Process for the preparation of bis-triphenylsilyl chromate

Abstract

This invention discloses a method for preparing bis(triphenylsilane) chromate. The method includes: using triphenylsilanol, triphenylchlorosilane, and potassium dichromate as raw materials, glacial acetic acid and cyclohexane as solvents, and acetic anhydride as a dehydrating agent, reacting under light-protected conditions, followed by filtration and vacuum distillation to obtain a crude bis(triphenylsilane) chromate product, which is then washed and dried to obtain the final bis(triphenylsilane) chromate. The method for preparing bis(triphenylsilane) chromate of this invention has advantages such as high atom utilization, high yield and purity, reduced Cr(VI) post-processing pressure, and low post-processing cost. The carbon content, as determined by elemental analysis, deviates from the theoretical value by as little as 3‰, and the yield can reach over 90%.

Description

Technical Field

[0001] This invention patent belongs to the field of polyolefin catalyst technology, specifically relating to a method for preparing bis(triphenyl)silane chromate. Background Technology

[0002] With the continuous development of the global economy, the application of polyethylene is constantly increasing. Catalysts are one of the key technologies in polyolefin synthesis. Currently, the most commonly used polyolefin catalysts on the market are Ziegler-Natta catalysts and chromium-based catalysts. Chromium-based catalysts, due to their simple preparation and high catalytic activity, can synthesize high-density polyethylene with a wide range of molecular weights. They are used in the manufacture of large hollow containers and have been widely used in the synthesis of polyolefins since the 1950s (MPMcDaniel, Chapter 3 - A Review of the Phillips Supported Chromium Catalyst and Its Commercial Use for Ethylene Polymerization, in: BCGates, H). (Eds.)Advances in Catalysis, Academic Press, 2010, pp. 123-606.) Chromium-based catalysts are used in many polyethylene plants, such as the widely used Unipol fluidized bed reactor process from Union Carbide Corporation (UCC). (FJKarol, in: Macromolecular Symposia, Wiley Online Library, 1995, pp. 563-575.) Bistriphenylsilane chromate is an important active component precursor for the preparation of chromium-based catalysts.

[0003] US Patent 2863891 discloses a method for synthesizing bis(triphenylsilane) chromate, which uses triphenylsilanol, chromium trioxide, and glacial acetic acid as raw materials to obtain the bis(triphenylsilane) chromate product with a yield of only 85%.

[0004] Union Carbide reported that a product of bis(triphenylsilane) chromate could be obtained by stirring at room temperature for 24 hours with triphenylsilanol and 4.5 molar amounts of chromium trioxide as raw material, carbon tetrachloride as solvent, and anhydrous magnesium sulfate as dehydrating agent, with a yield of 60% (LMBaker, WLCarrick, The Journal of Organic Chemistry, 35(1970)774-776).

[0005] Czech patent CS 175856 uses triphenylsilyl alcohol and chromium trioxide as raw materials and acetonitrile and butyronitrile as solvents. The yield is still not high, ranging from 41% to 71%, and the solvents acetonitrile and butyronitrile used are highly toxic.

[0006] The above methods use essentially the same raw materials, only the solvent is changed. Water is inevitably generated in the reaction products, and the post-processing requires water washing and desalting, which leads to the hydrolysis of bis(triphenylsilane)chromate, resulting in a decrease in product purity. Although Union Carbide's method can remove the water generated in the reaction, the solid mixture after the reaction contains unreacted chromium trioxide and magnesium sulfate containing water of crystallization, which are difficult to separate, resulting in a decrease in atom utilization. At the same time, chromium trioxide is also very harmful to the environment.

[0007] Soviet patent SU 689192 proposed an alternative synthetic route for bis(triphenylsilane) chromate, using triphenylchlorosilane and potassium dichromate as raw materials in a molar ratio of 2:1.1, with glacial acetic acid as solvent, and reacting at 50°C for 1.5 hours. The yield of bis(triphenylsilane) chromate was 94.2% based on chlorosilane stoichiometry. However, this technique suffers from difficult post-processing, affecting product purity, resulting in a low melting point, low atom utilization of Cr(VI), and the inability to recover the byproduct Cr(VI).

[0008] Wang Jun et al. used triphenylchlorosilane and potassium dichromate as raw materials in a molar ratio of 1:0.6, and a mixture of glacial acetic acid and cyclohexane as solvents. The reaction was carried out at 30°C for two hours. The reaction system was filtered to remove insoluble matter, and the filtrate was evaporated under reduced pressure at 50°C. The remaining solid was then washed with 5 ml each of glacial acetic acid and cyclohexane. The final product yield was only 80%, and the chromium atom utilization rate was far below 50% (Wang Jun, Li Yun, Li Cuiqin, Zhang Huaizhi, Chemical Industry and Engineering, 29(2012)21-25). However, this technology has low chromium atom utilization, and the byproduct chromium cannot be recovered.

[0009] Patent CN 1143082 also uses triphenylchlorosilane and potassium dichromate as raw materials, employing a mixture of glacial acetic acid and hydrocarbons (n-hexane, cyclohexane, etc.) as solvents. Alkali metal sodium or potassium oxides, hydroxides, or carbonates are added to promote the conversion of triphenylchlorosilane to triphenylsilanol. This triphenylsilane can react with the generated chromium trioxide to form chromate. Heating at 50-60°C for five hours with stirring generally yields over 90% of the triphenylchlorosilane, and the product's melting point range is relatively wide (from 154 to 161°C) depending on the reaction conditions. However, the carbon content of the samples obtained in this technical embodiment is generally more than 1% lower than the theoretical value, resulting in low product purity and a complex post-processing procedure. Summary of the Invention

[0010] The technical problem this invention aims to solve is the low utilization rate of chromium atoms in the preparation of bis(triphenylsilane) chromate, which affects product purity and yield. The purpose of this invention is to provide a method for preparing bis(triphenylsilane) chromate that has advantages such as high atom utilization, high yield and purity, reduced Cr(VI) post-processing pressure, and low post-processing cost. The carbon content, as determined by elemental analysis, deviates from the theoretical value by as little as 3‰, and the yield can reach over 90%.

[0011] To achieve the above objectives, the present invention provides a method for preparing bis(triphenylsilane) chromate. The method includes: using triphenylsilanol, triphenylchlorosilane, and potassium dichromate as raw materials, glacial acetic acid and cyclohexane as solvents, and acetic anhydride as a desiccant, reacting under light-protected conditions, filtering, and vacuum distilling to obtain crude bis(triphenylsilane) chromate, and then washing and drying to obtain bis(triphenylsilane) chromate.

[0012] The preparation method of the bis(triphenyl)silane chromate of the present invention is shown in the following reaction equation (1):

[0013]

[0014] In one embodiment of the method for preparing the bis(triphenylsilane) chromate of the present invention, the molar ratio of triphenylchlorosilane to triphenylsilanol can be, for example, 1:0.5-1.5, more preferably 1:0.8-1.2; the molar ratio of triphenylchlorosilane to potassium dichromate can be, for example, 1:0.1-1.1, more preferably 1:0.4-0.8; the volume ratio of glacial acetic acid to cyclohexane can be, for example, 1:0.2-1.2, more preferably 1:0.4-0.8; and the molar ratio of acetic anhydride to triphenylchlorosilane can be, for example, 1:0.2-4.5, more preferably 1:0.5-4. However, the present invention is not limited thereto. In other embodiments, the amounts of triphenylsilanol, triphenylchlorosilane, potassium dichromate, glacial acetic acid, cyclohexane, and acetic anhydride added can be adjusted according to actual production needs.

[0015] In one embodiment of the method for preparing the bis(triphenyl)silane chromate of the present invention, the reaction temperature may be, for example, 20–100°C, and the reaction time may be, for example, 0.5–24 hours; more preferably, the reaction temperature is 40–80°C, and the reaction time is 4–8 hours; the acetic anhydride may be added, for example, during the 0 to 2.5 hours of the reaction. However, the present invention is not limited thereto, and the reaction time may be appropriately extended as the amount of reactants increases.

[0016] The method for preparing bis(triphenyl)silane chromate of the present invention does not particularly limit the filtration operation steps or conditions, and the filtration can be carried out using steps and conditions commonly used in the art.

[0017] In one embodiment of the method for preparing bis(triphenyl)silane chromate of the present invention, the vacuum distillation is carried out at 20–80°C, more preferably at 40–60°C, but the present invention is not limited thereto.

[0018] In one embodiment of the method for preparing the bis(triphenyl)silane chromate of the present invention, the washing process involves sequentially washing with water, glacial acetic acid, and cyclohexane, wherein the amounts of water and cyclohexane are 1 to 5 times the volume of glacial acetic acid, more preferably 1 to 3 times, but the present invention is not limited thereto. The present invention does not particularly limit the washing temperature or the number of washing cycles; the washing can be performed at room temperature or at other temperatures, and the number of washing cycles can be 2 times, or 1 time, 3 times, or more.

[0019] In one embodiment of the method for preparing bis(triphenyl)silane chromate of the present invention, the drying is, for example, vacuum drying at 0–100°C for 0.5–12 hours, more preferably vacuum drying at 20–80°C for 2–8 hours, but the present invention is not limited thereto.

[0020] This invention discloses a method for preparing bis(triphenylsilane) chromate. Using triphenylsilanol, triphenylchlorosilane, and potassium dichromate as raw materials, glacial acetic acid and hydrocarbon compounds as solvents, and acetic anhydride as a desiccant, an active intermediate control technique is employed. The addition of the active intermediate triphenylsilanol disrupts the original reaction equilibrium, promoting the reaction and improving reaction efficiency and product yield. The utilization rate of chromium atoms is also significantly improved, reducing the processing cost of Cr(VI). Using acetic anhydride as a desiccant allows it to react with the generated water to produce acetic acid, avoiding product hydrolysis and preventing the introduction of inorganic salts, thus improving product purity. In this preparation method, when potassium dichromate is in excess at 5-20%, the yield of the bis(triphenylsilane) chromate product can reach over 90%, with a melting point reaching a maximum of 156-157°C. Detailed Implementation

[0021] The present invention will now be described in detail through embodiments. It should be noted that the following embodiments are only for further illustration of the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above description.

[0022] Source of raw materials or equipment:

[0023]

[0024] Evaluation and analysis methods: DSC differential thermal analysis, carbon and hydrogen elemental analysis.

[0025] Example 1

[0026] 1.47 g of triphenylchlorosilane, 0.88 g of potassium dichromate, and 1.38 g of triphenylsilanol were placed in a 100 ml flask. A mixed solvent of 20 ml glacial acetic acid and 10 ml cyclohexane was then added. The mixture was stirred at 50 °C for 5 h, and 0.5 ml of acetic anhydride was added at the 0.5 h mark. After the reaction was complete, the cyclohexane was removed by concentration using a rotary evaporator at 50 °C, followed by filtration to obtain crude bis(triphenylsilane) chromate. The crude product was washed twice successively with 10 ml of water, 5 ml of glacial acetic acid, and 5 ml of cyclohexane. Finally, the solid was vacuum dried at 50 °C for 5 h, all in the dark. 3.06 g of bis(triphenylsilane) chromate was obtained, with a yield of 96.3%. Its melting point was determined to be 157 °C. Elemental analysis showed C 68.41% and H 4.84%; calculated values ​​were C 68.12% and H 4.73%.

[0027] Example 2

[0028] 1.47 g of triphenylchlorosilane, 0.88 g of potassium dichromate, and 1.38 g of triphenylsilanol were placed in a 100 ml flask. A mixed solvent of 20 ml glacial acetic acid and 10 ml cyclohexane was then added. The mixture was stirred at 40 °C for 4 h, and 0.5 ml of acetic anhydride was added at the 0.5 h mark. After the reaction was complete, the cyclohexane was removed by rotary evaporation at 40 °C, followed by filtration to obtain crude bis(triphenylsilane) chromate. The crude product was washed twice successively with 15 ml of water, 5 ml of glacial acetic acid, and 10 ml of cyclohexane. Finally, the solid was dried under vacuum at 80 °C for 2 h, all in the dark. 2.93 g of bis(triphenylsilane) chromate was obtained, with a yield of 92.3%. Its melting point was determined to be 153 °C. Elemental analysis showed C 68.54% and H 4.71%; calculated values ​​were C 68.12% and H 4.73%.

[0029] Example 3

[0030] 1.47 g of triphenylchlorosilane, 0.59 g of potassium dichromate, and 1.66 g of triphenylsilanol were placed in a 100 ml flask. A mixed solvent of 20 ml glacial acetic acid and 10 ml cyclohexane was then added. The mixture was stirred at 50 °C for 8 h, and 0.5 ml of acetic anhydride was added at the 0.5 h mark. After the reaction was complete, the cyclohexane was removed by rotary evaporation at 55 °C, followed by filtration to obtain crude bis(triphenylsilane) chromate. The crude product was washed twice successively with 10 ml water, 10 ml glacial acetic acid, and 5 ml cyclohexane. Finally, the solid was dried under vacuum at 60 °C for 4.5 h, all in the dark. 2.43 g of bis(triphenylsilane) chromate was obtained, with a yield of 95.7%. Its melting point was determined to be 155 °C. Elemental analysis showed C 68.56% and H 4.69%; calculated values ​​were C 68.12% and H 4.73%.

[0031] Example 4

[0032] 1.47 g of triphenylchlorosilane, 1.18 g of potassium dichromate, and 1.10 g of triphenylsilanol were placed in a 100 mL flask. A mixed solvent of 20 mL glacial acetic acid and 16 mL cyclohexane was then added. The mixture was stirred at 50 °C for 5 h, and 0.5 mL of acetic anhydride was added at the 0.5 h mark. After the reaction was complete, the cyclohexane was removed by rotary evaporation at 50 °C, followed by filtration to obtain crude bis(triphenylsilane) chromate. The crude product was washed twice successively with 10 mL of water, 5 mL of glacial acetic acid, and 10 mL of cyclohexane. Finally, the solid was dried under vacuum at 45 °C for 7 h, all in the dark. 2.45 g of bis(triphenylsilane) chromate was obtained, with a yield of 96.5%. Its melting point was determined to be 156 °C. Elemental analysis showed C 68.42% and H 4.83%; calculated values ​​were C 68.12% and H 4.73%.

[0033] Example 5

[0034] 1.47 g of triphenylchlorosilane, 0.88 g of potassium dichromate, and 1.38 g of triphenylsilanol were placed in a 100 ml flask. A mixed solvent of 20 ml glacial acetic acid and 8 ml cyclohexane was then added. The mixture was stirred at 50 °C for 5 h, and 2 ml of acetic anhydride was added at the 2.5 h mark. After the reaction was complete, the cyclohexane was removed by rotary evaporation at 50 °C, followed by filtration to obtain crude bis(triphenylsilane) chromate. The crude product was washed three times successively with 5 ml of water, 5 ml of glacial acetic acid, and 5 ml of cyclohexane. Finally, the solid was dried under vacuum at 65 °C for 8 h, all in the dark. 2.88 g of bis(triphenylsilane) chromate was obtained, with a yield of 90.7%. Its melting point was determined to be 152 °C. Elemental analysis showed C 68.57% and H 4.68%; calculated values ​​were C 68.12% and H 4.73%.

[0035] Example 6

[0036] 1.47 g of triphenylchlorosilane, 0.88 g of potassium dichromate, and 1.38 g of triphenylsilanol were placed in a 100 ml flask. A mixed solvent of 20 ml glacial acetic acid and 8 ml cyclohexane was then added. The mixture was stirred at 40 °C for 5 h, and 0.5 ml of acetic anhydride was added at the 0.5 h mark. After the reaction was complete, the cyclohexane was removed by rotary evaporation at 60 °C, followed by filtration to obtain crude bis(triphenylsilane) chromate. The crude product was washed twice successively with 15 ml of water, 15 ml of glacial acetic acid, and 30 ml of cyclohexane. Finally, the solid was dried under vacuum at 45 °C for 7 h, all in the dark. 2.89 g of bis(triphenylsilane) chromate was obtained, with a yield of 91.2%. Its melting point was determined to be 152 °C. Elemental analysis showed C 68.53% and H 4.72%; calculated values ​​were C 68.12% and H 4.73%.

[0037] Example 7

[0038] 1.47 g of triphenylchlorosilane, 1.18 g of potassium dichromate, and 1.10 g of triphenylsilanol were placed in a 100 mL flask. A mixed solvent of 20 mL glacial acetic acid and 10 mL cyclohexane was then added, and the mixture was stirred at 80 °C for 5 h. At the 0 h mark, 0.5 mL of acetic anhydride was added. After the reaction was complete, the cyclohexane was removed by rotary evaporation at 45 °C, followed by filtration to obtain crude bis(triphenylsilane) chromate. The crude product was washed once each with 30 mL of water, 15 mL of glacial acetic acid, and 45 mL of cyclohexane. Finally, the solid was vacuum dried at 20 °C for 8 h, all in the dark. 2.34 g of bis(triphenylsilane) chromate was obtained, with a yield of 91.6%. Its melting point was determined to be 154 °C. Elemental analysis showed C 68.48% and H 4.77%; calculated values ​​were C 68.12% and H 4.73%.

[0039] Example 8

[0040] 2.94 g of triphenylchlorosilane, 1.76 g of potassium dichromate, and 2.76 g of triphenylsilanol were placed in a 100 ml flask. A mixed solvent of 40 ml glacial acetic acid and 20 ml cyclohexane was then added. The mixture was stirred at 50 °C for 8 h, and 1.00 ml of acetic anhydride was added at the 0.5 h mark. After the reaction was complete, the cyclohexane was removed by rotary evaporation at 50 °C, followed by filtration to obtain crude bis(triphenylsilane) chromate. The crude product was washed twice successively with 20 ml of water, 10 ml of glacial acetic acid, and 10 ml of cyclohexane. Finally, the solid was dried under vacuum at 50 °C for 5 h, all in the dark. 5.94 g of bis(triphenylsilane) chromate was obtained, with a yield of 93.7%. Its melting point was determined to be 156 °C. Elemental analysis showed C 68.46% and H 4.79%; calculated values ​​were C 68.12% and H 4.73%.

[0041] Comparative Example 1

[0042] 1.47 g of triphenylchlorosilane, 0.88 g of potassium dichromate, and 1.38 g of triphenylsilanol were placed in a 100 ml flask, followed by the addition of a mixed solvent of 20 ml glacial acetic acid and 10 ml cyclohexane. The mixture was stirred at 50 °C for 5 h. After the reaction was complete, the cyclohexane was removed by rotary evaporation at 50 °C, followed by filtration to obtain crude bis(triphenylsilane) chromate. The crude product was washed twice successively with 10 ml of water, 5 ml of glacial acetic acid, and 5 ml of cyclohexane. Finally, the solid was dried under vacuum at 50 °C for 5 h, with all operations conducted in the dark. 2.82 g of bis(triphenylsilane) chromate was obtained, with a yield of 89.0%. Its melting point was determined to be 153 °C. Elemental analysis showed C 66.46% and H 4.36%; calculated values ​​were C 68.12% and H 4.73%.

[0043] Comparative Example 2

[0044] 10.00 g of triphenylchlorosilane and 4.00 g of potassium dichromate were placed in a 100 ml flask, followed by the addition of 120 ml of glacial acetic acid. The mixture was stirred at 45 °C for 5 h. After the reaction was complete, the reactants were filtered to obtain crude bis(triphenylsilane) chromate. The crude product was washed twice, successively with 240 ml of water, 240 ml of glacial acetic acid, and 240 ml of hexane. Finally, the solid was dried under vacuum at 50 °C for 5 h, with all operations conducted in the dark. 8.83 g of bis(triphenylsilane) chromate was obtained, with a yield of 86.0%. Its melting point was determined to be 145 °C. Elemental analysis showed C 65.99% and H 4.90%; calculated values ​​were C 68.12% and H 4.76%.

[0045] Comparative Example 3

[0046] 10.00 g of triphenylchlorosilane, 4.30 g of potassium dichromate, and 1.00 g of potassium hydroxide were placed in a 100 ml flask, followed by the addition of a mixed solvent of 20 ml glacial acetic acid and 27 ml n-hexane. The mixture was stirred at 55 °C for 5 h. After the reaction was complete, the reactants were filtered to obtain crude bis(triphenylsilane) chromate. The crude product was washed sequentially with 47 ml of water, 24 ml of glacial acetic acid, and 24 ml of n-hexane via suction filtration. Finally, the product was collected by vacuum drying at 50 °C for 4 h. All operations were performed under light-protected conditions. A final yield of 8.80 g of bis(triphenylsilane) chromate was obtained, with a yield of 94.8%. Its melting point was determined to be 129 °C. Elemental analysis showed C 65.40% and H 4.50%; calculated values ​​were C 68.12% and H 4.73%.

[0047] Comparative Example 4

[0048] 5.53 g of triphenylchlorosilane and 1.00 g of chromium trioxide were placed in a 100 ml flask, followed by the addition of 20 ml of glacial acetic acid as the reaction solvent. The reaction was carried out at 50 °C for 5 h, and then filtered to obtain crude bis(triphenylsilane) chromate. The crude product was washed with water until neutral and dried, then recrystallized from carbon tetrachloride to obtain orange solid bis(triphenylsilane) chromate. All the above operations were carried out in the dark. 4.50 g of bis(triphenylsilane) chromate was obtained, with a yield of 70.9%. Its melting point was determined to be 152 °C. Elemental analysis showed C 61.38% and H 4.10%; calculated values ​​were C 68.12% and H 4.76%.

[0049] Comparative Example 5

[0050] 3.00 g of triphenylsilanol, 3.00 g of chromium trioxide, and 5.00 g of anhydrous magnesium sulfate were placed in a 250 ml round-bottom flask, followed by the addition of 90 ml of carbon tetrachloride. The mixture was stirred at room temperature in the dark for 24 h. After the reaction was complete, the filtrate was collected by filtration. The filtrate was evaporated to dryness at 50 °C and recrystallized from 30 ml of n-heptane to obtain dark red needle-like crystals of bis(triphenylsilane) chromate. All operations were performed in the dark. A final yield of 2.58 g of bis(triphenylsilane) chromate was obtained, with a yield of 75.0%. Its melting point was determined to be 151 °C. Elemental analysis showed C 65.27% and H 4.62%; calculated values ​​were C 68.12% and H 4.76%.

[0051] Comparative Example 6

[0052] 3.00 g of triphenylsilanol, 3.00 g of chromium trioxide, and 1.00 ml of acetic anhydride were placed in a 250 ml round-bottom flask, followed by the addition of 90 ml of carbon tetrachloride. The mixture was stirred at room temperature in the dark for 24 h. After the reaction was complete, the filtrate was collected by filtration. The filtrate was evaporated to dryness at 50 °C and recrystallized from 30 ml of n-heptane to obtain dark red needle-like crystals of bis(triphenylsilane) chromate. All operations were performed in the dark. A final yield of 2.75 g of bis(triphenylsilane) chromate was obtained, with a yield of 80.0%. Its melting point was determined to be 152 °C. Elemental analysis showed C 66.41% and H 4.74%; calculated values ​​were C 68.12% and H 4.76%.

[0053] Of course, the present invention may have other embodiments and variations. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and variations according to the present invention, but these corresponding changes and variations should all fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing bis(triphenyl)silane chromate, characterized in that, include: Triphenylsilanol, triphenylchlorosilane, and potassium dichromate were used as raw materials, glacial acetic acid and cyclohexane were used as solvents, and acetic anhydride was used as a dehydrating agent. The reaction was carried out under light-protected conditions, and the crude product of bis-triphenylsilane chromate was obtained by filtration and vacuum distillation. After washing and drying, bis-triphenylsilane chromate was obtained.

2. The preparation method according to claim 1, characterized in that, The molar ratio of triphenylchlorosilane to triphenylsilanol is 1:0.5~1.5; the molar ratio of triphenylchlorosilane to potassium dichromate is 1:0.1~1.1; the volume ratio of glacial acetic acid to cyclohexane is 1:0.2~1.2; and the molar ratio of acetic anhydride to triphenylchlorosilane is 1:0.2~4.

5.

3. The preparation method according to claim 2, characterized in that, The molar ratio of triphenylchlorosilane to triphenylsilanol is 1:0.8~1.2; the molar ratio of triphenylchlorosilane to potassium dichromate is 1:0.4~0.8; the volume ratio of glacial acetic acid to cyclohexane is 1:0.4~0.8; and the molar ratio of acetic anhydride to triphenylchlorosilane is 1:0.5~4.

4. The preparation method according to claim 1, characterized in that, The reaction is carried out at a temperature of 20~100℃ for a time of 0.5~24 hours.

5. The preparation method according to claim 4, characterized in that, The reaction is carried out at a temperature of 40-80°C for 4-8 hours.

6. The preparation method according to claim 1, characterized in that, The acetic anhydride was added during the first 0 to 2.5 hours of the reaction.

7. The preparation method according to claim 1, characterized in that, The vacuum distillation is carried out at 20~80℃.

8. The preparation method according to claim 1, characterized in that, The washing process involves sequentially using water, glacial acetic acid, and cyclohexane, with the amounts of water and cyclohexane being 1 to 5 times the volume of glacial acetic acid, respectively.

9. The preparation method according to claim 8, characterized in that, The amounts of water and cyclohexane used are 1 to 3 times the volume of glacial acetic acid, and the washing is performed twice.

10. The preparation method according to claim 1, characterized in that, The drying process involves vacuum drying at 0-100°C for 0.5-12 hours.

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