Improved electrolytic synthesis method of 3, 6-dichloropicolinic acid

Abstract

The invention 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 into which a certain flow of CO2 gas is introduced as a catholyte, taking an aqueous solution of alkali metal hydroxide as an anolyte, taking silver as a cathode and taking a nickel-based material as an anode to carry out an electrolytic reaction, and after the electrolytic reaction is completed, separating and purifying to obtain 3, 6-dichloropicolinic acid. And the 3, 6-dichloropicolinic acid is obtained. In the electrolysis process, the anode is separated from the cathode, so that corrosion is avoided; meanwhile, the introduced CO2 gas effectively controls the occurrence of hydroxylation side reaction, the molar yield of hydroxyl side products is reduced from 20%-35% to 2%-8%, and the molar yield of 3, 6-dichloropicolinic acid is increased from 45%-50% to 80% or above and can reach 92.2% to the maximum.

Description

(I) Technical field

[0001] The invention relates to an improved electrolytic synthesis method of 3,6-dichloropicolinic acid. (II) Background technology

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

[0003]

[0004] The synthesis methods reported so far mainly include hydrolysis, traditional chemical reduction and electrolytic reduction. The hydrolysis method has poor reaction selectivity and low yield, and the hydrazine used in the traditional chemical reduction method is highly toxic and environmentally unfriendly. The electrolytic reduction method improves the operating conditions at a relatively low cost, and the reducing agent used is electrons, which has relatively little environmental pollution. However, the electrolytic cells provided in the prior art also have many defects. For example, the diaphragm-free electrolytic cells provided by U.S. Patent No. 4217185 and our research group CN201198494Y use graphite as the anode, and there are voids in the graphite crystals. The electrolyte infiltrates into the voids, causing the graphite lattice to become loose, resulting in the falling off of graphite particles and thus damaging the graphite plates. Fan Qian CN100500945C et al. announced a high-temperature weak base dosing method in a diaphragmless electrolytic cell. The process increases the solubility of 3,4,5,6-tetrachloropyridine carboxylic acid (hereinafter referred to as 3,4,5,6-TCP) by increasing the temperature of the electrolyte, effectively solving the foaming problem. 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 suffer from severe corrosion in this system. Moreover, under high temperature and high concentration conditions, the pH of the cathode liquid gradually rises as the reaction proceeds. Excessive alkalinity is prone to hydroxylation side reactions, resulting in more impurities and reducing the quality of the product.

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

[0006] The purpose of the present invention is to provide an improved electrolytic synthesis method of 3,6-dichloropicolinic acid. The present invention adopts a diaphragm electrolytic cell, an aqueous solution containing 0.5-2 mol / L 3,4,5,6-tetrachloropicolinic acid and a certain flow of CO2 gas is used as a cathode liquid, an aqueous solution of an alkali metal hydroxide is used as an anode liquid, silver is used as a cathode, and a nickel-based material is used as an anode for electrolytic reaction. After the electrolytic reaction is completed, 3,6-dichloropicolinic acid is obtained. During the electrolysis process, the anode and the cathode are separated to avoid corrosion; at the same time, the CO2 gas introduced effectively controls the occurrence of hydroxylation side reactions, thereby improving the yield of the product.

[0007] The technical solution adopted by the present invention is:

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

[0009] Furthermore, the cathode liquid 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 rate of CO2 gas is continuously introduced.

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

[0011] Furthermore, 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] Furthermore, in the cathode liquid, the molar ratio of alkali metal carbonate or bicarbonate to 3,4,5,6-TCP is 0.1 to 2:1.

[0013] Furthermore, the CO2 gas flow rate is 10 to 30 mL / min, preferably 20 mL / min.

[0014] Furthermore, the diaphragm electrolyzer adopts an H-type electrolyzer, and the diaphragm is a Nafion cationic membrane.

[0015] Furthermore, the temperature of the electrolysis reaction is 25°C to 90°C, preferably 80-90°C.

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

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

[0018]

[0019] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in:

[0020] 1. The present invention adopts a diaphragm electrolytic cell, uses an aqueous solution containing 0.5-2 mol / L 3,4,5,6-TCP as a cathode liquid, an alkali metal hydroxide aqueous solution as an anode liquid, silver as a cathode, and a nickel-based material as an anode to carry out an electrolytic reaction. During the electrolytic experiment, the anode and the cathode are separated and no corrosion occurs.

[0021] 2. During the electrolysis process of the present invention, the hydroxide generated in the cathode chamber combines with the sodium ions permeated from the anode chamber to generate sodium hydroxide, which causes the pH of the cathode liquid to rise, thereby generating hydroxylated byproducts. The cathode liquid of the present invention combines with the generated sodium hydroxide to generate sodium carbonate through the continuous introduction of CO2 gas. The alkalinity of sodium carbonate is relatively weaker than that of sodium hydroxide, which can achieve the effect of controlling the pH of the cathode liquid, thereby controlling the generation of hydroxylated byproducts and improving the yield of the product. The molar yield of hydroxyl byproducts is reduced from 20% to 35% to 2% to 8%, and the molar yield of 3,6-dichloropicolinic acid is increased from 45% to 50% to more than 80%, and can reach up to 92.2%. (IV) Description of the drawings

[0022] Figure 1 Schematic diagram of an H-type electrolyzer with Nafion cationic membrane as the diaphragm.

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

[0024] Figure 3 Schematic diagram of 316 stainless steel after the electrolysis experiment in the diaphragm electrolytic cell of Example 1 is completed.

[0025] Figure 4 The effect of the presence or absence of CO2 gas on the pH of the electrolysis experiment during the experiments of Example 1 and Comparative Example 2. (V) Specific implementation methods

[0026] The present invention is further described below in conjunction with specific embodiments, but the protection scope of the present invention is not limited thereto:

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

[0028] The cathode liquid described in the embodiment of the present invention 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 rate of CO2 gas is continuously introduced.

[0029] Example 1: Electrolytic synthesis of 3,6-dichloropicolinic acid

[0030] use Figure 1The H-type electrolytic cell with Nafion cationic membrane as the diaphragm is shown, with 35 mL of 2M NaOH aqueous solution as the anode liquid, 35 mL of 0.7M Na2CO3+1M 3,4,5,6-tetrachloropyridine carboxylic acid aqueous solution as the cathode liquid (pH=8-10), silver as the cathode, and nickel-based material as the anode. At 80°C, the cathode liquid is stirred until it is clear, and CO2 gas is continuously introduced into the cathode liquid at a flow rate of 20 mL / min, and 4.16 A / dm 2 After 1.5 hours of reaction, 1.25A / dm 2 The electrolysis was stopped after 4.5 hours of reaction, and the pH of the cathode liquid 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-dichloropyridine carboxylic 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-dichloropicolinic 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 0.7 M Na2CO3 + 1 M 3,4,5,6-tetrachloropyridine carboxylic acid aqueous solution was used as the electrolyte, pH = 8-10. At 80 ° C, stirred until clear, CO2 gas was introduced at a flow rate of 20 mL / min, and the electrolytic reaction was carried out with silver as the cathode and nickel-based material as the anode. 4.16 A / dm 2 After 1.5 hours of reaction, 1.25A / dm 2 The electrolysis was stopped after 4.5 hours of reaction, and the pH of the cathode liquid 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-Dichloropyridine Carboxylic Acid Without the Introduction of CO2

[0039] The operation of introducing CO2 during the electrolysis process in Example 1 was cancelled, and other experimental conditions and operations were the same. The molar yield of hydroxyl byproducts was 33.5%, and the molar yield of 3,6-dichloropicolinic acid was 45.8%. Compared with Example 1, not introducing CO2 during the electrolysis process will significantly reduce the electrolysis efficiency. During the reaction, the pH change of the reaction solution is shown in Figure 4 .

Claims

1. An improved electrolytic synthesis method of 3,6-dichloropicolinic acid, characterized in that: The method comprises the following steps: using a diaphragm electrolytic cell, taking an aqueous solution containing 3,4,5,6-tetrachloropyridine carboxylic acid and a certain flow of CO2 gas as a cathode liquid, taking an alkali metal hydroxide aqueous solution as an anode liquid, taking silver as a cathode, and taking a nickel-based material as an anode, performing an electrolytic reaction, and obtaining 3,6-dichloropyridine carboxylic acid after the electrolytic reaction is completed.

2. The method according to claim 1, characterized in that The cathode liquid is prepared as follows: 3,4,5,6-tetrachloropyridine carboxylic acid is added to an aqueous solution of alkali metal carbonate or bicarbonate, stirred until a clear solution is formed, and a certain flow rate of CO2 gas is continuously introduced.

3. The method according to claim 2, characterized in that The alkali metal carbonate or bicarbonate includes Na2CO3, K2CO3, NaHCO3, KHCO3, NH4HCO3 or (NH4)2CO3.

4. The method according to claim 2, characterized in that 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-tetrachloropyridine carboxylic acid is 0.5-2 mol / L.

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

1.

6. The method according to claim 1, characterized in that The CO2 gas flow rate is 10 to 30 mL / min.

7. The method according to claim 1, characterized in that The diaphragm electrolyzer adopts an H-type electrolyzer, and the diaphragm is a Nafion cationic membrane.

8. The method according to claim 1, characterized in that The temperature of the electrolysis reaction is 25°C to 90°C.

9. The method according to claim 1, characterized in that The alkali metal hydroxide in the anolyte is NaOH or KOH, and the concentration of the alkali metal hydroxide is 0.5-4 mol / L.

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