A liquid indigo electrochemical reduction method with controllable alkali concentration

Abstract

The application discloses a liquid indigo electrochemical reduction method with controllable alkali concentration, which adopts a plate-and-frame electrolytic cell, uses a mixed alkali water solution of indigo and a chemical reducing agent as a cathode liquid, uses an acid water solution as an anode liquid, uses a three-dimensional porous carbon-based conductive material as a cathode, and uses a noble metal electrode as an anode; direct current or pulse current is sequentially passed through the anode liquid, a diaphragm and the cathode liquid from the anode to the cathode to perform an electrochemical reduction reaction, and indigo in the cathode liquid is electrochemically reduced into indigo white, the application uses sulfuric acid solution as the anode liquid to ensure that the alkali concentration in the cathode liquid is controllable, so that the alkali concentration and the indigo white concentration ratio in the electrolyte is below 3.4, which does not exceed the index in the industry, and under the condition of ensuring that the alkali concentration is controllable, the current efficiency of the electrochemical reduction of indigo can reach 58.7%, and the indigo conversion rate can reach 93.1%.

Description

(1) Technical field

[0001] The invention relates to a liquid indigo electrochemical reduction method with controllable alkali concentration. (2) Background technology

[0002] Indigo (I) is an important non-azo colorant with a unique hue. It is one of the oldest pigments known to mankind and is widely used in the food, pharmaceutical, and printing and dyeing industries. In particular, it is often used in the textile industry to make denim and blue jeans. Its production volume is huge. Therefore, it is of great significance to develop environmentally friendly and efficient indigo production processes. Indigo is poorly soluble in water, but the indigo white (II) obtained after reduction has good solubility in alkaline aqueous solutions. Therefore, before dyeing, indigo needs to be reduced to indigo white in an alkaline aqueous solution. The indigo white (golden yellow) dissolved in the alkaline aqueous solution is evenly adsorbed onto the cotton yarn. After exposure to air, the indigo white oxidizes to indigo, thus dyeing the cotton yarn evenly and firmly.

[0003]

[0004] Currently, the main method of indigo dyeing in industry is the hydrosulfite reduction dyeing method. Figure 1 The process flow and general material flow are as follows. In this process, a high concentration of indigo in the dye vat is reduced with hydrosulfite under alkaline conditions. The resulting indigo white solution is diluted in the dye bath before dyeing. The optimal concentration ratio of sodium hydroxide to indigo white in the dye vat is 3.4. Dyeing with this ratio produces excellent fabric. This is primarily reflected in: the fabric's color is purer and brighter, with ideal saturation and brightness; the fabric is evenly dyed, with no noticeable color variation. However, this process also poses significant environmental risks. The discharged wastewater contains high concentrations of sulfates and sulfites, which are highly corrosive and difficult to treat, causing significant environmental pollution.

[0005] Electrochemical indigo reduction is a safe and environmentally friendly technology that aligns with modern environmentally friendly economics. Currently, this technology uses an alkaline solution as the anode, but it suffers from the following issues: the ratio of sodium hydroxide to indigo white in the final electrolyte is significantly increased, exceeding the ratio used in industrial dyeing vats.

[0006] Excessive alkali concentration in the dye vat can have several adverse effects: (1) For fabrics, it may damage the fibers, reduce the degree of polymerization of cotton fibers, and make the fabric fragile and easily damaged; the high alkaline environment may cause the indigo to change color or overreact, preventing normal coloring. (2) In the dyeing process, high alkali concentration may cause dye precipitation, affecting the dyeing effect, and may also clog the dye vat's circulation system. (3) High alkali concentration also increases the difficulty and cost of wastewater treatment.

[0007] Therefore, finding a liquid indigo electrochemical reduction method with controllable alkali concentration is crucial in the textile industry. (3) Summary of the invention

[0008] The present invention aims to provide a liquid indigo electrochemical reduction method with controllable alkali concentration. This method ensures a controllable alkali concentration in the cathode chamber and a ratio of alkali solution to indigo white close to the industrially optimal ratio of 3.4:1, effectively improving indigo white yield and current efficiency. This method addresses issues such as poor dyeing results, low current efficiency, and difficult wastewater treatment.

[0009] The technical solution adopted in the present invention is:

[0010] The present invention provides a method for electrochemical reduction of liquid indigo with controllable alkali concentration. The method adopts a plate-and-frame electrolytic cell, uses a mixed alkaline aqueous solution of indigo (I) and a chemical reducing agent as a cathode liquid, an acidic aqueous solution as an anolyte, a three-dimensional porous carbon-based conductive material as a cathode, and a noble metal electrode as an anode; a direct current or a pulsed current is passed from the anode to the cathode through the anolyte, a diaphragm, and the cathode liquid in sequence to perform an electrochemical reduction reaction, thereby electrochemically reducing the indigo in the cathode liquid to indigo white (II).

[0011]

[0012] Preferably, the alkaline aqueous solution in the cathode liquid is an aqueous solution containing a supporting electrolyte, and the supporting electrolyte includes alkali metal hydroxides (such as NaOH, KOH, etc.), carbonates (such as Na2CO3, K2CO3, etc.), quaternary ammonium bases (such as tetramethylammonium hydroxide, tetraethylammonium hydroxide), and ammonia monohydrate.

[0013] Preferably, the chemical reducing agent includes any chemical reagent that can theoretically reduce indigo in an alkaline aqueous solution, more preferably a mixture of one or more of thiourea dioxide, sodium dithionite, sodium formaldehyde sulfoxylate (Rongalit C, BASF), glucose, and α-hydroxyaldehyde.

[0014] Preferably, the indigo concentration in the cathode liquid is 0.2-2 mol / L (preferably 0.34 mol / L), the chemical reducing agent concentration is 0.01-0.1 mol / L (preferably 0.05 mol / L); and the supporting electrolyte concentration is 0.1-10 mol / L (preferably 0.2-2 mol / L, more preferably 1.5-2 mol / L).

[0015] Preferably, the indigo concentration in the cathode solution is 0.34 mol / L, the chemical reducing agent concentration is 0.05 mol / L, and the supporting electrolyte concentration is 1.5-2 mol / L.

[0016] Preferably, after the cathode liquid is prepared, nitrogen is circulated for 40 minutes.

[0017] Preferably, the anolyte is an aqueous solution of sulfuric acid with a concentration of 0.1 to 10 mol / L (preferably 0.5 to 6 mol / L).

[0018] Preferably, the anode is a precious metal electrode such as a platinum electrode, a ruthenium-titanium electrode, or a titanium-iridium electrode, preferably a titanium-iridium electrode. The anode electrode can have various shapes, including a flat plate, a wire mesh, or a mesh, preferably a diamond-shaped mesh shape that facilitates gas discharge. The oxidation reaction occurs at the anode.

[0019] Preferably, the cathode is an electrode made of carbon-containing materials such as graphite felt, carbon felt or foamed glassy carbon, preferably graphite felt; the cathode is heat-treated at 300-1000° C. for 1-10 hours in a muffle furnace containing an oxygen atmosphere before use.

[0020] Preferably, the temperature range of the electrochemical reduction is 5 to 90°C, preferably 60 to 80°C; the current density range is 1 to 10 A / dm 2 , preferably 3 to 7 A / dm 2 The cathode liquid flow rate is 400 mL / min.

[0021] Preferably, the diaphragm of the plate-and-frame electrolytic cell is a cationic membrane (eg, Nafion 324) or a microporous membrane, preferably a cationic membrane.

[0022] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in:

[0023] The present invention uses an acidic aqueous solution (particularly a sulfuric acid aqueous solution) as the anolyte to ensure that the alkali concentration in the cathode liquid is controllable, so that the ratio of the alkali concentration to the indigo white concentration in the electrolyte is below 3.4, which does not exceed the industrial standards. While ensuring the controllable alkali concentration, the current efficiency of the indigo electroreduction can reach 58.7% and the indigo conversion rate can reach 93.1%. (IV) Description of the accompanying drawings

[0024] Figure 1 , schematic diagram of the indigo dyeing process and the general material flow.

[0025] Figure 2 , schematic diagram of the structure of the plate and frame electrolytic cell. (V) Specific implementation methods

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

[0027] Indigo white yield = (the amount of indigo white in the cathode liquid at the end of the reaction (mol) / the amount of indigo blue added to the cathode liquid at the beginning of the reaction (mol)) × 100%

[0028] Current efficiency = (effective power (Ah) / input power (Ah)) × 100%

[0029] Effective electricity (Ah) = the amount of indigo in the cathode liquid at the end of the reaction (mol) × 2 (e-) × 26.8 (Ah / (mol×e-)) - the amount of reducing agent added to the cathode liquid (mol) × 2 (e-) × 26.8 (Ah / (mol×e-))

[0030] Electricity input (Ah) = electrolysis current (A) × electrolysis time (h)

[0031] The optimal concentration ratio of sodium hydroxide to indigo white in industrial dyeing vats is 3.4:1.

[0032] The heat-treated graphite felt refers to graphite felt that is heat-treated at 500° C. for 2 hours in a muffle furnace containing an oxygen atmosphere before use.

[0033] Example 1: Electrochemical reduction of indigo (0.34 mol / L)

[0034] by Figure 2 The plate-and-frame electrolyzer shown in the figure is a reactor, with Nafion-324 cationic membrane as the diaphragm and heat-treated graphite felt (3×10×2 cm 3 ) is the cathode, and the diamond mesh titanium plated with iridium is the anode. Add the NaOH aqueous solution into the liquid storage tank and circulate it with nitrogen for 40 minutes, then add 35.7g of indigo and 2g of thiourea dioxide (reducing agent), stir evenly, and prepare 400mL of 2mol / L NaOH + 0.34mol / L indigo + 0.05mol / L thiourea dioxide aqueous solution, which is the cathode liquid; 400mL of 0.5mol / L sulfuric acid aqueous solution is the anode liquid. During the electrolysis process, the temperature is controlled at 60°C and the current density is controlled at 6A / dm 2 (current was 3.6 A) and the catholyte flow rate was 400 mL / min. Samples were taken at 0, 1, and 2 hours of electrolysis, and the indigo content in the catholyte was analyzed using a potentiometric titrator. Electrolysis was stopped after 2 hours, and the sodium hydroxide:indigo ratio in the catholyte was 3.3:1. Based on the titration results, the indigo yield was calculated to be 93.1%, and the current efficiency was 58.7%.

[0035] Examples 2-3: Effect of Alkali Concentration in Catholyte on Electrochemical Reduction of Indigo

[0036] The concentration of sodium hydroxide in the cathode liquid of Example 1 was changed to that shown in Table 1. Other operations were the same as in Example 1. The results are shown in Table 1. It can be seen that when the ratio of sodium hydroxide to indigo white is ensured to be at or below the industrial optimal ratio (3.4:1), the yield of indigo white and the current efficiency are the highest when the NaOH concentration in the cathode liquid is 2 M. Combined with Example 1, the optimal concentration is 2 M.

[0037] Table 1: Different concentrations of sodium hydroxide solution added to the cathode

[0038]

[0039] Examples 4-6: Effect of sulfuric acid concentration in the anolyte on the electrochemical reduction of indigo

[0040] The concentration of sulfuric acid in the anolyte of Example 1 was changed to that shown in Table 2. Other operations were the same as in Example 1. The results are shown in Table 2. It can be seen that the concentration of sulfuric acid in the anolyte has no significant effect on the yield of indigo white and the current efficiency.

[0041] Table 2 Sulfuric acid solution with different concentrations added to the anode

[0042]

[0043] Examples 7-18: Effects of current density, temperature, anode electrode, etc. on electrochemical reduction of indigo

[0044] 1. Temperature

[0045] The temperature of 60°C in Example 1 was changed to 5, 30, 50, 80, and 90°C, respectively. Other operations were the same as in Example 1. The results are shown in Table 3. It can be seen from Example 1 that with increasing temperature, the indigo yield and current efficiency first increase and then decrease. The indigo yield and current efficiency effects are within an acceptable range when the temperature is between 50-90°C. According to Example 1, the optimal temperature is 60°C.

[0046] Table 3 Effect of temperature on electrochemical reduction of indigo

[0047]

[0048] 2. Current density

[0049] The current density in Example 1 was changed to 1, 3, 7, and 10 A / dm 2 , other operations are the same, the results are shown in Table 4, it can be seen that the current density is 3-10A / dm 2 The indigo yield and current efficiency are within an acceptable range. Combined with Example 1, the optimal current density is 6A / dm 2 .

[0050] Table 4 Effect of current density on electrochemical reduction of indigo

[0051]

[0052] 3. Anode

[0053] The anode electrode in Example 1 was changed to a flat titanium-iridium, platinum sheet or ruthenium-titanium sheet, and other operations were the same. The results are shown in Table 5. It can be seen that the anode material has no significant effect on the indigo yield and current efficiency.

[0054] Table 5 Effect of anode electrode on electrochemical reduction of indigo

[0055]

[0056]

[0057] Comparative Example 1: Electrochemical Reduction of Indigo (0.34 mol / L) by Changing the Anode and Anolyte

[0058] The anode in Example 1 was replaced with 316 stainless steel, and the anolyte was replaced with 400 mL of a 2 mol / L sodium hydroxide aqueous solution. Other operations were the same as in Example 1. Electrolysis was stopped after 2 hours. According to the titration results, the indigo yield was calculated to be 94.4%, the current efficiency was 61%, and the concentration ratio of sodium hydroxide to indigo was 5.3:1. Although the current efficiency and indigo conversion rate in Comparative Example 1 were higher than those in Example 1, the final ratio of sodium hydroxide to indigo in the electrolyte was much higher than the industrial ratio.

[0059] Comparative Examples 2-4: Effects of Changing the Anode and Anolyte Catholyte Concentration on the Electrochemical Reduction of Indigo

[0060] In Example 1, the anode was replaced with 316 stainless steel, the anolyte was replaced with 400 mL of 2 mol / L aqueous sodium hydroxide solution, and the sodium hydroxide concentration in the catholyte was changed to 1.5, 1, and 0.8, respectively. All other operations were the same. The results are shown in Table 6. It can be seen that the sodium hydroxide to indigo white concentration ratios in Comparative Examples 3 and 4 were lower than the optimal concentration ratio in industry, but the indigo white yield and current efficiency were significantly lower than those in Example 1. Combined with Comparative Example 1, this demonstrates that, at industrial sodium hydroxide to indigo white ratios, the anode is not suitable for electrochemical reduction of indigo using alkaline solutions.

[0061] Table 6: Different concentrations of sodium hydroxide solution added to the cathode

[0062]

Claims

1. A liquid indigo electrochemical reduction method with controllable alkali concentration, characterized in that: The method adopts a plate-and-frame electrolytic cell, with a mixed alkaline aqueous solution of indigo and a chemical reducing agent as the cathode liquid, a sulfuric acid aqueous solution as the anolyte, a three-dimensional porous carbon-based conductive material as the cathode, and a noble metal electrode as the anode; a direct current or a pulsed current is passed from the anode to the cathode through the anolyte, the diaphragm, and the cathode liquid in sequence to perform an electrochemical reduction reaction, thereby electrochemically reducing the indigo in the cathode liquid to indigo white; The alkaline aqueous solution in the cathode liquid is an aqueous solution containing a supporting electrolyte, and the supporting electrolyte includes NaOH, KOH, Na2CO3, K2CO3, tetramethylammonium hydroxide, tetraethylammonium hydroxide, and ammonia monohydrate; The chemical reducing agent includes a mixture of one or more of thiourea dioxide, sodium dithionite, sodium formaldehyde sulfoxylate, glucose, and α-hydroxyaldehyde; The concentration of the sulfuric acid aqueous solution is 0.1 to 10 mol / L.

2. The method according to claim 1, wherein The indigo concentration in the cathode liquid is 0.2-2 mol / L, the chemical reducing agent concentration is 0.01-0.1 mol / L, and the supporting electrolyte concentration is 0.1-10 mol / L.

3. The method according to claim 2, wherein The indigo concentration in the cathode solution is 0.34 mol / L, the chemical reducing agent concentration is 0.05 mol / L, and the supporting electrolyte concentration is 1.5-2 mol / L.

4. The method according to claim 1, wherein After the cathode solution was prepared, nitrogen was circulated for 40 minutes.

5. The method according to claim 1, wherein The cathode is graphite felt, carbon felt or foamed glassy carbon; the cathode is used after heat treatment at 300-1000° C. for 1-10 hours in a muffle furnace containing an oxygen atmosphere.

6. The method according to claim 1, wherein The temperature range of the electrochemical reduction is 5 to 90°C; the current density range is 1 to 10 A / dm 2 , cathode liquid flow rate 400mL / min.

7. The method according to claim 1, wherein The diaphragm of the plate-and-frame electrolytic cell is a Nafion 324 cationic membrane.

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