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EP-4609015-B1 - METHOD OF ELECTROCHEMICAL CONVERSION OF AQUEOUS SOLUTIONS OF CARBONATES, BICARBONATES, CO2, C2-C5 ACIDS, SALTS OF C2-C5 ACIDS AND MIXTURES THEREOF

EP4609015B1EP 4609015 B1EP4609015 B1EP 4609015B1EP-4609015-B1

Inventors

  • Dytrych, Pavel
  • FAJGAR, RADEK
  • DRINEK, Vladislav
  • KOSTEJN, Martin
  • JANDOVA, Vera

Dates

Publication Date
20260506
Application Date
20231027

Claims (13)

  1. Method for electrochemically converting a starting electrolyte, selected from aqueous solutions of alkali metal carbonate, ammonium carbonate, alkali metal bicarbonate, ammonium bicarbonate, CO 2 , a C2-C5 carboxylic acid salt with an alkali metal or ammonium, or mixtures thereof, optionally a mixture thereof with ammonia, to form alcohols and/or carboxylic acid salts, characterized in that the said method is carried out in an electrochemical reactor in which the cathode is a catalyst comprising copper silicide, copper germanide and/or mixed copper germanide/silicide.
  2. The method according to claim 1, wherein the electrochemical reactor comprises a cathode compartment and an anode compartment, and these compartments are separated by a proton exchange membrane.
  3. The method according to claim 1 or 2, wherein the starting electrolyte is selected from aqueous solutions of alkali metal carbonate, ammonium carbonate, alkali metal bicarbonate, ammonium bicarbonate, CO 2 or a mixture thereof, optionally a mixture thereof with ammonia, wherein the reaction produces alcohols and salts of carboxylic acids, and wherein the cathode is a copper silicide and/or mixed germanide/copper silicide cathode.
  4. The method according to claim 3, wherein pH value of the starting electrolyte is between 6.7 and 9.
  5. The method of claim 3, wherein pH value of the starting electrolyte is greater than 9.
  6. The method according to claim 1 or 2, wherein the starting electrolyte is a solution of a C2-C5 carboxylic acid salt with an alkali metal or ammonium, wherein the reaction produces alcohols and alkali metal or ammonium salt(s) of carboxylic acid(s) having a higher carbon number than the starting carboxylic acid, and wherein the cathode is a copper germanide and/or a mixed copper germanide/silicide cathode.
  7. The method according to claim 1 or 2, wherein the starting electrolyte is a solution of a C2-C5 carboxylic acid, wherein the reaction produces an alcohol which may optionally further react in situ with the carboxylic acid to form an ester, and wherein the cathode is a copper silicide and/or mixed copper germanide/silicide cathode.
  8. The method according to claim 1 or 2, wherein the starting electrolyte is selected from aqueous solutions of alkali metal carbonate, ammonium carbonate, alkali metal bicarbonate, ammonium bicarbonate, CO 2 or a mixture thereof, optionally a mixture thereof with ammonia, optionally further containing a salt of C2-C5 carboxylic acid and alkali metal or ammonium, wherein salts of lower carboxylic acids, typically formate and acetate, are formed first, followed by a second in situ reaction of the C2-C5 carboxylic acid salts to carboxylic acid salts having more carbons than the number of carbons of the starting C2-C5 carboxylic acids, and wherein the cathode is a cathode made of mixture of copper silicide and copper germanide or a mixed germanide/copper silicide cathode.
  9. The method according to any one of the preceding claims, wherein the starting electrolyte contains ammonium cations.
  10. The method according to any one of the preceding claims, wherein the starting electrolyte contains ammonium carbonate, and optionally also ammonia.
  11. The method according to any one of the preceding claims, characterized in that the electrochemical reactor contains a platinum anode or stainless-steel anode.
  12. The method according to any one of claims 1 to 5 and 8, wherein the starting electrolyte, which is a solution selected from the solutions of alkali metal carbonate, ammonium carbonate, alkali metal bicarbonate, ammonium bicarbonate, CO 2 or a mixture thereof, optionally a mixture thereof with ammonia, is prepared by first preparing a saturated alkali metal hydroxide aqueous solution which is then saturated with a gas containing CO 2 , NH 3 , preferably to a pH of not lower than 6.7.
  13. The method according to any one of claims 1, 2 or 6, wherein the starting electrolyte, which is a solution of an alkali metal salt of C2-C5 carboxylic acid or ammonium salt of C2-C5 carboxylic acid, is prepared by adding a saturated solution of alkali metal hydroxide or ammonium hydroxide to a solution of the corresponding acid, completely neutralizing the acid.

Description

Field of Art The present invention relates to a method of electrochemically converting (reducing) aqueous solutions of alkali metal carbonates, ammonium carbonates, alkali metal bicarbonates, ammonium bicarbonate, CO2, C2-C5 carboxylic acids, salts of C2-C5 carboxylic acids with alkali metals or ammonium, and mixtures thereof, or mixtures thereof with ammonia, to form alcohols and/or salts of carboxylic acids. Background Art When a preparation of ethanol, methanol, propan-1-ol, isopropanol as a product of CO2 reduction is required in practice, the selection of the electrocatalytic system is problematic due to the short lifetime of the cathode material, low selectivity for ethanol and low yields, i.e., low conversion of CO2 in the form of carbonates, bicarbonates and free CO2 in a carbonate solution. Only electrocatalytic systems containing metallic copper are available Shanwen Wang, Electrochemical Reduction of CO2 to Alcohols: Current Understanding, Progress, and Challenges, Advanced Energy and Sustainability Research, 23 January 2022, Vol. 3, No. 1, DOI: 10.1002/aesr.202100131, R. Kortlever, Electrochemical carbon dioxide and bicarbonate reduction on copper in weakly alkaline media, Journal of Solid State Electrochemistry, 9 May 2013, Vol. 17, No. 7, pages 1843-1849, DOI: 10.1007/s10008-013-2100-9, CA 2864611). Copper is the only known material to allow the formation of C2+ products by direct reduction of CO2 in the form of carbonates, bicarbonates and free CO2 in carbonate solution, but it degrades after a short time (< 100 h) and the selectivity to ethanol and other products is minimal at the expense of hydrogen evolution. The present invention aims to overcome the drawbacks of the prior art. Disclosure of the Invention The present invention provides a method for electrochemically converting a starting solution, selected from aqueous solutions of alkali metal carbonate, ammonium carbonate, alkali metal bicarbonate, ammonium bicarbonate, CO2, a C2-C5 carboxylic acid, a C2-C5 carboxylic acid salt with an alkali metal or ammonium, or mixtures thereof, or mixtures thereof with ammonia, to form alcohols and/or carboxylic acid salts. The method is carried out in an electrochemical reactor in which the cathode is a catalyst comprising copper silicide, copper germanide and/or mixed copper germanide/silicide. Preferably, in the electrochemical reactor, the cathode compartment and the anode compartment are separated by a proton exchange membrane. The selectivity of the reaction is thus significantly increased. The aqueous solution of alkali metal carbonate, ammonium carbonate, alkali metal bicarbonate, ammonium bicarbonate, CO2, a C2-C5 carboxylic acid, a C2-C5 carboxylic acid salt with an alkali metal, or a mixture thereof, or a mixture thereof with ammonia, is used as the electrolyte in the electrochemical reactor. By the action of the catalyst cathode and by the passage of electric current, the conversions, in particular the conversion of carbonates, bicarbonates and CO2 to alcohols and/or salts of C1-C3 carboxylic acids, and/or the conversion of salts of carboxylic acids to salts of higher carboxylic acids, take place. In one preferred embodiment of the invention, the electrochemical conversion (reduction) process is carried out by submitting the starting electrolyte (starting solution), selected from aqueous solutions of alkali metal carbonate, ammonium carbonate, alkali metal bicarbonate, ammonium bicarbonate, CO2 or a mixture thereof, or a mixture thereof with ammonia, to electrochemical reaction, preferably using copper silicide and/or mixed copper germanide/silicide cathode, and the reaction produces alcohols and salts of carboxylic acids. Typical products are methanol, ethanol, acetate, propionate, valerate, lactate, hydrogen in the cathode compartment, and oxygen in the anode compartment. In this embodiment, the equilibrium of the reaction products can be influenced to some extent by the pH of the starting electrolyte. In the pH range of 6.7 to 9 (preferably 6.7 to 8.5), more alcohols (C1-C3 alcohols, in particular ethanol) are formed, and at pH higher than 9 (preferably 9.5 to 13.5), more C1-C3 salts of carboxylic acids (in particular acetate) are formed. The starting electrolyte may be prepared by dissolving alkali metal carbonate, ammonium carbonate, alkali metal bicarbonate, ammonium bicarbonate or ammonia in water, thus producing basic solutions. Saturating such a solution with carbon dioxide or saturating water with carbon dioxide decreases the pH. In another preferred embodiment, the electrochemical conversion (reduction) process is carried out by submitting the starting electrolyte (starting solution), which is an aqueous solution of an alkali metal or ammonium salt of C2-C5 carboxylic acid, to electrochemical reaction, preferably using copper germanide and/or mixed copper germanide/silicide cathode, and the reaction produces alcohols and carboxylic acid salts having a higher number of carbon a