EP3885470B1 - Method for producing alkaline metal alcaholates in a three-chamber electrolysis cell - Google Patents

Description

The present invention relates to a method for the electrochemical production of an alkali metal alkoxide solution. The process is carried out in an electrolytic cell which has three chambers, the middle chamber being separated from the cathode chamber by a cation-permeable solid electrolyte, for example NaSICON, and from the anode chamber by a diffusion barrier, for example a cation- or anion-selective membrane.

1. Background of the Invention

The electrochemical production of alkali metal alkoxide solutions is an important industrial process that is used, for example, in DE 103 60 758 A1 , the U.S. 2006/0226022 A1 and the WO 2005/059205 A1 is described. The principle of this process is an electrolytic cell in whose anode chamber there is a solution of an alkali salt, for example common salt or NaOH, and in whose cathode chamber there is the alcohol in question or a low-concentration alcoholic solution of the alkali metal alcoholate in question, for example sodium methoxide or sodium ethoxide. The cathode compartment and the anode compartment are separated by a ceramic which conducts the alkali metal ion used, for example NaSICON or its analogues for potassium or lithium. When a current is applied, chlorine is formed at the anode - if a chloride salt of the alkali metal is used - and hydrogen and alcohol ions are formed at the cathode. The charge equalization results from the fact that alkali metal ions migrate from the middle chamber into the cathode chamber via the ceramic that is selective for them. The charge equalization between the middle chamber and the anode chamber results from the migration of cations when using cation exchange membranes or the migration of anions when using anion exchange membranes or by migration of both types of ions when using non-specific diffusion barriers. This increases the concentration of the alkali alcoholate in the cathode chamber and the concentration of the sodium ions in the anolyte decreases. NaSICON solid electrolytes are also used in the electrochemical production of other compounds:
WO 2014/008410 A1 describes an electrolytic process for the production of elemental titanium or rare earths. This process is based on the fact that titanium chloride is formed from TiOz and the corresponding acid, this reacts with sodium alcoholate to form titanium alcoholate and NaCl and is finally converted electrolytically to form elemental titanium and sodium alcoholate.

WO 2007/082092 A2 and WO 2009/059315 A1 describe processes for the production of biodiesel in which triglycerides are first converted into the corresponding alkali metal triglycerides with the aid of alcoholates electrolytically produced via NaSICON and in a second step are converted into glycerol and the respective alkali metal hydroxide with electrolytically produced protons.

Accordingly, methods are described in the prior art which are carried out in electrolytic cells with an ion-permeable layer, such as, for example, NaSiCON solid electrolytes. However, these solid electrolytes typically have the disadvantage that they are not long-term stable to aqueous acids. This is problematic insofar as the pH drops in the anode chamber during electrolysis as a result of oxidation processes (for example when halogens are produced by disproportionation or by oxygen formation). These acidic conditions attack the NaSICON solid electrolyte, so the process cannot be used on an industrial scale. Various approaches have been described in the prior art to address this problem.

Three-chamber cells have been proposed in the prior art. Such are known in the field of electrodialysis, for example US 6,221,225 B1 .

WO 2012/048032 A2 and US 2010/0044242 A1 describe, for example, electrochemical processes for the production of sodium hypochlorite and similar chlorine compounds in such a three-chamber cell. The cathode chamber and the middle chamber of the cell are separated by a cation-permeable solid electrolyte such as NaSICON. In order to protect this from the acidic anolyte, the middle chamber is supplied with solution from the cathode chamber, for example. The US 2010/0044242 A1 also describes in Figure 6 that solution from the middle compartment can be mixed with solution from the anode compartment outside the compartment to obtain sodium hypochlorite.

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

That's how she describes it US 5,389,211A a process for the purification of alcoholate solutions in which a three-chamber cell is used, in which the chambers are separated from one another by cation-selective solid electrolytes or non-ionic partitions. The center compartment is used as a buffer compartment to prevent the purified alkoxide or hydroxide solution from the cathode compartment from mixing with the contaminated solution from the anode compartment.

The WO 2008/076327 A1 describes a process for preparing alkali metal alkoxides. A three-chamber cell is used, the middle chamber of which is filled with alkali metal alkoxide (See, for example, paragraphs [0008] and [0067] of the WO 2008/076327 A1 ). This protects the solid electrolyte separating the middle chamber and the cathode chamber from the solution in the anode chamber, which becomes more acidic during the electrolysis. However, this arrangement has the disadvantage that the alkali metal alkoxide solution is the desired product, but this is consumed as a buffer solution and is continuously contaminated. Another disadvantage of the in the WO 2008/076327 A1 The method described is that the formation of the alcoholate in the cathode chamber depends on the diffusion rate of the alkali metal ions through two membranes or solid electrolytes. This in turn slows down the formation of the alcoholate.

The WO 2009/073062 A2 describes a similar process for the production of alkali metal alkoxides in a three-chamber electrolytic cell. This includes a central chamber in which alkali metal alcoholate solution is also used as a buffer solution.

The object of the present invention was therefore to provide an improved process for the electrolytic production of alkali metal alkoxide which ensures protection of the cation-conducting solid electrolyte against acid but does not have the disadvantages mentioned above. In addition, the process should be distinguished by a more economical use of the educts compared to the prior art.

2. Brief Description of the Invention

Surprisingly, a process has now been found which achieves the object of the invention.

• wobei E <100> mindestens eine Anodenkammer K_A <101>, mindestens eine Kathodenkammer K_K <102> und mindestens eine dazwischen liegende Mittelkammer K_M <103> umfasst,
• wobei K_A <101 > eine anodische Elektrode E_A <104> und einen Ablauf A_KA <106> umfasst,
• wobei K_K <102> eine kathodische Elektrode E_K <105>, einen Zulauf Z_KK <107> und einen Ablauf A_KK <109> umfasst,
• wobei K_M <103> einen Zulauf Z_KM <108> umfasst, durch eine Diffusionsbarriere D <110> von K_A <101 > abgetrennt ist und durch einen alkalikationenleitenden Festelektrolyten F_K <111> von K_K <102> abgetrennt ist,
• wobei K_A <101 > und K_M <103> durch eine Verbindung V_AM <112> miteinander verbunden sind, durch welche Flüssigkeit aus K_M <103> in K_A <101 > geleitet werden kann,
• wobei das Verfahren die folgenden, gleichzeitig ablaufenden Schritte (a), (b) und (c) umfasst:
  1. (a) ein Lösung L₂ <113> umfassend den Alkohol ROH und bevorzugt umfassend mindestens ein Alkalimetallalkoholat XOR wird durch K_K <102> geleitet,
  2. (b) eine neutrale oder alkalische, wässrige Lösung L₃ <114> eines Salzes S umfassend X als Kation wird durch K_M <103>, dann über V_AM <112>, dann durch K_A <101 > geleitet,
  3. (c) zwischen E_A <104> und E_K <105> wird Spannung angelegt,
• wodurch am Ablauf A_KK <109> die Lösung L₁ <115> erhalten wird, wobei die Konzentration von XOR in L₁ <115> höher ist als in L₂ <113>,
• und wodurch am Ablauf A_KA <106> eine wässrige Lösung L₄ <116> von S erhalten wird, wobei die Konzentration von S in L₄ <116> geringer ist als in L₃ <114>,
• wobei X ein Alkalimetallkation ist und Rein Alkylrest mit 1 bis 4 Kohlenstoffatomen ist.

The method according to the invention is one for preparing a solution L ₁ <115> of an alkali metal alkoxide XOR in the alcohol ROH in an electrolysis cell E <100>,

• wherein E <100> comprises at least one anode chamber K _A <101>, at least one cathode chamber K _K <102> and at least one intermediate chamber K _M <103>,
• where K _A <101> comprises an anodic electrode E _A <104> and an effluent A _KA <106>,
• where K _K <102> comprises a cathodic electrode E _K <105>, an inlet Z _KK <107> and an outlet A _KK <109>,
• where K _M <103> comprises an inlet Z _KM <108>, is separated from K _A <101> by a diffusion barrier D <110> and is separated from _K K <102> by an alkali cation-conducting solid electrolyte F _K <111>,
• where K _A <101> and K _M <103> are connected to each other by a connection V _AM <112>, through which liquid can be conducted from K _M <103> to K _A <101>,
• the method comprising the following concurrent steps (a), (b) and (c):
  1. (a) a solution L ₂ <113> comprising the alcohol ROH and preferably comprising at least one alkali metal alkoxide XOR is passed through K _K <102>,
  2. (b) a neutral or alkaline, aqueous solution L ₃ <114> of a salt S comprising X as a cation is passed through K _M <103>, then over V _AM <112>, then through K _A <101>,
  3. (c) voltage is applied between E _A <104> and E _K <105>,
• whereby the solution L ₁ <115> is obtained at the outlet A _KK <109>, the concentration of XOR in L ₁ <115> being higher than in L ₂ <113>,
• and whereby an aqueous solution L ₄ <116> of S is obtained at the outlet A _KA <106>, the concentration of S in L ₄ <116> being lower than in L ₃ <114>,
• where X is an alkali metal cation and Rein is alkyl of 1 to 4 carbon atoms.

3. Pictures

illustration 1 shows the method according to the invention using a three-chamber cell E <100> comprising a cathode chamber K _K <102>, an anode chamber K _A <101> and a middle chamber K _M <103> located in between. The three chambers are delimited by an outer wall <117> of the three-chamber cell E <100>. The cathode chamber K _K <102> is also separated from the middle chamber K _M <103> by a NaSICON solid electrolyte F _K <111> that is selectively permeable for sodium ions. The middle chamber K _M <103> is additionally in turn separated from the anode chamber K _A <101> by a diffusion barrier D <110>. The NaSICON solid electrolyte F _K <111> and the diffusion barrier D <110> extend over the entire depth and height of the three-chamber cell E <100>.

A solution of sodium methoxide in methanol L ₂ <113> is passed through the cathode chamber K _K <102>. An aqueous solution of sodium chloride L ₃ <114> with a pH of 10.5 is added via the inlet Z _KM <108> in the same direction as gravity into the middle chamber _KM <103>. The connection V _AM <112>, which is formed between an outlet A _KM <118> of the middle chamber K _M <103> and an inlet Z _KA <119> of the anode chamber _KA <101>, creates the middle chamber K _M <103 > connected to the anode chamber K _A <101 >. Sodium chloride solution L ₃ <114> is conducted through this connection V _AM <112> from the middle chamber _KM <103> into the anode chamber _KA <101>. When a voltage is applied, methanol is reduced to methoxide and H ₂ in the cathode chamber K _K <102>. Sodium ions diffuse from the middle chamber K _M <103> through the NaSICON solid electrolyte F _K <111> into the cathode chamber K _K <102>. Overall, this increases the concentration of sodium methoxide in the cathode chamber K _K <102>, as a result of which a methanolic solution of sodium methoxide L ₁ <115> is obtained, the concentration of sodium methoxide in which is increased compared to L ₂ <113>. In the anode chamber K _A <101>, chloride ions from L ₃ <114> are oxidized to Clz.

Cl ₂ has an acidic reaction in aqueous solution. Due to the geometry of the three-chamber cell E <100> and the routing of the aqueous solution L ₃ <114>, the acid-sensitive NaSICON solid electrolyte becomes <111> protected from the increased acidity of the solution L ₄ <116> in the anode chamber K _A <101> compared to L ₃ <114>.

Figure 2 shows an embodiment of the method according to the invention, which corresponds to that in illustration 1 shown. The only difference is that the connection V _AM <112> from the middle chamber K _M <103> to the anode chamber K _A <101> is formed by a perforation in the diffusion barrier D <110>.

Figure 3 shows a diagram of the voltage profile of the electrolysis in a three-chamber cell according to the invention in comparison to a two-chamber cell. The measurement points of the comparative example are represented by triangles (▲), those of the example according to the invention by dots (•). The x-axis represents time in hours while the y-axis represents the measured voltage in volts. The comparison shows that a constant voltage profile is obtained with the cell according to the invention, while the voltage rises rapidly in the two-chamber cell due to the destruction of the solid electrolyte.

4. Detailed Description of the Invention 4.1 Electrolytic Cell E

The method according to the invention is carried out in an electrolytic cell E which comprises at least one anode chamber K _A , at least one cathode chamber K _K and at least one central chamber K _M located in between. This also includes electrolytic cells E which have more than one anode chamber K _A and/or cathode chamber K _K and/or middle chamber K _M . Such electrolytic cells, in which these chambers are joined together in a modular manner, are, for example, in DD 258 143 A3 , U.S. 2006/0226022 A1 described.

The anode chamber K _A includes an anodic electrode E _A . Any electrode familiar to a person skilled in the art that is stable under the conditions of the method according to the invention can be used as such an anodic electrode E _A . Such are in particular in WO 2014/008410 A1 , paragraph [024] or DE 10360758 A1 , paragraph [031]. This electrode E _A can consist of one layer or of several planar, mutually parallel layers, each of which can be perforated or expanded. The anodic electrode E _A comprises in particular a material which is selected from the group consisting of ruthenium oxide, iridium oxide, nickel, cobalt, nickel tungstate, nickel titanate, noble metals such as platinum in particular, which is deposited on a carrier such as titanium or ^Kovar® (an iron/nickel/ Cobalt alloy, in which the individual proportions are preferably as follows: 54% by mass iron, 29% by mass nickel, 17% by mass cobalt) is supported. Other possible anode materials are, in particular, stainless steel, lead, graphite, tungsten carbide, titanium diboride. E _A preferably comprises a titanium anode (RuO ₂ +IrO ₂ /Ti) coated with ruthenium oxide/iridium oxide.

The cathode chamber K _K includes a cathodic electrode E _K . Any electrode familiar to a person skilled in the art that is stable under the conditions can be used as such a cathodic electrode E _K . Such are in particular in WO 2014/008410 A1 , paragraph [025] or DE 10360758 A1 , paragraph [030]. This electrode E _K can be selected from the group consisting of mesh wool, a three-dimensional matrix structure or as "balls". The cathodic electrode E _K includes in particular a material selected from the group consisting of steel, nickel, copper, platinum, platinized metals, palladium, palladium supported on carbon, titanium. E _K preferably comprises nickel.

The at least one middle chamber K _M is located between the anode chamber K _A and the cathode chamber K _K .

The electrolytic cell E usually has an outer wall W _A . The outer wall W _A is in particular made of a material selected from the group consisting of steel, preferably rubberized steel, plastic, in particular Telene ^® (thermosetting polydicyclopentadiene), PVC (polyvinyl chloride), PVC-C (post-chlorinated polyvinyl chloride), PVDF (polyvinylidene fluoride ) is selected, is selected. W _A can be perforated in particular 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 the at least one middle chamber K _M lying between them.

K _M is separated from K _A by a diffusion barrier D and separated from _{K K} by an alkali cation-conducting solid electrolyte F K .

Any material which is stable under the conditions of the method according to the invention and which prevents or slows down the transfer of protons from the liquid in the anode chamber K _A into the middle chamber K _M can be used as the diffusion barrier D.

In particular, a non-ion-specific dividing wall or a membrane permeable to specific ions is used as the diffusion barrier D. The diffusion barrier D is preferably a membrane that is permeable to specific ions.

The material for the non-ionic partition is in particular selected from the group consisting of fabric, in particular textile fabric or metal fabric, glass, in particular sintered glass or glass frits, ceramic, in particular ceramic frits, membrane diaphragms.

If the diffusion barrier D is a “membrane that is permeable to specific ions”, this means according to the invention that the respective membrane favors the diffusion of certain ions through it over others. In particular, membranes are meant that favor the diffusion through them of ions of a certain type of charge compared to oppositely charged ions. More preferably, specific ion permeable membranes also favor the diffusion of certain ions having one charge type through them over other ions of the same charge type.

The diffusion barrier D is therefore preferably an anion-conducting membrane or a cation-conducting membrane.

According to the invention, anion-conducting membranes are those which selectively conduct anions, preferably selectively specific anions. In other words, they favor the diffusion of anions through them over that of cations, especially over protons, more preferably they additionally favor the diffusion of certain anions through them over the diffusion of other anions through them.

According to the invention, cation-conducting membranes are those which selectively conduct cations, preferably selectively specific cations. In other words, they favor the diffusion of cations through them over that of anions, more favorably they favor the diffusion of certain cations through them over the diffusion of other cations through them, much more favorably cations which are is not protons, more preferably sodium cations, over protons.

“Favour the diffusion of certain ions X over the diffusion of other ions Y” means in particular that the diffusion coefficient (unit m ² /s) of the ion type X at a given temperature for the membrane in question is higher by a factor of 10, preferably 100, preferably 1000 as the diffusion coefficient of the ionic species Y for the membrane in question.

The diffusion barrier D is more preferably an anion-conducting membrane, since this prevents the diffusion of protons from the anode chamber K _A into the middle chamber K _M particularly well.

In particular, a membrane which is selective for the anions comprised by the salt S is used as the anion-conducting membrane. Such membranes are known to those skilled in the art and can be used by them.

Salt S is preferably a halide, sulfate, sulfite, nitrate, bicarbonate or carbonate of X, more preferably a halide.

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

A membrane selective for halides, more preferably chloride, is preferably used as the anion-conducting membrane.

Anion-conducting membranes are described, for example, by MA Hickner, AM Herring, EB Coughlin, Journal of Polymer Science, Part B: Polymer Physics 2013, 51, 1727-1735 and Arges CG, Ramani V, Pintauro PN, Electrochemical Society Interface 2010, 19, 31-35 , WO 2007/048712 A2 as well as on Page 181 of the textbook by Volkmar M. Schmidt Electrochemical Process Engineering: Fundamentals, Reaction Engineering, Process Optimization, 1st edition (October 8, 2003 ).

Organic polymers, which are selected in particular from polyethylene, polybenzimidazoles, polyetherketones, polystyrene, polypropylene or fluorinated membranes such as polyperfluoroethylene, preferably polystyrene, are therefore even more preferably used as anion-conducting membrane, these covalently bonded functional groups selected from -NH ₃ ⁺ , - NRH ₂ ⁺ , -NR ₃ ⁺ , =NR ⁺ ;-PR ₃ ⁺ , where R is an alkyl group preferably having 1 to 20 carbon atoms, or other cationic groups. Preferably they have covalently bonded functional groups selected from -NH ₃ ⁺ , -NRH ₂ ⁺ , -NR ₃ ⁺ , more preferably selected from -NH ₃ ⁺ , -NR ₃ ⁺ , even more preferably -NR ₃ ⁺ .

If the diffusion barrier D is a cation-conducting membrane, it is in particular a membrane which is selective for the cations comprised by the salt S. The diffusion barrier D is even more preferably a membrane which conducts alkali cations, even more preferably a membrane which conducts potassium and/or sodium ions, most preferably a membrane which conducts sodium ions.

Cation-conducting membranes are described, for example, on page 181 of the textbook by Volkmar M. Schmidt Electrochemical Process Engineering: Fundamentals, Reaction Engineering, Process Optimization, 1st edition (October 8, 2003 ).

Accordingly, organic polymers, which are selected in particular from polyethylene, polybenzimidazoles, polyetherketones, polystyrene, polypropylene or fluorinated membranes such as polyperfluoroethylene, preferably polystyrene, polyperfluoroethylene, are even more preferably used as the cation-conducting membrane, with these covalently bonded functional groups selected from -SO ₃ - , -COO-, -PO ₃ ^2- , -PO ₂ H- , preferably -SO ₃ ^- , (described in DE 10 2010 062 804 A1 , US4,831,146 ) carry.

This can be, for example, a sulfonated polyperfluoroethylene (Nafion ^® with CAS number: 31175-20-9 ) be. These are the expert, for example, from WO 2008/076327 A1 , paragraph [058], US 2010/0044242 A1 , paragraph [0042] or the U.S. 2016/0204459 A1 known and under the Trade names Nafion ^® , Aciplex ^® F, Flemion ^® , Neosepta ^® , Ultrex ^® , PC-SK ^® commercially available. Neosepta ^® membranes are described, for example, by SA Mareev, D.Yu. Butylskii, ND Pismenskaya, C Larchet, L Dammak, VV Nikonenko, Journal of Membrane Science 2018, 563, 768-776 .

If a cation-conducting membrane is used as the diffusion barrier D , this can be, for example, a polymer functionalized with sulfonic acid groups, in particular of the following formula P _NAFION , where n and m independently of one another are an integer from 1 to 10 ⁶ , more preferably an integer from 10 to 10 ⁵ , more preferably is an integer from 10 ² to 10 ⁴ .

Any solid electrolyte which can transport cations, in particular alkali cations, more preferably sodium cations, from the central chamber K _M into the cathode chamber K _K can be used as the alkali cation-conducting solid electrolyte F _K . Such solid electrolytes are known to those skilled in the art and, for example, in DE 10 2015 013 155 A1 , in the WO 2012/048032 A2 , paragraphs [0035], [0039], [0040], in the US 2010/0044242 A1 , paragraphs [0040], [0041], in which DE 10360758 A1 , paragraphs [014] to [025]. They are sold commercially under the names NaSICON, LiSICON, KSICON. A sodium ion-conducting solid electrolyte F _K is preferred, this even more preferably having a NaSICON structure. NaSICON structures that can be used according to the invention are also described, for example, by N. Anantharamulu, K. Koteswara Rao, G. Rambabu, B. Vijaya Kumar, Velchuri Radha, M. Vithal, J Mater Sei 2011, 46 , 2821-2837.

NaSICON preferably has a structure of the formula M ^I 1 _+2w+x-y+z M ^II _w M ^III _x Zr ^IV _2-wxy M ^V _y (SiO ₄ ) _z (PO ₄ ) _3-z .

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

M" is a divalent metal cation preferably selected from Mg ²⁺ , Ca ²⁺ , Sr ²⁺ , Ba ²⁺ , Co ²⁺ , Ni ²⁺ , more preferably selected from Co ²⁺ , Ni ²⁺ .

M ^III is a trivalent metal cation, preferably selected from Al ³⁺ , Ga ³⁺ , Sc ³⁺ , La ³⁺ , Y ³⁺ , Gd ³⁺ , Sm ³⁺ , Lu ³⁺ , Fe ³⁺ , Cr ³⁺ , more preferably selected from Sc ³⁺ , La ³⁺ , Y ³⁺ , Gd ³⁺ , Sm ³⁺ , particularly preferably selected from Sc ³⁺ , Y ³⁺ , La ³⁺ .

M ^V is a pentavalent metal cation, preferably selected from V ⁵⁺ , Nb ⁵⁺ , Ta ⁵⁺ .

• w, x, y, z sind reelle Zahlen, wobei gilt, dass 0 ≤ x < 2, 0 ≤ y < 2, 0 ≤ w < 2, 0 ≤ z < 3,
• und wobei w, x, y, z so gewählt werden, dass gilt 1 + 2w + x - y + z ≥ 0 und 2 - w - x - y ≥ 0.

The Roman indices I, II, III, IV, V indicate the oxidation numbers in which the respective metal cations are present.

• 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.

More preferably, according to the invention, NaSICON has a structure of the formula Na _(1+v) Zr ₂ Si _v P _(3-v) O ₁₂ , where v is a real number such that 0≦v≦3. Most preferably v = 2.4

The cathode chamber K _K also comprises an inlet Z _KK and an outlet A _{KK ,} which makes it possible to add liquid, such as solution L ₂ , and liquid therein, such as solution L ₁ , to the cathode chamber K K removed. The inlet Z _KK and the outlet A _KK are attached to the cathode chamber K _K in such a way that the solution makes contact with the cathodic electrode E _K as it flows through the cathode chamber _{K K} . This is the prerequisite for the solution L ₁ being obtained at the outlet A _KK when the process according to the invention is carried out if the solution L ₂ of an alkali metal alkoxide XOR in the alcohol ROH is passed through _KK .

The anode chamber K _A also includes an outlet A _KA , which makes it possible to remove liquid, for example the aqueous solution L ₄ , located in the anode chamber K _A . In addition, the middle chamber K _M includes an inlet Z _KM , while K _A and K _M are connected to one another by a connection V _AM . As a result, a solution L ₃ can be added to K _M and this can then be passed through K _M and via V _AM into the anode chamber K _A , then through this K _A . V _AM and the outlet A _KA are attached to the anode chamber K _A in such a way that the solution L ₃ makes contact with the anodic electrode E _A as it flows through the anode chamber K _A . This is the prerequisite for the aqueous solution L ₄ being obtained at the outlet A _KA when the process according to the invention is carried out if the solution L ₃ is first passed through _KM , then V _AM , then _KA .

Inflows Z _KK , Z _KM , Z _KA and outflows A _KK , A _KA , A _KM can be attached to the electrolytic cell by methods known to those skilled in the art.

The connection V _AM can be formed within the electrolytic cell E and/or outside of the electrolytic cell E.

If the connection V _AM is formed within the electrolytic cell E , it is preferably formed by at least one perforation in the diffusion barrier D.

If the connection V _AM is formed outside the electrolytic cell E , it is preferably formed by a connection between K _M and K _A running outside the electrolytic cell E , in particular by the fact that in the middle chamber K _M a drain A _KM through the outer wall W _A is preferred is formed at the bottom of the middle chamber K _M , with the inflow Z _KM even more preferably being at the top of the middle chamber K _M , and in the anode chamber K _A an inflow Z _KA through the outer wall W _A , preferably at the bottom of the anode chamber K _A , is formed, and these are connected by a line, for example a pipe or a hose, which preferably comprises a material selected from rubber, plastic. The drain A _KA is then even more preferably at the top of the anode chamber K _A .

"Outflow A _KM at the bottom of the middle chamber K _M " means that the outflow A _KM is attached to the electrolytic cell E in such a way that the solution L ₃ leaves the middle chamber K _M in the same direction as gravity.

“Inlet Z _KA at the bottom of the anode chamber K _A ” means that the inlet Z _KA is attached to the electrolytic cell E in such a way that the solution L ₃ enters the anode chamber K _A against the force of gravity.

“Inlet Z _KM at the top of the middle chamber K _M ” means that the inlet Z _KM is attached to the electrolytic cell E in such a way that the solution L ₃ enters the middle chamber K _M in the same direction as gravity.

"Outlet A _KA at the top of the anode chamber K _A " means that the outlet A _KA is attached to the electrolytic cell E in such a way that the solution L ₄ leaves the anode chamber K _A against the force of gravity.

This embodiment is particularly advantageous and therefore preferred if the outlet A _KM is formed by the outer wall _WA at the bottom of the central chamber K _M and the inlet Z _KA is formed by the outer wall _WA at the bottom of the anode chamber _KA . This arrangement makes it particularly easy to separate gases produced in the central chamber K _M from L ₃ through the gas outlet G , while gases produced in the anode chamber K _A leave the anode chamber K _A with L ₄ and can then be further separated.

Accordingly, the direction of flow of L ₃ in K _M is the direction of flow of L ₃ in K _A in the opposite direction or in the same direction, preferably in the opposite direction, depending on how the connection V _AM is attached to the electrolytic cell E. Preferably, the direction of flow of L ₃ in K _M is in the same direction as gravity.

In a preferred embodiment of the present invention, connection V _AM is arranged between central chamber K _M and anode chamber K _A in such a way that at least part of the aqueous Solution L ₃ , more preferably the entire aqueous solution L ₃ , flows through the middle chamber K _M and the anode chamber K _A completely.

If the connection V _AM <112> is formed outside the electrolytic cell E <100>, this can be ensured in particular by the fact that Z _KM <108> and A _KM <118> are on opposite sides of the outer wall W _A <117> of the central chamber K _M <103> are arranged (i.e. Z _KM <108> on the bottom and A _KM <118> on the top of the electrolytic cell E <100> or vice versa) and Z _KA <119> and A _KA <106> on opposite sides of the outer wall W _A <117> of the anode chamber K _A <101> (i.e. Z _KA <119> on the bottom and A _KA <106> on the top of the electrolytic cell E <100> or vice versa), as is particularly the case in illustration 1 is shown. L ₃ <114> must flow through the two chambers _KM <103> and _KA <101> through this geometry. In this case, Z _KA <119> and Z _KM <108> can be formed on the same side of the electrolytic cell E <100>, with A _KM <118> and A _KA <106> then automatically also being formed on the same side of the electrolytic cell E <100> are. Alternatively, as in illustration 1 shown, Z _KA <119> and Z _KM <108> can be formed on opposite sides of the electrolytic cell E <100>, in which case A _KM <118> and A _KA <106> are then also automatically formed on opposite sides of the electrolytic cell E <100> are.

In particular, when the connection V _AM <112> is formed inside the electrolytic cell E <100>, this can be ensured by one side ("side A") of the electrolytic cell E <100>, which is the top or the bottom of the electrolytic cell E <100>, preferably as in Figure 2 shown is the top, includes the inlet Z _KM <108> and the outlet A _KA <106> and the diffusion barrier D <110> starting from this side A into the electrolytic cell <100>, but not quite to the of the Side A opposite side ("side B") of the electrolytic cell E <100>, which is then the bottom or the top of the electrolytic cell E <100>, and thereby 50% or more of the height of the three-chamber cell E <100>, more preferably 60% to 99% of the height of the three-chamber cell E <100>, more preferably 70% to 95% of the height of the three-chamber cell E <100>, even more preferably 80% to 90% of the height of the three-chamber cell E <100> , even more preferably 85% of the height of the three-chamber cell E <100>. Because the diffusion barrier D <110> does not touch side B of the three-chamber cell E <100>, a gap is created between the diffusion barrier D <110> and the outer wall W _A of side B of the three-chamber cell E <100>. The gap is then the connection V _AM <112>. Due to this geometry , L ₃ must flow completely through the two chambers K _M and K _A .

These embodiments best ensure that the aqueous salt solution L ₃ flows past the acid-sensitive solid electrolyte before it comes into contact with the anodic electrode E _A <104>, as a result of which acids are formed.

"Bottom of the electrolytic cell E" is, according to the invention, the side of the electrolytic cell E through which a solution (e.g. L ₃ <114> at A _KM <118> in illustration 1 ) exits the electrolytic cell E in the same direction as gravity or the side of the electrolytic cell E through which a solution (e.g. L ₂ <113> at Z _KK <107> in Figures 1 and 2 and L ₃ <114> at A _KA <119> in illustration 1 ) of the electrolytic cell E is fed against gravity.

According to the invention, "top side of the electrolytic cell E" is the side of the electrolytic cell E through which a solution (eg L ₄ <116> at A _KA <106> and L ₁ <115> at A _KK <109> in FIGS. 1 and 2) counteracts exits the electrolytic cell E under gravity or the side of the electrolytic cell E through which a solution (e.g. L ₃ <114> at Z _KM <108> in Figures 1 and 2) is fed to the electrolytic cell E in the same direction as gravity.

4.2 Process steps according to the invention

The method according to the invention comprises the following steps (a), (b) and (c), which are carried out simultaneously.

In step (a), a solution L ₂ comprising the alcohol ROH, preferably comprising an alkali metal alkoxide XOR in the alcohol ROH, is passed through K _K . X is an alkali metal cation and R is an alkyl group having 1 to 4 carbon atoms.

Preferably X is 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, methyl, more preferably selected from the group consisting of ethyl, methyl. Most preferably R is methyl.

The solution L ₂ is preferably free of water. According to the invention, “free of water” means that the weight of the water in the solution L ₂ based on the weight of the alcohol ROH in the solution L ₂ (mass ratio) is ≦1:10, more preferably ≦1:20, even more preferably ≦1:100 , more preferably ≤ 0.5:100.

If the solution L ₂ comprises XOR, the mass fraction of XOR in the solution L ₂ , based on the entire solution L ₂ , is in particular >0 to 30% by weight, preferably 5 to 20% by weight, even more preferably at 10 to 20% by weight, more preferably at 10 to 15% by weight, most preferably at 13 to 14% by weight, most preferably at 13% by weight.

If the solution L ₂ includes XOR, the mass ratio of XOR to alcohol ROH in the solution L ₂ is in particular in the range from 1:100 to 1:5, more preferably in the range from 1:25 to 3:20, even more preferably in the range 1:12 to 1:8, more preferably at 1:10.

In step (b) a neutral or alkaline aqueous solution L ₃ of a salt S comprising X as a cation is passed through K _M , then over V _AM , then through K _A .

The salt S is described above. The pH of the aqueous solution L ₃ is ≧7.0, preferably in the range from 7 to 12, more preferably in the range from 8 to 11, even more preferably from 10 to 11, most preferably at 10.5.

The mass fraction of the salt S in the solution L ₃ is preferably in the range > 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 at 20% by weight, based on the total solution L ₃ .

In step (c), a voltage is then applied between E _A and E _K .

This leads to a current transport from the charge source to the anode, to a charge transport via ions to the cathode and finally to a current transport back to the charge source. The charge source is known to those skilled in the art and is typically a rectifier that converts alternating current into direct current and can generate certain voltages via voltage converters.

• am Ablauf A_KK <109> wird die Lösung L₁ <115> erhalten, wobei die Konzentration von XOR in L₁ <115> höher ist als in L₂ <113>,
• am Ablauf A_KA <106> wird eine wässrige Lösung L₄ <116> von S erhalten, wobei die Konzentration von S in L₄ <116> geringer ist als in L₃ <114>.

This in turn leads to the following consequences:

• at the outlet A _KK <109> the solution L ₁ <115> is obtained, the concentration of XOR in L ₁ <115> being higher than in L ₂ <113>,
• at outlet A _KA <106> an aqueous solution L ₄ <116> of S is obtained, the concentration of S in L ₄ <116> being lower than in L ₃ <114>.

In the method according to the invention, such a voltage is applied in particular that such a current flows that the current density (= ratio of the current flowing to the electrolytic cell to the area of the solid electrolyte which contacts the anolyte located in the central chamber K _M ) is in the range of 10 to 8000 A/m ² , more preferably in the range of 100 to 2000 A/m ² , even more preferably in the range of 300 to 800 A/m ² , even more preferably 494 A/m ² . This can be determined by a person skilled in the art by default. The area of the solid electrolyte which contacts the anolyte located in the central chamber K _M is in particular 0.00001 to 10 m ² , preferably 0.0001 to 2.5 m ² , more preferably 0.0002 to 0.15 m ² , even more preferably 2.83 cm ² .

It goes without saying that step (c) is carried out in the method according to the invention when both chambers K _M and K _A are at least partially charged with L ₃ and K _K is at least partially charged with L ₂ .

The fact that in step (c) charge transport takes place between E _A and E _K implies that K _K , K _M and K _A are simultaneously charged with L ₂ and L ₃ , respectively, in such a way that they connect the electrodes E _A and E _K so far that the circuit is closed.

This is particularly the case when a liquid stream is continuously passed from L ₃ through _KM , V _AM and K _A and a liquid stream from L ₂ through K _K and the liquid stream from L ₃ passes the electrode _EA and the liquid stream from L ₂ the electrode E _K at least partially, preferably completely covered.

In a further preferred embodiment, the process according to the invention is carried out continuously, ie step (a) and step (b) are carried out continuously and voltage is applied in accordance with step (c).

After step (c) has been carried out, the solution L ₁ is obtained at the outlet A _KK , the concentration of XOR in L ₁ being higher than in L ₂ . If L ₂ already comprised XOR, the concentration of XOR in L ₁ is preferably 1.01 to 2.2 fold, more preferably 1.04 to 1.8 fold, even more preferably 1077 to 1.4 fold, even more preferably 1077 to 1077 fold 1.08-fold higher than in L ₂ , most preferably 1,077-fold higher than in L ₂ , more preferably with the mass fraction of XOR in L ₁ and in L ₂ being in the range 10 to 20% by weight, even more preferably 13 to 14% by weight.

At outlet A _KA an aqueous solution L ₄ of S is obtained, the concentration of S in L ₄ being lower than in L ₃ .

The concentration of the cation X in the aqueous solution L ₃ is preferably in the range from 3.5 to 5 mol/l, more preferably 4 mol/l. The concentration of the cation X in the aqueous solution L ₄ is more preferably 0.5 mol/l lower than that of the aqueous solution L ₃ used in each case.

In particular, the method according to the invention is carried out 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 a pressure of 0.5 bar to 1.5 bar, preferably 0.9 bar to 1.1 bar, more preferably 1.0 bar.

When the method according to the invention is carried out, hydrogen is typically produced in the cathode chamber K _K , which hydrogen can be removed from the cell via the outlet A _KK together with the solution L ₁ . The mixture of hydrogen and solution L ₁ can then in a particular embodiment of the present invention can be separated by methods known to those skilled in the art. In the anode chamber K _A , if the alkali metal compound used is a halide, in particular chloride, chlorine or another halogen gas can form, which can be removed from the cell via the outlet A _KK together with the solution L ₄ . In addition, oxygen and/or carbon dioxide can also be formed, which can also be removed. In a particular embodiment of the present invention, the mixture of chlorine, oxygen and/or CO2 and solution L ₄ can then be separated by methods known to those skilled in the art. Likewise, after the gases chlorine, oxygen and/or CO2 have been separated from the solution L ₄ , these can be separated from one another by methods known to those skilled in the art.

These results were surprising and not to be expected in the light of the prior art. The method according to the invention protects the acid-labile solid electrolyte from corrosion without having to sacrifice alcoholate solution from the cathode compartment as a buffer solution, as is the case in the prior art. The method according to the invention is thus more efficient than in WO 2008/076327 A1 described procedure in which the product solution is used for the middle chamber, which reduces the overall turnover.

Preferred Embodiments of the Invention

illustration 1 shows a preferred embodiment of the invention in a three-chamber cell E <100>. This comprises a cathode chamber K _K <102>, a middle chamber K _M <103> and an anode chamber K _A <101>. The anode chamber K _A <101> and the middle chamber K _M <103> are separated from one another by an anion exchange membrane extending over the entire cross section of the three-chamber cell E <100> as a diffusion barrier D <110>. The cathode chamber K _K <102> and the middle chamber K _M <103> are separated from one another by a permeable solid electrolyte (NaSICON) <111> that is selective for sodium ions and extends over the entire cross section of the three-chamber cell E <100>. The cathode chamber K _K <102> comprises a cathodic electrode E _K <105>, an inlet Z _KK <107> and an outlet A _KK <109>.

Anode chamber K _A <101> comprises an anodic electrode E _A <104> and drain A _KA <106> and is connected to middle chamber K _M <103> via connection V _AM <112>. The central chamber K _M <103> also includes an inlet Z _KM <108>. In the embodiment according to illustration 1 the connection V _AM <112> is formed outside the electrolytic cell E <100>, in particular by a tube or hose, the material of which can be selected from rubber, metal or plastic, with which liquid from the central chamber K _M <103> in the anode chamber K _A <101> can be routed outside the outer wall W _A <117> of the three-chamber cell E <100>. The connection V _AM <112> connects an outlet A _KM <118>, which breaks through the outer wall _WA <117> of the electrolysis cell E <100> at the bottom of the central chamber K _M <103>, with an inlet Z _KA <119>, which breaks through the outer wall W _A <117> of the electrolytic cell E <100> at the bottom of the anode chamber K _A <101>.

An electrolyte L ₂ <113> is conducted into the cathode chamber K _K <102> via the inlet Z _KK <107>. The electrolyte L ₂ <113> comprises methanol; a methanolic solution of sodium methoxide L ₂ <113> is preferably used as the electrolyte L ₂ <113>.

At the same time, an aqueous NaCl solution L ₃ <114> with pH 10.5 is introduced into the middle chamber _KM <103> via the inlet Z _KM <108>. This flows through the middle chamber K _M <103> and the connection V _AM <112> into the anode chamber K _A <101>.

A voltage is applied between the cathodic electrode E _K <105> and the anodic electrode E _A <104>. As a result, in the cathode chamber K _K <102>, methanol in the electrolyte L ₂ <113> is reduced to methoxide and H ₂ (CH ₃ OH + e → CH ₃ O ⁻ +½ H ₂ ). In the anode chamber K _A <101>, the oxidation of chloride ions to molecular chlorine takes place (Cl ^- → ½ Cl ₂ + e-). Chlorine gas Cl ₂ forms hypochlorous acid and hydrochloric acid in water according to the reaction Cl ₂ + H ₂ O → HOCl + HCl, which reacts acidically with other water molecules. The acidity damages the NaSICON solid electrolyte <111>, but is by the invention Arrangement in the anode chamber K _A <101> and thus kept away from the NaSICON solid electrolyte F _K <111> in the electrolytic cell E <100>. This increases its lifespan considerably.

Sodium ions diffuse from the middle chamber K _M <103> through the NaSICON solid electrolyte <111> into the cathode chamber K _K <102>. Overall, this increases the concentration of sodium methoxide in the cathode chamber K _K <102>, as a result of which a methanolic solution of sodium methoxide L ₁ <115> is obtained whose concentration of sodium methoxide is increased compared to L ₂ <113>. Due to the geometry of the three-chamber cell E <100> and the routing of the aqueous solution L ₃ <114> according to the invention, the acid-sensitive NaSICON solid electrolyte <111> is protected from the increased acidity compared to L ₃ <114> of the K _A <101> resulting in the anode chamber Solution L ₄ <116> protected.

In the Figure 2 The embodiment of the present invention shown corresponds to that in FIG illustration 1 shown. The only difference is that the connection V _AM <112> within the electrolytic cell E <100> is designed in such a way that the diffusion barrier D <110> does not extend over the entire cross section of the three-chamber cell E <100>. The connection V _AM <112> from the middle chamber K _M <103> to the anode chamber K _A <101> is thus formed by a gap in the diffusion barrier D <110>. In further preferred embodiments of the present invention, diffusion barriers D <110> with more than one gap can also be used, so that the connection V _AM <112> between central chamber K _M <103> and anode chamber K _A <101> extends through several gaps trains.

examples Inventive example

Sodium methylate (NM) was produced via a cathodic process, with 20% by weight NaCl solution (in water) being fed into the anode chamber and 10% by weight methanolic NM solution being fed into the cathode chamber. The electrolytic cell consisted of three chambers, as in illustration 1 shown with the anolyte being transferred through the middle compartment into the anode compartment. The connection between the middle and anode chamber is made by a hose that is attached to the bottom of the electrolytic cell. The anode compartment and middle compartment were separated by a 2.83 cm ² anion exchange membrane (Tokuyama AMX, ammonium groups on polymer). The cathode and middle chamber were separated by a ceramic of the NaSICON type with an area of 2.83 cm ² . The ceramic has a chemical composition of the formula Na _3.4 Zr _2.0 Si _2.4 P _0.6 O ₁₂ . The flow rate of the anolyte and that of the catholyte were each 90 mL/h and a current of 0.14 A was applied. The temperature was 35°C. The voltage curve (in V) over time (in hours) is in Figure 3 shown (●).

comparative example

The procedure was repeated with a two-chamber cell comprising only an anode and a cathode chamber, the anode chamber being separated from the cathode chamber by the NaSICON-type ceramic. Thus, this electrolytic cell did not contain a center chamber. This is reflected in a faster corrosion of the ceramic compared to the inventive example, which leads to a rapid increase in the voltage curve, see Figure 3 , (▲).

Result

By using a three-chamber cell as in the method according to the invention, corrosion of the solid electrolyte is prevented, while at the same time no alkali metal alkoxide product has to be sacrificed for the middle chamber.

Claims (15)

1. Process for preparing a solution L₁ <115> of an alkali metal alkoxide XOR in the alcohol ROH in an electrolysis cell E <100>,

   wherein E <100> comprises at least one anode chamber K_A <101>, at least one cathode chamber K_K <102> and at least one interposed middle chamber K_M <103>,

   wherein K_A <101> comprises an anodic electrode E_A <104> and an outlet A_KA <106>,

   wherein K_K <102> comprises a cathodic electrode E_K <105>, an inlet Z_KK <107> and an outlet A_KK <109>,

   wherein K_M <103> comprises an inlet Z_KM <108>, is separated from K_A <101> by a diffusion barrier D <110> and is separated from K_K <102> by an alkali metal cation-conducting solid-state electrolyte F_K <111>,

   wherein K_M <103> and K_A <101>are connected to one another by a connection V_AM <112> through which liquid can be routed from K_M <103> into K_A <101>,

   wherein the process comprises the following steps (a), (b) and (c) that proceed simultaneously:

   (a) a solution L₂ <113> comprising the alcohol ROH is routed through K_K <102>,

   (b) a neutral or alkaline, aqueous solution L₃ <114> of a salt S comprising X as cation is routed through K_M , then via V_AM , then through K_A <101>,

   (c) voltage is applied between E_A <104> and E_K <105>,

   which affords the solution L₁ <115> at the outlet A_KK <109>, wherein the concentration of XOR in L₁ <115> is higher than in L₂ <113>,

   and which affords an aqueous solution L₄ <116> of S at the outlet A_KA <106>, wherein the concentration of S in L₄ <116> is lower than in L₃ <114>,

   wherein X is an alkali metal cation and R is an alkyl radical having 1 to 4 carbon atoms.
2. Process according to Claim 1, wherein X is selected from the group consisting of Li⁺, Na⁺, K⁺.
3. Process according to Claim 1 or 2, wherein S is a halide, sulfate, sulfite, nitrate, hydrogencarbonate or carbonate of X.
4. Process according to any of Claims 1 to 3, wherein R is selected from the group consisting of methyl and ethyl.
5. Process according to any of Claims 1 to 4, wherein the diffusion barrier D <110> is selected from the group consisting of cation-conducting and anion-conducting membranes.
6. Process according to Claim 5, wherein the diffusion barrier D <110> is a sodium cation-conducting membrane.
7. Process according to any of Claims 1 to 6, wherein the flow direction of L₃ <114> in the middle chamber K_M <103> is the opposite of the flow direction of L₃ <114> in the anode chamber K_A <101>.
8. Process according to any of Claims 1 to 7, wherein the connection V_AM <112> is formed within and/or outside the electrolysis cell E <100>.
9. Process according to any of Claims 1 to 8, wherein the connection V_AM <112> between middle chamber K_M <103> and anode chamber K_A <101> is arranged in such a way that at least a portion of the aqueous solution L₃ <114> flows completely through the middle chamber K_M <103> and the anode chamber K_A <101>.
10. Process according to any of Claims 1 to 9, wherein the alkali metal ion-conducting solid-state electrolyte F_K <111> has a 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,

    where M^I is selected from Na⁺ and Li⁺,

    M^II is a divalent metal cation,

    M^III is a trivalent metal cation,

    M^V is a pentavalent metal cation,

    the Roman indices I, II, III, IV, V indicate the oxidation numbers in which the respective metal cations exist,

    and w, x, y, z are real numbers, where 0 ≤ x < 2, 0 ≤ y < 2, 0 ≤ w < 2, 0 ≤ z < 3,

    and where w, x, y, z are chosen such that 1 + 2w + x - y + z ≥ 0 and 2 - w - x - y ≥ 0.
11. Process according to any of Claims 1 to 10, wherein L₂ <113> comprises the alcohol ROH and an alkali metal alkoxide XOR.
12. Process according to Claim 11, wherein the mass ratio of XOR to alcohol ROH in L₂ <113> is in the range from 1:100 to 1:5.
13. Process according to Claim 11 or 12, wherein the concentration of XOR in L₁ <115> is 1.01 to 2.2 times higher than in L₂ <113>.
14. Process according to any of Claims 1 to 13, wherein the concentration of X in L₃ <114> is in the range from 3.5 to 5 mol/l.
15. Process according to any of Claims 1 to 14, which is performed at a temperature of 20 to 70°C and a pressure of 0.5 to 1.5 bar.

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