Method for electrochemical dechlorination synthesis of 2,3-dichloropyridine

Abstract

The application discloses a method for synthesizing 2,3-dichloropyridine through electrochemical dechlorination, which comprises the following steps: S1, constructing an electrolytic cell with an activated silver electrode as a cathode and a chemically inert material as an anode; S2, adding a solution containing 2,3,6-trichloropyridine into the electrolytic cell of S1 as a catholyte, and adding an aqueous solution of sulfuric acid or hydrochloric acid as an anolyte; S3, connecting a power supply, making the current pass through the solution containing 2,3,6-trichloropyridine from the anode to the cathode, and performing electrochemical reduction dechlorination reaction on the 2,3,6-trichloropyridine to generate the target product 2,3-dichloropyridine. The application replaces the platinum group noble metal electrode with the activated silver electrode, greatly reduces the cost of the electrode material, avoids the problems of poisoning and deactivation of the platinum group noble metal, and improves the current efficiency and selectivity.

Description

Technical Field

[0001] This invention belongs to the field of electrolytic synthesis technology, and particularly relates to a method for electrochemical dechlorination synthesis of 2,3-dichloropyridine. Background Technology

[0002] 2,3-Dichloropyridine is an important fine chemical intermediate. In the pharmaceutical field, it is mainly used to synthesize antibiotics and drugs for treating cardiovascular diseases. In the pesticide field, it is used to synthesize agricultural insecticides, herbicides, and fungicides. It is a key intermediate in the synthesis of the insecticide chlorantraniliprole and has wide applications in the pesticide, pharmaceutical, and fine chemical industries. With the gradual promotion of chlorantraniliprole, the demand for 2,3-dichloropyridine will also increase significantly. Its synthesis methods mainly include the following:

[0003] 1. Using nicotinamide as a starting material, 2,3-dichloropyridine is synthesized through Hoffmann degradation, chlorination, diazotization and Sandmeier reaction, as detailed in US Patent US20070161797A1. However, this process involves a large amount of acid and alkali reagents and generates a large amount of industrial waste, which does not meet the requirements of environmental protection.

[0004] 2. A mixture of 2,3-dichloropyridine and 2,5-dichloropyridine can be obtained through a three-step reaction using 2-chloropyridine as a raw material, as shown in US Patent 5380862. Although it can generate 2,3-dichloropyridine and 2,5-dichloropyridine simultaneously, the separation of the two is difficult and the overall yield is low, so it is not suitable for industrial production.

[0005] 3. Synthesis methods using 2-chloronicotinamide as raw material, such as Chinese patent CN102086174A, which discloses a method for producing 2,3-dichloropyridine. Although the side reaction caused by chlorination at the 2-position is avoided, the industrial production of this process is difficult because the raw material 2-chloronicotinamide is hard to obtain and has a high cost.

[0006] 4. Using 3-chloropyridine-N-oxide as raw material, such as Chinese patent CN115611803A, which discloses a method for synthesizing 2,3-dichloropyridine, this method can prepare 2,3-dichloropyridine in high yield. However, the reaction process is harsh and the steps are complex, making it difficult to promote and apply in industrial production.

[0007] 5. The synthesis method of 2,3-dichloropyridine by hydrogenation reduction using 2,3,6-trichloropyridine as raw material has the advantages of short process flow and low pollution of three wastes, and is a very promising method for the synthesis of 2,3-dichloropyridine. However, the traditional 2,3,6-trichloropyridine reduction dechlorination method mainly adopts the catalytic hydrogenation method with palladium as a noble metal catalyst. This method mainly has the following problems [see Rare Metal Materials and Engineering, 2018, 47(5): 1637-1642; Guangzhou Chemistry, 2019, 44(4): 65-68]:

[0008] (1) The reaction requires high temperature and high pressure conditions;

[0009] (2) The reaction requires the use of extremely dangerous hydrogen gas;

[0010] (3) The yield of the target product is low.

[0011] The precious metal palladium has a near-monopoly in the field of electrocatalytic hydrodechlorination (EHDC). Due to its unique electronic orbital structure, palladium can efficiently adsorb and activate C-Cl bonds, and has significant advantages in producing H* (referring to "atomic hydrogen adsorbed on the catalyst surface", more specifically written as H*ads). Therefore, it is widely regarded as the most effective catalyst for EHDC.

[0012] The closest existing technology, such as Chinese patent CN119776858A, involves a synthesis method for electrochemical dechlorination using electrodes modified with platinum group metals. Although this method achieved a good yield, the cost of using platinum group metals is high, and they are prone to poisoning or deactivation during long-term operation, which limits their industrial application. In the dechlorination process, the halide ions of the platinum group metal palladium reduce the electron cloud density of the d orbitals of Pd, weakening its ability to coordinate and activate reactants. The halide ions also strongly adsorb onto the active sites of Pd, physically shielding the reactants. Summary of the Invention

[0013] The purpose of this invention is to solve the above-mentioned technical problems existing in the prior art and to provide a method for electrochemical dechlorination synthesis of 2,3-dichloropyridine. This method uses an activated silver electrode to replace the platinum group metal electrode, which significantly reduces the cost of electrode materials and avoids the problems of platinum group metal poisoning and deactivation, while improving current efficiency and selectivity.

[0014] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0015] An electrochemical dechlorination method for synthesizing 2,3-dichloropyridine, characterized by comprising the following steps:

[0016] S1. Construct an electrolytic cell with an activated silver electrode as the cathode and a chemically inert material as the anode;

[0017] S2. Add a solution containing 2,3,6-trichloropyridine to the electrolytic cell of S1 as the catholy solution; add an aqueous solution of sulfuric acid or hydrochloric acid as the anolyte.

[0018] S3. Turn on the power supply so that the current flows from the anode to the cathode through a solution containing 2,3,6-trichloropyridine, and performs an electrochemical reduction and dechlorination reaction on 2,3,6-trichloropyridine to generate the target product 2,3-dichloropyridine.

[0019] Furthermore, in step S1, the activated silver electrode is obtained by surface activation of the silver electrode using a surface activation method.

[0020] Furthermore, the surface activation treatment method employs one of the following: electrochemical oxidation-reduction cycle treatment, chemical etching, or cyclic voltammetry.

[0021] Furthermore, in step S1, the anode material is one of the following: platinum mesh, platinum-plated titanium mesh, ruthenium oxide-coated electrode, or graphite electrode.

[0022] Furthermore, the solution containing 2,3,6-trichloropyridine in step S2 is an aqueous solution containing 2,3,6-trichloropyridine, an organic solvent, an electrolyte, and an acid-binding agent. The organic solvent is one of methanol, ethanol, isopropanol, acetonitrile, N,N-dimethylformamide, and 2,2,2-trifluoroethanol. The electrolyte is one of ammonium chloride, ammonium acetate, and tetrabutyltetrafluoroborate. The acid-binding agent is one of ammonia, trimethylamine, and triethylamine.

[0023] Furthermore, in the solution containing 2,3,6-trichloropyridine, the concentration of 2,3,6-trichloropyridine is 0.1-1 mol / L, the volume concentration of the organic solvent is 30-80%, the concentration of the electrolyte is 0.1-1 mol / L, and the volume concentration of the acid-binding agent is 1-5%.

[0024] Furthermore, the concentration of 2,3,6-trichloropyridine in the solution containing 2,3,6-trichloropyridine is 0.2-0.5 mol / L.

[0025] Furthermore, in step S2, the anolyte is a sulfuric acid aqueous solution with a mass concentration of 0.5-20% or a hydrochloric acid aqueous solution with a mass concentration of 1-10%.

[0026] Furthermore, the electrolytic reduction conditions in step S3 are 30-65℃ and -0.1 to -2.0V vs Ag / AgCl potential.

[0027] Furthermore, in step S3, electrolysis is stopped when the current drops to 5% or less of the initial current.

[0028] The present invention, by adopting the above-described technical solution, has the following beneficial effects:

[0029] (1) The use of activated silver electrode to replace platinum group noble metal electrode greatly reduces the cost of electrode material and avoids the problems of platinum group noble metal poisoning and deactivation. The activated silver electrode in this invention has excellent cycle stability and can maintain high catalytic activity after multiple electrolysis cycles, while the palladium modified electrode loses its activity after three electrolysis cycles, indicating that the activated silver electrode is more suitable for long-term industrial operation.

[0030] (2) Compared with the unactivated silver electrode, the activated silver electrode used in this invention has a significantly improved selective reduction and dechlorination performance of 2,3,6-trichloropyridine due to the increase in surface active sites, with a selectivity increase of more than 50%.

[0031] (3) Compared with existing platinum group noble metal electrode technologies (such as palladium-modified nickel foam), the present invention maintains a high yield while further improving selectivity by about 10%, current efficiency by about 15%, and the reaction conditions are milder and energy consumption is lower. Attached Figure Description

[0032] The present invention will be further described below with reference to the accompanying drawings:

[0033] Figure 1 The high-performance liquid chromatogram of the cathode liquid before electrolysis in Example 1 shows that the peak with a retention time of 10.798 min is the liquid chromatographic peak of 2,3,6-trichloropyridine.

[0034] Figure 2 The image shows the high-performance liquid chromatogram of the cathode liquid after electrolysis in Example 1. The peak with a retention time of 6.283 min is the liquid chromatographic peak of 2,3-dichloropyridine.

[0035] Figure 3 This is an electron microscope image of the silver electrode before activation in Example 1;

[0036] Figure 4 This is an electron microscope image of the activated silver electrode from Example 1;

[0037] Figure 5 This is an electron microscope image of the silver electrode after electrolysis in Example 1;

[0038] Figure 6 The effect of cathode potential on the synthesis of 2,3-dichloropyridine by electrolytic reduction of 2,3,6-trichloropyridine in Examples 2-6;

[0039] Figure 7 The effect of catholyte on the electrolytic reduction of 2,3,6-trichloropyridine to synthesize 2,3-dichloropyridine in Examples 12-21;

[0040] Figure 8 The effect of catholyte pH on the synthesis of 2,3-dichloropyridine by electrolytic reduction of 2,3,6-trichloropyridine in Examples 28-32. Detailed Implementation

[0041] An electrochemical dechlorination method for synthesizing 2,3-dichloropyridine includes the following steps:

[0042] S1. Construct an electrolytic cell with an activated silver electrode as the cathode and a chemically inert material as the anode.

[0043] The electrolytic cell is a diaphragm electrolytic cell. The diaphragm can be a mechanical diaphragm or an ion exchange diaphragm. The preferred ion exchange diaphragm is a cation exchange membrane, such as a perfluorosulfonic acid membrane.

[0044] The activated silver electrode is surface activated by one of the following methods: electrochemical oxidation-reduction cycle treatment, chemical corrosion, or cyclic voltammetry, which forms a rough or porous structure on the surface of the silver electrode, increasing the electrochemical active area and active sites. Compared with the unactivated silver electrode, the activated silver electrode has a significantly improved selective reduction and dechlorination performance of 2,3,6-trichloropyridine at the same potential.

[0045] like Figures 3 to 5 As shown, compared to the unactivated silver electrode, the activated silver electrode exhibits a large number of nanoscale defect sites on its surface, significantly increasing the actual specific surface area. These active sites provide more catalytic anchors, increasing the adsorption probability of 2,3,6-trichloropyridine and promoting electron injection into the C–Cl bond, thereby lowering the activation energy barrier and improving the reaction rate and conversion efficiency of reductive dechlorination. On the other hand, sites with specific geometries can force the substrate to adopt a unique adsorption orientation through spatial confinement effects, and combined with the differentiated activation ability of different sites for C–Cl bonds at different positions, guide the reaction to preferentially break the chloride bond at the target position.

[0046] The anode material is one of the following: platinum mesh, platinum-plated titanium mesh, ruthenium oxide-coated electrode, or graphite electrode.

[0047] S2. Add a solution containing 2,3,6-trichloropyridine to the electrolytic cell of S1 as the catholy solution; add a sulfuric acid aqueous solution with a mass concentration of 0.5-20% or a hydrochloric acid aqueous solution with a mass concentration of 1-10% as the anolyte.

[0048] The solution containing 2,3,6-trichloropyridine is an aqueous solution containing 2,3,6-trichloropyridine, an organic solvent, an electrolyte, and an acid-binding agent. The concentration of 2,3,6-trichloropyridine in the solution is 0.1-1 mol / L, preferably 0.2-0.5 mol / L. The organic solvent is one of methanol, ethanol, isopropanol, acetonitrile, N,N-dimethylformamide, or 2,2,2-trifluoroethanol, and the volume concentration of the organic solvent is 30-80%. The electrolyte is one of ammonium chloride, ammonium acetate, or tetrabutyltetrafluoroborate, and the electrolyte concentration is 0.1-1 mol / L. The acid-binding agent is one of ammonia, trimethylamine, or triethylamine, and the volume concentration of the acid-binding agent is 1-5%.

[0049] S3. Turn on the power supply and control the temperature of the catholyte to 30-65℃. Control the potential to -0.1 to -2.0V vs Ag / AgCl potential (relative to the Ag / AgCl reference electrode, where the concentration of chloride ions in the AgCl solution is 3mol / L). Let the current flow from the anode to the cathode through a solution containing 2,3,6-trichloropyridine to carry out an electrochemical reduction and dechlorination reaction of 2,3,6-trichloropyridine (A) in the solution to generate the target product 2,3-dichloropyridine (B). Stop the electrolysis when the current drops to 5% or less of the initial current. After the electrolysis is completed, recover the target product 2,3-dichloropyridine.

[0050]

[0051] The present invention can also use constant current or step-by-step constant current for electrolysis. Those skilled in the art can use the preferred electrode potential data provided by the present invention to determine the preferred electrolysis current density, thereby determining the preferred electrolysis current.

[0052] The present invention will be further described in conjunction with the following embodiments, but the scope of protection of the present invention is not limited thereto:

[0053] In the following embodiments, the H-type electrolytic cell, perfluorosulfonic acid membrane (using Nafion 324 membrane, Nafion 117 membrane, or Nafion 211 membrane), PDFV hydrophilic porous membrane, ceramic diaphragm, silver electrode, and Ag / AgCl reference electrode were all provided by Hangzhou Sai'ao Electrochemical Instrument Co., Ltd., and the unactivated silver electrode was a polished silver sheet.

[0054] The yield of 2,3-dichloropyridine is defined as Y=X / Z, where X is the amount (moles) of 2,3-dichloropyridine in the catholyte at the end of electrolysis, and Z is the amount (moles) of 2,3,6-trichloropyridine in the catholyte at the beginning of electrolysis.

[0055] Example 1: Electrolytic reduction of 2,3,6-trichloropyridine using an activated silver electrode to synthesize 2,3-dichloropyridine

[0056] An H-type electrolytic cell serves as the reactor, with Nafion 324 as the diaphragm. An activated silver electrode is used as the cathode (activated through an electrochemical oxidation-reduction cycle: in a mixed solution of 0.1 mol / L NaOH and 0.1 mol / L NaCl, cyclically scanned 20 times between -0.5 V and +0.5 V vs Ag / AgCl to activate the silver electrode surface). A platinum mesh is used as the anode, and Ag / AgCl is used as the reference electrode (the chloride ion concentration in the AgCl solution is 3 mol / L). The catholyte is 30 mL of an aqueous solution containing 0.2 mol / L 2,3,6-trichloropyridine, 0.1 mol / L tetrabutyltetrafluoroborate, 60% methanol, and 3% ammonia. The anolyte is 30 mL of a 5% sulfuric acid aqueous solution. The temperature of the catholyte is controlled at 30-35 °C, and the pressure at 0.1 MPa, with constant potential electrolysis (potential between -1.0 V and Ag / AgCl). (Ag / AgCl) Electrolysis is stopped when the current drops to 5% of the initial current.

[0057] 2,3,6-trichloropyridine and 2,3-dichloropyridine in the catholyte before and after electrolysis were analyzed by liquid chromatography. Figure 1 and Figure 2 As shown, the yield of 2,3-dichloropyridine was calculated to be 94.2%, and the current efficiency was 75.3%.

[0058] High-performance liquid chromatography (HPLC) analysis conditions: An HPLC system (UltiMate 3000) was used, with a Hypersil GOLD™ aQ C18 column (250 mm length × 4.6 mm id × 5 μm). Gradient elution was employed. The mobile phase was a mixed solution of V(H2O):V(CH3OH):V(CH3CN) = 4:3:2, containing 5 mL of H3PO4 per 1000 mL of mobile phase. The UV absorption wavelength was 230 nm, the mobile phase flow rate was 1.0 mL / min, the column temperature was 25 °C, and the injection volume was 20 μL.

[0059] Examples 2-6: Effect of Cathode Potential

[0060] With the cathode potential changed in Example 1 and other conditions remaining constant, the yields of 2,3-dichloropyridine are shown in Table 2 and [other tables not provided]. Figure 6 As shown, the results indicate that 2,3,6-trichloropyridine can be electrolytically reduced to 2,3-dichloropyridine within a potential range of -0.1 to -2.0 V, and the yield of 2,3-dichloropyridine is higher within a potential range of -0.5 to -1.5 V.

[0061] Table 2 shows the effect of cathode potential on the synthesis of 2,3-dichloropyridine from the electrolytic reduction of 2,3,6-trichloropyridine.

[0062] 2 -0.1V 80.5% 3 -0.5V 93.1% 4 -0.75V 93.5% 5 -1.5V 91.2% 6 -2.0V 86.3%

[0063] Examples 7-8: The Influence of Silver Electrode Activation Mode

[0064] By changing the activation method of the silver electrode in Example 1 while keeping other conditions unchanged, the yield of 2,3-dichloropyridine is shown in Table 3. The results show that the yield of 2,3-dichloropyridine decreased when the silver electrode was activated by electrochemical oxidation-reduction cycle treatment (the mechanically polished silver electrode was first oxidized and reduced in hydrochloric acid aqueous solution, and then oxidized and reduced in alkali metal hydroxide aqueous solution) and chemical corrosion (the mechanically polished silver electrode was placed in a treatment agent solution containing HNO3 and methanesulfonic acid, and a stable silver grain structure was formed on the surface of the silver electrode through the synergistic effect of directional growth and directional corrosion).

[0065] The reason why electrochemical oxidation-reduction cycling is superior to chemical etching lies in its "dissolution-redeposition" reconstruction mechanism rather than a simple "subtractive" dissolution. Electrochemical oxidation-reduction cycling, through precise control of the potential window and the number of cycles, enables the in-situ generation of highly active structures such as three-dimensional porous structures and nanoparticles on the silver surface. This significantly increases the number of catalytic sites and maintains a strong bond with the substrate, thereby improving selectivity, stability, and reproducibility. In contrast, chemical etching can only randomly roughen the surface, with limited and uncontrollable active sites, easily introducing impurities or causing structural detachment, making it difficult to obtain consistent high performance.

[0066] Table 3 shows the effect of silver electrode activation method on the electrolytic reduction of 2,3,6-trichloropyridine to synthesize 2,3-dichloropyridine.

[0067] 7 Electrochemical oxidation-reduction cycle treatment 84.8% 8 Chemical corrosion 72.4%

[0068] Examples 9-11: Influence of Anode Material

[0069] By changing the anode material in Example 1 while keeping other conditions unchanged, the yield of 2,3-dichloropyridine is shown in Table 4. The results show that the yield of 2,3-dichloropyridine is higher when using a platinum-plated titanium mesh and titanium-plated ruthenium oxide sheet as the anode. When using a graphite electrode as the anode, the yield of 2,3-dichloropyridine is still 88.9%, but corrosion and peeling of the graphite electrode were found after electrolysis.

[0070] Table 4 shows the effect of anode material on the synthesis of 2,3-dichloropyridine from the electrolytic reduction of 2,3,6-trichloropyridine.

[0071] 9 Platinum-plated titanium mesh 93.8% 10 Titanium-plated ruthenium oxide sheet 92.3% 11 Graphite electrode 88.9%

[0072] Examples 12-21: The Influence of Catholyte

[0073] By changing the composition of the catholyte in Example 1 while keeping other conditions unchanged, the yields of 2,3-dichloropyridine are shown in Table 5. Figure 7As shown, the results indicate that the yield of 2,3-dichloropyridine is relatively high when methanol, ethanol, acetonitrile, or DMF are used as organic solvents and ammonia, trimethylamine, or triethylammonium is used as an acid-binding agent. The reaction proceeds smoothly when tetrabutylammonium tetrafluoroborate, ammonium chloride, or ammonium acetate are used as electrolytes, with tetrabutylammonium tetrafluoroborate being the preferred electrolyte. The yield of 2,3-dichloropyridine is significantly reduced when no acid-binding agent is added. The suitable range for reactant concentration is 0.1-1 mol / L, with 0.2-0.5 mol / L being the preferred concentration.

[0074] Table 5 shows the effect of cathode liquid on the electrolytic reduction of 2,3,6-trichloropyridine to synthesize 2,3-dichloropyridine.

[0075] 12 An aqueous solution containing 0.2 mol / L 2,3,6-trichloropyridine + 0.1 mol / L tetrabutyltetrafluoroborate ammonium + 60 vol% ethanol + 3 vol% ammonia. 91.5% 13 An aqueous solution containing 0.2 mol / L 2,3,6-trichloropyridine + 0.5 mol / L tetrabutyltetrafluoroborate ammonium + 30 vol% isopropanol + 3 vol% ammonia. 92.8% 14 An aqueous solution containing 0.2 mol / L 2,3,6-trichloropyridine + 1 mol / L tetrabutyltetrafluoroborate + 30 vol% acetonitrile + 3 vol% ammonia 89.2% 15 An aqueous solution containing 0.2 mol / L 2,3,6-trichloropyridine + 0.1 mol / L tetrabutyltetrafluoroborate ammonium + 30 vol% DMF + 3 vol% trimethylamine 90.7% 16 An aqueous solution containing 0.2 mol / L 2,3,6-trichloropyridine + 0.1 mol / L ammonium chloride + 60 vol% ethanol + 1 vol% triethylamine 83.3% 17 An aqueous solution containing 0.5 mol / L 2,3,6-trichloropyridine + 0.1 mol / L ammonium acetate + 80 vol% methanol + 5 vol% ammonia. 82.9% 18 An aqueous solution containing 0.1 mol / L 2,3,6-trichloropyridine + 0.1 mol / L ammonium acetate + 80 vol% methanol + 5 vol% ammonia. 82.1% 19 An aqueous solution containing 1.0 mol / L 2,3,6-trichloropyridine + 0.1 mol / L ammonium acetate + 80 vol% methanol + 5 vol% ammonia. 82.8% 20 An aqueous solution containing 0.2 mol / L 2,3,6-trichloropyridine + 0.1 mol / L tetrabutyltetrafluoroborate ammonium + 60 vol% methanol + 3 vol% trimethylamine 92.6% 21 An aqueous solution containing 0.2 mol / L 2,3,6-trichloropyridine + 0.1 mol / L tetrabutyltetrafluoroborate ammonium + 60 vol% methanol. 61.5%

[0076] Examples 22-25: Effects of Anodic Solution

[0077] By changing the composition of the anolyte in Example 1 while keeping other conditions unchanged, the yields of 2,3-dichloropyridine are shown in Table 6. The results show that 2,3-dichloropyridine can be successfully produced with yields ranging from 90.5% to 94.0% when using 0.5% sulfuric acid aqueous solution, 20% sulfuric acid aqueous solution, 1% hydrochloric acid aqueous solution, and 10% hydrochloric acid aqueous solution.

[0078] Table 6 shows the effect of anolyte on the electrolytic reduction of 2,3,6-trichloropyridine to synthesize 2,3-dichloropyridine.

[0079] 22 A 0.5% (w / w) sulfuric acid aqueous solution 92.3% 23 20% sulfuric acid aqueous solution 94.0% 24 1% hydrochloric acid aqueous solution 90.5% 25 10% hydrochloric acid aqueous solution 93.1%

[0080] Examples 26-27: The Influence of the Diaphragm

[0081] By changing the type of membrane in Example 1 while keeping other conditions unchanged, the yield of 2,3-dichloropyridine is shown in Table 7. The results show that using Nafion 117 and Nafion 211 membranes has no significant effect on the reaction, but the yield of 2,3-dichloropyridine is slightly reduced.

[0082] Table 7 shows the effect of the diaphragm on the synthesis of 2,3-dichloropyridine from the electrolytic reduction of 2,3,6-trichloropyridine.

[0083] 26 Nafion 117 membrane 92.7% 27 Nafion 211 membrane 93.2%

[0084] Examples 28-32: The Influence of Cathode Fluid pH

[0085] By changing the pH of the catholyte in Example 1 while keeping other conditions unchanged, the yields of 2,3-dichloropyridine are shown in Table 8. Figure 8As shown, the results indicate that the addition of an acid-binding agent affects the reaction, meaning that pH is a crucial factor. Both excessively high and low pH values ​​can trigger side reactions, leading to reduced selectivity and current efficiency. For example, excessively high pH values ​​exacerbate side reactions such as the hydroxylation of 2,3,6-trichloropyridine, especially in methanol solution. Under strongly alkaline conditions, the methoxy anions generated have a stronger nucleophilicity than hydroxide anions, leading to side reactions and decreased selectivity. Conversely, excessively low pH values ​​exacerbate the hydrogen evolution side reaction, resulting in reduced current efficiency and even affecting the stability of the electrode material.

[0086] Table 8 shows the effect of cathode solution pH on the electrolytic reduction of 2,3,6-trichloropyridine to synthesize 2,3-dichloropyridine.

[0087] 28 3-5 61.5% 29 5-7 81.4% 30 7-9 93.1% 31 9-11 65.3% 32 11-13 50.3%

[0088] Comparative Example 1: Electrolytic reduction of 2,3,6-trichloropyridine using an unactivated silver electrode to synthesize 2,3-dichloropyridine

[0089] The cathode in Example 1 was replaced with an unactivated silver electrode, while other conditions remained unchanged. After electrolysis, the yield of 2,3-dichloropyridine was 62.5%, and the current efficiency was 50.1%.

[0090] The results showed that, compared with the unactivated silver electrode, the activated silver electrode increased the yield by 50.7%, the selectivity (in terms of yield, yield = conversion × selectivity) by about 51%, and the current efficiency by about 70%.

[0091] Comparative Example 2: Comparison with Palladium-Modified Nickel Foam Electrode

[0092] The cathode in Example 1 was replaced with palladium-modified nickel foam (palladium loading 1 mg / cm³). 2 Under the same conditions, the yield of 2,3-dichloropyridine was 85.6% and the current efficiency was 65.2% after electrolysis.

[0093] The results show that, compared with the palladium electrode, the activated silver electrode increases the yield by about 10%, the current efficiency by about 15%, and the electrode cost is significantly reduced.

[0094] Comparative Example 3: Electrode Stability Comparison Experiment

[0095] The activated silver electrode in Example 1 and the palladium-modified nickel foam electrode in Comparative Example 2 were subjected to three consecutive electrolysis experiments. The electrolysis conditions were exactly the same as in Example 1 each time (the cathode liquid was renewed after each electrolysis, and the electrode was used directly for the next electrolysis without any treatment). The yield of 2,3-dichloropyridine after each electrolysis was recorded, and the results are shown in Table 8.

[0096] Table 8 shows the comparison of electrode stability.

[0097] 1 94.2% 85.6% 2 93.5% 62.3% 3 92.8% 25.1%

[0098] The results showed that the yield of 2,3-dichloropyridine decreased slightly (from 94.2% to 92.8%) after three consecutive electrolysis cycles using the activated silver electrode, but it still maintained high catalytic activity. In contrast, the yield of 2,3-dichloropyridine using the palladium-modified nickel foam electrode decreased significantly to 62.3% after the second electrolysis, and only 25.1% after the third electrolysis, indicating a near loss of activity. This demonstrates that the activated silver electrode exhibits excellent cycle stability and is more suitable for long-term continuous operation, while the palladium-modified nickel foam electrode has poor stability and is difficult to meet industrial requirements.

[0099] Comparative Example 4: Comparison of different cathode potentials

[0100] The cathode potential in Example 1 was adjusted to -2.25V, while other conditions remained unchanged. After electrolysis, the yield of 2,3-dichloropyridine was 68.3%.

[0101] The results show that a further increase in the cathode potential will drastically accelerate the hydrogen evolution side reaction, and the electrode structure will be damaged or the surface will be covered by bubbles / passivation layers.

[0102] The key technical points of this invention are:

[0103] ① Prior to this invention, the mainstream technical route for the electrochemical synthesis of 2,3-dichloropyridine still relied on platinum group metals. Influenced by the technical concept that "only platinum group metals can achieve highly selective dechlorination," those skilled in the art might believe that the activity of silver electrodes is difficult to match the performance of palladium electrodes. Based on this technical background, those skilled in the art would not automatically rule out using silver electrodes as cathode materials, but would prioritize further optimizing the performance of platinum group metal electrodes (such as controlling the morphology, structure, and dispersion of palladium), rather than combining a silver electrode with a known relatively low electrocatalytic activity with an activation process to obtain an activated silver electrode and applying it to the specific selective dechlorination of the substrate 2,3,6-trichloropyridine. Therefore, those skilled in the art cannot easily solve the technical problems related to palladium electrodes using activated silver electrodes through conventional experiments, because the conventional approach in this field is to improve palladium-based catalysts, not to replace them with silver electrodes.

[0104] The mechanism of silver-catalyzed hydrodechlorination differs from that of palladium-catalyzed hydrodechlorination; it does not generate surface-adsorbed active hydrogen atoms (H*ads). Its reaction mechanism is generally considered to be a direct electron transfer pathway and may involve oxidative addition-reductive elimination steps. Therefore, the comparison with the electrolysis technology in the existing published patent CN119776858A is not simply a change in electrode materials, but also involves differences in the reaction mechanism.

[0105] Palladium electrode material:

[0106]

[0107] Silver electrode:

[0108]

[0109] Palladium catalysis belongs to the indirect reduction pathway, and its key active species is surface-adsorbed atomic hydrogen (H*ads). Under the influence of an electric field, water molecules are reduced on the palladium surface to generate H (H2O + e-). ⁻ →H+OH ⁻ These highly reactive hydrogen atoms then nucleophilically attack the C-Cl bonds of chlorinated organic compounds (R–Cl) adsorbed on the palladium surface, achieving hydrogen substitution of the chlorine atom through a process similar to nucleophilic aromatic substitution or hydrogenolysis, generating dechlorination products R–H and chloride ions Cl. ⁻ .

[0110] Silver catalysis follows a direct electron transfer pathway, without involving any hydrogen intermediates. Silver exhibits extremely weak hydrogen adsorption, making it difficult for water molecules to be reduced to hydrogen on its surface. The reaction mechanism is as follows: after the chlorinated organic compound R–Cl is adsorbed onto the silver surface, cathode electrons are directly injected to form the unstable free radical anionic intermediate [R–Cl]. ⁻ The intermediate rapidly undergoes C–Cl bond breaking (homogeneous or heterogeneous cleavage), simultaneously gaining a second electron and a proton (H+) from the solvent or electrode. ⁺ (from water or solvent), ultimately producing R–H and Cl. ⁻ .

[0111] ② From the perspective of technological development path, the application of silver electrodes in the existing technology is mainly concentrated in the following areas: (1) treating structurally simple chlorinated organic compounds (such as chloroacetic acid, chloroform, etc.); (2) when treating chloropyridine carboxylic acid compounds (such as 3,4,5,6-tetrachloropyridine carboxylic acid), the main target product is 3,6-dichloropyridine carboxylic acid, and its dechlorination site is guided by substituents such as carboxyl and amino groups to produce special selectivity. However, the pyridine ring in the structure of 2,3,6-trichloropyridine contains only three chlorine atoms without other directing substituents, and the electronic and steric effects are more complex. When those skilled in the art rely on their experience in treating substrates with similar structures using silver electrodes, they cannot reasonably predict its selectivity performance on the specific substrate of 2,3,6-trichloropyridine. In other words, even if silver electrodes are known to be used for the dechlorination reaction of other chloropyridine substrates, the difference in the substitution position of chlorine atoms in each chloropyridine substrate will significantly affect the selective path of the dechlorination reaction.

[0112] The above are merely specific embodiments of the present invention, but the technical features of the present invention are not limited thereto. Any simple changes, equivalent substitutions, or modifications made based on the present invention to solve essentially the same technical problems and achieve essentially the same technical effects are all covered within the protection scope of the present invention.

Claims

1. A method for the electrochemical dechlorination synthesis of 2,3-dichloropyridine, characterized in that... Includes the following steps: S1. Construct an electrolytic cell with an activated silver electrode as the cathode and a chemically inert material as the anode; S2. Add a solution containing 2,3,6-trichloropyridine to the electrolytic cell of S1 as the catholy solution; add an aqueous solution of sulfuric acid or hydrochloric acid as the anolyte. S3. Turn on the power supply so that the current flows from the anode to the cathode through a solution containing 2,3,6-trichloropyridine, and performs an electrochemical reduction and dechlorination reaction on 2,3,6-trichloropyridine to generate the target product 2,3-dichloropyridine.

2. The method for electrochemical dechlorination synthesis of 2,3-dichloropyridine according to claim 1, characterized in that: The activated silver electrode in step S1 is obtained by surface activation of the silver electrode using a surface activation method.

3. The method for electrochemical dechlorination synthesis of 2,3-dichloropyridine according to claim 2, characterized in that: The surface activation method employs one of the following: electrochemical oxidation-reduction cycle treatment, chemical etching, or cyclic voltammetry.

4. The method for electrochemical dechlorination synthesis of 2,3-dichloropyridine according to claim 1, characterized in that: The anode material mentioned in step S1 is one of the following: platinum mesh, platinum-plated titanium mesh, ruthenium oxide-coated electrode, and graphite electrode.

5. The method for electrochemical dechlorination synthesis of 2,3-dichloropyridine according to claim 1, characterized in that: The solution containing 2,3,6-trichloropyridine in step S2 is an aqueous solution containing 2,3,6-trichloropyridine, an organic solvent, an electrolyte, and an acid-binding agent. The organic solvent is one of methanol, ethanol, isopropanol, acetonitrile, N,N-dimethylformamide, and 2,2,2-trifluoroethanol. The electrolyte is one of ammonium chloride, ammonium acetate, and tetrabutyltetrafluoroborate. The acid-binding agent is one of ammonia, trimethylamine, and triethylamine.

6. The method for electrochemical dechlorination synthesis of 2,3-dichloropyridine according to claim 5, characterized in that: The solution containing 2,3,6-trichloropyridine has a concentration of 0.1-1 mol / L for 2,3,6-trichloropyridine, a volume concentration of 30-80% for the organic solvent, a concentration of 0.1-1 mol / L for the electrolyte, and a volume concentration of 1-5% for the acid-binding agent.

7. The method for electrochemical dechlorination synthesis of 2,3-dichloropyridine according to claim 6, characterized in that: The concentration of 2,3,6-trichloropyridine in the solution containing 2,3,6-trichloropyridine is 0.2-0.5 mol / L.

8. The method for electrochemical dechlorination synthesis of 2,3-dichloropyridine according to claim 1, characterized in that: The anolyte in step S2 is a sulfuric acid aqueous solution with a mass concentration of 0.5-20% or a hydrochloric acid aqueous solution with a mass concentration of 1-10%.

9. The method for electrochemical dechlorination synthesis of 2,3-dichloropyridine according to claim 1, characterized in that: The conditions for electrolytic reduction in step S3 are 30-65℃ and -0.1 to -2.0V vs Ag / AgCl potential.

10. The method for electrochemical dechlorination synthesis of 2,3-dichloropyridine according to claim 1, characterized in that: In step S3, electrolysis is stopped when the current drops to 5% or less of the initial current.

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