Reversible light driven gas absorbent solution and process

Abstract

The invention relates to a process for removing a target gas from a gas stream rich in the target gas and to an absorbent solution for absorbing a gas, such as carbon dioxide, from a gas stream. The invention involves the use of a photoactive compound that is convertible from a first state to a second state upon irradiation to facilitate removal or collection of a target gas from a gas stream.

Claims

1. A process for removing a target acidic gas from a gas stream rich in the target gas, comprising: contacting the gas stream with an absorbent solution comprising an amine selected from primary, secondary, or tertiary amines, alkanolamines, amino acids, and mixtures thereof, in which the target gas is soluble to form a target gas rich absorbent solution and a gas stream that is lean in the target gas; and converting a photoactive compound in the gas rich absorbent solution from a first state to a second state to bring about a reduction in pH of the absorbent solution wherein the gas rich absorbent solution comprising the photoactive compound in the first state has a higher absorption capacity for the target acid gas than the gas rich solution with the photoactive compound in the second state to cause desorption of the target gas from the absorbent solution comprising the target gas.

2. A process according to claim 1, wherein the amine is selected from the group consisting of monoethanolamine, ethylenediamine, 2-amino-2-methyl-ethanolamine and benzylamine, N-methylethanolamine, piperazine, piperidine and substituted piperidine, and diethanolamine, N-methyldiethanolamine, taurine, sarcosine, and alanine.

3. A process according to claim 1, wherein the photoactive compound is selected from the group consisting of naphthols and protonated merocyanine ring opened form of spiropyrans.

4. A process according to claim 1, wherein the photoactive compound is 1-(2-nitroethyl)-2-naphthol.

5. The process according to claim 1, wherein the photoactive compound is a photochromic compound.

6. The process according to claim 1, wherein the step of converting the photoactive compound from the first state to the second state involves irradiation with light.

7. The process according to claim 6, further comprising adding the photoactive compound to the target gas rich solution prior to exposing the target gas rich solution to light.

8. The process of claim 7, wherein the step of contacting the gas stream with the absorbent solution includes exposing the absorbent solution to light.

9. The process according to claim 1, wherein the amine selected from primary, secondary or tertiary amines, alkanolamines, amino acids, and mixtures thereof is in an amount to provide an initial pH of from about 9 to about 12 prior to absorption of CO.sub.2.

10. The process according to claim 9, wherein the pH of the absorbent solution falls within the operating range pK.sub.a of the photoactive compound, and wherein excitation of the photoactive compound provides a reduction in pH.

11. The process according to claim 1, wherein the converting of the photoactive compound from the first state to the second state comprises a change selected from the group consisting of: association or dissociation of moieties to or from the photoactive compound, a spatial rearrangement of at least a part of the compound, a steric change to the compound, forming or breaking bonds within the compound, ring formation, a change in the acid dissociation constant or base dissociation constant of at least a part of the compound, or a combination thereof.

12. The process according to claim 1, wherein the photoactive compound acts as a Brønsted base or weak Brønsted acid in the first state, and a strong Brønsted acid in the second state.

13. The process according to claim 1, wherein the conversion of the photoactive compound from the first state to the second state brings about a change in pK.sub.a of at least 1.

14. The process according to claim 1, wherein the gas rich absorbent solution comprises the photoactive compound in solution in a concentration of from about 0.1 mmol per liter of absorbent solution to 10 mol per liter of absorbent solution.

15. The process according to claim 1, wherein the photoactive compound includes at least one selected from the group consisting of spiropyrans, spirooxazines, dithienylethenes, fulgides, fulgimides, perimidinespirocyclohexadienones, quinones, benzodiazepines, naphthopyrans, dihydroindolines, protonated merocyanines, and naphthols.

16. The process according to claim 1, wherein the photoactive compound is a photoacid comprising a nucloeophilic moiety having a photodissociable proton, an electron accepting moiety, and a bridge between the nucleophilic and electron accepting moiety, wherein the electron accepting moiety bonds to a proton photodissociated from the nucleophilic moiety during reversible photoinduced intramolecular reaction to form a ring.

17. The process according to claim 1, wherein the target gas is selected from the group consisting of CO.sub.2, NO.sub.x (where x is between 0.5 and 2), SO.sub.2, H.sub.2S, and halogen gas.

18. The process according claim 1, wherein the target gas is carbon dioxide.

19. A process according to claim 1, wherein the light used to convert the photoactive from the first state to the second state is sunlight.

20. A process for separating carbon dioxide from a gas mixture comprising: providing an absorbent solution comprising an amine selected from the group consisting of primary amines, secondary or tertiary amines, alkanolamines, amino acids and mixtures thereof, and a photoactive compound having a first state and a second state of higher pK.sub.a than the first state formed on irradiation with light, wherein the amine is present in an amount to provide a pH of the absorbent solution in the range of from 9 to 12; contacting the absorbent solution with a gas stream rich in carbon dioxide to absorb carbon dioxide and provide an absorbent solution rich in absorbed carbon dioxide and a gas stream lean in carbon dioxide; irradiating the absorbent solution rich in absorbed carbon dioxide to convert the photoactive compound from the first state to the second state and produce a reduction in pH of the absorbent solution rich in absorbed carbon dioxide and reduce the absorption capacity of the absorbent solution rich in absorbed carbon dioxide; and collecting carbon dioxide desorbed from the absorbent solution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates a process for removing a target gas from a gas stream according to the present invention.

(2) FIG. 2 illustrates an alternative process for removing a target gas from a gas stream according to the present invention.

(3) FIG. 3 is a graph illustrating the relationship between absorbent solution pH and CO.sub.2 concentration.

(4) FIG. 4 is a graph illustrating the relationship between absorbent solution pH and SO.sub.2 concentration.

(5) FIG. 5 is a graph illustrating the relationship between CO.sub.2 absorption and pH by an absorbent solution containing monoethanolamine and MEH/spiropyran photoactive compound in the ground state and the excited state in accordance with an embodiment of the present invention.

(6) FIG. 6 is a graph illustrating the relationship between CO.sub.2 absorption and pH by an absorbent solution containing piperidine and MEH/spiropyran photoactive compound in the ground state and the excited state in accordance with an embodiment of the present invention.

(7) FIG. 7 is a graph illustrating the relationship between CO.sub.2 absorption and pH by an absorbent solution containing monoethanolamine and nitronaphthol photoactive compound in the ground state and the excited state in accordance with an embodiment of the present invention.

(8) FIG. 8 is a graph illustrating the relationship between CO.sub.2 absorption and pH by an absorbent solution containing piperidine and nitronaphthol photoactive compound in the ground state and the excited state in accordance with an embodiment of the present invention.

(9) FIG. 9 is a graph illustrating the relationship between pH and 1-(2-nitroethyl)-2-naphthol in either its ground or excited state as measured using a pH probe.

DETAILED DESCRIPTION

(10) The invention relates to the use of a photoactive compound in a solution for absorbing a target gas from a gas stream.

(11) FIG. 1 provides an illustration of an embodiment of a process for capture of a target gas from a flue gas stream. In this particular embodiment, the target gas is CO.sub.2. The process 100 includes an absorption reactor 102, for absorbing CO.sub.2 from a flue gas stream, and a desorption reactor 104 for desorbing CO.sub.2.

(12) The absorption reactor 102 includes a first inlet 106, a second inlet 108, a first outlet 110, and a second outlet 112, and a gas absorption contact region 114. The first inlet 106 of the absorption reactor 102 is a flue gas inlet through which a CO.sub.2 rich flue gas enters the absorption column 102. The second inlet 108 is an absorbent solution inlet through which a CO.sub.2 lean absorbent enters the absorption column 102. The CO.sub.2 rich flue gas and the CO.sub.2 lean absorbent contact in the gas absorption contact region 114. In this region the CO.sub.2 in the CO.sub.2 rich flue gas is absorbed into the absorbent solution where it is bound in solution.

(13) The absorbent solution includes a photoactive molecule, such as a photoactive acid or base. The photoactive acid or base undergoes photoisomerism, or other light induced structural changes, leading to a change in the pK.sub.a and a change in the pH of the solution. In this embodiment, the photoactive molecule exhibits a photo-induced change in Brønsted acid-base properties on exposure to light. In the first state, the ground state, the photoactive molecule acts as a Brønsted base or weak Brønsted acid, to provide an absorbent solution with a pH in the range 9-12, thus absorption of CO.sub.2 is carried out with the molecule in its basic or weakly acidic form. When in this state, the absorbent solution is able to absorb and retain CO.sub.2 from the CO.sub.2 rich flue gas with higher affinity and capacity than when the molecule is in a second state. In the second state, or excited state, which occurs when the photoactive molecule is exposed to light, the photoactive molecule is converted to a strong Brønsted acid resulting in a solution with a pH in the range 0-8.

(14) In this particular embodiment, the photo-induced change of the photoactive molecule, where the photoactive molecule is a photo-acid, is illustrated below:

(15) ##STR00003##

(16) where R is selected from the group consisting of hydrogen, C.sub.1 to C.sub.6 alkyl and —(CH.sub.2).sub.nW where n is from 1 to 6 (preferably 3 or 4) and W is —NH.sub.2, CO.sub.2.sup.− or SO.sub.3.sup.− (preferably SO.sub.3.sup.−).

(17) As can be seen, when the photoactive molecule is in the ground state, it is a weak acid or weak base (left hand molecule). The photoactive molecule in the ground state may be in protonated form. However, when converted to the excited state (right hand molecule), the photoactive molecule is a strong acid and acts as a proton source leading to a decrease in pH.

(18) In this embodiment, when R comprises NH.sub.2 the photo-induced change alters the affinity of the photoactive molecule for direct reaction with CO.sub.2 of the primary amine to form a carbamate. The ground state has a strong affinity for CO.sub.2, and the excited state has a lower affinity for reacting with CO.sub.2.

(19) In some embodiments the photoactive molecule may be a naphthol, for example the 1-(2-nitroethyl)-2-naphthol shown below:

(20) ##STR00004##

(21) The ground state of the naphthol has a pK.sub.a of 9.7 and the excited state has a pK.sub.a of 1.9.

(22) The absorption of CO.sub.2 from the CO.sub.2 rich flue gas into the absorbent solution results in a CO.sub.2 lean gas and a CO.sub.2 rich absorbent solution. The CO.sub.2 lean gas may still include some CO.sub.2, but at a lower concentration than the CO.sub.2 rich flue gas, for example a residual concentration of CO.sub.2.

(23) The CO.sub.2 lean gas leaves the absorption column 102 through the first outlet 110, which is a CO.sub.2 lean gas outlet. The CO.sub.2 rich absorbent solution leaves the absorption column through the second outlet 112, which is a CO.sub.2 rich absorbent outlet.

(24) The desorption reactor 104 includes an inlet 118, a first outlet 120, a second outlet 122, and a gas desorption region 124. The CO.sub.2 rich absorbent outlet 112 of the absorption column 102 forms the inlet 118 of the desorption column 104. Desorption of CO.sub.2 from the CO.sub.2 rich solution occurs in the gas desorption region 124. In the gas desorption region 124, the CO.sub.2 rich solution is exposed to light, which in this case is sunlight 126.

(25) On exposure to sunlight 126, the photoactive molecule in the CO.sub.2 rich solution undergoes a photo-induced change to the excited state which converts the photoactive molecule to a strong Brønsted acid, lowering the pH of the CO.sub.2 rich solution to within the range of pH 0-8. Broadly, in regard to photo-induced changes in Brønsted acid-base properties, in some cases, e.g. spiropyrans, leucohydroxide, the change in pH is a result of new ions being introduced to the solution. In other cases e.g. dithientlethenes, fulgides, the change in pH is due to a functional group in the molecule changing its pK.sub.a in response to photo-induced structural reorganisation. However, in the present embodiment, the photo-induced change is as a result of the photoactive molecule acting as a proton source, as illustrated above.

(26) In the excited state, the photoactive molecule itself has a low affinity for bonding with CO.sub.2, and the lower pH of the solution favours desorption of CO.sub.2 from the CO.sub.2 rich absorbent solution. The excited state of the molecule, and the lower pH of the solution, reduces the overall energy required to liberate CO.sub.2 from being bound in solution. Thus, exposure of the CO.sub.2 rich solution to light may cause desorption of CO.sub.2 to occur, or at least reduces the amount of heat energy required to drive CO.sub.2 out of the CO.sub.2 rich solution in comparison with the amount of heat that otherwise would have been required to remove CO.sub.2 from being bound by the photoactive molecule when the photoactive molecule is in the ground state. In brief, CO.sub.2 release is effected by conversion of the photoactive molecule to its acidic from through modulation of light exposure (e.g. controlling the amount and/or wavelength of exposed light). This is because the photo-induced change mimics the relationship between amine pK.sub.a and temperature to achieve the same effect rather than, or in addition to, heat induced changes. In this particular embodiment, further heat energy is required to effect desorption of CO.sub.2 from the CO.sub.2 rich solution.

(27) Removal of CO.sub.2 from the CO.sub.2 rich solution results in the formation of a CO.sub.2 gas stream and a CO.sub.2 lean absorbent solution. The CO.sub.2 lean absorbent solution may still include some CO.sub.2, but at a lower concentration than the CO.sub.2 rich solution, for example a residual concentration of CO.sub.2.

(28) The CO.sub.2 gas stream is taken off via the first outlet 120, which is a CO.sub.2 outlet. The CO.sub.2 lean absorbent solution is taken off via the second outlet 122, which is a CO.sub.2 lean absorbent solution outlet. The CO.sub.2 lean absorbent is then recycled and fed through the second inlet 108 to the absorption column 102.

(29) The above process is described with respect to a photoactive molecule that both binds with CO.sub.2, and changes the environment of the absorbent solution to enhance the uptake of CO.sub.2 by the solution, and to improve the ability of the photoactive molecule to bind with the CO.sub.2.

(30) In an alternative embodiment, the absorbent solution includes a CO.sub.2 absorbent molecule in addition to the photoactive molecule. In this case, the role of the photoactive molecule is largely to affect the environment of the absorbent solution. When the photoactive molecule is in the ground state, the environment of the absorbent solution is such that absorption of CO.sub.2 into the solution is enhanced. The CO.sub.2 then binds with the CO.sub.2 absorbent molecule. The photoactive molecule may or may not bind with CO.sub.2. When the photoactive molecule is in the excited state, the environment of the absorbent solution is such that the affinity or propensity of the solution for absorption and retention of CO.sub.2 is reduced. The CO.sub.2 absorbing molecule releases CO.sub.2 into solution, and CO.sub.2 is liberated from solution. As discussed above, some external heat energy may be required to drive the release and liberation of CO.sub.2 from the absorbent solution.

(31) An absorbent solution for absorbing an acid gas may comprise an amine such as a primary, secondary or tertiary amine, an alkanolamine, an amino acid or mixtures thereof. Suitable amines are known in the art and a skilled person will have no difficulty in selecting suitable amines having regard to the function known of the amine and the combination with the photoactive compounds described herein. Examples of alkanolamines are described in US2011/0116997 and US2013/0291724. Examples of alkyl amines and alkanolamines are also described in US2013/0291724. Suitable amines may be selected from the group consisting of alkyl amines, alkanolamines and amino acids, including but not limited to monoethanolamine, ethylenediamine, N-methylethanolamine, 2-amino-2-methyl-ethanolamine, N-methyldiethanolamine, piperazine, piperidine and substituted piperidine, benzylamine, diethanolamine, taurine, sarcosine and alanine. The amine compound may interact with a target gas such as CO.sub.2 and enhance the absorption of the target gas from a gas stream.

(32) In yet another alternative embodiment, a variation on the scheme above could use an absorbent solution including a non-photoactive amine to absorb the CO.sub.2, and then after a CO.sub.2 rich solution has formed, adding a photoactive molecule into the solution before exposing the solution to light. On exposure to light, the photoactive molecules are changed to the excited state, which promotes CO.sub.2 desorption. The photoactive molecules may be added to solution and removed once CO.sub.2 desorption is complete, or the photoactive molecules could be anchored onto solid materials exposed to the light. In this way the photoactive molecules are not involved in the absorption reaction, and are only mixed with the CO.sub.2 rich solution when removal of CO.sub.2 is desired.

(33) In another set of embodiments, the photoactive compound may combine the properties of a photoacid and an amine in a single molecule. An example of such a photoactive compound is protonated merocyanine (MEH) and its spiropyran form illustrated above, when R comprises NH.sub.2. A photoactive compound having combined properties may provide enhanced CO.sub.2 absorption through interaction of the target gas with the amine group whilst also be capable of undergoing a photo-induced change upon irradiation.

(34) FIG. 2 provides an illustration of another embodiment a process for capture of a target gas from a flue gas stream. In this particular embodiment, the target gas is CO.sub.2. This process 200 involves a similar layout to that in FIG. 1, where the process 200 includes an absorption reactor 202, for absorbing CO.sub.2 from a flue gas stream, and a desorption reactor 204 for desorbing CO.sub.2. However, in this process the photoactive molecule has a ground state in which the photoactive molecule acts as a strong Brønsted acid form with a solution pH in the range 0-8. When in the ground state, the absorbent solution has a low affinity, or low capacity to absorb and retain CO.sub.2. In the excited state, which occurs when the photoactive molecule is exposed to light, the photoactive molecule acts as a Brønsted base or weak Brønsted acid, to provide an absorbent solution with a pH in the range 9-12. When in the excited state, the absorbent solution is able to absorb and retain CO.sub.2 from a CO.sub.2 rich flue gas with higher affinity and capacity than when the molecule is in the ground state.

(35) In this process 200, a CO.sub.2 rich flue gas is introduced into the absorption reactor 202 via a flue gas inlet 206. A CO.sub.2 lean absorbent solution is introduced into the absorption reaction 202 via an absorbent solution inlet 208. The CO.sub.2 rich flue gas and the CO.sub.2 lean absorbent solution contact in a contact region 210 of the absorption reactor 202. The contact region 210 is exposed to light, such as UV light 212. The presence of the light 212 ensures that the photoactive molecule in the CO.sub.2 lean absorbent solution is in its excited state.

(36) As discussed above, in this embodiment, when the photoactive molecule is in the excited state, it acts as a Brønsted base or weak Brønsted acid, to provide an absorbent solution with a pH in the range 9-12. This environment is favourable for the absorption and retention of CO.sub.2. CO.sub.2 is absorbed from the CO.sub.2 rich flue gas into the CO.sub.2 lean absorbent solution to form a CO.sub.2 lean flue gas and a CO.sub.2 rich solution.

(37) In any event, the CO.sub.2 lean flue gas exits the absorption reactor 202 via the flue gas outlet 222 and the CO.sub.2 rich solution exits the absorption reactor 202 through a CO.sub.2 rich absorbent outlet 214.

(38) The CO.sub.2 rich absorbent outlet 214 forms the inlet 216 to the desorption reactor 204. The internal environment of the desorption reactor is such that the photoactive molecule is changed from the excited state to the ground state, where the photoactive molecule acts as a strong Brønsted acid. As discussed previously, when the photoactive molecule acts as a strong Brønsted acid, the solution environment is less energetically favourable to the absorption and retention of CO.sub.2. As above, CO.sub.2 may desorb from the solution, or some additional heat may be required to desorb CO.sub.2 from solution. Desorption of CO.sub.2 results in a CO.sub.2 gas stream and a CO.sub.2 lean absorbent solution. The CO.sub.2 gas stream is removed from the desorption column 204 via outlet 218, and the CO.sub.2 lean absorbent solution is removed from the desorption column 204 via outlet 220 and can be recycled to the absorption column 202 via inlet 208.

(39) In an alternative embodiment, the absorbent solution includes a CO.sub.2 absorbent molecule in addition to the photoactive molecule. In this case, the role of the photoactive molecule is largely to affect the environment of the absorbent solution. When the photoactive molecule is in the excited state, the environment of the absorbent solution is such that absorption of CO.sub.2 into the solution is enhanced. The CO.sub.2 then binds with the CO.sub.2 absorbent molecule. The photoactive molecule may or may not bind with CO.sub.2. When the photoactive molecule is in the ground state, the environment of the absorbent solution is such that the affinity or propensity of the solution for absorption and retention of CO.sub.2 is reduced. The CO.sub.2 absorbing molecule releases CO.sub.2 into solution, and CO.sub.2 is liberated from solution. As discussed above, some external heat energy may be required to drive the release and liberation of CO.sub.2 from the absorbent solution.

(40) In yet another alternative embodiment, a variation on the scheme above could use an absorbent solution including a non-photoactive amine to absorb the CO.sub.2, and a photoactive molecule into the solution and then exposing the solution to light to effect CO.sub.2 absorption. The photoactive molecules may be added to solution and removed once the CO.sub.2 absorption is complete, or the photoactive molecules could be anchored onto solid materials exposed to the light. In any event, after a CO.sub.2 rich solution has formed, the photoactive molecules are removed from the solution which promotes CO.sub.2 desorption. In this way the photoactive molecules are not involved in the desorption reaction, and are only mixed with the CO.sub.2 lean solution when absorption of CO.sub.2 is desired.

(41) A person skilled in the relevant art would appreciate that the absorption of CO.sub.2 and its removal from a CO.sub.2 rich flue gas could be influenced by a number of factors. These factors may include for example, the engineering of the process, the design of the reactors, the intensity of light used to irradiate the photoactive compound, the dimensions of the absorption and desorption reactors including the thickness of the gas contact region or the gas desorption region, the residence time of the absorbent solution in the gas absorption or gas desorption contact region, the efficiency of contact between the absorbent solution and the gas, as well as the type of photoactive compound and absorbent compound (if any) used. Each of these factors may influence the amount of target gas absorbed or desorbed. In one set of embodiments, the process of the invention removes at least 10% (v/v), preferably at least 20% (v/v) (for example at least 40% (v/v) or at least 50% (v/v)), of a target gas from a gas stream rich in the target gas.

(42) The invention will now be described with reference to the following examples. It is to be understood that the examples are provided by way of illustration of the invention and that they are in no way limiting to the scope of the invention.

EXAMPLES

Example 1

(43) This example describes a method of measurement of affinity for reaction with a target gas.

(44) As discussed previously, the target gas is preferably an acid gas of which CO.sub.2 is an example. The below discussion provides an embodiment of a methodology to measure the affinity or capacity of the absorbent solution for a target gas when the photoactive molecule is in the first state or the second state. In this embodiment, the target gas is CO.sub.2. However, it will be understood that this method may be generally applicable to determine the affinity or capacity of the absorbent solution for any acidic target gas, and is not limited to CO.sub.2.

(45) The affinity for reaction with CO.sub.2 is defined as the amount of CO.sub.2 absorbed by a solution in moles of CO.sub.2 absorbed/moles of absorbent molecules. The relative affinity of different solutions is compared under identical conditions of temperature, total pressure, CO.sub.2 partial pressure, gas composition, and concentration of absorbent molecules.

(46) A known mass of solution containing the photoactive molecule/s at the desired concentration/s is placed in a reaction vessel that is fully or partially transparent to UV-visible light (e.g. a vessel made of quartz or containing a quartz window). The photoactive molecule/s is/are then placed in the desired state for CO.sub.2 absorption (i.e. state which has greater or lower affinity for CO.sub.2). Depending upon the properties of the photoactive molecule/s this may be by exposure to UV-visible light of wavelengths matching the absorption bands of the molecule/s, or by exclusion of UV-visible light.

(47) At constant temperature and while maintaining exposure to/exclusion of UV-visible light a gas stream containing CO.sub.2 at a pressure of 101.3 kPa is slowly bubbled through the solution. This condition is maintained until the solution is saturated with CO.sub.2. The time required to achieve saturation will vary depending upon the amount of solution, the concentration of photoactive and other molecules and the gas flow rate used, but is typically a number of hours.

(48) Once saturated a liquid sample is taken from the reactor and added to a solution of 1 mol/L sodium hydroxide of at least 10 times the volume of the sample to ensure the absorbed CO.sub.2 remains in solution. This mixture of liquid sample and sodium hydroxide solution is then analysed for the CO.sub.2 content via a published and validated method to determine the CO.sub.2 content of an aqueous amine solution such as: measurement of the carbon-13 NMR spectrum and quantification of the concentration of all CO.sub.2 containing species; or heating under acidified conditions and measuring the amount of CO.sub.2 evolved using a CO.sub.2 analyser, or by absorption into an indicator solution.

(49) The affinity for reaction with CO.sub.2 is expressed in the moles of CO.sub.2 absorbed/moles of absorbent molecules as determined by this analysis.

Example 2

(50) This example demonstrates a process for removal of a target gas exemplified by CO.sub.2 using a photochromic compound exemplified by the merocyanine-spiropyran photochromic system shown in the following scheme.

(51) ##STR00005##

(52) The process of light driven stripping of CO.sub.2 from aqueous solution by action of photochromic molecule is demonstrated in the following way. An aqueous solution is prepared first with a concentration of protonated merocyanine (MEH) (1E.sup.−4 molar), to which was added an amine selected to operate in the pH range determined by MEH. The solution is then saturated with CO.sub.2 and the pH recorded to be 5. Photoexcitation is carried out to achieve the pH change using a 500 W HgXe arc lamp representing sun like illumination. The solution was passed through a photoreactor cell at a rate of flow past the irradiating light approximating the required time for exposure of a similar solution of MEH to produce the maximum pH change. The irradiation induces transformation of the photoactive compound, with the transformation producing a reduction in pH leading to desorption of CO.sub.2. The solution that exited the photoreactor cell was collected and the pH measurement of this solution was found to be 6.5. In the solution exiting the cell, CO.sub.2 has been desorbed from the solution and the photochromic compound has returned to its ground state form (MEH). The final pH of 6.5 represents the pH of the exiting solution after CO.sub.2 desorption. The results are shown below:

(53) TABLE-US-00001 Photoactive Light Amine name name and Initial CO.sub.2 wavelength/ CO.sub.2 and conc. conc. pH absorbed intensity Final pH desorbed Ethylenediamine MEH 5 7 mM Hg/Xe lamp 6.5 6.5 mM 0.1 mM 0.1 mM 500 W

(54) A quantitative relationship exists between the absorbent pH and its CO.sub.2 content. By taking into account the relevant reactions between CO.sub.2 and amine and the pK.sub.a of the photochromic molecule in the ground state, the CO.sub.2 content of the absorbent can be calculated using the published method Puxty, G., Maeder, M. Int. J. Greenh. Gas Control, 17 (2013), 215-224. A plot showing this relationship for 0.1 mM ethylenediamine and 0.1 mM MEH is shown in FIG. 3. The CO.sub.2 content at pH 5 and 6.5 is shown in FIG. 3, with the difference being the amount of CO.sub.2 removed.

Example 3

(55) This example demonstrates a process for removal of a target gas exemplified by SO.sub.2 using a photochromic compound exemplified by the merocyanine-spiropyran photochromic system shown below.

(56) ##STR00006##

(57) In this example, the process conditions of Example 2 is repeated, however the target gas is SO.sub.2 instead of CO.sub.2. The absorbent solution is an aqueous composition with 0.1 mM ethylenediamine and 0.1 mM MEH.

(58) The relationship between pH and SO.sub.2 content of the absorbent was determined is and shown in FIG. 4. The relevant reactions for SO.sub.2 are summarised in Goldberg, R. N., Parker, V. B. Journal of the National Bureau of Standards, 90 (1985), 341-390. The main difference between SO.sub.2 and CO.sub.2 is that the photoactive molecule needs to operate in a lower pH regime to achieve significant SO.sub.2 removal from the gas stream.

Example 4

(59) The requirement to select an amine for use with a photochromic molecule that results in an absorbent pH within the working range of the photochromic molecule can be illustrated by calculation. The pK.sub.a of MEH used in Example 2 is 7.8 in aqueous solution at room temperature. In its optically excited spiropyran form it becomes a strong acid (pK.sub.a<1). The pH of a mixture containing 0.5 M amine and 0.25 M MEH (ground state) as a function of absorbed CO.sub.2 and 0.5 M amine and 0.25 spiropyran (excited state) as function of absorbed CO.sub.2 was calculated using the published method Puxty, G., Maeder, M. Int. J. Greenh. Gas Control, 17 (2013), 215-224. The difference in pH of the absorbent at each concentration of absorbed CO.sub.2 is indicative of the ability of excitation to remove CO.sub.2 from the absorbent. The calculation was completed at 25° C. using the amines monoethanolamine (pK.sub.a=9.4) and piperidine (pK.sub.a=11.1) with the relevant reaction equilibrium constants taken from Fernandes, D., et al., J. Chem. Thermodynamics, 51 (2012), 97-102 and Fernandes, D., et al., J. Chem. Thermodynamics, 54 (2012), 183-191.

(60) The results of calculated CO.sub.2 absorption for absorbent solutions containing 0.25M MEH/spiropyran and either monoethanolamine and piperidine is shown in FIGS. 5 and 6, respectively.

(61) As seen in FIG. 5, a larger difference in pH is evident for the monoethanolamine sample indicating that when using this amine with a MEH/spiropyran system, more CO.sub.2 will be removed upon excitation than when using piperidine as the absorbent amine.

Example 5

(62) This example is identical to Example 4 except that the photoactive molecule is now 1-(2-nitroethyl)-2-naphthol which has a pK.sub.a of 9.7 in the ground state and 1.9 in the excited state. In this case piperidine is a better match for the operating pH range of the nitronapthol, compared to the MEH/spiropyran given in Example 4. The nitronapthol also extends the pH range over which CO.sub.2 removal could be achieved using monoethanolamine.

(63) The results of calculated CO.sub.2 absorption for absorbent solutions containing 0.25M nitronaphthol and either monoethanolamine and piperidine is shown in FIGS. 7 and 8, respectively.

Example 6

(64) To demonstrate the reversibility of the pH switching behaviour, a 1 mM solution of 1-(2-nitroethyl)-2-naphthol in 2% methanol aqueous solution was irradiated with a 500 W Xe/Hg lamp to simulate solar irradiation and the pH monitored with an electrode. The results are shown in FIG. 9. The operating range available for this molecule and those of other photoacids in this class extends to more basic (higher) pH than that of the merocyanine MEH. This means that more amines are available for use alongside this photoacid such as both piperidine and monoethanolamine.

(65) Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.