FRACTURE-RESISTANT PARTITION COMPRISING SOLID ELECTROLYTE CERAMICS FOR ELECTROLYTIC CELLS

Abstract

The present invention relates, in a first aspect, to an electrolysis cell E comprising a dividing wall W suitable for use in an electrolysis cell E. The dividing wall W encompasses at least two alkali metal cation-conducting solid-state electrolyte ceramics FA and FB separated from one another by at least one separating element T. Compared to the cases according to the prior art in which the dividing wall W encompasses the solid-state electrolyte in one piece, this arrangement is more flexible and the individual ceramics have more degrees of freedom available in order to react to fluctuations in temperature, for example by shrinkage or expansion. This increases stability with respect to mechanical stresses in the ceramic. The electrolysis cell E encompasses a cathode chamber KK divided by the dividing wall W from the adjacent chamber, which is a middle chamber KM of the electrolysis cell E. In a second aspect, the present invention relates to a process for producing an alkali metal alkoxide solution in the electrolysis cell E according to the first aspect of the invention.

Description

The present invention relates, in a first aspect, to an electrolysis cell E comprising a dividing wall W suitable for use in an electrolysis cell E. The dividing wall W encompasses at least two alkali metal cation-conducting solid-state ceramics F_A and F_B separated from one another by at least one separating element T. Compared to the cases according to the prior art in which the dividing wall W encompasses the solid-state electrolyte in one piece, this arrangement is more flexible and the individual ceramics have more degrees of freedom available in order to react to fluctuations in temperature, for example by shrinkage or expansion. This increases stability with respect to mechanical stresses in the ceramic.

The electrolysis cell E encompasses a cathode chamber K_K divided by the dividing wall W from the adjacent chamber, which is a middle chamber K_M of the electrolysis cell E.

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

1. BACKGROUND OF THE INVENTION

The electrochemical production of alkali metal alkoxide solutions is an important industrial process which is described, for example, in DE 103 60 758 A1, US 2006/0226022 A1 and WO 2005/059205 A1. The principle of these processes is reflected in an electrolysis cell in which the solution of an alkali metal salt, for example sodium chloride or NaOH, is present in the anode chamber, and the alcohol in question or an alcoholic solution with a low concentration of the alkali metal alkoxide in question, for example sodium methoxide or sodium ethoxide, is present in the cathode chamber. The cathode chamber and the anode chamber are separated by a ceramic that conducts the alkali metal ion used, for example NaSICON or an analogue for potassium or lithium. On application of a current, chlorine forms at the anode—when a chloride salt of the alkali metal is used—and hydrogen and alkoxide ions at the cathode. The charge is balanced in that alkali metal ions migrate from the middle chamber into the cathode chamber via the ceramic that is selective therefor. The balancing of charge between middle chamber and anode chamber results from the migration of cations when cation exchange membranes are used or the migration of anions when anion exchange membranes are used, or from migration of both ion types when non-specific diffusion barriers are used. This increases the concentration of the alkali metal alkoxide in the cathode chamber, and the concentration of the sodium ions in the anolyte is lowered.

NaSICON solid-state electrolytes are also used in the electrochemical production of other compounds:

WO 2014/008410 A1 describes an electrolytic process for producing elemental titanium or rare earths. The basis of this process is that titanium chloride is formed from TiO₂ and the corresponding acid, and this is reacted with sodium alkoxide to give titanium alkoxide and NaCl and finally converted electrolytically to elemental titanium and sodium alkoxide.

WO 2007/082092 A2 and WO 2009/059315 A1 describe processes for producing biodiesel, in which, with the aid of alkoxides produced electrolytically by means of NaSICON, triglycerides are first converted to the corresponding alkali metal triglycerides and are reacted in a second step with electrolytically generated protons to give glycerol and the respective alkali metal hydroxide.

However, these solid-state electrolyte ceramics typically have some disadvantages. In the operation of the electrolysis cell, there will inevitably be fluctuations in temperature in the cell, which result in expansion or shrinkage of the solid-state electrolyte ceramic. Since these ceramics are brittle, this can lead to fracture of the ceramic.

This problem arises particularly in the constantly repeating startup and shutdown processes that are unavoidable in the operation of electrolysis. During the heating and cooling, there are expansion and shrinkage phases, which results in movement of the ceramic back and forth in the electrolysis cell. These movements, on account of the uncontrolled distribution of forces in the ceramic, can cause it to break.

This can result in losses of integrity that can lead to leakage of brine into alcohol or vice versa. As a result, the product of the electrolysis—the alkoxide solution—is watered down. In addition, the electrolysis cell itself can also lose integrity and leak.

It was therefore the object of the present invention to provide an electrolysis cell that does not have these disadvantages.

A further disadvantage of conventional electrolysis cells in this technical field arises from the fact that the solid-state electrolyte does not have long-term stability with respect to aqueous acids. This is problematic in that, during the electrolysis in the anode chamber, the pH falls as a result of oxidation processes (for example in the case of production of halogens by disproportionation or by oxygen formation). These acidic conditions attack the NaSICON solid-state electrolyte to such a degree that the process cannot be used on an industrial scale. In order to counter this problem, various approaches have been described in the prior art.

For instance, three-chamber cells have been proposed in the prior art. These are known in the field of electrodialysis, for example U.S. Pat. No. 6,221,225 B1.

WO 2012/048032 A2 and US 2010/0044242 A1 describe, for example, electrochemical processes for producing sodium hypochlorite and similar chlorine compounds in such a three-chamber cell. The cathode chamber and the middle chamber of the cell are separated here by a solid-state electrolyte permeable to cations, for example NaSICON. In order to protect this from the acidic anolyte, the middle chamber is supplied, for example, with solution from the cathode chamber. US 2010/0044242 A1 also describes, in FIG. 6, the possibility of mixing solution from the middle chamber with solution from the anode chamber outside the chamber in order to obtain sodium hypochlorite.

Such cells have also been proposed in the prior art for the production or purification of alkali metal alkoxides.

For instance, U.S. Pat. No. 5,389,211 A describes a process for purifying alkoxide solutions in which a three-chamber cell is used, in which the chambers are delimited from one another by cation-selective solid-state electrolytes or else nonionic dividing walls. The middle chamber is used as buffer chamber in order to prevent the purified alkoxide or hydroxide solution from the cathode chamber from mixing with the contaminated solution from the anode chamber.

DE 42 33 191 A1 describes the electrolytic recovery of alkoxides from salts and alkoxides in multichamber cells and stacks of multiple cells.

WO 2008/076327 A1 describes a process for producing alkali metal alkoxides. This uses a three-chamber cell, the middle chamber of which has been filled with alkali metal alkoxide (see, for example, paragraphs and of WO 2008/076327 A1). This protects the solid-state electrolyte separating the middle chamber and the cathode chamber from the solution present in the anode chamber, which becomes more acidic in the course of electrolysis. A similar arrangement is described by WO 2009/073062 A1. However, this arrangement has the disadvantage that the alkali metal alkoxide solution which is consumed as buffer solution and continuously contaminated is the desired product. A further disadvantage of the process described in WO 2008/076327 A1 is that the formation of the alkoxide in the cathode chamber depends on the diffusion rate of the alkali metal ions through two membranes or solid-state electrolytes. This in turn leads to slowing of the formation of the alkoxide.

A further problem results from the geometry of the three-chamber cell. The middle chamber in such a chamber is separated from the anode chamber by a diffusion barrier and from the cathode chamber by an ion-conducting ceramic. During the electrolysis, this results unavoidably in development of pH gradients and in dead volumes. This can damage the ion-conducting ceramic and, as a result, increase the voltage demand of the electrolysis and/or lead to fracture of the ceramic.

While this effect takes place throughout the electrolysis chamber, the drop in pH is particularly critical in the middle chamber since this is bounded by the ion-conducting ceramic. Gases are typically formed at the anode and the cathode, such that there is at least some degree of mixing in these chambers. By contrast, no such mixing takes place in the middle chamber, such that the pH gradient develops therein. This unwanted effect is enhanced by the fact that the brine is generally pumped relatively slowly through the electrolysis cell.

It was therefore a further object of the present invention to provide an improved process for electrolytic production of alkali metal alkoxides, and also an electrolysis cell especially suitable for such a process. These are not to have the aforementioned disadvantages, and are especially to assure improved protection of the solid-state electrolyte prior to the formation of the pH gradient and more sparing use of the reactants compared to the prior art.

2. BRIEF DESCRIPTION OF THE INVENTION

The problem addressed by the invention is solved by an electrolysis cell E <1> according to the first aspect of the invention, comprising a dividing wall W. The dividing wall W <16> comprises one side S_KK <161> having the surface O_KK <163> and, opposite the side S_KK <161>, a side SA/MK <162> having the surface O_A/MK <164>. Said dividing wall also encompasses at least two alkali metal cation-conducting solid-state electrolyte ceramics F_A <18> and F_B <19> separated from one another by at least one separating element T <17>. The alkali metal cation-conducting solid-state electrolyte ceramics encompassed by the dividing wall W <16>, and especially also the separating element T <17>, are directly contactable here both via the surface O_KK <163> and via the surface O_A/MK <164>.

In a first aspect, the present invention accordingly relates to an electrolysis cell E <1> comprising

• • at least one anode chamber K_A <11> having at least one inlet Z_KA <110>, at least one outlet A_KA <111>, and an interior I_KA <112> comprising an anodic electrode E_A <113>,
  • at least one cathode chamber K_K <12> having at least one inlet Z_KK <120>, at least one outlet A_KK <121>, and an interior I_KK <122> comprising a cathodic electrode E_K <123>,
  • and at least one interposed middle chamber K_M <13> having at least one inlet Z_KM <130>, at least one outlet A_KM <131> and an interior I_KM <132>,
  • where I_KA <112> and I_KM <132> are then divided from one another by a diffusion barrier D <14>, and A_KM <131> is connected by a connection V_AM <15> to the inlet Z_KA <110>, such that liquid can be passed from I_KM <132> into I_KA <112> via the connection V_AM <15>,
    wherein
  • I_KK <122> and I_KM <132> are divided from one another by the dividing wall W <16>,
    characterized in that
    the alkali metal cation-conducting solid-state ceramics encompassed by the dividing wall W <16>, and especially also the separating element T <17>, directly contact the interior I_KK <122> on the S_KK side <161> via the surface O_KK <163>,
    and
    the alkali metal cation-conducting solid-state ceramics encompassed by the dividing wall W <16>, and especially also the separating element T <17>, directly contact the interior I_KM <132> on the side S_A/MK <162> via the surface O_A/MK <164>.

In a second aspect, the present invention relates to a process for producing a solution L₁ 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,

• • wherein the following steps (β1), (β2), (β3) that proceed simultaneously are conducted in an electrolysis cell E according to the first aspect of the invention:
    • (β1) a solution L₂ comprising the alcohol ROH is routed through K_K,
    • (β2) a neutral or alkaline, aqueous solution L₃ of a salt S comprising X as cation is routed through K_M, then via V_AM, then through K_A,
    • (β3) voltage is applied between E_A and E_K,
      which affords the solution L₁ at the outlet A_KK with a higher concentration of XOR in L₁ than in L₂,
      and which affords an aqueous solution L₄ of S at the outlet A_KA, with a lower concentration of S in L₄ than in L₃.

3. FIGS

3.1 FIGS. 1A and 1 B

FIG. 1A (=“FIG. 1 A”) shows a non-inventive electrolysis cell E. This comprises a cathode chamber K_K <12> and an anode chamber K_A <11>.

The cathode chamber K_K <12> comprises a cathodic electrode E_K <123> in the interior I_KK <122>, an inlet Z_KK <120> and an outlet A_KK <121>.

The anode chamber K_A <11> comprises an anodic electrode E_A <113> in the interior I_KA <112>, an inlet Z_KK <110> and an outlet A_KA <111>.

The two chambers are bounded by an outer wall <80> of the two-chamber cell E. The interior I_KK <122> is also divided from the interior I_KA <112> by a dividing wall consisting of a sheet of a NaSICON solid-state electrolyte F_A <18> which is selectively permeable to sodium ions. The NaSICON solid-state electrolyte F_A <18> extends over the entire depth and height of the two-chamber cell E. The dividing wall has two sides S_KK <161> and S_A/MK <162>, the surfaces OKK <163> and O_A/MK <164> of which contact the respective interior I_KK <122> or I_KA <112>.

An aqueous solution of sodium chloride L₃ <23> with pH 10.5 is introduced via the inlet Z_KA <110>, counter to the direction of gravity, into the interior I_KA <112>.

A solution of sodium methoxide in methanol L₂ <22> is routed into the interior I_KK <122> via the inlet Z_KK <120>.

At the same time, a voltage is applied between the cathodic electrode E_K <123> and the anodic electrode E_A <113>. This results in reduction of methanol in the electrolyte L₂ <22> to give methoxide and H₂ in the interior I_KK <122> (CH₃OH+e⁻→CH₃O⁻+½ H₂). At the same time, sodium ions diffuse from the interior I_KA <112> through the NaSICON solid-state electrolyte F_K <18> into the interior I_KK <122>. Overall, this increases the concentration of sodium methoxide in the interior I_KK<122>, which affords a methanolic solution of sodium methoxide L₁ <21> having an elevated sodium methoxide concentration compared to L₂ <22>.

In the interior I_KA <112>, the oxidation of chloride ions takes place to give molecular chlorine (Cl⁻→½ Cl₂+e⁻). In the outlet A_KA <111>, an aqueous solution L₄ <24> is obtained, in which the content of NaCl is reduced compared to L₃ <23>. Chlorine gas (Cl₂) in water, according to the reaction Cl₂+H₂O→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 F_A <18>.

FIG. 1 B (=“FIG. 1 B”) shows a further non-inventive electrolysis cell E. This three-chamber cell E comprises a cathode chamber K_K <12>, an anode chamber K_A <11> and an interposed middle chamber K_M <13>.

The cathode chamber K_K <12> comprises a cathodic electrode E_K <123> in the interior I_KK <122>, an inlet Z_KK <120> and an outlet A_KK <121>.

The anode chamber K_A <11> comprises an anodic electrode E_A <113> in the interior I_KA <112>, an inlet Z_KK <110> and an outlet A_KA <111>.

The middle chamber K_M <13> comprises an interior I_KM <132>, an inlet Z_KM <130> and an outlet A_KM <131>.

The interior I_KA <112> is connected to the interior I_KM <132> via the connection V_AM <15>.

The three chambers are bounded by an outer wall <80> of the three-chamber cell E. The interior I_KM <132> of the middle chamber K_M <13> is also divided from the interior I_KA <122> of the cathode chamber K_K <12> by a dividing wall consisting of a sheet of a NaSICON solid-state electrolyte F_A <18> which is selectively permeable to sodium ions. The NaSICON solid-state electrolyte F_A <18> extends over the entire depth and height of the three-chamber cell E. The dividing wall has two sides S_KK <161> and S_A/MK <162>, the surfaces O_KK <163> and O_A/MK <164> of which contact the respective interior I_KK <122> or I_KM <132>.

The interior I_KM <132> of the middle chamber K_M <13> is additionally divided in turn from the interior I_KA <112> of the anode chamber K_A <11> by a diffusion barrier D <14>. The NaSICON solid-state electrolyte F_A <18> and the diffusion barrier D <14> extend over the entire depth and height of the three-chamber cell E. The diffusion barrier D <14> is a cation exchange membrane (sulfonated PTFE).

In the embodiment according to FIG. 1B, the connection V_AM <15> is formed outside the electrolysis cell E, especially by a tube or hose, the material of which may be selected from rubber, metal and plastic. The connection V_AM <15> can guide liquid from the interior I_KM <132> of the middle chamber K_M <13> into the interior I_KA <112> of the anode chamber K_A <11> outside the dividing wall W_A <80> of the three-chamber cell E. The connection V_AM <15> connects an outlet A_KM <131> that penetrates the outer wall W_A <80> of the electrolysis cell E at the base of the middle chamber K_M <13> to an inlet Z_KA <110> that penetrates the outer wall W_A <80> of the electrolysis cell E at the base of the anode chamber K_A <11>.

An aqueous solution of sodium chloride L₃ <23> with pH 10.5 is introduced via the inlet Z_KM <130>, in the direction of gravity, into the interior I_KM <132> of the middle chamber K_M. The connection V_AM <15> formed between an outlet A_KM <131> from the middle chamber K_M <13> and an inlet Z_KA <110> to the anode chamber K_A <11> connects the interior I_KM <132> of the middle chamber K_M <13> to the interior I_KA <112> of the anode chamber K_A <11>. Sodium chloride solution L₃ <23> is routed through this connection V_AM <15> from the interior I_KM <132> into the interior I_KA <112>. A solution of sodium methoxide in methanol L₂ <22> is routed into the interior I_KK <122> via the inlet Z_KK <120>.

At the same time, a voltage is applied between the cathodic electrode E_K <123> and the anodic electrode E_A <113>. This results in reduction of methanol in the electrolyte L₂ <22> to give methoxide and H₂ in the interior I_KK <122> (CH₃OH+e⁻→CH₃O⁻+½ H₂). At the same time, sodium ions diffuse from the interior I_KM <132> of the middle chamber K_M <103> through the NaSICON solid-state electrolyte F_A <18> into the interior I_KK <122>. Overall, this increases the concentration of sodium methoxide in the interior I_KK<122>, which affords a methanolic solution of sodium methoxide L₁ <21> having an elevated sodium methoxide concentration compared to L₂ <22>.

In the interior I_KA <112>, the oxidation of chloride ions takes place to give molecular chlorine (Cl⁻→½ Cl₂+e⁻). At the outlet A_KA <111>, an aqueous solution L₄ <24> is obtained, in which the content of NaCl is reduced compared to L₃ <23>. Chlorine gas (Cl₂) in water, according to the reaction Cl₂+H₂O→HOCl+HCl, forms hypochlorous acid and hydrochloric acid, which give an acidic reaction with further water molecules. The acidity would damage the NaSICON solid-state electrolyte F_A <18>, but is restricted to the anode chamber K_A <11> by the arrangement in the three-chamber cell, and hence kept away from the NaSICON solid-state electrolyte F_K <18> in the electrolysis cell E. This considerably increases the lifetime thereof.

3.2 FIGS. 2A and 2 B

FIG. 2A (=“FIG. 2 A”) shows an inventive embodiment of the dividing wall W <16>. The dividing wall W <16> encompasses two NaSICON solid-state electrolyte ceramics F_A <18> and F_B <19> that are separated from one another by a separating element T <17> and are each secured gaplessly thereon. The separating element T <17> has the geometric shape of a cuboid, with F_A <18> and F_B <19> being secured gaplessly on the opposite sides thereof (for example by adhesive).

The side S_KK <161> with the surface O_KK <163> lies in the plane of the drawing, and the side SA/MK <162> with the surface O_A/MK <164> not visible in FIG. 2A lies behind the plane of the drawing.

FIG. 2 B (=“FIG. 2 B”) shows another inventive embodiment of a dividing wall W <16>. This encompasses four NaSICON solid-state electrolyte ceramics F_A <18>, F_B <19>, F_C <28>, F_D <29>, that are separated from one another by a separating element T <17> and are each secured gaplessly thereon. The separating element T <17> has the shape of a cross, with F_A <18>, F_B <19>, F_C <28> and F_D <29> being adhesively bonded on the opposite sides thereof. The side S_KK <161> with the surface O_KK <163> lies in the plane of the drawing, and the side SA/MK <162> with the surface O_A/MK <164> not visible in FIG. 2 B behind the plane of the drawing.

3.3 FIGS. 3A to 3 C

FIG. 3A (=“FIG. 3 A”) shows the detail view highlighted by a dotted circle in FIGS. 2A and 2 B. As described, the respective solid-state electrolyte ceramics F_A <18> and F_B <19> are secured on the separating element T <17>, for example by adhesive.

FIG. 3 B (=“FIG. 3 B”) illustrates a further inventive embodiment of the dividing wall W. Here, the separating element T <17> has two concave depressions (grooves) in which the two solid-state electrolyte ceramics F_A <18> and F_B <19> are fitted. For this purpose, the shape of the edges of the solid-state electrolyte ceramics F_A <18> and F_B <19> may be mechanically adjusted correspondingly. In addition, a seal Di <40> is used, which is mounted, for example, with an adhesive on the separating element T <17> and the respective solid-state electrolyte ceramic F_A <18> or F_B <19>. The separating element T <17> here may consist of two or more parts <171> and <172> which can be secured to one another as indicated by the dotted line in FIG. 3 B. Given an appropriate geometry and fit of the shape of the edges of the solid-state electrolyte ceramics F_A <18> and F_B <19>, the latter can be clamped between the two parts <171> and <172>, which further improves the stability of the connection of separating element T <17>/ceramic F_A <18> or F_B <19> and the integrity of the dividing wall W <16>.

FIG. 3 C (=“FIG. 3 C”) illustrates a further inventive embodiment of the dividing wall W. This corresponds to that described in FIG. 3 B, except that the depressions (grooves) in the separating element T <17> into which the two solid-state electrolyte ceramics F_A <18> and F_B <19> are fitted are not concave but run to a point.

3.4 FIGS. 4A to 4 D

FIG. 4A (=“FIG. 4 A”) to 4 D show further inventive embodiments of the dividing wall W <16>.

The dividing wall W <16> shown in FIG. 4A corresponds to the dividing wall W <16> shown in FIG. 2A, except that it also comprises a frame element R <20>. This completely covers all surfaces of the dividing wall W <16> except O_KK <163> and O_A/MK <164>. The frame element R <20> is not in one-piece form together with the separating element T <17>.

FIG. 4 B (=“FIG. 4 B”) shows a further inventive embodiment of the dividing wall W <16>. This corresponds to the embodiment shown in FIG. 4A, except that it comprises two frame elements R <20> that bound the upper and lower surfaces of the dividing wall W <16>.

FIG. 4 C (=“FIG. 4 C”) shows a further inventive embodiment of the dividing wall W <16>. The dividing wall W <16> shown in FIG. 4 C corresponds to the dividing wall W <16> shown in FIG. 2 B, except that it also comprises a frame element R <20>. This completely covers all surfaces of the dividing wall W <16> except O_KK <163> and O_A/MK <164>. The frame element R <20> is not in one-piece form together with the separating element T <17>.

FIG. 4 D (=“FIG. 4 D”) shows a further inventive embodiment of the dividing wall W <16>. This corresponds to the embodiment shown in FIG. 4 C, except that it comprises two frame elements R <20> that bound the upper and lower surfaces of the dividing wall W <16>.

3.5 FIGS. 5A and 5 B

FIG. 5A (=“FIG. 5 A”) shows a non-inventive electrolysis cell E. This corresponds to the electrolysis cell shown in FIG. 1A, with the difference that a dividing wall W <16> divides the interior I_KK <122> of the cathode chamber K_K <12> from the interior I_KA <112> of the anode chamber K_A <11>. The dividing wall is that shown in FIGS. 2A and 2 B.

FIG. 5 B (=“FIG. 5 B”) shows a non-inventive electrolysis cell E. This corresponds to the electrolysis cell shown in FIG. 1A, with the difference that a dividing wall W <16> divides the interior I_KK <122> of the cathode chamber K_K <12> from the interior I_KA <112> of the anode chamber K_A <11>. The dividing wall W <16> is that shown in FIGS. 4A to 4 D. The frame element R <20> forms part of the outer wall W_A <80>, such that the solid-state electrolyte ceramics encompassed by the dividing wall W <16> are protected from the pressure that would act thereon via the dividing wall W <16> if they were part of the dividing wall W <16>. In addition, the solid-state electrolyte ceramics are fully inserted within the electrolysis cell for separation of the interiors I_KK <122> and I_KA <112> since they are not partly concealed by the outer wall.

3.6 FIGS. 6A and 6 B

FIG. 6A (=“FIG. 6 A”) shows an electrolysis cell E <1> according to the first aspect of the invention. This corresponds to the electrolysis cell shown in FIG. 1 B, except that a dividing wall W <16> separates the interior I_KK <122> of the cathode chamber K_K <12> from the interior I_KM <132> of the middle chamber K_M <13>. The dividing wall W <16> is that shown in FIGS. 4A to 4 D.

FIG. 6 B (=“FIG. 6 B”) shows an electrolysis cell E <1> according to the first aspect of the invention. This corresponds to the electrolysis cell E <1> shown in FIG. 6A with the difference that the connection V_AM <15> from the interior I_KM <132> of the middle chamber K_M <13> to the interior I_KA <112> of the anode chamber K_A <11> is formed through a perforation in the diffusion barrier D <14>. This perforation may be made in the diffusion barrier D <14> or may already have been present therein from the outset in the production of the diffusion barrier D <14> (for example in the case of textile fabrics such as filter cloths or metal weaves).

3.7 FIGS. 7A and 7 B

FIG. 7A (=“FIG. 7 A”) shows a further embodiment of an inventive dividing wall W <16>. This comprises four NaSICON solid-state electrolyte ceramics F_A <18>, F_B <19>, F_C <28> and F_D <29> that are separated from one another by a separating element T <17> comprising two halves <171> and <172>. The dividing wall W <16> also comprises a frame element R <20> likewise consisting of two halves <201> and <202>.

The dividing wall W <16> consists of two collapsible portions in which half <171> of the separating element T <17> is in one-piece form together with half <201> of the frame element R <20>, and half <172> of the separating element T <17> is in one-piece form together with half <202> of the frame element R <20>. These two parts may optionally be connected to one another via a hinge <50> and may be locked in place via the lock <60> in the collapsed state.

The four NaSICON solid-state electrolyte ceramics F_A <18>, F_B <19>, F_C <28> and F_D <29> are clamped between these halves, with a ring functioning as a seal Di <40> in each case for sealing.

The left-hand side of FIG. 7A shows the front view of the side S_KK <161> with the surface OKK <163> of the dividing wall W <16>. The rings that function as seal Di <40> are indicated by dotted outlines. The right-hand side of the figure shows the lateral view of the dividing wall W <16>.

FIG. 7 B (=“FIG. 7 B”) shows a further embodiment of an inventive dividing wall W <16>. This corresponds to the embodiment described in FIG. 7A, except that it comprises nine NaSICON solid-state electrolyte ceramics F_A <18>, F_B <19>, F_C <28>, F_D <29>, F_E <30>, F_F <31>, F_G <32>, F_H <33>, F_I<34>.

4. DETAILED DESCRIPTION OF THE INVENTION

4.1 Dividing Wall W

The present invention relates in a first aspect to an electrolysis cell E comprising the dividing wall W. The dividing wall W is thus especially suitable as a dividing wall in an electrolysis cell, especially an electrolysis cell E.

The dividing wall W comprises at least two alkali metal cation-conducting solid-state electrolyte ceramics (“alkali metal cation-conducting solid-state electrolyte ceramic” is abbreviated hereinafter as “ASC”) F_A and F_B, separated from one another by a separating element T.

The dividing wall W comprises two sides S_KK and S_A/MK that are opposite one another, meaning that side S_A/MK is opposite side S_KK (and vice versa). The two sides S_KK and S_A/MK especially comprise planes that are essentially parallel to one another.

The geometry of the dividing wall W is otherwise subject to no further restriction, and may be matched in particular to the cross section of the electrolysis cell E in which it is used. For example, it may have the geometry of a cuboid and hence have a rectangular cross section, or the geometry of a frustocone or cylinder and accordingly a circular cross section.

Optionally, the dividing wall W may also have the geometry of a cuboid with rounded corners or bulges which may in turn have holes. The dividing wall W then has bulges (“rabbit's ears”) by which the dividing wall W can be fixed to electrolysis cells, or else frame parts of the dividing wall W can be fixed to one another.

The side S_KK of the dividing wall W has the surface OKK, and the side S_A/MK of the dividing wall W has the surface O_A/MK.

What is meant by the feature “dividing wall” is that the dividing wall W is liquid-tight. This means that the ASCs and the at least one separating element T gaplessly adjoin one another. Thus, no gaps exist between separating element T and the ASCs encompassed by the dividing wall W, through which aqueous solution, alcoholic solution, alcohol or water could flow from the S_KK side to the S_A/MK side or vice versa.

If there are two or more pairs of opposite sides via the surfaces of which the alkali metal cation-conducting solid-state electrolyte ceramics encompassed by the dividing wall W, and especially also the separating element T, are directly contactable, the pair of opposite sides referred to as S_KK and S_A/MK in the context of the invention is preferably that pair which encompasses the greatest surface areas O_KK and O_A/MK. If the surface areas encompassed by the respective pair of opposite sides are the same, the person skilled in the art can select one pair as S_KK and S_A/MK with surfaces OKK and O_A/MK.

Among dividing walls W where there are two or more pairs of opposite sides via the surfaces of which the alkali metal cation-conducting solid-state electrolyte ceramics encompassed by the dividing wall W and especially also the separating element T are directly contactable, preference is given to the dividing walls W where the surface areas encompassed by the respective pair of opposite sides are different, in which case the pair of opposite sides referred to as S_KK and S_A/MK in the context of the invention is that which encompasses the greatest surface areas O_KK and O_A/MK.

The inventive dividing wall W also includes embodiments in which the dividing wall W comprises more than two ASCs, for example four or nine or twelve ASCs, where at least two of the ASCs, but not all ASCs, are separated from one another by a separating element T, with the ASCs that are not separated from one another by a separating element T directly adjoining one another. However, this requires an exact fit of the respective adjacent ASCs, in order to rule out the formation of a gap between them, through which the liquid or water or alcohol or alcoholic solution could flow from the S_KK side to the S_A/MK side. It is therefore advantageous and preferable that, in the dividing wall W, all ASCs encompassed by the dividing wall W are separated from one another by at least one separating element T, meaning that no ASC directly adjoins another ASC, that is without a separating element T being in between.

The dividing wall W is further characterized in that the ASCs encompassed by the dividing wall W are directly contactable both via the surface O_KK and via the surface O_A/MK.

What is meant by “directly contactable” with regard to the ASCs encompassed by the dividing wall W is that some of the surfaces O_KK and O_A/MK are formed by the surface of the ASCs encompassed by the dividing wall W, meaning that the ASCs encompassed by the dividing wall W are directly accessible at the two surfaces O_KK and O_A/MK, such that they can be wetted at the two surfaces OKK and O_A/MK, for example, with aqueous solution, alcoholic solution, alcohol or water.

What this means for the arrangement of the ASCs in the dividing wall W is that, for each ASC encompassed by the dividing wall W, there is a route from the surface O_KK on the side S_KK to the surface O_A/MK on the side S_A/MK that leads completely through the respective ASC.

Typically, the at least one separating element T is also directly contactable both via at least a portion of the surface O_KK and via at least a portion of the surface OA/MK.

What is meant by “directly contactable” with regard to the at least one separating element T encompassed by the dividing wall W is that a portion of the surfaces O_KK and O_A/MK is formed by the surface of the separating element T, meaning that the separating element T is directly accessible at the two surfaces O_KK and O_A/MK, such that the separating element T can be wetted at the two surfaces O_KK and O_A/MK, for example, with aqueous solution, alcoholic solution, alcohol or water.

What this means more particularly for the arrangement of the separating element T in the dividing wall W is that, for the separating element T encompassed by the dividing wall W, there is a route from the surface O_KK on the side S_KK to the surface O_A/MK on the side S_A/MK that leads through the separating element T, and possibly through a seal Di, but not through an ASC.

In a preferred embodiment of the inventive dividing wall W, 50% to 95%, more preferably 60% to 90%, even more preferably 70% to 85%, of the surface O_KK is formed by the ASCs encompassed by the dividing wall W, with the rest of the surface O_KK even more preferably being formed by the separating element T and, if appropriate, the frame element R.

In a preferred embodiment of the inventive dividing wall W, 50% to 95%, more preferably 60% to 90%, even more preferably 70% to 85%, of the surface O_A/MK is also formed by the ASCs encompassed by the dividing wall W, with the rest of the surface O_A/MK even more preferably being formed by the separating element T and, if appropriate, the frame element R.

In a preferred embodiment, the dividing wall W comprises at least four ASCs F_A, F_B, F_C and F_D, and even more preferably comprises exactly four ASCs F_A, F_B, F_C and F_D.

In a further preferred embodiment, the dividing wall W comprises at least nine ASCs F_A, F_B, F_C, F_D, F_E, F_F, F_G, F_H and F_I, and even more preferably comprises exactly nine ASCs F_A, F_B, F_C, F_D, F_E, F_F, F_G, F_H and F_I.

In a further preferred embodiment, the dividing wall W comprises at least twelve ASCs F_A, F_B, F_C, F_D, F_E, F_F, F_G, F_H, F_I, F_J, F_K and F_L, and even more preferably comprises exactly twelve ASCs F_A, F_B, F_C, F_D, F_E, F_F, F_G, F_H, F_I, F_J, F_K and F_L.

This inventive arrangement of at least two ASCs alongside one another in the dividing wall W, compared to the conventional dividing walls in the prior art electrolysis cells, results in a further direction of spread for the ASCs in the event of the fluctuations in temperature that arise in the operation of the electrolysis cell. In the prior art electrolysis cells, the NaSICON sheets that function as dividing walls are framed by the outer walls of the electrolysis cell or by solid plastic frames. It is not possible in this way to dissipate the mechanical stresses that occur in the event of expansion within the NaSICON, which can lead to fracture of the ceramic.

By contrast, the individual ASCs within the inventive dividing wall W adjoin the separating element T, which leads to two advantageous effects, both of which increase the long-term stability of the ASC:

• • each ASC has a further available degree of freedom, i.e. a dimension in which it can expand. As well as expansion in z direction (i.e. beyond the thickness of the ceramic sheet at right angles to the plane of the dividing wall W), expansion in x and/or y direction is now also possible, i.e. in horizontal and vertical direction within the plane of the dividing wall W. This direction of expansion does not exist, or is at least greatly restricted, when the ASCs, for example as a solid sheet, span the cross section of the electrolysis cell and adjoin the solid wall of the electrolysis cell;
  • compared to a dividing wall of equal size consisting solely of one ASC, the division into multiple small ASCs has the effect that the stresses that occur within the smaller ASCs are also smaller in absolute terms, can be dissipated more rapidly and hence cannot as quickly build up to a stress that leads to the fracture of the ASC.

As a result, the tendency to fracture is distinctly reduced for the “divided” ASCs in the dividing wall W compared to the use of one sheet.

4.1.1 Alkali Metal Cation-Conducting Solid-State Electrolyte Ceramic “ASC”

A useful alkali metal cation-conducting solid-state electrolyte ceramic F_A, F_B etc. encompassed by the dividing wall W is any solid-state electrolyte through which cations, especially alkali metal cations, even more preferably sodium cations, can be transported from the S_A/MK side to the S_KK side. 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 to [025]. They are sold commercially under the NaSICON, LiSICON, KSICON name. A sodium ion-conducting solid-state electrolyte 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.

In a preferred embodiment of the dividing wall W, the alkali metal cation-conducting solid-state ceramics encompassed by the dividing wall W independently have an NaSICON structure of the formula M^I_1+2w+x-y+z M^II_w M^III_x Zr^IV_2-w-x-y M^V_y (SiO₄)_z (PO₄)_3-z.

M^I here is selected from Na⁺, Li⁺, preferably Na⁺.

M″ here is a divalent metal cation, preferably selected from Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Co²⁺, Ni²⁺, more preferably selected from Co²⁺, Ni²⁺.

M″ here is a trivalent metal cation, preferably selected from Al³⁺, Ga³⁺, Sc³⁺, La³⁺, Y³⁺, Gd³⁺, Sm³⁺, Lu³⁺, Fe³⁺, Cr³⁺, more preferably selected from Sc³⁺, La³⁺, Y³⁺, Gd³⁺, Sm³⁺, especially preferably selected from Sc³⁺, Y³⁺, La³⁺.

• • M^V here is a pentavalent metal cation, preferably selected from V⁵⁺, Nb⁵⁺, Ta⁵⁺.

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 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.

Even more preferably in accordance with the invention, the NaSICON structure has a structure of the formula Na_(1+v)Zr₂Si_vP_(3-v)O₁₂ where v is a real number for which 0≤v≤ 3. Most preferably, v=2.4.

In a preferred embodiment of the inventive dividing wall W, the ASCs encompassed by the dividing wall W have the same structure.

4.1.2 Separating Element T

According to the invention, the separating element T separates at least two alkali metal cation-conducting solid-state ceramics F_A and F_B encompassed by the dividing wall W, meaning that it is disposed between at least two alkali metal cation-conducting solid-state ceramics F_A and F_B encompassed by the dividing wall W.

A suitable separating element T which is encompassed by the dividing wall W is any body by means of which the respective ASCs can be arranged separately from one another. The ASCs here gaplessly adjoin the separating element T in order not to impair the function of the dividing wall which, in the electrolysis cell E, is to divide the cathode chamber in a liquid-tight manner from the adjacent middle chamber or anode chamber.

The shape of the separating element T can be chosen by the person skilled in the art, depending on the number of the ASCs encompassed by the dividing wall W.

If the dividing wall W comprises two or three ASCs, for example, these may each be separated by a land disposed between the ASCs as a separating element T (see FIG. 1A).

If the dividing wall W comprises four or more ASCs, these may be separated by a separating element T in the form of a cross (see FIGS. 1 B and 4 A) or grid (see FIG. 4 B).

It is particularly preferable that the dividing wall W comprises at least four ASCs, and even more preferable that the separating element T is then in the form of a cross or grid, since it is then ensured that all three dimensions are then fully available to the ASCs for thermal expansion/shrinkage.

The separating element T here may consist of one piece (FIGS. 2A, 2 B). In that case, the ASC is, for example, secured gaplessly to the separating element using a composition known to the person skilled in the art, for example by way of an adhesive, preference being given to using epoxy resins and phenol resins. Alternatively or additionally, the separating element T may also be shaped in such a way that the respective ASC can be fitted or clamped into the separating element. This can already be implemented in a corresponding manner in the production of the dividing wall W (Section 4.1.4).

In a preferred embodiment, the dividing wall W, especially between separating element T and the ASCs, comprises a seal Di (FIGS. 3 B, 3 C). This ensures in a particularly efficient manner that the dividing wall W is liquid-tight. The seal Di may be selected by the person skilled in the art for the respective ASC or the respective separating element T.

The seal Di especially comprises a material selected from the group consisting of elastomers, adhesives, preferably elastomers.

A useful elastomer is especially rubber, preferably ethylene-propylene-diene rubber (“EPDM”), fluoropolymer rubber (“FPM”), perfluoropolymer rubber (“FFPM”), or acrylonitrile-butadiene rubber (“NBR”).

In a further preferred embodiment, the separating element T comprises at least two portions T₁ and T₂ that can be secured to one another and hence clamp the ASCs between them.

In this embodiment, it is particularly preferable in that case to mount a seal Di between separating element T and ASC, in order to assure liquid-tightness.

The separating element T preferably comprises a material selected from the group consisting of plastic, glass, and wood. More preferably, the separating element T is composed of plastic. Even more preferably, the plastic is one selected from the group consisting of polypropylene, polystyrene, polyvinylchloride, post-chlorinated polyvinylchloride (“PVC-C”).

4.1.3 Frame Element R

In a further preferred embodiment, the dividing wall W also comprises a frame element R. The frame element R differs from the separating element T in that it is not disposed between the alkali metal cation-conducting solid-state electrolyte ceramics encompassed by the dividing wall W, i.e. does not separate these from one another. The frame element R bounds especially the surfaces O_KK and O_A/MK at least partly, preferably completely. What this means is more particularly: The frame element R surrounds the surfaces O_KK and O_A/MK at least partly, preferably completely.

The frame element R here may or may not be part of the surfaces O_KK and O_A/MK. The frame element R is preferably part of the surfaces O_KK and O_A/MK.

The frame element R is especially directly contactable or not directly contactable via the surfaces O_KK and O_A/MK, preferably directly contactable.

What is meant by “not directly contactable” with regard to the frame element R encompassed by the dividing wall W is that the frame element R is formed exclusively as at least part of the surfaces of those sides of the dividing wall W which are not the sides S_KK and S_A/MK. More particularly, the frame element R in that case forms at least 1%, more preferably at least 25%, more preferably at least 50%, even more preferably 100%, of the surface areas of the sides of the dividing wall W that are not the sides S_KK and S_A/MK.

What is meant by “directly contactable” with regard to the frame element R encompassed by the dividing wall W is that part of the surfaces O_KK and O_A/MK is formed by the surface of the frame element R, meaning that the frame element R encompassed by the dividing wall W is directly accessible at the two surfaces O_KK and O_A/MK, such that it can be wetted at the two surfaces O_KK and O_A/MK, for example, with aqueous solution, alcoholic solution, alcohol or water.

What this means for the arrangement of the frame element R in the dividing wall W is that there is a route from the surface O_KK on the side S_KK to the surface O_A/MK on the side S_A/MK that leads completely through the frame element R.

This includes the following embodiments:

• • a portion of the edge of the surfaces O_KK and O_A/MK is formed by the frame element R (as shown in FIGS. 4 B, 4 D);
  • the edge of the surfaces O_KK and O_A/MK is formed completely by the frame element R (as shown in FIGS. 4A, 4 C, 7 A, 7 B).

The frame element R here may additionally also be formed as at least part of the surfaces of those sides of the dividing wall W that are not the sides S_KK and SA/MK. More particularly, the frame element R forms at least 1%, more preferably at least 25%, more preferably at least 50%, even more preferably 100%, of the surface areas of the sides of the dividing wall W that are not the sides S_KK and S_A/MK.

FIG. 4 B and FIG. 4 D show, for example, embodiments in which the frame element R forms part of the surfaces of those sides of the dividing wall W which are not the sides S_KK and S_A/MK.

FIG. 4A and FIG. 4 C show, for example, embodiments in which the frame element R completely forms the surfaces of those sides of the dividing wall W which are not the sides S_KK and S_A/MK.

The frame element R is especially selected from a material selected from the group consisting of plastic, glass, wood. More preferably, the frame element R is composed of plastic.

Even more preferably, the plastic is one selected from the group consisting of polypropylene, polystyrene, polyvinylchloride, PVC-C.

In a further preferred embodiment, the frame element R and the separating element T are composed of the same material, and both are even more preferably composed of plastic, which is even more preferably selected from polypropylene, polystyrene, polyvinylchloride, PVC-C.

The frame element R here may consist of one piece. In that case, the ASC is secured gaplessly to the frame element R by a means known to the person skilled in the art, for example, via an adhesive, for which epoxy resins and phenolic resins are particularly suitable. Alternatively or additionally, the frame element R may also be shaped in such a way that the respective ASC can be fitted or clamped into the frame element R.

This also means that, in the preferred embodiment in which the dividing wall W encompasses a frame element R, the ASCs, the at least one separating element T and the frame element R adjoin one another in a gapless manner.

There thus exists between separating element T, frame element R and the ASCs encompassed by the dividing wall W no gaps through which aqueous solution or water could flow from the S_KK side to the S_A/MK side or vice versa.

In addition, especially when the dividing wall W comprises a frame element R with which the separating element T is formed at least partly in one-piece form, the frame element R may consist of at least two parts that are secured to one another and clamp the ASCs between them. For example, the dividing wall W, in that case, may have a hinge by which the two parts of the frame element R can be folded open and closed. In addition, the dividing wall W in that case may have a lock with which the two portions of the frame element R can be locked in place in the folded state (FIG. 7A).

In the folded state, it is then possible for the ASCs and, if it has not already been formed in one-piece form together with the frame element R, the separating element T to be clamped between the frame element R. In this embodiment, it is then possible for a seal to be mounted between separating element T and ASC or frame element R and ASC in order to assure liquid-tightness.

In a preferred embodiment, at least part of the separating element T <17> is formed in one piece with at least part of the frame element R <20>. What this means more particularly is that at least part of the separating element T merges into the frame element R.

Preferably, the separating element T <17> and the frame element R <20> are in one-piece form.

The embodiment of a frame element R has the advantage that it can function as part of the outer wall in the assembly of the electrolysis cell E. This part of the dividing wall W does not make contact with the solutions in the respective interior I_KK, I_KA or I_KM, and it would therefore be a waste the use the solid-state electrolyte ceramic F_A or F_B for this part. In addition, the part of the dividing wall W which is clamped between the outer wall or forms part thereof is subjected to pressures for which the brittle solid-state electrolyte ceramic F_A or F_B is unsuitable. Instead, a fracture-resistant and cheaper material is thus selected for the frame R.

4.1.4 Production of the Dividing Wall W

The dividing wall W can be produced by methods known to the person skilled in the art.

For example, the ASCs encompassed by the dividing wall may be inserted into a casting mould, optionally with seals, and the separating element may be cast by means of liquid plastic and then left to solidify (injection-moulding method). In the course of solidification, this then surrounds the ASCs.

Alternatively, the separating element T is cast separately (or in parts) and then secured gaplessly to the at least two ASCs (for example by adhesive bonding).

4.2 Electrolysis Cell E

The inventive dividing wall W is suitable as a dividing wall in an electrolysis cell E according to the first aspect of the invention.

In a first aspect, the present invention therefore relates to an electrolysis cell E comprising

• • at least one anode chamber K_A having at least one inlet Z_KA, at least one outlet A_KA, and an interior I_KA comprising an anodic electrode E_A,
  • at least one cathode chamber K_K having at least one inlet Z_KK, at least one outlet A_K, and an interior I_KK comprising a cathodic electrode E_K,
  • and at least one interposed middle chamber K_M having at least one inlet Z_KM, at least one outlet A_KM and an interior I_KM,
  • where I_KA and I_KM are then divided from one another by a diffusion barrier D, and A_KM is connected by a connection V_AM to the inlet Z_KA, such that liquid can be passed from I_KM into I_KA via the connection V_AM,
    where
  • I_KK and I_KM are divided from one another by the inventive dividing wall W,
    characterized in that
    the alkali metal cation-conducting solid-state ceramics encompassed by the dividing wall W, and especially also the separating element T, directly contact the interior I_KK on the side S_KK via the surface O_KK,
    and
  • the alkali metal cation-conducting solid-state ceramics encompassed by the dividing wall W, and especially also the separating element T, directly contact the interior I_KM on the side S_A/MK Via the surface O_A/MK.

The electrolysis cell E in the first aspect of the invention comprises at least one anode chamber K_A and at least one cathode chamber K_K, and optionally at least one interposed middle chamber K_M. This also includes electrolysis cells E having more than one anode chamber K_A and/or cathode chamber K_K and/or middle chamber K_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.

The electrolysis cell E in the first aspect of the invention, in a preferred embodiment, comprises an anode chamber K_A and a cathode chamber K_K, and optionally an interposed middle chamber K_M.

The electrolysis cell E typically has an outer wall W_A. The outer wall W_A 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_A may especially be perforated for inlets and outlets. Within W_A are then the at least one anode chamber K_A, the at least one cathode chamber K_K and, In the embodiments in which the electrolysis cell E comprises one, the at least one interposed middle chamber K_M.

4.2.1 Cathode Chamber KK

The cathode chamber K_K has at least one inlet Z_KK, at least one outlet A_KK, and an interior I_KK comprising a cathodic electrode E_K.

The interior I_KK of the cathode chamber K_K is divided from the interior I_KM of the middle chamber K_M by the inventive dividing wall W.

4.2.1.1 Cathodic Electrode E_K

The cathode chamber K_K comprises an interior I_KK which in turn comprises a cathodic electrode E_K. A useful cathodic electrode E_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 in the second aspect of the invention. These are described, in particular, in WO 2014/008410 A1, paragraph or DE 10360758 A1, paragraph [030]. This electrode E_K may be selected from the group consisting of mesh wool, three-dimensional matrix structure and “balls”. The cathodic electrode E_K especially comprises a material selected from the group consisting of steel, nickel, copper, platinum, platinized metals, palladium, carbon-supported palladium, titanium. Preferably, E_K comprises nickel.

In the embodiments of the electrolysis cell E according to the first aspect of the invention in which it comprises a middle chamber K_M, this is between the anode chamber K_A and the cathode chamber K_K.

4.2.1.2 Inlet Z_KK and Outlet A_KK

The cathode chamber K_K also encompasses an inlet Z_KK and an outlet A_KK. This enables addition of liquid, for example the solution L₂, to the interior I_KK of the cathode chamber K_K, and removal of liquid present therein, for example the solution L₁. The inlet Z_KK and the outlet A_KK are attached here to the cathode chamber K_K in such a way that the liquid comes into contact with the cathodic electrode E_K as it flows through the interior I_KK of the cathode chamber KK. This is the prerequisite for the solution L₁ to be obtained at the outlet A_KK in the performance of the process according to the invention in the second aspect of the invention when the solution L₂ of an alkali metal alkoxide XOR in the alcohol ROH is routed through the interior I_KK of the cathode chamber KK.

The inlet Z_KK and the outlet A_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 and corresponding connections (valves) that simplify the introduction and discharge of liquid.

4.2.2 Anode Chamber KA

The anode chamber K_A has at least one inlet Z_KA, at least one outlet A_KA, and an interior I_KA comprising an anodic electrode E_A.

The interior I_KA of the anode chamber K_A is divided from the interior I_KM of the middle chamber K_M by a diffusion barrier D.

4.2.2.1 Anodic Electrode EA

The anode chamber K_A comprises an interior I_KA which in turn comprises an anodic electrode E_A. A useful anodic electrode E_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 in the second aspect of the invention. These are described, in particular, in WO 2014/008410 A1, paragraph or DE 10360758 A1, paragraph [031]. This electrode E_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_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_A comprises a titanium anode coated with ruthenium oxide/iridium oxide (RuO₂+IrO₂/Ti).

4.2.2.2 Inlet Z_KA and Outlet A_KA

The anode chamber K_K also encompasses an inlet Z_KA and an outlet A_KA. This enables addition of liquid, for example the solution L₃, to the interior I_KA of the cathode chamber K_A, and removal of liquid present therein, for example the solution L₄. The inlet Z_KA and the outlet A_KA are attached here to the anode chamber K_A in such a way that the liquid comes into contact with the anodic electrode E_A as it flows through the interior I_KA of the anode chamber K_A. This is a prerequisite for the solution L₄ to be obtained at the outlet A_KA in the performance of the process according to the invention in the second aspect of the invention when the solution L₄ of a salt S is routed through the interior I_KA of the cathode chamber K_A.

The inlet Z_KA and the outlet A_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 and corresponding connections (valves) that simplify the introduction and discharge of liquid. The inlet Z_KA, in particular embodiments, may also be within the electrolysis cell, for example in the form of a perforation in the diffusion barrier D.

4.2.3 Middle Chamber K_M

The electrolysis cell E in the first aspect of the invention has at least one middle chamber K_M. The middle chamber K_M lies between cathode chamber and K_K anode chamber K_A. It comprises at least one inlet Z_KM, at least one outlet A_KM and an interior I_KM.

The interior I_KA of the anode chamber K_A is divided from the interior I_KM of the middle chamber K_M by a diffusion barrier D. A_KM is then also connected to the inlet Z_KA by a connection V_AM, such that liquid can be guided from I_KM into I_KA through the connection V_AM.

4.2.3.1 Diffusion Barrier D

The interior I_KM of the middle chamber K_M is divided from the interior I_KA of the anode chamber K_A by a diffusion barrier D and divided from the interior I_KK of the cathode chamber K_K by the dividing wall W.

The material used for the diffusion barrier D may be any which 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 interior I_KA of the anode chamber K_A to the interior I_KM of the middle chamber K_M.

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.

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”).

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.

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.

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.

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.

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: m²/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.

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_A into the middle chamber K_M.

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.

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

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

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

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).

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 —NH₃⁺, —NRH₂⁺, —NR₃⁺, ═NR⁺; —PR₃⁺, 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 —NH₃⁺, —NRH₂⁺ and —NR₃⁺, more preferably selected from —NH₃⁺ and —NR³⁺, even more preferably —NR₃⁺.

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.

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).

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 —SO₃⁻, —COO⁻, —PO₃²⁻ and —PO₂H⁻, preferably —SO₃⁻ (described in DE 10 2010 062 804 A1, U.S. Pat. No. 4,831,146).

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 or US 2016/0204459 A1, and are commercially available under the Nation®, 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.

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_NAFION below, where n and m may independently be a whole number from 1 to 106, preferably a whole number from 10 to 105, more preferably a whole number from 102 to 104.

4.2.3.2 Inlet Z_KM and Outlet A_KM

The middle chamber K_M also encompasses an inlet Z_KM and an outlet A_KM. This enables addition of liquid, for example the solution L₃, to the interior I_KM of the middle chamber K_M, and transfer of liquid present therein, for example the solution L₃, to the anode chamber K_A.

The inlet Z_KM and the outlet A_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 and corresponding connections (valves) that simplify the introduction and discharge of liquid. The outlet A_KM may also be within the electrolysis cell, for example in the form of a perforation in the diffusion barrier D.

4.2.3.3 Connection V_AM

In the electrolysis cell E according to the first aspect of the invention, the outlet A_KM is connected to the inlet Z_KA by a connection V_AM in such a way that liquid can be guided from I_KM into I_KA through the connection V_AM.

The connection V_AM may be formed within the electrolysis cell E and/or outside the electrolysis cell E, and is preferably formed within the electrolysis cell E.

1) If the connection V_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_AM.

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_AM is formed outside the electrolysis cell E, preferably formed by a connection of A_KM and Z_KA that runs outside the electrolysis cell E, especially in that an outlet A_KM through the outer wall W_A is formed from the interior of the middle chamber I_KM, preferably at the base of the middle chamber KM, the inlet Z_KM more preferably being at the top end of the middle chamber KM, and an inlet Z_KA through the outer wall W_A is formed in the interior I_KA of the anode chamber K_A, preferably at the base of the anode chamber K_A, 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_KA is then more preferably at the top end of the anode chamber K_A.

“Outlet A_KM at the base of the middle chamber KM” means that the outlet A_KM is attached to the electrolysis cell E in such a way that the solution L₃ leaves the middle chamber K_M in the direction of gravity.

“Inlet Z_KA at the base of the anode chamber KA” means that the inlet Z_KA is attached to the electrolysis cell E in such a way that the solution L₄ enters the anode chamber K_A counter to gravity.

“Inlet Z_KM at the top end of the middle chamber KM” means that the inlet Z_KM is attached to the electrolysis cell E in such a way that the solution L₄ enters the middle chamber K_M in the direction of gravity.

“Outlet A_KA at the top end of the anode chamber KA” means that the outlet A_KA is mounted on the electrolysis cell E in such a way that the solution L₄ leaves the anode chamber K_A counter to gravity.

This embodiment is particularly advantageous and therefore preferred when the outlet A_KM is formed by the outer wall W_A at the base of the middle chamber KM, and the inlet Z_KA by the outer wall W_A at the base of the anode chamber K_A. This arrangement makes it possible in a particularly simple manner to remove gases formed in the anode chamber K_A from the anode chamber K_A with LA, in order to separate them further.

When the connection V_AM is formed outside the electrolysis cell E, Z_KM and A_KM are especially arranged at opposite ends of the outer wall W_A of the middle chamber K_M (i.e., for example, Z_KM at the base and A_KM at the top end of the electrolysis cell E or vice versa) and Z_KA and A_KA are arranged at opposite ends of the outer wall W_A of the anode chamber K_A (i.e. Z_KA at the base and AKA at the top end of the electrolysis cell E or vice versa), as shown more particularly in FIG. 6A. By virtue of this geometry, L₃ must flow through the two chambers K_M and K_A. It is possible here for Z_KA and Z_KM to be formed on the same side of the electrolysis cell E, in which case A_KM and A_KA are automatically also formed on the same side of the electrolysis cell E. Alternatively, Z_KA and Z_KM may be formed on opposite sides of the electrolysis cell E, in which case A_KM and A_KA are then automatically also formed on opposite sides of the electrolysis cell E.

3) When the connection V_AM is formed within the electrolysis cell E, this may especially be ensured in that one side (“side A”) of the electrolysis cell E, which is the top end or the base of the electrolysis cell E, preferably the top end as shown in FIG. 6 B, comprises the inlet Z_KM and the outlet A_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 side (“side B”) of the electrolysis cell E opposite side A, which is then the base or the top end 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 outer wall WA of side B of the three-chamber cell E. In that case, the gap is the connection V_AM. By virtue of this geometry, L₃ must flow completely through the two chambers K_M and K_A.

These embodiments best ensure that the aqueous salt solution L₄ flows past the acid-sensitive solid-state electrolyte before it comes into contact with the anodic electrode E_A, which results in the formation of acids.

According to the invention, “base of the electrolysis cell E” is the side of the electrolysis cell E through which a solution (e.g. L₃ in the case of A_KM in FIG. 6A) 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. L₂ in the case of Z_KK in FIGS. 6A and 6 B) is supplied to the electrolysis cell E counter to gravity.

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. L₃ in the case of A_KA and L₁ in the case of A_KK in FIGS. 6A and 6 B) exits from the electrolysis cell E counter to gravity, or the side of the electrolysis cell E through which a solution (e.g. L₃ in the case of Z_KM in FIGS. 6A and 6 B) is supplied to the electrolysis cell E in the same direction as gravity.

4.2.4 Arrangement of the Dividing Wall W in the Electrolysis Cell E

The dividing wall W is arranged in the electrolysis cell E in such a way that the alkali metal cation-conducting solid-state electrolyte ceramics encompassed by the dividing wall W, and preferably also the separating element T, directly contact the interior I_KK on the side S_KK via the surface OKK. This means that the dividing wall W is arranged within the electrolysis cell E such that, when the interior I_KK on the side S_KK side is completely filled with solution L₂, the solution L₂, via the surface OKK, then makes contact with all alkali metal cation-conducting solid-state electrolyte ceramics encompassed by the dividing wall W, and preferably also the separating element T, such that ions (e.g. alkali metal ions such as sodium, lithium) from all ASCs encompassed by the dividing wall W can enter the solution L₂.

In addition, the dividing wall W is arranged in the electrolysis cell E such that the alkali metal cation-conducting solid-state electrolyte ceramics encompassed by the dividing wall W, and preferably also the separating element T, make direct contact with the interior I_KM on the side S_A/MK via the surface O_A/MK.

What this means is as follows: the dividing wall W adjoins the interior I_KM of the middle chamber KM. In these embodiments, the dividing wall W is arranged within the electrolysis cell E such that, when the interior I_KM on the side S_AM/K is completely filled with solution L₃, the solution La, via the surface O_A/MK, then makes contact with all alkali metal cation-conducting solid-state electrolyte ceramics encompassed by the dividing wall W, and preferably also the separating element T, such that ions (e.g. alkali metal ions such as sodium, lithium) from the solution L₄ can enter any ASC encompassed by the dividing wall W.

In a preferred embodiment of the electrolysis cell E in the first aspect of the invention, at least 50%, especially at least 70%, preferably at least 90%, most preferably 100%, of the portion of the surface O_KK which is formed by ASCs makes contact with the interior I_KK.

In a preferred embodiment of the electrolysis cell E with at least one middle chamber in the first aspect of the invention, at least 50%, especially at least 70%, preferably at least 90%, most preferably 100%, of the portion of the surface O_A/MK which is formed by ASCs makes contact with the interior I_KM.

4.3 Process According to the Invention

The present invention relates, in a second aspect, to a process for producing a solution L₁ 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 according to the second aspect of the invention is conducted in an electrolysis cell E in the first aspect of the invention.

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

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.

According to the invention, the electrolysis cell E comprises at least one middle chamber KM, and the steps (β1), (β2), (β3) that proceed simultaneously are then conducted.

4.3.1 Step (β1)

In step (β1), a solution L₂ comprising the alcohol ROH, preferably comprising an alkali metal alkoxide XOR and alcohol ROH, is routed through KK.

Solution L₂ 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 L₂ based on the weight of the alcohol ROH in solution L₂ (mass ratio) is ≤1:10, more preferably ≤1:20, even more preferably ≤1:100, even more preferably ≤0.5:100.

If solution L₂ comprises XOR, the proportion by mass of XOR in solution L₂, based on the overall solution L₂, 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.

If solution L₂ comprises XOR, the mass ratio of XOR to alcohol ROH in solution L₂ 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.

4.3.2 Step (2)

In step (B2), a neutral or alkaline, aqueous solution L₄ of a salt S comprising X as cation is routed through K_M, then via V_AM, then through K_A.

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

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

The pH of the aqueous solution L₄ 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.

The proportion by mass of salt S in solution L₄ 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 L₃.

4.3.3 Step (β3)

In step (β3), a voltage is then applied between EA and Ex.

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.

This in turn has the following consequences:

• • the solution L₁ is obtained at the outlet A_KK with a higher concentration of XOR in L₁ than in L₂,
  • an aqueous solution L₄ of S is obtained at the outlet A_KA, with a lower concentration of S in L₄ than in L₃.

In step (β3) of the process according to the second aspect the invention, in particular, such a voltage is applied that such a current flows such 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_M) is in the range from 10 to 8000 A/m², more preferably in the range from 100 to 2000 A/m², even more preferably in the range from 300 to 800 A/m², and even more preferably is 494 A/m². 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_M is especially 0.00001 to 10 m², preferably 0.0001 to 2.5 m², more preferably 0.0002 to 0.15 m², even more preferably 2.83 cm².

It will be apparent that step (β3) of the process according to the second aspect of the invention is conducted when the two chambers K_M and K_A are at least partly laden with L₃, and K_K is at least partly laden with L₂, such that both L₃ and L₂ come into contact with the solid-state electrolytes encompassed by the dividing wall W and especially also come into contact with the separating element T.

The fact that transfer of charge takes place between E_A and E_K in step (β3) implies that K_K, K_M and K_A are simultaneously laden with L₂ and L₄ such that they cover the electrodes E_A and E_K to such an extent that the circuit is complete.

This is the case especially when a liquid stream of L₃ is routed continuously through K_M, V_AM and KA and a liquid stream of L₂ through KK, and the liquid stream of L₃ covers electrode E_A and the liquid stream of L₂ covers electrode E_K at least partly, preferably completely.

In a further preferred embodiment, the process according to the second aspect of the invention is performed continuously, i.e. step (B1) and step (β2) are performed continuously, while applying voltage as per step (β3).

After performance of step (β3), solution L₁ is obtained at the outlet A_KK, wherein the concentration of XOR in L₁ is higher than in L₂. If L₂ already comprised XOR, the concentration of XOR in L₁ 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 L₂, most preferably 1.077 times higher than in L₂, where the proportion by mass of XOR in L₁ and in L₂ is more preferably in the range from 10% to 20% by weight, even more preferably 13% to 14% by weight.

An aqueous solution L₄ of S is obtained at the outlet A_KA, with a lower concentration of S in L₄ than in L₃.

The concentration of the cation X in the aqueous solution L₄ 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 L₄ is more preferably 0.5 mol/l lower than that of the aqueous solution L₄ used in each case.

More particularly, steps (β1) to (β3) of the process according to the second aspect of the invention 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.

In the course of performance of steps (β1) to (β3) of the process according to the second aspect of the invention, hydrogen is typically formed in the cathode chamber K_K, which can be removed from the cell together with solution L₁ via outlet A_KK. The mixture of hydrogen and solution L₁ 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_A, and this can be removed from the cell together with solution L₄ via outlet A_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 CO₂ and solution L₄ 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 CO₂ gases have been separated from solution La, to separate these by methods known to the person skilled in the art.

4.3.4 Additional Advantages of Steps (β1) to (β3)

This performance of steps (β1) to (β3) brings further surprising advantages that were not to be expected in the light of the prior art. Steps (β1) to (β3) 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, 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.

5. EXAMPLES

5.1 Comparative Example 1

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. The electrolysis cell consisted of three chambers that corresponded to those shown in FIG. 1B. 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 cm² 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 cm². The ceramic had a chemical composition of the formula Na_3.4Zr_2.0Si_2.4P_0.6O₁₂.

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.

In addition, there is expansion and shrinkage of the NaSICON ceramic on account of the heating and cooling effects when the electrolysis cell is repeatedly started up and shut down. In addition, the NaSICON membrane may be displaced within the cell. This is problematic since the tendency of the ceramic to fracture is enhanced and can lead to leakage of electrolyte from the middle chamber into the cathode chamber, which waters down the electrolysis product. In addition, this can lead to leaks in the outer wall of the cell, which leads to leakage of electrolyte to the outside.

5.2 Comparative Example 2

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 (FIG. 1A). 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.

5.3 Inventive Example 1

Comparative Example 1 is repeated using an electrolysis cell according to FIG. 6A in which a dividing wall comprising two NaSICON ceramics in a frame is inserted.

This arrangement reduces the extent of the expansion and shrinkage processes, which contributes to the service life of the ceramic and results in a cleaner product solution since the leakage is prevented.

5.4 Inventive Example 2

Comparative Example 2 is repeated using an electrolysis cell according to FIG. 6A in which a dividing wall comprising four NaSICON ceramics in a frame is inserted, in which frame element R and separating element T are combined (FIG. 7A, but without hinge and without lock).

This arrangement reduces the extent of the expansion and shrinkage processes, which contributes to the service life of the ceramic and results in a cleaner product solution since the leakage is prevented.

5.5 Result

The alleviation of the tensions within the ASCs by the expansion and shrinkage processes which result in the case of repeated electrolysis cycles leads to an extension of the lifetime of the electrolysis chamber. In the execution according to Inventive Examples 1 and 2, these effects are reduced, which increases the stability of the solid-state electrolyte.

The use of a three-chamber cell according to the invention 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 electrolysis cell with middle chamber and the corresponding process conducted therein.

6. REFERENCE SYMBOLS IN THE FIGS

Electrolysis cell	E	<1>
Anode chamber	KA	<11>
Inlet	ZKA	<110>
Outlet	AKA	<111>
Interior	IKA	<112>
Anodic electrode	EA	<113>
Cathode chamber	KK	<12>
Inlet	ZKK	<120>
Outlet	AKK	<121>
Interior	IKK	<122>
Cathodic electrode	EK	<123>
Middle chamber	KM	<13>
Inlet	ZKM	<130>
Outlet	AKM	<131>
Interior	IKM	<132>
Diffusion barrier	D	<14>
Connection	VAM	<15>
Dividing wall	W	<16>
Side	SKK	<161>
Side	SA/MK	<162>
Surface	OKK	<163>
Surface	OA/MK	<164>
Separating element	T	<17>
Part of a separating element		<171>
Part of a separating element		<172>
Solid-state electrolyte ceramic	FA	<18>
Solid-state electrolyte ceramic	FB	<19>
Solid-state electrolyte ceramic	FC	<28>
Solid-state electrolyte ceramic	FD	<29>
Solid-state electrolyte ceramics	FE, FF, FG,	<30>, <31>, <32>,
	FH, FI	<33>, <34>
Frame element	R	<20>
Frame part		<201>
Frame part		<202>
Seal	Di	<40>
Hinge		<50>
Lock		<60>
Outer wall	WA	<80>
Alkali metal alkoxide XOR in the	L1	<21>
alcohol ROH
Solution comprising the alcohol	L2	<22>
ROH
Neutral or alkaline, aqueous	L3	<23>
solution of a salt S comprising
X as cation
Aqueous solution of S, where	L4	<24>
[S]L4 < [S]L3.

Claims

1. An electrolysis cell E <1> comprising

at least one anode chamber KA <11> having at least one inlet ZKA <110>, at least one outlet AKA <111>, and an interior IKA <112> comprising an anodic electrode EA <113>,

at least one cathode chamber KK <12> having at least one inlet ZKK <120>, at least one outlet AKK <121>, and an interior IKK <122> comprising a cathodic electrode EK <123>,

and at least one interposed middle chamber KM <13> having at least one inlet ZKM <130>, at least one outlet AKM <131> and an interior IKM <132>,

where IKA <112> and IKM <132> are divided from one another by a diffusion barrier D <14>, and AKM <131> is connected by a connection VAM <15> to the inlet ZKA <110>, such that liquid can be passed from IKM <132> into IKA <112> via the connection VAM <15>, where IKK <122> and IKM <132> are divided from one another by a dividing wall W <16> comprising one side SKK <161> having the surface OKK <163> and, opposite the side SKK <161>, a side SA/MK <162> having the surface OA/MK <164>,

wherein the dividing wall W <16> encompasses at least two alkali metal cation-conducting solid-state electrolyte ceramics FA <18> and FB <19> separated from one another by at least one separating element T <17> in such a way that the alkali metal cation-conducting solid-state electrolyte ceramics encompassed by the dividing wall W <16> are directly contactable both via the surface OKK <163> and via the surface OA/MK <164>,

wherein

the alkali metal cation-conducting solid-state ceramics encompassed by the dividing wall W <16> directly contact the interior IKK <122> on the SKK side <161> via the surface OKK <163>, and the alkali metal cation-conducting solid-state electrolyte ceramics encompassed by the dividing wall W <16> directly contact the interior IKM <132> on the SA/MK <162> side via the surface OA/MK <164>.

2. The electrolysis cell E <1> according to claim 1, wherein the dividing wall W <16> comprises at least four alkali metal cation-conducting solid-state electrolyte ceramics FA <18>, FB <19>, FC <28> and FD <29>.

3. The electrolysis cell E <1> according to claim 2, wherein the separating element T <17> takes the form of a cross or grid.

4. The electrolysis cell E <1> according to claim 1, wherein the separating element T <17> comprises a material selected from the group consisting of plastic, glass, wood.

5. The electrolysis cell E <1> according to claim 1, wherein the dividing wall W <16> comprises a frame element R <20>.

6. The electrolysis cell E <1> according to claim 5, wherein at least a portion of the separating element T <17> is in one-piece form together with at least a portion of the frame element R <20>.

7. The electrolysis cell E <1> according to claim 1, wherein the alkali metal cation-conducting solid-state electrolyte ceramics encompassed by the dividing wall W <16> independently have a structure of the formula MI1+2w+x-y+z MIIw MIIIx ZrIV2-w-x-y MVy (SiO4)z (PO4)3-z,

where MI is selected from Na+ and Li+,

MII is a divalent metal cation,

MIII is a trivalent metal cation,

MV 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.

8. The electrolysis cell E <I> according to claim 1, wherein the connection VAM <15> is formed within the electrolysis cell E <1>.

9. A process for producing a solution L1 <21> 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,

wherein the following steps (β1), (β2), (β3) that proceed simultaneously are conducted in an electrolysis cell E <1> according to claim 1:

(β1) a solution L2 <22> comprising the alcohol ROH is routed through KK <12>,

(β2) a neutral or alkaline, aqueous solution L3 <23> of a salt S comprising X as cation is routed through KM <13>, then via VAM <15>, then through KA <11>,

(β3) voltage is applied between EA <113> and EK <123>,

which affords the solution L1 <21> at the outlet AKK <121>, with a higher concentration of XOR in L1 <21> than in L2 <22>,

and which affords an aqueous solution L4 <24> of S at the outlet AKA <111>, with a lower concentration of S in L4 <24> than in L3 <23>.

10. The process according to claim 9, wherein X is selected from the group consisting of Li+, Na+, K+.

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

12. The process according to claim 9, wherein R is selected from the group consisting of methyl and ethyl.