A high concentration indigo electrochemical reduction method without additional medium and chemical reducing agent

Abstract

The application discloses a high-concentration indigo electrochemical reduction method without additional medium and chemical reducing agent. A diaphragm electrolytic cell is used as a reactor, an alkaline aqueous solution containing indigo and indigo white is used as a cathode liquid, an alkaline aqueous solution is used as an anode liquid, a three-dimensional porous carbon-based conductive material is used as a cathode, and nickel, stainless steel or a nickel-containing alloy is used as an anode. Direct current or pulse current is sequentially passed through the anode liquid, the diaphragm and the cathode liquid from the anode to the cathode, and indigo in the cathode liquid is electrochemically reduced into indigo white. The application realizes high-concentration (≥0.6 mol / L) indigo high-efficiency (current density ≥6 A / dm 2 , high-current efficiency ≥71%, conversion rate ≥94%) electrochemical reduction into indigo white without using additional medium. The reduction reaction is carried out at normal pressure and without using hydrogen. The amount of sodium hydrosulfite is greatly reduced.

Description

(I)TECHNICAL FIELD

[0001] The present application relates to a high-concentration indigo electrochemical reduction method, in particular to a high-concentration indigo electrochemical reduction method without adding mediators and chemical reducing agents. (II)BACKGROUND

[0002] Indigo (I) is a blue powder, mainly used for dyeing cotton yarns of denim. Due to the long-lasting popularity of indigo-dyed jeans in the market, the annual consumption of indigo worldwide has remained above 60,000 tons. Indigo is difficult to dissolve in water, but the leucoindigo (II) obtained after reduction has good solubility in alkaline aqueous solution. Therefore, indigo needs to be reduced to leucoindigo in an alkaline aqueous solution before dyeing. After the leucoindigo (golden yellow) dissolved in the alkaline aqueous solution is uniformly adsorbed to the cotton yarn, the leucoindigo is oxidized to indigo by contacting with air, thereby uniformly and firmly dyeing the cotton yarn.

[0003]

[0004] The sodium dithionite reduction dyeing method is currently the most important method for indigo dyeing in industry. Figure 1 The process flow and approximate material flow are shown in the figure. The main problems of this process are: (1) a large amount of sodium dithionite is used, and sodium dithionite is very unstable and will quickly decompose or even burn when it contacts with water and / or air, thereby releasing a large amount of toxic gas; (2) the discharged wastewater contains high-concentration sulfate and sulfurous acid liquid, which is not only highly corrosive but also very difficult to treat.

[0005] In order to solve the above problems, many safe and environmentally friendly reducing agents have been used to replace sodium dithionite, such as sulfur dioxide, Rongalit C (BASF), glucose, α-hydroxy ketone, and α-hydroxy aldehyde. Due to high price or affecting dyeing effect, etc., these safe and environmentally friendly reducing agents have not been widely used in industry. The method of catalytic hydrogenation is used to reduce indigo to leucoindigo in alkaline aqueous solution, which can greatly reduce the amount of sodium dithionite used and can well alleviate the above two problems, but catalytic hydrogenation needs to be carried out under high pressure and a large amount of hydrogen gas is needed, which has the risk of explosion. Therefore, the catalytic hydrogenation technology for the reduction of indigo is also very limited in industrial use.

[0006] Electrochemical reduction is a safe and environmentally friendly technology for indigo reduction. Most of the current electrochemical reduction technologies only focus on the reduction of low-concentration indigo [Electrochimica Acta 47 (2002) 1989-1995; Journal of Applied Electrochemistry 32: 647-651, 2002; Journal of Cleaner Production 266 (2020) 121753; International journal of hydrogen energy 47 (2022) 27566-27578], because the electrochemical reduction of low-concentration indigo can theoretically achieve complete non-use of chemical reducing agents. Unfortunately, this technology has the problem of low current density (low utilization efficiency of electrolysis equipment) or (and) the need for external media (increasing cost and affecting dyeing quality). (III) Summary

[0007] The purpose of the present application is to provide a high-concentration indigo electrochemical reduction method without external media and chemical reducing agents, which realizes high-efficiency (high current density and high current efficiency) electrochemical reduction of high-concentration indigo to indigo white without the use of external media and chemical reducing agents, solving the problem of low current density or current efficiency or the need for external media or chemical reducing agents in the prior art.

[0008] The technical solution adopted by the present application is:

[0009] The present application provides a high-concentration indigo electrochemical reduction method without external media and chemical reducing agents, which uses a diaphragm electrolytic cell as a reactor, an alkaline aqueous solution containing high-concentration indigo (I) and a certain amount of indigo white (II) as a cathode liquid, an alkaline aqueous solution as an anode liquid, a three-dimensional porous carbon-based conductive material as a cathode, and a direct current or pulse current from the anode to the cathode through the anode liquid, the diaphragm and the cathode liquid in turn for electrochemical reduction reaction, and electrochemical reduction of indigo in the cathode liquid to indigo white.

[0010]

[0011] Preferably, the cathode is graphite felt, carbon felt or foamed glassy carbon, preferably graphite felt. The geometric area of the cathode can be adjusted according to the size of the reactor, and the more suitable thickness range is 1-30 mm, preferably 3-10 mm.

[0012] Preferably, the cathode is a three-dimensional porous carbon-based conductive material with a specific surface area of 2-100 m 2 / g, preferably a specific surface area of 35-85 m 2The specific surface area of the cathode has certain influence on the electrolysis effect, and it is not the larger the better, and is related to the heat treatment atmosphere, temperature and time.

[0013] More preferably, the cathode is a three-dimensional porous carbon-based conductive material with a specific surface area of 2-100 m 2 / g, which is obtained by heat treating a three-dimensional porous carbon-based conductive material in an oxygen-containing atmosphere at 300-1000°C for 1-10 h. The oxygen concentration in the oxygen-containing atmosphere is 0-100%, preferably air. The heat treatment temperature is preferably 400-800°C, and the time is preferably 2-5 h, more preferably 500°C and 2 h. If the three-dimensional porous carbon-based conductive material has stains on the surface, it can be cleaned with a solvent before heat treatment, and then dried.

[0014] Preferably, the total concentration of indigo blue and indigo white in the cathode solution is 0.15-2 mol / L, preferably 0.3-1 mol / L; and the concentration of indigo white is 0.015-1.0 mol / L, preferably 0.03-0.075 mol / L.

[0015] Preferably, the indigo white is added in the form of an indigo white solution, which is prepared by electrochemical reduction or catalytic hydrogenation or chemical reduction; and the indigo white solution is derived from one of the following: (1) by adding a certain amount of chemical reducing agent to an alkaline aqueous solution of indigo blue or by electrochemical reduction to obtain indigo white; (2) DyStar's pre-reduced liquid indigo white (obtained by catalytic hydrogenation); (3) by chemical reduction by adding a large amount of chemical reducing agent (such as sodium hydrosulfite) to an alkaline aqueous solution containing indigo blue.

[0016] More preferably, the indigo white solution is prepared as follows: an electrolytic cell with a cationic membrane as the separator (preferably an H-type electrolytic cell with a Nafion-324 cationic membrane as the separator) is used as the reactor, graphite felt (preferably heat-treated graphite felt) is used as the cathode, and a nickel-based material (preferably stainless steel) is used as the anode; an aqueous solution containing 0.3-1 mol / L indigo blue + 0.5-2 mol / L NaOH + 0.03-0.12 mol / L sodium hydrosulfite or indigo white is used as the cathode solution, and an aqueous solution of 0.5-7 mol / L sodium hydroxide is used as the anode solution; during electrolysis, the temperature is controlled at 60-80°C (preferably 70°C), and the current density is controlled at 4-6 A / dm 2 (preferably 6 A / dm 2 ); after electrolysis, the yield of indigo white in the cathode solution reaches 80-100% (preferably 95-100%), and the cathode solution is collected as the indigo white solution; the heat-treated graphite felt is obtained by placing the graphite felt in a muffle furnace in an air atmosphere at 500°C for 2 hours.

[0017] Preferably, the temperature of the electrochemical reduction ranges from 30 to 90℃, preferably from 60 to 80℃; the current density ranges from 2 to 10 A / dm 2 , preferably from 4 to 8 A / dm 2 . In the electrochemical reduction process, in order to improve the current efficiency, the current density can be gradually reduced according to the known rules, or even the electrochemical reduction is performed in a constant potential mode.

[0018] Preferably, before the indigo white is added into the cathode solution, the cathode solution is purged with nitrogen or pre-electrolyzed for 10 minutes or more, preferably for 10 to 20 minutes.

[0019] Preferably, the alkaline aqueous solution in the cathode solution and the alkaline aqueous solution in the anode solution are both aqueous solutions containing a supporting electrolyte, wherein the supporting electrolyte includes alkali metal hydroxide (such as NaOH, KOH, etc.), carbonate (such as Na2CO3, K2CO3, etc.), quaternary ammonium base (such as tetramethylammonium hydroxide, tetraethylammonium hydroxide), and ammonium bicarbonate; the concentration of the supporting electrolyte in the alkaline aqueous solution in the cathode solution ranges from 0.1 to 10 mol / L; the concentration of the supporting electrolyte in the alkaline aqueous solution in the anode solution ranges from 0.5 to 10 mol / L. Because the electrolyte in the anode solution is gradually consumed, the concentration of the electrolyte can be ensured by gradually adding the electrolyte in the anode solution. The alkaline aqueous solution in the cathode solution and the alkaline aqueous solution in the anode solution can be the same or different.

[0020] Preferably, the anode is made of nickel, stainless steel (such as 304, 316L, etc.), or a nickel-containing alloy (such as Hastelloy). If the cost or service life is not considered, titanium-based iridium oxide, titanium-based ruthenium oxide, or graphite material can also be used as the anode. The anode can have different shapes, such as a flat plate, a wire mesh, or a mesh shape, preferably a diamond mesh shape that is beneficial to the discharge of gas; the anodic oxidation reaction occurs on the anode. The separator is a cationic membrane (such as Nafion-324) and a microporous membrane, preferably a cationic membrane as the separator.

[0021] Preferably, the reactor is an H-type electrolytic cell or a plate-frame electrolytic cell, and the Nafion-324 cationic membrane is used as the separator.

[0022] Compared with the prior art, the present application has the following beneficial effects: (1) the method of the present application realizes the electrochemical reduction of high-concentration (≥0.6 mol / L) indigo blue into indigo white with high efficiency (current density ≥ 6 A / dm 2 , high current efficiency ≥ 71%, and yield ≥ 94%) without using an external medium; (2) the reduction reaction is performed at normal pressure and without using hydrogen; and (3) the amount of sodium dithionite is greatly reduced (compared with the traditional sodium dithionite reduction method for indigo blue dyeing process, about 800 kg of sodium dithionite is saved per ton of indigo blue dyeing). (IV) DESCRIPTION OF DRAWINGS

[0023] Figure 1 Fig. 1 is a schematic diagram of an indigo dyeing process and the general material flow.

[0024] Figure 2 Fig. 2 is the BET results of graphite felt before and after heat treatment.

[0025] Figure 3 Fig. 3 is a schematic diagram (A) and a photograph (B) of an H-type electrolytic cell.

[0026] Figure 4 Fig. 4 is a typical potential titration diagram.

[0027] Figure 5 Fig. 5 is an electrolysis system configured with a plate-frame electrolytic cell. (V) DETAILED DESCRIPTION

[0028] The present application will be further described in conjunction with specific examples, but the scope of protection of the present application is not limited thereto.

[0029] Yield of leucoindigo = (amount of substance (mol) of leucoindigo in catholyte at the end of reaction / total amount of substance (mol) of indigo and leucoindigo added in catholyte at the beginning of reaction) x 100%

[0030] Current efficiency = (effective electric quantity (Ah) / electric quantity (Ah) passed) x 100%

[0031] Effective electric quantity (Ah) = amount of substance (mol) of leucoindigo in catholyte at the end of reaction x 2 (e - ) x 26.8 (Ah / (mol x e - ) - amount of substance (mol) of leucoindigo added in catholyte x 2 (e - ) x 26.8 (Ah / (mol x e - )

[0032] Electric quantity (Ah) passed = electrolysis current (A) x electrolysis time (h)

[0033] Example 1, heat treatment of graphite felt

[0034] The graphite felt was placed in a muffle furnace in an air atmosphere and heat treated at 500°C for 2 hours to obtain the heat-treated graphite felt. The specific surface area of the graphite felt before and after the heat treatment was determined by BET, and the results are shown in Figure 2 From the graph, it can be calculated that the specific surface area of the graphite felt before and after the heat treatment was 2.12 m 2 / g and 87.1 m 2 / g, respectively.

[0035] Example 2, preparation of leucoindigo solution (containing 0.6 mol / L of leucoindigo)

[0036] TheFigure 3 The H-type electrolytic cell shown is the reactor, with a Nafion-324 cation exchange membrane as the diaphragm, and the graphite felt (0.3×2×3cm) after heat treatment according to the method of Example 1. 3 The cathode is a 0.6 mol / L indigo solution, and the anode is a 316 stainless steel sheet. A 40 mL aqueous solution is prepared by adding 6.3 g of indigo (0.6 mol / L), 3.2 g of NaOH (2 mol / L), and 0.21 g of sodium dithionite (0.03 mol / L) as a reducing agent to water; this solution serves as the cathode solution. A 60 mL aqueous solution of 5 mol / L sodium hydroxide is used as the anolyte. During electrolysis, the temperature is controlled at 70℃, and the current density is controlled at 6 A / dm³. 2 (Current is 360mA). Samples were taken at 0, 2, and 4 hours of electrolysis, and the indigo content in the catholyte was analyzed using a potentiometric titrator. The potentiometric titration results are as follows: Figure 4 As shown. Electrolysis was stopped after 5 hours, and the catholyte was collected to obtain an indigo solution. The yield of indigo was calculated to be 100% based on the titration results, and the concentration of indigo in the catholyte was 0.6 mol / L.

[0037] Example 3: Preparation of indigo solution (containing 2 mol / L indigo)

[0038] by Figure 3 The H-type electrolytic cell shown is the reactor, with a Nafion-324 cation exchange membrane as the diaphragm, and the graphite felt (0.3×2×3cm) after heat treatment according to the method of Example 1. 3 The cathode is made of 21g indigo (2mol / L), 0.8g NaOH (0.5mol / L), and 0.82g sodium dithionite (0.12mol / L) as a reducing agent, which is then added to water to prepare a 40mL aqueous solution. This solution serves as the cathode solution. A 60mL 0.5mol / L sodium hydroxide aqueous solution serves as the anolyte (NaOH solid is continuously added during electrolysis to maintain a concentration of 0.5mol / L). During electrolysis, the temperature is controlled at 70℃, and the current density is controlled at 6A / dm³. 2 (Current is 360mA). Electrolysis was stopped after 15 hours, and the catholyte was collected to obtain an indigo solution. The indigo content in the catholyte was analyzed using a potentiometric titrator. Based on the titration results, the indigo yield was calculated to be 100%, and the concentration of indigo in the catholyte was 2 mol / L.

[0039] Example 4: Electrochemical Reduction of Indigo—The total concentration of indigo and indigo white in the catholyte before electrolysis was 0.60 mol / L, and the concentration of indigo white was 0.075 mol / L.

[0040] by Figure 3 The H-type electrolytic cell shown is used as the reactor, with a Nafion-324 cation exchange membrane as the diaphragm, and the heat-treated graphite felt (0.3×2×3cm) as described in Example 1.3 The cathode was a 316 stainless steel sheet, and the anode was a 35 mL mixture of (A) an aqueous solution containing 0.6 mol / L indigo + 2 mol / L NaOH and (B) 5 mL of an indigo solution prepared by the method in Example 2 (containing 0.6 mol / L indigo). The anolyte was 60 mL of a 2 mol / L sodium hydroxide aqueous solution. During electrolysis, the temperature was controlled at 70°C and the current density was controlled at 6 A / dm³. 2 (Current: 360mA). Electrolysis was stopped after 4 hours, and the indigo content in the catholyte was analyzed using a potentiometric titrator. Calculations showed that the indigo yield was 95.1%, and the current efficiency was 73.8%.

[0041] Example 5: Electrochemical Reduction of Indigo—The total concentration of indigo and indigo white in the catholyte before electrolysis was 0.775 mol / L, and the concentration of indigo white was 0.25 mol / L.

[0042] by Figure 3 The H-type electrolytic cell shown is used as the reactor, with a Nafion-324 cation exchange membrane as the diaphragm, and the heat-treated graphite felt (0.3×2×3cm) as described in Example 1. 3 The cathode was a 316 stainless steel sheet, and the anode was a mixture of (A) 35 mL of an aqueous solution containing 0.6 mol / L indigo + 2 mol / L NaOH and (B) 5 mL of an indigo solution prepared by the method in Example 3 (containing 2 mol / L indigo). The anolyte was 60 mL of a 4 mol / L sodium hydroxide aqueous solution. During electrolysis, the temperature was controlled at 70°C and the current density was controlled at 6 A / dm³. 2 (Current: 360mA). Electrolysis was stopped after 4 hours, and the indigo content in the catholyte was analyzed using a potentiometric titrator. Calculations showed that the indigo yield was 95.9%, and the current efficiency was 73.4%.

[0043] Example 6: Electrochemical Reduction of Indigo—The total concentration of indigo and indigo white in the catholyte before electrolysis was 0.60 mol / L, and the concentration of indigo white was 0.075 mol / L—Nitrogen gas was purged before adding the indigo white solution.

[0044] by Figure 3 The H-type electrolytic cell shown is used as the reactor, with a Nafion-324 cation exchange membrane as the diaphragm, and the heat-treated graphite felt (0.3×2×3cm) as described in Example 1. 3) for the cathode, and 316 stainless steel sheet for the anode. 35 mL of 0.6 mol / L indigo + 0.2 mol / L NaOH aqueous solution was used as the catholyte, and 60 mL of 2 mol / L NaOH aqueous solution was used as the anolyte. Before electrolysis, the catholyte was bubbled with nitrogen for 10 min, and then 5 mL of the leucoindigo solution (containing 0.6 mol / L leucoindigo) prepared by the method of Example 2 was added. During electrolysis, the temperature was controlled at 70°C, and the current density was controlled at 6 A / dm 2 (360 mA of current). After 4 hours of electrolysis, the electrolysis was stopped, and the content of leucoindigo in the catholyte was analyzed by a potentiometric titrator. The calculation showed that the yield of leucoindigo was 98.2%, and the current efficiency was 76.6%.

[0045] Example 7, Electrochemical reduction of indigo - total concentration of indigo and leucoindigo in the catholyte before electrolysis was 0.60 mol / L, and the concentration of leucoindigo was 0.075 mol / L - electrolysis before adding the leucoindigo solution

[0046] Example 8, Electrochemical reduction of indigo - different concentrations of reaction substrate Figure 3 The H-type electrolytic cell shown in Fig. 1 was used as the reactor, Nafion-324 cationic membrane was used as the separator, and the graphite felt (0.3 x 2 x 3 cm 3 ) after heat treatment as described in Example 1 was used as the cathode, and 316 stainless steel sheet was used as the anode. 35 mL of 0.6 mol / L indigo + 1 mol / L NaOH aqueous solution was used as the catholyte, and 60 mL of 2 mol / L NaOH aqueous solution was used as the anolyte. During electrolysis, the temperature was controlled at 70°C, and the current density was controlled at 6 A / dm 2 (360 mA of current). After 20 min of electrolysis, 5 mL of the leucoindigo solution (containing 0.6 mol / L leucoindigo) prepared by the method of Example 2 was added to the catholyte. After another 4 hours of electrolysis, the electrolysis was stopped, and the content of leucoindigo in the catholyte was analyzed by a potentiometric titrator. The calculation showed that the yield of leucoindigo was 98.5%, and the current efficiency was 76.8% (not counting the electrolysis before adding the leucoindigo solution).

[0047] Example 8, Electrochemical reduction of indigo - different concentrations of reaction substrate

[0048] Except as specifically stated, the other reaction conditions were consistent with those of Example 5. The results showed that the total concentration of indigo and leucoindigo was 0.3-1.5 mol / L, and the concentration of leucoindigo was 0.049-1 mol / L, which was better for reduction.

[0049] Table 1, Electrochemical reduction conditions of indigo

[0050]

[0051]

[0052] a The concentration of NaOH in the anolyte was 0.5 mol / L

[0053] Examples 13-16: Electrochemical Reduction of Indigo—Different Indigo Concentrations in Catholyte

[0054] Unless otherwise specified, the other reaction conditions were the same as in Example 4, and the results showed that the reduction effect was better when the concentration of indigo in the catholyte was 0.03-0.3 mol / L.

[0055] Table 2. Electrochemical reduction conditions of indigo

[0056]

[0057] Examples 17-20: Electrochemical Reduction of Indigo—At Different Temperatures

[0058] Unless otherwise specified, the other reaction conditions were the same as in Example 4, and the results showed that the reduction effect was better when the electrochemical reduction temperature was 60-90℃.

[0059] Table 3. Electrochemical reduction conditions of indigo

[0060] Figure 3 Figure 3 Reaction temperature (°C) Yield of leucoindigo (%) 17 30 78.5 59.0 18 60 95.3 74.0 19 80 94.9 73.6 20 90 92.1 71.1

[0061] Examples 21-31: Electrochemical Reduction of Indigo—Different Cathodes

[0062] Unless otherwise specified, the preparation method of graphite felt is the same as in Example 1; the electrochemical reduction reaction conditions are the same as in Example 4. The results show that heat treatment at 400–800℃ for 0.5–2 hours yields a high indigo white yield.

[0063] Table 1. Electrochemical reduction conditions of indigo

[0064]

[0065]

[0066] a Oxygen atmosphere

[0067] Example 32: Electrochemical reduction of indigo (0.6 mol / L) – using a plate and frame electrolyzer scaled up 20 times – the total concentration of indigo and indigo white in the cathode solution before electrolysis was 0.61 mol / L, and the concentration of indigo white was 0.046 mol / L.

[0068] by Current efficiency (%) The plate and frame electrolytic cell shown is the reactor, with a Nafion-324 cation exchange membrane as the diaphragm, and the graphite felt (1.0×6.0×20cm) after heat treatment according to the method of Example 1. 3The cathode was a Hastelloy C-276 mesh, and the anode was a 750 mL aqueous solution containing 0.6 mol / L indigo + 2 mol / L NaOH, and the anolyte was 1200 mL aqueous solution containing 5 mol / L sodium hydroxide. Before electrolysis, nitrogen gas was passed through the catholyte for 40 min to remove oxygen, and then 50 mL of the catholyte solution from the electrolysis reaction in Example 5 (containing 0.74 mol / L indigo) was added. During electrolysis, the temperature was controlled at 70 °C, and the current density was controlled at 6 A / dm³. 2 (Current was 7.2A). Electrolysis was stopped after 4 hours, and the indigo content in the catholyte was analyzed using a potentiometric titrator. Calculations showed that the indigo yield was 96.2%, and the current efficiency was 80.3%.

[0069] Example 33: Electrochemical Reduction of Indigo – Cyclic Electrolysis – The total concentration of indigo and indigo white in the cathode solution before electrolysis was 0.60 mol / L, and the concentration of indigo white was 0.071 mol / L.

[0070] by Figure 5 The H-type electrolytic cell shown is used as the reactor, with a Nafion-324 cation exchange membrane as the diaphragm, and the heat-treated graphite felt (0.3×2×3cm) as described in Example 1. 3 The cathode was a 316 stainless steel sheet, and the anode was a 35 mL mixture of (A) an aqueous solution containing 0.6 mol / L indigo and 2 mol / L NaOH and (B) 5 mL of the catholyte (containing 0.57 mol / L indigo) after electrolysis according to the method in Example 4. The anolyte was 60 mL of a 2 mol / L sodium hydroxide aqueous solution. During electrolysis, the temperature was controlled at 70°C and the current density was controlled at 6 A / dm³. 2 (Current: 360mA). Electrolysis was stopped after 4 hours, and the indigo content in the catholyte was analyzed using a potentiometric titrator. Calculations showed that the indigo yield was 94.2%, and the current efficiency was 73.0%.

[0071] Example 34: Electrochemical Reduction of Indigo – Cyclic Electrolysis – The total concentration of indigo and indigo white in the cathode solution before electrolysis was 0.6 mol / L, and the concentration of indigo white was 0.071 mol / L.

[0072] by Figure 3 The H-type electrolytic cell shown is used as the reactor, with a Nafion-324 cation exchange membrane as the diaphragm, and the heat-treated graphite felt (0.3×2×3cm) as described in Example 1. 3) as anode. During electrolysis, the temperature was controlled at 70°C and the current density was controlled at 6 A / dm 2 After 4 hours of electrolysis, the electrolysis was stopped and the content of indigo white in the cathode solution was analyzed by potentiometric titration. The calculation showed that the yield of indigo white was 92.8% and the current efficiency was 71.8%.

[0073] Example 1, electrochemical reduction of indigo - total concentration of indigo and indigo white in cathode solution before electrolysis was 0.60 mol / L, concentration of indigo white was 0.075 mol / L (Comparative Example 4)

[0074] Example 1, electrochemical reduction of indigo - total concentration of indigo and indigo white in cathode solution before electrolysis was 0.60 mol / L, concentration of indigo white was 0.075 mol / L (Comparative Example 4) Figure 3 H-type electrolytic cell as shown in Figure 1 was used as reactor, Nafion-324 cationic membrane was used as separator, and the graphite felt (0.3 x 2 x 3 cm 3 ) after heat treatment as described in Example 1 was used as cathode, and 316 stainless steel sheet was used as anode. 40 mL of "aqueous solution containing 0.6 mol / L indigo + 2 mol / L NaOH" was used as cathode solution, and 60 mL of 2 mol / L sodium hydroxide aqueous solution was used as anode solution. During electrolysis, the temperature was controlled at 70°C and the current density was controlled at 6 A / dm 2 After 4 hours of electrolysis, the electrolysis was stopped and the content of indigo white in the cathode solution was analyzed by potentiometric titration. The calculation showed that the yield of indigo white was 92.8% and the current efficiency was 71.8%.

[0075] Example 1, electrochemical reduction of indigo - total concentration of indigo and indigo white in cathode solution before electrolysis was 0.60 mol / L, concentration of indigo white was 0.075 mol / L (Comparative Example 4)

[0076] Example 1, electrochemical reduction of indigo - total concentration of indigo and indigo white in cathode solution before electrolysis was 0.60 mol / L, concentration of indigo white was 0.075 mol / L (Comparative Example 4) Figure 3 H-type electrolytic cell as shown in Figure 1 was used as reactor, Nafion-324 cationic membrane was used as separator, and the graphite felt (0.3 x 2 x 3 cm 3 ) after heat treatment as described in Example 1 was used as cathode, and 316 stainless steel sheet was used as anode. 40 mL of "aqueous solution containing 0.6 mol / L indigo + 2 mol / L NaOH" was used as cathode solution, and 60 mL of 2 mol / L sodium hydroxide aqueous solution was used as anode solution. During electrolysis, the temperature was controlled at 70°C and the current density was controlled at 6 A / dm 2(Current: 360mA). Electrolysis was stopped after 4 hours, and the indigo content in the catholyte was analyzed using a potentiometric titrator. Calculations showed that the indigo yield was 15.5%, and the current efficiency was 13.8%.

[0077] Comparative Example 3: Electrochemical reduction of indigo (0.6 mol / L) – using a plate and frame electrolyzer scaled up 20 times – graphite particles (Comparative Example 32)

[0078] by Figure 3 Figure 5 The plate and frame electrolytic cell shown is the reactor, with a Nafion-324 cation exchange membrane as the diaphragm, and graphite particles (average particle size: 0.4 mm, packing volume: 1.0 × 6.0 × 20 cm) as the filling material. 3 The cathode was a Hastelloy C-276 mesh, and the anode was a 750 mL aqueous solution containing 0.6 mol / L indigo + 2 mol / L NaOH, and the anolyte was 1200 mL aqueous solution containing 5 mol / L sodium hydroxide. Before electrolysis, nitrogen gas was passed through the catholyte for 40 min to remove oxygen, and then 50 mL of the catholyte from the electrolysis reaction in Example 5 (containing 0.74 mol / L indigo) was added. During electrolysis, the temperature was controlled at 70 °C, and the current density was controlled at 6 A / dm³. 2 (Current was 7.2A). Electrolysis was stopped after 4 hours, and the indigo content in the catholyte was analyzed using a potentiometric titrator. Calculations showed that the indigo yield was 33.8%, and the current efficiency was 23.7%.

Claims

1. A high-concentration electrochemical reduction method for indigo without the addition of external media and chemical reducing agents, characterized in that, The method employs a diaphragm electrolytic cell as the reactor, using an alkaline aqueous solution containing indigo and indigo white as the cathode solution, an alkaline aqueous solution as the anolyte, and a three-dimensional porous carbon-based conductive material as the cathode to perform an electrochemical reduction reaction, electrochemically reducing indigo in the cathode solution to indigo white. The temperature range of the electrochemical reduction is 30~90℃, and the current density range is 2~10 A / dm³. 2 The cathode is a porous carbon-based conductive material with a specific surface area of ​​2~100 m² / g.

2. The high-concentration indigo electrochemical reduction method without external media and chemical reducing agents as described in claim 1, characterized in that, The cathode is a three-dimensional graphite felt, carbon felt, or foamed glass carbon.

3. The high-concentration indigo electrochemical reduction method without external media and chemical reducing agents as described in claim 1, characterized in that, The cathode is made by heat-treating a three-dimensional porous carbon-based conductive material at 300~1000℃ for 1~10h in an oxygen-containing atmosphere.

4. The high-concentration indigo electrochemical reduction method without external media and chemical reducing agents as described in claim 1, characterized in that, The total concentration of indigo and indigo white in the catholyte is 0.15~2.0 mol / L, and the concentration of indigo white is 0.015~1.0 mol / L.

5. The high-concentration indigo electrochemical reduction method without external media and chemical reducing agents as described in claim 1, characterized in that, The indigo is added in the form of an indigo solution, which is prepared by electrochemical reduction, catalytic hydrogenation, or chemical reduction.

6. The high-concentration indigo electrochemical reduction method without external media and chemical reducing agents as described in claim 5, characterized in that, The indigo solution is prepared according to the following steps: an electrolytic cell with a cation exchange membrane as the diaphragm is used as the reactor, with graphite felt as the cathode and stainless steel sheet as the anode; an aqueous solution containing 0.3~1.0 mol / L indigo + 0.5~2 mol / L NaOH + 0.03~0.12 mol / L sodium dithionite or indigo is used as the cathode solution, and a 0.5~7 mol / L sodium hydroxide aqueous solution is used as the anolyte; during electrolysis, the temperature is controlled at 60~80℃, and the current density is controlled at 4~6 A / dm³. 2 After electrolysis, the indigo yield in the catholyte reaches 80-100%, and the collected catholyte is recorded as an indigo solution.

7. The high-concentration indigo electrochemical reduction method without external media and chemical reducing agents as described in claim 1, characterized in that, The alkaline aqueous solutions are all aqueous solutions containing supporting electrolytes, including sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, tetramethylammonium hydroxide, tetraethylammonium hydroxide, and ammonia monohydrate; the concentration of supporting electrolytes in the catholyte is 0.1~10 mol / L; and the concentration of supporting electrolytes in the anolyte is 0.5~10 mol / L.

8. The high-concentration indigo electrochemical reduction method without external media and chemical reducing agents as described in claim 1, characterized in that, Before adding indigo to the catholyte, the catholyte is purged with nitrogen or pre-electrolyzed for 10 minutes or more.

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