Method for producing alkali metal alcoholates in an electrolysis cell

Abstract

The invention relates to a method for producing an alkali metal alcoholate solution L1 in an electrolysis cell E which comprises at least one cathode chamber KK, at least one anode chamber KA, and at least one central chamber KM lying therebetween. The interior IKK of the cathode chamber KK is separated from the interior IKM of the central chamber KM by a separating wall W comprising at least one alkali-cation-conductive solid ceramic electrolyte (=AFK) F (e.g. NaSICON). F has the surface OF.

Claims

1. A process for producing a solution L.sub.1<21> of an alkali metal alkoxide XOR in an alcohol ROH, in an electrolysis cell E<1>, where X is an alkali metal cation and R is an alkyl radical having 1 to 4 carbon atoms, said process comprising the following steps: (i) providing an alkali metal cation-conducting solid-state electrolyte ceramic F <19> having a surface O.sub.F<190>; (ii) removing a portion of the alkali metal cation-conducting solid-state electrolyte ceramic F<19> by etching the surface O.sub.F<190> with an etchant T<30>, wherein the etchant T<30> is applied only to a limited region of O.sub.F<190>, and not to the entire surface, thereby affording an alkali metal cation-conducting solid-state electrolyte ceramic F<18> with a higher conductivity than F<19> and having a surface O.sub.F<180> which differs from the surface O.sub.F<190> in at least one subregion O.sub.F<183>, wherein the surface O.sub.F<180> comprises the surfaces O.sub.A/MK<181> and O.sub.KK <182> and wherein O.sub.A/MK<181> and/or O.sub.KK <182> comprise at least a portion of O.sub.F<183>; (iii) arranging the alkali metal cation-conducting solid-state electrolyte ceramic F<18> in the electrolysis cell E<1>, wherein said electrolysis cell comprises at least one anode chamber K.sub.A<11>, at least one cathode chamber K.sub.K<12> and at least one interposed middle chamber K.sub.M<13>; wherein the at least one anode chamber K.sub.A<11> comprises: at least one inlet Z.sub.KA <110>; at least one outlet A.sub.KA <111>; and an interior I.sub.KA <112> with an anodic electrode E.sub.A<113>; the at least one cathode chamber K.sub.K<12> comprises: at least one inlet Z.sub.KK <120>; at least one outlet A.sub.KK <121>; and an interior I.sub.KK <122> with a cathodic electrode E.sub.K<123>; and the at least one middle chamber K.sub.M<13> comprises: at least one inlet Z.sub.KM <130>; at least one outlet A.sub.KM <131>; and an interior I.sub.KM <132>; wherein I.sub.KA <112> and I.sub.KM <132> are divided from one another by a diffusion barrier D<14>, and A.sub.KM <131> is connected by a connection V.sub.AM <15> to the inlet Z.sub.KA <110>, such that liquid can be passed from I.sub.KM <132> into I.sub.KA <112> via the connection V.sub.AM <15>; I.sub.KK <122> and I.sub.KM <132> are divided from one another by a dividing wall W<16> comprising the alkali metal cation-conducting solid-state electrolyte ceramic F <18>, wherein F<18> makes direct contact with the interior I.sub.KK <122> via the surface O.sub.KK <182> and with the interior I.sub.KM <132> via the surface O.sub.A/MK<181>, (iv-) and wherein the following steps (1), (2), (3) proceed simultaneously: (1) a solution L.sub.2<22> comprising the alcohol ROH is routed through I.sub.KK <122>, wherein, when O.sub.KK <182> includes at least a portion of O.sub.F<183>, the solution L.sub.2<22> makes direct contact with the subregion O.sub.F<183>; 2) a neutral or alkaline, aqueous solution L.sub.3<23> of a salt S comprising X as cation is routed through I.sub.KM <132>, then via V.sub.AM<15> through I.sub.KA <112>, where, when O.sub.A/MK<181> includes at least a portion of O.sub.F<183>, the solution L.sub.3<23> makes direct contact with the subregion O.sub.F<183>; (3) voltage is applied between E.sub.A<113> and E.sub.K<123>, thereby providing solution L.sub.1<21> at the outlet A.sub.KK <121>, with a higher concentration of XOR than in L.sub.2<22>, and providing aqueous solution L.sub.4<24> of S at the outlet A.sub.KA <111>, with a lower concentration of S than in L.sub.3<23>.

2. The process of claim 1, wherein: S.sub.MF<S.sub.MF; and wherein S.sub.MF is the mass-based specific surface area SM of the alkali metal cation-conducting solid-state electrolyte ceramic F before performance of step (ii) and S.sub.MF is the mass-based specific surface area SM of the alkali metal cation-conducting solid-state electrolyte ceramic F after performance of step (ii).

3. The process of claim 2, wherein the quotient S.sub.MF/S.sub.MF1.01.

4. The process of claim 1, wherein at least 1% of the surface O.sub.A/MK <181> is formed by OFA <183> and/or at least 1% of the surface O.sub.KK <182> is formed by O.sub.F <183>.

5. The process of claim 4, wherein the alkali metal cation-conducting solid-state electrolyte ceramic F<19> has a structure of the formula:
M.sup.I.sub.1+2w+xy+zM.sup.II.sub.wM.sup.III.sub.xZr.sup.IV.sub.2wxyM.sup.V.sub.y(SiO.sub.4).sub.z(PO.sub.4).sub.3z 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; wherein w, x, y, z are real numbers, where 0x<2, 0y<2, 0w<2, 0z<3; and wherein w, x, y, z are chosen such that 1+2w+xy+z 0 and 2wxy0.

6. The process of claim 5, wherein X is selected from the group consisting of Li.sup.+, Na.sup.+, and K.sup.+.

7. The process of claim 6, wherein X=Na.sup.+.

8. The process of claim 6, wherein S is a halide, sulfate, sulfite, nitrate, hydrogencarbonate or carbonate of X.

9. The process of claim 1, wherein the alkali metal cation-conducting solid-state electrolyte ceramic F<19> has a structure of the formula:
M.sup.I.sub.1+2w+xy+zM.sup.II.sub.wM.sup.III.sub.xZr.sup.IV.sub.2wxyM.sup.V.sub.y(SiO.sub.4).sub.z(PO.sub.4).sub.3z 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; wherein w, x, y, z are real numbers, wherein 0x<2,0y<2,0w<2,0z<3; and wherein w, x, y, z are chosen such that 1+2w+xy+z 0 and 2wxy0.

10. The process of claim 1, wherein X is selected from the group consisting of Li.sup.+, Na.sup.+, and K.sup.+.

11. The process of claim 10, wherein X=Na.sup.+.

12. The process of claim 1, wherein S is a halide, sulfate, sulfite, nitrate, hydrogencarbonate or carbonate of X.

13. The process of claim 12, wherein S is a chloride of X.

14. The process of claim 1, wherein R is selected from the group consisting of methyl and ethyl.

15. The process of claim 14, wherein R=methyl.

16. The process of claim 1, wherein connection VAM <15> is formed within the electrolysis cell E<1>.

17. The process of claim 1, wherein the connection V.sub.AM <15> is formed outside the electrolysis cell E<1>.

18. The process of claim 1, wherein O.sub.A/MK<181> and O.sub.KK <182> comprise at least a portion of O.sub.F <183>.

19. The process of claim 18, wherein at least 1% of the surface O.sub.A/MK <181> is formed by O.sub.F <183> and at least 1% of the surface O.sub.KK <182> is formed by OFA <183>.

20. The process of claim 1, wherein the etchant is applied using a nozzle.

Description

2. BRIEF DESCRIPTION OF THE INVENTION

(1) The objects of the invention are achieved by the process discussed herein.

3. FIGURES

(2) The figures show preferred embodiments of the process according to the invention.

(3) 3.1 FIGS. 1A to 1D

(4) FIGS. 1A (=FIG. 1A), 1B (=FIG. 1B), 1C (=FIG. 1C), and 1D (=FIG. 1D) illustrate steps (i) and (ii) of the process according to the invention.

(5) FIG. 1A shows an alkali metal cation-conducting solid-state electrolyte ceramic F <19>, as provided in step (i). This has a surface O.sub.F<190>. The surface O.sub.F<190> comprises, for example, two subregions; in FIG. 1A, these are the opposite areas identified by reference numerals <191> and <192>, of which the rear one <192> is hidden.

(6) The ASC F <19>, as shown in FIG. 1B, is disposed in a process chamber <90> with respect to a nozzle <40> such that the surface subregion <191> lies opposite it, while the surface subregion <192> faces away from it.

(7) FIG. 1C shows the step of contacting with the etchant T <30>, an aqueous acidic HCl solution (pH 5.5). The nozzle <40> applies T <30> to the surface O.sub.F<190>. This removes ASC as particles <185> from F <19>.

(8) This process step affords an ASC F <18>, the surface O.sub.F <180> of which has a part-surface O.sub.A/MK <181> facing the nozzle <40> and a part-surface O.sub.KK <182> facing away from the nozzle <40>. O.sub.A/MK <181> in turn comprises a subregion O.sub.F <183> that has formed by virtue of removal of ASC from F <19> by means of etching.

(9) Optionally, it is then possible to spray on an aqueous alkaline NaOH solution (pH 14) in order to neutralize the acidic aqueous solution and to end the etching operation in a controlled manner, such that no further pieces of ASC <185> are removed from F.

(10) FIG. 1D shows the ASC F <18> thus obtained. This has a surface O.sub.F <180>. The surface O.sub.F<180> comprises the two subregions O.sub.A/MK <181> and O.sub.KK <182>. F <18> corresponds to F <19>, or the surface O.sub.F <180> corresponds to the surface O.sub.F<190>, with the difference that O.sub.A/MK <181> includes the subregion O.sub.F <183>. On account of this subregion O.sub.F <183>, S.sub.MF, which is the mass-based specific surface area S.sub.M of the ASC F <18>, is greater than S.sub.MF, which is the mass-based specific surface area S.sub.M of the ASC F <19>.

(11) 3.2 FIGS. 2A and 2B

(12) FIG. 2A (=FIG. 2A) shows a noninventive embodiment in the form of an electrolysis cell which is a two-chamber cell.

(13) This electrolysis cell comprises a cathode chamber K.sub.K <12> and an anode chamber K.sub.A <11>.

(14) The anode chamber K.sub.A <11> comprises an anodic electrode E.sub.A <113> in the interior I.sub.KA <112>, an inlet Z.sub.KA <110> and an outlet A.sub.KA <111>.

(15) The cathode chamber K.sub.K <12> comprises a cathodic electrode E.sub.K <123> in the interior I.sub.KK <122>, an inlet Z.sub.KK <120> and an outlet A.sub.KK <121>.

(16) This electrolysis cell is bounded by an outer wall W.sub.A <80>.

(17) The interior I.sub.KK <122> is also divided from the interior I.sub.KA <112> by a dividing wall W <16> consisting of a sheet of an NaSICON solid-state electrolyte ceramic F <19> which is selectively permeable to sodium ions and has a surface O.sub.F<190>. The NaSICON solid-state electrolyte ceramic F <19> extends over the entire depth and height of the two-chamber cell. The NaSICON solid-state electrolyte ceramic F <19> makes direct contact with the two interiors I.sub.KK <122> and I.sub.KA<112> via the part-surfaces <192> and <191>, such that sodium ions can be routed from one interior into the other through the NaSICON solid-state electrolyte ceramic F <19>. In FIGS. 2A and 2B, the respective alkali metal cation-conducting solid-state electrolyte ceramic F <19> or F <18> is arranged in the respective electrolysis cell such that the area shaded in FIGS. 1A and 1D faces the user in FIGS. 2A and 2B.

(18) An aqueous solution of sodium chloride L.sub.3 <23> with pH 10.5 is introduced via the inlet Z.sub.KA <110>, counter to the direction of gravity, into the interior I.sub.KA <112>.

(19) A solution of sodium methoxide in methanol L.sub.2<22> is routed into the interior I.sub.KK <122> via the inlet Z.sub.KK <120>.

(20) At the same time, a voltage is applied between the cathodic electrode E.sub.K <123> and the anodic electrode E.sub.A <113>. This results in reduction of methanol in the electrolyte L.sub.2<22> to give methoxide and H.sub.2 in the interior I.sub.KK <122> (CH.sub.3OH+e.sup..fwdarw.CH.sub.3O.sup.+ H.sub.2). At the same time, sodium ions diffuse from the interior I.sub.KA <112> through the NaSICON solid-state electrolyte F <18> into the interior I.sub.KK <122>. Overall, this increases the concentration of sodium methoxide in the interior I.sub.KK <122>, which affords a methanolic solution of sodium methoxide L.sub.1<21> having an elevated sodium methoxide concentration compared to L.sub.2<22>.

(21) In the interior I.sub.KA <112>, the oxidation of chloride ions takes place to give molecular chlorine (CI- Cl.sub.2+e.sup.). In the outlet A.sub.KA <111>, an aqueous solution L.sub.4 <24> is obtained, in which the content of NaCl is reduced compared to L.sub.3 <23>. Chlorine gas (Cl.sub.2) forms in water, according to the reaction Cl.sub.2+H.sub.2O.fwdarw.HOCl+HCl, hypochlorous acid and hydrochloric acid, which give an acidic reaction with further water molecules. The acidity damages the NaSICON solid-state electrolyte ceramic F <19>.

(22) FIG. 2B (=FIG. 2B) shows a further embodiment of a noninventive process. The electrolysis cell corresponds to the arrangement shown in FIG. 2A, with the difference that a NaSICON solid-state electrolyte ceramic F <18> which has the surface O.sub.F <180> and has been pretreated by the method according to FIGS. 1A to 1D is utilized as dividing wall W <16>. F <18> has a part-surface O.sub.KK <182> that makes direct contact with the interior I.sub.KK <122> and a part-surface O.sub.A/MK <181> that makes direct contact with the interior I.sub.KA <112>. The NaSICON solid-state electrolyte F <18> is obtained by the method shown in FIGS. 1A to 1D from the NaSICON solid-state electrolyte F <19> and, by contrast with F <19>, has the subregion O.sub.F <183> that has formed as a result of the treatment in step (ii) on the part-surface O.sub.A/MK <181>. As a result, the NaSICON solid-state electrolyte ceramic F <18> has a greater mass-based specific surface area S.sub.MF than that (S.sub.MF) of the alkali metal cation-conducting solid-state electrolyte ceramic F <19>. This leads to better conductivity, i.e. to a reduction in voltage at the same current density during electrolysis, which leads to an energy-saving.

(23) 3.3 FIGS. 3A and 3B

(24) FIG. 3A (=FIG. 3A) shows one embodiment of the process according to the invention.

(25) It show the process according to the invention in the form of an electrolysis cell E <1> which is a three-chamber cell.

(26) The three-chamber electrolysis cell E <1> comprises a cathode chamber K.sub.K <12>, an anode chamber K.sub.A <11> and an interposed middle chamber K.sub.M <13>.

(27) The anode chamber K.sub.A <11> comprises an anodic electrode E.sub.A <113> in the interior I.sub.KA <112>, an inlet Z.sub.KA <110> and an outlet A.sub.KA <111>.

(28) The cathode chamber K.sub.K <12> comprises a cathodic electrode E.sub.K <123> in the interior I.sub.KK <122>, an inlet Z.sub.KK <120> and an outlet A.sub.KK <121>.

(29) The middle chamber K.sub.M <13> comprises an interior I.sub.KM<132>, an inlet Z.sub.KM<130> and an outlet A.sub.KM<131>. The interior I.sub.KA <112> is connected to the interior I.sub.KM<132> via the connection V.sub.AM<15>.

(30) The electrolysis cell E <1> is bounded by an outer wall W.sub.A <80>.

(31) The interior I.sub.KK <122> is also divided from the interior IKM <132> by a dividing wall W<16> consisting of a sheet of an NaSICON solid-state electrolyte ceramic F<18> which is selectively permeable to sodium ions and has the surface O.sub.F<180>. This surface has a part-surface O.sub.KK <182> that makes direct contact with the interior I.sub.KK <122> and a part-surface O.sub.A/MK<181> that makes direct contact with the interior I.sub.KM <132>. F<18> extends over the entire depth and height of the three-chamber cell E <1>. F<18> is obtained by an etching method corresponding to that shown in FIGS. 1A to 1D from the NaSICON solid-state electrolyte ceramic F<19> and, by contrast with F<19>, has the subregion O.sub.F <183> that has formed as a result of process step (ii) on the part-surface O.sub.A/MK<181>. Compared to the NaSICON solid-state electrolyte ceramic F<18> shown in FIG. 2 B, the subregion O.sub.F <183> obtained by process step (ii) is larger in the NaSICON solid-state electrolyte ceramic F<18> according to FIG. 3A. In addition, in step (ii), not only was the part-surface O.sub.A/MK<181> treated, but also part-surface O.sub.KK <182>, which still further increases the mass-based specific surface area S.sub.MF. This more extended processing is accordingly achieved by etching over a larger area, and in that F<19> is processed on both sides.

(32) As a result, the NaSICON solid-state electrolyte F <18> has an even greater mass-based specific surface area S.sub.MF than that (S.sub.MF) of the alkali metal cation-conducting solid-state electrolyte ceramic F <19>. This leads to a further improvement in conductivity.

(33) The NaSICON solid-state electrolyte ceramic F <18> makes direct contact with the two interiors I.sub.KK<122> and I.sub.KM<132>, such that sodium ions can be routed from one interior into the other through the NaSICON solid-state electrolyte ceramic F <18>.

(34) The interior I.sub.KM<132> of the middle chamber K.sub.M <13> is additionally divided in turn from the interior I.sub.KA <112> of the anode chamber K.sub.A <11> by a diffusion barrier D <14>. The NaSICON solid-state electrolyte ceramic F <18> and the diffusion barrier D <14> extend over the entire depth and height of the three-chamber cell E <1>. The diffusion barrier D <14> is a cation exchange membrane (sulfonated PTFE).

(35) In the embodiment according to FIG. 3A, the connection V.sub.AM <15> is formed outside the electrolysis cell E <1>, especially by a tube or hose, the material of which may be selected from rubber, metal and plastic. The connection V.sub.AM <15> can guide liquid from the interior I.sub.KM <132> of the middle chamber K.sub.M <13> into the interior I.sub.KA <112> of the anode chamber K.sub.A <11> outside the three-chamber cell E <1>. The connection V.sub.AM <15> connects an outlet A.sub.KM<131> that penetrates the outer wall W.sub.A <80> of the electrolysis cell E <1> at the base of the middle chamber K.sub.M <13> to an inlet Z.sub.KA <110> that penetrates the outer wall W.sub.A <80> of the electrolysis cell E at the base of the anode chamber K.sub.A <11>.

(36) An aqueous solution of sodium chloride L.sub.3 <23> with pH 10.5 is introduced via the inlet Z.sub.KM<130>, in the direction of gravity, into the interior I.sub.KM<132> of the middle chamber K.sub.M <13>. The connection V.sub.AM <15> formed between the outlet A.sub.KM <131> from the middle chamber K.sub.M <13> and an inlet Z.sub.KA <110> to the anode chamber K.sub.A <11> connects the interior I.sub.KM<132> of the middle chamber K.sub.M <13> to the interior I.sub.KA <11> of the anode chamber K.sub.A <11>. Sodium chloride solution L.sub.3 <23> is routed through this connection V.sub.AM <15> from the interior I.sub.KM<132> into the interior I.sub.KA<112>.

(37) A solution of sodium methoxide in methanol L.sub.2<22> is routed into the interior I.sub.KK <122> via the inlet Z.sub.KK <120>.

(38) At the same time, a voltage is applied between the cathodic electrode E.sub.K <123> and the anodic electrode E.sub.A <113>. This results in reduction of methanol in the electrolyte L.sub.2<22> to give methoxide and H.sub.2 in the interior I.sub.KK <122> (CH.sub.3OH+e.sup..fwdarw.CH.sub.3O.sup.+ H.sub.2). At the same time, sodium ions diffuse from the interior I.sub.KM <132> of the middle chamber K.sub.M <103> through the NaSICON solid-state electrolyte F <18> into the interior I.sub.KK <122>. Overall, this increases the concentration of sodium methoxide in the interior I.sub.KK <122>, which affords a methanolic solution of sodium methoxide L.sub.1<21> having an elevated sodium methoxide concentration compared to L.sub.2 <22>.

(39) In the interior I.sub.KA <112>, 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 <111>, an aqueous solution L.sub.4 <24> is obtained, in which the content of NaCl is reduced compared to L.sub.3 <23>. Chlorine gas (Cl.sub.2) forms in water, according to the reaction Cl.sub.2+H.sub.2O.fwdarw.HOCl+HCl, hypochlorous acid and hydrochloric acid, which give an acidic reaction with further water molecules. The acidity would damage the NaSICON solid-state electrolyte ceramic F <18>, but is restricted to the anode chamber K.sub.A <11> by the arrangement in the three-chamber cell, and hence kept away from the NaSICON solid-state electrolyte ceramic F <18> in the electrolysis cell E <1>. This considerably increases the lifetime thereof.

(40) FIG. 3B (=FIG. 3B) shows a further embodiment of the process according to the invention. This is conducted in an electrolysis cell E <1> which corresponds to the electrolysis cell E <1> shown in FIG. 3A, with the following difference:

(41) The connection V.sub.AM<15> from the interior I.sub.KM<132> of the middle chamber K.sub.M <13> to the interior I.sub.KA <112> of the anode chamber K.sub.A <11> is formed not outside the electrolysis cell E <1>, but rather inside through a perforation in the diffusion barrier D <14>. This perforation may be introduced into the diffusion barrier D <14> subsequently (for instance by stamping, drilling) or may already have been present therein from the outset on account of the production of the diffusion barrier D <14> (for example in the case of textile fabrics such as filter cloths or metal weaves).

(42) As in the embodiment according to FIG. 3A, not only is the part-surface O.sub.A/MK <181> treated, but also part-surface O.sub.KK <182>, which still further increases the mass-based specific surface area S.sub.MF. However, O.sub.F <183> is smaller than in the embodiment according to FIG. 3A.

4. DETAILED DESCRIPTION OF THE INVENTION

(43) 4.1 Step (i)

(44) In step (i) of the process according to the invention, an alkali metal cation-conducting solid-state electrolyte ceramic (=ASC) F having the surface O.sub.F is provided.

(45) The ASC F provided in step (i) is subjected to step (ii) in the process according to the invention and, after step (ii), the ASC F is obtained with the surface O.sub.F. Since F is essentially obtained from F by removing a portion of ASC from F in step (ii) in order to arrive at F, F and F have essentially the same chemical structure.

(46) A useful alkali metal cation-conducting solid-state electrolyte ceramic F, and especially also F, is any solid-state electrolyte by which cations, especially alkali metal cations, even more preferably sodium cations, can be transported from I.sub.KM into I.sub.KK. 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 ceramic F 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.

(47) In a preferred embodiment, the alkali metal cation-conducting solid-state electrolyte ceramic F, and especially also F, has an NaSICON structure of the formula
M.sup.I.sub.1+2w+xy+zM.sup.II.sub.wM.sup.III.sub.xZr.sup.IV.sub.2wxyM.sup.V.sub.y(SiO.sub.4).sub.z(PO.sub.4).sub.3z.

(48) M.sup.I here is selected from Na.sup.+, Li.sup.+, preferably Na.sup.+.

(49) M.sup.II here 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+.

(50) M.sup.III here 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+.

(51) M.sup.V here is a pentavalent metal cation, preferably selected from V.sup.5+, Nb.sup.5+, Ta.sup.5+.

(52) The Roman indices I, II, III, IV, V indicate the oxidation numbers in which the respective metal cations exist. w, x, y, z are real numbers, where 0x<2, 0y<2, 0w<2, 0z<3, and where w, x, y, z are chosen such that 1+2w+xy+z0 and 2wxy 0.

(53) Even more preferably in accordance with the invention, the NaSICON structure has a structure of the formula Na.sub.(1+v)Zr.sub.2Si.sub.vP.sub.(3v)O.sub.12 where v is a real number for which 0v3. Most preferably, v=2.4.

(54) In a preferred embodiment of the process according to the invention, the alkali metal cation-conducting solid-state electrolyte ceramics F and F have the same structure.

(55) 4.2 Step (ii)

(56) In step (ii) of the process according to the invention, a portion of the alkali metal cation-conducting solid-state electrolyte ceramic F is removed by etching the surface O.sub.F with an etchant T.

(57) The etchant T is preferably an acidic aqueous solution.

(58) This affords an alkali metal cation-conducting solid-state electrolyte ceramic F with the surface O.sub.F, where the surface O.sub.F differs from the surface O.sub.F in at least one subregion O.sub.F, and where the surface O.sub.F comprises the surfaces O.sub.A/MK and O.sub.KK, where O.sub.A/MK and/or O.sub.KK comprise at least a portion of O.sub.F, and where in particular O.sub.A/MK and O.sub.KK comprise at least a portion of O.sub.F.

(59) 4.2.1 Etching of the Surface O.sub.F with an Etchant T

(60) Etching of the surface O.sub.F with an etchant T can be effected by any method familiar to the person skilled in the art.

(61) Etchants T (also referred to as mordant or etching fluid) used may be agents familiar to the person skilled in the art. More particularly, the etchant T used is an acid or an oxidizing liquid.

(62) The etchant T used is more preferably an aqueous acidic solution. These are particularly suitable on account of the ease of handling of such solutions and on account of the acid sensitivity of the ASCs.

(63) In step (ii), the etching operation partly removes ASC from F.

(64) The extent to which ASC is removed from F can be controlled by the person skilled in the art via the selection of etchant T and via the time for which it is allowed to act on F. In the preferred embodiment, in which the etchant T is an acidic aqueous solution, this can be controlled, for example, via the pH of this acidic aqueous solution.

(65) What is meant by acidic aqueous solution is that the pH of this solution is <pH 7.

(66) The pH of the aqueous acidic solution is then preferably <6.9, more preferably <6.5, even more preferably <6.0, even more preferably <5.6, even more preferably <5.0, even more preferably <4.0, even more preferably <2.0, even more preferably <1.0, even more preferably <0.5.

(67) In another preferred embodiment, the pH of the aqueous acidic solution is then in the range of 0.5 to 6.9, more preferably in the range of 0.5 to 6.0, even more preferably in the range of 1.0 to 5.5, even more preferably in the range of 1.0 to 5.0, even more preferably in the range of 1.0 to 4.0, even more preferably in the range of 1.0 to 3.0.

(68) The temperature of the etchant T in step (ii) is preferably 10 C. to 90 C., preferably 30 C. to 70 C., more preferably 40 C. to 60 C.

(69) The aqueous acidic solution is especially a solution of an acid selected from the group of inorganic acids and organic acids, preferably selected from the group of the inorganic acids.

(70) Typical organic acids are formic acid, acetic acid, trifluoroacetic acid.

(71) The acid is preferably selected from the group consisting of H.sub.2SO.sub.4, HNO.sub.3, methanesulfonic acid, hydrohalic acids, especially HCl, HBr. Particular preference is given to HCl, H.sub.2SO.sub.4.

(72) The duration over which the surface O.sub.F is etched with the etchant T, especially the acidic aqueous solution, is especially >1 min, preferably more than 30 min, preferably more than 60 min, even more preferably more than 120 min.

(73) The etching of the surface O.sub.F with the etchant T, especially the acidic aqueous solution, can be conducted by contacting the surface O.sub.F with the etchant T, for example by immersing ASC F completely or partially in the etchant T, or by completely or partly spraying F with the etchant T.

(74) In the etching of the surface O.sub.F with the etchant T in step (ii), it is possible to treat the entire surface O.sub.F. This is possible in that, for example, a portion of the surface O.sub.F is first etched in a first sub-step, especially contacted with T, and ASC is removed in this part of the surface O.sub.F, and then, in a second sub-step, the side of F that has not been contacted in the first sub-step is contacted with T and ASC is removed there. In this embodiment, the entire surface O.sub.F is treated in step (ii), and the surface O.sub.F of the resultant ASC F differs completely from O.sub.F, meaning that O.sub.F and O.sub.F are identical.

(75) Alternatively and preferably, in step (ii), it is also possible to treat just part of the surface O.sub.F. This is possible, for example, in that just a portion of the surface O.sub.F of F is contacted with T, and ASC is removed only in this part of the surface O.sub.F. In addition, a portion of the surface O.sub.F may also be covered with a template, such that just part of the surface O.sub.F of the is removed in step (ii). In this case, an ASC F is obtained, the surface O.sub.F of which is still partly identical to the surface O.sub.F of F and differs from O.sub.F in the portion O.sub.F.

(76) In the embodiment of the present invention in which just a portion of the surface O.sub.F is treated in step (ii) and the surface O.sub.F of the ASC F obtained does not differ completely, but differs only partly, from O.sub.F, the shape of O.sub.F, if O.sub.F is encompassed only at least partly by O.sub.A/MK and/or O.sub.KK, is subject to no further restriction. For example, O.sub.F may form a coherent subregion on the surface O.sub.F. Alternatively, O.sub.F may be formed by multiple unconnected subregions on the surface O.sub.F, such that the shape thereof is reminiscent of the shape of the black spots (which would correspond to O.sub.F) on the white coat (which would correspond to O.sub.F) of the Dalmatian breed of dog.

(77) If the entire surface O.sub.F is treated in step (ii), the surface O.sub.F of the resultant ASC F differs completely from O.sub.F, meaning that O.sub.F and O.sub.F are identical.

(78) It is preferable that the surface O.sub.F of the ASC F is treated in step (ii) such that the area O.sub.F formed by etching in the resulting ASC F comprises at least one of the surface portions O.sub.A/MK and O.sub.KK, and preferably comprises both. It is very particularly preferable that the part O.sub.FA of the surface O.sub.F formed by etching coincides with at least one of the parts O.sub.A/MK, O.sub.KK (O.sub.F=O.sub.A/MK or O.sub.F=O.sub.KK). It is even more preferable that the part O.sub.FA of the surface O.sub.F formed by etching coincides with both parts O.sub.A/MK, O.sub.KK (O.sub.F=O.sub.A/MK+O.sub.KK).

(79) The etching operation in step (ii) of the process according to the invention can be ended when the etchant T is removed from the surface O.sub.F of F, for example by washing it off with water.

(80) In a preferred embodiment of step (ii) of the process according to the invention, the etching of the surface O.sub.F with the etchant T is ended by, after the etching of the surface O.sub.F with the etchant T, contacting the surface O.sub.F of the alkali metal cation-conducting solid-state electrolyte ceramic F with a neutral (pH 7) or alkaline (pH >7) aqueous solution.

(81) Preference is given to using an alkaline aqueous solution that preferably has a pH in the range of 7.1 to 14.0, more preferably in the range of 8.0 to 14.0, even more preferably in the range of 9.0 to 14.0, even more preferably in the range of 10.0 to 14.0, even more preferably in the range of 11.0 to 14.0, even more preferably in the range of 12.0 to 14.0, even more preferably in the range of 13.0 to 14.0, even more preferably in the range of 13.5 to 14.0.

(82) This stops the etching effect of the etchant T on the ASC.

(83) The pH of the aqueous alkaline solution is preferably >7.1, more preferably >7.5, even more preferably >8.0, even more preferably >9.0, even more preferably >10.0, even more preferably >11.0, even more preferably >12.0, even more preferably >13.0, even more preferably >13.5.

(84) The aqueous alkaline solution is especially an aqueous solution of a base selected from the group consisting of inorganic bases, especially alkali metal hydroxides, preferably NaOH, KOH.

(85) The ending of the etching operation in the preferred embodiment of step (ii) can be accomplished by contacting the ASC F with the aqueous alkaline solution, for example by complete or partial immersion of ASC F in the aqueous alkaline solution or by complete or partial spraying of F with the aqueous alkaline solution.

(86) After step (ii), i.e. once a portion of the alkali metal cation-conducting solid-state electrolyte ceramic F has been removed by etching the surface O.sub.F with an etchant T, the etching of the surface O.sub.F with the etchant T preferably having been ended by, after the etching of the surface O.sub.F with the etchant T, contacting the surface O.sub.F of the alkali metal cation-conducting solid-state electrolyte ceramic F with a neutral (pH 7) or alkaline (pH >7), an ASC F with the surface O.sub.F is then obtained. This is then disposed in the electrolysis cell E in step (iii).

(87) In a preferred embodiment of the present invention, step (ii) is preceded by step (iii) and the electrolytic process step (iv-), and thus takes place outside an electrolysis cell E.

(88) In this preferred embodiment, the ASC F is not subjected to step (iii) and (iv-) during the performance of step (ii).

(89) Accordingly, it is preferably not the ASC F that is disposed in an electrolysis cell E and subjected to step (iv-), but rather the ASC F according to step (iii) that has been obtained after step (ii).

(90) 4.2.2 Increase in Mass-Based Specific Surface Area

(91) The surprising advantage of the present invention is that step (ii) affords an ASC F having higher conductivity compared to the ASC F used in step (ii) when used in the electrolysis cell E according to step (iii) and step (iv-).

(92) More particularly, S.sub.MF<S.sub.MF, where S.sub.MF is the mass-based specific surface area S.sub.M of the alkali metal cation-conducting solid-state electrolyte ceramic F before performance of step (ii) and where S.sub.MF is the mass-based specific surface area S.sub.M of the alkali metal cation-conducting solid-state electrolyte ceramic F after performance of step (ii). This means that the mass-based specific surface area S.sub.M of the ASC F subjected to step (ii) is increased during the performance of step (ii) from S.sub.MF to S.sub.MF in the ASC F obtained.

(93) The mass-based specific surface area S.sub.M means the surface area A possessed by a material per unit mass m (S.sub.M=A/m, unit: m.sup.2/kg).

(94) The comparison of the mass-based specific surface areas S.sub.MF and S.sub.MF, i.e. the testing of the condition whether S.sub.MF<S.sub.MF, can be determined by methods known to the person skilled in the art for measuring the BET (Brunauer-Emmett-Teller) surface area, provided that both ASCs F and F are measured under the same conditions. Even if the ratio of the two parameters S.sub.MF and S.sub.MF is determined under different measurement conditions, the ratio S.sub.MF/S.sub.MF measured under particular measurement conditions will essentially be the same as the ratio S.sub.MF/S.sub.MF measured under different measurement conditions.

(95) More particularly, in the context of the present invention, the mass-based specific surface areas are carried out via BET measurements to ISO 9277:2010 with N.sub.2 (purity 99.99% by volume) as adsorbent at 77.35 K.

(96) Instrument: Quantachrome NOVA 2200e, Quantachrome Instruments.

(97) Sample preparation: degassing of the sample at 60 C. at 1 Pa. The evaluation is especially undertaken via the static volumetric method (according to point 6.3.1 of standard ISO 9277:2010).

(98) In respect of the ratio of S.sub.MF and S.sub.MF, i.e. the quotient S.sub.MF/S.sub.MF, the following applies in a preferred embodiment of the present invention:

(99) S.sub.MF/S.sub.MF1.01, preferably S.sub.MF/S.sub.MF1.1, preferably S.sub.MF/S.sub.MF1.5, preferably S.sub.MF/S.sub.MF2.0, preferably S.sub.MF/S.sub.MF3.0, preferably S.sub.MF/S.sub.MF5.0, preferably S.sub.MF/S.sub.MF10, more preferably S.sub.MF/S.sub.MF20, more preferably S.sub.MF/S.sub.MF50, more preferably S.sub.MF/S.sub.MF100, more preferably S.sub.MF/S.sub.MF150, more preferably S.sub.MF/S.sub.MF200, more preferably S.sub.MF/S.sub.MF500, more preferably S.sub.MF/S.sub.MF1000.

(100) In another preferred embodiment of the present invention, the quotient S.sub.MF/S.sub.MF is in the range from 1.01 to 1000, preferably in the range from 1.1 to 800, more preferably in the range from 1.5 to 600, more preferably in the range from 2.0 to 500, more preferably in the range from 3.0 to 400, more preferably in the range from 5.0 to 300, more preferably in the range from 15 to 250, more preferably in the range from 180 to 220.

(101) 4.3 Step (iii)

(102) In step (iii) of the process according to the invention, the ASC F obtained in step (ii) is disposed in an electrolysis cell E.

(103) 4.3.1 Electrolysis Cell E

(104) The electrolysis cell E comprises at least one anode chamber K.sub.A and at least one cathode chamber K.sub.K, and at least one interposed middle chamber K.sub.M. This also includes electrolysis cells E having more than one anode chamber K.sub.A and/or more than one cathode chamber K.sub.K and/or more than one middle chamber K.sub.M. 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.

(105) The electrolysis cell E, in a preferred embodiment, comprises an anode chamber K.sub.A and a cathode chamber K.sub.K, and an interposed middle chamber K.sub.M.

(106) The electrolysis cell E typically has an outer wall W.sub.A. The outer wall W.sub.A is especially made from a material selected from the group consisting of steel, preferably rubberized steel, plastic, especially selected from Telene (thermoset polydicyclopentadiene), PVC (polyvinylchloride), PVC-C (post-chlorinated polyvinylchloride), PVDF (polyvinylidenefluoride). W.sub.A may especially be perforated for inlets and outlets. Within W.sub.A are then the at least one anode chamber K.sub.A, the at least one cathode chamber K.sub.K and, in the embodiments in which the electrolysis cell E comprises one, the at least one interposed middle chamber K.sub.M.

(107) 4.3.1.1 Cathode Chamber K.sub.K

(108) The cathode chamber K.sub.K has at least one inlet Z.sub.KK, at least one outlet A.sub.KK, and an interior I.sub.KK comprising a cathodic electrode E.sub.K.

(109) The interior I.sub.KK of the cathode chamber K.sub.K is divided from the interior I.sub.KM of the middle chamber K.sub.M by the dividing wall W.

(110) The dividing wall W comprises the alkali metal cation-conducting solid-state electrolyte ceramic F, and F makes direct contact with the interior I.sub.KK via the surface O.sub.KK and with the interior I.sub.KM via the surface O.sub.A/MK.

(111) 4.3.1.1.1 Dividing Wall W

(112) The dividing wall W comprises the alkali metal cation-conducting solid-state electrolyte ceramic F. What is meant by the feature dividing wall is that the dividing wall W is liquid-tight. What this means more particularly is that the alkali metal cation-conducting solid-state electrolyte ceramic F encompassed by the dividing wall divides the interior I.sub.KK and the interior I.sub.KM completely from one another, or comprises multiple alkali metal cation-conducting solid-state electrolyte ceramics which join one another, for example in a gapless manner.

(113) There at least exist no gaps in the dividing wall W through which aqueous solution, alcoholic solution, alcohol or water could flow from I.sub.KK into I.sub.KM or vice versa.

(114) What is meant by makes direct contact in respect of the arrangement of the alkali metal cation-conducting solid-state electrolyte ceramics in the dividing wall W and in the electrolysis cell E and in respect of the surfaces O.sub.KK and O.sub.A/MK is that there is a theoretical pathway from I.sub.KK into I.sub.KM that leads completely from I.sub.KK via O.sub.KK through F to O.sub.A/MK and ultimately into I.sub.KM.

(115) It is preferable that at least 1% of the surface O.sub.A/MK is formed by O.sub.F and/or, especially and, at least 1% of the surface O.sub.KK by O.sub.F.

(116) Even more preferably, at least 10% of the surface O.sub.A/MK is formed by O.sub.F and/or, especially and, at least 10% of the surface O.sub.KK by O.sub.F.

(117) Even more preferably, at least 25% of the surface O.sub.A/MK is formed by O.sub.F and/or, especially and, at least 25% of the surface O.sub.KK by O.sub.F.

(118) Even more preferably, at least 40% of the surface O.sub.A/MK is formed by O.sub.F and/or, especially and, at least 40% of the surface O.sub.KK by O.sub.F.

(119) Even more preferably, at least 50% of the surface O.sub.A/MK is formed by O.sub.F and/or, especially and, at least 50% of the surface O.sub.KK by O.sub.F.

(120) Even more preferably, at least 60% of the surface O.sub.A/MK is formed by O.sub.F and/or, especially and, at least 60% of the surface O.sub.KK by O.sub.F.

(121) Even more preferably, at least 70% of the surface O.sub.A/MK is formed by O.sub.F and/or, especially and, at least 70% of the surface O.sub.KK by O.sub.F.

(122) Even more preferably, at least 80% of the surface O.sub.A/MK is formed by O.sub.F and/or, especially and, at least 80% of the surface O.sub.KK by O.sub.F.

(123) Even more preferably, at least 90% of the surface O.sub.A/MK is formed by O.sub.F and/or, especially and, at least 90% of the surface O.sub.KK by O.sub.F.

(124) Even more preferably, 100% of the surface O.sub.A/MK is formed by O.sub.F and/or, especially and, 100% of the surface O.sub.KK by O.sub.F.

(125) This is advantageous since, as a result, the surface of F that has been treated in accordance with the invention and takes part in the process according to the invention [step (iv-)], i.e. through which the ion flow takes place in the electrolysis, is particularly large.

(126) 4.3.1.1.2 Cathodic Electrode E.sub.K

(127) The cathode chamber K.sub.K comprises an interior I.sub.KK which in turn comprises a cathodic electrode E.sub.K. A useful cathodic electrode E.sub.K 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. These are described, in particular, in WO 2014/008410 A1, paragraph [025] or DE 10360758 A1, paragraph [030]. This electrode E.sub.K may be selected from the group consisting of mesh wool, three-dimensional matrix structure and balls. The cathodic electrode E.sub.K 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 comprises nickel.

(128) 4.3.1.1.3 Inlet Z.sub.KK and Outlet A.sub.KK

(129) The cathode chamber K.sub.K also encompasses an inlet Z.sub.KK and an outlet A.sub.KK. This enables addition of liquid, for example the solution L.sub.2, to the interior I.sub.KK of the cathode chamber K.sub.K, and removal of liquid present therein, for example the solution L.sub.1. The inlet Z.sub.KK and the outlet A.sub.KK are attached here to the cathode chamber K.sub.K in such a way that the liquid comes into contact with the cathodic electrode E.sub.K as it flows through the interior I.sub.KK of the cathode chamber K.sub.K. This is a prerequisite for the solution L.sub.1 to be obtained at the outlet A.sub.KK in the performance of the process according to the invention when the solution L.sub.2 of an alkali metal alkoxide XOR in the alcohol ROH is routed through the interior I.sub.KK of the cathode chamber K.sub.K.

(130) The inlet Z.sub.KK and the outlet A.sub.KK may be attached to the electrolysis cell E by methods known to the person skilled in the art, for example by means of holes in the outer wall W.sub.A and corresponding connections (valves) that simplify the introduction and discharge of liquid.

(131) 4.3.1.2 Anode Chamber K.sub.A

(132) The anode chamber K.sub.A has at least one inlet Z.sub.KA, at least one outlet A.sub.KA, and an interior I.sub.KA comprising an anodic electrode E.sub.A.

(133) The interior I.sub.KA of the anode chamber K.sub.A is divided from the interior I.sub.KM of the middle chamber KM by a diffusion barrier D.

(134) 4.3.1.2.1 Anodic Electrode E.sub.A

(135) The anode chamber K.sub.A comprises an interior I.sub.KA which in turn comprises an anodic electrode E.sub.A. A useful anodic electrode E.sub.A 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. These are described, in particular, in WO 2014/008410 A1, paragraph [024] or DE 10360758 A1, paragraph [031]. This electrode E.sub.A 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 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 comprises a titanium anode coated with ruthenium oxide/iridium oxide (RuO.sub.2+IrO.sub.2/Ti).

(136) 4.3.1.2.2 Inlet Z.sub.KA and Outlet A.sub.KA

(137) The anode chamber K.sub.A also encompasses an inlet Z.sub.KA and an outlet A.sub.KA. This enables addition of liquid, for example the solution L.sub.3, to the interior I.sub.KA of the anode chamber K.sub.A, and removal of liquid present therein, for example the solution L.sub.4. The inlet Z.sub.KA and the outlet A.sub.KA are attached here to the anode chamber K.sub.A in such a way that the liquid comes into contact with the anodic electrode E.sub.A as it flows through the interior I.sub.KA of the anode chamber K.sub.A. This is a prerequisite for the solution L.sub.4 to be obtained at the outlet A.sub.KA in the performance of the process according to the invention when the solution L.sub.3 of a salt S is routed through the interior I.sub.KA of the anode chamber K.sub.A.

(138) The inlet Z.sub.KA and the outlet A.sub.KA may be attached to the electrolysis cell E by methods known to the person skilled in the art, for example by means of holes in the outer wall W.sub.A and corresponding connections (valves) that simplify the introduction and discharge of liquid. The inlet Z.sub.KA, in particular embodiments, may also be within the electrolysis cell, for example in the form of a perforation in the diffusion barrier D.

(139) 4.3.1.3 Middle Chamber K.sub.M

(140) The electrolysis cell E has at least one middle chamber K.sub.M. The middle chamber K.sub.M lies between cathode chamber K.sub.K and anode chamber K.sub.A.

(141) The interior I.sub.KA of the anode chamber K.sub.A is divided from the interior I.sub.KM of the middle chamber K.sub.M by a diffusion barrier D. A.sub.KM is then also connected to the inlet Z.sub.KA by a connection V.sub.AM, such that liquid can be guided from I.sub.KM into I.sub.KA through the connection V.sub.AM.

(142) 4.3.1.3.1 Diffusion Barrier D

(143) The interior I.sub.KM of the middle chamber K.sub.M is divided from the interior I.sub.KA of the anode chamber K.sub.A by a diffusion barrier D and divided from the interior I.sub.KK of the cathode chamber K.sub.K by the dividing wall W.

(144) The material used for the diffusion barrier D may be any which is stable under the conditions of the process according to the invention and prevents or slows the transfer of protons from the liquid present in the interior I.sub.KA of the anode chamber K.sub.A to the interior I.sub.KM of the optional middle chamber K.sub.M.

(145) The diffusion barrier D used is especially a non-ion-specific dividing wall or a membrane permeable to specific ions. The diffusion barrier D is preferably a non-ion-specific dividing wall.

(146) 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 a textile fabric or metal weave, especially preferably a textile fabric. The textile fabric preferably comprises plastic, more preferably a plastic selected from PVC, PVC-C, polyvinylether (PVE), polytetrafluoroethylene (PTFE).

(147) If the diffusion barrier D 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.

(148) If the diffusion barrier D is a membrane permeable to specific ions, the diffusion barrier D is especially an anion-conducting membrane or a cation-conducting membrane.

(149) 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.

(150) 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.

(151) 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.

(152) If the diffusion barrier D 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 into the middle chamber K.sub.M.

(153) 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.

(154) The salt S is preferably a halide, sulfate, sulfite, nitrate, hydrogencarbonate or carbonate of X, even more preferably a halide.

(155) Halides are fluorides, chlorides, bromides, iodides. The most preferred halide is chloride.

(156) The anion-conducting membrane used is preferably one selective for halides, preferably chloride.

(157) Anion-conducting membranes are described, for example, by M. A. Hickner, A. M. 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).

(158) 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.sub.+, NRH.sub.2.sub.+, NR.sub.3.sub.+, NR.sup.+; PR.sub.3.sub.+, 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.sub.+, NRH.sub.2.sub.+ and NR.sub.3.sub.+, more preferably selected from NH.sub.3.sub.+ and NR.sub.3.sub.+, even more preferably NR.sub.3.sub.+.

(159) If the diffusion barrier D 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 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.

(160) 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).

(161) 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.sub., COO.sup., PO.sub.3.sup.2 and PO.sub.2H.sup., preferably SO.sub.3.sub. (described in DE 10 2010 062 804 A1, U.S. Pat. No. 4,831,146).

(162) 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.

(163) If a cation-conducting membrane is used as diffusion barrier D, 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.

(164) ##STR00001##
4.3.1.3.2 Inlet Z.sub.KM and Outlet A.sub.KM

(165) The middle chamber K.sub.M also encompasses an inlet Z.sub.KM and an outlet A.sub.KM. This enables addition of liquid, for example the solution L.sub.3, to the interior I.sub.KM of the middle chamber K.sub.M, and transfer of liquid present therein, for example the solution L.sub.3, to the anode chamber K.sub.A.

(166) The inlet Z.sub.KM and the outlet A.sub.KM may be attached to the electrolysis cell E by methods known to the person skilled in the art, for example by means of holes in the outer wall W.sub.A and corresponding connections (valves) that simplify the introduction and discharge of liquid. The outlet A.sub.KM may also be within the electrolysis cell, for example in the form of a perforation in the diffusion barrier D.

(167) 4.3.1.3.3 Connection V.sub.AM

(168) In the electrolysis cell E, the outlet A.sub.KM is connected to the inlet Z.sub.KA by a connection V.sub.AM in such a way that liquid can be guided from I.sub.KM into I.sub.KA through the connection V.sub.AM.

(169) The connection V.sub.AM may be formed within the electrolysis cell E and/or outside the electrolysis cell E, and is preferably formed within the electrolysis cell.

(170) 1) If the connection V.sub.AM is formed within the electrolysis cell E, it is preferably formed by at least one perforation in the diffusion barrier D. This embodiment is preferred especially when the diffusion barrier D used is a non-ion-specific dividing wall, especially a metal weave or textile fabric. This functions as a diffusion barrier D and, on account of the weave properties, has perforations and gaps from the outset that function as connection V.sub.AM.

(171) 2) The embodiment described hereinafter is preferred especially when the diffusion barrier D used is a membrane permeable to specific ions: In this embodiment, the connection V.sub.AM is formed outside the electrolysis cell E, preferably formed by a connection of A.sub.KM and Z.sub.KA that runs outside the electrolysis cell E, especially in that an outlet A.sub.KM is formed from the interior of the middle chamber I.sub.KM, preferably at the base of the middle chamber K.sub.M, the inlet Z.sub.KM more preferably being at the top end O.sub.EM of the middle chamber K.sub.M, and an inlet Z.sub.KA is formed in the interior I.sub.KA of the anode chamber K.sub.A, preferably at the base of the anode chamber K.sub.A, and these are connected by a conduit, for example a pipe or a hose, preferably comprising a material selected from rubber and plastic. This is advantageous especially since the outlet A.sub.KA, according to the invention, is formed at the top end O.sub.EA of the anode chamber K.sub.A.

(172) What is more particularly meant by outlet A.sub.KM at the base of the middle chamber K.sub.M is that the outlet A.sub.KM is attached to the electrolysis cell E in such a way that the solution L.sub.3 leaves the middle chamber K.sub.M in the direction of gravity.

(173) What is more particularly meant by inlet Z.sub.KA at the base of the anode chamber K.sub.A is that the inlet Z.sub.KA is attached to the electrolysis cell E in such a way that the solution L.sub.3 enters the anode chamber K.sub.A counter to gravity.

(174) What is more particularly meant by inlet Z.sub.KM at the top end O.sub.EM of the middle chamber K.sub.M is that the inlet Z.sub.KM is attached to the electrolysis cell E in such a way that the solution L.sub.3 enters the middle chamber K.sub.M in the direction of gravity.

(175) What is more particularly meant by outlet A.sub.KA at the top end O.sub.EA of the anode chamber K.sub.A is that the outlet A.sub.KA is attached to the electrolysis cell E in such a way that the solution L.sub.4 leaves the anode chamber K.sub.A counter to gravity.

(176) This embodiment is particularly advantageous and therefore preferred when the outlet A.sub.KM is formed by the base of the middle chamber K.sub.M, and the inlet Z.sub.KA by the base of the anode chamber K.sub.A. 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 with L.sub.4, in order to then separate them further.

(177) When the connection V.sub.AM is formed outside the electrolysis cell E, Z.sub.KM and A.sub.KM are especially arranged at opposite ends of the outer wall W.sub.A of the middle chamber K.sub.M (i.e., for example, Z.sub.KM at the base and A.sub.KM at the top end O.sub.EM of the electrolysis cell E or vice versa) and Z.sub.KA and A.sub.KA are arranged at opposite ends of the outer wall W.sub.A of the anode chamber K.sub.A (i.e. Z.sub.KA at the base and A.sub.KA at the top end O.sub.EA of the electrolysis cell E), as shown more particularly in FIG. 3A. By virtue of this geometry, L.sub.3 must flow through the two chambers K.sub.M and K.sub.A. It is possible here for Z.sub.KA and Z.sub.KM to be formed on the same side of the electrolysis cell E, in which case A.sub.KM and A.sub.KA are automatically also formed on the same side of the electrolysis cell E. Alternatively, Z.sub.KA and Z.sub.KM may be formed on opposite sides of the electrolysis cell E, in which case A.sub.KM and A.sub.KA are then automatically also formed on opposite sides of the electrolysis cell E.

(178) 3) When the connection V.sub.AM is formed within the electrolysis cell E, this may especially be implemented in that one side (side A) of the electrolysis cell E, which is the top end O.sub.E, comprises the inlet Z.sub.KM and the outlet A.sub.KA, and the diffusion barrier D extends proceeding from this side (side A) into the electrolysis cell E, but does not quite reach up to the opposite side (side B) of the electrolysis cell E from side A, which is then the base of the electrolysis cell E, and at the same time covers 50% or more of the height of the three-chamber cell E, preferably 60% to 99% of the height of the three-chamber cell E, more preferably 70% to 95% of the height of the three-chamber cell E, even more preferably 80% to 90% of the height of the three-chamber cell E, more preferably still 85% of the height of the three-chamber cell E. Because the diffusion barrier D does not touch side B of the three-chamber cell E, a gap thus arises between diffusion barrier D and the vessel B.sub.E on side B of the three-chamber cell E. In that case, the gap is the connection V.sub.AM. By virtue of this geometry, L.sub.3 must flow completely through the two chambers K.sub.M and K.sub.A.

(179) These embodiments best assure that the aqueous salt solution L.sub.3 flows past the acid-sensitive solid-state electrolyte before it comes into contact with the anodic electrode E.sub.A, which results in the formation of acids.

(180) According to the invention, base of the electrolysis cell E is the side of the electrolysis cell E through which a solution (e.g. L.sub.3 in the case of A.sub.KM in FIG. 3A) 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 in the case of Z.sub.KK in FIGS. 2A, 3A and 3B and L.sub.3 in the case of A.sub.KA in FIGS. 2A, 2B and 3A) is supplied to the electrolysis cell E counter to gravity.

(181) According to the invention, top end O.sub.E of the electrolysis cell E is the end of the electrolysis cell E through which a solution (e.g. L.sub.4 in the case of A.sub.KA and L.sub.1 in the case of A.sub.KK in all figures) exits from the electrolysis cell E counter to gravity, or the end of the electrolysis cell E through which a solution (e.g. L.sub.3 in the case of Z.sub.KM in FIGS. 3A and 3B) is supplied to the electrolysis cell E in the same direction as gravity.

(182) 4.3.1.4 Arrangement of the Dividing Wall Win the Electrolysis Cell E

(183) The dividing wall W is arranged in the electrolysis cell E such that the alkali metal cation-conducting solid-state electrolyte ceramic F encompassed by the dividing wall W makes direct contact with the interior I.sub.KK via the surface O.sub.KK.

(184) This means that the dividing wall W is arranged in the electrolysis cell E such that, when the interior I.sub.KK is completely filled with solution L.sub.2, the solution L.sub.2 contacts (i.e. wets) the surface O.sub.KK of the alkali metal cation-conducting solid-state electrolyte ceramic F encompassed by the dividing wall W, such that ions (e.g. alkali metal ions such as sodium, lithium) can enter the solution L.sub.2 from the alkali metal cation-conducting solid-state electrolyte ceramic F encompassed by the dividing wall W via the surface O.sub.KK.

(185) In addition, the dividing wall W is arranged in the electrolysis cell E such that the alkali metal cation-conducting solid-state electrolyte ceramic F encompassed by the dividing wall W makes direct contact with the interior I.sub.KM via the surface O.sub.A/MK.

(186) What this means is as follows: the electrolysis cell E comprises at least one middle chamber K.sub.M, and the dividing wall W adjoins the interior I.sub.KM of the middle chamber K.sub.M.

(187) The dividing wall W is arranged in the electrolysis cell E such that, when the interior I.sub.KM is completely filled with solution L.sub.3, the solution L.sub.3 contacts (i.e. wets) the surface O.sub.A/MK of the alkali metal cation-conducting solid-state electrolyte ceramic F encompassed by the dividing wall W, such that ions (e.g. alkali metal ions such as sodium, lithium) from the solution L.sub.3 can enter the alkali metal cation-conducting solid-state electrolyte ceramic F encompassed by the dividing wall W via the surface O.sub.A/MK.

(188) 4.4 Process Steps According to the Invention

(189) The present invention relates to a process for producing a solution L.sub.1 of an alkali metal alkoxide XOR in the alcohol ROH, where X is an alkali metal cation and R is an alkyl radical having 1 to 4 carbon atoms. The process is conducted in an electrolysis cell E.

(190) 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.+.

(191) 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.

(192) The steps (1), (2). (3) that proceed simultaneously are conducted.

(193) 4.4.1 Step (1)

(194) In step (1), a solution L.sub.2 comprising the alcohol ROH, preferably comprising an alkali metal alkoxide XOR and alcohol ROH, is routed through I.sub.KK. In the cases in which O.sub.KK encompasses at least a portion of O.sub.FA, the solution L.sub.2 makes direct contact here with the surface O.sub.FA. More particularly, the solution L.sub.2 then makes direct contact with the entire surface O.sub.FA encompassed by O.sub.KK.

(195) This means that, in the cases in which O.sub.KK encompasses the entire surface O.sub.FA, L.sub.2 makes direct contact with at least part of the surface O.sub.FA, and, in the cases in which O.sub.KK encompasses only part of the surface O.sub.FA, L.sub.2 makes direct contact with at least part of the surface O.sub.FA encompassed by O.sub.KK.

(196) This preferably means that, in the cases in which O.sub.KK encompasses the entire surface O.sub.FA, L.sub.2 makes direct contact with the whole surface O.sub.FA, and, in the cases in which O.sub.KK encompasses only part of the surface O.sub.FA, L.sub.2 makes direct contact with the whole part of the surface O.sub.FA encompassed by O.sub.KK.

(197) Solution L.sub.2 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 based on the weight of the alcohol ROH in solution L.sub.2 (mass ratio) is 1:10, more preferably <1:20, even more preferably <1:100, even more preferably <0.5:100.

(198) If solution L.sub.2 comprises XOR, the proportion by mass of XOR in solution L.sub.2, based on the overall solution L.sub.2, 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.

(199) If solution L.sub.2 comprises XOR, the mass ratio of XOR to alcohol ROH in solution L.sub.2 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.

(200) 4.4.2 Step (2)

(201) In step (2), a neutral or alkaline aqueous solution L.sub.3 of a salt S comprising X as cation is routed through I.sub.KM, then via V.sub.AM, then through I.sub.KA. In the cases in which O.sub.A/MK encompasses at least a portion of O.sub.FA, the solution L.sub.3 makes direct contact here with the surface O.sub.FA. More particularly, the solution L.sub.3 then makes direct contact with the entire surface O.sub.FA encompassed by O.sub.A/MK.

(202) This means that, in the cases in which O.sub.A/MK encompasses the entire surface O.sub.FA, L.sub.3 makes direct contact with at least part of the surface O.sub.FA, and, in the cases in which O.sub.A/MK encompasses only part of the surface O.sub.FA, L.sub.3 makes direct contact with at least part of the surface O.sub.FA encompassed by O.sub.A/MK.

(203) This preferably means that, in the cases in which O.sub.A/MK encompasses the entire surface O.sub.FA, L.sub.3 makes direct contact with the whole surface O.sub.FA, and, in the cases in which O.sub.A/MK encompasses only part of the surface O.sub.FA, L.sub.3 makes direct contact with the whole part of the surface O.sub.FA encompassed by O.sub.A/MK.

(204) The salt S is preferably a halide, sulfate, sulfite, nitrate, hydrogencarbonate or carbonate of X, even more preferably a halide.

(205) Halides are fluorides, chlorides, bromides, iodides. The most preferred halide is chloride.

(206) The pH of the aqueous solution L.sub.3 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.

(207) The proportion by mass of salt S in solution L.sub.3 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.

(208) 4.4.3 Step (3)

(209) In step (3), a voltage is then applied between E.sub.A and E.sub.K.

(210) 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.

(211) This in turn has the following consequences:

(212) the solution L.sub.1 is obtained at the outlet A.sub.KK with a higher concentration of XOR in L.sub.1 than in L.sub.2, an aqueous solution L.sub.4 of S is obtained at the outlet A.sub.KA, with a lower concentration of S in L.sub.4 than in L.sub.3.

(213) In step (3) of the process, 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 interior I.sub.KM) 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 interior I.sub.KM of the middle chamber K.sub.M 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.

(214) It is preferable that both O.sub.A/MK and O.sub.KK encompass part of the surface O.sub.FA. Step (3) of the process is more preferably performed when the interior I.sub.KM is laden with L.sub.3 and the interior I.sub.KK with L.sub.2 at least to such an extent that L.sub.3 and L.sub.2 make direct contact with the surface O.sub.FA of the alkali metal cation-conducting solid-state electrolyte ceramic F encompassed by the dividing wall W. This is preferable since, as a result, the surface O.sub.FA obtained as a result of the treatment in step (ii) within the two surfaces O.sub.A/MK and O.sub.KK takes part in the electrolysis process, and the advantageous properties of the alkali metal cation-conducting solid-state electrolyte ceramic F, which are manifested in an increase in conductivity and hence an increase in current density at the same voltage, have a particularly positive effect.

(215) The fact that transfer of charge takes place between E.sub.A and E.sub.K in step (3) implies that I.sub.KK, I.sub.KM and I.sub.KA are simultaneously laden with L.sub.2 and L.sub.3 respectively such that they cover the electrodes E.sub.A and E.sub.K to such an extent that the circuit is complete.

(216) This is the case especially when a liquid stream of L.sub.3 is routed continuously through I.sub.KM, V.sub.AM and I.sub.KA and a liquid stream of L.sub.2 through I.sub.KK, and the liquid stream of L.sub.3 covers electrode E.sub.A and the liquid stream of L.sub.2 covers electrode E.sub.K at least partly, preferably completely.

(217) In a further preferred embodiment, the process is performed continuously, i.e. step (1) and step (2) are performed continuously, while applying voltage as per step (3).

(218) After performance of step (3), solution L.sub.1 is obtained at the outlet A.sub.KK, wherein the concentration of XOR in L.sub.1 is higher than in L.sub.2. If L.sub.2 already comprised XOR, the concentration of XOR in L.sub.1 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, most preferably 1.077 times higher than in L.sub.2, where the proportion by mass of XOR in L.sub.1 and in L.sub.2 is more preferably in the range from 10% to 20% by weight, even more preferably 13% to 14% by weight.

(219) An aqueous solution L.sub.4 of S is obtained at the outlet A.sub.KA, with a lower concentration of S in L.sub.4 than in L.sub.3.

(220) The concentration of the cation X in the aqueous solution L.sub.3 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 is more preferably 0.5 mol/1 lower than that of the aqueous solution L.sub.3 used in each case.

(221) More particularly, steps (1) to (3) of the process are 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.

(222) In the course of performance of steps (1) to (3) of the process, hydrogen is typically formed in the interior I.sub.KK of cathode chamber K.sub.K, which can be removed from the cell together with solution L.sub.1 via the outlet A.sub.KK. The mixture of hydrogen and solution L.sub.1 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 interior I.sub.KA of the anode chamber K.sub.A, and this can be removed from the cell together with solution L.sub.4 via outlet A.sub.KK. 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 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, to separate these from one another by methods known to the person skilled in the art.

(223) 4.4.4 Additional Advantages of Steps (1) to (3)

(224) This performance of steps (1) to (3) brings further surprising advantages that were not to be expected in the light of the prior art. Steps (1) to (3) of the process according to the invention protect 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, these process steps are 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.

5. EXAMPLES

(225) 5.1 Comparative Example 1

(226) 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, as shown in FIG. 3A.

(227) 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 33 cm.sup.2 cation exchange membrane (Asahi Kasei, sulfonic acid groups on polymer). Cathode chamber and middle chamber were separated by a ceramic of the NaSICON type with an area of 33 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.

(228) The NaSICON ceramic used in Comparative Example 1, before being disposed within the cell, was cut with a further ceramic of equal dimensions from the same block.

(229) The anolyte was transferred through the middle chamber into the anode chamber. The flow rate of the anolyte was 1 l/h, and that of the catholyte 1 l/h. The temperature was 60 C.

(230) The voltage was varied within a range from 0 to 8 V while recording the current.

(231) It was also 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.

(232) 5.2 Comparative Example 2

(233) Comparative Example 1 is 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. The arrangement corresponds to that shown in FIG. 2A. This electrolysis cell thus does not contain any middle chamber.

(234) This is reflected in even faster corrosion of the ceramic compared to Comparative Example 1, which leads to a more rapid rise in the voltage curve when the electrolysis is to be conducted at constant current over a defined period of time.

(235) 5.3 Inventive Example 1

(236) Comparative Example 1 is repeated.

(237) For this purpose, the other NaSICON ceramic that was cut from the same block as that used in Comparative Example 1 is subjected to an etching process as follows:

(238) The ceramic is first placed in 2 M H.sub.2SO.sub.4 at 60 C. for 2 hours. In the course of this, the solution is mixed gently with the aid of a stirrer. Subsequently, the sheet is rinsed with water and introduced into an NaOH bath (20% by weight).

(239) Alternatively, the ceramic can also be installed directly into the electrolysis cell, which is then operated with NaOH as anolyte for 4 h.

(240) This increases the mass-based specific surface area of the NaSICON ceramic by a factor of 200.

(241) Thereafter, the electrolysis process according to Comparative Example 1 is repeated, with the surfaces that form on the ceramic in contact with the interiors of the cathode chamber and middle chamber.

(242) It is found that a significantly lower voltage has to be applied over the duration of the electrolysis at the same current as in Comparative Example 1.

(243) 5.4 Comparative Example 3

(244) Comparative Example 2 is repeated using an electrolysis cell in which the solid-state electrolyte ceramic treated according to Inventive Example 1 is used. Here too, a significant reduction in voltage compared to Comparative Example 2 is necessary at the same current.

(245) 5.5 Result

(246) The etching of the NaSICON ceramic and the exposure of the surfaces that result from the treatment in the interiors of the anode chamber or middle chamber and the cathode chamber surprisingly increases the conductivity of the solid-state electrolyte.

(247) The use of the three-chamber cell in the process according to the invention also 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 already apparent from the comparison of the two Comparative Examples 1 and 2 underline the surprising effect of the use of the electrolysis cell comprising at least one middle chamber in the process according to the invention.

(248) TABLE-US-00001 Reference symbols Component Abbreviation in figures Electrolysis cell E <1> Anode chamber K.sub.A <11> Inlet Z.sub.KA <110> Outlet A.sub.KA <111> Interior I.sub.KA <112> Anodic electrode E.sub.A <113> Cathode chamber K.sub.K <12> Inlet Z.sub.KK <120> Outlet A.sub.KK <121> Interior I.sub.KK <122> Cathodic electrode E.sub.K <123> Middle chamber K.sub.M <13> Inlet Z.sub.KM <130> Outlet A.sub.KM <131> Interior I.sub.KM <132> Diffusion barrier D <14> Connection V.sub.AM <15> Dividing wall W <16> Solid-state electrolyte ceramic after step (ii) F <18> Dust particles of the solid-state electrolyte <185> ceramic F Surface of F O.sub.F <180> Subregion of O.sub.F which, in the electrolysis O.sub.A/MK <181> cell E, makes direct contact with the interior I.sub.KM of the middle chamber K.sub.M Subregion of O.sub.F which, in the electrolysis O.sub.KK <182> cell E, makes direct contact with the interior I.sub.KK of the cathode chamber K.sub.K Subregion of O.sub.F which forms as a result of O.sub.F <183> etching in step (ii) of the process according to the invention; this subregion distinguishes O.sub.F from O.sub.F Solid-state electrolyte ceramic before step (ii) F <19> Surface of F O.sub.F <190> Subregion of O.sub.F <191> Subregion of O.sub.F <192> Alkali metal alkoxide XOR in the alcohol ROH L.sub.1 <21> Solution comprising the alcohol ROH L.sub.2 <22> Neutral or alkaline, aqueous solution of a salt L.sub.3 <23> S comprising X as cation Aqueous solution of S, where [S].sub.L4 < [S].sub.L3. L.sub.4 <24> Etchant T <30> Nozzle <40> Outer wall W.sub.A <80> Process chamber for etching treatment <90>