An improved electrolytic synthesis of 3,6-dichloropicolinic acid

Abstract

The application discloses an improved electrolytic synthesis method of 3,6-dichloropicolinic acid, which comprises the following steps: adopting a diaphragm electrolytic cell, taking an aqueous solution containing 3,4,5,6-tetrachloropicolinic acid and a certain flow of CO2 gas as a cathode liquid, taking an aqueous solution of alkali metal hydroxide as an anode liquid, taking silver as a cathode, and taking a nickel-based material as an anode to carry out an electrolytic reaction, and obtaining 3,6-dichloropicolinic acid after the electrolytic reaction is completed. In the electrolytic process, the anode is separated from the cathode, so that corrosion is avoided; meanwhile, the CO2 gas is introduced to effectively control the occurrence of a hydroxylation side reaction, the molar yield of a hydroxyl byproduct is reduced from 20% to 35% to 2% to 8%, the molar yield of 3,6-dichloropicolinic acid is increased from 45% to 50% to above 80%, and the highest can reach 92.2%.

Description

(I) TECHNICAL FIELD

[0001] The present application relates to an improved method for electrolytic synthesis of 3,6-dichloropyridine carboxylic acid. (II) BACKGROUND

[0002] 3,6-dichloropyridine carboxylic acid (hereinafter referred to as 3,6-DCP) can be used as herbicide and plant growth regulator, and is a new type of green pesticide with high efficiency, low toxicity and low residue, and has the following structural formula:

[0003]

[0004] The reported synthesis methods mainly include hydrolysis method, traditional chemical reduction method and electrolytic reduction method. The hydrolysis method has poor reaction selectivity and low yield, the traditional chemical reduction method uses hydrazine which is toxic and not friendly to the environment. The electrolytic reduction method improves the operation conditions at relatively low cost, and the reducing agent used is electron which has relatively small environmental pollution. However, the electrolytic cell provided in the prior art also has many defects. For example, the diaphragm-free electrolytic cell provided in US4217185 and CN201198494Y of our research group uses graphite as anode, and there are cavities in graphite crystal. The electrolyte will seep into the cavities, which will cause the graphite crystal lattice to be loose, resulting in the graphite particles falling off and thus the graphite plate being damaged. CN100500945C of Fan Qian discloses a high-temperature weak alkali dosing method in a diaphragm-free electrolytic cell. The process increases the temperature of the electrolyte to increase the solubility of 3,4,5,6-tetrachloropyridine carboxylic acid (hereinafter referred to as 3,4,5,6-TCP), effectively solves the problem of foaming, and unfortunately, the anode material of the reaction system is very difficult to select. The nickel alloy anode with good corrosion resistance under alkaline conditions will also be severely corroded in the system. Moreover, under the conditions of high temperature and high concentration, the pH of the cathode liquid gradually increases with the progress of the reaction, and the alkalinity is too strong to easily cause hydroxylation side reactions, resulting in more impurities and reducing the quality of the product.

[0005] Therefore, it is necessary to improve the method for electrolytic synthesis of 3,6-DCP. (III) SUMMARY

[0006] The present application aims to provide an improved method for electrolytic synthesis of 3,6-dichloropyridine carboxylic acid. In the present application, a diaphragm electrolytic cell is used, an aqueous solution containing 0.5-2 mol / L 3,4,5,6-tetrachloropyridine carboxylic acid and a certain flow rate of CO2 gas is used as cathode liquid, an aqueous solution of alkali metal hydroxide is used as anode liquid, silver is used as cathode, and nickel-based material is used as anode for electrolytic reaction. After the electrolytic reaction is completed, 3,6-dichloropyridine carboxylic acid is obtained. In the electrolysis process, the anode is separated from the cathode to avoid corrosion, and the CO2 gas is introduced to effectively control the occurrence of hydroxylation side reactions, thereby improving the yield of the product.

[0007] The technical scheme adopted by the present application is:

[0008] The present application provides an improved method for electrolytic synthesis of 3,6-dichloropicolinic acid, which comprises the following steps: using a diaphragm electrolytic cell, using an aqueous solution containing 3,4,5,6-tetrachloropicolinic acid (3,4,5,6-TCP) and a certain flow rate of CO2 gas as the cathode liquid, using an aqueous solution of alkali metal hydroxide as the anode liquid, using silver as the cathode, and using nickel-based material as the anode to carry out electrolytic reaction, and after the electrolytic reaction is completed, 3,6-dichloropicolinic acid is obtained.

[0009] Further, the cathode liquid is prepared by the following method: adding 3,4,5,6-TCP to an aqueous solution of alkali metal carbonate or bicarbonate, stirring until it becomes a clear solution, and continuously bubbling a certain flow rate of CO2 gas.

[0010] Further, the alkali metal carbonate or bicarbonate includes Na2CO3, K2CO3, NaHCO3, KHCO3, NH4HCO3, or (NH4)2CO3.

[0011] Further, the concentration of alkali metal carbonate or bicarbonate in the cathode liquid is 0.25-1 mol / L, and the concentration of 3,4,5,6-TCP is 0.5-2 mol / L.

[0012] Further, in the cathode liquid, the molar ratio of alkali metal carbonate or bicarbonate to 3,4,5,6-TCP is 0.1-2:1.

[0013] Further, the flow rate of CO2 gas is 10-30 mL / min, preferably 20 mL / min.

[0014] Further, the diaphragm electrolytic cell uses an H-type electrolytic cell, and the diaphragm is a Nafion cation membrane.

[0015] Further, the temperature of the electrolytic reaction is 25-90°C, preferably 80-90°C.

[0016] Further, the alkali metal hydroxide in the anode liquid is NaOH or KOH, and the concentration of alkali metal hydroxide in the anode liquid is 0.5-4 mol / L.

[0017] The reaction equation of the cathode chamber of the present application is as follows:

[0018]

[0019] Compared with the prior art, the present application has the following beneficial effects:

[0020] 1. The application adopts diaphragm electrolytic cell, uses 0.5-2 mol / L 3,4,5,6-TCP aqueous solution as catholyte, uses alkali metal hydroxide aqueous solution as anolyte, uses silver as cathode, and uses nickel-based material as anode to carry out electrolysis reaction, and in the electrolysis experiment, the anode is separated from the cathode, so corrosion does not occur.

[0021] 2. In the electrolysis process of the application, the hydroxide generated in the cathode chamber combines with the sodium ion permeated in the anode chamber to generate sodium hydroxide, which causes the pH of the catholyte to rise, and then generates hydroxylation by-products. In the application, the catholyte combines with the generated sodium hydroxide to generate sodium carbonate through continuous CO2 gas, and the alkalinity of sodium carbonate is relatively weak compared with sodium hydroxide, which can achieve the effect of controlling the pH of the catholyte, and then control the generation of hydroxylation by-products, thereby improving the yield of the product. The molar yield of the hydroxy by-product is reduced from 20% to 35% to 2% to 8%, the molar yield of 3,6-dichloropyridine carboxylic acid is increased from 45% to 50% to more than 80%, and the highest can reach 92.2%. (Four) Description of Drawings

[0022] Figure 1 It is a schematic diagram of H-type electrolytic cell with Nafion cation membrane as diaphragm.

[0023] Figure 2 It is a schematic diagram of 316 stainless steel after the end of the electrolysis experiment in the diaphragm-free electrolytic cell of Comparative Example 1.

[0024] Figure 3 It is a schematic diagram of 316 stainless steel after the end of the electrolysis experiment in the diaphragm electrolytic cell of Example 1.

[0025] Figure 4 It is the influence of CO2 gas on the pH of the electrolysis experiment during the experiment of Example 1 and Comparative Example 2. (Five) Specific Embodiments

[0026] The application will be further described below in conjunction with specific embodiments, but the protection scope of the application is not limited to this:

[0027] All aqueous solutions in the examples of the application are prepared with deionized water. The cathode is silver mesh, and the nickel-based material of the anode is 316 stainless steel.

[0028] The catholyte in the examples of the application is prepared as follows: 3,4,5,6-TCP is added to an aqueous solution of alkali metal carbonate or bicarbonate, stirred until it becomes a clear solution, and a certain flow of CO2 gas is continuously introduced.

[0029] Example 1, electrolytic synthesis of 3,6-dichloropyridine carboxylic acid

[0030] Use Figure 1The H-type electrolytic cell with a Nafion cationic membrane as the diaphragm was used as the anolyte, with 35 mL of a 2M NaOH aqueous solution as the anolyte, and 35 mL of an aqueous solution containing 0.7M Na2CO3 + 1M 3,4,5,6-tetrachloropyridine carboxylic acid as the catholyte (pH = 8-10). Silver was used as the cathode, and a nickel-based material was used as the anode. The catholyte was stirred at 80°C until it became clear. CO2 gas was continuously introduced into the catholyte at a flow rate of 20 mL / min, and a current of 4.16 A / dm was introduced. 2 After 1.5 hours of reaction, a current of 1.25A / dm 2 The electrolysis was stopped after 4.5 hours of reaction, at which time the cathode liquid pH was 13.5. The solution after the reaction was tested by HPLC, and the molar yield of the hydroxyl byproduct was 2.2%, and the molar yield of 3,6-dichloropicolinic acid was 92.2%. Figure 3 During the reaction, the pH of the reaction solution changes as shown in Figure 4 Compared with Comparative Example 2, the introduction of CO2 can achieve the effect of controlling the pH of the cathode liquid.

[0031] Examples 2 to 16: Electrolytic Synthesis of 3,6-Dichloropyridine Carboxylic Acid

[0032] The electrolysis reaction was carried out according to the method of Example 1 using the parameters in Table 1. The results are shown in Table 1.

[0033] Table 1. Electrolysis experimental parameters and results

[0034]

[0035] Comparative Example 1: Electrolytic Synthesis of 3,6-Dichloropyridine Carboxylic Acid in a Diaphragmless Electrolytic Cell

[0036] In a single electrolytic cell, 35 mL of an aqueous solution of 0.7 M Na2CO3 + 1 M 3,4,5,6-tetrachloropyridine carboxylic acid was used as the electrolyte, with a pH of 8 to 10. Stirring was performed at 80°C until the solution became clear. CO2 gas was introduced at a flow rate of 20 mL / min. The electrolysis reaction was carried out with silver as the cathode and nickel-based material as the anode. A current of 4.16 A / dm 2 After 1.5 hours of reaction, a current of 1.25A / dm 2 The electrolysis was stopped after 4.5 hours of reaction, at which time the cathode liquid pH was 13.5. The solution after the reaction was tested by HPLC, and the molar yield of the hydroxyl byproduct was 21.2%, and the molar yield of 3,6-dichloropicolinic acid was 75.1%. Figure 2 As shown, it is obvious that the anode has corroded.

[0037] Compared with Example 1, the electrolysis efficiency is significantly reduced.

[0038] Comparative Example 2. Electrolytic synthesis of 3,6-dichloropicolinic acid without CO2 bubbling

[0039] The CO2 bubbling during the electrolysis of Example 1 was eliminated, and the other experimental conditions and procedures were the same, to give a molar yield of 33.5% of the hydroxyl byproduct and 45.8% of 3,6-dichloropicolinic acid. Compared to Example 1, the electrolysis efficiency was significantly reduced without CO2 bubbling during the electrolysis. The pH change of the reaction solution during the reaction is shown in Table 2. Figure 4 .

Claims

1. An improved process for the electrolytic synthesis of 3,6-dichloropicolinic acid, characterized in that, The method comprises the following steps: using a diaphragm electrolytic cell, using an aqueous solution containing 3,4,5,6-tetrachloropyridine carboxylic acid and a certain flow rate of CO2 gas as a cathode solution, using an aqueous solution of alkali metal hydroxide as an anode solution, using silver as a cathode, and using a nickel-based material as an anode to carry out an electrolytic reaction, and obtaining 3,6-dichloropyridine carboxylic acid after the electrolytic reaction is completed; the cathode solution is prepared by the following method: adding 3,4,5,6-tetrachloropyridine carboxylic acid to an aqueous solution of alkali metal carbonate or bicarbonate, stirring until a clear solution is obtained, and continuously passing a certain flow rate of CO2 gas; the alkali metal carbonate or bicarbonate includes Na2CO3, K2CO3, NaHCO3, KHCO3, NH4HCO3 or (NH4)2CO3.

2. The method of claim 1, wherein, The concentration of the alkali metal carbonate or bicarbonate in the cathode solution is 0.25-1 mol / L, and the concentration of 3,4,5,6-tetrachloropyridine carboxylic acid is 0.5-2 mol / L.

3. The method of claim 2, wherein, The molar ratio of the alkali metal carbonate or bicarbonate to 3,4,5,6-tetrachloropyridine carboxylic acid in the cathode solution is 0.1-2:

1.

4. The method of claim 1, wherein, The flow rate of the CO2 gas is 10-30 mL / min.

5. The method of claim 1, wherein, The diaphragm electrolytic cell uses an H-type electrolytic cell, and the diaphragm is a Nafion cation membrane.

6. The method of claim 1, wherein, The temperature of the electrolytic reaction is 25-90℃.

7. The method of claim 1, wherein, The alkali metal hydroxide in the anode solution is NaOH or KOH, and the concentration of the alkali metal hydroxide is 0.5-4 mol / L.

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