co., Ltd.) was used for colorimetric analysis.
[0052] 2. Melting Range Analysis Take an appropriate amount of purified 2-phenylimidazoline and add it to the capillary. Tap it up and down so that the sample can tightly fill the capillary. Then use a melting range analyzer to perform melting range analysis on the purified 2-phenylimidazoline.
[0053] The performance test results of the purified 2-phenylimidazoline prepared in the present invention for the above Examples 1-3 and Comparative Example 1 are shown in Table 1.
[0054] Table 1, 2-phenylimidazoline performance test
[0055] L value: refers to black and white colors. The larger the value, the higher the brightness. a value: a value is represented by red and green. A positive a value is red, and a negative a value is green. b value: b value represents yellow and blue. A positive b value represents yellow, and a negative b value represents blue.
[0056] The recovery rates in the table are obtained based on the mass of solid produced / mass of solid added*100%.
[0057] The analysis results of Examples 1-3 and Comparative Example 1 show that water is the optimal solvent as a purification solvent, and methods such as mother liquor recovery and low-temperature cooling crystallization can further improve the product yield, with the highest yield reaching 98.2%. Performance tests on the purified 2-phenylimidazoline show that the product performance of the embodiments is greatly improved, reaching or even exceeding the industry's excellent product indicators, indicating that the present invention can purify crude products with poor color into high-quality white products in the industry, making them recognized by the market again and increasing product revenue.
[0058] The above-described embodiments merely represent several implementation methods of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.
Claims
1. A method for purifying 2-phenylimidazoline, characterized in that The following steps are involved: (1) Adding crude 2-phenylimidazoline to a solvent, stirring at 20-50°C, preferably 25-35°C, to fully dissolve impurities in the crude product, cooling the suspension containing the solvent for crystallization, and then filtering to collect the filtrate; (2) adding deionized water to the filtrate obtained in step (1), stirring at room temperature, cooling and crystallizing, and collecting the filtrate by filtration; (3) Drying the filtered product in step (2) to obtain the final purified product.
2. The purification process of 2-phenylimidazoline according to claim 1, wherein Further including: (4) Collecting the mother liquor of step (1) and / or step (2) as a solvent for purification of the next batch of crude 2-phenylimidazoline.
3. The purification process of 2-phenylimidazoline according to claim 2, wherein In step (4), the mass ratio of the crude 2-phenylimidazoline to the mother liquor is 1:(1-8), more preferably 1:(3-5), and the mixture is stirred at a temperature of 20-50°C, preferably 25-35°C, with a stirring rate of preferably 150-400 rpm, more preferably 200-300 rpm, and a stirring time of preferably 1-8 h, more preferably 3-5 h. After the stirring is completed, the suspension containing the solvent is cooled and crystallized, the cooling temperature is preferably 0-15°C, more preferably 5-10°C, the cooling time is preferably 5-40 min, more preferably 10-30 min, and the filtrate is collected by filtration; according to step (2), deionized water is added to the filtrate and stirred at room temperature, and then cooled and crystallized and filtered, and then dried according to step (3) to obtain a pure product.
4. The purification process of 2-phenylimidazoline according to any one of claims 1 to 3, wherein In step (1), the solvent is one or more of ethanol, methanol, acetone, and water, preferably water.
5. The method for purifying 2-phenylimidazoline according to any one of claims 1 to 4, wherein In step (1), the mass ratio of the crude 2-phenylimidazoline to the solvent is 1:(1-8), more preferably 1:(3-5); and / or The crude 2-phenylimidazoline is prepared by using benzonitrile and ethylenediamine as raw materials, using thioacetamide as a catalyst, and stirring under vacuum to generate 2-phenylimidazoline while releasing ammonia as a gaseous by-product. According to spectrocolorimetry, the L value of the crude product in colorimetric analysis is 83±2, further 83±1. The melting range determined according to GB / T617-2006 is 88-95°C.
6. The method for purifying 2-phenylimidazoline according to any one of claims 1 to 5, wherein In step (1), the stirring rate is 150-400 rpm, more preferably 200-300 rpm, and the stirring time is preferably 1-8 h, more preferably 3-5 h.
7. The method for purifying 2-phenylimidazoline according to any one of claims 1 to 6, wherein The cooling temperature in the cooling crystallization in step (1) is 0-15°C, more preferably 5-10°C, and the cooling time is preferably 5-40 min, more preferably 10-30 min.
8. The method for purifying 2-phenylimidazoline according to any one of claims 1 to 7, wherein In step (2), the mass ratio of deionized water to 2-phenylimidazoline is 1:(1-6), more preferably 1:(2-4); and / or In step (2), the stirring time at room temperature is 5-30 min, more preferably 10-20 min; and / or In step (2), the cooling temperature for cooling crystallization is 0-15°C, more preferably 5-10°C, and the cooling time is preferably 1-20 min, more preferably 10-15 min.
9. The method for purifying 2-phenylimidazoline according to any one of claims 1 to 8, wherein In step (3), the filtrate in step (2) is placed in an oven for drying at a temperature of 50-120° C., more preferably 70-90° C., and a drying time of preferably 10-60 min, more preferably 20-40 min.
10. The method for purifying 2-phenylimidazoline according to any one of claims 1 to 9, wherein Before washing and purification in step (1), pretreatment is performed to convert the contained benzoylethylenediamine impurity into 2-phenylimidazoline through a dehydration reaction. The pretreatment comprises heating the crude 2-phenylimidazoline at 100-160° C., preferably, the heating temperature is 120-140° C., and the heating time is preferably 2-5 h, more preferably 3-4 h. The pretreatment process is ensured to be carried out in a nitrogen environment. After the heating is completed, a dry powdered crude 2-phenylimidazoline is obtained and purified according to the steps (1)-(3) or steps (1)-(4).