WO2023194432A1

WO2023194432A1 - Method for preparing periodates via anodic oxidation in a steady state reactor

Info

Publication number: WO2023194432A1
Application number: PCT/EP2023/058931
Authority: WO
Inventors: Siegfried R. Waldvogel; Sebastian ARNDT; Ralf Gross
Original assignee: Pharmazell Gmbh
Priority date: 2022-04-05
Filing date: 2023-04-05
Publication date: 2023-10-12

Abstract

The present invention relates to a method for preparing a metal para-periodate by anodic oxidation of an iodine compound with an oxidation state from -I to +V in an electrolysis cell under specific conditions, and to a device for carrying out this reaction.

Description

Method for preparing periodates via anodic oxidation in a steady state reactor

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.

A general challenge in the production of periodates is the divergent solubility of iodic species, which strongly impedes the electrolysis performance. Iodine is well soluble in alkaline media by the disproportionation into iodide (l~) and hypoiodite (HIO), whereas it is insoluble in neutral to acidic media. In contrast, periodate is well soluble in aqueous acid wherein orthoperiodic acid (HsIOe) is formed, but insoluble in alkaline media, since para-periodate (Nasl^lOe) is formed. If iodine and para-periodate precipitate, this impairs the efficiency of the electrolysis by coating of the anodic surface and separator. In addition, blocking of the flow-electrolysis cell can occur, which immediately intercepts the electrolysis and may lead to damage of the electrolysis cell.

WO 2021/110928 relates to a method for preparing a metal periodate by anodic oxidation of a metal iodide using carbon-based anodes. The method is preferably carried out under basic conditions. Similarly, S. Arndt et al. describe in Angew. Chem. 2020, 132, 8112-8118 the electrochemical synthesis of sodium periodate by anodic oxidation of sodium iodide under basic conditions using a boron-doped diamond (BDD) electrode. In these documents, para-periodate is obtained in high yields and purities. The present inventors found however that clogging may occur to an undesirable extent when the reaction is scaled up under the conditions of these documents due to the formation of a solid periodate precipitate within the electrolysis cell. Clogging is not only a maintenance problem, requiring disassembling and cleaning of the cell, but may lead to high back pressure and even to the damage of the anode. US 4,687,565 relates to an electrolytic cell for producing periodates comprising a plurality of cathodes mounted vertically in parallel to the long sides of the cell body and spaced at equal intervals from each other, a plurality of open anodic compartment boxes with plate separators adapted to maintain equal intervals between the cathodes, one or more anodes inserted in the anodic compartment boxes, a plurality of diaphragms of polyvinyl chloride (PVC) mounted on both sides of the anodic compartment boxes in parallel to the cathodes, anolyte inlets mounted at the lower part of the anodic compartment boxes, anolyte outlets at the upper part of the opposite side of said boxes, a cell cover with air intake holes and cell gas exhaust pipes mounted on the cell body, and perforated PVC lids for the cathodic compartments positioned between the cell cover and the upper level of the catholyte.

US 5,074,975 relates to a process for the simultaneous electrosynthesis of alkaline peroxide and oxygenated halogen salts of alkali metals and to a reactor for carrying out said reaction. The reactor comprises an oxygen reduction cathode (which is advantageously porous) and a halogen-generating anode which are separated by an anion- permeable wall (which is advantageously an anion exchange membrane).

It was the object of the present invention to provide a method for the production of para-periodate which allows easy scale-up, can be carried out continuously or semi- continuously and avoids or at least reduces clogging. Moreover, the method should not be restricted to the oxidation of metal iodides, but should be applicable to all sorts of iodine starting materials. In particular embodiments, the method should furthermore allow the use of the starting materials in dispersed (e.g. dissolved or suspended) or in solid form, the in-line separation of the desired product and/or the recycling of electrolytes.

Summary of the invention

The present inventors found that clogging can be avoided or at least significantly reduced if in the anodic oxidation of an iodine compound to the desired (para-)periodate, an anolyte comprising a metal periodate and an iodine compound with an oxidation state from -I to +V is recirculated through an anolyte circuit into an anodic compartment, and the anolyte circuit comprises a continuous stirred tank reactor operated in a steady-state mode with respect to the overall molar ratio of metal periodate to iodine compounds with an oxidation state from -I to +V in the anolyte. In particular, the anolyte recirculated through the anodic compartment is not completely depleted of the targeted para-periodate or any periodate (i.e. any iodine salt with an oxidation state of the iodine atom of +VII; a more detailed definition is given below) when the anolyte is reintroduced into the anodic compartment, but contains substantial amounts thereof. The invention relates thus to a method for preparing a metal para-periodate by anodic oxidation of an iodine compound with an oxidation state from -I to +V in an electrolysis cell, said electrolysis cell comprising an anodic compartment having one or more anodes, a cathodic compartment having one or more cathodes, and a separator arranged between said anodic and cathodic compartments, said method comprising recirculating a catholyte through a catholyte circuit into said cathodic compartment, recirculating an anolyte through an anolyte circuit into said anodic compartment, said anolyte comprising a metal periodate and an iodine compound with an oxidation state from -I to +V selected from an iodine salt (with an oxidation state from -I to +V of course), elementary iodine, a mixture of different iodine salts (with an oxidation state from -I to +V of course) or a mixture of iodine with one or more iodine salts (with an oxidation state from -I to +V of course), carrying out said anodic oxidation of said iodine compound at a pH of at least 10, wherein said anolyte circuit comprises at least one continuous stirred tank reactor operated in a steady-state mode with respect to the overall molar ratio of metal periodate to iodine compounds with an oxidation state from -I to +V in said anolyte, wherein said continuous stirred tank reactor is operated in a “steady-state” mode by continuously or periodically feeding said iodine compounds with an oxidation state from -I to +V into said continuous stirred tank reactor and continuously or periodically removing metal periodate from said continuous stirred tank reactor.

The invention relates moreover to a device configured for preparing a metal para-perio- date by the method of the invention, i.e. for preparing a metal para-periodate by continuous anodic oxidation of an iodine compound comprising an electrolysis cell comprising an anodic compartment having one or more anodes, a cathodic compartment having one or more cathodes, and a separator arranged between said anodic and cathodic compartments, a catholyte circuit for recirculating catholyte through said cathodic compartment, said catholyte circuit comprising a catholyte reservoir, an anolyte circuit for circulating anolyte through said anodic compartment, said anolyte circuit comprising an at least one continuous stirred tank reactor, said at least one continuous stirred tank reactor being provided with feeding means for feeding an iodine compound with an oxidation state from -I to +V selected from an iodine salt (with an oxidation state from -I to +V of course), elementary iodine, a mixture of different iodine salts (with an oxidation state from -I to +V of course) or a mixture of iodine with one or more iodine salts (with an oxidation state from -I to +V of course) into said at least one continuous stirred tank reactor and withdrawing means for withdrawing metal periodate reaction product from said at least one continuous stirred tank reactor.

Detailed description of the invention

Definitions

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 (oxygen in an oxidation state of -II). Periodates include i.a. ortho-periodates (lOe^5-; the metal ortho-periodate thus having the formula MslOe), meta-periodates (IO₄“; the metal meta-periodate thus having the formula MIO₄), dimesoperiodates (I2O9⁴’; the metal dimesoperiodate thus having the formula M4I2O9), mesoperiodates (IO5³’; 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 M 104*2 MOH. M in the above formulae is a metal equivalent [(Mⁿ⁺)i/_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⁺)I/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 M 104*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 starting material is an iodine salt (e.g. an iodide or iodate) in which the counter cation of differs from the counter cation present in the base present during anodic oxidation or used during workup of the reaction product. For further details see below.

In terms of the present invention, iodine compounds with an oxidation state from -I to +V are inorganic iodine compounds in which the iodine atom has the indicated oxidation state, i.e. iodine salts with an oxidation state (of iodine) from -I to +V or elementary iodine. Inorganic iodine salts with an oxidation state (of the iodine atom) from -I to +V are for example iodides [(Mⁿ⁺)i/_n(l ); iodine atom in the oxidation state -I], hypoiodites [(Mⁿ⁺)i/_n(IO ); iodine atom in the oxidation state +l], iodites [(Mⁿ⁺)i/_n(IO2 )^_; iodine atom in the oxidation state +III], and iodates [(Mⁿ⁺)i/_n(IO3 )^_; iodine atom in the oxidation state +V], where M is a cation, generally a metal cation or an ammonium cation, and n is the charge number.

The anolyte is the liquid medium in the anode compartment(s); the catholyte is the liquid medium in the cathode compartment(s). In the terms of the present invention, a dispersion is a solution or a suspension. Analogously, “dispersed” means dissolved or suspended.

Operating the continuous stirred tank reactor in a “steady-state mode” with respect to the overall molar ratio of metal periodate to iodine compounds with an oxidation state from -I to +V in said anolyte means that a predefined molar ratio is selected and that during operation, the actual molar ratio of metal periodate to iodine compounds is kept at the predefined molar ratio (range) or close vicinity of the preselected molar ratio (range), wherein “close vicinity” means that the actual molar ratio (range) deviates from the preselected molar ratio (range) by at most ± 20 %, preferably by at most ± 10 %. As metal periodate is continuously formed during anodic oxidation of the iodine compound in the electrolysis cell, leading to a decrease of iodine compounds of a lower oxidation state (i.e. from -I to +V) and an increase of metal periodate in the anolyte, steady-state mode operation can only be maintained by adding iodine compound of a lower oxidation state (i.e. with an oxidation state from -I to +V) and/or removing metal periodate. In order to maintain the actual ratio within the above-defined range, up to 20 mol%, preferably up to 10 mol%, more or less of iodine compounds of a lower oxidation state can be added to the anolyte, when metal periodate is removed. The predefined molar ratio is expediently defined for a certain point in the electrolysis, e.g. for the anolyte before entering the anodic compartment.

The remarks made below concerning preferred embodiments of the invention are valid on their own as well as preferably in any combination with each other. As far as applicable, the remarks made in context with preferred embodiments of the method of the invention apply to the device of the invention, too, unless explicitly specified otherwise; and vice versa.

Embodiments (E.x) of the invention

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

E.1 . A method for preparing a metal para-periodate by anodic oxidation of an iodine compound with an oxidation state from -I to +V in an electrolysis cell, said electrolysis cell comprising an anodic compartment having one or more anodes, a cathodic compartment having one or more cathodes, and a separator arranged between said anodic and cathodic compartments, said method comprising recirculating a catholyte through a catholyte circuit into said cathodic compartment, recirculating an anolyte through an anolyte circuit into said anodic compartment, said anolyte comprising a metal periodate and an iodine compound with an oxidation state from -I to +V selected from an iodine salt (with an oxidation state from -I to +V, of course), elementary iodine, a mixture of different iodine salts (with an oxidation state from -I to +V, of course) or a mixture of iodine with one or more iodine salts (with an oxidation state from -I to +V, of course), carrying out said anodic oxidation of said iodine compound at a pH of at least 10, wherein said anolyte circuit comprises at least one continuous stirred tank reactor operated in a steady-state mode with respect to the overall molar ratio of metal periodate to iodine compounds with an oxidation state from -I to +V in said anolyte, wherein said continuous stirred tank reactor is operated in a “steadystate” mode by continuously or periodically feeding said iodine compounds with an oxidation state from -I to +V into said continuous stirred tank reactor and continuously or periodically removing metal periodate from said continuous stirred tank reactor.

E.2. The method according to embodiment E.1 , wherein said metal periodate comprised in said recirculated anolyte is predominantly a metal para-periodate.

E.3. The method according to any one of the preceding embodiments, wherein for operating the continuous stirred tank reactor in a steady-state mode with respect to the overall molar ratio of metal periodate to iodine compounds with an oxidation state from -I to +V in said anolyte a molar ratio or molar ratio range is defined and during operation, the actual molar ratio or molar ratio range of metal periodate to iodine compounds is kept at the defined molar ratio or molar ratio range or close vicinity of the defined molar ratio or molar ratio range, wherein “close vicinity” means that the actual molar ratio or molar ratio range may deviate from the defined molar ratio or molar ratio range by at most ± 20 %, where the molar ratio is calculated on the basis of iodine atoms present in the metal periodate and the iodine compound.

E.4. The method according to embodiment E.3, wherein the defined molar ratio or molar ratio range relates to the molar ratio or molar ratio range in the anolyte before entering the anodic compartment.

E.5. The method according to any one of the preceding embodiments, wherein in said recirculated anolyte the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :5 (meaning that some iodine compound with an oxidation state from -I to +V has to be present; but the overall amount of the iodine compound with an oxidation state from - I to +V must not exceed the amount of metal periodate by a factor of more than 5); where in case that the iodine compound has an oxidation state from -I to +111, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +111 is of from 1 :0 to 8:1 (meaning the metal periodate exceeds the overall amount of the iodine compound with an oxidation state from -I to +III by a factor of at least 8 - if the latter is at all present); and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :5 (meaning the overall amount of the iodine compound with an oxidation state of +V, if at all present, must not exceed the amount of metal periodate by a factor of more than 5).

E.6. The method according to embodiment E.5, wherein the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :4; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 10:1 , and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :4.

E.7. The method according to embodiment E.6, wherein the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 200:1 to 1 :4; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 10:1 , and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 200:1 to 1 :4.

E.8. The method according to any one of embodiments E.1 to E.4, wherein in said recirculated anolyte, when fed to the anode compartment, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :5 (meaning that some iodine compound with an oxidation state from -I to +V has to be present; but the overall amount of the iodine compound with an oxidation state from -I to +V must not exceed the amount of metal periodate by a factor of more than 5); where in case that the iodine compound has an oxidation state of from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of from -I to +III is of from 1 :0 to 8:1 (meaning the metal periodate exceeds the overall amount of the iodine compound with an oxidation state from -I to +III by a factor of at least 8); and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :5 (meaning the overall amount of the iodine compound with an oxidation state of +V, if at all present, must not exceed the amount of metal periodate by a factor of more than 5).

E.9. The method according to embodiment E.8, wherein the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :4; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 10:1 , and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :4.

E.10. The method according to embodiment E.9, wherein the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 200:1 to 1 :4; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 10:1 , and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 200:1 to 1 :4.

E.11 . The method according to any one of embodiments E.1 to E.4 and E.8 to E.10, wherein for operating the continuous stirred tank reactor in a steady-state mode with respect to the overall molar ratio of metal periodate to iodine compounds with an oxidation state from -I to +V, for the anolyte before entering the anodic compartment a molar ratio or molar ratio range is defined, and during operation, the actual molar ratio or molar ratio range of metal periodate to iodine compounds in the anolyte before entering the anodic compartment is kept at the defined molar ratio or molar ratio range ± 20 %; wherein said defined molar ratio or molar ratio range is selected thusly that in said anolyte, before entering the anodic compartment (i.e. when fed to the anodic compartment), the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :5; where in case that the iodine compound has an oxidation state of from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of from -I to +III is of from 1 :0 to 8:1 ; and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :5; where the molar ratio is calculated on the basis of iodine atoms present in the metal periodate and the iodine compound.

E.12. The method according to embodiment E.11 , wherein the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :4; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 10:1 , and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :4.

E.13. The method according to embodiment E.12, wherein the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 200:1 to 1 :4; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 10:1 , and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 200:1 to 1 :4.

E.14. The method according to any one of the preceding embodiments, for preparing an alkali metal para-periodate, where the iodine compound with an oxidation state from -I to +V is an alkali metal iodide, alkali metal iodate, iodine or a mixture thereof.

E.15. The method according to embodiment E.14, for preparing sodium para-periodate or potassium para-periodate, where the iodine compound with an oxidation state from -I to +V is sodium iodide, potassium iodide, sodium iodate, potassium iodate, iodine or a mixture thereof.

E.16. The method according to any of the preceding embodiments, where the anodic oxidation is carried out at a pH of at least 11 .

E.17. The method according to embodiment E.16, where the anodic oxidation is carried out at a pH of at least 12.

E.18. The method according to embodiment E.17, where the anodic oxidation is carried out at a pH of at least 13.

E.19. The method according to any of the preceding embodiments, 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.20. The method according to embodiment E.19, where the anodic oxidation is carried out in the presence of a metal hydroxide, where in case that said iodine compound with an oxidation state from -I to +V is an alkali metal iodine salt, the metal of the base corresponds to the metal in the metal iodine salt. E.21 . The method according to any of the preceding embodiments, where the catholyte comprises an aqueous medium with a pH of at least 10.

E.22. The method according to embodiment E.21 , where the catholyte comprises an aqueous medium with a pH of at least 11 .

E.23. The method according to embodiment E.22, where the catholyte comprises an aqueous medium with a pH of at least 12.

E.24. The method according to embodiment E.23, where the catholyte comprises an aqueous medium with a pH of at least 13.

E.25. The method according to any of the preceding embodiments, where the anodic oxidation is carried out at a current density in the range of from 10 to 1000 mA/cm².

E.26. The method according to embodiment E.25, where the anodic oxidation is carried out at a current density in the range of from 100 to 750 mA/cm².

E.27. The method according to embodiment E.26, where the anodic oxidation is carried out at a current density in the range of from 250 to 500 mA/cm².

E.28. The method according to embodiment E.27, where the anodic oxidation is carried out at a current density of 400 to 430 mA/cm².

E.29. The method according to any of the preceding embodiments, where said iodine compound with an oxidation state from -I to +V is introduced continuously or periodically into the continuous stirred tank reactor at essentially the same rate as metal periodate is removed continuously or periodically from the anode compartment, “essentially the same rate” meaning that the molar ratio of metal periodate to iodine compounds is maintained in the above-mentioned predefined range.

E.30. The method according to any of the preceding embodiments, where the metal periodate formed in the anode compartment is isolated from the anolyte removed from the anode compartment and the remainder of the anolyte is transferred to the continuous stirred tank reactor.

E.31 . The method according to embodiment E.30, where in case that the isolated periodate is not or not completely composed of metal para-periodate, the periodate different from para-periodate is converted into the para-periodate (e.g. by adding a base which converts periodates different from para-periodate into the latter).

E.32. The method according to any of the preceding embodiments, where said separator arranged between said cathodic and anodic compartments is selected from semipermeable membranes, diaphragms and frits.

E.33. The method according to embodiment E.32, where said separator arranged between said cathodic and anodic compartments is an ion-exchange membrane.

E.34. The method according to any of the preceding embodiments, where the one or more anodes comprise a carbon-based active layer. E.35. The method according to embodiment E.34, where the one or more anodes comprise a diamond layer doped with one or more I U PAC group 13, 15 or 16 elements of the periodic table.

E.36. The method according to embodiment E.35, where the one or more anodes comprise a boron-doped diamond layer.

E.37. The method according to any of the preceding embodiments, where said iodine compound with an oxidation state from -I to +V is fed into said continuous stirred tank reactor in form of an aqueous dispersion or in solid form.

E.38. A device for preparing a metal periodate, in particular a metal para-periodate, by continuous anodic oxidation of an iodine compound with an oxidation state from -I to +V, in particular a device configured for carrying out the method according to any embodiment of E.1 to E.37, comprising an electrolysis cell comprising an anodic compartment having one or more anodes, a cathodic compartment having one or more cathodes, and a separator arranged between said anodic and cathodic compartments, a catholyte circuit for recirculating catholyte through said cathodic compartment, said catholyte circuit comprising a catholyte reservoir, an anolyte circuit for circulating anolyte through said anodic compartment, said anolyte circuit comprising an at least one continuous stirred tank reactor, said at least one continuous stirred tank reactor being provided with feeding means for feeding an iodine compound with an oxidation state from -I to +V selected from an iodine salt, elementary iodine, a mixture of different iodine salts or a mixture of iodine with one or more iodine salts into said at least one continuous stirred tank reactor and withdrawing means for withdrawing metal periodate reaction product from said at least one continuous stirred tank reactor.

E.39. The device according to embodiment E.38, wherein the feeding means for feeding an iodine compound with an oxidation state from -I to +V comprise a starting material tank for housing a solution of the iodine compound with an oxidation state from -I to +V connected to the continuous stirred tank reactor.

E.40. The device according to any of embodiments E.38 and E.39, wherein the withdrawing means comprise a collection vessel connected to the continuous stirred tank reactor via a discharge line.

E.41 . The device according to embodiment E.40, wherein the feeding means for feeding an iodine compound comprise a feeding apparatus for a solid iodine compound connected to the continuous stirred tank reactor.

E.42. The device according to embodiment E.41 , wherein the withdrawing means comprise a collection vessel connected to the continuous stirred tank reactor via a discharge line and a recirculation line. E.43. The device according to any of embodiments E.38 to E.42, wherein the catholyte circuit comprises a catholyte tank, the catholyte tank being connected to the anolyte circuit via a transfer line for transferring a portion of the catholyte into the anolyte circuit.

E.44. The device according to any of embodiments E.38 to E.43, wherein the electrolysis cell comprises a cooling water system for controlling the temperature of the anodic compartment and/or the cathodic compartment of the electrolysis cell.

E.45. The device according to any of embodiments E.38 to E.44, wherein the device comprises a measurement system for determining the ratio of metal periodate to iodine compounds with an oxidation state from -I to +V in the anolyte.

E.46. The device according to embodiment E.45, wherein the measurement system comprises a potentiometer, an ammeter, a pH probe or combinations thereof.

The material to be subjected to the anodic oxidation is an iodine compound with an oxidation state from -I to +V. Iodine compounds with an oxidation state from -I to +V are actually iodine compounds with an oxidation state of -I, 0 (elementary iodine), +1, +111 or +V. The iodine compound with an oxidation state from -I to +V is selected from an iodine salt, elementary iodine, a mixture of different iodine salts or a mixture of iodine with one or more iodine salts.

As explained above, the iodine salts are inorganic iodine salts. Examples for suitable inorganic iodine salts with an oxidation state (of the iodine atom) from -I to +V are iodides [(Mⁿ⁺)i/_n(l ); iodine atom in the oxidation state -I], hypoiodites [(Mⁿ⁺)i/_n(IO ); iodine atom in the oxidation state +l], iodites [(Mⁿ⁺)i/_n(IO2 )^_; iodine atom in the oxidation state +III], and iodates [(Mⁿ⁺)i/_n(IO3 )^_; iodine atom in the oxidation state +V], where M is a cation, generally a metal cation or an ammonium cation, and n is the charge number. Among these, preference is given to the metal salts, i.e. to metal iodides, hypoiodites, iodites and iodates. Ideally, the metal counter cation corresponds to that desired for the targeted metal para-periodate, so that a cation exchange can be avoided.

Suitable metal counter cations for the iodine salts and for the targeted metal para-peri- odate are alkali metal cations, earth alkaline metal cations and transition metal cations.

Suitable alkali metal counter cations are for example the lithium, sodium, potassium or cesium cations.

Suitable earth alkaline metal counter cations are for example the magnesium or calcium cations. Suitable transition metal iodine salts 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 iodine salt, since the reaction (anodic oxidation) is carried out under 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 in the basic reaction medium 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 counter cations are the Sc(lll), Y(lll), La(lll), Co(ll), Ni(ll), Cu(l) and Zn(ll) cations. Among these, in view of their economic efficiency and availability, preference is given to Cu(l) and Zn(ll) cations.

Among the above-listed starting iodine compounds with an oxidation state of the iodine atom between -I and +V, preference is given to iodides, iodates, elementary iodine and mixtures thereof. More preference is given to alkali metal iodides, alkali metal iodates, earth alkaline metal iodides, earth alkaline metal iodates, Sc(lll) iodide, Sc(lll) iodate, Y(lll) iodide, Y(lll) iodate, La(lll) iodide, La(lll) iodate, Co(ll) iodide, Co(ll) iodate, Ni(ll) iodide, Ni(ll) iodate, Cu(l) iodide, Cu(l) iodate, Zn(ll) iodide, Zn(ll) iodate, elementary iodine and mixtures thereof. Even more preference of given to alkali metal iodides, in particular Lil, Nal, KI and Csl; alkali metal iodates, in particular Li(IC>3), Na(IC>3), K(IOs) and Cs(IC>3); earth alkaline metal iodides, in particular Mgl2 and Cal2; earth alkaline metal iodates, in particular Mg(IOs)2 and Ca(IC>3)2; Cui, Cu(IC>3), Znl2, Zn(IC>3)2, elementary iodine and mixtures thereof. Particular preference is given to alkali metal iodides, in particular Lil, Nal, KI and Csl; alkali metal iodates, in particular Li(IO3), Na(IO3), K(IOs) and Cs(IO3); earth alkaline metal iodides, in particular Mgl2 and Cal2; earth alkaline metal iodates, in particular Mg(IOs)2 and Ca(IO3)2; elementary iodine and mixtures thereof. More particular preference is given to alkali metal iodides, in particular Lil, Nal, KI and Csl; alkali metal iodates, in particular Li(IO3), Na(IO3), K(IOs) and Cs(IO3); elementary iodine and mixtures thereof. Specifically, Nal, KI, Na(IO3), K(IO3), iodine or a mixture thereof is used.

Accordingly, the method of the invention serves preferably for preparing an alkali metal para-periodate, an earth alkaline metal para-periodate, or the para-periodate of Sc(lll), Y(lll), La(lll), Co(ll), Ni(ll), Cu(l) or Zn(ll); or mixtures thereof. More preferably, the method of the invention serves for preparing an alkali metal para-periodate, in particular lithium, sodium or potassium para-periodate; an earth alkaline metal para-periodate, in particular magnesium or calcium para-periodate; Cu(l) para-periodate, Zn(ll) paraperiodate; or mixtures thereof. Even more preferably, the method of the invention serves for preparing an alkali metal para-periodate, in particular lithium, sodium or potassium para-periodate; or an earth alkaline metal para-periodate, in particular magnesium or calcium para-periodate; or mixtures thereof. Specifically, the method of the invention serves for preparing an alkali metal para-periodate, in particular sodium or potassium para-periodate; specifically sodium para-periodate.

In the method according to the invention, an anolyte is recirculated through an anolyte circuit into the anodic compartment. According to the invention, the anolyte comprises a metal periodate and an iodine compound with an oxidation state from -I to +V selected from an iodine salt, elementary iodine, a mixture of different iodine salts or a mixture of iodine with one or more iodine salts.

In the method according to the invention, the continuous stirred tank reactor is operated in a steady-state mode with respect to the overall molar ratio of metal periodate to iodine compound with an oxidation state from -I to +V in said anolyte by continuously or periodically feeding said iodine compound with an oxidation state from -I to +V into said continuous stirred tank reactor and continuously or periodically removing metal periodate from said continuous stirred tank reactor. Operating in a steady-state mode with respect to said overall molar ratio means that a suitable molar ratio or, more generally, a suitable molar ratio range is designated and during operation, the actual molar ratio or molar ratio range of metal periodate to iodine compounds is kept at said designated molar ratio or within said molar ratio range or in close vicinity to the designated molar ratio or molar ratio range, wherein “close vicinity” means that the actual molar ratio or (the limits of the) molar ratio range may deviate from the designated molar ratio or molar ratio range by at most ± 20 %. In this context, the molar ratio (range) is calculated on the basis of iodine atoms present in the metal periodate and the iodine compound. Values for suitable and preferred molar ratios or molar ratio ranges are defined below. It is expedient to define said molar ratios or molar ratio ranges for a certain point in electrolysis. One suitable point in electrolysis is the composition of the anolyte before it is fed to the anodic compartment. Accordingly, in a preferred embodiment, said molar ratio or molar ratio range relates to the molar ratio or molar ratio range in the anolyte before entering the anodic compartment. Thus, preferably, operating in a steady-state mode with respect to said overall molar ratio means that a suitable overall molar ratio (range) of metal periodate to iodine compound with an oxidation state from -I to +V is defined for the anolyte before entering the anodic compartment and during operation, the actual molar ratio (range) of metal periodate to iodine compounds in the anolyte before entering the anodic compartment is kept at said defined molar ratio or within said defined molar ratio range or in close vicinity to the defined molar ratio or molar ratio range, wherein “close vicinity” means that the actual molar ratio or molar ratio range may deviate from the designated molar ratio or molar ratio range by at most ± 20 %.

It has been found that clogging can be reduced or even suppressed if the amount of the metal periodate in the anolyte is present in substantial amounts. Clogging is mainly to be avoided in the anodic compartment, and therefore, it is important that the metal periodate be present in substantial amounts in the anolyte before this enters the anodic compartment (and of course also within the compartment, but the amount of periodate is adjusted before the anolyte is fed to the anodic compartment). Thus, in a preferred embodiment, in the recirculated anolyte, when fed to the anode compartment, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :5 (meaning that some iodine compound with an oxidation state from -I to +V has to be present; but the overall amount of the iodine compound with an oxidation state from -I to +V must not exceed the amount of metal periodate by a factor of more than 5); where in case that the iodine compound has an oxidation state of from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of from -I to +III is of from 1 :0 to 8:1 (meaning the metal periodate exceeds the overall amount of the iodine compound with an oxidation state from -I to +III (if the latter is at all present) by a factor of at least 8); and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :5 (meaning the overall amount of the iodine compound with an oxidation state of +V, if at all present, must not exceed the amount of metal periodate by a factor of more than 5).

More preferably, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :4; even more preferably from 200:1 to 1 :4; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 10:1 , and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :4, preferably from 200:1 to 1 :4.

Preferably, the above-given overall molar ratios are not limited to the anolyte before this enters the anodic compartment, but apply to the (re)circulated anolyte in general. Thus , preferably, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V in the (re)circulated anolyte is 1 : >0 to 1 :5; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 8:1 ; and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :5.

The molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V will differ depending on the position of the anolyte (just by way of example, the anolyte will of course contain more periodate after leaving the anodic chamber than before entering it), but the indicated molar ratio conditions will preferably apply independently of the position of the anolyte. As indicated above, the given molar ratios apply in particular to the anolyte before it enters the anodic compartment. The molar ratios at this position of the anolyte can for example be adjusted by the amount of the iodine compound with an oxidation state of -I to +V added to the stirred tank reactor and/or by the amount of periodate removed after the pass through the anodic chamber.

The method tolerates deviation from the above-given molar ratios to a certain degree, especially during the initial period when electrolysis is started, since a steady state is generally reached only after a certain start-up time.

A molar ratio of 1 : >0 means that a minimum amount of the iodine compound with an oxidation state from -I to +V is present.

Elementary iodine in context with the above molar ratios is calculated as containing 2 moles of I (i.e. 1 mol of l₂ is calculated as 2 mol for determining the above molar ratio). Actually, in the basic medium of the anolyte as used in the method of the invention with a pH of at least 10, elementary iodine disproportionates to iodide and hypoiodite, thus giving two iodine species: l₂ + 2 OH- l + IO + H₂O Also periodates containing x iodine atoms in the salt (e.g. dimesoperiodate (I2O9⁴’) contains two iodine atoms per dimesoperiodate anion) are calculated in this context as containing x moles of I (e.g. 1 mol of I2O9⁴’ is calculated as 2 mol for determining the above molar ratio of metal periodate to iodine compound of a lower oxidation state).

The overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V can be determined and adjusted, for example, by analysing continually or periodically the composition of the anolyte and feeding the iodine compound with an oxidation state from -I to +V in such an amount that the desired periodate/io- dine (-I to +V) species ratio is obtained. The anolyte can be quantitatively analysed, for example, by LC-PDA or titration. If LC-PDA is used, the impact factor of the analysed iodine species has to be determined to take account of the different sensitiveness of this method for different iodine species. Actually, if the anolyte in or discharged from the anodic compartment is analysed, it is generally sufficient to determine the ratio of periodate to iodate. As will be explained below, the oxidation of iodine species with an oxidation state of -I to +III to the periodate proceeds via the iodate, either by stepwise anodic oxidation or by comproportionation. This intermediate reaction to the iodate is fast compared to the oxidation of iodate to periodate, so that often the iodine species with an oxidation state of -I to +III are contained in amounts below the detection threshold. For analysing the anolyte via LC-PDA, the basic anolyte is expediently neutralized or acidified to convert any para-periodate (which has poor solubility in water) present in the anolyte into the water-soluble meta-periodate and thus assure a proper quantitative detection of all periodate species formed.

Alternatively, the composition can be calculated. When electrolysis is started, the composition of the anolyte is known. The continually or periodically removed periodate is isolated and weighed, from which the amount of consumed iodine compound with an oxidation state from -I to +V can be calculated. The amount necessary for the desired ratio is then replaced. A combination of these methods is of course also possible.

Analysis/calculation of the composition is suitably carried out at suitable time intervals, e.g. of some minutes or ca. one hour, at least at the beginning of the electrolysis until a stable state is obtained.

The metal periodate comprised in the recirculated anolyte can be any periodate in which the iodine atom has an oxidation state of +VII.

As explained above, 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 (lOe^5-; the metal orthoperiodate thus having the formula MslOe), meta-periodates (IO₄“; the metal meta-perio- date thus having the formula MIO4), dimesoperiodates (I2O9⁴’; the metal dimesoperiodate thus having the formula M4I2O9), mesoperiodates (IO5³’; 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 M 104*2 MOH. M in the above formulae is a metal equivalent [(Mⁿ⁺)i/_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⁺)I/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 M 104*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 iodine salt used as starting material differs from the counter cation present in the base present during anodic oxidation or used during workup of the reaction product.

Preferably, the metal periodate comprised in the recirculated anolyte is an ortho-perio- date, a meta-periodate, a para-periodate or a mixture thereof. More preferably, the metal periodate comprised in the recirculated anolyte comprises a metal para-perio- date. Even more preferably, the metal periodate is the predominating metal periodate species in said anolyte, i.e. it is comprised in a higher (molar) amount than other metal periodate species. Preferably, at least 50 mol-%, more preferably at least 70 mol-% of the metal periodates comprised in the recirculated anolyte are metal para-periodates.

The anodic oxidation is carried out at a pH of at least 10, e.g. from 10 to 14 or from 10 to 13, preferably of at least 11 , e.g. from 11 to 14 or higher, in particular of at least 12, e.g. from 12 to 14 or higher, and specifically of at least 13, e.g. 13 to 14 or higher; i.e. the anolyte has a pH of at least 10, e.g. from 10 to 14 or higher, preferably of at least 11 , e.g. from 11 to 14 or higher, in particular of at least 12, e.g. from 12 to 14 or higher, and specifically of at least 13, e.g. 13 to 14 or higher.

Hence, the anodic oxidation is preferably carried out in the presence of a base. Given the general use of aqueous media for the anodic oxidation, 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 iodine salt used. If elementary iodine is used as starting material, no such restriction is necessary, of course. The counter cation of the base corresponds however preferably to the counter cation of the desired para periodate. 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 iodine salt is not (sufficiently) water soluble. By way of example, CuO, CU(OH)2 and CuCOs are essentially insoluble in water. Therefore, if Cui or another Cu iodine salt is used as starting iodine salt, 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 iodines salts are used as starting materials; especially Mgl2 or Cal2 or other Mg or Ca iodine salts, 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 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 iodine salt, if such is used as starting material (and not elementary iodine); except for the above-described case where the corresponding base is not (sufficiently) water-soluble. In view of the preferred use of alkali metal iodine salts, earth alkaline metal iodine salts and certain transition metal iodine salts 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 iodine salt is used as starting material, the metal of the base corresponds preferably to the metal in the alkali metal iodine salt used.

The method of the invention preferably comprises subjecting an aqueous dispersion comprising an iodine compound with an oxidation state from -I to +V and a base as anolyte to anodic oxidation. Regarding suitable and preferred iodine compounds and suitable and preferred bases, reference is made to what has been said above. To be more precise, the method of the invention preferably comprises subjecting an anolyte, which is an aqueous dispersion comprising an iodine compound with an oxidation state from -I to +V in admixture with a metal periodate and a base, to anodic oxidation. In this anolyte, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is from 1 : >0 to 1 :5; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 8:1 ; and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :5. More preferably, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :4; even more preferably from 200:1 to 1 :4; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 10:1 , and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :4, preferably from 200:1 to 1 :4. As said above, this molar ratio is at least present in the anolyte before it enters the anodic chamber.

The processes in the anodic compartment depend on the starting material and are partially interconnected and complex. In theory, in a simplified mode, following processes occur under basic conditions depending on the iodine species to be oxidized:

I- + 8 OH- - IO₄- + 4 H₂O + 8 e- l⁺ + 8 OH- - IO₄- + 4 H₂O + 6 e- l₂ + 16 OH- - 2 IO₄- + 8 H₂O + 14 e- IO- + 6 OH- - IO₄- + 3 H₂O + 6 e- IO₃- + 2 OH- - IO₄- + H₂O + 2 e-

The first four starting iodine species are generally converted stepwise to the periodate via the subsequently higher oxidized species.

Moreover, as explained above, elementary iodine disproportionates under basic conditions to iodide and hypoiodite, thus giving two iodine species:

I2 + 2 OH- - l + IO + H₂O so that instead of the above 3^rd reaction scheme at least in part the first and the fourth occur.

Furthermore, the iodine species may also comproportionate with the also present periodate. By way of example, the reaction of iodide and elementary iodine is shown:

meaning that at least a part of the iodine or iodide starting material is not oxidized as such, but undergoes comproportionate with the also present periodate, so that iodate is the actually oxidized species. The ortho-periodate ICu- may then react further with hydroxyl ions to para-periodate:

IO₄- + 2 OH H₂IO₆ ³-

The overall concentration of metal periodate(s) and iodine compound(s) with an oxidation state from -I to +V in the anolyte is preferably from 0.01 to 12 mol/l, more preferably from 0.05 to 5 mol/l, even more preferably from 0.1 to 2 mol/l, in particular from 0.1 to 1 mol/l, specifically from 0.1 to 0.5 mol/l, and very specifically from 0.1 to 0.3 mol/l; where the concentration refers to the amount of iodine atoms, e.g. in case of a metal iodide starting material to the iodide anion, in case of a metal iodate starting material to the iodate anion, in case of elementary iodine as starting material to two I atoms (because of the disproportionation to iodide and hypoiodite in basic medium).

The base is used in a concentration to provide the desired pH.

The catholyte is generally an aqueous solution comprising an electrolyte. Preferably, the electrolyte is a base. Preferably, the base corresponds to that used in the anolyte. In case that the electrolyte is a base, the pH is preferably at least 8, more preferably at least 10, even more preferably at least 11 and in particular at least 12. 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 anolyte; i.e. they are thus 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. Preferably, the base used in the catholyte is the same as that used in the anolyte.

The reaction at the cathode(s) depends of course on the catholyte present in the cathode compartment(s). As said above, the catholyte is generally an aqueous medium. The reaction taking place at the cathode is in this case the reduction of water to hydrogen (under formation of hydroxyl anions):

2 H₂O + 2 e H2 + 2 OH-

The anolyte circuit comprises at least one continuous stirred tank reactor operated in a steady-state mode with respect to the overall molar ratio of metal periodate to iodine compounds with an oxidation state from -I to +V in said anolyte. As explained above, “steady state” in terms of the present invention includes a deviation of the absolute definition of up to 20 %. The anolyte is circulated between anodic compartment and stirred tank reactor.

The stirred tank reactor is fed with the materials for obtaining the desired composition and concentration of the anolyte to be fed to the anode compartment. To this purpose, the iodine compound(s) with an oxidation state from -I to +V which has/have been converted to periodate in the anodic oxidation process have to be supplemented; water and/or base, if depleted in the course of the reaction or product isolation process (see below) has/have to be supplemented, and if desired further additives or promotors (the use thereof is however not preferred) are added, too. Furthermore, anolyte from which a part of the formed periodate has been removed (see below details) is of course also returned to the stirred tank reactor.

At the start of the electrolysis, the stirred tank reactor is also fed with a metal periodate, preferably with a metal para-periodate. The amount of the metal (para-)periodate is such that the desired ratio of metal (para-)periodate to iodine compounds with an oxidation state from -I to +V in the desired concentrations is obtained. Once electrolysis has started, it is however not necessary anymore to add metal (para-)periodate, unless the desired ratio of metal (para-)periodate to iodine compounds with an oxidation state from -I to +V has to be adjusted because, for example, too much (para-)periodate has been isolated from the anolyte returned from the anodic compartment.

After electrolysis has started, metal periodate is formed at the anode from the iodine compounds with an oxidation state from -I to +V and is transported to the stirred tank reactor via the circulated anolyte, from where it is partially removed. Under the given conditions, especially due to the basic pH of at least 10, the periodate is at least partially composed of the desired para-periodate. Generally, the para-periodate is even the predominant periodate species in the recirculated anolyte; especially at higher pH values. Given the rather low water-solubility of metal para-periodates, a substantial amount thereof is generally present in solid form suspended in the anolyte and can be removed from the anolyte simply by precipitation; 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.

To this purpose, preferably, a part of the anolyte recirculated to the stirred tank reactor is removed from the stirred tank reactor and at least a part of the metal para-periodate is removed from this anolyte. If at least a part of the para-periodate is already present in solid form in this removed anolyte, isolation thereof can be carried out by usual means, such as filtration or decantation of the supernatant. If the anolyte needs first to be concentrated to generate sufficient solid matter for isolation, the anolyte is generally transferred into a suitable apparatus where a part of the solvent is removed, e.g. by evaporation, if desired under reduced pressure, or the anolyte is subjected to partial freeze-drying, partial reverse osmosis etc. The then obtained precipitate is isolated as described above.

If the anolyte has not been sufficiently basic, so that the para-periodate is not the predominating periodate species, the para-periodate can also be obtained after further base has been added. This converts meta-periodates and other periodate species into the desired para-periodates, which forces precipitation. Subsequent removal and workup can be carried out as described above.

Alternatively, for isolating the para-periodate, water can be removed more or less completely from the anolyte, 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.

If desired, the isolated para-periodate can be subjected to further purification steps in order to remove non-reacted iodine starting compound, 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. However, depending on the intended use of the para-periodate, it is not compulsory to submit it to purification steps. In many cases, especially if the para-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 residual 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 anolyte depleted of the (para-)periodate is then returned to the stirred tank reactor. Before, during or after returning said depleted anolyte to the stirred tank reactor, this is supplemented with an iodine compound with an oxidation state from -I to +V. The latter is added in an amount which is essentiality equimolar to the amount of the removed para-periodate. Preferably, the iodine compound with an oxidation state from -I to +V is fed to the stirred tank reactor.

The iodine compound with an oxidation state from -I to +V can be added in dispersed state (generally aqueous dispersion) or else in solid form. If the iodine compound is added in dispersed form (generally in water), the loss of water possibly occurring during removal of the (para-)periodate is simultaneously compensated at least in part. If the iodine compound is added in solid form, the possible loss of water is generally compensated by additionally adding water, preferably also to the stirred tank reactor. This can be carried out simultaneously with or time-independently of the addition of the solid iodine compound.

Removal of the (para-)periodate might also lead to a depletion of the base. This is also compensated by adding base, and can also be carried out simultaneously with or time- independently of the addition of the solid iodine compound. The added base can be a “fresh” material. In a particular embodiment, however, the base is added in form of a part of the catholyte from the cathode compartment. Since hydroxyl ions are formed at the cathode during electrolysis (by reduction of water), the catholyte is not necessarily supplemented with fresh base to compensate the transferred amount. Only the water transferred to the stirred tank reactor needs to be supplemented to the cathode compartment.

Removal of the anolyte from the anode compartment and from the stirred tank reactor as well as addition of the iodine compound with an oxidation state from -I to +V to the anolyte depleted of (para-)periodate can be carried out continuously or periodically.

Removal of the anolyte from the anode compartment and from the stirred tank reactor as well as addition of the iodine compound with an oxidation state from -I to +V to the anolyte depleted of para-periodate is preferably carried out at essentially the same rate, so that electrolysis occurs in an essentially steady state mode. This can be accomplished by simultaneously adding iodine compounds of a lower oxidation state and removing metal periodate and also intermittently, i.e. during certain time intervals iodine compounds are added and during other time intervals metal periodate is removed, as long as the molar ratio is kept in the predefined range.

Addition of the iodine compound with an oxidation state from -I to +V occurs preferably at such a rate and concentration that periodates are the predominating iodine species in the anolyte present in the stirred tank reactor and of course also in the anode compartment. Preferably, the addition occurs preferably at such a rate and with such a concentration that the overall molar ratio of all metal periodates and all iodine compounds with an oxidation state from -I to +V in the stirred tank reactor is from 1 : >0 to 1 :5; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 8:1 ; and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :5. More preferably, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :4; even more preferably from 200:1 to 1 :4; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 10:1 , and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :4, preferably from 200:1 to 1 :4. As said above, this molar ratio is at least present in the anolyte before it enters the anodic chamber.

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.

The anode material can principally be any one known in the art for this type of oxidation. If however the desired product is to be used for a specific purposes not tolerating specific impurities, such as heavy metals, in particular lead, as is the case for pharmaceutical processes, the anode material does preferably not contain such materials. Thus, in a preferred embodiment, the anode comprises a carbon-based active layer. 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 perse 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 (BDD).

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 (CFG) 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 L.J.J. Janssen et aL, Electrochimica Acta 2003, 48, 3959- 3964, 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.

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.

The separation of the anode compartment(s) from the cathode compartment(s) can be accomplished by usual dividing means (separators), such as semipermeable membranes or diaphragms 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. Diaphragms 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 diaphragm 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 e.g. ion exchange membranes, 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-copoly- mers, such as the Nation® 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 diaphragms. The electrolysis cell in which the process of the invention 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.

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 semi-continuous process or a continuous process.

In a continuous process design the electrolyte is passed continuously through the cell, i.e. the electrolyte containing the iodine compound to be oxidized 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 the batch and the continuous form: The process is principally continuous, but the anolyte containing the iodine compound to be oxidized is added at a certain point of time and the resulting reaction mixture is removed at a certain point of time.

For a semi-continuous or continuous process design, the anode and cathode compartments are generally designed as flow cells. Various designs and geometries of flow cells are known in the art and can be applied to the present method.

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 compartments). Moreover, means for continuous supply and removal, e.g. in the form of recirculation loops equipped with pumps, are provided. The stirred tank reactor is part of the anolyte circuit and generally also contains a loop to an apparatus for removing a part of the formed product and returning the remaining anolyte and a connection to a supply system for supplementing depleted starting material, water and base and any other desired components of the anolyte. As already explained above, the starting material can be supplemented in dispersed form or as solid to the stirred tank reactor. In the latter case, suitable supply systems for solids are required, such as screw conveyors and the like.

The device according to the invention is a preferred form of the electrolysis apparatus suitable for carrying out the method of the invention. Accordingly, the device configured for preparing a metal para-periodate by the method of the invention, i.e. for preparing a metal para-periodate by continuous anodic oxidation of an iodine compound comprises an electrolysis cell comprising an anodic compartment having one or more anodes, a cathodic compartment having one or more cathodes, and a separator arranged between said anodic and cathodic compartments, a catholyte circuit for recirculating catholyte through said cathodic compartment, said catholyte circuit comprising a catholyte reservoir, an anolyte circuit for circulating anolyte through said anodic compartment, said anolyte circuit comprising at least one continuous stirred tank reactor, said at least one continuous stirred tank reactor being provided with feeding means for feeding an iodine compound with an oxidation state from -I to +V selected from an iodine salt, elementary iodine, a mixture of different iodine salts or a mixture of iodine with one or more iodine salts into said at least one continuous stirred tank reactor and withdrawing means for withdrawing metal periodate reaction product from said at least one continuous stirred tank reactor.

For a given device according to the present invention, calibration tests can be conducted to determine operation conditions, under which the reactor is operated in a steady-state mode with respect to the molar ratio of metal periodate to iodine compounds with an oxidation state from -I to +V in the anolyte, for instance by using LC- PDA data or data obtained by other analytical or calculatory methods. Thus, by selecting suitable operating conditions, especially in terms of concentration (feeding rate) of the iodine compound, concentration (withdrawal rate) of the metal periodate, temperature, flow rate of the anolyte through the electrolysis cell, pH of the anolyte, operating current density of the electrolysis cell, the reactor can be maintained in a steady-state mode.

Preferably, the device of the present invention comprises a suitable measurement system for determining and/or controlling the operating conditions of the device. The measurement system typically comprises potentiometers and/or ammeters, pH meters, typically in-line pH meters, temperature sensors and flow rate sensors. Typically, there is a linear relationship between flow rate and current density, i.e. an increase in flow rate by a certain factor allows for a similar increase in current density while still maintaining steady-state conditions.

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.

When voltage is measured under galvanostatic control, reaction conditions can be regulated such that the voltage is essentially maintained at a desired value thus ensuring that the reactor is operated in a steady-state mode. Likewise, maintaining the current at the desired value under potentiostatic control can also ensure that the reactor is operated in a steady-state mode. The electrolysis system can be calibrated by measuring the current/voltage-relationship of various electrolytes with known compositions, especially known compositions of metal periodate and iodine compounds with an oxidation state from -I to +V. Based on the calibration data, the electrolysis can be carried out at a desired ratio of metal periodate to iodine compounds with an oxidation state from -I to +V by establishing and maintaining the appropriate current/voltage values during electrolysis.

Under galvanostatic control, an increase in cell voltage indicates that more metal periodate is produced. Therefore, in order to maintain a steady-state in the reactor, more iodine compound with an oxidation state from -I to +V should be added. Likewise, under potentiostatic control, a decrease in current density indicates an increase in metal periodate production, thus also requiring addition of iodine compound with an oxidation state from -I to +V to maintain steady-state.

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 voltage can be adjusted exactly to the redox potential of the iodine species to be oxidized.

The anodic oxidation is preferably carried out at a current density in the range of from 10 to 1000 mA/cm², more preferably from 100 to 750 mA/cm², in particular from 250 to 500 mA/cm² and specifically of 400 to 430 mA/cm². These values apply for a flow rate of ca. 7.5 L/h. If the flow rate is decreased or increased by a certain factor, the current density can be decreased or increased by the same factor.

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 45°C and specifically from 30 to 40°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 invention relates moreover to a device for preparing a metal para-periodate by continuous anodic oxidation of an iodine compound, to be more precise to a device configured for preparing a metal para-periodate by the method of the invention, comprising an electrolysis cell comprising an anodic compartment having one or more anodes, a cathodic compartment having one or more cathodes, and a separator arranged between said anodic and cathodic compartments, a catholyte circuit for recirculating catholyte through said cathodic compartment, said catholyte circuit comprising a catholyte reservoir, an anolyte circuit for circulating anolyte through said anodic compartment, said anolyte circuit comprising an at least one continuous stirred tank reactor, said at least one continuous stirred tank reactor being provided with feeding means for feeding an iodine compound with an oxidation state from -I to +V selected from an iodine salt, elementary iodine, a mixture of different iodine salts or a mixture of iodine with one or more iodine salts into said at least one continuous stirred tank reactor and withdrawing means for withdrawing metal periodate reaction product from said at least one continuous stirred tank reactor. Preferably, the feeding means comprise a starting material tank for housing a solution of the iodine compound with an oxidation state from -I to +V connected to the continuous stirred tank reactor.

The withdrawing means preferably comprise a collection vessel connected to the continuous stirred tank reactor via a discharge line.

In a particular embodiment, the feeding means for feeding an iodine compound with an oxidation state from -I to +V comprise a feeding apparatus for a solid starting material, here in particular for solid iodine compounds with an oxidation state from -I to +V connected to the continuous stirred tank reactor.

In another particular embodiment, the withdrawing means comprise a collection vessel connected to the continuous stirred tank reactor via a discharge line and a recirculation line.

Specifically, the device comprises a feeding apparatus for a solid starting material connected to the continuous stirred tank reactor and a collection vessel connected to the continuous stirred tank reactor via a discharge line and a recirculation line.

The catholyte circuit comprises preferably a catholyte tank, the catholyte tank being connected to the anolyte circuit via a transfer line for transferring a portion of the catholyte into the anolyte circuit.

Specifically, the device comprises a feeding apparatus for a solid starting material connected to the continuous stirred tank reactor, a collection vessel connected to the continuous stirred tank reactor via a discharge line and a recirculation line and a catholyte tank which is connected to the anolyte circuit via a transfer line for transferring a portion of the catholyte into the anolyte circuit.

The electrolysis cell preferably comprises a cooling water system for controlling the temperature of the anodic compartment and/or the cathodic compartment of the electrolysis cell.

Further preferred embodiments of the device of the invention will become apparent in context with the below figure description.

The method of the present invention combines the concepts of cyclic flow electrolysis and a continuous stirred tank reactor (e-CSTR), which is operated in a steady-state mode, which enables a fully continuous operation, if desired. The product can be separated in-line, and the remaining anolyte can be recycled to the process. The method allows the production of para-periodates in a quality suitable for demanding applications, such as pharmacy, cosmetics and nutrition. The method is versatile with respect to starting materials, tolerating arbitrary inorganic iodine compounds as starting compounds, including the start from readily available, economic and non-problematic, easy- to-handle iodides in a single step, but also from other iodine species, such as iodate (the species commonly afforded for recycling from oxidation processes in which periodates are used as oxidizing agent and thus also readily available) or iodine. The method is also versatile with respect to the physical form in which starting materials are added, allowing not only supply in dispersed state, but also in solid form. 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. Most importantly however, clogging can be significantly reduced. Furthermore, the method can be easily scaled-up to industrial design.

The method will now be further described with reference to schematic embodiments shown in the attached drawings and with reference to examples described below.

In the drawings:

Fig. 1 shows a first embodiment of a device for carrying out the present invention; and

Fig. 2 shows a second embodiment of a device for carrying out the present invention.

In Fig. 1 , a first embodiment of a device for preparing metal periodate in accordance with the method of the present invention is generally designated by reference sign 10. The device 10 comprises an electrolysis cell 11 having an anodic compartment 12, where an anode 13 is arranged and a cathodic compartment 14, where a cathode 15 is arranged. A separator 16, for instance a semipermeable membrane, a diaphragm or a frit, is arranged between the anodic compartment 12 and the cathodic compartment 14. Both compartments are provided with an anodic cooling water system 17 and a cathodic cooling water system 18, respectively, to allow for temperature regulation of the respective cells. The cathodic compartment 14 is part of a catholyte circuit 19, which comprises a catholyte tank 20 and a catholyte recirculation pump 21. The anolyte compartment is part of an anolyte circuit 22, which comprises a continuous stirred tank reactor 23, which acts as an anolyte tank and an anolyte recirculation pump 24. In the continuous stirred tank reactor 23, the predefined molar ratio of metal periodate to iodine compounds with an aggregation state from -I to +V is maintained. To this effect, the continuous stirred tank reactor 23 is fed with a starting material solution from starting material tank 25 via feed pump 26 and feed line 27. Anolyte can be withdrawn from the continuous stirred tank reactor 23 via discharge line 28 and discharge pump 29, which discharges a certain fraction of the anolyte into product collection vessel 30. In this embodiment, the rate at which iodine compounds are fed into the continuous stirred tank reactor 23 from starting material tank 25 corresponds to the rate at which metal periodate is generated via electrolysis in the anodic compartment 12, which in turn determines the rate at which metal periodate is removed from the continuous stirred tank reactor 23 via discharge line 28 into product collection vessel 30.

Fig. 2 shows a second embodiment of a device according to the present invention. In figure 2, elements which are identical or similar to elements described in connection with Fig. 1 are denoted by the same reference signs augmented by 100.

Accordingly, the device for preparing metal periodate is designated by reference sign 110 and comprises an electrolysis cell 111 having an anodic compartment 112 with an anode 113 and a cathodic compartment 114 with a cathode 115. Again, a separator 116 is arranged between the anodic compartment 112 and the cathodic compartment 114. Both compartments are provided with an anodic cooling water system 117 and a cathodic cooling water system 118, respectively. The cathodic compartment 114 is part of a catholyte circuit 119, which comprises a catholyte tank 120 and a catholyte recirculation pump 121 . The anolyte compartment 112 is part of an anolyte circuitl 22, which comprises a continuous stirred tank reactor 123 and an anolyte recirculation pump 124.

The device of Fig. 2 differs from the device of Fig. 1 in that the starting material, especially the iodine compounds, are fed as solids directly into the continuous stirred tank reactor 123. To this effect, a container 131 for the solid starting materials and a supply line 132 are provided. The container 131 can be provided with a motorized feeder, for instance a screw conveyor 133 driven by a motor 134. In this embodiment, the collection tank 130 is not a batch collection tank but a collection tank which allows removal of metal periodate from the anolyte fed into the collection vessel 130 via discharge line 128 and discharge pump 129. The depleted anolyte is recycled into the continuous stirred tank reactor 123 via recirculation line 135 and recirculation pump 136. As a further variation compared to the embodiment of Fig. 1 , the device 110 of Fig. 2 is provided with a transfer line 137 and a transfer pump 138 which allows the transfer of certain amount of catholyte into the continuous steel tank reactor 123 to allow for a pH adjustment of the anolyte during prolonged operation. As a certain amount of catholyte is transferred to the anolyte circuit, the catholyte circuit has to be resupplied with water (H2O) via water supply line 139.

Examples

Example 1 Anodic oxidation of sodium iodide to sodium para-periodate - supplementing of sodium iodide in solid form In a device 110 for preparing metal periodate as schematically depicted in Fig. 2, a flow-electrolysis cell 111 was equipped with 48 cm² electrodes 113, 115 in the anode and the cathode compartments 112, 114, respectively. The cell was divided (anode and the cathode compartment separated from each other through a cation exchange membrane 116 (Nation® N 324 from DuPont)) and tethered by 1 mm EPDM spacers. A boron-doped diamond electrode (DIACHEM® electrode from Condias GmbH; 10 pm BDD on silicium support (3 mm thickness); ca. 800 ppm of boron) was used as anode 113. Stainless steel was used as cathode 115. Each compartment was part of a respective electrolyte circuit 119, 122 which included respective reservoirs for the anolyte (V = 5 L) and catholyte (V = 2.5 L) and each circuit was powered by a membrane pump 121 , 124 (Ritmo R033 from Fink). The anolyte circuit included a continuous stirred tank reactor (CSTR) 123, which served as the anolyte reservoir and which was stirred at 350 rpm and was further connected to a collection vessel 130 via discharge line 128 for separation of a part of the formed periodate by sedimentation and decantation from the anolyte. Supernatant anolyte from the collection vessel 130 was returned to the CSTR 123 via recirculation line 135. The discharge line 128 and the recirculation line 135 were powered by peristaltic pumps 129, 136, respectively (ReglolCC from Ismatec). Moreover, the CSTR 123 was connected to a feed for solid starting material to supplement the consumed iodine compound (here sodium iodide), denoted by reference signs 131-134 in Fig. 2, as well as to the catholyte reservoir 120 to supplement consumed base. The catholyte reservoir 120 was moreover connected to a feed 139 to supplement water lost in the transfer of catholyte and consumed in electrolysis at the cathode. The cell was cooled by a thermostatically controlled anodic and cathodic cooling water system 117, 118 during the electrolysis using water.

For the electrolysis, para-periodate from previous electrolysis was suspended in caustic soda (4M NaOH) and added to the anolyte reservoir. pH of the anolyte was >12. The catholyte consisted of caustic soda of the same concentration. The anolyte was moreover fed continuously with solid Nal at an addition rate of 3.5 g/h Nal. The anolyte and catholyte were pumped through the electrolysis cell 111 with a flow rate of fr(an/cat) =7.5 L/h. The suspension was removed from the anolyte by decantation in a secondary cycle (fr(dec) = various). The composition of the anolyte was frequently controlled by LC-PDA analysis. The removed suspension was filtered after electrolysis, the filter residue was washed with acetone, dried under reduced pressure, and the mass of the product was determined to obtain the productivity.

After 29 h of electrolysis and after addition of a total amount of 103 g of Nal (0.68 mol), the para-periodate precipitated in the collector was filtered off, the filter residue was washed with water and acetone, and was dried under reduced pressure to obtain the product in 94% yield (497 g, 1.69 mol, 99.1 % purity by LC-PDA) considering the initial amount of para-periodate of 309 g (1 .05 mol). The productivity accounted to 6.5 g/h and the current efficiency to 24%. The IR spectra was in accordance with literature.

In the anolyte contained in the anolyte reservoir, the overall molar ratio of sodium periodate to sodium iodate was ca. 1 :2 to 1 :3, as determined by LC-PDA analysis of the anolyte returned to the CSTR. The iodide content in the anolyte returned to the CSTR was below the detection threshold of the LC-PDA. The amount of added iodide was such that the overall molar ratio of sodium periodate to sodium iodide (before entering the analytic compartment) was at least 15:1 .

No clogging occurred.

Example 2 Anodic oxidation of sodium iodide to sodium para-periodate - supplementing of sodium iodide in solid form

The proceeding of example 1 was repeated; the addition rate of Nal was however increased to 14.2 g/h. This increased the productivity to 16.8 g/h and the current efficiency to 61 %. However, the yield of sodium para-periodate after 29 h of electrolysis and after addition of a total amount of 411 g of Nal dropped to 61 %. For steady state reaction, this yield is however still very satisfactory. Purity was 98.7%.

No clogging occurred here, either.

Example 3 Anodic oxidation of sodium iodide to sodium para-periodate - supplementing of sodium iodide in solid form

The proceeding of example 1 was repeated; the flow rate was however increased to fr(an/cat)\o 700L/h (using FlujoOl pumps from Fink), and the addition rate of Nal was 8.5 g/h. This increased the productivity to 15.6 g/h and the current efficiency to 57%. The yield of sodium para-periodate after 29 h of electrolysis and after addition of a total amount of 247 g of Nal was 94%, purity 98.7%.

No clogging occurred here, either.

Example 4 Anodic oxidation of sodium iodide to sodium para-periodate - supplementing of sodium iodide in solid form The proceeding of example 1 was repeated, using however NaOH in a concentration of only 1 M. After 71 h of electrolysis and after addition of a total amount of 245 g of Nal at a feeding rate of 24 mmol/h (3.45 g/h) the yield of sodium para-periodate was 64% with a purity of 99.7%. Productivity was 4.3 g/h and the current efficiency was 16%.

No clogging occurred.

Example 5 Anodic oxidation of sodium iodate to sodium para-periodate - supplementing of sodium iodate in solid form

The proceeding of example 4 was repeated, using however sodium iodate as starting material. After 44 h of electrolysis and after addition of a total amount of 754 g of NalOs at a feeding rate of 86 mmol/h (17 g/h ) the yield of sodium para-periodate was 70% with a purity of 99.9%. Productivity was 17.9 g/h and the current efficiency was 16%. Similar results were obtained when increasing the addition rate of to 168 mmol/h (33 g/h).

In the anolyte contained in the anolyte reservoir, the overall molar ratio of sodium periodate to sodium iodate was ca. 1 :1 to 1 :2, as determined by LC-PDA.

No clogging occurred.

Example 6 Anodic oxidation of elementary iodine to sodium para-periodate - supplementing of iodine in solid form

The proceeding of example 4 was repeated, using however iodine as starting material. After 75 h of electrolysis and after addition of a total amount of 283 g of I2 the yield of sodium para-periodate was 44% with a purity of 99.6%. Productivity was 3.9 g/h and the current efficiency was 12%.

No clogging occurred.

In the anolyte contained in the anolyte reservoir, the overall molar ratio of sodium periodate to sodium iodate was ca. 1 :1 to 1 :2, as determined by LC-PDA. The amount of added iodine was such that the overall molar ratio of sodium periodate to iodine (before entering the analytic compartment) was at least 14:1.

Example 7 Anodic oxidation of sodium iodide to sodium para-periodate - supplementing of sodium iodide in dissolved form The set-up was simpler than in examples 1-6, containing no catholyte feed to the CSTR and not returning anolyte from the collector to the CSTR:

In a device 10 for preparing metal periodate as schematically depicted in Fig. 1 , a flowelectrolysis cell 11 was equipped with 48 cm² electrodes 13, 15 in the anode and the cathode compartments 12, 14, respectively. The cell was divided (anode and the cathode compartment separated from each other through a cation exchange membrane 16 (Nation® N324 from DuPont)) and tethered by 1 mm EPDM spacers. A boron-doped diamond electrode (DIACHEM® electrode from Condias GmbH; 10 pm BDD on silicium support (3 mm thickness); ca. 800 ppm of boron) was used as anode 13. Stainless steel was used as cathode 14. Each compartment was part of a respective electrolyte circuit 19, 22 which included respective reservoirs for the anolyte (V = 5 L) and catholyte (V = 2.5 L) and each circuit was powered by respective membrane pumps 21 , 24 (Ritmo R033 from Fink). The anolyte circuit included a continuous stirred tank reactor (CSTR) 23, which served as an anolyte reservoir and which was stirred at 300 rpm and was further connected to feedstock solution (starting material tank 25) via line 27 and to a collection vessel 30 via line 28. Both lines 27, 28 were powered by respective peristaltic pumps 26, 28 (ReglolCC from Ismatec). The cell was cooled by a thermostatically controlled anodic and cathodic cooling water system 17, 18 during the electrolysis using water.

For the electrolysis, the anolyte vessel was loaded with a para-periodate suspension from previous electrolysis suspended in caustic soda (1 M NaOH). pH of the anolyte was >10. The catholyte consisted of caustic soda of the same concentration. The anolyte and catholyte were pumped through the electrolysis cell with a flow rate of fr(an/cat) =7.5 L/h. The suspension was removed from the anolyte, while the feedstock solution was added at the same flow rate (fr(in/out) = various). The composition was frequently controlled by LC-PDA analysis. The removed suspension was filtered after electrolysis, the filter residue was washed with acetone, dried under reduced pressure, and the mass of the product was determined to obtain the productivity.

Electrolysis was carried out at a current of 20 A. Nal was added as an aqueous solution with an addition rate of 24 mmol/h. After 101 h of electrolysis and after addition of a total amount of 375 g of Nal the yield of sodium para-periodate was 52% with a purity of 99.6%. Productivity was 3.8 g/h and the current efficiency was 14%.

No clogging occurred. Example 8 Anodic oxidation of sodium iodate to sodium para-periodate - supplementing of sodium iodate in dissolved form

The proceeding of example 7 was repeated, using however sodium iodate as starting material. After 109 h of electrolysis and after addition of a total amount of 950 g of NalOs at an addition rate of 44 mmol/h the yield of sodium para-periodate was 68% with a purity of 99.9%. Productivity was 8.8 g/h and the current efficiency was 8%.

No clogging occurred. Nor did clogging occur when the addition rate of sodium iodate was increased to 179 mmol/h under otherwise analogous conditions.

List of reference signs:

10, 110 device of preparing metal periodate

11 , 111 electrolysis cell

12, 112 anodic compartment

13, 113 anode

14, 114 cathodic compartment

15, 115 cathode

16, 116 separator

1 , 117 anodic cooling water system

18, 118 cathodic cooling water system

19, 119 catholyte circuit

20, 120 catholyte tank

21 , 121 catholyte recirculation pump

22, 122 anolyte circuit

23, 123 continuous stirred tank reactor

24, 124 anolyte recirculation pump

25 starting material tank

26 feed pump

27 feed line

28, 128 discharge line

29, 129 discharge pump

30, 130 product collection vessel

131 starting materials container

132 supply line

133 screw conveyor

134 motor

135 recirculation line

136 recirculation pump

137 catholyte transfer line

138 transfer pump

139 water supply line

Claims

1 . A method for preparing a metal para-periodate by anodic oxidation of an iodine compound with an oxidation state from -I to +V in an electrolysis cell, said electrolysis cell comprising an anodic compartment having one or more anodes, a cathodic compartment having one or more cathodes, and a separator arranged between said anodic and cathodic compartments, said method comprising recirculating a catholyte through a catholyte circuit into said cathodic compartment, recirculating an anolyte through an anolyte circuit into said anodic compartment, said anolyte comprising a metal periodate and an iodine compound with an oxidation state from -I to +V selected from an iodine salt, elementary iodine, a mixture of different iodine salts or a mixture of iodine with one or more iodine salts, carrying out said anodic oxidation of said iodine compound at a pH of at least 10, wherein said anolyte circuit comprises at least one continuous stirred tank reactor operated in a steady-state mode with respect to the overall molar ratio of metal periodate to iodine compound with an oxidation state from -I to +V in said anolyte, wherein said continuous stirred tank reactor is operated in a steady-state mode by continuously or periodically feeding said iodine compound with an oxidation state from -I to +V into said continuous stirred tank reactor and continuously or periodically removing metal periodate from said continuous stirred tank reactor.

2. The method according to claim 1 , wherein said metal periodate comprised in said recirculated anolyte is predominantly a metal para-periodate.

3. The method according to any one of claims 1 or 2, wherein operating the continuous stirred tank reactor in a steady-state mode with respect to the overall molar ratio of metal periodate to iodine compounds with an oxidation state from -I to +V in said anolyte means that a predefined molar ratio or molar ratio range is selected and during operation, the actual molar ratio or molar ratio range of metal periodate to iodine compounds is kept at the predefined molar ratio or molar ratio range or in close vicinity of the preselected molar ratio or molar ratio range, wherein “close vicinity” means that the actual molar ratio or molar ratio range may deviate from the selected molar ratio or molar ratio range by at most ± 20 %, where the molar ratio is calculated on the basis of iodine atoms present in the metal periodate and the iodine compound.

4. The method according to claim 3, wherein the predefined molar ratio or molar ratio range relates to the molar ratio or molar ratio range in the anolyte before entering the anodic compartment.

5. The method according to any one of claims 1 to 3, wherein in said recirculated anolyte the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :5; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 8:1 ; and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :5; where preferably the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :4; more preferably from 200:1 to 1 :4; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 10:1 , and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :4, preferably from 200:1 to 1 :4.

6. The method according to any one of claims 1 to 4, wherein in said recirculated anolyte, when fed to the anodic compartment, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :5; where in case that the iodine compound has an oxidation state of from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of from -I to +III is of from 1 :0 to 8:1 ; and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :5; where preferably the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :4; more preferably from 200:1 to 1 :4; where in case that the iodine compound has an oxidation state from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +III is of from 1 :0 to 10:1 , and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :4, more preferably from 200:1 to 1 :4.

7. The method according to any one of claims 1 to 4 or 6, wherein for operating the continuous stirred tank reactor in a steady-state mode with respect to the overall molar ratio of metal periodate to iodine compounds with an oxidation state from -I to +V, for the anolyte before entering the anodic compartment a predefined molar ratio or molar ratio range is selected, and during operation, the actual molar ratio of metal periodate to iodine compounds in the anolyte before entering the anodic compartment is kept at the predefined molar ratio or molar ratio range ± 20 %; wherein said predefined molar ratio or molar ratio range is selected thusly that in said anolyte, before entering the anodic compartment (i.e. when fed to the anodic compartment), the overall molar ratio of the metal periodate and the iodine compound with an oxidation state from -I to +V is of from 1 : >0 to 1 :5; where in case that the iodine compound has an oxidation state of from -I to +III, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of from -I to +III is of from 1 :0 to 8:1 ; and where in case that the iodine compound has an oxidation state of +V, the overall molar ratio of the metal periodate and the iodine compound with an oxidation state of +V is of from 1 :0 to 1 :5; where the molar ratio is calculated on the basis of iodine atoms present in the metal periodate and the iodine compound.

8. The method according to any of the preceding claims, for preparing an alkali metal para-periodate, preferably sodium or potassium para-periodate, where the iodine compound with an oxidation state from -I to +V is an alkali metal iodide, alkali metal iodate, iodine or a mixture thereof, preferably sodium iodide, potassium iodide, sodium iodate, potassium iodate, iodine or a mixture thereof.

9. The method according to any of the preceding claims, where the anodic oxidation is carried out at a pH of at least 11 , in particular of at least 12.

10. The method according to any of the preceding claims, 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.

11 . The method according to claim 10, where the anodic oxidation is carried out in the presence of a metal hydroxide, where in case that said iodine compound with an oxidation state from -I to +V is an alkali metal iodine salt, the metal of the base corresponds to the metal in the metal iodine salt.

12. The method according to any of the preceding claims, where the catholyte comprises an aqueous medium with a pH of at least 10, preferably of at least 11 , in particular of at least 12.

13. The method according to any of the preceding claims, where the anodic oxidation is carried out at a current density in the range of from 10 to 1000 mA/cm², preferably from 100 to 750 mA/cm², in particular from 250 to 500 mA/cm² and specifically of 400 to 430 mA/cm².

14. The method according to any of the preceding claims, where said separator arranged between said cathodic and anodic compartments is selected from semi- permeable membranes, diaphragms and frits, and is in particular an ion-exchange membrane.

15. The method according to any of the preceding claims, where the one or more anodes comprise a carbon-based active layer.

16. The method according to claim 15, where the one or more anodes comprise a diamond layer doped with one or more I U PAC group 13, 15 or 16 elements of the periodic table.

17. The method according to claim 16, where the one or more anodes comprise a bo- ron-doped diamond layer.

18. The method according to any of the preceding claims, where said iodine compound with an oxidation state from -I to +V is fed into said continuous stirred tank reactor in form of an aqueous solution, aqueous suspension or in solid form.

19. A device configured for preparing a metal para-periodate by the method of any of claims 1 to 18, the device comprising said electrolysis cell comprising an anodic compartment having one or more anodes, a cathodic compartment having one or more cathodes, and a separator arranged between said anodic and cathodic compartments, said catholyte circuit for recirculating catholyte through said cathodic compartment, said catholyte circuit comprising a catholyte reservoir, said anolyte circuit for circulating anolyte through said anodic compartment, said anolyte circuit comprising at least one continuous stirred tank reactor, said at least one continuous stirred tank reactor being provided with feeding means for feeding said iodine compound with an oxidation state from -I to +V selected from an iodine salt, elementary iodine, a mixture of different iodine salts or a mixture of iodine with one or more iodine salts into said at least one continuous stirred tank reactor and withdrawing means for withdrawing said metal periodate reaction product from said at least one continuous stirred tank reactor.