EP1179612A1 - Cathode for electrolytic cells - Google Patents

Abstract

Cathode consists of an electrically conducting support with a coating of electrochemically deposited lead having a density of 0.001-2g/cm<3>. Independent claims are also included for: (1) a process for the production of a cathode comprising depositing a lead layer onto the cathode from an aqueous catholyte solution containing a lead salt; and (2) a process for the electrochemical reduction of organic compounds.

Description

The present invention relates to a cathode for electrochemical reduction of organic compounds is used, processes for their preparation and their Use for electrochemical reduction of organic Links.

The electrochemical reduction of organic compounds is an important method of representing a variety of Products. Lead is often used as the cathode, which is characterized by a high overvoltage against hydrogen distinguished.

For example, reductions of disulfide compounds (A. Aldaz et al. In Electrochemical Processing Technologies, 1997, Miami, Florida) to the corresponding thiols are carried out on lead plates. On an industrial scale, cysteine hydrochloride is obtained by reducing cystine on lead cathodes. (Review: TR Ralph et al., J. Electroanal. Chem. 1994 , 375, 17). High chemical yields are obtained with current densities up to 700 A / sqm. Due to the competing development of hydrogen, the electricity yields are limited to approx. 46%. Various authors describe a progressive passivation of the lead surface during this implementation (C. Daobao in Jingxi Huagong 1998, 15, p. 258ff. And p. 297ff., M. Li ibid., P. 294) even at current densities of 200-500 A / sqm.

Other examples of the use of lead cathodes are Reduction of aldehydes, ketones, carboxylic acids and carboxylic acid esters to alcohols, the reduction of heterocycles and Dehalogenations (overview: M.M. Baizer in Organic Electrochemistry, Marcel Dekker, New York 1991, p. 362 ff., I. Nishiguchi et al., Electroorg. Synth. 1991, p. 331). Numerous processes are also carried out on an industrial scale using lead cathodes (overview: "Industrial Electrochemistry" by Pletcher and Walsh, Chapman and Hall, London 1989, pp. 313-319).

Due to the low mechanical stability of the lead high weight and difficult to connect are used for industrial applications where large Electrode surfaces are needed, lead-plated carrier cathodes, for example made of copper or titanium (M.M. Baizer in Organic Electrochemistry, Marcel Verlag Dekker, New York 1991, p.274). Corresponding coating processes are expensive and complicated, however. in this connection you have to rely on flat geometries so that the Coating receives good adhesion to the carrier. It it is also known that lead surfaces are easily deactivated be (EP 0 931 856), whereby the current efficiency and the Area-time benefits decrease. Corrosion is also described by lead electrodes. To regenerate the The electrolytic cell must have a complete cathode surface be dismantled. The resulting high downtime severely restrict economic usability.

The patent specification DE 1 024 518 shows that Tin cathodes at a current density of 500 A / sqm cystine too Cysteine can be reduced with 100% electricity yield. own Experiments show that by increasing the current densities up to 2000 A / m2 of tin the electricity yields drop below 30%, because under these conditions the evolution of hydrogen starts much earlier. In addition, the electricity yield increases Follow-up attempts continue.

The object of the present invention is therefore to provide a high hydrogen overvoltage cathode, the is easy to manufacture and regenerate and which is the Reduction of organic compounds on an industrial scale suitable.

The object is achieved by a cathode consisting of an electrically conductive support with a coating of electrochemically deposited lead with a density between 0.001 and 2 g / cm ³ .

This lead layer has a loose structure with a density which is greatly reduced compared to solid lead in the range between 0.001 and 2 g / cm ³ , preferably between 0.01 and 1 g / cm ³ . The increase in surface area documented by the reduced density has an advantageous effect on the area-time performance of chemical processes.

The carrier of the cathode according to the invention consists of a electrically conductive material, preferably made of a metal excluding an alkali or alkaline earth metal or one Metal alloy or graphite.

The electrically conductive carrier particularly preferably consists of a metal or a metal alloy selected from the Group copper, nickel, tin, zinc, titanium, iron, steel, Stainless steel, cadmium and lead.

Elements can also be selected as alloy components from the group tungsten, chrome, cobalt, molybdenum, manganese, Bismuth, aluminum, mercury, zirconium, vanadium, silicon, Boron, niobium, tantalum, antimony, phosphorus and carbon, to be available.

In the following, the term metal also includes metal alloys, as stated above.

Such a carrier can be combined with one or more others of the metals mentioned. For the coating are all methods known to the person skilled in the art, e.g. Electroplate or sputtering.

The shape of the electrically conductive carrier is not critical. she is preferably selected from the large number of known or available electrode geometries, e.g. Plate, net, foam, Washer, tube, sieve plate, rod etc. selected.

The lead layer deposited on the electrically conductive carrier is distinguished by a reduced density in the range from 0.001 to 2 g / cm ³ , preferably between 0.01 and 1 g / cm ³ , compared to solid lead.

The invention further relates to a method of manufacture a cathode according to the invention.

The production of a cathode according to the invention is thereby characterized in that in a known electrolysis cell on an electrically conductive support that acts as a cathode is switched from an aqueous catholyte solution containing lead salt a layer of lead by means of electrochemical deposition is deposited with reduced density.

Any cell known to the person skilled in the art is an electrolytic cell suitable. A selection of frequently used cells can be found in textbooks, e.g. B. in "Industrial Electrochemistry" by Pletcher and Walsh, Chapman and Hall, London 1989, pp. 141-166.

Suitable as lead salts for the aqueous solution containing lead salt all lead salts, the lead in the second level included, e.g. Lead (II) carbonate, lead (II) chloride, Lead (II) fluoride, lead (II) acetate, lead (II) formate, Lead (II) oxalate, lead (II) nitrate and their basic salts, Lead (II) sulfate and lead (II) oxide.

It can also be a combination of a metallic lead corresponding acid, from which a lead salt containing is then obtained Solution forms can be used.

Water with the addition of is suitable as a solvent Mineral acids, preferably hydrochloric acid, sulfuric acid and Phosphoric acid in a concentration between 0.5 M and 12 M, in the case of hydrochloric acid up to 8.5 M (conc. hydrochloric acid). Other possible additives are organic acids, e.g. Acetic acid, formic acid, citric acid, lactic acid or their salts with ammonium, tetraalkylammonium, sodium or Potassium ions, or complexing agents for divalent lead ions, for example EDTA or NTA in amounts of 1 to 3 Equivalents based on the amount of lead present.

If necessary, the solution containing lead salt can additionally contain inorganic salts as conductive salts. Examples of this are all alkali, alkaline earth metal and ammonium salts, e.g. B. Sodium chloride, sodium sulfate, ammonium chloride, Lithium perchlorate.

If appropriate, the aqueous solution containing lead salt can also be used water-miscible cosolvents such as ethanol, methanol, i-propanol, n-propanol, acetonitrile, ethylene glycol, Ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, Tetrahydrofuran, 1,4-dioxane, methyl acetate, Ethyl acetate, formic acid, dimethylformamide, Sulfolane, ethylene carbonate, N, N'-dimethylpropyleneurea, N, N'-dimethylethylene urea, N-methylpyrrolidone, Tetramethylurea and hexamethylphosphoric triamide in Concentrations of 0.1 to 80% by weight, preferably 1 to 50% by weight. % contain.

The lead ion concentration lies in the lead salt-containing one aqueous solution preferably between 1 ppm and 20 g / l, especially preferably between 10 ppm and 10 g / l.

The pH of the electrolysis solution is preferably between 1 and 6, particularly preferably between 1 and 3.
The deposition of the lead layer is preferably achieved by applying a cathode potential between -0.1 V and the value at which hydrogen evolution starts at the cathode. Current densities of 0.1 A / m ² to 4000 A / m ² , preferably 10 to 3000 A / m ² are set here.

The temperature is when performing the procedure critical. It can be freely selected in a wide range. A lower temperature limit is due to the freezing point Given catholyte solution, an upper limit due to the stability the electrolytic cell. In the case of membrane cells, this is especially the thermal stability of the membrane. In this In this case, the temperature is limited to a maximum of 60 ° C.

The formation of the lead layer can be before or at the same time with the reduction of an organic compound.

The invention therefore also relates to a method for electrochemical Reduction of an organic compound, thereby characterized in that instead of a conventional cathode solid lead or a cathode with a solid Lead coating, e.g. a lead film, the invention Cathode is used.

These methods can be analogous to the known electrochemical ones Process for reducing such compounds under Use of the cathode according to the invention can be carried out. (Overview: M.M. Baizer in Organic Electrochemistry, publisher Marcel Dekker, New York 1991, pp. 362 ff.)

The compound to be reduced is preferably an organic sulfur compound, e.g. B. a disulfide compound, a sulfinic acid, sulfoxide or a thioether, or an organic carbonyl compound, for example an aldehyde or ketone.
Particularly preferred disulfide compounds are cystine, N-alkanoyl-cystine, homocystine and N-alkanoyl homocystine.
The concentration of the compound to be reduced is 0.01 M to 10 M, preferably 0.1 to 5 M. The solvent used is preferably an aqueous solution of mineral acid, for example hydrochloric acid, sulfuric acid or phosphoric acid, or of sodium or potassium hydroxide, sodium or potassium carbonate or acetate , Ammonia, ammonium chloride or ammonium acetate used in concentrations between 0.1 M and 12 M.
The electrochemical reduction of cystine and homocystine is preferably carried out in aqueous hydrochloric acid, sulfuric acid, sodium or potassium hydroxide solution or in an ammoniacal solution. The concentration of the cystine can be between 0.1 M and the saturation concentration. Saturated cystine or homocystine solutions are particularly preferred.
The reduction of N-acetylcystine is preferably carried out at pH values between 5 and 13.
The electrolysis is carried out at temperatures between 0 ° C and 70 ° C, preferably at temperatures between 10 ° C and 50 ° C.
Current densities are 0.1 A / m2 to 20,000 A / m2, preferably 10-5000 A / m2.

For the reduction of organic sulfur and Oxygen compounds can all be known to the person skilled in the art divided cells can be used (overview e.g. in "Industrial Electrochemistry" by Pletcher and Walsh, Chapman and Hall, London 1989, pp. 141-166.).

With the help of the cathode according to the invention, cysteine hydrochloride from cystine in a hydrochloric acid electrolysis solution the unusually high current density of 2000 A / qm with a Area-time performance of 7.3 kg of cysteine hydrochloride hydrate per hour and sqm of membrane area are produced (see G1.1 in scheme 1). This area-time performance is up to at least 5 Days of operation constant. In the electrolysis solution is at a detection limit of 0.5 ppm no lead was detectable.

A comparison experiment with a conventional cathode solid lead gave an area-time performance at 2000 A / sqm of only approx. 4 kg per hour and sqm membrane area. In particular with sales of over 90% occurs at the high current densities in the In the case of the conventional lead cathode, a strong slowdown implementation by prevailing hydrogen evolution on. This is also the case when using DE 1 024 518 to observe the tin cathode described (see Comparative Examples 7 and 8).

With the cathode according to the invention, others can also Reductions such as the homolysis of activated C-S bonds, the reduction of sulfinic acids or the reduction can be carried out very easily by keto groups.

The following schemes (scheme 1) illustrate examples of other possible uses.

Reduction of cysteine sulfinic acid leads to the formation of cystine (S, S) dioxide (Equation 2)
The reduction of 2,4-dicarboxy-2-methyl-thiazolidine leads to the formation of N- (1-carboxyethyl) cysteine (equation 3).

Pyruvic acid is reduced quantitatively to lactic acid (Equation 4).

Another advantage of the cathode according to the invention is its easy regenerability. The regeneration is without Disassembly of the cell possible. The lead layer is in an embodiment by rinsing the catholyte space with Water or mechanically removed with the catholyte.

In a further embodiment, the lead layer becomes partial or completely dissolved. Aqueous solutions are suitable for this of mineral acids or organic acids in concentrations between 0.1 and 10 M except for sulfuric acid. Examples of this are hydrochloric acid, phosphoric acid or acetic acid. Possibly can the aqueous acid solution complexing Increasing the solubility of the resulting lead salt in quantities between 0.5 and 5 equivalents based on that to be resolved Amount of lead can be added. Complexing agent for lead (II) ions are e.g. EDTA, NTA and citric acid. The temperatures are here within the working area of electrochemical Cell.

To protect the electrode from corrosion during the Maintain a protective voltage to remove the lead layer.

The following examples further illustrate the invention.

Example 1: Production of a cathode according to the invention

Cell used: a cell divided by means of a membrane made of Nafion 324 (electrode or membrane area 0.01 m ² ) with electrodes arranged plane-parallel and consisting of a copper cathode, an oxygen-developing DSA anode (DeNora Deutschland GmbH, anolyte: 20 percent sulfuric acid). The electrode-membrane distance was 1.8 cm.
Catholyte and anolyte solutions were pumped through the cell from reservoirs (batch recycle operation).

By pumping the catholyte consisting of a solution of 2.00 g of basic lead carbonate in 1.8 1 1N HCl Flow rates of 1-2 1 / h and a current I of 5 A. lead was deposited on the cathode in a loose structure. After a reaction time of 5 h, the residual content of lead in solution was 21.8 ppm.

Example 2: Production of a cathode according to the invention

Analogously to Example 1, the electrolytic deposition of the lead on the copper cathode was carried out from a solution of 2.00 g of basic lead carbonate in a solution of 360 g of cystine and 367 g of conc. HCl in 1314 g water at 6.9 A.
Response time: 15 h.
Residual content of lead in solution <0.5 ppm.
Conversion of cystine to cysteine hydrochloride: 99.7% (measured with HPLC and rotation value α).

Example 3: Reduction of cystine to cysteine hydrochloride

In the electrolysis apparatus according to example 1 with that according to example 1 prepared cathode was 1.00 kg cystine in 3.65 1 Water and 1019 g conc. HCl at a current density of 2000 A / qm and pumping rates of 10-12 1 / h to cysteine hydrochloride implemented. After 11 hours, 90% of the cystine was cysteine hydrochloride implemented. The reaction was complete after 20 hours. (Degree of conversion> 99.7%, determined by HPLC). Dry Yield> 99.9% (examination of the after several Crystallization obtained mother liquor). Calculated Area-time performance: 7.31 kg cysteine hydrochloride monohydrate per hour and sqm of membrane area.

Example 4: Reduction of cystine to cysteine hydrochloride

In a 0.01 square meter ElectroMPCell (ElectroCell AB, Tåby, Sweden) with tinned copper cathode, DSA anode (Parmascand, Sweden) and Nafion 324 cation exchange membrane, a loose lead layer was deposited as described in Example 2 (pumping rate 1 -2 l / h). Then, analogously to Example 3, 4.0 kg of cystine were converted to cysteine hydrochloride at 2000 A / m 2 (pumping rate 12 l / h). The reaction time was 79 hours.
Calculated area-time performance 7.41 kg cysteine hydrochloride monohydrate per sqm membrane area and per hour

Example 5: Reduction of cystine to cysteine hydrochloride

In a 0.01 square meter ElectroMPCell (ElectroCell AB, Tåby, Sweden) with nickel cathode, an oxygen-developing DSA anode (Parmascand, Sweden) and Nafion-324 cation exchange membrane, a loose was as described in Example 2 Lead layer deposited (pumping rate 1-2 l / h). Then 360 g of cystine were converted to cysteine hydrochloride analogously to Example 4. The reaction was complete after a reaction time of 7.0 hours. (Degree of conversion> 99.7%, determined by HPLC).
Calculated area-time output 7.52 kg of cysteine hydrochloride monohydrate per square meter of membrane area and per hour.

Example 6 : Reduction of cystine to cysteine hydrochloride

Analogously to Example 1, 750 mg of dissolved lead white was used loose lead deposited on a lead cathode. 1.00 kg As in Example 3, cystine became cysteine hydrochloride reduced. Total reaction time: 21.8 hours. Calculated area-time performance: 6.71 kg cysteine hydrochloride monohydrate per square meter Membrane area and per hour.

Example 7: Reduction of cystine to cysteine hydrochloride - comparative example

In an electrolytic cell with a lead plate as the cathode 360 g of cystine became cysteine hydrochloride as in Example 3 reduced. Total reaction time 9.4 hours. Calculated area-time performance 4 kg of cysteine hydrochloride monohydrate per square meter Membrane area and per hour

Example 8: Reduction of cystine to cysteine hydrochloride - comparative example to example 3

In an electrolysis apparatus according to Example 4 with a tinned copper plate as the cathode, a catholyte solution of 360 g cystine, 3.00 g tin (II) chloride dihydrate was reacted at a flow rate of 1-2 l / h and a current I of 5 A. , After a reaction time of 22 h, the conversion to cysteine was 98.1% (HPLC analysis). After this time, the tin content of the solution was still approximately 0.7 ppm (corresponding to approximately 0.6 mg).
The catholyte solution was drained from the cell and reservoir and through a solution of 1.00 kg of cystine, 1019 g of conc. HCl and 3651 g of water replaced. This was implemented at a current of 20 A and a flow of 11 l / h. After a reaction time of 15.2 hours, 90% cystine had been converted. After 24 hours, the solution still contained 2.7% cystine.

Example 9: Reduction of cysteine sulfinic acid to cystine (S, S) dioxide (Eq. 2)

In an electrolysis apparatus consisting of an electrolysis cell with disc-shaped electrodes (anode: DSA titanium expanded metal, cathode: lead sieve plate with loosely separated lead, electrodes used on both sides and each suspended with the help of concentrically welded rods, diameter 7.5 cm) and the As in the periphery constructed in Example 1, a solution of 1.523 g of L-cysteine sulfinic acid monohydrate in 600 ml of 2N hydrochloric acid was reacted at 10 A and a reaction temperature of 15 ° C. for 4.5 hours. The reaction solution contains 60% cystine (S, S) dioxide, 20% cystine sulfinic acid and 20% cystine ( ¹ H-NMR spectroscopic examination).

Example 10: Reduction of 2,4-dicarboxy-2-methyl-thiazolidine to N- (1-carboxyethyl) cysteine (Eq. 3).

A solution was found in the electrolysis apparatus according to Example 10 of 130.0 g of 2,4-dicarboxy-2-methyl-thiazolidine in one Mixture of 526 ml water and 144 g hydrochloric acid at 9.5 A and Temperatures of 16-20 ° C implemented. After a reaction time of 5 hours 66% N- (1-carboxyethyl) cysteine, 19% lactic acid and 15 % Cysteine received. The reaction solution was evaporated and the solid from 0.5 M hydrochloric acid with exclusion of oxygen recrystallized. N- (1-carboxyethyl) cysteine is obtained in the form of colorless crystals.

Claims (12)

Cathode consisting of an electrically conductive support with an electrodeposited lead coating with a density between 0.001 and 2 g / cm ³ . Cathode according to claim 1, characterized in that the carrier consists of a metal except an alkali or alkaline earth metal or a metal alloy or graphite. Cathode according to claim 2, characterized in that the electrically conductive support consists of a metal or a metal alloy selected from the group consisting of copper, nickel, tin, zinc, titanium, iron, steel, stainless steel, cadmium and lead. Cathode according to claim 3, characterized in that the metal alloy further selected elements from the group tungsten, chromium, cobalt, molybdenum, manganese, bismuth, aluminum, mercury, zirconium, vanadium, silicon, boron, niobium, tantalum, antimony, phosphorus and carbon contains. A method for producing a cathode according to one of claims 1 to 4, characterized in that in a known electrolysis cell on an electrically conductive support, which is connected as a cathode, from a lead salt-containing aqueous catholyte solution by means of electrochemical deposition with a lead layer reduced density is deposited. A method according to claim 5, characterized in that the aqueous solution containing lead salt is formed by means of a combination of metallic lead with an acid. A method according to claim 5 or 6, characterized in that the lead ion concentration in the lead salt-containing aqueous solution is between 1 ppm and 20 g / l. Method according to one of claims 5 to 7, characterized in that the lead layer is deposited by applying a cathode potential between -0.1 V and the value at which hydrogen evolution starts at the cathode, a current density of 0.1 A / m ² to 4000 A / m ^{2 is} set. Process for the electrochemical reduction of organic compounds, characterized in that a cathode according to one of claims 1 to 4 is used. A method according to claim 9, characterized in that the organic compound is an organic sulfur compound or an organic carbonyl compound. A method according to claim 10, characterized in that the sulfur compound is a disulfide compound. A method according to claim 11, characterized in that the disulfide compound is selected from the group cystine, N-alkanoyl-cystine, homocystine and N-alkanoyl homocystine.