FR2723107A1 - PROCESS FOR THE ELECTROLYTIC REDUCTION OF A DISULFIDE AND A PRODUCT THUS OBTAINED - Google Patents

Abstract

The invention relates to a method for electrolytic reduction of a disulfide in the cathode compartment of an electrolysis cell divided by a membrane into an anode compartment and a cathode compartment. The active surface of the cathode comprises a metal chosen from titanium, tantalum, niobium and zirconium or an alloy of at least two metals chosen from titanium, tantalum, niobium, zirconium, silver, l 'tin, aluminum, iron, molybdenum, gold, antimony, bismuth, palladium and zinc. The anode is an insoluble electrode comprising a corrosion resistant metal comprising titanium, tantalum, niobium or zirconium, or an alloy of two or more thereof, to which is applied a coating of an active substance containing iridium oxide. The invention also relates to a reduced product obtained by this process.

Description

The present invention relates to a method of electrolytic reduction of a

  disulfide and the reduced product thus obtained, and more particularly a process for producing cysteine stably for a prolonged period and with

  high yield by electrolytic reduction of cysteine.

  Compounds comprising an SH group in their molecule are widely used in particular as stabilizers for drugs, materials

  raw materials for agricultural chemicals, intermediates, resins.

  Cysteine, which is an important amino acid containing an SH group, is obtained by dissolving in hydrochloric acid or sulfuric acid the

  cystine which is a disulfide and by electrolytic reduction of cystine in solution.

  A process for obtaining a compound with an SH group from a disulfide by electrolytic reduction thereof has long been used, and US Pat. No. 2,907,703 describes a process for the synthesis by electrolytic reduction of a solution aqueous acid of such a disulfide. Recently, Japanese patent application published before examination JP-A-4- 9486 described such a process as being a high yield process for the electrochemical preparation of cysteine and

of its analogs.

  Application JP-A-4-9486 describes in detail the conditions of electrolysis, the anode and cathode materials for obtaining cysteine with a high yield by electrolytic reduction and also indicates that the choice of anode materials and cathode and membrane to prevent leakage of cystine or cysteine into the cathode compartment is important. In addition, it indicates that it is possible to carry out the synthesis of cysteine at low cost by electrolytic reduction thanks to the use as ammonia electrolyte.

or an aqueous amine solution.

  In these known technologies, the cathode material consists of tin, copper, silver, nickel, carbon, fluorinated carbon or glassy carbon and a silver-plated cathode is used for

industrial applications.

  However, the cathode materials described in the prior art

  are cathode materials with potential for hydrogen production relatively

  This is weak in alkaline electrolytes (for example containing ammonium hydroxide or an amine) and in acidic electrolytes (for example hydrochloric acid, sulfuric acid) and furthermore the state of the art indicates that

  uses a porous material to increase the yield of the desired reaction.

  However, when a carbonaceous material is used, such a material is fragile and becomes more porous in an electrolyte so that the hydrogen overpotential decreases and a release of hydrogen is favored which reduces the yield of the reaction. wanted. It therefore follows that the desired product is not obtained from

stably.

  In addition, a titanium plate is generally used as anode.

  platinum plating to avoid contamination by toxic heavy metals.

  Application JP-A-4-9586 also describes as an anode a compressed electrode.

  of titanium coated with platinum and indium, a lead or lead dioxide electrode and an electrode comprising titanium oxide Ti407 (EBONEX

  produced by Ebonex Technologies, Inc.).

  However, since lead and lead dioxide are toxic substances, it is not possible to use an electrode comprising these substances to avoid contamination by heavy metals. Also, when using a lead electrode or a lead dioxide electrode for electrolysis in an electrolyte containing halogen ions, ammonium ions, or an amine, the electrode is largely consumed and dissolves in the electrolyte in the form of lead ions, lead chloride, lead hydroxide, complex lead ions, etc., or results in the formation of

heavy metal in the electrolyte.

  In addition, a lead or lead dioxide electrode makes the electrolyte unstable because it electrolytically oxidizes substances.

organic such as amines.

  A platinum-plated or platinum-iridium-coated electrode (formed, for example, by plating a titanium plate with platinum or platinum and iridium) is undesirable because the noble metal has poor durability and is dissolved as complex ions so it is consumed

in a short time.

  When an electrode comprising titanium oxide (EBONEX) is used as the anode in order to reduce a disulfide, such an electrode has the disadvantage that the evolution of oxygen at the anode oxidizes the surface of the anode to form an electrically non-conductive oxide layer so that the

  voltage increases rapidly which makes electrolysis impossible.

  To overcome the drawbacks mentioned above, the aim of the present invention is to provide a process for the electrolytic reduction of a disulphide which can be implemented stably with a high yield for a prolonged period thanks to the use of an anode of great durability and a cathode allowing a high reduction efficiency. The present invention

  also relates to the product obtained by such a process.

  Thus, the present invention relates to a method of electro-

  lytic of a disulfide in a cathode compartment of an electrolysis cell divided by a membrane into an anode compartment and into a cathode compartment, which comprises carrying out the electrolytic reduction using a cathode constituted by an electrode having an active surface comprising a metal chosen from titanium, tantalum and zirconium or an alloy of at least two metals chosen from titanium, tantalum, niobium, zirconium, silver, tin, copper, aluminum, iron, molybdenum, gold, antimony, bismuth, palladium and zinc, and using an anode consisting of an insoluble electrode comprising a metal resistant to corrosion comprising titanium, tantalum, niobium or zirconium, or an alloy of two or more of these metals,

  coated with an active substance containing iridium oxide.

  In a preferred embodiment of the present invention, the coating of active substance on the anode also contains at least one metal

  chosen from titanium, tantalum, niobium, zirconium, tin, copper, anti-

  monk, ruthenium, platinum, cobalt, indium, molybdenum and tungsten or

at least one oxide of these metals.

  Other characteristics and advantages of the invention will appear better

  in the detailed description which follows and refers to the attached drawing, given

  by way of example only, and in which: the single figure is a schematic representation of an electrolysis cell which can be used in the electrolytic reduction process of the

present invention.

  In the cathode used in the method according to the present invention, the hydrogen production potential during the electrolytic reduction of a disulfide is as high as that of a conventional material such as lead, tin, copper, silver, niobium or carbon. Therefore, this cathode can be used for the electrolytic reduction of a disulfide. Furthermore, when using the alloy described above for the cathode, the production potential

  hydrogen increases which is advantageous.

  In addition, the cathode material used in the present invention

  has excellent corrosion resistance and is less soluble in electro

  trolyte. In addition, when using the alloy described above for the cathode, it is possible to further increase the corrosion resistance, which makes it possible to prevent

  contamination of the electrolyte by toxic heavy metals.

  The alloys which can be used as a cathode according to the present invention comprise Ti-5% Al-2.5% Sn (that is to say an alloy comprising 5% by mass of aluminum, 2.5% in mass of tin, the rest being titanium), Ti-5% Mo, Ti-45% Au, Ti-5% Nb, Zr-42% Zn, Ti-34 at 65% Zn, Zr-2, 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, Zr-32% Ag,

  Ti-65% Ag, Fe-65% AI, Sn-13% Bi and Ti-25% Bi.

  The cathode can be chosen according to its hydrogen production potential and its corrosion resistance according to the type of electrolyte used, but when it is a question of producing cysteine in a stable manner with a high yield during prolonged duration by electrolytic reduction of cystine, it is preferable to use in particular inexpensive titanium, zirconium, or an alloy

of these metals.

  The cathode can be in the form of a plate, a fabric, a bar or a porous body. From the angle of the reaction yield, it is preferable to use a large surface cathode such as a cathode in the form of a canvas or of porous body, and in particular a porous body of expanded metal, a porous body is preferred. made of perforated metal or a porous sintered body. The hydrogen production potential of a porous body formed by a conventional material is low so that when such a conventional porous body is used to improve the yield of the reaction, the amount of hydrogen which constitutes a sub- product, increases. However, since it is possible according to the present invention to use for the cathode materials having a high hydrogen production potential, when a porous body comprising such a material is used as cathode, the yield of the reaction is not lowered so that the reaction time can be shortened and the electrolysis machine can be short

  size, which makes electrolysis economical.

  Furthermore, since in the electrolytic reduction process according to the present invention an insoluble electrode is used as the anode comprising an electrode substrate comprising titanium, tantalum, niobium, zirconium or an alloy thereof on which is formed a coating comprising iridium oxide or iridium oxide and at least one metal chosen from titanium, tantalum, niobium, zirconium, tin, antimony, ruthenium, platinum, cobalt, indium, molybdenum and tungsten or at least one of their oxides, there is an anode with excellent corrosion resistance and great durability compared to a titanium electrode with platinum plating, a platinum and iridium plating electrode, a lead or lead alloy electrode, a lead dioxide electrode, or a Ti407 titanium oxide electrode (EBONEX) used conventional way, and which pr exhibits less dissolution of the constituents of the electrode in the electrolyte, so that it is possible to carry out the electrolysis of

  stably for an extended period of time.

  Although it is optionally possible to choose the anode substrate as a function of its resistance to corrosion with respect to the electrolyte, titanium has sufficient durability, is inexpensive as a metal forming thin layers of so it is preferably used as an anode substrate in

the present invention.

  Although an electrode obtained by coating the electrode substrate with iridium oxide as the sole electrode substance has sufficient durability, it is possible to further extend the durability of the electrode by applying a coating comprising iridium oxide and at least one metal chosen from titanium, tantalum, niobium, zirconium, tin, antimony, ruthenium, platinum, cobalt, indium, molybdenum and tungsten, or to

  minus one of their oxides, as an electrode substance.

  The metal or metal oxide used with iridium oxide as an electrode substance coating in the present invention can be selected by

  depending on the type of electrolyte and the conditions of electrolysis. When the reve-

  of electrode substances contains at least 20% by mass of iridium oxide, the potential of the electrode is stabilized. For example, an electrode having a coating of IrO2-Ta2s05 or IrO2-SnO2 can be used in an acid solution and in a neutral solution, and an electrode having a coating of IrO2-ln203, IrO2 - Co2-03 or IrO - SnO - In-203 can be used in an alkaline solution. These constituents of the coating can be chosen according to

  their resistance to corrosion.

  An intermediate layer for extending the durability of the electrode may be provided between the electrode substrate and the electrode active substance as described in Japanese patents No. 1296429 (JP-B-60-22074), 296432 ( JP-B-60-22075), 1472759 (JP-B-63-20313) and 1767891 (JP-B-4-43985) so as to further increase the durability of the electrode. As a method of applying the electrode substance to the electrode substrate, there can be used, for example, an electroplating method, a chemical plating method, a vapor deposition method, a method

  thermal decomposition, or a combination of these processes.

  The amount of iridium oxide applied to the anode substrate varies depending on the electrolysis conditions but it must be sufficient to stabilize the potential of the anode for several years of use and to avoid premature termination of the operation to replace the anode. Thus, it is preferable that the amount of electrode substance applied is between 10 g / m2 and

60 g / m2.

  The shape of the anode, which can be for example a plate, a cloth or a bar, is chosen according to the current density, the distribution of the

  current, heat generation, electrolyte flow.

  In the process according to the present invention, it is possible to use as membrane, for example, a cation exchange membrane, an anion exchange membrane, a neutral membrane, an amphoteric membrane, but it is preferred to use a cation exchange membrane, a membrane exchange membrane. anions, a neutral membrane or an amphoteric membrane based on fluorinated hydrocarbon compounds, from the point of view of handling and consumption

of electricity.

  To prevent losses of raw material constituted by the

  disulfide and to prevent the reaction product from entering the compartment

  Cathodic through the membrane, it is important to choose the membrane as a function of the properties of the disulfide and of the reaction product in the dissolved state in the electrolyte, that is to say in particular as a function of their charge and of the size of

their ions.

  For example, when the cystine is dissolved in an acidic aqueous solution having a pH less than or equal to 5, which corresponds to its isoelectric point, and when its electrolytic reduction is carried out by choosing as membrane cation exchange a membrane comprising carboxylic acid groups as ion exchange groups, it is possible to reduce losses of cystine by leaks in the anode compartment compared to the case of a membrane comprising sulfonic acid groups as ion exchange groups. It is preferred to use a membrane such as a SELEMION membrane (produced by the company Asahi Glass Co., Ltd.) or an ACIPLEX membrane (produced by the

  Asahi Chemical Industry Co., Ltd.).

  An amphoteric membrane having cation exchange groups and anion exchange groups can reduce losses of disulfide and reaction product by leakage thereof into the anode compartment, and it is preferred to use a membrane such as a membrane. NEOCEPTA (produced by Tokuyama Co., Ltd.). In the present invention, it is possible to stably produce for a prolonged period and with a high yield a cysteine less contaminated by impurities when produced by electrolytic reduction of a

  disulfide such as cystine in the cathode compartment of an electric cell

  trolysis divided by a membrane into an anode compartment and into a compar-

  cathode material, using as an electrode an electrode constituted for example by a titanium-tin alloy allowing a high yield of cysteine and as an anode an insoluble electrode obtained by applying an active substance containing iridium oxide on a metal resistant to corrosion including the

  titanium, tantalum, niobium, zirconium or an alloy thereof.

  The following nonlimiting examples are intended to illustrate the present

  invention more precisely.

Example 1

  A titanium plate with a length of 100 mm, a width of mm and a thickness of 3 mm, pickled with a hot aqueous solution of oxalic acid was covered with an aqueous solution containing acid chloroplatinic in an amount of 150 g / l in terms of Pt and iridium chloride in an amount of 50 g / l in terms of Ir. After drying, the titanium plate thus coated was subjected to baking in an oven muffle at 550 ° C in air for 15 min to form a coating of iridium oxide and platinum by thermal decomposition. The steps of coating with the solution, drying and cooking having been repeated 10 times, a coating of 15 g / m2 of iridium and g / m2 of platinum was formed to obtain an anode coated with iridium oxide and

of platinum.

  Furthermore, a tin plating with a thickness of 5 μm was applied to a titanium plate with a length of 100 mm, a width of 100 mm and a thickness of 3 mm by electroplating. The titanium plate thus coated was subjected to an electron beam under a reduced pressure of 1.3 x 10-2 Pa (10-4 torr) to form an alloy of titanium and tin containing 5% tin on the

  surface of the titanium plate which constitutes the cathode.

  The electrodes obtained above were used as anode and cathode, respectively, of a two-compartment electrolysis cell such as that shown in FIG. 1, an aqueous solution of sulfuric acid at 1 mol / A

  as anolyte and an aqueous solution of L-cystine at 0.5 molI and chlorine acid

  hydric at 2 mol / A as catholyte, and an amphoteric membrane (NEOCEPTA membrane) comprising quaternary ammonium groups as anion exchange groups and sulfonic acid groups as cation exchange groups on the opposite side of the membrane, to perform electrolysis by circulating the electrolytes from external electrolyte reservoirs, at a cathodic current density of 15 A / dm2 and at an electrolysis temperature of

C to produce L-cysteine.

  The cathodic current density was maintained and, after 4 h of electrolysis, the catholyte was analyzed by iodometry to assess the amount of L-cystine converted into L-cysteine. The current yield was calculated from the amount converted to L-cysteine and from the amount of electric current. The

  results obtained are presented in table 1 below.

  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 used successively as cathode, and calculated in each case the current efficiency. The results obtained are presented in Table 1 below.

  When using a graphite plate, we observe a disaggregation

  tion of graphite and we see that carbon is suspended in the catholyte. When a lead plate is used, it is found that the current density is satisfactory but that the lead dissolves during the electrolysis and that a white suspension appears in the catholyte so that it is not possible

to use this electrode again.

Table 1

  Cathode material Current efficiency Titanium and tin coated plate 92.6% Silver plate 42.2% Graphite plate 68.3% Lead plate 82.4%

Example 2

  A titanium plate as described in Example 1 was covered with an aqueous solution of tin chloride in an amount of 205 g / l in terms of Sn and iridium chloride in an amount of 150 g / l in terms of Ir. After drying, the titanium plate thus coated was subjected to firing in a muffle furnace at 550C in air for 15 min to form on the plate a coating of iridium oxide and d tin oxide by thermal decomposition. The coating, drying and cooking steps having been repeated 10 times, an anode coated with iridium oxide and tin oxide was obtained at the rate of

  g / m2 of iridium and 20.5 g / m2 of tin.

  On the other hand, a Zircaloy plate (Zr-2.5% Sn) with a length of 100 mm, a width of 100 mm and a thickness of 3 mm was used as the cathode, the same cell electrolysis as in Example 1, an aqueous solution of ammonium chloride at 1 mol / as anolyte, an aqueous solution of ammonia at 2 mol / 1 containing 0.5 mol / 1 of L-cystine as catholyte, and a NAFION 324 membrane (polymer based on tetrafluoroethylene with sulfonic groups produced by the company EI Du Pont de Nemours). Electrolysis was carried out for 6 h at a current density of

  A / dm2 and at a temperature of 45C. Then we analyzed the catholyte by iodo-

  metrics to get the amount of L-cystine converted to L-cysteine. The current density was calculated from the amount converted to L-cysteine and the amount of electric current. The results obtained are presented in Table 2

  below. In this case, the cathode and the anode 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 used successively as cathode, and calculated the current density in each case. The results obtained are presented in Table 2 below.

  When a graphite plate was used, a defect was observed

  graphite aggregation and the appearance of a carbon suspension in the catholyte. When lead plate was used, the appearance of a

  white suspension in the catholyte.

Table 2

  Cathode material Current efficiency Zircaloy 86.8% Silver plate 54.6% Graphite plate 62.4% Lead plate 76.3%

Example 3

  By the same method as in Example 1, five types of anodes were prepared, each containing iridium oxide and a metal or one or more other oxides. Using each of the anodes thus obtained, a long-term continuous durability test was carried out in an electrolysis cell without membrane, in an aqueous solution of 5% sulfuric acid and 3% ammonium chloride. %, using as counter-electrode a cathode prepared of the same

  so that in Example 1, at a current density of 10A / dm2, at a time

  electrolysis temperature of 45 C and for a distance between the electrodes of 10 mm.

  The duration at the end of which the electrolysis voltage has increased by 5 V compared to the initial voltage has been defined as durability of the anode. The durabilities of these anodes are presented in table 3 below as well as the compositions

anodes.

  Comparative Example 3 An evaluation was made in the same manner as in Example 3 using as anode a platinum-plated electrode, a platinum-iridium-plated electrode, a graphite electrode, a ferrite electrode,

  a lead-tin alloy or titanium oxide Ti407 (EBONEX) electrode.

  The results obtained are presented in Table 3.

Table 3

  Constituents ratio Durability Composition of the electrode (mass ratio) (h) IrO2-SnO2-Sb2O3 (40: 50: 10)> 3000 IrO2-Pt (70: 30)> 3000 IrO2-Ta205 (70: 30) > 3000 IrO2-RuO2-TiO2 (40: 20: 40)> 3000 IrO2Nb205 (90: 10)> 3000 Platinum plating (3, umn) 861 Pt-Ir (60: 40) 692 Graphite 223 Ferrite (NiO .Fe304) (20: 80) 426 Pb-Sn (95: 5) 1 352 Ti407 (EBONEXO) 165

Claims (10)

  1. Process for the electrolytic reduction of a disulfide in the compar-   cathode timent of an electrolysis cell divided by a membrane into an anode compartment and into a cathode compartment, characterized in that it comprises the implementation of electrolytic reduction by means of a cathode constituted by an electrode having a active surface comprising a metal chosen from titanium, tantalum and zirconium or an alloy of at least two metals chosen from titanium, tantalum, niobium, zirconium, silver, tin, copper, l aluminum, iron, molybdenum, gold, antimony, bismuth, palladium and zinc, and an anode constituted by an insoluble electrode comprising a corrosion-resistant metal comprising titanium, tantalum, niobium or zirconium, or an alloy of two or more thereof, to which is applied   a coating of an active substance containing iridium oxide.   2. Method according to claim 1, characterized in that the coating of active substance on the anode also contains at least one metal chosen from titanium, tantalum, niobium, zirconium, tin, copper, antimony, ruthenium, platinum, cobalt, indium, molybdenum and tungsten, or at   minus one of the oxides of these metals.   3. Method according to claim 1 or 2, characterized in that the active surface of the cathode comprises said alloy of at least two metals chosen from titanium, tantalum, niobium, zirconium, silver, tin , copper, aluminum, iron, molybdenum, gold, antimony, bismuth, palladium and zinc. 4. Method according to claim 3, characterized in that said alloy   is an alloy of titanium and tin.   5. Method according to any one of the preceding claims,   characterized in that the cathode is in the form of a porous body.   6. Method according to any one of the preceding claims,   characterized in that titanium, zirconium or an alloy thereof is used   as corrosion resistant metal for the anode.   7. Method according to any one of the preceding claims,   characterized in that the active substance of the anode contains at least 20% in mass of iridium oxide.   8. Method according to any one of the preceding claims,   characterized in that the active substance of the anode comprises lrO2-Ta2O5,   lrO2-SnO2, 1rO2-1n203, IrO2-Co-203 or IrO2-SnO2-In203.   9. Method according to any one of the preceding claims,   characterized in that the disulfide is cystine which is reduced to cysteine. 10. Reduced product characterized in that it is obtained by a process   according to any one of the preceding claims.