Improved preparation method of herbicide dicamba

Abstract

The invention discloses an improved herbicide dicamba preparation method, and relates to the technical field of compound synthesis, the method comprises the following steps: using 2, 5-dichlorophenol as an initial raw material, heating 2, 5-dichlorophenol and a potassium hydroxide solution in an organic solvent to form salt, dehydrating and purifying, then carrying out carboxylation reaction with CO2 under the action of a catalyst, acidifying, distilling, and drying to obtain 3, 5-dichlorophenol; 2, 6-dichlorosalicylic acid is added; according to the method, 2, 3, 6-dichlorosalicylic acid and dimethyl sulfate are subjected to an etherification reaction, a hydrolysis reaction and an acidification reaction, and pure dicamba is obtained. The catalyst comprises K2CO3 and KCl, the purity of the obtained 3, 6-dichlorosalicylic acid is greatly improved, and the consumption of hydrochloric acid is remarkably reduced; according to the method, secondary reaction is adopted in the etherification process, so that 3, 6-dichlorosalicylic acid is completely converted into dicamba, and the product prepared by the method is high in yield, high in purity and relatively strong in controllability.

Description

Technical Field

[0001] The invention relates to the technical field of compound synthesis, and in particular to a method for preparing an improved herbicide dicamba. Background Art

[0002] As is known to all, the scientific name of dicamba is 3,6-dichloro-2-methoxybenzoic acid, which is a green and low-toxic herbicide. In recent years, with the continuous environmental protection requirements, the demand for dicamba has been increasing, and the market prospects are good.

[0003] At present, the synthesis of dicamba from 2,5-dichlorophenol as raw material requires the synthesis of the intermediate 3,6-dichlorosalicylic acid first. In recent years, the synthesis method of 3,6-dichlorosalicylic acid is mainly reported as the synthesis of 2,5-dichlorophenol and CO2 under the action of a catalyst at high temperature and high pressure, and a series of studies have been carried out around the catalyst. The catalyst commonly used in this reaction is potassium carbonate or sodium carbonate, and the dosage is high. A large amount of hydrochloric acid will be consumed later, a large amount of wastewater will be generated, and the production cost is high. The patent publication number is CN102942474A, and the molar ratio of anhydrous potassium carbonate to 2,5-dichlorophenol is 1~2:1. The product yield and purity are both high, but the subsequent direct addition of hydrochloric acid for acid adjustment greatly increases the amount of waste acid and wastewater, reducing the economic benefits; the patent publication number is CN105801399A, and potassium carbonate and 3,6-dichlorosalicylic acid potassium salt are separated by dissolution and crystallization, but this method has large losses and is not easy to separate completely, resulting in a reduced product yield.

[0004] 3,6-Dichlorosalicylic acid is further etherified to synthesize dicamba. There are two main synthetic methods reported so far: (1) Dimethyl sulfate is added dropwise to 3,6-dichlorosalicylic acid in a sodium hydroxide aqueous solution. Since dimethyl sulfate is insoluble in water and easily decomposes, the reaction yield can only reach about 80%. The patent publication number is CN1830942A. By adding organic solvents such as acetone or tetrahydrofuran, the reaction yield can reach more than 85%, but it is still not ideal. (2) Methyl chloride is used instead of dimethyl sulfate as a methylating agent. The methods used in the patent publication numbers CN102942474A, CN109761786 A, and CN105801399A are all in which 3,6-dichlorosalicylic acid is dissolved in a strong alkaline solution in an organic solvent and reacts with methyl chloride gas under the action of a catalyst. The disadvantages of this route are that the production process is long, the operation is cumbersome, the energy consumption is high, and the efficiency is low. Summary of the invention

[0005] The object of the present invention is to provide an improved method for preparing the herbicide dicamba to solve the above-mentioned deficiencies in the prior art.

[0006] In order to achieve the above object, the present invention provides the following technical solution: comprising the following steps: S1. Preparation of potassium 2,5-dichlorophenol: Take 2,5-dichlorophenol as raw material, dissolve it in an organic solvent and heat it to 70-80°C, slowly add potassium hydroxide solution, heat it to 90°C after adding, react for 1-1.5h, heat it to boiling and dehydrate it to obtain an organic solution of potassium 2,5-dichlorophenol.

[0007] S2. Preparation of 3,6-dichlorosalicylic acid: put the dehydrated potassium 2,5-dichlorophenol organic solution into an autoclave, add a catalyst, pass CO2 to 2.0~2.5MPa at room temperature, stir and replace three times, pass CO2 to 2.6~2.8MPa at 25~35℃, stir for 1~1.5h, pass CO2 to 2.8~3.0 MPa, slowly heat to 155~165℃, react for 7~10h, and separate and purify to obtain 3,6-dichlorosalicylic acid.

[0008] S3. Preparation of dicamba: Dissolve the purified 3,6-dichlorosalicylic acid in a strong alkaline solution, slowly drop dimethyl sulfate at 15-20°C, raise the temperature to reflux for reaction for 6-8h, reduce the temperature to 15-20°C, add an equal amount of strong alkaline solution, slowly drop an equal amount of dimethyl sulfate, raise the temperature to reflux for secondary reaction for 6-8h, reduce the temperature to 60-80°C, add sodium hydroxide solution to the reaction solution, heat to reflux, hydrolyze for 4-6h, cool to room temperature, drop hydrochloric acid, adjust the pH to 1-2, let stand at 5-15°C for 1-1.5h, filter and dry to obtain dicamba.

[0009] As a further description of the above technical solution: in step S1, the organic solvent is toluene or xylene.

[0010] As a further description of the above technical solution: in step S1, the mass ratio of potassium 2,5-dichlorophenolate to toluene or xylene is 1:4-6, and the molar ratio of potassium 2,5-dichlorophenolate to potassium hydroxide is 1:1.0-1.2.

[0011] As a further description of the above technical solution: the catalyst in step S2 is K2CO3 and KCl.

[0012] As a further description of the above technical solution: in step S2, the mass ratio of potassium 2,5-dichlorophenolate to the catalyst is 1:0.8~1.2, and the mass ratio of K2CO3 to KCl is 1:0.1~0.4.

[0013] As a further description of the above technical scheme: the specific process of separation and purification of 3,6-dichlorosalicylic acid in step S2 is as follows: the reaction solution is cooled to 80°C, 500-700ml of deionized water is added, stirred to dissolve, and the liquid is separated while hot. After the upper organic phase is distilled to recover xylene, 200-300ml of deionized water is added to dissolve the substrate, and then the pH is adjusted to 2-3 with hydrochloric acid, and distilled at 138-145°C under normal pressure. The water vapor carries out the unreacted 2,5-dichlorophenol, which is filtered and collected for drying; the lower aqueous phase is cooled to room temperature, hydrochloric acid is added dropwise to PH=2-3, and distilled at 138-145°C under normal pressure. The raw material is evaporated with the water vapor until all the raw material is evaporated, cooled and allowed to stand to room temperature, filtered, and the filter cake is collected to obtain 3,6-dichlorosalicylic acid.

[0014] As a further description of the above technical solution: the strong alkaline solution in step S3 is a sodium hydroxide solution.

[0015] As a further description of the above technical solution: in step S3, the molar ratio of the strong alkaline solution to 3,6-dichlorosalicylic acid is 3-4:1, and the mass fraction of the solute in the strong alkaline solution is 8%-10%.

[0016] As a further description of the above technical solution: in step S3, the mass ratio of dimethyl sulfate to 3,6-dichlorosalicylic acid is 1.6~2:1.

[0017] In the above technical scheme, the present invention provides an improved method for preparing the herbicide dicamba, in which the molar ratio of the amount of sodium hydroxide solution added during the hydrolysis reaction in step S3 to 3,6-dichlorosalicylic acid is 2~4:1, and the mass fraction of the solute in the sodium hydroxide solution is 20%~25%.

[0018] Compared with the prior art, the technical effects of the present invention are as follows: 1. The present invention provides an improved method for preparing the herbicide dicamba. In the carboxylation process stage of preparing the intermediate 3,6-dichlorosalicylic acid from 2,5-dichlorophenol as a raw material, the amount of the original catalyst potassium carbonate is reduced while adding a potassium chloride catalyst of a suitable ratio, thereby greatly reducing the subsequent consumption of hydrochloric acid while ensuring the purity and yield of the 3,6-dichlorosalicylic acid product, thereby reducing the generation of three wastes, saving energy consumption, and reducing production costs.

[0019] 2. In the process of synthesizing dicamba from 3,6-dichlorosalicylic acid through etherification reaction in the present invention, dimethyl sulfate is used as a raw material, no solvent and catalyst are required, and a deep etherification method is used to perform a secondary etherification reaction, so that 3,6-dichlorosalicylic acid is completely converted into dicamba. The product prepared by this method has high yield and high purity, and is highly controllable, which solves the problem of low yield and purity of the product using dimethyl sulfate as a methylation reagent. Compared with the traditional methyl chloride as a methylation reagent, the process flow is short and the cost of production equipment is greatly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can also be obtained based on these drawings.

[0021] Figure 1 A flowchart provided for an embodiment of the present invention. DETAILED DESCRIPTION

[0022] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0023] See also Figure 1 The embodiment of the present invention provides an improved technical solution for preparing the herbicide dicamba, comprising the following steps: S1. Preparation of potassium 2,5-dichlorophenol: Take 2,5-dichlorophenol as raw material, dissolve it in an organic solvent and heat it to 70-80°C, slowly add potassium hydroxide solution, heat it to 90°C after adding, react for 1-1.5h, heat it to boiling and dehydrate it to obtain an organic solution of potassium 2,5-dichlorophenol.

[0024] In step S1, the organic solvent is toluene or xylene.

[0025] In step S1, the mass ratio of potassium 2,5-dichlorophenolate to toluene or xylene is 1:4-6, and the molar ratio of potassium 2,5-dichlorophenolate to potassium hydroxide is 1:1.0-1.2.

[0026] S2. Preparation of 3,6-dichlorosalicylic acid: put the dehydrated potassium 2,5-dichlorophenol organic solution into an autoclave, add a catalyst, pass CO2 to 2.0~2.5MPa at room temperature, stir and replace three times, pass CO2 to 2.6~2.8MPa at 25~35℃, stir for 1~1.5h, pass CO2 to 2.8~3.0 MPa, slowly heat to 155~165℃, react for 7~10h, and separate and purify to obtain 3,6-dichlorosalicylic acid.

[0027] The catalysts in step S2 are K2CO3 and KCl.

[0028] In step S2, the mass ratio of potassium 2,5-dichlorophenolate to the catalyst is 1:0.8-1.2, and the mass ratio of K2CO3 to KCl is 1:0.1-0.4.

[0029] The specific process of separation and purification of 3,6-dichlorosalicylic acid in step S2 is as follows: cooling the reaction solution to 80°C, adding 500-700 ml of deionized water, stirring and dissolving, separating the liquid while hot, distilling the upper organic phase to recover xylene, adding 200-300 ml of deionized water to dissolve the substrate, adjusting the pH to 2-3 with hydrochloric acid, distilling at 138-145°C under normal pressure, and removing unreacted 2,5-dichlorophenol with water vapor, filtering and collecting to dry; cooling the lower aqueous phase to room temperature, adding hydrochloric acid dropwise to pH=2-3, distilling at 138-145°C under normal pressure, and removing the raw material with the water vapor until all the raw material is evaporated, cooling and standing to room temperature, filtering, collecting the filter cake, and obtaining 3,6-dichlorosalicylic acid.

[0030] S3. Preparation of dicamba: Dissolve the purified 3,6-dichlorosalicylic acid in a strong alkaline solution, slowly drop dimethyl sulfate at 15-20°C, raise the temperature to reflux for reaction for 6-8h, reduce the temperature to 15-20°C, add an equal amount of strong alkaline solution, slowly drop an equal amount of dimethyl sulfate, raise the temperature to reflux for secondary reaction for 6-8h, reduce the temperature to 60-80°C, add sodium hydroxide solution to the reaction solution, heat to reflux, hydrolyze for 4-6h, cool to room temperature, drop hydrochloric acid, adjust the pH to 1-2, let stand at 5-15°C for 1-1.5h, filter and dry to obtain dicamba.

[0031] In step S3, the strong alkaline solution is a sodium hydroxide solution.

[0032] In step S3, the molar ratio of the strong alkali solution to 3,6-dichlorosalicylic acid is 3-4:1, and the mass fraction of the solute in the strong alkali solution is 8%-10%.

[0033] In step S3, the mass ratio of dimethyl sulfate to 3,6-dichlorosalicylic acid is 1.6-2:1.

[0034] The molar ratio of the amount of sodium hydroxide solution added during the hydrolysis reaction in step S3 to 3,6-dichlorosalicylic acid is 2-4:1, and the mass fraction of the solute in the sodium hydroxide solution is 20%-25%. Example

[0035] S1. Preparation of potassium 2,5-dichlorophenol: 2,5-dichlorophenol (80 g, 0.49 mol) was dissolved in xylene (400 ml), and 50 wt% KOH (0.49 mol) solution was slowly added dropwise at 70°C. After the addition was completed, the temperature was raised to 90°C and the reaction was continued for 1 hour. The mixture was refluxed and dehydrated using a water separator, and the reaction was continued for 4 hours until the water was completely removed to obtain a xylene solution containing only potassium 2,5-dichlorophenol.

[0036] S2, Preparation of 3,6-dichlorosalicylic acid: Transfer the solution from the previous step to an autoclave, add potassium carbonate (101g, 0.74mol), replace carbon dioxide 3 times (2MPa each time), pass CO2 to 2.0MPa at 25℃, stir for 1h, then pass to 2.8 MPa, start heating, 160℃, 5.5 MPa, react for 7h. After the end, reduce to 80℃, add 600ml of deionized water and stir to dissolve, separate the liquid while hot, take the upper and lower phases respectively, recover xylene at 140℃ in the upper organic phase, add 200ml of water to dissolve, adjust pH to 2-3 with 30% HCl, distill at 140℃, and the water vapor will take out the unreacted 2,5-dichlorophenol, collect by suction filtration, and dry to obtain 8g. The lower aqueous phase was cooled to room temperature, 30% HCl was added dropwise to pH = 2-3, and distilled at 140°C. The raw material was evaporated with the water vapor until all the raw material was evaporated. 19.6 g of unreacted 2,5-dichlorophenol was obtained by drying and weighing. The heating was turned off, and the mixture was cooled and allowed to stand to room temperature. The filter cake was collected by suction and dried in an oven at 60°C for 10 h to obtain 48.1 g of off-white solid with a product purity of 94.6%, namely 3,6-dichlorosalicylic acid. The product yield was 45%, the raw material recovery rate was 34.5%, and the total experimental yield was 79.5%.

[0037] S3. Preparation of dicamba: 5 g 3,6-dichlorosalicylic acid, 3.5 g sodium hydroxide and 22.5 g water were placed in a flask, reacted at 40°C for 30 min to form salt, cooled to 20°C, slowly added 10 g dimethyl sulfate (about 1.5 h), then heated to 100°C and refluxed for 7 h, cooled to 60°C, added 20 wt% NaOH solution (0.09 mol) and hydrolyzed for 6 h. After the reaction, 30% hydrochloric acid was used to adjust the pH to about 1, and the mixture was allowed to stand for 1 h. A precipitate was generated, which was filtered and dried to obtain 4.5 g of solid. The product purity was 82%, and the product yield was 73.8%. Example

[0038] S1. Preparation of potassium 2,5-dichlorophenol: 2,5-dichlorophenol (80 g, 0.49 mol) was dissolved in xylene (400 ml), and 50 wt% KOH (0.49 mol) solution was slowly added dropwise at 70°C. After the addition was completed, the temperature was raised to 90°C and the reaction was continued for 1 hour. The mixture was refluxed and dehydrated using a water separator, and the reaction was continued for 4 hours until the water was completely removed to obtain a xylene solution containing only potassium 2,5-dichlorophenol.

[0039] S2, Preparation of 3,6-dichlorosalicylic acid: Transfer the solution from the previous step to an autoclave, add potassium carbonate (68g, 0.49mol) and potassium chloride (7.5g, 0.1mol), replace carbon dioxide 3 times (2MPa each time), pass CO2 to 2.0MPa at 25℃, stir for 1h, then pass to 2.8MPa, start heating, 160℃, 5.5MPa, react for 7h. After the end, reduce the temperature to 80℃, add 600ml of deionized water and stir to dissolve, separate the liquid while hot, take the upper and lower phases respectively, recover xylene at 140℃ in the upper organic phase, add 200ml of water to dissolve, adjust the pH to 2-3 with 30% HCl, distill at 140℃, and the water vapor will take out the unreacted 2,5-dichlorophenol, collect by suction filtration, and dry to obtain 7g. The lower aqueous phase was cooled to room temperature, 30% HCl was added dropwise to pH = 2-3, and distilled at 140°C. The raw material was evaporated with the water vapor until all the raw material was evaporated. 19.1 g of unreacted 2,5-dichlorophenol was obtained by drying and weighing. The heating was turned off, and the mixture was cooled and allowed to stand to room temperature. The filter cake was collected by suction and dried in an oven at 60°C for 10 h to obtain 43.1 g of off-white solid with a product purity of 95.2%, namely 3,6-dichlorosalicylic acid. The product yield was 40.6%, the raw material recovery rate was 32.6%, and the total experimental yield was 73.2%.

[0040] S3. Preparation of dicamba: 5 g 3,6-dichlorosalicylic acid, 3.5 g sodium hydroxide and 22.5 g water were placed in a flask and reacted at 40°C for 30 min to form salt. After cooling to 20°C, 20 g dimethyl sulfate was slowly added dropwise (about 1.5 h), the temperature was raised to 100°C and refluxed for 7 h. After cooling to 60°C, 20 wt% NaOH solution (0.09 mol) was added for hydrolysis reaction for 6 h. After the reaction, 30% hydrochloric acid was used to adjust the pH to about 1. After standing for 1 h, a precipitate was produced. The solid was filtered and dried to obtain 4.4 g. The product purity was 85% and the product yield was 76.5%. Example

[0041] S1. Preparation of potassium 2,5-dichlorophenol: 2,5-dichlorophenol (80 g, 0.49 mol) was dissolved in xylene (400 ml), and 50 wt% KOH (0.59 mol) solution was slowly added dropwise at 70°C. After the addition was completed, the temperature was raised to 90°C and the reaction was continued for 1 hour. The mixture was refluxed and dehydrated using a water separator and the reaction was continued for 4 hours until the water was completely removed to obtain a xylene solution containing only potassium 2,5-dichlorophenol.

[0042] S2, Preparation of 3,6-dichlorosalicylic acid: Transfer the solution from the previous step to an autoclave, add potassium carbonate (54g, 0.39mol) and potassium chloride (15g, 0.20mol), replace carbon dioxide 3 times (2MPa each time), pass CO2 to 2.0MPa at 25℃, stir for 1h, then pass to 2.8MPa, start heating, 160℃, 5.5MPa, react for 7h. After the end, reduce to 80℃, add 600ml of deionized water and stir to dissolve, separate the liquid while hot, take the upper and lower phases respectively, recover xylene at 140℃ in the upper organic phase, add 200ml of water to dissolve, adjust pH to 2-3 with 30% HCl, distill at 140℃, and the water vapor will take out the unreacted 2,5-dichlorophenol, collect by suction, and dry to obtain 8g. The lower aqueous phase was cooled to room temperature, 30% HCl was added dropwise to pH = 2-3, and distilled at 140°C. The raw material was evaporated with the water vapor until all the raw material was evaporated. 21g of unreacted 2,5-dichlorophenol was obtained by drying and weighing. The heating was turned off, and the mixture was cooled and allowed to stand to room temperature. The filter cake was collected by suction and dried in an oven at 60°C for 10h to obtain 56g of off-white solid with a product purity of 95.2%, namely 3,6-dichlorosalicylic acid. The product yield was 52.8%, the raw material recovery rate was 33%, and the total experimental yield was 85.8%.

[0043] S3. Preparation of dicamba: 5 g 3,6-dichlorosalicylic acid, 3.5 g sodium hydroxide and 22.5 g water were placed in a flask, reacted at 40°C for 30 min to form salt, cooled to 20°C, slowly added 10 g dimethyl sulfate (about 1.5 h), heated to 100°C and refluxed for 7 h, cooled to room temperature, and then 3.5 g sodium hydroxide and 22.5 g water were added to the flask. After dissolving, the mixture was cooled to 20°C, slowly added 10 g dimethyl sulfate (about 1.5 h), heated to 100°C and refluxed for 7 h, cooled to 60°C, and 20 wt% NaOH solution (0.09 mol) was added for hydrolysis reaction for 6 h. After the reaction, the pH was adjusted to about 1 with 30% hydrochloric acid, and the mixture was allowed to stand for 1 h. A precipitate was produced, which was filtered and dried to obtain 4.5 g of solid. The product purity was 94%, and the product yield was 84%. Example

[0044] S1. Preparation of potassium 2,5-dichlorophenol: 2,5-dichlorophenol (80 g, 0.49 mol) was dissolved in toluene (400 ml), and 50 wt% KOH (0.59 mol) solution was slowly added dropwise at 70°C. The reaction was continued for 1 hour after the addition was completed. The mixture was refluxed and dehydrated using a water separator and the reaction was continued for 4 hours until the water was completely removed to obtain a toluene solution containing only potassium 2,5-dichlorophenol.

[0045] S2, Preparation of 3,6-dichlorosalicylic acid: Transfer the solution from the previous step to an autoclave, add potassium chloride (37g, 0.49mol), replace carbon dioxide 3 times (2MPa each time), pass CO2 to 2.0MPa at 25℃, stir for 1h, then pass to 2.4MPa, start heating, 160℃, 6.0MPa, react for 7h. After the end, reduce to 80℃, add 600ml of deionized water, stir and dissolve, separate while hot, take the upper and lower phases respectively, recover xylene at 140℃ in the upper organic phase, add 200ml of water to dissolve, adjust pH to 2-3 with 30% HCl, distill at 140℃, and the water vapor will take out the unreacted 2,5-dichlorophenol, collect by suction and dry to obtain 14g. The lower aqueous phase was cooled to room temperature, 30% HCl was added dropwise to pH = 2-3, and distilled at 140°C. The raw material was evaporated with the water vapor until all the raw material was evaporated. 29g of unreacted 2,5-dichlorophenol was obtained by drying and weighing. The heating was turned off, and the mixture was cooled and allowed to stand to room temperature. The filter cake was collected by suction and dried in an oven at 60°C for 10h to obtain 28g of off-white solid with a product purity of 89.2%, namely 3,6-dichlorosalicylic acid. The product yield was 24.78%, the raw material recovery rate was 53.88%, and the total experimental yield was 78.66%.

[0046] S3. Preparation of dicamba: 5 g 3,6-dichlorosalicylic acid, 3.5 g sodium hydroxide and 22.5 g water were placed in a flask, reacted at 40°C for 30 min to form salt, cooled to 20°C, slowly added 20 g dimethyl sulfate (about 1.5 h), heated to 100°C and refluxed for 7 h, cooled to room temperature, and then 3.5 g sodium hydroxide and 22.5 g water were added to the flask. After dissolving, the mixture was cooled to 20°C, slowly added 10 g dimethyl sulfate (about 1.5 h), heated to 100°C and refluxed for 7 h, cooled to 60°C, and 20 wt% NaOH solution () was added for hydrolysis reaction for 6 h. After the reaction, 30% hydrochloric acid was used to adjust the pH to about 1, and the mixture was allowed to stand for 1 h. A precipitate was produced, which was filtered and dried to obtain 4.2 g of solid. The product purity was 88.7%, and the product yield was 76.5%.

[0047] The above description is only by way of illustration of certain exemplary embodiments of the present invention. It is undoubted that those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the above drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A method for preparing an improved herbicide dicamba, characterized in that: The steps include: S1. Preparation of potassium 2,5-dichlorophenol: 2,5-dichlorophenol is used as a raw material, dissolved in an organic solvent, and then heated to 70-80°C, and potassium hydroxide solution is slowly added dropwise. After the addition is completed, the temperature is raised to 90°C for reaction for 1-1.5 hours, and heated to boiling for dehydration to obtain an organic solution of potassium 2,5-dichlorophenol; S2. Preparation of 3,6-dichlorosalicylic acid: put the dehydrated 2,5-dichlorophenol potassium organic solution into an autoclave, add a catalyst, pass CO2 to 2.0~2.5MPa at room temperature, stir and replace three times, pass CO2 to 2.6~2.8MPa at 25~35℃, stir for 1~1.5h, pass CO2 to 2.8~3.0MPa, slowly heat to 155~165℃, react for 7~10h, and separate and purify to obtain 3,6-dichlorosalicylic acid; S3. Preparation of dicamba: Dissolve the purified 3,6-dichlorosalicylic acid in a strong alkaline solution, slowly drop dimethyl sulfate at 15-20°C, raise the temperature to reflux for reaction for 6-8h, reduce the temperature to 15-20°C, add an equal amount of strong alkaline solution, slowly drop an equal amount of dimethyl sulfate, raise the temperature to reflux for secondary reaction for 6-8h, reduce the temperature to 60-80°C, add sodium hydroxide solution to the reaction solution, heat to reflux, hydrolyze for 4-6h, cool to room temperature, drop hydrochloric acid, adjust the pH to 1-2, let stand at 5-15°C for 1-1.5h, filter and dry to obtain dicamba.

2. The method for preparing an improved herbicide dicamba according to claim 1, characterized in that: In step S1, the organic solvent is toluene or xylene.

3. The method for preparing an improved herbicide dicamba according to claim 1, characterized in that: In step S1, the mass ratio of potassium 2,5-dichlorophenolate to toluene or xylene is 1:4-6, and the molar ratio of potassium 2,5-dichlorophenolate to potassium hydroxide is 1:1.0-1.

2.

4. The method for preparing an improved herbicide dicamba according to claim 1, characterized in that: The catalysts in step S2 are K2CO3 and KCl.

5. The method for preparing an improved herbicide dicamba according to claim 1, characterized in that: In step S2, the mass ratio of potassium 2,5-dichlorophenolate to the catalyst is 1:0.8-1.2, and the mass ratio of K2CO3 to KCl is 1:0.1-0.

4.

6. The method for preparing an improved herbicide dicamba according to claim 1, characterized in that: The specific process of separation and purification of 3,6-dichlorosalicylic acid in step S2 is as follows: cooling the reaction solution to 80°C, adding 500-700 ml of deionized water, stirring and dissolving, separating the liquid while hot, distilling the upper organic phase to recover xylene, adding 200-300 ml of deionized water to dissolve the substrate, adjusting the pH to 2-3 with hydrochloric acid, distilling at 138-145°C under normal pressure, and removing unreacted 2,5-dichlorophenol with water vapor, filtering and collecting to dry; cooling the lower aqueous phase to room temperature, adding hydrochloric acid dropwise to pH=2-3, distilling at 138-145°C under normal pressure, and removing the raw material with the water vapor until all the raw material is evaporated, cooling and standing to room temperature, filtering, collecting the filter cake, and obtaining 3,6-dichlorosalicylic acid.

7. The method for preparing an improved herbicide dicamba according to claim 1, characterized in that: In step S3, the strong alkaline solution is a sodium hydroxide solution.

8. The method for preparing an improved herbicide dicamba according to claim 1, characterized in that: In step S3, the molar ratio of the strong alkali solution to 3,6-dichlorosalicylic acid is 3-4:1, and the mass fraction of the solute in the strong alkali solution is 8%-10%.

9. The method for preparing an improved herbicide dicamba according to claim 1, characterized in that: In step S3, the mass ratio of dimethyl sulfate to 3,6-dichlorosalicylic acid is 1.6-2:

1.

10. The method for preparing an improved herbicide dicamba according to claim 1, characterized in that: The molar ratio of the amount of sodium hydroxide solution added during the hydrolysis reaction in step S3 to 3,6-dichlorosalicylic acid is 2-4:1, and the mass fraction of the solute in the sodium hydroxide solution is 20%-25%.

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