WO2010049739A2

WO2010049739A2 - Process for the capture of carbon dioxide

Info

Publication number: WO2010049739A2
Application number: PCT/GB2009/051470
Authority: WO
Inventors: Christopher Mark Rayner; Guillaume Robert Jean-Francois Raynel; Shashi Bala
Original assignee: University Of Leeds
Priority date: 2008-10-31
Filing date: 2009-10-31
Publication date: 2010-05-06
Also published as: WO2010049739A3; GB0820000D0

Abstract

The invention provides a method for the capture of carbon dioxide gas which comprises contacting the carbon dioxide with at least one acid or a salt thereof, wherein said acid has a pKa value in the range of from 0 to 14, and said acid does not include an amino group. The at least one acid or a salt thereof may be in a solid or liquid form, but preferably comprises an aqueous solution. The acid may be inorganic or organic. Preferred acids have pKa values in the range from 4 to 13. Suitable organic acids may comprise aliphatic, carbocyclic or heterocyclic acids and mono- or poly-acids. Preferably, the acid is present as a salt. The method offers a convenient and simple 10 process which uses inexpensive consumables which are preferably largely biocompatible and renewable, and thereby offers significant advantages over the methods of the prior art.

Description

PROCESS FOR THE CAPTURE OF CARBON DIOXIDE

Field of the Invention

The present invention is concerned with a novel approach to the capture of carbon dioxide, and provides alternative materials which may be more conveniently and efficiently applied to the absorption and release of carbon dioxide gas.

Background to the Invention

As a result of the increasing use of fossil fuels, the concentration of carbon dioxide in the atmosphere has risen from 280 ppm in pre-industrial times, to 377 ppm in 2004¹'², leading to rise in average global temperatures. This is expected to increase further in the short to mid-term until energy supplies which do not result in significant CO₂ emissions become established.³ According to the International Energy Agency World

Energy Outlook (2002), the predicted increase in combustion generated CO₂ emissions is around 1.8% per year and by 2030, if it continues at that rate, it will be 70% above

2000 levels.⁴

Hence, without significant abatement of CO₂ emissions, the global average temperature may increase by 1 .4-5.8 K by 2100.⁵ In view of the abundant global reserves of coal, this fuel is widely used for power generation in many countries around the world. However, for each unit of electricity generation, combustion of coal produces approximately double the amount of CO₂ when compared with natural gas. This problem is likely to be exacerbated in the future, because of the expected increase in coal burning for power generation units in order to sustain the economic growth of developing countries like China and India. Other substantial CO₂ producers include cement manufacturers, and ammonia production plants. Nevertheless, the major problem arises from coal fired power stations, with currently over 33% of global CO₂ emissions arising from such plants, and this high percentage offers a real opportunity for the reduction of CO₂ emissions by capturing CO₂ at source,⁶ concentrating it, and then handling it by storage in geological features (e.g. natural gas wells or the seabed), enhanced oil recovery, or sequestration - most likely by chemical or biochemical conversion into useful products (e.g. formic acid, methanol, polycarbonate plastics, polyhydroxyalkanoates, and biofuels).

The main current approach to absorption and stripping of CO₂ in packed columns is considered to be a mature technology, typically using aqueous monoethanolamine (30% w/w) as the absorption medium.⁴'⁵'⁷ However this approach has considerable problems, particularly when used to treat large volumes of flue gas with low CO₂ concentrations (typically 3-5% for natural gas and 10-15% for coal combustion), as the processes require the use of large sized equipment with high investment costs, and are also energy intensive. In a coal-fired power plant, typical energy consumption in the stripper reboiler can be as high as 15-30% of the power production. As a consequence, it has been calculated that application of current CO₂ capture technology to power plants would increase the price of electricity by as much as 70%.² In addition, the scale of CO₂ capture technology has to be potentially enormous to deal with the large volumes of flue gases to be processed. A large power station such as Drax in Yorkshire UK produces approximately 55,000 tonnes of CO₂ per day.⁸ This corresponds to a volume of around 28M m³ at atmospheric pressure which would require processing on a daily basis. On the basis that CO₂ represents 10-15% of a typical flue exhaust form coal firing² the actual volume of gas to be processed would be typically 7-10 times this amount.

In principle, the gas separation technologies which are currently used in the chemical industry, such as absorption in chemical solvents, adsorption using a solid adsorbent, membrane separation and cryogenic processes, can all be adapted for post-combustion capturing of CO₂ from thermal power plants. New technologies which could address this issue, including photocatalytic processes and chemical synthesis, are also under development. In addition, approaches such as pre-combustion CO₂ capture, as in an integrated gasification and combined-cycle (IGCC) plant, and combustion using pure oxygen instead of air (known as oxyfuel combustion) for the production of sequestration- ready CO₂, are also being developed for this purpose. Such technologies are reviewed in Industrial and Engineering Chemistry Research (Vol. 45, 2006), and provide a good insight into the current status and future developments of post-combustion CO₂ capture technologies.

However, such technologies are either not yet fully developed for deployment (a number of demonstrator plants are, in fact, being planned or constructed), or are not suitable for

CO₂ removal from flue gases emanating from large power plants. Consequently, the preferred option in the immediate future seems to be the post-combustion capture of

CO₂ via absorption (scrubbing) in amine-based solvents with solvent regeneration by steam stripping, because this is already a well-established process which finds widespread use in the chemical industry.⁴'⁵ The scrubbing technology is already in use for flue gas desulphurisation (FGD) in coal-fired power plants, and is also being used for CO₂ capture in a few CO₂ generating plants in use in the food industry.⁵

Although absorption/stripping is a mature technology, it suffers from considerable problems when used to treat large volumes of flue gas. Despite widespread use of this technology, the underlying chemistry is not fully understood, and it is likely that the process is quite complex, mainly because of the use of aqueous/amine based systems.⁹

The situation is further complicated by recent developments utilising mixed aqueous amine systems such as monoethanolamine (MEA, HOCH₂CH₂NH₂) and methyldiethanolamine (MDEA),¹⁰. However, whilst these more expensive materials give more favourable energy considerations, their stability may present a potential drawback.¹¹

Currently, aqueous MEA is widely used for CO₂ capture, and it typically serves as a benchmark for comparison with potential new systems; it also highlights some important issues with amine based approaches. Thus, it is known that MEA degrades after prolonged use, and the cost of solvent make-up cannot be excessive in a viable commercial process. There are a wide variety of other solvents also available, and their relative merits and other aspects have been recently assessed.¹² Other complex amines, have also been suggested,¹³ as well as ammonia,¹⁴ which would appear to offer some advantages over MEA and other amines in aqueous based systems, in terms of energy requirements, stability and disposal.

A consideration of the chemistry of amine-based solvents shows that there are three main routes by which amines can absorb CO₂, as illustrated in Scheme 1 .²'¹⁵ o

CO₂ U

R-NH 2₂ *- R -N^CH

H

Heat Carbarr c ac d

O CO₂ R -NH₂ 2 _f R -N ^cP R -NH₃

Heat

Arrrron urr carbarrate sa t

O CO₂ © Θ J^. R^{~N H}22 ^"TT* R -NH₃ O OH

H₂O

Arrrron urr b carbonate sa t Heat

Scheme 1 The particular mechanism which operates in any given situation depends on process considerations such as the presence of water or solvent, the concentration of amine and its structure, and CO₂ concentration and pressure. In aqueous based systems it is likely that all three mechanisms are operating, but that the overall mechanism involves predominantly the carbamate salt and ammonium bicarbonate.⁹ The carbamic acid is often favoured in solvents of high polarity (e.g. DMSO) but, otherwise, the ammonium carbamate is the dominant species in non-aqueous environments. All the C0₂-amine adducts decarboxylate on heating, liberating CO₂ and regenerating the amine. For example, in the case of aqueous MEA, decarboxylation is typically carried out at 120⁵C at 0.2 MPa, which has significant energy implications for the overall process. A process using ammonia operates at 82⁵C at 0.1 MPa, and is reported to be more efficient overall than MEA in terms of energy use.¹⁶ An alternative to thermal decarboxylation is to simply add an acid with a pKa < 5, such as concentrated sulphuric acid or glacial acetic acid, to give the corresponding ammonium salt and CO₂, as shown in Scheme 2. This is particularly useful for quantifying the amount of CO₂ captured as the bicarbonate or carbamate salt (vide infra), but is of limited use for commercial operation.

O

Θ

R-N Λ O R-N l-, CO₂ + 2 R-N l-;

H

O

Il <±>

R-NH₃ O Oh — — → CO₂ + R-NH, - H₂U

Scheme 2

Recent work using alcohols (or thiols) and appropriate bases shows considerable promise, but require anhydrous conditions, which is a major limitation for typical flue gas streams.¹⁷ Other alternative methods for CO₂ separation have been reviewed, and a comparison of these suggests that membrane diffusion is potentially the most powerful method but requires suitable membrane materials to be developed.¹⁸

Amongst other approaches to the capture of CO₂, US-A-2006/0154807 discusses a boronic acid-derived structure comprising a covalently linked organic network including a plurality of boron-containing clusters linked together by a plurality of linking groups which may be used to adsorb carbon dioxide. Similarly, WO-A-2008/091976 relates to the use of materials that comprise crystalline organic frameworks, including boronic acid derived- structures, which are useful for the storage of gas molecules, such as CO₂. GB-A- 1330604, on the other hand, is concerned with the separation of carbon dioxide from a gas stream by scrubbing with an aqueous solution of orthoboric acid and potassium hydroxide at 70 ° to 160⁰C at a pressure from atmospheric to 30 atmospheres.

In WO-A-2006/082436, there is disclosed a gas separation device for separating a reactive gas, such as CO₂, from a gaseous mixture, the device comprising a porous anode and cathode electrodes separated by an ionic membrane, the anode being impregnated with an absorbent compound or solvent, whilst the cathode is impregnated with an electrically conductive liquid. Amongst suitable absorbent compounds are amines, sulphonc acids and carboxylic acids. Absorption, desorption, or both are promoted by application of electric charge to the electrodes.

US-A-2005/0129598 teaches a process for separating CO₂ from a gaseous stream by means of an ionic liquid comprising an anion having a carboxylate function, which is used to selectively complex the CO₂. The ionic liquid, which is effectively a low melting molten salt made up entirely of ions, can subsequently be readily regenerated and recycled.

Thus, it is clear that current methods for CO₂ capture are expensive and far from ideal for large scale application, so the present invention attempts to address this problem by providing a solution which is relatively simple, and uses inexpensive processes and consumables, the latter of which are preferably largely biocompatible and renewable. Additionally, the process of the present invention seeks to provide lower temperatures of decarboxylation in many applications. Importantly, for rapid introduction, any process must also be compatible with existing sources of CO₂, such as power stations and cement works, and should also present opportunities for process intensification, as well as requiring significantly less energy than current methods. Surprisingly, the present inventors have found that a simple, convenient and inexpensive process for the capture of CO₂ is available by the use of acids and their salts which have a suitable pKa value, in the range of 0 < pKa < 14, and which are free from amino groups, and this method finds potential application in a series of associated processes.

Summary of the Invention Thus, according to the present invention, there is provided a method for the capture of carbon dioxide gas which comprises contacting the carbon dioxide with at least one acid or a salt thereof, wherein said acid has a pKa value in the range of from 0 to 14 and said acid does not include an amino group.

Preferably, said pKa value is in the range between 4 and 13, and most preferably in the range between 5 and 12.

Said at least one acid or a salt thereof may be an organic or inorganic acid or salt thereof, and may be in a solid or liquid form, and may comprise, for example, a powder, a slurry, a dispersion or a suspension. More preferably, said at least one acid or a salt thereof comprises a solution, optionally in an organic solvent, but most preferably an aqueous solution, which preferably has a concentration of at least 0.01 mol/L. Typically, contacting carbon dioxide with said organic or inorganic acid or salt thereof when said acid or salt thereof is in a liquid form may conveniently be achieved by bubbling the carbon dioxide through said liquid.

The term "acid" as used herein refers to a compound which, on treatment with a base such as hydroxide, forms a salt capable of playing an active role in a CO₂ capture process.

pK_a is defined as the -log of K₃, the acid dissociation constant, which is given as follows:

[H₃O⁺][A-]

pK_a = -log K

where AH represents the acid species and the quantities in square brackets are concentrations. All values quoted are measured in water.

Suitable inorganic acids may include, for example, aluminium hydroxide, trihydroxyoxovanadium, and phosphoric acid.

Typical organic acids may comprise aliphatic, carbocyclic or heterocyclic acids. Said acids may comprise mono- or poly-acids. Suitable polyacids comprise di-, tri- or tetra- acids, or may comprise polymeric acids. Preferably, said acids are present as acid salts. Examples of organic acids include carboxylic acids which may, for example, be carboxylic acids of the formula (I) or (II), or their salts:

R¹-CO₂H (I)

HO₂C-R²-CO₂H (II)

wherein R¹ is selected from a substituted or unsubstituted alkyl, alkenyl or alkynyl group, optionally including one or more chain heteroatoms, a substituted or unsubstituted carbocyclic group or a substituted or unsubstituted heterocyclic group; and

R² is selected from a substituted or unsubstituted alkylene, alkenylene or alkynylene group, optionally including one or more chain heteroatoms, a substituted or unsubstituted carbocyclic group or a substituted or unsubstituted heterocyclic group.

Typically, said chain heteroatoms are selected from oxygen, phosphorus and sulphur.

Suitable alkyl or alkylene groups may have up to 20, preferably up to 12 carbon atoms and may be linear or branched. Preferred groups are lower alkyl(ene) groups, especially CrC₄-alkyl(ene) groups, in particular methyl(ene), ethyl(ene), i-propyl(ene) or t- butyl(ene) groups, where alkyl(ene) may be substituted by one or more substituents. Unsubstituted alkyl(ene), preferably lower alkyl(ene), groups are especially preferred.

The term "alkenyl" or "alkenylene" as used herein refers to a straight or branched chain alkyl or alkylene moiety having from two to six carbon atoms and having, in addition, at least one double bond, of either E or Z stereochemistry where applicable. This term refers to groups such as ethenyl, 2-propenyl, 1 -butenyl, 2-butenyl, 3-butenyl, 1 -pentenyl, 2-pentenyl, 3-pentenyl, 1 -hexenyl, 2-hexenyl and 3-hexenyl and the like, and the corresponding alkenylene groups.

The term "alkynyl" or "alkynylene" as used herein refers to a straight or branched chain alkyl or alkylene moiety having from two to six carbon atoms and having, in addition, at least one triple bond. This term refers to groups such as ethynyl, 1 -propynyl, 2-propynyl, 1 -butynyl, 2-butynyl, 3-butynyl, 1 -pentynyl, 2-pentynyl, 3-pentynyl, 1 -hexynyl, 2-hexynyl and 3-hexynyl and the like, and the corresponding alkynylene groups.

The term "lower" when referring to alkyl(ene) substituents denotes a radical having up to and including a maximum of 7, i.e. C₁, C₂, C₃, C₄, C₅, C₆ or C₇ especially from 1 up to and including a maximum of 4, carbon atoms, the radicals in question being unbranched or branched one or more times.

Lower alkyl is, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, neopentyl, n-hexyl or n-heptyl.

Lower alkylene is, for example, methylene (-CH₂-), ethylene (-CH₂-CH₂-), propylene (- CH₂-CH₂-CH₂-) or tetramethylene (-CH₂-CH₂-CH₂-CH₂-).

Suitable carbocyclic group or heterocyclic groups may be aliphatic or aromatic, and can be mono- bi- or tri- cyclic. A monocyclic group comprises one ring in isolation, whilst a bicyclic group is a fused-ring moiety joined either at a common bond or at a common atom, thus providing a spiro moiety. A bicyclic group may comprise two aromatic moieties, one aromatic and one non-aromatic moiety or two non-aromatic moieties. A typical cyclic group is a cycloalkyl group.

Cycloalkyl is preferably C₃-Ci₀-cycloalkyl, especially cyclopropyl, dimethylcyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl or cycloheptyl, cycloalkyl being unsubstituted or substituted by one or more, especially 1 to 3, substituents.

Aromatic carbocyclic groups preferably have a ring system of not more than 16 carbon atoms and are preferably mono- bi- or tri- cyclic and may be fully or partially substituted, for example substituted by at least two substituents. Preferred aromatic carbocyclic groups include phenyl, naphthyl, indenyl, azulenyl, anthryl and phenanthryl groups, more preferably phenyl or naphthyl groups, most preferably phenyl groups. The carbocyclic group may be unsubstituted or substituted by one or more, especially from one to three, for example one, identical or different substituents.

Heterocyclic moieties may be aromatic or non aromatic, and preferably comprise an aromatic ring or ring system having 16 or fewer members, preferably a ring of 5 to 7 members. Heterocycles may also include a three to ten membered non-aromatic ring or ring system and preferably a five- or six-membered non-aromatic ring, which may be fully or partially saturated. In each case the rings may have 1 , 2 or 3 hetero atoms selected from the group consisting of nitrogen, oxygen and sulphur. The heterocycle is unsubstituted or substituted by one or more, especially from one to three, for example one, identical or different substituents.

Preferred heterocyclic moieties especially include radicals selected from the group consisting of thienyl, furyl, tetrahydrofuryl, pyranyl, thiopyranyl, benzofuranyl, pyrrolyl, pyrazolyl, pyrazinyl, thiazolyl, isothiazolyl, dithiazolyl, oxazolyl, isoxazolyl, pyridyl, pyrimidinyl, pyridazinyl, indolyl, triazolyl, tetrazolyl, isoquinolyl, quinolyl, benzofuranyl, dibenzofuranyl, benzothiophenyl, dibenzothiophenyl, phthalazinyl, quinoxalyl, acridinyl, phenothiazinyl and phenoxazinyl, each of these radicals being unsubstituted or substituted.

The term "substituted" as used herein in reference to a moiety or group means that one or more hydrogen atoms in the respective moiety are replaced independently of each other by the corresponding number of the described substituents. The substituents may be the same or different and may typically be selected from hydroxy, alkoxy, halogen, hydroxyalkyl (e.g. 2-hydroxyethyl), haloalkyl (e.g. trifluoromethyl or 2,2,2-trifluoroethyl), mercapto, carbonyl, acyl, acyloxy, sulfamoyl, carbamoyl, cyano, nitro and the like.

Substituents on carbocyclic or heterocyclic rings may also include alkyl groups, especially lower alkyl groups, which may be substituted or unsubstituted.

The term "alkoxy" as used herein refers to an unsubstituted or substituted straight or branched chain alkoxy group containing from one to six carbon atoms. This term refers to groups such as methoxy, ethoxy, propoxy, isopropoxy, butoxy, tert-butoxy, pentoxy, hexoxy and the like.

Halogen is especially fluorine, chlorine, bromine or iodine, more especially fluorine, chlorine or bromine, in particular chlorine.

Preferred organic monoacids include aliphatic acids incorporating substituents such as hydroxyl and carbonyl groups, and aromatic carbocyclic acids comprising optionally substituted phenyl rings, wherein the preferred substituent comprises a hydroxyl group, and the salts of these acids. Specific examples of preferred materials include mono- acids such as lactic acid, palmitic acid, pyruvic acid, glycolic acid, benzoic acid, A- hydroxybenzoic acid, ascorbic acid, and their sodium salts. Preferred organic polyacids include diacids, for example unsubstituted aliphatic diacids, especially aliphatic dicarboxylic acids such as oxalic acid, glutaric acid, succinic acid and adipic acid, and aliphatic diacids which are substituted with, for example, at least one hydroxyl group, for example dicarboxylic acids such as tartaric acid. Amongst suitable aromatic carbocyclic acids are phenyl-based acids such as terephthalic acid, resorcinol and gallic acid.

These compounds may be of synthetic or natural origin, and may be present as substantial components in industrial products or in waste products such as tannic acid, which may derive from industrial waste, such as that emitted by the paper industry, or consumer waste, including that from beverages high in polyphenol^ components, such as tea.

Further examples of suitable polyacids are various poly(alk)acrylic acids, for example poly(meth)acrylic acid, and poly(phenols), such as tannic acid. Amongst other examples of suitable organic acids may be included certain diketones, such as acetylacetone (2,4- pentanedione), and esters such as acetoacetate esters and malonic acid esters, for example diethyl malonate.

Suitable salts of the acids used in the method of the invention may include salts incorporating inorganic or organic cations. Thus, for example, suitable salts include metal salts, sulphonium salts or phosphonium salts. Suitable metal salts include alkali metal salts, for example, sodium and potassium salts, and alkaline earth metal salts such as calcium and magnesium salts.

In the method of the invention, the acids are preferably in the form of salts, and the method is most conveniently carried out by contacting CO₂ with the acids in salt form in aqueous solution at temperatures in the range of 10-60⁰C, more preferably 25-50⁰C, most preferably 35-45⁰C. Thus, adducts of the acids with CO₂ are typically obtained by passing CO₂ through aqueous solution of salts of the acids at 35-45⁰C.

Release of CO₂ from the adducts thus formed may then be achieved by heating the adducts under controlled conditions at temperatures of up to around 140⁰C at pressures in the range from 0.001 MPa to 100 MPa. Preferred temperatures are below 120⁰C, most preferably in the range of 20-120⁰C, with preferred pressure ranges of 0.01 MPa to 30 MPa. The method of the invention is simple and economic to implement, and involves contacting CO₂ with the acids in salt form in aqueous solution at the specified temperatures. Therefore, it is not an electrochemical process, and does not require the use of an electrical circuit or the passage of an electric current in order to facilitate capture of carbon dioxide.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Brief Description of the Drawings

The following description of the invention and examples include reference to the accompanying set of drawings, wherein:

Figure 1 is a schematic of a typical decarboxylation experiment set-up;

Figure 2 is a schematic of an NMR technique for determining species present in an aqueous solution (including other solvents, if required);

Figure 3 is a graph showing the evolution of gas and variation of temperature as a function of time for pure water;

Figure 4 is a graph showing the evolution of gas and variation of temperature as a function of time for ethanolamine (MEA); Figure 5 is a graph showing the evolution of gas and variation of temperature as a function of time for the decarboxylation of the sodium salt of lactic acid;

Figure 6 is a graph showing the evolution of gas and variation of temperature as a function of time for the decarboxylation of the sodium salt of malonic acid;

Figure 7 is a graph showing the evolution of gas and variation of temperature as a function of time for the decarboxylation of the sodium salt of maleic acid;

Figure 8 is a graph showing the evolution of gas and variation of temperature as a function of time for the decarboxylation of the sodium salt of palmitic acid;

Figure 9 is a graph showing the evolution of gas and variation of temperature as a function of time for the decarboxylation of the sodium salt of diethylmalonic acid;

Figure 10 is a graph showing the evolution of gas and variation of temperature as a function of time for the decarboxylation of the sodium salt of ascorbic acid;

Figure 1 1 is a graph showing the evolution of gas and variation of temperature as a function of time for the decarboxylation of the sodium salt of 4-hydroxybenzoic acid; and

Figure 12 is a graph showing the evolution of gas and variation of temperature as a function of time for the decarboxylation of the sodium salt of gallic acid.

Figure 13 is a graph showing the evolution of gas and variation of temperature as a function of time for the decarboxylation of the sodium salt of phosphoric acid; and

Figure 14 is a graph showing the evolution of gas and variation of temperature as a function of time for the decarboxylation of the sodium salt of 2,4-pentanedione.

Description of the Invention

The incorporation of carbon dioxide into a substrate is known as carboxylation; the removal of the same group is decarboxylation. This carboxylation/decarboxylation process is key to effective CO₂ capture and absorbent regeneration. A number of different chemical and/or physical methods of CO₂ capture may be operating and these are hereinafter discussed. The reaction of the sodium salt of a carboxylic acid with CO₂ may form sodium carbonic anhydride or an equivalent species, as shown in Scheme 3.

RCOOH+ NaOH

ydr ce

Scheme 3 - Formation of a sodium carbonic anhydride from sodium carboxylate with CO₂

In the presence of other functionality, such as an additional alcohol group, further hydrogen bonding and/or chelation would be expected, which would significantly influence the stability of the intermediate carboxylic acid-CO₂ adduct, in a similar fashion to that observed with an electron donor-acceptor (EDA) complex.

Alternatively, the sodium salt of an acid with the suitable pKa and concentration in a solvent, preferably an aqueous solution, reacts with dissolved CO₂ and/or carbonic acid to form the corresponding acid and sodium bicarbonate, as shown in Scheme 4.

R OH

Y 4 NaHCO₃

Scheme 4

The sodium salts of acids have good capacities for CO₂ absorption at room temperature. The acid may be deprotonated by a suitable base, preferably sodium hydroxide, to yield the resulting sodium salt. CO₂ is then bubbled through the solution, preferably an aqueous solution, to form the corresponding acid and sodium bicarbonate. The mixture may then be heated at a controlled rate from room temperature to l OO'€. The CO₂ evolved during this period probably results from weakly-bonded molecules of CO₂ or CO₂ dissolved in the solution. In order to determine the total amount of CO₂ absorbed, including that as bicarbonate, an acid with a pKa < 5 may be used, which frees CO₂ from its bicarbonate or carbonate (or carbamate with MEA) form. The temperature and the volume of gas evolved during such decarboxylation experiments were recorded as a function of time. The actual CO₂ generated is the difference between the amount of gas evolved and the amount of gas evolved with pure water (which is shown in Figure 3). Therefore, the curve of the gas evolved with pure water is reproduced on all the other graphs in order to better appreciate the net amount of CO₂ generated during the desorption process.

As has previously been disclosed, several prior art methods have relied on the use of amine-based systems, typically based on monoethanolamine (MEA), and comparative data relating to such systems are also presented in Figure 4.

The reaction of an amine (e.g. MEA) with CO₂ forms the corresponding carbamate salt when carried out in a non-polar solvent such as diethyl ether. These salts usually precipitate out of solution as soon as they are formed. However, when the reaction is carried out in polar solvents (e.g. DMSO), then carbamic acids are usually formed, probably via the carbamate salts which remain in the polar solution and are further carboxylated to the carbamic acids. Under aqueous conditions, however, the situation is rather different, as water can play a key role in the reaction, and an equilibrium between the carbamate, carbamic acid and bicarbonate species occurs.⁶ The actual process that occurs is complex, although decarboxylation studies suggest that the ammonium bicarbonate is the dominant species.

Referring to Scheme 5, it is seen that in solution, preferably an aqueous solution, CO₂ gas (g) is in equilibrium with its dissolved (d) and carbonic acid form. These species are themselves in equilibrium denoted by a hydration constant. In addition, carbonic acid and the dissolved CO₂ are in equilibrium with their respective bicarbonate form denoted by the acid dissociation constant (pKa = 3.6 and pKa = 6.3, respectively).

H₂CO₃ + H₂O

CO₂(C) + H₂O

Scheme 5 - Reaction of Carbon Dioxide with Water Therefore, if a sodium salt of an acid with a pKa lower than 3.6 is employed, little reaction should be observed if this is the only capture process operating. When the pKa of the acid is higher than 3.6 and lower than 6.3, although the sodium salt of the acid can deprotonate carbonic acid, the formed bicarbonate can react subsequently with the formed acid to give carbon dioxide dissolved in water, therefore leading to little or no intake of CO₂. The sum of these equilibria determines the efficiency of this capture process. Therefore, the salt from an acid which has a pKa close to, or higher than, 6.3 will drive these equilibria towards the formation of bicarbonate, and consequently result in a significant uptake of CO₂.

Results of experiments involving the decarboxylation of a selection of carboxylated solutions are presented in Table 1 .

Table 1 Decarboxylation of the solution of selected sodium salts of acids

With the sodium salt of lactic acid, the amount of CO₂ evolved is of the same order of magnitude as the baseline amount generated during control experiments. Therefore, under these conditions, there is little or no intake of CO₂ with this sorbent, which could be expected due to its relatively low pKa (3.86).

A small amount of CO₂ evolution is observed using the sodium salt of malonic acid, which can also be explained by the value of its second pKa (PKa₁ = 2.83 and pKa₂ = 5.69) being contained between 3.6 and 6.3. However, with palmitic acid (pKa = 4.78) and maleic acid (PKa₁ = 1 .92 and pKa₂ = 6.23), a reasonable amount of CO₂ is evolved - 65 and 51 ml_ respectively. In these cases, a solid precipitate or suspension is formed during the carboxylation. It is found that although the acid-base equilibrium is not favourable for these reactions, the resulting acid formed precipitates from solution and this drives the acid-base equilibrium towards the formation of bicarbonate, as shown in Scheme 6. The advantage of this capture process is that less energy is required to displace the solubility equilibrium, and thereby free CO₂ from its bicarbonate form.

Y + H₂CM CO_{2 ^} ' Y + NaI-CO₃ o o

prec p tate

Scheme 6

When the pKa of the absorbent is increased further, a significant quantity of CO₂ is evolved on heating the CO₂ adduct. Indeed, the CO₂ adducts of the sodium salts of diethylmalonic acid (pKa = 7.29), 4-hydroxybenzoic acid (pKa = 9.39), ascorbic acid (pKa = 1 1 .6) and gallic acid (estimated pKa of phenolic group ~ 10), phosphoric acid (pKa = 12.32) and 2,4-pentanedione (pKa = 8.95) evolve surprisingly large amounts of CO₂ - 41 , 155, 269, 145, 129 and 199 ml_ respectively. Interestingly, it is noted that the volume of CO₂ captured by ethanolamine (199 ml_, 0.98 mol/L) may be obtained using half of the concentration of the sodium salt of gallic acid (145 ml_, 0.30 mol/L).

The invention will now be further illustrated, though without in any way placing any limitation on its scope, by reference to the following examples.

Examples

General Experimental Procedures

Broadband proton-decoupled carbon-13 nuclear magnetic resonance spectra were recorded at 100 MHz on a Bruker Avance 400 spectrometer. Chemical shifts are expressed in parts per million (ppm).

Pure deionized water was obtained from a water purification system, Nanopure Diamond™ Barnstead. All other reagents were used as received. Carbon dioxide CP grade was purchased from BOC gases and delivered to the sample via an ISCO 260D controllable syringe pump.

General Procedure for the Preparation of the Sample Acid (10.0 mmol) and base - for a 1 :2 ratio (1 .96 eq., 19.6 mmol) or for a 1 :1 ratio (0.96 eq., 9.60 mmol) - were added to a 50 ml_ round-bottomed flask. Water (10 ml) was then added at room temperature and atmospheric pressure. In order to compensate for the exothermicity of the reaction, a water-bath at room-temperature was used as a heat sink. The mixture was stirred at room temperature (water bath) and atmospheric pressure for 30 minutes. Carbon dioxide was then bubbled through the mixture at a flow rate of 50 mL/min at room temperature and atmospheric pressure for 30 minutes. This gave a range of carboxylated samples suitable for decarboxylation studies.

General Procedure for the Decarboxylation of the Sample (Volumetric Technique) The flask containing the carboxylated mixture (obtained as described in the general procedure for the preparation of the sample) was subsequently connected to the decarboxylation set-up, as depicted in Figure 1 .

The decarboxylation system is composed of: • A paraffin oil bath;

• A water bath;

• A heating system, wherein the temperature of the hotplate, RCT basic IKA^® WERKE, was controlled by a temperature probe, ETS-D4 fuzzy IKA^® WERKE. On this hotplate, the temperature knob is fixed at 200⁰C, which is equivalent to providing a heating rate of room temperature to 100⁰C in approximately 30 minutes; the agitation knob is also fixed at 4 on a scale of 10;

• A gas container, wherein a water-filled up-side-down graduated glass cylinder (250 ml_) was employed to collect the gas evolved during the decarboxylation;

• A tube, wherein in order to minimise the dead volume, a 1/16" stainless-steel tubing (less than 1 metre long) was used. The tip in the flask was mounted with a ferrule to circumvent any possible disconnection during the decarboxylation procedure. The other tip was pushed to the top of the inverted graduated glass cylinder (250 ml_), which was filled with water, to prevent water flowing back to the flask; and • A seal, wherein a B14 suba seal was utilised to allow addition of other reagents, such as concentrated sulphuric acid, during the decarboxylation process. Cyanoacrylate sealant (Superglue) was employed to seal any potential leaks at the position where the stainless-steel tubing passes through the suba seal. Paraffin film was also used in addition to the glue as an extra precaution.

The flask was positioned in the oil bath such that the level of the oil was just above the level of the carboxylated solution. Subsequently, the mixture was heated from room temperature to 100⁰C. When the temperature and the volume of gas evolved became constant for at least 30 minutes, a small volume of strong acid, such as concentrated sulphuric acid (2 ml_) or glacial acetic acid (2.5ml_), was added to free carbon dioxide from its bicarbonate (and/or carbamate) form. The evolution of gas, and temperature, were recorded as a function of time and subsequently plotted on a graph.

General Procedure for the NMR Study of the Carboxylation (NMR technique) A small amount (0.5 ml_) of the carboxylated mixture (prepared as previously described) was added to a 7-inch long borosilicate glass NMR tube (Norell ST500), following which a NMR capillary tube containing 10 %w tert-butylalcohol in deuterium oxide was inserted, as shown in Figure 2.

Comparative Example - Decarboxylation of Water (Blank reaction) 10 ml_ of pure water were decarboxylated according to the general procedure for decarboxylation of samples. The results are set out in Figure 3.

Comparative Example - Decarboxylation of Ethanolamine

Ethanolamine (0.6 ml_, 9.84 mmol) was added to a 50 ml_ round-bottom flask, and then 10 ml_ of pure water was added at room temperature and atmospheric pressure. To compensate for the exothermicity of the reaction, a water-bath at room-temperature was used as a heat sink. The mixture was stirred at room temperature (water bath) and atmospheric pressure for 30 minutes. Carbon dioxide was then bubbled through the mixture at a flow rate of 50 mL/min at room temperature and atmospheric pressure for 30 minutes. Then, the mixture was decarboxylated according to the general procedure for decarboxylation of samples. The results are shown in Figure 4.

Example 1 - Decarboxylation of an Aqueous Solution of Sodium Salt of Lactic Acid The general procedure for the preparation of samples was followed using lactic acid (1 .10 ml_, 13.3 mmol) and sodium hydroxide (525 mg, 13.0 mmol). 1 ml_ of CO₂ was obtained, as illustrated in Figure 5. Example 2 - Decarboxylation of an Aqueous Solution of Sodium Salt of Malonic Acid The general procedure for the preparation of the sample was followed using malonic acid (1 .17 g, 1 1 .2 mmol) and sodium hydroxide (888 mg, 21 .9 mmol). 10 ml_ of CO₂ was obtained, as illustrated in Figure 6.

Example 3 - Decarboxylation of an Aqueous Solution of Sodium Salt of Maleic Acid The general procedure for the preparation of the sample was followed using maleic acid (1 .58 g, 13.6 mmol) and sodium hydroxide (1.08 g, 26.7 mmol). 51 ml_ of CO₂ was obtained, as illustrated in Figure 7.

Example 4 - Decarboxylation of an Aqueous Solution of Sodium Salt of Palmitic Acid The general procedure for the preparation of the sample was followed using palmitic acid (1 .10 g, 4.25 mmol) and sodium hydroxide (171 mg, 4.22 mmol). 65 ml_ of CO₂ was obtained, as illustrated in Figure 8.

Example 5 - Decarboxylation of an Aqueous Solution of Sodium Salt of Diethylmalonic

Acid

The general procedure for the preparation of the sample was followed using diethylmalonic acid (1 .70 g, 10.4 mmol) and sodium hydroxide (829 mg, 20.5 mmol). 47 ml_ of CO₂ was obtained, as illustrated in Figure 9.

Example 6 - Decarboxylation of an Aqueous Solution of Sodium Salt of Ascorbic Acid The general procedure for the preparation of the sample was followed using ascorbic acid (1.10 g, 4.25 mmol) and sodium hydroxide (171 mg, 4.22 mmol). 269 ml_ of CO₂ was obtained as illustrated in Figure 10.

Example 7 - Decarboxylation of an Aqueous Solution of Sodium Salt of 4- Hvdroxybenzoic Acid

The general procedure for the preparation of the sample was followed using A- hydroxybenzoic acid (1.51 g, 10.8 mmol) and sodium hydroxide (862 mg, 21.3 mmol). 155 ml_ of CO₂ was obtained as illustrated in Figure 1 1.

Example 8 - Decarboxylation of an Aqueous Solution of Sodium Salt of Gallic Acid The general procedure for the preparation of the sample was followed using gallic acid (519 mg, 3.05 mmol) and sodium hydroxide (366 mg, 9.05 mmol). 145 ml_ of CO₂ was obtained as illustrated in Figure 12. ¹³C NMR of the starting material: 173.1 (ArCO₂H), 147.4 (ArC), 141.0 (ArC), 123.7(ArC), 1 12.8 (ArC). ¹³C NMR of the sodium salt of gallic acid before the addition of CO₂: 180.0 (ArCO₂Na), 155.1 (ArC), 144.6 (ArC), 128.7 (ArC), 1 10.6 (ArC). ¹³C NMR of the sodium salt of gallic acid after the addition of CO₂: 177.7 (ArCO₂Na), 163.1 (HCO₃Na)J 47.3 (ArC), 139.1 (ArC), 130.2 (ArC), 1 12.2 (ArC). The latter ¹³C NMR showed the presence of bicarbonate, a peak at 163.1 ppm with an intensity of approximately 2 to 1 compared to the peak for the carboxylate group at 177.7 ppm.

Example 9 - Decarboxylation of an Aqueous Solution of Sodium Salt of Phosphoric Acid The general procedure for the preparation of the sample was followed using phosphoric acid (0.33 ml_, 4.87 mmol) and sodium hydroxide (579 mg, 14.3 mmol). 129 ml_ of CO₂ was obtained as illustrated in Figure 13.

Example 10 - Decarboxylation of an Aqueous Solution of Sodium Salt of 2,4- Pentanedione

The general procedure for the preparation of the sample was followed using 2,4- pentanedione (1 .06 ml_, 10.2 mmol) and sodium hydroxide (408 mg, 10.1 mmol). 199 ml_ of CO₂ was obtained as illustrated in Figure 14.

References

1 . Intergovernmental Panel on Climate Change Report, Climate Change 2007: The

Physical Science Basis, http://www.ipcc.ch. 2. Khatri, R.A. , Chuang, S.S.C., Soong, Y. and Gray, M., Energy and Fuels, 2006, 20, 1514.

3. Song, C, Catalysis Today, 2006, 115, 2.

4. Idem, R. and Tontiwachwuthikul, P., Ind. Eng. Chem. Res., 2006, 45, 2413.

5. Freund, P., Proc. Instn. Mech. Engrs. Part A: J. Power and Energy, 2003, 217, 1. 6. Steeneveldt, R., Berger, B. and Torp, T.A., Trans. IChemE, Part A, Chem. Eng.

Res. and Design, 2006, 84(A9), 739.

7. Calculated from http://www.planetark.com/dailynewsstory.cfm/newsid/40403/story.htm.

8. Jassim, M.S. and Rochelle, G.T., Ind. Eng. Chem. Res., 2006, 45, 2465. 9. Poplsteinova, J., Krane, J. and Svendsen, H. F., Ind. Eng. Chem. Res., 2005, 44,

9894; Yoon, S.Y., Lee, H., Chem. Lett, 2003, 32, 344; Park, J-Y., Yoon, S.J. and

Lee, H., Environ. Sci. Techno!., 2003, 37, 1670. 10. Idem, R.O., Wilson, M., Tontiwachwuthikul, P., Chakma, A., Veawab, A.,

Aronwilas, A. and Gelowitz, D., Ind. Eng. Chem. Res., 2006, 45, 2414. 1 1. BeIIo, A. and Idem, R.O., Ind. Eng. Chem Res., 2005, 44, 945; Uyanga, I.J. and

Idem, R.O., Ind. Eng. Chem. Res., 2007, 46, 2558.

12. Abanades, J. C, Rubin, E. S. and Anthony, E.J., Ind. Eng. Chem. Res., 2004, 43, 3462.

13. Ma'mun, S., Svendsen, H. F., Hoff, K.A. and Juliussen, O., Energy Conversion and Management, 2007, 48, 251 .

14. Yeh, JT. , Resnik, K.P, Rygle, K. and Pennline, H. W., Fuel Processing Technol., 2005, 86, 1533.

15. Dell'Amico, D. B., Calderazzo, F., Labella, L., Marchetti, F. and Pampaloni, G., Chem. Rev., 2003, 103, 3857 and refs. cited therein. 16. Yeh, J.T., Resnik, K.P., Rygle, K. and Pennline, H.W., Fuel Processing Technol., 2005, 86, 1533.

17. Delfort, B., Carrette, P. L, FR-A-2909010; Heldebrandt, D.J., Yonker, C.R., Jessop, P. G., and Phan, L., Energy Environ. ScL, 2008, 1 , 487.

18. Aaron, D. and Tsouris, C, Separation Science and Technol., 2005, 40, 321 .

Claims

1. A method for the capture of carbon dioxide gas which comprises contacting the carbon dioxide with at least one acid or a salt thereof, wherein said acid has a pKa value in the range of from 0 to 14 and said acid does not include an amino group.

2. A method as claimed in claim 1 wherein said acid has a pKa value in the range of from 4 to 13.

3. A method as claimed in claim 1 or 2 wherein said acid has a pKa value in the range of from 5 to 12.

4. A method as claimed in claim 1 , 2 or 3 wherein said at least one acid or a salt thereof is in a solid or liquid form.

5. A method as claimed in claim 4 wherein said liquid form comprises a solution, a slurry, a dispersion or a suspension.

6. A method as claimed in claim 5 wherein said solution comprises an aqueous solution.

7. A method as claimed in claim 5 wherein said solution comprises a solution in an organic solvent.

8. A method as claimed in any one of claims 1 to 7 wherein said acid comprises an inorganic acid.

9. A method as claimed in claim 8 wherein said inorganic acid comprises aluminium hydroxide, trihydroxyoxovanadium, or phosphoric acid.

10. A method as claimed in any one of claims 1 to 7 wherein said acid comprises an organic acid.

1 1 . A method as claimed in claim 10 wherein said organic acid comprises an aliphatic, carbocyclic or heterocyclic organic acid.

12. A method as claimed in claim 10 or 1 1 wherein said acid comprises a mono- or a poly-acid.

13. A method as claimed in claim 12 wherein said polyacid comprises a di-, tri- or tetra-acid or a polymeric acid.

14. A method as claimed in any one of claims 10 to 13 wherein said organic acid comprises a carboxylic acid.

15. A method as claimed in claim 14 wherein said carboxylic acid comprises a carboxylic acid of the formula (I) or (II), or a salt thereof:

R¹-CO₂H (I)

HO₂C-R²-CO₂H (II)

wherein R¹ is selected from a substituted or unsubstituted alkyl group, optionally including one or more chain heteroatoms, a substituted or unsubstituted carbocyclic group or a substituted or unsubstituted heterocyclic group; and R² is selected from a substituted or unsubstituted alkylene group, optionally including one or more chain heteroatoms, a substituted or unsubstituted carbocyclic group or a substituted or unsubstituted heterocyclic group.

16. A method as claimed in claim 15 wherein said chain heteroatoms are selected from oxygen, phosphorus and sulphur.

17. A method as claimed in claim 15 wherein said alkyl or alkylene groups have up to 20 carbon atoms and may be linear or branched.

18. A method as claimed in claim 15 wherein said carbocyclic group or heterocyclic groups may be aliphatic or aromatic.

19. A method as claimed in claim 15 or 18 wherein said heterocyclic group comprises an aromatic ring or ring system having 16 or fewer members.

20. A method as claimed in any one of claims 15 to 19 wherein the substituents may be the same or different and are selected from hydroxy, alkoxy, halogen, hydroxyalkyl, haloalkyl, mercapto, carbonyl, acyl, acyloxy, sulfamoyl, carbamoyl, cyano and nitro groups.

21 . A method as claimed in any preceding claim wherein said acid is present in the form of a salt.

22. A method as claimed in claim 21 wherein said salt of said acid comprises a salt with an inorganic cation.

23. A method as claimed in claim 22 wherein said salt of an inorganic cation comprises an alkali metal or alkaline earth metal salt.

24. A method as claimed in any one of claims 12 to 21 wherein said salt of said acid comprises a salt with an organic cation.

25. A method as claimed in claim 24 wherein said salt of an organic cation comprises a sulphonium or phosphonium salt.

26. A method as claimed in claim 10 wherein said organic acid comprises ascorbic acid, acetylacetone, an acetoacetate ester or a malonate diester.

27. A method as claimed in claim 15 wherein said carboxylic acid comprises malonic acid, maleic acid, palmitic acid, diethylmalonic acid, 4-hydroxybenzoic acid, or gallic acid.

28. A method as claimed in claim 13 wherein said polymeric acid comprises poly(meth)acrylic acid.

29. A method as claimed in any preceding claim wherein CO₂ is contacted with the acid in salt form in aqueous solution at temperatures in the range of 10-60⁰C.

30. A method as claimed in claim 29 wherein said temperature is in the range of 35- 45⁰C.

31 . A method as claimed in claim 29 or 30 wherein an adduct of an acid with CO₂ is obtained by passing CO₂ through an aqueous solution of a salt of the acid.

32. A method as claimed in claim 31 wherein release of CO₂ from the adduct thus formed is achieved by heating the adduct at temperatures of up to around 140⁰C.

33. A method as claimed in claim 32 wherein said temperature is in the range of 20- 12O⁰C.

34. A method as claimed in claim 32 or 33 wherein release of CO₂ from the adduct thus formed is achieved by heating the adduct at pressures in the range from 0.001 MPa to 100 MPa.

35. A method as claimed in claim 34 wherein said pressure is in the range of 0.01 MPa to 30 MPa.