WO2015139136A1

WO2015139136A1 - Co2 electro-reduction process

Info

Publication number: WO2015139136A1
Application number: PCT/CA2015/050206
Authority: WO
Inventors: Clive M H BRERETON; Colin Oloman
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-03-19
Filing date: 2015-03-19
Publication date: 2015-09-24
Anticipated expiration: 2016-09-19

Abstract

Improved processes for electro-reduction of CO2 (ERC) in alkaline condition by resolving the problems of CO2 feed gas composition, reactant consumption, bicarbonate disposal and formate product concentration. Processes for the electrochemical reduction of carbon dioxide in an electroreduction reactor that yields a catholyte product, an anolyte product, and bicarbonate salt, comprising a step of separating the bicarbonate salt from the catholyte product and using the recovered bicarbonate salt to concentrate a carbon dioxide gas feed to the electroreduction reactor. Further, processes for the electrochemical reduction of carbon dioxide in an alkaline catholyte solution comprising separating bicarbonates produced during the electrochemical reduction from the catholyte, converting the separated bicarbonates to carbonates, and using the carbonates from the previous step to concentrate a carbon dioxide feed gas to the process. In some embodiments, the process may further comprise recycling the bicarbonates to a catholyte feed.

Description

C0₂ ELECTRO-REDUCTION PROCESS

Field of the Invention

This invention pertains to the field of processes for the electro-chemical reduction of carbon dioxide, in particular, an improved process for the electro-reduction of carbon dioxide in alkaline conditions.

Background of the Invention

It is well known that carbon dioxide can be converted to useful products such as formate salts and carbon monoxide by electro-chemical reduction in processes such as those described in references 1 to 8 below:

1. Oloman et al., U.S. Patent Publication No. 2012/090052 A1

2. Kacsur et al., U.S. Patent Publication No. 2013/0105304 A1

3. Agarwal A., et al. "The electrochemical reduction of carbon dioxide to

formate/formic acid", ChemSusChem 201 1 , 9 1301 -1310

4 Oloman C, Li H., "Electrochemical processing of carbon dioxide",

ChemSusChem, 2008, 1 , 385-391.

5 Finn C, et al.."Molecular approaches to the electrochemical reduction of carbon dioxide", Chem.Comm. 2012,48, 1392-1399.

6 Jitaru M., "Electrochemical carbon dioxide reduction - fundamentals and applied topics", J.Univ.Chem.Tech. and Metalury, 2007, 42, 333-444. 7. Sanchez-Sanchez C, et al. "Electrochemical approaches to alleviation of the problem of carbon dioxide accumulation" Pure Appl. Chem. 2001 1 , 73(12), 1917-1927.

8. Delacourt C, et al. "Desigh of an electrochemical cell making syngas

(CO+H2) from C02", ECS Journal 2008, 155(1 ), B42-49.

9. Walas S., "Chemical Process Equipment", Butterworth, Boston, 1990. Page 1 14.

However the development of such processes for continuous operating in commercial use is hindered by inherent aspects of their chemistry. First, the C0₂ feed to the process must be recovered and concentrated from industrial waste gas streams. Second, the electro-chemical reaction taking place under alkaline conditions generates bicarbonate salts that consume reactants and present a disposal problem. Third, for practical purposes (e.g. to obtain useful concentrations of formate salts) the catholyte must be recycled, with resulting problems due to the accumulation of bicarbonate salts in the catholyte loop. These issues are partially recognized in the prior art, for example in reference 1 the formate salt is concentrated by secondary processes such as salt splitting, electro-dialysis or nano-filtration, while in reference 2 the bicarbonate salt is separated from the recycling catholyte by crystallization. However none of the prior art recognizes the commercial significance of the issue surrounding bicarbonate, or engages their resolution in a single integrated system. Summary of the Invention.

The present invention is an improved process for electro-reduction of CO2 (ERC) in alkaline condition, regarding its commercial application, by resolving the problems of CO₂ feed gas composition, reactant consumption, bicarbonate disposal and formate product concentration in a single integrated system.

In some aspects the present invention provides a process for the electrochemical reduction of carbon dioxide in an electroreduction reactor that yields a catholyte product, an anolyte product, and bicarbonate salt, the process comprising a step of separating the bicarbonate salt from the catholyte product and using the recovered bicarbonate salt to concentrate a carbon dioxide gas feed to the electroreduction reactor.

In some aspects, the present invention provides a process for the electrochemical reduction of carbon dioxide in an alkaline catholyte solution comprising the steps of: separating bicarbonates produced during the electrochemical reduction from the catholyte; converting the separated bicarbonates to carbonates; and using the carbonates from the previous step to concentrate a carbon dioxide feed gas to the process. In some embodiments, the process may further comprise recycling the bicarbonates to a catholyte feed. In some embodiments of the invention, the product of the electrochemical reduction of carbon dioxide may be a formate salt. In some embodiments, the product of the electrochemical reduction of carbon dioxide may be carbon monoxide. In some embodiments the step of separating the bicarbonates may be by crystallization.

In some embodiments of the invention, the step of concentrating a carbon dioxide gas feed may comprise converting bicarbonate salt to carbonate salt and using the carbonate salt to absorb carbon dioxide in a cyclic carbonate/bicarbonate C02 absorption/desorption system. In some embodiments, part of the bicarbonate salt from the absorption/desorption system may comprise the catholyte feed to the electroreduction reactor. In some embodiments, part of a supernatant solution from the bicarbonate crystallization may be recycled to the catholyte fed to the electroreduction reactor. In some embodiments, the desorbed carbon dioxide may be taken as the reactant feed to the electroreduction reactor. In some embodiments, part of the formate salt may be returned to the catholyte. In some embodiments, the separated bicarbonate salt may be converted to carbonate salt that is then recycled to the anolyte feed to the electoreduction reactor. In some embodiments, part of the anolyte product is recycled to the catholyte feed to the electroreduction reactor. In some embodiments the concentration of bicarbonate in the catholyte product from the electroreduction reactor may be in the range from about 1 to 4 molar. In some embodiments the concentration of formate in the catholyte product from the electroreduction reactor may range from about 0.5 to 6 molar. In some embodiments the bicarbonate crystallization may be done at a temperature ranging from about 40 °C to 90 °C under a carbon dioxide partial pressure ranging from about 0.5 to 2 bar. In some embodiments the catholyte pH may range from about 7 to 9. In some embodiments the bicarbonate salt may be one of ammonium lithium, sodium and potassium bicarbonate. In some embodiments the formate product concentration may be in the range of about 2 to 13 molar. In some embodiments the bicarbonate may be crystallized under an atmosphere comprising carbon dioxide. In some embodiments the concentration of carbon dioxide in the feed gas to the concentration step may be in the range of about 10 to 50 percent by volume. In some embodiments the concentration of carbon dioxide in the gas leaving the concentration step may be in the range of about 70 to 99 percent by volume.

Brief Description of the Drawings

Figure 1 shows a flowsheet of a generic continuous process for the electroreduction of C0₂.

Figure 2 shows a flowsheet of one manifestation of a process for the electro-reduction of C0₂ (ERC), according the present invention, to produce a solution of a formate salt such as potassium or sodium formate.

Figure 3 shows a flowsheet of a second manifestation of a process for the electroreduction of C02 according to the present invention.

Figure 4 shows experimental results for Example 2. Figure 5 shows a theoretical prediction of liquid phase composition from evaporation of a mixed potassium formate/bicarbonate solution.

Figure 6 shows experimental results for Example 3. Detailed Description of the Invention

Figure 1 shows a process for the electrochemical reduction of carbon dioxide to obtain C0₂ reduction products by cathode reactions with the generic form: xC0₂ + (y-2(z-2x))H⁺ + ye^" CxHyOz + (z-2x)H₂0 Reaction 1

where x, y and z may take integer values respectively of 1 to 3, 0 to 8 and 0 to 2, as exemplified in Table 1.

Table 1

The process of Figure 1 has an electrochemical reactor A where carbon dioxide (CO₂) is reduced according to Reaction 1 , along with the associated reactor feed, recycle and product separation systems.

In Figure 1 the electrochemical reactor A may have single or multiple electrochemical cells of parallel plate or cylindrical shape, wherein each cell is divided into an anode chamber with anode B and a cathode chamber with cathode C by a separator D. An electric power source E supplies direct current to the reactor at a voltage about 2 to 6 Volt/cell. The process uses anode and cathode feed tanks F and G along with the respective product separators H and I. In the continuous process an anode fresh feed J, optionally mixed with recycle U, forms anolyte liquid K which is passed to the anode chamber B where it is converted to anode output L, to be subsequently separated to products M and N and an optional anolyte recycle U. Meanwhile a cathode fresh feed O, optionally mixed with recycle V, forms catholyte liquid Q which is mixed with C0₂ gas P and passed to the cathode chamber C where the mixture (P+Q) is converted to cathode output R, to be subsequently separated to products S and T and an optional catholyte recycle V.

In the reactor A, the cathode C, where the C0₂ is reduced, includes a porous electrode with an electro-catalytic specific surface in the range about 100 to 100,000 m²/m³, which may include nano-structured surface embellishments, and may be in the form of a reticulate, foam, felt, matt, mesh, frit, fixed-bed, fluidized-bed, gas diffusion electrode (GDE), solid polymer electrode (SPE) or the like. The cathode is fed by a mixture of a C0₂ containing gas P and a catholyte liquid solution Q in a volumetric flow ratio from about 1 to 1000, measured at 1 bar(abs), 273 K. The gas P and liquid Q may be introduced separately to the cathode, or mixed before entering the cathode, then pass through the cathode in two-phase co-current flow. The co-current fluid (P+Q) flow path through the porous cathode may be preferably in the so-called "flow-by" mode with fluid flow orthogonal to the electric current or optionally in the so-called "flow-though" mode with fluid flow parallel to the electric current. The reactor may be oriented horizontally or sloped or preferably vertically, with the cathode fluid (P+Q) flow preferably upward but optionally downward. The separator D may be a layer of an electronically non- conductive material that is inherently ionically conductive, or made ionically conductive by absorption of an electrolyte solution. The preferred separator is an ion selective membrane such those under the trade names Nafion, Fumasep, VANADion, Neosepta and Selemion and PEEK as detailed in Table 4, and is preferably a cation exchange membrane (CEM) such as Nafion N424, with a selectivity above about 90%. The separator may also comprise a layer of porous hydrophilic material such as asbestos, Zirfon^RPerl (Agfa-Gevaert N.V.), Scimat (Freudenberg NonWovens), Celgard (Celgard LLC) and like materials used as separators in water electrolysers and electric batteries.

Depending on the desired anode products M,N,U and process conditions the electronically conductive anode material may be selected from those known to the art, including for example nickel, stainless steel, lead, conductive oxide (e.g. Pb0₂, Sn0₂), diamond, platinised titanium, iridium oxide and mixed oxide coated titanium (DSE), and the like. The anode may be a two-dimensional electrode or a three-dimensional (porous) electrode in the form of a reticulate, foam, felt, matt, mesh, frit, fixed-bed, fluidized-bed, gas-diffusion (GDE) or solid-polymer electrode (SPE).

The desired cathode products S,T,V and process conditions determine the choice of the electronically conductive cathode electro-catalyst material(s), which may be selected from the exemplary lists in Tables 2 and 3. The anode reaction is complimentary to the cathode electro-reduction reaction 1 and may be chosen from a wide range of electro-oxidations exemplified by reactions 2 to 10.

Product

40H^" 0₂ + 2H₂0 + 4e^~ Reaction 2 oxygen

2CI^" Cl₂ + 2e^~ Reaction 3 chlorine

2S0₄ ^2" S₂0₈ ^2" + 2e^" Reaction 4 persulphate

2C0₃ ^2" C₂0₆ ^2" + 2e^" Reaction 5 percarbonate

2H₂0 0₂ + 4H⁺ + 4e^" Reaction 6 oxygen

C₆H₆ + 2H₂0 C₆H₄0₂ + 2H⁺ + 2e^" Reaction 7 benzoquinone

C₈HioO + H₂0 C₈H₈0₂ + 4H⁺ + 4e^" Reaction 8 methoxybenzaldehyde

H₂ 2H⁺ + 2e^~ Reaction 9 proton

CH₄ + H₂0 CH₄0 + 2H⁺ + 2e^~ Reaction 10 methanol

The primary reactants at the anode may be soluble ionic species as in reactions 2 to 5, neutral species as in reactions 6 to 10, "immiscible" organic liquids as in reactions 7 and 8 or gases as in reactions 9 and 10. Immiscible liquid and gas reactants, along with an aqueous liquid anolyte, may engender multi-phase flow at the anode which may include respectively a gas/liquid foam or liquid/liquid emulsion.

The anolyte K may be a non-aqueous solution of an electrolyte, but preferably an aqueous solution of an acid or base and/or salt with alkali metal or ammonium cations. Corresponding reagents may be for example: sulphuric, hydrochloric, hydrobromic, phosphoric or methanesulphonic acid; sodium, potassium, rubidium, caesium or ammonium hydroxide or a sodium, potassium, rubidium, caesium, or ammonium salt of the above acids. The anolyte may optionally include species to be engaged in oxidative redox couples, such as Ag²⁺ / Ag^{1 +} ,Ce⁴7 Ce³⁺ , Co³⁺ / Co²⁺ , Fe³⁺ / Fe²⁺ , Mn³⁺ / Mn²⁺, V⁵⁺ / V⁴⁺ , organic couples such as quinone/hydroquinone and the like, in bare, complexed or chelated forms, with a redox potential matched to that of the desired anode process.

The desired cathode products S,T,V and process conditions determine the choice of the electronically conductive cathode electro-catalyst material(s), which may be selected from the exemplary lists in Table 2 or from organo-metal complexes of cobalt, copper, iron, nickel, palladium and rhenium such as those in Table 3, on electronically conductive supports.

Table 2. Cathode metal electro-catalyst materials

Cadmium/Antimony

Indium/Lead Alloy Tin/Aluminum Alloy Titanium/Tin Alloy Alloy

Cadmium/Tantalum Indium/Tantalum

Tin/Antimony Alloy

Alloy Alloy

CO PRODUCTION

Gallium/Aluminum Palladium/Silver Waspaloy Alloy High Purity Gallium Alloy Superalloy

Gallium/Antimony Palladium/Tantalum Zinc/Aluminum Alloy High Purity Gold Alloy Alloy

Gallium/Tantalum High Purity

Alloy Palladium Palladium/Zinc Alloy Zinc/Antimony Alloy

Gold/Aluminum Silver/Aluminum

Alloy High Purity Silver Alloy Zinc/Gallium Alloy

Silver/Antimony

Gold/Antimony Alloy High Purity Zinc Alloy Zinc/Nickel Alloy

Palladium/Aluminum

Gold/Gallium Alloy Alloy Silver/Gallium Alloy Zinc/Tantalum Alloy

Palladium/Antimony

Gold/Silver Alloy Alloy Silver/Nickel Alloy

Palladium/Gallium Silver/Tantalum

Gold/Tantalum Alloy Alloy Alloy

Palladium/Gold

Gold/Zinc Alloy Alloy Silver/Zinc Alloy

HYDROCARBON PRODUCTION

Copper/ Aluminum Copper/Tantalum Titanium/Nickel

Titanium Superalloy

Alloy Alloy Alloy

Copper/Antimony Titanium/Aluminum Titanium/Tantalum

High Purity Copper

Alloy Alloy Alloy

Titanium/Antimony

Copper/Nickel Alloy High Purity Titanium

Alloy

Copper/Nickel/Tin Titanium Metal Titanium/Copper

Alloy Matrix Composite Alloy

Table 3. Organo-metal electro-catalysts

formation of CO and HCOO-

Electrochemical C02 reduction catalyzed

by ruthenium complexes

[Ru(bpy)₂(CO)CI]⁺ [Ru(bpy)2(CO)2]2+ and CO, HCOO^"

[Ru(bpy)2(CO)CI]+. Effect of pH on the

formation of CO and HCOO-

Involvement of a Binuclear Species with

the Re-C(0)0-Re Moiety in

C0₂Reduction Catalyzed by Tricarbonyl

Rhenium(l) Complexes with Diimine

[Re(dmb)(CO)₃]₂ CO

Ligands: Strikingly Slow Formation of the

Re-Re and Re-C(0)0-Re Species from

Re(dmb)(CO)₃S (dmb = 4,4'-Dimethyl-2,2'- bipyridine, S = Solvent)

Electrocatalytic and Homogeneous

Iron Porphyrin Approaches to Conversion of C02 to liquid CO

Fuels

Electrocatalytic and Homogeneous

Re(bipy)(CO)₃CI Approaches to Conversion of C02 to liquid CO

Fuels

Electrocatalytic and Homogeneous

Ph₃PCo(tpfc) Approaches to Conversion of C02 to liquid CO

Fuels

Electrocatalytic and Homogeneous

CIFe(tpfc) Approaches to Conversion of C02 to liquid CO

Fuels

Electrocatalytic and Homogeneous

CIFe(tdcc) Approaches to Conversion of C02 to liquid CO

Fuels

Electrocatalytic and Homogeneous

[M(bpy)₂(CO)H]⁺

Approaches to Conversion of C02 to liquid CO, HCOO^" (M = Os, Ru)

Fuels

Electrocatalytic and Homogeneous

Rh(dppe)₂CI Approaches to Conversion of C02 to liquid HCOO^"

Fuels

Electrocatalytic and Homogeneous

[Pd(triphos)(PR₃)](BF₄)₂ Approaches to Conversion of C02 to liquid CO

Fuels

Electrocatalytic and Homogeneous

[ΝΙ₃(μ₃-Ι)(μ₃-ΟΝΜβ)(μ₂- Approaches to Conversion of C02 to liquid CO, C0₃ ^2" dppm)₃]⁺

Fuels

Electrocatalytic and Homogeneous

[Cu₂^-PPh₂bipy)₂- Approaches to Conversion of C02 to liquid CO, C0₃ ^2" (MeCN)₂[PF₆]₂

Fuels Electrocatalytic Reduction of Carbon

[Re(CO)₃(K²-N,N- Dioxide by a Polymeric Film of Rhenium CO

PPP)CI]

Tricarbonyl Dipyridylamine

Using a One-Electron Shuttle for the

Multielectron Reduction of C02 to

4-tert-butylpyridinium HCOO^", CH₃OH, CH₂0

Methanol: Kinetic, Mechanistic, and

Structural Insights

Molecular Approaches to the

[Ni(cyclam)]²⁺ Electrochemical Reduction of Carbon CO

Dioxide

Molecular Approaches to the

[Co(l)Porphyrin]^~ Electrochemical Reduction of Carbon CO

Dioxide

Silver Pyrazole Nitrogen Based Catalysts for the

CO

Supported on Carbon Electrochemical Reduction of C02

Silver Phthalocyanine Nitrogen Based Catalysts for the

CO

Support on Carbon Electrochemical Reduction of C02

Silver tris[(2- Nitrogen Based Catalysts for the

CO

pyridyl)methyl]amine Electrochemical Reduction of C02

A Local Proton Source Enhances C02

Iron Tetraphenyl

Electroreduction to CO by a Molecular Fe CO

Porphyrin

Catalyst

Iron 5, 10, 15, 20-

A Local Proton Source Enhances C02

terakis(2', 6'- Electroreduction to CO by a Molecular Fe CO dihydroxylphenyl)- Catalyst

porphyrin

Iron 5, 10, 15, 20-

A Local Proton Source Enhances C02

tetrakis(2', 6'- Electroreduction to CO by a Molecular Fe CO dimethoxyphenyl)- Catalyst

porphyrin

Table 4. Membrane materials

Sulphonated

Nafion N1 17 0.183 CEM

Fluoropolymer

Nafion Sulphonated

0.254 CEM

N1 1 10 Fluoropolymer

Sulphonated

Nafion N324 0.152 CEM Teflon Reinforced

Fluoropolymer

Sulphonated

Nafion N424 0.178 CEM Teflon Reinforced

Fluoropolymer

Sulphonated PTFE Monofilament

Nafion N438 CEM

Fluoropolymer Reinforced

VANADion:

Thickness

Name Type Base Material Note

(mm)

VANADion Fluoropolymer with

0.254 CEM

20 lonomer Coating

VANADion Fluoropolymer with Low Oil Composite

0.254 CEM

20L lonomer Coating Membrane

HYDRion:

Thickness

Name Type Base Material Note

(mm)

Fluoropolymer with

HYDRion

0.127 CEM Iridium or Platinum

N1 15

Coating

Fluoropolymer with

HYDRion

0.178 CEM Iridium or Platinum

N1 17

Coating

Fluoropolymer with

HYDRion

0.254 CEM Iridium or Platinum

N1 1 10

Coating

Fumatech:

Thickness

Name Type Base Material Note

(mm)

Fumasep Specifically for

0.050-0.070 CEM Fluoropolymer

FKE Electrolysis

Fumasep Polyethylene

0.1 10-0.130 CEM

FKS Terephthalate

Fumasep

0.080-0.100 CEM Fluoropolymer PEEK Reinforced FKB Fumasep

0.1 10-0.120 CEM Fluoropolymer PEEK Reinforced FKL

Very Low

Fumasep

0.100-0.130 AEM Fluoropolymer Resistance, PEEK FAB

Reinforced

Fumasep High Mechanical FAA-3-PK- 0.130 AEM Fluoropolymer Strength, PK 130 Reinforced

Very High

Fumasep

0.200-0.250 BPM Effectiveness, High FBM

Mechanical Strength

NEOSEPTA:

Thickness

Name Type Base Material Note

(mm)

Neosepta

0.150 CEM

CIMS

Neosepta

0.1 1 AEM

ACM

Neosepta High Mechanical

0.170 CEM

CMX Strength

Neosepta High Mechanical

0.140 AEM

AMX Strength

Neosepta

0.180 AEM

ACS

Neosepta Very Low Resistance

0.160 AEM

AFN (0.5 ohms.cm2)

SELEMION Hydrocarbon:

Thickness

Name Type Base Material Note

(mm)

SELEMION

0.120 CEM lonomer

CMV

SELEMION

0.120 AEM lonomer

AMV

SELEMION

0.200 AEM lonomer

AMT

SELEMION Very Low Resistance

0.100 AEM lonomer

DSV (~1 ohms)

SELEMION

0.120 AEM lonomer Low Proton Leakage AAV SELEMION Monovalent-lon-

0.120 AEM lonomer

ASV Selective

SELEMION

0.150 AEM lonomer Oxidant-Proof APS4

The catholyte Q may be a non-aqueous solution of an electrolyte, but preferably an aqueous solution of an acid or base and/or salt with alkali metal or ammonium cations. Corresponding reagents may be for example: sulphuric, hydrochloric, hydrobromic, phosphoric, methanesulphonic or formic acid; sodium, potassium, rubidium, caesium or ammonium hydroxide or a sodium, potassium, rubidium, caesium, or ammonium salt of the above acids, including the bicarbonate and carbonate salts. The catholyte may optionally include species to be engaged in reductive redox couples, such as, Cr³⁺ / Cr²⁺ , Cu²⁺ / Cu^{1 +} , Sn⁴⁺ / Sn²⁺ , Ti³⁺ / Ti²⁺ , V³⁺ / V²⁺ , organic couples such as quinone/hydroquinone and the like, in bare, complexed or chelated forms, with a redox potential matched to that of the desired cathode process. In some cases the catholyte may contain chelating and/or surface active agents (surfactants) such as for example amino-carboxylates (e.g. EDTA, DTPA), phosphonates and quaternary ammonium salts.

The feed gas P may contain about 1 to 100 volume % C0₂ and the cathode reactant mixture (P+Q) may enter and/or traverse the porous cathode in a two-phase flow pattern such as described in reference 3 as: "bubbly", "plug", "slug", "dispsersed" or "froth" (i.e. a foam). Methods for separating the anode and cathode products may be for example gas/liquid or liquid/liquid disengagement, crystallization, filtration, liquid extraction and distillation.

The electroreduction of C0₂ can be carried out in acid or alkaline conditions. The generic Equation 1 shows the reaction in acid but most practical ERC processes use an alkaline catholyte with pH in the range about 7 to 10. In this case two exemplary products of interest are formate salts and carbon monoxide, for which the cathode reactions are:

C0₂ +_H₂0 + 2e^" -> HC0₂ ^" + OH^" Reaction 1 1

C0₂ + H₂0 + 2e^" - CO + 20H^" Reaction 12

Each of these reactions is usually accompanied by the parasitic reduction of water: 2H₂0 + 2e^" - H₂ + 20H^" Reaction 13

The hydroxide (OH ) produced by reactions 1 1 ,12 and 13 subsequently reacts with carbon dioxide to generate bicarbonate:

C0₂ + OH^" HCO3^" Reaction 14

Depending on the Faradaic efficiency at the cathode each mole of the desired product (e.g. HC0₂ ^" or CO) may be accompanied by about 1 to 4 moles of undesired bicarbonate, in the form of a bicarbonate salt such as KHCO3. This bicarbonate is responsible for excess consumption of its associated counter-ions (e.g. K⁺) and of C0₂, both of which increase the process costs. A similar situation occurs with ERC under alkaline conditions to obtain a variety of products, such as those in Table 1. An objective of the present invention is to reduce, and preferably resolve, the problems associated with bicarbonate formation during the reduction of C0₂ in alkaline conditions.

The process of Figure 2, its main components and variants, are the basis for one manifestation of the present invention, which is described below.

For the sake of simplicity the cation invoked for the description for Figures 2 and 3 is potassium (K⁺), although in practise the K⁺ may be replaced by other alkali metal cations (e.g. Li⁺, Na⁺, Rb⁺, Cs⁺) or ammonium (NH ⁺). However, the invention may not be so readily applied to Rb⁺ and Cs⁺ due to the relatively high solubility of their bicarbonate salts in water.

In Figure 2 the items 1 to 12 are process units specified as follows: an electrochemical reactor 1 , a separator 2, a separator 3, a divider 4, a reactor/separator 5, a mixer 6, a reactor/separator 7, a divider 8, a reactor/separator 9, a mixer 10, a mixer 11 , and a mixer 12. The reference numbers 13 to 35 refer to process streams whose functions are described below.

For effective separation in unit 3 the charge should be held under a controlled partial pressure of carbon dioxide to promote crystallization of the bicarbonate by the reverse of reaction 14. The reactor/separator 7, divider 8 and reactor/separator 9 with respective process streams 13,14,15,16,17,19,20,30,31 constitute the conventional cyclic carbonate/bicarbonate C0₂ absorption/stripping process in which a waste gas stream 13, typically with about 10 to 40 volume % C0₂ is purified to above about 80 volume % C0₂ in stream 20, via absorption into a potassium carbonate solution 31 to form a potassium bicarbonate solution 15. The unwanted gases (e.g. N₂ , 0₂ ) are rejected in stream 14 then 15 is divided into 16 and 17 and C0₂ is stripped from the latter into 20. The remaining bicarbonate 16 is mixed with the catholyte product recycle 29 to give stream 18, an aqueous potassium bicarbonate-formate mixture which is combined with carbon dioxide gas 22 and fed to the cathode of the electrochemical reactor. With alkaline conditions in the cathode the electrochemical reactor converts C0₂ to a mixture of bicarbonate and formate salts 24 from which unconverted C0₂ 27 and potassium bicarbonate 26 are separated in separator 3 to give potassium formate solution 25 that is subsequently divided to a product 28 and the aforementioned recycle 29. The potassium bicarbonate 26 is decomposed by heat and separated into C0₂ gas 21 and a potassium carbonate solution 30. The former is mixed with stream 20 to feed the ERC reactor and the latter is mixed with the recycled potassium carbonate 19 to give a potassium carbonate solution 31 that feeds the aforementioned C0₂ concentration process. A controlled pH acid anolyte solution 32 enters the ERC anode where the anode reaction converts water to protons (H+) and oxygen gas by reaction 6 to give an anolyte exit stream 33. Oxygen 35 is separated in the recycle separator/tank 2 where a solution of potassium hydroxide (KOH) 34 joins the recycle loop to maintain the anolyte pH and process potassium balance. In this case the anolyte may include, for example, a solution of potassium sulphate (K₂S0₄) and sulphuric acid (H₂S0₄) and/or potassium hydrogen sulphate KHS0₄ or the analogous phosphates (H₃P0₄, K₃P0₄, K₂HP0₄, KH₂P0₄) or potassium salts with other anodically stable anions. Alternatively the anolyte pH and the potassium balance may be controlled through other electro-oxidation reactions, such as reactions 3 to 10 above, for example by generating chlorine by reaction 3 from an anolyte solution of potassium chloride (KCI) and hydrogen chloride (HCI) or by producing benzoquinone by reaction 7 in an emulsion with sulphuric acid.

The specifications of this process will depend, in part, on the available C0₂ source and the product requirements. Table 5 is presented for guidance on the choice of the main stream compositions by one skilled in the art and should not be construed to circumscribe these values.

Table 5. ERC bicarbonate recycle. Figure 2 example stream compositions.

Stream Approximate ranges of composition

C0₂ dry basis KHCOs K₂ C0₃ KHC0₂ KHS0₄

vol % of gas Molar Molar Molar Molar

13 10-50 0 0 0 0

14 1 - 10 0 0 0 0

15 0 2-5 1 -2 0.1 -0.5 0

16 0 2-5 1 -2 0.1 -0.5 0

17 0 2-5 1 -2 0.1 -0.5 0

18 0 1 -2 0.1 -0.5 1 -5 0

19 0 0.1 -0.5 2-5 0.1 -0.5 0

20 70-99 0 0 0 0

21 99 0 0 0 0

22 70-99 0 0 0 0

23 70-99 1 -2 0-0.1 1 -5 0

24 20-99 1 -4 0.1 -0.5 0.5-6 0

25 0 0.1 - 1 0-0.1 2-13 0

26 0 20 (solid) 0-0.1 0.1 -0.5 0

27 20-99 0 0 0 28 0 0.1 - 1 0 - 0.1 2 - 13 0

29 0 0.1 - 1 0 - 0.1 2 - 13 0

32 0 0 0 0 3

33 0 0 0 0 2

The carbonate recycle system of Figure 3 shows an alternative manifestation of this invention that serves to lower chemical feed (e.g. KOH) and disposal (e.g. KHCO₃) costs while maintaining an alkaline pH in the anolyte, which is useful to protect non- noble metal anodes from corrosion. Also, by increasing the concentration of C0₂ in the cathode feed the C0₂ recycle may improve the performance of the ERC reactor, for example by raising the Faradaic efficiency and/or current density.

In Figure 3 the items referenced by reference numerals 1 , 2, 3 and 4 have the same functions as in Figure 2. Items 5 to 9 are specified as follows: a divider 5, a reactor/separator 6, a mixer 7, a mixer 8, and a divider 9. In this case fresh carbon dioxide containing gas 10 is mixed with recycle carbon dioxide 21 in mixer 7, to give gas stream 11 which is fed to the cathode mixed (inside or outside the reactor) with a recycle potassium bicarbonate/formate liquid solution 12. The cathode product 13 is separated in unit 3 to a gas 14, potassium formate solution 15 and potassium bicarbonate recycle solids 16. In unit 4 the potassium formate solution 15 is divided into a potassium formate product solution 17 and a potassium formate solution recycle stream 18. In unit 8 the formate recycle 18 is mixed with a potassium bicarbonate solution 28, recycled from the anolyte loop then fed to the cathode in stream 12. The potassium bicarbonate stream 16 is divided in 5 to a reject bicarbonate 19 and a process bicarbonate 20, which is decomposed in unit 6 to potassium carbonate, carbon dioxide and water.

2KHC0₃ K₂C0₃ + C0₂ + H₂0 Reaction 14

Carbon dioxide 21 from unit 6 is mixed with fresh gas 10 and recycled to the cathode in stream 11 , while the potassium carbonate 22 is recycled to the anolyte loop via unit 2. The anolyte tank 2 mixes the recycle potassium carbonate 22 with fresh potassium hydroxide 23 and anolyte bicarbonate recycle 27 to give a potassium carbonate solution which is fed to the anode in stream 24. The anode reaction 15 converts water to oxygen gas plus protons and potassium carbonate to potassium bicarbonate, part of which is divided from the anode outlet stream 26 and recycled in stream 28 to the cathode feed mixer unit 8. The oxygen gas 29 disengages and may be separated from the anolyte loop in units 2 or 9. Analogous to Figure 2 and Table 5, depending on the separation efficiency in unit 3 the formate steams 17,18 will contain residual bicarbonate and the bicarbonate streams 16,19,20 will have residual formate.

2C0₃ ^" + H₂0^■> 2HC0₃ ^" + 0.5O₂ + 2e^" Reaction 15

Example 1 A single-cell continuous parallel plate trickle-bed electrochemical reactor was assembled with superficial area dimensions of 0.5 m long by 0.02 m wide for both the anode and the cathode. The 3D cathode, contained by a 3 mm thick gasket, was a bed of pure lead wool with a fibre diameter, porosity and specific surface respectively about 0.2 mm, 80% and 3000 m²/m³, contacted with a lead plate current collector and separated from a 316 stainless steel anode by a Nafion 1 1 10 cation membrane, which was supported in by 2 layers of a 8 mesh per inch polypropylene screen held in a 3mm thick anode gasket. The 3D cathode was fed with a [C0₂ gas + liquid electrolyte] mixture consisting of 100 vol% C0₂ gas at 150 Sml/minute and 2.3 ml/minute of 1 M aqueous potassium carbonate solution containing about 1 mM sodium DTPA, and which recycled into a catholyte batch volume of 230 ml. The anode was fed with a recycling flow of 1 M potassium carbonate solution at 30 ml/minute via a 1.5 litre pump tank. The reactor was operated at 120 kPa(abs), 295 K for 5 hours with a current of 5 A and voltage ranging from about 4.0 to 4.5 V.

During the above test the concentration of potassium formate in the catholyte product increased from about 0.15 M at 10 minutes to 0.3 M at 30 minutes to 0.58 M at 300 minutes, at which stage potassium bicarbonate crystals began to form in the catholyte recycle tank and to plug the 1/8 inch tubing in the catholyte loop. Virtually zero formate was detected in the anolyte.

Example 2.

A reactor was assembled as in Example 1 , but with a Fumasep FKB-130 cation membrane and a cathode 0.1 m high by 0.01 m wide consisting of 4 stacked layers of tin plated #30 stainless steel mesh with specific surface about 7000 m2/m3. The reactor was fed with an anolyte of 2.5 M KOH at 40 ml/min and catholyte recycling at 40 ml/min from a 2 litre batch of initial composition [0.5 M KHC03 + 2 mM sodium DTPA], plus 90 Sml/min pure C02 gas. The reactor was operated continuously for a period of 78 hours at 300 kPa(abs), 293 K at a superficial current density of 2000 A m². Figure 4 shows the accumulation of bicarbonate and formate in the catholyte batch over time, reaching concentrations of respectively 2.3 and 0.5 M. Near the end of this run KHCO3 began to crystallise from the catholyte and plug the process components.

This invention owes its novelty in part to the experimental observation of the crystallization of potassium bicarbonate from a recycling [potassium bicarbonate + potassium formate] aqueous solution (Example 1 ) and in part to subsequent theoretical predictions of the [bicarbonate + formate] solid/liquid phase relationships which are not available in the prior art. Figure 5 shows the result of such a theoretical prediction, based on the common-ion effect, of the composition of the liquid phase during evaporation of water from an aqueous solution with initial composition about [10 wt% potassium formate + 20 wt% potassium bicarbonate] under an atmosphere of C0₂. Such a solution may be obtained by recycling the alkaline catholyte in an ERC reactor, as exemplified by Example 2 and Figure 4. Example 3 recounts an experimental test of the concept of concentrating formate and crystallizing bicarbonate from a recycling catholyte.

Example 3.

Six hundred millilitres of a solution containing 10 wt% potassium formate, 21 wt% potassium bicarbonate, and water was placed in a rotating crystallizer flask, then sparged with C0₂ gas at a flow rate of about 6.2 SL/min, while being held at 80°C and about 105 kPa(abs) pressure over a period of about 3 hours. The gas sparge is required here to maintain a high partial pressure of C02 that suppresses the thermal decomposition of bicarbonate to the more soluble potassium carbonate. In this batch process water is lost by evaporation and potassium bicarbonate precipitates along with some potassium carbonate, while the potassium formate is concentrated in the liquid phase. Figure 6 shows the change in concentrations of bicarbonate, carbonate and formate in the supernatant solution over time. Figure 6 substantiates the concept illustrated by Figure 5. While Figure 5 and 6 show the concentrations of formate and bicarbonate in the liquid phase the amounts of these salts in the solid (crystalline) phase is readily found with a material balance by those skilled in the art. Experimental measurements on the solid phase from example 3 showed the concentrations of potassium bicarbonate, potassium carbonate and potassium formate respectively about 88, 7 and 5 percent by weight. The carbonate content is due to partial decomposition of bicarbonate by reaction 14.

With respect to Figure 2 such considerations lead to the novel concept of separating bicarbonate salts from highly concentrated solutions of formate salts, which opens the doors to recycling the bicarbonate via an integrated C0₂ concentration process and producing a marketable concentrated formate solution (e.g. 30+ wt% KHC0₂ ). Further, the bicarbonate separation has the un-expected consequence of providing a concentrated formate solution for recycle to the 3D cathode. Such a recycle is useful because it serves to raise the ionic conductivity of the catholyte and thus the performance of that ERC reactor, due to its positive effect of conductivity on the potential and current distribution in the 3D cathode.

With respect to Figure 3, the separation of bicarbonate salts from the recycling catholyte has the further advantage of replacing part of the costly potassium hydroxide feed with an alkaline recycling source of the potassium cations which are necessary to sustain the electric current driving the electrochemical reactions.

With suitable modifications by those skilled in the art the flowsheets of Figure 2 and 3 can be adapted for other products of C0₂ electroreduction in alkaline conditions, e.g. for carbon monoxide by reactions 12 and 14, which generates at least 2 moles of bicarbonate per mole of product CO.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A process for the electrochemical reduction of carbon dioxide in an electroreduction reactor that yields a catholyte product, an anolyte product, and bicarbonate salt, the process comprising a step of separating the bicarbonate salt from the catholyte product and using the recovered bicarbonate salt to concentrate a carbon dioxide gas feed to the electroreduction reactor.

2. A process for the electrochemical reduction of carbon dioxide in an alkaline catholyte solution comprising the steps of:

A. separating bicarbonates produced during the electrochemical reduction from the catholyte;

B. converting the separated bicarbonates to carbonates; and

C. using the carbonates from step B to concentrate a carbon dioxide feed gas to the process.

3. The process as claimed in claim 2 further comprising recycling the bicarbonates from step C to a catholyte feed.

4. The process as claimed in any one of claims 1 and 2 wherein a product of the electrochemical reduction of carbon dioxide is a formate salt.

5. The process as claimed in any one of claims 1 and 2 wherein a product of the electrochemical reduction of carbon dioxide is carbon monoxide.

6. The process as claimed in any one of claims 1 and 2 wherein the step of separating the bicarbonates is by crystallization.

7. The process as claimed in claim 1 wherein the step of concentrating a carbon dioxide gas feed comprises converting bicarbonate salt to carbonate salt and using the carbonate salt to absorb carbon dioxide in a cyclic carbonate/bicarbonate C02 absorption/desorption system.

8. The process as claimed in claim 7 wherein part of the bicarbonate salt from the absorption/desorption system comprises the catholyte feed to the electroreduction reactor.

9. The process as claimed in claim 6 wherein part of a supernatant solution from the bicarbonate crystallization is recycled to the catholyte fed to the electroreduction reactor.

10. The process as claimed in claim 7 wherein the desorbed carbon dioxide is taken as the reactant feed to the electroreduction reactor.

1 1. The process as claimed in claim 4 wherein part of the formate salt is returned to the catholyte.

12. The process as claimed in any one of claims 1 and 2 wherein the separated bicarbonate salt is converted to carbonate salt that is then recycled to the anolyte feed to the electoreduction reactor.

13. The process as claimed in claim 12 wherein the carbonate salt is converted to a bicarbonate salt in the anolyte product of the electroreduction reactor.

14. The process as claimed in claim 12 wherein part of the anolyte product is recycled to the catholyte feed to the electroreduction reactor.

15. The process as claimed in any one of claims 1 and 2 wherein the concentration of bicarbonate in the catholyte product from the electroreduction reactor ranges from about 1 to 4 molar.

16. The process as claimed in claim 4 wherein the concentration of formate in the catholyte product from the electroreduction reactor ranges from about 0.5 to 6 molar. 17. The process as claimed in claim 6 wherein the bicarbonate crystallization is done at a temperature ranging from about 40 °C to 90 °C under a carbon dioxide partial pressure ranging from about 0.5 to 2 bar.

18. The process as claimed in any one of claims 1 and 2 wherein the catholyte pH ranges from about 7 to 9.

19. The process as claimed in any one of claims 1 and 2 wherein the bicarbonate salt is one of ammonium lithium, sodium and potassium bicarbonate. 20. The process as claimed in claim 6 wherein the bicarbonate salt comprises one of ammonium, lithium, potassium and sodium bicarbonate. The process as claimed in claim 4 wherein the formate product concentration in the range of about 2 to 13 molar.

The process as claimed in claim 6 wherein the bicarbonate is crystallized under an atmosphere comprising carbon dioxide.

23. The process as claimed in any one of claims 1 and 2 wherein the concentration of carbon dioxide in the feed gas to the concentration step is in the range of about 10 to 50 percent by volume.

24. The process as in any one of claims 1 and 2 wherein the concentration of carbon dioxide in the gas leaving the concentration step is in the range of about 70 to 99 percent by volume.