GB2291887A - Use of insoluble electrode comprising an iridium oxide-containing coating as anode in electrolytic reduction of a disulphide compound - Google Patents

Description

2291887 METHOD FOR THE ELECTROLYTIC REDUCTION OF A DISULFIDE COMPOUND The

present invention relates to a method of the electrolytic reduction of a disulfide compound, and more particularly to a method of producing cysteine in a stable manner over a long period of time with a high yield by electrolytically reducing cystine.

Compounds having an HS-group have been widely used as stabilizers for medicaments, raw materials for agricultural chemicals, intermediate products, resins, etc.

Cysteine, which is an important amino acid ha ving an HS-group, is produced by forming a solution of cystine which is a disulfide compound in hydrochloric acid or sulfuric acid and electrolytically reducing the cystine in the solution.

A method of obtaining an HS-group-containing compound from a disulfide compound by electrolytic reduction has been practised for a long time and a method of synthesizing by electrolytically reducing an acidic aqueous solution thereof is disclosed in U.S. Patent 2,907,703. Also, recently, such a method is shown in "a high-yield method for the electrochemical preparation of cysteine and the analog thereof" described in JP-A-4-9486 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").

JP-A-4-9486 describes in detail the electrolytic conditions, the anode materials, and the cathode materials for obtaining cysteine in high yield by electrolytic reduction and 1 - also discloses that the selection of the anode materials and the cathode materials and the selection of a diaphragm f or preventing the loss by the leakage of cystine or cysteine in the cathode chamber are important. Furthermore, such patent publication discloses that by using a solution of aqueous ammonia, an aqueous amine, etc., as the electrolyte, the synthesis of cysteine by electrolytic reduction can be carried out at low cost.

In these known technologies, tin, copper, silver, nickel, carbon, fluorinated carbon or vitreous carbon is disclosed as a cathode material and a silver plated cathode is used for industrial purposes.

However, the cathode materials disclosed in the prior art are cathode materials having a relatively low hydrogen generating potential in both alkaline electrolytes (containing ammonium hydroxide, an amine, etc.), and acidic electrolytes (hydrochloric acid, sulfuric acid, etc.), and further it is disclosed therein that a porous material is used for the purpose of increasing the efficiency of the desired reaction.

However, for example, in the case of a carbonaceous material, the material itself is brittle and the material becomes more porous in an electrolyte. Accordingly, the hydrogen overpotential becomes lower and hydrogen generation becomes easy, whereby the efficiency of the desired reaction is lowered. As a result, there is a problem that the desired reduction product is not stably obtained.

2 - Furthermore, as the anode, a platinum-plated titanium plate is generally used for avoiding contamination by harmful heavy metals. Also, JP-A-4- 9586 discloses, inter alia, electrode comprising titanium carrying platinum and indium thereon, a lead or lead dioxide electrode and an electrode comprising titanium oxide (EBONEX, Ti407 trade name, made by Ebonex Technologies, Inc.) as the anode.

However, since lead or lead dioxide is a harmful' material, an electrode comprising such a material is unsuitable for the purpose of avoiding contamination by a heavy metal. Also, when a lead electrode or a lead dioxide electrode is used for electrolysis in an electrolyte containing a halogen ion, an ammonium ion, an amine, etc., the electrode is greatly consumed and becomes dissolved in the electrolyte as lead ions, lead chloride, lead hydroxide, lead complex ions, etc., or results in the formation of a heavy metal sludge in the electrolyte.

Since a lead or lead dioxide electrode electrolytically oxidizes an organic material such as amine, the electrode has the problem of making the electrolyte unstable.

On the other hand, a platinum-plated electrode or a platinum-iridiumcoated electrode (formed for example by plating a titanium plate with platinum or platinum and iridium) is undesirable since the coated noble metal is poor in durability and is dissolved as a noble metal or a complex ion, whereby the expensive noble metal is consumed in a short period of time.

When an electrode comprising titanium oxide (EBONEX, trade name) is used as an anode for the purpose of reducing a disulfide compound, the electrode has a disadvantage that the generation of oxygen at the anode causes the surface of the anode to be oxidized to form an electrically non-conductive oxide layer, thereby resulting in the voltage rising in a short period of time to make the electrolysis impossible.

An object of the present invention is to provide a method for the electrolytic reduction of a disulfide compound, which can be stably operated with a high efficiency over a long period of time by using an anode having a long life together with a cathode giving a high reduction yield.

According to the present invention, there is provided a method of electrolytically reducing a disulfide compound in a cathode chamber of an electrolytic bath partitioned by a diaphragm into an anode chamber and the cathode chamber, which comprises carrying out the electrolytic reduction using (1) an electrode having an electrode active surface comprising (a) a metal selected from titanium, tantalum and zirconium or (b) an alloy of at least two metals selected from titanium, tantalum, niobium, zirconium, silver, tin, copper, aluminum, iron, molybdenum, gold, antimony, bismuth, palladium and zinc, as the cathode, and (2) an insoluble electrode comprising a corrosionresistant metal comprising titanium, tantalum, niobium or zirconium, or an alloy of any two or more thereof having coated thereon an electrode active substance containing iridium oxide, - 4 is as the anode.

In a preferred embodiment of the present invention, the coating of the electrode active substance on the anode also contains at least one metal selected from titanium, tantalum, niobium, zirconium, tin, copper, antimony, ruthenium, platinum, cobalt, indium, molybdenum and tungsten, or at least one oxide of the oxides thereof.

In the accompanying drawing, Fig. 1 is a schematic view showing an electrolytic bath which is suitable for use in the electrolytic reduction method of the present invention.

The present invention will now be described in detail below.

In the cathode used in the present invention, the hydrogen generating potential when electrolytically reducing a disulfide compound is conventional material the same as or high compared with the such as lead, tin, copper, silver, niobium, carbon, etc., as disclosed in the prior art. Therefore, the cathode used in the present invention is useful for the electrolytic reduction reaction of a disulfide compound. Further, by using the alloy described above for the cathode, the hydrogen generating potential becomes higher, i.e., more advantageous.

Furthermore, the cathode material used in the present invention is excellent in corrosion resistance and is less soluble in the electrolyte. Also, by using the alloy described above for the cathode, the corrosion resistance is further increased, whereby contamination of the electrolyte by harmful heavy metals can be prevented.

The alloys which can be used as the cathode in the present invention includes Ti-5% Al-2.5% Sn (i.e., 5% by weight aluminum, 2.5% by weight tin, the remainder being titanium), Ti-5% Mo, Ti-45% Au, Ti-5% Nb, Zr-42% Zn, Ti-34 to 65% Zn, Zr2,5% Sn, Ta-25% Pd, Sn-23% Pd, Au-42% Zr, Sn-10% Sb, Mo-10% Sn, Sn-33 to 56% Ti, Cu-20% Sn, Cu-10% Au, Ti-5% Cu-20% Sn, Zr32% Ag, Ti-65% Ag, Fe-65% Al, Sn-13% Bi, and Ti-25% Bi.

The cathode can be selected considering the hydrogen generating potential and the corrosion resistance thereof depending on the kind of electrolyte used, but where cysteine is to be stably produced with a high ef f iciency over a long period of time by electrolytically reducing cystine, it is preferred to use, in particular, inexpensive titanium, zirconium, or an alloy thereof.

As the form of the cathode, a plate form, a mesh form, a rod form or a porous body, etc., can be used. From the point of the reaction efficiency, a mesh form, a porous body or other form having an extended surface area is preferred, and a porous body such as an expanded metal, a punched metal body or a porous sintered body, is particularly suitable. In a porous body comprising a conventionally used material, the hydrogen generating potential is low, and hence when such a conventional porous body is used for improving the reaction yield, the generated amount of by-product hydrogen is increased, but since cathode materials having a high hydrogen generating potential are usable in the present invention, when a porous body comprising such a material is used as a cathode, the desired reaction yield is not lowered. Accordingly, the reaction time can be shortened and the electrolysis apparatus can be smallsized, which make the electrolysis economical.

Furthermore, since in the electrolytic reduction method of the present invention, an insoluble electrode comprising an electrode substrate comprising titanium, tantalum, niobium, zirconium, or an alloy thereof having formed thereon a coating comprising iridium oxide or iridium oxide and at least one metal selected from titanium, tantalum, niobium, zirconium, tin, antimony, ruthenium, platinum, cobalt, indium, molybdenum and tungsten, or at least one of the oxides thereof, is used as the anode, the anode is excellent in corrosion resistance, has a long life as compared with a platinum-plated titanium electrode, a platinumiridium-plated electrode, a lead or lead alloy electrode, a lead dioxide electrode, or a titanium oxide (EBONEX, Ti407, trade name) conventionally used, and shows a less dissolution of the electrode components in the electrolyte, whereby the electrolysis can be stably carried out over a long period of time.

The electrode substrate of the anode can be optionally selected considering the corrosion resistance to the electrolyte, but titanium has a sufficient durability, is inexpensive in thin film-forming metals, and is preferably used 7 as the electrode substrate in the present invention.

Even the electrode f ormed by coating the electrode substrate with iridium oxide only as the electrode substance has a sufficient durability, but by applying a coating comprising iridium oxide and at least one metal selected from titanium, tantalum, niobium, zirconium, tin, antimony, ruthenium, platinum, cobalt, indium, molybdenum and tungsten, or at least one of the oxides thereof as the electrode substance, the life of the electrode can be further prolonged.

The metal or the metal oxide used together with iridium oxide as the coating of the electrode substance in the present invention can be optionally selected according to the kind of the electrolyte and the electrolytic conditions. When the coating of the electrode substance contains at least 20% by weight iridium oxide, the potential of the electrode is stabilized. For example, an electrode having a coating of IrO2-Ta205 or IrO2-SnO2 can be used in an acidic solution and a neutral solution, and an electrode having a coating of IrO2 In203. Ir02-CO203. or Ir02-Sn02-In203 can be used in an alkaline solution. These coating components can be selected considering the corrosion resistance of the components.

An intermediate layer coating for prolonging the life of an electrode may be formed between the electrode substrate and the electrode active substance as described in Japanese Patent No. 1296429 (JP-B-60-22074), Japanese Patent No. 296432 (JP-B-6022075), Japanese Patent No. 1472759 (JP-B-63-20313), - 8 and Japanese Patent No. 1767891 (JP-B-4-43985) (the term "JP-B" as used herein means an "examined published Japanese patent application), thereby to prolong the life of the electrode even further.

As a coating method of the electrode substance onto the electrode substrate, an electroplating method, a chemical plating method, a vapor deposition method, a thermal decomposi tion method, etc., or a combination thereof can be used.

The coating amount of iridium oxide onto the anode substrate varies depending on the electrolytic conditions, etc., but the coating amount should be sufficient to stabilize the anodic potential of the anode for several years of use and to avoid premature stopping of the operation for the purpose of anode replacement. Thus, it is preferred that the coating amount of the electrode substance is from 10 g/M2 to 60 g/M2.

The form of the anode may be a plate form, a mesh form, a rod form, etc., and is selected considering the current density, the current distribution, the heat generation, the flow rate of the electrolyte, etc.

As the diaphragm for use in the present invention, a cation-exchange membrane, an anion-exchange membrane, a neutral membrane, an amphoteric membrane, etc., can be used, but a f luorine-series or hydrocarbon- series cation-exchange membrane, anion-exchange membrane, neutral membrane or amphoteric membrane is preferred from the view of point of handling and electric power consumption.

9 - For preventing the loss of the disulfide raw material and the reaction product from leaking into the cathode chamber through the diaphragm, the diaphragm should be selected according to the dissolved states of the disulfide raw material and the reaction product in the electrolyte, that is, the charges, the sizes of the ions thereof, etc.

For example, when cystine is dissolved in an acidic aqueous solution having pH of 5 or less, which is the isoelectric point, and electrolytic reduction thereof is carried out, by selecting a membrane having a carboxylic acid group as the ion-exchange group as the cation-exchange membrane, the loss of cystine by leaking into the anode chamber can be reduced as compared with a membrane having a sulfonic acid group as the ion-exchange group. The use of a membrane such as a SELEMION membrane (trade name, made by Asahi Glass Co., Ltd.), ACIPLEX membrane (trade name, made by Asahi Chemical Industry Co., Ltd.), is preferred.

An amphoteric membrane having a cation-exchange group and an anionexchange group can reduce the loss of the disulfide compound and the reaction product by the leakage thereof into the anode chamber, and a membrane such as a NEOCEPTA membrane (trade name, made by Tokuyama Co., Ltd.) is preferably used.

In the present invention, when cysteine (a useful amino acid) is produced by the electrolytic reduction of a disulfide compound such as cystine, in a cathode chamber of an - electrolytic bath partitioned by a diaphragm into an anode chamber and the cathode chamber, using an electrode giving a high formation yield of cysteine, such as a titanium-tin alloy as the cathode, and an insoluble electrode formed by coating an electrode active substance containing iridium oxide on a corrosion resistant metal comprising titanium, tantalum, niobium, zirconium, or the alloy thereof as the anode, cysteine which is less contaminated by impurities can be produced stably over a long period of time at a high production efficiency.

The present invention is described in more detail below by the following examples.

Example 1

A titanium plate having a length of 100 mm, a width of 100 mm, and a thickness of 3 mm pickled with a hot aqueous oxalic acid solution was coated with an aqueous solution having chloroplatinic acid in an amount of 150 g/1 in terms of Pt and iridium chloride in an amount of 50 g/1 in terms of ir dissolved therein. After drying, the titanium plate thus coated was baked in a muffle furnace of 5501C in air for 15 minutes to form an iridium oxide-platinum coating by a thermal decomposition. By repeating 10 times the steps of coating the solution, drying, and baking, the coating of 15 g/m' as iridium and 5 g/M2 as platinum was formed to obtain an iridium oxideplatinum-coated anode.

on the other hand, tin plating having a thickness of 5 pm was applied on a titanium plate having a length of 100 mm, 11 - a width of 100 mm, and a thickness of 3 mm by electroplating.

The titanium plate thus plated was then irradiated with an electron beam under a reduced pressure of 10-4 torr to form a titanium-tin alloy composed of titanium-5% tin on the surface of the titanium plate as a cathode.

Using the electrodes obtained above as the anode and the cathode of the two-chamber type electrolytic bath shown in Fig. 1, using an aqueous solution of 1 mol/liter of sulfuriC acid as the anolyte and an aqueous solution of 0.5 mol/liter of L-cystine and 2 mols/liter of hydrochloric acid as the catholyte, and using an amphoteric membrane (NEOCEPTA membrane, trade name, made by Tokuyama Co., Ltd.) having a quaternary ammonium group as the anion-exchange group and a sulfonic acid group as the cation-exchange group at the opposite side as the diaphragm, electrolysis was carried out while circulating the electrolytes in external electrolyte tanks.

By carrying out the electrolysis at a cathode current density of 15 AMm' and an electrolysis temperature of 450C, Lcysteine was produced.

The cathode current density was maintained and the catholyte after passing an electric current for 4 hours was analyzed by an iodometric method to evaluate the amount of Lcystine converted to L-cysteine. The current efficiency was calculated from the amount converted to L-cysteine and the amount of electric current passed. The results obtained are shown in Table 1 below.

12 Comparative Example 1 The electrolysis was carried out in the same manner as in Example 1 except that a silver plate, a graphite plate and a lead plate were variously used as the cathode, and each current efficiency was obtained. The results obtained are shown in Table 1.

When the graphite plate was used, collapse of graphite was observed and carbon became suspended in the catholyte. The current density of the lead plate was good, but lead was dissolved during the electrolysis and white sludge became suspended in the catholyte, whereby the lead electrode could not further be used.

Table 1

Cathode Material Titanium-Tin-Plated Plate Silver Plate Graphite Plate Lead Plate Current Efficiency 92.6% 42.2% 68.3% 82.4% Example 2

A titanium plate as described in Example 1 was coated with an aqueous solution of tin chloride in an amount of 205 g/1 in terms of Sn and iridium chloride in an amount of 150 g/1 in terms of Ir. After drying, the titanium plate thus coated was baked in a muffle furnace of 5500C in air for 15 minutes to form an iridium oxide-tin oxide coating thereon by a thermal decomposition. By repeating 10 times the steps of coating the solution, drying, and baking, an iridium oxide-tin oxide-coated anode having the coating of 15 g/m' as iridium and 20.5 gln2 as tin was prepared.

On the other hand, zircaloy (Zr-2.5% Sn) having a length of 100 mm, a width of 100 mm, and a thickness of 3 mm was used as the cathode, the same electrolytic bath as used in Example 1 was used, an aqueous solution of 1 mol/liter of ammonium chloride was used as the anolyte, an aqueous solution of 2 mols/liter of ammonia containing 0.5 mol/liter of L cystine was used as the catholyte, and NAFION 324 (trade name, made by E.I. Du Pont de Nemours and Company) was used as the diaphragm.

The electrolysis was carried out and after passing an electric current for 6 hours at a current density of 10 A/dm2 and an electrolysis temperature of 45'C, the cathode was analyzed by an iodometric method, whereby the amount of L cystine converted to L-cysteine was obtained. The current density was calculated from the amount converted to L-cysteine and the amount of electric current passed. The results obtained are shown in Table 2 below. In this case, both the anode and the cathode were not changed.

Comparative Example 2 The electrolysis was carried out in the same manner as in Example 2 except that a silver plate, a graphite plate and a lead plate were variously used as the cathode, and each current density was obtained. The results obtained are shown in Table 2 below.

When the graphite plate was used, collapse of graphite was observed and carbon became suspended in the catholyte. Where the lead plate was used, white sludge became suspended in the catholyte.

Table 2

Cathode Material Current Efficiency Zircaloy Silver Plate Graphite Plate Lead Plate 86.8% 54.6% 62.4% 76.3% Example 3

Five kinds of anodes each containing iridium oxide and a metal or other oxide as shown in Table 3 below were prepared by the same method as in Example 1. Using each of the anodes thus prepared, a long-period continuous durability test was carried out in a non-diaphragm electrolytic bath in an aqueous solution of 5% sulfuric acid and 3% ammonium chloride, using the cathode prepared in the same manner as in Example 1 as the counter electrode, at a current density of 10 AMM2, at an electrolysis temperature of 451'C, and at a distance between the electrodes of 10 mm. The time when the electrolysis voltage rose to 5 volts higher than the initial voltage was defined as the life of the anode. The lives of these anodes thus obtained are shown in Table 3 below together with the compositions of the electrodes (anodes).

Comparative Example 3 Using a platinum-plated electrode, a platinum-iridiumplated electrode, a graphite electrode, a ferrite electrode, a lead-tin alloy electrode, or the oxide of titanium (EBONEX, Ti407) as the anode, the evaluation was made in the same manner as in Example 3. The results obtained are shown in Table 3.

Table 3,

Ratio of Component Composition of Electrode (weight ratio) Life (hour) IrC)2-Sn02-Sb203 IrO2-Pt Ir(32-Ta205 IrO2-RuO2-TiO2 (40:50:10) 7 0: 30 7 0: 30 4 0: 20:4 0 9 0: 10) >3000 >3000 >3000 >3000 IrO2-Nb205 >3000 Platinum-Plating (3 gm) 861 Pt-Ir (60:40) 692 Graphite 223 Ferrite (NiO-Fe304) (20:80) 426 Pb-Sn (95:5) 1352 Ti407 (EBONEX, trade name) 165 As described above, in a method of electrolytically reducing a disulfide compound (e.g., cystine), using a titanium-tin-plated electrode or a zirconium-tin electrode as the cathode, enables a reduced compound (e.g., cysteine) to be obtained in a high yield, and using an insoluble electrode incorporating an electrode catalyst containing iridium oxide as the anode, enables the electrolysis to be operated stably over a long period of time.

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Claims (11)

1. A method of electrolytically reducing a disulfide compound in a cathode chamber of an electrolytic bath partitioned by a diaphragm into an anode chamber and the cathode chamber, which comprises carrying out the electrolytic reduction using (1) an electrode having an electrode active surface comprising (a) a metal selected from titanium, tantalum and zirconium or (b) an alloy of at least two metals selected from titanium, tantalum, niobium, zirconium, silver, tin, copper, aluminum, iron, molybdenum, gold, antimony, bismuth, palladium and zinc, as the cathode, and (2) an insoluble electrode comprising a corrosion-resistant metal comprising titanium, tantalum, niobium or zirconium, or an alloy of any two or more thereof having coated thereon an electrode active substance containing iridium oxide, as the anode.

2. A method as claimed in claim 1, wherein the coating of the electrode active material on the anode also contains at least one metal selected from titanium, tantalum, niobium, zirconium, tin, copper, antimony, ruthenium, platinum, cobalt, indium, molybdenum and tungsten, or at least one of the oxides of these metals.

3. A method as claimed in claim 1 or 2, wherein the electrode active surface of the cathode comprises said alloy (b).

4. A method as claimed in claim 3, wherein the alloy is a titanium alloy containing tin.

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5. A method as claimed in any preceding claim, wherein the cathode is in the form of a porous body.

6. A method as claimed in any preceding claim, wherein titanium, zirconium or an alloy thereof is used as the corrosion-resistant metal of the anode.

7. A method as claimed in any preceding claim, wherein the electrode active substance of the anode contains at least 20% by weight-of iridium oxide.

8. A method as claimed in any preceding claim, wherein the electrode active substance comprises Ir02-Ta205, IrO2-SnO2., IrO2-1n203. 1r02-Co203 or IrO2-SnO2-1n203.

9. A method as claimed in any preceding claim, wherein the disulfide compound is cystine which is reduced to cysteine.

10. A method as claimed in claim 1, substantially as hereinbefore described in any one of Examples 1 to 3.

11. A reduced product when formed by a method as claimed in any preceding claim.

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