A powder electrode for electrochemically reducing and dechlorinating chloropicolinic acid, its preparation and application

Abstract

The present invention discloses a powder electrode for electrochemically reducing and dechlorinating chloropicolinic acid, and its preparation and application. The powder electrode consists of a current collector and a catalytic layer on the surface of the current collector. The catalytic layer includes a catalytic material, and the catalytic material is obtained by plating silver on the surface of an activated hydrogen storage alloy powder. After plating silver on the surface of the hydrogen storage alloy powder, the weight gain is 80-120 mg / g. The present invention provides the application of the powder electrode in the dechlorination of chloropicolinic acid by electrolytic reduction, effectively inhibiting the occurrence of hydrogen evolution side reactions, significantly improving the current efficiency of the dechlorination reaction of chloropicolinic acid and avoiding the large generation of hydrogen, and effectively reducing the electrode cost and maintenance cost.

Description

Technical Field

[0001] The present invention relates to the technical field of organic chemical industry, and particularly relates to a powder electrode for electrochemically reducing and dechlorinating chloropicolinic acid, and its preparation and application. Background Art

[0002] Chloropyridine compounds are important intermediates for "three drugs", mainly used in the synthesis of important chemical products such as pharmaceuticals, pesticides, and dyes. Among them, 4-amino-3,6-dichloropicolinic acid (AMINOPYRALID, abbreviated as 4-N-3,6-DCP) is an important highly active and low-toxic green pesticide, mainly used as a herbicide, insecticide, and plant growth regulator, and has great market value. 3,6-Dichloropicolinic acid (Clopyralid, abbreviated as 3,6-DCP) can be used as an important intermediate in organic synthesis, and can also be used as a herbicide and plant growth regulator.

[0003] At present, the preparation of chloropicolinic acid usually involves first performing full chlorination of pyridine and then using a method of selective reduction and dechlorination. Among various methods, the electrocatalytic hydrogenation dechlorination technology has become the current main industrial production route due to its advantages such as high reaction selectivity, good product quality, and less waste discharge.

[0004] In the electrocatalytic hydrodechlorination process, the electrode material has a decisive influence on the reaction efficiency. In order to find electrode materials with better performance, researchers at home and abroad have carried out a large number of studies. US4217185A first proposed a method for electrocatalytic hydrodechlorination of 3,4,5,6-tetrachloropicolinic acid using an activated silver electrode to prepare 3,6-dichloropicolinic acid, with a yield of up to 91.3%. CN1238565C proposed a method for electrocatalytic hydrodechlorination of 4-amino-3,6-dichloropicolinic acid using an activated silver electrode, with a yield of 61.9%. During the electrolysis process, the electrode activity decays significantly, resulting in low yields and current efficiencies. CN101522628A, CN102597328A, CN110656345A, etc. activated the silver electrode by using a method of periodically reversing the polarity during electrolysis, and successfully increased the yield of 4-amino-3,6-dichloropicolinic acid to over 90%, but the current efficiency of electrolysis was still low, between 30% and 40%. In addition, the frequent pole reversal activation is not only cumbersome to operate and has poor production stability, but also the electrode surface repeatedly undergoes oxidation and reduction, and the silver electrode shows serious powder shedding phenomenon, resulting in a short electrode life and high production costs. There are also reports of suppressing the hydrogen evolution reaction by increasing the alkalinity of the electrolyte to improve the efficiency of the hydrodechlorination reaction on the silver cathode. However, under high-alkaline conditions, the solubility of the raw material (3,4,5,6-tetrachloropicolinic acid) is low, the output per unit volume of the electrolyte decreases, and the emissions of three wastes increase. CN116288430A suppresses the occurrence of the hydrogen evolution side reaction and improves the selectivity of the dechlorination reaction on the cathode by increasing the substrate concentration and gradually reducing the electrolysis current to reduce the degree of electrode polarization. Under this condition, the current efficiency of 4-amino-3,6-dichloropicolinic acid can reach about 66%, but there are still obvious side reactions (mainly hydrogen evolution reaction). The evolution of a large amount of hydrogen not only affects the production cost, but also poses a safety hazard to production. CN115645814A proposed a low-loading palladium electrode, which utilizes the electrocatalytic activity of palladium for the hydrodechlorination reaction and the storage characteristics of palladium for atomic hydrogen to improve the dechlorination reaction efficiency and suppress the occurrence of the hydrogen evolution side reaction. However, considering the high price and limited reserves of precious metals, it is difficult to be widely used in industrial production. Summary of the Invention

[0005] The object of the present invention is to provide a powder electrode for the electrolytic reduction dechlorination of chloropicolinic acid, its preparation method and its application in the electrolytic reduction dechlorination of chloropicolinic acid. The hydrogen storage alloy powder electrode coated with silver particles is used to replace the pure silver electrode. By utilizing the performance of the hydrogen storage alloy to reversibly absorb and release hydrogen atoms, some of the active hydrogen atoms participating in the hydrogen evolution side reaction are stored and reused in the dechlorination reaction, while improving the dechlorination performance and solving the problems of low current efficiency and safety hazards of hydrogen evolution caused by the hydrogen evolution side reaction. In addition, the present invention can also solve the electrode loss problem caused by the regular activation of the silver electrode in the prior art and reduce the electrode cost.

[0006] To achieve the above-mentioned invention object, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a powder electrode for dechlorination of chloropicolinic acid by electrolytic reduction, which is composed of a current collector and a catalytic layer on the surface of the current collector. The catalytic layer includes a catalytic material, and the catalytic material is obtained by plating silver on the surface of an activated hydrogen storage alloy powder. After the surface of the hydrogen storage alloy powder is plated with silver, the weight gain is 80-120 mg / g.

[0008] The chloropicolinic acid described in the present invention can be 3,4,5,6-tetrachloropicolinic acid, 3,5,6-trichloropicolinic acid, 4-amino-3,6-dichloropicolinic acid, etc.

[0009] In the present invention, the current collector can be a current collector such as nickel foam, copper foam or aluminum foam. The current collector can be a commercially available product or can be prepared by itself according to the prior art. For example, the copper foam can be prepared by the following method: electrolyzing a copper sheet in an electrolyte of a diaphragm-free electrolytic cell containing 5-50 mmol / L copper sulfate and 0.5-2 mol / L sulfuric acid for 1-15 min, the electrolysis temperature is 20-50 °C, and the current density is 100-500 mA / cm 2 , obtaining a copper sheet with a porous copper foam thin layer, which is the copper foam current collector. Preferably, the concentration of copper sulfate in the electrolyte is 30-50 mmol / L, and the concentration of sulfuric acid is 0.5-1 mol / L. Preferably, the electrolysis temperature is 20-30 °C, and the electrolysis time is 10-15 min.

[0010] Preferably, the catalytic layer further includes PVA as a binder and a conductive agent. Further preferably, the mass ratio of the catalytic material to PVA is 10:0.4-5.

[0011] Preferably, the size of the hydrogen storage alloy powder is 5-30 μm.

[0012] Preferably, the hydrogen storage alloy is an AB 5 type rare earth nickel-based hydrogen storage alloy.

[0013] Preferably, the catalytic material is prepared by the following method: first, the hydrogen storage alloy powder is activated, and then the activated hydrogen storage alloy powder is added to a reaction system for preparing silver nanoparticles by reducing silver ammonia solution with formaldehyde as a reducing agent. After being dispersed evenly, a reduction reaction is carried out under ultrasonic oscillation conditions to deposit the generated silver particles on the surface of the hydrogen storage alloy powder, and then it is washed with water and dried to obtain the catalytic material.

[0014] As a further preference, the hydrogen storage alloy powder is subjected to activation treatment by the following method: adding the hydrogen storage alloy powder into an aqueous potassium hydroxide solution containing potassium borohydride, stirring at 50-85 °C for 4-10 h, filtering, washing with water until neutral, and then drying in vacuum to obtain the activated hydrogen storage alloy powder; in the aqueous potassium hydroxide solution containing potassium borohydride, the concentration of potassium borohydride is 0.4-1.2 M, and the concentration of potassium hydroxide is 3-6 M. More preferably, the vacuum drying conditions are: drying in a vacuum oven at 40-60 °C for 8-12 h. More preferably, in the aqueous potassium hydroxide solution containing potassium borohydride, the concentration of potassium borohydride is 0.5-1.0 mol / L, the concentration of KOH is 6 mol / L, the treatment temperature is 70-80 °C, and the stirring time is 6-10 h.

[0015] As a further preference, the reaction system for preparing silver nanoparticles by reducing silver ammonia solution with formaldehyde as a reducing agent is obtained by the following method: adding water, absolute ethanol, 25 wt% ammonia water, silver nitrate and 38 wt% formaldehyde aqueous solution into a reaction vessel, wherein the volume ratio of water, absolute ethanol and 25 wt% ammonia water is (3.5-4.5):5:(0.5-1.5) (more preferably 4-4.5:5:1-1.5), the feeding concentration of silver nitrate is 20-50 g / L (more preferably 35-40 g / L), and the feeding concentration of 38 wt% formaldehyde aqueous solution is 8-12 mL / L (more preferably 10-15 mL / L); and, adding the activated hydrogen storage alloy powder into the reaction system for preparing silver by reducing silver ammonia solution with formaldehyde as a reducing agent, so that the feeding concentration of the hydrogen storage alloy powder is 45-55 g / L.

[0016] As a further preference, the reduction reaction conditions in the preparation process of the catalytic material are: the ultrasonic output power is 300 W, the frequency is 38 kHz, and the reaction is carried out at 20-50 °C for 30 min-2 h until the plating is completed.

[0017] As a further preference, in the preparation process of the catalytic material, the drying is carried out in a vacuum oven at 80-120 °C.

[0018] In the second aspect, the present invention provides a preparation method of the powder electrode for electrocatalytic reduction dechlorination of chloropicolinic acid described in the first aspect, comprising the following steps:

[0019] Step 1: Activate the hydrogen storage alloy powder;

[0020] Step 2: Prepare the catalytic material according to the method described in the first aspect;

[0021] Step 3: Add the catalytic material obtained in step (2) into an aqueous PVA solution with a mass fraction of 2-10%. After stirring evenly, a paste-like substance is obtained. The paste-like substance is evenly coated on the current collector, and after standing for 5-10 min, it is dried in vacuum, and finally tableted to obtain a powder electrode for dechlorination by electrocatalytic reduction of chloropicolinic acid.

[0022] The implementation details of step 1 and step 2 are the same as those in the first aspect and will not be elaborated here.

[0023] Preferably, in step 3, the mass ratio of the catalytic material to PVA in the feed is 10:0.4-5, more preferably 10:0.8-1.2.

[0024] Preferably, in step 3, the mass fraction of PVA is 5%.

[0025] Preferably, in step 3, the vacuum drying conditions are: drying in a vacuum oven at 60 °C for 12 h.

[0026] In the third aspect, the present invention provides an application of the powder electrode described in the first aspect in dechlorination by electrocatalytic reduction of chloropicolinic acid.

[0027] Specifically, in the application, a diaphragm plate-and-frame electrolytic cell is used. The main structure of the diaphragm plate-and-frame electrolytic cell includes a cathode chamber and an anode chamber that are not connected to each other. The powder electrode serves as the cathode;

[0028] The application includes the following steps:

[0029] Step ①: Prepare the electrolyte: The cathode solution is a 0.5-2.5 mol / L aqueous sodium hydroxide solution containing 0.1-1.5 mol / L of the raw material, and the raw material is chloropicolinic acid; the anode solution is a 1.0 mol / ~3 mol / aqueous sodium hydroxide solution; and the cathode solution and the anode solution are respectively filled into the cathode chamber and the anode chamber;

[0030] Step ②: Connect the power supply, set the reaction temperature to 30-60 °C, and perform constant current electrolysis at different current densities in three stages. The current densities in the three stages are 10-15 A / dm 2 , 5-10 A / dm 2 , 0-5 A / dm 2 , and the electrolysis charges in the three stages are 1.0-1.6 times the theoretical charge, 0.4-0.8 times the theoretical charge, and 0.1-0.5 times the theoretical charge respectively. React continuously in this way until the reaction is complete. After the electrolysis is completed, filter out the electrolyte, cool and crystallize, filter and dry to obtain the dechlorinated product.

[0031] Preferably, the application further includes the following steps:

[0032] Before electrolysis in step ②, the electrode is first activated by constant current charge and discharge 4-5 times to make the electrode reach a stable hydrogen storage capacity, and the electrode is immersed in 0.5 mol / L glucose before use. As a further preference, the conditions for each activation are 10-20 A / dm 2 The lower charge and discharge activation is carried out for 3-5 min, and the hydrogen evolution situation of the electrode is observed before electrolysis to determine the electrode activity situation.

[0033] As a preference, in step ①, in the cathode liquid, the raw material feeding concentration is 0.8~1.2 mol / L, and the sodium hydroxide feeding concentration is 1.5~2.0 mol / L; in the anode liquid, the sodium hydroxide feeding concentration is 1.5~2.0 mol / L.

[0034] As a preference, in step ①, a catalytic material is added to the cathode liquid, and the catalytic material is the same as that in the powder electrode, so that the concentration of the catalytic material in the cathode liquid is 10~25 g / L, more preferably 20 g / L.

[0035] As a preference, in step ②, the total electrolysis time is 4.5~6 h.

[0036] In the electrolysis reaction at 30-60 °C of the present invention, the electrolysis of chloropicolinic acid is prone to over-dechlorination reaction and hydrogen evolution reaction, so there are relatively high requirements for the fluid distribution. As a preference, both the cathode chamber and the anode chamber are laterally fed and bottom-in and top-out. A plurality of liquid distribution plates are arranged at intervals in the horizontal direction in the cathode chamber, and a plurality of liquid inlet baffles are arranged at intervals in the vertical direction between the lowermost liquid distribution plate and the bottom of the cathode chamber. The liquid inlet baffles and the liquid distribution plates are both uniformly provided with a plurality of liquid permeable holes for liquid to pass through. The cathode chamber is divided into two regions by the lowermost liquid distribution plate, the lower part is the fluid distribution region, and the upper part is the reaction region. The present invention improves the fluid distribution uniformity and the liquid residence time by arranging the liquid inlet baffles and the liquid distribution plates, thereby effectively improving the reaction selectivity. As a further preference, the volume percentage of the reaction region in the fluid distribution region and the reaction region is 70-80%. As a further preference, the distribution density of the liquid distribution plates is 2-3 pieces / 10 cm reaction region height, and the distribution density of the liquid inlet baffles is 2-3 pieces / 10 cm fluid distribution region length; the size of the liquid permeable holes of the liquid distribution plates is 2-3 mm, and the spacing of the liquid permeable holes is 4-5 mm; the diameter of the liquid permeable holes of the liquid inlet baffles is 2-3 mm, and the spacing of the liquid permeable holes is 4-5 mm. As a further preference, both sides of the liquid inlet baffles and the liquid distribution plates are fixed on the wall surface of the tank body through grooves, and the widths of the liquid inlet baffles and the liquid distribution plates are both smaller than the width of the tank body so that there is a certain space between the non-fixed sides of the liquid inlet baffles and the liquid distribution plates and the wall surface of the tank body.

[0037] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0038] (1) The present invention uses a powder electrode for the electrolytic reduction dechlorination of chloropicolinic acid, effectively inhibiting the occurrence of the hydrogen evolution side reaction, significantly improving the current efficiency of the dechlorination reaction of chloropicolinic acid, and avoiding the large generation of hydrogen.

[0039] (2) The present invention uses a powder electrode for electrolysis, solving the electrode powder loss caused by the regular activation of the current silver electrode. At the same time, only a small amount of hydrogen storage alloy powder can achieve a good hydrogen storage effect, effectively reducing the electrode cost and maintenance cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is the main structure diagram of the electrolytic cell used in the electrolysis of the present invention; wherein 1 - end plate, 2 - current collector plate, 3 - cathode electrode, 4 - cathode plate frame, 5 - gasket, 6 - cation exchange membrane, 7 - gasket, 8 - anode plate frame, 9 - anode electrode, 10 - current collector plate, 11 - end plate.

[0041] Figure 2 It is the entire device flow chart of the electrolysis process of the present invention, wherein 12 - liquid storage tank, 13 - constant temperature water bath, 14 - magnetic circulation pump, 15 - liquid discharge port, 16 - magnetic circulation pump, 17 - buffer.

[0042] Figure 3 It is the internal structure schematic diagram of the cathode chamber of the present invention, wherein 18 - liquid inlet baffle, 19 - liquid distribution plate.

[0043] Figure 4 It is the liquid chromatography analysis chart of the electrolysis product in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0044] The following examples will further illustrate the present invention in conjunction with the accompanying drawings, but the protection scope of the present invention is not limited thereto:

[0045] For those not specified in the embodiments of the present invention, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments not specified for the manufacturer, they are all conventional products that can be obtained by conventional technical means or purchased commercially.

[0046] The ultrasonic instrument used in the embodiments of the present invention is a KQ5200DB type numerical control ultrasonic cleaner.

[0047] The hydrogen storage alloy powder used in the embodiments of the present invention is LaNi 5 type hydrogen storage alloy powder, with a size of 5 - 30 μm.

[0048] Such as Figure 1As shown in the figure, the main structure of the diaphragm plate and frame electrolytic cell includes a non-communicating cathode chamber 4 and anode chamber 8. A cation exchange membrane 6 is located between the two plate and frame electrolytic cells. On the cathode side, from the inside out, there are a gasket 5, cathode chamber 4, cathode electrode 3, current collector plate 2, and end plate 1 in sequence. On the anode side, from the inside out, there are a gasket 7, anode chamber 8, anode electrode 9, current collector plate 10, and end plate 11 in sequence. Both the cathode chamber 4 and anode chamber 8 have exhaust holes at the top. The cathode chamber 4 and anode chamber 8 are both ordinary plate and frame cells, with lateral liquid inlet and bottom-in and top-out liquid flow. Each component is fixed by bolts.

[0049] In the embodiment of the present invention, the main structure of the electrolytic cell is made of polypropylene material. The end plate is an aluminum alloy plate, the current collector plate is a copper sheet, and the gasket material is polytetrafluoroethylene. Polypropylene and polytetrafluoroethylene have excellent chemical stability, corrosion resistance, sealing performance, high lubricity and non-stickiness, electrical insulation, and good anti-aging endurance, and can work long-term at a temperature of 30 - 80 °C. Therefore, they are suitable as the materials for the electrolytic cell of the present invention.

[0050] The device and flow chart of the cathode electrolysis of the present invention are as Figure 2 shown in the figure. The device includes a cathode liquid storage tank 12, a magnetic circulation pump 14, a drain port 15, a magnetic circulation pump 16, a cathode chamber 4, and a buffer 17 arranged in sequence. Each device is connected by a PVC pipeline. The liquid inlet of the storage tank 12 is located at the top, and the liquid outlet is located at the bottom. The drain port is located at a tee joint. The storage tank 12 is temperature-controlled by a constant temperature water bath 13. The cathode liquid circulates among the above-mentioned devices during electrolysis. There are three bottle mouths at the top of the cathode liquid storage tank for adding alkali solution, measuring temperature, and exhausting gas. The anode electrolysis has a similar device and flow chart.

[0051] Figure 3 is a schematic diagram of the internal structure of the cathode chamber 4. A plurality of liquid distribution plates 19 are arranged at intervals in the horizontal direction in the cathode chamber 4. A plurality of liquid inlet baffles 18 are arranged at intervals in the vertical direction between the lowermost liquid distribution plate and the bottom of the cathode chamber 4. Both the liquid inlet baffles 18 and the liquid distribution plates 19 are evenly provided with a plurality of liquid permeable holes for liquid to pass through. The cathode chamber is divided into two regions by the lowermost liquid distribution plate, with the lower region being the fluid distribution region and the upper region being the reaction region. Preferably, the width of the liquid distribution plate 19 is less than the width of the cell body. Both sides of the liquid inlet baffles 18 and the liquid distribution plates 19 are fixed by grooves.

[0052] The overall dimensions of the cathode chamber and anode chamber adopted in the embodiment of the present invention (i.e., Figure 1The overall dimensions of the plate frame shown in Figures 4 and 8 are 14×14×2 cm. The size of the reaction area in the cathode chamber is 10×10×2 cm. The size of the liquid distribution plate is 10.5×1.6 cm, and they are arranged at equal intervals. When two liquid distribution plates are used, the spacing is 4.8 cm; when three liquid distribution plates are used, the spacing is 3.2 cm. The size of the liquid inlet baffle is 2.1×1.6 cm. When three liquid inlet baffles are used, the spacing is 2.2 cm, and the distance from the liquid inlet baffle closest to the liquid inlet to the liquid inlet is 2.2 cm; when two liquid inlet baffles are used, the spacing is 3.3 cm, and the distance from the liquid inlet baffle closest to the liquid inlet to the liquid inlet is 2.2 cm. Both the liquid inlet baffle 18 and the liquid distribution plate 19 are evenly provided with a plurality of liquid permeable holes for liquid passage. The aperture of the liquid permeable holes is 2 mm or 3 mm. The hole spacing of the liquid inlet baffle is 5 mm, and the hole spacing of the liquid distribution plate is 4 mm.

[0053] Example 1:

[0054] The parameters of the cathode chamber used in Example 1 are: 2 liquid inlet baffles; 2 liquid distribution plates; the aperture is 2 mm for both.

[0055] (1) Add 20 g of hydrogen storage alloy powder to a solution containing 0.5 mol / L potassium borohydride and 6 mol / L potassium hydroxide, stir at 80 °C for 6 h, filter, wash with distilled water until neutral, and then place in a vacuum oven at 60 °C for vacuum drying for 10 h.

[0056] (2) Mix 160 ml of water, 190 ml of absolute ethanol, and 40 ml of ammonia water (25 wt%) in a beaker. Add 15 g of silver nitrate and 5 ml of formaldehyde (38 wt%). Add 20 g of the hydrogen storage alloy powder treated in step (1) to the solution. After dispersing and mixing evenly for 5 min with an ultrasonic power of 300 W and a frequency of 38 kHz, then ultrasonically treat in a 25 °C water bath until the plating is completed; the reaction time is 1.5 h. After the plating is completed, take out the beaker, wash three times with deionized water, and place the obtained powder in a vacuum oven at 80 °C for drying. After weighing, the weight gain of the hydrogen storage alloy powder after silver plating is 87 mg / g.

[0057] (3) Electrolyze a copper sheet with dimensions of 10×10×0.15 cm in the electrolyte of a diaphragm-free electrolytic cell containing 50 mmol / L copper sulfate and 0.8 mol / L sulfuric acid for 10 min. The electrolysis temperature is 30 °C, and the current density is 300 mA / cm 2 , to obtain a thin layer with porous copper foam, which is the current collector.

[0058] (4)Using the washed and dried copper foam as the current collector, weigh 10 g of the treated alloy and add it to 20 mL of an aqueous PVA solution with a mass fraction of 5%. After stirring evenly, coat the paste-like substance evenly on the copper foam substrate. After standing for 5 min, place the electrode in a vacuum oven and dry it at 60 °C for 6 h. After drying, then press it into a tablet to obtain a powder electrode with dimensions of 10×10×0.15 cm.

[0059] (5)Prepare the electrolyte: The cathode solution is 500 ml of a 1.5 mol / L sodium hydroxide solution containing 0.6 mol / L of 4-amino-3,5,6-trichloropicolinic acid, and the anode solution is a 2.0 mol / L sodium hydroxide solution; Add the cathode solution and the anode solution to two electrode chambers respectively, with the flow rates of 2 L / min each. The powder cathode is charged and discharged at a constant current of 10 A / dm 2 for 3 min, and repeat 5 times for activation to make the cathode reach a stable hydrogen storage capacity. The anode is a Hastelloy electrode. Connect the power supply, set the reaction temperature to 50 °C, and perform constant current electrolysis with the current divided into three-stage current densities. The current densities of the three stages are 12 A / dm 2 , 6 A / dm 2 , 3 A / dm 2 respectively. The electrolysis times of the three stages are 1.87 h, 1.6 h, and 1.6 h respectively. The electrolyte is continuously circulated until the reaction ends. After electrolysis, filter out the electrolyte, cool it to crystallize, filter and dry it to obtain 4-amino-3,6-dichloropicolinic acid.

[0060] (6)After electrolysis, take a part of the electrolyte for filtration, dilution with the mobile phase and analysis by liquid chromatography. The mobile phase ratio is water:methanol:acetonitrile = 6:3:1, and the pH is adjusted to 2. Figure 4 It is the liquid chromatography analysis chart of the electrolysis product in Example 1. The results show that the yield of 4-amino-3,6-dichloropicolinic acid is 90.2%, and the current efficiency is 73.7%.

[0061] Example 2: Optimization of the preparation conditions of the powder electrode

[0062] The electrolysis device used in Example 2 is the same as that in Example 1.

[0063] (1)Add 20 g of hydrogen storage alloy powder to a solution containing 1.0 mol / L potassium borohydride and 6 mol / L potassium hydroxide, stir at 80 °C for 10 h, filter and wash with distilled water until neutral, and then place it in a vacuum oven at 60 °C for vacuum drying for 10 h.

[0064] (2) Mix 160 ml of water, 190 ml of absolute ethanol, and 40 ml of ammonia water (25%) in a beaker. Add 15 g of silver nitrate and 5 ml of formaldehyde (38%). Then add 20 g of the hydrogen storage alloy powder treated in step (1) to the solution. After dispersing and mixing evenly for 5 min with an ultrasonic power of 300 W and a frequency of 38 kHz, ultrasonically treat it in a 25°C water bath until the plating is completed. The reaction time is 3 h. After the plating is completed, take out the beaker, wash it three times with deionized water, and put the obtained powder into a vacuum oven at 80°C to dry. After weighing, the weight gain of the hydrogen storage alloy powder after silver plating is 95 mg / g.

[0065] (3) Electrolyze a copper sheet with dimensions of 10×10×0.15 cm in the electrolyte of a diaphragmless electrolytic cell containing 50 mmol / L of copper sulfate and 0.8 mol / L of sulfuric acid for 10 min. The electrolysis temperature is 30°C, and the current density is 300 mA / cm 2 , to obtain a thin layer with porous copper foam, which is the current collector.

[0066] (4) Using the washed and dried copper foam as the current collector, weigh 10 g of the treated alloy and add it to 20 mL of a 5% PVA aqueous solution. After stirring evenly, coat the paste evenly on the copper foam substrate. After standing for 5 min, put the electrode into a vacuum oven and dry it at 60°C for 6 h. After drying, then press it into a tablet to obtain a powder electrode with dimensions of 10×10×0.15 cm.

[0067] (5) Prepare the electrolyte: The cathode solution is a 1.5 mol / L sodium hydroxide solution containing 0.6 mol / L of 4-amino-3,5,6-trichloropicolinic acid, and the anode solution is a 2.0 mol / L sodium hydroxide solution; add the cathode solution and the anode solution to two electrode chambers respectively, with a flow rate of 2 L / min each. The powder cathode is charged and discharged at a constant current of 10 A / dm 2 for 3 min and repeated 5 times for activation to make the cathode reach a stable hydrogen storage capacity. The anode is a Hastelloy electrode. Connect the power supply, set the reaction temperature to 50°C, and perform constant current electrolysis with the current divided into three-stage current densities. The three-stage current densities are 12 A / dm 2 , 6 A / dm 2 , 3 A / dm 2 , and the electrolysis times for the three stages are 1.87 h, 1.6 h, and 1.6 h respectively. The electrolyte is continuously circulated until the reaction ends. After the electrolysis ends, filter out the electrolyte, cool and crystallize, filter and dry to obtain 4-amino-3,6-dichloropicolinic acid.

[0068] After the electrolysis is completed, take a part of the electrolyte for filtration, dilution with the mobile phase, and analysis by liquid chromatography. The mobile phase ratio is water: methanol: acetonitrile = 6:3:1, and the pH is adjusted to 2. The liquid chromatography results of the electrolysis products: the yield of 4-amino-3,6-dichloropicolinic acid is 91.6%, and the current efficiency is 74.8%.

[0069] Example 3: Electrolysis experiment with hydrogen storage alloy powder added to the cathode solution

[0070] The electrolysis device used in Example 3 is the same as that in Example 1.

[0071] (1)-(4): The powder electrode is prepared in the same process as in Example 2.

[0072] (5) Prepare the electrolyte: The cathode solution is a 1.5 mol / L sodium hydroxide solution containing 0.6 mol / L 4-amino-3,5,6-trichloropicolinic acid, and additionally add 10 g of the hydrogen storage alloy powder obtained in step (2). The anode solution is a 2.0 mol / L sodium hydroxide solution; add the cathode solution and the anode solution into two electrode chambers respectively, with the flow rate of each being 2 L / min. The powder cathode is charged and discharged at a constant current of 10 A / dm 2 for 3 min, and repeated 5 times for activation to make the cathode reach a stable hydrogen storage capacity. The anode is a Hastelloy electrode. Connect the power supply, set the reaction temperature to 50 °C, and perform constant current electrolysis with the current divided into three-stage current densities. The current densities of the three stages are 12 A / dm 2 , 6 A / dm 2 , 3 A / dm 2 , and the electrolysis times of the three stages are 1.87 h, 1.6 h, and 1.6 h respectively. The electrolyte is continuously circulated until the reaction ends. After the electrolysis is completed, filter out the electrolyte, cool and crystallize, filter and dry to obtain 4-amino-3,6-dichloropicolinic acid.

[0073] After the electrolysis is completed, take a part of the electrolyte for filtration, dilution with the mobile phase, and analysis by liquid chromatography. The mobile phase ratio is water: methanol: acetonitrile = 6:3:1, and the pH is adjusted to 2. The liquid chromatography results of the electrolysis products: the yield of 4-amino-3,6-dichloropicolinic acid is 92.3%, and the current efficiency is 75.4%.

[0074] Example 4: Electrolysis experiment with different current densities

[0075] The electrolysis device used in Example 4 is the same as that in Example 1.

[0076] (1)-(4): The powder electrode is prepared in the same process as in Example 3.

[0077] (5)Configure the electrolyte: The catholyte is a 1.5 mol / L sodium hydroxide solution containing 0.6 mol / L 4-amino-3,5,6-trichloropicolinic acid, and additionally add 10 g of the hydrogen storage alloy powder obtained in step (2). The anolyte is a 2.0 mol / L sodium hydroxide solution. Add the catholyte and anolyte into two electrode chambers respectively, with the flow rate of each being 2 L / min. The powder cathode is subjected to constant current charge and discharge at 10 A / dm 2 for 3 min, and repeat 5 times for activation to enable the cathode to reach a stable hydrogen storage capacity. The anode is a Hastelloy electrode. Connect the power supply, set the reaction temperature to 50 °C, and perform constant current electrolysis with the current divided into three-stage current densities. The current densities of the three stages are 10 A / dm 2 , 5 A / dm 2 , 2.5 A / dm 2 respectively. The electrolysis times of the three stages are 2.24 h, 1.9 h, and 1.9 h respectively. The electrolyte is continuously circulated until the reaction ends. After electrolysis, filter out the electrolyte, cool and crystallize, filter and dry to obtain 4-amino-3,6-dichloropicolinic acid.

[0078] (6)After electrolysis, take a part of the electrolyte for filtration, dilution with the mobile phase and analysis by liquid chromatography. The mobile phase ratio is water:methanol:acetonitrile = 6:3:1, and the pH is adjusted to 2. The liquid chromatography results of the electrolysis products: The yield of 4-amino-3,6-dichloropicolinic acid is 92.7%, and the current efficiency is 75.7%.

[0079] Examples 5 - 10: Electrolysis reactions with different electrolytic cell parameters

[0080] (1)-(4): The same as the process of Example 4 to prepare a powder electrode.

[0081] (5)-(6): The same as the process of Example 4 to carry out the electrolysis reaction.

[0082] The difference between the electrolysis device used and that of Examples 1 - 4 lies in the change of the internal structure of the cathode chamber. The electrolysis is carried out by changing the number and pore diameter of the liquid inlet baffle 18 and the number and pore diameter of the liquid distribution plate 19. The changed parameters and reaction results are shown in Table 1:

[0083] Table 1

[0084]

[0085] Example 11: Stability experiment of multiple electrolyses of the powder electrode

[0086] (1)-(5): The same as the process of Example 7 to prepare a powder electrode and add the hydrogen storage alloy powder to the catholyte, and carry out electrolysis and post-treatment under the same conditions. Five electrolysis experiments are carried out under the same conditions.

[0087] After the electrolysis was completed, a part of the electrolyte solution was taken for filtration, dilution with the mobile phase, and analyzed by liquid chromatography. The mobile phase ratio was water: methanol: acetonitrile = 6:3:1, and the pH was adjusted to 2. The results of liquid chromatography of the electrolysis products were as follows: the yield of 4-amino-3,6-dichloropicolinic acid was 92.8%, and the current efficiency was 75.7%. There was no obvious powder shedding on the electrode.

[0088] Example 12: Electrolysis experiment of substrate 3,4,5,6-tetrachloropicolinic acid

[0089] (1)-(4): The same as the process of Example 2, a powder electrode was prepared.

[0090] (5) Prepare the electrolyte solution: The cathode solution was 500 mL of 0.5 mol / L sodium hydroxide solution containing 0.1 mol / L 3,4,5,6-tetrachloropicolinic acid, and 10 g of the hydrogen storage alloy powder obtained in step (2) was additionally added. The anode solution was 500 mL of 1.0 mol / L sodium hydroxide solution; the cathode solution and the anode solution were respectively added to two electrode chambers, and the flow rates were each 2 L / min. The powder cathode was charged and discharged at a constant current of 10 A / dm 2 for 3 min, and repeated 5 times for activation to make the cathode reach a stable hydrogen storage capacity. The anode was a Hastelloy electrode. The power supply was turned on, the reaction temperature was set to 30 °C, and the current density was 50 A / dm 2 for constant current electrolysis for 0.67 h. The electrolyte solution was continuously circulated until the reaction ended. After the electrolysis was completed, the electrolyte solution was filtered out, cooled and crystallized, filtered and dried to obtain 3,6-dichloropicolinic acid.

[0091] (6) After the electrolysis was completed, a part of the electrolyte solution was taken for filtration, dilution with the mobile phase, and analyzed by liquid chromatography. The mobile phase ratio was water: methanol: acetonitrile = 5:2:3, and the pH was adjusted to 2. The results of liquid chromatography of the electrolysis products were as follows: the yield of 3,6-dichloropicolinic acid was 95.3%, and the current efficiency was 76.2%.

[0092] Comparative Example 1: Electrolysis experiment of silver mesh electrode

[0093] (1) Using a silver mesh as the working electrode; a nickel alloy of the same area as the counter electrode; mercury / mercuric oxide as the reference electrode. Using 0.5 mol / L NaCl + 0.5 mol / L NaOH aqueous solution as the working electrode solution and 1.5 mol / L sodium hydroxide aqueous solution as the counter electrode solution. The temperature of the working electrode solution was controlled at 25-30 °C, and an anodic oxidation current of 0.1 A / dm was applied to the silver mesh for 3 minutes; then the current direction was changed, and a cathodic reduction current of 0.1 A / dm was applied to the silver mesh for 3 minutes. The silver electrode was taken out and placed in deionized water to obtain an activated silver mesh for standby. 2 2

[0094] (2) Preparation of electrolyte: The catholyte is 500 mL of 1.5 mol / L sodium hydroxide solution containing 0.6 mol / L 4-amino-3,5,6-trichloropicolinic acid, and the anolyte is 500 mL of 2.0 mol / L sodium hydroxide solution; the catholyte and the anolyte are respectively added into two electrode chambers, and the flow rate is 2 L / min for each. The powder cathode is charged and discharged at a constant current of 10 A / dm 2 for 3 min and repeated 5 times for activation to enable the cathode to reach a stable hydrogen storage capacity. The anode is a Hastelloy electrode. The power supply is turned on, and the reaction temperature is set at 50 °C. The electrolysis is carried out at a constant current with the current divided into three-stage current densities. The current densities of the three stages are 12 A / dm 2 , 6 A / dm 2 , 3 A / dm 2 respectively. The electrolysis times of the three stages are 1.87 h, 1.6 h, and 1.6 h respectively. The reaction is continuously cycled until the reaction is complete. After the electrolysis, the electrolyte is filtered out, cooled and crystallized, filtered and dried to obtain 4-amino-3,6-dichloropicolinic acid.

[0095] (3) After the electrolysis, a part of the electrolyte is taken for filtration, dilution with the mobile phase and analyzed by liquid chromatography. The mobile phase ratio is water: methanol: acetonitrile = 6:3:1, and the pH is adjusted to 2. Under the same electrolysis with the same amount of electricity, the liquid chromatography results of the product: the yield of 4-amino-3,6-dichloropicolinic acid is 75.1%, and the current efficiency is 61.3%.

[0096] Comparative Example 2: Electrolysis experiment without activation treatment of the powder electrode

[0097] (1)-(4): The powder electrode is prepared in the same process as in Example 1.

[0098] (5): The powder electrode is not charged and discharged for activation before the reaction, and the electrolysis reaction is carried out in the same process as in Example 1.

[0099] (6) After the electrolysis, a part of the electrolyte is taken for filtration, dilution with the mobile phase and analyzed by liquid chromatography. The mobile phase ratio is water: methanol: acetonitrile = 6:3:1, and the pH is adjusted to 2. The liquid chromatography analysis results of the electrolysis product: the yield of 4-amino-3,6-dichloropicolinic acid is 83.3%, and the current efficiency is 68.1%.

[0100] Comparative Example 3: Electrolysis experiment without the inlet baffle and liquid distribution plate in the electrolytic cell

[0101] The cathode chamber of the electrolytic cell used in Comparative Example 3 has no inlet baffle and liquid distribution plate.

[0102] (1)-(4): The powder electrode is prepared in the same process as in Example 4.

[0103] (5) Preparation of electrolyte: The catholyte is a 1.5 mol / L sodium hydroxide solution containing 0.6 mol / L 4-amino-3,5,6-trichloropicolinic acid in 500 mL. Additionally, 10 g of the hydrogen storage alloy powder obtained in step (2) is added. The anolyte is a 2.0 mol / L sodium hydroxide solution in 500 mL. The catholyte and anolyte are respectively added to two electrode chambers, with flow rates of 2 L / min each. The powder cathode is subjected to constant current charge and discharge at 10 A / dm 2 for 3 min and repeated 5 times for activation to enable the cathode to reach a stable hydrogen storage capacity. The anode is a Hastelloy electrode. The power supply is turned on, and the reaction temperature is set at 50 °C. The current is subjected to constant current electrolysis at three-stage current densities. The current densities at the three stages are 12 A / dm 2 , 6 A / dm 2 , 3 A / dm 2 respectively. The electrolysis times at the three stages are 1.87 h, 1.6 h, and 1.6 h respectively. Such continuous cyclic reactions are carried out until the reaction is complete. After electrolysis, the electrolyte is filtered out, cooled and crystallized, filtered and dried to obtain 4-amino-3,6-dichloropicolinic acid.

[0104] (6) After electrolysis, a part of the electrolyte is taken for filtration, dilution with the mobile phase and analyzed by liquid chromatography. The mobile phase ratio is water: methanol: acetonitrile = 6:3:1, and the pH is adjusted to 2. The liquid chromatography results of the electrolysis products show that the yield of 4-amino-3,6-dichloropicolinic acid is 86.9%, and the current efficiency is 70.9%.

[0105] Comparative Example 4: Stability experiment of silver mesh electrode for multiple electrolyses

[0106] (1)-(5): Using a silver mesh electrode as the cathode electrode, electrolysis and post-treatment are carried out under the same conditions as in Example 11, and 5 electrolysis experiments are carried out under the same conditions.

[0107] (6) After electrolysis, a part of the electrolyte is taken for filtration, dilution with the mobile phase and analyzed by liquid chromatography. The mobile phase ratio is water: methanol: acetonitrile = 6:3:1, and the pH is adjusted to 2. The liquid chromatography results of the electrolysis products show that the yield of 4-amino-3,6-dichloropicolinic acid is 74.7%, and the current efficiency is 60.9%. The electrolysis effect can maintain good stability, but obvious powder detachment occurs.

[0108] Comparative Example 5: Electrolysis experiment of silver mesh electrode for substrate 3,4,5,6-tetrachloropicolinic acid

[0109] (1)-(5): Using a silver mesh electrode as the cathode electrode, electrolysis and post-treatment are carried out under the same conditions as in Example 12.

[0110] After the electrolysis is completed, a part of the electrolyte solution is taken for filtration, dilution with the mobile phase and analyzed by liquid chromatography. The mobile phase ratio is water: methanol: acetonitrile = 5: 2: 3, and the pH is adjusted to 2. The results of liquid chromatography of the electrolysis products are as follows: the yield of 3,6-dichloropicolinic acid is 78.2%, and the current efficiency is 62.5%. The electrolysis effect can maintain good stability, but obvious powder removal occurs.

Claims

1. A powder electrode for dechlorination of chloropicolinic acid by electrolytic reduction, characterized in that: The powder electrode is composed of a current collector and a catalytic layer on the surface of the current collector, wherein the catalytic layer includes a catalytic material, and the catalytic material is obtained by plating silver on the surface of a hydrogen storage alloy powder that has been activated. After the surface of the hydrogen storage alloy powder is plated with silver, the weight gain is 80-120 mg / g; the hydrogen storage alloy is an AB5 type rare earth nickel series hydrogen storage alloy, and the hydrogen storage alloy powder is activated by the following method: adding the hydrogen storage alloy powder to a potassium hydroxide aqueous solution containing potassium borohydride, stirring for 4-10 hours at 50-85°C, filtering, washing with water to neutrality, and then vacuum drying to obtain the activated hydrogen storage alloy powder; in the potassium hydroxide aqueous solution containing potassium borohydride, the potassium borohydride concentration is 0.4-1.2 M, and the potassium hydroxide concentration is 3-6 M.

2. The powder electrode for dechlorination of chloropicolinic acid by electrolytic reduction as claimed in claim 1, characterized in that: The catalytic layer also includes PVA as a binder and a conductive agent.

3. The powder electrode for dechlorination of chloropicolinic acid by electrolytic reduction as claimed in claim 1, characterized in that: The size of the hydrogen storage alloy powder is 5-30 μm.

4. The powder electrode for dechlorination of chloropicolinic acid by electrolytic reduction as claimed in claim 1, characterized in that: The catalytic material is prepared by the following method: firstly, hydrogen storage alloy powder is activated, then the activated hydrogen storage alloy powder is added to a reaction system in which silver nanoparticles are prepared by reducing silver ammonia solution with formaldehyde as a reducing agent, and after being evenly dispersed, a reduction reaction is carried out under ultrasonic vibration conditions to plate the generated silver particles onto the surface of the hydrogen storage alloy powder, and then the catalytic material is obtained by washing with water and drying.

5. A method for preparing a powder electrode for dechlorination by electrolytic reduction of chloropicolinic acid as claimed in any one of claims 1 to 4, characterized in that: The preparation method comprises the following steps: Step 1: Activating the hydrogen storage alloy powder; Step 2: obtaining a catalytic material by coating silver on the surface of the activated hydrogen storage alloy powder; Step 3: Add the catalytic material obtained in step (2) to a PVA aqueous solution with a mass fraction of 2-10%, stir evenly to obtain a paste, evenly coat the paste on the current collector, let it stand for 5-10 minutes, then vacuum dry, and finally press into sheets to obtain a powder electrode for dechlorination by electrolytic reduction of chloropicolinic acid.

6. Use of the powder electrode as claimed in any one of claims 1 to 4 in dechlorination by electrolytic reduction of chloropicolinic acid.

7. The use according to claim 6, characterized in that: The application adopts a diaphragm plate-frame electrolytic cell, the main structure of which includes a cathode chamber and an anode chamber that are not connected to each other, and the powder electrode is used as a cathode; The application comprises the following steps: Step ①: preparing an electrolyte: the cathode liquid is a 0.5-2.5 mol / L sodium hydroxide aqueous solution containing 0.1-1.5 mol / L raw material, wherein the raw material is chloropicolinic acid; the anolyte is a 1.0 mol / ~3 mol / L sodium hydroxide aqueous solution; and the cathode liquid and the anolyte are respectively placed in the cathode chamber and the anode chamber; Step ②: Turn on the power, set the reaction temperature to 30-60℃, and perform constant current electrolysis at different current densities in three stages. The current densities in the three stages are 10~15 A / dm 2 , 5~10 A / dm 2 , 0~5 A / dm 2 The electrolysis power in the three stages is 1.0~1.6 times the theoretical power, 0.4~0.8 times the theoretical power, and 0.1~0.5 times the theoretical power, respectively. The reaction is cyclically continued until the reaction is complete. After the electrolysis is completed, the electrolyte is filtered out, cooled and crystallized, filtered and dried to obtain the dechlorinated product.

8. The use according to claim 7, characterized in that: The application further comprises the following steps: Before electrolysis in step ②, use constant current charge and discharge 4-5 times for activation to allow the powder electrode to reach a stable hydrogen storage capacity.

9. The use according to claim 7, characterized in that: In step ①, a catalytic material is added to the cathode liquid, wherein the catalytic material is the same as the catalytic material in the powder electrode, so that the concentration of the catalytic material in the cathode liquid is 10-25 g / L.

10. The use according to claim 7, characterized in that: The cathode chamber and the anode chamber both have liquid inlet from the side and liquid inlet from the bottom and outlet from the top. The cathode chamber has a plurality of liquid distribution plates spaced apart in the horizontal direction, and a plurality of liquid inlet baffles spaced apart in the vertical direction between the lowest liquid distribution plate and the bottom of the cathode chamber. The liquid inlet baffles and the liquid distribution plates are both evenly provided with a plurality of liquid permeable holes for liquid to pass through. The cathode chamber is divided into two areas by the lowest liquid distribution plate, the lower area being a fluid distribution area, and the upper area being a reaction area.

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