Process for the preparation of 4,4'-dichlorodiphenyl sulfone

Abstract

The application discloses a preparation method of 4,4'-dichlorodiphenyl sulfone, comprising the following steps: (1) dissolving 4,4'-dichlorodiphenyl sulfoxide with a glycol ether solvent, and then adding 30% hydrogen peroxide to perform an oxidation reaction; (2) after the oxidation reaction is completed, the temperature is lowered to 20-25 DEG C, solid is filtered out, and 4,4'-dichlorodiphenyl sulfone is obtained by drying. The glycol ether / hydrogen peroxide system is used to perform the oxidation reaction of 4,4'-dichlorodiphenyl sulfone; the glycol ether solvent has low volatility, little smell, safety and environmental protection, good solubility for reaction substrates and hydrogen peroxide; the glycol ether / hydrogen peroxide system can form a high-activity peroxide catalytic species, 4,4'-dichlorodiphenyl sulfone can be prepared in high yield and high quality under mild conditions without refining, and the method has the advantages of a single reaction system, high recovery and reuse rate, no use of acid or metal catalyst, small equipment corrosion and less waste.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for preparing a monomer of polyether sulfone or polysulfone, in particular a method for preparing 4,4'-dichlorodiphenyl sulfone, and belongs to the field of preparing 4,4'-dichlorodiphenyl sulfone. BACKGROUND

[0002] 4,4'-dichlorodiphenyl sulfone (DDS) is a monomer for preparing engineering plastics such as polyether sulfone or polysulfone, and is also an intermediate for medicines, pesticides and dyes. In particular, with the wide application of polyether sulfone and other related engineering plastics, the demand for DDS is also increasing. The synthesis of DDS is not complex, and there are mainly two routes: sulfonation route and oxidation route. The sulfonation route is obtained by two series of sulfonation of chlorobenzene with a sulfonating agent, including sulfuric acid method, chlorosulfonic acid method and sulfur trioxide sulfonation method; and the oxidation route is obtained by oxidation of sulfide or sulfoxide. Industrially, both routes have been successfully used to produce DDS, but there are many specific processes. The sulfuric acid method has low cost and simple raw materials, but the reaction time is long, the product has low melting point and poor quality, and is only suitable for synthesis of raw materials for dyes and pesticides with low requirements for intermediates. The chlorosulfonic acid method is a mature process with good product quality, but the production cost is high, the equipment is severely corroded, and the three wastes are more. The sulfur trioxide method is a complex process, including three steps of sulfur trioxide sulfonation, chlorosulfoxide chlorination and Friedel-Crafts sulfonylation, and there is no actual production in China. The oxidation route is mainly sulfoxide oxidation, which first forms sulfoxide by reacting chlorosulfoxide and chlorobenzene, and then oxidizes it to DDS, with high conversion rate, simple process, less three wastes and better product quality than the sulfonation route, and is more suitable for modern chemical production. Figure 1

[0003] The high purity of the raw material monomer is required for the high molecular polymerization reaction of the synthesis of engineering plastics such as polyether sulfone, which is generally at least 99.5% or more. Therefore, even if the DDS synthesized by the sulfoxide oxidation method is not refined and purified, it cannot meet the requirements of the synthesis of engineering plastics. However, unlike the sulfonation route, the main impurity in the DDS product synthesized by the sulfoxide oxidation method is the residual raw material sulfoxide, which can be recycled and recycled. The main reason for incomplete oxidation of the raw material sulfoxide is that when the oxidizing agent such as hydrogen peroxide is added dropwise during the reaction, the product will continuously crystallize and precipitate, and part of the 4,4'-dichlorodiphenyl sulfoxide raw material will be wrapped in the crystalline interior in the form of inclusions or cocrystals, resulting in incomplete oxidation reaction. Since the boiling points of both are very high, the DDS purification cannot be carried out by distillation, and the traditional recrystallization refining method also cannot effectively remove the unoxidized 4,4'-dichlorodiphenyl sulfoxide residue due to the existence of cocrystals, and the product quality is difficult to guarantee, and the product yield is also reduced. The raw material 4,4'-dichlorodiphenyl sulfoxide for preparing DDS by the sulfoxide oxidation route can be conveniently obtained by Friedel-Crafts reaction of dichlorosulfoxide and chlorobenzene under the catalysis of aluminum chloride, so the key to the quality control of DDS lies in the sulfoxide oxidation unit. ​

[0004] As mentioned above, the main impurity in DDS products synthesized by the sulfoxide oxidation method is a small amount of sulfoxide from the raw material, which is trapped in the product through encapsulation or eutectic forms. This impurity is difficult to completely oxidize and remove. Therefore, developing a reaction system, especially a solvent system, to promote the complete oxidation of sulfoxide is crucial for improving the quality of DDS products. Chinese patents CN108047101A, CN102351758A, CN104402780A, and CN104557626A disclose a process for synthesizing DDS by the sulfoxide oxidation method. After preparing 4,4'-dichlorodiphenyl sulfoxide via Friedel-Crafts acylation, hydrogen peroxide is used as the initial oxidant in the presence of acetic acid to form peracetic acid, which has a stronger oxidizing ability, to oxidize 4,4'-dichlorodiphenyl sulfoxide and obtain the DDS product. In Chinese patents CN102351756A and CN102351757A, dichloromethane or dichloropropane is further used as a solvent to reduce the occurrence of DDS encapsulation or eutectic formation of the raw material during the oxidation of 4,4-dichlorodiphenyl sulfoxide. Chinese patent CN104557626A discloses a method of redissolving and oxidizing crude DDS to achieve complete reaction of the raw material. BASF, in patent CN114286815A (WO2021 / 037680), discloses a method for preparing DDS by oxidizing the corresponding sulfoxide in the presence of at least one peroxide, such as heptanoic acid, using alkyl carboxylic acids as solvents in the presence of 70% hydrogen peroxide, followed by slow, reduced-pressure, and cooled crystallization to obtain a high-purity product; however, this process is complex.

[0005] Most of the above-mentioned methods for preparing 4,4'-dichlorodiphenyl sulfone have problems such as low purity of the 4,4'-dichlorodiphenyl sulfone product, requiring additional purification steps, or even complex crystallization processes, which urgently need to be improved. Summary of the Invention

[0006] The main objective of this invention is to provide a method for preparing 4,4'-dichlorodiphenyl sulfone, which can improve the purity of the 4,4'-dichlorodiphenyl sulfone product without increasing the number of purification units.

[0007] The above-mentioned objective of the present invention is achieved through the following technical solution:

[0008] A method for preparing 4,4'-dichlorodiphenyl sulfone includes the following steps:

[0009] (1) Dissolve 4,4'-dichlorodiphenyl sulfoxide in a glycol ether solvent and then add hydrogen peroxide for oxidation reaction; (2) After the oxidation reaction is completed, cool down to 20-25℃, filter to precipitate solid, and dry to obtain 4,4'-dichlorodiphenyl sulfoxide.

[0010] In a preferred embodiment of the present invention, the glycol ether solvent includes, but is not limited to, any one of diethylene glycol dialkyl ether, triethylene glycol dialkyl ether, ethylene glycol dialkyl ether, or propylene glycol dialkyl ether, wherein the alkyl group is preferably a saturated alkyl group with a chain length of C1-C4; more preferably, the glycol ether solvent is selected from any one or more of diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, diethylene glycol methyl ether, ethylene glycol dimethyl ether, dipropylene glycol dimethyl ether, and triethylene glycol dimethyl ether.

[0011] As a preferred embodiment of the present invention, the ratio of 4,4'-dichlorodiphenyl sulfoxide to glycol ether solvent is 1:(1.5-4) based on g:ml; preferably, the ratio of 4,4'-dichlorodiphenyl sulfoxide to glycol ether solvent is 1:2.

[0012] In a preferred embodiment of the present invention, the hydrogen peroxide is 30% hydrogen peroxide; more preferably, the ratio of 4,4'-dichlorodiphenyl sulfoxide to 30% hydrogen peroxide is 1:0.45-0.6 by mass; most preferably, the ratio of 4,4'-dichlorodiphenyl sulfoxide to 30% hydrogen peroxide is 1:0.5.

[0013] As a preferred embodiment of the present invention, the filtrate obtained in step (2) is distilled and dehydrated and then used in the next batch of reaction.

[0014] In a preferred embodiment of the present invention, the temperature of the oxidation reaction is preferably 25°C-60°C; more preferably, the temperature of the oxidation reaction is preferably 50°C.

[0015] The main impurity in the 4,4'-dichlorodiphenyl sulfone product synthesized by the existing sulfoxide oxidation method is a small amount of sulfoxide from the raw material, which is trapped in the product through encapsulation or eutectic formation. This impurity is difficult to completely oxidize and remove. This invention uses a glycol ether / hydrogen peroxide system for the oxidation reaction of 4,4'-dichlorodiphenyl sulfone. Glycol ether solvents have low volatility, minimal odor, and are safe and environmentally friendly. They also exhibit good solubility for the reaction substrate and hydrogen peroxide. The glycol ether / hydrogen peroxide system used forms a highly active peroxide catalyst species, enabling the high-yield and high-quality preparation of 4,4'-dichlorodiphenyl sulfone under mild conditions without the need for purification. This method offers advantages such as a simple reaction system, high recovery and reuse rate, no use of acid or metal catalysts, minimal equipment corrosion, and reduced waste. Attached Figure Description

[0016] Figure 1 This is a flowchart of the existing synthetic process for 4,4'-dichlorodiphenyl sulfone.

[0017] Figure 2The HPLC purity analysis results are for the 4,4'-dichlorodiphenyl sulfone prepared in the embodiments of the present invention.

[0018] Figure 3 The results are HPLC purity analysis of 4,4'-dichlorodiphenyl sulfone prepared in Comparative Test Example 1 of this invention. Detailed Implementation

[0019] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer as a result. However, these embodiments are merely exemplary and do not constitute any limitation on the scope of the present invention. Those skilled in the art should understand that modifications or substitutions to the details and form of the present invention can be made without departing from the spirit and scope of the invention, but all such modifications and substitutions fall within the protection scope of the present invention.

[0020] Example 1: Preparation of 4,4'-dichlorodiphenyl sulfone

[0021] 100 g of 4,4'-dichlorodiphenyl sulfoxide and 200 mL of diethylene glycol dimethyl ether were added sequentially to a 500 mL three-necked flask equipped with a thermometer, mechanical stirrer, and constant pressure dropping funnel. The mixture was heated to 60 °C until completely dissolved. Then, 50 g of 30% hydrogen peroxide was slowly added dropwise through the dropping funnel. The mixture was kept at the same temperature and stirred. The reaction was monitored by chromatography. The reaction was completed in 4 hours. Heating was stopped, and the mixture was stirred and cooled to room temperature (25 °C). The solid was filtered out, and the mother liquor was recycled for the next batch. The filter cake was washed with methanol and dried to obtain 99.5 g of 4,4'-dichlorodiphenyl sulfoxide, with a yield of 94% and a chromatographic purity of 99.7%.

[0022] ¹H NMR (400MHz) δ (ppm): 7.88 (dt, J1 = 8Hz, J2 = 4Hz), 7.50 (dt, J1 = 8Hz, J2 = 4Hz). Purity analysis of the product was performed using HPLC, and the results are shown below. Figure 2 .

[0023] Example 2 Preparation of 4,4'-dichlorodiphenyl sulfone

[0024] To a 500 mL three-necked flask equipped with a thermometer, mechanical stirrer, and constant-pressure dropping funnel, 100 g of 4,4'-dichlorodiphenyl sulfoxide and 200 mL of diethylene glycol dimethyl ether were added sequentially. The mixture was heated to 60 °C until completely dissolved. Then, 45 g of 30% hydrogen peroxide was slowly added dropwise through the dropping funnel. The reaction was maintained at the specified temperature with stirring, and the reaction progress was monitored chromatographically. The reaction was completed in 4 hours. Heating was stopped, and the mixture was allowed to cool to room temperature (25 °C) with stirring. The solid was filtered off, and the mother liquor was recycled for the next batch. The filter cake was washed with methanol and dried to obtain 96.3 g of 4,4'-dichlorodiphenyl sulfoxide, with a yield of 91% and a chromatographic purity of 98.7%.

[0025] The structural identification parameters of the product are the same as those in Example 1.

[0026] Example 3 Preparation of 4,4'-dichlorodiphenyl sulfone

[0027] To a 500 mL three-necked flask equipped with a thermometer, mechanical stirrer, and constant-pressure dropping funnel, 100 g of 4,4'-dichlorodiphenyl sulfoxide and 200 mL of diethylene glycol dimethyl ether were added sequentially. The mixture was heated to 60 °C until completely dissolved. Then, 60 g of 30% hydrogen peroxide was slowly added dropwise through the dropping funnel while maintaining the temperature and stirring. The reaction was monitored chromatographically. The reaction was completed in 4 hours. Heating was stopped, and the mixture was allowed to cool to room temperature (25 °C) with stirring. The solid was filtered off, and the mother liquor was recycled for the next batch. The filter cake was washed with methanol and dried to obtain 99.3 g of 4,4'-dichlorodiphenyl sulfoxide, with a yield of 94% and a chromatographic purity of 99.4%.

[0028] The structural identification parameters of the product are the same as those in Example 1.

[0029] Example 4: Preparation of 4,4'-dichlorodiphenyl sulfone

[0030] To a 500 mL three-necked flask equipped with a thermometer, mechanical stirrer, and constant-pressure dropping funnel, 100 g of 4,4'-dichlorodiphenyl sulfoxide and 200 mL of diethylene glycol dimethyl ether were added sequentially. The mixture was heated to 50 °C until completely dissolved. Then, 60 g of 30% hydrogen peroxide was slowly added dropwise through the dropping funnel. The reaction was maintained at the specified temperature with stirring, and the reaction progress was monitored chromatographically. The reaction was completed in 5 hours. Heating was stopped, and the mixture was allowed to cool to room temperature (25 °C) with stirring. The solid was filtered off, and the mother liquor was recycled for the next batch. The filter cake was washed with methanol and dried to obtain 99.2 g of 4,4'-dichlorodiphenyl sulfoxide, with a yield of 94% and a chromatographic purity of 99.5%.

[0031] The structural identification parameters of the product are the same as those in Example 1.

[0032] Example 5 Preparation of 4,4'-dichlorodiphenyl sulfone

[0033] To a 500 mL three-necked flask equipped with a thermometer, mechanical stirrer, and constant-pressure dropping funnel, 100 g of 4,4'-dichlorodiphenyl sulfoxide and 200 mL of diethylene glycol dimethyl ether were added sequentially. The mixture was heated to 25 °C until completely dissolved. Then, 60 g of 30% hydrogen peroxide was slowly added dropwise through the dropping funnel. The reaction was maintained at the specified temperature with stirring, and the reaction progress was monitored chromatographically. The reaction was completed in 12 hours. Heating was stopped, and the mixture was allowed to cool to room temperature (25 °C) with stirring. The solid was filtered off, and the mother liquor was recycled for the next batch. The filter cake was washed with methanol and dried to obtain 72.6 g of 4,4'-dichlorodiphenyl sulfoxide, with a yield of 69% and a chromatographic purity of 98.4%.

[0034] The structural identification parameters of the product are the same as those in Example 1.

[0035] Example 6 Preparation of 4,4'-dichlorodiphenyl sulfone

[0036] To a 500 mL three-necked flask equipped with a thermometer, mechanical stirrer, and constant-pressure dropping funnel, 100 g of 4,4'-dichlorodiphenyl sulfoxide and 200 mL of diethylene glycol diethyl ether were added sequentially. The mixture was heated to 50 °C until completely dissolved. Then, 60 g of 30% hydrogen peroxide was slowly added dropwise through the dropping funnel. The reaction was maintained at the specified temperature with stirring, and the reaction progress was monitored chromatographically. The reaction was completed in 5 hours. Heating was stopped, and the mixture was allowed to cool to room temperature (25 °C) with stirring. The solid was filtered off, and the mother liquor was recycled for the next batch. The filter cake was washed with methanol and dried to obtain 103.2 g of 4,4'-dichlorodiphenyl sulfoxide, with a yield of 97.5% and a chromatographic purity of 99.5%.

[0037] For the structural identification of the product, please refer to Example 1.

[0038] Example 7 Preparation of 4,4'-dichlorodiphenyl sulfone

[0039] To a 500 mL three-necked flask equipped with a thermometer, mechanical stirrer, and constant-pressure dropping funnel, 100 g of 4,4'-dichlorodiphenyl sulfoxide and 200 mL of diethylene glycol dibutyl ether were added sequentially. The mixture was heated to 50 °C until completely dissolved. Then, 60 g of 30% hydrogen peroxide was slowly added dropwise through the dropping funnel. The reaction was maintained at the specified temperature with stirring, and the reaction progress was monitored chromatographically. The reaction was completed in 5 hours. Heating was stopped, and the mixture was allowed to cool to room temperature (25 °C) with stirring. The solid was filtered off, and the mother liquor was recycled for the next batch. The filter cake was washed with methanol and dried to obtain 100.1 g of 4,4'-dichlorodiphenyl sulfoxide, with a yield of 94.5% and a chromatographic purity of 99.8%.

[0040] The structural identification parameters of the product are the same as those in Example 1.

[0041] Example 8 Preparation of 4,4'-dichlorodiphenyl sulfone

[0042] To a 500 mL three-necked flask equipped with a thermometer, mechanical stirrer, and constant-pressure dropping funnel, 100 g of 4,4'-dichlorodiphenyl sulfoxide and 200 mL of diethylene glycol methyl ether were added sequentially. The mixture was heated to 50 °C until completely dissolved. Then, 60 g of 30% hydrogen peroxide was slowly added dropwise through the dropping funnel. The reaction was maintained at the specified temperature with stirring, and the reaction progress was monitored chromatographically. The reaction was completed in 12 hours. Heating was stopped, and the mixture was allowed to cool to room temperature (25 °C) with stirring. The solid was filtered off, and the mother liquor was recycled for the next batch. The filter cake was washed with methanol and dried to obtain 95.7 g of 4,4'-dichlorodiphenyl sulfoxide, with a yield of 90% and a chromatographic purity of 97.3%.

[0043] The structural identification parameters of the product are the same as those in Example 1.

[0044] Example 9 Preparation of 4,4'-dichlorodiphenyl sulfone

[0045] To a 500 mL three-necked flask equipped with a thermometer, mechanical stirrer, and constant-pressure dropping funnel, 100 g of 4,4'-dichlorodiphenyl sulfoxide and 200 mL of ethylene glycol dimethyl ether were added sequentially. The mixture was heated to 50 °C until completely dissolved. Then, 60 g of 30% hydrogen peroxide was slowly added dropwise through the dropping funnel. The reaction was maintained at the specified temperature with stirring, and the reaction progress was monitored chromatographically. The reaction was completed in 6 hours. Heating was stopped, and the mixture was allowed to cool to room temperature (25 °C) with stirring. The solid was filtered off, and the mother liquor was recycled for the next batch. The filter cake was washed with methanol and dried to obtain 89.5 g of 4,4'-dichlorodiphenyl sulfoxide, with a yield of 84.5% and a chromatographic purity of 99.5%.

[0046] The structural identification parameters of the product are the same as those in Example 1.

[0047] Example 10: Preparation of 4,4'-dichlorodiphenyl sulfone

[0048] To a 500 mL three-necked flask equipped with a thermometer, mechanical stirrer, and constant-pressure dropping funnel, 100 g of 4,4'-dichlorodiphenyl sulfoxide and 200 mL of ethylene glycol dibutyl ether were added sequentially. The mixture was heated to 50 °C until completely dissolved. Then, 60 g of 30% hydrogen peroxide was slowly added dropwise through the dropping funnel. The reaction was maintained at the specified temperature with stirring, and the reaction progress was monitored chromatographically. The reaction was completed in 6 hours. Heating was stopped, and the mixture was allowed to cool to room temperature (25 °C) with stirring. The solid was filtered off, and the mother liquor was recycled for the next batch. The filter cake was washed with methanol and dried to obtain 94.1 g of 4,4'-dichlorodiphenyl sulfoxide, with a yield of 89% and a chromatographic purity of 99.6%.

[0049] The structural identification parameters of the product are the same as those in Example 1.

[0050] Example 11 Preparation of 4,4'-dichlorodiphenyl sulfone

[0051] To a 500 mL three-necked flask equipped with a thermometer, mechanical stirrer, and constant-pressure dropping funnel, 100 g of 4,4'-dichlorodiphenyl sulfoxide and 200 mL of dipropylene glycol dimethyl ether were added sequentially. The mixture was heated to 50 °C until completely dissolved. Then, 60 g of 30% hydrogen peroxide was slowly added dropwise through the dropping funnel. The reaction was maintained at the specified temperature with stirring, and the reaction progress was monitored chromatographically. The reaction was completed in 5 hours. Heating was stopped, and the mixture was allowed to cool to room temperature (25 °C) with stirring. The solid was filtered off, and the mother liquor was recycled for the next batch. The filter cake was washed with methanol and dried to obtain 102.0 g of 4,4'-dichlorodiphenyl sulfoxide, with a yield of 96% and a chromatographic purity of 99.6%.

[0052] The structural identification parameters of the product are the same as those in Example 1.

[0053] Example 12 Preparation of 4,4'-dichlorodiphenyl sulfone

[0054] To a 500 mL three-necked flask equipped with a thermometer, mechanical stirrer, and constant-pressure dropping funnel, 100 g of 4,4'-dichlorodiphenyl sulfoxide and 200 mL of triethylene glycol dimethyl ether were added sequentially. The mixture was heated to 50 °C until completely dissolved. Then, 60 g of 30% hydrogen peroxide was slowly added dropwise through the dropping funnel. The reaction was maintained at the specified temperature with stirring, and the reaction progress was monitored chromatographically. The reaction was completed in 5 hours. Heating was stopped, and the mixture was allowed to cool to room temperature (25 °C) with stirring. The solid was filtered off, and the mother liquor was recycled for the next batch. The filter cake was washed with methanol and dried to obtain 101.7 g of 4,4'-dichlorodiphenyl sulfoxide, with a yield of 96% and a chromatographic purity of 99.4%.

[0055] The structural identification parameters of the product are the same as those in Example 1.

[0056] Comparative Test Example 1

[0057] To a 500 mL three-necked flask equipped with a thermometer, mechanical stirrer, and constant-pressure dropping funnel, 100 g of 4,4'-dichlorodiphenyl sulfoxide and 200 mL of glacial acetic acid were added sequentially, and the mixture was heated to 60 °C to dissolve. Then, 50 g of 30% hydrogen peroxide was slowly added dropwise through the dropping funnel, and the reaction was maintained at the specified temperature with stirring. The reaction was monitored chromatographically, and the reaction was completed in 4 hours. Heating was stopped, and the mixture was allowed to cool to room temperature (25 °C) with stirring. 200 mL of water was added, and a solid precipitated. The solid was filtered, and the filter cake was washed with acetic acid and dried to obtain 102.3 g of crude 4,4'-dichlorodiphenyl sulfoxide, with a yield of 97% and a chromatographic purity of 81.3%. HPLC analysis of the chromatographic purity of the prepared product is shown below. Figure 3 .

Claims

1. A method for preparing 4,4'-dichlorodiphenyl sulfone, characterized in that, include: (1) Dissolve 4,4'-dichlorodiphenyl sulfoxide in a glycol ether solvent and then add hydrogen peroxide for oxidation reaction; (2) After the oxidation reaction is completed, cool down to 20-25℃, filter to precipitate solid, and dry to obtain the product; The glycol ether solvent is selected from any one of diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, diethylene glycol methyl ether, ethylene glycol dimethyl ether, ethylene glycol dibutyl ether, dipropylene glycol dimethyl ether, or triethylene glycol dimethyl ether.

2. The preparation method according to claim 1, characterized in that, In step (1), the ratio of 4,4'-dichlorodiphenyl sulfoxide to glycol ether solvent is 1:1.5-4, calculated as g:ml.

3. The preparation method according to claim 2, characterized in that, The ratio of 4,4'-dichlorodiphenyl sulfoxide to glycol ether solvent is 1:

2.

4. The preparation method according to claim 1, characterized in that, In step (1), the hydrogen peroxide is 30% hydrogen peroxide; the ratio of 4,4'-dichlorodiphenyl sulfoxide to 30% hydrogen peroxide by mass is 1:0.45-0.

6.

5. The preparation method according to claim 4, characterized in that, The ratio of 4,4'-dichlorodiphenyl sulfoxide to 30% hydrogen peroxide is 1:0.

5.

6. The preparation method according to claim 1, characterized in that, The filtrate obtained in step (2) is dehydrated by distillation and then used in the next batch of reaction.

7. The preparation method according to claim 1, characterized in that, The oxidation reaction is carried out at a temperature of 25-60℃.

8. The preparation method according to claim 7, characterized in that, The oxidation reaction was carried out at a temperature of 50°C.

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