THREE-CHAMBER ELECTROLYTIC CELL FOR THE PRODUCTION OF ALKALI METAL ALKOXIDES

Abstract

The present invention relates, in a first aspect, to an electrolysis cell having three chambers, wherein the middle chamber is separated from the cathode chamber by a solid-state electrolyte permeable to cations, for example NaSICON, and from the anode chamber by a diffusion barrier, for example a membrane selective for cations or anions. The invention is characterized in that the middle chamber comprises a mechanical stirring device.

The electrolysis cell according to the invention solves the problem that a concentration gradient forms in the middle chamber of the electrolysis cell during the electrolysis, which leads to locally lowered pH values and hence to damage to the solid-state electrolyte. With the aid of the mechanical stirring device, it is possible to stir the electrolyte solution in the middle chamber during the electrolysis. This leads to mixing of the electrolyte solution in the middle chamber, which prevents the formation of a pH gradient.

In a second aspect, the present invention relates to a process for producing an alkali metal alkoxide solution in the electrolysis cell according to the invention.

Claims

1. An Electrolysis cell E <100> comprising at least one anode chamber K.sub.A <101>, at least one cathode chamber K.sub.K <102> and at least one interposed middle chamber K.sub.M <103>, wherein K.sub.A <101> comprises an anodic electrode E.sub.A <104> and an outlet A.sub.KA <106>, wherein K.sub.K <102> comprises a cathodic electrode E.sub.K <105>, an inlet Z.sub.KK <107> and an outlet A.sub.KK <109>, wherein K.sub.M <103> comprises an inlet Z.sub.KM <108>, is divided from K.sub.A <101> by a diffusion barrier D <110> and is divided from K.sub.K <102> by an alkali metal cation-conducting solid-state electrolyte F.sub.K <111>, wherein K.sub.M <103> and K.sub.A <101> are connected to one another by a connection V.sub.AM <112> through which liquid can be routed from K.sub.M <103> into K.sub.A <101>, wherein the middle chamber K.sub.M <103> comprises a mechanical stirring device <120>.

2. The electrolysis cell E <100> according to claim 1, wherein the alkali metal ion-conducting solid-state electrolyte F.sub.K <111> has a structure of the formula M.sup.I.sub.1+2w+x?y+z M.sup.II.sub.w M.sup.III.sub.x Zr.sup.IV.sub.2?w?x?y M.sup.V.sub.y (SiO.sub.4).sub.z (PO.sub.4).sub.3?z, where M.sup.I is selected from Na.sup.+ and Li.sup.+, M.sup.II is a divalent metal cation, M.sup.III is a trivalent metal cation, M.sup.V is a pentavalent metal cation, the Roman indices I, II, III, IV, V indicate the oxidation numbers in which the respective metal cations exist, and w, x, y, z are real numbers, where 0?x<2, 0?y<2, 0 ?w<2, 0 ?z<3, and where w, x, y, z are chosen such that 1+2w+x?y+z?0 and 2 ?w?x?y?0.

3. The electrolysis cell E <100> according to claim 1, wherein the mechanical stirring device <120> comprises a propeller aligned parallel to the alkali metal cation-conducting solid-state electrolyte F.sub.K <111>.

4. The electrolysis cell E <100> according to claim 1, wherein the connection V.sub.AM <112> is formed within the electrolysis cell E <100>.

5. The electrolysis cell E <100> according to claim 1, wherein the mechanical stirring device <120> accounts for a proportion ? of 1% to 99% of the volume encompassed by the middle chamber K.sub.M, wherein ?=[(V.sub.O?V.sub.M)/V.sub.O]*100, and wherein V.sub.O is the maximum volume of liquid that can be accommodated by the middle chamber K.sub.M <103> if it does not comprise a mechanical stirring device <120>, and wherein V.sub.M is the maximum volume of liquid that can be accommodated by the middle chamber K.sub.M<103> if it comprises the mechanical stirring device <120>.

6. The electrolysis cell E <100> according to claim 1, wherein the mechanical stirring device <120> interrupts the direct pathway in the middle chamber K.sub.M between inlet Z.sub.KM <108> and connection V.sub.AM <112> according to the thread test stated in the description.

7. The process for producing a solution L.sub.1 <115> of an alkali metal alkoxide XOR in the alcohol ROH in an electrolysis cell E <100> according to claim 1, wherein the process comprises the following steps (a), (b) and (c) that proceed simultaneously: (a) a solution L.sub.2 <113> comprising the alcohol ROH is routed through K.sub.K <102>, (b) a neutral or alkaline, aqueous solution L.sub.3 <114> of a salt S comprising X as cation is routed through K.sub.M <103>, then via V.sub.AM <112>, then through K.sub.A <101>, while the mechanical stirring device <120> stirs the solution L.sub.3 <114> in K.sub.M <103>, (c) voltage is applied between E.sub.A <104> and E.sub.K <105>, which affords the solution L.sub.1 <115> at the outlet A.sub.KK <109>, with a higher concentration of XOR in L.sub.1 <115> than in L.sub.2 <113>, and which affords an aqueous solution L.sub.4 <116> of S at the outlet A.sub.KA <106>, with a lower concentration of S in L.sub.4 <116> than in L.sub.3 <114>, wherein X is an alkali metal cation and R is an alkyl radical having 1 to 4 carbon atoms.

8. The process according to claim 7, wherein X is selected from the group consisting of Li.sup.+, Na.sup.+, K.sup.+.

9. The process according to claim 7, wherein S is a halide, sulfate, sulfite, nitrate, hydrogencarbonate or carbonate of X.

10. The process according to claim 7, wherein R is selected from the group consisting of methyl and ethyl.

11. The process according to claim 7, wherein L.sub.2 <113> comprises the alcohol ROH and an alkali metal alkoxide XOR.

12. The process according to claim 11, wherein the mass ratio of XOR to alcohol ROH in L.sub.2 <113> is in the range from 1:100 to 1:5.

13. The process according to claim 11, wherein the concentration of XOR in L.sub.1 <115> is 1.01 to 2.2 times higher than in L.sub.2 <113>.

14. The process according to claim 7, which is performed at a temperature of 20 to 70? C. and a pressure of 0.5 to 1.5 bar.

15. The process according to claim 7, wherein the stirrer speed of the mechanical stirring device <120> is varied during the performance of step (b).

Description

3. FIGURES

[0032] FIG. 1 shows a preferred embodiment of an electrolysis cell <100> according to the invention and of the process according to the invention. The three-chamber cell E <100> comprises a cathode chamber K.sub.K <102>, an anode chamber K.sub.A <101> and an interposed middle chamber K.sub.M <103>.

[0033] The cathode chamber K.sub.K <102> comprises a cathodic electrode E.sub.K <105>, an inlet Z.sub.KK <107> and an outlet A.sub.KK <109>.

[0034] The anode chamber K.sub.A <101> comprises an anodic electrode E.sub.A <104> and an outlet A.sub.KA <106> and is connected to the middle chamber K.sub.M <103> via the connection V.sub.AM <112>.

[0035] The middle chamber K.sub.M <103> comprises an inlet Z.sub.KM <108>.

[0036] The three chambers are bounded by an outer wall <117> of the three-chamber cell E <100>. The cathode chamber K.sub.K <102> is also separated from the middle chamber K.sub.M <103> by an NaSICON solid-state electrolyte F.sub.K <111> which is selectively permeable to sodium ions. The middle chamber K.sub.M <103> is additionally separated in turn from the anode chamber K.sub.A <101> by a diffusion barrier D <110>. The NaSICON solid-state electrolyte F.sub.K <111> and the diffusion barrier D <110> extend over the entire depth and height of the three-chamber cell E <100>. The diffusion barrier D <110> is made of glass.

[0037] In the embodiment according to FIG. 1, the connection V.sub.AM <112> is formed outside the electrolysis cell E <100>, especially by a tube or hose, the material of which may be selected from rubber, metal and plastic. Through the connection V.sub.AM <112>, it is possible to route liquid from the middle chamber K.sub.M <103> into the anode chamber K.sub.A <101> outside the outer wall W.sub.A <117> of the three-chamber cell E <100>. The connection V.sub.AM <112> connects an outlet A.sub.KM <118> that penetrates the outer wall W.sub.A <117> of the electrolysis cell E <100> at the base of the middle chamber K.sub.M <103> to an inlet Z.sub.KA <119> that penetrates the outer wall W.sub.A <117> of the electrolysis cell E <100> at the base of the anode chamber K.sub.A <101>.

[0038] An aqueous solution of sodium chloride L.sub.3 <114> with pH 10.5 is introduced via the inlet Z.sub.KM <108>, in the direction of gravity, into the middle chamber K.sub.M <103>. The connection V.sub.AM <112> formed between an outlet A.sub.KM <118> from the middle chamber K.sub.M <103> and an inlet Z.sub.KA <119> to the anode chamber K.sub.A <101> connects the middle chamber K.sub.M <103> to the anode chamber K.sub.A <101>. Sodium chloride solution L.sub.3 <114> is routed through this connection V.sub.AM <112> from the middle chamber K.sub.M <103> into the anode chamber K.sub.A <101>.

[0039] A solution of sodium methoxide in methanol L.sub.2 <113> is routed into the cathode chamber K.sub.K <102> via the inlet Z.sub.KK <107>.

[0040] At the same time, a voltage is applied between the cathodic electrode E.sub.K <105> and the anodic electrode E.sub.A <104>. This results in reduction of methanol in the electrolyte L.sub.2 <113> to give methoxide and H.sub.2 in the cathode chamber K.sub.K <102> (CH.sub.3OH+e.sup.?.fwdarw.CH.sub.3O.sup.?+? H.sub.2). At the same time, sodium ions diffuse from the middle chamber K.sub.M <103> through the NaSICON solid-state electrolyte F.sub.K <111> into the cathode chamber K.sub.K <102>. Overall, this increases the concentration of sodium methoxide in the cathode chamber K.sub.K <102>, which affords a methanolic solution of sodium methoxide L.sub.1 <115>, the sodium methoxide concentration of which is elevated compared to L.sub.2 <113>.

[0041] In the anode chamber K.sub.A <101>, the oxidation of chloride ions takes place to give molecular chlorine (Cl.sup.?.fwdarw.?Cl.sub.2+e.sup.?). In the outlet A.sub.KA <106>, an aqueous solution L.sub.4 <116> is obtained, in which the content of NaCl is reduced compared to L.sub.3 <114>. Chlorine gas (Cl.sub.2) in water, according to the reaction Cl.sub.2+H.sub.2O.fwdarw.HOCl+HCl, forms hypochlorous acid and hydrochloric acid, which give an acidic reaction with further water molecules. The acidity damages the NaSICON solid-state electrolyte <111>, but is restricted to the anode chamber K.sub.A <101> by the arrangement according to the invention, and hence kept away from the NaSICON solid-state electrolyte F.sub.K <111> in the electrolysis cell E <100>. This considerably increases the lifetime thereof.

[0042] In the middle chamber K.sub.M <103>, there is also a mechanical stirring device <120> in the form of a propeller stirrer <121> which is operated by an electric motor <122>, with the propeller stirrer connected to the electric motor via a transmission link <124>. The propeller stirrer <121> is freely suspended in the middle chamber K.sub.M <103>, but may also be secured on the inside of the outer wall W.sub.A <117>. The transmission link <124> reaches through a cutout <125> in the outer wall of the middle chamber K.sub.M <103> into the electrolysis cell E <100>. The aqueous solution L.sub.3 <114> supplied through the inlet Z.sub.KM <108> is mixed by the operation of the propeller stirrer <121>, which results in vortexing and turbulence. This turbulence L.sub.3 <114> in the solution prevents buildup of a pH gradient in the middle chamber K.sub.M <103> with progressive electrolysis, and prevents the development of a low pH in the solution directly adjoining the NaSICON solid-state electrolyte <111>. This further increases the service life of the NaSICON solid-state electrolyte <111>.

[0043] FIG. 2 shows a further embodiment of the electrolysis cell according to the invention and the process according to the invention corresponding to that shown in FIG. 1. The difference here is that the mechanical stirring device <120>, rather than the propeller stirrer <121> described in FIG. 1, comprises a magnetic stirrer bar <123-1> that can be operated with a magnetic stirrer system <123-2> disposed outside the middle chamber K.sub.M <103>. As in the embodiment shown in FIG. 1 too, the aqueous solution L.sub.3 <114> supplied through the inlet Z.sub.KM <108> is vortexed by this mechanical stirring device <120>. This turbulence in the solution L.sub.3 <114> destroys the pH gradient that builds up in the middle chamber K.sub.M <103> with progressive electrolysis.

4. DETAILED DESCRIPTION OF THE INVENTION

4.1 Electrolysis Cell E

[0044] The first aspect of the invention relates to an electrolysis cell E <100>. The electrolysis cell E <100> in the first aspect of the invention comprises at least one anode chamber K.sub.A <101>, at least one cathode chamber K.sub.K <102> and at least one interposed middle chamber K.sub.M <103>. This also includes electrolysis cells E <100> having more than one anode chamber K.sub.A <101> and/or cathode chamber K.sub.K <102> and/or middle chamber K.sub.M <103>. Such electrolysis cells in which these chambers are joined to one another in the form of modules are described, for example, in DD 258 143 A3 and US 2006/0226022 A1.

[0045] The anode chamber K.sub.A <101> comprises an anodic electrode E.sub.A <104>. A useful anodic electrode E.sub.A <104> of this kind is any electrode familiar to the person skilled in the art that is stable under the conditions of the process according to the invention in the second aspect of the invention. These are described, in particular, in WO 2014/008410 A1, paragraph [024] or DE 10360758 A1, paragraph [031]. This electrode E.sub.A <104> may consist of one layer or consist of multiple planar layers parallel to one another that may each be perforated or expanded. The anodic electrode E.sub.A <104> especially comprises a material selected from the group consisting of ruthenium oxide, iridium oxide, nickel, cobalt, nickel tungstate, nickel titanate, precious metals such as, in particular, platinum, supported on a support such as titanium or Kovar? (an iron/nickel/cobalt alloy in which the individual components are preferably as follows: 54% by mass of iron, 29% by mass of nickel, 17% by mass of cobalt). Further possible anode materials are especially stainless steel, lead, graphite, tungsten carbide, titanium diboride. Preferably, the anodic electrode E.sub.A <104> comprises a titanium anode coated with ruthenium oxide/iridium oxide (RuO.sub.2+IrO.sub.2/Ti).

[0046] The cathode chamber K.sub.K <102> comprises a cathodic electrode E.sub.K <105>. A useful cathodic electrode E.sub.K <105> of this kind is any electrode familiar to the person skilled in the art that is stable under the conditions. These are described, in particular, in WO 2014/008410 A1, paragraph [025] or DE 10360758 A1, paragraph [030]. This electrode E.sub.K <105> may be selected from the group consisting of mesh wool, three-dimensional matrix structure and balls. The cathodic electrode E.sub.K <105> especially comprises a material selected from the group consisting of steel, nickel, copper, platinum, platinized metals, palladium, carbon-supported palladium, titanium. Preferably, E.sub.K <105> comprises nickel.

[0047] The at least one middle chamber K.sub.M <103> is between the anode chamber K.sub.A <101> and the cathode chamber K.sub.K <102>.

[0048] The electrolysis cell E <100> typically has an outer wall W.sub.A <117>. The outer wall W.sub.A <117> is especially made from a material selected from the group consisting of steel, preferably rubberized steel, plastic, especially from Telene? (thermoset polydicyclopentadiene), PVC (polyvinylchloride), PVC-C (post-chlorinated polyvinylchloride), PVDF (polyvinylidenefluoride). W.sub.A <117> may especially be perforated for inlets and outlets. Within W.sub.A <117> are then the at least one anode chamber K.sub.A <101>, the at least one cathode chamber K.sub.K <102> and the at least one interposed middle chamber K.sub.M <103>.

[0049] K.sub.M <103> is separated from K.sub.A <101> by a diffusion barrier D <110> and from K.sub.K <102> by an alkali metal cation-conducting solid-state electrolyte F.sub.K <111>.

[0050] For the diffusion barrier D <110>, it is possible to use any material that is stable under the conditions of the process according to the invention in the second aspect of the invention and prevents or slows the transfer of protons from the liquid present in the anode chamber K.sub.A <101> into the middle chamber K.sub.M <103>.

[0051] The diffusion barrier D <110> used is especially a non-ion-specific dividing wall or a membrane permeable to specific ions. The diffusion barrier D <110> is preferably a non-ion-specific dividing wall.

[0052] The material of the non-ion-specific dividing wall is especially selected from the group consisting of fabric, which is especially textile fabric or metal weave, glass, which is especially sintered glass or glass frits, ceramic, especially ceramic frits, membrane diaphragms, and is more preferably glass.

[0053] If the diffusion barrier D <110> is a membrane permeable to specific ions, what this means in accordance with the invention is that the respective membrane promotes the diffusion of particular ions therethrough over other ions. More particularly, what this means is membranes that promote the diffusion therethrough of ions of a particular charge type over ions of the opposite charge. Even more preferably, membranes permeable to specific ions also promote the diffusion of particular ions of one charge type over other ions of the same charge type therethrough.

[0054] If the diffusion barrier D <110> is a membrane permeable to specific ions, the diffusion barrier D <110> is especially an anion-conducting membrane or a cation-conducting membrane.

[0055] According to the invention, anion-conducting membranes are those that selectively conduct anions, preferably selectively conduct particular anions. In other words, they promote the diffusion of anions therethrough over that of cations, especially over protons; even more preferably, they additionally promote the diffusion of particular anions therethrough over the diffusion of other anions therethrough.

[0056] According to the invention, cation-conducting membranes are those that selectively conduct cations, preferably selectively conduct particular cations. In other words, they promote the diffusion of cations therethrough over that of anions; even more preferably, they additionally promote the diffusion of particular cations therethrough over the diffusion of other cations therethrough, more preferably still that of cations that are not protons, more preferably sodium cations, over protons.

[0057] What is meant more particularly by promote the diffusion of particular ions X over the diffusion of other ions Y is that the coefficient of diffusion (unit: m.sup.2/s) of ion type X at a given temperature for the membrane in question is higher by a factor of 10, preferably 100, preferably 1000, than the coefficient of diffusion of ion type Y for the membrane in question.

[0058] If the diffusion barrier D <110> is a membrane permeable to specific ions, it is preferably an anion-conducting membrane since this particularly efficiently prevents the diffusion of protons from the anode chamber K.sub.A <101> into the middle chamber K.sub.M <103>.

[0059] The anion-conducting membrane used is especially one selective for the anions encompassed by the salt S. Such membranes are known to and can be used by the person skilled in the art.

[0060] The salt S is preferably a halide, sulfate, sulfite, nitrate, hydrogencarbonate or carbonate of X, even more preferably a halide.

[0061] Halides are fluorides, chlorides, bromides, iodides. The most preferred halide is chloride.

[0062] The anion-conducting membrane used is preferably one selective for halides, preferably chloride.

[0063] Anion-conducting membranes are described, for example, by M. A. Hickner, AM. Herring, E. B. Coughlin, Journal of Polymer Science, Part B: Polymer Physics 2013, 51, 1727-1735, by C. G. Arges, V. Ramani, P. N. Pintauro, Electrochemical Society Interface 2010, 19, 31-35, in WO 2007/048712 A2, and on page 181 of the textbook by Volkmar M. Schmidt, Elektrochemische Verfahrenstechnik-Grundlagen, Reaktionstechnik, Prozessoptimierung [Electrochemical Engineering: Fundamentals, Reaction Technology, Process Optimization], 1st edition (8 Oct. 2003).

[0064] Even more preferably, anion-conducting membranes used are accordingly organic polymers that are especially selected from polyethylene, polybenzimidazoles, polyether ketones, polystyrene, polypropylene and fluorinated membranes such as polyperfluoroethylene, preferably polystyrene, where these have covalently bonded functional groups selected from NH.sub.3.sup.+, NRH.sub.2.sup.+, NR.sub.3.sup.+, =NR.sup.+; PR.sub.3.sup.+, where R is alkyl groups having preferably 1 to 20 carbon atoms, or other cationic groups. They preferably have covalently bonded functional groups selected from NH.sub.3.sup.+, NRH.sub.2.sup.+ and NR.sub.3.sup.+, more preferably selected from NH.sub.3.sup.+ and NR.sub.3.sup.+, even more preferably NR.sub.3.sup.+.

[0065] If the diffusion barrier D <110> is a cation-conducting membrane, it is especially a membrane selective for the cations encompassed by the salt S. Even more preferably, the diffusion barrier D <110> is an alkali metal cation-conducting membrane, even more preferably a potassium and/or sodium ion-conducting membrane, most preferably a sodium ion-conducting membrane.

[0066] Cation-conducting membranes are described, for example, on page 181 of the textbook by Volkmar M. Schmidt, Elektrochemische Verfahrenstechnik: Grundlagen, Reaktionstechnik, Prozessoptimierung, 1st edition (8 Oct. 2003).

[0067] Even more preferably, cation-conducting membranes used are accordingly organic polymers that are especially selected from polyethylene, polybenzimidazoles, polyether ketones, polystyrene, polypropylene and fluorinated membranes such as polyperfluoroethylene, preferably polystyrene and polyperfluoroethylene, where these bear covalently bonded functional groups selected from SO.sub.3.sup.?, COO.sup.?, PO.sub.3.sup.2? and PO.sub.2H.sup.?, preferably SO.sub.3.sup.? (described in DE 10 2010 062 804 A1, U.S. Pat. No. 4,831,146).

[0068] This may be, for example, a sulfonated polyperfluoroethylene (Nafion? with CAS number 31175-20-9). These are known to the person skilled in the art, for example from WO 2008/076327 A1, paragraph [058], US 2010/0044242 A1, paragraph [0042] or US 2016/0204459 A1, and are commercially available under the Nafion?, Aciplex? F, Flemion?, Neosepta?, Ultrex?, PC-SK? trade names. Neosepta? membranes are described, for example, by S. A. Mareev, D. Yu. Butylskii, N. D. Pismenskaya, C. Larchet, L. Dammak, V. V. Nikonenko, Journal of Membrane Science 2018, 563, 768-776.

[0069] If a cation-conducting membrane is used as diffusion barrier D <110>, this may, for example, be a polymer functionalized with sulfonic acid groups, especially of the formula P.sub.NAFION below, where n and m may independently be a whole number from 1 to 10.sup.6, preferably a whole number from 10 to 10.sup.5, more preferably a whole number from 10.sup.2 to 10.sup.4.

##STR00001##

[0070] A useful alkali metal cation-conducting solid-state electrolyte F.sub.K <111> is any solid-state electrolyte that can transport cations, especially alkali metal cations, even more preferably sodium cations, from the middle chamber K.sub.M <103> into the cathode chamber K.sub.K <102>. Such solid-state electrolytes are known to the person skilled in the art and are described, for example, in DE 10 2015 013 155 A1, in WO 2012/048032 A2, paragraphs [0035], [0039], [0040], in US 2010/0044242 A1, paragraphs [0040], [0041], in DE 10360758 A1, paragraphs [014] to [025]. They are sold commercially under the NaSICON, LiSICON, KSICON name. A sodium ion-conducting solid-state electrolyte F.sub.K <111> is preferred, and this even more preferably has an NaSICON structure. NaSICON structures usable in accordance with the invention are also described, for example, by N. Anantharamulu, K. Koteswara Rao, G. Rambabu, B. Vijaya Kumar, Velchuri Radha, M. Vithal, J Mater Sci 2011, 46, 2821-2837.

[0071] NaSICON preferably has a structure of the formula

M.sup.I.sub.1+2w+x?y+zM.sup.II.sub.wM.sup.III.sub.xZr.sup.IV.sub.2?w?x?y M.sup.V.sub.y(SiO.sub.4).sub.z(PO.sub.4).sub.3?z.

[0072] M.sup.I is selected from Na.sup.+, Li.sup.+, preferably Na.sup.+.

[0073] M.sup.II is a divalent metal cation, preferably selected from Mg.sup.2+, Ca.sup.2+, Sr.sup.2+, Ba.sup.2+, Co.sup.2+, Ni.sup.2+, more preferably selected from Co.sup.2+, Ni.sup.2+.

[0074] M.sup.III is a trivalent metal cation, preferably selected from Al.sup.3+, Ga.sup.3+, Sc.sup.3+, La.sup.3+, Y.sup.3+, Gd.sup.3+, Sm.sup.3+, Lu.sup.3+, Fe.sup.3+, Cr.sup.3+, more preferably selected from Sc.sup.3+, La.sup.3+, Y.sup.3+, Gd.sup.3+, Sm.sup.3+, especially preferably selected from Sc.sup.3+, Y.sup.3+, La.sup.3+.

[0075] M.sup.V is a pentavalent metal cation, preferably selected from V.sup.5+, Nb.sup.5+, Ta.sup.5+.

[0076] The Roman indices I, II, III, IV, V indicate the oxidation numbers in which the respective metal cations exist.

[0077] w, x, y, z are real numbers, where 0?x<2, 0?y<2, 0?w<2, 0?z<3,

[0078] and where w, x, y, z are chosen such that 1+2w+x?y+z?0 and 2 ?w?x?y?0.

[0079] Even more preferably in accordance with the invention, NaSICON has a structure of the formula Na.sub.(1+v)Zr.sub.2Si.sub.vP.sub.(3?v)O.sub.12 where v is a real number for which 0?v?3. Most preferably, v=2.4.

[0080] The cathode chamber K.sub.K <102> also comprises an inlet Z.sub.KK <107> and an outlet A.sub.KK <109> that enables addition of liquid, for example the solution L.sub.2 <113>, to the cathode chamber K.sub.K <102> and removal of liquid present therein, for example the solution L.sub.1 <115>. The inlet Z.sub.KK <107> and the outlet A.sub.KK <109> are mounted on the cathode chamber K.sub.K <102> in such a way that the solution comes into contact with the cathodic electrode E.sub.K <105> as it flows through the cathode chamber K.sub.K <102>. This is a prerequisite for the solution L.sub.1 <115> to be obtained at the outlet A.sub.KK <109> in the performance of the process according to the invention in the second aspect of the invention when the solution L.sub.2 <113> of an alkali metal alkoxide XOR in the alcohol ROH is routed through K.sub.K <102>.

[0081] The anode chamber K.sub.A <101> also comprises an outlet A.sub.KA <106> that enables removal of liquid present in the anode chamber K.sub.A <101>, for example the aqueous solution L.sub.4 <106>. In addition, the middle chamber K.sub.M <103> comprises an inlet Z.sub.KM <108>, while K.sub.A <101> and K.sub.M <103> are connected to one another by a connection V.sub.AM <112> through which liquid can be routed from K.sub.M <103> into K.sub.A <101>. As a result, a solution L.sub.3 <114> can be introduced via the inlet Z.sub.KM <108> to K.sub.M <103>, and this can be routed through K.sub.M <103>, then via V.sub.AM <112>, into the anode chamber K.sub.A <101>, and finally through the anode chamber K.sub.A <101>. V.sub.AM <112> and the outlet A.sub.KA <106> are mounted on the anode chamber K.sub.A <101> in such a way that the solution L.sub.3 <114> comes into contact with the anodic electrode E.sub.A <104> as it flows through the anode chamber K.sub.A <101>. This is a prerequisite for the aqueous solution L.sub.4 <116> to be obtained at the outlet A.sub.KA <106> in the performance of the process according to the invention in the second aspect when the solution L.sub.3 <114> is routed first through K.sub.M <103>, then V.sub.AM <112>, then K.sub.A <101>.

[0082] The inlets Z.sub.KK <107>, Z.sub.KM <108>, Z.sub.KA <119> and outlets A.sub.KK <109>, A.sub.KA <106>, A.sub.KM <118> may be mounted on the electrolysis cell E <100> by methods known to the person skilled in the art.

[0083] The connection V.sub.AM <112> may be formed within the electrolysis cell E <100> and/or outside, preferably within, the electrolysis cell E <100>.

[0084] If the connection V.sub.AM <112> is formed within the electrolysis cell E <100>, it is preferably formed by at least one perforation in the diffusion barrier D <110>.

[0085] If the connection V.sub.AM <112> is formed outside the electrolysis cell E <100>, it is preferably formed by a connection of K.sub.M <103> and K.sub.A <101> that runs outside the electrolysis cell E <100>, especially in that an outlet A.sub.KM <118> through the outer wall W.sub.A <117> is formed in the middle chamber K.sub.M <103>, preferably at the base of the middle chamber K.sub.M <103>, the inlet Z.sub.KM <108> more preferably being at the top end of the middle chamber K.sub.M <103>, and an inlet Z.sub.KA <119> through the outer wall W.sub.A <117> is formed in the anode chamber K.sub.A <101>, preferably at the base of the anode chamber K.sub.A <101>, and these are connected by a conduit, for example a pipe or a hose, preferably comprising a material selected from rubber and plastic. The outlet A.sub.KA <106> is then more preferably at the top end of the anode chamber K.sub.A <101>.

[0086] What is meant by outlet A.sub.KM <118> at the base of the middle chamber K.sub.M <103> is that the outlet A.sub.KM <118> is attached to the electrolysis cell E <100> in such a way that the solution L.sub.3 <114> leaves the middle chamber K.sub.M <103> in the direction of gravity.

[0087] What is meant by inlet Z.sub.KA <119> at the base of the anode chamber K.sub.A <101> is that the inlet Z.sub.KA <119> is attached to the electrolysis cell E <100> in such a way that the solution L.sub.3 <114> enters the anode chamber K.sub.A <101> counter to gravity.

[0088] What is meant by inlet Z.sub.KM <108> at the top end of the middle chamber K.sub.M <103> is that the inlet Z.sub.KM <108> is attached to the electrolysis cell E <100> in such a way that the solution L.sub.3 <114> enters the middle chamber K.sub.M <103> in the direction of gravity.

[0089] What is meant by outlet A.sub.KA <106> at the top end of the anode chamber K.sub.A <101> is that the outlet A.sub.KA <106> is mounted on the electrolysis cell E <100> in such a way that the solution L.sub.4 <116> leaves the anode chamber K.sub.A <101> counter to gravity.

[0090] This embodiment is particularly advantageous and therefore preferred when the outlet A.sub.KM <118> is formed by the outer wall W.sub.A <117> at the base of the middle chamber K.sub.M <103>, and the inlet Z.sub.KA <119> by the outer wall W.sub.A <117> at the base of the anode chamber K.sub.A <101>. This arrangement makes it possible in a particularly simple manner to remove gases formed in the anode chamber K.sub.A from the anode chamber K.sub.A <101> with L.sub.4 <116>, in order to separate them further.

[0091] When the connection V.sub.AM <112> is formed outside the electrolysis cell E <100>, in particular, Z.sub.KM <108> and A.sub.KM <118> are arranged at opposite ends of the outer wall W.sub.A <117> of the middle chamber K.sub.M <103> (i.e. Z.sub.KM <108> at the base and A.sub.KM <118> at the top end of the electrolysis cell E <100> or vice versa) and Z.sub.KA <119> and A.sub.KA <106> are arranged at opposite ends of the outer wall W.sub.A <117> of the anode chamber K.sub.A <101> (i.e. Z.sub.KA <119> at the base and A.sub.KA <106> at the top end of the electrolysis cell E <100> or vice versa), as shown more particularly in FIG. 1. By virtue of this geometry, L.sub.3 <114> must flow through the two chambers K.sub.M <103> and K.sub.A <101>. It is possible here for Z.sub.KA <119> and Z.sub.KM <108> to be formed on the same side of the electrolysis cell E <100>, in which case A.sub.KM <118> and A.sub.KA <106> are automatically also formed on the same side of the electrolysis cell E <100>. Alternatively, as shown in FIG. 1, it is possible for Z.sub.KA <119> and Z.sub.KM <108> to be formed on opposite sides of the electrolysis cell E <100>, in which case A.sub.KM <118> and A.sub.KA <106> are automatically also formed on opposite sides of the electrolysis cell E <100>.

[0092] When the connection V.sub.AM <112> is formed within the electrolysis cell E <100>, this may especially be implemented in that one side (side A) of the electrolysis cell E <100>, which is the top end or the base of the electrolysis cell E <100>, preferably the top end as shown in FIG. 2, comprises the inlet Z.sub.KM <108> and the outlet A.sub.KA <106>, and the diffusion barrier D <110> extends proceeding from this side (side A) into the electrolysis cell E <100>, but does not quite reach up to the side (side B) of the electrolysis cell E <100> opposite side A, which is then the base or the top end of the electrolysis cell E <100>, and at the same time covers 50% or more of the height of the three-chamber cell E <100>, preferably 60% to 99% of the height of the three-chamber cell E <100>, more preferably 70% to 95% of the height of the three-chamber cell E <100>, even more preferably 80% to 90% of the height of the three-chamber cell E <100>, more preferably still 85% of the height of the three-chamber cell E <100>. Because the diffusion barrier D <110> does not touch side B of the three-chamber cell E <100>, a gap thus arises between diffusion barrier D <110> and the outer wall W.sub.A <117> of side B of the three-chamber cell E <100>. In that case, the gap is the connection V.sub.AM <112>. By virtue of this geometry, L.sub.3 <114> must flow completely through the two chambers K.sub.M <103> and K.sub.A <101>.

[0093] These embodiments best assure that the aqueous salt solution L.sub.3 <114> flows past the acid-sensitive solid-state electrolyte before it comes into contact with the anodic electrode E.sub.A <104>, which results in the formation of acids.

[0094] According to the invention, base of the electrolysis cell E <100> is the side of the electrolysis cell E <100> through which a solution (e.g. L.sub.3 <114> in the case of A.sub.KM <118> in FIG. 1) exits from the electrolysis cell E in the same direction as gravity, or the side of the electrolysis cell E through which a solution (e.g. L.sub.2 <113> in the case of Z.sub.KK <107> in FIGS. 1 and 2, and L.sub.3 <114> in the case of A.sub.KA <119> in FIG. 1) is supplied to the electrolysis cell E counter to gravity.

[0095] According to the invention, top end of the electrolysis cell E is the side of the electrolysis cell E through which a solution (e.g. L.sub.4 <116> in the case of A.sub.KA <106> and L.sub.1 <115> in the case of A.sub.KK <109> in FIGS. 1 and 2) exits from the electrolysis cell E counter to gravity, or the side of the electrolysis cell E through which a solution (e.g. L.sub.3 <114> in the case of Z.sub.KM <108> in FIGS. 1 and 2) is supplied to the electrolysis cell E in the same direction as gravity.

[0096] According to the invention, the middle chamber K.sub.M comprises a mechanical stirring device <120>. According to the invention, the mechanical stirring device <120> is in the solid state of matter. A suitable mechanical stirring device of this kind is any stirring device known to the person skilled in the art that is sufficiently inert to the electrolysis conditions.

[0097] The mechanical stirring device <120> especially comprises at least one material selected from rubber; plastic, which is especially selected from polystyrene, polypropylene, PVC, PVC-C; glass; porcelain; metal. The metal is especially a metal or an alloy of two or more metals selected from titanium, iron, molybdenum, chromium, nickel, platinum, gold, silver, preferably an alloy comprising at least two metals selected from titanium, iron, molybdenum, chromium, nickel, platinum, gold, silver, even more preferably a steel alloy comprising, as well as iron, at least one further metal selected from titanium, molybdenum, chromium, nickel, platinum, gold, silver, and is most preferably stainless steel.

[0098] Even more preferably, the mechanical stirring device <120> comprises magnetic material, such that it can be operated with a magnetic stirrer system.

[0099] The mechanical stirring device <120> is especially selected from propeller stirrer, pitched blade stirrer, disk stirrer, tumbling disk stirrer, hollow blade stirrer, impeller stirrer, crossbeam stirrer, anchor stirrer, paddle stirrer, gate stirrer, helical stirrer, toothed disk stirrer, low-volume stirrer, preferably a propeller stirrer.

[0100] The mechanical stirring device <120> is typically driven by a motor, which is preferably an electric motor outside the electrolysis cell E <100>. For example, this may be a motor <122> connected to the propeller stirrer <121> via a transmission link <124>, with the transmission link <124> extending into the electrolysis cell E <100> through a cutout <125> in the outer wall of the middle chamber K.sub.M <103>, as illustrated in FIG. 1.

[0101] Alternatively, the propeller stirrer may also be magnetic, such that it is a magnetic stirrer bar <123-1> which is driven by a magnetic stirrer system <123-2> disposed outside the middle chamber K.sub.M <103>, as illustrated in FIG. 2.

[0102] The magnetic stirring device <120> may be suspended loosely in the middle chamber K.sub.M <103>, as shown in FIG. 1.

[0103] Alternatively, the mechanical stirring device <120> may also be secured, for example on the solid-state electrolyte F.sub.K <111>, on the diffusion barrier D <110>, or on the outer wall <117> that bounds the inside of the middle chamber K.sub.M <103>. The securing can be effected by methods known to the person skilled in the art, for example by screw connection, clamping, adhesive bonding (polymer adhesive, PVC adhesive).

[0104] In a preferred embodiment of the electrolysis cell E <100> according to the first aspect of the invention, the mechanical stirring device <120> comprises a propeller aligned parallel to the alkali metal cation-conducting solid-state electrolyte F.sub.K <111>.

[0105] In a preferred embodiment of the electrolysis cell E <100> according to the first aspect of the invention, the mechanical stirring device <120> accounts for a proportion 4 of 1% to 99%, more preferably 2% to 50%, even more preferably 3% to 40%, even more preferably 4% to 30%, even more preferably 5% to 20%, most preferably 6% to 10%, of the volume encompassed by the middle chamber K.sub.M.

[0106] The proportion ? (in %) is calculated by ?=[(V.sub.O?V.sub.M)/V.sub.O]*100.

[0107] V.sub.O here is the maximum volume of liquid, for example the electrolyte L.sub.3 <114>, that can be accommodated by the middle chamber K.sub.M <103> if it does not comprise a mechanical stirring device <120>.

[0108] V.sub.M here is the maximum volume of liquid, for example the electrolyte L.sub.3 <114>, that can be accommodated by the middle chamber K.sub.M <103> if it comprises the mechanical stirring device <120>.

[0109] It has been found that, surprisingly, the mechanical stirring device <120> in the middle chamber K.sub.M <103> leads to turbulence and vortexing in the electrolyte L.sub.3 <114> that flows through the middle chamber K.sub.M <103> during the process according to the invention. This slows or entirely prevents the buildup of a pH gradient during the electrolysis, which protects the acid-sensitive solid-state electrolyte F.sub.K <111> and hence enables a longer run time for the electrolysis or extends the lifetime of the electrolysis cell E <100>.

[0110] It will be apparent that the mechanical stirring device <120> is mounted in the middle chamber K.sub.M <103> such that it sufficiently enables, or does not completely block, the flow of the electrolyte L.sub.3 <114> through the middle chamber K.sub.M <103> and the anode chamber K.sub.A <101>.

[0111] In a preferred embodiment of the electrolysis cell according to the invention, the mechanical stirring device <120> interrupts the direct pathway in the middle chamber K.sub.M between inlet Z.sub.KM <108> and connection V.sub.AM <112>.

[0112] Whether the direct route between inlet Z.sub.KM <108> and connection V.sub.AM <112> in the middle chamber K.sub.M is interrupted is ascertained by the following thread test: [0113] 1. A thread is run through the opening through which the inlet Z.sub.KM <108> opens into the middle chamber K.sub.M and out of the opening through which the connection V.sub.AM <112> opens into the middle chamber K.sub.M. The thread here is sufficiently long that its ends lie outside the middle chamber K.sub.M. [0114] 2. A force is exerted on the respective ends of the thread in opposing directions, such that the thread becomes taut without breaking. [0115] 3. If there is at least one thread that is touched by the mechanical stirring device <120> during operation thereof if it is introduced into the middle chamber and tautened in accordance with steps 1. and 2., the feature that the direct route between inlet Z.sub.KM <108> and connection V.sub.AM <112> in the middle chamber K.sub.M is interrupted is satisfied. [0116] 4. If there is no thread which, when introduced into the middle chamber and tautened according to steps 1. and 2., is touched by the mechanical stirring device <120> during operation thereof, the feature that the direct route between inlet Z.sub.KM <108> and connection V.sub.AM <112> in the middle chamber K.sub.M is interrupted is not satisfied.

[0117] The thread is especially made of sewing thread (for example from G?termann), fishing line, string.

[0118] Most preferably, a fishing line with a diameter of 0.2 mm is used for the thread test, as sold, for example, by Hemingway or Nexos.

4.2 Process Steps According to the Invention

[0119] The process according to the second aspect of the invention is one for producing a solution L.sub.1 <115> of an alkali metal alkoxide XOR in the alcohol ROH in an electrolysis cell E <100> according to the first aspect of the invention,

[0120] The process according to the second aspect of the invention comprises the steps (a), (b) and (c) that proceed simultaneously.

[0121] In step (a), a solution L.sub.2 <113> comprising the alcohol ROH, preferably comprising an alkali metal alkoxide XOR and alcohol ROH, is routed through K.sub.K <102>. X is an alkali metal cation and R is an alkyl radical having 1 to 4 carbon atoms.

[0122] X is preferably selected from the group consisting of Li.sup.+, K.sup.+, Na.sup.+, more preferably from the group consisting of K.sup.+, Na.sup.+. Most preferably, X=Na.sup.+.

[0123] R is preferably selected from the group consisting of n-propyl, iso-propyl, ethyl and methyl, more preferably from the group consisting of ethyl and methyl. R is most preferably methyl.

[0124] Solution L.sub.2 <113> is preferably free of water. What is meant in accordance with the invention by free of water is that the weight of water in solution L.sub.2 <113> based on the weight of the alcohol ROH in solution L.sub.2 <113> (mass ratio) is ?1:10, more preferably ?1:20, even more preferably ?1:100, even more preferably ?0.5:100.

[0125] If solution L.sub.2 <113> comprises XOR, the proportion by mass of XOR in solution L.sub.2 <113>, based on the overall solution L.sub.2 <113>, is especially >0% to 30% by weight, preferably 5% to 20% by weight, more preferably 10% to 20% by weight, more preferably 10% to 15% by weight, most preferably 13% to 14% by weight, at the very most preferably 13% by weight.

[0126] If solution L.sub.2 <113> comprises XOR, the mass ratio of XOR to alcohol ROH in solution L.sub.2 <113> is especially in the range of 1:100 to 1:5, more preferably in the range of 1:25 to 3:20, even more preferably in the range of 1:12 to 1:8, even more preferably 1:10.

[0127] In step (b), a neutral or alkaline, aqueous solution L.sub.3 <114> of a salt S comprising X as cation is routed through K.sub.M <103>, then via V.sub.AM <112>, then through K.sub.A <101>, while the mechanical stirring device <120> stirs the solution L.sub.3 <114> in K.sub.M <103>.

[0128] The salt S is preferably a halide, sulfate, sulfite, nitrate, hydrogencarbonate or carbonate of X, even more preferably a halide.

[0129] Halides are fluorides, chlorides, bromides, iodides. The most preferred halide is chloride.

[0130] The pH of the aqueous solution L.sub.3 <114> is ?7.0, preferably in the range of 7 to 12, more preferably in the range of 8 to 11, even more preferably 10 to 11, most preferably 10.5.

[0131] The proportion by mass of salt S in solution L.sub.3 <113> is preferably in the range of >0% to 20% by weight, preferably 1% to 20% by weight, more preferably 5% to 20% by weight, even more preferably 10% to 20% by weight, most preferably 20% by weight, based on the overall solution L.sub.3 <113>.

[0132] In step (c), a voltage is then applied between E.sub.A <104> and E.sub.K <105>.

[0133] This results in transfer of current from the charge source to the anode, transfer of charge via ions to the cathode and ultimately transfer of current back to the charge source. The charge source is known to the person skilled in the art and is typically a rectifier that converts alternating current to direct current and can generate particular voltages via voltage transformers.

[0134] This in turn has the following consequences: [0135] solution L.sub.1 <115> is obtained at the outlet A.sub.KK <109>, with a higher concentration of XOR in L.sub.1 <115> than in L.sub.2 <113>, [0136] an aqueous solution L.sub.4 <116> of S is obtained at the outlet A.sub.KA <106>, with a lower concentration of S in L.sub.4 <116> than in L.sub.3 <114>.

[0137] In the process according to the second aspect of the invention, in particular, such a voltage is applied that such a current flows that the current density (=ratio of the current supplied to the electrolysis cell to the area of the solid-state electrolyte in contact with the anolyte present in the middle chamber K.sub.M <103>) is in the range from 10 to 8000 A/m.sup.2, more preferably in the range from 100 to 2000 A/m.sup.2, even more preferably in the range from 300 to 800 A/m.sup.2, and even more preferably is 494 A/m.sup.2. This can be determined in a standard manner by the person skilled in the art. The area of the solid-state electrolyte in contact with the anolyte present in the middle chamber K.sub.M <103> is especially 0.00001 to 10 m.sup.2, preferably 0.0001 to 2.5 m.sup.2, more preferably 0.0002 to 0.15 m.sup.2, even more preferably 2.83 cm.sup.2.

[0138] It will be apparent that step (c) of the process according to the second aspect of the invention is performed when the two chambers K.sub.M <103> and K.sub.A <101> are at least partly laden with L.sub.3 <114> and K.sub.K <102> is at least partly laden with L.sub.2 <113>.

[0139] The fact that in step (c) transfer of charge takes place between E.sub.A <104> and E.sub.K <105> implies that K.sub.K <102>, K.sub.M <103> and K.sub.A <101> are simultaneously laden with L.sub.2 <113> or L.sub.3 <114> such that they cover the electrodes E.sub.A <104> and E.sub.K <105> to such an extent that the circuit is complete.

[0140] This is the case especially when a liquid stream of L.sub.3 <114> is routed continuously through K.sub.M <103>, V.sub.AM <112> and K.sub.A <101> and a liquid stream of L.sub.2 <113> through K.sub.K <102>, and the liquid stream of L.sub.3 <114> covers electrode E.sub.A <104> and the liquid stream of L.sub.2 <113> covers electrode E.sub.K <105> at least partly, preferably completely.

[0141] By virtue of the stream of the electrolyte L.sub.3 <114> being stirred by the mechanical stirring device <120> in the middle chamber K.sub.M <103>, there is no formation of a typical pH gradient in this chamber. This effect is even stronger when the mechanical stirring device <120> breaks the direct pathway in the middle chamber K.sub.M between inlet Z.sub.KM <108> and connection V.sub.AM <112>, since the mechanical stirring device <120> is then within the flow pathway of the electrolyte L.sub.3 <114> through the middle chamber K.sub.M <103> and disrupts unhindered flow.

[0142] This desired effect can be amplified in the process according to the second aspect of the invention by varying the stirring speed of the mechanical stirring device <120> during the performance of step (b), which can produce further turbulence that disrupts the formation of a pH gradient.

[0143] In a further preferred embodiment, the process according to the second aspect of the invention is performed continuously, i.e. step (a) and step (b) are performed continuously, while applying voltage as per step (c).

[0144] After performance of step (c), solution L.sub.1 <115> is obtained at the outlet A.sub.KK <109>, with the concentration of XOR in L.sub.1 <115> being higher than in L.sub.2 <113>. If L.sub.2 <113> already comprised XOR, the concentration of XOR in L.sub.1 <115> is preferably 1.01 to 2.2 times, more preferably 1.04 to 1.8 times, even more preferably 1.077 to 1.4 times, even more preferably 1.077 to 1.08 times, higher than in L.sub.2 <113>, most preferably 1.077 times higher than in L.sub.2 <113>, where the proportion by mass of XOR in L.sub.1 <115> and in L.sub.2 <113> is more preferably in the range from 10% to 20% by weight, even more preferably 13% to 14% by weight.

[0145] An aqueous solution L.sub.4 <116> of S is obtained at the outlet A.sub.KA <106>, with a lower concentration of S in L.sub.4 <116> than in L.sub.3 <114>.

[0146] The concentration of the cation X in the aqueous solution L.sub.3 <114> is preferably in the range of 3.5 to 5 mol/l, more preferably 4 mol/l. The concentration of the cation X in the aqueous solution L.sub.4 <116> is more preferably 0.5 mol/l lower than that of the aqueous solution L.sub.3 <114> used in each case.

[0147] In particular, the process according to the second aspect of the invention is conducted at a temperature of 20? C. to 70? C., preferably 35? C. to 65? C., more preferably 35? C. to 60? C., even more preferably 35? C. to 50? C., and at a pressure of 0.5 bar to 1.5 bar, preferably 0.9 bar to 1.1 bar, more preferably 1.0 bar.

[0148] In the course of performance of the process according to the invention, hydrogen is typically formed in the cathode chamber K.sub.K <102>, which can be removed from the cell together with solution L.sub.1 <115> via outlet A.sub.KK <109>. The mixture of hydrogen and solution L.sub.1 <115> can then, in a particular embodiment of the present invention, be separated by methods known to the person skilled in the art. When the alkali metal compound used is a halide, especially chloride, it is possible for chlorine or another halogen gas to form in the anode chamber K.sub.A <101>, and this can be removed from the cell together with solution L.sub.4 <116> via outlet A.sub.KK <106>. In addition, it is also possible for oxygen or/and carbon dioxide to form, which can likewise be removed. The mixture of chlorine, oxygen and/or CO.sub.2 and solution L.sub.4 <116> can then, in a particular embodiment of the present invention, be separated by methods known to the person skilled in the art. It is then likewise possible, after the chlorine, oxygen and/or CO.sub.2 gases have been separated from solution L.sub.4 <116>, to separate these by methods known to the person skilled in the art.

[0149] These results were surprising and unexpected in the light of the prior art. The process according to the invention protects the acid-labile solid-state electrolyte from corrosion without, as in the prior art, having to sacrifice alkoxide solution from the cathode space as buffer solution. Thus, the process according to the invention is more efficient than the procedure described in WO 2008/076327 A1, in which the product solution is used for the middle chamber, which reduces the overall conversion. In addition, the acid-labile solid-state electrolyte is stabilized in that the formation of a pH gradient is prevented on account of the mechanical stirring device <120>.

EXAMPLES

Comparative Example 1

[0150] Sodium methoxide (SM) was produced via a cathodic process, with supply of 20% by weight NaCl solution (in water) in the anode chamber and of 10% by weight methanolic SM solution in the cathode chamber. This electrolysis cell consisted of three chambers that corresponded to those shown in FIG. 1, except that the electrolysis cell did not have a mechanical stirring device <120> in the middle chamber, i.e. did not comprise the propeller stirrer <121> shown in FIG. 1 (and hence not the motor <122> and the transmission link <124> either). The connection between middle chamber and anode chamber was established by a hose mounted at the base of the electrolysis cell. The anode chamber and middle chamber were separated by a 2.83 cm.sup.2 anion exchange membrane (Tokuyama AMX, ammonium groups on polymer). Cathode chamber and middle chamber were separated by a ceramic of the NaSICON type with an area of 2.83 cm.sup.2. The ceramic had a chemical composition of the formula Na.sub.3.4Zr.sub.2.0Si.sub.2.4P.sub.0.6O.sub.12.

[0151] The anolyte was transferred through the middle chamber into the anode chamber. The flow rate of the anolyte was 1 l/h, that of the catholyte was 90 ml/h, and a current of 0.14 A was applied. The temperature was 35? C. The electrolysis was conducted for 500 hours at a constant voltage of 5 V. It was observed that a pH gradient developed in the middle chamber over a prolonged period, which is attributable to the migration of the ions to the electrodes in the course of the electrolysis and the spread of the protons formed in further reactions at the anode. This local increase in pH is undesirable since it can attack the solid-state electrolyte and can lead to corrosion and fracture of the solid-state electrolyte specifically in the case of very long periods of operation.

Comparative Example 2

[0152] Comparative Example 1 was repeated with a two-chamber cell comprising just one anode chamber and one cathode chamber, with the anode chamber separated from the cathode chamber by the ceramic of the NaSICON type. This electrolysis cell thus did not contain any middle chamber. This is reflected in even faster corrosion of the ceramic compared to the comparative example 1, which leads to a rapid rise in the voltage curve. With a starting voltage value of <5 V, this rises to >20 V within 100 hours.

Inventive Example 1

[0153] Comparative Example 1 is repeated, with the middle chamber comprising a propeller stirrer <121> aligned parallel to the NASICON solid-state electrolyte. This arrangement interrupts the uniform flow of the electrolyte through the middle chamber, resulting in turbulence in the electrolyte. This makes it difficult for a pH gradient to build up during the electrolysis.

Inventive Example 2

[0154] Comparative Example 1 is repeated, with the middle chamber K.sub.M <103> comprising a cross-shaped magnetic stirrer bar <123-1> which is operated by a magnetic stirrer system <123-2>. This arrangement too interrupts the uniform flow of the electrolyte through the middle chamber, resulting in turbulence. This makes it difficult for a pH gradient to build up during the electrolysis.

Result

[0155] The use of a three-chamber cell according to the invention in the process according to the invention prevents the corrosion of the solid-state electrolyte, and at the same time there is no need to sacrifice alkali metal alkoxide product for the middle chamber and the voltage is kept constant. These advantages that are apparent from the comparison of the two Comparative Examples 1 and 2 underline the surprising effect of the present invention.

[0156] In addition, the alleviation or destruction of the pH gradient that builds up with progressive electrolysis by the vortexing and turbulence in the electrolyte in the middle chamber leads to extension of the lifetime of the electrolysis chamber. This gradient, specifically in the case of very long operating periods, can make the electrolysis even more difficult and lead to corrosion and ultimately fracture of the solid-state electrolyte. In the execution according to Inventive Examples 1 and 2, this pH gradient is destroyed, which, in addition to the advantages mentioned that are provided by a three-chamber cell over a two-chamber cell, further increases the stability of the solid-state electrolyte.