Co 2 adsorbents

Abstract

A capture complex comprising a cucurbituril compound having the Formula (I), wherein n is 5, 6, 7, 8 or 10; and for each glycoluril unit, R1 is selected from hydrogen, halogen, optionally substituted -(C1-C6)alkyl, optionally substituted -(C1-C6)haloalkyl or -OH; and at least one guest molecule, wherein the complex is configured to selectively and reversibly adsorb CO2.

Description

[0001] C02Adsorbents

[0002] Field of the Invention

[0003] This invention relates to capture complexes that are configured to selectively and reversibly adsorb CO2. More specifically, the invention relates to capture complexes comprising a cucurbituril compound and at least one guest molecule. The invention also relates to the use of these capture complexes as adsorbents for capturing CO2.

[0004] Background

[0005] Escalating greenhouse gas concentrations in the atmosphere due to fossil fuel consumption has resulted in a ~1.0 °C increase in global mean surface temperature beyond pre-industrial levels. This global warming has been linked to increased instances of extreme weather and climate change, motivating the establishment of the Paris Climate Accord in 2015. To meet the ambitious target of the Paris Accord of confining further global temperature increases to 1 .5 °C, the capture and sequestration of the most prominent greenhouse gas - carbon dioxide (CO2) - is imperative.

[0006] One technique for capturing CO2 is chemical absorption, in which CO2 (either from a point source, such as flue gas, or directly from the atmosphere) is absorbed by a liquid solvent such as an amine. Monoethanolamine (MEA) solution has been widely used as a solvent for chemical absorption because it has a high affinity for CO2 and is relatively inexpensive. However, most CO2 capture systems based on MEA require a temperature of at least 80 °C to release CO2 after capture (for storage or use) and re-generate the amine. As a result, MEA suffers from solvent degradation due to the high temperatures employed for regeneration and also because of the presence of oxygen in the capture system. Thus, the reproducibility of the MEA system after the adsorption process is low. Accordingly, a huge amount of energy and cost is required to maintain MEA systems.

[0007] A viable alternative solvent to MEA is aqueous ammonia. Aqueous ammonia has a sufficient CO2 absorption capacity and advantageously does not degrade in the presence of oxygen, unlike MEA. However, ammonia is volatile and under high temperatures has been found to leak from capture systems. As a result, systems employing aqueous ammonia require additional cooling systems which substantially increases the running costs. Thus, the energy saving benefit of ammonia over MEA is partially offset by the additional energy requirements to keep the system at a low temperature. Alternatively, CO2 may be captured by solid adsorbents, such as porous materials, by chemical adsorption. The use of solid adsorbents for CO2 capture has become increasingly popular because they are efficient at adsorbing CO2, have relatively low maintenance costs in comparison to liquid solvents (due to considerably lower regeneration energies) and may be scaled up for commercial use.

[0008] However, existing solid adsorbents such as zeolites or Metal Organic Frameworks (MOFs), made of metal ions and organic building blocks, suffer from similar degradation issues as MEA. Further, these adsorbents are also either too expensive to be utilised in large scale or too unstable due to their sensitivity to moisture. There is, therefore, a need for an adsorbent that is able to efficiently adsorb CO2 and have sufficient reproducibility and renewability following the CO2 adsorption process.

[0009] Summary of the Invention

[0010] In a first aspect, the invention provides a capture complex comprising a cucurbituril compound having the Formula (I): wherein n is 5, 6, 7, 8 or 10; and for each glycoluril unit, R1is selected from hydrogen, halogen, optionally substituted -(Ci-Ce)alkyl, optionally substituted -(Ci-C6)haloalkyl or -OH; and at least one guest molecule, wherein the complex is configured to selectively and reversibly adsorb CO2.

[0011] Cucurbituril compounds are a group of organic compounds comprising repeating glycoluril monomer units linked by methylene bridges. Each glycoluril unit may be substituted at the ring fusion position, for example substituted with halogen, optionally substituted -(Ci-Ce)alkyl, optionally substituted -(Ci-C6)haloalkyl or -OH, or un-substituted at the ring fusion position, such that the cucurbituril compound has the Formula (II):

[0012]

[0013] (n = 5, 6, 7, 8, 10)

[0014] Preferably, for each glycoluril unit of the cucurbituril compound, R1is hydrogen and the cucurbituril compound has the Formula (II) above.

[0015] As used herein, the term cucurbituril (CB[n]) refers to cucurbituril compounds with unsubstituted glycouril units (where n denotes the number of repeating glycoluril units) according to Formula (II) above.

[0016] Cucurbituril compounds comprise a hydrophobic cavity accessed by two portals surrounded by polar carbonyl groups, at opposing ends of the cavity. The cavity is capable of forming strong, non-covalent complexes with guest molecules, herein referred to as a host-guest complex or a cucurbituril compound-guest complex.

[0017] Specifically, cucurbituril compounds such as cucurbiturils comprising 6, 7 or 8 repeating glycoluril monomers have been found to selectively adsorb CO2 gas within their cavities. Without wanting to be bound by theory, it is believed that the high selectively of cucurbiturils for CO2 is due to the high enthalpy of CO2 adsorption caused by the CO2 molecules and cucurbiturils interacting through hydrogen bonding and / or dipole-quadrupole interactions. However, due to the strong binding affinity between cucurbituril and CO2, a large amount of energy is required to release CO2 from the cucurbituril cavity. Thus, the temperature required for CO2 desorption from the cucurbituril cavity is higher than optimal for use as a commercial CO2 adsorbent.

[0018] The capture complex of the invention is configured to selectively and reversibly adsorb CO2. Preferably, the capture complex is configured to selectively and reversibly adsorb CO2 gas. Alternatively, the capture complex is configured to selectively and reversibly adsorb CO2 which is in a solution. For example, CO2 which is aquated (i.e., CO2(aq>).

[0019] As described above, CO2 has a strong binding affinity for cucurbituril compounds and may be selectively adsorbed by the cucurbituril compounds into the cavity. To reverse the adsorption (i.e., desorb or release the CO2 from the capture complex) the adsorbed CO2 molecules need to gain enough energy to overcome the binding energy barrier between CO2 and the cucurbituril compound.

[0020] As shown in the Figures and described below, the inventors have found that the inclusion of at least one guest molecule within the cucurbituril cavity decreases the temperature required for CO2 desorption in comparison to the temperature required for CO2 desorption from cucurbituril alone (i.e., without a guest molecule). Without wanting to be bound by theory, it appears that when at least one guest molecule is included within the cucurbituril cavity, CO2 is adsorbed and stored towards the entrance of the cavity, rather than towards the hydrophobic centre of the cavity. As a result, CO2 is desorbed from a complex comprising cucurbituril and at least one guest molecule at lower temperatures than from cucurbituril alone.

[0021] In addition, the inventors have also found that the capture complexes of the present invention have suitably high thermal decomposition temperatures (i.e., higher than those required for adsorption and desorption) and therefore exhibit desirable thermal stability, confirming their suitability for use as a CO2 adsorbent in methods of CCLIS or DAC. Further, the inventors have also found that the capture complexes of the present invention are chemically stable and therefore may be capable of performing under a wide range of conditions. In particular, the inventors have found that the inclusion of at least one guest molecule within the CB[7] cavity increases the thermal decomposition temperature in comparison to the thermal decomposition temperature of CB[7] alone.

[0022] Further, the capture complexes of the present invention may exhibit a high tolerance for moisture. Thus, the capture complexes of the present invention may be heat and moisture stable.

[0023] The abovementioned benefits may allow the capture complex of the present invention to be recycled and reused, and accordingly, have a longer life-span (e.g., up to 10 years) than known prior art CO2 adsorbents.

[0024] The or each guest molecule may be non-covalently bound to the cavity of the cucurbituril compound. As described above, the cavity of cucurbituril compounds is hydrophobic. Therefore, the or each guest molecule may bind to the cavity of the cucurbituril compound through hydrophobic interactions with the cavity, thus forming a host-guest complex with the cucurbituril compound (i.e., a cucurbituril compound-guest complex). Other interactions which may contribute to the binding of guest molecules to the cavity of cucurbituril compounds are ionic bonds, hydrogen bonds and van der Waals forces.

[0025] The phrase bound to the cavity of the cucurbituril compound refers to the or each guest molecule being at least partially within, and therefore partially bound to the cavity of the cucurbituril compound. The skilled person would appreciate that the or each guest molecule may be fully encapsulated by the cavity of the cucurbituril compound, wherein the or each guest molecule is completely within the cavity, or partially encapsulated by the cavity of the cucurbituril compound whereby part of the or each guest molecule is within the cavity.

[0026] The number of repeating glycoluril or substituted glycoluril monomer units in the cucurbituril compound defines the diameter of the portal and volume of the cavity. Therefore, as the number of repeating units in the cucurbituril compound increases, the volume of the cavity and consequently the size of the hydrophobic region of the cucurbituril compound increases.

[0027] Preferably, the cucurbituril compound comprises 6, 7 or 8 glycoluril or substituted glycouril units. The inventors have found that cucurbiturils comprising 6, 7 and 8 repeating glycoluril units and at least one guest molecule are particularly effective at selectively and reversibly adsorbing CO2. The size and shape complementarity shown by complexes comprising cucurbiturils with 6, 7 and 8 repeating glycoluril units and at least one guest molecule may contribute to the selective and reversible adsorption of CO2.

[0028] The or each guest molecule for the capture complex of the present invention may be an aromatic compound or a compound that comprise an aromatic group.

[0029] The or each guest molecules for the capture complex of the present invention may be an amine or a compound that comprises an amine group.

[0030] The or each guest molecule for the capture complex of the present invention may be a positively charged compound or a compound that comprises a positive charge. The source of the positive charge may be from an acetyl benzene cation, an alkyl ammonium cation, a pyridinium cation such as methylpyridinium, an imidazolium cation, or a quaternary ammonium cation.

[0031] The or each guest molecules for the capture complex of the present invention may be able to selectively and reversibly absorb CO2 themselves (i.e., by chemical reaction with CO2) The inventors have found that the inclusion of a guest molecule that is able to selectively and reversibly absorb CO2 (i.e., by chemical reaction with CO2) such as an amine or a compound that comprises an amine group, the selectivity of the capture complex can be varied and adsorption performance may be increased compared to the adsorption performance of a guest which is unable to selectively and reversibly absorb CO2 themselves.

[0032] Preferably, the or each guest molecule is complementary in size and shape to the cavity of the cucurbituril compound.

[0033] Examples of suitable guest molecules include 1 ,6 hexamethylenediamine, a cyclohexylammonium ion, an imidazolium cation, a viologen (i.e., compounds with the formula (CsH4NR)2n+) such as methyl viologen, a 1-ethyl-3-methylimidazolium cation, DL- tyrosine, a 2,7-dimethyl diazaphenanthrenium cation, 3H-indole, 2,6-dihydroxynaphthalene or trans-azobenzene.

[0034] The viologen may have the general formula (III): wherein is R2and R3are each independently selected from the group comprising of (e.g., the group consisting of) optionally substituted -(Ci-Ce)alkyl, optionally substituted -(Ci- Ce)haloalkyl or optionally substituted aryl. For methyl viologen, R2and R3are CH3. X may be any anion, such as chloride.

[0035] The imidazolium cation may have the general formula (IV): wherein R4is selected from the group comprising of (e.g., the group consisting of) halogen, optionally substituted -(Ci-Ce)alkyl, optionally substituted -(Ci-C6)haloalkyl or optionally substituted aryl.

[0036] Preferably, the guest molecule is methyl viologen (MV) or an imidazolium cation. When the cucurbituril compound is a cucurbituril and the guest molecule is methyl viologen, the capture complex may be referred to as CB[n]MV, where n denotes the number of repeating glycoluril units. When the cucurbituril compound is a cucurbituril and the guest molecule is an imidazolium cation, the capture complex may be referred to as CB[n]-imidazolium, where n denotes the number of repeating glycoluril units. Advantageously, the inventors have found that imidazolium cations, which are chemically unstable and therefore are not typically used as adsorbents, are stabilised when included as a guest within the cavity of cucurbituril. Herein, the term imidazolium refers to the imidazolium cation.

[0037] Preferably, the cucurbituril compound comprises one guest molecule within the cavity. Therefore, the capture complex may be a binary complex. Alternatively, the cucurbituril compound may comprise at least two guests molecule within the cavity. Therefore, the capture complex may be a ternary complex. Preferably, when the capture complex comprises at least two guest molecules each guest molecule is different.

[0038] Preferably, when the cucurbituril compound comprises 6 or 7 repeating glycoluril or substituted glycouril monomers, for example CB[6] or CB[7], the complex comprises one guest molecule within the cavity. Examples of suitable guest molecules for CB[6] include 1 ,6 hexamethylenediamine, a cyclohexylammonium ion or an imidazolium cation. Examples of suitable guest molecules for CB[7] include methyl viologen (MV), an imidazolium cation, trans-azobenzene or DL-tyrosine.

[0039] When the cucurbituril compound comprises 8 repeating glycoluril or substituted glycouril monomers, for example CB[8], the complex may comprise at least two guest molecules, for example two guest molecules, within the cavity. Preferably, each guest molecule is different. Examples of suitable guest molecules for CB[8] include a 2,7-dimethyl diazaphenanthrenium cation, 3H-indole, 2,6-dihydroxynaphthalene or trans-azobenzene.

[0040] As used herein, the phrase CO2 desorption temperature means the temperature at which the adsorbed CO2 begins to desorb from the capture complex. Preferably, the capture complex is configured to release or desorb CO2 at temperatures of less than 100 °C, for example less than 90 °C, for example less than 80 °C, for example less than 70 °C, for example less than 60 °C, for example less than 50 °C, for example less than 40 °C.

[0041] The CO2 desorption temperature of the capture complex may be less than 100 °C, for example less than 90 °C, for example less than 80 °C, for example less than 70 °C, for example less than 60 °C, for example less than 50 °C, for example less than 40 °C. The CO2 desorption temperature of the capture complex may be between 30 and 100 °C, for example between 30 and 70 °C, for example between 35 and 60 °C.

[0042] As shown in the Figures and described below, the inventors have also found that the capture complexes of the present invention have a low CO2 desorption temperature in comparison to the adsorbents of the prior art, whereby the average CO2 desorption temperature is 120 °C. MEA, for instance, has a CO2 releasing temperature of 80-110 °C. Therefore, to release CO2, the capture complexes of the present invention require less energy on average than the current commercially available adsorbents. In particular, the inventors have found that CB[6]MV has a CO2 desorption temperature of 57 °C and the desorption process of CB[7]- imidazolium starts at around 30 °C and reaches peak desorption around 35 - 40 °C

[0043] Preferably, the capture complex is a solid (e.g., a powder). By providing the capture complex as a powder, it may provide a large surface area for contact with molecules of CO2 when used as an adsorbent for capturing CO2. Optionally, the capture complex may be in the form of a tablet. By providing the capture complex in tablet form, it may be more easily incorporated into any existing CO2 capture system for use as an adsorbent for capturing CO2. Alternatively, the capture complex is provided in a solution.

[0044] In a further aspect, the invention provides the use of a capture complex comprising a cucurbituril compound having the Formula (I): wherein n is 5, 6, 7, 8 or 10; and for each glycoluril unit, R1is selected from hydrogen, halogen, optionally substituted -(Ci-Ce)alkyl, -(Ci-C6)haloalkyl or -OH; and at least one guest molecule, wherein the complex is configured to selectively and reversibly adsorb CO2, as an adsorbent for capturing CO2.

[0045] As discussed above, CO2 is attracted to the capture complex. Without wanting to be bound by theory, it is believed that the strong attraction between cucurbituril compounds and CO2 draws CO2 to the cucurbituril compounds. When CO2 is brought into contact with, or contacts the capture complex, hydrogen bonds and / or dipole-quadrupole interactions may occur between the CO2 molecules and the capture complex, to thereby cause the CO2 to physically adsorb onto the surface of the capture complex. This may occur at specific binding sites, such as within the cavity of the cucurbituril compound which requires the CO2 molecule to enter the cavity through the portals. Therefore, CO2 molecules may be adsorbed into the cavity of the cucurbituril compound-guest complex. Provided the temperature remains below the CO2 desorption temperature of the capture complex, the adsorbed C02 will remain within the cavity of the cucurbituril compound-guest complex.

[0046] The capture complex may have any of the features described above in relation to the first aspect of the invention. The capture complex may be used as an adsorbent for capturing CO2 in any existing CO2 capture system. For example, the capture complex of the present invention may be used as an adsorbent in a method of Direct Air Capture (DAC) of CO2 gas from air. Alternatively, the capture complex of the present invention may be used as an adsorbent in a method of Carbon Capture and Storage (CCS) of CO2 gas from a point source, such as flue gas. Following the capture of CO2, the adsorbent comprising adsorbed CO2 may then be heated to release CO2 (for storage or use) and re-generate the adsorbent.

[0047] As discussed above, the capture complex of the present invention is able to selectively and reversibly adsorb CO2. Other advantages of the invention may include high CO2 adsorption capacity, low CO2 desorption temperatures, high thermal stability and high tolerance for moisture. These benefits may make the capture complex of the present invention more environmentally-friendly (due to the long life-span of the adsorbent), more energy-efficient and lower cost than prior art techniques.

[0048] In a further aspect, the invention provides a method for capturing CO2, comprising contacting a capture complex with CO2, such that the CO2 is adsorbed by the capture complex; wherein the capture complex comprises a cucurbituril compound having the Formula (I): wherein n is 5, 6, 7, 8 or 10; and for each glycoluril unit, R1is selected from hydrogen, halogen, optionally substituted -(Ci-Ce)alkyl, -(Ci-C6)haloalkyl or -OH; and at least one guest molecule, and wherein the complex is configured to selectively and reversibly adsorb CO2.

[0049] The capture complex may have any of the features described above in relation to the first aspect of the invention. Preferably, the step of contacting the capture complex with CO2 may take place at room temperature and at ambient pressure. Preferably, the method for capturing CO2 is a method for capturing CO2 gas. The method may be a method of Direct Air Capture (DAC) of CO2 gas from air. Alternatively, the method may be a method of Carbon Capture and Storage (CCS) from a point source, such as flue gas.

[0050] The step of contacting the capture complex with CO2 may be performed in a variety of known methods, for example by gas-solid contactor. Various designs are known in the art, such as fixed-bed reactors, any of which would be suitable for use with the present invention.

[0051] When CO2 is brought into contact with, or contacts the capture complex, hydrogen bonds and / or dipole-quadrupole interactions occur between the CO2 molecules and the capture complex. As a result, the CO2 physically adsorbs onto the surface of the capture complex. This may occur at specific binding sites, such as within the cucurbituril compound cavity which requires the CO2 molecule to enter the cavity via a portal. Therefore, CO2 molecules may be adsorbed into the cavity of the cucurbituril compound-guest complex. Provided the temperature remains below the CO2 desorption temperature of the capture complex, then the adsorbed CO2 will remain within the cavity of the capture complex.

[0052] Following the step of contacting the capture complex with CO2, the capture complex comprising the adsorbed CO2 may then be heated to release CO2, such that the CO2 desorbs from the capture complex, (which may then utilised or further processed for storage and / or transportation). The desorption of CO2 from the capture complex re-generates the capture complex, such that CO2 is no longer adsorbed in the capture complex. Thus, the regenerated capture complex may be used again in the method of capturing CO2.

[0053] Preferably, the capture complex comprising the adsorbed CO2 is heated to less than 100 °C, for example less than 90 °C, for example less than 80 °C, for example less than 70 °C, for example less than 60 °C. Preferably, the capture complex comprising the adsorbed CO2 is heated to between 50 and 100 °C, for example between 60 and 70 °C.

[0054] The method for capturing CO2 is able to selectively and reversibly adsorb CO2. Other advantages of the method may include high CO2 adsorption capacity and reduced energy consumption due to low CO2 desorption temperatures. These benefits may make the method of the present invention more environmentally-friendly, more energy-efficient and lower cost than prior art techniques.

[0055] In a further aspect, the invention provides an adsorbent for capturing CO2, comprising a capture complex comprising a cucurbituril compound having the Formula (I):

[0056] wherein n is 5, 6, 7, 8 or 10; and for each glycoluril unit, R1is selected from hydrogen, halogen, optionally substituted -(Ci-Ce)alkyl, -(Ci-C6)haloalkyl or -OH; and at least one guest molecule, wherein the complex is configured to selectively and reversibly adsorb CO2. The capture complex may have any of the features described above in relation to the first aspect of the invention.

[0057] In a further aspect, the invention provides a capture system comprising a gas-solid contactor containing a capture complex comprising a cucurbituril compound having the Formula (I): wherein n is 5, 6, 7, 8 or 10; and for each glycoluril unit, R1is selected from hydrogen, halogen, optionally substituted -(Ci-Ce)alkyl, -(Ci-C6)haloalkyl or -OH; and at least one guest molecule therein; wherein the complex is configured to selectively and reversibly adsorb CO2 and wherein the gas-solid contactor is configured to contact CO2 with the capture complex, so as to allow for the adsorption of CO2 by the capture complex. The capture complex may have any of the features described above in relation to the first aspect of the invention.

[0058] The gas-solid contactor may comprise a chamber having an upstream end and an opposing downstream end and containing the capture complex therein; and an air flow system configured to create an air flow from the upstream end to the downstream end of the chamber and draw air through the chamber and into contact with the capture complex therein.

[0059] The gas-solid contactor may comprise a means for heating the capture complex, so as to allow for the desorption of CO2 from the capture complex. The heating means may be any means suitable for heating a gas-solid contactor, for example a heating element.

[0060] The upstream end of the chamber may comprise an inlet for receiving a gas comprising CO2 into the chamber and the downstream end may comprise an outlet for the removal of gas from the chamber.

[0061] During use of the capture system, a gas comprising CO2 (e.g., air or flue gas) is introduced into the gas-solid contactor and brought into contact with the capture complex, for example by the use of a system which generates an air flow, such as a fan. However, it will be appreciated that, due to the strong attraction between cucurbituril compounds and CO2, CO2 will be naturally drawn to the capture complex. Therefore, the use of an air flow system, such as a fan, may merely enhance the air flow of CO2 over the capture system to maintain a high concentration of CO2 on the surface of the capture complex. This may, in turn, increase the rate of CO2 adsorption, and subsequently the speed at which CO2 is captured by the complex.

[0062] Therefore, the strong attraction between cucurbituril compounds and CO2 may reduce the dependency on air flow systems, which in turn may reduce the overall energy consumption of the capture system.

[0063] As the gas contacts the capture complex, CO2 is adsorbed by the capture complex, thereby directly capturing CO2 from the gas. The gas may then be free or substantially free of CO2 and may be removed from the contactor, for example by the use of a fan. After a defined volume of gas has been introduced into the gas-solid contactor or once the capture complex is saturated with CO2 and is unable to adsorb any more CO2, no further gas is introduced into the gas-solid contactor.

[0064] The capture complex may then be heated, for example by a heating element, so as to allow for the desorption of CO2 from the capture complex. The capture complex may be heated in the gas-solid contactor or it may be removed from the gas-solid contactor and heated elsewhere.

[0065] The system may be a Direct Air Capture (DAC) system of CO2. Alternatively, the system may be a Carbon Capture and Storage (CCS) system. Detailed Description

[0066] Specific embodiments of the invention will now be described by way of examples, with reference to the accompanying drawings, in which:

[0067] Figure 1a is a schematic representation of the synthesis of glycoluril and 1b is a 1 H NMR spectrum of the synthesised glycoluril;

[0068] Figure 2a is a schematic representation of the synthesis of CB[n] and 2b is a 1 H NMR spectrum of the crude CB[n] reaction mixture;

[0069] Figure 3a is the molecular structure of CB[6] and 3b is a 1 H NMR spectrum of the CB[6] solid;

[0070] Figure 4 is a schematic representation of the condensation reaction between glycoluril and formaldehyde and the resulting structure of cucurbiturils;

[0071] Figure 5 shows the CO2 adsorption isotherm data for a) CB[6], b) CB[6]MV, c) CB[7], d) CB[7]MV, e) CB[8] and f) CB[8]MV;

[0072] Figure 6 shows the CO2 desorption TPD data for a) CB[6], b) CB[6]MV, c) CB[8] and d) CB[8]MV;

[0073] Figure 7 shows the TGA / DSC data for a) CB[6], b) CB[6]MV, c) CB[7], d) CB[7]MV and e) CB[8]MV;

[0074] Figure 8 is a schematic process flow diagram illustrating a capture system of the invention;

[0075] Figure 9 shows the CO2 adsorption for CB[7]-imidazolium;

[0076] Figure 10 shows the CO2 desorption TPD data for CB[n]-imidazolium.

[0077] Definition of Terms

[0078] “(Ci-Ce)” means a carbon radical having 1 , 2, 3, 4, 5 or 6 carbon atoms.

[0079] In this specification, unless stated otherwise, the term “halo” or “halogen” may be fluoro, chloro, bromo or iodo.

[0080] In this specification, unless stated otherwise, the term “alkyl” includes both straight and branched chain alkyl radicals and may be methyl, ethyl, n-propyl, / -propyl, n-butyl, / -butyl, s- butyl, f-butyl, n-pentyl, / -pentyl, f-pentyl, neo-pentyl, n-hexyl, / -hexyl or t-hexyl. The term “optionally substituted” means an alkyl radical as defined above which is substituted.

[0081] In this specification, unless stated otherwise, the term “haloalkyl” means an alkyl radical as defined above, substituted with one or more halo radicals. The term “(Ci-C6)haloalkyl” may include, but is not limited to, fluoromethyl, difluoromethyl, trifluoromethyl, fluoroethyl and difluoroethyl.

[0082] In this specification, unless stated otherwise, the term “aryl” refers to an optionally substituted monocyclic or bicyclic hydrocarbon ring system containing at least one unsaturated aromatic ring.

[0083] Examples

[0084] Example 1 : Method of Synthesis

[0085] 1.1 Synthesis of Glycol u I

[0086] To synthesis glycoluril, the repeating monomer and starting material for CB[n] synthesis, 300 g of urea was dissolved in 500 mL water followed by 250 g of a 40% aqueous solution of glyoxal and 43 mL concentrated hydrochloric acid, HCI (12 M). This mixture was heated at approximately 87 °C until a heavy precipitate formed. After cooling to room temperature, the precipitate was separated by centrifugation, and washed with deionized water and acetone. The resulting white solid was dried under a high vacuum. A typical example is shown below in Figure 1a. The 1 H NMR spectrum of the synthesised glycoluril is measured and shown below in Figure 1b.

[0087] 1.2 Synthesis and Purification of Cucurbiturils

[0088] The synthesis and purification of the cucurbiturils is well known in the art. All are readily synthesised using standard techniques and are commercially available.

[0089] CB[n] was synthesised by condensation reaction between glycoluril and formaldehyde according to Figure 2a, based on the method described by Younas, M. et al.

[0090] 196 g of the glycoluril and 86 g of paraformaldehyde were mixed thoroughly. 260 mL of ice- cold concentrated HCI solution was added gradually while stirring. After approximately 100 mL of HCI was added, stirring was no longer possible as the reactants were transformed into a brick-like material. This heterogeneous mixture was then gradually heated to 80 °C for 2.5 hours and kept at 80 °C for an additional 2.5 hours to dissolve the solids homogeneously. The resultant red solution was further heated to 100 °C for 19 hours to precipitate the CB[n] mixture of CB[5], CB[6], CB[7] and CB[8],

[0091] Figure 2b shows the 1 H NMR spectrum of the synthesised CB[n] mixture, confirming the formation of CB[5], CB[6], CB[7], and CB[8],

[0092] 1.3 Purification of Cucurbiturils

[0093] The CB[n] is purified based on their different solubilities in different solvents. Specifically, the CB[6] was obtained by purifying the CB[n] based on the solubility difference in water and ethyl alcohol. 3 L of deionized water was added into the as-synthesised CB[n] mixture and stirred for 48 hours to dissolve CB[5] and CB[7] into the water, whereas CB[6] and CB[8] remains in the solid precipitate. To separate CB[6] from the CB[6] and CB[8] mixture, 4.5 L of 3 M HCI was added into the CB[6] and CB[8] mixture, and then the excess of ethyl alcohol was added. This process precipitates the CB[6] and the CB[8] dissolved in the solution. The obtained CB[6] was washed 3 times by ethyl alcohol followed by acetone and then recrystallized from concentrated HCI.

[0094] The chemical structure and the proton NMR of the CB[6] are shown in Figure 3a and 3b, respectively.

[0095] 1.4 Synthesis of CB[n]MV

[0096] The mass of MV required for a molar mass ration of 1 :1 CB[n]:MV is dissolved into 40 mL of water to form an aqueous solution. CB[n] is added into the solution. The MV sets into cavity of the CB[n], To distribute the MV molecules evenly, the water mixture was sonicated for 20 minutes. The CB[n]MV was extracted from the water as a white powder by freeze drying.

[0097] The white powder may then be further processed into a tablet by a compression process. By compression, 400 mg of CB[n] is able to form a tablet of thickness 0.3 mm.

[0098] 1.5 Synthesis of CB[n]-lmidazolium

[0099] The mass of imidazolium required for a molar mass ration of 1 :1 CB[n]:imidazolium is dissolved into 40 mL of water to form an aqueous solution. CB[n] is added into the solution. The imidazolium sets into cavity of the CB[n], To distribute the imidazolium molecules evenly, the water mixture was sonicated for 20 minutes. The CB[n]-imidazolium was extracted from the water as a white powder by freeze drying.

[0100] The white powder may then be further processed into a tablet by a compression process. By compression, 400 mg of CB[n] is able to form a tablet of thickness 0.3 mm.

[0101] Example 2: Performance of CBfnlMV as an Adsorbent

[0102] 2. 1 CO2 Adsorbent Testing

[0103] An experiment was carried out to test the CO2 adsorption capacity of CB[n]MV (n = 6, 7, 8) and CB[n] (n = 6, 7, 8).

[0104] The CO2 adsorption capacity was tested in a high vacuum physisorption / chemisorption analyser (purchased from Anton Paar). 50 mg of the CB[n] or CB[n]MV to be tested was degassed under vacuum with a temperature of 120 °C for at least 5 hours in a glass cell. The cell was then connected to the gas analyser and the CO2 capturing program was run. The temperature was set at 25 °C and controlled by a thermostat tank. The pressure was raised from 0 to 1 Torr. CO2 gas was continuously purged into the system until equilibrium was reached and the maximum gas adsorption value (cc / g) was observed. The method was repeated 3 times for each CB[n] or CB[n]MV.

[0105] As shown in Figure 5, the CO2 adsorption value for CB[6], CB[7] and CB[8] is reduced after MV has been introduced into the cavity of the CB[6], CB[7] and CB[8], This is due to MV taking up space inside the cavities of the CB[6], CB[7] and CB[8] which reduces the space available for CO2 molecules. As a result, less CO2 molecules can be adsorbed by the complex. However, as shown in Figure 6, CB[6]MV, CB[7]MV and CB[8]MV all still adsorb an effective amount of CO2, meeting industry requirements. Therefore, CO2 continues to be adsorbed by CB[n]s even after the introduction of the guest molecule. CB[7]MV and CB[8]MV are particularly effective at adsorbing CO2. The CO2 adsorption performance of CB[8]MV is

[0106] 4.2 mmol / g.

[0107] 2.2 CO2 Desorption Testing

[0108] An experiment was carried out to test the CO2 desorption performance of CB[n]MV (n = 6, 7, 8) and CB[n] (n = 6, 7, 8) by measuring the CO2 releasing temperature.

[0109] The CO2 desorption capacity was measured by temperature programmed desorption (TPD) using a ChemBET PULSAR ™ TPR / TPD (Anton Paar). A 500 mg sample of the CB[n] or CB[n]MV to be tested was degassed in the sample holder at 140 °C under room pressure (approximately 1 bar) for 4 hours. The machine and sample were then cooled to room temperature (approximately 20 °C) and pure CO2 was purged into the sample holder until full absorption was achieved. The TPD program was then started and the temperature of the sample gradually increased from 20 to 120 °C at a rate of 2 °C per minute. The sharp rise in the TPD signal indicates the desorption temperature.

[0110] As shown in Figure 6, CB[6]MV and CB[8]MV have a lower desorption temperature than CB[6] and CB[8], For CB[6]MV specifically, a desorption temperature of 57 °C is required to start the release of CO2 which is lower than most current commercially available adsorbents, which have an average desorption temperature of 120 °C. For CB[8]MV, desorption of CO2 starts at a temperature of 35 °C. Without wanting to be bound by theory, it appears that when MV is within the cavity of CB[n], the adsorbed CO2 is stored towards the entrance of the CB[n] cavity close to the portals, rather than towards the centre of the cavity. As a result, the CO2 is more easily desorbed from CB[n]MV than CB[n] at lower temperatures.

[0111] Example 3: Performance of CBRI-imidazolium as an Adsorbent

[0112] 3. 1 CO2 Adsorbent Testing

[0113] An experiment was carried out to test the CO2 adsorption capacity of CB[7]-imidazolium.

[0114] The CO2 adsorbent capacity was tested using the same method as in Example 2.1.

[0115] As illustrated in Figure 9, the maximum CO2 uptake for CB[7]-imidazolium under 1 bar pressure is 1.5 mmol / g, which is around the same level as amorphous CB[6] (1.7mmol / g).

[0116] Similarly to CB[n]MV, it is believed that the presence of imidazolium takes up space within the CB[n] cavity which reduces the space available for CO2 molecules. As a result, less CO2 molecules can be adsorbed by the complex. However, CB[7]-imidazolium still adsorbs an effective amount of CO2, meeting industry requirements. Therefore, CO2 continues to be adsorbed even after the introduction of a guest molecule into the CB[n] cavity.

[0117] 3.2 CO2 Desorption Testing

[0118] An experiment was carried out to test the CO2 desorption performance of CB[7]-imidazolium.

[0119] The CO2 desorption performance was tested using the same method as in Example 2.2. As shown in Figure 10, the CO2 desorption of CB[7]-imidazolium starts at around 30 °C and reach peak desorption around 35 - 40 °C. This desorption temperature is much lower than the desorption temperature of both CB[n] molecules (80 °C) and imidazolium (60 °C) themselves.

[0120] Applications

[0121] The inventors have found that the adsorbents of the present invention maintain CO2 adsorption performance as pressure increases whilst decreasing their adsorption performance towards other gases (such as N2). Accordingly, the adsorbents of the present invention may be particularly useful in pressure swing adsorption (PSA) applications. In addition, the adsorbents of the invention demonstrate selectivity for CO2 over N20).

[0122] Moreover, the inventors have found that the adsorbents of the invention are also able to maintain CO2 adsorption performance under pressure less than 1 bar, whilst decreasing their adsorption performance towards other gases at this pressure. Therefore, due to the high selectively under low pressure, the adsorbents of the present invention may be particularly useful in vacuum swing adsorption (VSA) applications.

[0123] Example 4: Thermal Stability of CBfnlMV

[0124] An experiment was carried out to test the thermal stability of CB[n]MV (n = 6, 7, 8) and CB[n] (n = 6, 7) by using a Thermogravimetric Analyzer combined with Differential scanning calorimetry (TGA / DSC) to measure the change of weight of the sample whilst the temperature is increased. For each test, the temperature was increased from room temperature up to 600 °C.

[0125] As shown in Figure 7, CB[6] is the most thermally stable compound, with a decomposition temperature of 480 °C. All CB[n] and CB[n]MV have a peak at around 105 °C equating to water loss. The introduction of MV into CB[7] increases the decomposition temperature from 330 °C to 400 °C. These results suggests that the capture complexes of the present invention have high thermal decomposition temperature, and are therefore exhibit sufficient stability, confirming their suitability for use as an adsorbent in CO2 capture.

[0126] Example 5: CO2 Adsorbent Results

[0127] The CO2 desorption temperatures of two known solid adsorbents that are used in DAC systems, are 109.85 °C and 99.85 °C, respectively. These desorption temperatures are considerably higher than for CB[6]MV. Therefore, in addition to high CO2 adsorption capacity, the capture complex of the present invention may be more energy-efficient and have lower maintenance costs than the prior art adsorbents.

[0128] Other frequently used adsorbents, such as MOFs MIL-101 and MOF-5, are effective at capturing CO2 due to the high surface area available for adsorption but have a high desorption temperature of around 135 °C. Further, these adsorbents are also moisture sensitive and unstable at these temperatures. Thus, it is difficult to scale up these MOFs for commercial applications. Accordingly, while the capture complexes of the present invention may have a slightly lower CO2 adsorption capacity than MOFs, they have the additional advantage of being energy-efficient (due to lower desorption temperatures) and therefore lower maintenance costs.

[0129] Further, amine solvents require an average temperature of around 100 °C to release CO2 after capture. Thus, the capture complex of the present invention has a lower adsorption temperature than amine solvents and is without loss or degradation.

[0130] The above examples demonstrate that the CO2 capturing complex of the present invention is stable, has high reproducibility and a long-life cycle as a solid adsorbent.

[0131] Example 6: Direct Air Capture (DAC)

[0132] The invention will be described below in the context of a portable Direct Air Capture (DAC) system for capturing CO2 gas from air. However, the skilled person will appreciate that the capture complex, use and method of capturing CO2 of the present invention may be also be applicable to other Carbon Capture Utilisation and Storage (CCUS) technologies that require a solid CO2 adsorbent, such as the capture of CO2 from a point source (e.g., flue gas). Further, the skilled person will appreciate that the capture complex, use and method of capturing CO2 of the present invention may be also be applicable to capturing CO2 which is in a solution. For example, CO2 which is aquated (i.e., CO2 (aq>).

[0133] As shown in Figure 8, a gas-solid contactor (101) is arranged to receive a flow of gas, which comprises CO2 and to bring the gas into contact with the adsorbent (102), which comprises the capture complex of the present invention. Fans (103) guide the flow of gas into the contactor at room temperature and ambient pressure. A variety of gas-solid contactor designs are known in the art, such as fixed-bed reactors, any of which would be suitable for use with the present invention. As the gas is brought into contact with, or contacts the adsorbent, CO2 is adsorbed by the absorbent, thereby directly capturing CO2 from the gas. The gas may then be free or substantially free of CO2 and is guided out of the contactor by the fans (103).

[0134] After a defined volume of gas has been introduced into the gas-solid contactor or once the adsorbents are saturated with CO2 or are unable to adsorb any more CO2, the flow of gas is stopped and the gas-solid contactor heated to a temperature of at least 57 °C by the heating element (104) so that the CO2 desorbs from the adsorbent and collected for storage or used directly. The absorbent is then regenerated and the process of capturing CO2 repeated with the recycled adsorbent.

Claims

Claims1 . A capture complex comprising a cucurbituril compound having the Formula (I):wherein n is 5, 6, 7, 8 or 10; and for each glycoluril unit, R1is selected from hydrogen, halogen, optionally substituted -(Ci-Ce)alkyl, optionally substituted -(Ci- Ce)haloalkyl or -OH; and at least one guest molecule, wherein the complex is configured to selectively and reversibly adsorb CO2.

2. The capture complex according to claim 1 , wherein the cucurbituril compound has the Formula (II):wherein n is 5, 6, 7, 8 or 10.

3. The capture complex according to claim 1 or claim 2, wherein the or each guest molecule is non-covalently bound to the cavity of the cucurbituril compound.

4. The capture complex according to any preceding claim, wherein the cucurbituril compound comprises 6, 7 or 8 glycoluril or substituted glycouril units.

5. The capture complex according to any preceding claim, wherein the or each guest molecule is an aromatic compound or a compound that comprises an aromatic group.

6. The capture complex according to any preceding claim, wherein the or each guest molecule is an amine or a compound that comprises an amine group.

7. The capture complex according to any preceding claim, wherein the or each guest molecule is positively charged or a compound that comprises a positive charge.

8. The capture complex according to any preceding claim, wherein the or each guest molecule comprises an acetyl benzene cation, an alkyl ammonium cation, a pyridinium cation such as methylpyridinium, an imidazolium cation or a quaternary ammonium cation.

9. The capture complex according to any preceding claim, wherein the or each guest molecule is 1 ,6 hexamethylenediamine, a cyclohexylammonium ion, an imidazolium cation, a viologen such as methyl viologen, a 1-ethyl-3-methylimidazolium cation, DL- tyrosine, a 2,7-dimethyl diazaphenanthrenium cation, 3H-indole, 2,6- dihydroxynaphthalene or trans-azobenzene.

10. The capture complex according to any preceding claim, wherein the complex comprises one guest molecule.

11. The capture complex according to any preceding claim, wherein the complex comprises at least two guest molecules and wherein each guest molecule is different.

12. The capture complex according to any preceding claim, wherein the CO2 desorption temperature of the complex is less than 100 °C.

13. Use of a capture complex according to any preceding claim as an adsorbent for capturing CO2.

14. A method for capturing CO2, comprising: contacting the capture complex according to any of claims 1 to 12 with CO2, such that the CO2 is adsorbed by the capture complex.

15. The method according to claim 14, wherein the step of contacting the capture complex with CO2 takes place at room temperature and at ambient pressure.

16. The method according to claim 14 or claim 15, wherein the method is a method of Direct Air Capture (DAC) of CO2 gas from air.

17. The method according to claim 14 or claim 15, wherein the method is a method of Carbon Capture and Storage (CCS) of CO2 gas from a point source.

18. The method according to any of claims 14 to 17, comprising a further step of heating the capture complex comprising the adsorbed CO2, such that the CO2 desorbs from the capture complex.

19. The method according to claim 18, wherein the capture complex comprising the adsorbed CO2 is heated to less than 100 °C.

20. An adsorbent for capturing CO2 comprising the capture complex according to any of claims 1 to 12.

21. A capture system comprising: a gas-solid contactor containing the capture complex according to any of claims 1 to 12 therein; wherein the gas-solid contactor is configured to contact CO2 with the capture complex, so as to allow for the adsorption of CO2 by the capture complex.

22. The capture system according to claim 21 wherein the gas-solid contactor comprises: a chamber having an upstream end and an opposing downstream end and containing the capture complex therein; and an air flow system configured to create an air flow from the upstream end to the downstream end of the chamber and draw air through the chamber and into contact with the capture complex therein.

23. The capture system according to claim 21 or claim 22 wherein the gas-solid contactor comprises a means for heating the capture complex, so as to allow for the desorption of CO2 from the capture complex.

24. The capture system according to any of claims 21 to 23 wherein the upstream end of the chamber comprises an inlet for receiving a gas comprising CO2 into the chamberand the downstream end comprising an outlet for the removal of gas from the chamber.

25. The capture system according to any of claims 21 to 24, wherein the system is a Direct Air Capture (DAC) system or a Carbon Capture and Storage (CCS) system.

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