Title of the Invention

PROCESS FOR THE CONVERSION OF CARBON DIOXIDE TO FORMIC ACID

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 to produce formic acid.

Background of the Invention

References

1. Oloman et al., US 2012/090052 A1.

2. Kacsur et al., US 2013/0105304 A1.

3. Agarwal A., et al. "The electrochemical reduction of carbon dioxide to formate/formic acid", ChemSusChem 2011 , 9 1301-1310.

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

ChemSusChem, 2008, 1 , 385-391.

5. Hori Y., "Electrochemical C0 ₂ reduction on metal electrodes", Modern Aspects of Electrochemistry, No 42, 2008, Springer, New York.

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

7. Jitaru M., "Electrochemical carbon dioxide reduction - fundamentals and applied topics", J.Univ.Chem.Tech. and Metalury, 2007, 42, 333-444.

8. Walas S., "Chemical Process Equipment", Butterworth, Boston, 1990.

Page 114. l 9. _. WO03027018 A1 , EP1444166 A1 2004. "Chemical and thermal

decomposition of ammonium sulphate into ammonia and sulphuric acid".

10. US 4180550 A 1977. "Process for purifying sulfur dioxide containing gas by washing with ammonia aqueous solution"-

It is well known that carbon dioxide can be converted to formate salts by electrochemical reduction in processes such as those described in references 1 to 7. However, the development of such processes for continuous operating in commercial use is hindered by inherent aspects of their chemistry. First, the CO2 feed to the process must be recovered and concentrated from industrial waste gas streams. Second, the electro-chemical reaction generates bicarbonate salts that consume reactants and present a disposal problem. Third, to obtain practical concentrations of formate salts the catholyte must be recycled, with resulting problems due to the accumulation of bicarbonate salts in the catholyte loop. Further, the direct electro-reduction of C0 ₂ to formic acid (H ₂ C0 ₂ ) is difficult. Many authors erroneously report the production of "formic acid" from ERC (Reference 5, pages 102-109) when in fact the product is a formate salt, such as potassium formate KHC0 ₂ . This egregious misrepresentation comes from experimental work on ERC at pH > 6 where formic acid, with an acid dissociation constant of 1.6E-4 at 20 °C, cannot exist at equilibrium in aqueous solution at a concentration above about 1 E-8 molar.

These issues are partially recognized in the prior art; for example, in reference 1 , the bicarbonate salt is separated from the recycling catholyte by crystallization; while in reference 2, the formate salt from ERC is concentrated and/or converted to formic acid by secondary processes such as salt splitting, electro-dialysis, or nano-filtration. However none of the prior art recognizes the commercial significance of the issues surrounding bicarbonate, and engages their resolution in a single integrated system for the production of formic acid. Summary of the Invention

The present invention is an improved process for electro-reduction of CO ₂ (ERC) to obtain formic acid, which resolves the problems of C0 ₂ feed gas concentration and bicarbonate disposal, while consuming only carbon dioxide and water.

In some aspects, the present invention provides an electrochemical process producing formic acid wherein ammonium formate is used as an intermediary in the conversion of carbon dioxide and water to formic acid in an electrochemical reactor. In another aspect, the ammonium formate is generated in a catholyte in the electrochemical reactor.

In another aspect, the ammonium formate is reacted with an acid ammonium salt to produce said formic acid.

In another aspect, the ammonium formate is reacted with an acid ammonium salt to obtain formic acid and an ammonium salt, separating said formic acid and decomposing said ammonium salt to obtain ammonia.

In another embodiment, the invention comprises a process for the conversion of carbon dioxide and water to formic acid in an electrochemical reactor comprising the electrochemical conversion of carbon dioxide and water to ammonium formate; and the reaction of ammonium formate with an acid ammonium salt to produce formic acid.

In another aspect, the ammonium formate is accompanied by ammonium bicarbonate which is subsequently decomposed to ammonia and carbon dioxide for recycle within the process. In another aspect, the produced formic acid is accompanied by an ammonium salt which is subsequently decomposed to ammonia and an acid ammonium salt for recycle within the process.

In another embodiment, the invention comprises a process for producing formic acid from carbon dioxide that comprises the steps of: a. the electrochemical conversion of carbon dioxide to ammonium formate and ammonium bicarbonate in an electrochemical reactor; b. the separation and decomposition of ammonium bicarbonate to ammonia and carbon dioxide, which are recycled to said electrochemical reactor;

c. reaction of the ammonium formate with an acid ammonium salt to generate and separate the product formic acid and forming an ammonium salt;

d. decomposition of said ammonium salt from step c into an acid ammonium salt and ammonia;

e. recycle of the acid ammonium salt from step d to step c; and f. recycle of the ammonia from step d to the electrochemical reactor of step a. In another aspect, the acid ammonium salt is ammonium hydrogen sulphate or ammonium hydrogen phosphate. The ammonium salt is ammonium sulphate or ammonium phosphate.

In another aspect, the present invention provides an electrochemical process for conversion of carbon dioxide to ammonium formate comprising the separation of ammonium bicarbonate from a recycling catholyte, decomposing the ammonium bicarbonate to ammonia, and recycling the ammonia within the process.

In another aspect, the present invention provides an electrochemical process for conversion of carbon dioxide to ammonium formate comprising the separation of ammonium bicarbonate from a recycling catholyte, decomposing the ammonium bicarbonate to carbon dioxide, and recycling the carbon dioxide within the process.

In another aspect, the present invention provides an electrochemical process for the conversion of carbon dioxide to ammonium formate and formic acid that includes the separation of ammonium formate from a recycling catholyte and converting the ammonium formate to formic acid. In another aspect, the ammonium bicarbonate is separated from the recycling catholyte by crystallization.

Brief Description of the Drawings

Figure 1 shows a conceptual flowsheet of a generic continuous process for the electro-reduction of CO ₂ .

Figure 2 shows a conceptual flowsheet of a process for the electro-reduction of C0 ₂ (ERC), according the present invention, to produce formic acid. The process converts CO ₂ to ammonium formate by electrochemical reaction. The ammonium formate is subsequently converted to formic acid by thermochemical reaction. The process recycles all intermediate reaction products so as to effectively convert carbon dioxide and water to formic acid and oxygen.

Figure 3 shows the experimental result for Example 3.

Figure 4 shows the experimental result for Example 4.

Detailed Description of the Invention

Figure 1 shows a process for the electrochemical reduction of carbon dioxide to obtain CO ₂ reduction products by cathode reactions with the generic form: xC0 ₂ + (y-2(z-2x))H ⁺ + ye ^" CxHyOz + (z-2x)H ₂ 0 Reaction 1 where x, y and z may take integer values respectively of 1 to 3, 0 to 8 and 0 to 2, exemplified in Table 1.

Table 1

The process of Figure 1 has an electrochemical reactor A where carbon dioxide (C0 ₂ ) 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 C0 ₂ 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 CO ₂ is reduced, includes a porous electrode with an electro-catalytic specific surface in the range about 100 to 100,000 m ² /m ³ , 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 C0 ₂ containing gas P and a catholyte liquid solution Q in a volumetric flow ratio from about 10 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-through" 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 as those sold 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 include a layer of porous hydrophilic material such as asbestos, Zirfon ^R Perl (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. Pb0 ₂ , SnO^), 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- 0 ₂ + 2H ₂ 0 + 4e Reaction 2 oxygen

2CY - Cl ₂ + 2e ^" Reaction 3 chlorine

2S0 ₄ ² - - S ₂ 0 ₈ ² - + 2e ^" Reaction 4 persulphate

2C0 ₃ ^2" C ₂ 0 ₆ ^2" + 2e ^" Reaction 5 percarbonate

2H ₂ 0 0 ₂ + 4H ⁺ + 4e Reaction 6 oxygen

C ₆ H ₆ + 2H ₂ 0 C ₆ H ₄ 0 ₂ + 2H ⁺ + 2e ^" Reaction 7 benzoquinone

C ₈ H ₁₀ O + H ₂ 0 - C ₈ H ₈ 0 ₂ + 4H ⁺ + 4e Reaction 8 methoxybenzaldehyde

H ₂ 2H ⁺ + 2e Reaction 9 proton

CH + H ₂ 0 - CH ₄ 0 + 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 Ag ²⁺ / Ag ¹⁺ , Ce ⁴ 7 Ce ³⁺ , Co ³⁺ / Co ²⁺ , Fe ³⁺ / Fe ²⁺ , Mn ³⁺ / Mn ²⁺ , V ⁵⁺ / V ⁴⁺ , 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

Gold/Antimony Silver/Antimony

Alloy High Purity Zinc Alloy Zinc/Nickel Alloy

Palladium/Aluminum Zinc/Tantalum

Gold/Gallium Alloy Alloy Silver/Gallium Alloy Alloy

Palladium/Antimony

Gold/Silver Alloy Alloy Silver/Nickel Alloy

Gold/Tantalum Palladium/Gallium Silver/Tantalum

Alloy Alloy Alloy

Palladium/Gold

Gold/Zinc Alloy Alloy Silver/Zinc Alloy

HYDROCARBON PRODUCTION

Copper/Aluminum Copper/Tantalum Titanium Titanium/Nickel Alloy Alloy Superalloy Alloy

Copper/Antimony High Purity Titanium/Aluminum Titanium/Tantalum Alloy Copper Alloy Alloy

Copper/Nickel High Purity Titanium/Antimony

Alloy Titanium Alloy

Copper/Nickel/Tin Titanium Metal Titanium/Copper

Alloy Matrix Composite Alloy

Table 3. Organo-metal electro-catalysts

Electrocatalytic and Homogeneous

Re(bipy)(CO) ₃

Approaches to Conversion of C02 to liquid CO

CI

Fuels

Electrocatalytic and Homogeneous

Ph ₃ PCo(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

[M(bpy) ₂ (CO) Electrocatalytic and Homogeneous

H] ⁺ Approaches to Conversion of C02 to liquid CO, HCOO ^"

(M = Os, Ru) Fuels

Electrocatalytic and Homogeneous

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

Fuels

Electrocatalytic and Homogeneous

[Pd(triphos)(P

Approaches to Conversion of C02 to liquid CO

R ₃ )](BF ₄ ) ₂

Fuels

[Νί ₃ (μ ₃ -Ι)(μ ₃ - Electrocatalytic and Homogeneous

CNMe)(p ₂ - Approaches to Conversion of C02 to liquid co, co ₃ ² - dppm) ₃ ] ⁺ Fuels

[Cu ₂ ( -

Electrocatalytic and Homogeneous

PPh ₂ bipy) ₂ - Approaches to Conversion of C02 to liquid

(MeCN) ₂ [PF ₆ ] co, co ₃ ² - Fuels

2

Electrocatalytic Reduction of Carbon

[Re(CO) ₃ (K ² - Dioxide by a Polymeric Film of Rhenium CO

N,N-PPP)CI]

Tricarbonyl Dipyridylamine

Using a One-Electron Shuttle for the

4-tert- Multi electron Reduction of C02 to

butylpyridiniu HCOO ^" , CH ₃ OH, CH ₂ 0

Methanol: Kinetic, Mechanistic, and

m

Structural Insights

Molecular Approaches to the

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

Dioxide

Molecular Approaches to the

[Co(l)Porphyri

Electrochemical Reduction of Carbon CO n] ^" Dioxide

Silver

Pyrazole Nitrogen Based Catalysts for the

CO

Supported on Electrochemical Reduction of C02

Carbon

Silver

Phthalocyanin Nitrogen Based Catalysts for the

CO

e Support on Electrochemical Reduction of C02

Carbon Silver tris[(2-

Nitrogen Based Catalysts for the pyridyl)methyl CO

Electrochemical Reduction of C02 ]amine

Iron A Local Proton Source Enhances C02

Tetraphenyl Electroreduction to CO by a Molecular Fe CO Porphyrin Catalyst

Iron 5, 10, 15,

20-terakis(2', A Local Proton Source Enhances C02

6'- Electroreduction to CO by a Molecular Fe CO dihydroxylphe Catalyst

nyl)-porphyrin

Iron 5, 10, 15,

20-tetrakis(2', A Local Proton Source Enhances C02

6'- Electroreduction to CO by a Molecular Fe CO dimethoxyphe Catalyst

nyl)-porphyrin

Table 4. Membrane materials

Thickness

Name Type Base Material Note

(mm)

Fluoropolymer with

HYDRion

0.127 CEM Iridium or Platinum

N115

Coating

Fluoropolymer with

HYDRion

0.178 CEM Iridium or Platinum

N117

Coating

Fluoropolymer with

HYDRion

0.254 CEM Iridium or Platinum

N1110

Coating

Fumatech:

Thickness

Name Type Base Material Note

(mm)

Fumasep Specifically for

0.050-0.070 CEM Fluoropolymer

FKE Electrolysis

Fumasep Polyethylene

0.110-0.130 CEM

FKS Terephthalate

Fumasep

0.080-0.100 CEM Fluoropolymer PEEK Reinforced FKB

Fumasep

0.110-0.120 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

CI MS

Neosepta

0.11 AEM

ACM

Neosepta High Mechanical

0.170 CEM

CMX Strength

Neosepta High Mechanical

0.140 AEM

AMX Strength

Neosepta

0.180 AEM

ACS

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, Cr ³⁺ / Cr ² * , Cu ²⁺ / Cu ¹⁺ , Sn ⁺ / Sn ²⁺ , Ti ³⁺ / Ti ²⁺ , V ³⁺ / V ²⁺ , 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 % CO ₂ 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 8 as: "bubbly", "plug", "slug", "dispersed" 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.

Figure 2 shows an embodiment of the process of Figure 1 , in which formic acid is produced through the use of electrolytes comprising the ammonium cation (NH4 ⁺ ).The process of Figure 2, its main components and variants, are the basis for the present invention, which is described below.

In Figure 2 the items referenced by reference numerals 1 to 9 are process units specified as follows: an electrochemical reactor 1 , a mixer/separator 2, a separator 3, a divider 4, a thermochemical reactor 5, a separator 6, a mixer 7, a thermochemical reactor/separator 8, and a thermochemical reactor/separator 9. The reference numbers 10-27 refer to process streams whose functions are described below.

In this process the fresh C0 ₂ containing gas 10 is mixed with recycled C0 ₂ 19 in unit 7 , to give gas stream 11 which is fed to the cathode mixed (inside or outside the reactor) with a recycle ammonium bicarbonate/formate liquid cathol te solution 12. The cathode product 13 is separated in unit 3 to a gas 14, ammonium bicarbonate recycle solids 15 and ammonium formate solution 16, part of which may be recycled to the catholyte 12. In unit 5 the ammonium bicarbonate is decomposed to carbon dioxide, ammonia and water by reaction 11 which proceeds at a temperature above about 60 °C.

NH4HCO3 -» C0 ₂ + NH ₃ + H ₂ 0 Reaction 11 The mixed gas stream 18 is then separated to CC^ gas 19 and an aqueous ammonia solution 20. The CO2 gas 19 is mixed with the fresh C0 ₂ 10 for recycle to the cathode in 11, while the aqueous ammonia 20 is recycled to the anolyte loop via unit 2, where it is mixed with ammonia 24 from unit 9 and anolyte recycle 26 to give the anode feed stream 25. The anolyte 25 may include an acidic ammonium salt aqueous solution from which oxygen gas and protons are obtained by the anode reaction 6, while protons (H ⁺ ) and ammonium cations (NH ₄ ⁺ ) pass through the separator into the catholyte. For example, the anolyte may comprise sulphuric acid, ammonium sulphate and/or ammonium hydrogen sulphate (NH ₄ HS0 ₄ ) or the analogous phosphates (H3PO4, (NH ₄ ) ₃ P0 ₄ , (NH ) ₂ HP0 ₄ , NH ₄ H ₂ P0 ). In general the anolyte may comprise any acid and ammonium salt which undergoes thermal decomposition to ammonia in a reaction analogous to reaction 13 and whose anion is not electro-active under the prevailing conditions at the anode.

Alternatively the anolyte pH and the ammonium balance may be controlled through other electro-oxidation reactions, such as reactions 2 to 10 above, for example by generating chlorine in reaction 3 from an anolyte solution of ammonium chloride (NH ₄ CI) and hydrogen chloride (HCI) or by producing benzoquinone by reaction 7 in an emulsion with sulphuric acid and ammonium sulphate.

For convenience the process described below uses anolytes based on sulphuric acid (H ₂ S0 ₄ ) with the sulphate (S0 ₄ ^" ) and bisulphate (HS0 ₄ ^" ) anions.

In some embodiments manipulating the concentrations of H ⁺ and NH ₄ ⁺ in the anolyte may be used to control the bulk catholyte pH. Control of the bulk catholyte pH, for example in the range about 5 to 9, may be desirable to avoid ammonia vapour losses and to maintain a high Faradaic efficiency for C0 ₂ reduction in the cathode.

In unit 8 the ammonium formate solution 17 from unit 4 is converted to formic acid by reaction 12, with ammonium hydrogen sulphate 21 recycled from unit 9. The product formic acid 23 is separated from the reaction product mixture in unit 8 by means such as vacuum evaporation/distillation. NH4HCO2 + NH ₄ HS0 ₄ -» (NH ₄ ) ₂ S0 ₄ + H ₂ C0 ₂ Reaction 12

The ammonium sulphate from reaction 12 passes to unit 9 where it is decomposed to ammonium hydrogen sulphate and ammonia gas by reaction 13.

(NH ₄ ) ₂ S0 ₄ -=> NH ₄ HS0 ₄ + NH ₃ Reaction 13

The reactions 12 and 13 occur respectively at about 100 °C and 150 to 300 °C and are preferably driven by waste heat from an associated process such as cement manufacture or power generation.

Ammonium hydrogen sulphate from unit 9 is recycled to unit 8 in stream 21 while the ammonia 24 is recycled to the anolyte loop via unit 2. To promote reactions 2 and 13 streams 21 and 22 may include sulphuric acid and/or catalysts such as compounds of molybdenum or tungsten (e.g. ammonium molybdate or tungstate). The ammonia streams 20 and 24 together may be arranged to close the nitrogen balance (as NH ₃ and/or NH ₄ ⁺ ) and thus allow the overall conversion of carbon dioxide in stream 10 to formic acid in stream 23, according to the net reaction 14.

C0 ₂ + H ₂ 0 H ₂ C0 ₂ + I/2O2 Reaction 14

Optionally, the production of ammonium formate 17 according to the overall reaction 15 may be achieved by eliminating the process units 8 and 9 from the flowsheet of Figure 2.

C0 ₂ + NH ₃ + H ₂ 0 -> NH ₄ HC0 ₂ + ½ 0 ₂ Reaction 15 This method allows the production of ammonium formate with nearly stoichiometric conversion of CO2 and NH ₃ and H2O to NH4HCO2 i.e. one mole C0 ₂ plus one mole NH ₃ per mole NH ₄ HC0 ₂ .

Example 1

A single-cell continuous parallel plate trickle-bed electrochemical reactor was assembled with superficial area dimensions of 0.1 m long by 0.01 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 m ² /m ³ , contacted with a lead plate current collector and separated from a 316 stainless steel anode by a Nafion 1110 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 single pass [C0 ₂ gas + liquid electrolyte] mixture consisting of 100 vol% C0 ₂ gas at 150 Sml/minute and 1 ml/minute of an aqueous solution of 2M ammonium sulphate with about 0.1 M ammonium hydroxide and about 1 mM sodium DTPA. The anode was fed with a recycling flow of 2 M ammonium sulphate plus 0.1 M ammonium hydroxide solution at 30 ml/minute via a 1.5 litre pump tank. The reactor was operated at 120 kPa(abs), 295 K with a current of 0.5 A and voltage about 5.4 V. At 1 hour operating time the cathode product solution contained about 0.05 M ammonium formate, corresponding to about 30% Faradaic efficiency for formate.

In a similar test with an anolyte of 1 M potassium carbonate a 100 ml batch of ammonium sulphate cathoyte was recycled for 5 hours to give a final formate concentration of about 0.1 M.

Example 2

A single-cell continuous parallel plate trickle-bed electrochemical reactor was assembled with superficial area dimensions of 0.1 m long by 0.03 m wide for both the anode and the cathode. The 3D cathode, contained by a 3 mm thick gasket, was a packed bed of approximately 0.3 mm diameter tin granules, contacted with a tin plate current collector and separated from a platinised titanium anode by a Nation 117 cation membrane, which was supported in by polypropylene screen held in a 3mm thick anode gasket. The 3D cathode was fed with a single pass [C0 ₂ gas + liquid electrolyte] mixture consisting of 100 vol% C0 ₂ gas and an aqueous solution of 2M ammonium chloride + 0.5 M ammonium bicarbonate. The anode was fed with a recycling flow of 1.9 M ammonium sulphate + 0.8 M sulphuric acid. The reactor was operated at 120 kPa(abs), 295 K for a period of 60 minutes to give the results summarized in Table 5.

Table 5. Electroreduction of C0 ₂ to ammonium formate, using an acid anolyte

Example 3

About 5.1 grams of ammonium bicarbonate crystals in a glass reactor was continuously heated in a water bath at 70 °C. The contents of the reactor were weighed periodically to quantify ammonium bicarbonate decomposition. The results plotted in Figure 3 show an average rate of ammonium bicarbonate decomposition, ^■ · according to Reaction 11 , of 1.3 gram/hour. A similar experiment at 45 °C gave an average decomposition rate of about 0.6 gram/hour. Example 4

About 5.1 grams of ammonium sulfate crystals was heated on a hot-plate at 250 °C and weighed periodically to measure the decomposition. The results plotted in Figure 4 show an average rate of decomposition, according to Reaction 13, of about 1.8 gram/hour.

Example 5

US patent 857046 discloses the production of concentrated formic acid from formate salts using sodium bisulfate. In the present invention the sodium bisulphate (NaHS0 ₄ ) is replaced by ammonium bisulphate (NH4HSO4) to produce formic acid by Reaction 12.