Patent Grant
11174559
This application claims priority to European Patent Application No. 20165238.5, filed Mar. 24, 2020, incorporated herein by reference.
The present invention relates to a process for electrochemical preparation of an alkali metal alkoxide solution. The process is performed in an electrolysis cell having three chambers, wherein the middle chamber is separated from the cathode chamber by a solid-state electrolyte permeable to cations, for example NaSICON, and from the anode chamber by a diffusion barrier, for example a membrane selective for cations or anions.
The electrochemical preparation 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 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 analogues thereof 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 result of the balancing of charge is 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 preparation of other compounds:
WO 2014/008410 A1 describes an electrolytic process for preparing 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 prepared 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 toward 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 preparation 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 the 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 hypochlonte 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 preparation 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.
WO 2008/076327 A1 describes a process for preparing 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. 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.
It was therefore an object of the present invention to provide an improved process for electrolytic preparation of alkali metal alkoxide which ensures protection of the cation-conducting solid-state electrolyte from acid but does not have the aforementioned disadvantages. In addition, the process is to feature more sparing use of the reactants compared to the prior art.
A process which achieves the object of the invention has now surprisingly been found.
The process according to the invention is one for preparing a solution L1 <115> of an alkali metal alkoxide XOR in the alcohol ROH in an electrolysis cell E <100>,
The invention also includes the following embodiments:
1. Process for preparing a solution L1 <115> of an alkali metal alkoxide XOR in the alcohol ROH in an electrolysis cell E <100>.
2. Process according to Embodiment 1, wherein X is selected from the group consisting of Li+, Na+, K+.
3. Process according to Embodiment 1 or 2, wherein S is a halide, sulfate, sulfite, nitrate, hydrogencarbonate or carbonate of X.
4. Process according to any of Embodiments 1 to 3, wherein R is selected from the group consisting of methyl and ethyl.
5. Process according to any of Embodiments 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 Embodiment 5, wherein the diffusion barrier D <110> is a sodium cation-conducting membrane.
7. Process according to any of Embodiments 1 to 6, wherein the flow direction of L3 <114> in the middle chamber KM <103> is the opposite of the flow direction of L3 <114> in the anode chamber KA <101>.
8. Process according to any of Embodiments 1 to 7, wherein the connection VAM <112> is formed within and/or outside the electrolysis cell E <100>.
9. Process according to any of Embodiments 1 to 8, wherein the connection VAM <112> between middle chamber KM <103> and anode chamber KA <101> is arranged in such a way that at least a portion of the aqueous solution L3 <114> flows completely through the middle chamber KM <103> and the anode chamber KA <101>.
10. Process according to any of Embodiments 1 to 9, wherein the alkali metal ion-conducting solid-state electrolyte FK <111> has a structure of the formula
M11+2w+x−y+zMIIwMIIIxZrIV2−w−x−yMVy(SiO4)z(PO4)3−z,
where M1 is selected from Na+ and Li+,
MII is a divalent metal cation,
MIII is a trivalent metal cation,
MV is a pentavalent metal cation,
the Roman indices I, II, III, IV, V indicate the oxidation numbers in which the respective metal cations exist,
and w, x, y, z are real numbers, where 0≤x<2, 0≤y<2, 0≤w<2, 0≤z<3,
and where w, x, y, z are chosen such that 1+2w+x−y+z≥0 and 2−w−x−y≥0.
11. Process according to Embodiments 1 to 10, wherein L2 <113> comprises the alcohol ROH and an alkali metal alkoxide XOR.
12. Process according to Embodiment 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 Embodiment 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 Embodiments 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 Embodiments 1 to 14, which is performed at a temperature of 20 to 70° C. and a pressure of 0.5 to 1.5 bar.
A solution of sodium methoxide in methanol L2 <113> is routed through the cathode chamber KK <102>. An aqueous solution of sodium chloride L3 <114> with pH 10.5 is introduced through 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> of the middle chamber KM <103> and an inlet ZKA <119> of 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>. On application of a voltage, methanol is reduced to methoxide and H2 in the cathode chamber KK <102>. 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>, chloride ions from L3 <114> are oxidized to Cl2.
Cl2 gives an acidic reaction in aqueous solution. Owing to the geometry of the three-chamber cell E <100> and the guiding of the aqueous solution L3 <114>, the acid-sensitive NaSICON solid-state electrolyte <111> is protected from the elevated acidity, compared to L3 <114>, of the solution L1 <116> that results in the anode chamber KA <101>.
Electrolysis Cell E
The process according to the invention is performed in an electrolysis cell E comprising at least one anode chamber KA, at least one cathode chamber KK and at least one interposed middle chamber KM. This also includes electrolysis cells E having more than one anode chamber KA and/or cathode chamber KK and/or middle chamber KM. 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, US 2006/0226022 A1.
The anode chamber KA comprises an anodic electrode EA. A useful anodic electrode EA of this kind is any electrode familiar to the person skilled in the art that is stable under the conditions of the process according to the invention. These are described, in particular, in WO 2014/008410 A1, paragraph [024] or DE 10360758 A1, paragraph [031]. This electrode EA 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 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. EA comprises a titanium anode coated with ruthenium oxide/iridium oxide (RuO2+IrO2/Ti).
The cathode chamber KK comprises a cathodic electrode EK. A useful cathodic electrode EK 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 10251 or DE 10360758 A1, paragraph 10301 This electrode EK may be selected from the group consisting of mesh wool, three-dimensional matrix structure and “balls”. The cathodic electrode EK especially comprises a material selected from the group consisting of steel, nickel, copper, platinum, platinized metals, palladium, carbon-supported palladium, titanium. Preferably, EK comprises nickel.
The at least one middle chamber KM is between the anode chamber KA and the cathode chamber KK.
The electrolysis cell E typically has an outer wall WA. The outer wall WA is especially selected 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 may especially be permeated for inlets and outlets. Within WA are then the at least one anode chamber KA, the at least one cathode chamber KK and the at least one interposed middle chamber KM.
KM is separated from KA by a diffusion barrier D and from KK by an alkali metal cation-conducting solid-state electrolyte FK.
The diffusion barrier D used may be any material that is stable under the conditions of the process according to the invention and prevents or slows the transfer of protons from the liquid present in the anode chamber KA into the middle chamber KM.
The diffusion barrier D used is especially a non-ion-specific dividing wall or a membrane permeable to specific ions. The diffusion barrier D is preferably a membrane permeable to specific ions.
The material for the non-ionic 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.
If the diffusion barrier D is a “membrane permeable to specific ions”, what this means in accordance with the invention is that the respective membrane promotes the diffusion of particular ions therethrough over others. 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.
Preferably, the diffusion barrier D is accordingly 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 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.
More preferably, the diffusion barrier D is an anion-conducting membrane since this particularly efficiently prevents the diffusion of protons from the anode chamber KA into the middle chamber KM.
The anion-conducting membrane used is especially one selective for the anions included in the salt S. Such membranes are known to and can be used by the person skilled in the art.
Salt S is preferably a halide, sulfate, sulfite, nitrate, hydrogencarbonate or carbonate of X, more preferably a halide.
Halides are fluorides, chlorides, bromides, iodides. The most preferred halide is chloride.
The anion-conducting membrane used is preferably a membrane which is selective for halides, more preferably chloride.
Anion-conducting membranes are described, for example, by M. A. Hickner, A. M. Herring, E. B. Coughlin, Joumal of Polymer Science, Part B: Polymer Physics 2013, 51, 1727-1735 and C. G. Arges, V. Ramani, P. N. Pintauro, Electrochemical Society Interface 2010, 19, 31-35, 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+.
When the diffusion barrier D is a cation-conducting membrane, it is especially a membrane which is selective for the cations included in the salt S. Even more preferably, the diffusion barrier D is an alkali metal cation-conducting membrane, even more preferably still a potassium ion-conducting 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 —SO−, —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 10581, US 2010/0044242 A1, paragraph 100421 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, 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 w % bole 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 is any solid-state electrolyte that can transport cations, in particular alkali metal cations, even more preferably sodium cations, from the middle chamber KM into the cathode chamber KK. Such solid-state electrolytes are known to the person skilled in the art and are described, for example, in DE 10 2015 013 155 A1, in WO 2012/048032 A2, paragraphs 100351, [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 is preferred, wherein the latter 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−yMVy (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 A13+, 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+.
My 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 also comprises an inlet ZKK and an outlet AKK that enables addition of liquid, for example the solution L2, to the cathode chamber KK and removal of liquid present therein, for example the solution L1. The inlet ZKK and the outlet AKK are mounted on the cathode chamber KK in such a way that the solution comes into contact with the cathodic electrode EK as it flows through the cathode chamber KK. This is a prerequisite for the solution L1 being obtained at the outlet AKK in the performance of the process according to the invention when the solution L2 of an alkali metal alkoxide XOR in the alcohol ROH is routed through KK.
The anode chamber KA also comprises an outlet AKA that enables removal of liquid present in the anode chamber KA, for example the aqueous solution L4. In addition, the middle chamber KM comprises an inlet ZKM, while KA and KM are connected to one another by a connection VAM. As a result, it is possible to add a solution L3 to KM and then route it through KM, and to route it via VAM into the anode chamber KA, then through this KA. VAM and the outlet AKA are mounted on the anode chamber KA in such a way that the solution L3 comes into contact with the anodic electrode EA as it flows through the anode chamber KA. This is a prerequisite for the aqueous solution L4 being obtained at the outlet AKA in the performance of the process according to the invention when the solution L3 is routed first through KM, then VAM, then KA.
Inlets ZKK, ZKM, ZKA and outlets AKK, AKA, AKM may be mounted on the electrolysis cell by methods known to the person skilled in the art.
The connection VAM may be formed within the electrolysis cell E and/or outside the electrolysis cell E,
If the connection VAM is formed within the electrolysis cell E, it is preferably formed by at least one perforation in the diffusion barrier D.
If the connection VAM is formed outside the electrolysis cell E, it is preferably formed by a connection of KM and KA that runs outside the electrolysis cell E, especially in that an outlet AKM is formed in the middle chamber KM through the outer wall WA, preferably at the base of the middle chamber KM, the inlet ZKM more preferably being at the top end of the middle chamber KM, and an inlet ZKA is formed in the anode chamber KA through the outer wall WA, preferably at the base of the anode chamber KA, and these are preferably connected by a conduit, for example a pipe or a hose, preferably comprising a material selected from rubber and plastic. The outlet AKA is then more preferably at the top end of the anode chamber KA.
“Outlet AKM at the base of the middle chamber KM” means that the outlet AKM is mounted on the electrolysis cell E in such a way that the solution L3 leaves the middle chamber KM in the direction of gravity.
“Inlet ZKA at the base of the anode chamber KA” means that the inlet ZKA is mounted on the electrolysis cell E in such a way that the solution L3 enters the anode chamber KA counter to gravity.
“Inlet ZKM at the top end of the middle chamber KM” means that the inlet ZKM is mounted on the electrolysis cell E in such a way that the solution L3 enters the middle chamber KM in the direction of gravity.
“Outlet AKA at the top end of the anode chamber KA” means that the outlet AKA is mounted on the electrolysis cell E in such a way that the solution L4 leaves the anode chamber KA counter to gravity.
This embodiment is particularly advantageous and therefore preferred when the outlet AKM is formed by the outer wall WA at the base of the middle chamber KM, and the inlet ZKA by the outer wall WA at the base of the anode chamber KA. This arrangement makes it possible in a particularly simple manner to separate gases formed in the middle chamber KM from L3 via the gas outlet G, while gases formed in the anode chamber KA leave the anode chamber KA with L4 and can then be separated off further.
Accordingly, the flow direction of L3 into KM is the opposite of or the same as the flow direction of L3 into KA, preferably the opposite, according to how the connection VAM is mounted on the electrolysis cell E. Preferably, the flow direction of L3 into KM is in the direction of gravity.
In a preferred embodiment of the present invention, connection VAM between middle chamber KM and anode chamber KA is arranged such that at least part of the aqueous solution L3, preferably the entire aqueous solution L3, flows completely through the middle chamber KM and the anode chamber KA.
When the connection VAM <112> is formed outside the electrolysis cell E <100>, this may especially be implemented in that ZKM <108> and AKM <118> are arranged at opposite ends of the outer wall WA <117> of the middle chamber KM <103> (i.e. ZRM <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 upper 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 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” is the side of the electrolysis cell E 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
Process Steps According to the Invention
The process according to the invention comprises steps (a), (b) and (c) as follows, which are performed simultaneously.
In step (a), a solution L2 comprising the alcohol ROH, preferably comprising an alkali metal alkoxide XOR in the alcohol ROH, is routed through KK. X is an alkali metal cation and R is an alkyl radical 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 and methyl, more preferably from the group consisting of ethyl and methyl. R is most preferably methyl.
Solution L2 is preferably free of water. What is meant in accordance with the invention by “free of water” is that the weight of water in solution L2 based on the weight of the alcohol ROH in solution L2 (mass ratio) is ≤1:10, more preferably ≤1:20, even more preferably ≤1:100, even more preferably ≤0.5:100.
If solution L2 comprises XOR, the proportion by mass of XOR in solution L2, based on the overall solution L2, is especially >0% to 30% by weight, preferably 5% to 20% by weight, more preferably 10% to 20% by weight, more preferably 10% to 15% by weight, most preferably 13% to 14% by weight, at the very most preferably 13% by weight.
If solution L2 comprises XOR, the mass ratio of XOR to alcohol ROH in solution L2 is especially in the range of 1:100 to 1:5, more preferably in the range of 1:25 to 3:20, even more preferably in the range of 1:12 to 1:8, even more preferably 1:10.
In step (b), a neutral or alkaline, aqueous solution L3 of a salt S comprising X as cation is routed through KM, then via VAM, then through KA.
The salt S is described above. The pH of the aqueous solution L3 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 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.
In step (c), it is then possible to apply a voltage between EA and EK.
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 leads in turn to the following consequences:
solution L1 <115> is obtained at the outlet AKK <109>, wherein the concentration of XOR in L1 <115> is higher than in L2 <113>,
an aqueous solution L4 <116> of S is obtained at the outlet AKA <106>, wherein the concentration of S in L1 <116> is lower than in L3 <114>.
In the process according to the invention, in particular, such a voltage is applied that such a current flows that the current density (=ratio of the current which flows to the electrolysis cell to the area of the solid-state electrolyte in contact with the anolyte present in the middle chamber KM) 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, 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 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, in the process according to the invention, step (c) is performed when the two chambers KM and KA are at least partly laden with L3 and KK is at least partly laden with L2.
The fact that transfer of charge takes place between EA and EK in step (c) implies that KK, KM and KA are simultaneously laden with L2 and L3 such that they cover the electrodes EA and EK to such an extent that the circuit is complete.
This is the case especially when a liquid stream of L3 is routed continuously through KM, VAM and KA and a liquid stream of L2 through KK, and the liquid stream of L3 covers electrode EA and the liquid stream of L2 covers electrode EK at least partly, preferably completely.
In a further preferred embodiment, the process according to 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 is obtained at the outlet AKK, wherein the concentration of XOR in L1 is higher than in L2. If L2 already comprised XOR, the concentration of XOR in L1 is preferably 1.01 to 2.2 times, more preferably 1.04 to 1.8 times, even more preferably 1.077 to 1.4 times, even more preferably 1.077 to 1.08 times, higher than in L2, most preferably 1.077 times higher than in L2, where the proportion by mass of XOR in L1 and in L2 is more preferably in the range from 10% to 20% by weight, even more preferably 13% to 14% by weight.
At the outlet AKA, an aqueous solution L4 of S is obtained, where the concentration of S in L4 is lower than in L3.
The concentration of cation X in the aqueous solution L3 is preferably in the range of 3.5 to mol/1, more preferably 4 mol/1. The concentration of cation X in the aqueous solution L4 is more preferably 0.5 mol/1 lower than that of the aqueous solution L3 used in each case.
In particular, the process according to the invention is performed 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 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, which can be removed from the cell together with solution L1 via outlet AKK. The mixture of hydrogen and solution L1 can then, in a particular embodiment of the present invention, be separated by methods known to the person skilled in the art. If the alkali metal compound used is a halide, especially chloride, chlorine or another halogen gas can form in the anode chamber KA, which can be removed from the cell together with solution L4 via outlet AKK. In addition, there may also be formation of oxygen and/or carbon dioxide, which can likewise be removed. The mixture of chlorine, oxygen and/or CO2 and solution L4 may 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 gases chlorine, oxygen and/or CO2 have been separated from solution L4, to separate these from one another 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.
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 VAH<112>. The middle chamber KM <103> additionally comprises an inlet ZKM <108>. In the embodiment according to
An electrolyte L2 <113> is routed into the cathode chamber KK <102> via the inlet ZKK <107>. The electrolyte L2 <113> comprises methanol; the electrolyte L2 <113> used is preferably a methanolic solution of sodium methoxide L2 <113>.
At the same time, an aqueous NaCl solution L3 <114> with pH 10.5 is introduced into the middle chamber KM <103> via the inlet ZKM <108>. This flows through the middle chamber KM <103> and the connection VAM <112> into the anode chamber KA <101>.
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−+½H2). In the anode chamber KA <101>, the oxidation of chloride ions takes place to give molecular chlorine (Cl−→½Cl2+e−). 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.
At the same time, sodium ions diffuse from the middle chamber KM <103> through the NaSICON solid-state electrolyte <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>. Owing to the geometry of the three-chamber cell E <100> and the guiding of the aqueous solution L3 <114> according to the invention, the acid-sensitive NaSICON solid-state electrolyte <111> is protected from the elevated acidity, compared to L3 <114>, of the solution L4 <116> that results in the anode chamber KA <101>.
The embodiment of the present invention shown in
Sodium methoxide (SM) was prepared via a cathodic process, wherein 20% by weight NaCl solution (in water) is supplied in the anode chamber and 10% by weight methanolic SM solution in the cathode chamber. The electrolysis cell consisted of three chambers, as shown in
The process was repeated with a two-chamber cell comprising solely an anode chamber and a cathode chamber, with separation of the anode chamber from the cathode chamber by the ceramic of the NaSICON type. This electrolysis cell thus did not contain a middle chamber. This results in more rapid corrosion of the ceramic compared to the inventive example, which leads to a rapid rise in the voltage curve, see
Result
The use of a three-chamber cell as 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.
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