Patent Application
20240295034
The present invention relates, in a first aspect, to an electrolysis cell having three chambers, wherein the middle chamber is separated from the cathode chamber by a solid-state electrolyte permeable to cations, for example NaSICON, and from the anode chamber by a diffusion barrier. The invention is characterized in that the middle chamber comprises a mechanical stirring device. The electrolysis cell according to the invention solves the problem that a concentration gradient forms in the middle chamber of the electrolysis cell during the electrolysis, which leads to locally lowered pH values and hence to damage to the solid-state electrolyte. With the aid of the mechanical stirring device, it is possible to stir the electrolyte solution in the middle chamber during the electrolysis, which prevents the formation of a pH gradient.
In a second aspect, the present invention relates to a process for producing an alkali metal alkoxide solution in the electrolysis cell according to the invention.
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.
The prior art accordingly describes processes that are performed in electrolysis cells with an ion-permeable layer, for example NaSICON solid-state electrolytes. However, these solid-state electrolytes typically have the disadvantage that they lack long-term stability towards 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 preparing 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
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 [0008] and [0067] 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 is the desired product, but this is consumed and continuously contaminated as buffer solution. 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 an object of the present invention to provide an improved process for electrolytic production of alkali metal alkoxides, and also an electrolysis chamber 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.
What have now been found, surprisingly, are an electrolysis cell and a process that solve the problem addressed by the invention.
The electrolysis cell E <100> in the first aspect of the invention comprises at least one anode chamber KA <101>, at least one cathode chamber KK <102> and at least one interposed middle chamber KM <103>,
In a second aspect, the present invention relates to a process for producing a solution L1 <115> of an alkali metal alkoxide XOR in the alcohol ROH in an electrolysis cell E <100> according to the first aspect of the invention,
which affords the solution L1 <115> at the outlet AKK <109>, with a higher concentration of XOR in L1 <115> than in L2 <113>,
and which affords an aqueous solution L4 <116> of S at the outlet AKA <106>, with a lower concentration of S in L4 <116> than in L3 <114>,
wherein X is an alkali metal cation and R is an alkyl radical having 1 to 4 carbon atoms.
The cathode chamber KK <102> comprises a cathodic electrode EK <105>, an inlet ZKK <107> and an outlet AKK <109>.
The anode chamber KA <101> comprises an anodic electrode EA <104> and an outlet AKA <106> and is connected to the middle chamber KM <103> via the connection VAM <112>.
The middle chamber KM <103> comprises an inlet ZKM <108>.
The three chambers are bounded by an outer wall <117> of the three-chamber cell E <100>. The cathode chamber KK <102> is also separated from the middle chamber KM <103> by an NaSICON solid-state electrolyte FK <111> which is selectively permeable to sodium ions. The middle chamber KM <103> is additionally separated in turn from the anode chamber KA <101> by a diffusion barrier D <110>. The NaSICON solid-state electrolyte FK <111> and the diffusion barrier D <110> extend over the entire depth and height of the three-chamber cell E <100>. The diffusion barrier D <110> is made of glass.
In the embodiment according to
An aqueous solution of sodium chloride L3 <114> with pH 10.5 is introduced via the inlet ZKM <108>, in the direction of gravity, into the middle chamber KM <103>. The connection VAM <112> formed between an outlet AKM <118> from the middle chamber KM <103> and an inlet ZKA <119> to the anode chamber KA <101> connects the middle chamber KM <103> to the anode chamber KA <101>. Sodium chloride solution L3 <114> is routed through this connection VAM <112> from the middle chamber KM <103> into the anode chamber KA <101>.
A solution of sodium methoxide in methanol L2 <113> is routed into the cathode chamber KK <102> via the inlet ZKK <107>.
At the same time, a voltage is applied between the cathodic electrode EK <105> and the anodic electrode EA <104>. This results in reduction of methanol in the electrolyte L2 <113> to give methoxide and H2 in the cathode chamber KK <102> (CH3OH+e−→CH3O−+½ H2). At the same time, sodium ions diffuse from the middle chamber KM <103> through the NaSICON solid-state electrolyte FK <111> into the cathode chamber KK <102>. Overall, this increases the concentration of sodium methoxide in the cathode chamber KK <102>, which affords a methanolic solution of sodium methoxide L1 <115>, the sodium methoxide concentration of which is elevated compared to L2 <113>.
In the anode chamber KA <101>, the oxidation of chloride ions takes place to give molecular chlorine (Cl−→½Cl2+e−). In the outlet AKA <106>, an aqueous solution L4 <116> is obtained, in which the content of NaCl is reduced compared to L3 <114>. 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 <111>, but is restricted to the anode chamber KA <101> by the arrangement according to the invention, and hence kept away from the NaSICON solid-state electrolyte FK <111> in the electrolysis cell E <100>. This considerably increases the lifetime thereof.
In the middle chamber KM <103>, there is also a mechanical stirring device <120> in the form of a propeller stirrer <121> which is operated by an electric motor <122>, with the propeller stirrer connected to the electric motor via a transmission link <124>. The propeller stirrer <121> is freely suspended in the middle chamber KM <103>, but may also be secured on the inside of the outer wall WA <117>. The transmission link <124> reaches through a cutout <125> in the outer wall of the middle chamber KM <103> into the electrolysis cell E <100>. The aqueous solution L3 <114> supplied through the inlet ZKM <108> is mixed by the operation of the propeller stirrer <121>, which results in vortexing and turbulence. This turbulence L3 <114> in the solution prevents buildup of a pH gradient in the middle chamber KM <103> with progressive electrolysis, and prevents the development of a low pH in the solution directly adjoining the NaSICON solid-state electrolyte <111>. This further increases the service life of the NaSICON solid-state electrolyte <111>.
The first aspect of the invention relates to an electrolysis cell E <100>. The electrolysis cell E <100> in the first aspect of the invention comprises at least one anode chamber KA <101>, at least one cathode chamber KK <102> and at least one interposed middle chamber KM <103>. This also includes electrolysis cells E <100> having more than one anode chamber KA <101> and/or cathode chamber KK <102> and/or middle chamber KM <103>. Such electrolysis cells in which these chambers are joined to one another in the form of modules are described, for example, in DD 258 143 A3 and US 2006/0226022 A1.
The anode chamber KA <101> comprises an anodic electrode EA <104>. A useful anodic electrode EA <104> of this kind is any electrode familiar to the person skilled in the art that is stable under the conditions of the process according to the invention in the second aspect of the invention. These are described, in particular, in WO 2014/008410 A1, paragraph [024] or DE 10360758 A1, paragraph [031]. This electrode EA <104> may consist of one layer or consist of multiple planar layers parallel to one another that may each be perforated or expanded. The anodic electrode EA <104> especially comprises a material selected from the group consisting of ruthenium oxide, iridium oxide, nickel, cobalt, nickel tungstate, nickel titanate, precious metals such as, in particular, platinum, supported on a support such as titanium or Kovar® (an iron/nickel/cobalt alloy in which the individual components are preferably as follows: 54% by mass of iron, 29% by mass of nickel, 17% by mass of cobalt). Further possible anode materials are especially stainless steel, lead, graphite, tungsten carbide, titanium diboride. Preferably, the anodic electrode EA <104> comprises a titanium anode coated with ruthenium oxide/iridium oxide (RuO2+IrO2/Ti).
The cathode chamber KK <102> comprises a cathodic electrode EK <105>. A useful cathodic electrode EK <105> of this kind is any electrode familiar to the person skilled in the art that is stable under the conditions. These are described, in particular, in WO 2014/008410 A1, paragraph [025] or DE 10360758 A1, paragraph [030]. This electrode EK <105> may be selected from the group consisting of mesh wool, three-dimensional matrix structure and “balls”. The cathodic electrode EK <105> especially comprises a material selected from the group consisting of steel, nickel, copper, platinum, platinized metals, palladium, carbon-supported palladium, titanium. Preferably, EK <105> comprises nickel.
The at least one middle chamber KM <103> is between the anode chamber KA <101> and the cathode chamber KK <102>.
The electrolysis cell E <100> typically has an outer wall WA <117>. The outer wall WA <117> is especially made from a material selected from the group consisting of steel, preferably rubberized steel, plastic, especially from Telene® (thermoset polydicyclopentadiene), PVC (polyvinylchloride), PVC-C (post-chlorinated polyvinylchloride), PVDF (polyvinylidenefluoride). WA <117> may especially be perforated for inlets and outlets. Within WA <117> are then the at least one anode chamber KA <101>, the at least one cathode chamber KK <102> and the at least one interposed middle chamber KM <103>.
KM <103> is separated from KA <101> by a diffusion barrier D <110> and from KK <102> by an alkali metal cation-conducting solid-state electrolyte FK <111>.
For the diffusion barrier D <110>, it is possible to use any material that is stable under the conditions of the process according to the invention in the second aspect of the invention and prevents or slows the transfer of protons from the liquid present in the anode chamber KA <101> into the middle chamber KM <103>.
The diffusion barrier D <110> used is especially a non-ion-specific dividing wall or a membrane permeable to specific ions. The diffusion barrier D <110> is preferably a non-ion-specific dividing wall.
The material of the non-ion-specific dividing wall is especially selected from the group consisting of fabric, which is especially textile fabric or metal weave, glass, which is especially sintered glass or glass frits, ceramic, especially ceramic frits, membrane diaphragms, and is more preferably glass.
If the diffusion barrier D <110> is a “membrane permeable to specific ions”, what this means in accordance with the invention is that the respective membrane promotes the diffusion of particular ions therethrough over other ions. More particularly, what this means is membranes that promote the diffusion therethrough of ions of a particular charge type over ions of the opposite charge. Even more preferably, membranes permeable to specific ions also promote the diffusion of particular ions of one charge type over other ions of the same charge type therethrough.
If the diffusion barrier D <110> is a “membrane permeable to specific ions”, the diffusion barrier D <110> is especially an anion-conducting membrane or a cation-conducting membrane.
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 <110> is a “membrane permeable to specific ions”, it is preferably an anion-conducting membrane since this particularly efficiently prevents the diffusion of protons from the anode chamber KA <101> into the middle chamber KM <103>.
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, AM. Herring, E. B. Coughlin, Journal of Polymer Science, Part B: Polymer Physics 2013, 51, 1727-1735, by C. G. Arges, V. Ramani, P. N. Pintauro, Electrochemical Society Interface 2010, 19, 31-35, in WO 2007/048712 A2, and on page 181 of the textbook by Volkmar M. Schmidt, Elektrochemische Verfahrenstechnik-Grundlagen, Reaktionstechnik, Prozessoptimierung [Electrochemical Engineering: Fundamentals, Reaction Technology, Process Optimization], 1st edition (8 Oct. 2003).
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 <110> is a cation-conducting membrane, it is especially a membrane selective for the cations encompassed by the salt S. Even more preferably, the diffusion barrier D <110> is an alkali metal cation-conducting membrane, even more preferably a potassium and/or sodium ion-conducting membrane, most preferably a sodium ion-conducting membrane.
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 [0042] or US 2016/0204459 A1, and are commercially available under the Nafion®, Aciplex® F, Flemion®, Neosepta®, Ultrex®, PC-SK® trade names. Neosepta® membranes are described, for example, by S. A. Mareev, D. Yu. Butylskii, N. D. Pismenskaya, C. Larchet, L. Dammak, V. V. Nikonenko, Journal of Membrane Science 2018, 563, 768-776.
If a cation-conducting membrane is used as diffusion barrier D <110>, this may, for example, be a polymer functionalized with sulfonic acid groups, especially of the formula PNAFION 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.
A useful alkali metal cation-conducting solid-state electrolyte FK <111> is any solid-state electrolyte that can transport cations, especially alkali metal cations, even more preferably sodium cations, from the middle chamber KM <103> into the cathode chamber KK <102>. Such solid-state electrolytes are known to the person skilled in the art and are described, for example, in DE 10 2015 013 155 A1, in WO 2012/048032 A2, paragraphs [0035], [0039], [0040], in US 2010/0044242 A1, paragraphs [0040], [0041], in DE 10360758 A1, paragraphs [014] to [025]. They are sold commercially under the NaSICON, LiSICON, KSICON name. A sodium ion-conducting solid-state electrolyte FK <111> is preferred, and this even more preferably has an NaSICON structure. NaSICON structures usable in accordance with the invention are also described, for example, by N. Anantharamulu, K. Koteswara Rao, G. Rambabu, B. Vijaya Kumar, Velchuri Radha, M. Vithal, J Mater Sci 2011, 46, 2821-2837.
NaSICON preferably has a structure of the formula
MI1+2w+x−y+zMIIwMIIIxZrIV2−w−x−y MVy(SiO4)z(PO4)3−z.
MI is selected from Na+, Li+, preferably Na+.
MII is a divalent metal cation, preferably selected from Mg2+, Ca2+, Sr2+, Ba2+, Co2+, Ni2+, more preferably selected from Co2+, Ni2+.
MIII 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+.
MV 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, NaSICON has a structure of the formula Na(1+v)Zr2SivP(3−v)O12 where v is a real number for which 0≤v≤3. Most preferably, v=2.4.
The cathode chamber KK <102> also comprises an inlet ZKK <107> and an outlet AKK <109> that enables addition of liquid, for example the solution L2 <113>, to the cathode chamber KK <102> and removal of liquid present therein, for example the solution L1 <115>. The inlet ZKK <107> and the outlet AKK <109> are mounted on the cathode chamber KK <102> in such a way that the solution comes into contact with the cathodic electrode EK <105> as it flows through the cathode chamber KK <102>. This is a prerequisite for the solution L1 <115> to be obtained at the outlet AKK <109> in the performance of the process according to the invention in the second aspect of the invention when the solution L2 <113> of an alkali metal alkoxide XOR in the alcohol ROH is routed through KK <102>.
The anode chamber KA <101> also comprises an outlet AKA <106> that enables removal of liquid present in the anode chamber KA <101>, for example the aqueous solution L4 <106>. In addition, the middle chamber KM <103> comprises an inlet ZKM <108>, while KA <101> and KM <103> are connected to one another by a connection VAM <112> through which liquid can be routed from KM <103> into KA <101>. As a result, a solution L3 <114> can be introduced via the inlet ZKM <108> to KM <103>, and this can be routed through KM <103>, then via VAM <112>, into the anode chamber KA <101>, and finally through the anode chamber KA <101>. VAM <112> and the outlet AKA <106> are mounted on the anode chamber KA <101> in such a way that the solution L3 <114> comes into contact with the anodic electrode EA <104> as it flows through the anode chamber KA <101>. This is a prerequisite for the aqueous solution L4 <116> to be obtained at the outlet AKA <106> in the performance of the process according to the invention in the second aspect when the solution L3 <114> is routed first through KM <103>, then VAM <112>, then KA <101>.
The inlets ZKK <107>, ZKM <108>, ZKA <119> and outlets AKK <109>, AKA <106>, AKM <118> may be mounted on the electrolysis cell E <100> by methods known to the person skilled in the art.
The connection VAM <112> may be formed within the electrolysis cell E <100> and/or outside, preferably within, the electrolysis cell E <100>.
If the connection VAM <112> is formed within the electrolysis cell E <100>, it is preferably formed by at least one perforation in the diffusion barrier D <110>.
If the connection VAM <112> is formed outside the electrolysis cell E <100>, it is preferably formed by a connection of KM <103> and KA <101> that runs outside the electrolysis cell E <100>, especially in that an outlet AKM <118> through the outer wall WA <117> is formed in the middle chamber KM <103>, preferably at the base of the middle chamber KM <103>, the inlet ZKM <108> more preferably being at the top end of the middle chamber KM <103>, and an inlet ZKA <119> through the outer wall WA <117> is formed in the anode chamber KA <101>, preferably at the base of the anode chamber KA <101>, and these are connected by a conduit, for example a pipe or a hose, preferably comprising a material selected from rubber and plastic. The outlet AKA <106> is then more preferably at the top end of the anode chamber KA <101>.
What is meant by “outlet AKM <118> at the base of the middle chamber KM <103>” is that the outlet AKM <118> is attached to the electrolysis cell E <100> in such a way that the solution L3 <114> leaves the middle chamber KM <103> in the direction of gravity.
What is meant by “inlet ZKA <119> at the base of the anode chamber KA <101>” is that the inlet ZKA <119> is attached to the electrolysis cell E <100> in such a way that the solution L3 <114> enters the anode chamber KA <101> counter to gravity.
What is meant by “inlet ZKM <108> at the top end of the middle chamber KM <103>” is that the inlet ZKM <108> is attached to the electrolysis cell E <100> in such a way that the solution L3 <114> enters the middle chamber KM <103> in the direction of gravity.
What is meant by “outlet AKA <106> at the top end of the anode chamber KA <101>” is that the outlet AKA <106> is mounted on the electrolysis cell E <100> in such a way that the solution L4 <116> leaves the anode chamber KA <101> counter to gravity.
This embodiment is particularly advantageous and therefore preferred when the outlet AKM <118> is formed by the outer wall WA <117> at the base of the middle chamber KM <103>, and the inlet ZKA <119> by the outer wall WA <117> at the base of the anode chamber KA <101>. This arrangement makes it possible in a particularly simple manner to remove gases formed in the anode chamber KA from the anode chamber KA <101> with L4 <116>, in order to separate them further.
When the connection VAM <112> is formed outside the electrolysis cell E <100>, in particular, ZKM <108> and AKM <118> are arranged at opposite ends of the outer wall WA <117> of the middle chamber KM <103> (i.e. ZKM <108> at the base and AKM <118> at the top end of the electrolysis cell E <100> or vice versa) and ZKA <119> and AKA <106> are arranged at opposite ends of the outer wall WA <117> of the anode chamber KA <101> (i.e. ZKA <119> at the base and AKA <106> at the top end of the electrolysis cell E <100> or vice versa), as shown more particularly in
When the connection VAM <112> is formed within the electrolysis cell E <100>, this may especially be implemented in that one side (“side A”) of the electrolysis cell E <100>, which is the top end or the base of the electrolysis cell E <100>, preferably the top end as shown in
These embodiments best assure that the aqueous salt solution L3 <114> flows past the acid-sensitive solid-state electrolyte before it comes into contact with the anodic electrode EA <104>, which results in the formation of acids.
According to the invention, “base of the electrolysis cell E <100>” is the side of the electrolysis cell E <100> through which a solution (e.g. L3 <114> in the case of AKM <118> in
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. L4 <116> in the case of AKA <106> and L1 <115> in the case of AKK <109> in
According to the invention, the middle chamber KM comprises a mechanical stirring device <120>. According to the invention, the mechanical stirring device <120> is in the solid state of matter. A suitable mechanical stirring device of this kind is any stirring device known to the person skilled in the art that is sufficiently inert to the electrolysis conditions.
The mechanical stirring device <120> especially comprises at least one material selected from rubber; plastic, which is especially selected from polystyrene, polypropylene, PVC, PVC-C; glass; porcelain; metal. The metal is especially a metal or an alloy of two or more metals selected from titanium, iron, molybdenum, chromium, nickel, platinum, gold, silver, preferably an alloy comprising at least two metals selected from titanium, iron, molybdenum, chromium, nickel, platinum, gold, silver, even more preferably a steel alloy comprising, as well as iron, at least one further metal selected from titanium, molybdenum, chromium, nickel, platinum, gold, silver, and is most preferably stainless steel.
Even more preferably, the mechanical stirring device <120> comprises magnetic material, such that it can be operated with a magnetic stirrer system.
The mechanical stirring device <120> is especially selected from propeller stirrer, pitched blade stirrer, disk stirrer, tumbling disk stirrer, hollow blade stirrer, impeller stirrer, crossbeam stirrer, anchor stirrer, paddle stirrer, gate stirrer, helical stirrer, toothed disk stirrer, low-volume stirrer, preferably a propeller stirrer.
The mechanical stirring device <120> is typically driven by a motor, which is preferably an electric motor outside the electrolysis cell E <100>. For example, this may be a motor <122> connected to the propeller stirrer <121> via a transmission link <124>, with the transmission link <124> extending into the electrolysis cell E <100> through a cutout <125> in the outer wall of the middle chamber KM <103>, as illustrated in
Alternatively, the propeller stirrer may also be magnetic, such that it is a magnetic stirrer bar <123-1> which is driven by a magnetic stirrer system <123-2> disposed outside the middle chamber KM <103>, as illustrated in
The magnetic stirring device <120> may be suspended loosely in the middle chamber KM <103>, as shown in
Alternatively, the mechanical stirring device <120> may also be secured, for example on the solid-state electrolyte FK <111>, on the diffusion barrier D <110>, or on the outer wall <117> that bounds the inside of the middle chamber KM <103>. The securing can be effected by methods known to the person skilled in the art, for example by screw connection, clamping, adhesive bonding (polymer adhesive, PVC adhesive).
In a preferred embodiment of the electrolysis cell E <100> according to the first aspect of the invention, the mechanical stirring device <120> comprises a propeller aligned parallel to the alkali metal cation-conducting solid-state electrolyte FK <111>.
In a preferred embodiment of the electrolysis cell E <100> according to the first aspect of the invention, the mechanical stirring device <120> accounts for a proportion 4 of 1% to 99%, more preferably 2% to 50%, even more preferably 3% to 40%, even more preferably 4% to 30%, even more preferably 5% to 20%, most preferably 6% to 10%, of the volume encompassed by the middle chamber KM.
The proportion ζ (in %) is calculated by ζ=[(VO−VM)/VO]*100.
VO here is the maximum volume of liquid, for example the electrolyte L3 <114>, that can be accommodated by the middle chamber KM <103> if it does not comprise a mechanical stirring device <120>.
VM here is the maximum volume of liquid, for example the electrolyte L3 <114>, that can be accommodated by the middle chamber KM <103> if it comprises the mechanical stirring device <120>.
It has been found that, surprisingly, the mechanical stirring device <120> in the middle chamber KM <103> leads to turbulence and vortexing in the electrolyte L3 <114> that flows through the middle chamber KM <103> during the process according to the invention. This slows or entirely prevents the buildup of a pH gradient during the electrolysis, which protects the acid-sensitive solid-state electrolyte FK <111> and hence enables a longer run time for the electrolysis or extends the lifetime of the electrolysis cell E <100>.
It will be apparent that the mechanical stirring device <120> is mounted in the middle chamber KM <103> such that it sufficiently enables, or does not completely block, the flow of the electrolyte L3 <114> through the middle chamber KM <103> and the anode chamber KA <101>.
In a preferred embodiment of the electrolysis cell according to the invention, the mechanical stirring device <120> interrupts the direct pathway in the middle chamber KM between inlet ZKM <108> and connection VAM <112>.
Whether the direct route between inlet ZKM <108> and connection VAM <112> in the middle chamber KM is interrupted is ascertained by the following “thread test”:
The thread is especially made of sewing thread (for example from Gütermann), fishing line, string.
Most preferably, a fishing line with a diameter of 0.2 mm is used for the thread test, as sold, for example, by Hemingway or Nexos.
The process according to the second aspect of the invention is one for producing a solution L1 <115> of an alkali metal alkoxide XOR in the alcohol ROH in an electrolysis cell E <100> according to the first aspect of the invention,
The process according to the second aspect of the invention comprises the steps (a), (b) and (c) that proceed simultaneously.
In step (a), a solution L2 <113> comprising the alcohol ROH, preferably comprising an alkali metal alkoxide XOR and alcohol ROH, is routed through KK <102>. X is an alkali metal cation and R is an alkyl radical having 1 to 4 carbon atoms.
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.
Solution L2 <113> is preferably free of water. What is meant in accordance with the invention by “free of water” is that the weight of water in solution L2 <113> based on the weight of the alcohol ROH in solution L2 <113> (mass ratio) is ≤1:10, more preferably ≤1:20, even more preferably ≤1:100, even more preferably ≤0.5:100.
If solution L2 <113> comprises XOR, the proportion by mass of XOR in solution L2 <113>, based on the overall solution L2 <113>, is especially >0% to 30% by weight, preferably 5% to 20% by weight, more preferably 10% to 20% by weight, more preferably 10% to 15% by weight, most preferably 13% to 14% by weight, at the very most preferably 13% by weight.
If solution L2 <113> comprises XOR, the mass ratio of XOR to alcohol ROH in solution L2 <113> is especially in the range of 1:100 to 1:5, more preferably in the range of 1:25 to 3:20, even more preferably in the range of 1:12 to 1:8, even more preferably 1:10.
In step (b), a neutral or alkaline, aqueous solution L3 <114> of a salt S comprising X as cation is routed through KM <103>, then via VAM <112>, then through KA <101>, while the mechanical stirring device <120> stirs the solution L3 <114> in KM <103>.
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 L3 <114> is ≥7.0, preferably in the range of 7 to 12, more preferably in the range of 8 to 11, even more preferably 10 to 11, most preferably 10.5.
The proportion by mass of salt S in solution L3 <113> is preferably in the range of >0% to 20% by weight, preferably 1% to 20% by weight, more preferably 5% to 20% by weight, even more preferably 10% to 20% by weight, most preferably 20% by weight, based on the overall solution L3 <113>.
In step (c), a voltage is then applied between EA <104> and EK <105>.
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:
In the process according to the second aspect of the invention, in particular, such a voltage is applied that such a current flows that the current density (=ratio of the current supplied to the electrolysis cell to the area of the solid-state electrolyte in contact with the anolyte present in the middle chamber KM <103>) 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 KM <103> 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 (c) of the process according to the second aspect of the invention is performed when the two chambers KM <103> and KA <101> are at least partly laden with L3 <114> and KK <102> is at least partly laden with L2 <113>.
The fact that in step (c) transfer of charge takes place between EA <104> and EK <105> implies that KK <102>, KM <103> and KA <101> are simultaneously laden with L2 <113> or L3 <114> such that they cover the electrodes EA <104> and EK <105> to such an extent that the circuit is complete.
This is the case especially when a liquid stream of L3 <114> is routed continuously through KM <103>, VAM <112> and KA <101> and a liquid stream of L2 <113> through KK <102>, and the liquid stream of L3 <114> covers electrode EA <104> and the liquid stream of L2 <113> covers electrode EK <105> at least partly, preferably completely.
By virtue of the stream of the electrolyte L3 <114> being stirred by the mechanical stirring device <120> in the middle chamber KM <103>, there is no formation of a typical pH gradient in this chamber. This effect is even stronger when the mechanical stirring device <120> breaks the direct pathway in the middle chamber KM between inlet ZKM <108> and connection VAM <112>, since the mechanical stirring device <120> is then within the flow pathway of the electrolyte L3 <114> through the middle chamber KM <103> and disrupts unhindered flow.
This desired effect can be amplified in the process according to the second aspect of the invention by varying the stirring speed of the mechanical stirring device <120> during the performance of step (b), which can produce further turbulence that disrupts the formation of a pH gradient.
In a further preferred embodiment, the process according to the second aspect of the invention is performed continuously, i.e. step (a) and step (b) are performed continuously, while applying voltage as per step (c).
After performance of step (c), solution L1 <115> is obtained at the outlet AKK <109>, with the concentration of XOR in L1 <115> being higher than in L2 <113>. If L2 <113> already comprised XOR, the concentration of XOR in L1 <115> is preferably 1.01 to 2.2 times, more preferably 1.04 to 1.8 times, even more preferably 1.077 to 1.4 times, even more preferably 1.077 to 1.08 times, higher than in L2 <113>, most preferably 1.077 times higher than in L2 <113>, where the proportion by mass of XOR in L1 <115> and in L2 <113> is more preferably in the range from 10% to 20% by weight, even more preferably 13% to 14% by weight.
An aqueous solution L4 <116> of S is obtained at the outlet AKA <106>, with a lower concentration of S in L4 <116> than in L3 <114>.
The concentration of the cation X in the aqueous solution L3 <114> is preferably in the range of 3.5 to 5 mol/l, more preferably 4 mol/l. The concentration of the cation X in the aqueous solution L4 <116> is more preferably 0.5 mol/l lower than that of the aqueous solution L3 <114> used in each case.
In particular, the process according to the second aspect of the invention is conducted at a temperature of 20° C. to 70° C., preferably 35° C. to 65° C., more preferably 35° C. to 60° C., even more preferably 35° C. to 50° C., and at a pressure of 0.5 bar to 1.5 bar, preferably 0.9 bar to 1.1 bar, more preferably 1.0 bar.
In the course of performance of the process according to the invention, hydrogen is typically formed in the cathode chamber KK <102>, which can be removed from the cell together with solution L1 <115> via outlet AKK <109>. The mixture of hydrogen and solution L1 <115> can then, in a particular embodiment of the present invention, be separated by methods known to the person skilled in the art. When the alkali metal compound used is a halide, especially chloride, it is possible for chlorine or another halogen gas to form in the anode chamber KA <101>, and this can be removed from the cell together with solution L4 <116> via outlet AKK <106>. In addition, it is also possible for oxygen or/and carbon dioxide to form, which can likewise be removed. The mixture of chlorine, oxygen and/or CO2 and solution L4 <116> can then, in a particular embodiment of the present invention, be separated by methods known to the person skilled in the art. It is then likewise possible, after the chlorine, oxygen and/or CO2 gases have been separated from solution L4 <116>, to separate these by methods known to the person skilled in the art.
These results were surprising and unexpected in the light of the prior art. The process according to the invention protects the acid-labile solid-state electrolyte from corrosion without, as in the prior art, having to sacrifice alkoxide solution from the cathode space as buffer solution. Thus, the process according to the invention is more efficient than the procedure described in WO 2008/076327 A1, in which the product solution is used for the middle chamber, which reduces the overall conversion. In addition, the acid-labile solid-state electrolyte is stabilized in that the formation of a pH gradient is prevented on account of the mechanical stirring device <120>.
Sodium methoxide (SM) was produced via a cathodic process, with supply of 20% by weight NaCl solution (in water) in the anode chamber and of 10% by weight methanolic SM solution in the cathode chamber. This electrolysis cell consisted of three chambers that corresponded to those shown in
The anolyte was transferred through the middle chamber into the anode chamber. The flow rate of the anolyte was 1 l/h, that of the catholyte was 90 ml/h, and a current of 0.14 A was applied. The temperature was 35° C. The electrolysis was conducted for 500 hours at a constant voltage of 5 V. It was observed that a pH gradient developed in the middle chamber over a prolonged period, which is attributable to the migration of the ions to the electrodes in the course of the electrolysis and the spread of the protons formed in further reactions at the anode. This local increase in pH is undesirable since it can attack the solid-state electrolyte and can lead to corrosion and fracture of the solid-state electrolyte specifically in the case of very long periods of operation.
Comparative Example 1 was repeated with a two-chamber cell comprising just one anode chamber and one cathode chamber, with the anode chamber separated from the cathode chamber by the ceramic of the NaSICON type. This electrolysis cell thus did not contain any middle chamber. This is reflected in even faster corrosion of the ceramic compared to the comparative example 1, which leads to a rapid rise in the voltage curve. With a starting voltage value of <5 V, this rises to >20 V within 100 hours.
Comparative Example 1 is repeated, with the middle chamber comprising a propeller stirrer <121> aligned parallel to the NASICON solid-state electrolyte. This arrangement interrupts the uniform flow of the electrolyte through the middle chamber, resulting in turbulence in the electrolyte. This makes it difficult for a pH gradient to build up during the electrolysis.
Comparative Example 1 is repeated, with the middle chamber KM <103> comprising a cross-shaped magnetic stirrer bar <123-1> which is operated by a magnetic stirrer system <123-2>. This arrangement too interrupts the uniform flow of the electrolyte through the middle chamber, resulting in turbulence. This makes it difficult for a pH gradient to build up during the electrolysis.
The use of a three-chamber cell according to the invention in the process according to the invention prevents the corrosion of the solid-state electrolyte, and at the same time there is no need to sacrifice alkali metal alkoxide product for the middle chamber and the voltage is kept constant. These advantages that are apparent from the comparison of the two Comparative Examples 1 and 2 underline the surprising effect of the present invention.
In addition, the alleviation or destruction of the pH gradient that builds up with progressive electrolysis by the vortexing and turbulence in the electrolyte in the middle chamber leads to extension of the lifetime of the electrolysis chamber. This gradient, specifically in the case of very long operating periods, can make the electrolysis even more difficult and lead to corrosion and ultimately fracture of the solid-state electrolyte. In the execution according to Inventive Examples 1 and 2, this pH gradient is destroyed, which, in addition to the advantages mentioned that are provided by a three-chamber cell over a two-chamber cell, further increases the stability of the solid-state electrolyte.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21182468.5 | Jun 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/066937 | 6/22/2022 | WO |
