METHOD FOR PREPARING PERIODATES

Abstract

In various aspects and embodiments the invention provides a method for preparing a metal periodate by anodic oxidation of a metal iodide in an electrolysis cell comprising one or more anodes and one or more cathodes, characterised in that the one or more anodes are carbon-comprising electrodes. In certain embodiments the method is characterised in that the one or more anodes comprise a diamond layer doped with one or more IUPAC group 13, 15 or 16 elements of the periodic table.

Description

The present invention relates to a method for preparing a metal periodate by anodic oxidation of a metal iodide using carbon-based anodes.

TECHNICAL BACKGROUND

Periodates are powerful oxidizing agents widely used in chemical syntheses. There is a constant need for economic methods for producing them and for production methods complying with certain regulations, especially if the periodates are used at some point in the synthesis of products for use in the pharmaceutical, cosmetic or nutrition field.

The periodates or periodic acid can be produced electrochemically by anodic oxidation starting from elemental iodine or from iodine salts in which iodine has an oxidation state of below +VII.

For instance, the anodic oxidation of iodine to periodic acid using PbO₂ anodes is described in U.S. Pat. No. 2,830,941 or by Y. Aiya et al. in Journal of the Electrochemical Society 1962, 109, 419.

The anodic oxidation of iodic acid or iodates to periodic acid or periodates is described, for example, by Y. Aiya et al. in Journal of the Electrochemical Society 1962, 109, 419, H. H. Willard et al. in Trans. Electrochem. Soc. 1932, 62, 239, A. Hickling et al. in J. Chem. Soc. 1940, 256 or C. W. Nam et al. in Journal of the Korean Chemical Society 1971, 16, 324. The oxidation is carried out on PbO₂ anodes.

Metal-based electrodes, such as lead dioxide, ruthenium dioxide, iridium dioxide, etc. may however dissolve under anodic conditions and may contribute to metal impurities in the desired product. In particular, lead-based anodes are well known to dissolve under anodic conditions and mass losses of 0.05 g/Ah or even 2.5 g/Ah may occur, as reported by C. W. Nam et al. in Journal of the Korean Chemical Society 1974, 18, 373 or by Y. Aiya et al. in Journal of the Electrochemical Society 1962, 109, 419. The presence of such metal impurities is however not acceptable in certain applications, such as pharmaceutics, cosmetics or nutrition. To comply, for example, with pharmaceutical guideline regulations, it is thus necessary to remove these impurities, which is cumbersome and non-economic.

Another disadvantage of the above-mentioned methods is the use of iodine or iodates as starting materials. Iodine sublimes readily at ordinary temperatures and generates hazardous vapors. Moreover, it is poorly soluble in water (the usual medium for such electrolyses). Iodate in comparison is safe to handle, but the molar cost is significantly higher.

Iodides as starting material are interesting both from an economic and handling view.

The direct electrochemical oxidation of iodide (I⁻) to periodate was investigated on lead dioxide as anode material by Kim and Nam (C. W. Nam and H. J. Kim, Journal of the Korean Chemical Society 1974, 18, 373-380). Herein, a current density of 150 mA/cm² was applied to electrolyze a 1 molar aqueous solution of potassium iodide at 60° C. in an undivided cell. Potassium dichromate (K₂Cr₂O₇, 0.5 g/L) was added to prevent cathodic reduction.

However, here again, the lead dioxide electrode may dissolve under anodic conditions and lead to metal impurities in the desired product, which for certain applications is not acceptable. Moreover, the presence of chromium is not acceptable in certain applications, such as health, personal care and nutrition.

U.S. Pat. No. 5,520,793 relates to an electrochemical process for producing high purity grades of hydrogen iodide by cathodic reduction of solubilized iodine. Concomitantly, the anode is used for generating a product of value selected from periodic acid, oxygen or protons. For this purpose, the anodic compartment contains and aqueous solution comprising an oxidation agent. The aqueous solution is preferably acidic. The anode may be any of those commercially used. Among many others, graphite is mentioned as suitable anode material. The process at the anode is however not used for providing metal periodates. Actually, the presence of any foreign cations, such as sodium and potassium, is undesired, since this would have the potential for contaminating the catholyte by back-migration or passing through the cell divider.

WO 2004/055243 relates to a process for electrolytic production of inorganic peroxy compounds, such as perhalogenic acids, perhalogenates or permanganates starting from halogen or magnesium compounds in a lower oxidation state using an electrode with a doped diamond layer as anode. Specifically described is only the oxidation of sodium chlorate or chlorine to sodium perchlorate. The oxidation of chlorine is carried out at pH 0 using an electrolyte containing NaCl. NaCl is said to be oxidized first to chlorine and then to perchlorate. The current efficiency of 65% is not satisfactory.

Janssen et al. described in Electrochimica Acta 2003, 48, 3959 and in NL1013348C2 the electrochemical synthesis of lithium periodate from lithium iodate using a boron-doped diamond (BDD) anode. The importance of using lithium salts is stressed both in context with the starting material as well as the product.

However, as already explained, iodates are rather expensive starting materials, and even more so if used as lithium salts. Lithium salts in general are rather expensive.

It was an object of the present invention to provide an electrochemical method for the production of periodates which avoids the drawbacks of the prior art processes. To be more precise, the method should yield the periodates devoid of metals undesired in certain applications, such as pharmaceutics, cosmetics or nutrition, and especially devoid of lead, and should start from low-priced, readily available and easy-to-handle starting materials. The method should allow high current efficiency and should be suitable for scale-up. Moreover, it should be possible to avoid the use of toxic anti-reducing agents.

SUMMARY OF THE INVENTION

The object is achieved by an electrochemical method using metal iodides as starting material and a carbon-based anode.

The present invention thus relates to a method for preparing a metal periodate by anodic oxidation of a metal iodide in an electrolysis cell comprising one or more anodes and one or more cathodes, characterised in that the one or more anodes are carbon-comprising electrodes.

DETAILED DESCRIPTION OF THE INVENTION

The remarks made below concerning preferred embodiments of the invention are valid on their own as well as preferably in combination with each other concerning.

Embodiments (E.x) of the Invention

General and preferred embodiments E.x are summarized in the following, non-exhaustive list. Further preferred embodiments become apparent from the paragraphs following this list.

• E.1. A method for preparing a metal periodate by anodic oxidation of a metal iodide in an electrolysis cell comprising one or more anodes and one or more cathodes, characterised in that the one or more anodes are carbon-comprising electrodes.
• E.2. The method as defined in embodiment E.1, characterised in that the one or more anodes comprise a diamond layer doped with one or more IUPAC group 13, 15 or 16 elements of the periodic table.
• E.3. The method as defined in embodiment E.2, characterised in that the one or more anodes comprise a boron-doped diamond layer.
• E.4. The method as defined in any of embodiments E.2 or E.3, where the doped diamond layer is connected to a support material, where the support material is selected from the group consisting of elemental silicon, germanium, zirconium, niobium, titanium, tantalum, molybdenum, tungsten, carbides of the eight afore-mentioned elements, graphite, glassy carbon, carbon fibre and combinations of the afore-mentioned materials.
• E.5. The method as defined in any of the preceding embodiments, where the metal iodide is selected from the group consisting of alkali metal iodides, earth alkaline metal iodides and transition metal iodides.
• E.6. The method as defined in embodiment E.5, where the metal iodide is selected from the group consisting of alkali metal iodides, earth alkaline metal iodides, Cu(I) iodide and Zn(II) iodide.
• E.7. The method as defined in embodiment E.6, where the metal iodide is selected from the group consisting of lithium iodide, sodium iodide, potassium iodide, caesium iodide, magnesium iodide, calcium iodide, Cu(I) iodide and Zn(II) iodide.
• E.8. The method as defined in embodiment E.7, where the metal iodide is selected from the group consisting of sodium iodide, potassium iodide and Cu(I) iodide.
• E.9. The method as defined in embodiment E.8, where the metal iodide is selected from the group consisting of sodium iodide and potassium iodide.
• E.10. The method as defined in any of the preceding embodiments, where the periodate is a para-periodate, meta-periodate, ortho-periodate or a mixture of two or three of these periodates.
• E.11. The method as defined in embodiment E.10, where the periodate is a para-periodate, a meta-periodate or a mixture of a para-periodate and a meta-periodate.
• E.12. The method as defined in any of the preceding embodiments, for preparing sodium para-periodate, sodium meta-periodate or a mixture of sodium para-periodate and sodium meta-periodate by anodic oxidation of sodium iodide; or for preparing potassium para-periodate, potassium meta-periodate or a mixture of potassium para-periodate and potassium meta-periodate by anodic oxidation of potassium iodide.
• E.13. The method as defined in embodiment E.12, for preparing sodium para-periodate, sodium meta-periodate or a mixture of sodium para-periodate and sodium meta-periodate by anodic oxidation of sodium iodide.
• E.14. The method as defined in any of the preceding embodiments, comprising subjecting an aqueous solution comprising the metal iodide to anodic oxidation, where the aqueous solution comprises the metal iodide in a concentration of from 0.001 to 12 mold, where the concentration refers to the amount of iodide.
• E.15. The method as defined in embodiment E.14, where the aqueous solution comprises the metal iodide in a concentration of from 0.05 to 2 mold, where the concentration refers to the amount of iodide.
• E.16. The method as defined in embodiment E.15, where the aqueous solution comprises the metal iodide in a concentration of from 0.1 to 1 mold, where the concentration refers to the amount of iodide.
• E.17. The method as defined in embodiment E.16, where the aqueous solution comprises the metal iodide in a concentration of from 0.2 to 0.6 mold, where the concentration refers to the amount of iodide.
• E.18. The method as defined in embodiment E.17, where the aqueous solution comprises the metal iodide in a concentration of from 0.3 to 0.5 mold, where the concentration refers to the amount of iodide.
• E.19. The method as defined in any of the preceding embodiments, where the anodic oxidation is carried out at a pH of at least 8.
• E.20. The method as defined in embodiment E.19, where the anodic oxidation is carried out at a pH of at least 10.
• E.21. The method as defined in embodiment E.20, where the anodic oxidation is carried out at a pH of at least 12.
• E.22. The method as defined in embodiment E.21, where the anodic oxidation is carried out at a pH of at least 14.
• E.23. The method as defined in any of embodiments E.19 to E.22, where the anodic oxidation is carried out in the presence of a base, where the base is selected from the group consisting of metal hydroxides, metal oxides and metal carbonates.
• E.24. The method as defined in embodiment E.23, where the base is a metal hydroxide, where in case that the metal iodide is an alkali metal iodide, the metal of the base corresponds to the metal in the metal iodide.
• E.25. The method as defined in any of embodiments E.23 or E.24, where the method comprises subjecting an aqueous solution comprising the metal iodide and a base to anodic oxidation, where the metal iodide and the base are used in a molar ratio of from 1:2 to 1:30, where the molar ratio relates to moles of iodide present in the metal iodide and moles of hydroxide present in or obtainable from the base.
• E.26. The method as defined in embodiment E.25, where the method comprises subjecting an aqueous solution comprising the metal iodide and a base to anodic oxidation, where the metal iodide and the base are used in a molar ratio of from 1:2 to 1:20, where the molar ratio relates to moles of iodide present in the metal iodide and moles of hydroxide present in or obtainable from the base.
• E.27. The method as defined in embodiment E.26, where the metal iodide and the base are used in a molar ratio of from 1:5 to 1:15, where the molar ratio relates to moles of iodide present in the metal iodide and moles of hydroxide present in or obtainable from the base.
• E.28. The method as defined in embodiment E.27, where the metal iodide and the base are used in a molar ratio of from 1:8 to 1:12, where the molar ratio relates to moles of iodide present in the metal iodide and moles of hydroxide present in or obtainable from the base.
• E.29. The method as defined in embodiment E.28, where the metal iodide and the base are used in a molar ratio of approximately 1:10, where the molar ratio relates to moles of iodide present in the metal iodide and moles of hydroxide present in or obtainable from the base.
• E.30. The method as defined in any of the preceding embodiments, where the anodic oxidation is carried out at a current density in the range of from 10 to 500 mA/cm².
• E.31. The method as defined in embodiment E.30, where the anodic oxidation is carried out at a current density in the range of from 50 to 150 mA/cm².
• E.32. The method as defined in embodiment E.31, where the anodic oxidation is carried out at a current density in the range of from 80 to 120 mA/cm².
• E.33. The method as defined in embodiment E.32, where the anodic oxidation is carried out at a current density of ca. 100 mA/cm².
• E.34. The method as defined in any of the preceding embodiments, where the electrolysis cell in which the anodic oxidation is carried out comprises one or more anodes in one or more anode compartments and one or more cathodes in one or more cathode compartments, where the anode compartments are separated from the cathode compartments, where in particular the electrolysis cell is a divided cell in which the anode compartment(s) is/are separated from the cathode compartment(s) by usual dividing means (separators), such as semipermeable membranes or diaphragmas.
• E.35. The method as defined in embodiment E.34, where the one or more cathode compartments comprise an aqueous medium with a pH of at least 8.
• E.36. The method as defined in embodiment E.35, where the one or more cathode compartments comprise an aqueous medium with a pH of at least 10.
• E.37. The method as defined in embodiment E.36, where the one or more cathode compartments comprise an aqueous medium with a pH of at least 12.
• E.38. The method as defined in embodiment E.37, where the one or more cathode compartments comprise an aqueous medium with a pH of at least 14.
• E.39. The method as defined in any of the preceding embodiments, comprising
  • introducing into the one or more anode compartments an aqueous solution containing a metal iodide and optionally a base;
  • subjecting said aqueous solution to an electrolysis to obtain anodic oxidation of the iodide; and
  • isolating the metal periodate formed in the anodic oxidation of the iodide from the one or more anode compartments.
• E.40. The method as defined in embodiment E.39, where the aqueous solution contains an alkali metal iodide in a concentration of from 0.01 to 5 mold; and further contains a base which is an alkali metal hydroxide, where the alkali metal of the base corresponds to the alkali metal in the alkali metal iodide; where the alkali metal iodide and the base are contained in a molar ratio of from 1:2 to 1:30.
• E.41. The method as defined in embodiment E.40, where the aqueous solution contains an alkali metal iodide in a concentration of from 0.05 to 2 mold.
• E.42. The method as defined in embodiment E.41, where the aqueous solution contains an alkali metal iodide in a concentration of from 0.1 to 1 mold.
• E.43. The method as defined in embodiment E.42, where the aqueous solution contains an alkali metal iodide in a concentration of from 0.2 to 0.6 mold.
• E.44. The method as defined in embodiment E.43, where the aqueous solution contains an alkali metal iodide in a concentration of from 0.3 to 0.5 mold.
• E.45. The method as defined in any of embodiments E.39 to E.44, where the alkali metal iodide and the base are contained in a molar ratio of from 1:2 to 1:20.
• E.46. The method as defined in embodiment E.45, where the alkali metal iodide and the base are contained in a molar ratio of from 1:5 to 1:15.
• E.47. The method as defined in embodiment E.46, where the alkali metal iodide and the base are contained in a molar ratio of from 1:8 to 1:12.
• E.48. The method as defined in embodiment E.47, where the alkali metal iodide and the base are contained in a molar ratio of approximately 1:10.
• E.49. The method as defined in any of embodiments 39 to 48, where the alkali metal iodide is sodium iodide and the base is sodium hydroxide; or the alkali metal iodide is potassium iodide and the base is potassium hydroxide.
• E.50. The method as defined in any of the preceding embodiments, where the anodic oxidation is carried out in the absence of promoters and additives.
• E.51. The method as defined in any of the preceding embodiments, which is carried out as a semi-continuous process.
• E.52. The method as defined in any of embodiments E.1 to E.50, which is carried out as a continuous process.
• E.53. The method as defined in any of embodiments E.1 to E.50, which is carried out as a batch process.

Metal periodates are the metal salts of the various periodic acids. In the periodic acids the corresponding anions are composed of iodine in oxidation state+VII and oxygen. Periodates include i.a. ortho-periodates (IO₆^5-; the metal ortho-periodate thus having the formula M₅IO₆), meta-periodates (IO₄⁻; the metal meta-periodate thus having the formula MIO₄), dimesoperiodates (I₂O₉^4-; the metal dimesoperiodate thus having the formula M₄I₂O₉), mesoperiodates (IO₅^3-; the metal mesoperiodate thus having the formula M₃IO₅) and para-periodates. Para-periodates are salts of the formula M₃H₂IO₆ and are also known as the corresponding double salt MIO₄*2 MOH. M in the above formulae is a metal equivalent [(Mⁿ⁺)_1/n, where n is the charge number]; in case of, for example, an alkali metal periodate M is thus an alkali metal cation; and in case of an earth alkaline metal periodate M is (M²⁺)_1/2. In periodates with more than one negative charge, the more than one metal equivalents M can have the same or different meanings. By way of example, in the para-periodates M₃H₂IO₆ or MIO₄*2 MOH all three metal equivalents M can have the same meaning or can be derived from different metals; a situation which can for example occur if the counter cation of the iodide used as starting material differs from the counter cation present in the base optionally present during anodic oxidation or used during workup of the reaction product. For further details see below.

The metal iodide is preferably selected from the group consisting of alkali metal iodides, earth alkaline metal iodides and transition metal iodides.

Suitable alkali metal iodides to be used as starting materials in the method of the present invention are for example lithium, sodium, potassium or cesium iodide.

Suitable earth alkaline metal iodides to be used as starting materials in the method of the present invention are for example magnesium or calcium iodide.

Suitable transition metal iodides to be used as starting materials in the method of the present invention are those which are stable under atmospheric conditions (air, moisture) and are at least partly soluble at the desired concentration in the reaction medium. Generally, the reaction medium for the anodic oxidation is aqueous. A low solubility in pure water does however not necessarily disqualify a transition metal iodide, since the reaction (anodic oxidation) can for example be carried out under acidic or—preferably— basic conditions which may drastically enhance solubility in the reaction medium. By way of example, CuI, which is essentially non-soluble in water at a pH of ca. 7, is nevertheless a suitable starting compound if the reaction medium is basic and especially if the reaction medium contains as base an inorganic basic salt in which the cation forms water-soluble iodides, such as is the case, for example, for NaOH or KOH.

Suitable transition metal iodides are the Sc(III), Y(III), La(III), Co(11), Ni(II), Cu(I) and Zn(II) iodides. Among these, in view of their economic efficiency and availability, preference is given to Cu(I) iodide (CuI) and Zn(II) iodide (ZnI₂).

The method of the invention thus serves preferably for preparing a metal periodate by anodic oxidation of an alkali metal iodide, an earth alkaline metal iodide or a transition metal iodide which is stable under atmospheric conditions (air, moisture) and is soluble at the desired concentration in the reaction medium, where the transition metal iodide is preferably selected from the Sc(III), Y(III), La(III), Co(II), Ni(II), Cu(I) and Zn(II) iodides.

The method of the invention serves more preferably for preparing a metal periodate by anodic oxidation of an alkali metal iodide, an earth alkaline metal iodide, Cu(I) iodide or Zn(II) iodide.

Even more preferably, the method of the invention serves for preparing a metal periodate by anodic oxidation of an alkali metal iodide or an earth alkaline metal iodide. As said above, suitable alkali metal iodides are those of lithium, sodium, potassium or cesium. Among these, preference is given to the iodides of lithium, sodium or potassium. More preference is given to the iodides of sodium or potassium. Suitable earth alkaline metal iodides are those of magnesium or calcium. Among these, preference is given to calcium iodide.

The method of the invention serves in particular for preparing an alkali metal periodate by anodic oxidation of an alkali metal iodide. Suitable and preferred alkali metal iodides are listed above. Suitable alkali metal periodates are those of lithium, sodium, potassium or cesium. Among these, preference is given to the periodates of lithium, sodium or potassium. More preference is given to the periodates of sodium or potassium. Specifically, the method of the invention serves for preparing a sodium periodate by anodic oxidation of sodium iodide, or for preparing a potassium periodate by anodic oxidation of potassium iodide. Very specifically, the method of the invention serves for preparing a sodium periodate by anodic oxidation of sodium iodide.

The periodate to be prepared according to the invention is preferably a para-periodate, meta-periodate, ortho-periodate or a mixture of two or three of these periodates. Thus, in a preferred embodiment, the method of the invention serves for preparing a metal para-periodate, meta-periodate, ortho-periodate or a mixture of two or three of these periodates (of course by anodic oxidation of a metal iodide). Suitable and preferred metal iodides are listed above. More preferably, the method of the invention serves for preparing a para-periodate, meta-periodate, ortho-periodate or a mixture of two or three of these periodates by anodic oxidation of an alkali metal iodide, an earth alkaline metal iodide, Cu(I) iodide or Zn(II) iodide. Even more preferably, the method of the invention serves for preparing a para-periodate, meta-periodate, ortho-periodate or a mixture of two or three of these periodates by anodic oxidation of an alkali metal iodide or earth alkaline metal iodide. Particularly preferably, the method of the invention serves for preparing an alkali metal para-periodate, meta-periodate, ortho-periodate or a mixture of two or three of these periodates by anodic oxidation of an alkali metal iodide. In particular, the method of the invention serves for preparing sodium para-periodate, sodium meta-periodate, sodium ortho-periodate or a mixture of two or three of these periodates by anodic oxidation of sodium iodide, or for preparing potassium para-periodate, potassium meta-periodate, potassium ortho-periodate or a mixture of two or three of these periodates by anodic oxidation of potassium iodide. Specifically, the method of the invention serves for preparing sodium para-periodate, sodium meta-periodate, sodium ortho-periodate or a mixture of two or three of these periodates by anodic oxidation of sodium iodide.

The periodate to be prepared according to the invention is more preferably a para-periodate, a meta-periodate or a mixture of a para-periodate and a meta-periodate. Thus, in a more preferred embodiment, the method of the invention serves for preparing a metal para-periodate, or a metal meta-periodate or a mixture of a metal para-periodate and a metal meta-periodate (of course by anodic oxidation of a metal iodide). Suitable and preferred metal iodides are listed above. Even more preferably, the method of the invention serves for preparing a metal para-periodate, a metal meta-periodate or a mixture of a metal para-periodate and a metal meta-periodate by anodic oxidation of an alkali metal iodide, an earth alkaline metal iodide, Cu(I) iodide or Zn(II) iodide. Even more preferably, the method of the invention serves for preparing a metal para-periodate, a metal meta-periodate or a mixture of a metal para-periodate and a metal meta-periodate by anodic oxidation of an alkali metal iodide or earth alkaline metal iodide. Particularly preferably, the method of the invention serves for preparing an alkali metal para-periodate, an alkali metal meta-periodate or a mixture of an alkali metal para-periodate and an alkali metal meta-periodate by anodic oxidation of an alkali metal iodide. In particular, the method of the invention serves for preparing sodium para-periodate, sodium meta-periodate or a mixture of sodium para-periodate and sodium meta-periodate by anodic oxidation of sodium iodide, or for preparing potassium para-periodate, potassium meta-periodate or a mixture of potassium para-periodate and potassium meta-periodate by anodic oxidation of potassium iodide. Specifically, the method of the invention serves for preparing sodium para-periodate, sodium meta-periodate or a mixture of sodium para-periodate and sodium meta-periodate by anodic oxidation of sodium iodide.

Carbon-comprising anodes (or electrodes, more generally speaking) or carbon-based anodes/electrodes, as they are also termed in the following, are well known in the art and include for example graphite electrodes, vitreous carbon (glassy carbon) electrodes, reticulated vitreous carbon electrodes, carbon fiber electrodes, electrodes based on carbonized composites, electrodes based on carbon-silicon composites, graphene-based electrodes and diamond-based electrodes.

The carbon-comprising anodes are not necessarily composed entirely of the carbonaceous material. While graphite electrodes are often composed of graphite as only or as main material, other carbonaceous materials may be present just as or as a part of the outer layer of the electrode, i.e. of that part which is in direct contact with the electrolyte. Further details are given below.

Among the above anodes, preference is given to diamond-based electrodes. Such electrodes are characterized by a very high overpotential for both oxygen and hydrogen evolution leading to a wide potential window.

Because of its large bandgap of more than 5 eV, diamond per se is normally an electric insulator and thus not suitable as electrode material. However, diamond can be made conductive by doping with certain elements. Another alternative of making diamond conductive is annealing thin undoped diamond films in vacuum at temperatures above 1550° C. These drastic conditions presumably result in the formation of a network of conducting carbon phases within the diamond film.

The former method is however more practical and reliable. Therefore, diamond-based electrodes are preferably electrodes comprising electroconductively-doped diamond.

Suitable dopants are selected from IUPAC groups 13, 15 or 16 elements of the periodic table.

Accordingly, in a preferred embodiment, the one or more anodes used in the method of the present invention comprise diamond doped with one or more IUPAC group 13, 15 or 16 elements of the periodic table.

A suitable dopant of group 13 is boron. Suitable dopants of group 15 are nitrogen and phosphorus. A suitable group 16 dopant is sulfur. Boron doping leads to p-type semiconductors, whereas nitrogen-, phosphorus- and sulfur-doping results in n-type conductivity. It is also possible to use two or more different dopants, resulting in, for example, boron-nitrogen-co-doping or boron-sulfur-co-doping. If in case of co-doping the two or more dopants are of different conductivity, such as is the case with B—N—or B—S-co-doping, the type of the resulting conductivity in the co-doped diamond depends inter alia on the concentration of the single dopants and can be tuned to the desired type.

Among the above dopants, preference is given to boron doping. Thus, in a more preferred embodiment, the one or more anodes used in the method of the invention comprise boron-doped diamond.

The boron-doped diamond comprises boron in an amount of preferably 0.02 to 1% by weight (200 to 10,000 ppm), more preferably of 0.04 to 0.2% by weight, in particular of 0.06 to 0.09% by weight, relative to the total weight of the doped diamond.

As already indicated above, such electrodes are generally not composed of doped diamond alone. Rather, the doped diamond is attached to a substrate. Most frequently, the doped diamond is present as a layer on a conducting substrate, but diamond particle electrodes, in which doped diamond particles are embedded into a conducting or non-conducting substrate are suitable as well. Preference is however given to anodes in which the doped diamond is present as a layer on a conducting substrate.

Thus, in particular, the one or more anodes used in the method of the invention comprise a boron-doped diamond layer.

Suitable support materials for electrodes comprising a boron-doped diamond layer are silicon, self-passivating metals, metal carbides, graphite, glassy carbon, carbon fibers and combinations thereof.

Suitable self-passivating metals are for example germanium, zirconium, niobium, titanium, tantalum, molybdenum and tungsten.

Suitable combinations are for example metal carbide layers on the corresponding metal (such an interlayer may be formed in situ when a diamond layer is applied to the metal support), composites of two or more of the above-listed support materials and combinations of carbon and one or more of the other elements listed above. Examples for composites are siliconized carbon fiber carbon composites (CFC) and partially carbonized composites.

Preferably, the support material is selected from the group consisting of elemental silicon, germanium, zirconium, niobium, titanium, tantalum, molybdenum, tungsten, carbides of the eight aforementioned metals, graphite, glassy carbon, carbon fibers and combinations (in particular composites) thereof.

More preference is given to elemental silicon, germanium, zirconium, niobium, titanium, tantalum, molybdenum, tungsten and a combination of one of the seven afore-mentioned metals with the respective metal carbide.

Doped diamond electrodes and methods for preparing them are known in the art and described, for example, in the above-mentioned Janssen article in Electrochimica Acta 2003, 48, 3959, in NL1013348C2 and the references cited therein. Suitable preparation methods include, for example, chemical vapour deposition (CVD), such as hot filament CVD or microwave plasma CVD, for preparing electrodes with doped diamond films; and high temperature high pressure (HTHP) methods for preparing electrodes with doped diamond particles. Doped diamond electrodes are commercially available.

Suitably, the electrochemical oxidation of the iodide is carried out in aqueous medium. Thus, the method of the invention comprises subjecting an aqueous solution comprising the metal iodide to anodic oxidation.

The aqueous solution comprises the metal iodide in a concentration of preferably from 0.001 to 12 mold, more preferably from 0.01 to 5 mold, even more preferably from 0.05 to 2 mold, in particular from 0.1 to 1 mold, specifically from 0.2 to 0.6 mold, and very specifically from 0.3 to 0.5 mold; where the concentration refers to the amount of iodide. “The concentration refers to the amount of iodide” means to express that in order to obtain an aqueous solution containing, for example, of 1 mold of iodide, 1 mold of the iodide (M+)(I⁻) are used, but only 0.5 mold of the iodide (M²⁺)(I⁻)₂ and only 0.33 mold of the iodide (M³⁺)(I⁻)₃.

If the reaction is carried out in batch, the above concentrations refer of course to the concentrations at the beginning of the reaction, since, as a matter of course, the concentration of the iodide decreases in the course of its conversion into the periodate. In case of a continuous design of the reaction, the above concentrations refer to the concentration in the aqueous medium continually introduced into the reaction. In case of a semi-continuous design of the reaction, the above concentrations refer to the concentration in the aqueous medium introduced in the course of the reaction.

It is principally possible to carry out the anodic oxidation in acidic, neutral or basic medium. However, under neutral or acidic conditions the intermediately formed iodine is not immediately oxidized further, but may evaporate or precipitate and thus elude from further reaction. In acidic medium, this is aggravated by the existence of three additional reaction paths to the formation of iodine, which distinctly enhances the problem of iodine elusion:

IO⁻+I⁻+2H+→I₂+H₂O;

IO₃⁻+5I⁻+6H+→3I₂+3 H₂O; and

IO₄⁻+7I⁻+8H+→4I₂+4H₂O.

In basic medium, by contrast, the iodine stage directly reacts further by disproportionation with OH⁻to hypoiodic acid and iodide (I₂+OH⁻→I⁻+HIO). Basic reaction conditions are therefore preferred. Further advantages of an alkaline pH are the lower open circuit voltage and hence, the higher efficiency of the electrolysis.

Preferably, the anodic oxidation is carried out at a pH of at least 8, preferably of at least 10, in particular of at least 12 and specifically of at least 14.

Accordingly, the anodic oxidation is preferably carried out in the presence of a base. Suitable bases are all those which are water-soluble and form hydroxyl ions in aqueous phase. Preferred are inorganic bases, such as metal hydroxides, metal oxides and metal carbonates.

In order to avoid any separation problems, in these bases, the counter cation corresponds preferably to the metal cation in the iodide used. Given the general use of aqueous media for the anodic oxidation, exceptions to this preference are required if the base the cation of which corresponds to the metal cation in the iodide is not (sufficiently) water soluble. By way of example, CuO, Cu(OH)₂ and CuCO₃ are essentially insoluble in water. Therefore, if CuI is used as iodide, it is indicated to use a water-soluble base which is not Cu-based; such as NaOH or KOH. The same applies if certain alkaline earth metal iodides are used; especially MgI₂ or Cal₂, since the corresponding hydroxides, oxides and carbonates are scarcely water-soluble. In this case, too, it is indicated to use a water-soluble base which is not Mg— or Ca-based; such as NaOH or KOH. “Not sufficiently water-soluble” means that the base is not soluble at the concentration required to obtain the desired iodide-to-base ratio (at a given iodide concentration) or the desired pH.

Preferably, the base is a metal hydroxide. More preferably, the base is a metal hydroxide, where the metal of the base corresponds to the metal in the metal iodide used; except for the above-described case where the corresponding base is not (sufficiently) water-soluble. In view of the preferred use of alkali metal iodides, earth alkaline metal iodides and certain transition metal iodides as starting materials and seeing the solubility of the corresponding hydroxides in the aqueous reaction medium, preference is given to the use of alkali metal hydroxides as base. In case an alkali metal iodide is used as starting material, the metal of the base corresponds preferably to the metal in the alkali metal iodide used.

Preferably, the metal iodide and the base are used in a molar ratio of from 1:2 to 1:30, more preferably from 1:2 to 1:20, even more preferably from 1:5 to 1:15, particularly preferably from 1:8 to 1:12 and in particular in a molar ratio of from 1:10 to 1:11, where the molar ratio is calculated based on moles of iodide present in the metal iodide and moles of hydroxide present in or obtainable from the base. In case of metal iodides the metal cation of which forms water-soluble hydroxides (such as the alkali metal iodides), the molar ratio is very specifically approximately 1:10, whereas in case of iodides the metal cation of which forms scarcely water-soluble or insoluble hydroxides (such as the earth alkaline metal iodides, Cu(I) iodide and Zn(II) iodide), the molar ratio is very specifically approximately 1:11.

The method of the invention thus preferably comprises subjecting an aqueous solution comprising the metal iodide and a base to anodic oxidation, where the metal iodide and the base are used in a molar ratio of from 1:2 to 1:30, more preferably from 1:2 to 1:20, even more preferably from 1:5 to 1:15, particularly preferably from 1:8 to 1:12 and specifically in a molar ratio of from 1:10 to 1:11, where the molar ratio relates to moles of iodide present in the metal iodide and moles of hydroxide present in or obtainable from the base. In case of metal iodides the metal cation of which forms water-soluble hydroxides (such as the alkali metal iodides), the molar ratio is very specifically approximately 1:10, whereas in case of iodides the metal cation of which forms scarcely water-soluble or insoluble hydroxides (such as the earth alkaline metal iodides, Cu(I) iodide and Zn(II) iodide), the molar ratio is very specifically approximately 1:11.

“Molar ratio calculated based on moles of iodide present in the metal iodide and moles of hydroxide present in or obtainable from the base” is to be understood as follows: In case x moles of the iodide M^a+(I⁻)_a and y moles of the base M^b+(OH⁻)_b are used, the relevant molar ratio is calculated as (x·a):(y·b). In case x moles of the iodide M^a+(I⁻)_a and y moles of the base (M⁺)₂(CO₃^2-) or (M⁺)₂(O^2-) are used, the relevant molar ratio is calculated as (x·a):(y·2), since 1 mol of oxide or carbonate gives rise to the formation of 2 moles of hydroxide. Analogously, in case x moles of the iodide M^a+(I⁻)_a and y moles of the base (M²⁺)(CO₃^2-) or (M²⁺1 (O^2-)₃ are used, the relevant molar ratio is calculated as (x·a):(y·2). In case x moles of the iodide M^a+(I⁻)_a and y moles of the base (M³⁺)₂(CO₃^2-)₃ or (M³⁺)₂(O^2-)₃ are used, the relevant molar ratio is calculated as (x·):(y·6).

A molar ratio of “approximately” 1:10 or 1:11 means to include uncertainties, such as due to weighing errors and the like and generally includes a deviation of ±15%.

The specific 1:10 ratio corresponds to the theoretical optimum stoichiometry for the formation of the para-periodate, as can be seen from the reaction at the electrodes under basic conditions:

$\frac{\begin{array}{c}\mathrm{Anode}:I^{-}+8{\mathrm{OH}}^{-}⟶{\mathrm{IO}}_4^{-}+4H_{2}O+8e^{-}\\ \mathrm{Cathode}:8H_{2}O+8e^{-}⟶4H_2+8{\mathrm{OH}}^{-}\end{array}}{\sum :I^{-}+4H_{2}O+2{\mathrm{OH}}^{-}⟶H_2{\mathrm{IO}}_6^{3-}+4H_2}$

8 equivalents of OH⁻are thus required for optimum conversion at the anode. To obtain the para-periodate, which is the particularly desired target periodate to be produced with the method of the present invention, another 2 OH⁻are needed.

In case of the use of iodides the metal cation of which forms scarcely water-soluble or insoluble hydroxides, another hydroxyl equivalent per iodide equivalent is necessary to compensate for hydroxyl ions precipitated by the formation of scarcely or insoluble salt with said cation and thus withdrawn from the reaction, requiring thus the specific 1:11 ratio as optimum stoichiometry in case of this type of iodides.

The electrolysis can be carried out under galvanostatic control (applied current is controlled; voltage may be measured, but is not controlled) or potentiostatic control (applied voltage is controlled; current may be measured, but is not controlled), the former being preferred.

In case of the preferred galvanostatic control, the observed voltage is generally in the range of from 0 to 30 V, more frequently from 1 to 20 V and in particular from 1 to 10 V.

In case of potentiostatic control, the applied voltage is generally in the same range, i.e. from 1 to 30 V, preferably from 1 to 20 V, in particular from 2 to 10 V.

The anodic oxidation is preferably carried out at a current density in the range of from 10 to 500 mA/cm², more preferably from 50 to 150 mA/cm², in particular from 80 to 120 mA/cm² and specifically of ca. 100 mA/cm².

To maximize the conversion of iodide to periodate, a charge amount of preferably at least 660,000 C (˜7 F), more preferably of at least 772,000 C (˜8 F), in particular of at least 868,000 C (˜9 F), and specifically of at least 964,000 (˜10 F) per mol of iodide anions to be oxidized is applied; e.g. a charge amount of preferably from 660,000 to 1,928,000C (˜7-20 F), more preferably from 772,000 to 1,446,000 C (˜8-15 F), in particular from 868,000 to 1,156,000 C (˜9-12 F), and specifically from 964,000 to 1,060,000 C (˜10-11 F) per mol of !-anions to be oxidized.

The anodic oxidation is preferably carried out at a temperature of from 5 to 80° C., more preferably from 10 to 60° C., in particular from 20 to 30° C. and specifically from 20 to 25° C.

The reaction pressure is not critical. The anodic oxidation is therefore generally carried out at ambient pressure. Higher pressures can however be indicated if the reaction is to be carried out at a temperature above the normal boiling point of the aqueous medium in order to avoid ebullition.

The electrolysis cell in which the anodic oxidation is carried out comprises one or more anodes in one or more anode compartments and one or more cathodes in one or more cathode compartments, where the anode compartments are preferably separated from the cathode compartments.

The separation of the anode compartment(s) from the cathode compartment(s) can be accomplished by using different electrolysis cells for cathode(s) and anode(s) and connecting these cells by a salt bridge for charge equalization; preference is however given to using a common electrolysis cell in which the anode compartment(s) is/are separated from the cathode compartment(s) by usual dividing means (separators), such as semipermeable membranes or diaphragmas or frits. Alternatively expressed, the electrolysis cell is a divided cell. The separators separate the anolyte [liquid medium in the anode compartment(s)] from the catholyte [liquid medium in the cathode compartment(s)], but allow charge equalization. Diaphragmas are separators comprising porous structures of an oxidic material, such as silicates, e.g. in the form of porcelain or ceramics. Due to the sensitivity of diaphragma materials to harsher conditions, semipermeable membranes are however generally preferred, especially if the reaction is carried out at basic pH, as it is preferred. The semipermeable membrane is preferably one which resists such conditions, especially basic pH. Suitable semipermeable membranes are in particular cation exchange membranes, i.e. membranes composed of materials which allow the passage of cations [and the fluid (which is generally water)], but not of anions. More specifically, the semipermeable membrane is a proton exchange membrane (PEM). Membrane materials which resist harsher conditions, especially basic pH, are based on fluorinated polymers. Examples for suitable materials for this type of membranes are sulfonated tetrafluoroethylene based fluoropolymer-copolymers, such as the Nafion® brand from DuPont de Nemours or the Gore-Select® brand from W. L. Gore & Associates, Inc. In case of the use of iodides with bi- or higher valent counter cations, preference is however given to the use of separators different semipermeable membranes which are permeable only for monovalent cations. In this case, preference is given to diaphragmas.

The reaction at the cathode(s) depends of course on the catholyte present in the cathode compartment(s). As already indicated above, the one or more cathode compartments typically comprise an aqueous medium as catholyte. The reaction taking place at the cathode is in this case the reduction of water to hydrogen (under formation of hydroxyl anions; see above equation). Preferably, the one or more cathode compartments comprise an aqueous medium with a pH of at least 8, preferably of at least 10, in particular of at least 12 and specifically of at least 14.

Suitably, an inorganic base is used to obtain the basic pH in the aqueous medium in the cathode compartment. Suitable and preferred inorganic bases have already been described above in context with the medium in the anode compartments; i.e. they are preferably selected from metal hydroxides, metal oxides and metal carbonates. Preference is given to the hydroxides. More preference is given to alkali metal hydroxides, in particular to sodium and potassium hydroxide.

The cathode material is not very critical, and any material commonly used is suitable, such as stainless steel, chromium-nickel steel, platinum, nickel, bronze, tin, zirconium or carbon. In a specific embodiment, a stainless steel electrode is used as cathode.

Preferably, the method of the invention comprises

• • introducing into the one or more anode compartments an aqueous solution containing the metal iodide and optionally a base;
  • subjecting said aqueous solution to an electrolysis to obtain anodic oxidation of the iodide; and
  • isolating the metal periodate formed in the anodic oxidation of the iodide from the one or more anode compartments.

The aqueous solution which is introduced into the one or more anode compartments preferably contains the metal iodide in a concentration of from 0.001 to 12 mold, more preferably from 0.01 to 5 mold, even more preferably from 0.05 to 2 mold, in particular from 0.1 to 1 mold, specifically from 0.2 to 0.6 mold, and very specifically from 0.3 to 0.5 mold; where the concentration refers to the amount of iodide; see above explanation. As regards preferred metal iodides, reference is made to what has been said above. As already indicated above, if the reaction is carried out in batch, the concentrations refer of course to the concentrations at the beginning of the reaction, since, as a matter of course, the concentration of the iodide decreases in the course of its conversion into the periodate. In case of a continuous or semi-continuous design of the reaction, the above concentrations refer to the concentration in the aqueous medium continually or portion-wise introduced in the course of the reaction.

The aqueous solution preferably contains a base. As regards preferred bases, reference is made to what has been said above. Preferably, the counter cation in the base corresponds to the metal cation in the iodide; exception from this preference being the case where the corresponding base is not (sufficiently) water-soluble; see above explanations.

The aqueous solution as introduced into the one or more anode compartments preferably contains the metal iodide and the base in a molar ratio of from 1:2 to 1:30, more preferably from 1:2 to 1:20, even more preferably from 1:5 to 1:15, particularly preferably from 1:8 to 1:12 and in particular in a molar ratio of from 1:10 to 1:11. In case of metal iodides the metal cation of which forms water-soluble hydroxides (such as the alkali metal iodides), the molar ratio is very specifically approximately 1:10, whereas in case of iodides the metal cation of which forms scarcely water-soluble or insoluble hydroxides (such as the earth alkaline metal iodides, Cu(I) iodide and Zn(II) iodide), the molar ratio is very specifically approximately 1:11. The molar ratio relates to moles of iodide present in the metal iodide and moles of hydroxide present in or obtainable from the base; see above explanation.

More preferably, the aqueous solution contains an alkali metal iodide in a concentration of preferably from 0.01 to 5 mold, more preferably from 0.05 to 2 mold, in particular from 0.1 to 1 mold, specifically from 0.2 to 0.6 mold, and very specifically from 0.3 to 0.5 mold; and further contains a base which is an alkali metal hydroxide, where the alkali metal of the base corresponds to the alkali metal in the alkali metal iodide; where the alkali metal iodide and the base are contained in a molar ratio of preferably from 1:2 to 1:30, more preferably from 1:2 to 1:20, even more preferably from 1:5 to 1:15, particularly preferably from 1:8 to 1:12 and in particular in a molar ratio of approximately 1:10.

Even more preferably, the aqueous solution contains sodium or potassium iodide in a concentration of from 0.01 to 5 mold, preferably from 0.05 to 2 mold, in particular from 0.1 to 1 mold, specifically from 0.2 to 0.6 mold, and very specifically from 0.3 to 0.5 mold; and further contains a base which is sodium or potassium hydroxide, where the alkali metal of the base corresponds to the alkali metal in the alkali metal iodide (i.e. is sodium hydroxide if the iodide is sodium iodide; and is potassium hydroxide if the iodide is potassium iodide); where the sodium or potassium iodide and sodium or potassium hydroxide are contained in a molar ratio of preferably from 1:2 to 1:30, more preferably from 1:2 to 1:20, even more preferably from 1:5 to 1:15, particularly preferably from 1:8 to 1:12 and in particular in a molar ratio of approximately 1:10. Specifically, the aqueous solution contains sodium iodide in a concentration of from 0.01 to 5 mold, preferably from 0.05 to 2 mold, in particular from 0.1 to 1 mold, specifically from 0.2 to 0.6 mold, and very specifically from 0.3 to 0.5 mold; and further contains sodium hydroxide, where sodium iodide and sodium hydroxide are contained in a molar ratio of preferably from 1:2 to 1:30, more preferably from 1:2 to 1:20, even more preferably from 1:5 to 1:15, particularly preferably from 1:8 to 1:12 and in particular in a molar ratio of approximately 1:10.

In another more preferred embodiment, the aqueous solution contains an earth alkaline metal iodide in a concentration of preferably from 0.005 to 2.5 mold, more preferably from 0.025 to 1 mold and in particular from 0.05 to 0.5 mold; and further contains a base which is an alkali metal hydroxide; where the earth alkaline metal iodide and the base are contained in a molar ratio of preferably from 1:4 to 1:60, more preferably from 1:4 to 1:40, even more preferably from 1:10 to 1:30, particularly preferably from 1:16 to 1:24 and in particular in a molar ratio of approximately 1:22.

Even more preferably, the aqueous solution contains calcium iodide in a concentration of from 0.005 to 2.5 mold, preferably from 0.025 to 1 mold and in particular from 0.05 to 0.5 mold; and further contains a base which is sodium hydroxide; where the calcium iodide and sodium hydroxide are contained in a molar ratio of preferably from 1:4 to 1:60, more preferably from 1:4 to 1:40, even more preferably from 1:10 to 1:30, particularly preferably from 1:16 to 1:24 and in particular in a molar ratio of approximately 1:22.

In yet another more preferred embodiment, the aqueous solution contains Cu(I) iodide in a concentration of preferably from 0.01 to 5 mold, more preferably from 0.05 to 2 mold, in particular from 0.1 to 1 mold, specifically from 0.2 to 0.6 mold, and very specifically from 0.3 to 0.5 mold; and further contains a base which is an alkali metal hydroxide; where the Cu(I) iodide and the base are contained in a molar ratio of preferably from 1:2 to 1:30, more preferably from 1:2 to 1:20, even more preferably from 1:5 to 1:15, particularly preferably from 1:8 to 1:12 and in particular in a molar ratio of approximately 1:11.

In the method of the present invention the anodic oxidation is preferably carried out in the absence of promoters and additives. Promoters in terms of the present invention are understood as anti-reducing agents and oxidation promoters, such as polarizing substances. Additives are understood to refer to any substance different from the starting compounds, products formed in the course of reaction, acids, bases, the electrolyte medium (generally water) and promoters. In prior art methods, the presence of promoters or additives is often necessary for obtaining periodates in satisfactory yields. For instance, the method of Nam et al. as described in Journal of the Korean Chemical Society 1974, 18, 373 requires the presence of potassium dichromate as anti-reducing agent. It is obvious that chromium is to be avoided in certain applications, such as health, personal care or nutrition. Fluorides, such as lithium fluoride or silicium fluoride are also often used; they are said to enhance the overpotential of oxygen at the anode and improve the oxidation efficiency. In particular, in the method of the present invention the anodic oxidation is preferably carried out in the absence of any promoters, and especially in the absence of any chromium salts and any fluorides such as lithium fluoride or silicium fluoride.

If more than one anode is used, the two or more anodes can be arranged in the same anode compartment or in separate compartments. If the two or more anodes are present in the same compartment, they can be arranged next to each other or on top of each other. If one or more anode compartments are used, they, too, can be arranged next to each other or on top of each other.

The same applies to the case that one or more cathodes are used.

Alternatively, if the electrolysis apparatus comprises more than one anode and more than one cathode, the electrolysis apparatus may comprise more than one electrolysis cell, each cell comprising an anode and a cathode compartment. The electrolysis cells can be arranged next to each other or on top of each other.

Suitable geometries and arrangements of electrolysis apparatuses containing more than one anode and/or cathode are known to those skilled in the art.

The method of the invention is suitable for reactions on laboratory scale as well as on industrial scale.

The method of the invention can be carried out as a discontinuous (batch) process, a semi-continuous process or a continuous process, but is preferably carried out as a semi-continuous or continuous process.

While in the batch (or discontinuous; time-related) method the electrolyte containing the iodide is subjected to electrolysis and after a certain time this is stopped and the product is isolated from the anode compartment, in a continuous process design the electrolyte is passed continuously through the cell, i.e. the electrolyte containing the iodide is continually added and reacted and the resulting reaction mixture is continually removed from the process. The semi-continuous process design contains elements of both forms. The process is principally continuous, but the electrolyte containing the iodide is added at a certain point of time and the resulting reaction mixture is removed at a certain point of time.

If the reaction is carried out in batch, the anode and cathode compartments are generally designed as batch cells. If the reaction is carried out semi-continuously or continuously, the anode and cathode compartments are generally designed as flow cells.

Various designs and geometries of batch and flow cells are known in the art and can be applied to the present method.

The suitable design of the electrolysis apparatus depends on whether the reaction is carried out as batch, continuous or semi-continuous process and can be determined by the skilled person. Generally, the electrolysis apparatus is equipped with a heat exchanger, a thermometer, a mixing means and a gas outlet off the cathode and also the anode compartment(s). In case of continuous or semi-continuous processes, means for continuous supply and removal, e.g. in the form of recirculation loops equipped with pumps, are provided.

After anodic oxidation has been carried out, the periodate is isolated from the anodic compartment. If desired, the periodate is then isolated from the aqueous medium removed from the anodic compartment. Isolation methods and work-up depend inter alia on the desired product and the reaction conditions and are principally known to those skilled in the art.

For instance, to obtain the para-periodate, the water-solubility of which is not high, this can simply be precipitated from the aqueous reaction medium if the latter has been sufficiently basic; if necessary after concentration. Concentration, if required, can be carried out by usual means, such as evaporation of a part of the solvent, if desired under reduced pressure, partial freeze-drying, partial reverse osmosis etc. The precipitated product can be isolated by usual means, such as filtration or decantation of the supernatant. If desired, the precipitate can then be subjected to further purification steps in order to remove non-reacted iodide, excess base, undesired side products etc., if any, such as washing with water or water-containing solvent mixtures, digestion with water or water-containing solvent mixtures or recrystallization. Alternatively, water can be removed from the aqueous reaction medium, for example by evaporation of the solvent, if desired under reduced pressure, freeze-drying, reverse osmosis, if necessary followed by evaporation of residual water, etc., and, if desired, the residue can be purified as described above for the precipitate. If the reaction medium has not been sufficiently basic, the para-periodate is obtained after further base has been added to the reaction medium. Subsequent workup can be carried out as described above.

To obtain the meta-periodate, the reaction medium needs to be neutralized if the reaction medium has been sufficiently basic to form the para-periodate. Principally, any acid can be used if the corresponding anion is not susceptible to oxidation. Suitable acids are for example sulfuric acid, hydrogensulfates, phosphoric acid, dihydrogenphosphates, hydrogenphosphates, nitric acid, silicic acid and the like. If a hydrogensulfate, dihydrogenphosphate or hydrogenphosphate is used, the counter cation thereof is preferably the same as that of the iodide used as starting product or the same as that of the base, if counter cations in base and iodide differ—provided the corresponding hydrogensulfate, dihydrogenphosphate or hydrogenphosphate is water-soluble. The meta-periodate can then be isolated from the aqueous medium by usual means, such as described above. Given the good water solubility of meta-periodates, it is however generally necessary to concentrate the aqueous medium before the meta-periodate precipitates.

Depending on the intended use of the periodate, it is not compulsory to submit the desired periodate to purification steps. In many cases, especially if the periodate is to be used as oxidizing agent, it is sufficient to evaporate the water from the aqueous medium obtained after neutralization. In some cases, it is not even necessary to evaporate water (i.e. the aqueous medium obtained from the anodic compartment can be used as such in subsequent oxidation processes), or at least not to dryness.

The exact composition of the periodate obtained in the method of the invention depends on the reaction and work-up conditions. If for example anodic oxidation is carried out under neutral pH and if the reaction is not followed by basic or acidic work-up in which a cation different from the counter cation in the iodide used as starting material is introduced, then a periodate is obtained in which the counter cation corresponds to that of the iodide. If anodic oxidation is carried out under basic conditions and the base used is an inorganic basic salt in which the counter cation corresponds to the counter cation in the iodide and if further basic or acidic work-up does not introduce a cation different from the counter cation in the iodide used as starting material, again a periodate is obtained in which the counter cation corresponds to that of the iodide. This is for example the case if an alkali metal iodide is reacted in the presence of an alkali metal basic salt having the same alkali metal as counter cation. If a basic work-up follows the reaction, this uses the same base as that used in the reaction or at least a base with the same alkali metal cation. If an acidic work-up is applied, the acid is either not a metal salt or is a salt containing the same alkali metal as the iodide, such as alkali metal hydrogensulfate, hydrogenphosphate or dihydrogenphosphate. If however a metal iodide is reacted in the presence of a base which contains a different cation (e.g. the combination CuI/NaOH) and/or the work-up is carried out with a base or an acid which introduces a cation different from the cation of the iodide, generally various periodates differing in their counter cations may form. If desired, these can be separated from each other, e.g. by fractionated crystallization, but for most application purposes the periodates can be subjected to further use as obtained; i.e. without further separation from each other.

The method of the present invention allows the production of periodates in a quality suitable for certain demanding applications, such as pharmacy, cosmetics and nutrition, starting from readily available, economic and non-problematic, easy-to-handle iodides in a single step. High conversion and space-time yields coupled with high atom-economy and the lack of necessity of promoters and additives make this method particularly attractive.

The method will now be further illustrated by the following examples.

EXAMPLES

Example 1 Anodic Oxidation of Sodium Iodide to Sodium Periodate in a Semi-Continuous Process

A flow cell with an anode and a cathode compartment, separated from each other through a cation exchange membrane (Nafion® from DuPont) tethered with 0.5 mm Teflon spacers, the compartments containing a 12 cm² electrode surface, was connected with two Ritmo®033 pumps from Fink (Germany) for each compartment. A boron-doped diamond electrode (DIACHEM® electrode from Condias GmbH; 10 μm BDD on silicium support (3 mm thickness); ca. 800 ppm of boron) was used as anode and stainless steel was used as cathode. Nal (1.49 g, 10.0 mmol, 0.4 M) was dissolved in 25 mL of aqueous NaOH (4 M) and the solution was circulated through the anodic chamber (flow rate=7.5 L/h). An aqueous solution of NaOH (4 M) was circulated through the cathodic chamber. A current density j of 100 mAcm⁻² and a charge amount of Q=10 F=9649° C. were applied. The reaction was carried out at room temperature for ca. 2.2 h. After electrolysis, the suspension formed in the anodic compartment was removed therefrom and the flow system was rinsed with aqueous NaHSO₄ and water. For analytical purpose, a sample of the suspension was mixed with aqueous NaHSO₄ until complete dissolution of the precipitate and the resulting solution was analyzed by liquid chromatography with a photo diode array (LC-PDA; LC stationary phase: C18 reversed phase), showing a yield of 94% of sodium para-periodate. The suspension was mixed with aqueous NaOH to complete precipitation and the precipitate was filtered, washed with cold water and dried to yield the desired sodium para-periodate in a purity of 97% (90% yield of isolated para-periodate).

A part of the precipitated sodium para-periodate was converted into the meta-periodate by acidification with nitric acid, concentration of the obtained solution and recrystallization of the precipitate in water (130° C.→room temperature) to obtain sodium meta-periodate in a yield of 65%.

Example 2 Anodic Oxidation of Sodium Iodide to Sodium Periodate in a Semi-Continuous Process

The process was carried out in analogy to example 1, using however 0.3 mol of Nal and 5 mol of NaOH; 0 was 9 F. The yield according to LC-PDA was 93%.

Example 3 Anodic Oxidation of Sodium Iodide to Sodium Periodate in a Semi-Continuous Process

The process was carried out in analogy to example 1, using however 0.1 mol of Nal and 3 mol of NaOH. The flow rate was 4 L/h, j was 90 mAcm⁻² and Q=12 F. The yield according to LC-PDA was 90%.

Example 4 Anodic Oxidation of Potassium Iodide to Potassium Periodate in a Semi-Continuous Process

The process was carried out in analogy to example 1, using however potassium iodide instead of sodium iodide and potassium hydroxide instead of sodium hydroxide. The yield according to LC-PDA was 89%.

Example 5 Anodic Oxidation of Cu(I) Iodide to Cu(I) Periodate in a Semi-Continuous Process

The process was carried out in analogy to example 1, using however Cu(I) iodide instead of sodium iodide. Sodium hydroxide was used as a base. The yield according to LC-PDA was 76%.

Example 6 Anodic Oxidation of Sodium Iodide to Sodium Periodate in a Batch Process

A divided beaker cell equipped with a Nafion® membrane, a BDD anode, and a stainless steel cathode (both 3×1 cm²) was used. Both chambers were filled with an aqueous solution of NaOH (3.0 M, 6 mL). Nal (640 μmol, 0.33 M) was added to the anodic chamber and the electrolysis was started using a charge amount of Q=9 F=1719 C (t=1.59 h) and a current density of j=100 mA/cm². After the electrolysis was completed, the content of the anode chamber was acidified with an aqueous NaHSO₄ solution and the solution was analyzed by LC-PDA, showing a yield of 90%.

Comparative Examples—Anodic Oxidation of Sodium Iodide in a Batch Process on Pb or PbO₂ Anodes

The process was carried out in analogy to example 6, using however Pb or PbO₂ anodes. The PbO₂ anode was prepared from a Pb anode by subjecting the same to electrolytical oxidation in 30% H₂SO₄ at 100 mA/cm² and 3000 C. Anodic oxidation of sodium iodide using a Pb or PbO₂ anode and otherwise under the same conditions as in example 6 resulted in the formation of 53% or <54% of sodium periodate, respectively.

Claims

1. A method for preparing a metal periodate by anodic oxidation of a metal iodide in an electrolysis cell comprising one or more anodes and one or more cathodes, characterised in that the one or more anodes are carbon-comprising electrodes.

2. The method as claimed in claim 1, characterised in that the one or more anodes comprise a diamond layer doped with one or more IUPAC group 13, 15 or 16 elements of the periodic table.

3. The method as claimed in claim 2, characterised in that the one or more anodes comprise a boron-doped diamond layer.

4. The method as claimed in claim 2, where the doped diamond layer is connected to a support material, where the support material is selected from the group consisting of elemental silicon, germanium, zirconium, niobium, titanium, tantalum, molybdenum, tungsten, carbides of the eight afore-mentioned elements, graphite, glassy carbon, carbon fibre and combinations of the afore-mentioned materials.

5. The method as claimed in claim 1, where the metal iodide is selected from the group consisting of alkali metal iodides, earth alkaline metal iodides and transition metal iodides.

6. The method as claimed in claim 5, where the metal iodide is selected from the group consisting of alkali metal iodides, earth alkaline metal iodides, Cu(I) iodide and Zn(II) iodide; preferably from lithium iodide, sodium iodide, potassium iodide, caesium iodide, magnesium iodide, calcium iodide, Cu(I) iodide and Zn(II) iodide; in particular from sodium iodide, potassium iodide and Cu(I) iodide; and specifically from sodium iodide and potassium iodide.

7. The method as claimed in claim 1, where the periodate is a para-periodate, meta-periodate, ortho-periodate or a mixture of two or three of these periodates, and is in particular a para-periodate, a meta-periodate or a mixture of a para-periodate and a meta-periodate.

8. The method as claimed in claim 1, for preparing sodium para-periodate, sodium meta-periodate or a mixture of sodium para-periodate and sodium meta-periodate by anodic oxidation of sodium iodide; or for preparing potassium para-periodate, potassium meta-periodate or a mixture of potassium para-periodate and potassium meta-periodate by anodic oxidation of potassium iodide;

and in particular for preparing sodium para-periodate, sodium meta-periodate or a mixture of sodium para-periodate and sodium meta-periodate by anodic oxidation of sodium iodide.

9. The method as claimed in claim 1, comprising subjecting an aqueous solution comprising the metal iodide to anodic oxidation, where the aqueous solution comprises the metal iodide in a concentration of from 0.001 to 12 mol/l, preferably from 0.01 to 5 mol/l, more preferably from 0.05 to 2 mol/l, in particular from 0.1 to 1 mol/l, specifically from 0.2 to 0.6 mol/l, and very specifically from 0.3 to 0.5 mol/l; where the concentration refers to the amount of iodide.

10. The method as claimed in claim 1, where the anodic oxidation is carried out at a pH of at least 8, preferably of at least 10, in particular of at least 12 and specifically of at least 14.

11. The method as claimed in claim 10, where the anodic oxidation is carried out in the presence of a base, where the base is selected from the group consisting of metal hydroxides, metal oxides and metal carbonates.

12. The method as claimed in claim 11, where the base is a metal hydroxide, where in case that the metal iodide is an alkali metal iodide, the metal of the base corresponds to the metal in the metal iodide.

13. The method as claimed in claim 11, where the method comprises subjecting an aqueous solution comprising the metal iodide and a base to anodic oxidation, where the metal iodide and the base are used in a molar ratio of from 1:2 to 1:30, preferably 1:2 to 1:20, more preferably from 1:5 to 1:15, even more preferably from 1:8 to 1:12 and in particular in a molar ratio of approximately 1:10; where the molar ratio relates to moles of iodide present in the metal iodide and moles of hydroxide present in or obtainable from the base.

14. The method as claimed in claim 1, where the anodic oxidation is carried out at a current density in the range of from 10 to 500 mA/cm2, preferably from 50 to 150 mA/cm2, in particular from 80 to 120 mA/cm2 and specifically of ca. 100 mA/cm2.

15. The method as claimed in claim 1, where the electrolysis cell in which the anodic oxidation is carried out comprises one or more anodes in one or more anode compartments and one or more cathodes in one or more cathode compartments, where the anode compartments are separated from the cathode compartments.

16. The method as claimed in claim 15, where the one or more cathode compartments comprise an aqueous medium with a pH of at least 8, preferably of at least 10, in particular of at least 12 and specifically of at least 14.

17. The method as claimed in claim 15, comprising

introducing into the one or more anode compartments an aqueous solution containing the metal iodide and optionally a base;

subjecting said aqueous solution to an electrolysis to obtain anodic oxidation of the iodide; and

isolating the metal periodate formed in the anodic oxidation of the iodide from the one or more anode compartments.

18. The method as claimed in claim 17, where the aqueous solution contains an alkali metal iodide in a concentration of from 0.01 to 5 mol/l, preferably from 0.01 to 5 mol/l, more preferably from 0.05 to 2 mol/l, in particular from 0.1 to 1 mol/l, specifically from 0.2 to 0.6 mol/l, very specifically from 0.3 to 0.5 mol/l; and further contains a base which is an alkali metal hydroxide, where the alkali metal of the base corresponds to the alkali metal in the alkali metal iodide; where the alkali metal iodide and the base are contained in a molar ratio of from 1:2 to 1:30, preferably 1:2 to 1:20, more preferably from 1:5 to 1:15, even more preferably from 1:8 to 1:12 and in particular in a molar ratio of approximately 1:10.

19. The method as claimed in claim 17, where the alkali metal iodide is sodium iodide and the base is sodium hydroxide; or the alkali metal iodide is potassium iodide and the base is potassium hydroxide.

20. The method as claimed in claim 1, where the anodic oxidation is carried out in the absence of promoters and additives.

21. The method as claimed in claim 1, which is carried out as a semi-continuous or continuous process.