Powder electrode for electrochemical reductive dechlorination of chloropicolinic acid and preparation and application of powder electrode

Abstract

The invention discloses a powder electrode for reducing and dechlorinating chloropicolinic acid by an electrochemical method as well as preparation and application of the powder electrode. The powder electrode is composed of a current collector and a catalytic layer on the surface of the current collector, the catalytic layer comprises a catalytic material, the catalytic material is obtained by plating silver on the surface of hydrogen storage alloy powder subjected to activation treatment, and after the surface of the hydrogen storage alloy powder is plated with silver, the weight increment is 80-120 mg / g. The invention provides the application of the powder electrode in chloropicolinic acid electrolytic reduction method dechlorination, the generation of hydrogen evolution side reaction is effectively inhibited, the current efficiency of chloropicolinic acid dechlorination reaction is obviously improved, the generation of a large amount of hydrogen is avoided, and the electrode cost and the maintenance cost are effectively reduced.

Description

Technical Field

[0001] The invention relates to the technical field of organic chemical industry, and in particular to a powder electrode for electrochemical reduction dechlorination of chloropicolinic acid, and a preparation method and application thereof. Background Art

[0002] Chloropyridine compounds are important "three-drug" intermediates, mainly used in the synthesis of important chemical products such as medicines, pesticides, and dyes. Among them, 4-amino-3,6-dichloropyridine carboxylic acid (AMINOPYRALID, referred to as 4-N-3,6-DCP) is an important highly active, low-toxic green pesticide, mainly used as a herbicide, insecticide and plant growth regulator, and has huge market value. 3,6-Dichloropyridine carboxylic acid (Clopyralid, referred to 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 full chlorination of pyridine and then selective reduction dechlorination. Among various methods, electrocatalytic hydrogenation dechlorination technology has become the most important industrial production route due to its advantages of high reaction selectivity, good product quality, and low emission of three wastes.

[0004] In the process of electrocatalytic hydrodechlorination, the electrode material has a decisive influence on the reaction efficiency. In order to find electrode materials with better performance, domestic and foreign researchers have carried out a lot of research. US4217185A first proposed a method for preparing 3,6-dichloropicolinic acid by electrocatalytic hydrodechlorination of 3,4,5,6-tetrachloropicolinic acid using an activated silver electrode, with a yield of 91.3%. CN1238565C proposed a method for preparing 4-amino 3,6-dichloropicolinic acid by electrocatalytic hydrodechlorination of activated silver electrode, with a yield of 61.9%. The electrode activity decayed significantly during the electrolysis process, resulting in low yield and current efficiency. CN101522628A, CN102597328A, CN110656345A, etc., by using the method of periodic polarity reversal to activate the silver electrode during the electrolysis process, successfully increased the yield of 4-amino 3,6-dichloropicolinic acid to more than 90%, but the current efficiency of electrolysis is still relatively low, between 30% and 40%. In addition, frequent polarity reversal activation is not only cumbersome to operate and has poor production stability, but also the electrode surface undergoes repeated oxidation and reduction, and the silver electrode exhibits serious powdering phenomenon, resulting in a short electrode life and high production cost. There are also reports that the efficiency of the hydrogenation dechlorination reaction on the silver cathode is improved by inhibiting the hydrogen evolution reaction by increasing the alkalinity of the electrolyte, but under high alkalinity conditions, the solubility of the raw material (3,4,5,6-tetrachloropicolinic acid) is low, the output per unit volume of electrolyte decreases, and the discharge of three wastes increases. CN116288430A suppresses the occurrence of hydrogen evolution side reaction by increasing substrate concentration, gradually reducing electrolysis current, and reducing electrode polarization degree, thereby improving the selectivity of dechlorination reaction on cathode. 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). Large amounts of hydrogen evolution not only affect production costs, but also bring hidden dangers to production safety. CN115645814A proposes a low-loaded palladium electrode, which utilizes the electrocatalytic activity of palladium for hydrogenation dechlorination reaction and the storage characteristics of palladium for atomic hydrogen to improve the efficiency of dechlorination reaction and suppress the occurrence of hydrogen evolution side reaction. However, considering the high price and limited reserves of precious metals, it is difficult to apply them to industrial production on a large scale. Summary of the invention

[0005] The present invention aims to provide a powder electrode for dechlorination by electrolytic reduction of chloropicolinic acid and a preparation method thereof and application thereof in dechlorination by electrolytic reduction of chloropicolinic acid, wherein a hydrogen storage alloy powder electrode coated with silver particles is used instead of a pure silver electrode, and the performance of the hydrogen storage alloy in reversibly absorbing and releasing hydrogen atoms is utilized to store some active hydrogen atoms participating in the hydrogen evolution side reaction and reuse them in the dechlorination reaction, thereby achieving the improvement of the dechlorination performance and solving the safety hazards of low current efficiency and 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] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical scheme:

[0007] In the 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, wherein 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 may be a current collector such as nickel foam, copper foam or aluminum foam. The current collector may be a commercially available product or may be prepared by itself according to the prior art. For example, the copper foam may be prepared by the following method: electrolyzing a copper sheet in an electrolyte containing 5-50 mmol / L copper sulfate and 0.5-2 mol / L sulfuric acid in a membrane-free electrolytic cell for 1-15 min, with an electrolysis temperature of 20-50°C and a current density of 100-500 mA / cm 2 , a copper sheet with a porous copper foam thin layer is obtained, which is a copper foam current collector. Preferably, the copper sulfate concentration in the electrolyte is 30-50 mmol / L, and the sulfuric acid concentration 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 catalyst layer further comprises PVA as a binder and a conductive agent. Further preferably, the mass ratio of the catalyst material to the 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 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 in which silver nanoparticles are prepared by reducing a silver ammonia solution with formaldehyde as a reducing agent, and after being evenly dispersed, a reduction reaction is carried out under ultrasonic oscillation 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.

[0014] As a further preference, 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 to 10 hours at 50 to 85°C, filtering and 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 to 1.2 M, and the potassium hydroxide concentration is 3 to 6 M. Further preferred vacuum drying conditions are: placing in a vacuum oven at 40 to 60°C for vacuum drying for 8 to 12 hours. Further preferred potassium hydroxide aqueous solution containing potassium borohydride, the potassium borohydride concentration is 0.5-1.0 mol / L, the KOH concentration is 6 mol / L, the treatment temperature is 70 to 80°C, and the stirring time is 6 to 10 hours.

[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: water, anhydrous ethanol, 25wt% ammonia water, silver nitrate and 38wt% formaldehyde aqueous solution are added into a reaction container, wherein the volume ratio of water, anhydrous ethanol and 25wt% 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 38wt% formaldehyde aqueous solution is 8-12 mL / L (more preferably 10-15 mL / L); and, hydrogen storage alloy powder after activation treatment is added to the reaction system for preparing silver by reducing silver ammonia solution with formaldehyde as a reducing agent, so that the feeding concentration of hydrogen storage alloy powder is 45-55 g / L.

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

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

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

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

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

[0021] 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.

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

[0023] Preferably, in step 3, the mass ratio of the catalytic material to PVA 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 condition is: placing in a vacuum oven and drying at 60° C. for 12 h.

[0026] In a third aspect, the present invention provides the use of the powder electrode described in the first aspect in the dechlorination of chloropicolinic acid by electrolytic reduction.

[0027] Specifically, 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 serves as a cathode;

[0028] The application comprises the following steps:

[0029] 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;

[0030] 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.

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

[0032] Before the electrolysis in step ②, the electrode is activated by charging and discharging at a constant current for 4-5 times to achieve a stable hydrogen storage capacity. The electrode is soaked in 0.5 mol / L glucose before use. As a further preference, the activation condition is 10-20 A / dm 2 Charge and discharge for 3-5 min to activate the electrode, and observe the hydrogen evolution at the electrode before electrolysis to determine the electrode activity.

[0033] Preferably, 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] Preferably, in step ①, a catalytic material is added to the cathode liquid, and 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, more preferably 20 g / L.

[0035] Preferably, in step ②, the total electrolysis time is 4.5 to 6 h.

[0036] In the electrolysis reaction of 30-60°C of the present invention, the electrolysis of chloropicolinic acid is prone to excessive dechlorination reaction and hydrogen evolution reaction, so there are high requirements for the distribution of fluid. Preferably, the cathode chamber and the anode chamber are both lateral liquid inlet and bottom-in and top-out, and the cathode chamber is provided with a plurality of liquid distribution plates at intervals in the horizontal direction, and a plurality of liquid inlet baffles are provided at intervals in the vertical direction between the lowest liquid distribution plate and the bottom of the cathode chamber, and the liquid inlet baffles and the liquid distribution plates are 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 part is the fluid distribution area, and the upper part is the reaction area. The present invention improves the uniformity of fluid distribution and the liquid residence time by setting the liquid inlet baffle and the liquid distribution plate, thereby effectively improving the selectivity of the reaction. As a further preference, the reaction area accounts for 70-80% of the volume percentage of the fluid distribution area and the reaction area. As a further preference, the distribution density of the liquid distribution plate is 2-3 pieces / 10cm reaction area height, and the distribution density of the liquid inlet baffle is 2-3 pieces / 10cm fluid distribution area length; the liquid permeable hole size of the liquid distribution plate is 2-3mm, and the spacing of the liquid permeable holes is 4-5mm; the liquid permeable hole diameter of the liquid inlet baffle is 2-3mm, and the spacing of the liquid permeable holes is 4-5mm. As a further preference, both sides of the liquid inlet baffle and the liquid distribution plate are fixed to the wall of the tank body through grooves, and the width of the liquid inlet baffle and the liquid distribution plate is smaller than the width of the tank body so that a certain space is left between the two sides of the liquid inlet baffle and the liquid distribution plate that are not fixed and the wall of the tank body.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] (1) The present invention uses powder electrodes for electrolytic reduction dechlorination of chloropicolinic acid, effectively inhibits the occurrence of hydrogen evolution side reaction, significantly improves the current efficiency of chloropicolinic acid dechlorination reaction and avoids the large-scale generation of hydrogen.

[0039] (2) The present invention uses powder electrodes for electrolysis, which solves the problem of electrode powder loss caused by regular activation of silver electrodes. At the same time, only a small amount of hydrogen storage alloy powder is needed to achieve a good hydrogen storage effect, and the electrode cost and maintenance cost are effectively reduced. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0042] Figure 3 Schematic diagram of the internal structure of the cathode chamber in the present invention, wherein 18 is a liquid inlet baffle, and 19 is a liquid distribution plate.

[0043] Figure 4 This is a liquid chromatography analysis diagram of the electrolysis product in Example 1. DETAILED DESCRIPTION

[0044] The following embodiments 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] If no specific conditions are specified in the examples of the present invention, the experiments were carried out under conventional conditions or conditions recommended by the manufacturer. If no manufacturer is specified for the reagents or instruments used, they are all conventional products that can be obtained by conventional technical means or purchased commercially.

[0046] The ultrasonic instrument used in the embodiment of the present invention is a KQ5200DB digitally controlled ultrasonic cleaner.

[0047] The hydrogen storage alloy powder used in the embodiment of the present invention is LaNi 5 Type hydrogen storage alloy powder, size is 5-30 μm.

[0048] like Figure 1As shown, the main structure of the diaphragm plate-frame electrolyzer includes a cathode chamber 4 and an anode chamber 8 which are not connected to each other, a cationic membrane 6 is provided in the middle of the two plate-frame electrolyzers, the cathode side is composed of a gasket 5, a cathode chamber 4, a cathode electrode 3, a current collecting plate 2, and an end plate 1 from the inside to the outside, and the anode side is composed of a gasket 7, an anode chamber 8, an anode electrode 9, a current collecting plate 10, and an end plate 11 from the inside to the outside, the cathode chamber 4 and the anode chamber 8 are both provided with exhaust holes at the top, the cathode chamber 4 and the anode chamber 8 are both ordinary plate-frame tanks, both of which are liquid-inlet sideways and liquid-inlet-up and liquid-outlet-down, and the components are all 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 collecting plate is a copper sheet, and the gasket material is polytetrafluoroethylene. Polypropylene and polytetrafluoroethylene have excellent chemical stability, corrosion resistance, sealing, high lubricity and non-stickiness, electrical insulation and good anti-aging resistance, and can work for a long time at a temperature of 30-80°C, so they are suitable for the electrolytic cell material of the present invention.

[0050] The device and flow chart of cathode electrolysis of the present invention are as follows Figure 2 As shown, 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, which are arranged in sequence. The devices are connected by PVC pipes. The liquid inlet of the storage tank 12 is located at the top, the liquid outlet is located at the bottom, and the drain port is located at the tee. The storage tank 12 is temperature-controlled by a constant temperature water bath 13. The cathode liquid circulates between the above-mentioned devices during the electrolysis process. The cathode liquid storage tank has three bottle ports on the top for adding alkali solution, measuring temperature, and exhausting. Anode electrolysis has similar devices and flow charts.

[0051] Figure 3 Schematic diagram of the internal structure of the cathode chamber 4, wherein the cathode chamber 4 is provided with a plurality of liquid distribution plates 19 spaced apart in the horizontal direction, and a plurality of liquid inlet baffles 18 spaced apart in the vertical direction between the lowest liquid distribution plate and the bottom of the cathode chamber 4, and 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 areas by the lowest liquid distribution plate, the lower area is the fluid distribution area, and the upper area is the reaction area. Preferably, the width of the liquid distribution plate 19 is less than the width of the tank body. Both sides of the liquid inlet baffle 18 and the liquid distribution plate 19 are fixed by grooves.

[0052] The overall dimensions of the cathode chamber and the anode chamber used in the embodiment of the present invention (i.e. Figure 1The overall size of the plate frame shown in 4 and 8 is 14×14×2 cm, the reaction area in the cathode chamber is 10×10×2 cm, 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 interval is 4.8 cm; when three liquid distribution plates are used, the interval 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 interval is 2.2 cm, and the distance between the liquid inlet baffle closest to the liquid inlet and the liquid inlet is 2.2 cm; when two liquid inlet baffles are used, the interval is 3.3 cm, and the distance between the liquid inlet baffle closest to the liquid inlet and the liquid inlet is 2.2 cm. The liquid inlet baffle 18 and the liquid distribution plate 19 are evenly arranged with multiple liquid permeable holes for liquid to pass through, the aperture of the liquid permeable holes is 2 mm or 3 mm, the interval between the liquid inlet baffle holes is 5 mm, and the interval between the liquid distribution plate holes is 4 mm.

[0053] Embodiment 1:

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

[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, rinse with distilled water until neutral, and then place in a vacuum oven at 60°C for vacuum drying for 10 h.

[0056] (2) 160 ml of water, 190 ml of anhydrous ethanol, and 40 ml of ammonia water (25 wt%) were mixed in a beaker, along with 15 g of silver nitrate and 5 ml of formaldehyde (38 wt%). 20 g of the hydrogen storage alloy powder treated in step (1) was added to the solution, and the mixture was dispersed by ultrasonic treatment at a power of 300 W and a frequency of 38 kHz for 5 min to make it uniform. The mixture was then ultrasonicated in a 25°C water bath until the coating was completed. The reaction time was 1.5 h. After the coating was completed, the beaker was removed, and the powder was washed three times with deionized water. The powder was dried in a vacuum oven at 80°C. After weighing, the weight gain of the hydrogen storage alloy powder after silver plating was 87 mg / g.

[0057] (3) A copper sheet with a size of 10 × 10 × 0.15 cm was electrolyzed in an electrolyte containing 50 mmol / L copper sulfate and 0.8 mol / L sulfuric acid in a membraneless electrolytic cell for 10 min at a temperature of 30 °C and a current density of 300 mA / cm 2 , a thin layer with porous foam copper is obtained, which is the current collector.

[0058] (4) Use the washed and dried copper foam as the current collector, weigh 10 g of the treated alloy and add it into 20 mL of 5% PVA aqueous solution, stir evenly and evenly coat the paste on the copper foam substrate. After standing for 5 min, put the electrode into a vacuum oven at 60 ° C and dry it for 6 h. After drying, press and form a powder electrode with a size of 10 × 10 × 0.15 cm.

[0059] (5) Prepare the electrolyte: the cathode liquid 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 2.0 mol / L sodium hydroxide solution. Add the cathode liquid and the anolyte into the two electrode chambers respectively, with a flow rate of 2 L / min each. The powder cathode is heated at 10 A / dm 2 The cathode was activated by constant current charging and discharging for 3 min and repeated 5 times to achieve a stable hydrogen storage capacity. The anode was a Hastelloy electrode. The power was turned on and the reaction temperature was set to 50°C. The current was divided into three stages for constant current electrolysis. The current density of the three stages was 12 A / dm 2 、6A / dm 2 , 3 A / dm 2 The three-stage electrolysis time was 1.87 h, 1.6 h, and 1.6 h, respectively. The electrolyte was continuously circulated until the reaction was completed. After the electrolysis was completed, the electrolyte was filtered out, cooled and crystallized, filtered and dried to obtain 4-amino-3,6-dichloropicolinic acid.

[0060] (6) After the electrolysis is completed, a portion of the electrolyte is filtered, diluted with 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. Figure 4 This is a 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 powder electrode preparation conditions

[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, rinse with distilled water until neutral, and then place in a vacuum oven at 60°C for vacuum drying for 10 h.

[0064] (2) 160 ml of water, 190 ml of anhydrous ethanol, and 40 ml of ammonia water (25%) were mixed in a beaker, and 15 g of silver nitrate and 5 ml of formaldehyde (38%) were added. 20 g of the hydrogen storage alloy powder treated in step (1) was added to the solution, and the mixture was dispersed by ultrasonic treatment at a power of 300 W and a frequency of 38 kHz for 5 min to make it uniform. Then, the mixture was ultrasonically treated in a 25°C water bath until the coating was completed. The reaction time was 3 h. After the coating was completed, the beaker was removed, and the powder was washed three times with deionized water. The powder was dried in a vacuum oven at a temperature of 80°C. After weighing, the weight gain of the hydrogen storage alloy powder after silver plating was 95 mg / g.

[0065] (3) A copper sheet with a size of 10 × 10 × 0.15 cm was electrolyzed in an electrolyte containing 50 mmol / L copper sulfate and 0.8 mol / L sulfuric acid in a membraneless electrolytic cell for 10 min at a temperature of 30 °C and a current density of 300 mA / cm 2 , a thin layer with porous foam copper is obtained, which is the current collector.

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

[0067] (5) Prepare the electrolyte: the cathode liquid is a 1.5 mol / L sodium hydroxide solution containing 0.6 mol / L 4-amino-3,5,6-trichloropicolinic acid, and the anolyte is a 2.0 mol / L sodium hydroxide solution. The cathode liquid and the anolyte are added to the two electrode chambers respectively, with a flow rate of 2 L / min each. The powder cathode is heated at 10 A / dm 2 The cathode was activated by constant current charging and discharging for 3 min and repeated 5 times to achieve a stable hydrogen storage capacity. The anode was a Hastelloy electrode. The power was turned on and the reaction temperature was set to 50°C. The current was divided into three stages for constant current electrolysis. The current density of the three stages was 12 A / dm 2 , 6 A / dm 2 , 3 A / dm 2 The three-stage electrolysis time was 1.87 h, 1.6 h, and 1.6 h, respectively. The electrolyte was continuously circulated until the reaction was completed. After the electrolysis was completed, the electrolyte was filtered out, cooled and crystallized, filtered and dried to obtain 4-amino-3,6-dichloropicolinic acid.

[0068] (6) After the electrolysis, a portion of the electrolyte was filtered, diluted 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 liquid chromatography results of the electrolysis product showed that the yield of 4-amino-3,6-dichloropicolinic acid was 91.6%, and the current efficiency was 74.8%.

[0069] Example 3: Electrolysis experiment with adding hydrogen storage alloy powder to cathode liquid

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

[0071] (1)-(4): A powder electrode was prepared in the same manner as in Example 2.

[0072] (5) Prepare the electrolyte: the cathode liquid is a 1.5 mol / L sodium hydroxide solution containing 0.6 mol / L 4-amino-3,5,6-trichloropyridine carboxylic acid, and 10 g of the hydrogen storage alloy powder obtained in step (2) is added. The anode liquid is a 2.0 mol / L sodium hydroxide solution. The cathode liquid and the anode liquid are added to the two electrode chambers respectively, with a flow rate of 2 L / min each. The powder cathode is heated at 10 A / dm 2 The cathode was activated by constant current charging and discharging for 3 min and repeated 5 times to achieve a stable hydrogen storage capacity. The anode was a Hastelloy electrode. The power was turned on and the reaction temperature was set to 50°C. The current was divided into three stages for constant current electrolysis. The current density of the three stages was 12 A / dm 2 , 6 A / dm 2 , 3 A / dm 2 The three-stage electrolysis time was 1.87 h, 1.6 h, and 1.6 h, respectively. The electrolyte was continuously circulated until the reaction was completed. After the electrolysis was completed, the electrolyte was filtered out, cooled and crystallized, filtered and dried to obtain 4-amino-3,6-dichloropicolinic acid.

[0073] (6) After the electrolysis, a portion of the electrolyte was filtered, diluted 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 liquid chromatography results of the electrolysis product showed that the yield of 4-amino-3,6-dichloropicolinic acid was 92.3% and the current efficiency was 75.4%.

[0074] Example 4: Electrolysis experiments at different current densities

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

[0076] (1)-(4): A powder electrode was prepared in the same manner as in Example 3.

[0077] (5) Prepare the electrolyte: the cathode liquid is a 1.5 mol / L sodium hydroxide solution containing 0.6 mol / L 4-amino-3,5,6-trichloropyridine carboxylic acid, and 10 g of the hydrogen storage alloy powder obtained in step (2) is added. The anode liquid is a 2.0 mol / L sodium hydroxide solution. The cathode liquid and the anode liquid are added to the two electrode chambers respectively, with a flow rate of 2 L / min each. The powder cathode is heated at 10 A / dm 2 The cathode was activated by constant current charging and discharging for 3 min and repeated 5 times to achieve a stable hydrogen storage capacity. The anode was a Hastelloy electrode. The power was turned on and the reaction temperature was set to 50°C. The current was divided into three stages for constant current electrolysis. The current density of the three stages was 10 A / dm 2 , 5 A / dm 2 , 2.5 A / dm 2 The three-stage electrolysis time was 2.24 h, 1.9 h, and 1.9 h, respectively. The electrolyte was continuously circulated until the reaction was completed. After the electrolysis was completed, the electrolyte was filtered out, cooled and crystallized, filtered and dried to obtain 4-amino-3,6-dichloropicolinic acid.

[0078] (6) After the electrolysis, a portion of the electrolyte was filtered, diluted with 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 liquid chromatography results of the electrolysis product showed that the yield of 4-amino-3,6-dichloropicolinic acid was 92.7% and the current efficiency was 75.7%.

[0079] Example 5-10: Electrolysis reaction with different electrolytic cell parameters

[0080] (1)-(4): A powder electrode was prepared in the same manner as in Example 4.

[0081] (5)-(6): The electrolysis reaction is carried out in the same manner as in Example 4.

[0082] The difference between the electrolysis device used and the embodiment 1-4 is that the internal structure of the cathode chamber is changed, and the number and aperture of the liquid inlet baffle 18 and the number and aperture of the liquid distribution plate 19 are changed to perform electrolysis. The changed parameters and reaction results are shown in Table 1:

[0083] Table 1

[0084]

[0085] Example 11: Powder electrode multiple electrolysis stability test

[0086] (1)-(5): The process is the same as in Example 7, the powder electrode and cathode liquid are prepared, hydrogen storage alloy powder is added, and electrolysis and post-treatment are carried out under the same conditions. Five electrolysis experiments are carried out under the same conditions.

[0087] (6) After the electrolysis, part of the electrolyte was filtered, diluted with 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 liquid chromatography results of the electrolysis product showed that the yield of 4-amino-3,6-dichloropicolinic acid was 92.8% and the current efficiency was 75.7%. The electrode did not lose powder significantly.

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

[0089] (1)-(4): A powder electrode was prepared in the same manner as in Example 2.

[0090] (5) Prepare the electrolyte: the cathode liquid is 500 mL of 0.5 mol / L sodium hydroxide solution containing 0.1 mol / L 3,4,5,6-tetrachloropyridine carboxylic acid, and 10 g of the hydrogen storage alloy powder obtained in step (2) is added. The anode liquid is 500 mL of 1.0 mol / L sodium hydroxide solution. The cathode liquid and the anode liquid are added to the two electrode chambers respectively, with a flow rate of 2 L / min each. The powder cathode is heated at 10 A / dm 2 The cathode was activated by constant current charging and discharging for 3 min and repeated 5 times to achieve a stable hydrogen storage capacity. The anode was a Hastelloy electrode. The power was turned on and the reaction temperature was set to 30°C and the current density was 50 A / dm 2 The constant current electrolysis was carried out for 0.67 h, and the electrolyte was continuously circulated until the reaction was completed. After the electrolysis was completed, the electrolyte was filtered out, cooled and crystallized, filtered and dried to obtain 3,6-dichloropicolinic acid.

[0091] (6) After the electrolysis, a portion of the electrolyte was filtered, diluted with 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 liquid chromatography results of the electrolysis product showed that the yield of 3,6-dichloropicolinic acid was 95.3% and the current efficiency was 76.2%.

[0092] Comparative Example 1: Silver mesh electrode electrolysis experiment

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

[0094] (2) Prepare the electrolyte: the cathode liquid 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. Add the cathode liquid and the anolyte into the two electrode chambers respectively, with a flow rate of 2 L / min each. The powder cathode is heated at 10 A / dm 2 The constant current charge and discharge was repeated for 3 minutes, and the activation was repeated 5 times to make the cathode reach a stable hydrogen storage capacity. The anode was a Hastelloy electrode. The power was turned on, the reaction temperature was set to 50°C, and the current was divided into three stages of current density for constant current electrolysis. The current density of the three stages was 12A / dm 2 , 6 A / dm 2 , 3 A / dm 2 The three-stage electrolysis time is 1.87 h, 1.6 h, and 1.6 h, respectively. The reaction is continued in this cycle until the reaction is complete. After the electrolysis is completed, 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 portion of the electrolyte was filtered, diluted 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 liquid chromatography results of the product under the same electrolysis amount were: the yield of 4-amino-3,6-dichloropicolinic acid was 75.1%, and the current efficiency was 61.3%.

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

[0097] (1)-(4): A powder electrode was prepared in the same manner as in Example 1.

[0098] (5): The powder electrode was not activated by charge and discharge before the reaction, and the electrolysis reaction was carried out in the same manner as in Example 1.

[0099] (6) After the electrolysis, a portion of the electrolyte was filtered, diluted with 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 analysis of the electrolysis product showed that the yield of 4-amino-3,6-dichloropicolinic acid was 83.3% and the current efficiency was 68.1%.

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

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

[0102] (1)-(4): A powder electrode was prepared in the same manner as in Example 4.

[0103] (5) Prepare electrolyte: The cathode liquid is 500 mL of 1.5 mol / L sodium hydroxide solution containing 0.6 mol / L 4-amino-3,5,6-trichloropyridine carboxylic acid, and 10 g of hydrogen storage alloy powder obtained in step (2) is added. The anode liquid is 500 mL of 2.0 mol / L sodium hydroxide solution. The cathode liquid and the anode liquid are added to the two electrode chambers respectively, with a flow rate of 2 L / min each. The powder cathode is heated at 10 A / dm 2 The cathode was activated by constant current charging and discharging for 3 min and repeated 5 times to achieve a stable hydrogen storage capacity. The anode was a Hastelloy electrode. The power was turned on and the reaction temperature was set to 50°C. The current was divided into three stages for constant current electrolysis. The current density of the three stages was 12 A / dm 2 , 6 A / dm 2 , 3 A / dm 2 The three-stage electrolysis time is 1.87 h, 1.6 h, and 1.6 h, respectively. The reaction is continued in this cycle until the reaction is complete. After the electrolysis is completed, the electrolyte is filtered out, cooled and crystallized, filtered and dried to obtain 4-amino-3,6-dichloropicolinic acid.

[0104] (6) After the electrolysis, a portion of the electrolyte was filtered, diluted 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 liquid chromatography results of the electrolysis product showed that the yield of 4-amino-3,6-dichloropicolinic acid was 86.9% and the current efficiency was 70.9%.

[0105] Comparative Example 4: Multiple Electrolysis Stability Tests of Silver Mesh Electrodes

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

[0107] (6) After the electrolysis, part of the electrolyte was filtered, diluted with 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 liquid chromatography results of the electrolysis product showed that the yield of 4-amino-3,6-dichloropicolinic acid was 74.7% and the current efficiency was 60.9%. The electrolysis effect was able to maintain good stability, but obvious powder loss occurred.

[0108] Comparative Example 5: Silver mesh electrode electrolysis experiment of substrate 3,4,5,6-tetrachloropyridine carboxylic acid

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

[0110] (6) After the electrolysis, part of the electrolyte was filtered, diluted with 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 liquid chromatography results of the electrolysis product showed that the yield of 3,6-dichloropicolinic acid was 78.2% and the current efficiency was 62.5%. The electrolysis effect was able to maintain good stability, but obvious powder loss occurred.

Claims

1. A powder electrode for dechlorination of chloropicolinic acid by electrolytic reduction, characterized in that: 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. 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.

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 claims 1 to 4, characterized in that: The preparation method comprises the following steps: Step 1: Activating the hydrogen storage alloy powder; Step 2: preparing a catalytic material according to the method described in the first aspect; 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 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.

Images